Powder Processing, Densification Behaviour, Microstructure and Mechanical Properties of Al2o3- 50 vol% zro2 Composites

Abstract

Al2O3-ZrO2 composites containing nominally equal volume fraction of Al2O3 and ZrO2 were prepared through solution chemistry route using inorganic precursors of AlCl3 and ZrOCl2. It was expected that the solution chemistry route will yield highly reactive fine powders which could be sintered into a dense composite with a small grain size. Such a microstructure is expected to retain t-ZrO2 without the use of stabilizers. Three different processing routes, viz gelation (ROUTE 1), precipitation (ROUTE 2) and precipitation followed by washing (ROUTE 3) (consisting of hot water and alcohol washing) were employed. However, the ROUTE 1 powders could be sintered only up to 72% relative density and the highest density was 88% for ROUTE 3. FTIR and DSC/TG studies revealed that the washing process not only removed soluble salts but also helped to reduce the extent of agglomeration and produced softly agglomerated powders.However, the washing process could not completely remove chlorides and the presence of the residual chlorine hindered densification by creating residual pores during the final densification stage. The sintered samples had nearly equal grain sizes (1.9 μm and 1.5 μm for Al2O3 and ZrO2 respectively). However, extensive microcracking resulted during spontaneous t→m transformation of ZrO2 during cooling from the sintering temperature. The combined effect of microcracking and low sintered density resulted in poor mechanical properties (σf = 64 MPa, KIC = 1.74 MPam1/2, HV = 4.1 GPa). The partial substitution of chloride precursor by nitrate precursor (Al(NO3)3 for AlCl3) (ROUTE 4) resulted in improved sintered density (95% relative density) but grain size was more than the critical size for prevention of spontaneous t→m transformation during cooling of sintered sample and once again microcracking resulted in lowering of strength and toughness (σf = 88.7 MPa, KIC = 1.9 MPam1/2, HV = 7.9 GPa). The initial stage densification mechanism of these composites was studied from non-isothermal and isothermal sintering behavior. The densification was controlled by volume diffusion with the activation energy for densification being 236-258 KJmol-1.