Contact fatigue failure modes of HVOF coatings

Abstract

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 (∆ = 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.

Durability of plasma-sprayed Cr3C2-NiCr coatings under rolling contact conditions

“…Previous studies by Ahmed [7] showed that delamination was the commonly failure mode of the coated specimens in rolling contact. The coating is either delaminated at a location within the coating or just at coating/substrate interface.…”

“…Previous studies showed that the RCF failure modes of the spray coatings could be categorized into four types and named as surface abrasion, spalling, cohesive delamination (i.e., delamination within coating), and bulk failure (i.e., interfacial delamination) [4]. Table 1 summarizes the typical RCF test results from the published literature [1,4,[7][8][9][10][11][12][13][14][15][16][17]. These results are present in terms of deposition process, coating hardness, coating material, thickness and the contact stress.…”

Failure Analysis of Cr7C3 Coating for Helicopter Transmission System

“…The rough contact between the coating and the surface of the work piece and the micro sliding in the contact region make the coating prone to plastic deformation and micro shear. Therefore, it is necessary to study the lubrication state between the wedges and the inner and outer rings [5] . An empirical formula for calculating the oil film thickness, m h , between wedges and the inner and outer rings is presented as follows: …”

Effects of laser carburised striation units on rail steel fatigue wear

“…As a result, pitting will appear owing to the surface material falling out. 20,21 Rollers of the tester are made of GCr15 bearing steel. It attaches to high-carbon chromium bearing steel, which has good performance and wide applications.…”