Superhard single‐phase (Ti,Cr)B₂ ceramics

Abstract

A nominally pure and dense (Ti0.9Cr0.1)B2 ceramic was produced by spark plasma sintering of powders synthesized by boro/carbothermal reduction of oxides. The synthesized powders were a single phase and had an average particle of 0.4 ± 0.1 μm and an oxygen content of 1.2 wt%. Average Vickers hardness values of the resulting ceramics increased from 25.9 ± 0.8 GPa at a load of 9.81 N, to 46.3 ± 0.8 GPa at a load of 0.49 N. Compared to the nominally pure TiB2 ceramic obtained under the same processing conditions, the (Ti0.9Cr0.1)B2 ceramic had higher values under the same load due to the finer average grain size (2.4 ± 1.0 μm), higher relative density, and solid solution hardening. The results indicated that the Cr addition promoted densification, suppressed grain growth, and improved the hardness of TiB2 ceramics. This is the first report for dense and single‐phase (Ti,Cr)B2 ceramics as superhard materials.

B4C–(Ti0.9Cr0.1)B2 composites with excellent specific hardness and enhanced toughness

“…The reliability factors of refinement are R p = 7.14% and R wp = 10.52% for BTC, and R p = 8.49% and R wp = 11.25% for BT, demonstrating a high reliability of refinement results. It can be seen that the lattice parameters of B 4 C, TiB 2 , and (Ti 0.9 Cr 0.1 )B 2 phases in both composites deviate lightly from those given by JCPDS cards and of pure (Ti 0.9 Cr 0.1 )B 2 reported by Feng et al 18 Such deviations may be related to the existence of residual stress in B 4 C, TiB 2 , and (Ti 0.9 Cr 0.1 )B 2 phases of both composites due to their different thermal expansion coefficients (TECs). 25,26 Moreover, carefully analyzing the lattice parameters of B 4 C in BT and BTC, one can find that the lattice parameter a is smaller in BTC (5.6027 Å) than that in BT (5.6100 Å), suggesting that B 4 C grains in BTC are subject to greater compressive stress in a direction, which will be proven in latter sections through residual stress measurement.…”

“…Young's modulus E and Shear's modulus G of BTC are slightly lower than those of BT, which could be ascribed to relatively lower modulus of CrB 2 (E, 417 GPa) compared to TiB 2 (E, 560 GPa). 18,28,30 Both hardnesses at different loads and flexural strength of BTC, despite coarser-grained microstructure, are higher than those of BT, which may be attributed to solid-solution strengthening effects. Especially, BTC exhibits superhardness (43.2 ± 3.0 GPa at 9.8 N and 52.8 ± 3.9 GPa at 4.9 N), which is obviously higher than those of BT (33.4 ± 0.4 GPa at 9.8 N and 42.4 ± 1.3 GPa at 4.9 N), B 4 C (28.5 ± 1.2 GPa at 9.8 N), (Ti 0.9 Cr 0.1 )B 2 (25.9 ± 0.8 GPa at 9.8 N), and even some newly developed superhard materials such as (Mo 0.2 Ta 0.2 Ni 0.2 Cr 0.2 W 0.2 )B (48.5 ± 4.1 GPa at 0.49 N).…”

“…Incorporating Cr in TiB 2 accelerated the sintering behavior of BTC, possibly resulting from lower melting point (T m , 2200 • C) and weaker covalent nature of CrB 2 compared to TiB 2 (T m , 3225 • C). 18,[27][28][29] The bulk densities (ρ) and relative densities (R.D.) of as-prepared BTC and BT are listed in Table 2, suggesting that they are both nearly fully dense.…”

“…It has come to light that the improvement of hardness/strength could be achieved by solid-solution strengthening effect. [16][17][18][19][20][21][22][23][24] Previous studies conducted by Feng et al demonstrated that the solid solution of Cr could improve the hardness of TiB 2 . 18 (Cr 0.2 Hf 0.2 Ta 0.2 Ti 0.2 Zr 0.2 )B 2 also exhibited the highest Vickers hardness (48.3 GPa at 0.49 N) among a series of high-entropy diboride ceramics with different compositions.…”

“…[16][17][18][19][20][21][22][23][24] Previous studies conducted by Feng et al demonstrated that the solid solution of Cr could improve the hardness of TiB 2 . 18 (Cr 0.2 Hf 0.2 Ta 0.2 Ti 0.2 Zr 0.2 )B 2 also exhibited the highest Vickers hardness (48.3 GPa at 0.49 N) among a series of high-entropy diboride ceramics with different compositions. 19 Inspired by such exciting results and simultaneously considering the low density of CrB 2 (∼5.21 g cm −3 ) merely higher than that of TiB 2 (∼4.52 g cm −3 ) among all the diborides, the introduction of (Ti 0.9 Cr 0.1 )B 2 solid-solution to B 4 C matrix is expected to be able to acquire lightweight composites with better performances.…”