Wear resistant alumina coatings produced by a sol–gel process

Aluminium oxide films were prepared by a dip-coating procedure from solutions of hydrolysed aluminium-tri-sec-butoxide in methoxyethanol on austenitic steel as well as hard metal tools. The heat treatment was performed in an argon atmosphere or vacuum at temperatures up to 1100 °C. Characterisation of the coatings by SEM, showed nearly crack free coatings with a dense morphology which completely cover the substrate. The chemical composition of the coatings was investigated by ESCA, GD-OES and XRD measurements. This analysis detected a multi-layered arrangement of spinel phases, γ- and α-alumina in the coatings. The scratch test showed sufficient adhesion of the coatings comparable to CVD layers. Pin-on-disc and oscillating sliding tests were carried out on sol–gel coated samples and also on uncovered reference samples. The determined wear rates and friction coefficients showed significant improvement in wear resistance of sol–gel coatings containing a corundum phase. The strong adhesion to the substrates is due to a layered assembly formation and gradient composition of the coating caused by an interdiffusion of metal cations and oxygen in the metal/oxide interface during heat treatment.