ICMCTF2015 Session E1-3: Friction, Wear, Lubrication Effects, and Modeling

Solid lubricant coatings are commonly employed for use in mechanical systems operating in environments where oil or grease lubrication is not feasible. In particular, solid lubricant coatings are currently being used in rolling element bearings operating in vacuum environments and/or at high temperatures. Although numerous studies have reported the tribological performance of these materials in sliding contact, few if any studies have reported their performance in rolling contact. Since functional lifetimes of solid lubricant coatings in rolling element bearings are derived from sliding contact experiments, it is beneficial and important to understand if the actual performance of the coatings in rolling contact bears a relationship to their sliding contact performance. In this work, MoS₂-Sb₂O₃-Au coatings are deposited onto AISI 52100 and M50 specimens by magnetron sputtering. Mechanical properties are studied using scratch tests and nano-indentation techniques whereas microstructural analysis is done with Raman Spectroscopy and Transmission Electron Microscopy. Friction and wear measurements are performed on specimens in both unidirectional and reciprocating sliding using ball on flat tribometers in both vacuum and humid environments. Friction coefficient values as low as ~0.03 and as high as ~0.15 are measured at 1.2 GPa Hertzian contact stress, and depend greatly upon the test environment. Functional lifetimes of the coatings are obtained from rolling contact tests performed in a thrust ball bearing tribometer and five ball on rod tribometers in both humid and vacuum atmospheres.

Increasing awareness of the impact of climate change promotes the political will to reduce greenhouse gas emissions in the European Union. With regard to mobility and transportation, an efficient power train technology contributes to sustainable lowering of CO₂ emissions in terms of reducing mechanical losses and saving fossil fuels. Due to the periodic or continuous operation in mixed and boundary friction, minimization of mechanical losses of frictional components in automobile gear box offers massive potential for target achievement. Recent developments in downsizing contribute to increasing demands on highly loaded rolling contacts in terms of friction reduction and load capacity. In close cooperation between Surface Engineering Institute (IOT) and Gear Research Center (FZG), the aim of this work was to reduce mechanical losses in power train by diamond-like carbon (DLC) coatings and chromium based nitride coatings on highly loaded gear wheels of automobile gear box under severe rolling conditions. The zirconium based DLC coatings ZrC_g (a:C-H/ZrC_g) and nanocomposite ZrC (a-C:H/ZrC) as well as the nitride hard coating (Cr,Al)N were deposited by physical vapor deposition (PVD) at IOT. Two industrial DLC coatings (a-C:H) served as reference. Tribological model tests of sliding contacts in pin-on-disc tribometer and investi­gations on the wetting behavior between PVD coated components and commercial gear oils revealed a correlation between wetting and frictional behavior. Application-related friction behavior of the coatings in lubricated rolling-sliding contacts was tested in twin-disc test rig at FZG. Calculations and measurements of average lubrication film thickness using uncoated discs verified operation in fluid friction regime. Despite complete separation of the coated surfaces, coefficient of friction was reduced by 35 % using ZrC_g coated discs compared to uncoated steel discs. Practical investigations of DLC coated gear wheels in FZG gear efficiency test rig confirmed the great potential for efficiency improve­ment. Besides mini­mizing mechanical losses in mixed and boundary friction, the DLC coatings contribute to significant friction reduction under elasto-hydrodynamic lubrication (EHL). Mechanical losses in EHL were reduced by up to 23 % using the industrial DLC coatings and 34 % using the DLC coating ZrC_g compared to uncoated gear wheels, especially at higher loads and higher circumferential speed. This yet widely unknown favorable effect of DLC coatings under EHL conditions can be attributed to the thermo physical properties of DLC coatings opening doors to new and enriching possibilities in tribology and efficiency improvement.

Increasing demands on the performance of intricate parts like bearings in biomedical and aerospace systems have created the necessity for improved coatings that can achieve enhanced performance in challenging environments. In this work, we have developed and applied a process to deposit wear resistant (WC/aC:H, CrN_x, TiC/aC) and solid lubricant (Ti-MoS₂) coatings onto spherical rolling elements. The coatings were deposited in a closed-field unbalanced magnetron sputtering system and their mechanical, compositional, and microstructural properties have been examined. The tribological performance of coated specimens has been evaluated under boundary layer lubrication in rolling and sliding contact tribometers, and the functional performance of the coatings on spherical rolling elements has been examined in full-scale thrust ball bearing testing.

In the automotive industry, electronic connectors guarantee most of the features of vehicles. The main requirement of materials used in electrical connectors is to allow low and stable electrical contact resistance (ECR). However due to engine vibrations and thermal fluctuations, fretting wear damages can be activated, decaying the electrical conductance endurance. One solution to increase the ECR endurance consists to apply noble coatings like silver and gold. In addition to small fretting oscillations, the connectors are subjected to large clipping/unclipping operations and this must also be considered to predict the ECR endurance.

Previous work has been realized to investigate this aspect, combining large reciprocating sliding (± 1000µm) and small fretting oscillations (± 9µm) on a silver coating interface (Fig.1). The study showed that the fretting electrical performance is highly influenced by the application of large reciprocating sliding. Indeed, the fretting endurance is multiplied by two compared to plain fretting situations (Fig.2). We demonstrated that periodic reciprocating sliding (i.e., clipping/unclipping), by removing the oxide debris layer trapped in fretting scar and transferring noble material from the reciprocating track to the fretting zone, can improve the electrical endurance of contacts.

The purpose of this research work is to quantify this aspect by investigating the influence of the reciprocating sliding amplitude (D) on the ECR performance. The studied interface involves in an homogeneous Ag-Ni / Ag-Ni (2 µm) contact subjected to constant fretting loading conditions (P=3N, f=30Hz, RH=10%, T=25°C and δ_g=±9µm) combined with the reciprocating slidings amplitude (D=± 250µm to ± 2000µm). The sliding sequence consists in a single reciprocating sliding imposed every N_B=10000 fretting cycles (Fig.1). The results showed a link between the reciprocating sliding amplitude (D) and the electrical failure (Nc). Indeed, the increase of the reciprocating amplitude (D) promotes a linear increase of the ECR endurance (Fig.3). SEM, EDX and 3D profiles suggest that the ECR degradation is due to a wear process of the silver layer. The application of large reciprocating sliding by promoting silver transfer from the reciprocating track to the fretting scar tends to increase the silver volume available in the fretting interface and therefore increases the ECR endurance. However, we demonstrated that if the reciprocating amplitude D is too small (D<D_th≈±500µm), close to the fretting sliding domain, the acceleration of wear process compensates the gain induced by silver transfer, and therefore promotes a reduction of ECR endurance (Fig.3).

In the present work we combined in-situ tribometry with atomistic simulations in order to improve our understanding of nanoscale interfacial processes of a tungsten-carbon-hydrogen tribo couple. Sliding induces severe changes of the material with respect to topography, composition and microstructure. All these effects can be summarized by the term "third body" formation [1]. Experiments were performed using a novel experimental platform for the on-line correlation of friction, wear and topography under lubricated sliding [1]. Then, in order to elucidate the atomistic level processes which contribute to the observed microstructural evolution in the experiments, classical molecular dynamics are performed employing a bond order potential for the Tungsten-Carbon-Hydrogen system. The combined experimental and simulation data allowed a look at the third body formation of tungsten and tungsten containing-coatings [3,4] under dry and lubricated conditions sliding against metals and diamond-like carbon coatings [5].

[1] M. Godet, Wear, 100 (1984) 437-452.

[2] S. Korres, M. Dienwiebel, Rev. Sci. Instrum., 81 (2010) 063904.

[3] P. Stoyanov, P.A. Romero, T.T. Järvi, L. Pastewka, M. Scherge, P. Stemmer, A. Fischer, M. Dienwiebel, M. Moseler, Tribol. Lett., 50 (2013) 67-80.

[4] P. Stoyanov, P. Stemmer, T.T. Järiv, R. Merz, P.A. Romero, M. Scherge, M. Kopnarski, M. Moseler, A. Fischer and M. Dienwiebel, ACS Appl. Mat. Int., 5 (2013) 6123-6135.

[5] P. Stoyanov, P. A. Romero, R. Merz, M. Kopnarski, M. Stricker, P. Stemmer, M. Dienwiebel,M. Moseler, Acta mater. 67 (2014) 395-408.

Natural gas is considered to be critical for our transportation needs in the 21st Century and beyond. Current economics make natural gas a feasible alternative fuel with lower emission, however there are some technical challenges that will arise not only in refueling the cars but also in the combustion chamber of an engine. This paper focuses on tribological challenges in refueling systems specifically in compressors. In order to fill vehicle’s tank quickly, compressors should increase the gas pressure from very low levels to 5000 psi levels, and they should also be reliable for more than 10,000 hours of operation with little or no maintenance. Use of oil mist in the gas may help solve some of the tribological challenges however oil has severe negative effects on the whole system. Accordingly, in this paper, we concentrate on the tribological performance of hydrogenated diamond like carbon (DLC) and nitride based coatings against PTFE type seal materials filled with different fillers (Carbon, MoS2, glass, etc.). A series of vanadium nitride – copper and –nickel composite and hydrogenated DLC films were prepared using high power impulse magnetron sputtering (HPIMS) system. The films were grown on 52100 steel substrates for tribological tests. X-Ray Diffraction (XRD), raman Spectroscopy, nano-indentation techniques were used to characterize the structural, mechanical and chemical nature of the resultant coatings. Bench-top tribological testing was done using a sealed vacuum tribometer under 900 torr methane gas pressure. Overall, the effect of temperature, contact pressure, type of PTFE fillers on the tribolocigal behavior of coatings were investigated. Both nitride and carbon based coatings improved the wear performance of the system depending on the type PTFE composite counter-face used. PTFE filled with abrasive fillers such as glass resulted in abrasive wear on DLC due to low hardness of the coating, however PTFE seal material with carbon based fillers performed very well when tested against the DLC coatings. At the end of each test, confocal raman was used to evaluate sliding interfaces and any structural changes resulting from the tribological tests to shed more insight inton the possible mechanisms responsible for such tribological failures and-improvements.

Continuous contact of hard materials leads to high friction between the two surfaces sliding against each other, and can result in high temperatures, scarring the life and quality of the parts. To provide low friction and improve wear resistance, solid lubricants have an edge over conventional choice. However from the recent studies it was attained that concentration of solid lubricant mixture is highly effecting the friction and wear characteristics of the material. Hence in the present investigation, an attempt has made to investigate the tribological properties of solid lubricant (MoS_2, graphite, boric acid) concentration (solid lubricant + SAE oil) in the fluid mixture were varied to weight ratio. Experiments are carried out on a ball-on-disc tribometer at constant load and varying speed conditions, where STAVAX ESR steel is used as a disc material, and WC as a ball material. Outcome of this study shows that the presence of solid lubricant film on disc specimen will greatly influence the sliding performance of ball material in lubricating and preventing wear through reduction in the friction and wear in sliding interaction.

The existence of oil additives can offer so me possible reduction in friction and wear of sliding contact in various applications. Solid lubricants are materials which despite being in the solid phase are able to reduce friction between two surfaces sliding against each other without the need for a liquid media. Layered solid lubricants like boric acid, molybdenum disulphide and graphite reduce friction by the mode of easy shear between their layers. In this context, the main objective of the current research work is to study the effectiveness and also to investigate and evaluate the friction and wear behaviour of MoS₂, boric acid, and graphite in depth at different sliding speeds. The results have been compared with dry sliding conditions. It has been revealed from the experiments that the friction coefficient reduces with increase in sliding speeds for the all the considered environments. Results also indicate the effectiveness of solid lubricant coated samples in terms of lowest frictional temperature compared to other conditions. The improved performance of solid lubricant coated samples can be attributed to the sliding resistance offered with predominant asperities interaction. Further, in comparison with all the other conditions molybdenum disulphide performed much superior results in terms of reduction in friction coefficient. This could be possibly due to higher temperature and shear force causing reorientation of lubricating layers. The results presented in this wok expected to form a scientific basis toward to select the best solid lubricant in various industrial applications for possible minimization of friction and wear.

Keywords: solid lubricants, MoS₂, graphite, boric acid, friction, wear