Contact fatigue failure modes of HVOF coatings

The objective of this experimental study was to investigate the influence of coating thickness and contact stress fields on the performance and fatigue failure modes of thermal spray (WC–12%Co) HVOF coatings. Results of this study indicate that a non-dimensional coating thickness parameter (Δ=ξ/Ψ), where ξ is the coating thickness and Ψ the depth of maximum shear stress, can be used as a useful index to optimise coating delamination resistance during Hertzian contact loading. Apart from the detection of a new failure mode (termed spalling), which is a rare failure mode in thermal spray coatings, results indicate that by appropriate control of coating thickness, and tribological conditions, it is possible to achieve a fatigue life in excess of 70 million stress cycles, without failure. This improvement in coating performance was attributed to improved fracture toughness of liquid fuel HVOF (JP5000) coatings. Coating failure was attributed to micro- and macrocracking within the coating microstructure. Thermal spray coatings were deposited by a JP5000 system in three different thicknesses on the surface of 440-C steel substrate cones to vary the depth of shear stress within the Hertzian stress field. Rolling contact fatigue (RCF) tests were conducted using a modified four-ball machine under various tribological conditions of contact stress, configuration and lubrication. Surface observations were made using scanning electron microscope (SEM), surface interferometry and light microscopy, whereas subsurface observations were made using die penetrant investigations.