Two dimensionless parameters controlling the occurrence of stick-slip motion in a 1-DOF system with Coulomb friction

The condition for the occurrence of stick-slip motion is examined analytically based on a 1-DOF system with Coulomb friction which includes seven independent parameters. There exist two dimensionless parameters which control the occurrence of stick-slip motion, one denotes the easiness for the occurrence of stick-slip motion and the other is the damping ratio acting to suppress the motion. A simple non-occurrence inequality of stick-slip motion is derived with the two dimensionless parameters through dimensionless analysis and numerical simulation, which has both high accuracy and high usability. Its validity is confirmed by the comparison with the experimental results reported by Singh.     

Influence of surface roughness on friction and scuffing behaviour of cast iron under sparse lubrication

Friction and scuffing behaviour of grey cast iron as influenced by the surface roughness under sparse lubrication conditions is studied. The studies are carried out on a three-shoe-on-disc machine under the conditions of parallel sliding. The experiments were conducted at three oil supply rates of 0.10, 0.22 and 0.36 μg/cm² per contact obtained through a mist oiling system and with specimens of cast iron shoes of different roughness values, Ra, between 0.04 and 2.0 μm against a 0.55% C steel disc. In a step load procedure, the friction torque at the end of each load step and the scuffing load are the major parameters measured. Results of friction and scuffing behaviour as a function of roughness and oil supply rates have been discussed.     

Effects of lubricants on friction and wear of Ti(CN)/1045 steel sliding pairs

The friction and wear properties of Ti(CN)/1045 steel rubbing pairs were investigated under dry and lubricated conditions by using a pin-on-disk tribometer. The selected speed range was 0.8 to 3.2 m/s and the load range was 58.8 to 235.2 N. Distilled water and a mineral oil (no additives) were used for lubrication, respectively. The wear of Ti(CN) ceramic under dry conditions was caused mainly by adhesion between the rubbing surfaces and the microfracture of Ti(CN). With the load and speed increasing, the adhesion and diffusion between rubbing surfaces increased and resulted in wear increasement of Ti(CN). Because of the brittleness of ceramics, the microfracture wear of Ti(CN) increased rapidly when the load was raised to some high values. The lubricating and cooling effects of the lubricants could improve the frict on and wear. Compared with water, oil was much better in improving the tribological properties. The analysis results obtained from XPS and AES examinations showed that ferrous oxide was produced on the wear scars, which could reduce the adhesion between the rubbing surfaces to some extent. The lubricating effects of the oil under boundary lubrication conditions were attributed to the formation of carbon films on the rubbing surfaces.     

Friction-induced vibrations of reciprocating contacts

The tangential and normal oscillations for three reciprocating and sliding pairs of different materials are experimentally obtained at different normal loads and reciprocating frequencies. The magnitude and phase shift of the transfer function have been measured to correlate the normal and tangential induced oscillations. The results show a remarkable effect of the properties of sliding pairs on both the normal and tangential vibration amplitude during the prescribed motion. The maximum normal amplitude of oscillation is found to vary linearly with the maximum contact pressure regardless of the material type of the sliding surfaces. It is also found that at high normal loads, the tangential amplitude is proportional to the sonic speed in the sliding pair material. Analytical justification has been introduced throughout the work to substantiate and explain the experimental findings.     

Super-low friction of MoS2 coatings in various environments

The ultra-low friction coefficient (typically in the 10⁻² range) of MoS₂-based coatings is generally associated with the friction-induced orientation of ‘easy-shear’ planes of the lamellar structure parallel to the sliding direction, particularly in the absence of environmental reactive gases and with moderate normal loads. We used and AES/XPS ultra-high vacuum tribometer coupled to a preparation chamber, thus allowing the deposition of oxygen-free MoS₂ PVD coatings and the performance of friction tests in various controlled atmospheres. Friction of oxygen-free stoichiometric MoS₂ coatings deposited on AISI 52100 steel was studied in ultra-high vacuum (UHV: 5 × 10⁻⁸ Pa), high vacuum (HV: 10⁻³ Pa), dry nitrogen (10⁵ Pa) and ambient air (10⁵ Pa). ‘Super-low’ friction coefficients below 0.004 were recorded in UHV and dry nitrogen, corresponding to a calculated interfacial shear strength in the range of 1 MPa, about ten times lower than for standard coatings. Low friction coefficients of about 0.013–0.015 were recorded in HV, with interfacial shear strength in the range of 5 MPa. Friction in ambient air leads to higher friction coefficients in the range of 0.2. Surface analysis performed inside the wear scars by Auger electron spectroscopy shows no trace of contaminant, except after friction in ambient air where oxygen and carbon contaminants are observed. In the light of already published results, the ‘super-low’ friction behaviour (10⁻³ range) can be attributed to superlubricity, obtained for a particular combination of cystallographic orientation and the absence of contaminants, leading to a considerable decrease in the interfacial shear strength.     

Reduction of the sliding friction of metals by the application of longitudinal or transverse ultrasonic vibration

The influence on sliding friction of ultrasonic vibration both parallel and perpendicular to the sliding direction has been studied for samples of aluminium alloy, copper, brass and stainless steel sliding against tool steel. Experiments were performed at a mean sliding speed of 50 mm s⁻¹, and at mean contact pressures up to 0.7 MPa, with vibration amplitudes up to 10 μm at 20 kHz. Significant reduction in sliding friction was observed (up to >80%) and good agreement was found between the measured values and the predictions of two simple models for the effects of longitudinal and transverse vibrations. Longitudinal vibration produces greater reduction in friction than transverse vibration at the same amplitude and frequency. At high vibration amplitudes, the reduction in friction was less than that predicted by the models, because significant metallic transfer occurred from the softer metals to the tool steel counter surface.     

Dependence of the friction process on the molecular structure and architecture of thin polymer films

The frictional properties of a homologous series of poly(n-alkyl methacrylates) (PnAMA) and a series of poly(methyl methacrylate) (PMMA) films, cast from a variety of solvents, are characterized. The choice of polymer film was driven by the consideration of the possible mechanisms for the accommodation of a macroscopically applied shear stress by molecular entities. Two possible mechanisms are proposed: (i) the relative flexibility of the polymer backbone chain. For this purpose the PnAMAs have been chosen. By varying the length of the substituent chain, the relative molecular freedom around the backbone chain is altered. These molecular differences are sensed in the frictional properties at the macroscopic level, and (ii) the molecular organization is also proposed to be a factor in determining the friction response of a particular polymer film. For this purpose, the frictional properties of PMMA films cast from different solvents are investigated. There is observed to be a strong influence of the molecular organization on the frictional properties of the solvent cast PMMA films. The molecular probe employed to characterize the molecular environment is vibrational spectroscopy. Conformationally sensitive vibrational modes are used to determine the relative flexibility of the backbone chain and the organization of the chain network.     

A procedure for evaluating the positioning accuracy of reciprocating friction drive systems

The dynamic characteristics of stick-slip motion in reciprocating friction drive systems are investigated under dry contact using a 0.45% carbon steel pair. Based on this dynamic analysis, the stick-slip motion can be eliminated under certain experimental conditions depending upon driver speed, normal load and spring constant. The effects of normal load, driver speed, and spring constant on the positioning accuracy of the reciprocating friction drive system are examined under harmonic oscillation without stick-slip motion. Results show that at very low spring constants, the slip increases with increasing driver speed such that higher normal load has better positioning accuracy or smaller slip than does lower normal load. However, at high spring constants and high normal loads, there exhibits severe wear or peak at two limit positions on the sliding region. As a result, smaller normal loads have a better positioning accuracy than larger normal loads due to severe wear at high normal loads.     

Investigation of the friction effect at pin joints for the five-point double-toggle clamping mechanisms of injection molding machines

In this study, an analytical formulation for the necessary thrust of the crosshead of the five-point double-toggle clamping mechanism during real-mold clamping operation is proposed in order to investigate the effect of the friction at pin joints. The friction is considered to be a Coulomb friction. The inertia effects are not considered.Numerical examples are studied for different values of coefficient of friction to investigate the effects of friction at pin joints. In order to estimate the kinetic friction coefficient at pin joints, the motor torques required for real-mold clamping obtained by the present study are adjusted by using different values of coefficient of friction and mechanical efficiency to fit the experimental data.     

The composition of lubricant-derived surface layers formed in a lubricated cam/tappet contact II. Effects of adding overbased detergent and dispersant to a simple ZDTP solution

The surface films formed by a set of oils of related additive chemistry, but with differing detergent and dispersant contents, have been chemically characterized using a combination of surface analysis techniques. The films were formed in a cam/tappet friction apparatus with a direct acting bucket tappet geometry. In the absence of overbased detergent, the amorphous films were composed essentially of inorganic zinc phosphates formed by the ZDTP anti-wear additive, and evidence of higher molecular weight phosphates (e.g. metaphosphates) was found. Adding overbased detergent and dispersant resulted in partial replacement of zinc by the detergent metal and loss of the higher molecular weight phosphates in favour of ortho- and pyro-phosphates.     

Coefficient of friction for aluminum in contact with a carbon fiber epoxy composite

In bolted joints, a large part of the load is transferred by friction. The objective of this investigation is to measure the coefficient of friction for carbon fiber epoxy matrix composite, HTA-6376, in contact with aluminum, 3637-77, in reciprocal sliding. During testing, the coefficient of friction increased initially with number of cycles and after reaching a maximum, slowly decreased. The initial coefficient of friction is approximately 0.23 and the peak coefficient of friction after wear in is approximately 0.68. The coefficient of friction is independent of normal load. During wear, cracks are formed at the fiber–matrix interface, which causes the matrix layer on the original composite surface to break off in pieces. It also causes single fibers or groups of fibers to be broken off and removed from the surface. Pieces of carbon fiber caused depressions in the aluminum surface. The wear debris is reattached to the composite surface but not to the aluminum surface.     

Elastoplastic finite element analyses of an upsetting-sliding test for the determination of friction at medium and high contact pressure

The upsetting-sliding test is a friction test employed for the determination of the tangential stress-contact pressure relationship at a specimen-tool or target part-contactor contact surface. It is used in addition to the analysis of forming sequences or mechanical design. The specimen, or target part, used in the test is the same that the one involved in the studied forming sequence so that the physical and chemical characteristics of the contact involved by the test and sequence are the same. In the test, the contact pressure is exerted by an inclined front surface indenter which takes the place of the tool or the contactor. Then, the indenter is moved in contact with the generator line surface of the specimen or the target part. The computation of the tangential stress, contact pressure and effective plastic strain firstly with respect to the penetration, sizing length and front face angle and, secondly, with respect to the frictional variations are performed with a three-dimensional finite element model. An initial guide for the determination of the indenter length and front angle and indenter penetration in the specimen is proposed in order to define the most realistic tangential stress-contact pressure curves.     

Characteristics of wear results tested by pin-on-disc at moderate to high speeds

The use of a pin-on-disc configuration for investigating the wear mechanism or behaviour of solid materials is examined carefully since the results of such configurations differ from published data and some existing theories cannot be applied to such a configuration directly. The obvious contradictions include the following. The results between the arrangements of the rotating pin and the stationary pin under the same load and speed are different. The bulk temperatures of the rubbing specimens increase with the duration of testing, which may eventually arrive at a steady state. However, before the wear condition reaches a steady state, it will have continuously varied. Moreover, the friction coefficient increases with sliding speed when the applied load on the rubbing specimens is over certain levels. All these contradictions can be reasonably explained with the accurate prediction of bulk and flash temperatures at the contact area. To this end, this paper provides a more reasonable method for the calculation of temperatures and the real and apparent contact areas.     

The friction and wear properties of nanometre SiO2 filled polyetheretherketone

Nanometre SiO₂ filled-polyetheretherketone (PEEK) composite blocks with different filler proportions were prepared by compression moulding. Their friction and wear properties were investigated on a block-on-ring machine by running a plain carbon steel (AISI 1045 steel) ring against the composite block. The morphologies of the wear traces and the transfer film were observed by scanning electron microscopy (SEM). It was found that nanometre SiO₂ filled-PEEK exhibited considerably lower friction coefficient and wear rate in comparison with pure PEEK. The lowest wear rate was obtained with the composite containing 7.5 wt.% SiO₂. The SEM pictures of the wear traces indicated that with the frictional couple of carbon steel ring/composite block (fillec with 7.5 wt.% filler), a thin, uniform, and tenacious transfer film was formed on the ring surface. It was inferred that the transfer film contributed largely to the decreased friction coefficient and wear rate of the filled PEEK composites.     

Microtribological applications of probe microscopy

This paper introduces the new field of microtribology; it gives a general overview and then presents some typical research results. Micromachines use very lightweight sliding parts, and their wear is primarily due to surface interaction forces rather than load or weight. The ultimate goal of microtribology is to create practical zero-wear devices. Microtribological evaluations of surfaces have started using new tools, such as the scanning probe microscope. Quasi-static indentation tests, impact indentation tests, line scratch tests, and scanning scratch tests have been performed using an atomic force microscope with a diamond tip. Frictional force distributions and adhesive force distributions have also been obtained using atomic force microscopes. Water clusters adsorbed onto solid surfaces have been observed using a scanning tunnelling microscope. The configuration, adsorption, and mobility of lubricant molecules have also been evaluated using a scanning tunnelling microscope.     

Si_3N_4陶瓷旋转超声磨削加工的表面摩擦特性

为了探索结构陶瓷材料在摩擦过程中表面形貌的变化规律及其对摩擦特性影响,分析了摩擦过程中材料的接触过程及力学关系,并对旋转超声磨削加工的Si3N4陶瓷试样开展了摩擦表面形貌、摩擦因数等特性的试验研究。首先根据接触特点和材料特性,基于分形理论推导出接触面总载荷计算公式,基于该公式建立了结构陶瓷摩擦因数分形模型。分析结果表明:当初始表面轮廓分形维数分别为1.4,1.45,1.5和1.55时,摩擦因数与摩擦后表面轮廓分形维数呈类似正态分布曲线。然后通过旋转超声磨削加工的Si3N4陶瓷试样面面接触摩擦试验,研究了摩擦后陶瓷材料表面微观形貌和摩擦因数变化规律,分析了各因素对摩擦因数的影响。试验结果表明:产生微观裂纹是Si3N4陶瓷摩擦后表面微观形貌的显著特点;温度值等于160℃是Si3N4陶瓷摩擦因数由下降转为上升的拐点;当施加载荷为360N和往复频率为80Hz时,摩擦因数最大。得到的结果为通过表面形貌控制提高结构陶瓷耐磨性能提供了技术支撑。     

The lubricated wear of ceramics : Part II: The wear and friction of silicon nitride and steel in the presence of mineral oil or an ester based lubricant

The friction and wear characteristics of combinations of silicon nitride, alumina and AISI 52100 steel in the presence of mineral oil containing anti-wear, dispersant and detergent additives have been investigated in a tri-pin-on-disc machine. The tests were carried out at a nominal temperature of 100°C for a range of sliding speeds, loads and total sliding distances. In Part II of this two-part paper a comparison will be made between the tribological performance of these sliding pairs of materials in mineral oil and ester based lubricant environments. The results of the investigation showed that the alumina performed relatively poorly under these test conditions, whereas silicon nitride showed good potential as an improved wear resisting material compared with 52100 steel. Wear factors of the order of 10⁻¹⁰ mm³/Nm were deduced for the alumina, while values as low as 10⁻¹¹ mm³/Nm were typical of the silicon nitride sliding against 52100 steel discs. The alumina pins wore by a process of brittle fracture at the surface, whereas the silicon nitride pins wore primarily by a tribochemical polishing mechanism. The rate of tribo-chemical wear was found to be proportional to the nominal contact area.     

Improvement in dynamic characteristics of surface grinding machines with a horizontal spindle and a reciprocating work-table by means of a new type of dynamic damper

A number of types of dynamic damper for surface grinding machines with a horizontal spindle and a reciprocating work-table are developed and their effectiveness is investigated experimentally. In all cases, the amplitudes of the tuning-fork mode vibrations of the machines were reduced to about one-fifth of the values in the original states, and chatter marks due to these vibrations have been suppressed completely. In particular, a multi-degrees-of-freedom two-directional type dynamic damper has excellent adaptability to the variation in the dynamic characteristics of the surface grinding machines during machining operations and is also effective in suppressing the horizontal vibration of the wheel-head.     

Tribofragmentation and antiwear behaviour of isogeometric phosphorus compounds

The tribofragmentation behaviour of organic phosphorus compounds is very important for their application as antiwear and extreme-pressure additives. The objective of this review therefore is to summarize and evaluate the results of some recent research into the thermal and tribological properties of a series of mono- and dithiophosphate esters and amides by a systematic comparison of their structure, thermal stability and friction and wear properties. The experimental data show that the chemical structure of an additive greatly influences its physico-chemical and tribological properties. For high temperature performance it is essential that the additive is sufficiently stable to degrade only in the friction zone and not at the temperature of the bulk fluid.     

A comparative study on the microstructure and tribological properties of Si3N4 and TiN films produced by the IBED method

The tribological properties of Si₃N₄ and TiN thin films produced by ion beam enhanced deposition (IBED) were compared on a SRV friction and wear testing machine. The friction coefficient of all thin films shows a descending tendency with increase in load, and is lower than that of 52100 steel. All the IBED films show a much better wear resistance than 52100 steel, especially in the higher load and frequency ranges; it can reach six times that of the latter. In order to understand the reasons for their excellent properties, the microstructure, microhardness and bonding strength with the substrate were analysed by SEM, X-ray diffraction, Knoop hardness and scratching test methods separately. The results show that the TiN(1) film exhibits the best tribological properties, which are closely related with its greater hardness and bonding strength.