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.     

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.     

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     

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.     

Gradation of oxidational wear of metals

This paper considers mild-oxidational wear of metals by studying their behavior under friction with different loads. Low carbon, steel and copper are chosen as the model materials. We show that tribo-oxidation and the structure of surface layers of materials, both formed in the process of plastic deformation during friction, provide the boundary conditions of mild and severe wear. Oxidational wear is predominant when structural changes are minimal. As the load increases, oxidational wear is at first accompanied by metallic wear and afterwards the oxidational wear accompanies the metallic wear. The structure of the metal surface layers changes gradually during these processes, so that the strengthening of the metal is high enough to withstand friction forces. When the magnitude of frictional forces becomes higher than the maximal strength of the plastically deformed metal, the transition to severe wear occurs.The composition of different types of oxides and the fineness of wear particles varies with the friction conditions. Under light load friction conditions, fine wear particles are formed. These particles contain oxides of high oxygen content. As the friction conditions become tougher, in particular when the load increases, large-sized wear particles are formed. These particles contain oxides of a higher metal content. Phase composition and fineness of wear particles are used for gradation of mild wear.Analyses of phase composition of oxides and estimation of the fineness of wear particles are suggested as a method of wear character diagnostics. The electron diffraction method of the study of wear particles is used for this analysis in order to evaluate and choose appropriate friction and wear conditions.     

Transition of the lubricated wear of carbon steel

To study the transition of the lubricated wear of 0.53% C steel in the steady state, wear tests were carried out by rubbing the annular surfaces of two cylindrical test pieces in machine oil with no additives. The ratio of mating areas was varied to approach actual contact conditions. Three regions of variation in the coefficient of friction with contact load were determined. Fatigue wear, characterized by a friction coefficient μ ≈ 0.05 and a specific wear rate ω_s ≈ 0.005 × 10^?6 mm³ N^?1 m^?1, occurs in the first region.A transition from fatigue wear to adhesive wear, with μ ≈ 0.05?0.12 and ω_s ≈ (0.005?0.05) × 10^?6 mm³ N^?1 m^?1, takes place gradually within a specific load range. Finally, adhesive wear predominates above the load level that marks the end of the transition. The same behaviour was analysed through stepwise loading tests. The onset of transition and seizure occurs at constant mean surface temperatures. However, the end of transition is also affected by factors other than temperature. The results are compared with the transitions reported by the International Research Group on Wear of Engineering Materials of the Organization for Economic Cooperation and Development.     

The effect of surface topography on the wear of steel

The pattern of roughness and wear has been obtained by varying the speed, the number of passes and the pre-roughness of the samples in both dry and lubricated sliding conditions. Three-dimensional graphs of wear versus speed versus sliding time and roughness of the worn surfaces were produced and a linear relation between the variation of the roughness and the amount of wear at a high number of passes has been determined. The initial dry wear rate is influenced by the prior surface roughness. In the case of lubricated conditions the influence of prior roughness is not pronounced. In dry sliding the smooth surfaces in the running-in stage roughen and the rough surfaces become smoother, while in lubricated conditions both surfaces become smoother in the first few passes. Sliding perpendicular to the lay produces rougher surfaces and more wear than is the case in the parallel direction.     

Influence of surface preparation on roughness parameters,friction and wear

The aim of the present research was to investigate influence of surface preparation on roughness parameters and correlation between roughness parameters and friction and wear. First the correlation between different surface preparation techniques and roughness parameters was investigated. For this purpose 100Cr6 steel plate samples were prepared in terms of different average surface roughness, using different grades of grinding, polishing, turning and milling. Different surface preparation techniques resulted in different R_a values from 0.02 to 7 μm. After this, correlation between surface roughness parameters and friction and wear was investigated. For this reason dry and lubricated pin-on-disc tests, using different contact conditions, were carried out, where Al₂O₃ ball was used as counter-body. It was observed that parameters R_ku, R_sk, R_pk and R_vk tend to have influence on coefficient of friction.     

Wear of Silicon Carbide in Sliding Contact with Lubricated Thin-Film Rigid Disk

Silicon carbide (SiC) is a potential ceramic material for recording heads, yet its tribological performance against lubricated thin-film rigid disks is not fully known. Square pins with a 100 mm radius spherical surface were made from hot pressed SiC, chemical vapor deposited (CVD) SiC, and Al₂O₃TiC, and tested with lubricated thin-film disks. The pin-on-disk tests showed that the region of contact on the spherical surface of the SiC and CVD-SiC pins wears away to form a circular wear plateau with smears in and around the plateau. The wear plateau is formed rapidly in the first 1000 drag revolutions and then very gradually grows in size with further revolutions. Analysis of the smears showed that a large fraction of the smears contained SiO₂ which had been oxidized from SiC due to high temperatures generated at the pin surface in contact with the disk. In contrast, tests with Al₂O₃. TiC pins did not show any formation of a wear plateau on the pins.     

Abrasive wear of some commercial polymers

Cylindrical test pins of some commercial polymer-based bearing materials (comprising two nylons 6, a filled nylon 6/6, a filled ultra-high molecular weight polyethene (uhmwpe) and three polyurethanes) were rotated, in dry conditions and at constant load and sliding speed, on circular tracks on stationary discs of steel gauze and abrasive paper.Wear against run-in steel gauze was proportional to the sliding time (distance), with the specific wear rate, v_sp, (wear volume per unit area per unit sliding distance) varying with the nominal pressure, p, according to v_sp = Kp^α. Values of K and α are presented enabling comparison of the fatigue wear of the materials at various loads against steel (or a counterface with rounded asperities) in non-transfer film conditions. Nylon 6 showed the least wear and the polyurethanes showed the greatest wear, up to pressures of 3.43 MN m^?2 (500 lbf in^?2).With abrasive paper, the circular path became progressively clogged with transfer films and wear debris, and the wear volume, ΔW, diminished with time, t, throughout the test duration, following the relationship ΔW = Dt^c, where both c and D are functions of the wear path diameter. c appears to be related to the film transfer capability of the polymer. The best overall abrasive wear resistance (in transfer film conditions) was exhibited by the filled uhmwpe, followed by two polyurethanes. Nylon 6 showed relatively poor abrasion resistance under these conditions. The mechanical properties indicate, with one exception, a similar ranking order for non-transfer film conditions     

Experimental extrapolation model for friction and wear of polymers on different testing scales

The friction and wear behaviour for polyoxymethylene homopolymers (POM-H) and polyethylene terephthalate with teflon additives (PET/PTFE) is compared on small-scale cylinder-on-plate tests (50-200 N normal loads) and large-scale flat-on-flat tests (190-3880 kN normal loads). A common parameter to characterise tribological data is the contact pressure×sliding velocity (pv-value), but its use seems restricted to a single testing scale. Four experimental models are presented to extrapolate tribological data from one to another testing scale, based on (i) one single mechanical parameter (normal load or contact pressure), (ii) two mechanical parameters (normal load and sliding velocity), (iii) the contact pressure-sliding velocity model (pv-temperature limit), (iv) macroscopic geometry model. The latter model is most extensive, considering the influences of thermal effects (frictional heat generation and dissipation), sample geometry (geometry factor G) and visco-elastic contact (critical contact pressure p₀). For unfilled polymers, the introduction of macroscopic scaling factors allows for the extrapolation of coefficients of friction obtained on different testing scales. Specific or volumetric wear rates cannot be extrapolated because they strongly depend on the sample geometry, while linear wear rates are in better agreement when considering the transitions between mild wear, softening and melting. For internally lubricated polymers, extrapolation is more difficult. The differences depending on the testing scale are attributed to contact stress concentrations near the sample borders and limited wear debris mobility within large contact areas, promoting a homogeneous film formation onto the polymer surface.     

Mild wear of a low alloy steel at temperatures up to 500°C

Wear behaviour of 52100 low alloy steel has been studied on a pin on disc wear machine at disc temperatures ranging from room temperature to 500°C. Transitions occur in the wear rate versus load curves at certain critical loads, the magnitude of which increase with temperature. These transitions were found to be associated with change in surface oxide, lower wear rates being recorded when the predominant oxide was the spinel Fe₃O₄ for all temperatures. At disc temperatures above 300°C out of contact oxidation appears to be the most important wear limiting factor. A surface model was developed enabling contact temperature, numbers and size of contacts and critical oxide film thickness to be deduced. Remarkable agreement was found between oxide thicknesses estimated from this model and measured values using a scanning electron microscope     

Studies of high temperature sliding wear of metallic dissimilar interfaces II: Incoloy MA956 versus Stellite 6

The development of wear surfaces formed during limited debris retention sliding wear of Incoloy MA956 against Stellite 6 between room temperature and 750 °C, and sliding speeds of 0.314 and 0.905 m s⁻¹ (7 N applied load, 4522 m sliding distance) were investigated. At 0.314 m s⁻¹, mild oxidational wear was observed at all temperatures, due to oxidation of Stellite 6-sourced debris and transfer to the Incoloy MA956; this debris separated the Incoloy MA956 and Stellite 6 wear surfaces. Between room temperature and 450 °C, the debris mainly took the form of loose particles with limited compaction, whilst between 510 °C and 750 °C the debris were compacted and sintered together to form a Co–Cr-based, wear protective ‘glaze’ layer. The behaviour was identical to that previously observed on sliding Nimonic 80A versus Stellite 6 at 0.314 m s⁻¹.At 0.905 m s⁻¹, mild oxidational wear was only observed at room temperature and 270 °C and dominated by Incoloy MA956-sourced debris. At 390 and 450 °C, the absence of oxide debris allowed ‘metal-to-metal’ contact and resulted in intermediate temperature severe wear; losses in the form of ejected metallic debris were almost entirely Incoloy MA956-sourced. This severe wear regime was also observed from 510 up to 630 °C, but increasingly restricted to the early stages of wear by development of a wear protective Incoloy MA956-sourced ‘glaze’ layer. This ‘glaze’ layer formed so rapidly at 690 °C and 750 °C, that severe wear was all but eliminated and wear levels were kept low.The behaviour observed for Incoloy MA956 versus Stellite 6 at 0.905 m s⁻¹ contrasts sharply with that previously observed for Nimonic 80A versus Stellite 6, in that the Incoloy MA956-sourced high Fe–Cr debris formed a protective oxide ‘glaze’, whilst the Nimonic 80A-sourced Ni and Cr oxides formed an abrasive oxide that at high sliding speeds assisted wear. The data indicates that the tendency of oxide to form a ‘glaze’ is readily influenced by the chemistry of the oxides generated.     

The effect of temperature on the unlubricated sliding wear of 5 CrNiMo steel against 40 MnB steel in the range 400–600°C

5 CrNiMo steel is used traditionally as hot forging die material in China. High temperature wear is a common failure mode of the steel. This paper deals with the sliding wear behavior of the steel in the temperature range 400°C to 600°C. The composition and features of the worn surface were analyzed using SEM, EDS and XRD. The oxidation of 5 CrNiMo steel under sliding wear condition at elevated temperature indicated that the oxide transfer layer formed on the sliding surface consisted of Fe₃O₄ and Fe₂O₃. The wear mechanism changed with the test temperature and the oxide transfer layer played an important part in the change in wear mechanism. At lower temperatures, wear was due to abrasive wear. From 500°C to 550°C, the oxide transfer layer presented a relatively compact morphology and the oxidational wear was the principal wear mechanism resulting in low wear rate at 500°C. When the test temperature was at 600°C, adhesive wear was predominant, and the wear rate increased greatly.     

Dry sliding wear of epoxy/cenosphere syntactic foams

Dry sliding wear behavior of epoxy matrix syntactic foams filled with 20, 40 and 60 wt% fly ash cenosphere is reported based on response surface methodology. Empirical models are constructed and validated based on analysis of variance. Results show that syntactic foams have higher wear resistance than the matrix resin. Among the parameters studied, the applied normal load (F) had a prominent effect on wear rate, specific wear rate (w_s) and coefficient of friction (μ). With increasing F, the wear rate increased, whereas w_s and μ decreased. With increase in filler content, the wear rate and w_s decreased, while the μ increased. With increase in sliding velocity as well as sliding distance, the wear rate and w_s show decreasing trends. Microscopy revealed broken cenospheres forming debris and extensive deformation marks on the wear surface.     

Tribological Behavior of Composites Based on ZA-27 Alloy Reinforced with Graphite Particles

The objective of this investigation is to assess the influence of graphite reinforcement on tribological behavior of ZA-27 alloy. The composite with 2 wt% of graphite particles was produced by the compocasting procedure. Tribological properties of unreinforced alloy and composite were studied, using block-on-disk tribometer, under dry and lubricated sliding conditions at different specific loads and sliding speeds. The worn surfaces of the samples were examined by the scanning electron microscopy (SEM). The obtained results revealed that ZA-27/graphite composite specimens exhibited significantly lower wear rate and coefficient of friction than the matrix alloy specimens in all the combinations of applied loads (F _n ) and sliding speeds (v) in dry and lubricated tests. The positive tribological effects of graphite reinforcement of ZA-27 in dry sliding tests were provided by the tribo-induced graphite film on the contact surface of composite. In test conditions, characterized by the small graphite content and modest sliding speeds and applied loads, nonuniform tribo-induced graphite films were formed leading to the increase of the friction coefficient and wear rate, with increase of the sliding speed and applied load. In conditions of lubricated sliding, the very fine graphite particles formed in the contact interface mix with the lubricating oil forming the emulsion with improved tribological characteristics. Smeared graphite decreased the negative influence of F _n on tribological response of composites, what is manifested by the mild regime of the boundary lubrication, as well as by realization of the mixed lubrication at lower values of the v/F _n ratio, with respect to the matrix alloy.     

Sliding wear behavior of Fe-Al and Fe-Al/WC coatings prepared by high velocity arc spraying

A comparative study was carried out to investigate the microstructure and tribological behavior of Fe-Al and Fe-Al/WC iron aluminide based coatings against Si₃N₄ under dry sliding at room temperature using a pin-on-disc tribotester. The coatings were prepared by high velocity arc spraying (HVAS) and cored wires. The effect of normal load on friction coefficient and wear rate of the coatings was studied. The microstructure and the worn surfaces of the coatings were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion spectroscope (EDS). The results showed that, the main phases in both coatings were iron aluminide (Fe₃Al and FeAl) and α. WC/W₂C particles were embedded in the matrix of the composite coating. With adding WC hard particles, the Fe-Al/WC composite coating exhibited higher wear-resistance than Fe-Al coating. But the friction coefficient of both coatings showed little difference. As the load increased, the friction coefficient decreases slightly due to a rise of friction contact temperature and larger areas of oxide film formation on the worn surface, which act as a solid lubricant. Increasing load causes the maximum shear stress occurring at the deeper position below the surface, thereby aggravating the wear. The coating surface is subjected to alternately tensile stress and compression stress during sliding, and the predominant wear mechanism of the coatings appears to be delamination.     

The effect of abrasive grain size on the transition between abrasive and adhesive wear

Experiments were carried out in which SiC abrasives with a grain size range of 3–150 μm were inserted between sliding metals. The metals were pure aluminium, copper, iron, nickel and zinc. The test geometry was a tube end against a flat surface. The effect of grain size can be classified into three regimes. In the first, where abrasive grains are larger than a critical size d_c (about 50 μm), the wear rate is independent of grain size. In the second regime the wear rate decreases as abrasive grains become smaller than d_c to a limit at a transition grain size d_t (about 10 μm). In the third regime the wear rate is high and independent of abrasive grain size. The wear debris consists of large metal flakes with abrasive particles mixed in. Although abrasive particles are present, the wear is primarily adhesive, and the action of the abrasive particles is to promote the removal of metallic wear debris from the contact region.