Diamondlike-carbon (DLC) coatings have received a lot of research attention by physicists and engineers, especially in the past 25 years. Attempts to use such materials in tribological applications have achieved variable success. The rationale for this work was to investigate the wear durability of three types of DLC coatings applied to hardened and tempered bearing steel and subject them to realistic high pressure cyclic loading under oil lubricated conditions for long duration. A thrust bearing design was deployed for this purpose. The wear and friction behavior of the DLC coated materials relative to uncoated materials was compared when using base (additive free) oils and typical autoengine formulated oils. The type of oil used made no difference to the dynamic friction and oil temperature for all the material and oil combinations used. Durability of the coated and uncoated roller bearings was determined by the type of material. For the uncoated bearings, life was limited after very many test cycles (approaching a billion) via classical rolling contact fatigue pitting. For all the DLC coated rollers life was governed by wear of their coatings. In the case of the tungsten doped DLCs (a-C:H:W), these were worn progressively and uniformly via microabrasion, whereas the nondoped ta-C and a-C DLC coatings were principally worn via delamination and tearing. The latter effect was relatively rapid and was considered to be initiated by blistering of the coating, a process that was probably driven by the high elastic energy/internal stress within the nondoped coating materials. The durability to delamination and tearing of the ta-C coatings was slightly lowered in formulated oil compared to tests made in base oil. Overall, for the test conditions used here, there was no apparent benefit in using DLC coatings.

The paper employs a rough-surface numerical elastic contact method designed to analyze Hertzian elastic contact effects of surface coatings. In particular the paper explores the differences in the surface contact mechanics and the resulting sub-surface stresses experienced over a range of differing coating material-properties, thickness, and machined roughness levels in a quantitative manner. The effect of a range of surface roughness properties and in particular root mean square roughness (σ) and correlation length β * , on the magnitude and depth of maximum shear stresses in the layer under individual asperities is investigated. This is done for a hard and stiff, and also for a soft and compliant coating, and for two coating thicknesses in each case. The results suggest that the magnitude of the local shear stress increases with increasing ratio σ / β * approximately linearly. The depth of the maximum local shear stress is found to correlate best with β * , however a further clear trend is observed between this depth and the number of profile peaks. The depth also shows a relation to the ratio σ / β * but the correlation in this case is weaker with significant deviations. Neither the magnitude nor the depth of shear stresses shows any significant trend in relation to the roughness (σ) alone. The tensile stresses at the interface, and the subsequent potential for delamination, are also investigated and found to be significant. Approximate correlation between the magnitude of interface tensile stress and root mean square roughness is achieved, but no clear trend in relation to correlation length is evident.

This is the second part of two companion papers, the first of which reported the empirical data on wear properties in carbon nitride coatings by a spherical diamond counter-face in repeated sliding contacts through in situ examination, with an emphasis on the effect of friction cycles and normal load. The second part will concentrate on wear mechanisms for the transition from “No observable wear particles” to “Wear particle generation.” The relationship between the critical number of friction cycles, Nc, and the representative plastic strain, Δ ε p , at asperity contact region was confirmed to follow the Manson-Coffin equation with two empirical constants, β and C . The observed generation of wear particles in carbon nitride coatings is therefore concluded to be a low cycle fatigue wear by surface flow and surface delamination in the ploughing mode. For further predicting lifespan, a simplified theoretical expression, combining the Manson-Coffin equation with the analytical solution of a proposed elastic perfectly-plastic indentation model, gives the relation between the critical number of friction cycles, Nc, and the coating thickness h , with respect to the contact pressure P , and the radius R of the asperity on the tip of the diamond pin.

The objective of this work has been to study the origin of surface failures in UHMWPE tibial bearings of total knee replacements. Earlier examination of hundreds of retrieved prostheses, along with analysis of the properties of UHMWPE material from retrieved bearings, had shown that a large number of the bearings suffered from subsurface oxidation. This oxidation was related to the gamma irradiation used to sterilize the tibial bearings. Mechanical properties of the polymer were significantly deteriorated in the oxidized region, with the most severe reduction of strength and ductility occurring about 1 mm beneath the contact surface. In this work the contact stress distribution in the bearings was analyzed, and tribotesting of the bearing materials was carried out under simulated service conditions. Fatigue cracks and delamination developed in bearings tested in a knee simulator and in rolling/sliding test specimens, and the damage was similar to that found in retrieved tibial bearings. The fatigue cracks invariably initiated in the embrittled oxidized layer, and the depth of that layer determined the depth at which the cracks began. The stress analysis showed that the maximum shear stress and von Mises equivalent stress reached high levels in the subsurface oxidized zone where the delamination and contact fatigue failures initiated. [S0742-4787(00)04401-5]

The friction and wear properties of unmodified ultra-high molecular weight polyethylene (UHMWPE) were investigated experimentally. Dinks of semicrystalline UHMWPE were slid against polished CoCrWNi pins in bovine serum at ranges of contact pressure and sliding speed typical of those encountered in total joint replacements. The coefficient of friction was monitored continuously during testing, and the wear rate was determined from surface profilometry measurements of worn disk surfaces accounting for strain relaxation. Scanning electron microscopy (SEM) results demonstrated that surface deterioration comprises adhesion, third-body abrasion by polyethylene wear debris, and delamination wear. The contribution of these mechanisms to the overall wear rate and the formation of wear debris depends predominantly on the contact pressure and secondarily on the sliding speed. Transmission electron microscopy (TEM) yielded new insight into the evolution of the microstructure morphology of UHMWPE during sliding. Cross sections parallel to the wear tracks obtained from various depths were analyzed with the TEM to develop a spatial mapping of the subsurface microstructure as a function of contact pressure. Alignment of crystalline regions (lamellae) in the polyethylene microstructure parallel to the sliding surface was found to occur during sliding even at relatively low contact pressures. SEM observations suggested that the highly oriented microstructure is the precursor to delamination wear, leading to the formation of wear particles larger than those produced by adhesion and third-body abrasion at the contact interface.

Two flat layered elastic half-spaces, of different material properties, are pressed together and slide against each other with a constant coefficient of friction. Although a nominally steady-state solution exists, an analysis of the dynamic motion yields complex eigenvalues with positive real parts, i.e., a flutter instability. These results demonstrate that self-excited (unstable) motion occurs for a wide range of material combinations. The physical mechanism responsible for this instability is that of slip-wave destabilization. The influence of the properties of the layers on the destabilization of sliding motion is investigated. These dynamic instabilities lead either to regions of stick-slip or to areas of loss-of-contact. Finally the dynamic stresses at the interfaces between the layers and the semi-infinite bodies are determined and compared to the nominally steady-state stresses. These dynamic stresses are expected to play an important role in delamination.

Cutting forces were measured during turning AISI 4340 steel with uncoated, two-layer coated (TiC/Al 2 O 3 ), and three-layer coated (TiC/Al 2 O 3 /TiN) cemented WC-Co inserts possessing chip-breaker grooves in order to investigate the progression of tool wear. The variations of cutting force signals are interpreted in light of predominant wear mechanisms. A characteristic feature of wearing coated tools is shown to be the rapid increase of the radial and axial cutting forces when the tool life is reached. This behavior is associated with both the increase of the real contact area on the nose and the flank face due to coating delamination and the relatively less severe wear on the rake face. Normalizing the cutting forces by those of unworn tools yields a single curve for each cutting force. The instant at which either the radial or the axial force deviates from these curves determines the wear life of coated tools. The good agreement between results for the wear life of coated tools obtained from cutting force and nose wear measurements suggests that the wear life of multilayer ceramic coated tools can be accurately determined by monitoring the cutting force response.

Turning experiments were performed with cemented WC-Co cutting tools coated with two-layer and three-layer overcoats of TiC/Al 2 O 3 and TiC/Al 2 O 3 /TiN, respectively. For comparison, uncoated WC-Co tools were also tested under similar cutting conditions. The predominant wear mechanisms of the various ceramic overcoats and cemented WC-Co were investigated using surface profilometry, scanning electron microscopy, and energy dispersive X-ray analysis. Representative results of the tool wear behavior are presented, and the significance of each ceramic layer on the overall tool wear resistance is interpreted in light of the identified dominant wear mechanisms. Delamination wear characterized by the propagation and linkage of surface, subsurface, and interfacial cracks, abrasion, surface plastic shearing, plucking of carbide grains, and dissolution/diffusion are shown to occur depending on the tool material. These wear processes are not mutually exclusive; they may occur simultaneously at different positions on the same tool surface. Based on nose wear data, correlations between wear lives of coated and uncoated tools and feedrate are established.

Seizure phenomena in pin-on-disk tests have been studied for “soft” and “hard” steel specimens. Differences in competing and dominant wear mechanisms under steady state friction have been preserved for “soft” and “hard” specimens in the region of transition to seizure or galling. Severe wear was observed for “soft” specimens under all loads tested, while adhesion and splitting off of wear particle conglomerates (microseizure) were identified for “hard” specimens. The contact temperature, calculated in accordance with the temperature model of plastically deformed contact spots (Kuhlmann-Wilsdorf), has appeared to be low for “soft” specimens and not sufficient for adhesion interaction. The effect of oxide films on the friction of “hard” specimens has been estimated in accordance with the temperature model for a coated semi-infinite body (Tian and Kennedy). The insulated oxide films on the surface of “hard” specimens create the “skin effect” and lead, therefore, to raising the temperature up to the temperature of adhesion interaction. Temperature instability of hard surfaces has been demonstrated to result from the “skin effect” and from a disturbance in equilibrium of formation and failure of oxide films. It has been shown that for “soft” specimens the prime cause of transition to seizure was the mechanical interlocking between the wear particles and the soft disk surface combined with mechanical instability, while for “hard” specimens the cause was temperature instability. A more realistic temperature model of the contact has been considered, which takes into account some competing wear mechanisms (oxidational wear, ploughing, delamination) and the effect of wear particles.

The friction and wear mechanisms of particulate (metal particle and barium ferrite) and metal evaporated (ME) magnetic tapes were investigated. We conducted tests on these tapes in contact with metal-in-gap (MIG) video heads, using a rotary head recorder in the still (pause) mode. We measured signal degradation and friction during the tests. We conducted chemical and surface analyses of the interface components after the tests. We found discernible differences between the tribological behavior of particulate tapes, and that of the ME tape. The particulate tapes exhibited a more stable friction and head output than the ME tape. We attributed this to a cleaner contact region, due to effective action of the head cleaning agents (HCAs) found in the particulate tapes. The particulate tapes exhibited wear lifetime longer by an order of magnitude, than that of the ME tape. Mild continuous adhesive wear occurred on particulate tapes followed by catastrophic failure. Tape fatigue possibly led to the catastrophic failure. On the ME tape surface, damage initiated at high points or bumps, which resulted in localized delaminations of the tape coating. This led to a catastrophic removal of the entire magnetic coating over the rubbing track. The major difference between the particulate and ME tapes was that signal dropouts concurrent with increases in friction, which resulted from debris accumulation on the video head, preceded the catastrophic failure in the case of ME tapes. We investigated the running-in process of the video head. We found that the durability of a tape and the initial head output increased, and the initial friction force on a tape decreased, as the head ran-in with the tape. We attributed this result to the tape forming a favorable contour on the head rubbing surface. Deposits on the head surface consisted of binder for the particulate tapes, and lubricant and the magnetic coating for ME tape. Tape materials transferred preferentially to the recessed metal core and the recessed glass of the MIG head.

A finite element model previously described is used to analyze indentation of hard-face materials. The materials (substrate and coating) are supposed elastoplastic, the spherical indenter is elastic. The state of stress during indentation (including unloading) of hard-face materials (chromium layer on steel) is investigated in details. The tensile stresses which might lead to failure are especially discussed, and related with experiments showing fracture or delamination of the coating. The model also shows the influence of film thickness on surface and interface stresses.

The processes of tribofilm formation and disruption and the predominant tribo-mechanisms of unlubricated ceramic materials were investigated experimentally. Sliding experiments in humidity controlled atmospheres revealed that the formation of interfacial tribofilms significantly affects the steady-state friction and wear properties of ceramics. Scanning electron microscopy and various composition analysis techniques demonstrated that although tribochemical reactions might occur, the principal mechanisms of tribofilm formation were the generation, agglomeration, and compaction of fine wear debris produced from both sliding surfaces. The tribofilms exhibited different tribological characteristics, depending on their elemental compositions and the humidity. For all the ceramic pairs tested, the steady-state coefficients of friction decreased with relative humidity. In contrast to the conventional fracture toughness approach, surface profilometry and microscopy studies showed that the highest wear rates were encountered with the toughest ceramic. Plowing grooves parallel to the direction of sliding, fine wear debris of round and cylindrical shapes, microcracking, and localized delamination of the tribofilms were identified. Microscopic observations suggested that damage of the subsurface material adjacent to the interface of the tribofilms was immeasurable. Qualitative comparison of the topographical features of worn surfaces indicated that, depending on the humidity and the type of ceramic, microplasticity, microfracture, and delamination of the tribofilms were the prevailing steady-state tribomechanisms.

This paper deals with the problem of an asperity-excited thermo-mechanical field in a medium with a thin surface layer and a near surface void defect. The solutions for the temperature distribution and stress state in the vicinity of the void are obtained. Numerical results are obtained using the material properties of Stellite III for both the coating and the substrate. The parametric effects of material properties in the coating and the substrate are considered by varying the corresponding property in the coating or the substrate. The stress field solutions of the coated medium with a cavity are compared with the solutions for a single material with a cavity. In the case of the single material with a cavity, while the thermal stress is much larger than the thermal stress of the no cavity case, the principal directions of the thermal stress are not significantly changed. However, for the case of a coated medium with a cavity, not only is the thermal stress much larger than that of the no cavity case, but also the principal directions of the thermal stress are changed. The principal directions of the thermal stress field depend on both the material properties and the ligament thickness (thickness between the wear surface and the top edge of the cavity). When the angle of the principal directions becomes larger, shear delamination will initiate at the coating/substrate interface.

This paper presents the results of a preliminary analysis of the branching behavior of a horizontal subsurface crack subjected to a moving compressive surface load. The purpose of the analysis is to provide some fundamental understanding concerning the behavior of subsurface contact fatigue cracks, including the onset of pitting or delamination. The analysis is approximate inasmuch as the traction-free boundary condition at the surface is only partially satisfied, and the history dependent effects arising from crack face friction are not included. The results show that mode I branching crack growth is possible even though the stress field is largely compressive. Nevertheless, based on a simple competing failure mode model the results indicate that within the limits of the analysis, nonbranching mode II growth is likely to be the dominant mode of propagation.

The wear rate data of glass fiber reinforced polyethylene terephthalate (PET) has been analyzed statistically using the wear lifetime model together with the two-parameter Weibull distribution function. The specific wear rate versus normal load curve has been given a theoretical explanation, the empirical model being derived from the concepts of crack propagation and delamination physics.

The results of wear tests and surface analyses confirmed that the wear behavior in the presence of ZDDP is controlled by chemically reacted surface films. Surface analysis by Auger microprobe showed that the reacted layer is composed of iron sulfide and zinc phosphate and that it extends to several hundred Å below the contact surface. It was found that the effectiveness of ZDDP in wear reduction was dependent on the test conditions. Under low loads and temperatures, it decreased wear by two orders of magnitude. However, at higher loads and temperatures, wear was increased by one order of magnitude. Scanning electron microscopy confirmed that at low loads, ZDDP changes the wear mechanism from plowing to a mild form of surface deformation, whereas at high loads plowing is changed to delamination of the surface layer. Surface analysis by Auger showed that higher loads and temperatures had resulted in reduction of sulfur content in the surface films. This reduction is attributed to changes in the reaction kinetics at higher contact temperatures caused by high contact loads and elevated oil temperatures. Since sulfides are usually harder than phosphates, the higher wear rate by delamination is explained by the loss of film strength due to reduction of sulfide content.

Lubricated and dry experiments on titanium and steel surfaces with and without TiN sputtered coatings of various thicknesses have been conducted. The significance of the layer thickness, interfacial “friction”, magnitudes of normal and tangential surface tractions, and the mechanical properties of the layer and of the substrate (e.g., elastic modulus and hardness) are critically examined. The conditions under which the deformation mode at the solid-solid contacts is elastic or plastic are addressed in light of the experimental evidence and a finite element analysis. It is shown that surfaces with very low friction, especially for unlubricated sliding, and practically zero wear rates can be obtained in both lubricated and dry sliding by coating the surfaces with sufficiently thick TiN layers. Removal of the protective TiN layer resulted in plowing, severe damage, and delamination.

The influence of laser surface transformation hardening on the sliding wear characteristics and mechanisms of ASTM class-40 gray and 80-55-06 ductile cast irons was investigated. A 1.2 kw, continuous wave, CO 2 gas laser was employed to scan the beam successively across the surfaces of cast irons to generate hardened and tempered layers with various case depths. A pin-on-disk wear test system was then used to study the wear behavior as functions of case depth, microstructure, hardness, and surface roughness. As expected, a dramatic improvement in resistance to scuffing and sliding wear was obtained. However, the most significant result was the occurrence of negligible oxidational wear for a load range that increased with an increase in case depth. Resistance to mild and severe wear, mild-to-severe wear transition load, and frictional heating were increased with an increase in case depth. Analysis of worn surfaces and wear debris revealed that negligible oxidational wear in laser-hardened irons is due to two mechanisms: oxidation and adhesion of oxide to the substrate. In contrast, the mild oxidational wear of untreated irons occurs through the formation of loose oxide debris. The mechanisms of severe wear were plastic deformation, delamination, and adhesion; the rate process was controlled by adhesion for laser hardened irons and delamination for untreated irons.

The effects of ion implantation on the friction and wear behavior of metals were investigated. Experiments were conducted with iron, titanium, and copper implanted with nitrogen ions, iron implanted with aluminum ions, and copper implanted with zinc ions. The significant reduction in friction and wear of the iron and titanium systems is attributed to a hard layer formed during the ion implantation process. This hard layer minimizes plowing and subsurface deformation and hence reduces the delamination wear process, i.e., crack nucleation, crack propagation, and the formation of delamination wear sheets. The implanted copper specimens did not appear to have a hard surface layer and showed little improvement in their tribological behavior over the unimplanted copper. Iron implanted with nitrogen ions was compared with ion nitrided iron. Under dry sliding conditions and high loads ion nitriding appears to be superior to ion implantation. However, at lower loads and under lubricated conditions, ion implantation is more advantageous due to its superior surface finish. A finite element model of an elastic semi-infinite solid under the contact of a stationary rigid asperity showed that the hard layer does not change the subsurface stress distribution. Therefore, the role of this thin layer is not to support the load, but to decrease the plowing component of friction, which, in turn, substantially reduces subsurface deformation and thus wear.

The wear mechanisms of metals under low-speed sliding conditions have been investigated experimentally in order to check the validity of the delamination theory of wear. The subsurface damage done under the wear track is documented using nine different metals with various chemical compositions and microstructures under an argon atmosphere. The experimental results show that the predominant wear mechanism is the wear by delamination of sheets. The wear sheet is generated by a subsurface crack extending over a distance which is in general, one to two orders of magnitude longer than the diameter of the asperity contact. Voids are nucleated around inclusions during plastic deformation. In metals without inclusions the void nucleation rate is low and so is the wear rate, even when they have very low hardness. All these observations substantiate the predictions of the delamination theory of wear. It is postulated that a tangential load which is greater than that predicted by the adhesion theory can be transmitted by a “plowing” mechanism between the sliding surfaces. A new wear equation is presented, which complements the model presented in a previous paper.