Stick‐slip in the presence of a normal vibration

This paper presents a dynamic analysis of a contact system subjected to an applied normal vibration. The tangential motion of the system is found to exhibit stick‐slip behaviour dependent on the applied vibration, even in the absence of a difference in static and kinetic friction or a negative gradient in the friction‐velocity relation. The analysis leads to the development of a criterion for stick‐slip in the form of an equation defined in terms of the system and input parameters. This criterion is also presented graphically and is assessed through a series of experiments. The analysis and measurements show that stick‐slip occurs for a significantly wider drive velocity range when the normal vibration applied is near the natural frequency of the drive system. In addition, when stick‐slip occurs in the presence of a normal vibration, the frequency of stick‐slip is found to lock in to the frequency of the applied vibration.     

Occurrence limit of stick‐slip: dimensionless analysis for fundamental design of robust‐stable systems

In most practical mechanical systems, sliding surfaces are utilised under the assumption that they operate smoothly. Stick‐slip motion can therefore be a serious nuisance that interferes with achieving high performance in mechanical systems. The present paper describes the nature of stick‐slip based on an analysis of a 1‐DOF sliding system. The dimensionless parameters controlling the stick‐slip are clarified by deriving the dimensionless forms of the governing equations. For a friction model that considers the dependence of the kinetic friction coefficient on the relative velocity, we find three types of sliding systems with regard to stick‐slip: the unstable system, the stable system and the robust‐stable system. A criterion is proposed for the fundamental design of robust‐stable systems; if a sliding system is robust stable, no matter how large a disturbance is, the energy of the disturbance is dissipated perfectly, and steady sliding without any vibration is ensured. Copyright © 2009 John Wiley & Sons, Ltd.     

Stick–slip friction in dissimilar polymer pairs used in automobile interiors

The static and dynamic friction of dissimilar pairs of plastics used in automotive interiors was measured as a function of normal load, system stiffness, and surface roughness. Glass fiber filled polypropylene (FPP) was slid on polycarbonate (PC) and glass fiber filled styrene–maleic–anhydride copolymer (SMAC) in a single pass, unidirectional sliding test. The friction was characterized by the value of static coefficient of friction (COF) and the number of stick–slip cycles during sliding. It was found that the FPP/PC and FPP/SMAC pairs had fewer instances of stick slip than FPP/FPP, PC/PC, and SMAC/SMAC pairs except for one of the SMAC polymers. The surface texture which had the smallest average radius of peak curvature, had the lowest value of static COF. The decrease in the static COF of polypropylene (PP) caused by the addition of glass fiber was most likely caused by the increase in elastic modulus and hardness.     

A study of fiber-capstan friction. 2. Stick–slip phenomena

A study was made of the friction of nylon fiber (monofilament fishing line) rubbing against spinning aluminum and nylon capstans (cylinders), in the presence of various lubricants. Use of variable speed drives with appropriate gear reducers allowed variation of sliding speed over four orders of magnitude and monitoring of the entire Stribeck curves, from boundary through mixed to hydrodynamic sliding, for these systems. The effects on friction of seven system variables—cylinder material, sliding speed, system elasticity, input tension, lubricant viscosity, lubricant additives and cylinder surface roughness—were investigated. Special attention was given to the effects of elasticity on friction and the character of sliding. Factors which determine the critical speed where stick–slip sliding just appears or disappears are defined.     

Molecular dynamics study of the interfacial slip phenomenon in ultrathin lubricating films

The tribological performance of thin films of a liquid alkane has been studied through a molecular dynamics simulation with particular attention being paid to the phenomenon of interfacial slip. The model system for the simulation consists of two solid walls, with the lubricant molecules confined between them. Molecules of n‐decane (C₁₀H₂₂) were chosen to represent the lubricant molecules. The results of the simulation show: the average velocity of decane molecules in a Couette flow exhibits largely a linear distribution, but with a slip velocity at the solid‐liquid interface; when the simulations are performed at different temperatures, the slip ratios were found to vary with temperature; slip behaviour depends strongly on the solid‐fluid interaction; and slip ratios increase with decreasing film thickness, suggesting that slip in the thin films is a confinement‐related phenomenon.     

Speed‐ and topography‐dependent boundary friction characteristics of steel

A novel tribometer that undergoes significant changes at ultra‐slow (>5.0 µm s⁻¹) to moderate (<20 cm s⁻¹) sliding speeds was developed in order to study the friction‐speed characteristics of steel lubricated with oil. Three different surface topographies were applied to the specimens, and the friction characteristics with an additive‐free base oil (MO91) and a stearic acid‐formulated oil (StA/MO91) were studied to understand the effects of surface textures on the lubrication performance of an adsorbed molecular layer formed by StA. Friction reduction behaviour of the adsorbed layer observed in the transverse direction was attributed to microscopic hydrodynamic action that maintained the load‐carrying performance of the adsorbed layer. Copyright © 2010 John Wiley & Sons, Ltd.     

High‐speed optical imaging of liquid film flow and liquid macro‐slip over free surfaces with different surface energies

High‐speed optical imaging is introduced as a visualization method to investigate the film‐flow properties of liquids with different surface tensions and viscosities (water, poly‐alpha‐olefin oil, and glycerol) over free rotating surfaces with different surface energies and polarities (steel, and two different diamond‐like‐carbon ‐ DLC coatings, i.e. DLC, and F‐DLC). It was found that the polar surface energy strongly influences the structural dynamics of the liquid film's flow and the film's slip. Namely, a decrease in the polar surface energy results in a less stable film with de‐wetting areas and breakups into streams, as well as in a larger amount of film slip, which was most clearly expressed by the F‐DLC. It was also found that the combination of a high surface tension and a low viscosity provides the largest amount of liquid slip, with the most obvious breakup of the liquid film being observed with water, which clearly exhibits these properties.     

Alkyloxy‐s‐triazine derivatives with and without fluorine‐containing substituents as lubricants for steel–steel contact

Three alkyloxy‐s‐triazine derivatives were synthesized and their tribological properties as lubricants for steel–steel contact were evaluated using an Optimol SRV tester at 20°C and 100°C. Their thermal stabilities were also investigated by thermogravimetric analysis. The results show that the three alkyloxy‐s‐triazine lubricants have good thermal stability. Moreover, 2,4,6‐tris(1,1,5‐tri‐H‐octafluoropentyloxy)‐1,3,5‐s‐triazine (FPOT) possesses the best anti‐wear property and good load‐carrying capacity both at 20°C and 100°C. At 20°C the anti‐wear effectiveness of 2,4,6‐tris(n‐pentyloxy)‐1,3,5‐s‐triazine (POT) is the worst, while at 100°C that of the 1,1,5‐tri‐H‐octafluoropentyloxy and/or 1,1,7‐tri‐H‐dodecafluoroheptyloxy tri‐substituted s‐triazine mixture (FMOT) is the worst. In addition, the friction‐reducing properties of the two fluoroalkyloxy‐s‐triazines, FPOT and FMOT, are not as good as those of the non‐fluorine‐containing alkyloxy‐s‐triazine POT. Scanning electron spectroscopy with an energy dispersive analyzer of X‐ray and X‐ray photoelectron spectroscopy analyses of the worn surface indicate that during the rubbing process, tribochemical reactions occur between the lubricants and the metal surface to generate a complex boundary lubrication film comprised of FeF₂, Fe(OH)₂, organofluorine and organonitrogen compounds. Copyright © 2006 John Wiley & Sons, Ltd.     

Geometric effects in high‐temperature vapour‐phase lubrication using hydrocarbon feed gases

The effectiveness of ‘far‐field’ vapour‐phase lubrication, in which areas of a bearing surface that are cycled through the contact are exposed to vapour while outside the contact, has been demonstrated in both sliding and combined roll slide tests using acetylene vapours to deposit pyrolytic graphite. Friction coefficients as low as μ = 0.008 have been measured for steel at 540°C with far‐field acetylene concentrations as low as 5%. Effective vapour‐phase lubrication depends on solid lubricant deposition that exceeds the contact's capacity to remove solid lubricant through wear. While the rate of removal is increased by increasing the sliding velocity, in far‐field vapour‐phase lubrication the rate of lubricant deposition, and therefore the lubrication effectiveness, is augmented by increased areas available for far‐field deposition, such as those provided by performing wear tests with increased wear‐track diameters. These geometric concepts may be considered in rolling‐element bearing and gear set applications where vapour‐phase lubrication is to be employed.     

Biobased dry‐film metalworking lubricants

Metalworking lubricants must allow the manufacture of acceptable products at competitive cost without causing harm to operators or the environment. One way of attaining such a goal is through the use of biobased raw materials in lubricant formaulations. Biobased materials are generally non‐toxic, easily biodegradable, and abundantly available from renewable agricultural sources. However, successful application of biobased marterials in lubrication requires a thorough understanding of the tribochemical properties of these agricultural products. Recent studies have shown that biobased lubricants comprising starch and vegetable oils have promising lubrication properties. This paper discusses investigations into the effect of film thickness on the friction properties of dry‐film lubricants formulated from starch‐soybean oil composites.     

An analytical approach on the tribological behaviour of pocketed slider bearings with boundary slip including cavitation

Slider bearing performance depends on the boundary conditions at the interface between the solid surfaces and the fluid. This paper presents the combined effect of pockets and boundary slip on the load support and friction of parallel sliding systems using analytical solutions for a simple pocketed bearing. The effect of cavitation was of particular interest with respect to the inlet suction mechanism. It was demonstrated that applying boundary slip in a pocketed slider bearing gives a reduction in load support compared with the textured bearing without wall slip. Adding slip over the whole surface could retard the presence of cavitation. The influence of boundary slip is explored, and was found to significantly affect the frictional behaviour. Copyright © 2016 John Wiley & Sons, Ltd.     

Investigation of mechanical and tribological properties of tribo‐layer of Ni3Al matrix composites

The sliding wear of Ni₃Al matrix composites with addition of 1.5 wt.% graphene nanoplates was studied through pin‐on‐disc wear testing. The spontaneous formation of a tribo‐layer produced during sliding wear was found to result in a deviation from Archard scaling and an unexpected high wear resistance that was not based on hardness alone. The tribo‐layer exhibited specific microstructural evolution with significant severe deformation and grain refinement after wear. In the grain refinement area, the accumulation of dislocations and an increase in misorientations were found to lead to strain hardening. For the plastic deformed area, reduction in the dislocation density inside the elongated ultrafine grains reduced strain hardening compared with the grain refinement area. It can be concluded that the deviation from Archard scaling occurred primarily as a result of the microstructural evolution of the tribo‐layer, resulting in the specific performance of mechanical and tribological properties of Ni₃Al matrix composites under cyclic sliding wear process. Copyright © 2016 John Wiley & Sons, Ltd.     

Anti‐wear performance of organomolybdenum compounds as lubricant additives

A four‐ball tester was used to evaluate the anti‐wear performance of three kinds of organomolybdemun compounds in engine oils and lithium soap grease, i.e. molybdenum dialkyldithiophosphate (MoDDP), molybdenum dialkyldithiocarbamate (MoDTC), and sulphur‐ and phosphorus‐free organomolybdeum (molybdate). The results indicate that a low concentration of MoDDP does not improve the anti‐wear properties of the commercial engine oils, but a high concentration of MoDDP can obviously improve the anti‐wear properties of the engine oils. MoDTC does not improve the anti‐wear properties of the engine oils, but worsens the anti‐wear properties of the oils. Significant improvement of the anti‐wear properties is obtained with molybdate added in the four fully formulated oils. Both MoDDP and molybdate have good anti‐wear properties in the lithium soap grease, especially MoDDP with a high concentration. Copyright © 2006 John Wiley & Sons, Ltd.     

Frictional and load‐carrying behaviours of micro‐textured sector shape pad thrust bearing incorporating the cavitation and thermal effects

Frictional and load‐carrying behaviours of micro‐textured sector shape pad thrust bearing have been explored and reported herein. The textured pad surfaces have been generated employing different cross‐sectional shapes (circular, square, trapezoidal and triangular) of grooves. Based on the thermohydrodynamic lubrication analysis, it is observed that when the adopted texture pattern is placed on the pad towards the entry region, it produces substantial reduction in friction coefficient. The texture involving the square cross‐sectional shape of grooves has yielded substantial reductions in the friction coefficient in comparison with the conventional plain pad. Copyright © 2016 John Wiley & Sons, Ltd.     

Friction and wear behavior of AZ91D magnesium alloy against steel lubricated with N‐ and S‐containing organic borates

The friction reduction and antiwear properties of a range of nitrogen‐ and sulfur‐containing organic borates as oil additives in mineral oil were assessed using a Timken tester with a bearing steel ring against an AZ91D magnesium alloy block. X‐ray photoelectron spectrometry and scanning electron microscopy were employed to examine the boundary film formed on the surface of the magnesium alloy block. The results show that the borate additives were effective at reducing the wear of magnesium. The acting mechanism of borate, which acted as additive in the magnesium‐on‐steel tribological system, was proposed. Copyright © 2006 John Wiley & Sons, Ltd.     

Load‐carrying capacity of lubricated and dry sliding elements

The factors influencing the load‐carrying capacity of tribological contacts are reviewed. The load‐carrying capacity depends in part on the lubrication regime, and hydrodynamic, hydrostatic, and elastohydrodynamic lubrication are discussed. In some circumstances, lubrication is not possible. The parameters of dry rubbing are described, and approaches to the design and operation of dry bearings are discussed, particularly the choice of suitable non‐metallic materials for use in the wear couple.     

Reduction of adhesion and friction of silicon oxide surface in the presence of <Emphasis Type="Italic">n</Emphasis>-propanol vapor in the gas phase

The equilibrium adsorption of gas phase alcohol molecules has been proposed as a new means of in-use anti-stiction and lubrication for MEMS devices. Adhesion and friction of silicon oxide surfaces as a function of n-propanol vapor pressure in the ambient gas were invesitigated using atomic force microscopy. As the vapor pressure increases, the adsorbed n-propanol layer thickness increases. The adhesion and friction significantly decrease with very little addition of n-propanol vapor.     

Si3N4 as a biomaterial and its tribo‐characterization under water lubrication

Hip implant wear is recognised as the main cause of hip implant failure therefore has been widely investigated both experimentally and clinically, demonstrating the coexistence of abrasive, adhesive, fatigue and corrosive wear. Many clinical in vivo and bulk material wear rate data from published literature have been presented for non‐oxide ceramic implants. Several studies have shown that the coefficient of friction of self‐mated silicon nitride in water decreases from an initially high value to about 0.002 after a certain run‐in period. Since the worn surfaces become extremely smooth, the low friction is attributed to the initiation of hydrodynamic lubrication by a thin water film at the interface. The possibility of mixed lubrication, i.e. hydrodynamic lubrication by water and boundary lubrication due to the presence of colloidal silica on the wearing surfaces, has also been proposed. Influence of load, speed and surface roughness on the duration of the run‐in period of silicon nitride under water lubrication was investigated in this study. The results confirmed that a low coefficient of friction is obtained following a run‐in period when a wear scar of sufficient size is developed to reduce the contact stress. The run‐in period, during which the coefficient of friction is fairly high, is shorter for smoother surfaces and at higher loads and speeds. The striations that appeared to be associated with the high‐friction spikes can be formed as a result of surface film breakdown. Although the results are consistent with the proposed mechanisms of hydrodynamic lubrication or mixed lubrication, it is proposed that the low‐friction behaviour may also be related to fundamental interactions between two hard and elastically deforming surfaces covered with hydrogen‐terminated oxide films. Copyright © 2016 John Wiley & Sons, Ltd.     

Dynamic behavior of fluid‐structure coupling of hydrostatic spindle under effect of oil film slip

To better understand the dynamic characteristics of a hydrostatic spindle in fluid‐structure coupling, the impact of oil film slip on the 4 dynamic stiffness and damping coefficients of the spindle is studied. On the basis of modified Reynolds equation, which considers the microscale velocity slip effect, rotation error of the spindle is calculated. To solve the rotor axis orbit under the existence of eccentric quality, 4 dynamic stiffness and damping coefficients of the oil film, which describe the dynamics of a rotor axis orbit, are calculated by using load increment method and the pressure perturbation method. The research results show that velocity slip caused a certain impact on dynamics of bearing stiffness and damping performance. The experiment of the measuring path of the shaft verifies the correct and effect of the orbit of shaft center model.     

An indirect electric field effect on the friction of boundary‐lubricated contacts

To achieve significant friction variations by means of an externally applied voltage, it is found to be more effective to impose an electric field on the area surrounding a tribopair rather than directly through the contact. Using a new applied electric field setup, the friction behaviour of ceramic‐metal tribopairs, lubricated with an aqueous emulsion, is investigated. The potential affects the surface characteristics during the instant that the surface is out of rubbing contact. Consequently, the nominal frictional behaviour differs from that with no potential effect. The unexcited, excited, and post‐excited friction results for six different ceramic‐metal couples using a pin‐on‐disc tester are presented, and their characteristics are summarised.