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Processing of fine-grained aluminum foam by spark plasma sintering
Abstract
cessed by a spark plasma sintering method. Therefore, it is worthwhile to investigate the microstructure and mechanical properties of aluminum foam processed by spark plasma sintering. In the present paper, microstructure and compressive properties of an aluminum foam processed by spark plasma sintering are compared with those of an aluminum foam processed by electric furnace sintering. Commercially available aluminum powder (purity = 99.9%, powder size < =3 µm) was used as a starting material. Sodium chloride (NaCl) particles with a spheroidal shape (mean size = 520 µm) were prepared as the space-holding material. The weight ratio of the aluminum powder to the space-holding powder was determined, for the required porosity, to be 80%. Fig. 1 shows the schematic illustration of processing of an aluminum foam by spark plasma sintering. The process consisted of four steps; mixing, pressing, sintering and leaching. First, the aluminum powder and the spaceholding powder were thoroughly blended in an agate mortar. After the ingredients were homogeneously mixed, the mixed powder was uniaxially pressed in a carbon die at a pressure of 20 MPa. Then, spark plasma sintering was conducted at 773 K with a pressure of 20 MPa for 5 min by using an on-off pulsed DC voltage. The sintered specimen was placed into a running hot water bath to leach out the imbedded NaCl particles, leaving behind the aluminum foam with porous structure. A scanning electron micrograph of the aluminum foam processed by spark plasma sintering is shown in Fig. 2. It can be seen that completely interconnected networks of cell edges were formed and the space-holding particles were leached out. Sintering in an electric furnace was carried out for comparison with spark plasma sintering. For the sintering in the electric furnace, the mixed powder was uniaxially pressed in a steel mold at a pressure of
... An enormous potential of porous materials for structural and functional applications stimulates the development of the processing methods of these materials [1][2][3][4][5][6]. Processing of porous metals through bubbling gas into a molten metal lacks control over the percentage of porosity, size of pores, and the structure of the pore walls [7]. By powder sintering, porous structures can be produced using removable space holders and pore formers or by allowing only partial densification of the powder by limiting the pressure and/or temperature. ...
... If further heat treatment is necessary, it can be performed after the space holder has been removed. Using NaCl as a space holder, aluminum [7,34,35], titanium [33], and titanium alloys [52] have been fabricated. ...
Spark plasma sintering (SPS), a sintering method that uses the action of pulsed direct current and pressure, has received a lot of attention due to its capability of exerting control over the microstructure of the sintered material and flexibility in terms of the heating rate and heating mode. Historically, SPS was developed in search of ways to preserve a fine-grained structure of the sintered material while eliminating porosity and reaching a high relative density. These goals have, therefore, been pursued in the majority of studies on the behavior of materials during SPS. Recently, the potential of SPS for the fabrication of porous materials has been recognized. This article is the first review to focus on the achievements in this area. The major approaches to the formation of porous materials by SPS are described: partial densification of powders (under low pressures, in pressureless sintering processes or at low temperatures), sintering of hollow particles/spheres, sintering of porous particles, and sintering with removable space holders or pore formers. In the case of conductive materials processed by SPS using the first approach, the formation of inter-particle contacts may be associated with local melting and non-conventional mechanisms of mass transfer. Studies of the morphology and microstructure of the inter-particle contacts as well as modeling of the processes occurring at the inter-particle contacts help gain insights into the physics of the initial stage of SPS. For pre-consolidated specimens, an SPS device can be used as a furnace to heat the materials at a high rate, which can also be beneficial for controlling the formation of porous structures. In sintering with space holders, SPS processing allows controlling the structure of the pore walls. In this article, using the literature data and our own research results, we have discussed the formation and structure of porous metals, intermetallics, ceramics, and carbon materials obtained by SPS.
... In addition, the porous Al in the previous studies often had pores as large as millimeters. [3][4][5][6][7][8] The spacer method (or replication method), which has been employed since the 1960s, is one of the methods for fabricating porous Al. [14][15][16][17][18][19] Features of the spacer method are: (1) strict control of the porosity and pore size, and (2) distribution of homogeneous pores and small pores. Hence, porous metals fabricated by the spacer method have been used for investigations of the effects of the porosity and pore size on various properties of porous metals. ...
... [20][21][22] Also, small pore sizes of approximately 100 mm can be attained by the spacer method. [14][15][16][17][18][19] In the present study, porous Al specimens with porosities ranging from 55.7 to 68.2% and pore sizes ranging from 53-106 to 425-500 mm are fabricated by the spacer method, and the damping capacity is examined in order to understand the effect of the porosity and pore size. ...
The room-temperature damping properties of porous aluminum fabricated by the spacer method were investigated using the method of lateral resonant vibration in cantilever holding. In particular, the effects of the porosity and pore size, which are the representative parameters of porous metals and can be controlled well by spacer method, on the damping properties were focused on. The damping capacity increased with increasing porosity and pore size. Local stress concentration arising from the heterogeneity of porous structures seems responsible for the enhanced damping capacity under the condition in which the main damping mechanism is amplitude-dependent dislocation damping. The present results point out the importance of the porous structure control m damping properties.
... 1) In particular, porous metals with high porosity (to 95%) and small pore size (less than 1 mm) have been fabricated by the spacer (or replication) method. [11][12][13][14][15][16][17][18][19][20] Through this method, the pore size and porosity can be uniformly controlled, and formation of structural defects such as missing cell walls, coupled pores and irregularly elongated pores etc. can be successfully suppressed. Porous aluminum has been fabricated via the powder-metallurgical spacer method comprising the sintering of mixed (Al/NaCl) powder and dissolution of the NaCl particles in water. ...
... Porous aluminum has been fabricated via the powder-metallurgical spacer method comprising the sintering of mixed (Al/NaCl) powder and dissolution of the NaCl particles in water. 14,[17][18][19][20] The spacer method having good adjustability of porosity and pore size makes it possible to properly evaluate the effect of the porosity and pore size on the electrical properties of porous metals. In the present study, porous Al specimens with pore size range from 212-300 to 850-1000 mm and porosity from 77 to 90% are produced by the spacer method, and their electrical resistivities are experimentally investigated. ...
Porous Al specimens with a pore size range from 212-300 to 850-1000 mu m and a porosity range from 77 to 90% were produced by the powder-metallurgical spacer method, and their electrical properties were experimentally investigated. The electrical resistivity increased with an increase in porosity; on the other hand, it was negligibly affected by the pore size when the pore size was sufficiently small. The experimental results agreed with the theoretical results obtained using the unit-cell model in which size of apertures at cell walls are taken into consideration. However, at the maximum pore size in the range investigated, the measured value was much higher than the calculated one. This is likely to be related to the large variation in the local density of the cross section.
... Because of their special size, a large number of particles are present at the surface compared to that of the bulk. For this reason, nanostructured materials have found potential utility in a wide range of catalytic and gas sensing, applications [19,20]. During these last decades, several techniques of development were implemented to manufacture nanopowders [21]. ...
... According to the type of acoustic model, the conditions of optimization and controlled production of foam, which were described above, metal foam was prepared by Spacer and SPS methods [23,24]. This process consists of four steps: mixing, pressing, sintering and leaching. ...
Determining the structural properties of aluminum metal foam is essential to predicting its acoustic behavior. Acoustic models are presented that show the relationship between the morphology of the absorber and the sound absorption coefficient (SAC). Optimizing the parameters affecting the SAC can be the maximum theoretically SAC achieved at each frequency. In the previous article (https://doi.org/10.32604/sv.2021.09729) the parameters of porosity percentage (Ω), pore size (D) and pore opening size (d) were optimized by the genetic algorithm and Lu model. In this study, the optimal aluminum metal foam was synthesized using Spark Plasma Sintering (SPS), with the maximum temperature of 420 °C and final pressure of 20 MPa in samples with thicknesses of 5, 10, 15 and 20 mm in different frequencies from 1000 to 6300 Hz. The crystal structure and microstructure of samples were investigated using XRD and SEM. Optimized metal foam SAC (0.67, 0.9, 1 and 1) and experimental peak SAC (0.44, 0.67, 0.76 and 0.82) were compared with the optimized SAC in 5, 10, 15 and 20 mm thicknesses, respectively. The values of the coefficient of determination (R2) according to multiple linear regression (MLR) for the two optimized SAC and experimental in thicknesses of 5, 10, 15 and 20 mm were 0.90, 0.95, 0.96 and 0.90, respectively. The results of this study show that porous metal foam can have a high absorption coefficient in any desired thickness and frequency by using the optimal morphology.
... Porous materials became an interest matter for researchers both in the industrial and scientific field due their combination of unique physical, mechanical, thermal, electrical, and acoustic properties in conjunction with low density and high specific strength [1][2][3][4][5]. The idea that inspired the creation of these cellular materials finds its roots in natural elements such as wood, pumice stone, bones and other materials which have been used for different applications for a long period of time [6]. ...
This paper presents the usage of spark plasma sintering (SPS) as a method to obtain aluminum-expanded perlite syntactic foams with high porosity. In the test samples, fine aluminum powder with flaky shape particles was used as matrix material and natural, inorganic, granular, expanded perlite was used as a space holder to ensure high porosity (35–57%) and uniform structure. SPS was used to consolidate the specimens. The structures were characterized by scanning electron microscopy and compression tests. Energy absorption (W~7.49 MJ/m3) and energy absorption efficiency (EW < 90%) were also determined.
... Solid-state methods include the sintering of metal powder. It is well known that spark plasma sintering (SPS) decreases the sintering time and temperature, compared with sintering in an electric furnace, resulting in suppression of grain growth during sintering [22][23][24][25]. Therefore, it is worthwhile to investigate the microstructure and mechanical properties of aluminum foam processed by SPS. ...
The present paper illustrates a comparison of open-cell aluminum foams. The foams were fabricated by two different methods: spark plasma sintering and replication on a polyurethane template. The influence of pressure, temperature, and diameter of space holding material on foam obtained by the spark plasma sintering method was investigated. Additionally, the aluminum powder content in slurry and atmosphere during thermal processing of foam prepared by the replication technique were studied. The morphology and structure of obtained samples were analyzed by scanning electron microscopy and X-ray diffraction analysis. Supplementarily, mechanical properties and electrical conductivity were studied. The porosity of obtained samples was 83% for the SPS sample and 85% for the replication sample. The results of the studies carried out gave us an understanding that the SPS method is more promising for using the obtained foams as cathode current collectors in lithium-ion batteries due to excessive aluminum oxidation during sintering in the furnace.
... Sun and Zhao [32] studied the sintering temperature effect, Mg percentage, cell size on energy absorption of Al foam and sintering time. Wen et al. [33] fabricated Al foam using finely-graded Al powder of 3-micron size and 520 micron NaCl particles such as porosity of the foam was 80%. High melting point metals like Copper, NiTi, and Ti-6Al-4 V foam have also been fabricated using SDP. ...
A new sintering method is developed for fabricating large, 3-dimensional, open-cell metal foam parts using milling or friction stir welding machines. Temperature and pressure required for sintering are achieved by plunging and moving a rotating tool against a sacrificial top sheet in the setup. Predefined tool paths are used to compress and sinter the powder mixture in a die cavity. In this paper, the method was used to fabricate open-cell copper foam plates using sintering and dissolution process (SDP). Samples with different porosities were fabricated. Produced foams have uniform mechanical properties, pore morphology, and porosity. Further, their mechanical properties are seen to be on par with that for copper foams fabricated using other sintering methods reported in literature. Except for top sheet, the setup can be reused any number of times. The developed process is robust, cost-effective, operator friendly, and easy to use.
... If formed, such microvoids are able to render the cell walls brittle and consequently generate lower plateau stress values. 85 ...
Metallic or ceramic micro/nanoporous materials have attracted particular attention due to some interesting structural and functional properties. There exist a variety of methods for producing porous materials by which optimized features can be reached. Spark plasma sintering (SPS) is one of these new-emerging approaches. This technique is often combined with conventional technologies and produce a variety of porous structures with tailorable microstructure and physicomechanical properties. This review addresses SPS and obtainable porous materials with nanoscale and microscale microstructural features. The processing methods, microstructural phenomena, and physicomechanical properties of these materials are discussed in some details, along with the likely future directions required for this novel field.
... Aluminum foams with closed cell structure exhibit high stiffness-to-weight ratio and novel energy dissipation ability [1][2][3][4][5][6][7] , leading to a variety of applications fields, including damping, electromagnetic shielding, sound insulation, heat exchange, sound absorption, and energy absorption [8][9][10][11] . ...
Closed-cell aluminum foams with different morphology were fabricated under both increased and reduced pressure. It is shown that the cell size distribution and cell wall thickness is significantly influenced by foaming pressure. Quasi-static compressive tests were performed on foam specimens with similar density but different cellular structure. Results indicates that compressive stress of aluminum foams with homogenous structure is much higher than that of specimen with wide cell size distribution.
... It is capable of processing conductive and non conductive materials. The basic principle of SPS process is based on the electrical spark discharge phenomenon where low voltage pulse current generates spark plasma in fine local areas between the particles at high energy [25][26]. The SPS material processing time is 5 to 20 min and the temperature is lower than the conventional sintering temperature [27][28]. ...
... 8 Distribution of porosity evaluated from X-ray CT images for height direction of compression test specimens corresponding to positions A～F. 3・3 圧縮試験結果 図 9 に各圧縮試験片 A～F の圧縮試験から得られた圧縮応力-ひずみ曲線を示す． 圧縮初期に急激に応力が上昇 する弾性領域，その後の応力がほぼ一定で推移するプラトー領域，および圧縮の最後の段階で応力が急激に上昇 する緻密化領域の 3 つの領域が確認され，従来の発泡金属等のポーラス金属の応力-ひずみ曲線 (Gibson, 2000)(Miyoshi, et al., 2000)と同様の傾向を示した．焼結スペーサー法において焼結が不足する場合，崩れるような脆性 的な変形挙動を示し，低いプラトー応力で応力-ひずみ曲線が不安定に上下に振動することが知られている(Hakamada, et al., 2005)． 本研究の場合は， どの圧縮試験片においても崩れるような変形挙動はほとんど見られず， 得られた応力-ひずみ曲線は， 焼結スペーサー法の先行研究(Sun and Zhao, 2003) (Wen, et al., 2003) (Hakamada, et al., 2005)，および摩擦粉末焼結法におけるスポット FSP(Hangai, et al., 2012) (半谷他, 2014)や 1 列走査 (Hangai, et al., 2014) (圖子田他, 2014)において作製されたポーラス Al の応力-ひずみ曲線とほぼ同様の傾向を示している． 図 10 に，図 9 に示した応力-ひずみ曲線から算出したプラトー応力値を，本研究で作製した全ての各圧縮試験 片について示す．ここでプラトー応力は JIS-H-7902「ポーラス金属の圧縮試験方法」に基づき，ひずみ 20-30%に おける応力の平均値とした (日本工業規格, 2008)．圧縮試験片採取位置に関わらず，ほぼ同程度のプラトー応力 を有していることが分かる． 以上，図 9 と図 10 からマルチパス摩擦粉末焼結法によりツール走査時の摩擦熱と押込荷重のみで Al 粉末同士 の強固な焼結を伴ったポーラス Al 板を作製でき， ツール走査方向によらず全体で均一な強度を有していることが ...
Multi-pass friction powder sintering process was proposed for fabricating a large plate of porous aluminum by the sintering and dissolution process. In this process, sintering of a powder mixture of aluminum and NaCl was achieved only the rotating tool plunged into the die filled with the powder mixture was made to traverse as in the case of multi-pass friction stir processing. In this study, porous aluminum plates of 60 mm × 30 mm × 5 mm with porosity of 70% were successfully fabricated by multi-pass friction powder sintering process. From measurements of temperature and torque, it was found that the entire sample had almost same sintering condition during the multi-pass friction powder sintering process. From X-ray computed tomography (CT) and scanning electron microscopy (SEM) observations of the pore structures of the fabricated porous aluminum plates, it was found that the entire sample had almost same pore shape and porosity that was similar to the NaCl morphology and proportion. From compression tests of the fabricated porous aluminum plates, it was found that entire sample exhibited a similar stress-strain curve to that of the previous work regardless of the position. This is considered to be attributed to the good bonding between aluminum particles during friction powder sintering process. From these results, it was indicated that multi-pass friction powder sintering process had a potential to fabricate the porous aluminum plate that was similar to the size of the tool traversing area.
... Up to now, most metal foams fabricated via the SDP using NaCl as space holders have been limited to aluminum (Al) foam [4][5][6][7][8][9][10]. This is mainly due to the relationship between the melting points of the metal and NaCl. ...
Open-celled metal foams have received considerable attention in various fields and are expected to be used as engineering materials where heat exchange, sound absorption and filtration are required. In this study, Cu foam specimens with NaCl volume fractions of 60%, 70% and 80% were successfully fabricated by the friction powder compaction (FPC) process with the sintering and dissolution process (SDP) using NaCl as space holders. In the FPC process, no external heat source was used for fabricating Cu foam except for the friction heat generated by the rotating tool plunged into the die and powders. From the X-ray CT and SEM observation of the pore structures of the fabricated Cu foam, it was found that almost the entire specimen had a pore structure similar to the NaCl morphology, regardless of the NaCl volume fraction. This is mainly because the sintering process for Cu particles in the FPC process was achieved at a temperature lower than the melting point of NaCl. From compression tests of the fabricated Cu foam, Cu foam exhibited ductile fracture regardless of its NaCl volume fraction, which is considered to be attributed to the good bonding between Cu particles. The plateau stress and energy absorption decreased with increasing NaCl volume fraction, indicating strong relationships between them. The Cu foam with the highest energy absorption per unit volume up to the specific stress changed from the high-NaCl-volume-fraction Cu foam to the low-NaCl-volume-fraction Cu foam with increasing compression stress. Consequently, it was shown that the mechanical properties of Cu foam can be controlled by adjusting the volume fraction of NaCl.
... Recently, porous Al has been prepared by a combined method of spark plasma sintering (SPS) and the dissolution of space-holding sodium chloride (NaCl) particles [8]. This method is effective for two reasons [9,10]: one is that sufficient connection of Al powder can be obtained by SPS because of the breakdown of the aluminum oxide film which impedes Al sintering. ...
Porous Al with controlled pore size was prepared by the spacer method including spark plasma sintering and the dissolution of space-holding NaCl particles. The NaCl of the controlled pore size (particle diameter control range of 5˰ڌm~20˰ڌm) were prepared by precipitation method. The effects of sintering condition such as the sintering electric current intensity, voltage and the size, morphology and content of NaCl powder on the porosity and size of porous Al are investigated. The porous Al with higher porosity of 69.41% and smaller pore size of 5 ڌm was obtained.
... Time consuming production in the case of HIP or HUP (Hot Uniaxial Pressing) is critical for cost reasons. The Spark Plasma Sintering (SPS) is an innovative method achieving limited grain growth and maximum densification within several minutes at lower temperatures compared with conventional sintering techniques [14,15]. The main issue encountered with ZnS densification is the fact that this compound tends to sublimate when the temperature is more than 1200°C. ...
The sintering of two different polycrystalline zinc sulphide powders has been investigated by two different techniques. Conventional sintering technique (Hot Uniaxial Pressing, HUP) and the Spark Plasma Sintering (SPS) also known as electric field-assisted sintering technique (FAST) have been compared in terms of sintering parameters (temperature, pressure) and optical properties of the prepared ZnS ceramics. This study demonstrates the potentiality to sinter ZnS by SPS in very short times, at lower sintering temperatures, compared with HUP, with good densification (close to the theoretical density) and good optical transmission in the infrared range 8–12μm.
... (2) Preparation of sieved spacer particles can narrow the pore size distribution. Recently, spark plasma sintering (SPS) and dissolution of spacing NaCl particles have been combined for efficient production of porous Al. 13) In the previous work, 14) the effects of the sintering temperature on compressive properties of porous Al fabricated by SPS and NaCl dissolution have been investigated. In the present paper, the effects of other conditions such as the sintering pressure, sintering time and raw Al powder size on the compressive properties of porous Al are investigated. ...
Porous aluminum with a porosity of 78% and pore size of 850-10001 mu m was fabricated under various sintering pressure, sintering time and raw Al powder size conditions by the spacer method consisting of spark plasma sintering (SPS) and sodium chloride (NaCI) dissolution. The effects of the fabrication conditions on compressive properties of the porous Al were investigated. The sintering pressure of 20 MPa and sintering time of 10 min were needed to fabricate robust porous Al under the sintering temperature of 843 K and raw Al powder size of 3 Pm. Also, the porous Al specimen fabricated from Al powder of 300 pm exhibited much lower flow stress than those fabricated from Al powder of 3 and 20 Pm when employing the temperature of 843 K, the pressure of 20 MPa and the duration time of 10 min. This indicates that the raw Al powder size is needed to be much smaller than the spacer size. This is because the Al particle cannot touch with adjacent Al particles when the At powder size is comparable to the spacer size.
... Secondly, small amounts of sintering aids, e.g., transition metal oxides, are added to increase the densification rate [16]. For the application in oxygen separation, a reducing atmosphere or vacuum is required to consolidate a CGO membrane on a metallic substrate, for example, an aluminum foam plate [17], to protect the metal from oxidation. The sintering behavior of CGO in a reducing atmosphere is expected to be different from that in air due to the partial reduction of Ce 4+ to Ce 3+ at low oxygen partial pressures and the corresponding change in oxygen vacancy concentration might have a significant influence on the sintering kinetics [18]. ...
The present work investigates the processes of densification and grain growth of Ce0.9Gd0.1O1.95−δ (CGO10) during sintering under reduced oxygen partial pressure. Sintering variables were experimentally characterized and analyzed using defect chemistry and sintering constitutive laws. Based on the results achieved, the grain size–relative density relationship, the densification rate and the grain-growth rate were determined. The activation energies for densification and grain growth were evaluated, and the dominant densification mechanism was indicated. For comparison, the densification behavior of CGO10 sintered in air was also studied. Accelerated densification was observed in early-stage sintering of CGO10 in a reducing atmosphere. This might be attributed to the oxygen vacancies generated by the reduction of Ce4+ to Ce3+ in the reducing atmosphere, which facilitate the diffusion of ions through the lattice. The densification activation energy of CGO10 in the reducing atmosphere was evaluated to be 290±20kJmol−1 in the relative density range of 0.64–0.82, which was much smaller than that of CGO10 sintered in air (770±40kJmol−1). The grain-growth activation energy of CGO10 sintered in the reducing atmosphere was evaluated to be 280±20kJmol−1 in the grain size range of 0.34–0.70μm. The present work describes a systematic investigation of sintering behavior of CGO10 under reduced oxygen partial pressure, which contributes to the first known determination of the fundamental parameters associated with densification and grain growth during early-stage sintering of CGO10 in a reducing atmosphere.
... 1,7,8) Recently, it has been reported that porous Al can be fabricated by a combined method of spark plasma sintering (SPS) and the dissolution of space-holding sodium chloride (NaCl) particles. 9) This method is a spacer method, and is effective for two reasons: One is that sufficient connection of Al powder can be attained by SPS because of the breakdown of the aluminum oxide film which impedes Al sintering. 10,11) The other is that the adjustment of the weight ratio of starting materials and preparation of NaCl particles with an intended particle size can easily control pore size and porosity. ...
Porous aluminum with a porosity of 78% and pore sizes of 850-1000 mu m was produced by the spacer method including spark plasma sintering (SPS) at various sintering temperatures of 773-853 K and the dissolution of space-holding sodium chloride particles. The effect of sintering temperature on the compressive properties of porous Al was investigated. The stress-strain curves for the specimens fabricated at 8 13 K and higher exhibited a plateau region with a nearly constant flow stress to a large strain of about 40%. However, the specimens fabricated at less than 813 K showed no plateau regions, although there were no significant differences in the characteristics of the pores by macroscopic observation between the porous At specimen fabricated at 843 K and that fabricated at 773 K. Microscopic observation revealed that there were large voids in the cell wall of the specimens fabricated at 773 K, showing that sintering was insufficient and the connection of At powder was poor. Furthermore, the fully-dense Al specimen fabricated at 793 K exhibited poor ductility in tension. Therefore, poor ductility in tension of cell walls may be responsible for the lack of plateau regions in the specimens fabricated at less than 813 K.
Ti-6Al-4V alloy has been used for bone implant materials. The porous Ti-6Al-4V lowers the mechanical misfit and prevents the implant loosening between bone and metal implant. The influence of sintering parameters, i.e., heating rate, sintering temperature, pressure, space holder size, and its distribution to the formation of both micro and macro- porosity is investigated. The initially developed image analysis of samples was used to quantitatively analyze the influence of process control agent (PCA) on the distribution of macro-pores. From the SEM results and samples relative density, it is found that the main parameter to control in the solid-state space holder method is space holder distribution, sintering pressure, and sintering temperature. Additionally, the most effective condition to produce less micro-porosity and more interconnectivity between macro-pore is the utilization of 10 wt% ethanol as PCA and sintering at 700 °C, 60 MPa.
The dense uniformed joints between molybdenum disilicides (MoSi 2 ) and stainless steel powders were prepared using the spark plasma sintering (SPS) technique with nine graded materials and their welding behavior was investigated. The results showed that such joints can be achieved using graded interlayers because the coefficient of thermal expansion (CTE) of each interlayer closely matched over a wide temperature range. Furthermore, the compatibility between the graded interlayers prevented MoSi 2 with low toughness from the occurrence of microcracks resulted from the residual stresses formed during cooling of the joint. Moreover, the 9-layer joint with a thickness of 1.0 mm for each layer exhibited the minimum residual stress if the compositional exponent was 0.8.
Retraction: AKM Asif Iqbal, Muhamad Zami Bin Abdul Hamid, A. K. M. Parvez Iqbal (2020) Effect of porosity on the microstructure and mechanical properties of YSZ thermal barrier coating densified by spark plasma sintering (SPS). International Journal of Applied Ceramic Technology : https://doi.org/10.1111/ijac.13599
The above article, published online on 9 August 2020 in Wiley Online Library (wileyonlinelibrary.com), has been retracted by agreement between the authors, the outgoing Editor‐in‐Chief, Monica Ferraris, the incoming Editor‐in‐Chief, Young‐Wook Kim, and Wiley Periodicals, LLC. The retraction has been agreed due to unattributed overlap between images presented as original data in this article and the following article published in Ceramics International , “Relationship between mechanical properties and microstructure of yttria stabilized zirconia ceramics densified by spark plasma sintering” by Arnaud Fregeac, Florence Ansart, Serge Selezneff, Claude Estournès, Volume 45, 2019, pages 23740‐23749.
Functionally graded aluminum foam (FG Al foam) is a new class of Al foam in which the pore structure varies over the foam, resulting in corresponding variations in the mechanical properties of the foam. In this study, FG Al foam plates were fabricated by a friction powder sintering (FPS) process with a traversing tool that is based on a previously developed sintering and dissolution process. The variation of the mechanical properties was realized by setting the volume fraction φ of NaCl in the mixture to 60, 70, and 80%. Long FG Al foam plates were fabricated with a length equal to the tool traversing length with φ varying in the tool traversing direction. From x-ray computed tomography observation, it was shown that the density of the Al foam decreased with increasing φ. In contrast, almost uniform pore structures were obtained in each area. According to the results of compression tests on each area, the plateau stress and energy absorption tended to decrease with increasing φ. Therefore, it was shown that FG Al foam plates with varying mechanical properties can be fabricated by the FPS process with the traversing tool.
Functionally graded (FG) porous copper (Cu) consisting of layers with porosity p = 80% and 60% in a single porous Cu specimen was fabricated by a friction powder compaction (FPC) process via a sintering and dissolution process route. The FPC process is very simple and energy efficient since it only requires a rotating tool to be plunged into an oxygen-free Cu plate with a hole filled with a mixture of Cu powder and NaCl powder, and no external heat source is necessary. The sintering of the powder is mainly achieved by the friction heat and pressing load generated by the rotating tool in the oxygen-free Cu plate and powder. It was shown that the fabricated FG porous Cu has the potential to exhibit two plateau regions with almost the same plateau stresses as uniform porous Cu with p = 80% and 60%.
This paper presents for the first time the processing of aluminum (Al)-carbon nanotube (CNT) open-cell foams. Al-2wt pct CNT and Al foams were successfully produced using a spark plasma sintering and dissolution process. Al-CNT foams with porosity levels of ~78 pct were produced. The mechanical response of the open-cell foams reveals initial evidence of enhanced damage tolerance of Al-CNT foams over Al foams produced in this study. © 2016 The Minerals, Metals & Materials Society and ASM International
The vesicant problem during the process of preparing closed-cell aluminum foam by molten body transitional foaming process was discussed and the effect of granularity and addition of TiH2 on porosity of closed-cell aluminum foam was investigated. The static compressive behavior of closed-cell aluminum foam and the influence of porosity on static compressive property of closed-cell aluminum foam were researched as well. The results show that with increasing granularity of TiH2, the porosity of closed-cell aluminum foam firstly increases and then decreases gradually, the granularity should be controlled in the range of 38-74 mu m which can result in higher porosity. The porosity of closed-cell aluminum foam increases with the increasing addition of TiH2, and the addition of TiH2 should be controlled from 1.5% to 2.5% which can result in homogeneous cell and moderate strength of closed-cell aluminum foam. The compressive process of closed-cell aluminum foam obviously displays linear elastic phase, plastic collapse phase, and densification phase, and the compressive strength grows with decreasing porosity.
Open-cell foam aluminum with porosity more than 70% was prepared by means of penetrating casting technology using NaCl in different particle sizes as precursor. The surface morphology, element content, porosity and pore diameter distribution were characterized with SEM, ICP-MS, and mercury injection apparatus. The results show that open-cell foam aluminum with different pore diameters can be obtained through adjusting the particle size of NaCl. The samples present uniform distribution of pore diameters. The porosity of the foam aluminum with small pore diameter is higher than the large one. Its pore diameter and porosity are 41.95 mu m and 72.50%, respectively.
Using spark plasma sintering technique, a bulk Al90Mn9Ce1 alloy, reinforced by quasicrystal particles, has been sintered under the pressure of 50 MPa at 400°C. The density and the compressive strength of the as-prepared bulk alloy are above 98% and 1000 MPa, respectively. It has been found that during the spark plasma sintering process, under the condition of large current, short sintering time and lower sintering temperature, the metastable icosahedron quasicrystal phase in primary powder particles is kept. And the strong discharge plasma among particles can decompose or break the oxide film on the primary powder's surface, resulting in the increment of their combination strength. So the as-prepared bulk alloy possesses high density and ultra-high strength.
This is a review of the processing, structure and properties of metals containing a significant volume fraction of distributed internal porosity. These materials serve in a variety of applications, some of which place emphasis on their mechanical properties, while others are driven by transport processes made possible by the accessibility of open pores to the ingress and flow of fluid. Both classes of properties are reviewed after presenting the making and the structure of these materials. Coverage thus includes the processing and structure of highly porous metals, and their properties including conduction, fluid flow, convective heat and mass transfer, thermal expansion, elastic deformation, followed by plasticity, creep, fracture and fatigue.
The sintering of Recently, a friction powder compaction process by the sintering and dissolution process route for fabricating porous aluminum (Al) has been developed.the mixture of Al and NaCl powders was conducted only by a rotating tool plunged into the mixture. In this process, elongation of pores, which was caused by shear deformation generated by rotating tool, was observed. In this study, shear deformation was observed using X-ray computed tomography (CT) by fabricating Al-Fe compact. It was shown that the shear deformation was marked at the vicinity of the rotating tool, but largely decreased as the distance from the rotating tool became large. By comparing with the pore structures of porous aluminum with porosity of 60%, elongation of pores were only observed at the vicinity of the rotating tool. From these results, it was indicated that although the slight shear deformation was generated at the remote part from the rotating tool, the NaCl remained its shape and therefore no deformation of pore structures was observed.
This chapter describes the processing and properties of metals containing significant fractions of porosity, processed using powders. The basic concepts used in porous materials research are introduced and the different types of processing techniques that have been explored are surveyed. The reported property data for different foams are collated and used to illustrate the range of properties that have been achieved and methods to predict the properties of porous metals from elementary knowledge about their structure are discussed. Finally, the outlook for porous metals research and some likely future directions of fruitful enquiry are suggested.
A new friction powder compaction process by the sintering and dissolution process route for fabricating open-cell porous aluminum (Al), which requires no external heat sources except friction heat, was developed. The sintering of the mixture of Al and NaCl powders was conducted only by a rotating tool plunged into the mixture. Namely, the sintering of the mixture was achieved because of the friction heat and pressing load generated by the rotating tool plunged into the mixture. In this study, porous Al with porosities of 60%, 70% and 80% were successfully fabricated. Their pore structures were observed by SEM and X-ray computed tomography (CT), and their mechanical properties were investigated by compression tests. It was found that almost the entire specimen had pore structures similar to the NaCl morphology regardless of the porosity. Plateau stress decreased with increasing porosity indicating strong relationships between them. Porous Al exhibited ductile fracture regardless of its porosity, which is considered to be attributed to the good bonding between Al particles.
Porous Al was fabricated by combining a sintering dissolution process and a friction powder compaction process. Porous Al fabricated using a Cu die exhibited superior mechanical properties to that fabricated using an Al die and had a fine pore structure, which is attributed to the high load and temperature during the sintering process when using the Cu die.
In this study, a porous Al/NaCl composite was fabricated by a sintering and dissolution process, and the possibility of controlling the mechanical properties of porous Al by varying the amount and distribution of residual NaCl was investigated. It was shown that the range of the plateau region decreased and the stress–strain curves became similar to those of dense materials as the amount of residual NaCl increased. The deformation started from the layers where NaCl was removed, then the layers where NaCl remained started to deform. This indicates that the strength of each region can be controlled by varying the amount and distribution of residual NaCl.
Open-cell porous aluminum with a controlled pore structure can be fabricated by sintering and dissolution process. To overcome the size limitation of porous aluminum fabricated by the sintering and dissolution process an enhanced friction powder compaction (FPC) process for fabricating porous aluminum was proposed. In this process the rotating tool plunged into the powder mixture and die during the FPC process is made to traverse perpendicular to the direction of plunging. It was found that long porous aluminum can be fabricated with a length equal to the tool traversing length. By scanning electron microscopy (SEM) observation of the pore structures it was found that although the region in the vicinity of the traversing rotating tool had an elongated pore structure almost the entire sample had a pore structure that was similar to the NaCl morphology regardless of the traversing direction. From compression test fabricated porous aluminum exhibited ductile fracture which is considered to be attributed to the good bonding between aluminum particles.
Fe-2Cu-2Ni-1Mo-0.8C (wt pct) elemental mixed powders were rapidly sintered within 6 min by spark plasma sintering, and the effects of sintering parameters on the densification degree and performance of the as-sintered materials were investigated. Results showed that when a proper combination of pulse electric current and constant electric current was employed for sintering, the density and bend strength of the as-sintered material reached the maxima, being 7.61×10 3 kg/m 3 and 1540 MPa, respectively. Its corresponding fracture morphology was characterized as the mix of ductile, intergranular and cleavage fractures.
In this study, porous titanium samples were manufactured by space holder methods using two kinds of urea and sodium chloride space holders. Three-dimensional pore structures were obtained by a computed-tomography (CT) technique and utilized for finite element analysis in order to investigate the mechanical properties. The CT-based finite element analyses were in better agreement with the experimental results than unit cell model-based analyses. Both the experimental and CT-based results showed the same tendency that the elastic modulus decreased with increasing the porosities. The total porosity of the bulk body plays a key role in determining the elastic modulus of porous materials.
A new friction powder compaction (FPC) process by the sintering and dissolution process (SDP) route for fabricating open-cell aluminum (Al) foam, which requires no external heat sources, was developed. Foams with porosities of 74 and 83 pct were successfully fabricated and their compressive responses were investigated. The sintered mixture during the removal process was observed nondestructively by X-ray computed tomography (CT) to reveal the progress of the removal of soluble particles and to confirm that they were completely dissolved.
Al-Si closed-cell aluminum foams of different densities were prepared by molten body transitional foaming process. The tensile behavior of Al-Si closed-cell aluminum foam was studied and the influence of relative densities on the tensile strength and elastic modulus was also researched. The results show that the fracture surfaces of Al-Si closed-cell aluminum foam display quasi-cleavage fracture consisting of brittle cleavages and ductile dimples. The tensile strength and elastic modulus are strictly affected by the relative density of Al-Si closed-cell aluminum foam. With increasing relative density, the tensile strength increases and the strain at which the peak strength is measured also increases; in addition, the elastic modulus increases with increasing relative density.
Porous Al specimens with a pore size range from 212-300 to 610-700 mu m, a porosity from 85 to 95% and a specimen thickness from 2 to 20 mm were produced by the spacer method, and their sound absorption capacity was investigated. For these specimens, sound absorption coefficient increased with increasing porosity. On the other hand, sound absorption coefficient varied inconsistently with the variation of pore sizes. The latter may be attributed to variation of aperture sizes of each specimen because the porous Al specimens with differerent pore sizes produced by the spacer method should have different aperture sizes. Sound absorption coefficient increased at the frequency below 2000 Hz with increasing specimen thickness.
Porous metals, or metallic foams, are emerging ecomaterials that can be applied to structural use, shock absorber, filter, heat exchanger, etc. Their very low densities and peculiar deformation behaviors will facilitate the application. The control of pore characteristics such as porosity and pore size distribution can be successfully achieved by spacer method. In this paper, fabrication of porous aluminum via the spacer method is introduced and their excellent properties due to homogeneous pore characteristics are exhibited.
In the present study, porous nickel foam samples with pore sizes of 20 μm and 150 μm and porosities of 60 % and 70 % were fabricated by the space-holding sintering method via powder metallurgy. Electron scanning microscopy (SEM) and Image-Pro Plus were used to characterise the morphological features of the porous nickel foam samples. The anisotropic mechanical properties of porous nickel foams were investigated by compressive testing loading in different directions, i.e. the major pore axis and minor pore axis. Results indicated that the nominal stress of the nickel foam samples increases with the decreasing of the porosity. Moreover, the foam sample exhibited significantly higher nominal stress for loading in the direction of the major pore axis than loading in direction of the minor pore axis. It is also noticeable that the nominal stress of the nickel foams increases with the decreasing of the pore size. It seems that the deformation behaviour of the foams with a pore size in the micron-order differs from those with a macro-porous structure.
Porous copper specimens with relative densities of 0.22-0.96 were produced by spacer method and their compressive properties were investigated. In the low relative density range (relative density < 0.5-0.6), porous copper showed a density exponent n of 2.3, where n represents the relative density dependence of yield strength. In this range, the bending and buckling of cell walls and the formation of macroscopic deformation bands were observed. On the other hand, porous copper with a higher relative density (0.5-0.6 < relative density < 0.9-1) had an n value of approximately 1, where the dominant deformation mode of cell walls was yielding and no clear deformation band was observed. Also, in the highest relative density range (relative density is very close to 1), the compressive properties degraded markedly with decreasing density, indicating that stress concentration around the minimal pores occurred in this density range.
The mechanical properties of a closed-cell aluminium foam were investigated by compressive tests, and the deformation behaviours of the aluminium foams were studied using X-ray microtomography. The results indicate that the deformation of the aluminium foams under compressive loading was localized in narrow continuous deformation bands having widths of order of a cell diameter. The cells in the deformation bands collapsed by a mixed deformation mechanism, which includes mainly bending and minor buckling and yielding. Different fractions of the three deformation modes led to variations in the peak stress and energy absorption for different foam samples with the same density. It was also found that the cell morphology affects the deformation mechanism significantly, whilst the cell size shows little influence.
Spacer method is excellent technique of processing porous metals with well-controlled pore characteristics such as porosity (up to 90%) and pore size (as small as several hundred micrometers). Compressive properties of porous aluminum fabricated by the spacer method are investigated. They were subjected to monotonic compression tests at room temperature, and showed less fluctuated flow stress during their compressive deformation than conventional porous aluminum alloy, reflecting their homogeneous pore characteristics. Also, shortening behavior of the porous aluminum fabricated by the spacer method during cyclic compression was significantly differed from that of conventional porous aluminum alloy. Therefore, it can be concluded that the homogeneity of pore characteristics is responsible for compressive properties of porous metals. Monotonic compression tests on porous copper specimens with various porosities, which were made by the spacer method, were also conducted. The yield stress of the porous copper with high porosity (or low relative density) depended on the relative density more strongly than that of the porous copper with low porosity (or high relative density). It is presumed that porous metals with high porosity and ones with low porosities have different deformation mechanisms.
Deformation behavior under monotonic and cyclic compressions of porous Al specimens, whose porosities were 80 and 90%, produced by the spacer method was investigated at room temperature. The deviation in flow stress in the strain range of the plateau region was very low under monotonic compression. Also, under cyclic compression, the strain gradually increased with cycles. These deformation characteristics of the porous Al produced by the spacer method were related to a decrease in the intensity of deformation localization. From the comparison of closed-cell porous Al (ALPORAS) specimens of different sizes, it was suggested that the sample size effect is responsible for the decrease in the intensity of deformation localization under monotonic compression. However, the decrease in the intensity of deformation localization under cyclic compression was due to the uniform cell structure, which is an advantage of the spacer method, and not due to the sample size effect.
The compressive properties of porous copper with relative densities, rho/rho(s), of 0.22-0.96 were investigated. In the low relative density range (rho/rho(s) < 0.5-0.6), porous copper showed a density exponent n of 2.3, where n represents the relative density dependence of yield strength. In this range, the bending and buckling of cell walls and the formation of macroscopic deformation bands were observed. However, porous copper with a higher relative density (0.5-0.6 < rho/rho(s) < 0.9-1) had an n value of similar to 1, where the dominant deformation mode of cell walls was yielding, and no clear deformation band was observed. Also, in the highest relative density range (rho/rho(s), very close to 1), the compressive properties degraded markedly with decreasing density, indicating that stress concentration around the minimal pores occurred in this density range. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Porous aluminum with well-adjusted porosity and pore size was fabricated by a powder-metallurgical spacer method, and its fluid conductivity was investigated. As the porosity and pore size increased, the Darcian permeability increased. However, the permeability of the porous aluminum was significantly lower than the expected value based on those of other similar porous materials. The low fluid conductivity is attributed to the presence of small apertures at cell walls, which is peculiar to the porous metals fabricated by the powder-metallurgical spacer method.
Closed-cell aluminum foams were prepared by molten body transitional foaming process. The anisotropic damping property of closed-cell aluminum foam was measured in two directions using the forced vibration method. The measured results show that the loss factors of the TD specimens are higher than that of the LD specimens. The loss factors ratio βL/βT depends linearly on the shape-anisotropy ratio R. Anisotropic damping behavior is due to the variation of Young’s modulus resulting from anisotropic cell morphology.
In the present study, porous nickel foams with three different porosities (i.e. 50%, 60% and 70%) were fabricated using the space-holding sintering method. Ammonium bicarbonate particles with sizes ranging from 1 ̃ 2 mm were chosen as the space-holding material. The anisotropic behaviours of the nickel foam samples were investigated by compressive testing loading at different directions, i.e., in both directions of the major and minor axis of ellipsoidal cells. Electron scanning microscopy (SEM) and Image-Pro Plus was used to characterise the morphological characteristics of the porous nickel foam samples. Results indicated that the porous nickel foam samples exhibited obvious anisotropic mechanical properties. The foam sample shows significantly higher nominal stress for loading in the direction of the major axis of the pores than loading in the direction of the minor axis of pores. The nominal stress increases with the decreasing of the porosity.
Solid-state foaming of metals can be achieved by hot-isostatic pressing of powders in presence of argon followed by expansion of the resulting high-pressure argon bubbles at ambient pressure and elevated temperature. This foaming technique was first demonstrated by Kearns et al. [1] for Ti-6Al-4V, but is limited by its low creep rate and ductility, which lead to early cell wall fracture. We address these issues by performing the foaming step under superplastic conditions. Rather than using microstructural superplasticity (requiring fine grains which are difficult to achieve in porous powder-metallurgy materials), we used transformation superplasticity, which occurs at all grain sizes by biasing with a deviatoric stress (from the pore pressure) of internal stresses (from the allotropic mismatch during thermal cycling about the allotropic temperature range). As compared to control experiments performed under isothermal creep conditions, superplastic foaming under temperature cycling of unalloyed titanium and alloyed Ti-6Al-4V led to a significantly higher pore volume fraction and higher foaming rate.
The possibilities for manufacturing metal foams or other porous metallic structures are reviewed. The various manufacturing processes are classified according to the state of matter in which the metal is processed-solid, liquid, gaseous or ionised. Liquid metal can be foamed directly by injecting gas or gas-releasing blowing agents, or by producing supersaturated metal-gas solutions. Indirect methods include investment casting, the use of space-holding filler materials or melting of powder compacts which contain a blowing agent. If inert gas is entrapped in powder compacts, a subsequent heat treatment can produce cellular metals even in the solid state. The same holds for various sintering methods, metal powder slurry foaming, or extrusion and sintering of polymer/powder mixtures. Finally, electro-deposition or metal vapour deposition also allow for the production of highly porous metallic structures. The various ways for characterising the properties of cellular metals are reviewed in second section of this paper. Non-destructive as well as destructive methods are described. Finally, the various application fields for cellular metals are discussed. They are divided into structural and functional applications and are treated according to their relevance for the different industrial sectors. (C) 2001 Elsevier Science Ltd. All rights reserved.
Cellular solids include engineering honeycombs and foams (which can now be made from polymers, metals, ceramics, and composites) as well as natural materials, such as wood, cork, and cancellous bone. This new edition of a classic work details current understanding of the structure and mechanical behavior of cellular materials, and the ways in which they can be exploited in engineering design. Gibson and Ashby have brought the book completely up to date, including new work on processing of metallic and ceramic foams and on the mechanical, electrical and acoustic properties of cellular solids. Data for commercially available foams are presented on material property charts; two new case studies show how the charts are used for selection of foams in engineering design. Over 150 references appearing in the literature since the publication of the first edition are cited. It will be of interest to graduate students and researchers in materials science and engineering. © Lorna J. Gibson and Michael F. Ashby, 1988 and Lorna J. Gibson and Michael F. Ashby, 1997.
The symposium, Porous and Cellular Materials for Structural Applications, was held on April 13--15 at the 1998 MRS Spring Meeting in San Francisco. Recent developments were presented and discussed in the area of porous and cellular materials, including polymer-, ceramic-, and metal-based materials. The general focus of the symposium was on porous materials that are being developed, at least in part, for structural applications. Theoretical aspects of the mechanical behavior of porous and cellular materials were discussed, as well as the specific mechanical properties of a wide variety of solid foam materials. Design principles for the use of solid foams in structures were presented, and a number of promising applications for porous and cellular materials were shown. Papers on the manufacture of solid foams and the production of parts containing solid foams were presented. A significant portion of the symposium was devoted to new porous materials that cannot be classified as foams, such as hollow spheres, hot isostatically compacted and expanded (HICE) materials and GASAR materials. Thirty nine papers were processed separately for inclusion on the data base.
A sintering-dissolution process (SDP) is described for manufacturing net-shape, open-cell Al foams. These foams are characterized based on porosity, microstructure and compressive properties under a range of SDP conditions. The capabilities of SDP are discussed.
Recently, there is a high interest in using light-weight metallic foams (e.g., Al and Mg) for automotive, railway and aerospace applications where weight reduction and improvement in comfort are needed. Metallic foams also have a potential for absorbing impact energy during the crashing of a vehicle either against another vehicle or a pedestrian. In this study, enhancement of absorption energy in a closed-cell structure has been performed by an increase in the aspect ratio of cell-wall thickness against the cell-edge length with the reduction of cell size. The absorbed energy in a modified foam is estimated comparing with that in a conventional ALPORAS with the same relative density.
Nb3Al powder prepared by hydriding–dehydriding was sintered using spark plasma sintering. Experiments on high temperature strength and superconductivity show that the sintered samples have superior properties than those reported elsewhere.
Compressive behaviour of CYMAT aluminium foams with relative densities ranged from 5% to 20% has been studied experimentally in this paper. An MTS machine is employed to apply a compressive load at strain rates of 10−3–10+1 s−1 to these closed-cell aluminium foams. It has been found that the plateau stress is insensitive to the strain rate and is related to the relative density by a power law. Deformation is not uniform over the whole sample: it first occurs in the weakest band, followed by the next weakest bands after the first one has been completely crushed.
The graphite die set in spark plasma system (SPS) is heated by a pulse direct current. Weak plasma, discharge impact, electric field and electric current, which are based on this current, induce good effects on materials in the die. The surface films of aluminum and pure WC powders are ruptured by the spark plasma. Pure AlN powder is sintered without sintering additives in the electric field. The spark plasma leaves discharge patterns on insulators. Organic fibers are etched by the spark plasma. Thermosetting polyimide is consolidated by the spark plasma. Insoluble polymonomethylsilane is rearranged into the soluble one by the spark plasma. A single crystal of CoSb3 is grown from the compound powders in the electric field by slow heating. Coupled crystals of eutectic powder are connected with each other in the electric field.
This study primarily concerns the role of cell wall microstructure in influencing the mechanical behaviour of metallic foams. Three closed-cell foams have been examined, having rather similar relative densities and cell structures but significant differences in cell wall microstructure. It is concluded that these differences can substantially affect the micro-mechanisms of deformation and failure under different types of loading and can also have an influence on the macroscopic mechanical response. Cell wall ductility and toughness are impaired by high volume fractions of coarse eutectic, fine oxide films and large brittle particles, all of which were present in one or more of the foams studied. This impairment can lead to extensive brittle fracture of cell walls, with little energy absorption, even under nominally compressive loading conditions. The influence of cell wall ductility tends to become more significant when the loading state is such that local tensile stresses are generated.
Spark plasma sintering is a new process by which ceramics and composites can be consolidated very rapidly to full density. In the present study, piezoelectric Nd2Ti2O7 second phase toughening nanocrystalline alumina composites with higher toughness were successfully developed at relatively low temperatures through this technique.