ICMCTF2013 Session E1-1: Friction, Wear, and Lubrication; Effects & Modeling

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.

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

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.

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.

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.

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.