Phase stabilization and characterization of plasma sprayed alumina based coatings prepared by mechanically blended alumina-chromia powders

Abstract

Atmospheric plasma spray process is used to produce Al2O3 based coatings. The feedstock sprayed to fabricate such coatings contained 1-6wt.% Cr2O3 powder mechanically blended with Al2O3 feedstock, in either of agglomerated and crushed form. The coatings display good microstructural integrity. The local mechanical properties of the agglomerated Al2O3 coating are estimated by nanoindentation based hardness testing with a very low load. The possibility of identification of αAl2O3 phase (stable phase) present in the coating with the help of nanoindentation has been discussed. Nanoindentation technique is found useful in the estimation of local properties of plasma sprayed Al2O3 coating. It is observed that the coating composition, annealing temperature, process parameter and feedstock particle size bear significance over the type and quantity of phases of Al2O3 formed in the coatings. Quantification of such phases is done by Rietveld analysis. Studies revealed that with a fixed set of process parameters and a specific type of Al2O3 powder, there is a limit of Cr2O3 addition, up to which, stabilization of α-Al2O3 occurs. For agglomerated Al2O3, the maximum stabilization (stable phase) is observed at 4wt.% Cr2O3 addition. The Al2O3-4wt.% Cr2O3 coating, containing the highest amount of α-Al2O3 phase possesses superior tribomechanical properties as compared to other Al2O3 based coatings. The improved microhardness, reduced surface roughness and improved wear resistance of the coatings are observed with an increase in α-Al2O3 quantity in the coating. The stabilization of α-Al2O3 is found to occur through the formation of Al2O3-Cr2O3 solid solution. Being isostructural with α-Al2O3, the (Alx-Cr1-x)2O3 solid solution induces further formation of α-Al2O3 in the solidifying Al2O3 splats at its surrounding. Further, on annealing, the residual metastable phases get converted to α-Al2O3. The absolute temperature required for this conversion is also estimated in this study. With an increase in annealing temperature, the quantity of α-Al2O3 in the coating is found to increase. Considering the improvement in mechanical properties, 1150 oC is found to be the suitable annealing temperature for all types of Al2O3-Cr2O3 coatings. Formation of α-Al2O3 in the Al2O3-Cr2O3 coatings can be enhanced by selecting proper process parameters and particle sizes of Al2O3 feedstock. The effect of secondary gas (H2) flow rate on the formation of α-Al2O3 in the Al2O3-Cr2O3 coatings is studied. It is observed thatwith an increase in H2 flow rate the formation of α-Al2O3 in the coating reduces due to the improved melting of the particles and their higher cooling rates. However, with changing H2 flow rates, the maximum stabilization point in terms of Cr2O3 addition does not change. Effect of particle sizes of Al2O3 feedstock on phase stabilization of α-Al2O3 in Al2O3-Cr2O3 coatings is also studied by spraying three different sizes of crushed Al2O3. For different particle sizes, the maximum stabilization points obtained are also different. For 25 μm, 30 μm and 34 μm sizes of Al2O3 the maximum stabilization occurs at 3wt.%, 4wt.% 5wt.% Cr2O3, respectively. Al2O3 powders of larger sizes require higher amounts of Cr2O3 powder to be blended with to obtain maximum phase stabilization in the final coatings. Finally, this Al2O3-Cr2O3 composite coating produced by plasma spraying has considerable potential in tribomechanical applications. The knowledge and information thus generated are not only of academic importance but also of relevance to actual users and industries involved in the production of hard and wear resistant coatings.