ICMCTF2014 Session E1-1: Friction, Wear, and Lubrication: Effects and Modeling

Diamond-like carbon coatings offer a unique combination of high hardness, low friction and low tendency to stick. Among the diversity of DLC films, the hydrogen-free ta-C films are becoming more and more important. Besides superhardness this coating type shows ultra-low and super-low friction in combination with specific lubricant types. Several authors have been reported on extreme low friction of ta-C lubricated with glycerol and corresponding esters. In these reports, wear was not investigated in detail. However, recent investigations have shown relatively high wear of some ta-C coatings with fatty-acid based lubricants. Surprisingly, high wear and ultra-low friction occurred at the same time. Though all of the described ta-C coatings were similar by composition regarding sp3- and hydrogen content, different coating processes were used. These different processes, covering lab-scale to industry-scale technologies, could lead to different properties and cause the discrepancy in wear behavior under ultra-low or super-low friction.

In this study, the influence of sp3-content, intrinsic stress and particle density on the low-friction-high-wear phenomenom was systematically investigated measuring the wear and friction coefficients. Tribological testing was performed with an oscillating test rig, using glycerol and glycerol mono-oleate as lubricants to achieve the low friction state. A fully formulated engine oil was used for reference. The hybridization state and depth-resolved chemical composition of the tribolayer on the coating and counter part was studied using Raman spectroscopy, TEM, and NRA. The results are correlated and an attempt is made to describe the influence of specific ta-C coating properties on friction and wear.

This contribution presents a tribological study of magnetron sputtered amorphous carbon-based thin films containing Si (a-C:Si) compared to a common sputtered carbon thin film (a-C). Molecular dynamics simulations predict tetrahedral bonds between C and Si in the amorphous carbon matrix. Ball-on-disk-tests carried out at room temperature against Al₂O₃ revealed a coefficient of friction (COF) of 0.1 for both film types. Between 250 and 325°C, Si addition decreases the COF and the wear rate to < 0.05 and < 3*10^-18 m³/ N*laps, respectively. For comparison, the a-C reference shows a COF of 0.15 and a wear rate of 9*10^-17 m^3/N*laps. This low friction behavior was further investigated by combining X-ray photoelectron spectroscopy, transmission electron microscopy and Fourier transform infrared spectroscopy techniques of the wear tracks and the worn material on the counterbody. Besides friction-induced graphitization of the coating surface, a reaction layer based on Si-O compounds was formed, being responsible for the excellent thermal stability of a-C:Si coatings.

Diamond-like carbon (DLC) films cover a wide range of different carbon based coatings, starting from soft hydrogenated films to extremely hard hydrogen-free films. Due to the varying characteristics of DLC coatings, they have differing tribological properties that influence on their performance and applicability. Coating properties, such as elasticity and fracture toughness, greatly influence on the practical performance of the coatings in real applications. An interesting aspect is also the influence of tempering or aging of the coating on the performance.

In order to study how aging influences the tribological performance of DLC coatings we have evaluated 20 years old DLC coatings and compared the results to earlier results reported. It seems that the effect on the wear resistance has not changed greatly. However, variation in the friction performance for the a-C:H type coatings was observed. When studying the temperature effects on a-C:H type DLC coatings similar type of effects were observed.

These aging and temperature effects were further elaborated by multi-scale modeling. In multiscale modelling the integrated approach combining material microstructural features modelled by molecular dynamic simulation (MDS) on nanoscale with FE modeling was applied. The tribological aspects of aging and elevated temperatures on DLC films will be reviewed with the validation of the models in different scaled and the arguments for aging mechanisms are presented.

Nowadays, it is widely accepted that nanoscale phenomena of sliding couples determine the friction and wear performance of a macroscopic system. Understanding these processes can lead to optimal tribological performance by developing novel materials and optimizing the sliding conditions (e.g. contact pressure, sliding velocity, etc.). However, studying these phenomena can be quite challenging due to the confined nature of the sliding interfaces and the dimensions of third bodies. In this study, macroscopic tribometry is linked with classical atomistic simulations in order to improve our understanding of the nanoscale interfacial processes during sliding of hydrogenated DLC (a-C:H) against a metal (W) in dry and lubricated conditions. Experimentally, using an on-line tribometer, wear and roughness measurements are performed after each sliding cycle, which are then correlated to the frictional resistance. Ex situ analysis is also performed on the worn surfaces (i.e. plates and counterfaces) using X-ray photoelectron spectroscopy (XPS), Auger Electron Spectroscopy (AES), and cross-sectional transmission electron microscopy (TEM) imaging of the near-surface region. Then, in order to elucidate the atomistic level processes which contribute to the observed microstructural evolution in the experiments, classical molecular dynamics are performed employing a bond order potential for the Tungsten-Carbon-Hydrogen system. Macroscopic tribometry shows that dry sliding of a-C:H against W results in higher frictional resistance and significantly more material transfer compared to lubricated conditions. Similarly, the molecular dynamic simulations exhibit higher average shear stresses and clear material transfer for dry conditions compared to simulations with hexadecane as a lubricant. In the lubricated simulations the lower shear stress and the absence of a material transfer is attributed to hexadecane monolayers that are partially tethered to the a-C:H surface and that significantly reduce adhesion and mechanical mixing between the sliding partners.

The first scratch method was Mohs scale; it has been used to characterize the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs. Now a day the Scratch Testing in micro scale is run out with constant, progressive or with increment load. The track is monitored with optical microscopy and acoustic sensor to evaluate the failure, the first failure is named critical load CL [1]. In nanoscale the scratch test is run out with constant normal load applied with a pyramidal diamond tip connected at atomic force microscopy.

In this paper we present the scratch test results in micro and nano scale from Silver diamond coatings (DLAg). The scratch test in micro scale was run out in progressive normal load with an acoustic Emission (AE) sensor connected with Rockwell C tip. The critical load sound was simultaneously measured to confirm the failure in scratch test. The failure was evaluated with an optical microscope. The load at which such failure of the coating occurred was plotted as a function of time.

In nano-scale the scratch test was run out in the same silver diamond coatings (DLSi) and it was used in constant normal load applied under diamond corner of a cube tip connected at atomic force microscopy. The scratch was performed to analyze the depth profile and the grain morphology in the track. Also was used as a supplementary analyzes Kelvin probe force microscopy (KPFM), also known as surface potential microscopy. It is a noncontact variant of atomic force microscopy (AFM). The KPFM was used to analyze the track surface potential and the results showed information about the surface composition and electronic state of the local structures on the surface track and some grains self-healing the track.

[1] - ASTM Standard G171 (03) – Standard Test Method for Scratch Hardness of Materials Using a Diamond Stylus.

Titanium alloys and Co-Cr based alloys are very promising materials for the femoral components of total hip and knee replacements. The metallic joint replacement components should have some common features, including not only high static and dynamic fatigue strength and biocompatibility bust also high corrosion and wear resistance. In this study, pin-on-disc and cyclic impact-sliding wear test methods were used to study tribological behaviour of both DLC coated and uncoated Ti-6Al-4V and ASTM F-75 CoCr bio-materials. The tests were carried out in both ambient dry and Hank’s balanced salt solution (HBSS). The potentiodynamic polarization test was also conducted which showed that uncoated Ti-6Al-4V and CoCr substrates had excellent corrosion resistance in the HBSS environment. Pin-on-disc test results indicated that the HBSS provided a lubricate effect with less concern of corrosion in the pure sliding condition and during the impact-sliding wear tests. The CoCr substrate was superior to Ti alloy substrate in the cyclic impact-sliding tests at both dry and HBSS test conditions,. The lubricating effect also appeared for both DLC-coated substrates. The DLC coating performed better on CoCr than on Ti alloy in both test environments, which matched the behaviours of uncoated substrates. The phenomena were discussed with consideration of a stronger strain-induced work hardening effect of CoCr substrate.