Effect of annealing on tribo-mechanical properties of plasma sprayed Alumina-Chromia coatings

Abstract

Plasma sprayed Al2O3 (alumina) is used as a heat resistant, corrosion resistant and hard coating. While spraying the Al2O3 feedstock powder of α-phase it forms different phases in the coatings. Considering the mechanical properties stable α-Al2O3 is desirable in coating. However the other metastable phases formed due to the complexity of the spraying process and rate of solidification. It is expected that the prevention of these phases in the coatings will significantly improve the mechanical, electrical, and other properties of Al2O3 coatings. The result regarding the possibility of stabilization of Al2O3 through addition of Cr2O3 (chromia) has been already reported. When Cr2O3 is added in the feedstock of Al2O3, a solid solution (AlxCr1-x)2O3 forms in the deposit which is superior to other phases of Al2O3. Annealing of Al2O3 deposit at the temperature higher than its recrystallization temperature also improves the quantity of α-Al2O3 content.This work is carried out on plasma sprayed Al2O3-Cr2O3 coatings to study the effect of annealing temperature on the phase transformation and other properties with different compositions. Plasma sprayed Al2O3 coatings with 1 to 6 wt.% Cr2O3 content is annealed at different temperature from 900 oC to 1200 oC and then characterized along with as-sprayed coatings. Analysis of phase formation of the coatings is studied with X-ray diffraction patterns. SEM micrographs are used to study the microstructure of the coatings. The effect of annealing on the coating properties is investigated in terms of vickers microhardness, wear resistance and coefficient of friction.Cr2O3 addition influences the phase formation, and maximum solid solubility of Al2O3- Cr2O3 is found at Al2O3-4wt.% Cr2O3. It is observed with the XRD patterns that complete transformation in to stable α-Al2O3 phase obtain at 1200 oC and it has the maximum hardness among coatings annealed at lower temperatures. Annealing at 1200 oC leads to formation and propagation of cracks at very large extent which decreases its wear resistance. Hence the maximum hardness is observed for Al2O3-4wt.% Cr2O3 coating annealed at 1200 oC and maximum wear resistance for Al2O3-4wt.% Cr2O3 coating annealed at 1100 oC among all the coatings.