Investigation of Scuffing Resistance of Piston Rings Run against Piston Ring Grooves

The hot scuffing phenomenon of a piston ring in a ring groove is one of the important failure mechanisms in an automotive engine. The dynamic motion of a piston ring against the ring groove is very complicated. Five motions are usually involved, including radial sliding, circumferential sliding, impacting, rocking, and tilting. Experimental or theoretical studies in this field are rarely reported. To investigate the hot scuffing behavior of piston rings run against various coated ring grooves, a previously developed tester with the capability of simulating most of the relative motions between the ring and the ring groove at an elevated temperature has been further modified and engineered. Several coating materials on piston top ring grooves have been evaluated in terms of their scuffing resistance using the tester. The results have indicated that the anodized aluminum piston groove has the best scuffing resistance when run against a stainless steel ring, and the Ni-based composite coated aluminum ring groove is better than the tin plated ring groove or the bare ring groove but not as good as the hard anodized one.     

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.     

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.     

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.     

缸套/活塞环摩擦学性能试验机的设计研究

根据缸套 /活塞环胶合失效试验研究的需求 ,设计了一台集试验、测量和控制于一体的往复式摩擦磨损试验机。该设备具有加热和温度控制功能 ,采用快速响应速度的微细热偶测量接触区温度 ,同时通过四臂电桥测电阻法测量缸套 -油膜 -活塞环之间的动态接触电阻 ,以反映接触区的润滑状态。试验表明该试验机性能稳定 ,可测物理量丰富 ,是胶合试验研究可靠的实验工具     

Piston Assembly Design for Improved Thermal-Tribological Performance

The tribological system in the piston assembly of an internal combustion engine includes contacts at interfaces of piston/piston ring/cylinder liner, piston skirt/cylinder wall, and piston/piston pin/ connecting rod. The thermal and tribological properties of the piston, piston rings, and cylinder wall are critical to the life and quality of the engine. Severe wear and scuffing failure, especially at the ring/ring groove and ring/liner interfaces, may present a major problem if the piston temperature is too high. Temperature considerations for the piston often limit the effort to increase the engine power.

A new engine piston incorporating the heat pipe cooling technology has been developed for reducing the piston temperature, especially in the ring land and along the piston wall. The current work aims at investigating the effect of reciprocating heat pipes on heat conduction in the piston, and thus the tribological behavior of the piston assembly. Due to the high thermal conductance of the reciprocating heat pipe, a considerably large amount of combustion heat, which is conventionally conducted through the piston wall, is transferred through heat pipes. This new design will result in a lower temperature on the piston wall and a reasonably low temperature distribution in the piston.     

Influence of DLC coating thickness on tribological characteristics under sliding rolling contact condition

Diamond-like-carbon (DLC) coating of thickness 3 and 10 μm were developed with and without radical nitriding pretreatment on steel rollers and spur gear pair. The friction coefficient and wear amount were evaluated under sliding rolling contact condition in vacuum and under oil lubrication. Delamination of coatings was observed at the interface of the substrate. The wear resistance of coatings improved with the thickness of the coating. In vacuum both the roller and the gear pair of 10 μm coating thickness with radical nitriding showed identical wear behavior. The radical nitriding seemed to enhance the life of DLC coatings.     

铀表面铝镀层热应力的有限元分析

对铀表面磁控溅射沉积铝镀层的热应力进行了热弹塑性有限元分析。结果表明：镀层内的热应力较大,达到铝的屈服强度,镀层界面两侧存在明显应力梯度,试样侧边存在由于边缘效应引起的应力分布不均匀性,至侧边约4倍镀层厚度处,应力分布不均匀性逐渐消失；沉积温度升高,界面塑性应变明显增大,镀层弹性模量和泊松比对镀层界面热应力和塑性应变的影响较小,而屈服强度的影响较大,减薄镀层厚度有利于改善镀层界面的热应力和塑性应变。     

水力加压器组合密封结构设计及参数优化

为了提高水力加压器密封性能,设计一种由滑环与O形密封圈组成的组合密封;利用流体压力渗透载荷的加载方法对密封结构进行有限元仿真,得到单因素滑环结构参数对密封性能的影响规律;利用正交试验,分析多因数滑环结构参数综合作用对活塞密封性能的影响。研究结果表明:滑环沟槽底部厚度、滑环侧边宽度、滑环高度、活塞单边径向密封间隙对动密封面接触压力影响依次减弱,新型密封结构选择滑环高度6.5 mm、滑环侧边宽度2.65 mm、滑环沟槽底部的厚度0.7 mm、单边径向间隙0.25 mm时,其最大接触应力比常规O形密封圈结构提高了245%;新型密封结构中的动密封面接触应力比常规O形密封圈结构有了显著的提高,提高了水力加压器的密封性能。     

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.     

The maximum tensile stres on a hard coating under sliding friction

In the friction of a hard coating the maximum tensile stress in the sliding direction generated at the friction surface is important for predicting crack propagation in the coating. The finite element method is employed to evaluate the stress field in the hard coating and the substrate under frictional loads, and the ratio between the values of maximum tensile stress and the maximum contact pressure is calculated under various contact conditions. Finally, a simple equation is introduced for the calculation of the maximum tensile stress at the friction surface. This equation is a function of friction coefficient, maximum contact pressure, coating thickness, contact width and elastic moduli of coating and substrate, and gives the value of the maximum tensile stress which is affected by the existence of the substrate.     

A review of scuffing models

The mechanism of wear known as scuffing has been a research topic of great interest for many years. However, the question of how scuffing is initiated and the factors that contribute to its occurrence are still poorly understood. In general, it can be said that scuffing manifests itself as the sudden failure of lubricating films in mechanical equipment operating under extreme conditions of load and/or speed. Components such as cams, tappets, gears and piston rings are all prone to scuffing failure. Understanding the mechanisms that initiate failure would enable the development of criteria for scuffing prediction. This paper reviews the numerous scuffing models and theories that exist today and, in doing so, defines the important factors involved in scuffing initiation. Emphasis is given to existing theoretical scuffing models which have been correlated by a wealth of research data obtained from experimental test rigs.     

An integrated approach to evaluating the tribo-contact for coated cutting inserts

The orthogonal machining process when end turning medium carbon and austenitic stainless steels with cemented WC-Co tools coated with single-layer (TiC), two-layer (TiC/TiN), and three-layer (TiC/Al₂O₃/TiN) hard thin films was investigated. Extensive experimental investigations including the thermal, mechanical and tribological responses of the tribo-contact between the coating–substrate system and the chip, under different cutting conditions, were carried out. The study sheds light on the cutting forces, the interface temperatures and the tribo-contact conditions, including the friction energy dissipated at the tool–chip interface, the frictional heat flux conducting into either the chip or the insert, the mean coefficient of sliding friction and the contact loads exerted on the tool rake face. Finally, it was demonstrated how the intrinsic coating properties control the heat flux flowing into both components of such a closed tribo-system and the mechanical stresses on the contact area.     

In situ studies of the lubricant chemistry and frictional properties of perfluoropolyalkyl ethers at a sliding contact

In situ analyses of lubricated sliding contacts were performed by interfacing an ultraviolet Raman spectrometer to a ball-on-flat tribotester. The sliding contact was simulated by rotating a sapphire window that is transparent to ultraviolet radiation against a stationary ball. Various loads were transmitted to the contact center through the ball. A branched perfluoropolyaklyl ether (Krytox 479) and two linear perfluoropolyalkyl ethers (Fomblin 491 and Fomblin 497) have been studied under various loads at a 10 cm/s sliding speed. Krytox and a Fomblin of lower viscosity, Fomblin 497, decomposed to amorphous carbon upon sliding on a chrome steel ball but no amorphous carbon was detected from Fomblin 491. The amount of amorphous carbon at the contact area during sliding was a balance of formation and removal rates. It is postulated that surface activity of the chrome steel ball was the main cause for the lubricant degradation. The lubricant degradation at the chrome steel/sapphire interface was found to slightly increase the kinetic coefficient of friction at the contact center. However, catastrophic scuffing was not observed.     

Durability of chromium plating under dry sliding conditions

Experimental data are presented to show that engineering chromium plating on cast iron fails catastrophically after a given number of sliding cycles depending upon the applied load and the coating thickness. The failure mechanism involves the initiation of cracks in the chromium both at its sliding surface and at the coating-substrate interface adjacent to graphite flakes. The cracks propagate through the coating to give fracture and mechanical breakdown of the coating. The use of durability limit curves to identify ‘fail-safe’ conditions is discussed.     

Effect of some material and operating variables on scuffing

This paper summarizes the results of scuffing tests performed on AMS 6260 steel disks, covering three oils (a MIL-L-7808G oil, a MIL-L-23699A oil, and a straight mineral oil), two oil supply temperatures, a variety of sliding and sum velocities, and two modes of operating the test disks such that the potential failure sites on the disk surfaces either do or do not synchronize precisely in repeated cycles of operation. It is shown that, under otherwise comparable situations, (a) different oil-steel combinations allow the operation to penetrate by different degrees into the boundary lubrication regime before scuffing occurs, (b) an increase in the sliding velocity, at constant sum velocity, decreases the scuff failure load and the critical temperature, (c) an increase in the sum velocity, at constant sliding velocity, increases the scuff failure load and the critical temperature, (d) the effect of changing the sliding velocity or sum velocity, at a constant sliding-to-sum velocity ratio, depends on the balance of the opposing effects of sliding ans sum velocities at the particualar velocity ratio of interest, and (e) the scuff failure load and the critical temperature are markedly increased when the potential failure sites on the disk surfaces do not precisely synchronize on repeated cycles of operation.It is further demonstrated that the variations of the oil film thickness at scuffing, the coefficient of friction at scuffing, and the critical temperature with respect to all surface and operating variables correlate satisfactorily with a dimensionless parameter ξ_f.     

A numerical/experimental study of contact damage in non-planar porcelain/metal bilayers

In this report, we investigate and visualize the effect of shape irregularity on contact damage in a brittle coating on a stiff metal substrate. Hertzian contact damage in a dental porcelain layer of thickness between 0.25 and 0.75 mm, fused onto a Ni–Cr alloy substrate in both curved and planar geometries was studied with the aid of the finite element method and experimental investigation. Three failure modes were examined with varying porcelain layer thickness: cone cracking at the upper surface of the porcelain, median or interface cracking at the layer/substrate interface and plastic deformation below the contact area in the substrate. It is shown that curvature has very little effect on the initiation of surface cone cracks in this system, but substantial effect on the initiation of interface radial cracks. In particular, curvature reduces the critical load for the onset of interface cracks.     

A study of thermal and lubrication phenomena in the piston‐ring and cylinder‐sleeve tribosystem of an internal combustion engine and flash temperature calculation

The piston ring is one of the main elements in an internal combustion engine. Together with the cylinder sleeve, the piston ring has two basic functions: (1) contacting the cylinder sleeve to prevent the gases formed above the piston from migrating to the crankcase and (2) as a translation couple, formed of the piston‐ring and cylinder‐sleeve assembly. Complex tribological phenomena occur in the piston‐ring and cylinder‐sleeve tribosystem, according to variations in sliding speed, gas pressure, and temperature. This paper presents a method of calculation of the thickness of the lubricant film in the piston‐ring and cylinder‐sleeve tribosystem, using the Reynolds equation, integrated in specific conditions. According to the Newtonian behaviour of the lubricant, the shear stresses in the lubricant film between the piston ring and cylinder sleeve are determined. A computational procedure to determine flash temperatures in the piston‐ring and cylinder‐sleeve tribosystem is presented. The theoretical results, including film thickness, sliding speed, gas pressure, and flash temperatures for a complete crankshaft cycle are also presented.     

摆动活齿传动中激波器胶合原因研究

分析了摆动活齿传动中激波器与活齿的相对速度及其变化规律。从相对摩擦和相对约束滑动两个方面，探讨了激波器发生胶合失效的原因，并提出减轻激波器胶合的技术方案。     

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。