SURFACE MODIFICATION OF HVOF THERMAL SPRAYED WC-CoCr COATINGS BY LASER TREATMENT

In this work the affects of laser characteristics on microstructure and microhardness of high velocity oxygen fuel sprayed (HVOF) WC-CoCr coatings were investigated. The coating was deposited with a Sulzer Metco WokaJet (TM)-400 kerosene fuel and the laser surface treatments were applied using CO2 laser with 10.6 mu m wavelength. Large variations in surface properties were produced from variation in the laser processing parameters. In total, four levels of peak power (100, 200, 300 and 350 W), four levels of spot diameter (0.2, 0.4, 0.6 and 1 mm) and three levels of pulse repetition frequency (PRF) were investigated. An initial set of tests were followed by a more detailed 33 factorial design of experiments. Pulse repetition frequency and duty cycle were set in order to maintain the same overlap in the x and y directions for the raster scanned sample spot impact dimensions. Overlaps of 30% were used in the initial tests and 10% in the more detailed trials. The results have shown that care must be taken to keep the irradiance at a relatively low level compared to uncoated surfaces. High irradiance can in this case result in rough and porous surfaces. Lower levels of irradiance are shown to provide more uniform microstructures, reduced porosity and increased microhardness.