Effect of Heat Treatment Temperature on the Pore Size and Density in Plasma Sprayed Aluminum Oxide Parts

Abstract

We have studied the effect of heat treatment conditions on the size and number of pores in plasma-sprayed aluminum oxide parts. Samples 100 × 100 × 2 mm in dimensions have been characterized by X-ray computer tomography, qualitative phase analysis, and scanning electron microscopy. The results demonstrate that the number and size of pores vary nonmonotonically with increasing heat treatment temperature, which is due to the heating-induced polymorphic transformations of aluminum oxide, accompanied by changes in the average density of the material and the associated changes in its volume. The temperature dependences of the number of both small and large pores have extrema at ~1100°C. At this temperature, the number of small pores decreases drastically, whereas the number of large pores increases. Further raising the temperature to 1300°C leads to an inverse process: the number of small pores increases drastically, whereas the number of large pores drops sharply. This is due to the transformation of δ-Al2O3 into α-Al2O3, which occurs very rapidly and is accompanied by a large increase in the density of the material of the plasma-sprayed parts and its embrittlement.