Preliminary investigation of the effect of dimple size on friction in line contacts

The scale of surface texture is becoming an important issue of surface texture design, particularly for the condition of low speed and high load. Experiments were carried out to investigate the effect of dimple size on friction under line contact condition. The patterns of dimples distributed as square array were fabricated on the surface of brass disks. Each pattern has the same area density of 7%, the same depth over diameter ratio h/d of 0.03, and dimple diameter d varying from 20 to 60 μm. The frictional tests of the brass disk sliding against a stationary cylindrical surface of bearing roller were conducted. It was found that the pattern with dimple diameter of 20 μm presented the effect of friction reduction. For the further understanding of the effect of dimple size under line contact condition, numerical simulations were also carried out to evaluate the hydrodynamic pressure within the contact of cylindrical and plane surfaces. The effects of dimple size and radius of the cylinder on the load carrying capacity were evaluated and discussed.     

Optimization of Surface Texturing for Contact Between Steel and Ultrahigh Molecular Weight Polyethylene Under Boundary Lubrication

In this study, we developed a design method for surface texturing optimization and evaluated it experimentally. Square patterns composed of different fractions of pore shapes were prepared by using a nanosecond pulse laser on AISI 1045 steel surfaces. A pore diameter of 75 μm was used to design the area densities. The area density was 5–25 %, and the pore depth was 20–75 μm. Different area densities and pore depths were examined for optimization, and pin-on-disk sliding tests were performed using ultrahigh molecular weight polyethylene pins. The results showed that all of the coefficients of friction of the textured specimens were reduced regardless of the area density and aspect ratio. Optimization was performed according to the measured dimensions of the pores, and the optimized area density and pore depth under the current sliding conditions were found to be 25 % and 25 μm, respectively.     

凹坑织构对石墨材料水润滑性能的影响

为提高核主泵屏蔽电机用水润滑石墨轴承的摩擦润滑性能,采用高速雕铣机在石墨试样表面加工不同形状、深度和面积占有率的凹坑织构,通过水润滑条件下的销盘摩擦试验,测试分析凹坑织构结构参数对石墨材料水润滑性能的影响。结果表明：在水润滑条件下凹坑织构具有一定的减摩效果,随着转速的增加,凹坑织构的动压效应增强,石墨试样摩擦因数减小;随着载荷的增加,石墨试样摩擦因数减小;随着凹坑深径比、面积占有率的增加,石墨试样摩擦因数均呈现先减小后增大的变化趋势,当凹坑深径比为0.5、凹坑面积占有率为3%左右时,石墨试样摩擦因数最小;相较三角形凹坑织构,正方形凹坑和圆形凹坑织构的减摩效果更好。     

Experimental investigation of tribological performance of laser textured stainless steel rings

A series of experiments is conducted to examine the frictional characteristics of laser surface-textured, heat-treated 17-4 PH stainless steel specimens. Two dimple shapes are tested: circular and elliptical. The circular dimples are designed with different sizes, dimple densities and dimple depth-to-diameter ratios. It is found that the cavitation pressure (P_cav) in a circular dimple is strongly affected by the operational speed. The surface textured specimens provide low coefficient of friction compared with plain (dimple-free) surfaces. However, for the material used in the current experiments, the surface texture decreases the surface's resistance to wear.     

Simulation of piston ring tribology with surface texturing for internal combustion engines

Design of piston rings is a very crucial subject in the field of internal combustion engines. In the present paper, a numerical model is created using the Navier–Stokes equations. Fluid–structure interaction analysis is performed in order to calculate the structural integrity of the ring for several engine operational conditions and texturing patterns. This paper illustrates the hydrodynamic friction force under various surface artificial texturing in terms of spherical and rectangular microdimples. Piston ring stress analysis is also investigated due to gas leakage. Results show a substantial reduction of the friction using rectangular texturing and less reduction using spherical texturing. The rectangular microdimple parameters were considered to obtain a better friction reduction with the following configurations: H_d = 4 µm, ρ_τ = 0.61, λ = 20 and s = 0.004. Each rectangular texture cell is defined by the dimple depth, H_d; the texture density, ρ_τ; the dimple aspect ratio, λ; and the relative dimple depth, s. Copyright © 2014 John Wiley & Sons, Ltd.     

Sliding velocity dependency of the friction coefficient of Si-containing diamond-like carbon film under oil lubricated condition

In this study we investigated the sliding velocity dependency of the coefficient of friction for a Si-containing diamond-like carbon (DLC-Si) film in an automatic transmission fluid (ATF) under a wide range of contact pressures. The DLC-Si film and a nitrided steel with a surface roughness, Rz_JIS, of around 3.0 μm were used as disk specimens. A high-carbon chromium steel (JIS-SUJ2) bearing ball was used as a ball specimen. Friction tests were conducted using a ball-on-disk friction apparatus under a wide range of sliding velocites (0.1-2.0 m/s) and contact pressures (P_max: 0.42-3.61 GPa) in ATF. The friction coefficients for the nitrided steel had a tendency to decrease with an increase in sliding veloicity under all the contact pressure conditions; however, the friction coefficients for the DLC-Si film were stable with respect to sliding velocities under all the contatct pressures. These results indicate that the DLC-Si film suppresses the stick-slip motion during sliding againt steel in ATF, which is a desired frictional characteristic for the electromagnetic clutch disks used under lubrication. Furthermore, the DLC-Si film showed a higher wear resistance and lower aggression on the steel ball specimen than the nitrided steel. There were less hydrodynamic effects on the friction coefficient for the DLC-Si film possibly due to maintenance of the initial surface roughness and its poorer wettability with the fluid. X-ray photoelectron spectroscopy (XPS) analysis of the sliding surfaces revealed that the adsorption film derived from the succinimide on the sliding surfaces of the DLC-Si film and the mating steel ball also contributed to the sufficient and less sliding-velocity-dependant friction coefficients.     

The effect of flat-on-ring sample alignment on sliding friction break-in curves for aluminum bronze on 52100 steel

The effects of test sample fixturing on the interpretation of frictional break-in behavior are described for dry sliding flat-on-ring tests of CDA 688 bronze on 52100 steel. It is demonstrated that for otherwise similar test conditions (i.e. 10 N load, 20 cm s^?1 velocity, 1 μm polished block surfaces and flowing argon gas environment) tilt of the fixed flat block can affect the break-in duration for friction and for wear because of the rate at which a balance of steady state sliding surface contact conditions is achieved.     

Tribological Characteristics of Nitrogen (N+) Implanted Iron

The effect of implantation of nitrogen ions (1.5 MeV) on the friction and wear characteristics of pure iron sliding against M-50 steel (unimplanted) was studied in a pin-on-disk sliding friction apparatus. Test conditions included room temperature (～25°C), a dry air atmosphere, a load of ½ kg (4.9 N), sliding velocities of 0.043 to 0.078 m/s (～15 to 25 rpm), a pure hydrocarbon lubricant (n-hexadecane), or a USP mineral oil and nitrogen ion implantation doses of 5 × 10¹⁵ and 5 × 10¹⁷ ions/cm².

No differences in wear rates were observed in the low-dose (5 × 10¹⁵ ions/cm²) experiments. In the high-dose experiments (5 × 10¹⁷ ions/cm²), small reductions in initial (～40 percent) and steady-state (～20 percent) wear rates were observed for nitrogen-implanted iron riders as compared with unimplanted controls. No differences in average friction coefficients were noted for either dose.

Auger electron spectroscopy combined with argon ion bombardment revealed a subsurface Gaussian nitrogen distribution with a maximum concentration of 6 atomic percent at a depth of 8 × 10^?7 m (0.8 μm). Similar analysis within the wear scar (～2.0 × 10^?5 m subsurface) of an implanted rider after 20 μm of wear yielded only background nitrogen concentration. No inward migration of nitrogen ions was observed.     

Effect of surface laser texture on friction properties of nickel-based composite

Laser surface texturing (LST) was performed on the nickel-based composites by a Nd:YAG pulsed laser and the regular-arranged dimples with diameter of 150 μm were fabricated on their surfaces. The textured surfaces were smeared with molybdenum disulfide powder. The tribological properties of the textured and filled composites were investigated by carrying out sliding wear tests against an alumina ball as a counterface using a high temperature ball-on-disk tribometer. The tests were conducted at a sliding speed of 0.4 m/s and at normal loads ranging from 20–100 N and from room temperature to 600 °C. The friction coefficient of nickel-based composite textured and smeared with molybdenum disulfide was found to reduce from 0.18 to 0.1 at the temperature range from 200 to 400 °C. The texture with a dimple density of 7.1% was observed to prolong wear life of MoS2 film by more than four times in comparison to the texture with other dimple densities. The lubricious oxide particles stored in the dimples reduce friction coefficient at elevated temperatures and compensate for the extra lubricant owing to the degradation of MoS₂ caused by its oxidation at high temperatures.     

Sliding and rolling of individual micrometre sized glass particles on rough silicon surfaces

Abstract

Access to the tribological contact behaviour of individual small particles is crucial with respect to many applications in particle technology. In this paper, we present a simple nanoindentation based approach to study the rolling friction of micrometre sized spherical particles. The results are compared to sliding friction tests, which are carried out by a nanoindentation based colloid probe technique. The potential of the approach is evaluated for borosilicate glass spheres featuring nominal radii of 2·5 and 10 μm in contact with silicon surfaces. The roughness of the latter is modified by a plasma etching process. Significant differences of sliding and rolling friction with respect to roughness are observed even though the variation of root mean square roughness only ranges from 0·3 to 2·7 nm respectively.     

Frictional Performance of a Textured Surface with Elliptical Dimples: Geometric and Distribution Effects

Geometric and distribution effects of inclined dimples on the friction and wear performance of a textured surface were experimentally investigated. The frictional coefficients and face temperatures of four different kinds of elliptical surfaces were measured under various conditions of rotation speed, load, and viscosity. Experimental results showed that distribution parameters significantly affected frictional performance. From the experiments it was found that surfaces with double-row elliptical dimples have smaller frictional coefficients and lower face temperatures than that of the single-row elliptical dimples and had a 59% decrease in frictional coefficient and a 58% decrease in face temperature rise. This means that double-row elliptical dimples can lead to a more profound hydrodynamic effect as well as possess a lower chance of surface contact and wear. Moreover, a larger dimple diameter and slender ratio further improved the frictional performance of the textured surface with double-row elliptical dimples.     

Dimples shape and distribution effect on characteristics of Stribeck curve

A purpose of this research is to study the influence of geometrical characteristics of the surface texture on the Stribeck curve in lubricating sliding.The tribosystem consists of the stationary block pressed at the required constant load 1800 N against the ring rotating at the defined speeds. Tests were conducted at increasing sliding speed of range 0.08–0.69 m/s. Every speed was maintained for 2 min. The test was carried out under conformal contact conditions. The sliding was unidirectional. The block was a part of a bearing sleeve hardened EN-GJS 400-15 cast iron with a hardness value of 50 HRC. The ring samples, 35 mm in diameter, were made from hardened 42CrMo4 steel of hardness 32 HRC. Some variants of specimen surfaces were created by burnishing technique. The area density of oil pockets S was in the range 7.5–20%. The dimples depth to length ratios were between 0.03 and 0.08. Ring surfaces with oil pockets of short drop, long drop and spherical shapes were tested.It was shown that with proper shape and dimensions as well as suitable area density of oil pockets the friction characteristic of the sliding pairs could be improved in comparison to non-textured surfaces.     

Effect of the degree of crystallinity on the friction and wear of poly(ethylene terephthalate) under water lubrication

By means of a pin-on-disk type of wear-testing apparatus, coefficients of friction and wear depths of various poly(ethylene terephthalate) (PET) specimens with different degrees of crystallinity were measured at sliding speeds of 0.01 and 0.1 m s⁻¹ under a constant load of 10 N with water lubrication. The coefficients of friction with lubrication were lower than those for unlubricated conditions and there was little dependence on the degree of crystallinity. However, the wear rates with lubrication were higher than those obtained under dry conditions and decreased with increasing degree of crystallinity, in contrast with the unlubricated wear rates. These results are discussed on the basis of the plasticization of worn polymer surfaces by water.The worn surfaces and frictional tracks were observed by optical and electron microscopy. The frictional tracks for low crystallinity PET were smooth with many fine scratches and characteristic of transfer wear. For high crystallinity, frictional tracks were somewhat different from those for low crystallinity PET.     

Evaluation of friction coefficient using indentation model of Brinell hardness test for sheet metal forming

This work presents an indentation model of the Brinell hardness test, which is a rigid ball-deformable plane contact model (RB-DP model), to elucidate the sliding friction mechanism of sheet metal forming. In the proposed model, the friction force can be defined as a combination of shear (shearing effect) and plough (ploughing effect) forces. The real contact area ratio α is determined from the RBDP model under sliding condition. Moreover, the lateral contact area ratio A _c /A _r can be specified as a function of the real contact area ratio α. Based on Meyer’s law and Hertz contact problem, the maximum contact area ratio α _u , a limiting condition of the real contact area ratio α, can be described as a function of the strain hardening exponent n. Additionally, a limiting condition applies: the strain hardening exponent n must be less than 0.64 in the present model. The present friction model reveals that the friction coefficient μ _d is a function of strain hardening exponent n, the real contact area ratio α and the maximum contact area ratio α _u . The calculated friction coefficient μ _d agrees with the published experimental results.     

Flow characteristics along and above dimpled surfaces with three different dimple depths within a channel

The effects of dimples in altering time-averaged flow behavior occur mostly within one-half of one dimple print diameter from the surface, and the dimples within the arrays periodically eject a primary vortex pair from each dimple, which exists in conjunction with edge vortex pairs that form along the spanwise edges of staggered dimples regardless of three dimple depths. As the dimple depth increases, deeper dimples eject stronger primary vortex pairs, with hig her levels of turbulence transport due to larger deficits of time-averaged, normalized total pressure and streamw ise velocity as the surfaces with deeper dimples are approached. Primary vortex pair ejection frequencies range about 7–9 Hz, and edge vortex pair oscillation frequencies range about 5–7 Hz forR _eH =20,000, regardless of dimple depths.     

Adhesive wear and frictional characteristics of UHMWPE and HDPE sliding against different counterfaces under dry contact condition

Abstract

The current work evaluates the wear and frictional performance of ultrahigh molecular weight polyethylene (UHMWPE) and high density polyethylene (HDPE) sliding against different metal counterfaces, stainless steel(SS), mild steel (MS) and aluminium (Al), under dry contact condition. The experiments were conducted using pin on disc machine at different sliding distances (0–40·32 km), 15 N applied load and 2·8 m s^–1 sliding velocity. Interface temperatures and frictional forces were measured simultaneously during the sliding, while specific wear rates were determined for every 1·68 km sliding distance. Based on the optical microscopy of the worn surface and wear track, frictional and wear results were analysed and discussed. The experimental results showed that the type of counterface material significantly influences both frictional and wear performances of the selected polymers. This was mainly due to the film transfer characteristics. Higher temperature and friction coefficient for UHMWPE and HDPE were evident when sliding took place against Al counterface. Sliding the polymers against stainless steel showed low friction coefficients compared to other counterfaces.     

Contact Mechanics and Friction on Dry and Wet Human Skin

The surface topography of the human wrist skin is studied using an optical method and the surface roughness power spectrum is obtained. The Persson contact mechanics theory is used to calculate the contact area for different magnifications, for both dry and wet condition of the skin. For dry skin, plastic yielding becomes important and will determine the area of contact observed at the highest magnification. The measured friction coefficient [M.J. Adams et al., Tribol Lett 26:239, 2007] on both dry and wet skin can be explained assuming that a frictional shear stress σ_f ≈ 15 MPa acts in the area of real contact during sliding. This frictional shear stress is typical for sliding on polymer surfaces, and for thin (nanometer) confined fluid films. The big increase in the friction, which has been observed for glass sliding on wet skin as the skin dries up, can be explained as resulting from the increase in the contact area arising from the attraction of capillary bridges. This effect is predicted to operate as long as the water layer is thinner than ～14 μm, which is in good agreement with the time period (of order 100 s) over which the enhanced friction is observed (it takes about 100 s for ～14 μm water to evaporate at 50% relative humidity and at room temperature). We calculate the dependency of the sliding friction coefficient on the sliding speed on lubricated surfaces (Stribeck curve). We show that sliding of a sphere and of a cylinder gives very similar results if the radius and load on the sphere and cylinder are appropriately related. When applied to skin the calculated Stribeck curve is in good agreement with experiment, except that the curve is shifted by one velocity-decade to higher velocities than observed experimentally. We explain this by the role of the skin and underlying tissues viscoelasticity on the contact mechanics.     

Thermoelastic Analysis of Grooved Friction Clutches Using Finite Element Method

High temperatures appear on friction clutch surfaces due to the heat generated between the contacting surfaces during the slipping period. In some cases the maximum temperatures will exceed the allowable working temperature; friction clutches will fail rapidly when working under these conditions for a long time. Therefore, it is necessary to study the effect of design parameters (e.g., grooves) on the thermoelastic behavior of friction clutches to avoid these kinds of failures or at least increase the lifetime of friction clutch. A finite element method will be used to investigate the effect of the circumferential groove on the thermoelastic behavior of a single-disc clutch during a sliding phase. Axisymmetric models have been developed in this analysis to simulate the dry clutch systems during the sliding period. The effect of the groove area ratio G.R (defined as the groove area divided by the nominal contact area) on thermal and elastic behaviors is investigated. The results showed that the groove size has a significant effect on the magnitude and distribution of contact pressure, temperature field and heat generated along the frictional surfaces.     

Investigation of the tribological properties of the different textured DLC coatings under reciprocating lubricated conditions

In this study, we propose the use of laser surface texturing methods to improve the wear resistance of diamond-like-carbon (DLC) coatings. First, the application of dimples perpendicular to an engineering surface is introduced by using laser in a controlled manner. The solid DLC lubricant is subsequently deposited on the textured surface using the magnetron sputtering technique. In the experiments, the dimple densities were varied from 0 to 30%, and the dimple diameters were varied from 40 to 300 μm. The effect of the geometric parameters on the wear performance was studied using a reciprocating sliding-wear tester under oil lubrication conditions. The results showed that the DLC coatings with the appropriate dimple density (10%) and diameter (∼100 μm) demonstrated an obvious improvement in terms of the friction coefficient and wear rate compared with that of the un-textured DLC coatings. The experimental treatment produced respective reductions of 20% in friction and nearly 52% reduction of wear rate. This improvement could be explained by the action of reservoirs that enhance lubricant retention and trap the wear particles during sliding motions. In addition, the relationship between the dimple diameter, the contact width ratio and the wear performance is discussed.     

The genesis of friction

The genesis of friction is explained in terms of a new theory. Contrary to the postulates of the adhesion theory of friction, this theory postulates that the frictional force (and thus the friction coefficient μ) is affected by the sliding distance and the environment because of the changing contributions of three components of friction, i.e. that due to the deformation of surface asperities (denoted μ_d), that due to plowing by wear particles and hard asperities (denoted μ_p) and that due to the adhesion of the flat portions of the sliding surface (denoted μ_a). Therefore the coefficient of friction is not a simple material property. There are four or six stages of the friction regime depending on the sliding conditions. The initial friction coefficient μ_i can range from 0.1 to 0.2 for most machined surfaces. μ_i is largely independent of environmental conditions (including lubricants), materials and surface topography. μ_p varies from 0 to 1.0 and μ_a from 0 to 0.4. Only μ_a depends on the quality of surface adhesion. The history-dependent frictional behavior of materials is represented in the “friction space” diagram. According to this theory, the compatibility of sliding surfaces is dictated more by the mechanical properties of materials such as hardness than by their relative solubility at low temperatures.