Contact fatigue failure evaluation of post-treated WC–NiCrBSi functionally graded thermal spray coatings

Abstract

Functionally graded thermal spray coatings applied to industrial components, which are subjected to contact fatigue or repeated impact loading, can reduce components weight and internal stresses whilst improving the adhesive strength to combat surface and sub-surface crack propagation. However, defects within the coating microstructure, which cannot be removed by the functionally graded approach, can compromise components reliability in high stress tribological applications. Post-treatments such as vacuum heating and Hot Isostatic Pressing (HIPing) have been shown in scientific studies to improve the coating microstructure, however, the influence of post-treatment on thermal spray coatings in rolling/sliding contacts have been seldom reported. This paper reports the rolling contact fatigue (RCF) analysis of functionally graded WC–NiCrBSi coatings deposited by a JP5000 system and subjected to post-treatment. HIPing was carried out at two different furnace temperatures of 850 and 1200 °C, whilst vacuum heating was performed at the elevated temperature of 1200 °C. The rate of heating and cooling was kept constant at 4 °C/min. RCF tests were conducted using a modified four-ball machine under various tribological conditions of contact stress and configuration, in both full film and mixed elasto-hydrodynamic lubrication (EHL). Test results reveal that the performance of coatings was highly dependent on the changes within the coating microstructure. Coatings HIPed at 1200 °C displayed relatively improved RCF performance over the as-sprayed coatings at stress levels of 2 and 2.7 GPa in full film lubrication. Improvement in RCF performance was attributed to the densification and homogeneity within the coating microstructure.