Behavior of free-standing atmospheric plasma sprayed Al2O3-Cr2O3 solid solution-based coatings under cyclic heating at 1000 °C

This paper investigates the cyclic heating behavior of free-standing atmospheric plasma sprayed Al2O3 - Cr2O3 coatings containing a solid-solution phase. The coatings are deposited on steels by spraying mechanically blended Al2O3 - Cr2O3 powder mixture with variable Cr2O3 content (2-6 wt%) in the feedstock. The tests are performed up to 50 cycles at 1000 degrees C temperature in a muffle furnace in a normal atmosphere. After each cycle, the weight gain of the coatings is measured, and the oxidation rate constant (Kp) for each material is estimated. The microstructure and phases of the coatings are studied by using scanning electron microscopy (SEM) and Xray diffractometry (XRD) techniques respectively. The porosity and crack length are estimated by image analysis (Image J) technique. The hardness of the coatings is measured by Vickers microhardness tester. After 50 cycles, all the free-standing coatings are found to be intact without fragmentation. At higher cycles, the Al2O3 - Cr2O3 coatings form alpha-Al2O3 and (Al0.9Cr0.1)2O3 solid-solution phase only. The Al2O3 - Cr2O3 coatings gain much less weight than the pure Al2O3 coating due to the major presence of oxidation resistant alpha-Al2O3 phase in the former. However, among the Al2O3 - Cr2O3 coatings with an increase in Cr2O3 content, the Kp value tends to increase due to the high presence of metastable (Al1.8Cr0.05)2O3 solid-solution phase. On the other hand, the (AlxCr1-x)2O3 phase helps to reduce crack formation due to its high fracture toughness properties. With the highest amount of solid-solution phase content, the Al2O3-4 wt% Cr2O3 coating shows the highest fracture toughness among all. The micro-porosity of the coatings is increased with an increase in the heating cycle, and the coating hardness is reduced accordingly.