Pressure-assisted densification of non-homogeneous ceramic compacts
Finite element techniques were used to study the densification behavior and residual stress development during hot pressing of ceramic compacts containing density or composition gradients. A new processing methodology is investigated for it's feasibility to generate residual stress strengthened ceramic compacts. For this purpose, pressure-assisted densification by power law creep and diffusion have been modeled. Existing two stage densification models were found to be inadequate to model compaction of ceramic compacts containing either density or composition gradients due to discontinuities in the density-time and stress-time behavior predicted by these models. A unified blending densification model was developed based on the hypothesis that porosity is likely to change gradually from being completely open at the beginning of compaction to completely closed at full density. This unified blending model eliminated both the density and the stress discontinuities. The material constants for the unified model were obtained by fitting to experimental data from hot pressing of homogeneous alumina powders.
Finite element analysis simulations of isothermal hot pressing of alumina compacts with density gradients indicate that hot pressing with a constant axial load is not likely to lead to any significant residual stresses. However, hot pressing of such compacts with a constant axial displacement rate shows that significant compressive residual stresses develop on the surface of the compacts. Numerical results also indicate that hot pressing of alumina powders in dies with non-uniform cross-section will result in residual surface compressive stresses in the compact.
Numerical simulations of hot pressing of layered cylindrical alumina/3Y-TZP shows that stress gradients can be generated during hot pressing, with higher compressive stresses in the outer surface of the compact. However, the significantly lower elastic modulus of the inner 3Y-TZP core results in tensile surface stresses on elastic unloading of the compact. These tensile stresses reduce the compressive residual stresses resulting from the mismatch in thermal expansion coefficients of the two layers on cooling of the compact. Based on these results, it is recommended that in order to add to the residual stresses generated due to thermal expansion mismatch, a layered composite compact should have comparable elastic modulus, lower densification rate of the powders in the outer layer, and a lower thermal expansion coefficient in the outer layer than in the inner core. Such a system should aide in maximum outer surface compressive residual stresses.