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 and friction behavior of Al6061 alloy reinforced with carbon nanotubes

Friction and wear behavior of Al6061 monolithic alloy and 1 wt% CNTs reinforced Al6061 composite prepared through ball milling and spark plasma sintering was investigated. It was found that, under mild wear conditions, the composite displayed lower wear rate and friction coefficient compared to the monolithic alloy. However, for severe wear conditions, the composite displayed higher wear rate and friction coefficient compared to the monolithic alloy. Analysis of worn surfaces revealed that, at lower loads, abrasion was the dominant wear mechanism for both materials. At higher loads, adhesion was found to be dominant for the monolithic alloy while excessive sub-surface fracturing and delamination were mainly observed for the composite. Also, it was clarified that the friction and wear behavior of Al–CNT composites is largely influenced by the applied load and there exists a critical load beyond which CNTs could have a negative impact on the wear resistance of aluminum alloy.     

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.     

Friction and wear behavior of Ni-P coated Si3N4 reinforced Al6061 composites

Al6061 matrix composite reinforced with nickel coated silicon nitride particles were manufactured by liquid metallurgy route. Microstructure and tribological properties of both matrix alloy and developed composites have been evaluated. Dry sliding friction and wear tests were carried out using pin on disk type machine over a load range of 20-100 N and sliding velocities of range 0.31-1.57 m/s. Results revealed that, nickel coated silicon nitride particles are uniformly distributed through out the matrix alloy. Al6061-Ni-P-Si₃N₄ composite exhibited lower coefficient of friction and wear rate compared to matrix alloy. The coefficient of friction of both matrix alloy and developed composite decreased with increase in load up to 80 N. Beyond this, with further increase in the load, the coefficient of friction increased slightly. However, with increase in sliding velocity coefficient of friction of both matrix alloy and developed composite increases continuously. Wear rates of both matrix alloy and developed composites increased with increase in both load and sliding velocity. Worn surfaces and wear debris was examined using scanning electron microscopy (SEM) for possible wear mechanisms. Energy dispersive spectroscope (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscope (XPS) techniques were used to identify the oxides formed on the worn surfaces and wear debris.     

High stress abrasive wear behaviour of aluminium hard particle composites: Effect of experimental parameters, particle size and volume fraction

High stress abrasive wear behaviour of aluminium alloy (ADC-12)–SiC particle reinforced composites has been studied as a function of applied load, reinforcement size and volume fraction, and has been compared with that of the matrix alloy. Two different size ranges (25–50 and 50–80 μm) of SiC particles have been used for synthesizing ADC-12–SiC composite. The volume fraction of SiC particles has been varied in the ranges from 5 to 15 wt%. It has been noted that the abrasive wear rate of the alloy reduced considerably due to addition of SiC particle and the wear rate of composite decreases linearly with increase in SiC content. It has also been noted that the wear resistance of composite varies inversely with square of the reinforcement size. The wear rate of the alloy and composite has been found to be a linear function of applied load but invariant to the abrasive size; at critical abrasive size, transition in wear behaviour is noted. This has been explained through analytically derived equations and wear–surface examination.     

A study on friction and wear characteristics of nanometer Al2O3 /PEEK composites under the dry sliding condition

The friction and wear properties of the polyetheretherketone (PEEK) based composites filled with 5 mass% nanometer or micron Al₂O₃ with or without 10 mass% polytetrafluroethylene (PTFE) against the medium carbon steel (AISI 1045 steel) ring under the dry sliding condition at Amsler wear tester were examined. A constant sliding velocity of 0.42 m s⁻¹ and a load of 196 N were used in all experiments. The average diameter 250 μm PEEK powders, the 15 or 90 nm Al₂O₃ nano-particles or 500 nm Al₂O₃ particles and/or the PTFE fine powders of diameter 50 μm were mechanically mixed in alcohol, and then the block composite specimens were prepared by the heat compression moulding. The homogeneously dispersion of the Al₂O₃ nano-particles in PEEK matrix of the prepared composites was analyzed by the atomic force microscopy (AFM). The wear testing results showed that nanometer and micron Al₂O₃ reduced the wear coefficient of PEEK composites without PTFE effectively, but not reduced the friction coefficient. The filling of 10 mass% PTFE into pure PEEK resulted in a decrease of the friction coefficient and the wear coefficient of the filled composite simultaneously. However, when 10 mass% PTFE was filled into Al₂O₃/ PEEK composites, the friction coefficient was decreased and the wear coefficient increased. The worn scars on the tested composite specimen surfaces and steel ring surfaces were observed by scanning electron microscopy (SEM). A thin, uniform, and tenacious transferred film on the surface of the steel rings against the PEEK composites filled with 5 mass% 15 nm Al₂O₃ particles but without PTFE was formed. The components of the transferred films were detected by energy dispersive spectrometry (EDS). The results indicated that the nanometer Al₂O₃ as the filler, together with PEEK matrix, transferred to the counterpart ring surface during the sliding friction and wear. Therefore, the ability of Al₂O₃ to improve the wear resistant behaviors is closely related to the ability to improve the characteristics of the transfer film.     

Plate-like wear particle formation in a lubricated ball-on-plate friction pair

The mechanism of formation of plate-like wear particles in a ball-on-plate lubricated friction pair has been examined for wear constants of K < 10⁻¹⁰ (mm³ mm⁻¹ N⁻¹). The plate Vicker's hardness was 2.80–3.00 kN/mm², the sliding speed 1.74 m s⁻¹ and the load 50 N. The following mechanism is suggested: scratching of the surface and formation of ridges at the scratch border, lateral deformation of ridges and formation of thin sheets, and cracking and separation of plate-like particles from these sheets.     

Dry-sliding self-lubricating ceramics: CuO doped 3Y-TZP

Dense 8 mol% CuO doped 3Y-TZP ceramics prepared by pressureless sintering at 1500 °C exhibits a good wear-resistance (specific wear rate k < 10⁻⁶ mm³ N⁻¹ m⁻¹) and promisingly low friction (coefficient of friction f = 0.2–0.3) when sliding against an alumina ball under unlubricated conditions. It was recognized that a self-lubricating mechanism is the most important contribution to the reduction of friction. During operation of the tribosystem, a soft interfacial patchy layer is generated in the contact area. As confirmed by calculations, based on a deterministic friction model, this soft interfacial patchy layer reduces friction. It was demonstrated that the presence of copper oxide is important for the formation of such an interfacial layer. The mechanism of the transition from mild to severe wear was also investigated. Detachment of a top layer in the wear track was proven to be the main reason for this tribological change.     

Surface potential effects on friction and abrasion of sliding contacts lubricated by aqueous solutions

This study has investigated the relationship between applied interfacial electrical potentials and friction and abrasion for steel/steel contacts in alkaline aqueous-based solutions. The potential at a steel-aqueous lubricated surface is important since it determines a number of important surface properties which influence the overall friction and resistance to abrasion. The experimental approach used a pin-on-disc rig incorporating potentiostatic control of the disc between −1.0 and +1.0 V overpotential.Tests employed a load of 50 N at a sliding speed of 0.03 m s⁻¹. EN 24 grade steel, equivalent to AISI 4340, was used as the material for the pin and disc. Tests were conducted in electrolyte that contained electroactive species, namely octanoate ions, which could be “switched on” to the surface. Coefficient of friction measurements were carried out throughout testing and later linked to potential or current density behaviour to assess the mechanical and electrochemical interactions and its effect on wear and friction. The effects of lubricity of the adsorbed layers are discussed and used to explain the performance.     

Dry sliding wear behaviour of polyamide 66 and polycarbonate composites

A study has been made of the reciprocating dry sliding wear behaviour of polyamide 66 and polycarbonate containing glass fibres, ultra high molecular weight polyethylene (UHMWPE) and polytetrafluoroethylene (PTFE/2% Si oil). Studies have been conducted at sliding loads of 2 kg and 10 kg at an average velocity of 0.33 m s⁻¹ against a hardened stainless steel counterface with a surface roughness of 0.3 μm.It has been shown that additions of 10–15% of filler/reinforcement lead to greatly improved sliding wear behaviour. PTFE/2% Si oil filled polyamide 66 has been shown to have the best overall wear performance whilst the high glass filled variants of polyamide 66 and polycarbonate have the best combination of wear resistance and mechanical strength. These findings are discussed with reference to composite constitution and properties, thermal effects and counterface interactions. Explanations are advanced to account for the differences in behaviour inter alia the composite materials.     

Dry sliding wear characteristics of some industrial polymers against steel counterface

In this investigation, the influence of test speed and applied pressure values on the friction and wear behaviour of polyamide 66 (PA 66), polyoxymethylene (POM), ultrahigh molecular weight polyethylene (UHMWPE), 30% glass fibre reinforced polyphenylene-sulfide (PPS+30%GFR) and aliphatic polyketone (APK) polymers were studied. Friction and wear tests of PA 66, POM, UHMWPE, PPS+30%GFR and APK versus AISI D2 steel were carried out at dry condition on a pin-on-disc arrangement. Tribological tests were performed at room temperature at different pressures (0.35–1.05 MPa) and sliding speeds (0.5–2.0 m/s). The results showed that, for all polymers used in this investigation, the coefficient of friction decreases linearly with the increase in pressure. The specific wear rate for UHMWPE, PPS+30%GFR and APK were in the order of 10⁻⁵ mm³/N m, while the wear rate value for PA 66 was in the order of 10⁻⁶ mm³/N m. In addition to this, the wear rate value for POM was in the order of 10⁻³ mm³/N m. Furthermore, as the results of this investigation, the wear rate showed very little sensitivity to the applied pressures and test speed.     

Tribological behavior of metal matrix composites

The wear and friction behavior of continuous graphite fiber reinforced metal matrix composites was investigated. Composite materials were tested against 4620 steel at 54 m s^?1 at room temperature in air without lubricant. The graphite fibers studied included rayon-, pitch- and polyacrilonitrile (PAN)-based fibers. Both high modulus and high strength PAN-based fibers were examined. The fibers were incorporated into copper- and silver-based alloys by means of a liquid metal infiltration technique. The results of this study indicate that the type of graphite fiber in the composite is the most significant factor in the wear and friction behavior of metal matrix composites. In some high modulus fiber tin-bronze composites the fiber fraction influences the wear rate but not the coefficient of friction. Neither the matrix alloy nor the composite tensile strength per se correlate with the friction and wear properties; however, there are specific trends for the various matrix alloys.     

Boundary lubrication additives for aluminium: A journey from nano to macrotribology

The tribology of two potential additive molecules for aluminium has been studied in the self assembled monolayer state at low loads (10⁻⁹–10⁻² N) and in dispersion state in n-hexadecane at high load (60 N). The two additive molecules are 1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane (FOTS), which has a rigid rod-like helical structure and octadecyltrichlorosilane (OTS), which has a zigzag backbone configuration. The adhesion, friction and wear performance have been investigated. Molecular conformational order and stiffness were found to play major roles in frictional dissipation and wear resistance, recorded in a nanotribometer and a pin-on-disc machine, while for very low load measurements (LFM), adhesion was found to have a significant role on friction along with molecular entropy.     

The friction and wear of electroless Ni–P matrix with PTFE and/or SiC particles composite

In this paper, the friction behaviour and wear mechanism of electroless Ni–P matrix with PTFE and/or SiC particles composite coating are investigated by virtue of ring-on-disk wear machine at a high load of 150 N. The worn surface, wear debris and the composition changes after wear were characterized using scanning electron microscopy (SEM) and energy-dispersive analysis of X-ray (EDAX). By comparison with Ni–P and Ni–P–SiC coatings, the results indicated that the combination of a PTFE-rich mechanical mixed layer (PRMML) formed on the worn surface and hard SiC were responsible for the good tribological properties of the hybrid Ni–P–PTFE–SiC composites at high load. After heat treatment at 400 °C for 1 h, the wear rate of Ni–P matrix composites decreased with corresponding increase in microhardness. During sliding, an obvious decrease in the temperature rise with PTFE addition was attributed to the good anti-friction of PTFE.     

A comparative study on the wear properties of coarse-grained Al6061 alloy and nanostructured Al6061–Al2O3 composites

Wear behavior of nanostructured Al6061 alloy and Al6061–Al₂O₃ nanocomposites produced by milling and hot consolidation were investigated. The samples were characterized by hardness test, pin-on-disk wear test, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Nanocomposites containing 3 vol% Al₂O₃ showed a maximum hardness of 235 HV and optimum wear rate of 4×10⁻³ mg/m. Increasing the amount of Al₂O₃ up to 5 vol% resulted in decrease in hardness values (∼112 HV) and a sharp rise in wear rate (∼18×10⁻³ mg/m).     

Tribological behaviour of duplex treated Ti–6Al–4V: combining nitrogen PSII with a DLC coating

The chemical structure and tribological behaviour of Ti–6Al–4V plasma source ion implanted with nitrogen then DLC-coated in an acetylene plus hydrogen-glow discharge (bias voltage −10 to −30 kV) were investigated. The as-modified samples have a TiN/H:DLC multilayer architecture (coating resistivity 1.6×10⁹ to 2.4×10¹¹ Ω/cm) and exhibit higher hardness, especially at low loads or plastic penetrations in the order of deposition bias voltage −10, −20 and −30 kV. At a lower contact load (1 N) and higher sliding speed (0.05 m/s), frictional properties in most cases improved, as did wear properties. At a higher contact load (5 N) and lower sliding speed (0.04 m/s), friction showed almost no improvement, and wear properties deteriorated. When the material of the counterbody was then changed from AISI 52100 to Ti–6Al–4V modified as the disc (contact load 5 N unchanged, sliding speed decreased), the friction coefficient decreased (but showed no improvement compared with the unmodified sample), while wear properties deteriorated further, and wear was changed from just the disc to both disc and ball, abrasive and adhesive dominated. Transfer films, mainly made up of wear debris transferred from the disc wear surfaces, were formed on the wear scars of the counterbodies. The deterioration of wear properties of the modified samples at the higher contact load is considered to be caused by the “thin ice” effect.     

Effect of the Graphite Content on the Tribological Behavior of Al/Gr and Al/30SiC/Gr Composites Processed by In Situ Powder Metallurgy (IPM) Method

The influence of graphite content on the dry sliding wear characteristics of Al6061/Gr composites along with Al6061/30SiC/Gr hybrid composites has been assessed using a pin-on-disc wear test. The composites with different volume fraction of graphite particles up to 13% were processed by in situ powder metallurgy (IPM) technique. The porosity and hardness of the resultant composites were also examined. It was found that an increase in the graphite content reduced the porosity, hardness, and friction coefficient of both types of composites. The hybrid composites were more porous and exhibited higher hardness and lower coefficient of friction at identical graphite contents. The increased graphite content in the range of 0–13 vol.% resulted in increased wear rate of Al/Gr composites. The Al/30SiC composite exhibited a lower wear rate as compared with the base alloy and graphite addition up to 9 vol.% improved the wear resistance of these hybrid composites. However, more graphite particles addition resulted in increased wear rate. SEM micrographs revealed that the wear mechanism was changed from mostly adhesive in the base alloy sample (Al/0Gr) to the prominently abrasive and delamination wear for Al/Gr and Al/SiC/Gr/composites.     

Measurements of tribological properties of poly(pyrrole) thin film bearings

We report the results of a recent study on the tribological properties of electropolymerised thin films at light loads and low speeds. Poly(pyrrole) films incorporating different counter-ions have been electrochemically deposited onto gold electrodes on the plano-convex glass substrates and studied extensively. The measuring apparatus has been greatly improved from that reported earlier and now provides simultaneous monitoring of frictional force and wear. High precision capacitive gauging is employed to provide high resolutions of frictional force of better than 100 μN and height variation (wear) of 2 nm. A large number of specimens of poly(pyrrole) grown from five different counter-ions were prepared and their performances evaluated. The film morphology of each type of film was examined by atomic force microscopy (AFM) for control of the variability of film formation. Results are presented for the friction coefficients and wear rates observed for the films typically at a load of 2 N and a sliding speed of 5 mm s⁻¹. The effects of normal loading force and sliding speed on the friction coefficient are also discussed with a load range of 0.2–5 N and a sliding speed up to 30 mm s⁻¹.     

Friction and wear behavior of electroless Ni-based CNT composite coatings

Ni-based carbon nanotube (CNT) composite coatings with different volume fraction (from 5 to 12 vol.%) of CNTs were deposited on medium carbon steel substrates by electroless plating. The friction and wear behavior of the composite coatings were investigated using a pin-on-disk wear tester under unlubricated condition. Friction and wear tests were conducted at a sliding speed of 0.0623 m s⁻¹ and at an applied load of 20 N. The experimental results indicated that the friction coefficient of the composite coatings decreased with increasing the volume fraction of CNTs due to self-lubrication and unique topological structure of CNTs. Within the range of volume fraction of CNTs from 0 to 11.2%, the wear rate of the composite coatings showed a steadily decreasing trend with increasing volume fraction of CNTs. Because of the conglomeration of CNTs in the matrix, however, the wear rate of the composite coatings increased with further increasing the volume fraction of CNTs.