Friction, Wear, and Lubrication; Effects & Modeling

Monday, April 29, 2013 1:30 PM in Room Golden West

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1:30 PM E1-1-1 Tribological Comparison Between a Commercial DLC and an Experimental TaSiN Thin Films
Marco Figueroa, Ernesto García (SEPI, ESIME-Zacatenco, Instituto Politécnico Nacional, Mexico); Giovanni Ramírez (Instituto de Investigaciones en Materiales,); Stephen Muhl, Sandra Rodil (Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de Mexico, México); Albano Cavaleiro, Almícar Ramalho (University of Coimbra, Portugal)

The aim of this work is to compare the tribological properties, friction coefficient and wear rate, of a commercial DLC thin film and a nanocomposite thin film of tantalum nitride (TaN) nanocrystals embedded in an amorphous silicon nitride (SiNx) phase, both coating were deposited on a cp-titanium substrates polished to roughness of 70 nm.

The structure and hardness of the thin films were evaluated by x-ray diffraction and nanoindentation. The adhesion of the film to the substrate was evaluated using scratch testing, using a 100Cr6 steel ball from 0 to 80 N along 8 mm of scratch. The friction coefficient of both coatings was studied in a CETR reciprocating ball-on-flat tribometer using a 10 mm diameter polycrystalline alumina ball as the counter-body, with a 2 N normal load and a stroke of 10 mm at 5 Hz. The wear properties were studied by micro-scale abrasive ball cratering using a 25.4 mm diameter 100Cr6 steels ball, a constant velocity of 75 RPM and applying two different loads 0.1 and 0.05 N and a wide range of sliding distances from 3 to 300 laps. For all the tests a controlled flow of slurry of 8.4 µm diameter SiC micro particles at 10% wt. concentration in distilled water was used. The wear scars were analyzed by profilometry, optical microscope and scanning electron microscopy to understand the wear mechanisms involved.

The results indicated that the hardness of TaSiN was higher than the DLC thin film. We observed considerable variation in the different critical loads for each coating and testing conditions. In terms of friction coefficient the DLC thin film had a lower value than the TaSiN thin film, but the TaSiN had a significantly better wear resistance than the DLC samples. Finally, we report the details of the wear mechanisms that occur for the two types of thin film.

2:10 PM E1-1-3 In-situ Synthesis of DLC Boundary Films From Base Lubricating Oils at Sliding Tribological Interfaces
Ali Erdemir, Osman Eryilmaz (Argonne National Laboratory, US)
Nanocomposite coatings are made of nano-scale multilayers and/or composite architectures that make them very hard, resilient, and tough. These properties are much desired in most tribological applications since they often insure longer durability and better performance even under very stringent operating conditions. In this study, we explored the possibility of further functionalizing such nanocomposite coatings by making them catalytically very active so that they can also derive or extract their own carbon-based boundary films in-situ from the base lubricating oils (including mineral, synthetic, or vegetable base oils). In our nanocomposite coating architectures, we identified and combined a variety of catalytically active hard (nitrides, carbides, or oxides of Mo, W, V, Re, etc.) and soft phases (such as Ag, Ni, Pd, Au, Cu, etc.) at some optimum concentrations so that when tested under lubricated sliding conditions, they would be able to crack base oil molecules (mainly long-chain hydrocarbons) into dimers and trimers and then deposit them as protective boundary films on sliding contact surfaces. Using UV Raman and a variety of other surface and structure analytical techniques, we elucidated the structural chemistry of these boundary films and confirmed that they were similar to those diamonlike carbon (DLC) films that are typically produced by CVD and PVD methods. Under the very severe contact conditions of our tribological test systems, these DLC boundary films were able to afford friction coefficients of as low as 0.02 and provide some of the highest resistance to wear and scuffing. In this paper, we will concentrate on the structural and chemical nature of these DLC boundary films and ascertain fundamental mechanisms that are responsible for their impressive tribological behaviors.
2:50 PM E1-1-5 Critical Role of Tribofilm in the Performance of Electrical Contacts Involving Cu-DLC Nanocomposite Coating
Ryoichi Hombo (Denso Corporation, Japan); Takanori TAKENO (Tohoku University, Japan); Julien Fontaine (LTDS, France); Hiroyuki Miki (Tohoku University, Japan); Naoki Kato, Takahiro Nozu, Naruhiko Inayoshi (Denso Corporation, Japan); Michel Belin (Ecole Centrale de Lyon, France)
Metal containing diamond-like carbon coatings have unique tribological characteristics that depend on various factors such as combination of materials, sliding conditions, environments and so on. In this study, electrical and tribological properties of a copper containing diamond-like carbon (Cu-DLC) nanocomposite coating deposited on copper-based substrates were investigated. A hybrid deposition process composed of plasma enhanced chemical vapor deposition and DC magnetron co-sputtering of copper target was used for the deposition. The tribological behavior was investigated by using a ball-on-flat reciprocating tribometer, up to 2000 sliding cycles. The counterpart of the Cu-DLC plate was a ball of copper alloy. The four-terminal method was used for the measurement of the electrical contact resistance between the ball and the Cu-DLC plate during the tribo-test. While initial value of the electrical contact resistance was hundreds of milliohms, it gradually decreased with increasing number of sliding cycles, reaching about 4 to 6 milliohms after 200 cycles. The friction coefficient was approximately 0.25, one third of the one for an uncoated substrate, and was stable from the beginning of the test to 2000 cycles. A tribofilm was built up on the sliding surface of the ball as the sliding cycle increased, consisting mainly of copper according to energy dispersive X-ray spectroscopy. Surprisingly, the Cu-DLC coating on the plate was almost worn out after less than 200 cycles, without detrimental effects neither on the coefficient of friction nor on the electrical contact resistance. The good electrical and tribological characteristics of the contact were thus provided by the tribofilm on the ball. The unique structure and properties of the tribofilm, resulting from a selective transfer process of nano-sized copper grains, will be discussed.
3:10 PM E1-1-6 Influence of the Coating Structure of a-C:H-W Coatings on their Wear-performance: a Theoretical Approach and its Practical Confirmation
Astrid Gies (OC Oerlikon Balzers AG, Liechtenstein); Thomas Chudoba (ASMEC GmbH, Germany); Norbert Schwarzer (Saxonian Institute of Surface Mechancis, Germany); Jürgen Becker (Oerlikon Balzers Coating Germany GmbH, Germany)

Diamond-like Carbon – coatings are widely used in the automotive industry due to their extraordinary wear and friction reducing properties which can mainly be attributed to their high hardness, low affinity to metals and very low coefficients of friction. Depending on the application, DLC-coated surfaces might experience severe mechanical interactions with the counterpart which determine the lifetime and reliability of the coated surface and therefore the entire tribological system. In order to well adapt a DLC-coating to a tribological system, it is necessary to know the stress- and strain-fields arising in the coating-substrate system under mechanical loads.

A few years ago, a theoretical approach was proposed with the aim of modelling contact problems in order to optimize the coating properties in a way to avoid any plastic deformation or fracture of the coating-substrate system for a given load range [1]. As a result, it was shown that the use of a layered coating structure with a gradual transition of the Young’s modulus from the substrate to the coating surface might protect both, substrate and coating plus the often rather sensitive interface area from plastic deformation. In addition, a final top layer with a lower modulus or a gradient with decreasing Young’s modulus should avoid any coating fracture due to tensile stresses.

In this work, a graduated a-C:H-W coating was deposited on a steel substrate according to the above described optimized layer system. The mechanical properties of this graduated coating system were determined using nanoindentation. In addition, a multiaxial, 3-dimensional nanoindentation test (reciprocating wear test with nanometre resolution) was carried out in order to analyse the wear-performance of the graduated coating system. By comparing the wear-performance of the graduated coating system to a non-graduated one we were able to confirm the above described theoretical approach predicting a better wear-performance for a graduated coating system.

[1] N. Schwarzer: "Coating Design due to Analytical Modelling of Mechanical Contact Problems on Multilayer Systems", Surface and Coatings Technology 133 –134 (2000) 397 - 402

3:30 PM E1-1-7 From Predictive Modelling via Optimized Testing to Applied Coating Development: DLC Coatings Durability under Nano-fretting Conditions
Tomasz Liskiewicz (Leeds University, UK); Ben Beake (Micro Materials Ltd., UK); Norbert Schwarzer (Saxonian Institute of Surface Mechanics, Germany); Mike Davies (Micro Materials Ltd, UK)

Improved integration of measurement data obtained from mechanical testing is required to provide reliable inputs for predictive wear models. In this work, a new global increment nano-fretting wear model based on the effective indenter concept has been used and the results were compared with experimental data. A series of DLC coatings with varied mechanical properties was deposited using industrial scale PECVD system and characterised on low-drift nano-indentation platform. Successive nano-scale fretting measurements have been performed with a variety of probes at different contact loads in order to examine the effect of different contact conditions on coating wear. A physical analysis of the nanoindentation test allowed us not only to extract the true coating Young’s Modulus (E) but also the coating yield strength (Y). In comparison to the hardness (H) this is the basis for a more generic understanding of the mechanical coating behavior. This allowed direct examination of the influence of the variation of Y/E in the coatings on the observed nano-fretting wear. Correlation with H/E, evaluation of the stress field evolution during the test and the extraction of wear and fretting parameters now gives us the opportunity to actually discuss the effects possibly being dominant within the performed nano-tribo-tests. The model does not only allow to analyze nano-fretting tribological experiments but also to forward simulate such tests and it gives hints for better component life-time predictions.

3:50 PM E1-1-8 Microwear Investigations of DLC Coatings with Nanometer Resolution in Normal and Lateral Direction
Thomas Chudoba, Kavita Mayekar (ASMEC Advanced Surface Mechanics GmbH, Radeberg, Germany)

In the last years several techniques have been developed to investigate the wear of surfaces with nanometer resolution. This mainly includes the Atomic Force Microscopy (AFM) in the lowest force range. However, measurement techniques to determine the displacement with nanometer resolution in the higher force range, approximately between 1mN and 1N, hardly exist. On the other hand it is interesting to investigate single asperity contacts with contact radii between 0.5µm and 50µm and to understand the dominating wear mechanisms in this load range. This is now possible with the Universal Nanomechanical Tester of ASMEC which has both in normal and lateral direction nanometer and micro-newton resolution during reciprocating wear experiments. The instrument was used for wear measurements on diamond like carbon coatings (DLC) of different hardness, produced by chemical and physical vapor deposition (CVD/PVD) methods. Two conical diamond indenters with a tip radius of 6µm and 70µm and a hard metal sphere with 100µm radius have been applied. For the lateral displacement, 80µm oscillation length at 0.17Hz oscillation frequency was set for every cycle of 6s. A s uitable applied force range was selected for each indenter type to study the wear behavior. Additionally some of these samples were measured in an oil bath to study the influence on the friction coefficient. The average friction coefficient and the average normal displacement per cycle were calculated for both conditions. The values of force and contact pressure limit were detected when the wear process begins. Additionally it was analyzed how the wear rate per cycle develops. The results showed that for the lower pressures less than one atomic layer was removed per slide and no correlation was found between the friction and the wear rate.

4:10 PM E1-1-9 Failure Mechanisms of DLC and TiN Biomedical Coatings on SS316L and M2 Substrates under Cyclic Impact-sliding Loads
Ying Chen, Xueyuan Nie (University of Windsor, Canada); Jonathan Housden (Tecvac, Ltd., UK); Allan Matthews (University of Sheffield, UK)
Owing to the superior tribological and mechanical properties with corrosion resistance, biocompatibility and hemocompatibility, a-Si:H DLC and TiN coatings have emerged as promising materials for biomedical (implant) applications. A cyclic inclined impact-sliding test was utilized in dry and Hank’s balanced salt solution (HBSS) conditions to study the fatigue wear behaviors of DLC and TiN coatings and the supporting capability of different substrates—soft SS316L and hardened M2. Effects of substrate hardness on the coating failure mechanisms were studied. This test simulates the reciprocal combined impact-sliding motion by using a steel counterface. In each impact-sliding cycle, the forces comprised a dynamic impact load, Fi, and a pressing load, Fp, (Fi/Fp=120 N/300 N). It was found that HBSS didn’t cause corrosion degradation because dense Si (for DLC) and Ti (for TiN) interfaces dominated the corrosion behaviors. Instead, the solution provided a lubricate effect and enhanced coating durability. The main coating failure mechanisms under very high contact stress in this test were fatigue cracking, chipping and peeling. Softer substrate exhibited energy absorption against impacts and caused less failure percentage on coating surface. But hard substrate provided a better loading support to the coating which altered the coating failure evaluation procedure.
4:30 PM E1-1-10 Tribological Behavior of the Superhard Coatings of Ta-N-Si and Nb-N-Si
Giovanni Ramirez (Argonne National Laboratory, US); Sandra Rodil (Universidad Nacional Autónoma de México - Instituto de Investigaciones en Materiales, Mexico); Ali Erdemir, Osman Eryilmaz (Argonne National Laboratory, US); Stephen Muhl (Universidad Nacional Autónoma de México - Instituto de Investigaciones en Materiales, Mexico)

In this work, two different superhard coatings were prepared; Nb-N-Si and Ta-N-Si. The coatings were deposited using a reactive dual magnetron sputtering system, using two targets, one metallic target (Nb or Ta) and a silicon target, in a reactive mixture of gases (argon and nitrogen). By changing the power in the silicon target, the amount of incorporated Si could be changed from 0 to about 15 at%. For the tribological tests, the films presenting the maximum hardness were chosen, which correspond to Si contents about 5 at% for both films. The films were grown on M2 high speed steel to evaluate the tribological behavior.

The microstructural properties, measured using X-ray diffraction, showed the growth of crystalline coatings presenting the FCC phase of the metallic nitrides (NbN or TaN). The composition of the films was measured using X-ray photoelectron spectroscopy (XPS). The mechanical properties were obtained using the nanoindentation technique and were ~35 GPa for the Nb46N44Si5, and ~40 GPa for the Ta45N44Si5.

The tribological behavior was evaluated using two techniques, Ball on Disk (BoD) to evaluate the behavior in boundary lubricated sliding conditions, and High Frequency Reciprocating Rig (HFRR) to evaluate the behavior in the mixture of hydrodynamic and boundary condition regimes. The tests were made in two conditions: air to evaluate the coating as solid lubricant, and in engine oils to evaluate the performance of the coatings to application in the automotive industry.

The BoD tests were carried out in air, base oil (PAO 4), and fully formulated 5W30 grade full synthetic engine oil. The results of the test in air showed an improvement of the wear resistance of the coated steel for both coatings. The coefficient of friction of the Ta-N-Si coating was lower than for the bare substrate, while the Nb-N-Si did not present any significant reduction. Similarly, the tests on engine oils showed that the Nb-N-Si coatings were inert and did not improve the tribological behavior in comparison to the M2 steel. Meanwhile, a reduction in the CoF was observed for the Ta-N-Si.

The HFRR test results showed a similar trend to those obtained using BoD. The Raman technique was used to understand the mechanism that improved the tribological behavior of the coatings.

Acknowledgements: S.E. Rodil and G. Ramirez wish to acknowledge the financial support from DGAPA-UNAM IN103910.

4:50 PM E1-1-11 Friction Reduction by Thermal Post-deposition Treatment of Arc Evaporated TiAlTaN Coatings in Methane
Nina Schalk (Materials Center Leoben Forschung GmbH, Austria); Christian Mitterer (Montanuniversität Leoben, Austria); Christoph Czettl (CERATIZIT Austria GmbH, Austria); Bernhard Sartory (Materials Center Leoben Forschung GmbH, Austria); Marianne Penoy, Claude Michotte (CERATIZIT Luxembourg S.àr.l., Luxembourg)

For severe cutting applications, the reduction of friction between work piece and coated tool surface is of vital importance to limit the thermal load. In the last decade, several low friction coatings like diamond like carbon and MoS2 have been suggested. Within this work, the surface of arc evaporated TiAlTaN coatings was modified by thermal treatment in methane. The modified coatings were investigated with respect to their gain in weight, composition and structure by a precision balance, GDOES, SEM-FIB, and TEM. The determined gain in weight could be attributed to an up to ~200 nm thick discontinuous carbon layer on the coating surface. This carbon layer was identified as graphite using Raman spectroscopy. SEM and TEM revealed a few nm thick carbon diffusion zone in the TiAlTaN coating. The friction coefficient determined by ball-on-disc tests was reduced from 0.66 for the as-deposited coating to 0.25 after the treatment in methane, where the low friction behaviour is stable over 1000 m.

5:10 PM E1-1-12 Dangling Bonds Induced Cross-linking Model in Nanoscratched Graphene Layers
Qi. Zhang (Key Laboratory of Education Ministry for Modern Design and Rotor-Bearing System, School of Mechanical Engineering, Xi'an Jiaotong University, China); Dongfeng Diao (Shenzhen University and Key Lab. Of Ed. Ministry for Modern Design and Rotor-Bearing Sys., Xi’an Jiaotong Univ., China); Lei Yang (Key Laboratory of Education Ministry for Modern Design and Rotor-Bearing System, School of Mechanical Engineering, Xi’an Jiaotong University, China)
Dangling bonds induced cross-linking between interlayer graphene during nanoscratch is simulated by molecular dynamics method. The normal stress over 74 GPa leads to broken of the hexagonal ring of intralayer graphene, producing unstable dangling bonds which easily make up sp2 or sp3 with neighbor layers. The cross-linking density increases with the scratching depth, causing higher scratch hardness and reaching the peak of 90 GPa. The cross-linking is reversible after scratch when the normal stress is less than 90 GPa, beyond which the atoms from different graphene layers will be mixed together forming amorphous, making the scratch hardness decrease sharply. Keywords: Dangling Bonds; Cross-linking; Graphene Layers; Scratch Hardness