Sintering of glass matrix composites with small rigid inclusions

We investigated the effect of dispersed crystalline particle volume content Φ on sintering of glass matrix composites (GMC) for low-temperature co-fired ceramics (LTCC) applications. Such composites typically consist of alumo-borosilicate glass and α-Al2O3 powders of similar average particle size (D50 ≈ 3 μm). Sintering shrinkage was observed by dilatometry and heating microscopy and was backed up by glass viscosity measurements. Microstructure analysis revealed that α-Al2O3 particles do neither show significant dissolution into the liquid phase nor detectable crystallization throughout LTCC firing schedules. Therefore, in this study α-Al2O3 particles were treated as small rigid inclusions. It was found that Φ lowers the shrinkage rate of GMC. While the lowering is small for small Φ and at the early stage of densification it progressively increases during sintering, and final shrinkage shifts up to 170 K to higher temperatures for Φ = 0.45. The behaviour observed could be explained assuming that sintering is controlled by the effective viscosity, which progressively increases non-linearly during densification due to the gradually wetting of the surface area of corundum particles. We could demonstrate that Al2O3 cluster can cause residual pores and reduce the attainable shrinkage. The reduction of attainable shrinkage is found to depend on Φ3, reaching about 8% at Φ = 0.45.