No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
The Toughening of Alumina with Iron—Effects of Iron Distribution on Fracture Toughness
Abstract
Two composites have been fabricated by hot pressing powder blends of alumina with 20 volume percent of ductile iron particles. The composites differ in the shape, size and distribution of the iron particles. The fracture toughness of each composite has been obtained in situ, by testing inside a scanning electron microscope, using a double cantilever beam technique modified specifically for small ceramic specimens. Observation of the crack-particle interactions has enabled information to be gained about the toughening mechanisms occurring and hence the parameters for microstructural tailoring of these materials have been deduced. Results showed that the fracture toughness of the composites differed greatly due to the distribution of the iron throughout the microstructure, which in turn affected the type and degree of observed toughening mechanism. These material-specific toughening parameters were then used to fabricate a third alumina/iron composite with a more optimised fracture toughness.
... Among structural ceramics, alumina (Al 2 O 3 ) is appreciated for high stiffness, excellent thermal stability and relatively low density, but its extreme brittleness restricts this promising ceramic from many advanced structural applications, such as in military armour systems, as aircraft engine parts and in space engineering [1][2][3][4][5][6]. In order to overcome the brittleness issue, carbon nanotubes (CNTs) with extremely high strength and exceptional flexibility have inspired researchers to investigate the possibility of incorporating them into Al 2 O 3 . ...
... Several reports on the CNTs-reinforced Al 2 O 3 have demonstrated large enhancements in fracture toughness, hardness and other mechanical properties [7][8][9][10][11][12][13]. After years of rigorous research, the role of CNTs in Al 2 O 3 ceramics has been well-documented, such as hampering the densification process, refining the matrix by grain pinning, increasing the fracture toughness by crack bridging/crack deflection, improving the wear resistance by sliding/rolling, and improving the electrical/thermal properties [2][3][4][5][6][7][8][9][10][11][12][13][14][15]. In literatures, issues such as the homogenous dispersion of CNTs, achieving higher densities of the nanocomposites, understanding the toughening mechanism and Al 2 O 3 -CNT interfacial bonding have been intensively discussed, whilst the microstructural tuning is hardly attempted [7][8][9][10][11][12][13][14][15]. ...
... The weak grain bonding, large grain sizes, residual flaws and intergranular fracture mode are generally attributed to the low fracture toughness/strength of monolithic Al 2 O 3 . Our hot-pressed plain Al 2 O 3 samples also exhibited similar results (shown in Table 1), well in line with those literature reports [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. ...
Nanocomposites of Y2O3 doped Al2O3 reinforced with 2wt% MWCNTs were fabricated by hot-pressing. The influence of a small Y2O3 addition (300ppm) on the microstructure and mechanical properties of CNT-reinforced Al2O3 was investigated.The resulting Y2O3 doped Al2O3-CNT nanocomposites showed near theoretical densities(>99%) with an astounding 5 times finer grain size and substantial improvements in the fracture toughness(40%), flexural strength(20%) and hardness(18%) than pure Al2O3, prepared under identical experimental conditions. The Y2O3 addition improved the density of the matrix, eliminated the residual flaws and formed YAG (yttrium–aluminium–garnet) particles at the grain boundaries in the nanocomposites. The submicron YAG particles, together with the CNTs, promoted the Al2O3 matrix to form fine grained microstructures and altered the fracture mode, thus leading to higher toughness and other mechanical properties.The improved mechanical properties allows for the Y2O3 doped Al2O3-CNT nanocomposites to be used for applications in load bearing structures.
... Iron has been identified as a possible dopant for alumina substrates for such an application. Fe doping has been reported to decrease the transformation temperature of the amorphous alumina g phase to the crystalline a alumina [1], to possibly improve alumina-metal adhesion [2] and to improve the alumina's fracture toughness by means of a crack-bridging mechanism due to the presence of a ductile iron phase [1,[3][4][5]. However, the presence of a ductile iron phase may compromise the substrate's electrical insulating properties. ...
... It is well known that the presence of a second phase can improve the fracture toughness of a ceramic [20]. The presence of a ductile phase, in this case Fe, could induce strengthening by means of a crack-bridging mechanism [4]. For this mechanism to work, a sufficient quantity of the ductile phase of Fe metal must be available after sintering. ...
We present a method to improve the overall properties of sintered alumina substrate via iron doping that has a higher fracture toughness, lower thermal conductivity, lower thermal expansion coefficient and comparable dielectric constant than pure alumina. Such properties are beneficial for harsh environment electronic packages. X-ray and electron probe microanalyses concluded that toughening is likely caused by multiple phase strengthening and ruled out crack bridging or grain boundary strengthening.
... Studies on the preparation and characterization of aluminum oxide (Al 2 O 3 ) nanocomposites to form metal matrix composites have been attracting scientific interest because of the nanocomposites possessing special physical, chemical, mechanical and magnetic properties [2][3][4][5][6]. The techniques used in the synthesis of nanoparticles also play a remarkable role on the properties of the nanocomposites, which in turn have a significant impact on their applications [5,[7][8][9]. ...
The finite size effect of CuO nanoparticles and their effect on the magnetic behavior of a CuO–Al 2 O 3 nanocomposite synthesized via a solution combustion method are discussed here. It is observed that the introduction of CuO nanoparticles of ∼20 nm in size on the surfaces of Al 2 O 3 nanosheets strongly influences the magnetic moment of the nanocomposite. The X-ray diffraction pattern reveals the presence of a CuO phase in the CuO–Al 2 O 3 nanocomposite. Electron microscopy imaging shows the presence of CuO nanoparticles distributed over the surfaces of Al 2 O 3 nanosheets. Magnetic measurements performed at 5 and 300 K revealed a strong enhancement in the saturation magnetization of the CuO–Al 2 O 3 nanocomposite at both temperatures. This increase is due to the formation of CuO nanoparticles on the surfaces of the Al 2 O 3 nanosheets. These CuO nanoparticles show a ferromagnetic nature due to the uncompensated surface Cu ²⁺ spins of the copper ions. The presence of the Cu ²⁺ state in the CuO nanoparticles in the CuO–Al 2 O 3 nanocomposite was confirmed by X-ray photoelectron spectroscopy (XPS) measurements.
... The increase in the magnitude of toughness is directly proportional to the deformation of the ductile particle. It spans the face of a propagating crack and imposes closure tractions which reduces the intensity of stress at the tip of crack (Trusty and Yeomans, 1997). The influence of the reinforcing phase is directly linked to its volume fraction and the geometrical parameters such as reinforcement size, shape, orientation and distribution (Li et al., 2003). ...
... Some studies were done on alumina-zirconia composites [5][6][7], alumina-SiC composites [8][9][10] and many other alumina-ceramic composites. In addition to this, some studies have focused on alumina-metal composites such as alumina-copper composites [11][12][13][14], alumina-silver composites [15][16], alumina-tungsten composites [17], alumina-iron composites [18], alumina-molybdenum composites [19], alumina-chromium composites [20][21][22], alumina-cobalt composites [23], and alumina-nickel composites [1][2][3][4], however alumina-metal solid solution is comparatively less investigated. ...
Alumina-spinel(NiAl 2 O 4 , CoAl 2 O 4 solid solution)-metal(nickel, cobalt solid solution) composites were fabricated by partial reduction of spinel in carbon-bed and sintered with three different processes namely, pressureless, hot pressing, and spark plasma sintering (SPS). The microstructural features and mechanical properties of composites were investigated. The pressureless samples, SPS samples, and hot pressed samples reached > 91%, > 97% and > 98% theoretical density, respectively. The flexural strength of SPS, hot pressed, and pressureless samples were about 415 MPa, 367 MPa, and 247 MPa, respectively. Vickers Microhardness of SPS, hot pressed, and pressureless sintering were about 15.3, 14.5, and 10.98 GPa, respectively. The fracture toughness of SPS and hot pressed samples did not have a significant difference, and they were about 7.2 and 7.8 MPa.m 1/2 , repectively.
... In the synthesis of the iron-based binder, metallic iron powder is carbonated only to a small fraction (necessitated by limitations in reaction kinetics [12]), which results in the presence of large amounts of residual metallic powder in the microstructure. The presence of this phase, a significant fraction of which is elongated, will likely render notable increase in the toughness of this binder because of the energy dissipation by plastic deformation [13] imparted by the metallic particulate phase. In addition, the matrix contains other processing additives including harder fly ash particles, softer limestone particles, and ductile clayey phases which influence the overall fracture performance of the novel binder significantly. ...
... Alumina is a kind of material widely used in industry, but its application is constricted by its brittleness. Motivated by the potential for use of metal-ceramic composites as structural and functional materials, numerous studies on metal-ceramics have been performed such as the nickel-toughenedalumina [1,2], the iron-toughened-alumina [3,4], the silver-toughened-alumina [5], the molybdenumtoughened-alumina [6,7], etc. ...
... Wider applications of ceramics is limited by their low fracture toughness. A method to improved the fracture toughness is to introduce a ductile metallic phase into the ceramic matrix [1][2][3]. Such novel ceramic -metal composites find a range of applications, e.g. as materials for energy technology or auto mobile industry. ...
The present paper is focused on ceramic–metal composites obtained via different
technologies which leads to different microstructures in terms of size and distribution of metal phase. Composites analysed in paper were produced by the following methods:(a) infiltration of porous ceramics by metal, (b) consolidation under high pressure and (c) sintering of ceramic powder coated by metal. Their microstructures were investigated by scanning and transmission electron microscopy methods. The three methods of composite fabrication employed in the present study result in specific spatial distribution and dispersion of metal phase. Presureless infiltration of porous ceramics by liquid metal is driven by capillary force and make it possible to produce microstructure with percolation of metal phase in ceramic matrix. The volume fraction of metal phase in this case depends on the size
of pores. The size of pores influence also the kinetics and extent of infiltration. Ceramic preforms with small size of pore are not fully infiltrated. This method is useful for composite with size of metal phase in the range of micrometers. Hot pressing under high pressure produces microstructures of composites with metal phase grain size in the range from nano to micrometers. Moreover, it allows to achieve the nanometric size of ceramic grains. In the case of ceramic powders covered by metal, compression and hot pressing preserves nanometric size of metal. The grain growth of ceramic grains is suppressed.
... Combining two different materials in one homogeneous composite results in obtaining a material with compromising properties. For example, addition of metal phase to ceramics resulted in increasing of fracture toughness and simultaneously in decreasing of hardness and Young modulus [1]. ...
This paper describes the technology and microstructure of Al2O3-Fe functionally graded composites, FGM, obtained by slip-casting under magnetic field. Alumina a-Al2O3, provided by Alcoa (symbol A16SG), with average grain size of 0.5 µm, and iron powder, (symbol Distaloy AB) from Hoganas, with average grain size of 35 µm, were used to produce a series of specimens which differed in contents of Fe particles in Al2O3. As a source of magnetic force a permanent magnet was used. Preforms were sintered in a vacuum at temp. 1470oC. The microstructures of the specimens were quantitatively described via stereological methods. Sections, parallel to the magnetic field lines were analyzed using special image analysis software. Stereological methods presented in this work have been used to determine gradient in the volume fraction of the Fe particles and variation in their size and dispersion. These parameters are essential for controlling the technological process of interest and to design microstructure for needed properties (fracture toughness).
... Much of this research focuses on incorporating one or more reinforcing phase(s) into an alumina matrix because the applications for monolithic alumina are limited by its low fracture toughness, brittle fracture behavior, and poor sinterability [1]. Various types of relatively soft metallic particles such as Ni, Fe, Al, Cu, Mo, and Nb have been added to alumina to improve its ductility [2][3][4][5][6][7]; on the other hand, alumina-based ceramic composites can be reinforced with hard secondary phases such as carbides, borides, and nitrides to produce novel ceramic materials with high strength and toughness for use in cutting tools [1,[8][9][10][11]. ...
Al2O3–TiB2 composites were synthesized using high-energy ball milling from starting powders containing Al, TiO2, and B2O3. To explore the effect of the addition of another ductile metallic phase during milling, 15 wt.% Ni was added to a sample of the starting powders. The phase transformations and microstructure of the milled powder mixtures were investigated using X-ray diffraction and electron microscopy. The results showed that the Ni addition facilitated the mechanochemical reaction between the Al, TiO2, and B2O3. Before the appearance of the Al2O3–TiB2 composite, the intermediate product NiAl was formed by a gradual exothermic reaction. With continued milling, the final phases of Al2O3–TiB2 and Ni were obtained.
... However, slow crack growth resulted in a failure of alumina ceramic component with time in service [3]. In order to increase the toughness of alumina which has been reported as approximately 3 MPa·m 1/2 in many studies, many techniques have been investigated based on the addition of fibers, whiskers and hard particulates [4,5]. ...
Al2O3 and Al2O3–YSZ composites containing 3 and 5 wt.% TiO2 were prepared by spark plasma sintering at temperatures
of 1350–1400 °C for 300 s under a pressure of 40MPa. The grain growth of aluminawas suppressed by
the addition of YSZ. Al2O3–YSZ composites showed higher hardness than monolithic Al2O3. There was not a considerable
difference in hardness values for Al2O3–YSZ composites containing 10 and 20 vol.% YSZ and the addition
of TiO2 decreased the hardness of the composites. The fracture toughness of Al2O3 increased from
2.8 MPa·m1/2 to 4.3 MPa·m1/2 with the addition of 10 vol.% YSZ, further addition resulted in higher fracture
toughness values. The fracture toughness valueswere increasedwith TiO2 addition and the highest value of fracture
toughness, 5.3 MPa·m1/2, was achieved with the addition of 20 vol.% YSZ and 5 wt.% TiO2. Preliminary
in vivo tests demonstrated the biocompatibility and osseointegration of the composites after 6 week postimplantation
in femur of Wistar rats.
... This was possible because the toughening mechanism activated in the Al 2 O 3 -Mo nanocomposite is not crack bridging or plastic deformation (due to the small size of the Mo nanoparticles), but it is a result of the stresses generated by the differential thermal expansion between alumina and Mo. Additionally, Trusty et al. [17] Some authors added intermetallic elements to achieve a combination of specific properties such as high ductility and strength. Sglavo et al. [18] fabricated an Al 2 O 3 -Ni 3 Al nanocomposite and reported that the fracture toughness was about 7 MPam½ at room temperature for a 10 vol% composite hot pressed at 1350 o C. Gong et al. [19] prepared an Al 2 O 3 -5 vol% Fe 3 Al nanocomposite and reported that the bending strength and the fracture toughness were 832 MPa and 7.34 MPam½ respectively. ...
Alumina has received considerable attention and has been historically well-accepted as biomaterials for dental and medical applications. This article reviews the applications of this material in dentistry. It presents a brief history, dental applications and methods for improving the mechanical properties of alumina-based materials. It also offers perspectives on recent research aimed at the further development of alumina for clinical uses, at their evaluation and selection, and very importantly, their clinical performance. This article also stated about the Functionally Graded Materials (FGMs) which has been conceived as a new material design approach to improve performance compared to traditional homogeneous and uniform materials. This technique allows the production of a material with very different characteristics within the same material at various interfaces. The importance of the FGM concept in biological applications and functions was highlighted. Fundamentally, the combination of mechanical properties and biocompatibility are very important factors in application of any biomaterial to medical or dental fields. The characteristics of the surface govern the biocompatibility of the material, and the mechanical strength is determined by the average mechanical strength of the materials. However, the fabrication of FGMs is most often hindered by the variation of elastic, plastic, thermal, chemical, and kinetic properties within the composite. Across a material interface, these discontinuities in material properties lead to the formation of residual stresses. Despite these challenges, compositional gradient structures offer significant benefits. Notable research literature is highlighted regarding (1) applications of alumina in various fields in dentistry; (2) improvement of the mechanical properties of alumina by microstructural manipulation, FGM as well as composite formulations involving metallic, intermetallic elements and bioceramics.
... Al 2 O 3 is widely used as a ceramic material because of its low cost, high strength, and relatively large coefficient of thermal expansion (CTE) that makes it suitable for use with metals. Various types of dispersant powder such as Ni, Fe, Al, Ag, Cu, W, Mo, Pt, and Nb have been used with Al 2 O 3 to improve its ductility [5][6][7][8][9][10][11][12][13]. Ni is a good choice for metal-ceramic composite fabrication because it is highly ductile, thermally stable at high temperature, and corrosion resistant. ...
The effect of sintering profiles on densification of nanosized Ni/Al2O3 composite was investigated. Ni powder (100-nm average grain diameter) was combined with Al2O3 powder in a ratio of 60:40 by weight (40.1:59.9 by volume) and sintered for 3 h at 1350 °C. To observe the variation in sintering behavior of the Ni powder in the composite, two different H2 reduction profiles were used: H2 gas added at 700 °C for 2 h (Profile #1) during sintering and H2 gas added from room temperature to 1000 °C with no holding time (Profile #2). The microstructural changes in the Ni/Al2O3 composite, which consisted mainly of Ni dispersed in an Al2O3 matrix, were observed for these two sintering profiles. For Profile #1, incomplete sintering of the Al2O3 matrix was observed due to the extensive growth of Ni dispersants, and the overall dispersion of nanosized Ni powder was poor but coarsened. In contrast, in the composite sintered using Profile #2, the overgrowth of Ni dispersants was reduced, the sintering of the Al2O3 matrix was much more complete, and the density was 10% greater. In addition, a large difference in the Vickers’ hardness values between those two samples was measured. The significant difference in density differences for those two composites is possibly due to the rearrangement of Al2O3 powders caused by the overgrowth of Ni powders, and the increase in the non-contact area of the Al2O3 powders during the coarsening of Ni dispersion that resulted in a less dispersed and more porous sample. This illustrates that the sintering profile dramatically controls the density and the hardness of composites formed from nanosized metal powders.
... However, the application of Al 2 O 3 ceramics as engineering parts is handicapped by their brittleness. Numerous studies have focused on methods to reinforce alumina with secondary phases in order to improve specific mechanical properties [1][2][3][4][5], where the reinforcing phase takes the form of particles, fibers, whiskers or laminates. ...
Al2O3/Ni composite ceramics toughened by metallic particles was fabricated by the vacuum reduction of Al2O3 and nickel nitrate (Ni(NO3)2·6H2O) mixed powders at 800-1000° for 2-4h and hot pressed at 1400° ∼ 1500°C for 1-2h. Three-point bending strength and facture toughness of the composite were studied. With the increase of Ni content, its flexural strength and facture toughness increases significantly. Microstuctural investigations of the composite revealed that fine nickel particles dispersed homogeneously at the matrix grain boundaries, forming the intergranular nanocomposite.
... reported. [5][6][7][8][9][10][11][12][13] Recent advances in nanotechnology have emerged along with numerous new nanomaterials possessing extraordinary properties. It is postulated that these nanomaterials could be incorporated into brittle ceramics to generate highly toughened composites that will be suitable for advanced engineering applications. ...
This paper describes the mechanical properties of carbon nanotube-reinforced Al2O3 nanocomposites fabricated by hot-pressing. The results showed that compared with monolithic Al2O3 the fracture toughness, hardness and flexural strength of the nanocomposites were improved by 94%, 13% and 6.4% respectively, at 4vol.% CNT additions. For 10vol.% CNT additions, with the exception of the fracture toughness, which was improved by 66%, a decrease in mechanical properties was observed when compared with those for monolithic Al2O3. The toughening mechanism is discussed, which is due to the uniform dispersion of CNTs within the matrix, adequate densification, and proper CNT/matrix interfacial connections.
... and mullite (with Mo 15 ). With alumina, nickel [16][17][18][19][20][21][22][23][24] , silver [25][26][27][28][29] , molybdenum 30-32 , copper 33,34 , iron [35][36][37] and to a lesser extent niobium 32,38 , chromium 39 and chromium-nickel 39 alloys were popular choices for the metal inclusions, as they offered the dual benefits of favourable properties and compatibility with the matrix. ...
This review is concerned with ductile particle ceramic matrix composites, which are a group of materials comprising micro- or nano-scale metallic particles in a ceramic matrix. The most studied materials have an alumina matrix; nickel, iron, molybdenum, copper, and silver are some of the more frequently used metals. In contrast to conventional cermets and composites containing an interconnected metallic phase, the particles are discrete. The larger particles provide a toughening increment by deforming plastically and bridging an advancing crack. For the nanoscale composites significant improvements in strength have been reported. Improvements in strength and toughness, coupled with changes to elastic properties and thermal conductivity, have led to improved thermal shock resistance and a consideration of these materials for wear applications.
... Mechanical properties of structural ceramics can be improved usually by adding second reinforcements , such as brittle particles, whisker, fiber, and metallic particles. Previous studies indicated that the toughening improvement by a dispersed addition was based on several mechanisms, such as transformation toughening [1], crack deflection [2], thermal residual stresses [3,4] , microcrack forma- tion [5], debonding [6], pull-out [7], crack bridging [8], as well as plastic deformation of the metallic particles [9,10]. Multiple toughening mechanisms usually operate in second phase reinforced and toughened ceramics [11,12] . ...
Coexisting LiTaO3/Al2O3 ceramic composite was hot pressed at 1573 K. The highest values of flexural strength and fracture toughness reached 492.9 MPa and 5.3 MPam1/2, respectively, for the composite containing 15 vol.% LiTaO3. Domain switching which was clearly discerned near the crack tip in LiTaO3 grains is suggested as a new toughening mechanism in structural ceramics.
... In recent years metal and metal-oxide-ceramic composites have been researched extensively due to their unusual properties and application potentials, but past research mostly focused on their characterization as structural materials [1][2][3]. With the development of material science, more and more attention is being paid to improve ceramic's performance both as a structural material and as a functional material. ...
Fe2O3–Al2O3 nano-composites were synthesized by sol–gel means. The properties of samples sintered at various thermal treatment temperatures were investigated by X-ray diffraction (XRD) and Mössbauer spectroscopy (MS). The experimental results show that the γ- to α-Al2O3 transformation occurs at lower temperature after iron oxide doping. The samples obtained at 1173K contained poorly crystallized γ-Al2O3 phase and an amorphous iron oxide. When the temperature of heating was increased to 1373K, the sample was composed of α-Fe2O3/α-Al2O3 nano-composite and some solid solution. A superparamagnetic phenomenon was observed until the thermal treatment temperature reached 1373K.
Ceramics are used in ballistic protection applications for their high hardness and compressive strength; however, they are brittle and shock-sensitive. Co-continuous ceramic composite (C4) infused with a ductile phase can compensate for the brittleness of the ceramic and increase energy absorption with a potential weight reduction in comparison with present multilayer armours. Nevertheless, the research on C4 for armour applications is in its infant stage. This article aims to investigate the energy absorption characteristics of C4 composites with ballistic grade Al5083 infiltrated into different volume fractions of SiC foam (10PPI, 20PPI and 30PPI). The infiltrated C4 samples have 13, 15 and 18 volume percent of SiC in 10PPI-C4, 20PPI-C4 and 30PPI-C4 respectively. The uniaxial compression test of C4 samples revealed that 30PPI-C4 samples have 135 times high compression strength than ceramic preforms and hardness of C4 also enhanced by 13% for 30PPI-C4 samples. The gas gun impact and Split Hopkinson Pressure Bar (SHPB) tests on the 30PPI-C4 specimens reveals that their specific energy absorption (SEA) are 78% and 30% higher than those of 10PPI-C4 and 20PPI-C4 respectively.
The application of additive manufacturing using liquid material extrusion is inherently linked to material-related deformations and limitations in the choice of component geometry. This empirical study addresses the question of how the plasticity of a ceramic composite material can be utilized for a new integrated design and manufacturing process. In the exploratory approach, the liquid material is not limited in its soft plastic state, but its malleability is harnessed for a design-oriented approach. For this purpose, soft magnetic particles are added to a liquid clay mass. The developed composite material can be controlled, stabilized, and shaped by magnetic fields directly in the additive manufacturing process using modified equipment. In this study a permanent magnet and an electromagnet were compared while the distance between the printed part and the magnet was controlled by an optical sensor.
Ceramic Matrix Composites (CMCs) have many interesting properties, mainly light weight, cost efficiency, low density, high compressive strength, high hardness and durability. Hence, they emerged as a boon to the development of personnel armors in the past. The current work aims to review various new methodologies adapted for the reinforcement of Alumina (Al2O3) CMCs in recent times, including some of the interesting results obtained with respect to mechanical properties, suitability of the synthesized composites for armor applications, and the upcoming reinforcement trends. Finally, studies related to reinforcement in Al2O3 CMCs, specifically towards armor applications have been consolidated to arrive at some of the important inferences for concluding reasonably.
Alumina (Al2O3) ceramics have today become a multibillion dollar global industry which has changed the world in the last few decades. Alumina uniquely combines low cost with extreme hardness, extreme electrical resistivity, extreme corrosion resistance, and high refractoriness, and it is the most biocompatible material in current clinical use. Of the 1.3 million hip replacements implanted annually in the global $7 Billion hip replacement industry, 55% now use alumina or zirconia-toughened alumina (ZTA) bearings. A decade ago, alumina or ZTA bearings were rare and still considered experimental. This meteoric rise has led to strong interest in ceramic hip resurfacing and a ceramic knee, a field currently solely serviced by metal bearings. This is not possible with alumina, and is right at the limit of what is possible with ZTA. A quantum leap in toughness could bring alumina ceramic hip resurfacing and an alumina ceramic knee to the mass market. As discussed in Chapter 2, metal microfiber reinforcement can provide this quantum leap in toughness as it can produce up a 600-fold toughness enhancement, and commonly 100-fold or more (two orders of magnitude) increase in work of fracture over the parent ceramic. Zirconia toughening of alumina can only give a threefold work of fracture enhancement of alumina. Alumina is the number one wear-resistant ceramic used in wear-resistant linings in the global $500 Billion mining industry. For high impact applications, such as jaw crushers and heavy-duty ore-chute liners, the tough cermet tungsten carbide (Chapter 8) is the material of choice, but it is more than four times more expensive than alumina. Metal microfiber-reinforced alumina could potentially compete with tungsten carbide in this role. However metal fiber-reinforced SiC composite technology developed by author Ruys, and discussed in Chapter 4, is a much lower cost high-impact wear-resistant ceramic technology, much better suited to competing with tungsten carbide. This chapter outlines the background to alumina bearings in orthopedics and alumina in the mining industry. This is followed by a comprehensive literature review of alumina ceramic matrix composites. The chapter then concludes with an overview of research done by the authors on metal microfiber-reinforced alumina as a biomaterial, and a synopsis of the proposed manufacturing process for such a material.
The cathodic exfoliation of graphite in molten salts can be considered as a low-cost and efficient method for the scalable production of carbon nanostructures with various applications including anode materials for Li ion batteries, supercapacitors, ceramic-based composites and adsorbents for removal of organic pollutants. Various nanostructured carbon materials, such as molten salt-produced graphene nanosheets decorated with SnO2 nanocrystals, Sn-filled carbon nanostructures and graphene-wrapped Si nanoparticles can be fabricated for the application as active materials for lithium-ion batteries. As another application, interconnected graphene nanostructures comprising of nanosheets and nanoscrolls were found to exhibit an excellent performance in electrochemical supercapacitor studies. Moreover, slip cast alumina ceramics containing a low amount of molten salt graphene demonstrated higher values of mechanical properties in comparison with that of bare alumina. As another example, 3D graphene nanosheets produced in molten salts exhibit a high dye adsorption performance in a wide range of the solution pH from 2 to 11. The current chapter reviews these applications.
In order to improve the toughness of the Ni-P coatings, Ti particles were introduced into the plating solution to produce Ni-P-Ti composite coating (on API-X100 steel), which is subsequently annealed to form superelastic NiTi precipitates within the coating. Effect of Ti content on deposition rate and surface morphology of Ni-P-Ti coatings has been studied. A detailed investigation of the influence of Ti content and annealing on microstructure, microhardness and scratch behavior of Ni-P-Ti composite coatings was carried out. Further, scratch resistance as well as toughening mechanisms, namely, crack deflection, crack bridging and crack shielding were discussed. The NiTi precipitates greatly improve the cracking and scratch resistance as a result of superelastic NiTi toughening effect.
Les composites alumine-chrome sont obtenus par pressage à chaud de poudres provenant du broyage réactif entre des poudres d'aluminium, d'alumine et d'oxydes de chrome. Différents types de composites ont été caractérisés : ils correspondent à des fractions volumiques de chrome comprises entre 5 et 36% et différentes voies d'élaboration. Une étude microstructurale a été menée pour mieux connaître les différents constituants et les différentes phases des poudres mais aussi des échantillons massifs (notamment ceux qui sont minoritaires). Une approche micromécanique est utilisée pour déterminer le champ de contraintes internes, générées lors du refroidissement après le pressage des poudres, du fait des différences existant entre les coefficients de dilatation thermique et les propriétés thermo-mécaniques des phases. Ces différences et ces contraintes pouvant conduire à la détérioration et à la fissuration du composite pourraient peut-être aussi être utilisées avec avantage et renforcer le composite, du moins localement selon les tailles et dispositions relatives de la phase métallique. Les résultats des calculs par éléments finis ont été confrontés aux résultats expérimentaux de détermination des contraintes moyennes dans les phases, obtenus en utilisant le rayonnement synchrotron. Pour évaluer le comportement de la microstructure et des contraintes internes vis à vis de la fissuration, une procédure d'indentation de type hertzien (indentation d'un échantillon plan par une sphère) a été mise en place pour analyser le mode de fissuration des échantillons massifs. Les résultats issus de l'étude de la microstructure, des calculs et de la fissuration sont comparées pour analyser l'influence de la microstructure et des champs locaux de contraintes sur le comportement et les trajectoires des fissures dans des composites biphasés alumine-chrome. Des voies concrètes d'amélioration voire d'optimisation des microstructures au regard de la résistance à la fissuration en sont ainsi dégagées.
Des poudres composites alumine-métal (Fe, Cr, Ni, Nb, V… ) ont été élaborées par mécanosynthèse dans le but de réaliser ultérieurement des matériaux denses et massifs. Ces produits, à grains nanocristallins, sont obtenus par broyage réactif à haute énergie d'un mélange de poudres d'aluminium et d'oxydes métalliques. Un large domaine de composition (de 0 à 36% en volume) de la phase métallique (fer ou chrome) est exploré. Deux voies distinctes de mise en forme et de consolidation ont été suivies (pressage uniaxial à chaud sur poudres sèches et suspensions filtre-pressées). La mise en place progressive de la microstructure lors de la densification des poudres réactives et ayant complètement réagi a été étudiée. Des hypothèses sur les mécanismes qui régissent l'évolution de la microstructure ont été émises. Les propriétés physiques, électriques et mécaniques des différents cermets obtenus ont été mesurées. Le renforcement par différents mécanismes de la matrice d'alumine par la seconde phase métallique est mis en évidence ainsi que l'importance de l'interface dans ces composites et sa possible modification par l'addition d'élément d'addition comme le titane.
In the current work, ceramic-matrix composites based on coarse-grained alumina with additions of magnesia-partially stabilized zirconia and/or fine-grained TRIP steel have been prepared via pressure slip casting. The slip development is described in detail, covering additive selection and recipe formulation. Rheological properties and filtration behavior were investigated in a falling sphere viscometer and in a compression permeability filtration (CPF) cell, respectively. The results from laboratory scale were successfully transferred to a commercial pressure slip casting device. The casted samples were pressureless-sintered at 1450°C in an inert gas atmosphere. The distribution of the steel within the matrix was studied using X-ray tomography. Additionally, the formation of an interface between alumina and steel was evaluated by EBSD analysis. The reinforcing phases changed the mechanical and thermal properties of the material and therefore had a positive effect on the thermal shock performance. In particular, the presence of both, zirconia and 16-7-3 steel particles, resulted in a higher R parameter. The main responsible mechanism for this improvement was found to be crack deflection.
Highly crystalline graphene nanosheets were reproducibly generated by the electrochemical exfoliation of graphite electrodes in molten LiCl containing protons. The graphene product has been successfully applied in several applications. This paper discusses the effect of molten salt produced graphene on the microstructure and mechanical properties of alumina articles produced by slip casting and pressureless sintering, wich is one of the most convenient methods used for the commercial production of alumina ceramics. Apart from graphene, graphite powder and multi-walled carbon nanotubes (CNTs) were also used in the preparation for comparison. The strengthening effect of graphene was realized by the microstructural refinement and by influencing the formation of alumina nanorods during sintering of α-Al2O3 articles. The fracture toughness of the sintered alumina articles increased to an impressive value of 6.98 MPa m1/2 by adding 0.5wt% graphene nanosheets. It was attributed to the unique microstructure obtained comprising of micrometer sized alumina grains separated by alumina nanorods.
Alumina, like ceramics in general, exhibits good wear properties, but its uses are limited due to its brittleness. Experiments have been carried out in order to improve the ductility and toughness of alumina through powder metallurgy, by dispersing stainless steel powder in an alumina matrix. Sintering of alumina with a stainless steel (316L) has been studied, using parameters such as the forming pressure, thermal cycle, steel volumic fraction in the composite and binders removing atmosphere (air or argon with 10 percent of hydrogen). Experiments have shown that the residual carbon content from binders has a great impact on both microstructures (macro segregation of chromium) and final density of the material. Dilatometry has been used to understand the ability of stainless steel to clean the residual carbon from alumina during thermal cycle, thus improving its sintering.
Alumina-(0-20 vol. pct) iron composites were fabricated by hot-pressing well-mixed alumina and iron powders at 1400°C and 30MPa for 30 min. Hot-pressed bodies with nearly theoretical density were obtained for addition up to 10 vol. pct Fe, but the relative density decreased gradually with the further increase in Fe addition. The materials exhibited a homogeneous dispersion of Fe. The fracture strength of the composites exhibited a maximum value of 604 MPa at 15 vol. pct Fe, which is 1.5 times that of alumina alone. The fracture toughness increases with the increase in Fe content, reaching 7.5 MPa·m1/2 at 20 vol. pct Fe. The theoretical value of fracture toughness was calculated and compared with the experimental one. The toughening mechanism of the composites was also discussed.
Andreas MORTENSEN Professeur Président Tanguy ROUXEL Professeur Rapporteur Georges CAILLETAUD Professeur Rapporteur Anke PYZALLA Privat Dozent Examinatrice Sabine DENIS Professeur Directrice de thèse Alain MOCELLIN Professeur Directeur de thèse
Polycrystalline alumina, doped with MgO below the solubility limit, was reinforced with sub-micron particles of Ni by infiltration of Ni-nitrate into fired alumina green bodies, followed by reduction and sintering. The Ni particle size and location were monitored both after reduction and after sintering by transmission electron microscopy. Particle occlusion was found to increase with sintering time and temperature, and is correlated with experimentally detected Mg segregation to the Ni-alumina interfaces, resulting in partial depletion of Mg at the alumina grain boundaries and thus their increased mobility. Occlusion of Ni particles reduces the fracture strength and Weibull modulus of the composites, indicating that particle location is a key microstructural parameter for reaching high fracture strengths, and that this can be controlled via grain boundary and interface adsorption.
Oxidization of diamond in the sintering process of diamond/borosilicate glass composites would result in low compressive fracture strength (CFS) of the grit and uncontrolled expansion with many irregular pores in the composites, causing low bending strength of the tools. In this paper diamond/borosilicate glass composites were prepared by cold pressing and sintering at 850 C for 120 min in air. An active element Zn was incorporated into the composites in order to resolve the above issues. The effects of Zn contents on the properties of the composites was investigated by the bending strength tests, the volume expansion rate tests, differential scanning calorimeter test (DSC), thermogravimetry (TG), X-ray diffraction analysis (XRD), and scanning electron microscope (SEM). The results showed Zn was oxidized and then converted to ZnAl2O4 and Zn2SiO4 phases during sintering. The bending strength improved and the expansion phenomenon was inhibited for the composites with various Zn additions. The maximum bending strength and minimum volume expansion rate were obtained for the composite GZ8. This Zn content resulted in a decrease of volume expansion rate from 8.57% to -20.53%, and an increase in bending strength from 28.49 MPa to 74.02 MPa compared with the composite GZ0. The CFS results of the diamond grits separated from GZ0 and GZ8 was 21N and 26N, respectively.
In this work, we present systematical characterizations of iron doped alumina substrates produced by solid state sintering of ball milled powders. It was found that the doped samples have higher fracture toughness, lower thermal conductivity, smaller coefficient of thermal expansion and higher relative dielectric constant than undoped ones. A reduction in thermal conductivity could arguably give extra protection to the package chip in a high temperature application environment and can be attributed to an increase in phonon scattering. Furthermore, the decrease in coefficient of thermal expansion also helps to reduce thermal induced stress between the substrates and device chip. The observed improvement in fracture toughness cannot be explained by the common toughening mechanism, such as crack bridging or due to the increase in crystallite size, and is the subject of further investigation.
Both ceramics and intermetallics are potential materials for high temperature applications. Recent studies indicated that the toughness of ceramics can be improved by adding intermetallics. Among the intermetallics studied, NiAl is an attracting material for its low density and high melting point. In the present study, the toughening behaviour of Al2O3–NiAl composites is investigated. In order to determine the contribution from the matrix and reinforcement, the composites containing 0–100 vol% NiAl are prepared by hot-pressing. The toughness of the Al2O3–NiAl composites is higher than the values predicted by the rule of mixtures, i.e. the contribution from only the matrix and reinforcement. The toughness enhancement is contributed by the combination of crack deflection and the plastic deformation of NiAl grains. The toughening mechanism for the Al2O3-rich composites is mainly crack deflection. The contribution of the plastic deformation of NiAl to toughening effect increases with increasing NiAl content. It is due to the fact that more NiAl grains are interconnected with each other as NiAl content is increased.
The mechanical properties and thermal shock behavior of the hot pressed alumina matrix ceramics, with added 6 vol.% tungsten carbide particles, was investigated. The thermal shock resistance of the materials was evaluated by water quenching and subsequent three-point bend testing of flexural strength diminution. The hot-pressed composite exhibited 70°C higher temperature differential of thermal shock resistance than the monolith, as well as increased flexural strength and fracture toughness. The calculation of thermal shock resistance parameters for the composite and the monolith gives a possible explanation for the differences in thermal shock behaviour.
Cobalt-coated Al2O3 and TiC powders were prepared using an electroless method to improve resistance to thermal shock. The mixture of cobalt-coated Al2O3 and TiC powders (about 70 wt.% Al2O3–Co + 30 wt.% TiC–Co) was hot-pressed into an Al2O3–TiC–Co composite. The thermal shock properties of the composite were evaluated by indentation technique and compared with the traditional Al2O3–TiC composite. The composites containing 3.96 vol.% cobalt exhibited better resistance to crack propagation, cyclic thermal shock and higher critical temperature difference (ΔTc). The calculation of thermal shock resistance parameters (R parameters) shows that the incorporation of cobalt improves the resistance to thermal shock fracture and thermal shock damage. The thermal physic parameters are changed very little but the flexure strength and fracture toughness of the composites are improved greatly by introducing cobalt into Al2O3–TiC (AT) composites. The better thermal shock resistance of the composites should be attributed to the higher flexure strength and fracture toughness.
It has been known for many years that the incorporation of metallic particulates into a ceramic matrix can bring about improvement on ceramic mechanical properties, but little is known how the addition of metallic particles into a ceramic base affects the thermal shock resistance of this sort of material. The present work is concerned with the thermal shock behavior of an alumina base ceramic matrix composite containing 5vol% of copper particles. The composite, hot-pressed at 1550°C, exhibited increased thermal conductivity, enhanced toughness, decreased modulus and higher resistance to thermal shock compared with monolithic alumina. Some mechanical and thermal properties relevant to thermal shock are discussed which give plausible explanations for the differences between the composite and the monolith.
Direct observation of crack propagation in LiTaO3/Al2O3 composite ceramics was carried out using in situ transmission electron microscopy (TEM). Domain switching induced by crack propagation, crack deflection and branching at domain boundaries and ripples similar to the contrasts of 180° domains at the microcrack tip inside LiTaO3 grains were detected evidently. Domain switching, crack deflection, branching and energy dissipation resulting from the formation of contrasts similar to the 180° domains at the microcrack tip, were proposed as the toughening mechanisms in LiTaO3/Al2O3 ceramics.
A number of researchers demonstrated some of the characteristics and the effect of carbon content on sintering of stainless steel or alumina composites in a paper presented at the European Powder Metallurgy Association's EuroPM09 conference, held in Copenhagen, Denmark in 2009. The two materials used in the study were α-alumina and 316L HC stainless steel. This choice was made so that there was a common sintering temperature for the two components of the composite. Different steel fractions were tested for the alumina / steel composite and the powders were mixed using soft and short planetary milling, so that no milling took place. The powders were uniaxially pressed in a cylindrical carbide die at 600, 400, 200 and 100 MPa. The sintering cycle also consisted of a higher heating rate of 3°C/min up to 1410°C, where the temperature was maintained for four hours.
The purpose of the present study was to describe the change of the crack profile and as a consequence its length due to the metal particles introduced into the ceramic matrix. The results are based on the experimental observations of the crack propagation in ceramicmetal composites (CMCs). Changes of crack profile by the metal particles affect the fracture toughness of the composite. In the method of measuring the fracture toughness which is based on the Vickers indentation technique the changes of the crack length by the metal particles lead to increase the fracture toughness. The real crack length measured according to the crack profile deflection by the metal particles is longer than that measured as a straight line and can be divided into two parts. One part is the length of the crack which is moving through the ceramic matrix. The second part is equal to the length of the crack profile that results from the interaction between the metal particles and the crack. Knowing the path of crack propagation it is possible to predict the fracture of the composites and to design composites with the desired fracture toughness.
The properties of silver toughened alumina composites as a cutting tool were estimated. The relative density and apparent porosity of the composites were estimated as 1.72 and 94.2% respectively while volume fraction of the silver was experimentally determined as 5.01% by point counting method. The results revealed that for commercial exploitation of this composite as cutting tool material would depend on further optimization of the chemistry and the gradient microstructure of the system.
Alumina-iron nanocomposite powders containing 5vol.% of iron were fabricated by high-energy ball milling with different ball-to-powder weight ratios (BPRs) as part of the study of ceramic-metal nanocomposite magnetic materials. The microstructure and morphology of the composite powders were characterized using the X-ray diffraction, optical microscopy and scanning electron microscopy. XRD analysis and SEM examination in combination with energy dispersive X-ray spectrometry confirmed that the nanocomposite structure of the powder particles formed only after 8 hours milling for both BPRs used. With a higher BPR of 16:1, Fe-Cr alloy material was broken from the stainless steel balls and incorporated into the nanocomposite powder. However, such a problem did not occur with a lower BPR of 5:1. The mechanism for formation of the alumina matrix nanocomposite powder is found to be dependent on BPR and milling time.
This paper aims to study the sintering of 316L stainless steel and alumina composites. Compositions range from 0 to 100 vol.-% steel, and the experimental procedures involve density and microstructure analysis of the samples, as well as dilatometric measurements. In this study, it is shown that reducing atmosphere debinding can lead to carbon residues. These have a negative effect on alumina densification by delaying the sintering onset. For metal-ceramic composites, densification is modified by a complex interaction involving carbon (which lowers alumina density), chromium oxide (which is documented in literature to diminish alumina densification) and stainless steel phase. Chromium carbide formation is possible for some experimental conditions (1-30% stainless steel and hydrogenated argon debinding); this mechanism, locking both carbon and chromium outside alumina phase, leads to higher sintered densities.
A new process has been developed to produce sintered α-Al2O3 reinforced with sub-micron Ni particles. The process is based on the infiltration of ceramic preforms with liquid salts, which are reduced during pressureless sintering to form metal particles. The reduction occurs within the open pores of the ceramic preform, after evaporation of the metal salts into the gas phase. By controlling the partial pressure of oxygen during the sintering process, a reaction between the Ni particles and the alumina matrix can be invoked to form Ni-spinel (NiAl2O4) particles. Three-point bending experiments show an increase in strength for the nickel-reinforced alumina and a significant increase for the spinel-reinforced alumina.
Several metals have been proposed as second phases in ceramic matrix composites in order to improve their fracture toughness. Unfortunately, the use of metals is limited by low melting temperature, as for Al and Ag, poor oxidation resistance, as for Ni, Mo and W, and decrease of mechanical strength as temperature increases. In these respects, high temperature structural intermetallics show better properties. This work presents the preparation and the characterization of a Ni3Al reinforced-alumina. A ceramic composite containing 10 vol% Ni3Al powder was prepared by hot-pressing at 1350°C for 1.5 h green compacts of the mixture of ceramic and intermetallic powder. Microstuctural features were investigated by scanning electron microscopy (SEM). Elastic modulus, flexural strength and fracture toughness were measured at room and high temperatures and correlated to the microstructural characteristics of the material. A toughening mechanism due to plastic deformation of the intermetallic particles during crack propagation was seen to operate both at room and at high temperature.
A chemical electroless method for Ni–P deposition on the surface of Al2O3 powder with the use of phosphate baths has been presented. The possibility of ceramic/metal nanocomposite formation by sintering the coated powders at temperatures of 1200 and 1400°C is described. The results of the sintering process showed that at both sintering temperatures, Ni–P becomes a liquid. Moreover, the beginning of the sintering process of the ceramic matrix at 1400°C was observed. This temperature is lower than required for the sintering of pure Al2O3 (1600°C). This result indicates the possibility to decrease the sintering temperature of Al2O3/Ni–P composites.
The transverse rupture strength of hot-pressed and annealed composites of magnesium oxide and dispersed metallic phases (nickel, iron, cobalt) increases with increasing volume fraction of metal and annealing temperature. The strengthening effect of the metal is attributed to an inhibition of grain growth while flaw healing occurs during the annealing of the composites.
Advanced structural ceramics are a family of materials that exhibit a combination of high strength at elevated temperatures, high hardness, dimensional stability, good corrosion and erosion behavior, high elastic modulus, low mass density, and generally low coefficients of friction. Major efforts are under way to apply structural ceramics to automotive engines for wear components and turbochargers; in gas turbines as rotary and stationary components, recuperators, and regenerators; and ceramic bearings for both rotary and reciprocating engines.
Two microstructural characteristics, particle morphology and size, have been examined with respect to toughening by crack deflection. Particle morphology effects were evaluated in a series of hot-pressed silicon nitrides comprised of rod-shaped grains of various aspect ratios and a bariumsilicate glass ceramic containing spherulites. Lithium-alumino-silicate glass ceramics containing Li2Si2O5 lath-shaped crystals were studied for particle size effects. Independent measures of the fracture toughness and the crack deflection process were performed and results were correlated with a crack deflection model.
Brittle solids can be toughened by introducing ductile inclusions. In the present study, nickel inclusions are incorporated into an alumina matrix. Three processing routes are investigated, namely (1) a powder metallurgy route, (2) a gas reduction route, and (3) a reaction sintering route. Optimal processing conditions for each route are explored. The properties of the resulting composites are measured and compared in the light of the respective routes and microstructures.ZusammenfassungEine Zähigkeitssteigerung bei spröden Werkstoffen kann durch das Einbringen duktiler Einschlüsse erreicht werden. In der vorliegenden Arbeit wurden Nickel-Einschlüsse in eine Aluminiumoxid-Matrix eingelagert. Es wurden drei verschiedene Herstellungsmethoden untersucht, und zwar erstens ein pulvermetallurgischer Prozess, zweitens ein Prozess über Gasreduktion und drittens die Herstellung über Reaktionssintern. Die optimalen Herstellungs-bedingungen für jede der genannten Herstellungs-methoden wurden erarbeitet. Die Eigenschaften der sich ergebenden Verbundwerkstoffe wurden bestimmt und unter dem Aspekt des Herstellungs-prozesses und der Gefüge miteinander verglichen.RésuméDes solides fragiles peuvent être renforcés en introduisant des inclusions ductiles. Dans cette présente étude, des inclusions de nickel sont incorporées dans une matrice d'alumine. Trois procédés différents sont étudiés: (1) un procédé par mélange de poudres métalliques, (2) un procédé de réduction en phase gazeuse, et (3) un procédé de réaction par frittage. Pour chaque procédé les conditions expérimentales optimales sont recherchées. Les propriétés des composites obtenus sont mesurées et comparées à la lumière des procédés respectifs et des microstructures.
A fracture mechanics approach has been used to predict fracture toughness increases due to crack deflection around second phase particles. The analysis is based on a determination of the initial tilt and the maximum twist of the crack front between particles, which provides the basis for evaluating the deflection-induced reduction in crack driving force. Features found to be important in determining the toughness increase include the volume fraction of second phase, the particle morphology and aspect ratio, and the distribution of interparticle spacing. Predictions are compared with expected surface area increases.
Theoreticalanalyses of small-scale bridging of crack surfaces by elastic-ideally plastic springs are presented and applied to the study of the fracture toughness of ceramics reinforced by small particles. The dependence of toughening on particle size, concentration, strength, and ductility is explored, and relations between toughening and bridge length at fracture are given. Available experimental information is examined in the light of the analyses.
In this paper, we describe a framework for the processing of niobium-reinforced MoSi2 composites. As a part of the program, composites containing coated and uncoated niobium reinforcements were produced. Chemical compatibility between the coatings and the matrix was studied and the effect of the interface modification by the coating on the fracture toughness of the composites was investigated via four-point bending tests on chevron-notched samples. The results indicated that the coatings have a significant effect on the debonding at the reinforcement-matrix interface, which in turn can affect the damage tolerance of the composite. Also observation of the crack propagation in the composites suggests that the matrix failed at the early stages of loading. The results are discussed in terms of the mismatch between the elastic constants of the composite constituents.
Finite element analysis was used to study the fracture toughening of a ceramic by a stress induced dilatant transformation of second phase particles. The finite element method was based on a continuum theory which modelled the composite as subcritical material. Transient crack growth was simulated in the finite element mesh by a nodal release technique. The crack's remote tensile opening load was adjusted to maintain the near-tip energy release rate at the level necessary for crack advance. The transformation zone surrounding the crack developed as the crack propagated through the composite. Resistance curves were computed from the analysis; and the results show that during crack advance maximum toughness is achieved before a steady state is reached.
The toughening effect of a crack-bridging ductile phase in a brittle material may be predicted if ligament deformation is characterized. A plastically deforming ligament constrained by surrounding elastic matrix material is modelled using finite elements and the relevant toughness enhancement information extracted. Comparison is made to model experiments as well as to toughness measured for technologically important materials. The results suggest that debonding along the interface between the ligament and the matrix may enhance the toughening effect of a ductile phase.
The toughening of Al2O3 by Al has been investigated, using electron microscopy to provide in sit measurements of the microstructural parameters that govern ductile ligament toughening. The results are used to compare the measured toughness with values calculated for ligaments that exhibit limited debonding during ductile rupture. Good agreement confirms that toughening is dominated by the plastic work expended in the ductile rupture of ligaments stretched between the crack surfaces. Furthermore, it is demonstrated that in situ measurements are needed to assign the appropriate flow stress, because of the occurrence of coupled precipitation/solution hardening, and to ascertain the extent of the plastic stretch.
The role of elastic, thermoelastic, and interfacial properties in the toughening of a brittle matrix by metallic second-phase particles was studied. Two composites were studied: glass+partly oxidized Ni particles (thermal expansion coefficient of the glasses lower than, equal to, and higher than that of Ni) and glass+partly oxidized Al particles (thermal expansion and elastic moduli equal). Weak interfacial bonding between the nickel and its oxide and developed stress concentrations are the major toughness limitations found in the glass/Ni composites. When the thermal expansion coefficient and elastic modulus of the second phase are sufficiently greater than that of the glass matrix, a propagating crack will bypass the particles. When the thermal and elastic stresses are minimized and satisfactory bonding is achieved (glass/Al composites), a 60x toughness increase was realized.
An in situ study is made of crack interfaces in composites of alumina reinforced with silicon carbide whiskers. Both qualitative observations of the whisker-bridging micro-mechanisms and quantitative measurements of the crack profile are made to assess the specific role of the whiskers on the toughness curve (T-curve or R-curve). At small crackwall separations the whiskers act as elastic restraints to the point of rupture. In some cases the whiskers remain in frictional contact with the alumina matrix over large pullout distances (more than 1 μm) corresponding to a bridging zone approaching 1 mm. The results are discussed in relation to existing models of whisker reinforcement and published long-crack T-curve data.
An expression is derived for the change of localK
1 value of a crackfront near circular and spherical inclusions with elastic moduli and thermal expansion coefficient different from those of the matrix. The derivation is based on the concept of an image stress which is imposed on the crack, to illustrate the interaction between elastic and thermal stress concentrations developed around inclusions in a composite material and the crack-tip stress field.
The transverse rupture strength of hot-pressed and annealed composites of magnesium oxide and dispersed metallic phases (nickel, iron, cobalt) increases with increasing volume fraction of metal and annealing temperature. The strengthening effect of the metal is attributed to an inhibition of grain growth while flaw healing occurs during the annealing of the composites.The strength of magnesium oxide hot-pressed with nickel fibres is not affected by the volume fraction of fibre or the annealing temperature, and is comparable to the strength of porous magnesia. However, the work of fracture, though insensitive to heat-treatment, increases by at least two orders of magnitude for a moderate volume fraction of randomly oriented fibres. Mechanisms of energy absorption during the fracture of composites containing weakly bonded, non-aligned fibres are discussed. They include the work done in plastically deforming the fibre as it is withdrawn from its socket. It is concluded that this mechanism may be of importance in composites containing very weakly bonded ductile fibres.
Morphology effect of ductile reinforcements was evaluated using a four-point bend test on chevron-notched MoSi2 composites reinforced with 20 vol. % niobium. The niobium used had three different morphologies, i.e., fibre, foil and particle. The thickness of the foils was 250 m, while the fibres and particles had diameters of 250 and 200 m, respectively. Toughness of MoSi2 composites was increased from 3.3 MPa m1/2 for the matrix to 15 MPa m1/2 with the incorporation of the Nb fibres or foils. The particulate composites also exhibited an increase in toughness (7 MPa m1/2). The toughening achieved was mainly attributed to ductile phase bridging in all the composites tested. The relatively small toughness improvement in the particulate composites was ascribed to the embrittlement of the Nb particles. The results indicate that toughening by crack bridging depends mainly on the intrinsic properties of the ductile bridging ligaments rather than on their morphology, and that the embrittlement of the bridging ligament is detrimental to the toughening of the composites.
A model for the fracture toughness of cobalt reinforced tungsten carbide hardmetals is presented. The fracture energy of the composite is obtained from the sum of the energies dissipated during fracture along the four crack paths that are available in this composite (i.e. dimple rupture across the binder or in the binder near a binder/carbide interface, brittle trans- and intergranular matrix fracture). While specific energies for matrix cracking are known from previous work, the plastic work spent during formation of unit area of binder rupture has been calculated on the basis of a recent stereometric evaluation of the dimple morphology in the binder. The specific work of fracture in the reinforcing binder phase scales with its mean flow stress and the depth that suffers plastic deformation. Estimates for the mean flow stress of submicrometer binder layers, as present in WC-Co, and for the mean size of the plastic zone are obtained subsequently by combination with the yield stress and the work hardening parameters of the binder and the microstructure of the composite respectively. Combining these results with recent experimental data for the distribution of the four crack paths on the fracture surface renders eventually the fracture energy of the composite. It is recognized that the effects of the depth of plastic deformation and the distribution of fracture paths combine to yield the well known effects of microstructural geometry (i.e. volume fraction and mean linear intercept of the metal phase) on composite toughness in accordance with experimental observations of crack propagation in WC-Co. It is emphasized that beside geometric effects both the matrix toughness and the plastic properties of the binder play a dominant role for the fracture resistance of metal reinforced brittle matrices.
Brittle solids can be toughened by incorporating ductile inclusions into them. The inclusions bridge the crack and are stretched as the crack opens, absorbing energy which contributes to the toughness. To calculate the contribution to the toughness it is necessary to know the force-displacement curve for an inclusion, constrained (as it is) by the stiff, brittle matrix. Measured force-displacement curves for highly constrained metal wires are described and related to the unconstrained properties of the wire. The constraint was achieved by bonding the wire into a thick-walled glass capillary, which was then cracked in a plane normal to the axis of the wire and tested in tension. Constraint factors as high as 6 were found, but a lesser constraint gives a larger contribution to the toughness. The diameter of the wires (or of the inclusions) plays an important role. Simple, approximate, models for the failure of the wires are developed. The results allow the contribution of ductile particles to the toughness of a brittle matrix composite to be calculated.
The addition of a dispersed ductile phase to a brittle material can lead to significant increases in fracture resistance compared to the untoughened matrix material. Often the important mechanism appears to be bridging by intact ductile ligaments behind the advancing crack tip. Although a framework for predicting toughness enhancements from bridging mechanisms exists, the required detailed model of ligament deformation which would provide the load-extension relation for a typical ligament has not been available. In this paper, numerical modeling of a plastically deforming ligament constrained by surrounding elastic matrix material is performed and the relevant toughness enhancement information extracted. Comparison is made to model experiments as needed to investigate such deformation processes as well as to toughnesses measured for technologically important composites. The results suggest that debonding along the interface between the ligament and the matrix may enhance the toughening effect of a ductile phase.
The fracture micromechanics and underlying physical processes of fracture in Al2O3-based ceramic specimens have been studied as a function of grain size by instrumented in situ dynamic scanning electron microscopy (SEM) using the double torsion technique. The toughness is found to increase with grain size. Crack bridging is found to extend over hundreds of grain diameters behind the crack tip, resulting in R-curve behaviour. Evidence is amassed which points to frictional energy dissipation, rather than distrubuted microcracking or crack-closure due to elastic ligaments, as the dominant contribution to toughening. The friction occurs at grains which bridge the crack faces and are pulled out as the faces separate. Restraining stresses, which constrain the bridging grains in their sockets, are believed to be the result of both grain morphology and the thermal expansion anisotropy of the material. Simple modelling indicates that only a few percent of the grains need be involved in the frictional process to account for the toughening. The conclusion is supported by hysteresis measurements.
Brittle solids can be toughened by the introduction of ductile metallic inclusions. In the present study, the mechanical properties and oxidation resistance of Al2O3/Ni composites are investigated. The oxidation resistance of the ceramic/metal composite (8 × 10-11 g2 cm-4 s-1 at 1300° C) is comparable to that of many silicon nitrides. The fracture toughness of the composite containing 13 vol.% nickel is twice that of alumina alone. The square of the toughness enhancement for composites containing various amounts of nickel exhibits a linear relationship with the product of volume fraction and inclusion size, as predicted in theoretical models. For the alumina/nickel composite system, it is demonstrated that dissolved oxygen in the nickel increases the yield strength of nickel and enhances the toughness of the composite.
A fracture mechanics approach has been used to predict fracture toughness increases due to crack deflection around second phase particles. The analysis is based on a determination of the initial tilt and the maximum twist of the crack front between particles, which provides the basis for evaluating the deflection-induced reduction in crack driving force. Features found to be important in determining the toughness increase include the volume fraction of second phase, the particle morphology and aspect ratio, and the distribution of interparticle spacing. Predictions are compared with expected surface area increases.
Thesis (Ph. D.)--University of Surrey, 1994.
Crack-particle interac-tions in alumina-iron composites Small-scale crack bridging and the fracture toughness of partic-ulate-reinforced ceramics Deformation of crack-bridging ductile reinforcements in toughened brittle materiab
- P A Trusty
- J A Yeomans
- B Budiansky
- J C Amazigo
- A G Evans
Trusty, P. A. & Yeomans, J. A., Crack-particle interac-tions in alumina-iron composites. Ceram. Eng. Sci. Proc., 14 (1993) 908. Budiansky, B., Amazigo, J. C. & Evans, A. G., Small-scale crack bridging and the fracture toughness of partic-ulate-reinforced ceramics. J. Mech. Phys. Solids, 36 (1988) 167. Mataga P. A., Deformation of crack-bridging ductile reinforcements in toughened brittle materiab. Acta. Met-all., 37 (1989) 3349.
The fabrication and properties of alumina-ductile metal particle composites
- X Sun
- P A Trusty
- J A Yeomans
- H R Shercliff
Sun, X., Trusty, P. A., Yeomans, J. A. & Shercliff, H. R., The fabrication and properties of alumina-ductile metal particle composites. In Proceedings of ICCM8, ed. S. W. Tsai & G. S. Springer. SAMPE, Hawaii, 1991. Paper
The fabrication and properties of alumina-ductile metal particle composites
- Sun