Abstract
Changes in the phase composition and porosity of plasma-sprayed Al2O3 ceramics before and after its treatment by hot isostatic pressing (HIP) were studied. The treatment was carried out using a HIP unit with a graphite resistance heater in an argon atmosphere at a temperature of 1600 °C at a gas static pressure of 200 MPa for 3 hours. The phase composition of the material was determined by quantitative phase analysis. The porosity of ceramics was determined by X-ray computed tomography. It was found that immediately after the end of plasma spraying, the ceramic material contained a set of α, γ, and δ modifications of Al2O3, which differed in density and type of crystal lattice. The total porosity of the ceramics was 18.9 vol. %. After HIP, the phase composition of the plasma-sprayed ceramics changed. The metastable phase modifications γ and δ-Al2O3 disappeared, and the stable modification α-Al2O3 occupied the entire volume of the ceramic material. The total porosity of the ceramics decreased to 9.7 vol. %. The obtained results do not coincide with the literature data, which states that HIP of pressed ceramic samples from α-Al2O3 (corundum) at a temperature of 1300 – 1350 °С and a gas-static pressure of 150 MPa for 30 minutes leads to an increase in the density of these samples to values of more than 98 % on their theoretical density i.e. up to porosity values less than 2 %. The results of this study showed that HIP of plasma-sprayed samples of ceramics from pure alumina in the more severe above-mentioned regime (temperature 1600 °С, gas-static pressure 200 MPa, holding time 3 hours), although it leads to a 2-fold decrease in the porosity of these samples compared to the initial value, but still the value of the porosity of ceramics remains more than 9 %. According to the authors, the reasons for these discrepancies can be associated with polymorphic transformations of aluminum oxide occurring in the material of plasma-sprayed ceramic samples during their heating and accompanying these transformations with changes in the density and volume of the ceramic material, which affects its porosity.
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