Steel Counterface Research Articles

Effect of Transfer Films on Friction of PTFE/PEEK Composite

Abstract This paper investigates the effect of transfer films on friction coefficient of polytetrafluoroethylene (PTFE)/polyetheretherketone (PEEK) composite. Friction experiments were carried out first to investigate transfer-film development during sliding contact of PTFE/PEEK composite with different PTFE volume fractions on a steel counterface. Quantitative relationships between PTFE/PEEK composite friction coefficient and constituent material mechanical properties are then established to facilitate the subsequent investigation of friction mechanisms and influence of transfer films on the composite friction. A micromechanics-based friction theory is developed for predicting PTFE/PEEK composite friction coefficient. The effect of transfer films on composite friction is accounted for based on two distinctly different mechanisms—one with solid-state film lubrication and the other with PTFE as a solid-state lubricant on the top surface of transfer films. The friction theory is first validated through the excellent agreement obtained between the theoretical predictions and the in-house experimental results on PTFE/PEEK composite with up to 20% PTFE (by volume). The validity of the theory is further demonstrated by comparing the theoretical predictions with the test data reported by other researchers in the literature.

Study of pressure dependence on sinterable zirconia nanoparticle tribofilm growth

Tribological sintering of antiwear films as a function of applied load has been investigated using a novel zirconia nanoparticle antiwear additive. Spherical five nanometer diameter zirconium oxide (ZrO2) nanoparticles are dispersed in polyalphaolefin (PAO) synthetic base oil and tested between AISI 52100 steel counterfaces in a ball-on-disk tribometer with a slide-to-roll ratio of 50%. The apparatus allows tribofilm thickness data to be tracked in-situ at set intervals, and tribofilms reaching a maximum thickness of 160 nm have been measured. A tribofilm formation dependence on load has been found, as higher loads assist initial growth and final thickness. Contrary to the chemical tribofilm formation processes of traditional antiwear additives such as zinc dialkyldithiophosphate (ZDDP), testing suggests the primary mechanism of tribofilm growth for zirconium oxide is nanoparticle adsorption followed by particle accumulation and sintering. A strong correlation between friction force, tribofilm width and Hertzian contact theory has been found. Initial tribofilm growth rate grew exponentially with applied load, whereas isolation of the linear growth phase yielded a linear relationship. Mechanical stylus, scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) are employed to investigate the roughness, morphology and chemical nature of the sintered tribofilms, respectively.

Composite Materials in the TiN–Cr3C2–C/Matrix System

The influence of modifying additions on the strength and tribological properties of titanium nitride composites has been examined. The lowest contact angle (25–30°) is shown by the PG12N-01 alloy, containing chromium, boron, silicon, and carbon. Relatively porousless structures can be produced in the TiN–PG12N-01 system when chromium carbide and carbon are introduced into the composite as graphite, contributing to the formation of TiCN to improve wetting and adhesion. Graphite introduced additionally as an impurity remains after sintering as an individual uniformly distributed phase integrated into the overall composite structure. This graphite can successfully act as a dry lubricant, improving the service characteristics. In addition, it activates sintering of the composite allowing it to become almost porousless. The optimum sintering temperature falls in the range 1450–1500°C depending on carbon and metal matrix contents. The resultant composites have 300–500 MPa strength and 16.1 GPa hardness. The highest strength can be reached through an optimum combination of graphite and PG12N-01 matrix. The mechanical characteristics deteriorate when the amount of these components is higher or inadequate. The antifriction characteristics of the materials have been tested by dry end friction (at a speed of 8 m/sec, 50 MPa pressure in the contact area, and 10 km sliding path) against a 65G steel counterface with 60 HRC hardness: friction coefficient has been found to be 0.27–0.3 and wear resistance 0.2 ± 0.02 μm/km. The developed ceramic materials thus have high strength and antifriction characteristics and can be used in dry friction conditions at high speeds and loads.

Tribological Characteristics of Commercial Metals

This project conducts a study on wear performance and frictional behaviour of selected metals against stainless steel counterface under dry contact condition. The chosen materials for conducting this study are mild steel, copper and aluminium. The parameters used for inspection and analysis of this project are applied load (0-90N) and sliding distance (0-14 km). Block on ring machine was used to conduct the adhesive wear testings. The worn surfaces are examined and wear mechanisms are categorized using scanning electron microscopy. The results reveal that copper shows better wear properties and aluminium shows less friction. Mild steel exhibits a high rate of wear and material removal. All three materials revealed three different wear mechanisms; aluminium (abrasive and adhesive), mild steel (abrasive and ploughing), copper (adhesive).

Microstructural evolution, mechanical and nanoindentation studies of stir cast binary and ternary aluminium based composites

The superior qualities offered by aluminium matrix composites (AMC) over the years has made it a widely used material for the fabrication of various components in the engineering sector. The effect of silicon carbide and ferrotitanium particles addition on the properties of stir cast aluminium-based composite was investigated. Stir casting technique was employed to ensure homogeneous dispersion of the reinforcement particles within the aluminium matrix. The microstructural, mechanical and tribological properties of the specimens were further assessed to determine the integrity of the fabricated composites. Results from microstructural examination showed a uniform dispersion of the reinforcement particles within the aluminium matrix. The reduced wear rate recorded by the composite reinforced with 5 wt% SiC +2 wt% TiFe resulted from the sliding of the stainless steel counterface over the oxide layers formed on the specimen surface. The specimen reinforced with 5 wt% TiFe +2 wt% SiC exhibited an enhanced tensile and nanomechanical properties, while the binary composite system with 5 wt% TiFe reinforcement had its wear resistance and modulus of elasticity improved.

A complex interdependence of thermal conductivity and lubricity of two solid lubricants to control the tribo-performance of PAEK based composites

In this work, a series of four composites was developed by incorporation of two solid lubricants (SLs) (total amount 20 wt % and added in a complementary manner) viz. graphite (well known for thermal conductivity as well as good lubricity) and PTFE (Poly tetra fluoroethylene) (well known for excellent lubricity, but poor thermal conductivity) into PAEK (Poly aryl ether ketone) reinforced with short glass fibres. It was of interest to investigate how these two SLs work together, especially when with increasing contents of PTFE, thermal conductivity (TC) of the composites would decrease and lubricity should increase. The composites were characterized for density, TC and mechanical properties. Tribo-testing was done by varying load, velocity and against mild steel counterface. The composites showed very low wear rates (K0 ≈ 10−16 m3/Nm) and ultra-low coefficient of friction (μ ≈ 0.03). With increase in PTFE contents, wear resistance and also TC decreased indicating TC was a dominating parameter to control the wear of composites. Worn surfaces were studied with scanning electron microscopy (SEM), energy dispersive X ray analysis (EDAX), Raman spectroscopy, XPS (X-ray photoelectron spectroscopy) and wettability analysis to understand the wear mechanisms.

Effect of vacuum annealing treatment on dry sliding wear behavior of TiC/Ti-1100-0.5Nb composite under different operating temperatures

Dry sliding wear tests at 25 °C and 500 °C were performed for TiC/Ti-1100-0.5Nb composites on the quenched and tempered GCr15 steel counter-face disks in this work. Meanwhile, the TiC/Ti-1100-0.5Nb composites were treated at the annealing temperature of 1050 °C, 1100 °C, 1150 °C, respectively. Results demonstrate that the TiC quantity and α-laths spacing increase with the increase of annealing temperature. The composites have better high temperature wear properties. The composites under 1100 °C vacuum annealing treatment possess the best wear performance, which benefited from the coordination of granular TiC particles dispersion strengthening and the solid solution strengthening of carbon atoms. The wear mechanism of the composite changes from the adhesive and abrasive wears at room temperature to oxidative mild wear at 500 °C. The formation of nano Fe2O3 and TiO2 oxide particles on worn surface plays a protective role during the wear process, which improved the high temperature wear properties.

Effect of amount of TiB2 and B4C particles on tribological behavior of Al7075/B4C/TiB2 mono and hybrid surface composites produced by friction stir processing

In this study, by friction stir processing, we fabricated several defect-free Al7075/B4C/TiB2 mono and hybrid surface composites containing various amounts of B4C and TiB2 reinforcements with high quality interfacial bonding with substrate, fine recrystallized matrix grains, and homogenous dispersion of ceramic particles through the matrix. The composite region of hybrid composite samples was more constricted compared with that of mono composite samples. All hybrid composites had more hardness and wear resistance in comparison to Al/B4C mono composite sample. Among different hybrid composites, the most optimal dispersion of ceramic particles, minimum size of B4C and TiB2 ceramic particles, the lowest friction coefficient, and the highest hardness and wear resistance belonged to the hybrid composite containing equal weight percentage of B4C and TiB2 ceramic particles (named 50%B4C-50%TiB2 hybrid composite). The hardness and wear resistance of the Al7075/B4C, Al7075/TiB2 mono composites and the 50%B4C-50%TiB2 hybrid composite were respectively enhanced by (91%, 121%, 107%) and (67%, 82%, 87%) regarding Al7075 base alloy. During wear, an unstable-simple tribolayer was formed on the surfaces of Al7075 base alloy and unreinforced FSPed samples; however, a more stable mechanically mixed tribolayer containing a mixture of oxygen, ceramic particles, and alloying elements of Al7075 base alloy and steel counterface was formed on the surface of the composite samples. The most stable tribolayer with the highest self-lubricating behavior belonged to the 50%B4C-50%TiB2 hybrid composite owing to the high amount of iron and very low amount of B4C in the tribolayer, resisting the subsurface of tribolayer to crack propagation.

Large-scale tribological characterisation of eco-friendly basalt and jute fibre reinforced thermoset composites

The present research aims at understanding the tribological behaviour of advanced unsaturated polyester/vinyl ester based thermoset composites reinforced by inorganic (mineral-based) or organic (vegetal) fibres such as basalt and jute. These fibres are non-toxic and widely available in nature. Thermosets have limitations in the formation of a uniform transfer layer during sliding wear. To surpass these limitations, tribo-fillers such as polytetrafluoroethylene, polyoxymethylene or molybdenum disulphide (PTFE/POM/MoS2) are added into the contact surface. The composites developed for the current research are characterised for their friction and wear behaviour, using a large-scale (sample size typically 50 × 50 × 7 mm) linear reciprocating sliding flat-on-flat test configuration. In order to simulate real scale application, 50 mm/s sliding speed and 10 kN normal force which corresponds to 4 MPa contact pressure, are applied under dry contact conditions. In this research work 12 different tribocomposites are developed and tested against AISI 100Cr6 steel counterface. It was evidenced that composites blended with PTFE have the lowest coefficient of friction and longest service life. MoS2 filled tribocomposites have the highest coefficient of friction. The dominant wear mechanisms for the failure of all investigated composites are thermal degradation and delamination, and abrasion for the counter surface.

Effects of particle size and surface modification of SiC on the wear behavior of high volume fraction Al/SiCp composites

Abstract Aluminum matrix composites with a high SiC particle volume fraction (55 vol%) have been fabricated by utilizing an innovative liquid pressing process. The microstructure of the fabricated composites was investigated. The effects of interface oxidation and reinforcement particle size on sliding wear behavior were studied using the pin-on-disk test against SAE 52100 steel counterfaces. The different wear behaviors of high volume fraction Al/SiC composites were compared to low volume fraction composites. Larger reinforcement shows improved wear resistance because it helps composites maintain rigidity under severe shear deformation. In addition, the SiC interface modification increased the wear resistance of the composites by preventing the formation of interfacial cracks between the matrix and reinforcement. The results of this study present the enhancement of the wear properties through interface modification of high volume fraction-reinforced metal matrix composites.

Tribological Studies of Epoxy Composites With UHMWPE and MoS2 Fillers Coated on Bearing Steel: Dry Interface and Grease Lubrication

Abstract Epoxy with ultra-high molecular weight polyethylene (UHMWPE) and MoS2 fillers was coated on a bearing steel (SAE 52100). Frictional and wear properties of the coated samples in sliding contact were investigated on a pin-on-disc tribometer under a normal load of 10 N and a linear sliding speed of 1 m/s against a bearing steel ball. The optimized coating composition (72 wt% Epoxy + 7 wt% hardener + 18 wt% UHMWPE + 3 wt% MoS2) showed highly improved tribological properties compared to pure epoxy and other epoxy-based composites. There was 75% reduction in the coefficient of friction (COF) in the dry interfacial condition (COF reduced from 0.2 to 0.05) over pure epoxy and 80% reduction with grease as the lubricant. The specific wear-rate of the composite was lower by five orders of magnitude over that of pure epoxy. Other mechanical properties such as hardness, tensile strength, and Young's modulus of the composite showed increments of 86%, 121%, and 43%, respectively, with respect to those of pure epoxy. 2–3 wt% of MoS2 had drastic effects on improving strength and reducing friction and wear of the composites. For dry sliding, initial abrasive and adhesive wear mechanisms led to transfer film formation on the steel counterface, and the shearing was mainly within the transfer film. For the grease-lubricated case, a thin layer of grease helped in easy shearing, and the transfer film formation was avoided. This epoxy-based composite will have applications as tribological coatings for journal bearings.

Hybrid Wear-Reducing Micro-pits Counterface Texture Against Polymeric Solid Lubricants

We report a hybrid wear-reducing surface by sliding steel counterfaces with pre-textured micro-pits pattern against polymeric solid lubricants. When being slid against a freshly prepared pin of the same polymer material, the hybrid surface significantly reduced wear rate of the polymer and steel counterface by up to 69% and 550% compared with the untextured unconditioned surface for the four different polymers surveyed in this study. Polymer wear generally decreased with increased iteration round of preconditioning while counterface wear was more dependent on transfer film and wear track topography. Worn counterface profilometry analysis suggested the preconditioned hybrid surface improved debris retention within the micro-pits which might mitigate wear by acting as debris reservoirs at the sliding interface and promoting transfer film formation.

Perfluoroalkoxy (PFA)-α-Alumina Composites: Effect of Environment on Tribological Performance

Perfluoroalkoxy polymer (PFA)-α-phase alumina (α-Al2O3) composites have dramatically (~ 10,000 ×) improved wear resistance compared to unfilled PFA when tested in air. In this study, unfilled PFA and PFA-α-Al2O3 composites were tested using a linear reciprocating tribometer in dry nitrogen (< 2.5 ppm O2 and H2O), humidity-controlled air (15%, 30%, and 45% relative humidity), and submerged water. PFA-α-Al2O3 tested in dry nitrogen (N2) and in submerged water exhibited wear rates that were two to three orders of magnitude higher than those of PFA-α-Al2O3 composites tested in humidity-controlled air. Infrared spectra of the worn PFA-α-Al2O3 surfaces tested in air exhibited peaks attributed to carboxylate salts. These peaks were severely diminished on the PFA-α-Al2O3 sample tested in dry N2 and not observed for submerged water environments. The high wear rate of PFA-α-Al2O3 tested in dry N2 and submerged water was attributed to the lack of carboxylate salts, which require oxygen and water form. These carboxylate salts bond to the α-Al2O3 filler within the PFA matrix which reinforces the polymer wear surface. Additionally, these carboxylate salts form a transfer film on the stainless steel counterface, which protects both the composite and counterface from wear and abrasion.

Wear rate at RT and 100 °C and operating temperature range of microalloyed Cu50Zr50 shape memory alloy

The effect of microalloying with Co on the wear rate and on the operating temperature range of Cu50Zr50 shape memory alloy against 304 stainless steel counterface has been investigated by studying the mass loss and wear behaviour of Cu50Zr50, Cu49.5Zr50Co0.5 and Cu49Zr50Co1 at. % at room temperature (RT) and 100 °C. For the alloys tested at 15 N, maximum wear resistance is achieved at RT for the alloy with 0.5 at. % Co compared to the parent Cu50Zr50 at. % alloy. This is mostly attributed to the effect of Co in promoting stress-induced martensitic transformation (i.e., work-hardening). For wear tests at 100 °C (100 °C plus friction temperature for 1 h), the mass loss is higher than that at RT since martensite partly reverts into soft austenite through an isothermal process. In addition, the alloys are more prone to oxidation with formation of thick oxide layers that can easily get fragmented and detached from the surface thus resulting is higher mass loss than at RT. The effect of Co in promoting martensitic transformation is negligible when testing at 100 °C, since the stress-induced martensite partly reverts into austenite and the thick oxide layer formed on the surface not only masks the effect of the underlaying substrate for it can also easily detach upon wear.

Influence of carbon fiber content on bio-tribological performances of high-density polyethylene

The present study aims to investigate the effect of short carbon fiber (SCFs) content on wear performance of high-density polyethylene (HDPE). SCFs reinforced composites with different weight fractions (5 wt%, 10 wt%, 15 wt% and 20 wt%) were fabricated by melt compounding and compression molding. To evaluate bio- tribological performance of the samples, three different loads (20 N, 40 N and 60 N) were applied to simulate body fluid (SBF) environment against stainless steel counterface. The scanning electron microscope was used to investigate the morphology of composite granules and worn surfaces of samples. Also, the hardness test was conducted for all samples. The results show that the hardness of high-density polyethylene increases significantly depending on the short carbon fiber content. 20 wt% SCFs reinforced composite exhibited the highest hardness, which is 34% improvement compared to the pure HDPE. However, the same trend was not observed for wear resistance of composites. Composites containing 10 wt% SCFs showed best wear performance in SBF fluid conditions.

Tribo-mechanical interpretation for advanced thermoplastics and the effects of wear-induced crystallization

Despite the available knowledge in tribology of thermoplastic polymers the relation between the dominating wear mechanisms and the influencing material factors are still questionable. The present research aims to relate the tribological properties of thermoplastics to their mechanical behaviour and morphological features. Wear-induced further crystallisation of semi-crystalline polymers has been analysed in relation to measured wear and friction/bulk temperature. In this paper nine different polymers (polyamide-imide – PAI, polyether-imide – PEI, polycarbonate – PC, polyphenylsulfone – PPSU, polyethylene terephthalate – PET, ultra-high molecular weight polyethylene – UHMWPE, polyvinylidene fluoride – PVDF, polyphenylene sulfide – PPS, polyamide 6 – PA6) were compared. All specimens were tested with a large scale linear reciprocating flat-on-flat tribo-tester in dry contact condition against 100Cr6 steel counterface. A contact pressure of 4 MPa and 50 mm/s sliding speed were chosen for all the experiments. Wear testing resulted an increase in crystallinity for the semi-crystalline grades. Amongst them, PET and PPS showed a high relative increase in crystallinity. In case of these two materials, the frictional heating was sufficiently high to modify the morphology of the contact surface but still it was below the melting range. Results of instrumented indentation tests also confirm the significant crystallinity increase by showing higher hardness/spring stiffness after wear testing.

High-Temperature Tribo-Oxidative Wear of a Cu-Based Metal-Matrix Composite Dry Sliding Against Heat-Treated Steel

The tribo-oxidative wear of a Cu-based metal-matrix composite, dry sliding against a steel counterface, was investigated at 400 °C by means of pin-on-disc sliding tests. The applied pressure was 0.5 and 1 MPa, and the sliding velocity was 1.57 and 7 m/s. Special emphasis was given to the determination of the characteristics of the friction layers, whose constituents were identified and quantified by means of SEM and XRD measurements, and using the Rietveld analysis of the XRD patterns. The pin friction layers were found to be mainly made by compacted CuO/Cu2O oxides and by Fe oxides (mainly Fe3O4) transferred from the disc surface. The disc friction layers were made by CuO transferred from the pin surface and Fe3O4-rich regions, formed by direct asperity oxidation, due to the quite high (600 °C or more) flash temperatures achieved during sliding. This information was used to explain the experimental dependency of the coefficient of friction and the wear rate with the adopted tribological parameters.

Tribological behavior of glass/sisal fiber reinforced polyester composites

AbstractThe present study addresses the tribological behavior of polyester composites reinforced with a natural fiber (sisal), a synthetic fiber (glass) and their combination (glass/sisal hybrid). The composites were obtained by compression molding with an overall fiber content of 20 or 40 vol%. The composites were rubbed against a steel counterface using a tribometer. Pure sisal composite exhibited superior wear resistance, mainly due to the formation of a concise tribofilm. It also showed the lowest coefficient of friction (CoF) because of the lubricating action of the water present in the natural fiber. Pure glass composite showed the lowest wear resistance due to the strong abrasive effect of the glass fibers. The increase in fiber content lead to an increase in CoF and its effect on wear was complex and discussed throughout the article.

Effect of material hardness, counter-face hardness and load on the tribological properties of virgin and glass filled PTFE using Taguchi Approach

The present paper aims at evaluating the tribological performance of virgin and glass fiber filled PTFE against different steel counterfaces to co-relate the effect of material and counterface hardness on friction and wear performance. The influence of load on friction and wear of virgin and glass fiber filled PTFE is also studied. The experiments were designed according to Taguchi’s L18 orthogonal array and conducted on a pin on disc reciprocating tribometer. Analysis of variance was used to determine the effect of various parameters on coefficient of friction and wear rate. The results revealed that the counterface hardness is the dominant factor affecting the COF and the material hardness is the dominant factor influencing the wear rate. Regression analysis was carried out to predict the outcomes of the experiments. The predicted and measured values show a good degree of proximity. Further, confirmation tests were also conducted with the random parameter combinations for validation of the regression equations. Furthermore, contour plot analysis has also been carried out to ascertain the ranges of COF and wear rate for different settings of control factors.

Comparative studies of the tribological behaviors and tribo-chemical mechanisms for AlMgB14-TiB2 coatings and B4C coatings lubricated with molybdenum dialkyl-dithiocarbamate

Abstract We compared the tribological behaviors and the tribo-chemical mechanisms of AlMgB14-TiB2 coatings and B4C coatings under lubrication with molybdenum dialkyl-dithiocarbamate (MoDTC). In unidirectional sliding contact, the boride coatings exhibited an unstable tribological performance, while the friction coefficients of these coatings maintained a very steady low level in reciprocating sliding contact. No severe wear occurred to the boride coatings. However, significant abrasive wear was observed on the steel balls rubbing against the boride coatings under unidirectional sliding contact. Analysis indicated incomplete dissociation of MoDTC at coating wear tracks and no formation of MoS2 tribofilms on steel counterfaces in the case of unidirectional sliding contact, whereas well-developed MoS2 tribofilms were formed on AlMgB14-TiB2 coatings under reciprocating sliding condition.