Dependence of the rolling contact fatigue of HVOF-sprayed WC-17%Co hardmetal coatings on substrate hardness
The continual demand for higher load-bearing capacity and improved performance and reliability of gears and other components increases the role of high endurable Hertz pressures and low friction. Coatings prepared by high velocity oxy-fuel spraying (HVOF), one of the most important thermal spray processes, have the potential to serve as a technical solution for highly loaded components such as gears. In this work the rolling contact fatigue (RCF) behaviour of HVOF-sprayed WC17%Co coatings was investigated on roller specimens with coating thicknesses of 600 µm and 100 µm deposited on 16 MnCr 5 steel substrates in the unhardened, quenched and tempered, and case-hardened states with hardness values of 190 HB, 400 HB and 5862 HRc, respectively. The endurable Hertz pressur es of the coated specimens were benchmarked against uncoated case-hardened 16 MnCr 5 steel. The failed coatings and coating damages before failure were thoroughly studied by optical and SEM microscopy. The highest durability was realised for a coating thickness of 600 µm through the use of the quenched and tempered substrate (about 130% of the benchmark). Coatings on these substrates showed a minimal amount of micropits and no other damage such as large cracks after testing. It can be assumed that differences in specimen preparation by grit blasting prior to spraying and different substrate strengths were responsible for the different results for the coated unhardened, quenched and tempered, and case-hardened substrates. The durability for a coating thickness of 600 µm was higher than for a coating thickness of 100 µm when unhardened and case-hardened substrates were used. Further improvements in the rolling contact fatigue behaviour could be achieved by optimisation of the combination of coating thickness and substrate strength and by use of other WC-based coatings.