ICMCTF2012 Session E1-3: Friction Wear Lubrication Effects & Modeling

Thanks to their unique tribological properties, Diamond-Like Carbon coatings are increasingly used in industrial applications. They indeed combine low friction with good wear resistance, while most tribological coatings are either hard wear resistant materials but exhibiting relatively high friction, or soft with easy shear and thus low friction but exhibiting higher wear rates. How to account then for the paradoxical combination of hard DLC coatings exhibiting low wear together with low friction?

Thanks to high hardness, high strain tolerance and smoothness achieved with most DLC deposition techniques, plastic flow is not likely to occur in the contact area, neither as abrasive phenomena nor as shearing of contacting asperities. Nevertheless, adhesive interactions with DLC surfaces can vary significantly depending on counterface nature and environment, from strong covalent bonding to weak Van-der-Waals interactions between passivated surfaces, especially with hydrogen. Solid lubrication processes of DLC films are thus mostly governed by adhesion, which in turn affects the velocity accommodation modes.

From various experimental results obtained with different DLC coatings in several environments, the key-role of adhesive phenomena will be highlighted. These results will point out that formation, behavior and release of adhesive junctions are critical for contact evolution. The strength of adhesive junctions is affected not only by tribochemical reactions with counterface and/or environment, but also by their size and hence by surface topography. The release of these junctions will finally depend on mechanical properties of the contacting surfaces.

The tribological properties of CrAlN, CrAlYN and CrAlZrN coatings deposited by d.c. reactive magnetron sputtering are studied during pin-on-disk experiments at room temperature and under heating at 250, 500 and 650 ºC using alumina balls as counterparts. The influence of the metallic composition (Al, Y and Zr) in terms of friction and wear properties and oxidation resistance is studied by means of cross-sectional scanning electron microscopy (X-SEM), energy dispersive X-ray analysis (EDAX) and Raman analysis of the contact region after friction tests. The wear mechanism is characterized by the formation of an overcoat rich in chromium and aluminium oxides whose composition is determined by the initial chemical characteristics of the coating and the testing temperature. The increase in temperature above 500ºC produces a change in wear mechanism from abrasive to adhesive concomitantly with the formation of surface layer rich in chromium oxides. Thus, the addition of Y, and particularly Zr, favours the preferential formation of Cr₂O₃ versus CrO₂ leading to a reduction of friction and wear of the counterpart. Conversely, the tribological behaviour of pure CrAlN coatings is characterized by higher friction but lower film wear rates as a result of higher hardnesses and major presence of aluminium oxides on the coating surface.

Thin film coatings are indispensable in all current design for cutting tool applications. True mechanical properties like hardness, elastic modulus, adhesion and wear resistance are essential parameters which need to be understood in the development of tool design. The general interest of mechanical characterization at small scales is to evaluate the properties of the hard coating as it becomes thinner and to optimize it to be able to fit the increasing and challenging demands of industry.

The objective of this study is to find a test procedure which supports the optimization of coatings for cutting tools by the measurement of the mechanical properties by nanoindentation of the substrate and the coating. An extended Hertzian approach is utilized to derive the specific Young moduli and yield strengths of substrate and coating based on the data obtained from nanoindentation. These mechanical constants are utilized to simulate the stress profiles in the coating-substrate system and to dimension the scratch test as well as the pin-on-disk test at elevated temperatures.

Scratch testing will give information about the critical failure loads and adhesion of the coating, whilst high temperature tribology testing can explain the evolution of the friction force and wear rate versus the working temperature.

Nitride-based nanocomposite coatings that consist of inclusions of silver, bismuth in vanadium, molybdenum, or tungsten nitride matrices was investigated as potential adaptive coatings to reduce friction in the temperature range from 25 to 1000 °C. These nanocomposite structures were selected based on the premise that a binary metal oxide layer with low shear strength will form on the surface of the coatings at elevated temperatures. The nitride-based coatings were produced using unbalanced magnetron sputtering and their elemental composition was evaluated using x-ray photoelectron spectroscopy (XPS). The tribological properties of the materials against Si₃N₄ balls were investigated at different temperatures. Reduced friction coefficients in the 0.1 to 0.2 range were recorded at high temperatures (600 °C to 1000 °C range) due to the formation of silver- and bismuth-based binary metal oxides as determined by Raman spectroscopy and x-ray diffraction (XRD) measurements on the surface of these coatings after testing. In addition, real-time Raman spectroscopy and HT-XRD (High Temperature XRD) provided valuable insight into the processes that the coating undergoes as a result of thermal and/or mechanical stresses upon heating at temperatures up to 1000 °C.

This study is one part of a series of efforts to optimize the mechanical properties and tribological properties of the Ti-Si-C-N nanocomposite coatings. In this study Ti-Si-C-N coatings were deposited on Ti-6Al-4V and Custom 450, a stainless steel used in turbine industry, by Plasma Enhanced Magnetron Sputtering (PEMS) using Ti targets in an argon-nitrogen- trimethylsilane gaseous mixture. The discharge current generated by thermionic emission from W filaments, the bias voltage, the flow rate of N₂ and the flow rate of TMS were the main parameters varied during the deposition process. Scanning Electron Microscopy (SEM) and X-Ray Diffractometry (XRD) were used to study the microstructure and morphology of these coatings. Erosion resistance were determined using 50 µm alumina at 30° and 90° at a velocity of 14 m/s, while the coefficient of friction and the wear resistance were evaluated using a ball-on-disc tester. It was found that a nanocomposite microstructure with a small crystallite size (5.5-6.8 nm) and a good morphological quality free from any delamination, columnar structure or any defects was achieved at relatively high ion bombardment. The micro hardness of the coatings increases with the increase of both the nitrogen flow rate and the TMS flow rate, and the highest hardness of 46GPa was reached. These coatings also exhibit excellent erosion and wear resistance.

In this work, we have investigated the metal-polymer sliding properties of micro-tube-forming tool surfaces coated with various low adhesive thin films. The selected polymer substrates were Polyamide-6, polyethylene terephtalathe and polytetreafluorethylene, and the tested coatings: commercial CrN and Cr₂N as deposited by electron beam techniques and diamond like carbon, as deposited by plasma activated chemical vapor deposition.

Ball-on-disc tests have been chosen to gain information about the film wear, polymer surface degradation and material transfer at the tool/polymer contact zone. The tests have been conducted at RT and at 150ºC. It has been shown that the coatings decrease the tool surface wear. In addition, the polymer adhesive wear, i.e. the material transfer from the polymer to the tool surface decrease in overall when low adhesion films are deposited on the tool surface, both at RT and at 150ºC. More specifically, Cr-based coatings provide better antiadhesion properties than diamond like carbon. No clear difference between RT and 150ºC in terms of amount of sticking (within the limits of the approach).

PA-6 is found to be the polymer with the lowest tendency to sticking, probably because of its superior mechanical properties. PTFE exhibited the largest tendency to sticking due to its low mechanical strength. Finally, it has been observed that the COF of all the tool coating / polymer tribopair depend only on the polymer nature, regardless the film properties deposited.

Gold coatings that are ideally suited for low electrical contact resistance (ECR) applications are mechanically soft and exhibit unacceptable amounts of adhesion and friction. In the current study, we explored novel pathways for balancing these fundamentally opposing phenomena, i.e., friction and ECR, through microstructral control at the nanoscale. Using a Triad e-beam evaporation system with capability for co-depositing three elements or compounds, Au-ZnO nanocomposite thin films with 0.1 to 10 vol.% ZnO were synthesized. The sheet resistivity of the bulk film was characterized using a four point probe technique and correlated to the surface ECR value. The films were post annealed to obtain a range of grain sizes and characterized by nanoindentation and high resolution TEM. ECR-friction measurements were made using a pin-on-disk ECR-tribometer where a hemispherically tipped noble metal alloy pin slid on the Au-ZnO nanocomposite film while current was passed through the sliding contact. Friction force and electrical contact resistance data were acquired simultaneously as a function of applied load. Cross-sections of wear scars suitable for TEM were prepared by focused ion beam microscopy. Changes to the grain structure arising from frictional contact with passage of current were analyzed by TEM analysis on cross-sections of wear scars. The role of nanoscale ceramic inclusions on the stability of grain structures during sliding electrical contacts will be discussed.

* Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000

Mo₂N-Ag and MoN-Ag nanocomposite films were deposited by using a hybrid deposition system composed of cathodic arc and magnetron sputtering. Molybdenum is evaporated by cathodic arc, and silver is introduced into the structure through magnetron sputtering. Sputtering power is used as a variable for changing the silver content of the films. The crystal structure of the films was evaluated using a glancing angle X-ray diffractometer with a thin film attachment. The cross-sectional film morphology and elemental analyses were conducted using a Field emission scanning electron microscope (FE-SEM) equipped with energy dispersive spectroscopy (EDS) unit. The tribological properties of films were investigated under atmospheric conditions against different counterface materials, namely 440C and Al₂O₃. The morphology of coatings and wear tracks of both side were examined using scanning electron microscopy (SEM), light microscopy, and 3D optical profilometry. Wear debris and the coatings were analyzed by micro-Raman system. The results revealed the positive role of silver addition both on film and counterbody wear. Low friction coefficients (0.28 and 0.31) were observed for MoN+8at. % Ag and Mo₂N+10at.% Ag coatings, respectively. Raman investigation showed that silver -molybdate compounds were formed in the wear tracks. The decrease in the friction coefficients was attributed the formation of silver - molybdate phases at the surface during the friction test. Higher Ag content (>22 at. %) caused deterioration of the mechanical properties and wear resistance of the coatings.