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 $\text{ξ}{}_{\mathrm{?}}$.     

Temperature—The Key to Lubricant Capacity

Failure of a nonreactive mineral oil can be predicted by Blok's formula for determining the maximum temperature between two bodies in rolling and sliding contact. Evaluation of many lubricants on a geared roller lest machine revealed that the lubricant failure for any particular lubricant-material combination occurs at a constant, critical contact temperature over wide ranges of load, sliding velocity, surface velocity, specimen temperature, film thickness, and viscosity grade. Coefficient of friction can be predicted by a parameter involving the unit load, inlet viscosity, sum velocity, and sliding velocity. The load capacity of a lubricant varies inversely with specimen temperature for a constant set of lest conditions. Electrical resistance measurements across the contact zone aided in identifying the lubricant failure point and in revealing the action of two deposit-forming additives.     

Failure criteria in thin film lubrication with EP additives

The scuffing behaviour of sliding couples made from 12 NiCr 13 steel, lubricated with SAE 90 and SAE 20 W30 oils with and without additives, was studied as a function of relative sliding speed and bulk oil temperature. It was found that the scuffing load decreases almost inversely with sliding speed. Calculations which take into account the decrease in hardness at increasing temperature show a fairly good constancy of total contact temperature (bulk + flash temperature) at scuffing (i.e. values ranging from 550 ° to 650 °C for all oils and test conditions). At low speeds (up to 1 m s^?1) a well-defined increase in scuffing load was found when EP additives were used; at higher speeds this effect was found to have vanished completely. Variations in nominal contact pressure in the range 1 to 3 had no appreciable influence on the scuffing load, indicating that, in the present case, scuffing was associated with a transition from the boundary lubrication regime to the severe wear regime.     

The Effect of Lubricant Traction on Scuffing

This paper presents the results of a disc machine gear simulation investigating the influence of lubricant traction characteristics and formulation on the load at which scuffing occurs. Scuffing theories in general link the onset of scuffing to the amount of heat generated in the contact and the authors hypothesized that reduced heat generation with low traction lubricants should lead to an increase in scuffing load. The study compared low traction PAO-based lubricants with mineral oils in basestock, antiwear and EP formulations and at both high (>6) and moderate (approximately 1.2) specific film thickness, λ. At λ > 6, the benefits of the synthetics over their mineral counterparts ranged from 25 percent to 220 percent and at λ ? 1.2, the benefits were a uniform 40 percent. It was particularly interesting to observe that the antiwear PAO-based oil gave a similar scuff load per unit contact width to an EP mineral gear oil. In addition, it was shown that scuffing load decreased with increasing traction coefficient to the power of approximately ? 1.85, close to the ?2.00 power predicted by the frictional power intensity concept. The agreement with flash temperature theory, with a predicted power of ?1.33, was less close.     

Failure criteria in thin film lubrication: Investigation of the different stages of film failure

The mechanisms of failure of sliding lubricated concentrated steel contacts have been studied using a newly developed tribometer which allows fast separation of the sliding surfaces at the different stages of film failure. The appearance of the corresponding contact topography was investigated by means of optical interference microscopy. The results indicate a gradual transition from full to partial elastohydrodynamic (EHD) lubrication as a function of increasing load and increasing bulk oil temperature until complete failure of the EHD film occurs at critical triplets of normal load, sliding velocity and bulk oil temperature.     

Scuffing of cast iron and Al390-T6 materials used in compressor applications

Scuffing can lead to catastrophic failure in engineering components, but the surface mechanism that leads to scuffing is not well understood. In this experimental study, the surface and subsurface changes on gray cast iron and Al390-T6 were investigated under starved lubrication (a mixture of R410A refrigerant combined with a polyolester lubricant) and pure sliding. Controlled tribological experiments were conducted using a high-pressure tribometer to simulate the contact conditions found in typical air conditioning compressors. Tribological tests were conducted on several sets of tribo-pairs consisting of gray cast iron disks and gray cast iron pins, for periods of 25, 50, and 75% of the time-to-scuff in order to investigate the progression of surface and subsurface changes leading to scuffing. The evolution of the chemical composition in the topmost surface layer of the disks was investigated as the disk/pin tribo-pairs progressed towards scuffing. Important changes in chemical composition were observed on the disks that had scuffed. Specifically, a marked decrease in manganese compounds was measured, which indicates a significant loss of the protective coating that was applied to the disks. Vickers microhardness tests were also performed on the disks at all stages, and showed a reduction in the means of hardness measurements as the disk neared the scuffed stage, in agreement with other studies. Similar scuffing experiments were also performed with tribo-pairs of Al390-T6/52100 steel; surface and subsurface changes were also characterized using chemical analysis and cross-section scanning electron microscope (SEM). In this case, depletion of silicon, which is embedded in the aluminum matrix to increase the hardness of the alloy compounds, was observed at scuffing. This observed reduction in hardness not only agrees with the cast iron hardness study, but also was consistent with the chemical analysis of the cast iron where a chemical component associated with hardening is depleted at the instance of scuffing.     

Scuffing as a Desorption Process—An Explanation of the Borsoff Effect

The hypothesis that scuffing of surfaces lubricated by a plain mineral oil results from desorption is tested for high speed cases using two types of apparatus. One uses a medium speed, 1-inch steel ball, 4-ball machine, and a simulated oil consisting of hexadecanoic acid in hexadecane. This lubricant is shown to behave in the same fashion at medium speeds as it does at very slow speeds. Thus, a graph of logarithmic concentration of acid against reciprocal absolute scuffing temperature gives a good straight line, just as at slow speeds. In the second high speed gear tester, the rise in scuff load of gears at very high speeds, the Borsoff effect, can be explained semiquantitatively by considering the residence time of molecules. These two types of results suggest that the desorption-failure mechanism may be valid over the whole rubbing speed range, and not merely at the very low speeds normally tested.     

Signature Response for Friction and Wear

An experimental study of signature response for friction and wear is carried out in a triplane ball-on-peg machine under varying load conditions. Photographs of worn roller test specimens at critical temperatures were taken to determine the nature of the wear surface under boundary lubricated conditions. Acoustic response in the beginning and at the point of scuffing has demonstrated a definite trend which follows a similar pattern as that of friction. The experiment was repeated with a combination of steel and brass and with steel and mild steel using as the lubricants hexadecane, MoS₂ and light stock oil. The scuffing load was accompanied by severe wear of the surface. A critical value for friction and that of acoustic response was observed with variations in temperature. Results obtained contribute to the assesment in the change in friction and wear values on sliding surfaces. This help comes from a knowledge of acoustic emission and the known condition of the surface and both components of the pair. The findings of these experimental results and the mode of signature response provides a good scope for further investigations aimed at understanding the mechanism and establishing a method for diagnostic maintenance.     

Scuffing failure analysis and experimental simulation of piston ring–cylinder liner

The scuffing failure phenomenon of piston ring–cylinder liner is studied theoretically and experimentally. The load and bulk temperature when scuffing failure occurs are measured under different engine speed, lubricant, and environmental temperature in a bench test. Based on the experimental results, the asperity capacity when scuffing occurs is evaluated. Surface contact temperature is determined with the measured bulk temperature and the surface flash temperature calculated by Blok theory. The scuffing failure threshold of piston ring–cylinder liner is established by using specific oil film thickness. This revised version was published online in June 2006 with corrections to the Cover Date.     

Influence of surface roughness on friction and scuffing behaviour of cast iron under sparse lubrication

Friction and scuffing behaviour of grey cast iron as influenced by the surface roughness under sparse lubrication conditions is studied. The studies are carried out on a three-shoe-on-disc machine under the conditions of parallel sliding. The experiments were conducted at three oil supply rates of 0.10, 0.22 and 0.36 μg/cm² per contact obtained through a mist oiling system and with specimens of cast iron shoes of different roughness values, Ra, between 0.04 and 2.0 μm against a 0.55% C steel disc. In a step load procedure, the friction torque at the end of each load step and the scuffing load are the major parameters measured. Results of friction and scuffing behaviour as a function of roughness and oil supply rates have been discussed.     

Comparative analysis based on adiabatic shear instability for scuffing failure between unidirectional and reciprocating sliding motion

The principal goal of the experiments described here is to study the sliding motion effects on the scuffing life on the basis of adiabatic shear plastic instability. Experimentally we observed that the load capacity of the surface decreased and micro-scuffing initiated frequently under the reciprocating sliding motion as compared to under the unidirectional sliding motion. According to the adiabatic shear instability model, the scuffing initiation occurs when the rate of thermal softening exceeds that of work hardening due to plastic deformation. In order to ascertain the thermal softening in sliding surfaces, the contact temperatures were calculated. We found that the higher friction coefficient under the reciprocating sliding motion caused the higher contact temperature than that under the unidirectional motion. Therefore, the rate of thermal softening could exceed that of work hardening easily under the reciprocating sliding motion owing to frictional heating. We speculated that the scuffing initiation could roughen the sliding surfaces rapidly under the reciprocating sliding motion and confirmed that our assumption demonstrated above, was consistent with the experimental observation. In conclusion, there is a synergy effect in relation to scuffing failure because the frictional heating, surface roughening, and scuffing initiation function together to enhance each other, and consequently, the load capacity of surfaces could decrease under reciprocating sliding.     

On data dispersion in pin-on-disk wear tests

Ten pin-on-disk sliding wear tests for each experimental condition were carried out with a commercial tungsten carbide (WC) pin on silicon carbide (SiC) disks in order to determine the wear and friction data dispersion. The tests were repeated using two sliding speeds (v), 0.1 and 1.0 m/s, and two applied loads (P), 5 and 50 N. The wear data showed a dispersion in the range of 28–47 and 32–56%, for disk and pin, respectively. For the disk, the dispersion decreased when increasing both sliding speed and applied load; for the pin, no clear relationship was found. The friction values spread in the range of 5–15%, with a lower dispersion at high applied load, independent of the sliding speed. From a statistical point of view, it was found that, in all the experimental conditions adopted, about 20% of the wear and friction values can be considered outliers.     

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。     

润滑油对滑动摩擦副咬死性能的影响

在ＭＨＫ－５００型环块磨损试验机上对Ｎ４６＃机械油在不同供同温度下由４５＃淬火试环、试块组成的滑动摩擦副进行了抗咬死性能试验研究。试验结果表明：供油温度对摩擦副的抗咬死性能有较大的影响，在一定的范围内提高润滑油的供油温度有利于提高摩擦副的抗咬死能力。试验还发现：随着润滑油中磨粒的增多，摩擦副咬死极限载荷逐渐下降，且摩擦副失效后表面温度表现出不确定性。润滑同中磨粒状况对摩擦副咬死影响不可忽视。对摩擦     

供油温度对 45~# 淬火钢滑动摩擦副咬死影响的试验研究

在ＭＨＫ－５００型环块式磨损试验机上，在Ｌ－ＡＮ４６全损耗系统用油的不同供油温度下，对由４５＃淬火钢试环、试块组成的滑动摩擦副进行了抗咬死性能试验研究。试验结果表明：供油温度对摩擦副的抗咬死性能有较大的影响。在一定的范围内，提高润滑油的供油温度，有利于提高摩擦副的抗咬死能力。试验还发现：摩擦副失效后表面温度表现出不确定性。润滑油中的磨粒状况对摩擦副咬死影响不可忽视。对摩擦副咬死的研究应考虑润滑油中磨粒随时间改变的因素，基于时变的观点来研究摩擦副咬死机理更具有实际意义。     

Scuffing of Area Contacts Under Starved Lubrication Conditions

The scuffing behavior of 390-T6 and DHT3 aluminum alloys, Si-Pb brass and gray cast iron, sliding against 1018 carburized steel, is experimentally studied under starved lubrication conditions. The major emphasis of the study is on the 390-T6 aluminum alloy. All tests are conducted in a high pressure tribometer (HPT) under RI34a (tetrafluoroethane) environment with polyalkylene glycol (PAG) and polyolester (POE) lubricants to simulate failures of critical tribocontacts in refrigerant compressors. An area contact, pin-on-disc geometry, is used to examine the effects of degree of lubricant starvation, sliding velocity, materials, and lubricant/refrigerant mixtures on scuffing. The scuffing transitions characteristics of 390-T6 aluminum as a function of lubricant supply rate are also examined. The processes leading to scuffing and failure mechanisms are studied by examination of scuffed surfaces and subsurfaces. Based on the experimental observations, it is hypothesized that bulk material failure during scuffing is due to plastic shearing or smearing.     

Abrasive wear behavior of high speed steel and hard metal coated with TiAlN and TiCN

The wear behavior of M2 high speed HSS steel and WC hard metal coated with TiAlN and TiCN were investigated and compared, using the pin on disk standard test with different loads. The coating PVD process has been done by two different suppliers, using an industrial equipment unit with optimized conditions. The coated layers were measured and characterized. The load, sliding distance and velocity of 0.5 m/s were kept constant during the abrasion test in order to control these variables. The counterface disks used were electric steel sheets from three different suppliers. The lost volume and temperature at the pin end have been measured during the wear test. Comparisons of tribological performance for the coated HSS and hard metal were done, using a plot of lost volume versus sliding distance for substrates and coatings. The pin worn surfaces were observed using a scanning electron microscope. A significant increase in the wear resistance of M2 steel and WC hard metal when coated with TiAlN and TiCN was observed. Quality of these coatings depended upon the supplier. Excessive porosity has diminished the TiAlN counting wear resistance from one supplier. However, in general the performance of TiAlN is superior to TiCN. The pin wear rate depended on the disk microstructure.     

点接触弹流润滑性能及应用分析

牵引式无级变速器的传动零件间处于点接触状态，在某一传动比时，相对自转速度为零。本文研究了该状况下各种工况参数如滑动率、滚动速度和载荷等对点接触的弹流润滑性能的影响。研究表明：随着滑动率的增大，摩擦因数增大，油膜最大温升增大；在相同滑动速度下，随着滚动速度的增大，油膜厚度增大，但摩擦因数减小；随着载荷的增大,油膜厚度减小,摩擦因数增大，油膜最大温升增大。     

Antiwear Tribofilm Formation on Steel Surfaces Lubricated With Gear Oil Containing Borate,Phosphorus, and Sulfur Additives

The formation of antiwear tribofilms plays a critical role in the longevity of automotive gears. The focus of this experimental study was on the lubrication efficacy of gear oils with different contents of borate-, phosphorus-, and sulfur-containing additives leading to the formation of protective tribofilms. Experiments were performed with AISI 52100 steel balls sliding against AISI 52100 steel disks in baths of different oils at ambient (～32 °C) and elevated (～100 °C) temperatures under load and speed conditions favoring sliding in the boundary lubrication regime. Friction coefficient responses accompanied by electrical contact voltage measurements provided real-time information about the formation and durability of the antiwear tribofilms. The wear resistance of the tribochemical films was quantified by wear rate data obtained from surface profilometry measurements of wear tracks on the disk specimens and sliding tests performed at ambient temperatures after the formation of the tribofilms during elevated-temperature sliding. Results indicate a strong dependence of tribofilm formation on temperature and type of additives. The slightly lower friction and higher wear resistance obtained at elevated temperatures with blended oils is attributed to the increased chemical reactivity of additives containing borate, phosphorus, and sulfur, leading to the formation of durable tribofilms. Relatively higher wear resistance and faster tribofilm formation were obtained with the borate-enriched gear oil formulations.     

Boundary lubricant adsorption data from high speed four-ball scuffing temperatures

A high speed (1475 r.p.m.) four-ball machine, lubricated with solutions of stearic acid in both paraffin oil and more refined liquid paraffin, has been used to determine scuffing temperatures as a function of load and stearic acid concentration. Frictional heating was reduced to a minimum by using a very short (3 s) running time.A characteristic S-shaped plot of scuffing temperature against load was observed for each solution and solvent. The form of these curves is explained in terms of a gradual change, with increase in load, from predominant elastohydrodynamic lubrication to predominant boundary friction, accompanied by surface damage. By selecting the load at the point of inflection, scuffing temperatures show maximum sensitivity to stearic acid concentration and optimum conditions are attained for the study of boundary adsorption phenomena. The calculated heat of adsorption for stearic acid on steel at this point is about 30 % higher than the value calculated from transition temperatures determined using a low rotational speed of 0.75 r.p.m. It is considered that the higher value reflects that the boundary film functions at higher temperatures at high speeds.