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Sintering of zirconium carbide-based composites
Abstract
Zirconia (ZrO2), titanium carbide (TiC), or molybdenum (Mo) was used to create a series of composite materials aimed to improve the densification, refractory hardness, and fracture toughness of zirconium carbide (ZrC). ZrC powder was produced from a cost-effective, in situ reactive heat treatment of nano-sized zirconium dioxide and graphite. The composites were pressureless sintered in vacuum at temperatures up to 1900 °C. The final densities of the composites varied between 96 -98%. Hardness ranged from (HV10) 17 GPa, 19 GPa, and 20.2 GPa for ZrC-Mo, ZrC-ZrO2, and ZrC-TiC respectively. Fracture toughness ranged from 5.6 MPa*m1/2 for ZrC-Mo, 10.2 MPa*m1/2 for ZrC-ZrO2, and 7.1 MPa*m1/2 for ZrC-TiC. However, ZrC-TiC composites was able to be sintered at just 1800 °C compared to all other samples, which needed at least 1900 °C for highest refractory results.
... In addition, SPSed cermets contained more eta phases in the microstructure for WC-Co [23][24][25]. Previous studies performed by the authors of this paper demonstrated that a composition of ZrC-20 wt% Mo ( $ ZrC-14 vol%Mo) was the optimal ratio for the cermet sintered under vacuum up to 1900°C with mechanical properties HV 10 1690 and 5.6 MPa*m 1/2 [26]. In this study, the same ZrC-20 wt%Mo composition is produced and undergoes a series of SPS experiments at temperatures ranging from 1600 to 2100°C using two compaction pressures: 50 MPa or 100 MPa. ...
The microstructure analysis and mechanical characterisation were performed on a ZrC-20wt.%Mo cermet that was spark plasma sintered at various temperatures ranging between 1600–2100 °C under either 50 or 100 MPa of compaction pressure. The composite reached ~98% relative density for all experiments with an average grain size between 1–3.5 μm after densification. The nature of SPS technology caused a faster densification rate when higher compaction pressures were applied. The difference in compaction pressures produced different behaviors in densification and grain structure: 1900 °C, 100 MPa produced excessive grain growth in ZrC; 1600 °C, 50 MPa revealed a very clear ZrC grain structure and Mo diffusion between carbide grains; and 2100 °C, 50 MPa exhibited the highest overall mechanical properties due to small clusters of Mo phases across the microstructure. In fact, this particular sintering regime gave the most optimal mechanical values: 2231 HV10 and 5.4 MPa⁎m1/2, and 396 GPa Young's modulus. The compaction pressure of SPS played a pivotal role in the composites’ properties. A moderate 50 MPa pressure caused all three mechanical properties to increase with increasing sintering temperature. Conversely, a higher 100 MPa pressure caused fracture toughness and Young modulus to decrease with increasing sintering temperature.
Majority of the fracture toughness studies of metallic materials typically use the Chevron notch technique, compact specimens and round notched tensile specimens. These methods require considerable time for sample preparation and for the notch geometry control. Only a few of them can be applied for cemented carbides due to the very high brittleness of the hard phase. This is why various indentation fracture toughness (IF T) techniques have been developed (1,2), which are more rapid and simple. Indentation toughness measurements results depend critically on the assumption about the crack type (Palmqvist or median/radial cracks), on the equations used for the calculation of fracture toughness and on the material-dependent and material-independent constants ( 3). In the present work a comparative study of IFT calculation methods was carried out to find a reliable technique for studied materials (WC-Co, TiC-Fe/Ni). Several IFT equations for ceramic materials, recommended by standards and publications, were used for the evaluation of the fracture toughness and compared with published conventional fracture toughness data ( 4,5). Only few of the equations give reliable estimation of the fracture toughness of cemented carbides.
Zirconium carbide, titanium carbide, zirconium nitride and titanium nitride, which are promising candidates for the ceramic matrix of inert matrix fuel (IMF) to transmute long-lived actinides were sintered using the spark plasma sintering (SPS) technique. Dy2O3(20wt%) was added as a surrogate for Am2O3 and the sintering behaviours of Dy2O3 dispersed carbide or nitride matrix composites were compared with that of the matrix. The spark plasma sintering conditions consisted of a rapid heating rate of 75Kmin−1 and a very short holding time of 1–4min at maximum temperatures ranging from 1773 to 2000K. Dy2O3 dispersed carbides and nitrides with about 80% of theoretical density were obtained by spark plasma sintering with a rapid heating rate and a short dwelling time. When Dy2O3 was added to the matrix, the shrinkage of the carbide or of nitride composites was initiated from a lower temperature than its matrix material during the heating stage. In the case of TiC and TiN, microstructural observation exhibited that Ti is soluble in dysprosium oxide and densification is enhanced around the oxide phase.
The excess energies for A1−xBxC mixed carbides (where A and B are metals) have been calculated using ab-initio calculations, for 14 systems. A thorough comparison has been made with experimentally assessed excess energies. The comparison shows that conventional ab-initio calculations applied to rather simple structural models can be used to predict the sign, magnitude and symmetry of the excess energy for A1−xBxC mixed carbides. The calculated excess energies have also successfully been used to describe several AC–BC systems where the experimental information does not give a unique determination of the excess energy in traditional CALPHAD modelling. The systems that have been studied are CrC–TiC, HfC–NbC, HfC–TaC, HfC–TiC, HfC–VC, NbC–TaC, NbC–VC, NbC–ZrC, TaC–VC, TaC–ZrC, TiC–VC, TiC–ZrC and VC–ZrC.
Zirconium carbide containing up to 40 wt% yttria-stabilized zirconia was sintered at 1800 to 2200° C in argon and carbon monoxide atmospheres in a graphite tube furnace. Well-dispersed fine powders containing 20 wt% ZrO2 or more, when formed into compacts with green densities of 55% or above, could be sintered to theoretical density in 1 h at 2000° C. The resulting microstructure consisted of a dispersed ZrO2 phase in a continuous zirconium oxi-carbide matrix. The fracture toughness and four-point bend strength were a maximum at 40 wt% ZrO2 and were 5.8 MPa m1/2 and 320 MPa, respectively. The fracture surface was irregular and primarily intergranular.
The sinterability of ZrC was enhanced by high-energy ball milling as well as introduction of graphite and SiC as sintering additives. Densification process and microstructure development were investigated for ZrC-based ceramics densified by pressureless sintering. As-received ZrC powder showed poor sinterability. After high-energy ball milling, ZrC powder can be sintered to 98.4% theoretical density at 2100 °C. The obtained ceramic had fine microstructure and fewer entrapped pores. Introduction of 2 wt.% graphite combined with high-energy ball milling lowered the densification temperature of ZrC. The relative density of obtained ceramic reached up to 95% at 1900 °C. Introduced SiC inhibited ZrC grain growth during sintering and consequently avoided the entrapped pores within the grains. The relative density of ZrC–SiC reached up to 96.7% at 2100 °C. ZrC–SiC composite formed an interesting intragranular structure and had high fracture strength at room temperature.Research Highlights► High-energy ball milled ZrC was pressurelessly sintered to 98.4% theoretical density at 2100°C. ► High-energy ball milled ZrC was pressurelessly sintered to 95% theoretical density at 1900°C using 2wt% graphite as sintering aid. ► High-energy ball milled ZrC + 20vol.%SiC was pressurelessly sintered to 96.7% theoretical density at 2100°C. ► The obtained ZrC + 20vol.%SiC ceramic formed an intragranular microstructure and had excellent mechanical properties.
ZrC-based composites were produced by pressureless sintering thanks to the addition of MoSi2 as sintering aid. After preliminary tests, a baseline ZrC material and two mixed ZrC–HfC and ZrC–ZrB2 composites with 20 vol% MoSi2 were densified at 1900 to 1950 °C reaching final relative densities of 96%–98%. Mean particle size of the dense bodies ranged from 5 to 9 μm. Secondary phases were found to form during sintering, such as SiC and Zr–Mo–Si-based compounds. Room-temperature mechanical properties were in the range of the values reported in the literature for similar materials densified by pressure-assisted techniques. The flexural strength was tested at room temperature, 1200 and 1500 °C.
Pressureless sintering of ZrC–Mo cermets was investigated in a He/H2 atmosphere and under vacuum. A large density increase was observed for specimens with >20 vol% Mo after heating at 2150°C for 60 min in a He/H2 atmosphere. The increase in density was attributed to the formation of Mo2C during heating and its subsequent eutectic reaction with Mo, which produced rounded ZrC grains in a Mo–Mo2C matrix. Sintering in vacuum did not produce the same increase in density, due to the lack of Mo2C formation and subsequent lack of liquid formation, which resulted in a microstructure with irregular ZrC grains with isolated areas of Mo. Mechanical properties testing showed a decrease in Young's modulus with increasing Mo content that was consistent with the models presented. Flexure strength of ZrC–Mo cermets sintered in He/H2 atmosphere materials increased with increasing Mo content from 320 MPa at 20 vol% Mo to 410 MPa at 40 vol% Mo. Strength was predicted by adapting theories developed previously for WC–Co cermets.
Zirconium carbide–tungsten (ZrC–W) cermets were prepared by a novel in situ reaction sintering process. Compacted stoichiometric zirconium oxide (ZrO2) and tungsten carbide (WC) powders were heated to 2100°C, which produced cermets with 35 vol% ZrC and 65 vol% W consisting of an interpenetrating-type microstructure with a relative density of ∼95%. The cermets had an elastic modulus of 274 GPa, a fracture toughness of 8.3 MPa·m1/2, and a flexural strength of 402 MPa. The ZrC content could be increased by adding excess ZrC or ZrO2 and carbon to the precursors, which increased the density to >98%. The solid-state reaction between WC and ZrO2 and W–ZrC solid solution were also studied thermodynamically and experimentally.
The hot-pressing kinetics of zirconium carbide were studied between 1700 and 2400 C in argon. The validity of different theoretical models due to Murray, Koval'chenko, Skorokhod, Scholz and Lersmacher was tested. For temperatures exceeding 2200 C, there is reasonably good agreement between kinetics and the whole set of models, but it has not been possible to classify them in order to draw conclusions on the sintering mechanism. The activation energy of ZrC hot-pressing was calculated, starting from the viscosity calculated by Murray's formula, as 41 kcal mol–1 (171.7 kJ mol–1).