Faqiang Shi,Limeng Liu,Jiandong Wu,Junfeng Hou,Yujin Wang
TaC x (x = 0.5, 0.55, 0.6 and 0.7) ceramics with 20 mol% Al addition were prepared by hot pressing TaC, Ta and Al powder mixtures. Effects of the overall C:Ta ratios and the Al addition on constituent phases, microstructures and mechanical properties of the consolidated materials were examined. Results showed that O impurity contained in the starting powders was collected in form of Al 2 O 3 due to strong affinity of O with Al. The TaC x -Al composites were easy to densify due to presence of molten Al at its melting point ( T m = 660 °C) and a transient Al–Ta eutectic liquid at higher temperatures. After densification, the metallic liquids were eliminated ultimately by formation of refractory Ta 4 AlC 3 via reaction of Al, Ta and TaC. Reactions between the remnant Ta and TaC after Ta 4 AlC 3 formation resulted in ζ-Ta 4 C 3-z or h-Ta 2 C depending on the initial C:Ta ratio. The Ta 4 AlC 3 compound was compatible with the tantalum carbides. Microstructures of the TaC x -Al (x = 0.5–0.7) composites were consisted of ζ-Ta 4 C 3-z and Ta 4 AlC 3 fine lamellar grains to benefit the mechanical properties of the composites. The TaC 0.7 –20mol% Al composition had satisfying mechanical properties (flexural strength 613 ± 28 MPa and fracture toughness 11.2 ± 0.8 MPa m 1/2 ). TG-DSC evidenced an increase in oxidation resistance of the TaC x ceramics by the addition of 20 mol% Al.
Zhuang Lu,Limeng Liu,Guihong Geng,Alexander M. Laptev,Zhaoping Hou,Yujin Wang
TaC ceramics with 0.03–0.60 wt% of boron additions were prepared by hot pressing at 2100 °C for 1 h under a pressure of 40 MPa. Effects of boron content on densification, phase composition, microstructure, mechanical properties and oxidation resistance of the TaC ceramics were investigated. When the boron content was 0.12 wt% and above, full density was obtained due to reactions between boron and oxygen impurity at presence of TaC. Minor phases of TaB 2 and C were formed in the 0.24 and 0.60 wt% B compositions after gas-out of the oxygen impurity. Microstructure of the TaC ceramics was refined with increasing in boron content. The TaC ceramic with 0.24 wt% of boron showed the best mechanical properties with a Vickers hardness , flexural strength and fracture toughness of 17.7 GPa, 534 MPa and 4.6 MPa m 1/2 , respectively. When more boron was added, interfacial bonding of the TaC grains was strengthened causing a decrease in fracture toughness. Oxidation resistance of the TaC ceramics increased with boron content. Particularly, the 0.60 wt% B composition showed a weight gain of 0.0018 g/cm 2 after oxidization at 800 °C in air for 3 h.
Limeng Liu,Jozef Vleugels,Shuigen Huang,Jie Wei,Yujin Wang
TaC ceramics with 0-0.237 wt% B addition were prepared by hot pressing. The effect of B addition on the phase constitution, interfacial chemistry/bonding and mechanical properties of the TaC ceramics were investigated. Upon B addition, the elimination of O impurity and segregation of B at grain boundaries were evidenced, accompanied by an increase in bonding strength of the TaC grains, to result in a fracture mode change from intergranular to transgranular and a reduced fracture toughness. Addition of excessive B resulted in the formation of TaB 2 and C within TaC ceramics. Further, TaC-TaB 2 -SiC composites were prepared by Si addition. Coherent bonding between TaB 2 and TaC was preserved in the TaC-TaB 2 -SiC composites, and residual stresses due to thermal expansion mismatch of the different phases increased flexural strength and fracture toughness of the composites.
The Vickers hardness of the bulk TaB and nano-TaC ceramics sintered at high pressure and high temperature were measured by using a diamond indentation method. Our tests gave a maximum Vickers hardness of 13.9 GPa for TaB, and 19.2 GPa for nano-TaC ceramics under a load of 29.4 N. The hardness of nano-TaC-5 wt% Co composites was also measured under the same load for comparison, which was found to be much lower than that of TaB and nano-TaC. The relative densities calculated by the Archimedes method are 13.5 g/cm 3 for TaB and 13.6 g/cm 3 for nano-TaC. The averaged fracture toughness is 6.8 ± 0.3 MPa m 1/2 for TaB and 5.0 ± 0.2 MPa m 1/2 for nano-TaC. Our results show that the hardness of the TaB may not be as high as previous reports.
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