Fine-particle slurry wear resistance of selected tungsten carbide thermal spray coatings

The surface degradation of tungsten carbide based thermal spray coatings when exposed to fine-particle slurry abrasion has been investigated. The coatings that were studied contain binder-phase constituents consisting of either nickel or cobalt. The coatings were deposited onto test cylinders using a detonation gun device. After applying approximately 0.15 mm thickness of thermal spray coating, the coatings were ground, then diamond polished to achieve surface roughnesses of 0.03 μm Ra or less. The coatings were exposed to a three-body abrasive wear test involving zirconia particles (less than 3 μm diameter) in a water-based slurry. Results show that preferential binder wear plays a significant role in the wear of these tungsten carbide coatings by fine abrasives. In the comparison presented here, the coating containing nickel-based binder with a dense packing of primary carbides was superior in terms of retaining its surface finish upon exposure to abrasion. The coating containing a cobalt binder showed severe surface degradation.     

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.     

The effect of counterface surface roughness on the wear of UHMWPE in water and oil-in-water emulsion

The South African gold mining industry is at present involved in a programme whereby the hydraulic stoping machinery currently operating on a water-based fluid will be modified to run on mine service water. The wear of the UHMWPE seals is an area of particular concern. This paper examines the effect of type of lubricant and counterface surface roughness on the wear of UHMWPE. A reciprocating sliding wear rig was used with UHMWPE sliding against AISI 431 at an average speed of 0.25 m/s. The contact pressure was 10 N/mm². Tests were conducted in water and in a 5% oil-in water emulsion (5:95). The surface roughness of the steel was varied in the range 0.1–1.0 μm (centreline average). The results of the tests in water showed that the logarithm of the specific wear rate is proportional to the surface roughness. The results of the tests in 5:95 showed that there is a significant transition in specific wear rate at a surface roughness of approximately 0.35 μm. At surface roughness less than 0.35 μm the specific wear rate in 5:95 is considerably lower than the specific wear rate in water, while at surface roughness greater than 0.35 μm the specific wear rate in 5:95 approaches that in water. However, SEM examination of the surfaces from both series of tests showed that, irrespective of lubricant, there was a change in the mode of material removal at a surface roughness of approximately 0.35 μm. The wear mechanisms are discussed as a function of type of lubricant and surface roughness. It is believed that the topography of the counterface is responsible for the change in the mode of material removal.     

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     

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。     

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.     

Effect of Gear Surface and Lubricant Interaction on Mild Wear

In this study, a twin-disc test machine was used to simulate a rolling/sliding gear contact for three surface finishes, each run with two types of lubricants, thus seeking to develop insight into the tooth flank/lubricant tribological system. The test disc surfaces were case-carburised before the surfaces were produced by: transverse grinding followed by a mechanical abrasive polishing process; transverse grinding only; and transverse grinding followed by preheating as a final finishing step (intended to enhance the build-up of an easily sheared surface boundary layer using a sulphur additive). The twin-disc contact was lubricated with an ester-based environmentally adapted lubricant or a polyalphaolefin-based commercial heavy truck gearbox lubricant. To obtain information about the composition of chemically reacted surface layers, the specimens used were analysed using glow discharge-optical emission spectroscopy. The results indicate that the interactions between different surface finishes and lubricants have different impacts on friction behaviour, wear and the reacted surface boundary layer formed by the lubricant. Running a smooth (polished) surface with the appropriate lubricant drastically reduces the friction. Surface analysis of the ground surfaces gives clear differences in lubricant characteristics. The commercial lubricant does not seem to react chemically with the surface to the same extent as the EAL does. Micropitting was found on all ground discs with both lubricants, though at different rates. The highest amount of wear but less surface damage (i.e. micropits) was found on the preheated surface run with the commercial lubricant.     

Surface lay effect on rough friction in roller contact

Surface lay describes the direction of the predominant surface pattern. A properly designed surface texture configuration has been recognised as a vital issue affecting lubrication and sliding in machinery applications in the literature. Gaining understanding of this tribological phenomenon is no doubt beneficial in facilitating the production of more efficient machine parts and thus reduces production cost. This paper describes an experimental method to investigate the effect of surface lay on lubricated rolling/sliding of ground roller surfaces. By using the rough friction test rig, different surface lay contacts can be simulated and the friction can be measured. Friction behaviour was interpreted in terms of Stribeck curves (friction coefficient as the function of Hersey parameter [ην/p]). Results show that an optimal contact lay angle that provides a minimum friction value is achievable through rig testing. The relative sliding speed direction has a symmetrical effect on friction at the same lay orientation; for sliding speed angles less than about 80°, the larger the angle, the lower the friction, and vice versa.     

DLC coating of boundary lubricated components—advantages of coating one of the contact surfaces rather than both or none

The influence of conventional extreme-pressure (EP) and anti-wear (AW) additives on the wear and friction behaviour of DLC coatings has been investigated. Special emphasis was put on exploring if it is most beneficial to coat only one or both the contacting surfaces and on when and how the coatings may improve the friction situation in sliding contact boundary lubricated systems. Tests were performed in a load-scanning test rig, which allows the normal load to gradually increase during the forward stroke and to correspondingly decrease during the reverse stroke. The sliding speed was set to 0.1 m/s, while the normal load was in the range between 140 and 1700 N (2.4–5.6 GPa).This investigation indicates that, under boundary lubrication conditions, addition of commercial AW and EP additives to PAO oil may significantly improve the tribological properties of DLC coatings. Furthermore, the DLC/steel combination was found to give a smoother running-in process and a better tribological performance than the DLC/DLC and steel/steel combination.     

The detection of subsurface zones in aluminium-based alloys 2017A and 6101A using a positron annihilation technique

This paper presents positron lifetime studies of the subsurface region in aluminium-based alloys 2017A and 6101A, the surfaces of which were exposed to dry sliding. In the case of the 2017A alloy, the total range of the subsurface zone below the worn surface detected using the positron annihilation technique was larger than 120 μm and this value was hardly affected by load and sliding distance. For the 6101A alloy, the subsurface zone ranged from 30 to 500 μm and depended on the applied load and only to some extent on the sliding distance. The obtained results are significantly different from the results achieved for pure aluminium, thus alloying has considerable effect on the subsurface zone formation. The positron lifetime depth distribution characterizing the subsurface zone was correlated with the microhardness profile.     

Super-precision cylindrical machining

A study has been made of the theory and techniques of turning precise circular components. The possibility of such high accuracy cylindrical machining was analysed according to the principle of making a precise circle. In the study, a form of chucking-type preciison cylindrical machining was developed by combining an insensitive vibration cutting mechanism - using a main spindle system which features an air bearing - with superposition superfinishing. In the first process, the work is chucked on the main spindle and machined using a continuously and systematically pulsating cutting force. In the second process, the work is finished by a newly developed superposition superfinishing device which features equivalent grades of ultrasonic vibration stone. Key points of the techniques are a torsional vibration mode tool for producing accurate, high amplitude vibration of the cutting point, and a contrivance for making accurate movements of the superposition superfinishing device. Machined roundness of 0.1–0.2 μm and surface roughness of 0.03–0.09 μm R_max were obtained with plain carbon steels, stainless steels and hardened steels (H_RC 41, 53, 60). It is considered that turning to roundness ≈ 0, cylindricity ≈ 0 and surface roughness ≈ 0 can be realized by means of this new machining process and its lathe.     

Influence of counter material on friction and wear performance of PTFE–metal binary coatings

The study presented in this paper concerns the influence of the counter materials on wear and friction performance of polytetrafluoroethylene (PTFE) reservoirs arranged in distinct patterns on coated surfaces. Al-bronze and Mo coatings were deposited on a mild steel substrate using an atmospheric plasma spray process. Three patterns of PTFE reservoirs were used. Pins, which served as counter surface, were made of three different materials. Wear tests were carried out in a pin-on-disc test rig at room temperature and under dry contact conditions. The tests were carried out at a constant pressure of 10 MPa. An average linear speed of the test disc was 0.036 m/s. The importance of appropriately matched hardness of two surfaces in sliding contact is emphasised. Due to inadequate hardness of the counter material, performance of Mo coating was adversely affected and expected beneficial action of PTFE reservoirs severely hindered. Al-bronze coating proved to be performing far better than Mo coating. Serious deterioration of Mo coating occurred faster than that for Al-bronze coating.     

Wear and scuffing characteristics of composite polymer and nickel/ceramic composite coated piston skirts against aluminum and cast iron cylinder bores

The purpose of this research was to evaluate the tribological behavior and compatibility between coated piston skirts and aluminum or cast iron bore counterfaces. Aluminum piston skirts with either composite polymer coatings (CPCs) or nickel/ceramic composite coatings (NCCs) were evaluated. Among the NCC coated piston skirts, Ni–P–BN showed consistent low wear on either cast iron or the aluminum bores. The tin plated piston skirt generated low wear depths on cast iron or 390 Al bore surfaces, but higher wear depths on 413 Al or 356 Al bore. All the CPCs generated much less wear on cast iron or aluminum cylinder bores compared with the Ni–P–SiC or Ni–P–Si₃N₄ skirt coatings. Even the wear tests using 413 Al and 356 Al bores showed very low wear depths. Among the CPCs, two coatings with different percentages of molybdenum disulfide and graphite particles dispersed in the resin generated the lowest wear on 390 Al bore. Using a CPC over a hard-anodized surface, the bore wear depth was further reduced and became much more consistent compared with using a CPC alone. The response of the coatings to a simulation of the oil starvation associated with scuffing conditions revealed that the CPCs had intrinsic resistance to scuffing. However, the durability was not very good. The Ni–P–BN coating had intrinsic resistance to scuffing and good durability when sliding against 390 Al bore in the unlubricated conditions. The hard anodized surfaces with the CPCs showed much improved coating durability with good scuffing resistance.     

接触应力和相对滑动速度对金属表面自修复膜生成的影响及机制

采用羟基硅酸镁粉体作为润滑油添加剂,在MMU-5G材料端面摩擦磨损试验机上,研究了不同接触应力和相对滑动速度对45^#钢/45^#钢摩擦副磨损表面自修复膜生成的影响及其机制,借助SEM及EDS测试分析摩擦副的表面形貌及表面成分组成。结果表明,接触应力和相对滑动速度对羟基硅酸镁粉体添加剂在磨损表面形成自修复膜影响显著。在接触应力为1.53,3.06,4.59,7.64MPa和在相对滑动速度0.416m/s的工况条件下,试样磨损表面有自修复膜生成。接触应力为3.06MPa和相对滑动速度为0.416m/s时,易于在短时间内达到磨损-自修复动态平衡,自修复效果最为理想。自修复膜的生成过程包含磨粒磨损和摩擦化学反应2个阶段。自修复膜的生成使得试样摩擦磨损表面平整光滑,可以有效降低金属磨损。     

Compositional changes in lubricated sliding metal surfaces related to seizure

This paper presents an investigation into the mechanisms of failure in lubricated sliding metal contacts. Reciprocated sliding with cylinder-on-disc geometry was performed with three types of lubricants based on polyalphaolefin (PAO) oils and three sets of additives. The normal force and sliding speed were chosen to give partial scuffing or seizure within a few hours. The chemical surface films which form through reactions between additives in the lubricants and the metal surfaces were analysed by scanning electron microscopy and Auger electron spectroscopy, before and after the onset of seizure. It is concluded that all three oils formed a rigid surface film as a result of a combination of chemical and mechanical actions in the contact surface. Seizure was initiated by mechanical fatigue and disruption of the film which exposed the metal surfaces to severe scuffing. It Was Also Noticed that different additives gave different friction and wear properties to the contact system.     

The role of transfer film and back transfer behavior on the tribological performance of polyoxymethylene in sliding

The role of transfer films formed during the sliding of polymer composites against steel counterfaces was studied in terms of the tribological behaviors of the composites. The composites were prepared by compression molding and sliding tests were run in pin-on-disk sliding configuration. The counterface was made of tool steel hardened to 55–60 HRC and finished to a surface roughness of 0.09–0.10 μm Ra. Wear tests were run for 6 hrs at the sliding speed of 1.0 m/s and contact pressure of 0.65 MPa. Transfer films formed on the counterfaces during sliding were investigated using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The results showed that as the transfer film became smooth and uniform, the wear rate decreased. The examination of worn surfaces using Energy Dispersive Spectroscopy (EDS: dot mapping mode) showed the back-transfer of the steel counterface material to the polymer pin surface. This behavior is believed to strengthen the polymer pin surface during sliding thereby contributing to the decrease in wear rate. This paper was recommended for publication in revised form by Associate Editor Jae Cheon Lee Minhaeng Cho received his B.S. and M.S. degrees in Mechanical Engineering from Chung-Ang University, Seoul in 1993 and 1995, respectively. He received his M.S. degree in Materials Science and Engineering from Oregon State University in 2000, and his Ph.D. in Mechanical Engineering from Iowa State University in 2004. Dr. Cho is currently an Assistant Professor at the School of Mechanical Engineering at Chung-Ang University in Seoul, Korea. His research interests are in the area of tribology, surface phenomena, and functional surfaces such as laser surface texturing and ultra-thin coatings.     

Tribological behavior of boundary lubricated sliding surfaces using three different spacing of surface profiles

The ball-on-disk type sliding tests with boundary lubricated steels were carried out to verity the effect of initial spacing in surface profiles on wear and scuffing. Three kinds of surface spacing, which are closely related with initial surface micro-cracks on sliding surfaces, were produced onAISI 1045 steel surfaces using different grinding and polishing processes. Frictional forces and time to scuffing were measured, and the shape and amount of wear particles were analyzed to compare the with original surface profiles. From the tests, it was confirmed that the size of wear particles are related closely to the original spacing of the surface profile. The time to failure and amount of wear were sensitive to the surface spacing. The wider surface spacing shows much longer sliding life and smaller amount of wear than the others. Time to scuffing was increased with increasing surface profile spacing. The size of wear particles increased while the wear and wear rateK were decreased with an increase in surface spacing. After the sliding tests, surface cracks of inner parts of the wear track formed due to scuffing were observed and compared among the specimens having the different surface spacing.     

Defect characterization of roller bearing surfaces with laser Doppler vibrometry

Laser Doppler vibrometry is presented as an effective method of surface profile measurement and surface crack detection in rolling bearing balls. For both inspections, a surface velocity profile is used in addition to surface displacement profile. An experimental setup and a signal-processing algorithm are used to obtain digitized surface profiles and power spectral density measurements of ball surfaces. Profile measurements are presented that match lobed ball frequency results with a resolution of 0.025 μm. Velocity measurement provides a much higher bandwidth for defect detection. Cracks down to 22 μm in width can be detected with a 150 μm by 500 μm wide laser spot size. Cracks smaller than 22 μm can be detected by decreasing the laser spot size. The method is well suited for a manufacturing environment because of its durability, simplicity, and high speed.     

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.     

Frictional properties of some additives for sliding guide way lubricants in a sliding speed range between 0.002 and 1.5 m/s with a thrust collar friction tester

Frictional properties of some additives used in sliding guide way lubricants were studied for a pair of mild steels under sliding speeds ranging from 0.002 to 1.5 m/s with a thrust collar type friction tester. As the sliding speed was decreased, the coefficient of friction first decreased, then after reaching the minimum value, it increased. The minimum friction coefficient μ_min and the transition sliding speed V_min showing μ_min depended upon the additive. That is, μ_min for the oil containing sulfurized lard was lower than those for the other oils tested and its V_min was as low as 0.005–0.1 m/s compared with the transition speeds of 0.2–0.5 m/s for the other oils tested. Observations of the surface profile and surface analyses of the test specimen after the tests indicated that the unique phenomena for sulfurized lard were due to the formation of a boundary film containing a long carbon chain with a chemical bond between the sulfur and carbon.