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.     

The effect of phosphating on the friction and wear properties of grey cast iron

The friction and wear resistance of two commercial manganese phosphate coatings have been evaluated. Grey cast iron wear pins were treated by the two processes and were tested by sliding against a steel disc, under both lubricated and dry sliding wear conditions.Phosphating increases the sliding distance to scuffing as well as the scuffing load, whilst marginally reducing the coefficient of friction. No advantage was found in phosphating dry sliding surfaces.Phosphating reduces the likelihood of adhesive wear in marginal or poorly lubricated sliding couples. The choice of phosphate coating is primarily dependent on the surface finish of the sliding counterface; thin coatings are suitable if the counterface is smooth but thicker coatings are superior against rougher surfaces.     

Scuffing Performance of Amorphous Carbon During Dry-Sliding Contact

Scuffing is a major problem that limits the life and reliability of sliding tribo-components. When scuffing occurs, friction force rises sharply and is accompanied by an increase in noise and vibration; severe wear and plastic deformation also occur on the damaged surface. Attempts have been made over the years to combat scuffing by enhancing the surface properties of the machine elements, and by methods involving lubricant formulation and coating application.

In this study, the authors evaluated the scuffing performance of an amorphous, near-frictionless carbon (NFC) coating that provides super-low friction under dry sliding conditions. The test configuration used a ball-on-flat contact in reciprocating sliding. The coating was deposited on HI3 steel. An uncoated 52100 steel ball was tested against various coated flats in room air. Compared to uncoated surfaces, the carbon coating increased the scuffing resistance of the sliding surfaces by two orders of magnitude. Microscopic analysis shows that scuffing occurred on coaled surfaces only if the coating had been completely removed. It appears that depending on coating type, the authors observed that coating failure occurs before scuffing failure by one of two distinct mechanisms: the coating failed in a brittle manner and by spoiling, or by gradual wear.     

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 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.     

Condition monitoring of marine diesel engines through ferrographic oil analysis

The effect of surface hardness on scuffing was investigated using a two-disc machine. The changes in the surface conditions, especially surface roughness, surface hardness and the formation of an oxide film, in the running-in process have a marked influence on the scuffing resistance. Therefore a hard disc does not always have a high resistance to scuffing. In this investigation, quenched discs of the highest surface hardness, in which the improvement in surface roughness and the formation of an oxide film are expected to be small, offered the poorest resistance to scuffing. By contrast, annealed discs of the lowest hardness, in which a diminished surface roughness and an increased surface hardness caused by the work hardening that necessarily accompanies the formation of an oxide film are expected, exhibited a considerably higher resistance to scuffing.     

The Durability of Gear and Disc Specimens—Part II: Post Failure Examination and Gear-Disc Correlation

In this paper the authors present a post failure examination of the gear and disc tests reported in Ref (I) on a standard carburized steel (BS SI56), a temper resistant carburized steel (AISI M50NiL), and a carburized and nitrided temper-resistant steel (AISI M50NiL Duplex).

An analysis of the relationship between the gear and disc tests is also presented. The fatigue lives of the gear tests correlated well with the chamfered disc tests. However, this was thought to be fortuitous, due to the different stressing of the specimens. The fatigue lives of the crowned disc tests gave longer lives than the gears for equivalent stresses. As the slide-roll ratio was increased towards that of the highest point of single tooth contact (?0.26) the correlation improved. At a slide-roll ratio of ?0.28 on the disc specimens, where the correlation of pitting life might have been expected to be the best, scuffing of the disc occurred, possibly due to the higher temperatures. Several factors thought to be responsible for the difference in lives between gear and disc specimens have been discussed. The roughness and slide-roll ratio have been deemed the most important.     

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.     

Scuffing resistance of phosphate esters

Scuffing tests with phosphate esters were conducted under very severe conditions of lubrication using a reconstructed Timken tester. Aryl phosphates were more effective as extreme pressure agents than alkyl phosphates. The difference in scuffing resistance between aryl and alkyl phosphates is attributed to the difference in the surface films produced by the reaction between the phosphate esters and the steel surface. Aryl phosphates formed iron phosphide and alkyl phosphates formed iron phosphate.     

渗氮GCr15钢在含氮硼酸酯添加剂润滑油作用下的摩擦学性能

针对提高滚动轴承使用寿命的问题,提出离子渗氮处理与添加剂结合的解决办法。用等离子渗氮炉在GCr15轴承钢表面制备了渗氮层,并用扫描电子显微镜(Scanning electron microscope, SEM )、X射线衍射仪(X-ray diffractometer, XRD)和显微硬度计观测了渗层的形貌、相结构和硬度变化。利用四球摩擦磨损试验机对比考察渗氮钢和未渗氮钢在含氮硼酸酯润滑条件下的摩擦学性能,并用X射线光电子能谱仪(X-ray photoelectron spectroscope, XPS)分析摩擦反应膜的成分与化学结合态。结果表明,渗氮层与润滑油添加剂之间发生了意想不到的良好交互作用,渗氮钢在质量分数为1.25%氮硼酸酯的润滑条件下表现出最低的摩擦因数和磨斑直径,比未渗氮钢分别降低了34%和45%;分析证明,渗氮层的摩擦表面生成了高BN质量分数的摩擦反应膜是获得优异摩擦学性能的主要原因,而在未渗氮钢摩擦表面未检测到BN。     

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.     

Effect of nitrogen on the corrosion-erosion synergism in an austenitic stainless steel

High temperature gas nitrided AISI 304L austenitic stainless steel containing 0.55 wt% N in solid solution, was corrosion, erosion and corrosion-erosion tested in a jet-like device, using slurry composed of 3.5% NaCl and quartz particles. Scanning electron microscopy analysis of the damaged surfaces, mass loss measurements and electrochemical test results were used to understand the effect of nitrogen on the degradation mechanisms. Increasing the nitrogen content improved the corrosion, erosion and corrosion-erosion resistance of the AISI 304L austenitic stainless steel. Smoother wear mark contours observed on the nitrided surfaces indicate a positive effect of nitrogen on the reduction of the corrosion-erosion synergism.     

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.     

Surface Deteriorations During Scuffing Process of Steel and Analysis of Their Contribution to Wear Using In Situ Synchrotron X-Ray Diffraction and Optical Observations

In a previous study, we developed a novel in situ analysis and observation system that allows for simultaneous synchrotron X-ray diffraction (XRD) and optical observations of a frictional surface. This in situ system was used to investigate the scuffing phenomena of SUJ2 bearing steel (AISI 52100); characteristic surface deteriorations occurred during the scuffing process, including plastic flow, heat-spot formation, austenite transformation, and a decrease in the width of the XRD peaks (indicating a decrease of dislocations and strain). These surface deteriorations are not observed during normal wear, hence it is possible that they cause catastrophic wear during the scuffing of steel. In this study, to elucidate the scuffing mechanism of steel, we focused on the following two points: (1) whether the above surface deteriorations are unique to SUJ2 steel or whether they occur in general steels as well, and (2) the extent to which these surface deteriorations contribute to the wear amount. To achieve these objectives, we performed scuffing tests on four types of steel using the previously developed in situ system. In particular, we focused on the first stage of the scuffing process. The present test results suggest that these surface deteriorations also occur in general steels, and that plastic flow and heat-spot formation, which originate from the same phenomenon, are the dominant contributors to the wear amount during the scuffing of steel. Furthermore, the wear amount per unit plastic flow appears to be independent of steel composition.     

Galling resistance evaluation of tool steels by two different laboratory test methods for sheet metal forming

Adhesive accumulation of work material on the tool surface is today a major problem in many sheet metal‐forming applications. Different laboratory test methods are used to investigate galling with respect to different tool materials, lubricants and process conditions. In the present study, the galling resistance of a modern nitrogen‐alloyed powder metallurgy tool steel and an conventional ingot cast D2 type tool steel was evaluated under lubricated sliding against ferritic stainless steel sheets using a commercial pin‐on‐disc (POD) and an in‐house made slider‐on‐flat‐surface (SOFS) tribotester. The investigated tool steels ranked similarly in terms of galling resistanc in both test methods. However, sliding distances to galling were longer for the SOFS equipment due to continuous sliding on new lubricated sheet surface. Best performance was demonstrated by the powder metallurgy tool steel treated to 65 HRC. Differences in friction behaviour and galling initiation were analysed on the basis of the two different working conditions, i.e. open (SOFS) and closed (POD) tribosystems. Copyright © 2012 John Wiley & Sons, Ltd.     

High temperature wear resistance of (TiAl)N PVD coating on untreated and gas nitrided AISI H13 steel with different heat treatments

The wear resistance of a PVD (Ti_0.7Al_0.3)N coating deposited on an as-received and gas nitrided AISI H13 has been examined by using ball-on-disc tests at room temperature and at 600 °C. In order to determine the influence of a previous heat treatment on this type of steel on the wear resistance of the (Ti_0.7Al_0.3)N coating, two commercial heat treatments were employed which gave rise to the same substrate hardness. Surface microhardness measurements have been carried out to determine the load-carrying capacity of the coated systems. In general, the wear behavior was found to be independent of the nature of the heat treatment applied to the substrate prior to the nitriding process but strongly dependent on the testing temperature. At room temperature, there were small variations between the different systems tested, whereas at high temperatures, clear differences were found between them. At 600 °C, a typical temperature that could be achieved during the aluminum extrusion processes, the nitrided H13 steel/(Ti_0.7Al_0.3)N PVD duplex coating shows a satisfactory wear resistance compared to both the nitrided steel and the steel substrate only coated with (Ti_0.6Al_0.4)N, which exhibited the worst performance. The satisfactory wear resistance observed for the duplex coating system at high temperature is mainly a consequence of two different aspects. Firstly, its higher load-carrying capacity due to the existence of a hard nitrided layer, as well as its high H/E ratio. Both parameters allow the presence of higher elastic strains without the failure of the ceramic layer, which would normally occur in the case of TiAlN PVD coatings deposited directly on the AISI H13 steel. Secondly, the intrinsic characteristics of the coating, i.e. its chemical constitution, which allows the formation of a dense oxide mixture inside the wear track that impedes both its further oxidation and the deterioration of the mechanical properties as consequence of nitrogen diffusion.     

Wear resistance and anti-sticking properties of duplex treated forming tool steel

The aim of this work was to investigate the potential of using hard physical vapour deposition (PVD) coatings on forming tools, as well as to determine the influence of plasma nitriding on the load-carrying capacity and wear resistance of coated tool surfaces. A load-scanning test rig was used for evaluation, where duplex treated cold work tool steel samples were loaded against soft austenitic stainless steel and hardened ball bearing steel, respectively. Four different coatings (TiN, TiB₂, TaC and DLC) and two substrate treatments (hardening and plasma nitriding in two different gas mixtures) were included.Plasma nitriding alone significantly improved the friction, wear, and anti-sticking properties of the tool steel. PVD coating, and especially PVD coating of nitrided tool steel further improved the performance. Therefore, from the point of view of tool life as well as work peace surface quality, the DLC coating with its excellent anti-sticking properties and sufficiently good wear resistance represent the best solution for forming tool applications of austenitic stainless steel.     

A comparative study on the tribological behaviors of nitrided and sulfur-nitrided 35CrMo steel lubricated in PAO base oil with MoDTC additive

A nitrided layer on 35CrMo steel was prepared by the ion nitriding process, and then a sulfur-nitrided layer was obtained by low temperature ion sulfuration. The results showed that both the nitrided and sulfur-nitrided surfaces improved the wear resistance efficiently under PAO lubrication, and exhibited the best wear resistance and friction-reducing property under PAO with 0.75% MoDTC lubrication. Compared with the plain and nitrided surfaces, the sulfur-nitrided surface exhibited the best synergistic effect with MoDTC displaying the lowest friction coefficient and wear volume. The mechanism of the best effect was due to MoS₂ and FeS formed on the sulfur-nitrided surface.     

A comparative study on the tribological behaviors of nitrided and sulfur-nitrided 35CrMo steel lubricated in PAO base oil with MoDTC additive

A nitrided layer on 35CrMo steel was prepared by the ion nitriding process, and then a sulfur-nitrided layer was obtained by low temperature ion sulfuration. The results showed that both the nitrided and sulfur-nitrided surfaces improved the wear resistance efficiently under PAO lubrication, and exhibited the best wear resistance and friction-reducing property under PAO with 0.75% MoDTC lubrication. Compared with the plain and nitrided surfaces, the sulfur-nitrided surface exhibited the best synergistic effect with MoDTC displaying the lowest friction coefficient and wear volume. The mechanism of the best effect was due to MoS₂ and FeS formed on the sulfur-nitrided surface.     

Improving the Properties of Low-Alloy and Carbon Steel Tools by Cyclic Nitriding

Cyclic nitriding characterized by uniform alternation of cycles with high and low nitrogen potential is considered. In the active stage of the process, the surface layer of the steel is saturated with nitrogen, forming a nitride zone. In the passive stage, the nitrides are resorbed and the internal nitriding zone expands. The phase composition of the nitrided layer in У8 and ХВГ tool steels may be regulated so as to increase the wear resistance of the steel and form a smooth hardness gradient over the surface layer. The length of the active and passive stages such that the wear resistance of the steel is 2.5 times greater than in traditional nitriding by ammonia is determined.