ICMCTF2016 Session E1-4: Friction, Wear, Lubrication Effects, and Modeling

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.

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.

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.

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.

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 R_a 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.

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.