ICMCTF2010 Session E3-2: Tribology of Nanostructured and Amorphous Films

Amorphous MoS₂/Sb₂O₃/Au composite sputtered coatings are well known for their solid lubricating behavior and environmental robustness. In the current study, we have investigated the fundamental mechanisms of friction and interfacial shear strength, and the role of contact stress and environment on their tribological behavior. Friction and wear measurements were made from 0.3 to 1.1 GPa contact pressures in dry (<1% RH) nitrogen or humid (50% RH) air, with precise control of dew point and oxygen content. The friction coefficient of MoS₂/Sb₂O₃/Au in dry nitrogen was extremely low, ~0.007, whereas in humid air it increased to ~0.16, with minimal amount of wear in both environments with wear factors of 1.2-1.4 x 10^-7 mm³/Nm. The coatings also exhibited non-Amontonian friction behavior, with friction coefficient decreasing with an increase in Hertzian contact stress. The main mechanism responsible for low friction and wear in both dry and humid environments is governed by the interfacial sliding between the wear track and the friction-induced transfer film on the counterface ball. The interfacial shear strength, computed from friction coefficient - inverse Hertzian contact stress plots, was found to be 20 MPa in dry nitrogen and 38 MPa in humid air (both very low values for solid lubricant coatings). Cross-sectional transmission electron microscopy (XTEM) with Automated eXpert Spectral Image Analysis (AXSIA) software for X-ray spectral images and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) were used to study the solid lubrication mechanisms of the wear tracks and corresponding transfer films. It was determined that that in dry nitrogen, frictional contact transformed the amorphous coating into crystalline 2H-MoS₂ basal (0002) planes oriented parallel to the sliding direction inside the wear track. This amorphous to crystalline transformation resulted in a continuous MoS₂ film across the entire wear track, while in the humid air, the wear track was also enriched with Au islands leading to the slightly higher friction coefficient and interfacial shear strength. In both environments, the transfer films also exhibit (0002)-orientated basal planes along their entire section resulting in predominantly self-mated ‘basal-on-basal’ interfacial sliding, and thus low wear. This presentation will also discuss the interrelationships between interfacial structure/tribochemsitry and the pressure-induced transfer film shear strength to explain the observed friction and wear behaviors in the two environments.

MoS₂ and WS₂ are widely used as solid lubricants in coatings for tribological applications and have shown super-low friction in non-humid atmospheres and vacuum. They are known for their non-Amontonian behaviour, meaning a decreased coefficient of friction μ with increasing load. The big drawback is their sensitivity to humidity, leading to tribo-induced oxidation of WO₃ and MoO₃, respectively. These oxides are much harder to shear, thus resulting in higher μ. Inorganic Fullerene like nanoparticles (IFLs) of WS₂ and MoS₂ have been proposed to perform better, based on that their closed structure would make them more stable against oxidization.

This paper compares the friction behaviour of three MeS₂ based coatings in various atmospheres under different loads and sliding velocities against ball bearing steel balls.

A novel electrodeposited composite coating with IFL-WS₂ in a Ni-P-matrix is compared with a PVD coating with a columnar WS₂ structure and with a commercial MoS₂ PVD coating. The tests were performed in a pin-on-disc setup. Wear tracks and the transfer films were analysed with EDX, XPS and TEM.

In dry nitrogen atmosphere the IFL-WS₂ coating shows μ below 0.01, which is as good as the commercial PVD coating. The IFL-WS₂ coating shows very little wear under these conditions. In more humid atmospheres the IFL-coating performs better then the planar WS₂ coating but not as good as the commercial MoS₂ coating. Tests at elevated temperature, when no water is adsorbed to the surface, show some very promising results for the IFL-WS₂ coating. It has also been observed that the IFL-WS₂ coating performs better under higher normal load. Transfer films were observed for all contacts and their characteristics were investigated and are believed to have an important role for the friction behaviour.

The results clearly show that coatings containing IFL-nanoparticles can become an attractive alternative for specific tribological situations.

^*Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.

Among the different examples that one can imagine, decorative thin film systems are certainly among the most important examples, since this kind of materials are subjected to simultaneous wear and corrosion environments whenever they are put in service. In order to gain further knowledge, both in the test method itself,, as well as in the study of the future behavior of a particular decorative-like system, ZrN_xO_y thin films were tested for their tribocorrosion properties on a standard reciprocating sliding tribometer (TE 67/E) in an environment of artificial sweat solution. The study was conducted as a function of load (3-6-9N) and electrode potential (-0.5, 0, +0.5 V (E vs SCE)). The evolutions of the friction coefficient and current have been measured during the test. Electrochemical impedance test were used to comprehend the changes in the surface chemistry before and after the sliding test. For the in-depth understanding of the tribocorrosion process, wastage map, mechanisms map and synergism map were constructed as a function of electrode potentials and load.

The vacuum cathodic arc evaporation (CAE) process is now widely used on an industrial scale essentially to prepare protective hard coatings on cutting tools, metal molds etc… It has been recognized that high ionization levels of cathodic arc discharges and high ions energy can provide advantages as, for example, enhanced adhesion which is required for mechanical applications involving high loads. CAE allows deposition of a wide range of hard compounds as nitrides or carbonitrides. However, CAE generates macrodroplets that lead to degrade the surface roughness of coatings. The recent trend is to develop advanced nanostructured hard coatings in order to enhance properties as hardness, toughness, oxidation resistance etc…

This paper deals with the recent developments in both CAE technology and nanostructured thin films synthesis. Technological progresses on macrodroplets filtering or new cathodes are discussed. Elaboration and characterization of both nanocomposite and nanolayered hard coatings are described. Thus, relationships are established between deposition parameters, nanostructure and tribological behaviours of the coatings.

High performance turbine engines, aircraft-engine control components, and air-foil bearings are required to operate at increasing temperatures, which increases system efficiency. Liquid lubrication systems have limited temperature capability and reaction with steel surfaces can result in corrosion. Development of wide-temperature-range solid lubrication that offers low coefficient of friction (COF) and low wear factor (WF) is a key technology for future mechanical systems. Engineered Coatings and Southwest Research Institute deposited nanomultilayer coatings, adopted from the dry-machining industry, for tribological-property characterization at room temperature (RT), 260°C (M50 steel substrates vs. M50 balls), 485°C (Pyrowear substrates vs. Si₃N₄ balls), and 648°C (Pyrowear substrates vs. Si₃N₄ balls). From the sliding-wear tests conducted in air on coated Pyrowear substrates, nanomultilayer coatings consisting of TiAlCrN / (Ti-W-Ti)N exhibited the best combination of low COF (~0.4 at RT and ~0.37 at 485° C) and low ball WF at RT and 485° C, while a nanomultilayer coating of TiAlCrN / TiN exhibited the best performance at 648°C (COF ~0.37). A TiAlCrN / (Ti-W-Ti)N multilayer coating with a WS₂ cap layer exhibited lowest COF at RT on Pyrowear, but the low COF (~0.2) lasted for only about 50% of the test duration; at 485°C the COF was ~0.4. At lower temperatures on M50 steel substrates, the best performing coating was a monolayer of AlTiCrSiCN at RT, while at 260°C the best performer was the nanomultilayer coating TiAlCrN / TiN. However in both cases COF on coated M50 steel was higher than for coatings on Pyrowear. Photographs of the ball wear scars were taken to characterize the wear behavior.

Oil and gas drilling represents a unique industry that demands innovation and efficiency and where many physical limitations and technological challenges exist. The materials used in drilling oil and gas wells have to balance between different opposing attributes and are usually pushed to their maximum mechanical capability. This presentation will discuss the applications of various thick and thin film coatings in drill bits that are used to make the well for extracting oil and gas. The drilling environment can be corrosive or abrasive or both. Thus, keeping a rock bit drilling in the hole for an extended period of time with a desirable penetration rate requires substantial protection on the exterior surface and sustaining wear resistance of the internal components. Functional coatings, such as tungsten carbide hardfacing, silver plating, thermally sprayed coating, and diamond-like carbon coating, have been attractive engineering solutions as they can be applied to almost any geometry without adding substantial dimensions. Careful material selection, testing, and application of the coating can not only enhance the performance but also extend the life of a rock bit. A comprehensive test program will be demonstrated to show how tungsten carbide hardfacing and diamond-like carbon coating were successfully brought to commercialization in oil and gas drilling.

Titaniun carbide (TiC) thin films are widely used for tribological applications due to their high hardness (H) and wear resistance. TiC films prepared by low temperature Plasma Enhanced Chemical Vapor Deposition (PECVD) exhibit an unusual simultaneous combination of properties such as very high H, low Young’s modulus (E), and low friction coefficient (µ). Tailoring such properties is well suited for a wide range of tribological applications, particularly for the protection against solid particle erosion in different components of aircraft and helicopters. In order to further improve the properties of TiC, in the present work, we incorporated silicon (Si) as an alloying element to obtain ternary nanostructured Ti-Si-C films. Si addition into the base Ti-C resulted in significant microstructural, mechanical and tribological modifications. By controlling the Si content in the films, we succeeded to obtain transition from films consisting of fine nano-sized TiC crystallites embedded in a an amorphous SiC/a-C:H matrix to a microstructure formed by nano-sized SiC crystallites encapsulated in a TiC/a-C:H matrix. This allowed one to control the main mechanical characteristics, namely H, E, and µ, in the range of 14-32 GPa, 140-240 GPa, and 0.16-0.6, respectively. For films prepared under optimized conditions, high resistance to elastic and plastic deformation of the Ti-Si-C films, expressed by high values of H/E and H³/E² ratios, respectively, resulted in 8 times higher erosion resistance at 90° compared to bare steel substrate. At the same time, erosion resistance at 30° impact angle increased by a factor of 22 due to a simultaneous combination of high H and low µ. Taking into consideration the severe erosion test conditions and the Ti-Si-C film thickness of only 2.5 µm in this work, further improvement is expected for thicker films.