Relation between scuffing resistance and the increase in surface hardness during tests under conditions of rolling/sliding

The effect on scuffing resistance of a change in the condition of the rubbing surfaces, especially surface hardness, was investigated using a two-disc machine. Scuffing resistance increased with increasing surface hardness during testing. The increase of surface hardness by work hardening is more beneficial in preventing scuffing than is a change in carbon content or heat treatment. Work-hardened disc surfaces allow surface film formation as a result of plastic deformation. Materials with large Meyer indices and high surface hardness are less prone to scuffing.     

Scuffing Behavior of Al-Si Alloy Modified by Fine Particle Bombarding and Powder Impact Plating Hybrid Surface Treatment

The occurrence of scuffing is widely observed on tribological components made of Al-Si alloys. The object of this study is to investigate the scuffing behavior of Al-Si alloy modified by fine particle bombarding (FPB) and powder impact plating (PIP) hybrid surface treatment. FPB treatment, PIP treatment, and hybrid treatment consisting of FPB and PIP were conducted on Al-Si alloy samples. The surface morphology, microstructure, and hardness of the samples were examined. Block-on-ring tests were performed to investigate the scuffing resistance of the samples. The test results show that the scuffing resistance of Al-Si alloy depends on the surface hardness and friction coefficient. Scuffing resistance is improved by FPB treatment, which increases the surface hardness by nanocrystallization and reduces the friction coefficient by decreasing the surface roughness and producing a microdimpled surface. PIP treatment reduces the friction coefficient by generating an Sn coating on the surface of the sample, thereby improving the scuffing resistance. After FPB and PIP hybrid surface treatment, the surface hardness is increased and the friction coefficient is further reduced. Therefore, the sample modified by hybrid surface treatment exhibits the highest scuffing resistance.     

Scuffing resistance of phosphate esters II: Effect of applied voltage

The scuffing resistance of tricresyl phosphate under severe conditions of lubrication was investigated using a reconstructed Timken machine. Although the iron phosphate formed on rubbing surfaces is effective in preventing scuffing under mild lubricating conditions, transformation to iron phosphide is essential to prevent scuffing under severe lubricating conditions. The friction polymer produced during tests is beneficial in increasing scuffing resistance.The formation of surface films is influenced by the electric current flowing between the rubbing surfaces as a result of the formation of a galvanic cell or an externally applied voltage. To prevent scuffing it is beneficial to promote film formation on a surface operating under more severe lubricating conditions than those of the mating surface. However, excessive promotion of surface film formation by the applied voltage decreases scuffing resistance by preventing film formation on the mating surface.     

Analysis of micro-elastohydrodynamic lubrication for engineering contacts

The paper describes methods which have been developed for theoretical analysis of elastohydrodynamic lubrication (EHL) in situations where the nominal oil film thickness is of the same order as the height of roughness asperities on the surfaces in contact. In such contacts the roughness significantly affects the distribution of pressure, and pressures much higher than predicted on the basis of smooth surface theory are obtained even under conditions where a full fluid film is present. This has consequences for surface fatigue processes such as pitting, and the presence of roughness is also considered to be a crucial factor in the occurrence of scuffing in which hydrodynamic film failure leads to adhesion and severe distress of the surfaces. The major aim of the work is to develop a better undertanding of the physics of scuffing based upon models of film failure in the presence of roughness.     

Temperature Analysis in Lubricated Simple Sliding Rough Contacts

A temperature analysis of dry sliding fully plastic contact is extended to calculate the asperity temperatures between a sliding lubricated rigid smooth plane and a stationary elastic rough surface. First, surface roughness is generated numerically to have a Gaussian height distribution and a bilinear autocorrelation function. Lai and Cheng's elastic rough contact computer program is then used to determine the asperity contact loads and geometries of real contact areas. Assuming different frictional coefficients for shearing the lubricant film at the noncontact areas, shearing the surface film at the asperity contacts and shearing the oxide film as the asperity temperature exceeds a critical temperature, asperity temperature distributions can be calculated. Eight cases in Durkee and Cheng's scuffing tests of lubricated simple sliding rough contacts are simulated by using 20 computer-generated rough surfaces. The results show that scuffing is correlated to high-temperature asperities which are above the material-softening temperature.     

Effect of surface roughness and surface texture on scuffing

This paper discusses the results of disk tests designed to examine the effect of surface roughness and surface texture on scuffing and related behavior. AISI 9310 steel disk of varying surface roughness and surface texture, along with a MIL-L-7808G lubricant, were used. It is shown that, under otherwise comparable situations, (a) an increase in the composite surface roughness increases the coefficient of friction at scuffing, decreases the scuff failure load, and decreases the critical temperature, (b) the cross-ground disks give a lower coefficient of friction at scuffing and a higher scuff failure load than the circumferentially-ground disks, but nearly the same critical temperature, and (c) an increase in the sliding velocity or sum velocity, at a constant sliding-to-sum velocity ratio, decreases the coefficient of friction at scuffing, decreases the scuff failure load, and decreases the critical temperature.The superior performance of the cross-ground disks compared with the circumferentially-ground disks is attributed to the effect of surface texture on microelastohydrodynamic action. The variations of the oil film thickness ratio at scuffing, the coefficient of friction at scuffing, and the critical temperature with respect to surface roughness, surface texture, and operating variables have been shown to correlate well with a dimensionless parameter $\text{ξ}{}_{\mathrm{?}}$.     

Improvement of wear resistance of working surfaces of direct-current machine commutator by impact machining

The paper discusses the results of studying the influence of commutator working surface roughness and hardness on the rate of bar wear. The obtained dependences describe the effect of impact machining variables on bar surface roughness and surface layer hardness. The conditions for impact machining (IM) commutator copper are determined. The composition of the oxide film that arises on the commutator surface upon impact machining accompanied by the introduction of a graphite suspension is analyzed and the effect of the oxide film on the commutation of direct current machines (DCM) is evaluated.     

Global and local analysis of gear scuffing tests using a mixed film lubrication model

Gear tests were performed in a FZG test rig in order to evaluate the influence of the operating conditions (torque, speed and oil bath temperature), gear geometry and base oil viscosity on gear scuffing.A mixed film lubrication model was used to evaluate the normal pressures and shear stresses in several points along the gear meshing line, for each load stage and for all the gear scuffing tests performed.The gear scuffing results were analyzed using two different approaches: one considering global gear parameters defined at the meshing line scale and another based on local parameters at the roughness asperity scale, determined using the mixed film lubrication model.The analysis at the roughness asperity level was used to complete the scuffing study performed with global gear contact parameters, explaining the occurrence of scuffing during ‘running-in’, justifying the zones in teeth flanks where the first scuffing marks appear and supplying indicators for low scuffing resistance at high oil bath temperatures.     

A review of scuffing and running-in of lubricated surfaces, with asperities and oxides in perspective

The slow progress in the understanding of scuffing (scoring) and runningin of most lubricated surfaces is probably due to an inadequate understanding of the details of asperity deformation and oxide formation. The thickness and properties of oxides influence the stress states imposed on asperities as much as does the liquid lubricant, but the oxides are ignored in theories. Present theories also focus on adhesion as the cause of scuffing and they usually do not take account of the changing surface roughness during sliding. There may indeed be some evidence of adhesion in the later stages of damage but adhesion has not been demonstrated to be the initiating mechanism of scuffing. Plastic fatigue is the more likely explanation, and this can occur without atomic contact between the sliding surfaces.     

Scuffing behaviour of salt-bath nitrided steel in loaded lubricated sliding/rolling contacts

The modified 3 in (76.2 mm) David Brown disc machine has been used to investigate the effect on scuffing performance of grinding away various amounts from the surface of nitrided steel discs. The results revealed a systematic deterioration in scuffing resistance as the surface nitrogen concentration decreased, but quantitative correlation between these parameters was hindered by the inadequacies of electron probe X-ray microanalysis as a tool for measuring low nitrogen concentrations. It was concluded that there is no safe depth to which Tenifer-treated (salt-bath nitrided) C15 steel can be ground without impairing its scuffing performance. Other disc tests were carried out to assess the effect on scuffing of phosphate treatment and oil formulation. Results showed that, while phosphate treatment is valuable when running against untreated steel, it is of no benefit against Tenifer-treated steel. On the other hand, the formulation of the lubricant was found to improve the scuffing resistance of Tenifer-treated steel     

The effect of the addition of phosphate esters to paraffinic base oils on their lubricating performance under sliding conditions

Using a modified Timken machine, the effect of the addition of different phosphate esters to paraffinic base oils on their lubricating performance was investigated. The aryl phosphate ester tricresyl phosphate (TCP) has a greater resistance to scuffing than the alkyl phosphate ester trioctyl phosphate (TOP) when used as a lubricant. In contrast, base oils containing TOP are superior to those containing TCP, although phosphate esters are effective only in base oils of higher viscosity than the esters.The lubricating performance of high viscosity base oils containing TOP is improved by coupling the ability of TOP to form a beneficial surface film with the ability of the base oils to build up thick oil films. The addition of TCP, which has a higher adsorption ability but a lower reactivity than TOP, seems, however, to prevent the formation of the oxide film which is formed with base oils alone owing to the marked ability of TCP to adhere to a metal surface. In addition, because of the low reactivity of TCP, iron phosphate is hardly formed. Consequently, the addition of TCP increases the scuffing load of the base oils but has the disadvantage of increasing the coefficient of friction.     

Structural, optical and mechanical properties of nanostructure diamond synthesized by chemical vapor deposition

Nanostructure diamond (NSD) films on Si substrate are prepared by microwave plasma enhanced chemical vapor deposition (MPECVD) using methane and hydrogen as the reactants with two-step negative substrate bias (SB). The dependencies of the NSD film morphology, grains, surface roughness, crystal and bonding structures and hardness on the negative SB at the bias-enhanced growth (BEG) step and substrate temperature during growth have been investigated by conducting atomic force microscopy (CAFM), X-ray diffraction (XRD), Raman spectroscopy and nanoindentation. The hardness of the NSD film is found to be as high as 80 GPa with CAFM average and root mean square roughness of 7 and 9 nm, respectively, under optimal negative SB at the BEG step. From the studies of substrate temperature effect, the hardness of the NSD film is as high as 70 GPa, with average and root mean square CAFM roughness of 9 and 11 nm, respectively, which were obtained at a substrate temperature of 500 °C. In both cases, the film hardness was found to be affected by the size of clusters, which are composed of many small NSD particles, the amount of NSD in an amorphous matrix as well as surface roughness. We also synthesized transparent NSD films by MPECVD under optimized single-step growth conditions on quartz substrates, which are scratched with several micrometers diamond powder. A hardness as high as 60 GPa and a maximum transmittance of 60% in the visible light region are achieved for an NSD coating of 1.0 μm thickness with small surface roughness.     

Effect of different surface treatments and coatings on the scuffing performance of hardened steel discs at very high sliding speeds

The paper describes the results of scuffing experiments using a high speed two disc rig, in which a range of different surface treatments and coatings was investigated. The rig used is capable of simulating the operation of heavily loaded gear contacts in terms of maximum Hertzian contact pressure (up to 1.7 GPa), temperature (100 °C oil feed), and sliding speed (up to 25 m/s). The reference scuffing performance was that of axially ground case-carburised or nitrided steel discs having a surface finish of 0.4 μm Ra. The different surface conditions were as follows. The ground discs were superfinished to less than 0.1 μm Ra using a proprietary polishing method. Two different super-hard coatings were investigated. The main conclusions to be drawn from the work are: nitrided surfaces are superior to case-carburised surfaces, but it is essential to remove the compound layer (‘white layer’) to achieve durability at high sliding speeds. The benefits of superfinishing are clearly demonstrated; this improves the scuffing performance and also gives lower friction and bulk temperatures. Hard coatings show promise, and the triple combination of nitriding, superfinishing and hard coating gave particularly impressive scuffing resistance in these tests.     

Effect of real longitudinal surface roughness on lubrication film formation within line elastohydrodynamic contact

In many applications (e.g. roller, barrel or needle bearings) surface features exhibit longitudinal alignment to the direction of motion. These features are produced by surface finishing techniques in the circumferential direction and are associated with line or very wide elliptical contact geometries. In such a case, the contact length in the direction of motion is considerably shorter comparing its width and the effect of a longitudinal roughness could significantly influence the lubrication film formation. Recent experimental studies have indicated less severe effect of a longitudinal roughness on lubrication film formation in the comparisons with that observed with transversely orientated roughness caused by the inlet perturbation. Nevertheless, these experimental studies have been focused on the behaviour of artificially produced asperities within a circular contact. The quantitative experimental study of longitudinal real surface roughness within a line contact has not been realized yet. That is why, in this study, the line contact formed between a steel tapered roller and glass disc is observed within an optical test rig and the effects of real surface roughness on lubrication film formation are studied. Experiments carried out under pure rolling conditions have shown that the depth is the key parameter that influences the effect on the film thickness. If the roughness features are shallow, the lubrication film shape within the contact follows the shape of the surface closely. However, the groove having only about 800 nm in depth divided the line contact into two parts that behave as two separate line contacts. Such an effect can increase the risk of the wear of rubbing surfaces as the lubrication film thickness between the real machine components can be significantly lower than expected.     

The break-in stage of cylinder-ring wear: A correlation between fired engines and a laboratory simulator

The wear of the piston ring-cylinder wall contact area in fired engines has not been satisfactorily simulated in bench testers so far. This paper reports the development of a successful test device in which the same progression of surface change occurs as in fired engines. These changes were observed by microscopy, by hardness indentations and by the use of the stylus roughness tracer. The test device uses ring and cylinder segments and it oscillates at 350 cycles min^-1 over a stroke of 19 mm. It is apparently not necessary to duplicate the temperature, atmosphere, fluid film thickness and other obvious conditions in an engine to achieve simulation. At least, the materials in the engine were not affected by the conditions in the engine that were not duplicated in the laboratory tester.The purpose in developing the tester was to study the role of the honed roughness pattern found on most cylinder walls. Several engine manufacturers attempt to achieve “fplateau honing”. No manufacturer polishes new cylinder walls. The reason for the honed roughness is to allow a high wear rate, without catastrophic scuffing, in locations of high stress between poorly conforming parts.     

A Study of Torn and Folded Metal (TFM) on Honed Cylinder Bore Surfaces

Torn and folded metal (TFM) on honed cylinder bore surfaces has been observed and studied for decades. However, there is no objective and quantitative method for ranking the severity of TFM. Studies on its effect on the scuffing resistance of a cylinder bore and piston skirt/piston ring are also very limited. Machined surfaces are currently evaluated and ranked qualitatively based on fax film images. The results may vary greatly depending upon the individual's experience and also the image quality. In this study, an attempt has been made to develop a method that can be used to evaluate cylinder bore surfaces objectively. A “surface index” has been developed for classifying honed bore surfaces. To further examine the effect of TFM on a cylinder bore performance, bench tests have been conducted to evaluate cylinder bore scuffing resistance and its correlation with the bore surface quality in terms of the value of the surface index.     

有机硼酸酯添加剂对摩擦副抗咬死性能影响研究

通过对稀土金属硼系列有机硼酸酯（Ｃ９Ｈ２１ＢＯ３）添加剂的润滑性能抗咬死性能试验研究,发现有机硼酸酯添加剂能显著改善滑动摩擦副的抗咬死性能及润滑油的润滑性能,通过分析磨斑表面Ｘ射线衍射图谱（ＸＰＳ）,发现该种添加剂作用下,摩擦副表面形成了聚合物膜,使得摩擦力减小了,随着载荷的增大,添加剂在摩擦热产生的高温作用下,部分原子如Ｃ、Ｂ同表面金属发生化学反应,生成了ＦｅＣ,Ｂ２Ｏ３,Ｈ３ＢＯ３等物质。结合     

滑动速度对油润滑表面胶合磨损的影响

在油润滑条件下,钢对钢摩擦副的胶合摩损不仅取决于润滑油膜是否破裂,而且取决于在摩擦表面上化学反应膜的形成情况。本文研究了在油润滑条件下滑动速度对钢摩擦副胶合的影响。在低滑动速度下摩擦表面易于形成反应膜,油膜破裂后并不直接发生胶合。胶合发生在高温、高摩擦系数的恶劣条件下。在高滑动速度下油膜破裂后很容易发生胶合,发生胶合前的表面温度和摩擦系数都比较低。     

Friction and Wear Performance of Boron Doped,Undoped Microcrystalline and Fine Grained Composite Diamond Films

Chemical vapor deposition(CVD) diamond films have attracted more attentions due to their excellent mechanical properties. Whereas as-fabricated traditional diamond films in the previous studies don’t have enough adhesion or surface smoothness, which seriously impact their friction and wear performance, and thus limit their applications under extremely harsh conditions. A boron doped, undoped microcrystalline and fine grained composite diamond(BD-UM-FGCD) film is fabricated by a three-step method adopting hot filament CVD(HFCVD) method in the present study, presenting outstanding comprehensive performance, including the good adhesion between the substrate and the underlying boron doped diamond(BDD) layer, the extremely high hardness of the middle undoped microcrystalline diamond(UMCD) layer, as well as the low surface roughness and favorable polished convenience of the surface fine grained diamond(FGD) layer. The friction and wear behavior of this composite film sliding against low-carbon steel and silicon nitride balls are studied on a ball-on-plate rotational friction tester. Besides, its wear rate is further evaluated under a severer condition using an inner-hole polishing apparatus, with low-carbon steel wire as the counterpart. The test results show that the BD-UM-FGCD film performs very small friction coefficient and great friction behavior owing to its high surface smoothness, and meanwhile it also has excellent wear resistance because of the relatively high hardness of the surface FGD film and the extremely high hardness of the middle UMCD film. Moreover, under the industrial conditions for producing low-carbon steel wires, this composite film can sufficiently prolong the working lifetime of the drawing dies and improve their application effects. This research develops a novel composite diamond films owning great comprehensive properties, which have great potentials as protecting coatings on working surfaces of the wear-resistant and anti-frictional components.     

Factors Influencing the Abrasion Resistance of Thin-Film Magnetic Media

Abrasion testing was performed on DC sputtered hydrogenated carbon films on commercial thin-film disks processed under several conditions. Four film characteristics were found to influence the abrasion resistance of the disk, i.e., roughness, film adhesion, carbon toughness, and coefficient of friction. It was found that both fine-scale surface topography from the sputtered layer and substrate texture degraded abrasion resistance. Excellent abrasion resistance was observed for carbon films as thin as 10 nm on polished substrates when the magnetic film topography was minimized. Film adhesion was degraded by exposure to temperature and humidity. Hydrogen incorporation into the carbon films reduced film hardness which degraded abrasion resistance.