Zhongrong Zhou

Mixed fretting regime

Abstract The mixed fretting regime (MFR) has been proven to be the most dangerous regime for crack nucleation and service failure. It can be identified from the evolution of the “tangential load-displacement” loops, i.e. when there is a variation in the shape of the loops. Usually elliptical (partial slip) and quasi-rectangular (gross slip) loops are encountered in MFR. This regime has been obtained in fretting wear and fretting fatigue tests on numerous metal- base alloys. In contrast with the classic stick-slip condition in Mindlins elastic theory, MFR depends strongly on the normal load, the imposed amplitude and material properties such as elongation. During fretting testing under MFR, the material can fail due to overstraining or overstressing and the location of the maximum varies. For ductile materials, a competition exists between the formation of the tribologically transformed structure, particle detachment and the nucleation of fatigue cracks. In the case of brittle materials, MFR is generally avoided due to the rapid formation of debris and the establishment of a third body layer in which easier velocity accommodation favours the gross slip condition. A mechanical and material approach is used to analyse the specific behaviour of materials submitted to MFR. The study of fretting crack nucleation and propagation is emphasized.

Surface treatment by high current pulsed electron beam

Abstract Electron beams are becoming an increased subject of interest for materials processing. While continuous electron beams have already found wide applications in drilling, hardening, cutting and welding, the advantage of a pulsed electron beam has just emerged. It generates a high power density up to 108–109 W/cm2 at the target surface. Such a high energy is deposited only in a very thin layer within a short time, and causes superfast processes such as heating, melting and evaporation. A dynamic stress field induced in these processes leads to significant modification effects in the material. The combination of these processes provides the material with improved physicochemical and mechanical properties unattainable with ordinary surface treatment techniques. The present paper reports our recent research work on surface treatment by high-current pulsed electron beam (HCPEB). HCPEB is produced on system ‘Nadezhda-2’ with an energy range of 20–40 kV. A series of pure Al and mold steels were studied. Some of them were pre-coated with C, Cr, Ti or TiN powders. A strong enhanced diffusion effect was revealed: the surface elements diffuse approximately several micrometers in depth into the substrate only after several bombardments. Tribological behaviors of these samples were characterized and significant improvement in wear resistance was found. Finally, TEM analysis reveals the presence of stress waves generated by the coupling of thermal and stress fields, which constitutes the main cause of the enhanced diffusion.

Tribology of dental materials: a review

The application of tribology in dentistry is a growing and rapidly expanding field. Intensive research has been conducted to develop an understanding of dental tribology for successful design and selection of artificial dental materials. In this paper, the anatomy and function of human teeth is presented in brief, three types of current artificial dental materials are summarized, and their advantages and disadvantages, as well as typical clinical applications, are compared based on the literature. Possible tribological damage of tooth structure, which is induced by complex interfacial motion, and friction-wear test methods are reported. According to results obtained by the authors and from the literature, the main progress in the area of dental tribology on both natural teeth and artificial dental materials is reviewed. Problems and challenges are discussed and future research directions for dental tribology are recommended.

Lubrication in fretting—a review

Abstract Lubrication is one of the important methods of preventing fretting damage owing to a remarkable reduction in the coefficient of friction. This review attempts to bring up-to-date the known facts concerning the lubrication in fretting situations. Results obtained by the authors in combination with results from literature are examined in detail. The effects of many lubricants on fretting behaviour have been investigated and some general features have been found although the majority of experimental data was obtained under quite differing conditions. In order to select successfully the right lubricant for a specific fretting application, representative lubricants, oil, grease, polymeric film and molybdenum disulphide (MoS 2 ) with different physical states were used. Results indicate that the palliative effect on fretting should be analyzed using the fretting regimes concept. In particular, solid lubricant is most efficient in the partial slip regime while grease and oil, particularly the latter, are more suitable in the gross slip regime. Nevertheless, it has been noted that the lubrication mechanism in fretting is poorly understood and the need for further work is discussed.

New model to explain tooth wear with implications for microwear formation and diet reconstruction

Significance Dental microwear is among the most common proxies paleontologists use for diet reconstruction. Recent models have suggested that while quartz grit adherent to food produces wear of tooth enamel, softer particles, such as silica phytoliths found in many plants, do not. Some have therefore suggested that microwear patterns better reflect habitat than diet. This is important to paleobiologists because reconstructions of species from the earliest vertebrates to human ancestors have relied on dental microwear as a proxy for diet. Here we present an in vitro study demonstrating that softer particles produce microwear under conditions mimicking chewing. Enamel wear occurs not because an abrasive is hard but because it exceeds the binding force of proteins that hold together hydroxyapatite crystallites. Paleoanthropologists and vertebrate paleontologists have for decades debated the etiology of tooth wear and its implications for understanding the diets of human ancestors and other extinct mammals. The debate has recently taken a twist, calling into question the efficacy of dental microwear to reveal diet. Some argue that endogenous abrasives in plants (opal phytoliths) are too soft to abrade enamel, and that tooth wear is caused principally by exogenous quartz grit on food. If so, variation in microwear among fossil species may relate more to habitat than diet. This has important implications for paleobiologists because microwear is a common proxy for diets of fossil species. Here we reexamine the notion that particles softer than enamel (e.g., silica phytoliths) do not wear teeth. We scored human enamel using a microfabrication instrument fitted with soft particles (aluminum and brass spheres) and an atomic force microscope (AFM) fitted with silica particles under fixed normal loads, sliding speeds, and spans. Resulting damage was measured by AFM, and morphology and composition of debris were determined by scanning electron microscopy with energy-dispersive X-ray spectroscopy. Enamel chips removed from the surface demonstrate that softer particles produce wear under conditions mimicking chewing. Previous models posited that such particles rub enamel and create ridges alongside indentations without tissue removal. We propose that although these models hold for deformable metal surfaces, enamel works differently. Hydroxyapatite crystallites are “glued” together by proteins, and tissue removal requires only that contact pressure be sufficient to break the bonds holding enamel together.

Friction-induced nanofabrication on monocrystalline silicon

Fabrication of nanostructures has become a major concern as the scaling of device dimensions continues. In this paper, a friction-induced nanofabrication method is proposed to fabricate protrusive nanostructures on silicon. Without applying any voltage, the nanofabrication is completed by sliding an AFM diamond tip on a sample surface under a given normal load. Nanostructured patterns, such as linear nanostructures, nanodots or nanowords, can be fabricated on the target surface. The height of these nanostructures increases rapidly at first and then levels off with the increasing normal load or number of scratching cycles. TEM analyses suggest that the friction-induced hillock is composed of silicon oxide, amorphous silicon and deformed silicon structures. Compared to the tribochemical reaction, the amorphization and crystal defects induced by the mechanical interaction may have played a dominating role in the formation of the hillocks. Similar to other proximal probe methods, the proposed method enables fabrication at specified locations and facilitates measuring the dimensions of nanostructures with high precision. It is highlighted that the fabrication can also be realized on electrical insulators or oxide surfaces, such as quartz and glass. Therefore, the friction-induced method points out a new route in fabricating nanostructures on demand.

Effect of curved track support failure on vehicle derailment

In order to investigate the effect of curved track support failure on railway vehicle derailment, a coupled vehicle–track dynamic model is put forward. In the model, the vehicle and the structure under rails are, respectively, modelled as a multi-body system, and the rail is modelled with a Timoshenko beam rested on the discrete sleepers. The lateral, vertical, and torsional deformations of the beam are taken into account. The model also considers the effect of the discrete support by sleepers on the coupling dynamics of the vehicle and track. The sleepers are assumed to move backward at a constant speed to simulate the vehicle running along the track at the same speed. In the calculation of the coupled vehicle and track dynamics, the normal forces of the wheels/rails are calculated using the Hertzian contact theory and their creep forces are determined with the nonlinear creep theory by Shen et al [Z.Y. Shen, J.K. Hedrick, and J.A. Elkins, A comparison of alternative creep-force models for rail vehicle dynamic analysis, Proceedings of the 8th IAVSD Symposium, Cambridge, MA, 1984, pp. 591–605]. The motion equations of the vehicle/track are solved by means of an explicit integration method. The failure of the components of the curved track is simulated by changing the track stiffness and damping along the track. The cases where zero to six supports of the curved rails fail are considered. The transient derailment coefficients are calculated. They are, respectively, the ratio of the wheel/rail lateral force to the vertical force and the wheel load reduction. The contact points of the wheels/rails are in detail analysed and used to evaluate the risk of the vehicle derailment. Also, the present work investigates the effect of friction coefficient, axle load and vehicle speed on the derailments under the condition of track failure. The numerical results obtained indicate that the failure of track supports has a great influence on the whole vehicle running safety.

An investigation of molybdenum disulfide bonded solid lubricant coatings in fretting conditions

Fretting wear tests with a large range of displacement amplitudes have been carried out using molybdenum disulfide (MoS2) bonded solid lubricant coatings. Dynamic analyses in combination with microscopic examinations through SEM, EDX and XPS have been performed. Experimental results showed that such MoS2 coatings influenced greatly the fretting regimes and reduced the coefficient of friction. Further studies on comparisons between MoS2 bonded solid lubricant and magnetron-sputtered MoS2 coatings have been made in fretting conditions. MoS2 bonded solid lubricant, as one of the simple and cheap modification techniques is also considered as an effective palliative for fretting.

A comparative study on fretting wear-resistant properties of ion-plated TiN and magnetron-sputtered MoS2 coatings

This paper studied the fretting properties and mechanism of TiN and MoS2 coatings on 1045 steel. The results showed that the fretting-resistant property of ion-plated TiN coating is obviously better than that of magnetron-sputtered MoS2 coating on SRV fretting tests. Under the given test parameters (normal load 10 N, slip amplitude 100 μm and frequency 10 Hz), the fretting wear mechanism of TiN coating is particle detachment through the propagation and fracture of fine microcracks on surface, and the fretting wear mechanism of MoS2 coating showed its damage process by degradation and abrasion of materials.

Towards a deeper understanding of the formation of friction-induced hillocks on monocrystalline silicon

Friction-induced hillocks can be produced on monocrystalline silicon by scratching under given conditions. Results show that the height of these hillocks increases with the applied normal load or number of scratching cycles, but decreases with the sliding velocity. Transmission electron microscope (TEM) and energy dispersive x-ray (EDX) analysis show that the hillock contains a thin superficial oxidation layer and a thick disturbed (amorphous and deformed) layer in the subsurface. Although the formation of the silicon hillock is the combined results of mechanical interaction and tribochemical reaction, the mechanical interaction plays a more dominant role. Further analysis indicates that the formation of hillock is mostly induced by the amorphization of crystal silicon during scratching. Low sliding speed is found to facilitate the formation of a thick amorphization layer under the same loading condition. Since the friction-induced hillock is the initial surface damage on the nanoscale, the results will shed new light on understanding and controlling the nanowear process of silicon in micro/nanoelectromechanical systems.