Review of oxidational wear: Part I: The origins of oxidational wear

This is the first part of a two-part in-depth review of the oxidational wear of metals. It discusses the parallelism between the formation of an oxide film for dry contact conditions and of other surface films for lubricated contacct. Wear modes are unified into two major classes of mild and severe wear, including both lubricated both dry and conditions. Oxidational wear is a mechanism of mild wear in which protective oxide films are formed at the real areas of contact (during the time of a contact) at the contact temperature, T_cc. When the oxide reaches a critical thickness ξ, usually 1 to 3 μm, the oxide breaks up and eventually appears as wear particles. These oxides are preferentially formed on plateaux, which alternately carry the load - as they reach their critical thickness - and are removed. Temperature is important in determining the structure of the oxide film present, which in turn affects the wear properties of the sliding interfaces. Hence, this part of the review concludes with a thorough treatment of the thermal aspects involved during the sliding of a typical laboratory simulation of the oxidational wear of steel specimens without lubrication. This treatment shows how the general surface temperature (T_s) and the division of heat (?)_at the interface can be calculated and used, in conjunction with the measured wear rate (w), to give information about a possible surface model consisting of N contracts on the (thermally expanded) operative plateau, the height of the plateau being identical to the critical oxide film thickness (ξ) mentioned above.Part II outlines recent research to determine the oxidational constant, ie the activation energy and the Arrhenius constant, relevant to oxidational wear. It is found that the Arrhenius constant for oxidational wear is different from that for static oxidation tests. Some typical values of N, ξ and T_c are calculated from oxidational wear experiments. A new oxidational wear theory designed to take into account the oxide growth which occurs at the general surface temperature, T_s (where T_s < T_c) , whilst operation plateau is out-of-contact. This theory is most relevant to weat at elevated temperatures, where it is not permissible to assume that out-of-contact oxidation is negligible. After a brief review of the small amount of work done on the effects of partial oxygen pressures on oxidational wear, Part II concludes with a discussion of the possible connection between the general oxidational wear theory for dry contacts and the wear which occurs at lubricated contacts.     

The role of atmospheres and lubricants in the oxidational wear of metals

The formation of oxides during wear is reported to reduce wear and friction of metals by preventing severe metal-to-metal contact. However, the oxides formed at the contact areas are not always effective in reducing the wear of metals. There are many factors that affect the formation of oxides at the contact areas. The effect of atmospheres is a key factor since it controls the oxidation kinetics and oxide morphology. The oxides are also formed under lubricated conditions. The effect of dissolved oxygen in the lubricants plays a very important role in the wear of metals. In this paper, the role of atmospheres and lubricants in the oxidational wear of metals is reviewed.     

Review of oxidational wear Part II: Recent developments and future trends in oxidational wear research

This is the second part of a two-part in-depth review of the oxidational wear of metals. In the first part, the origins of the oxidational theory of wear were described, with special emphasis on the role of oxide films in the wear of metals at relatively low ambient temperatures. Under these conditions, most of the oxidation occurs at the temperature of the real areas of contact between sliding surfaces. In this part of the review, recent developments involving determination of the activation energies and Arrhenius constants for oxidation during wear are described together with an account of some of the research into the use of measurements of oxide film thickness to deduce the contact temperatures at the real areas of contact. Although the effect of partial oxygen pressures upon the wear of metals is covered in this review, it is in the effects of elevated temperatures upon wear that the future of oxidational wear research would appear to lie. Under these conditions, the effect of oxidation upon a given contact area whilst ‘out-of-contact’ must be taken into account. An oxidational wear theory relevant to elevated ambient temperatures is described and the most likely trends in oxidational wear research in the 1980's are discussed.     

Computational methods applied to oxidational wear

Theories of oxidational wear tend to involve the number of contacts (N) between sliding tribo-elements as well as the contact temperature (T_F), i.e. the temperature at the real areas of contact between these tribo-elements. This paper describes how these important parameters can be found for a given tribo-system, namely high-chromium ferritic steel pins sliding against rotating austenitic stainless steel disks, by the application of a numerical method to the analysis of the theoretical and experimental divisions of heat (DTH and DE) between tribo-elements undergoing oxidational wear. The paper then shows how one can use these values of N and T_F as inputs to a similar numerical method in order to deduce credible and consistent values for the tribological oxidation constants required for the theoretical oxidational wear rates (W_RTH) to be equal to the experimentally measured oxidational wear rates (W_REXPT) of the given tribo-system.     

Developments in the oxidational theory of mild wear

Previous work in which the oxidational theory of the mild wear of metals has been applied to low alloy steels has revealed discrepancies between theoretical predictions and experimentally derived data. These were due to the incorrect assumption that oxidational constants measured under static conditions could be applied without change to the very different conditions which exist at the real areas of contact between two sliding surfaces. It is shown that although the activation energies (Q_p) for static and sliding conditions are likely to be the same, the accompanying Arrhenius constants (A_p) will be very different, leading to very different oxide growth rates.The authors have used previously reported friction, wear and heat flow results (for pin-on-disc wear experiments with a low alloy medium carbon steel) to obtain the best possible values of A_p (using published values of Q_p for static oxidation) consistent with an explanation of these results in terms of the oxidational theory of mild wear. These values of A_p and Q_p were then used to explain the wear behavior of a low alloy carbon steel when used in similar experiments. An essential feature of the successful correlation between theory and experiment was the analysis of the structure of the wear debris by X-ray diffraction, which enabled the appropriate values of Q_p and A_p to be assigned according to the interfacial temperatures indicated by these structures     

Origins and development of oxidational wear at low ambient temperatures

The origins and the development of the oxidational theory of mild wear under conditions where the ambient temperatures are sufficiently low that no significant oxidation can occur outside the instantaneous real areas of contact between two sliding surfaces are reviewed in this paper. Emphasis is placed on the importance of heat flow analysis for calculating surface temperatures and the division of heat at the sliding interface, especially in so far as it is used for checking the surface model used for explaining the wear rates obtained in some pin-on-disc experiments with low alloy steels. It is shown that it is possible to deduce values for the oxidation constants during wear that are different from those obtained from static oxidation tests and which are relevant to a range of low alloy steels. The implications of some of the computed values of the number N of asperities beneath the pin at any given time, the temperature T_c within the real areas of contact and the critical oxide film thickness ξ are discussed.     

The response surface method and the analysis of mild oxidational wear

Instead of using the conventional oxidation theory to depict a disk’s wear rate as a function of contact temperature, the response surface method (RSM) is herein introduced to relieve the one-factor-at-a-time defect in portraying tribological characteristics. By means of a central composite design (CCD) technique, fewer operating conditions are needed to establish expressions for the wear rate parameter, the contact temperature and the friction coefficient as a function of sliding speed and applied load. A second degree polynomial was used to represent a curved surface which fits the experimental data. In addition to results for the designated operating conditions, wear rate parameters and contact temperatures obtained from the polynomials were compared with the experimental results. The activation energy in the wear rate expression can thus be derived as a function of sliding speed, applied load and contact temperature. The experimental data for the wear rate parameter can be expressed by smooth curves, instead of two different straight lines in two temperature subdivisions.     

Abrasive wear of metals

Abrasive wear resistance, abrasive wear mode and abrasive wear rate are discussed with experimental results, and abrasive wear theories are introduced and explained from the viewpoints of effective work for plastic deformation and fracture.     

High-temperature sliding wear of metals

Temperature can have a considerable effect on the extent of wear damage to metallic components. During reciprocating sliding, under conditions where frictional heating has little impact on surface temperatures, there is generally a transition from severe wear to mild wear after a time of sliding that decreases with increase in ambient temperature. This is due to the generation and retention of oxide and partially-oxidized metal debris particles on the contacting load-bearing surfaces; these are compacted and agglomerated by the sliding action, giving protective layers on such surfaces. At low temperatures, from 20 to 200°C, the layers generally consist of loosely-compacted particles; at higher temperatures, there is an increase in the rates of generation and retention of particles while compaction, sintering and oxidation of the particles in the layers are facilitated, leading to development of hard, very protective oxide ‘glaze’ surfaces. This paper reviews some of the main findings of extensive research programmes into the development of such wear-protective layers, including a model that accounts closely for the observed effects of temperature on wear rates during like-on-like sliding.     

The abrasive wear of metals

A quite general relationship exists between abrasive wear and the ploughing contribution to friction. Tensile stresses are developed in the material displaced by the ploughing indenter and these give rise to wear particles.     

An explanation of the wear of metals

Mathematical analysis establishes that the well-known empirical linear wear law for the adhesive wear of metals is the consequence of the statistics of surface roughness and is almost independent of the assumed contact model. A strain ratio fatigue failure criterion (i.e. the Manson-Coffin low cycle fatigue law) coupled with a probabilistic treatment of surface asperity height distribution and surface contact provides, for the first time, a fundamental explanation for the formation of wear particles.     

Contact mechanics and the wear of metals

It is commonly observed that metallic wear debris takes the form of thin platelets, leading to the term ‘delamination wear’. Modelling this phenomenon has proved a stiff challenge in Contact Mechanics since the fractures which give rise to wear particles lie parallel, or nearly so, to the surface; i.e. on planes of maximum compressive stress. Sectioning the surface layer beneath a wear track has revealed it to have acquired severe plastic strains, which suggests that the cracks are ductile fractures, driven by plastic strain rather than elastic stress intensity. The paper reviews recent research into the progressive plastic deformation of surfaces in repeated sliding: the process known as ‘ratchetting’. Included is an analysis of ‘running-in’ of rough surfaces by repeated sliding and a discussion of the criterion of rupture under cyclic plastic strain.     

Strain rate response and wear of metals

Models and mechanisms proposed for wear of metals are reviewed. It has been observed earlier that in severe wear if the microstructure, which evolves in the subsurface in response to the traction, is deleterious and unstable, cracks may nucleate at the site of these instabilities leading to the generation of debris. It is postulated here that, given the traction, metals respond to the local strain rate and temperature by evolving a microstructure in a unique fashion. If the response is deleterious a crack is nucleated. Further, if there is sufficient energy available locally the crack is propagated to generate debris particle. The strain rate responses of titanium and copper are recorded in compression and used to interpret the wear characteristics of these metals in the sliding speed range of 0.1 to 4 ms⁻¹.     

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.     

金刚石切削黑色金属时刀具磨损机理的摩擦磨损试验

为了降低黑色金属金刚石切削过程中的刀具磨损,提高表面加工质量和精度,对刀具磨损机理进行了研究.通过黑色金属金刚石摩擦磨损试验,模拟了实际切削过程中的刀具磨损行为;分别采用扫描电镜(SEM) 、X射线能谱仪(EDS)以及拉曼光谱仪(RS)对工件表面形貌、实验前后工件表面化学组分变化以及金刚石磨损表面的晶体结构转变进行了检测,同时提出了用石墨化程度作为试验过程中评价金刚石磨损的指标.试验结果表明:金刚石的磨损主要与机械力和温度有关,摩擦速度和工件材料中的含碳量对其影响相对较小;石墨化磨损、扩散磨损和氧化磨损等磨损机理共存,其中石墨化为导致金刚石磨损的主要原因.结合红外热像仪测温和热传导理论推算,近似获得了摩擦界面的真实温度,且随着温度升高15％,金刚石石墨化程度显著加剧83％.作者提出,应当综合考虑热-力耦合作用下的刀具磨损机理,以便进一步探寻抑制刀具磨损的工艺措施.     

An experimental study of wear of ferrous metals

An experimental study of tracks generated by point contact sliding surfaces under different loads has been carried out on a Bowden-Laben machine. The damage to mild steel, cast iron and carburized steel under repeated rubbing was studied by microhardness testing and microscope examination.The microhardness value at various depths below the track can be an indication of severe wear by sliding action. A critical value for a combination of materials was determined. The mechanism of wear and its gradual change at the contact surface with increased load and cycles of reciprocating sliding motion are discussed.     

The magnetic effect on the wear of metals

The effect of a magnetic field on the wear of ferromagnetic metals (nickel and iron) has been investigated. Wear tests were carried out under an external magnetic field applied horizontally or vertically to the rubbing surface in atmospheres of air, nitrogen and argon. In air a magnetic field horizontal to the surface enhances specific wear. However, a magnetic field vertical to the surface accelerates the severe-to-mild-wear transition, so that it gives a remarkable reduction in specific wear. In nitrogen or argon a vertical magnetic field does not have this effect. The reduction in specific wear is thought to be due to the chemisorption of oxygen onto the ferromagnetic metals activated by the application of a vertical magnetic field.