Microstructure and high-temperature strength of pressureless-sintered silicon carbide

Abstract

Silicon carbide (SIC) cermic has been recognized as a prime candidate material for structural applications such as heat engine components, heat exchangers etc., due to its good strength, excellent resistance to corrosion and oxidation, and excellent thermomechanical as well as thermophysical properties. The densification of SiC via pressureless sintering has been an important subject for years, and the first work was pioneered by Prochazka [1] who obtained a dense SiC with the addition of boron and carbon as sintering aids by means of a liquid-phase sintering mechanism. In spite of sufficient flexural strength, SiC ceramics in the SiC + B + C system showed no improvement in the resistance to fracture [2, 3]. A number of investigations indicate the importance of sintering aids, which play a crucial role in changing the mechanical as welt as high-temperature properties of the SiC ceramic [3-6]. Omori and Takei [7], who used alumina and yttria as sintering aids, obtained a dense SiC ceramic with an improved mechanical strength of greater than 6 5 0 MPa. More recently, Lee et al. [3] obtained a dense SiC ceramic with a fracture toughness as high as -8 .3 MPam 1/2, by allowing exaggerated growth of the SiC grains at 2000 °C to form a microstructure containing considerable amounts of plate-like a-SiC grains. Unfommately, the fracture strength of their SiC ceramics decreased to -450 MPa. A recent investigation by the present authors has demonstrated that dense SiC ceramics with a four-point flexural strength above 600 MPa and a fracture toughness as good as 6 . 0 M P a m 1/2 are achievable through the control of microstructure evolution in monolithic SiC ceramic, by starting with a mixture of ozand /3-SIC powders via a two-step pressureless sintering procedure [8]. The requirement of high-temperature strength is a prime consideration in the development of SiC ceramics, since one of their principal uses is for heat-engine applications where the high-tempera~tre properties are critically important. Although studies on the high-temperature strength of SiC ceramics are extensive, most of the investigations involving sintering aids were focused on boron and carbon systems. For employing A1203 and 57203 as sintering aids, the available data on the strength at elevated temperatures are not extensive. In a number of patent references [6,9] the data for high-temperature strength of SiC ceramics, in spite of their high room-temperature strength, showed a considerable reduction in strength by over 25% at 1200 °C.