Spark Plasma Sintering of fine alpha-silicon nitride ceramics with LAS for spatial applications

作者：Reveron Helen^*; Blanchard Laurent; Vitupier Yann; Riviere Emmanuelle; Bonnefont Guillaume; Fantozzi Gilbert

来源：Journal of the European Ceramic Society, 2011, 31(4): 645-652.

DOI：10.1016/j.jeurceramsoc.2010.10.026

• Si(3)N(4)
• Sintering
• Mechanical properties
• Thermal properties
• Spark Plasma Sintering

Many space systems such as satellite mirrors and their supporting structures require to be made from very low-thermal expansion materials combining both high hydrostability and relatively high mechanical properties. in this study, we have applied the "composite concept" in order to explore the possibility of fabricating near zero thermal expansion silicon nitride based ceramics. Consequently, a negative thermal expansion material belonged to the lithium aluminosilicate family (LAS powder crystallized under de beta-eucryptite structure) was introduced in an alpha-silicon nitride fine powder (5 and 20 vol% of LAS) and the resulting composite system was sintered by Spark Plasma Sintering (SPS) at 1400 and 1500 degrees C. In the case of 20 vol% LAS compositions, relatively well-densified ceramics (94.4% of the theoretical density) were produced without adding any further sintering additive. The addition of yttria and alumina oxides allowed enhancing the densification level up to 98.2% (20 vol% LAS compositions) or from 62.3% up to 96.7% of the theoretical density in 5 vol% LAS materials. Nevertheless, it was impossible to full consolidate silicon nitride/LAS composite ceramics at temperatures lower than the temperature at which beta-eucryptite melts, even by using SPS technology. Moreover, because of the relatively low temperatures involved in SPS, the alpha to beta-Si(3)N(4) transformation was avoided, resulting in microstructures composed of fine equiaxed alpha-Si(3)N(4) grains (<200 nm) and of a glassy phase. Even if the effect of having a very large negative thermal expansion material was lost during the sintering step (because of the beta-eucryptite melting), ceramics containing only 20 vol% of LAS-based phase exhibited very interesting values as regards of mechanical properties (strength, hardness, toughness, and Young's modulus), thermal conductivity and thermal expansion coefficient. We discuss in this work why we are so interested in developing dense silicon nitride/LAS ceramics sintered without any further additive addition, even though beta-eucryptite is melted during the process and the transformation to the beta phase is avoided.

• 出版日期2011-4