The understanding of surface phenomena is a big challenge due to the presence of concomitant effects. In order to control and reduce the friction coefficient of two surfaces in relative motion, several mechanisms and theories have been developed. The macroscopic properties influencing friction such as hardness, elastic modulus and shear stress are not obviously correlated to fundamental properties of materials such as bonding energy, phonons and electrical (thermal) conductivity. Furthermore, few studies were conducted with the aim of correlating the nanoscale friction and the surface chemical structure/composition.

In this work, we have focused on the friction behavior between a conical diamond tip sliding over different modified flat steel and iron surfaces at indentation depths from 20 to 200 nm, relating the influence of the chemical bonds at the outermost layers to the sliding resistance. The surface was also characterized by means of glow discharge - emission optical spectroscopy (GD-OES), scanning electron microscopy (FEG-SEM), glancing angle X-ray diffraction (GA-XRD) and atom probe tomography (APT). Experimentally, the friction coefficient decreases when nitrogen atoms are substitutedby oxygen in the iron network of AISI 1045 steel. Theoretically, this effect is expected due to energy dissipation mechanisms through phonons. According to phononic friction mechanisms, the vibrational frequency mode, single or collective, of atoms that constitute the material influences strongly the friction coefficient of the system. However, there are different theoretical phononic models with different levels of accuracy when compared to the experimental results. In order to analyze which is the theoretical model with the best approximation to the experimental results, we have applied five phononic models.

Our latest results are showing that the friction coefficient is only modified when an oxide layer (pure magnetite) is grown in AISI 1045 steel and it is independent of the oxide layer thickness. Moreover, pure iron samples were nitrided and post-oxidized. Surprisingly, the friction coefficient does not change with the presence of magnetite (oxide layer). The APT analysis showed a preferential oxygen diffusion perpendicular to the surface and preferentially at the grain boundaries, which leads to form oxynitrided structures instead of an oxide-nitride bilayer system. This information may open new pathways to increase the accuracy of theoretical models including the morphology and growth mechanisms of the outermost layers that prompt nonconservative forces.

In nuclear power plants, tubes of the rod cluster control assemblies undergo impacts at low contact pressures against guides, which lead to a specific wear on the contact surfaces. Fretting wear and impact-fretting wear behaviours of nuclear power materials were investigated in this study. An original double impact-fretting system allowing independent control of impact energy and fretting sliding, was developed to investigate a nitrided 316L SS rod / 304L SS plate contact (Figure 1).

First, plain fretting-wear tests were conducted in air under different displacement amplitudes ranged from 20 to 80 µm, different normal loads from 10 to 50 N and different temperatures from 20 to 320 °C, in order to evaluate how a phase transformation of the 304L SS alloy induced by temperature can affect wear.

On the other hand, the load ratio (defined by R_p=P_min/P_max), and the impact ratio (IR, number of impacts during on fretting cycle) were investigated in air and in a solution composed of 1 000 ppm of bore and 130 ppm of lithium reproducing nuclear environment at atmospheric pressure and room temperature. The normal load ratio was decreased until that impact-fretting condition was reached and various impact ratios were applied to evaluate how the impact loading can increase the fretting wear rate (Figure 2).

Surface damage evolution was followed by 3D profilometry and several analyses (SEM, µ-hardness) were conducted on the surface and on cross-sections. Based on these results, fretting-wear and impact-fretting-wear mechanisms of the 304L SS plate and nitrided 316L SS tube were investigated. First results showed that worn surfaces were smoother in solution than in air: due to water acting as lubricant, the wear particles were ejected resulting in a “U” shape instead of a “W” shape in air [1]. Thus, wear volume was more important during impact-fretting test than during simple reciprocating fretting-test, due to the accumulation of both wear mechanisms [2][3]. A modified energy wear approach is implemented to quantify the impact-fretting wear rate depending on the ambient conditions.

References

[1] X. Mi and Al.“Investigation of fretting wear behavior of Inconel 690 alloy in tube/plate contact configuration,” Wear, vol. 328–329, pp. 582–590, 2015

[2] Y. Sato, a. Iwabuchi, M. Uchidate, and H. Yashiro, “Dynamic corrosion properties of impact–fretting wear in high-temperature pure water,” Wear, pp. 1–11, 2014

[3] D. Kaczorowski, J. J. M. Georges, S. Bec, A. Tonck, A. B. Vannes, and J. P. Vernot, “Wear of a stainless steel in pressurised high temperature water,” Comptes Rendus l’Academie des Sci. - Ser. IV Physics, Astrophys., vol. 2, no. 01, pp. 739–747, 2001

Wear resistant lubricious coatings based such as DLC are being used increasingly in high technology applications in many industrial sectors. The effective design and processing of these coatings increasingly depends on developing an understanding of their behaviour in tribological contacts that can be used as a basis for predicting their performance in applications.

This paper describes the results of in situ microtribology experiments carried out on a range of steel samples prepared with 3 different topographies, and both uncoated and coated with DLC. The in situ microtribometer was used to carry out multi-pass experiments in the SEM that gave continuous high resolution observation of the wear scar that was developed. Experiments were carried out where the passes were made in the same position on the sample to give information on the build-up of damage when the same area was subjected to repeated stressing by the test probe, and also where random positions for the passes were selected giving a good simulation of abrasion.

The images and videos that have been obtained show details of the formation of the wear damage and the generation of debris, and also show the clear reduction of damage for the coated samples in comparison with the uncoated samples.

Friction and wear mitigation is typically accomplished by introducing a shear accommodating layer (e.g., a thin film of liquid) between surfaces in sliding and/or rolling contacts. When the operating conditions are too extreme for the liquid realm, attention turns to solid coatings and engineered surfaces. The focus of this talk is an overview of how contacting solid surfaces change structurally at the atomic and nanoscales in order to accommodate interfacial shear. In particular, evidence of activating multiple, parallel slip planes, the so-called ‘deck-of-cards’ type shear, will be shown for many material classes including deformation hardened Co-Cr alloys, nanocrystalline ZnO coatings, and nanocomposite amorphous and crystalline MoS₂ and WS₂-based solid lubricant coatings. While the structure of the mating surfaces is paramount in controlling the friction behavior, high resolution microscopy also reveals that multiple, parallel slip planes are active due to the subsurface shear stresses generated during sliding and rolling. In addition, the importance of defective structure will be shown for both metallic alloys and ceramics that exhibit low stacking fault energies and single slip systems in order to achieve low friction surfaces. Obtaining these low interfacial shear stress surfaces along with load supporting underlayers that minimize the contact area, collectively support the ‘Bowden and Tabor’ concept for achieving long life tribological contacts.

The objective of this study is to investigate the effect of heat treatment routes involving carburizing and quenching techniques on the surface mechanical properties and tribological behavior of carburized AISI 8620 steel. This particular material is commonly used in drivetrain components and undergoes sliding or rolling under high contact pressure conditions. In this study, carburizing was carried out at carbon potentials ranging from 0.45% to 1.05 % while maintaining the same carburizing temperature, followed by different end quenching routes with the goal of varying hardness and retained austenite (RA) levels. End quenching routes utilized included deep freezing at -100°F for 1 hour and cooling in ambient air. The samples were then evaluated for their surface mechanical properties, and tribological behavior under dry sliding. X-Ray diffraction was used to quantify RA percentage. Our samples yielded RA values between 3-15%. It was observed that a higher carbon potential during carburizing hindered transformation of austenite to martensite during quenching which resulted in higher RA%. Microhardness tests were conducted and residual stress on the samples were measured using X-ray diffraction. Reciprocating friction and wear tests were carried out using ball on flat micro-tribometer designed to produce hertzian pressures between 6-7.5 GPa between a diamond probe and the samples under dry sliding conditions. It was observed that above a certain hertzian contact pressure level, samples with a higher carbon potential carburized sample resulted in increased abrasive wear resistance. This increased wear resistance is attributed to the higher hardness and conditions conducive to transformation of RA into martensite via ploughing-induced work-hardening. Analysis of the wear tracks using field emission scanning electron microscopy revealed microploughing and wedge formation to be the dominant abrasive wear mechanisms observed on the samples. EDS analysis confirmed the higher carbon content for the samples exhibiting higher hardness. All carburized samples showed significantly higher compressive residual stress on their surface which was beneficial for wear resistance. Hardness and RA had comparatively significant effect on wear resistance as compared to residual stress. The findings indicate that surface treatments resulting in a combination of high hardness and moderately high RA levels result in enhanced abrasive wear resistance for AISI 8620 steels.

The application of PVD coatings to work rolls used in the hot rolling processes can aid in increasing the roll life, controlling friction and reducing aluminum adhesion to the roll’s surface. Aluminium pick-up on the work roll surface is known to influence the surface quality of the rolled aluminum products. This pick-up is affected by the tribological conditions between the steel roll and the aluminum sheet, including the work roll surface condition and lubrication. A better understanding of the buildup of aluminum adhesion to various coatings surfaces under rolling conditions is therefore necessary.

In the present study, material transfer and adhesion from Al-Mg alloy samples to various PVD coatings deposited on AISI M2 steel rolls were examined using a hot rolling tribo-simulator. The coatings applied to the work rolls included Cr, ZrN, TiN and TiCN, which were compared with an uncoated M2 steel work roll after 1, 10 and 20 lubricated hot rolling passes. The work rolls possessed an average surface roughness (Ra) of 0.2 µm. Material transfer to the work rolls was examined after 1, 10 and 20 passes under a rolling schedule similar to industrial break down rolling. Scanning electron microscopy (SEM) and focus ion beam (FIB) microscopy were employed to investigate material adhesion and the surface conditions of the work rolls. Aluminum and magnesium transfer were observed on all work roll surfaces from the first hot rolling pass. It was most severe on the uncoated M2 steel work roll, while the TiN coating performed better with much less material transfer observed on its surface during the initial stage of hot rolling. The material transfer on both Cr-coated and uncoated M2 steel rolls contained more magnesium than aluminum, while an opposite trend was observed for the nitride coatings.

Shot peening is a surface mechanical treatment which induces compressive residual stresses and work hardens the surface. This increases the resistance to crack initiation and propagation, and several studies have shown that this prolongs the bending fatigue life of components. However, very few works have been conducted on the tribological behaviour of austempered ductile iron (ADI) after shot peening, and it is thus not clear whether shot peening actually improves the tribological characteristics.

Hence, this work was carried out to study the scuffing performance of upper ausferritic Cu-Ni ADI, before and after shot peening. A conventional pin-on-disk tribometer capable of maintaining a constant unidirectional, sliding velocity between the pin and disk was used. Tests on both as-austempered ADI and shot peened ADI were carried out under starved lubrication conditions. Results show that shot peened specimens resulted in an improved scuffing wear resistance of the ADI. Shot peened specimens survived 21×10³ cycles, while the as-austempered specimens endured 2.3×10³ cycles before failure.

The improved scuffing performance due to shot peening might seem anomalous, since rough surfaces generally create low values of the specific film thickness λ_sand induce scuffing. However, the superposition of indentations arising from the shot peening process acted as oil reservoirs by dragging the oil into them and creating a load-bearing hydrodynamic pressure. This resulted in a reduction of surface traction forces, which rendered an extremely low coefficient of friction of 0.08. This can be considered as an advantage in starved lubricated moving components. The higher scuffing resistance of the shot peened specimens was also partly attributed to the high hardness and high induced compressive stresses present in the surface of these specimens. On the other hand, the lower resistance to scuffing attested by the specimens tested in the as-austempered condition was attributed to plastic flow and micro-fracture of the asperities.