ICMCTF2014 Session E1-2: Friction, Wear, and Lubrication: Effects and Modeling

Fretting wear is a complex phenomenon related to interactions between two sliding bodies under a low displacement amplitude. It can often occur in joints, oscillating splines, coupling, bearings… At the contact areas, liquid lubrication can be easily squeezed out, resulting in metal-to-metal contact and consequently an important local wear. To prevent from wear damage, therefore, many efforts have been done. Solid lubrication has been proven as an effective way to protect the surface of materials and to prolong the service lifetime of elements. Previous studies focused on the tribological behavior of various solid lubricants on the same substrate and then the best functional solid lubricant can be selected. In this study, in contrast, the research is performed on the effect of different substrates on the tribological behavior of a solid lubricant, with consideration of contact pressure, displacement amplitude, contact configuration, coating position (coating on flat substrate, coating on ball or cylinder substrate or coating on both substrates), and thickness of coating. The purpose of this study is to investigate the effect of different substrate structures on the lifetime and friction coefficient of a coating and to assess its importance when compared with the importance of other test conditions, with the use of statistical methods.

An aerosol sprayed MoS₂-based varnish was deposited on a cleaned and polished 304 stainless steel or AISI M2 flat surface and AISI52100 ball or cylinder surface at room temperature. The thickness of the coating was about 10 µm or 20 µm. The normal force was varied from 100 N to 700 N and the displacement amplitude was from ± 10 µm to ± 40 µm.

Some results could be drawn from this study. Firstly, the structure of the substrate does not affect the friction coefficient of the coating but it could change the coating lifetime greatly. The porosity in the substrate can greatly increase the lifetime of the coating by about a factor three. This is because the porosity in the substrate can increase the adhesion force between the coating and the substrate. The effect of structure of substrate on coating lifetime can be ranked in the first place, followed by the effect of contact configuration and displacement amplitude. Secondly, the friction coefficient is mainly dependent on the contact pressure which is mainly determined by both contact configuration and contact force. The friction coefficient is inversely proportional to the contact pressure. Thirdly, the thickness of the coating has no significant effect on the friction coefficient and lifetime of the coating.

Hence in this paper we propose novel thermal spray lubricious oxide coatings based on a crystal chemical approach as proposed by Ali Erdemir. The crystal-chemical approach can be used to predict the extent of adhesive interactions between two or more oxides at a sliding interface; hence, it can be used to predict frictional performance. Different combinations of the oxide materials are chosen based on their ionic potential differences and plasma sprayed to a thickness of 150 to 200 microns. Their composition, microstructure and room temperature and high temperature wear characteristics are reported in this paper.

Among the various compounds of the transition metal dichalcogenides (TMD) family, MoS₂ is the most known member. Being used as a solid lubricant for several decades, it has been intensively studied both theoretically and experimentally. Nevertheless, there are still many unclear points regarding the lubrication mechanism.

Classical simulations have the potential of providing decisive insights into the tribological behavior of MoS₂. The key point of this kind of calculations is the particles interaction potential (also called force field, FF). In fact, the reliability of the results depends in large part on the FF ability to reproduce interactions with sufficient accuracy. This calls both for a carefully choice of the particular functional form of the FF and for an optimization of the parameters that comes into play in the FF.

The force matching (FM) approach[1] is a powerful technique to develop new FFs. Briefly, it consists of a least-squares fitting of the FF potentials to positions/forces data obtained from reference calculations at a higher level of description using sophisticated ab initio methods. Given these data as input, one optimizes the value of FF parameters minimizing the difference between reference and classical forces. In this way it is possible to parameterize FFs that not only allows to carry out classical molecular dynamics simulations (with reduced computational costs), but also permits to reach a high level of accuracy comparable with the one of the reference simulations.

In this contribution, we present the novel FF for MoS₂ developed using the FM approach and based on many-body interaction potentials. We then use the FF for simulating a sliding of TMD crystals, allowing us to understand the microscopic events that take place during the dynamical process. Moreover, this knowledge could lead to get insights into the tunability of factors involved in the tribological behavior.

[1] F. Ercolessi and J. B. Adams, Europhys. Lett., 26, 583 (1994).

When macroscopic bodies are pressed together, it is widely acknowledged that contact only occurs at localized spots between surface asperities. Friction thus involves the shearing of a myriad of micro-contacts which are distributed over length scales ranging from micrometers to nanometers. Although widely debated, the manner in which these micro-contacts locally dissipate energy remains obscure. In order to get more insights into this widely debated problem, spatially resolved measurements of frictional stresses are much needed. Unfortunately, most experiments only rely on measurements of friction force and of its dependence on load and velocity which are averaged quantities of local frictional properties. Then, validation of the local friction law and a fortiori of the related models remains rather indirect. In order to overcome these limitations, we have developped a method to measure local friction of rubbers by means of a contact imaging approach. Silicon rubber substrates marked on their surface are prepared in order to measure the lateral displacement field induced by the steady state friction of a glass lens. Then, deconvolution of this displacement field provides a spatially resolved measurement of the actual shear stress distribution at the contact interface. Using this technique, we will discuss how local friction of rubbers with randomly rough rigid surfaces depends on the details of surface topography. Some new developement with patterned surfaces will also be presented where crack like motions at the interface are evidenced during stick-slip motions.

(Ti, Al, Si)N nanocomposite coatings have been extensively studied due to their good tribological performances. The application of (Ti, Al, Si)N coatings on cutting tools has dramatically extended the tool lifetime and enables higher speed machining. However, the tribological behavior of a coating system is determined by both the coating and substrate. Generally, the superhard coatings have a high Young's modulus yielding the elastic mismatch between the coating and substrate. This elastic mismatch is possible to induce an early delamination or substrate plastic flow in the coating system and weakens the tribology behavior.

In this paper, the (Ti, Al, Si)N superhard coatings as well as traditional TiN and (Ti, Al)N coatings were deposited on high speed steel (HSS) and WC-Co by sputtering. The friction measurements were carried out to determine the failure load and stress of different coating systems. The cross-sections of the wear scars were prepared by focused ion beam (FIB) microscopy to analyze the failure modes in the friction tests. The electron back-scattering diffraction (EBSD) were used to determine the plastic flow underneath the wear scar. It was found that the failure mechanism of a superhard coating system is mainly due to the substrate yielding and interfacial shear. The large elastic mismatch between coating and substrate can weakens the tribology behavior of a coating system even though the coating is superhard.

The TiSiN coatings were deposited using Pulsed Laser Deposition such that the samples had a range of silicon contents. The films were characterized by SEM-EDS (chemical composition and surface morphology), X-ray diffraction (crystalline structure and grain size) and Nanoindentation (hardness and Young’s modulus). Other tribological properties of the coatings were evaluated using scratch testing, this was carried out without causing severe cracking or total spallation of the coatings, using two counter materials (1/16” balls of 100CR6 and Al2O3). To study the plastic deformation caused by the application of the load during the scratch measurements we used 3d profilometry. Finally, micro-Raman spectroscopy was employed to study any deformation-induced morphology changes and the chemical reactions produced, under the different loads, by the contact with the counterpart materials. The results showed that the silicon content increased the mechanical properties of the coatings but the effect was greatest around to 8 at Si%, with this resulting in a hardness of 35Gpa. The Raman spectroscopy after the scratch test showed that the load produced a lattice deformation in the crystalline structure of the coatings and that the formation of titanium oxide was dependent on the applied load and the counterpart material.

Hydrophobically functional coatings can be used to protect surfaces and therefore improve the performance and lifetime of a broad range of applications such as optoelectronics and touchscreens. Organic-inorganic hybrid materials such as silica sol-gel coatings are particularly effective and allow for advantageous low-temperature processing and substrate compatibility. However, the functional molecules in these coatings are susceptible to abrasive wear and thus lose their performance over time in the harsh environments typically encountered. Moreover, the matrix of the coating itself is inherently porous and susceptible to wear.

To combat these problems, a silica nanoparticle reinforced matrix was developed to increase hardness and wear resistance of the overall coating. This study involved the abrasive wear analysis of fluorinated composite silica particle reinforced sol-gel silica coatings dip-coated on glass substrates. Varying amounts of silica nanoparticles from 0.5 to 10 wt% of the precursor weight were added to examine the structural dependence of abrasive wear mechanisms to elucidate strengthening mechanisms that could lead to improvements of coating properties. Abrasion was conducted using an in-house built reciprocating polishing wear apparatus. Characterization of the water contact angle of the coating was conducted to determine the hydrophobic functionality after wear cycles. Atomic force microscopy, lateral force microscopy, nanoindentation, nanoscratch, contact angle goniometry, XPS, SEM, and optical microscopy were performed at intervals of abrasive wear testing to characterize these wear mechanisms and the functional degradation of the coating.