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.     

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.     

Wear transition diagram for silicon carbide

To obtain information on the tribological behaviour of silicon carbide at elevated temperatures, unlubricated ball-on-flat wear tests were conducted on sintered silicon carbide in self-mated sliding in air. The contact load was varied from 3.2 to 98.0 N, and a temperature range of 23°C to 1000°C was used. Scanning electron microscopy, Fourier transform infrared spectroscopy and energy-dispersive spectroscopy were used to elucidate the wear mechanisms. The results of the tests and observations were employed to construct a wear transition diagram, which provides a summary of tribological information including friction coefficient, wear coefficient and wear mechanisms as a function of temperature and load. The wear transition diagram for the sintered silicon carbide studied is divided into four regions plus one transition zone. At room temperature, under high loads and high environmental humidity, the tribological behaviour is controlled by tribochemical reactions between the silicon carbide surface and water vapour in the environment. Under low loads and at temperatures below 250°C, wear occurs by ploughing and polishing. At temperatures about 250°C and under low loads, tribooxidation and formation of cylindrical wear particles control the tribological behaviour. Wear occurs by microfracture when the load is increased above a critical value; and both the friction coefficient and the wear coefficient increase.     

Studies of high temperature sliding wear of metallic dissimilar interfaces

The evolution of microstructures in the glaze layer formed during limited debris retention sliding wear of Nimonic 80A against Stellite 6 at 750 °C and a sliding speed of 0.314 m s⁻¹ (7 N applied load, 4522 m sliding distance) was investigated using scanning electron microscopy (SEM), energy dispersive analysis by X-ray (EDX), X-ray diffraction (XRD), scanning tunnelling microscopy (STM) and transmission electron microscopy (TEM). The collected data indicate the development of a wear resistant nano-structured glaze layer. The process of ‘fragmentation’ involving deformation, generation of dislocations, formation of sub-grains and their increasing refinement causing increasing misorientation was responsible for the formation of nano-structured grains. The rapid formation of this glaze layer from primarily cobalt–chromium debris transferred from (and also back to) the surface of the Stellite 6, kept wear of both the Nimonic 80A and Stellite 6 to very low levels.However, increasing the sliding speed to 0.905 m s⁻¹ (750 °C) suppressed glaze formation with only a patchy, unstable glaze forming on the Stellite 6 counterface and an absence of glaze development on the Nimonic 80A sample (the Nimonic 80A surface was covered with at most, a very thinly smeared layer of oxide). The high levels of oxide debris generated at 0.905 m s⁻¹ instead acted as a loose abrasive assisting wear of especially the Nimonic 80A. This behaviour was attributed to a change in oxide chemistry (due to the dominance of nickel and chromium oxides generated from the Nimonic 80A) resulting in poor oxide sintering characteristics, in combination with increased mobility and reduced residency of the oxide debris at 0.905 m s⁻¹.     

Fretting at high temperatures

The fretting damage to an austenitic stainless steel, type 321, in CO₂ is much reduced at temperatures above 400°C by the formation of a glaze type oxide. Increasing the normal pressure from 2 to 6.9 MN m⁻² at 650°C greatly increased the extent and quality of the glaze. The nickel-based alloy, Inconel 718, developed glaze oxide when fretted at 540°C in air, as indicated by a low coefficient of friction and wear rate. At 280°C, the glaze was only found at greater amplitudes of slip. Although the titanium alloy Ti-6Al-4V in air at 200 to 400°C developed a surface oxide which had some of the superficial features of a glaze, it nevertheless did not reduce the coefficient of friction to values characteristic of glaze. The common feature of high-temperature alloys which develop protective glaze oxides is that they are capable under conditions of sliding and fretting of forming a spinel type oxide which, however, must be adequately supported by a creep-resistant substrate at the operating temperature     

Wear transitions and stability of polyoxymethylene homopolymer in highly loaded applications compared to small-scale testing

Wear and failure mechanisms of polyoxymethylene homopolymer (POM-H) loaded above its yield strength are studied on test samples with a 22500 mm² sliding area at 8–150 MPa contact pressures. Test results are compared to small-scale cylinder-on-plate tests. Plastification of the sliding surfaces at high loads is favourable for low friction, while different wear mechanisms compared to small-scale testing are induced. Small-scale tests show a transition from mild adhesive/abrasive wear to severe wear due to softening, which is characterised by the formation of shear lips. Softening of large-scale sliding surfaces does not cause overload but it contributes to stable wear rates. Overload of large-scale samples is characterised by the transition from softening to melting and degradation. The dimensional stability of polymer elements is influenced by creep and it is verified that deformation of small-scale samples and large-scale samples loaded at 8 MPa is recovered after sliding, while it remains as permanent deformation for large-scale tests at 16–150 MPa. The wear transitions are further analysed by optical microscopy and available temperature models. The flash temperature concept can be applied for small-scale tests and large-scale tests up to 8 MPa. Calculated flash temperatures indicate softening and are in agreement with visual observations of the polymer surfaces. Flash temperatures for large-scale tests at 16–150 MPa indicate melting and degradation that was not visually observed on the polymer surfaces. The bulk temperature model prevails during large-scale sliding and only indicates melting at 150 MPa. Thermal analysis of the worn polymer surfaces confirms that crystallisation happened during small-scale sliding and large-scale sliding up to 55 MPa, occurring between 120 and 150 °C. Thermo-oxidative degradation above 200 °C is evidenced at 150 MPa.     

Characterization of wear scar surfaces using combined three-dimensional topographic analysis and contact resistance measurements

In this paper, a technique for the quantitative characterization of wear scar surfaces, using combined three-dimensional topographical analysis and contact resistance measurements, is introduced. Parameters for the characterization of wear surfaces, developed during sliding of pin-on-disk specimens in oxygen at high temperature, such as wear volume, roughness, average wear depth on the disk specimen, surface coverage by wear-protective oxide layers and their distributions over the wear surface, are presented and calculated. Such analyses provide more effective data for the analysis of wear processes and wear mechanisms.This method has been applied to the analysis of dry reciprocating sliding wear of a nickel-base alloy, N80A, at temperatures to 600°C. It was found that there was usually a difference between the wear rates of the pin and the disk. This difference increased with increase in temperature, the wear of the pin being much less than that of the disk at the higher temperatures. Although the total wear of both the pin and the disk decreased considerably with increase in temperature, the damage to the disk, judged by the wear depth of the scar, was much higher at elevated temperatures than at low temperatures. The roughnesses of the wear surfaces generally increased with increase in temperature. Less than 50% coverage of the scar surfaces by wear-protective oxide layers was sufficient for the severe-to-mild wear transition. However, the distribution of the wear-protective layers over the wear surfaces was non-uniform. Most of them were concentrated near the centre of the scar, along the sliding direction, under the present conditions. These features of the wear scar surfaces were mainly related to the adhesion and compaction of wear debris particles onto the wear surfaces, leading to development of the wear-protective layers at the various temperatures.     

Contact and thermal analysis of transfer film covered real composite-steel surfaces in sliding contact

For composite-steel surfaces in sliding contact an anisotropic numerical contact algorithm has been developed to study the ‘layer type’ problems. An FE contact analysis was applied to evaluate the contact parameters (real contact area, contact pressure distribution and normal approach). The contact temperature rise was determined by using both a numerical thermal algorithm for stationary and a FE transient thermal technique for ‘fast sliding’ problems.The effect of a continuous transfer film layer (TFL), that had built up during wear of the PEEK matrix material on the steel counterpart, was considered. Its thickness was assumed to be t=1 μm, and its material properties were that of PEEK at room temperature or, in the case of frictional heating, at a temperature of 150°C (i.e. above the glass transition temperature of the polymer matrix).Results are presented for a spherical steel asperity, with/without TFL, sliding over composite surfaces of different fibre orientation, and in addition, for real composite-steel surfaces (based on measured surface roughness data) in sliding contact. The TFL has an effect on the contact parameters especially at higher operating temperatures (i.e. 150°C); it results in the production of a larger contact area and a lower contact pressure distribution. The contact temperature rise is clearly higher if a TFL is present. Due to the low thermal conductivity of PEEK, the TFL is close to the melting state or it even gets molten within a small vicinity of the contact area.     

Influence of grain refinement on the high temperature fretting behaviour of IN 738 LC

The wear and friction behaviour of Inconel 738 LC in contact with SiSiC was studied for the case of oscillating sliding motion. The test temperature was varied in the range from room temperature up to 700 °C. A large-grain IN 738 LC and a grain-refined modification were compared. In both cases, the wear dropped drastically at temperatures above 400–500 °C and remained low for the grain-refined modification, but increased again for the large-grain modification at temperatures above 600 °C. The high wear/low wear transition was accompanied by a transfer of metal on the ceramic surface.     

Tribological characteristics of low-pressure plasma-sprayed A12O3 coating from room temperature to 800 °C

The tribological characteristics of low-pressure plasma-sprayed (LPPS) Al₂O₃ coating sliding against alumina ball have been investigated from room temperature to 800 °C. These friction and wear data have been compared quantitatively with those of bulk sintered alumina to obtain a better understanding of wear mechanisms at elevated temperatures. The friction and wear of Al₂O₃ coating show a strong dependence on temperature, changing from a mild to a severe wear regime with the increase of temperature. The coefficient of friction at room temperature is approximately 0.17 to 0.42, depending on applied load. The tribochemical reaction between the coating surface and water vapor in the environment and the presence of the hydroxide film on the Al₂O₃ coating reduce the friction and wear at room temperature as contrasted to those of bulk sintered alumina. At intermediate temperatures, from 400 to 600 °C, the friction and wear behavior of Al₂O₃ coating depends on the inter-granular fracture and pull-out of Al₂O₃ grains. At above 700 °C, formation and deformation of fine grain layer, and abrasive wear in the form of removal of fine alumina grains further facilitate the friction and wear process of Al₂O₃ coating.     

Several plasma diffusion processes for improving wear properties of Ti6Al4V alloy

Different kinds of diffusion processes, plasma nitriding, oxidizing and oxynitriding as of a combination of other two, have been applied to Ti6Al4V alloy to evaluate the effect of treatment times (1 and 4 h) and temperatures (650 and 750 °C) on wear properties of the alloy. It was observed that a hard modified layer was produced on the surface of the alloy after each diffusion process. While TiN and Ti₂N phases form in the modified layer with plasma nitriding, mainly TiO₂ phase forms after plasma oxidizing treatment. The wear tests performed at different normal loads showed that all treated samples, except for nitrided and oxidized at 650 °C for 1 h, exhibited higher wear resistance than untreated Ti6Al4V alloy. The plasma nitrided samples showed adhesive wear. On the other hand, while the plasma oxidizing samples displayed adhesive wear at lower loads, wear mechanism changed to abrasive wear as the load increased because the oxide film which covers the surface was broken during the sliding at higher loads.     

The tribological behaviour of nickel and nickel-chromium alloys at temperatures from 20° to 800 °C

Measurements are presented of friction and wear during sliding of specimens of Ni-Cr alloys containing 0% to 40% Cr on like specimens in air at 20°, 400° and 800 °C. The worn specimens have been examined by optical and scanning electron microscopy, electron probe microanalysis and electron diffraction and microhardness measurements have been made.Under the sliding conditions used, all the alloys show a transition temperature above which a low coefficient of friction and usually relatively low wear are observed after a time and below which these parameters remain relatively high throughout. Above the transition temperatures, the frictiontime loci show sharp reproducible changes from relatively high to low coefficients of friction. Such changes can be associated with the formation of a thermally softened oxide layer (termed a glaze) on the bearing areas during sliding. Once the glaze is formed, very little further wear occurs for the high chromium-content alloys, although further damage does take place with the weaker low chromium-content alloys, especially at temperatures just above the transition temperature. These tribological properties of the glaze are associated with its low shear strength and the strength of the underlying alloy substrate.During sliding at temperatures below the transition temperatures, metal-to-metal contact takes place, although oxide is formed on the bearing area of the low chromium-content alloys even at 20 °C. The friction and wear behaviour is largely determined by the strength and work-hardenability of the alloy.Correlations between the tribological behaviour of these binary Ni-Cr alloys and commercial Nimonic alloys indicate that the trace elements in the latter play only a relatively minor role in determining this behaviour. It is concluded that high strengths and relatively rapid transient oxidation rates of the alloys, and appropriate physical properties of the resulting oxide films, are important qualities of the alloys under the conditions used.     

Studies on high temperature wear and its mechanism of Al-Si/graphite composite under dry sliding conditions

Wear behaviours of aluminum silicon alloy and Al-Si/graphite composite were investigated at ambient and elevated temperatures. The trend showed a decrease in wear rate with increase in temperature. The reduction in wear rate was mainly attributed to the formation of glazing layer and oxide layer at higher temperature. This was invariably observed in alloy and composites. In addition, the presence of graphite in composite offered better wear resistance for all temperatures under consideration. The wear due to oxidation was predominant during high temperature sliding.     

The transition from severe to mild sliding wear for Fe-12%Cr-base alloys at low temperatures

A study of the friction and wear behaviour of two commercial Fe-12%Cr-base alloys Jethete M152 and Rex 535 during like-on-like reciprocating sliding in air at ambient temperatures up to 200 °C has been carried out. As expected from practical experience, the overall wear resistance of Rex 535 is superior to that of Jethete M152. In all cases, the wear processes are characterized by an initial period of primary severe wear with associated high, but irregular, coefficients of friction, followed by a transition to a steady state period of secondary mild wear with associated reduced and steady friction values. The time of this transition is load independent and decreases with increasing ambient temperature but occurs more rapidly for Rex 535 than for Jethete M152, which accounts entirely for the former's superior overall wear performance. The wear rate during sliding in the primary severe wear period is independent of alloy, of applied load and, possibly, of temperature while the secondary wear rate is independent of alloy but is dependent on temperature, although not in a regular manner. The transition from severe to mild wear can be correlated with the generation and comminution of metal wear debris particles during the severe wear period until the particles are small enough for substantial oxidation of the exposed surfaces to take place at the ambient temperature of sliding. The subsequent temperature dependence of the secondary mild wear rate is probably related to changes in the adhesive properties of this tribogenerated wear debris. The faster transition from severe to mild wear for Rex 535 compared with that for Jethete M152 is associated with easier comminution of the metal wear particles of Rex 535 owing to their lower ductility. Hence the significance of oxidation of the debris surfaces becomes important at an earlier stage in the sliding process.     

The sliding wear of 316 stainless steel in air in the temperature range 20–500°C

The friction and reciprocating wear of 316 stainless steel in air has been investigated in the temperature range 20–500°C at constant load using a standard pin and flat geometry. A marked change in wear behaviour occurred above 300°C. From room temperature to 300°C the wear rate decreased slowly with increasing temperature. This was accompanied by an increasing fraction of oxide in the wear debris. At 300°C the debris consisted entirely of oxide with the α Fe₂ O₃ structure. In this temperature range wear can be explained essentially in terms of mild wear. Above 300°C the wear rate decreased by an order of magnitude and was accompanied by a severely distorted wear surface. There was a high proportion of metallic particles in the wear debris. The surface roughening occurs at an early stage of wear and stops when glazed oxide regions form. The low wear rate is explained in terms of the high hardness of the glazed load-bearing areas and re-incorporation of wear debris into the wear scar.     

Prediction of Scuffing Failure Based on Competitive Kinetics of Oxide Formation and Removal: Application to Lubricated Sliding of AISI 52100 Steel on Steel

A method for predicting scuffing failure based on the competitive kinetics of oxide formation and removal has been developed and applied to the sliding of AISI 52100 steel on steel with poly-α-olefin as the lubricant. Oxide formation rates were determined using static oxidation tests on coupons of 52100 steel covered with poly-α-olefin at temperatures of 140°C to 250°C. Oxide removal rates were determined at different combinations of initial average nominal contact pressures (950 MP a to 1578 MP a) and sliding velocities (0.4 m/s to 1.8 m/s) using a ball-on-disk vacuum tribotester. The nominal asperity flash temperatures generated during the wear tests were calculated and the temperatures corresponding to the intersection of the Arrhenius plots of oxide formation and removal rates were determined and taken as the critical failure temperatures. The pressure-velocity failure transition diagram was constructed by plotting the critical failure temperatures along isotherms of average nominal asperity flash temperatures calculated at different combinations of contact stress and sliding speed. The predicted failure transition curve agreed well with experimental scuffing data.     

The Effects of Elevated Ambient Temperatures on the Friction and Wear Behavior of Some Commercial Nickel Base Alloys

Measurements of friction and wear during sliding of specimens of Nimonic 75, C263, Nimonic 108 and Incoloy 901 on like specimens in air at temperatures from 20 to 800 C are presented. Under the sliding conditions used, all the alloys show a transition temperature, above which low wear and a low coefficient of friction during sliding are observed after a time and below which these parameters remain relatively high throughout. These temperatures are about 150 C for N75, about 200 C for C263 and N108 and between 200 and 300 for Incoloy 901. At given temperatures above the transition temperatures, the coefficient of friction-time loci show sharp, generally very reproducible, changes from relatively high to low coefficients of friction. The times at which these occur decrease with increasing temperature for a given alloy. Such changes can be closely correlated to the formation of a stable, adherent, thermally softened, oxide layer or glaze on the load-bearing areas during sliding. Once the glaze is established, very little further wear takes place. These tribological properties of the glaze are associated with its low shear strength and the high strength of the underlying alloy substrate. They depend more on its physical properties than on its precise chemical composition. It is concluded that high strength, relatively rapid transient oxidation rates, and appropriate physical properties of the resulting oxide films are important qualities in alloys employed under sliding conditions in air at elevated temperatures.     

The effect of oxide-forming alloying elements on the high temperature wear of a hot work steel

A great deal of research has been conducted to clarify the role of oxide films in the wear of metals. Oxides formed during dry sliding of steels at high temperatures determine their tribological behavior. The present work deals with the influence of the oxide-forming alloying elements aluminum and silicon on the oxidation and wear of three selected hot work steels. For this investigation, ball-on-disc experiments were carried out in ambient air and 500 °C. Wear tracks on the disks and balls were characterized using both a scanning electron microscope and an optical profiler. The oxidation products were characterized using X-ray photoelectron spectroscopy and energy-dispersive electron probe microanalysis. The results show that the alloying elements aluminum and silicon yield a reduction of the oxide film thickness and thus lead to an increase in mechanical wear as temperature rises.     

Hot wear properties of ceramic and basalt fiber reinforced hybrid friction materials

In the present study, hybrid friction materials were manufactured using ceramic and basalt fibers. Ceramic fiber content was kept constant at 10 vol% and basalt fiber content was changed between 0 to 40 vol%. Mechanical properties and friction and wear characteristics of friction materials were determined using a pin-on-disc type apparatus against a cast iron counterface in the sliding speeds of 3.2–12.8 m/s, disc temperature of 100–350 °C and applied loads of 312.5–625 N. The worn surfaces of the specimens were examined by SEM. Experiments show that fiber content has a significant influence on the mechanical and tribological properties of the composites. The friction coefficient of the hybrid friction materials was increased with increasing additional basalt fiber content. But the specific wear rates of the composites decreased up to 30 vol% fiber content and then increased again above this value. The wear tests showed that the coefficient of friction decreases with increasing load and speed but increases with increasing disc temperature up to 300 °C. The most important factor effecting wear rate was the disc temperature followed by sliding speed. The materials showing higher specific wear rates gave relatively coarser wear particles. XRD studies showed that Fe and Fe₂O₃ were present in wear debris at severe wear conditions which is indicating the disc wear.     

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.