Tribological characterization of alumina and silicon carbide under lubricated sliding

The aim of this study was to investigate the tribological properties of four different advanced ceramics - monolithic alumina, SiC whisker-reinforced alumina, monolithic silicon carbide and SiSiC-during lubricated sliding. Advanced techniques of electron microscopy and spectroscopy have been used to characterize the materials before and after testing. Tests have been performed where two flat discs were rotated against each other under closed contact in an environment of oil and water. The main wear mechanisms of the four ceramics are identified and discussed. What clearly emerges from these studies is the much more reliable performance of the silicon carbides than the aluminas. The silicon carbides have a low wear rate where microfracture and oxidation are the main deteriorating mechanisms. The capability to maintain smooth surfaces and thus also a high degree of hydrodynamic lubrication is largely due to the potential of the water to dissolve formed wear debris. The main wear mechanism of the aluminas is surface fracture. The rough fracture surfaces and the fact that the wear fragments form a discontinuous surface film will reduce the effect of the lubrication, thus accelerating the wear. Furthermore, a deformation layer with microcracks develops in the contact which decreases wear resistance.     

Two-body abrasive wear of electroless nickel composite coatings

The two-body abrasive wear of electroless nickel (EN), EN-silicon carbide, and EN-alumina composite coatings have been investigated using a scratch test with a diamond indenter. The coatings were heat treated at temperatures of 100–500° C. The hardness of the coatings increased with heat treatment temperature from 500 HV100 for the as-deposited condition to 1008 HV100 when fully hardened. Scratch testing showed that the as-deposited coating had scratch tracks with a high degree of plasticity, signs of microploughing and tensile cracking and was characterised as a ductile failure. On the other hand, the heat-treated coatings showed chipping and cracking on the edge of the scratch tracks, failing in a brittle manner. The heat-treated EN-silicon carbide coatings, however, exhibited no cracking nor chipping, believed to be due to its higher fracture toughness than the other heat-treated coatings, attributable to its lower phosphorus content. The volume of material removed from the silicon carbide scratch track was 1/3 of the volume removed from the steel substrate at a 20 N load, and showed the best wear/ scratch resistance of any of the coatings tested.     

Friction and wear properties of metal powder filled PTFE composites under oil lubricated conditions

Four kinds of polytetrafluoroethylene (PTFE)-based composite, pure PTFE, PTFE+30vol.%Cu, PTFE+30vol.%Pb and PTFE+30vol.%Ni composite, were prepared. The friction and wear properties of these metal powder filled PTFE composites sliding against GCr15 bearing steel under both dry and lubricated conditions were studied using an MHK-500 ring-block wear tester. The worn surfaces of the PTFE composites and the transfer films formed on the surface of GCr15 bearing steel were examined using scanning electron microscopy (SEM) and optical microscopy respectively. Experimental results show that the friction and wear properties of the PTFE composites can be greatly improved by liquid paraffin lubrication. The wear of these PTFE composites can be decreased by at least 1 to 2 orders of magnitude compared with that under dry friction conditions, while the friction coefficients can be decreased by 1 order of magnitude, SEM and optical microscopy investigations of the rubbing surfaces show that metal fillers of Cu, Pb and Ni not only raise the load carrying capacity of the PTFE composites, but also promote transfer of the PTFE composites onto the counterfaces, so they greatly reduce the wear of the PTFE composites. However, the transfer of these PTFE composites onto the counterfaces can be greatly reduced by liquid paraffin lubrication, but transfer still takes place.     

The effect of silicon content and morphology on the wear of aluminium-silicon alloys under dry and lubricated sliding conditions

Improvement in the efficiency of the internal combustion engine has resulted in the increased usage of aluminium alloys and, in particular, aluminium-silicon as a substitute for cast iron. Despite the wide use of such materials in tribological environments little knowledge is available on the wear resistance of aluminium-silicon alloys. This paper investigates the wear performance of a range of binary aluminium-silicon alloys produced by a novel melt-spray technique. In addition, samples of the 11wt% silicon alloy were produced by conventional casting methods to elucidate the influence of silicon morphology on wear resistance. Pin-on-ring wear tests were carried out under dry and boundary-lubricated conditions. Surface analysis showed a similar wear mechanism under both conditions, these being: (1) oxidative and (2) metallic wear. Under boundary-lubricated conditions the load at which the transition to metallic wear occurred was increased. Raising the silicon content of the alloy was reflected in an increase in both wear resistance and transition load. Under dry sliding conditions the wear rate of the 11wt% alloy increased with a reduction in the silicon particle size, whereas under boundary-lubricated conditions the reverse was observed and the sand-cast alloy exhibited superior performance.     

Effect of surface etching on the lubricated sliding wear of an eutectic aluminium–silicon alloy

The paper presents a study of the formation of wear grooves on near-eutectic aluminium–silicon alloy flats, by sliding a steel ball. The formation of the grooves are tracked on etched and unetched flats as functions of normal load and sliding distance. The groove is initially formed by plastic flow, and then expanded by micro-abrasion as the ball continues to slide on the groove. Etching causes surface hardening of the alloy, but, more importantly creates a surface topology that reduces the peak contact pressure, which discourages further plastic flow in the subsurface. This effect is rationalised using an existing contact mechanical model of indentation of rough surfaces.     

Hydrodynamic lubrication of textured steel surfaces under reciprocating sliding conditions

The influence of surface topography on lubricant film thickness has been investigated for the reciprocating sliding of patterned plane steel surfaces against cylindrical counterbodies under conditions of hydrodynamic lubrication. Patterns of circular depressions, grooves and chevrons were used, and the fractional area coverage, depth, width and sliding orientation relative to the texture were systematically varied. Textured samples with features much larger than the elastic contact width gave film thicknesses, which were smaller than those for non-textured samples. This effect was more significant for larger features. For patterns composed of circular pockets, maximum film thickness was achieved for an area coverage fraction f≈0.11. Chevron patterns pointing along the sliding direction gave higher film thicknesses than those pointing across. For an area coverage ratio of ca. 0.06, maximum film thickness was achieved for a feature depth to width ratio of about 0.07. Among the patterns investigated, chevrons were the most effective and grooves the least effective in increasing hydrodynamic film thickness.     

Friction and wear characteristics of lead and its compounds filled polytetrafluoroethylene composites under oil lubricated conditions

The friction and wear properties of Pb, PbO, Pb₃O₄, or PbS filled polytetrafluoroethylene (PTFE) composites sliding against GCr15 bearing steel under both dry and liquid paraffin lubricated conditions were studied by using an MHK-500 ring-block wear tester. The worn surfaces and the transfer films of these PTFE composites formed on the surface of GCr15 bearing steel were then investigated by using a scanning electron microscope (SEM) and an optical microscope, respectively. Experimental results show that filling Pb, PbO, Pb₃O₄ or PbS to PTFE can greatly reduce the wear of the PTFE composites, but the wear reducing action of Pb₃O₄ is the most effective. Meanwhile, PbS increases the friction coefficient of the PTFE composite, but Pb and Pb₃O₄ reduce the friction coefficients of the PTFE composites. However, the friction and wear properties of lead or its compounds filled PTFE composites can be greatly improved by lubrication with liquid paraffin, and the friction coefficients of the PTFE composites can be decreased by one order of magnitude. Optical microscope investigation of transfer films shows that Pb, PbO, Pb₃O₄ and PbS enhance the adhesion of the transfer films to the surface of GCr15 bearing steel, so they greatly reduce the wear of the PTFE composites. However, the transfer of the PTFE composites onto the surface of GCr15 bearing steel can be greatly reduced by lubrication with liquid paraffin, but the transfer still takes place. SEM examination of worn surfaces shows that the interaction between liquid paraffin and the PTFE composites creates some cracks on the worn surfaces of the PTFE composites; the creation and development of the cracks reduces the load-carrying capacity of the PTFE composites, and this leads to deterioration of the friction and wear properties of the PTFE composites filled with lead or its compounds under higher loads in liquid paraffin lubrication.     

Friction and wear performance of MoDTC‐containing and ester‐containing lubricants over steel surfaces under reciprocating conditions

Lubricant oil can be regarded as a complex mixture of base oils and additives, each one with its specific functions and behaviour. In this paper, the interaction of a molybdenum dialkyldithiocarbamate (MoDTC)‐based additive and combinations of a polyalphaolefin and a synthetic ester is investigated. A reciprocating ball‐on‐disc configuration was used for tribological tests. The effect of MoDTC is seen as a sharp drop in the coefficient of friction. This friction reduction is affected by the base fluid: the effect is more intense and lasts longer when the ester content is decreased. The applied normal force also affects the MoDTC effect, which is not sustainable at higher loads. Copyright © 2014 John Wiley & Sons, Ltd.     

The transition between high and low wear regimes under multidirectional reciprocating sliding

The wear phenomena and wear characteristics of reciprocating sliding wear with superimposed lateral vibrations were investigated using a ball-on-disc tribometer. The tribometer enabled two orthogonal oscillations, whereas one oscillation had a constant amplitude of 1 mm (primary oscillation) and the other one had a variable amplitude from 0 to 20.2 μm (secondary oscillation). Ball and disc were made of AISI 52100 steel. The ball surface was polished and the disc surface was unidirectionally grinded parallel to the direction of primary oscillation. Two regimes with different wear rates were found, being separated by a characteristic transition amplitude of 2.7 ± 0.4 μm in the secondary oscillation. This transition correlated with a change of wear mechanisms from tribochemically to mechanically dominated wear. A wear model based on surface topography and particle motion was developed. The wear model is able to predict the value of the transition amplitude by means of characteristic topographical data and the size of wear particles.     

极限环境温度对柴油机气缸套磨损的影响

为了研究不同环境温度对柴油机寿命的影响,以某型柴油机为对象,通过模拟极限环境温度下柴油机实际工况,利用受力分析和热平衡计算得到边界条件,基于雷诺方程和改进的Holm-Achard黏着磨损公式建立润滑磨损数值计算模型.经验证上止点附近模型的计算值与实测值误差不超过5%.计算表明,油膜厚度随外界温度升高而逐渐变薄.高温319K时气缸套径向最大磨损深度为41.21μm,低温230 K时为35.8μm.     

Effect of geometry change of rough point contact due to lubricated sliding wear on lubrication

The geometry change of a single asperity due to lubricated wear was studied by an experimental simulation with a ball‐on‐disc set up. The wear leads to the formation of a tilted section at the tip of the ball, which is proved to be due to the presence of oil during the process. The effect of the geometry change of rough surface contacts due to wear was examined by a micro‐EHL analysis. A non‐Newtonian visco‐plastic fluid model which includes the effect of a limiting shear strength was used. This revised version was published online in June 2006 with corrections to the Cover Date.     

Metallic sliding friction under boundary lubricated conditions: Investigation of the influence of lubricant at the start of sliding

Experiments are described in which a hard half-wedge, representing a scaled up model asperity, was indented into the horizontal surface of a relatively soft specimen with the specimen then moved in a direction parallel to its surface and normal to the wedge edge. Forces measured at the wedge-specimen interface are used to investigate the state of lubrication at the interface at all stages of the experiment, i.e. from initial indentation to final steady-state wave formation. The results are used to offer possible explanations for the nature of boundary lubrication and of stick-slip motion.     

Some observations on the role of oxygen in lubricated wear

The role of oxygen in lubricated wear is discussed. Previous studies have shown that at low concentrations, oxygen can reduce wear, but at higher concentrations, it can increase wear. The situation is complicated by ZDTP antioxidants. The present paper reports experimental work aimed at elucidating the role of hydroperoxides - whether they participate in the formation of wear-resistant oxide films, if they (or peroxy radicals) are the main source of oxygen, and whether ZDTP-hydroperoxide interactions in the wear zone are important in controlling wear.     

Effect of wear on EHD film thickness in sliding contacts

A theoretical solution to the elastohydrodynamic (EHD) lubrication problem in sliding contacts, which takes into consideration the effect of the change in shape of the gap due to wear on the load‐carrying capacity, is presented. The model of such a contact is based on assumptions of Grubin and Ertel (von Mohrenstein). The resultant dimensionless Reynolds and film profile equations have been solved numerically for a number of cases with several values of thickness of the worn layer. Iteration of the EHD film thickness is performed by means of the secant method. Values of the calculated dimensionless film thickness are presented as a function of dimensionless wear. The conclusions concern the influence of the linear wear on the film thickness in heavily loaded sliding contacts. Copyright © 2006 John Wiley & Sons, Ltd.     

Effect of pH on wear and friction of silicon nitride sliding against alumina in water

The influence of the pH in water lubricated sliding contacts was evaluated in terms of friction and wear. The experiments were carried out using a ball-on-disc setup. Si₃N₄ balls and Al₂O₃ discs were tested at temperature of (22±2) °C, sliding speed of (1.00±0.03) m/s and normal load of (54.25±0.17) N. Eight types of water with pHs varying from 3 to 12 were used as lubricant. The running-in period, friction coefficient and wear-volume were shown to be nearly independent of the initial pH values within the DLVO range (4≤pH≤10), since at these range the water׳s pH tends to the same value (7.6±0.3) during the test. Superlubricity could be reached with negligible wear by properly setting the electrochemical properties and operating conditions of the tribosystem.     

Effect of the microstructure of thermally sprayed coatings on friction and wear response under lubricated and dry sliding conditions

Five thermally sprayed coatings, considered for application in earth moving vehicle undercarriage components, undergoing consecutively lubricated and dry sliding wear, were prepared and investigated. Four different wear tests were employed to reveal the factors controlling the wear response under different contact conditions. An approach considering the local and overall strength of the coating in connection with the knowledge of contact stresses helped to understand the major wear modes under specific contact conditions for the materials tested. The testing illustrated that in situations with lower contact stresses the contact response of the coating may not be directly related to the intersplat binding strength and hardness, but rather to the local splat properties. It has been shown that a porous structure although limiting the wear resistance of nickel aluminum coating in dry under high contact stress may be beneficial in quasi-lubricated sliding. High local splat hardness measured for the molybdenum carbide coatings appeared to be crucial for its enhanced wear resistance. The testing configuration ‘rotating ring on flat’ newly employed in this study has been proven to be a highly reproducible and efficient way of wear testing for thermal spray coatings.     

Effect of geometrical parameters in micro-grooved crosshatch pattern under lubricated sliding friction

Tribological test was carried out using a pin-on-disc geometry with textured SKD11 pin on bearing steel disc, under sliding in paraffin oil. Micro-grooved crosshatch pattern has been fabricated with various angles and widths. The effects of geometrical parameters on friction were mainly examined in mixed and elastohydrodynamic lubrication. The results show that friction control can be achieved by fabricating the micro-grooved crosshatch pattern on a contact surface. It is observed that each geometrical parameter of texture influence on friction, especially decrease of groove aspect ratio and increases of groove sliding length show friction reduction performance. Crucial parameter G_l was proposed for micro-grooved crosshatch texture. The friction mechanism is explained by micro fluid flow with limited theoretical approach.     

Tribological properties of metal-plastic multilayer composites under oil lubricated conditions

Three kinds of metal-plastic multilayer composites, which were composed of a steel backing, a middle layer of sintered porous bronze and a surface layer of polytetrafluoroethylene (PTFE) filled by Pb or Cu₂O powders, were prepared. The friction and wear properties as well as the limiting pressure times velocity (PV) values of these metal-plastic multilayer composites sliding against 45 carbon steel under both dry and oil lubricated conditions were evaluated on a MPV-1500 friction tester with a steel axis rotating on a journal bearing. The worn surfaces of these metal-plastic multilayer composites and the transfer films formed on the surface of steel axis were examined by electron probe microscopy analysis (EPMA). Experimental results show that filling of Pb to PTFE reduces the friction coefficient and wear of the composite, while filling of Cu₂O to PTFE increases the friction coefficient but decreases the wear of the composite. The friction and wear properties as well as the limiting PV values of these metal-plastic multilayer composites can be greatly improved with the oil lubrication. EPMA investigations show that Pb and Cu₂O fillers preferentially transfer onto the surfaces of steel axis, which may enhance or deteriorate the adhesion between transfer films and steel surfaces. Meanwhile the transfer of these metal-plastic multilayer composites onto the steel surface can be greatly reduced with oil lubrication, which results in the remarkable decrease of the wear of these metal-plastic multilayer composites.     

Wear transitions and mechanisms in lubricated sliding of a molybdenum coating

Wear tests were performed for a Mo coating sliding against bearing steel specimen under boundary lubrication conditions. Results were compared with (i) hardened carbon steel sliding against bearing steel and (ii) Mo coating sliding against boron cast iron. Tests indicated that the wear resistance of the Mo coating was superior to that of the uncoated hardened steel. The initial surface topographies of the coatings were suitable to facilitate the transfer of the applied load directly onto the phases and prevented the softer phase directly involved in the wear process. The morphology of the transfer layer formed on the Mo coating was identified by X-ray diffractometry. And the layers were expected to supply an in situ lubrication effect. The wear rates of the coating against a steel slider were lower compared with those worn against a cast iron slider. With increasing applied load, the probability of the harder phases crack and fracture increased until the fraction of the unfragmented phases on the contact surfaces was no longer adequate to support the load. The dominant wear mechanisms in each wear regime were discussed.     

Tribological characterization of thin hard coatings by reciprocating sliding tests

Thin hard coatings on metal or ceramic surfaces offer a large spectrum of improvements of the friction and/or wear behaviour of tribosystems. The development of coatings and the tailoring of their properties require test methods providing information about their friction and wear behaviour. A new wear test standard (ASTM) is under development for the evaluation of friction and wear quantities for sliding motions using the reciprocating sliding mode. The applicability of this test method to coated specimens was checked by testing uncoated and coated steel specimens in contact with alumina balls, whereby lower loads were used than in the ASTM proposal for bulk materials. Additionally, the influence of the relative humidity of the surrounding air at room temperature on friction and wear results was examined.