Abstract: The influence of additive silica on the microstructure of plasma sprayed Al2O3 and Al2O3+13wt pct TiO2 ceramic coatings at laser melting has been investigated in this study. At the laser melting, additive silica in Al2O3 ceramic coating can reduce the stress of cooling shrinkage generated during solidification. Moreover, silica can render finer size of grains of the melting layer and form continuous glassy matter around the grain boundaries so as to reduce further the cooling stresses and to suppress the formation and spreading of cracks. On the other hand, at the laser melting, TiO2 reacts with Al2O3and transforms into TiAl2O5. The latter new phase has great and anisotropic coefficients of thermal expansion leading to big and asymmetrical stresses and thus to form cracks in the melting layer of Al2O3+13wt pct TiO2 coating. Due to the fact that the influence of additive silica on the suppression of the formation of cracks is rather limited and cannot counterbalance the negative effect of TiAl2O5, thus the melting layer of Al2O3+13wt pct TiO2 coating doped with 3wt pct SiO2 cracks also. Nevertheless, TiO2 can greatly develop the wear resistance of the ceramic coating as sprayed or laser melted.

Key words: Plasma spraying, Laser remelting, Additive silica, Stress, crack