The relevance of wear-mechanism maps to mild-oxidational wear

The presence of oxygen in the environment in which a steel sliding system operates will promote a mild form of wear with wear debris consisting mainly of iron oxides. Of the oxidation-dominated mechanisms, mild-oxidational wear (the prefix describes the extent of oxidation and not the wear rate) has been most extensively investigated. In this paper, examples will be used to show that the wear-mechanism map for the unlubricated sliding of steels can adequately predict the occurrence of mild-oxidational wear and the trend of wear rates as well as describe the resultant features on the worn surfaces. It is also shown that this map is relevant to delamination wear and to test geometries other than the pin-on-disk configuration. It is suggested that the more-recently constructed wear maps for aluminium and magnesium alloys could similarly be used to predict the wear characteristics of these alloys during sliding.     

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.     

Friction and wear behaviour of sintered steels submitted to sliding and abrasion tests

Fe–C–Mo and Fe–C–Cr steels were sintered by PM processes carried out using different values of temperature and pressure, leading to different microstructures and density values. Flat specimens were submitted to tribological tests in order to evaluate their behaviour under both dry sliding and abrasive wear conditions. A flat-on-cylinder tribometer was used for the sliding tests, while a micro-scale ball cratering device was used for the abrasion tests. The dry sliding wear resistance of the PM steels was mainly influenced by the composition and sintering conditions. In this regard, the best behavior was observed for the more hardenable Fe–C–Mo steels with higher Mo content, sintered under conditions giving rise to bainitic microstructures. A determining role was also played by the porosity content and pore shape: reduction in porosity (obtained by increasing the sintering temperature and the compacting pressure), as well as an increase in pore roundness, led to a significant improvement in the resistance to sliding wear. A mild oxidative wear regime were observed for all the sintered steels under relatively low values of the applied load, while an increase of the applied load led to a delamination wear regime. The resistance to abrasive wear was low for all the tested steels, irrespective of composition and sintering cycle.     

Delamination wear of metal injection moulded 316L stainless steel

The wear behavior of metal injection moulded (MIM) stainless steels was studied using a pin-on-disc apparatus under dry sliding conditions. Pin specimens were MIM 316L stainless steel, while disc specimens were wrought 316L stainless steel. At low sliding speeds (0.2–0.6 m/s), the wear rates gradually decreased with increasing sliding speed, but then increased at high sliding speeds (0.6–2 m/s). The adhesive-induced delamination wear dominated at low sliding speeds, while abrasive-induced delamination wear dominated at high sliding speeds. At low sliding speeds, the surface densification occurred on the worn surface of pin specimens, hence no difference was found between the wear resistances of MIM pins containing 2% and 6% porosity. In contrast, the abrasive-induced delamination wear at high sliding speeds was enhanced by porosity; therefore the wear rates of MIM pins containing 6% porosity were higher than those of MIM pins containing 2% porosity.     

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.     

Wear characteristics of a diffusion bonded sintered steel with short term surface treatments

A pin on disc machine was used to investigate the tribological behavior of a diffusion bonded sintered steel, with and without surface treatments of steam oxidation and manganese phosphating, over a wide range of speed (0.2–4 m/s) and applied load (4–500 N) in conditions of dry sliding and starved lubrication by oil impregnation of the porous structure of the materials. Besides the calculated wear rates, the wear mechanisms were determined by examination of the components of the rubbing system (sintered pin, disc and generated debris). A transition from a mild to a severe wear regime was identified, denoted by sharp changes of the wear rate. A transient wear regime, interposed between the mild and severe wear regimes, was detected. The rubbing surface quality degradation was in terms of material displacement around the pin circumference due to a delamination wear mechanism. Such regime was detected for the base sintered steel in dry sliding at 1 m/s for the load range 60–80 N and for both surface treatments in oil impregnated sliding at 0.5 m/s for the load range 200–300 N. Oil impregnation of the base sintered steel expanded the mild wear regime towards higher loads throughout the whole sliding speed range compared to dry sliding. For the lower speeds of 0.2 and 0.5 m/s, manganese phosphated samples in dry sliding exhibited higher transition loads compared to the base sintered steel. The lower oil impregnability of the surface treated samples, due to the sealing of porosity by steam oxidation, led to slightly lower transition loads in oil impregnated sliding, compared to the base sintered steel.     

Effect of Thermochemical Treatments on the Sliding Wear Mechanisms of Steels Under Boundary Lubrication

The effect of heat treatment, carburizing and plasma nitriding on the material-removal mechanisms and wear behavior of steels under boundary-lubricated conditions has been studied. A controlled procedure using mild abrasives under a light load was adopted in order to avoid long-term effects from severe wear in the running-in stage of pin-on-disc testing. Electron microscopy of the sliding surfaces showed that wear of the untreated steel took place by abrasive and adhesive mechanisms. Heat treatment by austenitizing, quenching and tempering (through-hardening) reduced the wear rate and carburizing produced a further reduction. The principal wear mechanisms in the both through-hardened and carburized steels was abrasive and delamination wear with adhesive wear being unimportant. Electron microscopy on cross-sections through the sliding surfaces of the carburized and through hardened steels showed extensive plasticity, cracks, and delaminated wear phenomena. The elimination of adhesive wear as a major wear mechanism is attributed to the influence of hardness on junction growth and the emergence of delamination wear to the effect of nano-crystalline carbides on fatigue life. Plasma nitriding resulted in an additional reduction in wear rate and the effective elimination of delamination wear. Electron microscopy on the plasma-nitrided steel revealed the presence of a dispersion of white layer regions, which raised the yield strength but also - resulted in micro-pitting due to a deformation mismatch with the matrix.     

Transition of elevated-temperature wear mechanisms and the oxidative delamination wear in hot-working die steels

Dry sliding wear tests of H13 and H21 steels were performed at 400 °C. The wear mechanisms and their transitions were studied, and an oxidative delamination wear was suggested. A mild oxidative wear prevailed with oxide fatigue delamination under less than 3.54 MPa. Under 3.54-5.31 MPa, the oxidative wear prevailed with oxide fatigue delamination and the oxidative delamination wear. As the pressure surpassed 5.31 MPa, a severe wear prevailed with the oxidative delamination wear and the plastic extrusion. The oxidative delamination wear meant that the delamination occurred inside the matrix underneath tribo-oxides with long-ditch delamination and belt-like wear debris.     

Macroscopic effects of delamination wear

Measurement using a pin-on-disc machine of periodic changes in the energy parameters of sliding surfaces under conditions of unlubricated contact has provided experimental evidence for the delamination theory of wear. The experimental procedure is suitable for the investigation of lubricated frictional contact.     

Computational simulations of delamination wear in a coating system

Delamination wear constitutes one relevant wear process in many materials. In particular, wear failure through delamination is relevant for material systems where a coating is present on a substrate material. A computational modeling approach is presented that aims to describe the processes of formation and growth of wear delaminations. The model is based on the cohesive zone model approach. An irreversible cohesive zone model is used in a parametric numerical study of delamination wear in a coating system. The model predicts trends in agreement with the trends emerging from Archard's law. The proposed modeling approach has the advantage that details of the delamination wear process can numerically be studied, and that a unified framework from delamination initiation and propagation is provided.     

Research on oxidation wear mechanism of the cast steels

The uni-directional pin-on-disk wear tests were performed in elevated-temperature air at 400 °C for the Cr–Mo–V cast steels with different compositions. Morphology, composition and structure of worn surfaces, oxidation films and matrix were examined using SEM, EDS, XRD and TEM. The relations between oxidation wear rate and matrix were studied. The mechanism of wear was clarified. Under elevated-temperature air at 400 °C, typical oxidation wear was presented in the cast steels. Oxidation of worn surface and fatigue delamination of oxide film proceed alternatively during sliding. As there are not coarse second phases in steel, oxide film is main factor in determining wear rate, which conforms to Quinn's oxidation wear theory. In this case, delamination of oxide film was found to take place inside oxide film or at interface of matrix and oxide film. This is classified as mild oxidation wear with lower wear rate. As coarse second phases exist in steel, the wear rate is strongly dependant on microstructures of matrix. In this case, oxide film delaminates from the inside of matrix under oxide film. This is classified as severe wear with high wear rate.     

Simulation of wear and contact pressure distribution at the pad-to-rotor interface in a disc brake using general purpose finite element analysis software

Passenger car disc brakes are safety-critical components whose performance depends strongly on contact conditions at the pad-to-rotor interface. The interface can be classified as a conformal dry sliding contact. During braking both brake pad and rotor surfaces are worn, affecting the useful life of the brake as well as its behavior. This paper discusses how wear of the pad-to-rotor interface can be predicted using general purpose finite element analysis software. A three-dimensional finite element model of the brake pad and the rotor is developed to calculate the pressure distribution in the pad-to-rotor contact. A wear simulation procedure based on a generalized form of Archard's wear law and explicit Euler integration is used to simulate the wear of the brake pad under steady-state drag conditions.     

Friction and wear of sintered iron

Using a wear test rig of the disc and pin type the influence of the sliding velocity V and contact pressure P on the friction and wear of sintered iron under unlubricated (dry) conditions was studied. The results showed that for a fixed test duration the coefficient of friction and wear varied according to the law KP^aV^b.     

Microstructure and Wear Behaviors of Plasma-Sprayed FeCrNiMoCBSi Coating with Nano-Grain Dispersed Amorphous Phase in Reciprocating Sliding Contact

A novel FeCrNiMoCBSi amorphous/nanocrystalline coating was fabricated using a plasma spraying process. The coating was dense with a low porosity of approximately 0.99%. The coating consisted of a 67.8 vol% amorphous phase coupled with many nanocrystalline grains that were approximately 5?nm in diameter. The mechanical properties of the as-sprayed coating were determined by nanoindentation measurement, and the tribological behaviors were systematically investigated in a reciprocating sliding contact. The results show that FeCrNiMoCBSi coatings possess superior wear resistance compared to other typically similar Fe-based amorphous coatings. The tribological behaviors evolve with the combination of normal load and sliding velocity. Herein, the dominant wear mechanisms are delamination wear and oxidation wear. With an increase in normal load and sliding velocity, the abrasive wear is gradually weakened, the formation of oxide films on the worn surfaces is facilitated, and wear debris is ground to powder. The oxide films suffer from fatigue wear with induced cracks undergoing reciprocating sliding effects.     

Dry sliding wear in injection molded 17-4 PH stainless steel powder with nickel boride additions

Dry sliding wear behavior of injection molded 17-4 PH stainless steel powder with nickel boride additions has been studied on a pin-on-disc wear tester using an alloy steel pin and disc of hardness 63 HRC. The PIM alloys in the as sintered as well as in the precipitate-hardened conditions were investigated for their wear behavior. Wear rate was found to be initially decreased with the increasing nickel boride amount. Optical microscopy and XRD analysis were preformed to characterize the basic microstructures for all samples. SEM observations of the worn surfaces revealed plastics deformations with delamination of surface layers by subsurface cracks as the mechanism in the as sintered and precipitate-hardened conditions.     

Factors influencing the performance of sintered carbides under arduous conditions of rolling contact

Using an accelerated service simulation test the performance of sintered carbides under arduous conditions of unlubricated and elevated temperature rolling contact has been assessed and compared with that of conventional ball bearing materials and other potentially suitable wear resistant materials. Metallurgical and electronmicroscopical investigation of tested specimens has provided fine-scale information of use in the elucidation of failure mechanisms and their controlling factors.Under the conditions of test, sintered tungsten carbide was superior to the other materials tested unlubricated at both ambient and elevated temperature. Grain size of the carbide as well as the amount of matrix material can have a considerable influence on performance and on the wear mechanism of failure.     

Friction and Wear of Metals in Gases Up to 600 C

Apparatus is described for measuring friction and wear in controlled atmospheres. A comparison is made of the room temperature behavior of copper, mild steel and brass, rubbed against a hardened tool steel, in four environments—vacuum (10^?3 mm Hg), dry helium, dry carbon dioxide, and dry air. The effect of varying the water vapor content in air is also discussed.

The initial selection of rubbing pairs for service at elevated temperatures is gaseous environments under unlubricated conditions, is based on their long-term resistance to corrosion and their ability to give low wear rates. In general, therefore, the materials must be hard. Several alloys having chromium contents of between 1% and 30%, and hardness values in the range 200–1000 vpn have been investigated. These included two low-chromium nitrided steels, a tungsten-chromium tool steel and a series of four Co-Cr-W alloys. Specific wear rates and friction coefficients varied markedly with temperature, and values in the ranges 10^?13–10^?8 cm³/cm kg and 0.1–0.8, respectively, were obtained in both dry carbon dioxide and dry helium. Lowest wear rates were observed with the nitrided steels. The diverse characteristics observed are discussed on the basis of current theories of adhesive wear.     

Sliding wear characteristics of gas boronized steel

The wear characteristics and the mechanism of sliding wear of boronized steel under unlubricated conditions were studied. Characteristic wear curves of FeB and Fe₂B boride layers formed on SAE 1045 steel were similar in form. The maximum wear rates were obtained under a sliding velocity of 0.30 m s^?1 for FeB specimens and 0.50 m s^?1 for Fe₂B specimens. Under such conditions both mechanical wear caused by scratching and oxidative wear occurred. Under conditions of mild wear the wear loss was caused mainly by oxidative wear. Under conditions of heavy wear destruction of the sliding surface was caused by thermal stress. The wear debris was composed principally of iron oxides (α Fe₂O₃, Fe₃O₄) formed by oxidative wear, α iron and borides (FeB, Fe₂B) produced by mechanical wear and B₂O₃ produced by the preferential oxidation of boron in the boride layer.     

Fretting of WC/a-C:H and Cr2N Coatings Under Grease-Lubricated and Unlubricated Conditions

The fretting phenomenon was investigated experimentally in contacts between coated and uncoated steel rod and ball specimens generating a circular Hertzian contact. A fretting wear test rig equipped with a video camera was used to observe the effects of fretting on coated steel surfaces in both grease-lubricated and unlubricated environments. Tungsten carbide reinforced amorphous hydrocarbon (WC/a-C:H) and chromium nitride (Cr₂N) coatings were tested and compared. Fretting wear volumes and surface profiles are presented for both grease-lubricated and unlubricated conditions. Videos of a coated ball fretting against a transparent sapphire flat were recorded and screen captures are presented. The role of normal load, lubrication, frequency, and amplitude of motion on the fretting wear of coatings is discussed. The lubricant released from the grease was observed to flow through channels in the stick zone of the fretting contacts. Both coatings were found to reduce fretting wear. WC/a-C:H was more effective at reducing wear under unlubricated conditions. WC/a-C:H decreased fretting wear more than Cr₂N when delamination was avoided in grease-lubricated contacts.     

Tribological performance of PBT/PC/TPE ternary polymer alloy

Polybutylene terephthalate (PBT)/Polycarbonate (PC)/thermoplastic elastomer (TPE) ternary polymer alloy (TPA) is one of a new generation of PBT based polymer alloys with good thermal conductive, mechanical, and physical properties. TPA-59 was chosen as representative material factorially-de-signed experiment and has been studied, in the present work, to evaluate its potential as a tribological material. Its sliding, friction, and wear behaviour under unlubricated conditions was investigated with a pin-on-disc machine using various loads, sliding velocities, sliding distances, and disc surface roughnesses. The specific wear rates were found to be comparable to those of commercial materials currently used in bearings.