ICMCTF2014 Session E1-3: Friction, Wear, and Lubrication: Effects and Modeling

Silver tantalate coatings were produced by reactive unbalanced magnetron sputtering from silver and tantalum sources as potential high temperature solid lubricants. The films were wear-tested at 750 °C under normal loads of 1 , 2 , 5 , and 10 N against a Si₃N₄ counterface. These sliding tests revealed that the frictionmonotonically increased as the load was increased. A systematic investigation of the surface and sub-surface region of the wear track, using techniques such as scanning Auger nanoprobe, atom probe tomography, and cross-sectional transmission electron microscopy, revealed the following trends with increasing load: (1) a decrease in the amount of Ag on the surface of the wear track; (2) a decrease in the thickness of the mechanically mixed layer that forms as a result of the reconstruction of AgTaO₃ to form Ta₂O₅ and Ag; and, (3) the formation of a porous structure throughout the tribofilm as a result of the segregation and migration of Ag from the original AgTaO₃ matrix. 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 paper intends to present the development of a new tribometer (TUC), which allows producing complex mechanical stresses on samples. The literature is wide on tribological behavior of thin coatings, focusing in particular on micro-scale abrasion tests [1]. In order to characterize different stress distributions during the test, pin-on-disk test and the scratch test are also used to complement the assessment of tribological behavior. This new instrument aims to overcome the need to use three different conventional methods, providing additional features to the characterization.

The underlying idea of this tribometer is based on the cratering method [2]. The instrument is also provided with crater depth sensor and load cells, which permit to have an instantaneous measurement of the friction coefficient and the wear rate.

In order to validate the results, a broad experimental activity was carried out using both bulk and coated samples – SiC wafer and TiN coating respectively. Special care has been devoted to the calibration of the machine and the frequency content of the acquired signals. The instrument can produce both standardized micro-scale abrasion tests and tests characterized by more complex mechanical stresses. This feature allows to have more consistent results. In addition, a comparison with commercial scratch testers was performed in order to assess the response of the samples to different shear stress components.

Finally, the results of the tests are presented. In particular, the instantaneous Archard’s wear coefficient is discussed, showing that useful information can be extrapolated regarding run-in and coating delamination.

[1] Rutherford, K.L., Hutchings, I.M. (1996) “A micro-abrasive wear test, with particular application to coated systems”, Surface and Coatings Technology, 79 (1-3), pp. 231-239.

This paper overviews the studies on surface films at tribo-interface that governs friction and wear behaviors in hydrogen gas environment. The studies include friction and wear of a range of metals, polymeric materials and coatings, and rolling contact fatigue of steels. Dry sliding of metals is governed by the formation of thin surface films, which depends on hydrogen itself but more significantly, for some important transition metals, on trace oxygen and water in hydrogen. Oxide films formed on steel surfaces show resistance against hydrogen uptake that accelerates hydrogen-assisted flaking failure under rolling-sliding contact. Tribo-chemical films at metal surfaces also affect friction and wear of polymers used in dynamic seals. These effects are enhanced in hydrogen at ultra-high pressure. Diamond like coatings are candidates for use in hydrogen applications, and some findings including delamination caused by hydrogen uptake and the effect of trace impurities on friction are shown.

The phenomenon of friction is present at every moment from to provide the braking action in vehicles up to avoid/minimize its action in mechanical devices for energy efficiency issues. Despite of the accumulated knowledge after centuries of research in such area, the phenomenon of friction is not fully understood. From a tribological point of view, the friction coefficient depends on shear strength, toughness fracture, hardness, and elastic modulus where all of them are macroscopic properties of the involved materials divided in adhesive and ploughing friction components. From a physicochemical point of view, the macroscopic properties cited before are understood in terms of attractive forces (Van der Waals and/or Casimir), strength and stiffness of chemical bonds, grain boundary interactions, and thermal conductivity involving phonons and electronic band structures. However, there is a lack of a theory that allows to connect the macroscopic behavior with nanoscopic aspects and an integrated model of friction still remains as a challenge.

The aim of this study is to investigate the friction behavior at the nanoscale of a plain steel after plasma nitriding and different processing times of plasma post-oxidation. Nanoindentation experiments under different low normal loads were performed in order to analyze the friction behavior between a conical tip (diamond) and the modified steel surface from 20 nm up to 300 nm in-depth. The chemical structure of the outermost layers was determined by GD-OES. The crystalline phases and microstructure were analyzed by GA-XRD and SEM, respectively. Moreover, hardness and elastic modulus at those depths were measured. One can see that the friction coefficient (CoF) decreases when higher oxygen content are detected on surface. However, the lower the hardness, the higher the CoF. According to recent models that correlate the friction behavior with phonons, a stiffer chemical bond must dissipate more energy increasing the CoF, which is in opposition to our results. In our system, we propose that not only phonons contribute to thermal conductivity but also the electronic behavior is important to dissipate energy. Finally, the friction behavior will be discussed taking into account the chemical structure and physical models for thermal conductivity of materials.

Wear resistant overlays are widely used in abrasive wear applications in the mining, construction and agricultural industries. In the present study, new overlays were formed using a submerged arc welding (SAW) process using mixtures of ferro-alloy powders, cast iron chips or stainless steel shots as alloying sources on a low carbon steel substrate. These overlay coatings are essentially metal matrix composites consisting of in-situ formed (Fe,Cr)_x(C,B)_y type hard phases in a metallic matrix. Microstructural analysis (optical and scanning electron microcopy) along with macro- and micro-hardness measurements were performed on the overlays to evaluate the mechanical properties of the different microstructures developed. ASTM G65 dry sand/rubber wheel abrasion testing (Procedure A) was done on samples to assess their wear resistance. Microstructural results indicate that both hypoeutectic and hypereutectic structures of Fe-Cr-C and Fe-Cr-C-B systems with and without primary hard phases could be produced using appropriate powder mixture ratios and through controlling the dilution levels. ASTM G65 test results shows that the overlays produced using lower cost materials show mass losses comparable to those produced using commercially available high alloy wires and rods.

By improving tribological properties, especially wear resistance of contact surfaces, hard coatings provide great opportunity for improving performance, durability and efficiency of mechanical systems, including forming tools and machine components. However, for the successful application of hard coatings, coated surfaces have to perform adequately under dry and oil-lubricated conditions, with the majority of forming tools and components still being oil-lubricated. In the case of metallic surfaces action of extreme-pressure (EP) and anti-wear (AW) additives, used to reduce friction and wear, is well understood and described in detail. However, this is not the case for coated surfaces, especially when it comes to the influence of additive type and contact conditions.

The effects of work roll material on the surface quality of aluminum during hot and cold rolling has long been a subject of intense research. The roll itself is subject to wear, as well as galling or scuffing from interaction with the aluminum work piece. These defects to the roll and aluminum alloy are affected by the tribological conditions between the roll and the aluminum piece; as such lubrication and roll surface conditions are parameters of particular interest to researchers. A rolling simulator, with a roll-on-block configuration, developed at the University of Windsor has been used to study the effects of rolling conditions on the surface quality of rolled aluminum products. It simulates the tribological surface deformation experienced by the work piece during industrial rolling, allowing for the variation of rolling parameters such as roll surface conditions (e.g. surface roughness and coating), temperature, rolling load, lubrication and forward slip to examine their effects on the roll and the work piece.

In this study, the effect of roll coatings on the life time of the rolls and the rolled aluminum surface quality were of particular interest. AISI 52100 steel roll surfaces were treated with eight coatings which included non-hydrogenated DLC, chromium and six nitride based coatings; which were run against AA1100 aluminum pieces at high and low temperatures to emulate hot and cold rolling conditions. Scanning electron microscopy (SEM), focused ion beam (FIB) microscopy, and high resolution transmission electron microscopy (HR-TEM) were employed to investigate adhesion and surface conditions of the rolls and work pieces. The severity of aluminum adhesion on the rolls was found to be a function of the coating type. During dry tests aluminum adhesion was most severe on the uncoated and chromium coated rolls, with corresponding severe surface damage in the form of deep groves covering the mating aluminum pieces. Nitride coatings performed better, especially the ZrN, showing less aluminum adhesion to the roll and surface damage to the work piece. However, the non-hydrogenated DLC coatings displayed the best performance of all the coatings with no aluminum adhesion and little surface damage to the work piece surface. These observations had good correlation with the coefficient of friction plots. This research looks to evaluate adhesion-mitigating coatings for the improvement of rolled aluminum surface quality and the roll life and minimizing the use of harmful lubricants.

Tribochemical reactions can highly influence the tribological properties of materials as they chemically modify the surfaces in contact with consequent modification of their adhesion, resistance to wear and friction.

One example is the environmental dependence of the tribological behavior of carbon films. We report the real-time atomistic description of the tribochemical reactions occurring at the interface between two diamond films in relative motion, by means of large scale ab initio molecular dynamics. We show that the load-induced confinement is able to catalyze diamond passivation by water dissociative adsorption. Such passivation decreases the energy of the contacting surfaces and increases their electronic repulsion. At sufficiently high coverages, the latter prevents surface sealing, thus lowering friction. Our findings elucidate effects of the nanoscale confinement on reaction kinetics and surface thermodynamics, which are important for the design of new lubricants. [1]

A second example is the functionality of chemical additives included in engine oils to reduce friction in conditions of boundary lubrication. We identified the reaction paths for phosphorus release from the dissociative adsorption of organo-phosphorous molecules and analyzed its effects in reducing the adhesion and shear strength of iron interfaces in comparison with sulfur.[2]

Lamellar materials like graphite and molybdenum disulfide are known to be good solid lubricants. In micro- and nano-scale applications, the thickness of a solid lubricant can be a very important factor. We considered few-layer films of graphene and MoS2 and studied the effects of an applied load by analyzing interlayer electronic charge displacements. [3,4]

[1] G. Zilibotti, S. Corni and M. C. Righi, Phys. Rev. Lett., in printing.

[2] M. I. De Barros-Bouchet, M. C. Righi, D. Philippon, S. Mambingo-Doumbe, T. Le Mogne, J. M. Martin, A. Bouffet, submitted.

[3] M. Reguzzoni, A. Fasolino, E. Molinari and M. C. Righi, Phys. Rev. B 86, 245434 (2012).

[4] G. Levita, A. Cavaleiro, E. Molinari, T. Polcar, and M.C. Righi, submitted.