ICMCTF2015 Session E1-2: Friction, Wear, Lubrication Effects, and Modeling

Textured surface with low friction have been prepared in air by using CO₂ and their friction performance and their chemical composition were explored. For textured surface production, a layer of graphite powder was placed onto titanium alloy (Ti-6Al-4V) and a CO₂ laser beam (CO₂LB) was irradiated on the covered surface. The CO₂LB produced graphite integration on titanium surface, and at the same time, changes graphite chemical structure. Textured surface showed low friction coefficient in air environment. The results shown textured surface as a good candidate for anchoring cells in biomedical application due to roughness and carbon presence. The CO₂LB could be used to write or draw textured lines, or letters producing lubricated surfaces. Also could be used to prepare lubricated areas to reducing fretting during satellite launching and avoiding cold welding in many satellite devices in satellite orbit.

In this work we focus our interest in the observation of the PVD hard coating growth defects in a tribological contact. The contact area between two sliding bodies is not constant at all times. In the beginning, only the asperities in the form of growth defects are in real contact with the counter body. Under the load these asperities break, spall and form small particles. The real contact area is increasing for about a couple of hundred cycles before it stabilizes. In terms of friction, we recognize this behavior as the running-in period. The coefficient of friction also increases until it reaches a steady state value. How does this transition from the run-in to the steady state friction occur, and more important how can the growth defects affect the tribological performance, is still not well understood.

To investigate the behavior in microscale in the contact area we used tool steel coated with TiAlN hard coating prepared by unbalanced magnetron sputtering. A set of pin-on-disc tribological experiments with a low number of cycles was conducted. As a counted body either the alumina (Al₂O₃ as a hard, ideal surface) or the softer 100Cr6 ball was used. The scope is that we managed to track individual nodular growth defects before and after the tribological test. This enabled us a more detailed view on the role of growth defects in the tribological contact. The scanning electron microscopy, 3D profilometry and focused ion beam cross sections were done in order to follow the sliding “cycle to cycle” dependent changes.

Successful operation of satellites and launch vehicles requires using multiple moving mechanical assemblies (MMAs). The correct choice of lubricants and tribocoatings is critical for the operation of spacecraft MMAs. However, the space environment is challenging. Examples include vibration during launch, thermal cycling on orbit, and the need to work effectively for missions up to twenty years in duration without lubricant replenishment. Especially challenging is the need for tribomaterials to withstand the vacuum of space during lengthy missions. As such, they must exhibit low vapor pressures, since evaporation of lubricants can result in loss from and premature failure of devices, as well as contamination of sensitive spacecraft components. Although unique synthetic liquid lubricants are used heavily in spacecraft for a variety of applications, solid lubricants are used with many devices because of their low vapor pressure, lack of migration, relative insensitivity to temperature changes, and low contamination potential. They also exhibit boundary protection and very low friction. Soft solid lubricants such as molybdenum disulfide (MoS₂) and polytetrafluoroethylene (PTFE) have been used traditionally. More recently, hard low friction coatings such as hydrogenated diamond-like carbon have shown promise for operation in vacuum with existing spacecraft lubricants, or even unlubricated operation in vacuum. In addition, increasing interest in low friction nanoparticles has highlighted their potential utility. Tribomaterials show performance in vacuum that differs from that in air, and even in nitrogen, which contains small partial pressures of oxygen and water. This issue is important for spacecraft hardware, because it is often prohibitive to test them in a space-like environment, including vacuum, before launch. In this talk, results will be presented from studies done at The Aerospace Corporation that elucidate the effects of vacuum and temperature extremes on the tribological performance of important spacecraft tribomaterials. Emphasis will be on correlating surface chemical and tribological properties.

This work was funded in part by The Aerospace Corporation's Sustained Experimentation and Research for Program Applications program.

© The Aerospace Corporation 2014

Silver tantalate (AgTaO₃) coatings have been found to exhibit outstanding tribological properties at elevated temperatures. To understand the mechanisms involved in the tribological behavior of the Ag-Ta-O system, tantalum oxide coatings with a small content of Ag were produced to investigate the metastable nature of this self-lubricating material. The coatings were produced by unbalanced magnetron sputtering, ball-on-disk wear tested at 750 °C, and subsequently characterized by X-ray diffraction (XRD), Scanning Auger Nanoprobe (SAN), cross-sectional Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM). Complementary molecular dynamic simulations were carried out to investigate changes in the chemical and structural properties at the interface due to sliding for films with varying Ag content. Both the experimental characterization and the theoretical modeling showed that silver content affects friction and wear, through the role of Ag in film reconstruction during sliding. The results suggest that the relative amount of Ag may be used to tune film performance for a given application.

Pin-on-disk measurements were performed on a variety of polished 410 stainless steel samples with and without 2 micron hard coatings prepared by magnetron sputtering. The coatings were of NbN, NbSiN and TaSiN which had been previously characterized in terms of their composition (EDX), structure (XRD), topography (perfilometry), hardness and Young’s modulus (nanoindentation), coating adhesion (scratch testing). The room temperature tribological testing was carried out at atmospheric pressure, high vacuum and in pure argon at 2 torr. The pin was a 1/8” diameter 52100 steel ball and an applied force of 2 N, a tangential velocity of 8 m/min and 5000 revolutions were used for all of the tests. We report the variation of the friction coefficient and the wear rate as function of the testing conditions, together with a selection of SEM images of the wear track to aid the description of the wear mechanisms. The wear rate of the uncoated sample was much larger than the samples with hard coatings and in general there was no relationship between the wear rate and the friction coefficient. For the coated samples there was significantly more production of debris for the argon and vacuum tests compared to the atmospheric conditions.

Most hard coatings are based on ceramic materials and are generally brittle. It is desirable to have coatings that are both hard and tough. Here, we will review several strategies that can be employed to increase coating toughness while maintaining hardness. Various nanocomposite and multilayer coatings (Ti/TiB₂, FeMn/TiB₂, Fe/VC and W/VC) were synthesized to explore three such toughening strategies: coherency strain, transformation toughening, and nanograined metals. Practical methods used to measure coating toughness in this work will be presented: scratch testing, nanoindentation, and modified Vickers. Results demonstrate that coating systems that exploit these strategies show significantly enhanced toughness compared with those that do not . In particular, the strategy of using nanolayers of a metal with high elastic modulus alternating with spacer layers much thinner than the metal appears to be the most effective. In principle, one can reach hardness values up to 10% of the elastic modulus, while attaining toughness comparable to most nanocrystalline metals. Given that most metals with high elastic moduli are refractory materials, such coatings may also be useful for high-temperature applications.

Copper possess excellent ductility and conductivity owing to which they are very popular in electrical contacts. However, copper is soft leading to seizure of sliding contacts at high loads leading to failure of these components. Researchers have attempted to address these issues by developing self lubricating copper based composites using graphite as a lubricant. The surface hardness of copper has been improved by developing copper based ceramic reinforced composite coatings to minimize wear mainly by electroplating techniques. There are few reports on development of hard coatings on copper by use of thermal spray method and in particular plasma spraying. Plasma spray technology is currently gaining widespread popularity in most of the industries in developing effective wear resistant coatings. In the light of the above, the present work focuses on development of hard composite coatings of TiO₂-30 wt% Inconel 718 on commercially pure copper substrate using atmospheric plasma spray technique.

Metallographic studies, hardness and adhesive wear studies have been carried out on both copper substrate and the developed composite coatings. A pin on disc tribometer has been used for dry sliding wear tests. Loads were varied from 5-20N in steps of 5 while sliding velocity was varied between 0.628 m/s to 1.57 m/s . A sliding distance of 1500m was maintained for all the tests. Under all the studied test conditions, the developed composite coatings possessed lower coefficient of friction and higher wear resistance. SEM studies of worn samples have been studied to enunciate the material removal mechanism.

In this study, Ni–MWCNT metal matrix composite (MMC) coatings were prepared from a modified Watt's type electrolyte by using pulse current (PC) plating technique under constant density of 0.30 mA.cm-². The influence of MWCNT content in the electrolyte on the tribological properties were tested by using a reciprocating ball-on-disk apparatus, sliding against to M50 steel ball (Ø 10 mm). The wear tests were carried out at sliding speed of 100, 200 and 300 mm/s under a constant load. Microstructural, morphological and microhardness features were studied. Coatings were characterized using Raman microscopy, X-ray diffraction and SEM analysis before and after wear testing. Characterizations were especially concentrated on the clarifying the wear mechanisms. Results showed that the amount of MWCNT, co-deposited in the deposited layer can significantly affect the microstructure and tribological behavior of Ni–MWCNT nanocomposite coatings.