Influence of Nitride on Sinterability of the Composite of Lithium Aluminum Silicate and Silicon Carbide

Abstract

Ceramics with zero thermal expansion coefficients at room temperature (293K) were investigated. We found the thermal expansion coefficient was controlled by a compounding ratio of lithium aluminum silicate (LAS) and silicon carbide (SiC), which have negative and positive thermal expansion coefficients respectively. Although it was difficult to densify the composite of the LAS and SiC (LAS/SiC) in the sintering process, an addition of nitride improved the sinterability of the LAS/SiC. In order to examine the effect of the nitride additive, at first, the melting point of the LAS with silicon nitride (Si3N4) or aluminum nitride was measured by TG-DTA. The melting point of the LAS decreased with existence of nitride. It is believed that the densification of the LAS/SiC was promoted by the nitride, because the nitride causes the LAS/SiC to form a liquid phase, thereby decreasing the melting point. Next, the lattice constant of the LAS with Si3N4 was measured by XRD and it was verified that the a-axis was longer and the c-axis was shorter than those of the LAS without additive. It is supposed that this phenomenon is due to the substitution of nitrogen for oxygen in the LAS lattice, and the decrease of the melting point of the LAS with nitride seems to be influenced by this substitution of nitrogen.

Conventional sintering of LAS–SiC nanocomposites with very low thermal expansion coefficient

“…This is in well agreement with the melting points reported in [6] for the LAS matrix (1417 ºC) and for the LAS-20%SiC composite (1416 ºC). The addition of 20% SiC to the LAS matrix doesn't affect the melting point (only 1 ºC).…”

“…Thermal expansion slightly increases from 1380 to 1390 ºC. Specific rigidity values over 40 GPa/g/cm 3 are reached for the highest temperatures which is in good agreement with other values other LAS-SiC composites [3] and can be improved either using sintering aids [6], which can affect the …”

“…This option is very interesting as together with the CTE design for a certain temperature range, also other properties can be design, adding the proper amount and type of second phases in the matrix. This has been tested successfully in [6] were they have obtained a zero expansion material at room temperature with high elasticity for a LAS-SiC composite similar to the one proposed in our study. In that study, it was reported the influence of the silicon nitride addition on the sinterability of LAS-SiC composites which resulted in close to theoretical density materials with excellent thermal and mechanical properties.…”

“…Post-HIP treatments on the sintered can be applied to improve this density and consequently, the elastic modulus of the samples. Table 1 shows the values of the Young's modulus and it can be seen that they are below the theoretical values which would be around 150-165 GPa, and also below the 150 GPa reported in [6]for a LAS-SiC composite with added Si 3 N 4 . Nevertheless, our results go beyond the elastic properties of the most commonly material now in use in the satellite mirror industry, Zerodur ® with a Young's modulus of 90 GPa.…”

“…If the melting point of the matrix and the composite is very similar, and as we have pointed out, the densification takes place in the solid state, the sintering temperature should be the same. But this is not the case, and it is well known the difficulty to obtain dense LAS-SiC composites without additives [6], (which low the sintering temperature by the formation of a glass phase), and with pressureless methods. The explanation is that SiC nanoparticles, which are situated at the LAS grain boundaries, have a high melting point (~2700 ºC) and, consequently, a very low diffusion capability at the sintering temperatures we are dealing with (< 1400 ºC).…”