Effect of wear particles on the contact of unlubricated sliding metals

The behaviour of tin bronze when rubbed at high speed against a case hardened steel, was studied. Removal of the particles of tin bronze which adhere to the steel affects the process of transfer, wear rate, mean surface temperature, coefficient of friction and surface topography.     

A practical interpretation of unlubricated wear data for some non-ferrous metals

An attempt is made to indicate how the results from simple unlubricated pin-on-disc type wear tests can be meaningfully interpreted by a designer in his approach to the selection of materials for practical applications.     

The unlubricated wear of iron

The unlubricated wear of pure iron has been studied under two different sliding conditions using a dynamic wear rig operating in the scanning electron microscope (SEM) and a simple pin-on-disk apparatus operating in air. The formation of wear debris under these sliding conditions suggested a delamination mechanism. The structure and amount of wear debris produced by the pin in the SEM tests were strongly influenced by small variations in surface roughness of the hardened disks. The use of normalized variables, incorporating key parameters such as the contact geometry and hardness, introduces a new method for presenting wear rates. Using the normalized variables, the measured wear rates were found to agree with Archard's law. Furthermore, this method of presentation allowed useful comparisons to be made between wear rate data from different investigations.     

The influence of atmospheric humidity on the abrasive wear of metals

A pin-on-disc apparatus has been used to obtain continuous simultaneous measurements of the wear and friction (sliding force) behaviour of metals on bonded silicon carbide abrasive paper under conditions of controlled humidity. Iron, mild steel and copper exhibit qualitatively similar wear behaviour: the wear rate decreases progressively with the number of passes over the same track. In contrast, the wear rate of titanium remains constant. Variation in atmospheric humidity has little effect on the wear rates of copper or titanium, although a slight effect was found in mild steel and iron. A stronger dependence on humidity was found in the friction behaviour of all four metals, as well as a corresponding relationship between humidity and the specific energy required for metal removal by abrasion. Preliminary results from single-particle scratch tests reveal changes in the contact between a single silicon carbide particle and a polished iron surface at different humidity levels. Although only tentative explanations can be made at this stage for these effects, it is evident that any proposed mechanism must account for the behaviour of both the metal and the abrasive together, rather than of one component of the system alone.     

The unlubricated wear of sintered iron

The unlubricated wear of sintered pure iron has been investigated under different sliding conditions using a dynamic wear rig operating in the scanning electron microscope and a simple pin-on-disk apparatus operating in air. Delamination and ploughing mechanisms were found to be responsible for wear during sliding under the different conditions. Using normalized variables, the measured wear rates agreed well with the predictions of Archard's law and correlated with wear data from other sources. The effect of porosity in the iron has also been studied. Open pores on sliding surfaces were found to be important in generating and trapping wear debris. When the trapping mechanism was bypassed by cleaning, or rendered ineffective by pore closure, the wear behaviour of high porosity sintered iron approached that of the lower porosity specimens.     

The unlubricated wear of sintered steels

The unlubricated wear of 10.3% porosity sintered medium-carbon and 12.8% porosity sintered low-alloy steels was investigated under different sliding conditions. Their wear characteristics were found to be similar to their non-sintered counterparts. Within this range of sliding conditions, both mild-oxidational and delamination wear took place, with the former dominating the wear processes. The oxide debris produced changed with the applied load: switching from a low-temperature oxide to a high-temperature one at higher loads. The wear rates agreed reasonably well with Archard's law and with data from other sources, suggesting that Archard's law can account for the wear rates produced by both mild-oxidational wear and delamination wear. This agreement also suggests that within the range of porosites investigated an overall framework can be established in the unlubricated wear of both sintered and non-sintered steels.     

Effect of varying load on wear resistance of carbon steel under unlubricated conditions

Pin-on-disk type wear tests of mild steel in contact with itself were conducted under unlubricated conditions in moist air to investigate the effect of changes in load on the transition behavior between severe and mild wear. Machines in operation are usually subjected to varying loads. In this study, two contact load levels were used as a simple varying load condition. The load was changed in a step-wise manner between the low and high levels either once or twice during certain tests.Severe wear appears at the high load levels and mild wear at the low load levels in wear tests under constant loading. After oxidized, work-hardened wear surfaces have been formed under mild wear at the low load during the first stage, the wear mode changes to “quasi-mild wear” having a low rate at the high load in the final stage. The load range, where the low wear rate can be maintained under quasi-mild wear, extends to the higher load level after the change(s) in load. Once the quasi-mild wear surface has been generated, the wear mode can be maintained even at the high load for a very long sliding distance.     

The unlubricated wear of flake graphite cast iron

The wear behaviour of flake graphite cast iron was correlated with the microstructural parameters of graphite volume fraction and flake size using a pin-on-ring specimen configuration. Pin specimens of cast iron were prepared under carefully controlled melting and casting conditions to provide microstructures with variation in either carbon content or flake size but with the same type A graphite structure and pearlitic matrix.Mild and severe modes of equilibrium wear were identified, the predominant effect of both microstructural parameters being in the severe wear regime. Decrease in flake size and increase in carbon content are detrimental to the wear behaviour resulting in a marked increase in the severe wear rate and a decrease in the mild-to-severe transition load.     

Comments on “The unlubricated wear of cast irons”

A numerical solution is developed for the equations governing the laminar hydrodynamic flow in a sector-shaped thrust bearing with its axis parallel to but offset from the rotational axis. The lubricant viscosity is assumed to be a function of the temperature distribution in the fluid film. The rotating plate is assumed to be an isothermal component and the heat conduction equation in the stationary component is solved simultaneously with the governing equations of the fluid film. Thermal effects are shown to be pronounced especially at large values of offset from the rotational axis.     

Oxidative wear kinetics in unlubricated steel sliding contact

A tools lifetime absolutely depends on the material properties and lubrication. By a lubricant absence, the direct contact between the materials might cause an increase of friction and wear, which might lead to a tool's failure. That is why wear prediction and control are important requirements in industry.The wear prediction experiments in this study were performed for an unlubricated (dry) bushing/shaft system of relatively high load and low sliding speed. Two different commercially available tool steels were used for tribological investigations. Normal load and temperature were considered as independent variable in the wear testing. The results showed that the extensive oxidation of metals have a positive factor on wear. When the temperature raises formation of the protective oxide layers consequently change the wear transition to the mild oxidative wear.Based on the classical wear equations and on the chemical oxidation kinetics concepts a wear prediction model was developed and statistically evaluated in the case when metallic contact wear as the predominant wear mechanism was combined with the oxidation of metals.     

Transfer during unlubricated sliding wear of selected metal systems

Results of pin-on-disk sliding tests in vacuum, with copper, CuNi, nickel and molybdenum sliding against iron, indicate that initial transfer events involve discrete fragments. We propose that the process involved in the initial transfer events is a consequence of local shear instabilities which develop at large plastic strains. In copper samples the initial transfer elements are lamellar, with layer thickness equal to the cell thickness in the highly deformed base material adjacent to the sliding interface.Prolonged sliding gives rise to the formation of transfer layers or patches on the specimen surface. These layers or patches are composed of finely mixed material derived from the two sliding counterparts as well as scattered pieces of more recently transferred fragments. Typical wear debris particles are generated from the transfer layers or patches.Transfer tendencies for different materials combinations can be predicted from an adhesion point of view if geometrical effects are properly considered. Combining theoretical predictions of transfer tendencies and experimental observations of transfer and geometrical effects, one finds that for a pin-and-disk system the disk should be made of the material having the higher cohesive strength.     

Tribochemical effects on the friction and wear behaviors of diamond-like carbon film under high relative humidity condition

Friction and wear behaviors of diamond-like carbon (DLC) film in humid N₂ (RH-100%) sliding against different counterpart ball (Si₃N₄ ball, Al₂O₃ ball and steel ball) were investigated. It was found that the friction and wear behaviors of DLC film were dependent on the friction-induced tribochemical interactions in the presence of the DLC film, water molecules and counterpart balls. When sliding against Si₃N₄ ball, a tribochemical film that mainly consisted of silica gel was formed on the worn surface due to the oxidation and hydrolysis of the Si₃N₄ ball, and resulted in the lowest friction coefficient and wear rate of the DLC film. The degradation of the DLC film catalyzed by Al₂O₃ ball caused the highest wear rate of DLC film when sliding against Al₂O₃ ball, while the tribochemical reactions between DLC film and steel ball led to the highest friction coefficient when sliding against steel ball.     

Influence of environment on the unlubricated wear of 316 stainless steel at room temperature

The wear of a type 316 austenitic stainless steel has been studied in a variety of gaseous atmospheres at constant load and slidind speed under reciprocating conditions. The environments reported include air, CO₂, argon and partial vacuum. The wear debris in all cases was essentially metallic and its rate of production per unit length was constant with sliding distance. On this evidence the wear process was classified as being of the severe type. There was, however, a wide range of specific wear rates which varied by a factor 30 extending from 1 × 10⁻¹³ m³/N m in air to 3 × 10⁻¹⁵ m³/N m in argon, decreasing with decreasing oxygen partial pressure.It is thought that the wear process in all the atmospheres examined was basically the same, being one of prow formation with the subsequent breakdown of these features to wear particles. The initial stage in prow growth is the formation of transfer elements or platelets by asperity interactions and their accumulation to form plateaux. There was a correlation between wear rate and platelet size, the wear rate decreasing with decreasing platelet size. The prow formation stage is considered to be the rate limiting step in the severe wear of ductile materials, but the influence of oxygen partial pressure on transfer element size needs further investigation.     

Wear of some F.C.C. metals during unlubricated sliding part IV: Effects of atmospheric pressure on wear

The dependence of wear on atmospheric pressure was studied in relation to the fatigue of materials. Ni, Cu and Au specimens were fatigued at various atmospheric pressures and the results were compared with their wear behaviours. Similar curves were obtained for each material when the reciprocal of the fatigue life and the number of wear fragments were plotted against the atmospheric pressure. It was concluded that changes in the number of wear fragments or in the amount of wear are governed by the resistance of the materials to fatigue. On the basis of the findings obtained, mechanisms of wear are discussed.     

Acetal gear wear and performance prediction under unlubricated running condition

Tests have shown that the acetal gear wear rate will be increased dramatically when the load reaches a critical value for a specific geometry. The gear surface will wear slowly with a low specific wear rate if the gear is loaded below the critical condition. The sudden increase in wear rate may be due to the gear operating temperature reaching the material melting point under the critical load condition. Gear surface temperature has been investigated in detail by studying three components: ambient, bulk and flash temperatures. Through extensive experimental investigations and modelling on gear surface temperature variations, a general relation has been built up between gear surface temperature and gear load capacity. A new design method for acetal gear has been proposed based on the link between polymer gear wear rate and its surface temperature. The method has been related to test results under different operating ambient temperature and gear geometries. Good agreements have been achieved between the proposed method predictions and experimental test results. The maximum torque the acetal gears can transmit is 9 Nm for both the driver and driven with a 2 mm module, 30 teeth and 17 mm face width. Copyright © 2006 John Wiley & Sons, Ltd.     

Effect of boronising on friction and wear of ferrous metals

Boronising is a surface diffusion treatment analogous to carburising, but produces harder surfaces without recourse to quenching. Friction and wear resistance of boronised coatings have been evaluated on steels and grey cast iron under dry sliding conditions. Boronising inhibits adhesive wear during running-in and at loads above the mild/severe transition load. The onset of severe wear is prevented until the boronised layer has been worn away, its life expectancy is extremely long and is inversely related to applied load.Boronising has considerable advantages over other similar surface conversion treatments under adhesive wear conditions and is likely to find application at elevated temperatures and under non-oxidising conditions.     

Wear of some F.C.C. metals during unlubricated sliding Part III: A mechanical aspect of wear

The relations between the size of wear fragments and the sliding velocity or normal load were studied from a mechanical point of view for the unlubricated wear of nickel. It was found that the size of mean wear fragments has a definite quantitative correlation with the thickness of the plastically deformed substrate layer, irrespective of the experimental conditions. The thickness of the deformed layer is determined by the actual forces working on real contact points, which are dependent on the dynamic properties of the specific sliding system. These mechanical factors govern wear-velocity and wear-normal load characteristics.     

An experimental investigation into the friction and wear characteristics of unlubricated roller bearings

Lubricated rolling element bearings usually fail by fatigue. In the case of unlubricated rolling element bearings failure is due to wear of the rolling elements and cage. As the amount of wear increases so the friction torque of the bearing increases until catastrophic failure occurs and the bearing seizes.     

Comments on the sliding wear of metals

The author reviews selected experimental results which have contributed to improved understanding of sliding wear processes. The emphasis is on the chemical and structural changes which occur at and near the surface of metallic materials during sliding in different environments. The importance of plastic deformation, fracture, transfer, mechanical mixing, phase transformations and oxidation is discussed. Examples of transitions are described, and interesting correlations noted. In selecting the content of this paper, the author includes controversial results and conclusions and raises questions about the development of wear equations, interpretations of the wear coefficient, the importance of adhesion, the roles of hardness, the causes of transitions and the location of debris-producing cracks.