Binderless WC with high strength and toughness up to 1500 °C

A tungsten carbide ceramic containing 5 vol% silicon carbide was hot pressed to full density at 1820 °C. A small amount of transient liquid phase based on W-C-Si-O facilitated oxide removal in the reducing environment and favoured the development of a bimodal microstructure containing sub-micrometric grains with square or rod-like morphologies. These microstructural features led to outstanding mechanical properties from room to elevated temperatures. For the first time, WC-materials were characterized up to 1500 °C exhibiting flexural strength over 1 GPa in the whole temperature range and fracture toughness from 7 to 15 MPa⋅√m.     

Microstructure and properties of HfC and TaC-based ceramics obtained by ultrafine powder

HfC and TaC-based ceramics were hot pressed at 1900 °C for 5-20 min starting from synthesized ultrafine powders. The addition of 5 vol.% of MoSi₂ improved the densification, which increased from around 85-90% for the pure matrices to 95% for the composites. The flexural strength was measured at room temperature and at 1500 °C under protective atmosphere. The added value of this work consists in the utilization of cheap synthesized powders for the realization of the composites with properties comparable to those obtained using more expensive commercial powders. The effect of the nanometric size of the starting powder showed to have the potential to improve the densification behavior and the mechanical properties, however it is necessary a further optimization of the synthesis condition in order to avoid the formation of agglomerates of unreacted powder.     

Chemical conversion during transient liquid-phase hot pressing of TaSi2–TaC–SiC SHS-powder

This article presents a study on the manufacturing of three-phase TaSi₂–TaC–SiC ceramics through self-propagating high-temperature synthesis (SHS) and their subsequent chemical conversion to TaC–SiC carbide composites during transient liquid-phase hot pressing (HP). The effect of carbon black doping, ranging from 0% to 7%, on the degree of chemical conversion, structure, mechanical, and thermophysical properties of the ceramics was investigated. Our results showed that the proportionate increase of carbide content and decrease of TaSi₂ content in hot-pressed samples was achieved through carbon black doping. The increase of TaSi₂ content during hot pressing led to an increase in porosity from 4.3% to 23.8%, while the density decreased from 6.3 to 4.6 g/cm³. Superior mechanical properties were obtained when SHS-powder was doped with 1.5% carbon black (HV = 15.2 GPa, K_IC = 4.8 MPa × m^1/2, and σ_bend = 331 MPa). The structure of the ceramics was characterized by a TaSi₂–SiC matrix and highly dispersed TaC grains predominantly residing inside TaSi₂, with the TaC–TaSi₂ and TaSi₂–SiC interface being incoherent, as demonstrated through TEM studies. Complete conversion of TaSi₂ to TaC and SiC was achieved through 7% carbon black doping, resulting in the hot-pressed sample consisting solely of carbide grains. Two-stage hot pressing was employed to enhance the relative density of the two-phase TaC–SiC sample, resulting in ceramics characterized by HV up to 22.3 GPa, K_IC up to 6.1 MPa × m^1/2, σ_bend up to 256 MPa, and λ up to 36 W/(m × K).     

High-performance B4C matrix composites fabricated from the B4C–Si–CeO2 system via reactive hot pressing

Boron carbide (B₄C) matrix composites had the advantages of high hardness, high melting point and low density. However, due to the low relative density and poor fracture toughness of B₄C, its comprehensive properties were limited in engineering applications. In this work, in order to improve the comprehensive properties of B₄C composites, B₄C–SiC–SiB₆–CeB₆ composites were designed and fabricated via reactive hot pressing at 2050 °C and 20 MPa with B₄C matrix and novel additives (Double doping of Si and CeO₂) as raw materials. The effects of additive CeO₂ content on the microstructures and mechanical properties of composite were investigated, and reaction mechanisms of B₄C, Si and CeO₂ at different temperatures were studied in detail. The work showed that liquid phase Si and SiB₆ greatly improved the densification of composites. CeB₆ played an indispensable role in the formation of SiC–SiB₆ agglomerate structure, increasing strength and supplementing toughness. When the content of CeO₂ was 6 wt%, the relative density, hardness, flexural strength and fracture toughness reached to 99.7%, 34.9 GPa, 461.46 MPa and 5.57 MPa m^1/2, respectively. Our strategy benefited from the formation of two liquid phases and SiC–SiB₆ agglomerate structure, showing great potential in promoting sintering and improving fracture toughness.     

Stacking behavior of the close-packed Ta-atom planes and its effects on formation of “hybrid grains” in TaC0.66-0.7 ceramics

In previous works, it was found hard to synthesize “phase pure” ζ-Ta₄C_3-z at relatively low temperatures even by prolonged heating, though ζ-Ta₄C_3-z was believed stable till decomposition at ~2130°C. When the samples were subjected to TEM, vast richness of locally disordered structures in close relation with stacking of the close-pacted Ta-atom planes was observed. Although kinetic factors including diffusion of C atoms/vacancies and re-stacking of the Ta-atom planes explain the densely disordered structures, the richness of local disorders is a scenario that shows cohabitant of the cubic, rhombohedral, and hexagonal structures in a single grain, i.e. formation of a “hybrid grain” consisted of the three symmetries, indicating a transitional or intermediate stage before complete formation of the final phase of rhombohedral ζ-Ta₄C_3-z. This time tantalum carbide ceramics TaC_x with C:Ta atomic ratios x = 0.66 and 0.7 were prepared by reaction hot pressing of TaC and Ta powder mixtures. 5–30 mol% Cu/Ag additives and heat treatments were used to reproduce “hybrid grains” to facilitate further TEM and HRTEM observations on the disordered hybrid grains to argue for the transitional/intermediate stage. The cohabitant cubic, rhombohedral, and hexagonal structures in single grains may also help explain the difficulty in identification of the various phases by XRD in the transitional/intermediate stage of ζ-Ta₄C_3-z reaction. Microstructural evolution and fracture toughness of the composites were also investigated.     

Effects of pressure on densification behaviour,microstructures and mechanical properties of boron carbide ceramics fabricated by hot pressing

Effects of pressure, from ordinary (30 MPa) to high pressure (110 MPa), on densification behaviour, microstructures and mechanical properties of boron carbide ceramics sintered by hot pressing are investigated. With increasing pressure, the relative density sharply increases within 30–75 MPa, slowly increases within 75–100 MPa and finally stagnates. For samples within 75–100 MPa, densification begins at approximately 1000 °C, and the dominant densification process ends before the soaking stage. High relative densities of 98.49% and 99.76% are achieved. For samples within 30–50 MPa, densification begins at approximately 1500 °C, and the soaking stage (initial 20 min) is still important for the dominant densification process. The final relative densities are only 87.90% and 92.32%. The above-mentioned differences are derived from contributions of pressure, and the dominant densification mechanism under high pressure is plastic deformation. The average grain size of the samples slightly increases with increasing soaking time. The grain size under higher pressure is larger than that under lower pressure at corresponding periods because grains grow easily with reduced pores. Vickers hardness and fracture toughness increase as grain size decreases in fully dense samples. However, when the samples do not achieve full density, relative density becomes more influential than grain size in hardness and toughness. A soaking time of 30 min is enough for samples under 100 MPa. Prolonging the soaking time has deleterious effects on mechanical properties. The relative density, grain size, hardness and fracture toughness of the samples under 100 MPa for 30 min are 99.73%, 1.96 µm, 37.85 GPa and 3.94 MPa m^1/2, respectively.     

Formation, densification, and selected mechanical properties of hot pressed Al4SiC4, Al4SiC4 with 30 vol.% WC, and Al4SiC4 with 30 vol.% TiC

Powders of Al₄C₃ and SiC were combined by high-energy milling to produce Al₄SiC₄, Al₄SiC₄ + 30 vol.% TiC, and Al₄SiC₄ + 30 vol.% WC. Five different temperatures were used to hot press the constituents. XRD, SEM, relative density, and hardness measurements showed that formation of single-phase Al₄SiC₄ occurred at 1450 °C and full densification (99%) was achieved at 1500 °C. Both of these temperatures are lower than previously reported. Adding TiC and WC increases hardness, while WC improves densification (99.5%).     

Investigating the effect of SPS‎ parameters on densification ‎and fracture toughness of ZrB2-SiC nanocomposite‎

In this study, the effect of sintering parameters on densification and fracture toughness of spark plasma sintering ZrB₂-SiC nanocomposites was evaluated. For this purpose, ZrB₂-??30?vol% SiC nanocomposites in the conditions of ?1600?°C-4?min, 1700?°C-4?min, 1800?°C-4?min, 1800?°C-8?min, 1800?°C-12?min? were sintered.? Scanning Electron Microscopy (SEM) was used in order to investigate the ?microstructural variations. The bulk density was measured accoring to ASTM C 373–88. Single edge notch beam (SENB) method was used to ?determine the fracture toughness of samples. Microstructural observations showed that ?an increase in sintering temperature led to slight ?increase in SiC grains size but no sensitive variation in ZrB₂. However, increasing the sintering time resulted to increase both ZrB₂ and SiC grain size. Also, it was found, temperature and time ascent always increases the relative density. In addition, it was concluded that optimal temperature and time to reach the highest fracture toughness are 1800?°C and 8?min, respectively. Investigation of SEM images of the Vickers indent and their path propagation showed that the deviation and branching of crack are the most important toughening ?mechanisms in ZrB₂-SiC nanocomposites.?     

Enhancing the sinterability and fracture toughness of Al2O3–TiN0.3 composites

We introduce a new and effective method for improving the fracture toughness of Al₂O₃-based composites through the addition of a nonstoichiometric material. Al₂O₃–TiN_0.3 composites were sintered by spark plasma sintering with different TiN_0.3 content at temperatures between 1300 and 1600 °C for 10 min and a micro-region diffusion phenomenon was observed at the Al₂O₃–TiN_0.3 interface. Ti atoms from TiN_0.3 diffused into Al₂O₃ to occupy Al sites, which led to the formation of Al vacancies that enabled the transport of aluminum by a vacancy mechanism. The optimal densification temperature of the Al₂O₃–30vol% TiN_0.3 composite was approximately 1400 °C. The maximum fracture toughness measured was 6.91 MPa m^1/2, from the composite with 30 vol% TiN_0.3 sintered at 1500 °C.     

Densification and properties of zirconia prepared by three different sintering techniques

Densification of nanocrystalline yttria stabilized zirconia (YSZ) powder with 8 mol% Y₂O₃, prepared by a glycine/nitrate smoldering combustion method, was investigated by spark plasma sintering, hot pressing and conventional sintering. The spark plasma sintering technique was shown to be superior to the other methods giving dense materials (≥96%) with uniform morphology at lower temperatures and shorter sintering time. The grain size of the materials was 0.21, 0.37 and 12 μm after spark plasma sintering, hot pressing and conventional sintering, respectively. Total electrical conductivity of the materials showed no clear correlation with the grain size, but the activation energy for spark plasma sintered materials was slightly higher than for materials prepared by the two other densification methods. The hardness, measured by the Vickers indentation method, was found to be independent on grain size while fracture toughness, derived by the indentation method, was slightly decreasing with increasing grain size.     

High pressure synthesis of tungsten carbide–cubic boron nitride (WC–cBN) composites: Effect of thermodynamic condition and cBN volume fraction on their microstructure and properties

Tungsten carbide (WC) with different amounts of Cubic boron nitride (cBN) were synthesized by High Pressure-High Temperature (HPHT) method. The mapping correlation between thermodynamic condition, cBN addition, and microstructure, mechanical properties of WC–cBN composites was established and analyzed by response surface methodology. The main factors affecting the properties of composites were identified by ANOVA. The optimum thermodynamic condition was calculated. It was found that a minor phase transformation of cBN into the low-hardness hBN occurred at a temperature of 1300 °C and intensified at 1500 °C. The homogeneously dispersed cBN particles in the WC matrix promoted an improvement of hardness and fracture toughness, but the phase transition of cBN and its truss effect can dramatically reduce the mechanical properties. The Vickers hardness and fracture toughness of the well-sintered WC-cBN bulks reached a high value of 34 GPa and 13.6 MPa·m^1/2, which are improved by 17% and 52% respectively compared with the pure WC samples sintered under similar high-pressure level.     

碳化硅晶须增韧氧化铝复合材料制备中的几个问题

本文就ＳｉＣｗ－Ａｌ２Ｏ３复合材料制备的主要环节，介绍了国内外的最新研究动态，讨论了该研究领域存在的主要问题，提出了今后努力的方向。     

Effect of mechanical properties on the ballistic resistance capability of Al2O3-ZrO2 functionally graded materials

Because Al₂O₃ exhibits high strength and hardness, it is prevalently used as a ceramic material. ZrO₂ is often added to increase the toughness of such a material. Therefore, this study mixed Al₂O₃ and ZrO₂ to formulate functionally graded materials (FGMs). four-layer and eleven-layer Al₂O₃-ZrO₂ FGM_S were produced from Al₂O₃ and ZrO₂ mixtures by sintering at 1500 °C. Moreover, testing sheets were created by mixing various ratios of Al₂O₃ and ZrO₂ to analyze their fracture toughness and hardness. The results revealed the 90% Al₂O₃-10% ZrO₂ sheet to exhibit a hardness of 15.12 GPa, and the 50% Al₂O₃-50% ZrO₂ sheet to attain a fracture toughness as high as 4.7 MPa m^0.5. The impact resistance test involved analyzing various types of testing sheets, including the four-layer Al₂O₃-(0%, 10%, 20%, 30%) ZrO₂ FGM, eleven-layer Al₂O₃-(0–100%) ZrO₂ FGM, 100% Al₂O₃ composite material, 90% Al₂O₃-10% ZrO₂ composite material, 70% Al₂O₃-30% ZrO₂ composite material, and 50% Al₂O₃-50% ZrO₂ composite material. The ballistic tests showed that FGMs of the same areal density (4.64 g/cm²) or thickness (11 mm) attained the highest energy absorption. The experimental results confirmed that FGMs can delay the formation and propagation of ceramic cones. Specifically, toughened alumina materials prevent the growth of radial and circumferential cracks, delay the formation of ceramic cones, decrease cones hitting against the back plane, and increase the penetrating resistant capability of the ceramic materials experiencing bullet impact, features important for applications in fields such as aerospace, aviation, automobile, the military industry, and biomedicine     

Influence of Ni and ZrO2 contents on sintering and mechanical properties of WC-2wt.%ZrO2-1wt.%Ni composites

WC-2wt.%ZrO₂-1wt.%Ni composites were prepared by vacuum pressureless sintering (VPS) and post-hot isostatic pressing (post-HIP). The microstructure, phase composition, densification, hardness, fracture toughness, and flexural strength of composites with different Ni and ZrO₂ contents were systematically investigated. The results show that WC-Ni and WC-ZrO₂-Ni composites prepared by VPS can be densified by the addition of a small amount of Ni as the binder phase. Moreover, the densities of the composites can be further enhanced i.e. the composites are rendered nearly fully dense following HIP while the grains remain fine without obvious growth. The binder Ni and ZrO₂ phases are uniformly distributed in the WC matrix and exhibit high bonding strength with it. The hardness, fracture toughness, and flexural strength of the WC-ZrO₂-Ni composites following HIP could reach 22.4?GPa, 12.0?MPa?m^1/2, and 1101.2?MPa, respectively. Based on the influence of the Ni and ZrO₂ contents on the microstructures and mechanical properties of the WC-2wt.%ZrO₂-1wt.%Ni composites, the fracture mechanism was determined to be governed by the phase transformation of ZrO₂ that led to the development of some micro-cracks followed by deflection, bridging, and branching of the cracks to improve fracture toughness. The composites are mainly composed of elongated triangular prismatic WC grains and ZrO₂ phases, and hence the fracture mode can change such that transgranular fracture becomes the main fracture mode accompanied by a small amount of intergranular fracture. Thus, the flexural strength of the composites can be improved.