Heat treatment of plasma sprayed alumina-chromia composite coatings

Abstract

Atmospheric plasma spray technique is used to develop alumina-based coatings by spraying mechanically blended alumina-chromia feedstock with chromia content varying from 1–6 wt-%. The influence of annealing temperature (900–1200°C) on the phases formed in the coatings is also studied. Phases present in the coatings are analysed and quantified using X-ray diffraction and Rietveld refinement, respectively. Microstructures of the coating cross sections are studied under scanning electron microscope. Wear rate of the annealed coatings are evaluated by linear reciprocating wear tests. The adhesion strength of the coatings is also estimated by ASTM C1624 standard. The results indicate that the increase in annealing temperature causes rise in transformation of metastable phases of alumina into stable α-alumina. The minimum annealing temperature required to complete this transformation is estimated for each coating. The lowest specific wear rate is observed at 1150°C temperature and with 4 wt-% chromia content. The significant change in the trend of wear rate observed at the annealing temperature of 1200°C can be attributed to formation of large amount of cracks in the coatings. Adhesion strength of the coating improves with increasing chromia content. However, with increase in annealing temperature the coating adhesion decreases significantly due to the formation of cracks.