Guo Jingkun

Hot isostatic pressing of α-silicon carbide ceramics

Abstract A submicron α-SiC powder was densified by hot isostatic pressing using a Vycor glass container. The effect of different additives such as B 4 C, AlN and Al 2 O 3 on densification, mechanical properties and oxidation behaviour was investigated. The results showed that the densification mechanism was attributed to liquid-phase sintering via the formation of a eutectic between SiO 2 and additive, and Al 2 O 3 was confirmed to be an effective additive for hot isostatic pressing of SiC. Hot isostatically pressed, Al 2 O 3 -doped SiC had an ultrafine-grained microstructure with grain size varying from 0·5 to 2·5 μm, and exhibited a flexural strength above 570 MPa and a fracture toughness higher than 5·17 MPa m 1 2 .

Microstructure and properties of hot isostatically pressed SiC ceramics with an Al2O3 addition

Abstract Dense Al 2 O 3 -doped SiC ceramics with an average grain size of 1.2 μm were obtained by hot isostatic pressing. The microstructure has been analyzed by TEM, EDX, EELS, SAD and SEM. The results show that the non-crystalline secondary phase, an alumino-silicate glass, is predominantly located at triple grain junctions in Al 2 O 3 -doped SiC ceramics. The influence of the glassy phase on sintering behaviour, high-temperature strength and oxidation resistance is investigated and discussed.

SiC-Si 3N4 Gradient Composite Ceramics by Special Hip Processing

α-SiC fine powder clad with special glass were sintered by HIP processing under 200 MPa at 1800 °DC. The density of material obtained is close to 97.5% of theoretical density, bending strength reached 582 MPa, which is about 50% higher than that of common pressureless sintered silicon carbide. After post-HIP treatment, density can be further increased to above 98.5% of theoretical density. The material with different composition can also be got in various atmospheres. According to thermodynamic calculation, SiC can be transformed into Si3N4 under optimum temperature and high N2-pressure. Experimental work was successful in fabricating SiC-Si3N4 gradient composite material. This kind of new SiC-Si3N4 gradient composite material were also identified by X-ray diffraction analysis. The strength and fracture toughness of this material is 900± 100 MPa and 8.4 MPa.m 1/2 respectively, which are about one times higher than those of hot-pressed SiC material. This study offered a new HIP processing technology for preparing high performance SiC-Si3N4 gradient composite ceramics directly from pure SiC powder.

INFLUENCE OF ZrO 2 PARTICLE SIZE ON TRANSFORMATION TOUGHENING IN Y-TZP

The influence of particle size on the fraction of tetragonal ZrO_2 transformable as induced by stress as well as on the mechanical properties has been studied. The fraction of tetragonal ZrO_2 particles does not increase monotonously as the particle size decreases in Y-TZP with a constant Y_2O_3 content. The experimental results show that there is an optimum particle size range, at which the fraction of tetragonal ZrO_2 in the material and the volume fraction of tetragonal ZrO_2 transformable as induced by stress are both the highest while at the same time both the flexual strength and the fracture toughness of the material reach the optimum values. For Y-TZP samples containing constant Y_2O_3 contents, there is a linear relationship between the fracture toughness and the volume fraction of transformable tetragonal grO_2 under tensile stress. All these above-mentioned results are discussed by the rationalized thermodynamical formulae and a revised expression for the critical stress intensity factor.