Xiaming Zhou

Microstructure of alumina-3wt.% titania coatings by plasma spraying with nanostructured powders

Abstract The microstructure of plasma-sprayed alumina–3wt.% titania coatings with the nanostructured powders was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the nanostructured coating exhibited a bimodal microstructure: splat lamellae similar to the conventional coating and equiaxed grains originating from the starting feedstock. The splat lamellae consisted of γ-Al2O3 grains and most of them were less than 200 nm in diameter. Equiaxed grains took the modification of α-Al2O3 and ranged from 150 to 800 nm in size. In addition, the modifications of TiO2 disappeared and Ti element was dissolved in γ-Al2O3 grains. The bimodal microstructure contributed to the improvement in mechanical properties of the nanostructured coating.

Evaluating microhardness of plasma sprayed Al2O3coatings using Vickers indentation technique

In this work, the microhardness of plasma sprayed Al2O3 coatings was evaluated using the Vickers indentation technique, and the effects of measurement direction, location and applied loads were investigated. The measured data sets were then statistically analysed employing the Weibull distribution to evaluate their variability within the coatings. It was found that the Vickers hardness (VHN) increases with decreasing applied indenter load, which can be explained in terms of Kicks law and the Meyer index k of 1.93, as well as relating to the microstructural characteristics of plasma sprayed coatings and the elastic recovery taking place during indentation. In addition, VHN, measured on the cross section of coatings, was obviously higher than that on its top surface. The obtained Weibull modulus and variation coefficient indicate that the VHN was less variable when measured at a higher applied load and on the cross section of coating. The obvious dependence of the VHN on the specific indentation location within through-thickness direction was also realized. These phenomena described above in this work were related to the special microstructure and high anisotropic behaviour of plasma sprayed coatings.

Sliding Wear Performance of Plasma-Sprayed Al2O3-Cr2O3 Composite Coatings Against Graphite under Severe Conditions

Al2O3, Cr2O3, and Al2O3-Cr2O3 composite coatings were produced by plasma spraying. Their tribological properties were evaluated at high load conditions. The average friction coefficients, wear rates, and worn surface temperatures of the coating/graphite pairs were measured. Compared with the single coating/graphite pairs, the friction coefficients of composite coating/graphite pairs are more stable. The corresponding wear rates and worn surface temperatures are lower, which may be conducive to the formation of more effective and stable graphite transfer film on the surface of the coating subjected to abrasion. Especially, 10wt.%Al2O3-90wt.%Cr2O3 (AC90) composite coating shows better anti-wear performance, which may be attributed to its higher thermal conduction.

Microstructural Characterization and Strengthening-Toughening Mechanism of Plasma-Sprayed Al2O3-Cr2O3 Composite Coatings

In this study, Al2O3, Cr2O3, and Al2O3-Cr2O3 coatings were fabricated by plasma spraying. X-ray diffraction was employed to determine the phase composition of powders and coatings. The morphologies and microstructures of the coatings were characterized using electron probe microanalyzer and transmission electron microscopy. Vickers hardness, fracture toughness, and bending strength of the coatings were measured. Al2O3-Cr2O3 composite coatings show better comprehensive mechanical properties than the individual Al2O3 and Cr2O3 coatings, which are attributed to the formers larger intersplat adhesion or interlamellar cohesion and lower porosity. Solid solution strengthens the phase interfaces and grain boundaries, which is beneficial to improve the mechanical performance of the composite coatings.