M. Hadfield

The influence of ring crack location on the rolling contact fatigue failure of lubricated silicon nitride: experimental studies

This paper presents an experimental study of the influences of ring crack location within the contact path on the rolling contact fatigue failure. The rolling contact tests are performed on silicon nitride/steel elements. Silicon nitride ball surfaces are examined before testing using a dye-penetrant technique and light microscopy. The surfaces during testing, post-test and after failure are examined using light microscopy. Research shows that fatigue failure under rolling contact loading is markedly sensitive to the location of ring crack on the contact track. Only a few locations on the contact track can lead to fatigue failure at the maximum Hertz contact pressure of 5.58 GPa.

Parameter optimization by Taguchi methods for finishing advanced ceramic balls using a novel eccentric lapping machine

Abstract The final finishing process of advanced ceramic balls used in hybrid precision bearings constitutes two-thirds of the total manufacturing cost, and hence effective and economic finishing methods and processes are critical to their widespread application. A novel eccentric lapping machine is designed and manufactured. Hot isostatically pressed silicon nitride ball blanks (diameter 13.25mm) are used to investigate the feasibility of accelerating the ball finishing process while maintaining high surface quality. Taguchi methods are used during the first step of finishing to optimize lapping parameters; the L9 (34) four-parameter, three-level orthogonal array is used to design the experiment. Experimental results reveal that this novel eccentric lapping method is very promising; a material removal rate of 40 μm/h is achievable. The optimum lapping condition is found to be high speed, high load and high paste concentration with 60 μm diamond particles. The analysis of variance shows that the most significant lapping parameter is lapping load, which accounts for 50 per cent of the total, followed by lapping speed (31 per cent); the particle size and paste concentration only account for 12 per cent and 7 per cent respectively. A comparison with previous lapping experiments and the mechanism of material removal are also discussed briefly.

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.

Failure modes of ceramic elements with ring-crack defects

Abstract A hybrid ceramic/steel angular contact ball bearing is experimentally modelled using a modified four-ball machine. Ceramic ball surfaces are artificially damaged with ring pre-cracks. Rolling contact fatigue failure modes are studied after testing with high contact stresses and speeds. The ceramic ball surface and subsurface are analysed in detail using electron microscopy and a dye-penetration technique.

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.

Rolling contact fatigue failure modes of lubricated silicon nitride in relation to ring crack defects

The use of silicon nitride applied to rolling element bearings shows some practical advantages over traditional bearing steels. The contribution of this present study is to provide understanding of surface crack defect characteristics and the subsequent rolling fatigue modes of failure. Surface defects such as pressing faults and ring cracks are characterised using light microscopy. Experimental bench testing using an accelerated rolling contact fatigue rig produces results which identify the relationship between fatigue failure modes and surface defects. The rolling contact tests are performed on silicon nitride/steel elements using a variety of lubricants. Ball surfaces are examined before testing using a dye-penetrant technique and light microscopy. Post-test surfaces and failures are examined using light and scanning electron microscopy. A model of surface crack propagation in lubricated contact is described. The boundary element model is used to investigate the growth mechanism of ring crack defects during rolling contact.

Failure of silicon nitride rolling elements with ring crack defects

Hot isostatically pressed silicon nitride rolling elements are currently used in hybrid rolling element bearings. Surface defects such as ring cracks are difficult to detect during high volume production processes and hence there is a need to assess their influence on the rolling contact performance of the material. An accelerated rolling contact fatigue bench test is used to examine failure mode and performance of silicon nitride rolling elements with and without surface ring cracks. The surface ring cracks are found on silicon nitride as purchased from manufactures and are not produced artificially. Experimental test results are presented with pre-test surface analysis using a dye penetration technique. Theoretical calculations of the lubrication regime, contact stress fields and finite element analysis are presented. Scanning electron microscopy is employed to observe the failure modes of the silicon nitride rolling elements.

The influence of ring crack location on the rolling contact fatigue failure of lubricated silicon nitride: fracture mechanics analysis

The influence of ring crack location within the contact path on rolling contact fatigue failure has been studied using numerical fracture analysis. The numerical calculations are based on a three-dimensional model for the ring crack propagation. The ring crack is considered as a conic shape with a curved line as the crack front. The rolling contact loading is simulated by repeated Hertzian point contact load with normal pressure and tangential traction. Fracture mechanics analysis is utilised to determine the stress intensity factors (SIFs) along the crack front and the SIFs are analysed using a three-dimensional boundary element model. The analytical results are verified by experimental studies, which show that present predictions of ring crack location influence are consistent with the experimental observations.

The effect of the test machine on the failure mode in lubricated rolling contact of silicon nitride

The experimentally assessed performance of technical ceramics in lubricated rolling contact is presented. Special emphasis is on the failure modes resulting from the test conditions encountered in the model rolling contact used.