CAVITATION EROSION OF LASER-SURFACE ALLOYED COATINGS ON AL-12-PERCENT-SI

Laser surface alloyed coatings on Al-12%Si have been eroded in distill ed water at 20 degrees C using a 20 kHz ultrasonic facility operating with a 50 mu m peak-to-peak amplitude. Erosion was measured by weight loss and the damage examined using microscopical techniques. Eight coa tings were selected for examination on the basis of high hardness and structural integrity. These consisted of a series of Fe and Ni alloys each with additions of either Mn, Cu or Cr. The linear erosion rate of Al-12%Si was 27.6 mg h(-1). This was decreased in the iron alloy coat ings to 7.2-3.8 mg h(-1) and in the nickel coatings to 2.47-0.71 mg h( -1). These rates show the erosion resistance of the coatings are bette r than some structural ceramics. Improvements in the incubation period were generally less substantial and the results varied greatly from a lloy to alloy. The main reason for the large increase in resistance of the coatings to erosion is the densely packed intermetallic compounds which act as a fine fibre-reinforced composite which restricts the si ze of the cracks during erosion. It appears that the matrix strengthen ing through rapid solidification and increased solid solubility is lim ited. The weakness of the matrix leads to failure by plastic flow and ductile fracture.