This paper presents a biotribological study on cartilage wear and friction, using a system of cartilage-on-stainless steel. This study is a part of the ongoing biotribology research by Dr. Furey at the Virginia Polytechnic Institute and State University. Two loads (65 N and 20 N) and three lubricants (saline reference, reference + hyaluronic acid, and bovine synovial fluid) were tested and evaluated using several analysis techniques. These techniques included wear analysis by hydroxyproline measurement, scanning electron microscopy (SEM), histologic sectioning and staining, numerical analysis of friction and specimen displacement data, and Fourier transform infrared (FTIR) analysis. Biochemical wear analysis showed that, under high load, the saline reference generated the most wear, hyaluronic acid produced less wear, and bovine synovial fluid produced the least. Wear was sensitive to load with all three lubricants, but was not significantly affected by the lubricant under low load. SEM photographs and histologic sections showed evidence of plowing and surface delamination, as well as another wear mechanism that produced wear markings perpendicular to the direction of sliding. Opaque films remained on the polished stainless steel disks after saline and hyaluronic acid tests, but not after synovial fluid tests. FTIR analysis of these films, as well as fresh and worn cartilage, showed that the cartilage experienced chemical changes during sliding. / Master of Science

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Tribology is the science of interacting surfaces in relative motion. It is specifically concerned with the friction, wear and lubrication of these surfaces. Although tribology has conventionally been associated with the surface interaction of mechanical systems, concepts of tribology have also been important in the study of biological systems. Biotribology is one of the newest fields to emerge in the discipline of tribology. It can be described as the study of friction, wear and lubrication of biological systems, mainly synovial joints such as the human hip and knee. Osteoarthritis (OA) is partially characterized by the loss of articular cartilage from the contacting surface of the articulating bones in synovial joints. Although it has been studied extensively, the exact pathways and pathogenesis of OA have yet to be determined. Several factors have been cited as possible contributors to the condition. These factors can primarily be grouped into two categories of mechanical or biochemical abnormalities. Research in biotribology enables the examination of both the mechanical and biochemical factors involved in joint lubrication and OA. This research has focussed on the mechanisms of normal joint lubrication, as well as the possible connections between biotribology and osteoarthritis. Particular emphasis is placed on the effects of biochemical changes and environment on cartilage wear and damage. Studies were carried out using a test device developed to study the tribological properties of articular cartilage, in vitro. A cartilage-on-cartilage test configuration was used with bovine articular cartilage and a cartilage-on-stainless steel configuration used with lapine articular cartilage. Articulating surfaces were put in sliding contact under a normal load. Natural and biochemically modified environments were created to simulate possible normal and pathologic in vivo conditions. Wear and friction of the articular cartilage were measured and related to biochemical environments which are suspected in clinical cases of OA. Quantitative measurement of cartilage wear was achieved through hydroxyproline assay of the post-test lubricants. Surface and subsurface damage were also examined through the use of scanning electron microscopy and histological staining techniques. The results of four separate studies demonstrated that: (1) exposure of bovine cartilage to collagenase-3, an enzyme suspected in the cartilage degeneration seen in OA, significantly increased cartilage wear (p = 0.001); (2) lapine cartilage with surgically induced OA exhibited higher coefficients of friction, but no significant increase in wear over normal cartilage from the same animal; (3) the addition of white blood cell lysate, comparable to what would be seen in mild joint inflammation, to synovial fluid significantly increased cartilage wear over normal synovial fluid (p = 0.002); (4) the removal of "boundary lubricating" surface-active phospholipids (SAPLs) from normal synovial fluid had no significant effect on cartilage wear. These results demonstrate that biochemical changes in the cartilage, as well as the synovial fluid, can lead to increased wear of and damage to the articular cartilage surface. How these changes may occur in living systems remains to be determined. The use of the tribological test device developed and various analytical techniques has made it possible to quantitatively evaluate the effects of biochemical changes and environment on the wear and damage of articular cartilage. These studies have demonstrated that research in biotribology has the potential to make significant contributions to the current knowledge not only of normal joint lubrication but of joint pathology as well. / Ph. D.

Advanced ceramic materials have found many new applications in the automotive and other industries. To satisfy demands of higher temperatures and inert surfaces, new lubrication methods for these ceramics need explored and evaluated. This thesis focuses on a boundary lubrication method termed tribopolymerization -- the formation of polymers at the tribological interphase. The research evaluated new high-temperature classes of anti-wear additives. The work involved experiments on steel and alumina material pairs with a pin-on-disk tribometer used to explore the anti-wear capabilities of selected additives in the liquid phase at concentrations of 1% by weight in hexadecane. New additives included aromatic compounds with various pendant groups adding the design functionality necessary for in-situ polymerization. The amino, hydroxyl, acid, and ester functional groups underwent studies across several aromatic molecular compositions while new heterocyclic additives, in particular the readily available lactams, underwent exploratory tests as a new class under the tribopolymerization design approach. In concentrations of 1%, additives showed significant wear reductions of up to 99.9 %. Anti-wear behavior persisted in select cases at concentrations as low as 0.1% by weight. Compounds from two new classes demonstrated anti-wear behavior at 6x the frictional heat generation of standard exploratory conditions. This surprising effect partially filled a void in the effective range of operating conditions between 0.25 m/s, 40 N and 1.0 m/s, 160 N. Earlier work by Tritt found a complete absence of anti-wear behavior for the previous additive classes at the high-speed conditions. In addition, several individual compounds constituent to an A-R-A + B-R'-B condensation polymerization reaction demonstrated significant anti-wear behavior when used alone. In particular, the compound BTDA from DuPont's Kapton® exhibited higher wear reductions than any other new additive. These findings support tribopolymerization as an effective approach to boundary lubricant design. Low wear was often associated with an attached reaction debris layer. This finding is consistent with previous work involving tribopolymerization anti-wear additives with ceramics. Further research into the roles of the debris layer and tribochemistry will help in understanding the complex anti-wear behavior of these new high-temperature additive classes. / Master of Science

A study has been conducted to determine whether or not tribopolymerization can occur under conditions of fretting contact. Using a laboratory test consisting of oscillating hard steel balls loaded against flat steel or aluminum disks, effects of various monomers on friction, wear, and surface film formation were determined. Monomers, capable of polymerizing either by polycondensation or addition processes, were used at 1 % concentration in hexadecane. Under the conditions used (90N load, 65Hz frequency, 200μm peak-to-peak amplitude, 1 hour), the monomers tested reduced friction or wear or both. Fourier Transform Infrared Microscopy (FTIRM) analysis of the test specimens shows that organic material is present in the wear scar regions and depends on the metal used, the monoester structure, location on the disk, and the method of cleaning the surface after a test. With aluminum-on-steel the addition of I % styrene to hexadecane reduced the wear volume of the disk by over 650/0; furthermore, positive evidence of polystyrene was found via FTIRM. These results support the hypothesis proposed by Kajdas that addition-type tribopolymerization can be initiated by exo-electron emission. Under these conditions of expected relatively low surface temperatures, tribopolymerization does not occur with polycondensation-type compounds such as the C₃₆ dimer acid/ethylene glycol monoester. With this additive, known to be extremely effective in reducing wear at higher surface temperatures, effects on wear were small and no positive evidence of polymerization was found; however, metal soaps formed under these conditions. / Master of Science

No / This paper introduces a method to study the degree of change that affected a prehistoric context as the result of environmental processes. It is based on the direct examination of a representative sample of stone tool by-products, and on the identification of all surface alteration features. We summarize the theoretical bases for the formation of some wear features and the main results of a number of experiments involving interaction between chert flakes and sediments. Experimental results include: (1) the wear rate of flakes is not constant; (2) the wear rate increases as the size of the grains that compose the matrix increases; (3) fine grained chert resists wear better then coarse grained chert; and (4) the presence of moisture will trigger some chemical reactions that promote wear and the formation of films on chert surfaces. We apply these findings to the cave site of Grotta di Pozzo, Italy, and conclude that, strictly within the area sampled, there is low degree of disturbance and low intensity of chemical processes that may, however, confound the reconstruction of human activities in this part of the cave.

As technology advances, mechanical and electrical systems are subjugated to intense temperature fluctuations through their service life. Designing coatings that operate in extreme temperatures is, therefore, a continuing challenge within the tribology community. Silver tantalate was chosen for investigation at the atomic level, the physical and chemical properties that influence the thermal, mechanical, and tribological behavior for moving assemblies in high temperature tribological applications. By correlating behavior of internal physical processes to the macro tribological behavior, the tribological community will potentially gain improved predicative performance of solid lubricants in future investigations. Three different approaches were explored for the creation of such materials on Inconel substrates: (1) powders produced using a solid state which were burnished on the surface; (2) monolithic silver tantalate thin films deposited by magnetron sputtering; and, (3) an adaptive tantalum nitride/silver nanocomposite sputter-deposited coating that forms a lubricious silver tantalate oxide on its surface when operated at elevated temperatures. Dry sliding wear tests of the coatings against Si3N4 counterfaces revealed friction coefficients in the 0.06 - 0.15 range at T ~ 750 °C. Reduced friction coefficients were found in nanocomposite materials that contained primarily a AgTaO3 phase with a small amount of segregated Ag phase, as suggested by structural characterization using X-ray diffraction. The presence of nanoparticles of segregated Ag in the thin films further enhanced the performance of these materials by increasing their toughness. Additional characterization of the AgTaO3 films at 750 °C under normal loads of 1, 2, 5, or 10 N revealed that the friction monotonically increased as the load was increased. These results were complemented by molecular dynamics simulations, which confirmed the increase of friction with load. Further, the simulations support the hypothesis that this trend can be explained in terms of decreased presence of Ag clusters near the sliding surface and the associated decreased porosity. The results suggest that the relative amount of Ag in a TaN or Ta2O5 mastrix may be used to tune film performance for a given application.

The purpose of this thesis was to compare graphene nanoplatelets (GNP) and WS2 as solid lubricant additives to aluminum in order to reduce friction and wear. The central hypothesis of this work relied on lubricating properties of 2D materials, which consist layers that slip under a shear force. Two aluminum composites were made (Al-2 vol.% GNP and Al-2 vol.% WS2) by spark plasma sintering. Tribological properties were evaluated by ball-on-disk wear tests at room temperature (RT) and 200°C. WS2 not only presented the lowest COF (0.66) but also improved the wear resistance of aluminum by 54% at RT. Al-2 vol.% GNP composite displayed poor densification (91%) and low hardness resulting in poor wear resistance. The wear rate of Al-2 vol.% GNP composite increased by 233% at RT and 48% at 200°C as compared to pure aluminum. GNP addition also resulted in lower COF (0.79) as compared to pure aluminum (0.87).

<p dir="ltr">Rolling contact fatigue (RCF) is one of the leading causes of failure in critical tribological components such as rolling element bearings (REBs), gears, cam and followers, etc. This is especially paramount for advanced aerospace applications where REB components need to operate for billions of RCF cycles before routine maintenance or inspection is performed. The rolling motion between the rolling elements and raceway produces RCF, wherein a complex, non-proportional, alternating contract stress is applied over a small material volume. Moreover, the highly localized stress occurs on the same length scale as microstructural features such as carbides, inclusions, grain size, hardness gradients from carburization, surface roughness, thereby amplifying their effect on fatigue performance. Therefore, the objective of this dissertation is to investigate critical microstructural modifications and their effects on RCF performance via experiments and computational modeling.</p><p dir="ltr">Initially, an investigation was undertaken to investigate surface roughness effects on RCF. The surface roughness of various REBs was measured through optical surface profilometry and used to construct rough surface pressure distributions, which were then used in a continuum damage mechanics (CDM) finite element (FE) framework. The results demonstrated that life is reduced as lambda ratio decreases. It was also observed that a 2-parameter Weibull cumulative distribution function can describe the relationship between the near surface orthogonal shear stress concentration and ratio of surface failures.</p><p dir="ltr">Next, the enhancement to RCF life from grain size refinement of through hardened bearing steels was studied. To capture the effects of grain refinement, torsion stress-life data of various grain size were used in the RCF model. A predictive life equation for different grain sizes was constructed based on the exponential trend observed between grain size and life from the simulation data. The life equation was then used to calculate the quotient of RCF at two different grain sizes. This quotient was defined as the life improvement ratio and it was observed that this investigation’s ratios compared well with existing life improvement ratios from RCF experiments.</p><p dir="ltr">Hardness gradient is a common microstructural modification to improve RCF life of tribo-components. Variation of hardness gradients is prevalent in case hardened (i.e. case carburized) bearing materials. Therefore, the CDM-FE RCF model was modified to investigate the effects of various hardness gradient types and depths on fatigue life improvement. The simulation results enabled the identification of potentially optimal gradients aimed to mitigate manufacturing challenges and provided the foundation for the construction of a general fatigue life equation.</p><p dir="ltr">A fundamental study to understand the impact various common RCF failure criteria have on RCF life estimation was then conducted using computational modeling. To capture the variation of a material’s resistance to fatigue, the critical CDM damage parameters were assumed to follow a probabilistic distribution instead of a singular value. The CDM-FE model was modified to consider the shear reversal, the octahedral shear stress, the maximum shear stress, the Fatemi-Socie criteria, and the Dang Van multi-axial fatigue parameter as failure criteria. Simulation life results revealed that the CDM-FE model with shear reversal and Fatemi-Socie criteria best match empirical predictions from well-established RCF life theory. Notably, the Fatemi-Socie exhibited the best agreement over all operating conditions.</p><p dir="ltr">The next investigation focused on the cleanliness of aerospace-quality bearing steels. Torsion fatigue experiments established the stress-life (S-N) relation for three common aerospace quality bearing steels. The S-N data was later used to calibrate the RCF model’s damage equation, which considered the Fatemi-Socie criteria following conclusions from a previous investigation. Simulation results were observed to corroborate well with RCF experiments that were conducted for all three materials, while noting the simulations offered a significant time saving. As a result, a subsequent investigation focused on establishing the stress-life relationship for one of the aerospace quality bearing steels through a combined experimental and analytical approach. Good corroboration was observed between simulations and experiments at three contact pressures. This finding is particularly significant as it strengthens the reliability of computational RCF model as an efficient means to assess the RCF performance of bearing materials.</p><p dir="ltr">Furthermore, the detailed investigation on RCF performance of each critical microstructural modifications and their respective effect greatly improves the state-of-the-art. The findings emanating from the various investigations offer informed fatigue design recommendations that aid in the selection of rolling element bearings for critical tribological and aerospace applications.</p>

Nowadays sealing systems are commonly designed by means of hydrodynamic and elastohydrodynamic theories. Although the analytical as well as the computational approaches have improved in meaning full manner since the last decades: For small sealing gaps, in the order of micrometers and below, a discrepancy between experimental investigated and theoretically predicted leakage flows occur. As a cause for the discrepancy a breakdown of the no slip boundary condition is suspected. Since in small sealing gaps the continuum hypothesis is violated and molecular effects have to be considered. One fundamental quantity to take molecular affects into account is the slip length. Within this paper a new measurement apparatus to evaluate the slip length for hydraulic applications is presented. The adjustable gaps between two planar surfaces are in the order of magnitude of 1 μm. In a first step the slip length for the system steel-oil –steel is investigated at three different temperatures: 18°C, 22°C and 25°C. The measured slip lengths are in the order of magnitude of ~100 nm.