M.H. Zhu

An investigation on fretting wear life of bonded MoS2 solid lubricant coatings in complex conditions

Abstract Compared to MoS2 coatings obtained by magnetron sputtering or ion beam assisted deposition, the MoS2 bonded solid lubricant coatings has been widely applied in sliding wear due to a cheap technical process and good thickness. However, such coatings technology has been rarely investigated in complex fretting condition. In this work, fretting wear tests of the six novolac epoxy resin bonded molybdenum disulfide solid lubricant coatings have been carried out in various normal loads and amplitudes. The effects of various process preparations, substrate materials, and environmental conditions on the fretting wear life were investigated. Main results show that enhancing substrate hardness, increasing coating thickness, sandblasting substrate surface, and heat curing coating will improve fretting wear life to some extent while it decreases with increasing relative humidity. Oxidation effect is one of the important factors for coating degradation at high temperature.

An experimental investigation on composite fretting mode

An apparatus for composite fretting tests on a steel ball opposite to the inclined steel flat with different inclined angles (60° and 45°) have been carried out. During the tests, the maximum imposed loads were varied from 200 N to 400 N and 800 N at a constant loading speed of 12 mm/min. Dynamic analysis in combination with microscopic examinations through optical microscopy, profilometery, and SEM have been performed. Composite fretting behaviour has been analyzed and compared with the conventional fretting.

An experimental study on radial fretting behaviour

Abstract A new test apparatus has been developed for radial fretting test. Main experimental conditions are as follows: the amplitude of normal load from 200 to 800N, the number of cycles from 1 to 3×105 cycles. Three contact pairs (a 52100 ball against 52100, 1045, 1045 steel with TiN coating) were used for the test. Variations of normal load vs indentation depth between two contact surfaces have been analyzed as a function of cycles. Contact degradation was examined through metallographic expertise on the flat specimen and radial fretting behaviour was compared in the paper.

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.

Macrodeformation Twins in Single-Crystal Aluminum.

Deformation twinning in pure aluminum has been considered to be a unique property of nanostructured aluminum. A lingering mystery is whether deformation twinning occurs in coarse-grained or single-crystal aluminum at scales beyond nanotwins. Here, we present the first experimental demonstration of macrodeformation twins in single-crystal aluminum formed under an ultrahigh strain rate (∼10^{6}  s^{-1}) and large shear strain (200%) via dynamic equal channel angular pressing. Large-scale molecular dynamics simulations suggest that the frustration of subsonic dislocation motion leads to transonic deformation twinning. Deformation twinning is rooted in the rate dependences of dislocation motion and twinning, which are coupled, complementary processes during severe plastic deformation under ultrahigh strain rates.

A study on high-speed rolling contact between a wheel and a contaminated rail

A 3-D explicit finite element model is developed to investigate the transient wheel–rail rolling contact in the presence of rail contamination or short low adhesion zones (LAZs). A transient analysis is required because the wheel passes by a short LAZ very quickly, especially at high speeds. A surface-to-surface contact algorithm (by the penalty method) is employed to solve the frictional rolling contact between the wheel and the rail meshed by solid elements. The LAZ is simulated by a varying coefficient of friction along the rail. Different traction efforts and action of the traction control system triggered by the LAZ are simulated by applying a time-dependent driving torque to the wheel axle. Structural flexibilities of the vehicle–track system are considered properly. Analysis focuses on the contact forces, creepage, contact stresses and the derived frictional work and plastic deformation. It is found that the longitudinal contact force and the maximum surface shear stress in the contact patch become obviously lower in the LAZ and much higher as the wheel re-enters the dry rail section. Consequently, a higher wear rate and larger plastic flow are expected at the location where the dry contact starts to be rebuilt. In other words, contact surface damages such as wheel flats and rail burns may come into being because of the LAZ. Length of the LAZ, the traction level, etc. are varied. The results also show that local contact surface damages may still occur as the traction control system acts.

An investigation of fretting behaviour of several metallic materials under grease lubrication

Abstract Fretting wear tests under grease lubrication have been carried out on an aluminium alloy, 52100 steel and low-alloy steel. The sphere–flat contact configuration is used. The influence of the displacement amplitude and normal load is investigated. Comparison between dry and lubricated contact of aluminium alloy, between 52100/52100 steel and 52100/low-alloy steel contact with grease lubrication has been carried out. Results show that grease lubrication strongly affects fretting behaviour. Base oil that separated from the grease during friction may result in accelerated contact wear by fretting.

Tribological and electric-arc behaviors of carbon/copper pair during sliding friction process with electric current applied

The tribological behaviors of carbon block sliding against copper ring with and without electric current applied were investigated by using an advanced multifunctional friction and wear tester, and the electric-arc behaviors were analyzed in detail. The results show that the normal load is one of the main controlling factors for generation of electric arc during friction process with electric current applied. The strength of electric arc is enhanced with the decrease of normal loads and the increase of electric currents. The unstable friction process and the fluctuated dynamic friction coefficients are strongly dependent upon the electric arc. The wear volumes and the wear mechanism of carbon brush were affected by the electric arc obviously. As no electric arc occurs, no clear discrepancy of the wear volumes of the carbon samples with and without electric current applied could be detected. While the wear mechanisms are mainly mechanical wear. However, under the condition of the electric arc appearance, the wear volume of carbon with electric current applied increases much more rapidly than that without electric current applied and also increases obviously with the increase of electric current strengths and the decrease of normal loads. The wear mechanisms of carbon block are mainly electric arc ablation accompanying with adhesive wear and material transferring.

Fretting wear behavior of Inconel 690 in hydrazine environments

The friction and wear behaviors of Inconel 690 flat against Si3Ni4 ball were investigated using a hydraulic fretting test rig equipped with a liquid container device. The loads of 20–80 N, reciprocating amplitudes of 80–200 μm and two different environments (distilled water and hydrazine solution at temperatures from 25 to 90 °C) were selected. The results show that the ratio of Ft/Fn is lower in distilled water than that in hydrazine solution at the same temperature in the slip regime. Both the ratio of Ft/Fn and wear volume gradually increase with increasing medium temperature under the given normal load and displacement amplitude. Besides the displacement amplitude and load, temperature also plays an important role for wear behavior of Inconel 690 material. The increase of temperature could reduce the concentration of dissolved oxygen, and promote the absorption reaction of hydrazine and dissolved oxygen. As a result, the oxidative corrosion rate is obviously lowered. Abrasive wear and delamination wear are the main mechanisms of Inconel 690 in distilled water. However, in hydrazine solution the cracks accompanied by abrasive wear and delamination wear are the main mechanisms.

Effect of laser quenching on wear and damage of heavy-haul wheel/rail materials

In order to alleviate the wear of heavy-haul wheel/rail materials, the wear experiments of wheel/rail rollers undergoing laser quenching were carried out using rolling-sliding testing apparatus. The results indicate that laser quenching produces a hard martensite layer with a certain depth. The improvement of wear resistance of materials resulting from laser quenching markedly decreases the wear volume of wheel/rail rollers. The maximal decrease rate of wear volume of wheel and rail materials is about 92.9% and 89.5%, respectively. Furthermore, the worn surfaces of wheel/rail rollers present different surface morphology and damage mechanisms due to difference of hardness matching. When the surface hardness of wheel/rail rollers undergoing laser quenching is high, the spalling damage and adhesion wear are dominating. When a single wheel or rail roller undergoes laser quenching, the damage mechanism of wheel and rail rollers are significantly different. According to the severe wear region of the heavy-haul wheel/rail, it is proposed that the rail side and wheel flange may undergo laser quenching treatment for alleviating wear of heavy-haul wheel/rail materials. However, further work should be carried out for clarifying the fatigue characteristic of wheel/rail with laser quenching.