ICMCTF2013 Session E1-2: Friction, Wear, and Lubrication; Effects & Modeling

The significant development in the mechanical industry requires a constant evolution of skills, particularly in the area of functionalization and protection of metallic alloys surface. The wear of materials is one of the causes of loss of profitability. This study aims to develop solutions to extend the lifetime of stainless steel 316L or 304L whose raw materials are relatively expensive . The main objective in this study is to increase their durabilility by covering these stainless steels with a protecting thin ceramic film. Thanks to their high hardness values, ceramic coatings would present the best properties to fight against wear. Apart from the conventional deposition way such as PVD or CVD, the sol-gel method allows many choices of formulations avoiding a thermal treatment step at very high temperature.

In this work, we have achieved to optimize protective coatings of stainless steel against wear, using sol-gel method associated with dip-coating technique. We have chosen the silica and alumina compounds which are well known as good candidates to tackle this problem because of their intrinsic properties [1,2]. The combination of both oxides is a very promising way for that kind of application even if the literature is not very abundant on these topics. Several synthesis pathways have been investigated to reach this objective. We propose three new routes to achieve this type of coating. First a mixture of sols precursors of both oxides, second with the previous mixture with the incorporation of suspended particles of silica or alumina and the third one corresponding to a single sol precursor of silica or alumina loaded with oxide particles (silica and/or alumina).

This article investigates the various ratios alumina / silica. Processing parameters such as: i) the speed of withdrawal (that controls the coating thickness) and ii) the temperature of the thermal treatment after deposition, have been studied. Structural properties were evaluated by XRD, NMR and Raman. Tribological behavior of the coating was studied in order to select the suitable composition for the targeted application. The friction and the wear between a couple of materials were investigated using a pin on disk standardized test, ASTM G-99.

[1] W. Zhang, Characterization and tribological investigation of SiO2 and La2O3 sol–gel films, Applied Surface Science. 185 (2001) 34-43.

[2] T. Hübert, J. Schwarz, B. Oertel, Sol-gel alumina coatings on stainless steel for wear protection, Journal of Sol-Gel Science and Technology. 38 (2006) 179-184.

Surfaces that provide low friction as well as mechanical stability are desired in many technical applications. As these two requirements are usually fulfilled by changing material properties in opposite directions, a promising route is to design mechanically stable coatings that are tribochemically active, meaning that they have the ability to become modified by the tribological contact in a way that lowers the friction. A well-known example is nanocomposite coatings such as TiC/a-C, from which carbon can be graphitized to create a lubricating surface layer. Another class of materials is the transition metal dichalcogenides (TMDs), such as WS₂ and MoS₂. When subjected to the pressure and shear in a tribological contact, the TMD planes become horizontally aligned and thereby enable easy sliding. In the current work, the concept of tribochemically active coatings is applied in a new way by doping TiC/a-C coatings with sulfur, which can be released in the tribological contact to form low-friction compounds.

Coatings were deposited by magnetron sputtering from titanium and carbon targets, with incorporation of sulfur by reactive sputtering using H₂S gas. It was found that sulfur goes into the carbide phase, creating a doped TiC_xS_y phase in a matrix of amorphous carbon. The incorporation of sulfur leads to a lattice expansion of more than 10%. The possibility of release of sulfur was investigated by annealing, which lead to sulfur removal and lattice contraction.

First-principles calculations using supercells and the exact muffin-tin orbitals method were performed for TiC_xS_y, and the calculated lattice expansions were found to match the experimental ones. Furthermore, the density of states and charge density for TiC and TiC_xS_y were calculated, showing that the incorporation of sulfur leads to filling of antibonding states and that the sulfur atoms occupy larger volumes than carbon atoms.

Tribological testing against steel balls in an unlubricated pin-on-disk setup showed that increasing sulfur content lead to lower friction and longer lifetimes, an improvement that can be due to changes in microstructure as well as chemistry. However, one of the S-rich coatings was also tested against a W-coated steel ball, leading to a distinct friction reduction (to µ≈0.05). Transmission electron microscopy in the wear track confirmed the local formation of horizontally aligned WS₂. It has thus been shown that nanocomposite coatings containing the newly discovered TiC_xS_y phase are tribochemically active, and offer new possibilities of designing self-lubricating material systems.

In tribological one of the main environment parameters that affect the behavior of the surfaces is the temperature since this can cause surfaces to undergo changes in their mechanical and structural properties. At relative high temperatures, the oxidation layer can affect the tribological characteristics because this is the contact between two surfaces. The oxide layer can reduce or increase the wear rate and the friction coefficient because of a reduction in the contact or the generation of debris. Boriding is a technique used to increase the hardness and the wear resistance of the surface, particularly at high temperatures.

This work presents the results of a study of the tribological performance of a borided layer on the stainless steel AISI 410 at different temperatures under sliding motion wear testing. The borided layer was generated by the package boriding method at 950 °C for 1 hour. X-ray diffraction showed the presence of crystalline CrB, FeB and Fe₂B phases. The thickness of the boride layer was 11.9 ± 0.86 μm and the hardness was 24 GPa with a roughness of 120 nm. The sliding tests were performed on a pin on disk tester using an alumina ball counter-body (Al₂O₃), with an applied load of 1N. The friction coefficient was seen to increase by approximately by 29% with increasing the temperature from Room temperature to 450 °C, with the wear rate showing a similar behavior.

Ti-4Al-4Mo-2Sn is a high strength, forgeable alpha-beta alloy with good high temperature mechanical properties. This alloy has many potential applications in the aerospace industry, which is currently dominated by the widespread use of Ti-6Al-4V. It is increasingly understood that surface engineering treatments need to be designed for the specific substrate material to be treated (this being particularly important for Ti-alloys) and in this respect no study has yet been carried out for the improvement of the tribological properties of Ti-4Al-4Mo-2Sn by surface engineering means. Triode-plasma oxygen and/or nitrogen diffusion treatments have recently been shown to be extremely effective in reducing surface damage and increasing the wear life of several Ti alloys subjected to dry sliding, abrasion and impact wear.

This work focuses on the development of cost-effective enhanced-plasma processing techniques, which will permit the use of Ti-4Al-4Mo-2Sn in tribological applications – even under relatively high loading. The use of low pressure intensified plasmas (at various temperatures in the range of 600-800 ^oC) is shown to allow the diffusion of (for example) nitrogen, to generate deep hardened layers. The process parameters for the surface modification technique are selected such as to minimize degradation of mechanical properties and corrosion resistance in the Ti- alloy. The treated specimens were tested and characterized using a variety of techniques, including X-ray diffraction, electrochemical corrosion testing, optical and scanning electron microscopy, nanoindentation, and nanoscratch testing. Furthermore, the effects on the bulk substrate material were also studied by uniaxial tensile and fatigue testing. The results obtained indicate that the proposed technique is capable of significantly improving the tribological characteristics of the chosen Ti alloy without detriment to the substrate microstructure and mechanical properties. The treated surfaces demonstrate improved corrosion behaviour when compared to untreated control samples.

In this work three commercial coatings were tested in a pin on disc setup against UNS S32750super duplex stainless steel. In the first part of this project, adhesion and nano indentation measurements were performed in PVD coated cemented carbide substrates. The pin on disc tests were conducted in a CSM tribometer. The tribological pair consisted of PVD coated cemented carbide and super duplex stainless steel. The adopted load and speed, in the pin on disc tests, were 20N and 20 cm/s, respectively. The pin on disc test was conducted with and without lubrication. The results show that the friction coefficient and wear rate is linked to the coating chemical composition. Moreover, the silicon based coating show the lowest interaction with the stainless steel ball resulting in a low wear rate.

The purpose of this work is to study the influence of the normal force (N) and abrasive slurry concentration (C) on the coefficient of friction of thin films in micro-abrasive wear tests by rotative ball. Initially, a micro-abrasive wear testing by free rotative ball equipment was designed and constructed, able to measure the coefficient of friction on the tribo-system "thin-film – abrasive slurry – ball". After, experiments were conducted with thin films of TiN, TiC, CrN, TiAlN, HfN, ZrN, TiZrN, TiN/TiAl (multi-layer), TiHfC and TiHfCN, balls of AISI 52100 steel and abrasive slurries prepared with green silicon carbide (SiC) particles + distilled water. All tests were conducted without interruption, and the abrasive slurry was continuously agitated and fed between the ball and specimen. The tangential (T) and normal forces were monitored throughout the tests and the coefficient of friction (μ) was calculated by the equation μ = T/N. The results obtained have shown that:

i) The hardness of the thin films presented influence on the coefficient of friction: high hardness was related with low coefficient of friction;

ii) The coefficient of friction behavior and magnitude were independent of the normal force: for the same thin film and different values of normal force, the coefficient of friction remained, approximately, unchanged;

iii) The concentrations of abrasive slurries affected the magnitude of the coefficient of friction: high abrasive slurry concentration was related with low coefficient of friction.