PLASMA SPRAYED COATINGS OF MECHANOFUSED ALUMINA-STEEL PARTICLES

Steel particles (Fe + 19 wt% Cr and 11 wt% Ni) with a size distribution in the range between 50 and 100 mu m were coated with pure alpha-alumina (99.99%) particles whose mean size was 0.6 mu m using a mechanofusion process. It was possible to obtain particles with alumina shells with a mean thickness of 2-4 mu m. EDS analysis showed that, according to the detection limit, no alumina was imbedded in the steel core and reciprocally no steel was included in the alumina shell. The composite powder was then plasma sprayed using a PTF4 type plasma gun (nozzle internal diameter 7 mm) with the internal injection of the powder 3 mm upstream of the nozzle exit. The arc current was 500 A and plasma forming gas mixture was 53 slm Ar and 7 slm H-2. The coatings were sprayed on cast iron samples (diameter: 25 mm, thickness: 5 mm) located on a rotating substrate holder (110 mm diameter). The plasma torch was moved parallel to the holder axis. The substrates were preheated with the plasma gun and their surface temperature was controlled by air jets monitored by an IR monochromatic pyrometer. The coatings showed a uniform distribution of alumina in the steel matrix for two substrate preheating temperatures of 300 and 550 degrees C.