ICMCTF2015 Session E1-1: Friction, Wear, Lubrication Effects, and Modeling

In this experimental study periodic line-like patterns with different periodicities (5, 9 and 18 µm) were fabricated by laser-interference patterning on stainless steel samples (AISI-304) and on 100 Cr 6 steel balls. Linearly reciprocating dry sliding tests with a ball-on-disk configuration were performed on patterned pairs and on patterned-substrate/unpatterned-ball pairs. Alignment effects were studied by means of a newly developed positioning method, which ensures an optimal alignment. The number of cycles was set to 200 and 20.000 to study the run-in and stability behaviour of the patterned surfaces, all of which exhibited a lower kinetic coefficient of friction than the unpatterned reference. Depending on the respective alignment, different friction and wear mechanisms can be distinguished. Differences in the real contact area, the possibility to store wear particles and the time needed to remove the native or laser-induced oxide layer can be well correlated with the temporal evolution of the coefficient of friction.

A finite element analysis was used in order to investigate the elastic contact of a coated sphere compressed by a rigid flat. Different coating and substrate geometrical and mechanical properties were analyzed to obtain a universal dimensionless model for the load–displacement relation. Both hard and soft coatings were considered. Dimensionless parameters, which control the behavior of the elastically loaded coated sphere, were identified. A proper normalization of the dimensional load and displacement was found resulting in a universal model. This model also provides a universal expression for the effective modulus of elasticity that is based only on mechanical properties of the coating and the substrate.

Load carrying properties of titanium nitride (TiN) and diamond-like carbon (DLC-7Zr) coatings were studied by scratch testing and by three-dimensional finite element modelling. TiN and DLC coatings were deposited on titanium substrates without and with high-pressure torsion (HPT) processing. The high-pressure torsion process decreased the grain size of titanium from 8.6 µm to 130 nm and increased the hardness from 1.83 GPa to 3.05 GPa.

The scratch tests were carried out with a spherical diamond tip sliding on a coated flat surface with increasing normal load. The contact was modelled with three-dimensional finite element method (3D FEM) and the generated stresses on and below the surface were simulated at the location where first principal cracks were formed . In the model, the diamond tip was considered as elastic and the coating behaved in an elastic-plastic manner.

The scratch test results revealed that both TiN and DLC coatings had a higher critical load on HTP-processed titanium compared to those on the titanium substrates without HTP processing. The increased load-carrying property of hard coatings on harder titanium substrates is related to generated stresses inside the substrate and its deformation behaviour. The harder substrate carries greater stresses compared to the softer substrate which experiences more elastic and plastic deformations. The deformation of the substrate generates strain between the coating and the substrate which is most notable in the perpendicular direction to the scratch direction.

Natural fiber reinforced plastics (NFRP) constitute a real challenge for academia and industry since they are becoming a viable alternative to synthetic fiber ones in many industrial applications which not require high structural performances. Machining of NFRP such as milling process is still practically an unavoidable operation to facilitate the parts assembly in addition to the finishing of final products. Unfortunately, the effect of tool surface, while had a significant impact on the tribological performance during machining had not yet studied.

This work aims to investigate tool coating effect on NFRP surface finish and tool wear during profile milling process with particular emphasis on shearing mechanisms efficiency. The cutting experiments were carried out on two NFRP specimens with treated and untreated flax fibers using three different cutting tools. Uncoated carbide monograin (MG₁₀), titanium diboride coated (TiB₂) and diamond coated (Dia) were used to conduct profile milling tests. Cutting forces, cutting temperatures, and tool wear were measured during each test. Interfaces states and surface topography of both the insert and NFRP surfaces after machining process were acquired by Scanning Electronic Microscope (SEM) and White Light Interferometry (WLI). Machined NFRP surface finish is characterized using a multi-scale analysis based on wavelets decomposition. The tool coating signature on the machined surface quality is hence identified at all scales from roughness to waviness.

Results demonstrate the tribological effect of contact stiffness on NFRP surface finish and interfaces behaviors after profile milling process.

Satellites and other moving mechanical assemblies in the space environment have numerous mission-critical sliding interfaces. Low ambient pressures, ultraviolet radiation, atomic oxygen and hot and cold thermal extremes are critical aspects of the low earth orbit (LEO) space environment that collude to make traditional lubrication strategies difficult. As such there is an applied need to develop materials for viable lubrication and test these materials in the actual space environment. This opportunity was presented with the Materials on the International Space Station Experiments (MISSE) program.

Eight pin-on-disk tribometers were delivered to the International Space Station in November 2009 to evaluate candidate space materials. They were exposed directly to the extreme conditions of Low Earth Orbit (LEO), including atomic oxygen, ultrahigh vacuum, radiation (including UV radiation), microgravity, micrometeoroids and thermal ranges from -40°C to 60°C. The experiments were returned from the space station to earth in May 2011 where postflight analysis began. As expected, these conditions proved extremely harsh on candidate space lubricant materials and coatings, including MoS2/Sb2O3/Au, MoS2/Sb2O3/C, YSZ/Au/MoS2/C, SiOx doped DLC, PTFE/alumina nanocomposite and gold. This research presents the effects of the LEO environment on the composition and tribological performance of resulting from the MISSE space tribometer experiments.

The contribution of the micro-scale roughness of the engineered surface tooth flanks, even though it is essential in the fundamental mechanics of gear contact, has not yet been well understood.

This paper covers the study of the simultaneous effect of micro-finish tooth flank surface and lubricant on vibratory and wear behavior of gears contacts. Different gears were hence manufactured with using three industrial finishing processes (shaving, grinding and power-honing) while having the same macro-scale characteristics. Then, experimental tests of the vibration performances of the gears were carried out on an instrumented single-stage low power rig in both dry and wet conditions using two lubricants viscosities. Furthermore, a new non-destructive sensory measurement technique was developed to characterize the interfacial thin layer of the tooth flank surface at various length scales from micro to macro wavelength. Results show the ability of this new method to rate the contribution of the tooth flank asperities during the meshing of the teeth on the gear in terms of induced vibration responses and wear performances.

Recent studies reveal that the liquid nitrogen (LN2) possesses a micro-hydrostatic lubrication effect owing to its lower friction and wear values when compared to dry environment. However, it is still uncertain whether the lubrication effect is responsible for reduction in friction coefficient or by its cooling effect at the contact interface. In order to ascertain the effect of LN2 as lubricant under tribological conditions, in this research work sliding wear tests are conducted on Al-TiC composite to characterize the friction coefficient, wear rate and heat transfer under a range of sliding velocities and loads. A customized setup has been proposed to supply the LN2 into sliding contact with a restricted flow rate and pressure for better cooling and lubrication. Comparative studies have been carried out between LN2, cryogenic compressed air cooling and dry environment. The results indicate that the presence of LN2 in sliding contact offers significant reduction in friction and wear values when compared to other environments. The positive results with LN2 could be due to the hydraulic pressure of trapped LN2 into the sliding contact which takes away a part of normal load resulted in reduced friction coefficient and wear rate.

In this work, the micro-scale abrasion wear resistance of CoB/Co₂B coating was evaluated by ball cratering test. The coating was formed on the surface of a CoCrMo alloy using the powder-pack boriding method. The boriding process was carried out at 1173 K during 6 h of treatment resulting in the formation of CoB/Co₂B phases with a total thickness of 23 µm, approximately. The hardness–depth profile was obtained by Berkovich depth-sensing indentation across the boride layer using a load of 50 mN. The wear rate in the boride phases on CoCrMo alloy was evaluated by a Plint TE-66 micro-scale abrasion tester using SiC particles in distilled water as abrasive slurry. Different duration tests were performed in order to analyze the wear evolution. The cobalt boride improved wear resistance of CoCrMo alloy. Furthermore, a wear-mode map was developed to identify the two and three body abrasion regimens (grooving and rolling) and the mix wear zone.