Contact fatigue failure modes in hot isostatically pressed WC-12%Co coatings

The objective of this experimental study was to investigate the influence of the post-treatment, Hot Isostatic Pressing (HIPing), on the Rolling Contact Fatigue (RCF) performance of thermal spray (WC-12%Co) coatings. Thermal spray coatings were deposited using a JP5000 High Velocity Oxy Fuel (HVOF) system in three different thicknesses on the surface of 440-C steel substrate cones to vary the depth of the shear stress within the Hertzian stress field. The furnace temperature during the HIPing process was varied at 850 °C and 1200 °C. RCF tests were conducted using a modified four ball machine under identical tribological conditions of contact stress, configuration and lubrication. Surface observations were made using Scanning Electron Microscopy (SEM) and Light Microscopy. Results of this preliminary study, which is the first of its kind in published literature to evaluate the RCF of HIPed cermet coatings, indicate that variation in HIPing temperature can have a significant influence on a coatingʹs delamination resistance. These results are discussed to comprehend the performance and ascertain the fatigue failure modes in HIPed HVOF coated rolling elements. Apart from comparing the failure modes between HIPed and As-Sprayed coatings, results indicate that by increasing the HIPing temperature to 1200 °C, and maintaining full film lubrication, it is possible to achieve a fatigue life in excess of 70 million stress cycles without failure in relatively thin (50 μm) cermet coatings. Coating failure was attributed to maximum shear stress occurring either at the coating/substrate interface or within the coating microstructure, resulting in delamination due to cyclic loading.