ICMCTF2018 Session E1-2: Friction, Wear, Lubrication Effects, and Modeling

Relocation profilometry was used to evaluate the damage that occurred in micro-tribology experiments on a range of DLC coated and uncoated steel samples. Three roughness conditions were tested ranging from a ground surface to a smooth surface for both the coated and uncoated materials. The effect of varying orientation of the micro-tribology experiments with the directionality of the finished surfaces. The micro-tribology experiments were carried out using single pass scratch tests using a diamond indenters. The same areas were examined with an Olympus Lext confocal microscope which gave image and height maps from each area examined. The same areas were examined before and after scratching, and the resulting pairs of height maps registered and subtracted using Image J. The true volume of damage was thereby calculated for all experiments. Friction measurements were also made. This enabled the energy dissipated in the damage formation to be calculated.

Little effect of orientation on the damage was observed. What was quite surprising was that there was also no effect of roughness on the damage that was observed.

Nickel-based self-lubricating claddings with the addition of Ag and MoS₂ were prepared by means of laser cladding on stainless steel substrates, aiming at their implementation in metal forming applications involving demanding tribological conditions at high temperature. The novelty of our approach relies in the addition of MoS₂ with the aim achieve a uniform silver distribution within the resulting cladding by means of an encapsulation mechanism, preventing it from floating to the surface during the deposition process and being subsequently lost during surface preparation. The role of Ag and MoS₂ concentration on the encapsulation process is discussed in terms of phase composition and resulting microstructures. The tribological behaviour of the resulting laser claddings was evaluated at high temperature under unidirectional sliding. The encapsulation of Ag led to outstanding tribological properties while keeping the amount of Ag used at lower concentrations, thus increasing the economic feasibility of the claddings. The improvement in terms of both friction and wear was observed for the self-lubricating claddings compared to the reference alloy, making them good candidates for use in high temperature applications such as metal forming.

Plasma electrolytic oxidation (PEO) is an attractive technology for improving wear resistance and environmental protection of aluminum alloys. PEO results in the hard alumina based ceramic coatings of up to 100-150 micrometer thickness which are well adhered to the surface with morphology graded from a dense region near the coating-substrate interface to a porous outer region [1]. Such properties may provide PEO as an ideal underlying layer for the application of solid lubricants which can be entrapped in outside porous and provide reservoirs for the tribological contact lubrication, however the relevant work is scarce. This study investigates the fretting wear behavior and adaptive mechanisms for the PEO produced Al₂O₃ surface of about 11-12 GPa hardness with a top layer of an MoS₂-Sb₂O₃-C chameleon solid lubricating coating, which is named such for its ability to self-adapt tribological contact surface and provide friction and wear reduction in variable humidity [2]. Coupons of AA 6082 alloy were coated by the PEO process and then were over-coated by a burnishing process with a MoS₂-Sb₂O₃-C chameleon coating to prepare such duplex coating combination. The coated surfaces were then subjected to over 10,000 cycles of fretting wear against steel and alumina balls with variable amplitude (0 to 100 micrometers) and loads (10-100 N) in both humid air and in dry nitrogen, including cycled (air/nitrogen) environment conditions. The tests demonstrated low friction coefficients, considerable reduction in critical amplitude for the stick-slip transition, and self-adaptive tribological behavior in the cycled environment tests. Friction coefficients of the order of 0.10 to 0.15 in humid air and 0.06 to 0.09 in dry nitrogen were recorded and linked with the surface self-adjustment from graphite to MoS₂ lubrication, respectively. Raman, SEM and cross-sectional FIB/SEM/EDX analysis of the wear tracks were used to investigate the mechanisms of the adaptation and fretting wear performance. The study demonstrate the effectiveness of the adaptive PEO-Chameleon coating system performance for the fretting wear mitigation in changing environment.

[1] A.L. Yerokhin et al., Surface and Coatings Technology, 122 (1999) 73.

[2] J.S. Zabinski et al., Tribology Letters, 23 (2006) 155.

The ability to model and predict friction and wear performance in boundary-lubricated conditions is limited by the lack of qualitative and quantitative information on transient tribochemical reactions between the lubricant additives and surfaces. It is these reactions that define tribofilms ’ physical and chemical properties, essential for friction and wear performance of industrial boundary lubricated systems, such as valve train, piston ring/liner and pumps. In addition to tribofilm formation, recent work has shown that some of the lubricant additives, such as Molybdenum dialkyl dithiocarbamate (MoDTC), can have a detrimental impact on hydrogenated Diamond-Like-Carbon (DLC) coating durability.

In the current paper, a bespoke tribometer has been designed to be coupled with the Raman spectroscopy and synchrotron X-ray Absorption Spectroscopy for in-situ study of the tribochemical reactions between the lubricant additives and the surface. The focus will be on two typical lubricant additives: Zinc dialkyl dithiophosphate (ZDDP) used as an anti-wear, and MoDTC used as a friction modifier additive. The growth of tribofilms on heat-treated and DLC coated surfaces, in relation to friction and wear performance, will be discussed in detail.

Titanium (Ti) films were deposited by DC Magnetron Sputtering by changing deposition times and substrate temperature on AISI 316L steel to evaluate different thicknesses and its properties. Cristal orientation was determined by grazing angle X-ray diffraction (XRD). Scanning electronic microscope (SEM) was used to determine the surface composition and deposition characteristics through elemental analysis, and also to measure film thickness. Ellipsometry measurements were performed to compare film thicknesses estimated with SEM. Surface topography was obtained by atomic force microscope (AFM). Using nanoindentation test with a spherical indenter, mechanical properties were estimated. Furthermore, failure mechanisms and critical loads were determined by progressive load scratch tests. Wear behavior was studied through pin-on-disk tests with a 6 mm-diameter WC ball. SEM and optical profilometry were used to examine wear tracks; wear rates and coefficient of friction were analyzed.