On thin film lubrication

Various phenomena are revealed under EHL and micro-EHL conditions, such as the properties of the lubricant under high pressure, traction, and the load-bearing capacity of the lubricant film, and are discussed in the present paper. A new lubrication regime, thin film lubrication, has been discussed. The theoretical and practical significance of research on thin film lubrication is elaborated. Finally, the characteristics describing thin film lubrication and its main research directions are suggested.     

Rolling contact fatigue in lubricated contacts

This work presents and discusses the results of rolling contact fatigue tests (IP-300) with six different lubricants using a four-ball E.P. lubricant tester. These results tended to confirm the mechanism for rolling fatigue proposed by Jin and Kang. The lubricants tested were two mineral oils (SN 350 and SN 600) and four synthetic oils (PAG-9, PAG-12, PAO 6, and PAO X), the test machine used was a Four-Ball E.P., and the IP 300/87 standard was applied. The results indicated that: 1) lubrication has an important influence on the rolling fatigue life of mechanical components; 2) normally, in oils of the same family, the higher the viscosity is, the higher the rolling fatigue life is; 3) besides viscosity, other lubricant properties such as the pressure–viscosity coefficient (α), compressibility (B), and the EHL friction coefficient (γ), should all be taken into account for lubricated contact design.     

A continuous viscosity model for thin film lubrication

The thickness of adsorbed molecular layers is the most critical factor in studying thin film lubrication, and is a key feature that distinguishes thin from thick film lubrication analysis. A method is presented that enables the adsorbed layer thickness to be calculated. This is based on adsorption theory and expressed in terms of molecular interaction energies. A continuous cross-gap viscosity model incorporating the layer thickness is introduced and used to calculate the load capacity and frictional characteristics of a simple bearing operating in the thin film regime.     

Rolling contact fatigue of case-hardened chromium steel

The rolling contact fatigue of case-hardened steel surfaces in lubricated heavily loaded contact was studied. Three different case-hardening treatments were tested with a ratio of slide to roll of ?5%. Other ratios of slide to roll were used to determine the effect of tangential traction on the rolling contact fatigue endurance. When the actual contact width was measured after testing, the scatter of the fatigue results was reduced. The depth of the maximum plastic strain was determined by measuring the hardness before and after testing and was found to correspond to the occurrence of the lowest ratio of the critical shear stress to the amplitude of the load-induced orthogonal shear stress. The role of residual stresses in rolling contact fatigue is discussed. It was found that a more detailed knowledge of lubricant behaviour in heavily loaded contacts is needed to reveal the true distribution of tangential traction on the contact surface. This affects the angle of the plane and the value of the maximum amplitude of the shear stress beneath the contact zone.     

Failure criteria in thin film lubrication with EP additives

The scuffing behaviour of sliding couples made from 12 NiCr 13 steel, lubricated with SAE 90 and SAE 20 W30 oils with and without additives, was studied as a function of relative sliding speed and bulk oil temperature. It was found that the scuffing load decreases almost inversely with sliding speed. Calculations which take into account the decrease in hardness at increasing temperature show a fairly good constancy of total contact temperature (bulk + flash temperature) at scuffing (i.e. values ranging from 550 ° to 650 °C for all oils and test conditions). At low speeds (up to 1 m s^?1) a well-defined increase in scuffing load was found when EP additives were used; at higher speeds this effect was found to have vanished completely. Variations in nominal contact pressure in the range 1 to 3 had no appreciable influence on the scuffing load, indicating that, in the present case, scuffing was associated with a transition from the boundary lubrication regime to the severe wear regime.     

季戊四醇油酸酯膜厚测量及薄膜润滑研究

运用光干涉法相对光强原理,通过纳米级润滑膜厚度测量仪测量了不同温度、不同载荷下纳米间隙中环境友好润滑剂季戊四醇油酸酯的油膜厚度。探究了膜厚与速度、载荷、温度之间的关系,观察了薄膜润滑现象。结果表明：在对数坐标系下,膜厚与速度具有一定的线性关系,在速度较高时,线性关系更强,润滑剂具有流体效应。载荷对膜厚的影响远小于速度对膜厚的影响,温度对膜厚的影响主要表现在温度越高时,润滑剂的黏度越低,润滑剂的膜厚越薄。     

Rolling contact fatigue endurance of powder-extruded bearing steel

The mechanical and toughness properties and rolling contact fatigue endurance of powder-extruded bearing steel made from nitrogen atomized remelted turning powder in as-extruded and as-heat treated state are presented. The mechanical and toughness properties are comparable with those of wrought bearing steel. Rolling contact fatigue endurance was tested by the AXMAT method in as-extruded and as-spheroidizing annealed states depending on the austenitizing temperature, which ranged from 790 °C to 870 °C. Powder-extruded bearing steel in both states showed slightly higher hardness (63 HRC), higher contact fatigue resistance (L₁₀ = 80.7 and 55.2 million load cycles, respectively) in comparison with wrought steel (L₁₀ = 31.8 million load cycles). The results demonstrate the advantageous properties of powder-extruded bearing steel for high dynamic loading.     

Velocity analysis for layered viscosity model under thin film lubrication

The lubrication of bearings is directly affected by the flow properties of the liquid lubricant. One key feature of lubricant is its viscosity distribution in the bearing. The distribution is dependent on the adsorbent properties of the surface and the velocity distribution of the lubricant. This paper is only connected with the velocity analysis on the cross-gap flow. Because of increasing the speed, apart from increasing the temperature, the other features of lubricant will be changed. When fluid film is thin, the speed effects become important due to the increased shear rate. This paper describes the fluid velocity profiles produced for the case where layers of greater viscosity than the bulk occur on the bearing surfaces.     

Measurements of friction in strip drawing under thin film lubrication

Strip drawing is used to investigate the friction behaviour under thin film lubrication in metal forming with plastic deformation. Friction coefficients are measured under a wide range of tribological conditions. The surface roughness is measured on an interferometric profilometer. The results show that the friction coefficient decreases with increasing oil film thickness h_w, as estimated using a formula appropriate for smooth tool and workpiece. Measurements of the surface topography show that change in friction is associated with a change in contact ratio between the tool and strip. The effect of strip reduction, strip roughness and die roughness on the friction coefficient is also investigated.     

Rolling contact fatigue performance of detonation gun coated elements

Rolling contact fatigue performance of thermal spray coatings has been investigated using an experimental approach. A modified four ball machine which simulates a rolling element bearing was used to examine the coating performance and failure modes in a conventional steel ball bearing and hybrid ceramic bearing configurations. Tungsten carbide (WC-15% Co) and aluminium oxide (Al₂O₃) were thermally sprayed using a super D-Gun (SDG2040) on M-50 bearing steel substrate in the geometrical shape of a cone. A coated cone replaced the upper ball that contacts with three lower balls. The rolling contact fatigue (RCF) tests were performed under immersed lubricated conditions using two different lubricants. Fatigue failure modes were observed using a scanning electron microscope. Microhardness measurements of the coating and the substrate and elastohydrodynamic fluid film thickness results are included. The results show the requirement for significant optimization of the coating before use in rolling element bearing applications. The coating was fractured in a delamination mode. Test results show an optimization in coating process is required before these coatings can be used for rolling contact applications. WC-Co coatings perform better than Al₂O₃ coatings in rolling contact.     

Rolling contact fatigue life model incorporating residual stress scatter

There has been a considerable body of research concerned with predicting the service life of rolling contact components. It is well known that the prevailing cause of failure in rolling contact components is fatigue crack. Although the trend has been toward the use of surface integrity in fatigue life prediction, one aspect missing in most models has been a consideration of the residual stress scatter in the subsurface. Since the fatigue life of a manufactured component depends on its weakest point, the local extreme is most relevant for the fatigue life prediction. This study thus incorporates the residual stress scatter into the rolling contact fatigue life model to predict the fatigue life closer to the experimental life. It is noted that residual stresses vary significantly at the same depth. After the residual stress scatter is incorporated into the model, the predictions agree well with the experimental lives.     

Rolling contact fatigue properties of Tuffritrided 0.2% C steel

The effects of initial surface roughness and lubricant viscosity on the rolling contact fatigue behaviour of a Tufftrided plain 0.2% C steel are reported. The Tufftriding treatment substantially improves the rolling contact performance. Improvement in the surface finish before Tufftriding is beneficial. Changes in lubricant bulk viscosity have no significant effect on the fatigue properties.     

Modeling and analysis of interfacial electro-kinetic effects on thin film lubrication

At the interface between a solid and a following liquid, charge migration within the electric double layer causes an electro-viscous force to develop which manifests itself as an enhanced fluid viscosity. Based on the Poisson–Boltzmann equation, a new mathematical model of the electro-viscosity is developed. Numerical analysis is carried out taking into account the influence of the electric double layer on thin film lubrication. Analysis results show that the electro-viscosity leads to a significant increase in the film thickness in the thin film regime (below 100 nm).     

Failure criteria in thin film lubrication: Investigation of the different stages of film failure

The mechanisms of failure of sliding lubricated concentrated steel contacts have been studied using a newly developed tribometer which allows fast separation of the sliding surfaces at the different stages of film failure. The appearance of the corresponding contact topography was investigated by means of optical interference microscopy. The results indicate a gradual transition from full to partial elastohydrodynamic (EHD) lubrication as a function of increasing load and increasing bulk oil temperature until complete failure of the EHD film occurs at critical triplets of normal load, sliding velocity and bulk oil temperature.     

Failure of thin film lubrication — A detailed study of the lubricant film breakdown mechanism

The mechanism of breakdown of the lubricant film in a concentrated steel contact has been studied by examining friction-time diagrams as functions of sliding speed (ranging from 0.0007 to 0.7 m s^?1) and normal force on the contact. It was found that, even at the lower speed of 0.0007 m s^?1, the load-carrying capacity of the contact still depends strongly on lubricant viscosity. Over the whole range of speeds, lubricant film breakdown can be explained in terms of a major disturbance in the steady state condition, which is characteristic of (partial) elastohydrodynamic lubrication. In that condition, oxidation of microasperities which enter the surface zone keeps abreast of metal-to-metal junction formation.     

Theory analysis of asymmetrical electric double layer effects on thin film lubrication

This paper presents a theoretical research concerning the effects of the asymmetrical electric double layers on thin film lubrication. A mathematical model of electro-viscosity considering the asymmetrical electric double layers is developed, which acts as an extensional study of the symmetrical cases. Then, the numerical analyses based on this model are performed to evaluate the influence of electric double layers on the thin water lubricating film with and without consideration of the elastic deformation of the opposing surfaces. The results show that the electric double layers lead to significant variations in the properties both of hydrodynamic and elasto-hydrodynamic lubricating film.