HVO/AF (High-Velocity-Oxygen/Air-Fuel) WC-17Co and WC-10Co4Cr coatings exhibit great potential in the replacement of electrolytic hard chrome (EHC) coating, and comprehensive properties of such coatings should be not worse than those of electrolytic hard chrome coating. The impingement-resistance of HVAF coatings sprayed on 300M ultra-high strength steel was studied in this paper. As an important property index, the fracture toughness of HVAF WC coatings was measured using micro-indentation method at the load of 9.8, 19.6, 24.5, 29.4 and 49.0N respectively. The cracks resulted from stress concentration in the micro-indentation were analyzed. The impingement-resistance for two HVAF WC coatings and EHC was evaluated according to the ASTM D3170 standard, and steel ball dropping experimentation was performed at the height of 0.61, 1.52, 1.83, 2.36 and 2.59m respectively. The cracks caused by both impingements were analyzed using SEM and optical microscopy in comparison with cracks in micro-indentation test.

The influence of the high velocity air fuel (HVAF) sprayed coating on the fatigue behavior of the low alloy steel was studied at different stress levels. It was observed that only one single main crack initiator existed in the substrate after fatigue at low stress levels, but there were multi-cracks on the substrate surface at high stress level. Detailed investigations showed that the cracks in the HVAF coatings sinuously extended to the interface and deflected thereat along the interface. Consequently, free-standing coating was formed due to its limited bond strength to the substrate and the lower elastic modulus than that of the substrate. The gap between the free-standing coating and the substrate surface was found to be correlated with the stress level. The high stress can greatly degrade the adhesion to the substrate causing the delamination of the coating. The cracks in the HVAF coating had no significant effect on the fatigue life of the substrate.