Interconnected Porosity Modification of Plasma Sprayed Alumina Coating Using Excimer Laser Surface Treatment

Thermally sprayed ceramic coatings such as plasma-sprayed alumina exhibit a composite microstructure actually due to the presence of defects such as pores, interlamellar and intra-lamellar cracks. These second phase typed features influence the mechanical behaviour of the coating dramatically. In this study, an excimer laser surface treatment of plasma-sprayed alumina surface was developed for the optimizing of component properties such as wireline tool used in the oil industry. On the contrary to liquid phase treatment realized with C02  or YAG laser, an excimer laser processing presents short wavelength means that for ceramic materials, the energy is absorbed in a region of the surface. Moreover the short pulse duration of excimer laser does not affect the temperature distribution below the surface. These conditions lead to surface treatment free of cracks. Effect of laser operating parameters, i.e. wavelength, pulse number and power density, on microstructure and the sealing quality of the coating are discussed. First, surface and cross section of the microstructures were studied using image analysis of scanning electron microscope (SEM). Surface roughness and coating ablation were characterized according to laser treatment.  Then, three dimensional (3D) microstructures were obtained using X-ray microtomography to evaluate the 3D porosity after laser treatment. Finally, nanoindentation and Electrochemical Impedance Spectroscopy (EIS) were carried out to characterize respectively the mechanical and electrical properties of coating. Using this approach excimer laser surface processing was shown to be an innovative process to modify insulating characteristics of plasma-sprayed alumina.