New cylinder liner surfaces for low oil consumption

Anticipated emission legislation and reduced fuel consumption are the main driving forces when developing new engines. Optimization of the active surfaces in the piston system is one possible way to meet the above demands. In this study the effects of surface topography and texture direction of the ring/liner contact on oil film thickness and friction were simulated and experimentally tested. “Low wear” results from the experimental wear tests with “glide honed” smooth liner surfaces supported the “low friction” simulation results. In addition a new wear volume sensitive surface roughness parameter, R_ktot, based on the Abbot–Firestone bearing area curve was introduced.     

Effect of lubricants on micropitting and wear

Micropitting was studied using a three-contact disc machine having a central roller in contact with three harder, annular counter-discs (“rings”) of precisely controlled roughness. Roughness, running conditions, base stock and additive concentration were varied. The response of the same lubricants in a reciprocating sliding wear test operating in the boundary regime was also studied.Results of experimental studies of the rolling contact behaviour of carburised steel rollers are reported. All the tests with the additive present led to micropitting. However, severe micropitting wear was only observed when the calculated film thickness exceeded 12% of the centre-line average roughness of the rings.It was found that there was an approximately inverse correlation between the micropitting damage in the disc machine test and the mild wear in the reciprocating sliding test. This was attributed to the tendency of anti-wear additives to prevent running-in of the rough surface.     

Modeling of fretting wear evolution in rough circular contacts in partial slip

This paper presents a numerical model that maps the evolution of contact pressure and surface profile of Hertzian rough contacting bodies in fretting wear under partial slip conditions. The model was used to determine the sliding distance of the contacting surface asperities for one cycle of tangential load. The contact pressure and sliding distance were used with Archard's wear law to determine local wear at each surface asperity. Subsequently, the contact surface profile was updated due to wear. The approach developed in this study allows for implementation of simulated and/or measured real rough surfaces and study the effects of various statistical surface properties on fretting wear. The results from this investigation indicate that an elastic–perfectly plastic material model is superior to a completely elastic material model. Surface roughness of even small magnitudes is a major factor in wear calculations and cannot be neglected.     

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.     

The effect of surface topography on the wear of steel

The pattern of roughness and wear has been obtained by varying the speed, the number of passes and the pre-roughness of the samples in both dry and lubricated sliding conditions. Three-dimensional graphs of wear versus speed versus sliding time and roughness of the worn surfaces were produced and a linear relation between the variation of the roughness and the amount of wear at a high number of passes has been determined. The initial dry wear rate is influenced by the prior surface roughness. In the case of lubricated conditions the influence of prior roughness is not pronounced. In dry sliding the smooth surfaces in the running-in stage roughen and the rough surfaces become smoother, while in lubricated conditions both surfaces become smoother in the first few passes. Sliding perpendicular to the lay produces rougher surfaces and more wear than is the case in the parallel direction.     

The roughness effect on the frequency of frictional sound

Dry sliding of two bodies in contact generates a wide range of effects like friction, wear, heat and sound among others. The main interest of this study is in the frequency characteristics of the generated sound.In the past, frequency spectrum and sound pressure level with relation to surface topography (surface roughness in particular), have been studied mainly for concentrated contacts like stylus or hemispherical tip pin on a rough surface. Studies on flat–flat contacts were mainly focused on the topography of contacting surfaces and its relation to occurrence or non-occurrence of squeal (high pitch, high sound pressure level sound) in brake systems.The present study aims to clarify the effect of surface roughness on the frequency of non-squealing frictional sound generated in dry flat–flat sliding contact.Sound was generated by the dry contact in rubbing by hand of two rectangular cross-section stainless-steel plates having similar surface roughness. The roughness of the contacting surfaces varied in the range R_z=0.8–12.4 μm. The sound spectra had 5 peaks (P₁, P₂, P₃, P₄ and P₅) in order of increasing frequency and it was found that the peak frequency was shifted when the roughness of the rubbed surfaces changed. The first peak P₁ was most sensitive to change of surface roughness and it shifted from 3.0 to 4.5 kHz when the maximum surface roughness changed from R_z=10.9 to . When the surface was relatively rough, this peak was close to the first bending natural frequency of the plate at 2.377 kHz.     

磨合阶段的机械密封端面形貌特征研究

表面粗糙度对机械密封性能有着显著影响。利用MMU-2摩擦磨损试验机和表面轮廓仪,对3个不同粗糙程度的密封端面进行了摩擦磨损试验。引入特征粗糙度,研究了3个不同粗糙程度的密封端面在摩擦磨损过程中的特征粗糙度变化规律,并与表面的轮廓算术平均偏差粗糙度变化规律进行了比较。研究表明,不同初始粗糙程度的端面在跑合过程结束后,粗糙度趋于一致。这一结果为利用特征粗糙度正确设计机械密封端面,合理选择端面加工方法提供了依据。     

Analysis of plane and rough contacts, subject to a shearing force

We analyse the surface traction conditions induced in plane contact between two bodies whose surfaces are rough. It is assumed that the roughness may be idealised by a surface of regularly spaced cylindrical “bumps”, and the overall geometry may be in the form of a cylinder, flat ended punch or wedge. The stick-slip regime experienced by each individual asperity contact is found, and hence it is shown how the applied shearing force produces concentrated regions of surface damage. Conditions for crack initiation are then discussed, and compared with equivalent results found for nominally smooth contacts.     

The machinability and tribological characteristics of aluminum alloys with improved elevated temperature properties using rapidly solidified powder

In this investigation the tribological characteristics of rapidly solidified Al–8Fe–4Ce with improved elevated temperature properties were studied. Such characteristics were compared with cast aluminum–silicon alloy and cast zinc–aluminum alloy. These materials included Al–13Si, Zn–35Al, Zn–35Al–Si, Zn–35Al–3.75Si and Zn–35Al–5.8Si. The wear rates of all materials were tested on a crossed-cylinders wear machine against 440C stainless steel counterface lapped by random abrasion using diamond paste to the desired average surface roughness. The effects of sliding distance on both the worn volume and the coefficient of friction were examined. The aluminum–iron–cerium alloy (Al–8Fe–4Ce) showed the lowest wear rate. The experiments were then extended on this material to examine the effect of varying the applied load and sliding speed on its wear rate. It was found that increasing the applied load increased the wear rate while it was slightly sensitive to the change in sliding speed. As the wear results showed that the Al–8Fe–4Ce alloy has the lowest wear rates, its machinability during turning operation was studied. Statistically-based experimental design (response surface methodology) using central composite second-order rotatable design technique was used to improve the experimentation design without loss of accuracy of the results. The interaction of cutting parameters (cutting speed, feed rate and depth of cut) was examined and their effect on the average surface roughness was reported. It was found that employing a combination of high cutting speed and small depth of cut with small feed rate causes a significant reduction in R_a. The data were represented in three-dimensional and contour graphs for selecting the appropriate machining conditions required to achieve desired values of surface roughness.     

A partial elastohydrodynamic theory for the running-in process of hobbed gears

The running-in process of hobbed gears has been studied experimentally. The initial wear was found to be small although it increased with running speed and load. To explain the results, a simulation procedure was developed that was based on a random surface model and a proposed relation between film thickness, surface roughness and running conditions. A new oil model was used for the analysis of conditions between meshing rough flanks. The wear and deformation of asperities during the running-in period is due to direct contact between the asperities. The parameters determining wear and deformation are the adhesion and temperature between the asperities in contact.     

Laboratory assessment of wear on nitrided surfaces of dies for hot extrusion of aluminium

A newly designed “block-on-cylinder” wear resistance testing rig, which allows testing at higher contact pressures than conventional testing methods, was used to elucidate the effect of an iron nitride compound layer (white layer) on the wear resistance of nitrided dies used for hot extrusion of aluminium (Al). The tested dies (AISI H13) with various nitrided microstructures were provided by different manufacturers of equipment for plasma and gas nitriding. The wear surfaces were analysed by SEM, BEC, micrography and roughness measurements. It was found that the iron nitride compound layer is chemically more stable against hot Al in comparison to die material. Deterioration of the compound layer begins with cracking, and as a consequence, its spalling from the nitrided surface. A high thickness of the compound layer combined with a low nitriding depth leads to its earlier spalling and vice versa. Due to the increased roughness at the removal sites, accelerated chemical attack by hot Al takes place. Comparative and simultaneous testing of two nitrided samples was carried out.     

Surface topography and integrity effects on sliding wear

The effect of surface roughness and integrity on the sliding wear of metals was investigated experimentally. The results are consistent with the delamination theory of wear. The initial wear rate was influenced by the surface roughness and the applied load but the steady state wear rate was independent of the initial roughness. Under low applied loads delamination of smooth surfaces commences soon after sliding is initiated, whereas the delamination of rough surfaces is delayed until the original asperities are worn. Consequently, under low loads the initial wear rate of a smooth surface is higher than that of a rougher surface. The opposite is found under high loads since original asperities are removed immediately. It is also shown that machining damage to the surface or the subsurface (in the form of deformation, voids and cracks) accelerates the initial wear rate of the machined surface.     

考虑表面粗糙度的面齿轮齿面接触应力分析

根据齿轮啮合原理,得出了面齿轮的齿面方程,求得面齿轮齿面曲率。结合赫兹接触理论,推导了点接触面齿轮传动接触点的接触应力及应力沿齿面的分布规律,从齿根到齿顶,齿面接触应力先增大后减小,在靠近齿面中点处达到最大值。由粗糙表面接触理论,分析了面齿轮齿面微观弹塑性变形时的接触面积,并得到粗糙齿面接触时面齿轮齿面接触应力及其分布。对比分析了几种不同粗糙度条件下面齿轮齿面接触应力的变化规律,结果表明:齿面接触应力随表面粗糙度的增大而增大,与齿根处相比,齿顶接触应力受表面粗糙度影响更大。文中的分析可为面齿轮磨损及润滑机理的研究提供依据。     

表面粗糙度对边界摩擦与磨损性能的影响

本文实验研究了表面粗糙度对线接触ＧＣｒ１５钢摩擦副的边界摩擦与磨损性能的影响。实验所采用的润滑剂为３０号机械油加１％的硬脂酸。实验结果表明：存在一最佳表面粗糙度级别，在此级别下，摩擦副的边界摩擦系数最小，耐磨性能最好。     

Friction variation in the cold-rolling process

Friction variation along the rolling contact interface was measured by a ‘sensor roll’ embedded with sensors on an experimental rolling mill. The measured results show that the friction coefficient is not constant along the contact length. Empirical formulae describing the friction variation for certain rolling condition were obtained. Experiments were also carried out to study the surface roughness of the strip. It was found that the roll and strip surface roughness can be transferred to each other along the rolling direction. Only slight change of roughness in the transverse direction was observed.     

Coating thickness effects on initial wear of nitrogen-doped amorphous carbon in nano-scale sliding contact: Part I—in situ examination

This paper, the first of a two-part series, presents the empirical data obtained from in situ examination on the generation of wear particles on carbon nitride coatings by a spherical diamond counter-face during repeated sliding contacts. In particular, the effect of coating thickness, varying from 1 to 500 nm, on the generation of wear particles was examined.Based on the in situ examination, the shape transition maps for generated wear particles were obtained for carbon nitride coatings of various thickness. The results show that the critical number of friction cycles, Nc, for the transition from “no observable wear particles” to “wear particle generation” generally increased with increasing coating thickness. It was noted that up to 20 friction cycles, the maximum Hertzian contact pressure, P_max, for “no observable wear particles” regime can be increased from 1.39Y to 1.53Y if silicon was coated with carbon nitride coating thicker than 10 nm, where Y is defined as the yield strength of silicon.     

A model of liquid-mediated adhesion with a 2D rough surface

A computational analysis is conducted on a solid–liquid–solid interface: a rough, elastic surface is brought into contact with rigid flat while being influenced by an intervening liquid film of fixed volume. The problem is solved (iteratively) by satisfying the equations of elasticity and capillarity, while conserving liquid volume and honoring the constraints of no surface interpenetration and no negative pressures at dry contact points. As the liquid volume is increased from low values, a critical condition is reached whereby there are sudden increases in the values of certain output parameters, such as tensile force and wetted length. For the smaller values of surface roughness, the critical point is associated with a near complete collapse of the surface topography.     

Friction characteristics of a ferritic-austenitic stainless steel during lubricated reciprocating motion

This report describes friction measurements of stainless steel against stainless steel during lubricated, small-amplitude reciprocating motion. The experimental investigation was divided into two parts. First, four different lubricants were evaluated using a response surface design, during which the average contact pressure and the sliding velocity were varied. Secondly, a 2⁴ factorial design with three replicate runs was performed. Here, the coefficient of friction in the initial stage and the duration of that stage were studied. The independent variables were the average contact pressure, sliding velocity, surface roughness and type of lubricant. In the early state (stage I), the value of the frictional force is controlled by plowing of the surfaces by asperities. In many lubricated contacts, this is the practically useful stage. The experimental results from the response surface design show that the duration of stage I depends on the type of lubricant. Adhesive wear can take place before 100 cycles. The factorial design indicates that the coefficient of friction in the initial stage is affected by the type of lubricant, surface roughness and the simultaneous change of the surface roughness and type of lubricant. The duration of the initial stage is affected by a change in the surface roughness, average contact pressure and a simultaneous change in average contact pressure and surface roughness. A two-parameter Weibull analysis was performed on the data from the factorial design. For the tests where lubricant no. 3 was used, a mixed distribution was indicated for the duration of stage I. This mixed distribution indicates that a weakest-link process as well as a healing process were involved.     

INFLUENCE OF MACHINING ON FRICTION AND WEAR OF LUBRICATED CERAMICS IN SLIP-ROLLING CONTACTS

The slip-rolling friction and wear tests were performed in a twin disc tribometer of the Amsler-type at a Hertzian pressure of 3 GPa over 2 million revolutions. Paraffinic oil with no additives was used as lubricant. The ceramics were machined with different processes, resulting in different surface roughness (i.e., rough and fine honed, rough and fine ground, rough and fine lapped, and rough and fine polished). Ceramic materials like HIP-Si₃N₄, S-SiC, HIP-ZrO₂, and GPS-Si₃N₄-TiN were investigated as self-mated couples. This paper summarizes the results. Si₃N₄, Si₃N₄-TiN, and ZrO₂ generally exhibit a small wear coefficient in the range of 10^-9 mm³/Nm in paraffinic oil and their wear coefficients correlate with the initial surface roughness and the material removal rate. The lowest wear coefficients were exhibited by ZrO₂- With a reduction of the Hertzian pressure to 1.5 GPa, S-SiC exhibits the same tribological behavior as the other ceramics.