Tribological performance of graphite-like carbon films with varied thickness

Graphite-like carbon (GLC) films with different thickness were deposited on 316 L stainless steel using closed field unbalanced magnetron sputtering system to investigate the influence of film thickness on the microstructure, mechanical and tribological properties. The results showed that the surface of the deposited films exhibited granular-like morphology, and the sp2 content, surface roughness increase with the increase of film thickness, leading to the lower of hardness and higher of the internal stress. Both of the friction curves obtained by nano-tribological tests and fretting wear experiments revealed a three-stage evolution tendency with the same wear mechanism for the first two stages. The intermediate thick GLC film had the lowest specific wear rate, whilst the fretting fatigue life increased with film thickness

Failure mechanism and protective role of ultrathin ta-C films on Si (100) during cyclic nano-impact

Complex mechanical behavior with phase transformation and high brittleness limits the reliability of silicon-based microelectromechanical systems. Although very hard ultra-thin films are being considered as protective overcoats to improve the service life of substrate materials, their resistance to fatigue can be at least as important as hardness when exposed to cyclic loading. In this study repetitive nano-impact tests with a spherical diamond probe have been used to investigate the fatigue behavior and protective role of 5 and 80 nm tetrahedral amorphous carbon (ta-C) films on silicon. At the lowest load there was delamination of the 80 nm film but not for the 5 nm film. At higher loads failure involved lateral cracking of the silicon substrate. Single impact tests showed that this was preceded by ring and radial cracking. Changing contact pressure during the test provided further support for the degradation mechanism and the influence of phase transformation in the Silicon substrate. Under repetitive contact the thin film systems showed lower impact depth and greater impact cycles before substrate fracture than the uncoated Silicon. This is related to their enhanced load support which affects phase transformation in the substrate, with potentially delamination providing an additional impact energy dissipation mechanism

Dynamic changes of mechanical properties induced by friction in the Archard wear model

© 2019 Elsevier B.V. Fretting is small-amplitude, oscillatory motion between two bodies leading to surface damage. During the fretting process, a tribologically transformed structure (TTS) which has different mechanical properties and microstructure than the starting material is formed on the surface. The commonly-used Archard wear equation relating wear volume to the hardness of the worn material does not account for changes in mechanical properties induced by friction in fretting. To investigate that effect, ball-on-plane fretting tests were conducted on three engineering materials (type 316 stainless steel, pure copper, and titanium alloy Ti-6Al-4V) against an alumina ball to generate TTS layers. The evolution of mechanical properties and microstructures of TTS layers were investigated using nanoindentation and focused ion beam-scanning electron microscope (FIB-SEM). Wear volumes after different fretting cycles were measured with a white light interference microscope. Results show that the mechanical properties of TTS layers evolve differently on different materials during the fretting process. Microstructures of TTS layers also vary from one material to the other. A modified wear model that accounts for friction-induced dynamic changes in mechanical properties is proposed. In these tests the modified model was able to predict the wear volume of 316 steel and pure copper more accurately than the classical Archard model, but it was less successful in predicting wear on Ti6Al4V where there is added complexity from changing microstructure, oxidation, porosity and cracking

Micro-scale impact testing - A new approach to studying fatigue resistance in hard carbon coatings

© 2019 Elsevier Ltd Improving the fatigue resistance of DLC coatings under highly loaded repetitive contact is an important step to increasing their performance in demanding applications. The nano-impact test has been shown to be effective at highlighting differences in resistance to contact damage in thin hard carbon coatings deposited on hardened steel. A novel micro-scale rapid impact test capability capable of producing repetitive impacts at significantly higher strain rate and energy than in the nano-impact test has been developed recently enabling the study of coating fatigue with less sharp spherical indenters than in the nano-impact test. Results with the new micro-impact technique on two commercial hard carbon coatings (Graphit-iC and Dymon-iC from Teer Coatings) on tool steel are presented. The role of coating mechanical properties on the fatigue resistance and the load-sensitivity of the impact failure mechanism is discussed. The harder coating with higher sp 3 /sp 2 bonded C (Dymon-iC) was found to be significantly less durable under fatigue loading than the softer Graphit-iC. Reasons for the observed differences are discussed

Analysis of surface roughness morphology with TRIZ methodology in automotive electrical contacts: Design against third body fretting-corrosion

Electrical connectors for motor vehicles are essential for the safe and efficient operation of a vehicle. However, their durability is limited by fretting induced corrosion. This type of surface damage is observed between two interconnected surfaces exposed to vibration and temperature variations. Such conditions occur during normal vehicle operation and cause two parts of an electrical contact to move at high frequency and with a small amplitude of movement relative to each other. This damages both surfaces, creates wear particles and then oxidizes them in the air. This causes an oxide layer to form at the interface, isolating the two surfaces and increasing the electrical resistance, resulting in contact failure. This study shows how the service life of electrical contacts can be extended by a surface design approach that controls the metal interface and ensures low contact resistance. The approach combines surface morphology with the progressive process of interfacial oxidation. A strong relationship between surface roughness and electrical contact resistance was observed and is elucidated in this study. Theory of Inventive Problems Solving (TRIZ) was used to identify surface texturing as a viable option to increase durability of automotive electrical connectors

Micro-scale impact resistance of coatings on hardened tool steel and cemented carbide

© 2020 Elsevier B.V. Micro-impact, a novel accelerated test method for assessing coating durability under repetitive contact, has been developed to concentrate impact-induced stresses close to the interfaces in coating systems. Test results are described for carbon coatings on hardened tool steel and nitride-based coatings on cemented carbide. At higher load it was possible to show the increasing contribution of the substrate properties (load support and ductility) to the coating system response whilst retaining high sensitivity to the coating properties. Hard and elastic carbon coatings on hardened tool steel displayed very low impact resistance under these conditions. Relatively soft carbon-based coatings with more metallic character and high plasticity (low H/E) deposited on hard but tough tool steel were resistant to radial cracking and lateral fracture at high load. Lateral fracture at high load and extensive substrate cracking was observed at higher load for hard nitrides on cemented carbide. The micro-impact test has the potential to significantly speed up the pace of coating system selection for durability under highly loaded repetitive contacts, as occur in coatings applications in engine components and in discontinuous cutting operations

Macroscale structural superlubricity: Dynamic evolution of tribolayers in two-dimensional materials under extreme pressure

Achieving macroscale structural superlubricity with two-dimensional (2D) materials under ultrahigh contact pressure in ambient condition is particularly challenging. Furthermore, the mechanisms underlying the disparate trans-scale tribological behaviors of 2D materials continue to be a subject of debate. Here, we propose a novel principle concerning pressure-induced dynamic structural evolution and tribochemical behaviors of tribolayers to broaden the macroscale structural superlubricity. For the first time, robust macroscale structural superlubricity with ultralow wear rate is realized by 2D material coating in ambient condition by sliding steel counterparts under ultrahigh contact pressure. The results reveal that macroscale structural superlubricity of 2D materials is highly dependent on the dynamic evolution of tribolayers nanostructures, as well as the adsorption and tribochemical behaviors governed by extreme pressure. These findings shed light on achieving robust macroscale structural superlubricity with 2D materials for harsh engineering conditions

Tailoring the corrosion and tribological performance of Ti-modified MoS2-based films in simulated seawater

Film protection has become a crucial means to improve the corrosion and wear performance of key components in aggressive environment. In this study, the feasibility of using MoS2-based modified films in artificial seawater (3.5% NaCl solution) was evaluated by co-deposition of Ti to produce Ti-MoS2 composite and Ti/MoS2 multilayer films. The microstructure, wettability, mechanical, tribological and corrosion behavior of the Ti-modified MoS2-based films was contrasted to pure MoS2 film. The results show that the incorporation of Ti not only improves densification, but also promotes a transformation from a columnar to an amorphous film structure, leading to the improvement of mechanical properties of Ti-MoS2 composite film and Ti/MoS2 multilayer film. The friction coefficient curves of all of the three MoS2-based film in 3.5 wt% NaCl solution show stable values during the sliding process. The advantage of the preferential (002) growth orientation, improved mechanical properties and reduced hydrophobicity for both of the Ti-modified MoS2 films caused the decreased friction coefficient and wear rate in NaCl solution. The electrochemical results before and after friction show that the ranking of corrosion resistance is Ti/MoS2 multilayer > Ti-MoS2 composite film > pure MoS2 film, which is attributed to the compact microstructure and the presence of surface passive films

Fretting wear behavior of duplex PEO/chameleon coating on Al alloy

Plasma electrolytic oxidation (PEO) is an attractive technology for improving resistance to wear, heat and corrosion of aluminum alloys. PEO results in a hard, well-adhered alumina ceramic coating with a morphology which is graded from a dense region near the substrate interface to a porous outer region. Such properties mean that PEO can be an ideal underlying layer for the application of solid lubricants which can be entrapped in outer pores and provide reservoirs for the tribological contact lubrication. This study investigates the fretting wear behavior and adaptive mechanisms for a PEO-produced alumina surface of about 11–12 GPa hardness with a top layer of an MoS2/Sb2O3/C chameleon solid lubricating coating, the composition of which was designed to self-adapt in variable humidity environments for friction and wear reduction. 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 a duplex coating combination. The coated surfaces were investigated using nanoindenation, Raman spectroscopy and scanning electroscopy and were then subjected to fretting wear tests against steel and alumina balls with variable amplitude (0–100 μm) and loads (10–100 N) in both humid air and in dry 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 cycled environment tests. Friction coefficients of the order of 0.10–0.15 in humid air and 0.06–0.09 in dry nitrogen were recorded and linked with the surface self-adjustment from graphite to MoS2 lubrication, respectively, which was confirmed by Raman spectroscopy. The studies demonstrate the effectiveness of the PEO/chameleon duplex coating system for the friction reduction of and fretting wear in the gross-slip regime, as well as significantly reducing the critical amplitude of stick-slip transition for fatigue wear mitigation