Laser remelting of plasma sprayed zirconia based ceramic coating for pyrochemical reprocessing applications

Type 316L stainless steel rod was coated with 300 μm thick yttria stabilised zirconia (≈8%Y₂O₃) by plasma spraying over a 50 μm metallic bond coating of NiCrAlY. However, the presence of porosity and microcracks in the as sprayed coating is a drawback. In the present work an attempt was made to study the plasma spraying of such coatings and to enhance their properties through laser surface treatment. A continuous wave CO₂ laser of 10·6 μm wavelength with a beam diameter of 1·5 mm was used to eliminate porosity and microcracks. Powers of 50, 75 and 100 W with 2500 μm s^&minus1 scan rate and 500 μm step size were applied to melt the surface. The as coated and laser treated samples were characterised using X-ray diffraction, optical microscopy and SEM. The porosity was decreased from 10% in the as coated sample to 0·5% in the laser treated samples. Segmented cracks were formed due to residual stresses developed during laser remelting and solidification. Delamination occured near the bond coat/ceramic interface in the coating as the melted thickness was higher for higher laser power. Microhardness measurements indicated higher hardness in the laser treated region. Optical and SEM examination revealed polygonal grains of zirconia formed during laser remelting in all the laser treated samples. The X-ray diffraction analysis of laser treated samples revealed the formation of tetragonal, non-transformable tetragonal and cubic phases. With an increase of laser power, the non-transformable tetragonal phase increased marginally.