STUDY OF PROCESSING VARIABLES IN LASER GLAZING PLASMA-SPRAYED THERMAL BARRIER COATINGS

A continuous-wave CO2 laser with a defocused beam has been employed to glaze the surface layer of plasma-sprayed (ZrO2-20wt.%Y2O3)/(M-Cr-Al-Y) coatings. The effects of laser parameters, such as power and travel speed, on the melting width, melting depth, microhardness, coupling efficiency, microstructure and process defects have been evaluated. The results of this investigation showed that the melting width was between the two values of the 1/e and the 1/e2 models when the input energy was less than 100 J mm-2. The melting depth increased with increasing input energy when the input energy was less than 20 J mm-2. However, the melting depth remained at about 150 mum as the input energy was larger than 20 J mm-2. The coupling efficiency increased with increasing laser travel speed, while the power density had no significant influence on the coupling efficiency. The laser parameters had no significant influence on the microhardness for the as-glazed layer. The melted layer glazed with different parameters revealed exclusively a cubic phase with a columnar structure which grew perpendicular to the surface with a [100] preferred orientation. A region of appropriate processing parameters of high travel speed and high power density has been suggested.