Stephen M. Hsu

Ceramic wear maps

Abstract Ceramic wear maps have been developed to elucidate the complex interactions of the operating parameters, environments, and wear mechanisms. This paper summarizes these interactions for four ceramics, alumina, yttria-doped zirconia, silicon carbide and silicon nitride. Wear maps of these ceramics are systematically constructed using measured data under dry sliding, water, and paraffin lubricated conditions. For each material, different wear level regions and wear transition zones are identified as a function of operating conditions and lubrication conditions. Wear mechanism studies performed within each wear region give rise to the wear mechanism maps. These maps facilitate material comparison and selection. The knowledge of wear, wear transitions, and wear mechanisms for a material pair enables realistic wear model development. One outcome of this approach is the recognition that a single wear model for a material pair cannot cover all operating conditions and environments. As wear maps are constructed today, they are material pair specific. Within a material pair, there are microstructural dependence and surface properties influence. These parameters can change substantially for a given chemical composition of the material. How to incorporate these factors into the wear map research remains an issue. The search for a universal parameter such as the “asperity temperature” in Ashbys wear map continues in spite of mounting evidence that this may not be practical or feasible. But the hope remains that some parameters can be identified to normalize a large number of materials, operating conditions, and environments for tribological applications. Systematic wear maps are the first steps in this direction.

Diverse cellular and apoptotic responses to variant shapes of UHMWPE particles in a murine model of inflammation.

The wear of orthopaedic prostheses results in the release of a markedly heterogeneous assortment of particulate debris, with respect to both size and shape. Although particle size has been extensively examined, the role of particle shape in adverse inflammatory reactions to debris remains unclear. Using an in vivo murine model of inflammation, we assessed tissue responses to globular and to elongated ultra-high molecular weight polyethylene (UHMWPE) particles with a similar surface area, and investigated whether inflammation and cellular apoptosis varied with particle shape in the debris-tissue interaction. Histological changes of UHMWPE-stimulated pouch membrane were assessed using a computerized image analysis system. Quantitative real time PCR and ELISA were performed to assess mRNA expression and protein level of the cytokines, and TUNEL assays were conducted to quantify apoptotic cells. The data revealed that elongated particles generated more active inflammatory air pouches, stimulated more severe membrane proliferation and the inflammatory cellular infiltration compared to globular particles. Increased levels of IL-1beta and TNFalpha were detected in the lavage and homogenate of pouches stimulated with elongated particles in comparison to pouches with globular particles, and the apoptotic assay indicated more severe apoptotic changes within the inflammatory membrane provoked with elongated particles. Our results suggest that cellular responses to UHMWPE wear debris are dependent on the shape of the particles.

Wear and wear transition mechanisms of ceramics

Abstract The combination of indentation, inclined-plane sliding, and wear tests provides useful and unique way to investigate ceramic wear and wear transition mechanisms. The experimental results show that wear mechanisms of ceramics are predominantly dependent on the tribological contact stresses. At low contact stress, the removal of material is controlled by plastic deformation induced microfracture on the asperity contact scale. Wear debris are produced when the plastic deformation exceeds the plasticity limit of the material, which is very limited for ceramics. As the tribological stress increases and reaches a critical point, various kinds of cracks (such as partial cone cracks, lateral/shallow cracks, and radial cracks etc.) are initiated. These cracks can propagate owing to subsequential contact or at higher contact stress. When these cracks intersect each other, chunks of material are detached from the bulk material and crushed by the subsequential tribological contact into fine particles and carried away from the contact region as wear debris. Wear transitions from deformation- to crack/fracture-controlled wear in ceramics can be attributed to the change of wear mechanisms during the sliding contact due to the tribological stress exceeding the critical microcrack stress or the fracture stress. With the inclined plane sliding test, a wide range of stresses can be applied progressively under one single sliding and the critical stress of the transition can be estimated. By using the results of repeated sliding test on an inclined plane, the wear and wear transition mechanisms have been demonstrated. Although ceramic wear mechanisms are dominated by fracture processes, tribochemical reaction products may be present to moderate the stress distribution by providing a reaction layer for wear protection.

Wear prediction for metals

In spite of the large number of wear models found in the literature, no model can predict metal wear a priori based only on materials property data and contact information. The complexity of wear and the large number of parameters affecting the outcome are the primary reasons for this situation. This paper summarizes the current understanding of wear modelling for metals. Several recent approaches such as wear mapping and wear transition diagrams have suggested some future possible directions for improvement. Some success has been achieved in describing severe wear of steels under unlubricated conditions using thermomechanical approaches. However, modelling of mild wear remains problematic, especially under lubricated conditions. In mild wear, asperity contact events dominate the wear processes. A single asperity collision simulation apparatus has been used to study asperity-asperity contact phenomena. Shear strain and strain accumulation were found to be the dominant underlying causes for wear. It is proposed that future research in wear prediction for metals incorporate the following aspects: wear mapping, temperature, shear strain response, boundary lubricating film strength, and surface roughness.

Distinct gene expression of receptor activator of nuclear factor-κB and rank ligand in the inflammatory response to variant morphologies of UHMWPE particles

Recent studies have examined the role of wear debris-induced bone resorption in the aseptic loosening of orthopedic prostheses. Research has shown that inflammation depends not only on the amount of particulate debris, but also the shape and size of the accumulated wear particles. Our previous studies have demonstrated that variant shapes of ultra-high molecular weight polyethylene (UHMWPE) particles induce diverse cellular and apoptotic responses in a murine inflammation model. Since enhanced osteoclastogenesis is recognized as a hallmark of bone loss in prosthetic loosening, we have now investigated the gene expression of receptor activator of nuclear factor-kappaB (RANK) and receptor activator of nuclear factor-kappaB ligand (RANKL) during the inflammatory response to different shapes of UHMWPE particles. Two shapes of UHMWPE particles (globular or elongated) were implanted in established air pouches on BALB/c mice, and pouches harvested 7 days after stimulation with UHMWPE particles. Gene levels of RANK, RANKL, TNFalpha, IL-1beta, and cathepsin K (CK) were quantified by real time RT-PCR, and TRAP staining of pouch membrane was used to evaluate osteoclastogenesis. We found that (i) elongated particles generated significantly higher RANK and RANKL gene expression than globular particles in pouch tissue; (ii) elongated particles provoked significantly higher IL-1beta and TNFalpha gene expression; (iii) a positive association was found between tissue inflammation status and the gene level of RANK/RANKL; and (iv) elongated particles stimulated significantly higher CK gene expression in comparison with globular particles. Histology revealed that clusters of TRAP+ cells were located in regions in contact with elongated particles. Overall, these data suggest that the morphology of wear debris may be a critical factor in the pathogenesis of prosthetic loosening.

Quantitative wear maps as a visualization of wear mechanism transitions in ceramic materials

Abstract The materials properties of advanced structural ceramics are providing new technological opportunities for improved wear-resistant components in heat engines. Use of ceramics could result in higher efficiency, increased power output and longer lifetimes. However, the successful application of these new materials may be inhibited by the need for evaluated materials properties and the availability of appropriate design criteria. Among the crucial properties of ceramics for use in new engine designs are the friction and wear characteristics of the wear couples. The screening of materials for friction and wear performance usually requires empirical field trials which may be very expensive. Laboratory testing of tribological properties would be much less expensive. What is lacking in laboratory tests, however, is a good correlation between the specific wear test conducted in the laboratory and the specific operating conditions of the industrial application. Basic to such a correlation is an understanding of the mechanisms of the wear process and how the mechanisms change with variations in the operating conditions. This paper describes a new methodology for the wear testing of ceramics that is intended to enable materials screening and designing to be based on laboratory results. The methodology prescribes a systematic effort to measure and represent the wear characteristics of ceramics in a uniform and unified manner. The result is a set of wear maps that collectively provide a comprehensive representation of the wear properties of the materials. Presentation of the wear results in three-dimensional representations allows simultaneous parametric dependencies to be visualized more readily than the traditional two-dimensional graphs. The resulting visual structures of different regions of wear space may indicate the effective limits of competing wear mechanisms and, hence, may provide a basis for wear model development.

Adhesion and Friction Studies of Silicon and Hydrophobic and Low Friction Films and Investigation of Scale Effects

Tribological properties are crucial to the reliability of microelectromechanical systems/ nanoelectromechanical systems (MEMS/NEMS). In this study, adhesion and friction measurements are made at micro and nanoscales on single-crystal silicon (commonly used in MEMS/NEMS) and hydrophobic and low friction films. These include diamondlike carbon (DLC), chemically bonded perfluoropolyether (PFPE), and self-assembled monolayer (SAM) films. Since MEMS/NEMS devices are expected to be used in various environments, measurements are made at a range of velocities, humidities, and temperatures. The relevant adhesion and friction mechanisms are discussed. It is found that solid films of DLC, PFPE, and SAM can reduce the adhesion and friction of silicon. These films can be used as and-adhesion films for MEMS/NEMS components under different environments and operating conditions. Finally, the adhesion and friction data clearly show scale dependence. The scale effects on adhesion and friction are also discussed in the paper.

Effect of microstructure on the wear transition of zirconia-toughened alumina

Abstract The mechanical properties of alumina ceramics can be improved by the addition of pure or partially stabilized zirconia particles. In the present study, the wear characteristics of zirconia-toughened alumina (ZTA) composites under a non-reactive fluid (paraffin oil) lubricated condition were investigated. The wear transition load (i.e. the load at which a rapid increase in wear occurs) increased with increasing zirconia content up to 20 vol.%. The transition from mild to fracture-coritrolled wear of ZTA depends on the material properties (e.g. hardness, elastic modules), the contact conditions (e.g. hertzian stress, coefficients of friction) and the microstructure of the material. The effect of the microstructure on wear was demonstrated and a Hall-Petch-type relationship between the microfracture stresses and the grain size was found. The effect of the grain size distribution on the wear transition load was also shown.

Effect of selected chemical compounds on the lubrication of silicon nitride

Successful use of advanced ceramics in many tribological applications requires an understanding of the physical, chemical, and mechanical properties of the material. Physical and mechanical data are relatively abundant for most ceramics. However, information on the chemical interactions of ceramics is scarce. This is especially true for chemical interactions with regard to lubrication of these materials. This paper investigates the influence of selected chemical compounds on the friction and wear of silicon nitride under boundary lubrication conditions. A ball-on-three-flat modification of the four-ball wear tester was utilized to evaluate the tribological characteristics of a hot pressed silicon nitride lubricated with a paraffinic base oil containing 1 weight percent additives. Friction, wear, and. film formation tendencies were observed for a range of oil soluble chemical compounds containing oxygen, sulfur, nitrogen, chlorine, and phosphorous. A wide range of additive response was observed. Friction c...

The Nature and Origin of Tribochemistry

Tribochemistry can be defined as the chemical reactions that occur between the lubricant/environment and the surfaces under boundary lubrication conditions. The precise nature of the chemical reactions is not well understood. What causes the reactions to take place is also a subject of speculation. Surface analysis of the surface and the reaction products yields insufficient information for a full understanding of the nature and origin of the chemical reactions. The reaction mixture is complex and the quantity is very small. Classical analytical techniques provide elemental concentration but the organic portion of the reaction products eludes definition. Recent observation of surface emissions of electrons, charged particles, from rubbing surfaces invites speculation that this emission provides the source of energy causing tribochemical reactions to take place. This study reviews our current knowledge of tribochemistry and examines the issue of the source of tribochemistry in boundary lubrication. Experiments were designed to examine the issue of whether mechanical disruption of surface bonds can lead to the formation of reaction products. The second issue is whether these reaction products correspond to those observed under normal rubbing conditions. Thermally induced reaction products were also examined for comparison. Surfaces covered with a monolayer of stearic acid molecules were scratched by a glass lens covered with a layer of diamond particles. Unidirectional scratches were made at 20-30 nm depth. The reaction products were monitored by a grazing angle FTIR capable of detecting the spectrum of a monolayer. The spectrum was then compared with the spectrum from a thermally heated case and a rubbed case. Results suggest that such scratching indeed produces similar reaction products.