Effects of initial α‐phase content on properties of pressureless solid‐state sintered SiC ceramics

Abstract

α‐ and β‐SiC starting powders of similar particle sizes were used to investigate the effect of initial α‐phase content on the electrical, thermal, and mechanical properties of pressureless solid‐state sintered (PSS) SiC ceramics with B4C and C additives. For β‐SiC starting powders, a coarse‐grained microstructure with elongated platelet grains was formed by the transformation of 3C to 6H and finally to 4H‐SiC phase. In contrast, materials prepared from α‐SiC powders exhibited a fine‐grained microstructure with platelet grains. This study revealed the beneficial effect of α‐SiC starting powders in achieving low electrical resistivity and high thermal conductivity in PSS SiC ceramics, which was attributable to their higher sinterability, lower impurity content, and lower 6H to 4H‐SiC phase transformation rate compared with β‐SiC powders. The electrical resistivity decreased by an order of magnitude and the thermal conductivity increased by 32% with an increase in initial α‐phase content from 0 to 100%. The flexural strength increased by approximately 16% with increasing initial α‐phase content due to a decreased flaw size with decreasing grain size. However, the fracture toughness and hardness were insensitive to the change in initial α‐phase content.

Electrical, thermal, and mechanical properties of porous silicon carbide ceramics with a boron carbide additive

“…The microstructures of both the argon‐sintered specimens and nitrogen‐sintered specimens consist of relatively large spherical pores originating from the PMMA templates and small pores formed by the interparticle spaces. It is well‐known that B atoms diffuse into the SiC lattice and reduce the grain boundary energy (γ gb ), whereas C increases the surface energy (γ sv ) of SiC by removing the native silica film via carbothermal reduction 64,65 . The combined action of the B 4 C‐derived B and C reduce the grain‐boundary‐energy‐to‐surface‐energy ratio to a subcritical value, and a densification of the SiC strut occurs via the solid‐state sintering in both the argon and nitrogen atmospheres 65,66 …”

“…Approximately 46 vol% (24 wt%) of the sacrificial template was added to each specimen; thus, roughly 2/3 of the residual porosity originated from the templates, and 1/3 of the porosity was caused by the incomplete densification of struts in both the SCA0 and SCN0 specimens. The decrease in porosity with increasing B 4 C content in the SCA and SCN specimens is attributed to the partial densification of the porous SiC ceramic struts, as B 4 C is considered as an effective sintering additive for the solid‐state sintering of SiC ceramics 65,70 …”

“…The PMMA‐derived carbon content in each specimen is ∼0.02 wt% (the carbon residue of PMMA after decomposition in an argon or nitrogen atmosphere is ∼0.08 wt%). All the carbon contained in each specimen has been removed by reacting with the native SiO 2 film via SiO 2 + 3C → SiC + 2CO (g) reaction during sintering 37,65 . The required carbon content for removing 1.18 wt% SiO 2 contained in the starting SiC powder via the above reaction is 0.71 wt%.…”

Effect of solid loading and carbon additive on microstructure and mechanical properties of 3D‐printed SiC ceramic

“…The densification of the sintered SiC was attributed to not only C (from the carbonization of sucrose) but also B 4 C. 43,44 B 4 C could promote the densification of SiC ceramics by segregating at the grain boundaries, thereby reducing the grain boundary energy, whereas C could promote densification by increasing the surface energy of SiC by removing the native SiO 2 film via a carbothermal reduction during heating. 44 However, too high sintering temperature would cause the abnormal growth in the SiC ceramic (as shown in Figure 3E), which decreased the mechanical properties of the SiC parts. Figure 4 shows EDS elemental maps of a fracture surface of the sample sintered at 2200 • C. A small number of pores and residual carbon were observed in the boundaries of SiC grains.…”

“…Compared to the sample sintered at 2050 • C, the fracture surfaces of the samples prepared at 2100-2250 • C (Figure 3B-E) were smoother and the number of pores decreases. The densification of the sintered SiC was attributed to not only C (from the carbonization of sucrose) but also B 4 C. 43,44 B 4 C could promote the densification of SiC ceramics by segregating at the grain boundaries, thereby reducing the grain boundary energy, whereas C could promote densification by increasing the surface energy of SiC by removing the native SiO 2 film via a carbothermal reduction during heating. 44 However, too high sintering temperature would cause the abnormal growth in the SiC ceramic (as shown in Figure 3E), which decreased the mechanical properties of the SiC parts.…”