ICMCTF2017 Session E1-3: Friction, Wear, Lubrication Effects, and Modeling

Adhesion properties of hard thin coatings were strong l y affected from used substra t e material. In this study, TiNbVN coa t ings were d e posited on 2024 A l -alloy, M2 and H13 steels substrates using react iv e magnetron sputtering method. Structural and m e chanical properties of the coat i ngs were anal y zed by SEM, EDS, XRD, and nanohardness. Adhesion properties of the coatings were deter m ined via scratch test under progressive load. It was observed that critical load ( Lc) values of TiNbVN coa t ings varied in each substra t e. The coati n g m icrostruc tu re is dense a nd the fi l m thickness is about 440n m . The nanohardness values were taken under 1mN load and affec t ed by the substr a tes hardness due to l o ad c arr y ing capacit y . While the softest Al- alloy substrate hardness 1.25GPa, the coating hardness was measu re d 12GPa. On the other hand, the highest coa t ing hardness (39 G Pa) value taken fr o m the hardest M 2 substrate h a rdness (5.7GPa). The results of all the adh e sion tests s h ow a linear relationship between the hardness of th e base mater i al and the adhesion values. While adhesion value fr o m the co ated M2 w a s Lc≈65N, as function of the substrate eff e ct, the measured adhesion values are Lc≈15N and Lc≈50N from 20 2 4 Al-alloy and H13 st e el respectively.

The process of deposition of thin films can induce large compressive stresses (up to a few GPa) that combined with the low adhesion of the film to the substrate may result in simultaneous buckling and delamination of the film, leading experimentally to a large variety of buckled structures including straight-sided buckles, telephone cords or circular blisters. The mechanism of nucleation and propagation of elastic blisters has been widely investigated and described, either with analytical ^[1,2] or numerical methods ^[3]. In particular, the relationship between the mode mixity dependent interfacial toughness on the morphology of the wavy buckles has been highlighted ^[3]. The response of ductile thin films deposited on rigid substrates remains an open issue. For instance, it has been evidenced experimentally ^[4] that circular blisters exhibit folding angles at their base larger to that estimated from an elastic model (figure s1). In addition, recent experimental observations of 400nm gold films deposited on silicon wafers showed straight buckles with higher deflections compared to elastic predictions (figure 2). These differences in morphology are thought to originate from the elastic-plastic response of the film but the governing features need to be clarified. This is the purpose of the present study.

In this context, we carry out Finite Element simulations with a model that accounts for isotropic yielding and the non-linearity of the film. This approach aims at identifying the elastic-plastic constitutive model that is able to capture the experimental observations. A mode mixity dependent cohesive zone model is used to describe the thin film/substrate interface, that enables us to study the effect of plasticity on the stability and growth of straight and circular blisters.

[1] Hutchinson et al., Adv. in Appl. Mech. 29 (1992) 63.

[2] Hutchinson et al. Acta Metallurgica Materialia, 40 (1992) 295

[3] Faou et al., J. Mech. Phys. Sol., 75 (2015) 93.

[4] Coupeau et al., Thin Solid Films 469 (2004) 221.

Thin films coatings are used in many high technology applications particularly in microelectronics devices. Using flexible polymers as a substrate on which thin films (metal, oxides or organic) are bonded can give rise to new industrial applications such as OLED, flexible electronics or flexible photovoltaics devices. The mechanical stability and failure behavior of multilayer structures deposited on flexible substrate has been extensively studied both experimentally and theoretically [1-3]. Several relaxation mechanisms in thin films have been identified such as channel cracks, debonding or buckle delamination.

The objective of this study is to understand the multi-cracking of the silver and/or zinc oxide layers of various thicknesses coated on elastoplastic substrates (ETFE). In the process of cracking many parameters should be taken into account such as the elastic modulus mismatch between the film and the substrate, the plasticity of the substrate and the ductile or brittle nature of the film.

In the literature several experimental and analytical studies can be found. In [4], after experimental investigations, the existence of three different fracture stages was confirmed, the third one being a saturation stage of the cracks density at high strain with large opening of the existing cracks. Conventional models in literature such as Xia & Hutchinson model [3] and the “Shear lag” formalism [1,2] do not account for those experimental observations. We show that taking into account the plasticity in the substrate allows for capturing the crack density at the saturation regime.

To further validate this new model and the experimental observations, we present a numerical study which uses a cohesive zone model for the interface and to simulate the cracking of the film. This model also takes into account the plastic behavior of the substrate (Fig1). The different stages of cracking observed experimentally, including the nucleation stage were simulated (Fig2). A relationship between the properties of the film toughness, the saturation stress level in the film and the saturation distance between the cracks under deformation has been evidenced.

In modern industry, mechanical parts are subjected to friction and wear, leading to heat generation, which effect the reliability, life and power consumption of machinery. Solid lubricant additives have demonstrated better tribological performance in terms of reducing the machining zone temperature by creating friction without polluting the environment. With an appropriate application of solid lubricant additives in the sliding interface, the friction reduction and wear resistance properties of the lubricant have been successfully improved. Therefore, an attempt has been made in this research work with an investigation of using molybdenum disulphide suspension to reduce the friction at machining zone. To achieve this, in the present work, Electrostatic charged spray lubricant (ECSL) system has been envisaged for effective supply of solid lubricant mixture at an extreme low flow rate to the sliding interface of WC pin and Ti-6Al-4V alloy as disk materials. Excessive tribological measurements with SAE 40 oil concentrated with 20wt% of MoS₂ with micron size particles showed friction coefficient as low as 0.001 and negligible wear. It is proposed that negatively charged sprayed MoS₂ solid lubricant mixture at nozzle tip has found remarkable influence on their tribological behavior.