Fabrication of reaction-bonded Cr2O3 ceramics

Reaction-bonding to form Cr2O3 can be achieved by gaseous oxidation of a Cr phase. The reaction-bonding process is best conducted by complete oxidation before sintering. Below approximate to 800 degrees C, the activation energy for oxidation is 220 kJ mol(-1), indicating the predominance of Cr3+ outward diffusion along high diffusivity paths, e.g., grain boundaries and dislocations. At higher temperatures, the activation energy is reduced to 52 kJ mol(-1) as a result of oxygen transport along lower-energy paths, e.g., along microcracks, and the internal and external surfaces. In spite of the decrease in activation energy, the access of oxygen to the inside of the powder compact is hindered by the progressive densification of the oxidizing powder compacts. Maximum densification is achieved for fully oxidized Cr/Cr2O3 compacts when the oxygen partial pressure is close to that of the Cr-Cr2O3 equilibrium. 0.1 wt% MgO addition increases the density and reduces the grain size of the reaction-bonded Cr2O3 samples due to the possible formation of the spinel phase MgCr2O4. ZrO2 and MgO additions improve the fracture strength and toughness of conventionally sintered Cr2O3 and change its fracture mode from intergranular to intragranular. For reaction-bonded Cr2O3 samples with or without MgO addition, their fracture strength and toughness data are roughly the same as those of sintered Cr2O3 doped with ZrO2 and MgO and their fracture surfaces are predominantly intragranular. (C) 1999 Elsevier Science Limited. All rights reserved.