Rehan Ahmed

Rolling contact fatigue of surface coatings—a review

Abstract 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.

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 (∆ = 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.

Rolling contact fatigue performance of detonation gun coated elements

Abstract Rolling contact fatigue performance of thermal spray coatings has been investigated using an experimental approach. A modified four ball machine which simulates a rolling element bearing was used to examine the coating performance and failure modes in a conventional steel ball bearing and hybrid ceramic bearing configurations. Tungsten carbide (WC-15% Co) and aluminium oxide (Al 2 O 3 ) were thermally sprayed using a super D-Gun (SDG2040) on M-50 bearing steel substrate in the geometrical shape of a cone. A coated cone replaced the upper ball that contacts with three lower balls. The rolling contact fatigue (RCF) tests were performed under immersed lubricated conditions using two different lubricants. Fatigue failure modes were observed using a scanning electron microscope. Microhardness measurements of the coating and the substrate and elastohydrodynamic fluid film thickness results are included. The results show the requirement for significant optimization of the coating before use in rolling element bearing applications. The coating was fractured in a delamination mode. Test results show an optimization in coating process is required before these coatings can be used for rolling contact applications. WC-Co coatings perform better than Al 2 O 3 coatings in rolling contact.

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

Abstract 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 coatings 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.

Mechanisms of fatigue failure in thermal spray coatings

The aim of this experimental study was to ascertain the fatigue failure modes of thermal spray coatings in rolling/sliding contact. These failure modes outline the design requirements of thermal spray coatings for high-stress tribological applications including impact and point or line contact loading. Recently, a number of scientific studies have addressed the fatigue performance and durability of thermal spray coatings in rolling/sliding contact, but investigations on the mechanisms of these failures are seldom reported. The understanding of such failure mechanisms is, however, critical in optimizing the generic design of these overlay coatings. This study takes a holistic approach to summarize the results of ongoing research on various cermet (WC-Co) and ceramic (Al2O3) coatings deposited by detonation gun (D-Gun), high-velocity oxyfuel (HVOF), and high-velocity plasma spraying (HVPS) techniques, in a range of coating thickness (20–250 µm) on various steel substrates to deliver an overview of the various competing failure modes. Results indicate four distinct modes of fatigue failure in thermal spray cermet and ceramic coatings: abrasion, delamination, bulk failure, and spalling. The influences of coating process, thickness, materials, properties of substrate materials, and prespray conditions on these fatigue failure modes are also discussed. A modified four-ball machine was used to investigate these failure modes under various tribological conditions of contact stress and lubrication regimes in conventional steel and hybrid ceramic contact configurations. Results are discussed in terms of pre- and post-test surface examination of rolling elements using scanning electron microscopy (SEM), electron probe microscopy analysis (EPMA), and surface interferometry, as well as subsurface observations using x-ray diffraction (XRD), residual stress analysis, and dye-penetrant investigations.

Failure modes of plasma sprayed WC–15%Co coated rolling elements

Abstract This experimental study addresses the failure modes of plasma sprayed coatings in rolling contact. A high velocity plasma spraying system was used to deposit WC–15%Co coatings on the surface of 15 mm diameter 440-C bearing steel cones. These coatings were deposited in two different thickness. Rolling contact fatigue (RCF) tests were conducted using a modified four ball machine in conventional steel ball bearing and hybrid ceramic bearing configurations. These tests were conducted under various tribological conditions of contact stress and lubrication regimes at room temperature. Failure modes were investigated on the basis of surface and subsurface observations of failed coated rolling elements. Surface observations were made using conventional scanning electron microscopy (SEM) and light microscopy. Subsurface observations were made using fluorescent dye penetrant technique. Observations of debris generated during the RCF tests, changes in topography of lower planetary balls, electron probe microscope analysis (EPMA), microhardness/fracture toughness investigations and, coating microstructural studies are also included to aid the discussion. Two modes of failures, i.e., surface wear and coating delamination, were observed during this investigation. Coated rolling elements failed in either one or a combination of these two modes depending upon the tribological conditions during the RCF test. Surface wear was associated with asperity contact in the presence of microslip/sliding within the contact region. The process was accelerated in the later stages of RCF tests in the presence of wear debris due to additional mechanism of three body abrasion. Coating delamination was associated with the initiation/propagation of subsurface cracks, which resulted due to defects in the coating microstructure. These cracks propagated at the depths of orthogonal shear stress and maximum shear stress under the surface of wear track.

Identification of surface features on cold-rolled stainless steel strip

Abstract A novel method based on three-dimensional profilometry data and Matlab analysis software is described to identify surface features on cold-rolled stainless steel strip. The aim of the method is to detect automatically pits and roll marks that can be observed in optical or SEM micrographs. Pits are identified by locating regions which are significantly deeper than the immediately adjacent surface. Deep or steep features which extend a significant distance in the direction of rolling are identified as roll marks. Results for typical cold-rolled stainless steel sheet show that the algorithms are effective in identifying the more obvious pits and roll marks. By suitable adjustment of the tolerances used in the analysis, the method can be tailored to detect less severe features. Application of the method, either for research purposes or routine industrial inspection, will require tuning of these tolerances to detect pits of the severity relevant to the end use of the strip. The methodology has been applied to a series of rolled strip samples to track the evolution of pits and roll marks during a schedule. Results show how the initially large area of deep pits is rapidly eliminated and transformed into shallow pits. The pit identification method is used to estimate the effect of trapped oil on lubrication. Results suggest that this expelled oil will contribute significantly to the lubrication of the surrounding area. Finally, a good correlation is demonstrated between strip surface reflectance measurements and the estimated pit area.

An Experimental Investigation of Surface Pit Evolution During Cold-Rolling or Drawing of Stainless Steel Strip

This paper presents an experimental investigation of the mechanisms of pit elimination in strip drawing and rolling of stainless steel strips. Strip drawing tests with artificial indents confirm the role of micro-plasto-hydrodynamic lubrication (MPHL) in allowing pits to be reduced in size and depth. The similarity of results for two oils, which differ in viscosity by a factor of 10, is attributed to the fact that oil is drawn out of the pits rather easily, so that the behavior tends to the unlubricated case. Similar behavior is observed for strip drawing of shot blast white hot band. For much smoother bright anneal strip, it is suggested that the presence of an oil film in the unpitted region prevents generation of pressure differences between the pits and the unpitted regions. A comparison of stripdrawn and cold-rolled stainless steel samples show that the change in pit area and R roughness varies with overall reduction in a remarkably similar way. The reason for such similar behavior is attributed to the absence of hydrodynamic action in preventing pit elimination, albeit for opposite reasons. The similar rate of pit evolution in both cases confirms the usefulness of the strip drawing rig as a tool to model the change of surface topography during rolling, as long as care is taken in matching the regimes of lubrication. DOI: 10.1115/1.1327580

Experimental measurement of the residual stress field within thermally sprayed rolling elements

Abstract A non-destructive experimental approach using an X-ray diffraction technique was used to investigate the generation of residual stresses in thermally sprayed rolling elements. The rolling elements were detonation gun coated balls and high velocity oxy-fuel coated cones. A modified four ball machine was used to perform rolling contact fatigue (RCF) tests on tungsten carbide cobalt (WC-Co) coated samples on steel substrate. RCF tests were conducted in conventional steel ball bearing and hybrid ceramic configurations. Residual stress measurements were performed at different sample orientations on different coating thicknesses and various substrate geometries. Residual stress measurements on as-sprayed samples, pre-tested samples, and after the RCF tests were performed during this study. This enabled the measurement of residual stresses generated during the thermal spraying process and due to the RCF tests. Residual stress measurements are also made on the failed areas of the coatings. Residual stress measurement results are presented in the form of principal stress values, using complex stress/strain relationships. These results indicate that residual stresses are critical to the performance of coatings. The generation of residual stresses is not only dependent upon the coating process but also on the coatings thickness and substrate geometry. RCF tests induce tensile residual stresses within the contact area and coating microstructure. Residual stress magnitude depends upon the test configuration and time of failure. Compressive residual stresses caused by the coating process are helpful as they improve the RCF life of the coatings. A multiple-cause diagram relating to the generation of residual stresses within the coatings has been presented and stress measurements have been explained with the aid of figures and scanning electron microscopy observations.

Indentation testing and its acoustic emission response: applications and emerging trends

Abstract This review summarises the state of knowledge on acoustic emission (AE) techniques applied to material property evaluation during indentation (e.g. hardness) testing. There are two aspects of application of AE technique to indentation which makes it unique, i.e. (1) enhancing the understanding of the evolution of material accommodation mechanisms under loading and (2) qualitative and quantitative evaluation of mechanical properties such as fracture toughness and bond strength from the AE signal. Both of these aspects have the potential to improve our understanding of the structure property relationships of current and future generation materials. In addition, the knowledge developed here can be incorporated to improve the AE based condition monitoring systems for stress critical applications. This review concentrates on the phenomena which occur during indentation and how its examination can be used to study more fundamental behaviour of materials such as deformation and fracture. The uncertainty in quantifying and measuring the total crack surface in indentation makes a simple fracture mechanics based assessment of toughness difficult. It is therefore expected that correlation between AE and fracture patterns will lead to an improved method for material’s quality evaluation. The main part of this review is presented on AE of material classifications. These classifications include ceramics, glasses, composites, metals and metallic foams, thin solid films and thermally sprayed coatings. Apart from quasi‐static indentation testing, attention has also been paid to studies on various AE instrumented indentation systems so that information can be derived about the progress of deformation and cracking processes. This review discusses the studies summarising those aspects that have so far been established and the areas of controversy and/or lack of knowledge. The prospect of using AE techniques to monitor indentation tests is also assessed, taking into account those few studies that have been reported so far in different groups of materials. Although with some limitations, it is concluded that AE monitored indentation testing has considerable scope to assess in much more detail the deformation and cracking properties of materials under localised stress condition. It is possible to construct empirical relationships and develop theoretical understanding linking mechanical parameters with AE signal characteristics and its derived features. However, the occurrence of multiple events at different locations superimposing the AE signal requires more advanced signal processing techniques. With the advancement of very thin films and nanomaterials, it is anticipated that AE response measured during nanoindentation will be critical for enhancing our understanding of future generation applications, as it allows individual events to be investigated without resorting to more complex signal processing techniques. In terms of material accommodation, the understanding of physical mechanisms generating AE require a multiscale approach, e.g. correlations exist between fracture and sudden release of AE energy, dislocations and Bremsstrahlung and Frank–Reid sources, and maternsitic phase transformation with rapid variation in the shape of deformation volume generating AE exist; however, integration of continuum elastic–plastic and molecular dynamics models is necessary to enhance our understanding of the physical mechanisms generating AE response. This multiscale approach can be further helped by the experimental data using AE instrumented nanoindentation as it allows very localised and fine scale measurement in load or displacement control.