Surface remelting treatment of plasma-sprayed Al2O3+13 wt.% TiO2 coatings

Oxide Al2O3 + 13 wt.% TiO2 coatings, formed by means of plasma spraying on steel surfaces, were remelted using high-energy heat sources, i.e., the welding TIG (Tungsten Inert Gas) method and a cw-CO2 laser. The treated and untreated samples were subjected to comparative structural examination using both optical and scanning electron microscopes. For quantitative investigation of porosity, a computer image analyser was used. To analyse the phase transformations in oxide coatings, the X-ray diffraction method was utilised. Due to the polymorphic nature of Al2O3, the phase transformation of the Al2O3 + 13% TiO2 coating material was investigated at every stage of the performed modifications following the scheme: powder -> sprayed coating -> remelted coating. Additionally, the effect of the remelting treatment on the wear resistance and hardness of plasma-sprayed coatings was studied. The microstructural examination of the remelted coatings revealed numerous structurally positive and technologically beneficial changes such as reduction in porosity, chemical composition homogenisation, surface smoothing and increase in the structure dispersion. Further investigations provided evidence that the coatings improved by remelting possess enhanced mechanical properties. Also, the investigations confirmed the possibility of replacing laser techniques by the TIG method with a guarantee of obtaining similar results. (c) 2006 Elsevier B.V. All rights reserved.