Effect of laser surface melting upon the devitrification of plasma sprayed cordierite

Devitrification behavior of plasma sprayed cordierite (2MgO–2Al2O3–5SiO2) modified by addition of 6.3 wt.% TiO2 was studied in the as-sprayed condition and after laser surface melting. Laser treatment was performed using a pulsed Nd:YAG laser and the melt depth was maintained within the plasma sprayed coating. Laser surface melting produced a chemically homogeneous glass surface that contained extensive microcracks in both the melted and unmelted portions of the coating. Laser surface melting was found to be effective in reducing porosity and surface roughness. Roughness of the as-sprayed surface was reduced to Ra 2.5 μm or better from a roughness of Ra 14 μm. Devitrification of as-sprayed cordierite was dependent upon TiO2 distribution in the coating, whereas crystallization in the laser melted coating was dominated by nucleation at microcracks. Microcrack density was significantly reduced by viscous glass flow and transformation induced volume expansion during devitrification.