ICMCTF2016 Session E1-4: Friction, Wear, Lubrication Effects, and Modeling
Wednesday, April 27, 2016 1:30 PM in Room California
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E1-4-1 Tribochemical Aspects of Superlubricity on Hydrogen-free Amorphous Carbon and Diamond
Stefan Makowski, Frank Schaller, Volker Weihnacht, Gregor Englberger (Fraunhofer Institute for Material and Beam Technology IWS, Dresden, Germany); Michael Becker (Fraunhofer USA – Center for Coatings and Diamond Technology, East Lansing, MI, USA); Andreas Leson (Fraunhofer Institute for Material and Beam Technology IWS, Dresden, Germany)
Hydrogen-free tetrahedral amorphous carbon, also known as ta-C, is an excellent tribological coating for both lubricated and unlubricated conditions due to its superhardness and low reactivity with metal surfaces. Of particular interest are also superlow-friction properties that can be obtained with lubricants like glycerol and some fatty acid-based esters. Such low friction of µ < 0.01 is attributed to water-based shear bands (hydration lubrication) and has been found for several tribological systems for ta-C and diamond. Similar behavior is observed of oily lubricants like fatty acid esters on ta-C. Yet, the role of the carbon coating and its structure in the process is not fully understood.
We studied the tribological behavior of fatty acid-based lubricants on sp3-rich ta-C coatings and single crystalline diamond with regard to ultra-low friction and wear. We report on the observation of tribochemical processes influenced by the amorphicity of the carbon phase, lubricant chemistry, counter body material as well as technological aspects like surface preparation. Under certain conditions a tribochemical reaction occurs and a state of ultra-low friction is achieved. In some cases of low-friction increased wear of these superhard coatings can be found which is associated with unsaturated fatty acids.
In conclusion, both favorable and detrimental effects of tribochemical processes on ta-C coatings were identified, which can be influenced by some parameters of the tribosystem.
E1-4-2 Ultralow Friction of a-C:H Coating Lubricated with a Mesogenic Fluid
Tobias Amann, Bernhard Blug, Andreas Kailer, Stefan Schnakenberg, Natalie Oberle (Fraunhofer Institute for Mechanics of Materials IWM, Germany)
Diamond-like carbon coatings (DLC) are widely used in tribological applications due to their high hardness and chemical inertness. These material properties lead to a high wear resistance combined with low friction at specific tribological parameters. Especially in today’s big challenge to increase the energy efficiency and reliability of mechanically systems DLC-coatings are a promising technique.
Hydrogenated amorphous carbon coatings (a-C:H) are commonly used because of their low friction and wear resistance. Coatings based on a-C:H with a high hydrogen content are known to show ultralow friction in high vacuum or nitrogen atmosphere. Until recently only hydrogen free ta-C coatings showed ultralow friction in combination with certain lubricants (e.g. poly-alpha-olefin) and additives (e.g. glycerol mono-oleate). The low friction usually comes along with higher wear rates.
The aim of this investigation is to identify a suitable lubricant for an a-C:H coating to reduce friction at low wear rates in contrast to standard lubricants. Friction tests using a rotating ball-on-3-plates geometry were performed to analyze the tribological behavior of uncoated iron in relation to an a-C:H and Si-a-C:H coating. Therefore fully additivated commercial available lubricants and an additive free special mesogenic fluid were used. This study shows for the first time the possibility to realize ultralow friction of an a-C:H coating under lubricated conditions. Stribeck curves with the mesogenic fluid with the material combination 100Cr6(ball)- a-C:H(plates) reached ultralow friction values (µ ≈ 0.005) from 1.0 m/s sliding velocity. In contrast using the standard lubricant a coefficient of friction of µ ≈ 0.07 was achieved. But not only the friction value was improved, also lower wear was measured with the mesogenic fluid.
The results of this study demonstrate that mesogenic fluids are promising lubricants to realize ultralow friction with a-C:H coatings. Comparative friction and wear measurements were analyzed and the formation of a surface active tribofilm and the influence of the polarity are discussed.
E1-4-3 Molecular Dynamics Study of Triboinitiated Chemistry and Graphene Coatings on Iron Surfaces
David Schall (Oakland University, USA)
Tribosystems containing both iron and hydrocarbon-based lubricants are ubiquitous and an understanding of the chemistry that takes place in such systems is essential. In this study, the triboinitiated chemistry of alkane and alkene lubricants and iron surfaces will be presented using molecular dynamics simulation and a REAX-FF interatomic potential function for Fe, O, C and H (Zou, JOM, 64, 2012, 1426). The REAX-FF potential includes terms for chemical reactivity with charge transfer which enables investigation of tribochemistry in the sliding interface. The carbons surrounding double bonds in unsaturated hydrocarbon molecules have been show to form a metastable bond bridging between adjacent Fe atoms on surfaces. When conditions are right, mechanical interactions between the two iron surfaces in the interface can be enough to sever carbon bonds in the lubricant leading to reduced molecular weight and the formation of free radicals. Such a mechanism could be a mechanism for sludge and varnish formation in engine components, a very costly problem for the automotive industry. In straight chain alkanes, no surface chemical reactivity was observed. Recent experiments have shown that even single layers of graphene on steel components can greatly reduce wear (Berman, Carbon, 54, 2013, 454). Berman et al have hypothesized that graphene forms a conformal protective layer on the steel surface with or without additional lubrication. In this talk molecular simulations of graphene on iron surfaces with and without additional lubricants will be presented and examined with respect to triboinitated mechanochemistry.
E1-4-5 Numerical Modelling of Thermal Fatigue Damage in Coatings with Heterogeneous Microstructure
Newton Fukumasu, Izabel Machado (University of São Paulo, Brazil); Mário Boccalini Jr. (Institute for Technological Research, Brazil); Roberto Souza (University of São Paulo, Brazil)
Thermal fatigue is a phenomenon observed in many mechanical applications, ranging from power generation to manufacturing processes. For coated components, thermal fatigue cycles may induce regular patterns of cracks, which may lead to the spallation of the coating and damage of the overall wear behavior of the system. Among several possibilities of studying thermal fatigue resistance, this work is focused on the presence of second-phase particles inside a monolayer coating, such that the coating is not homogeneous at the microstructural scale. Hard precipitates may improve the tribological behavior in terms of abrasive wear, as well as result in distinct patterns of thermal fatigue cracks. Soft precipitates may induce residual stress relief, auto-healing characteristics or lubricant properties. In this work, the Finite Element Method (FEM) was applied to simulate cohesive and adhesive failures of the coating submitted to a finite number of thermal cycles. Parametric mechanical properties of the particles were analyzed (Young’s modulus, yield stress, fracture toughness) while keeping constant both coating and substrate material properties. Results allowed the evaluation of the influence of second-phase particles on crack density and cyclic crack propagation in the coating during the fatigue process.
E1-4-6 High Temperature Tribology of Plasma Nitrided T91 Alloy Steel
Hadiansyah Kausar (Bandung Institute of Technology, Indonesia); Ladislav Cvrcek (Czech Technical University in Prague, Czech Republic); Husaini Ardy (Bandung Institute of Technology, Indonesia)
The ferritic steel (T91) are often used as material for super heater tubes in thermal power plants. Surface of the tubes is during combustion at temperature around 750 ºC exposed to erosion and wear which are caused by the presence of abrasive particles, e.g. quartz and FeS2 from coal fuel or Al2O3 from mineral constituents in the fly ash. Therefore the study of wear resistance at high temperature is one of the essential criteria to determine the life of component made from T91 alloy steel.
The research was focused on the effect of elevated temperature on tribology properties. Steel was tested in as received condition, and after plasma nitriding. The testing samples were cut out from the original tube commonly used in power plants. The experiments were conducted using high temperature pin-on-disc tribometer. Each sample was tested at a constant linear speed and at constant load. Testing temperature was varied from 25 °C to 750 ºC. Plasma nitrided samples were produced using industrial system. The microstructure of the samples was examined using light microscopy and electron microscopy equipped with EDS and WDS. The profile of wear tracks and surface roughness were analysed by optical profilometer. The hardness of the samples was measured by micro and nano hardness tester.
The microstructure of the tested specimens was not changed before and after tribology tests. Base material specimens have the highest coefficient of friction and wear rate at the room temperature. As temperature increases, the formation of the oxide layer reduces the wear rate and friction of the specimens. Plasma nitrided sample which had high hardness at room temperature shows that the nitrided layer reduced wear rate significantly at room temperature, but this effect decreases with increasing temperature testing. To understand this behaviour was analysed chemical composition from cross-section of nitrided layer depending on the increasing temperature. Changes of the tribological behaviour for nitrided steel at temperatures above 400 ºC reflected changes of the nitrogen distribution in the nitrided layer and changes in the formation of surface oxide layer and its roughness.
E1-4-7 Modeling Intrinsic Residual Stresses Built-up during Growth of Nanostructured Multilayer NbN/CrN Coatings
Juliano Araujo (University of Sao Paulo, Brazil); Rafael Giorjao, Jefferson Bettini (CNPEM, Brazilian National Center for Research on Energy and Materials, Brazil); Roberto Souza, André Tschiptschin (University of São Paulo, Brazil)
15 µm thick nanostructured multilayer NbN/CrN coatings with different periodicities have been deposited onto martensitic stainless steel, in an industrial-size cathodic arc, physical vapor deposition (CAPVD) chamber. Three rectangular cathodes (two Cr and one Nb) in alternate positions, working independently were fed with its own power supplies. The multilayer periodicities (≤ 20 nm) were controlled by varying the rotating speed of the table in the center of the chamber and the time the specimens pass in front of each Nb or Cr target . The obtained coatings were characterized by high resolution transmission electron microscopy (HRTEM), selected area diffraction (SAD) and electron energy loss spectroscopy (EELS) in order to assess the crystal structure, epitaxy, degree of coherency of the NbN/CrN interface and Cr and Nb cross contamination during deposition.
The results showed that a uniform NbN/CrN nanostructured coating formed along the entire coating thickness, constituted by alternate individual layers of NbN and CrN and separated by a highly coherent interface. Compositional variations measured across the multilayers showed that the higher the rotating speed of the table, the greater is the cross-contamination built-up in the NbN and CrN individual layers. Based on these results, a finite element analysis (FEA) model was developed for assessing intrinsic residual stresses for the coatings with different periodicities, based on the assumption that differences in lattice parameters and lattice strains are proportional to chemical composition inside individual layers. Modeled intrinsic residual stress profiles show stress gradients developed from the center of individual layers towards the interface, differently from the results published in literature, obtained using different assumptions in terms of stress constraints.
E1-4-8 Topographical Orientation Effects on Friction and Wear in Sliding DLC and Steel Gear Contacts
Timo Hakala, Anssi Laukkanen, Kenneth Holmberg, Helena Ronkainen (VTT Technical Research Centre of Finland Ltd, Finland); Gwidon Stachowiak, Pawel Podsiadlo, Marcin Wolski (Curtin University, Australia); Mark Gee (National Physical Laboratory, UK); Carsten Gachot (Saarland University, Germany); Lawrence Li (Hong Kong City University, China)
The effect of surface roughness and topographical orientation of diamond like carbon (DLC) coated surfaces on friction and wear has been investigated. Three levels of surface roughness in the range of 0.004 – 0.11 μm Ra value and topographical orientations at 0°, 45° and 90° angles from grinding marks were used. The surface topography was characterized by the fractal, the texture aspect ratio and the texture direction signatures calculated using the variance orientation transform method. A finite element (FE) model including the signatures was developed and used to study the effect of roughness and orientation in a single scratch test and a gear contact. The study showed that the topographical orientation affects considerably friction and wear in DLC vs DLC contacts. For the gear contact it was found that contact stresses and strains calculated, where the first cracks are expected to occur, highly depend on sliding direction and local surface topography.
The modelling results explain why the coatings with lower surface roughness have better wear resistance in the gear applications, considering the location and propensity of wear damage initiation at different scales of roughness topographies. The initiation of surface damage in gear contacts is quantified for surface design purposes by introducing a wear resistance FE model. The FE model can be applied to estimate the time the gear can operate before initiation of damage becomes likely for different surface loadings and topographies. Links to other characteristics, such as DLC coating and substrate properties as well as coating defect structure are also discussed.
E1-4-9 Crumpled Graphene Balls for Efficient Lubrication
Xuan Dou, Andrew Koltonow, Xingliang He (Northwestern University, USA); Hee-Dong Jang (Korea Institute of Geoscience & Mineral Resources, Korea); Qian Wang, Yip-Wah Chung, Jiaxing Huang (Northwestern University, USA)
Ultrafine particles are widely used as lubricant oil additives since they can infiltrate and separate tribological contacts, thereby reducing the friction and wear between surfaces. They are more thermally and mechanically stable compared to molecular additives under high friction, and the good dispersibility of ultrafine particles without molecular ligands is highly desirable. Based on our previous report, ultrafine particles with miniaturized crumpled structure should self-disperse in lubricant oil, just like how crumpled paper balls do not readily stick to each other. Meanwhile, they could reduce friction and wear like miniature ball bearing. Here we report the use of crumpled graphene balls as a high-performance additive that can significantly improve the lubrication properties of polyalphaolefin base oil. In friction test, crumpled graphene’s tribological performance excels that of other carbon additives including graphite, reduced graphene oxide, and carbon black. Notably, polyalphaolefin base oil with only 0.01–0.1 wt. % of crumpled graphene balls outperforms a fully formulated commercial lubricant in terms of friction and wear reduction.