The aim of this review is to survey the state of the art relating to the rolling contact fatigue (RCF) investigation of various overlay coatings and also, to ascertain the influence of design parameters such as the type of deposition process, coating material and thickness on the RCF performance. Rolling contact fatigue is a significant factor in the failure of components in rolling/sliding contact. Although, sintered ceramics have provided improvements in RCF life of components in rolling/sliding contact, e.g. hybrid ceramic bearings, the economic and technological constraints associated have so far limited their use to specialist applications. Physical and chemical vapor deposition (PVD, CVD) as well as thermal spraying are methods of depositing overlay coatings. The designer must thus choose a deposition method based on economic and technical flexibility, e.g. material choice, functional grading, etc. Amongst this family of overlay coatings, PVD coatings are already finding commercial use whilst others are at a research and development stage. The available literature on the RCF testing of various types of overlay coatings is considerable, but it is generally difficult to synthesize all of the results to obtain a comprehensive understanding of the parameters which can have a significant effect on a coating's resistance to rolling contact fatigue. This review thus compares the RCF performance of these overlay coatings and discusses the results in terms of coating processes, materials, thickness, residual stress and tribological conditions of contact stress and lubrication.

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The impact and fatigue resistance of overlay coatings is significantly influenced by the residual strain (or stress) field induced during coating deposition, post-treatment, and in-service loading. Optimization of the residual strain field is therefore critical to the life and performance of components. Nondestructive measurement of these strain fields in relatively thin 300-400 m thermal spray coatings, however, poses a challenge because conventional techniques, such as deep hole drilling, x-ray diffraction, synchrotron diffraction, and changes in beam curvature either make these techniques destructive and/or provides only a very near-surface strain measurement. This particularly complicates the strain analysis in cermet coatings, e.g., WC-Co deposited by the thermal spraying process, where the low penetration depth of x-ray and synchrotron-diffraction ray can only provide a through thickness measurement of stress or strain profile via the destructive layer removal technique. Recent investigations have therefore concentrated on the use of neutron diffraction technique for such analysis, and this paper reports some of the early findings of the comparison of through thickness strain measurements in relatively thin 400 m as-sprayed and post-treated WC-12wt. % Co coatings via the neutron diffraction technique. Since neutrons are not charged, they do not interact with the electron cloud surrounding the atom (unlike x-ray); hence, diffraction results from the interaction with the atomic nucleus. Neutrons therefore have greater penetration depth in most engineering materials, and therefore provide a nondestructive through thickness strain measurement. Results of strain measurement are discussed with the structure property relationships and contact fatigue performance, and indicate that post-treatment of these coatings results in harmonization of the strain field within the coating, and at the coating substrate interface. This significantly influences the contact fatigue performance by improving both the cohesive and adhesive strength of these coatings.

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.