In-situ laser pelletization of advanced ternary thermal barrier coating system

Powder technology is considered as a product cost effective technology to produce advanced ceramics. A 1 kW CO₂ pulsed laser, working at 0.4 and 1.0 kW, 2–200 ms pulsed lengths and 1 mm laser beam diameter, was used to produce pellets from the loose powder of 8 wt% yttria partially stabilized zirconia (YPSZ) containing 8 wt% alumina. SEM equipped with EDS and WDS, x-ray diffraction, x-ray step scanned and Vickers microhardness were employed to evaluate and analyze the dimensions, quality, microstructure, chemical composition, phases, hardness and fracture toughness of the pellets. Homogeneous distribution of a fine scale duplex structure of nontransformable tetragonal (t′) + α-alumina was obtained. The rapid solidification which followed the pelletization process after laser melting of the powder mixture prevented thermal transformation of cubic (c) phase to tetragonal (t) and monoclinic (m) phases. Complete t′ phase is formed by athermal transformation with a small amount of rhombohedral (r) phase. Hardness higher than 19 GPa with a fracture toughness higher than 10 MPa.m^1/2 was achieved. The results show the possibility of using a pulsed laser to produce completely re-melted and re-solidified spherical pellets with smooth surfaces under wide ranges of processing parameters. Also, it was found that there is a specific interaction time (pulsed length) required for melting and producing pellets at any level of power density. The pulsed lengths needed are in the range of 10–200 ms at power densities and specific energies values of 510 to 1275 W mm⁻² and 5–255 J mm⁻² respectively. Pellets diameters of 1000–4000 μm were effectively produced. The results clearly demonstrate a high efficiency of laser coupling with ceramic powders. (paper)