On sliding friction and wear of polyetheretherketone (PEEK) composites at elevated temperatures

The friction and wear behaviour of polyetheretherketone (PEEK) composites, incorporating different amounts of short carbon fibres with different surface treatments, was studied under dry sliding conditions against smooth steel on a pin-on-disc apparatus at different temperatures. Wear of the composites was reduced considerably in all cases, but, whatever the surface treatment, wear increased with increasing temperature for all proportions off fibres. For minimum friction coefficient there was an optimum proportion of fibre volume fraction of about 10 vol.%. The effect of the fibre surface treatment was not significant for the tribological behaviour of the PEEK composites. To predict wear performance, a wear model proposed by Friedrich and Voss seemed to work properly, and, furthermore, a friction model was developed to predict the friction behaviour of PEEK composites with short carbon fibres.     

Friction and dry sliding wear behavior of carbon and glass fabric reinforced vinyl ester composites

Friction and dry sliding wear behavior of glass and carbon fabric reinforced vinyl ester composites have been presented. The results show that the coefficient of friction and wear rate increased with increase in load/sliding velocity and depends on type of fabric reinforcement and temperature at the interphase. The excellent tribological characteristics were obtained with carbon fiber in vinyl ester. It is believed that a thin film formed on counterface was seems to be effective in improving the tribological characteristics. The worn surfaces examined through SEM, showed higher levels of broken glass fiber in vinyl ester compared to carbon-vinyl ester composites.     

固体自润滑涂层摩擦磨损特性研究

随着机械加工技术和空间技术的发展,固体润滑涂层在比较恶劣的润滑环境下能起到独特的润滑效果,得到高速发展和广泛应用.在综合国内外研究和报道的基础上,对一般固体润滑涂层和纳米固体润滑涂层的摩擦磨损特性的研究现状进行归纳,指出固体润滑涂层摩擦磨损研究的发展方向.     

Effect of sliding speed on friction and wear behaviour of nanocrystalline nickel tested in an argon atmosphere

The sliding speed dependence of the coefficient of friction (COF) and wear rate (W) of a nanocrystalline (nc) Ni with a grain size of 15 ± 3 nm and a hardness of 5.09 ± 0.11 GPa was compared to that of a microcrystalline (mc) Ni with a grain size of 20 ± 5 μm and a hardness of 1.20 ± 0.05 GPa. The sliding wear tests were performed in an argon environment under a constant normal load of 2 N using three different sliding speeds of 0.2 × 10⁻², 0.8 × 10⁻² and 3.0 × 10⁻² m/s. The lesser wear damage in the nc Ni at any given speed was attributed to its higher hardness and its greater elastic depth recovery ratio compared to the mc Ni. The mc Ni's COFs and wear rates were independent of the sliding speed over the relatively small range used. However, the same small increase in sliding speed caused an 86% reduction in the nc Ni's wear rate, from 3.44 × 10⁻³ to 0.47 × 10⁻³ mm³/m, and a 31% increase in its COF, from 0.49 ± 0.05 to 0.64 ± 0.06. A modified Archard equation was proposed to predict wear rates of Ni as a function of grain size and sliding speed. Increasing the sliding speed made it increasingly difficult for surface damage by plastic deformation to occur in the nc Ni, because the grain-boundary-induced deformation mechanisms are more difficult to operate at higher strain rates. At the highest speed, the smallest amount of debris was generated, which was not sufficient to form protective tribolayers leading to a high COF value.     

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.     

Sliding wear behaviour of ZrN and (Zr, 12 wt% Hf)N coatings

In the present study, the sliding wear resistances of ZrN and (Zr, 12 wt% Hf)N coatings deposited on a hardened AISI D2 tool steel by arc-physical vapor deposition (PVD) technique were examined by a ball-on-disc wear tester. Alloying of ZrN coating with 12 wt% Hf did not change the hardness significantly, but achieved an improvement on adhesion strength and dry sliding wear resistance against steel (AISI 52100-55HRC) and Al₂O₃ balls.     

Abrasive wear mechanisms of fluoropolymer based composite coatings on aluminum substrates

Experimental studies on wear performance and wear mechanisms of fluoropolymer based composite, non-stick coatings on aluminum substrates were carried out by using particular wear testing methods, i.e. the “mechanical tiger paw (MTP) Test” and the “Nord Test”. Both are supposed to simulate household abrasive operating conditions in a laboratory environment. It was found that the wear process involved in the MTP Test was an accelerated one, but that in the Nord Test was decelerated due to a transition from severe three-body abrasion to milder two-body sliding wear. In addition, both effects of the microstructure of the coatings and of the surface treatment of the substrates were discussed.     

Dry sliding friction and wear properties of Al-25Zn-3Cu-3Si alloy

Friction and wear properties of Al-25Zn-3Cu-3Si alloy were investigated over a range of pressure and sliding speed using a pin-on-disc test machine. The friction coefficient of the alloy increased with sliding speed, but decreased with increasing pressure up to 1.5 MPa, above which the trend reversed. However, the temperature and wear volume of the alloy increased continuously with increasing pressure and sliding speed. A fine-grained layer and a region with flow lines were observed underneath the surface of the wear samples. The formation of these regions was related to smearing of wear particles and heavy deformation of surface material, respectively.     

On the effect of counterface material and aqueous environment on the sliding wear of various PEEK compounds

Polyetheretherketone (PEEK) compounds containing carbon fibres (CF), glass fibres (GF), PTFE, and graphite, respectively, were exposed to unidirectional sliding against various counterparts (100Cr6, X5CrNi18-10, alumina, and bronze). Some of these tests were repeated in water. The stainless steel revealed the best results under dry conditions, whereas alumina was the best counterpart in water. The compound containing GF plus PTFE performed best under dry conditions. Under wet conditions, CF were superior to GF, which react very susceptibly to water. The aqueous environment usually accelerated the compound wear. Only in case of CF containing compounds sliding against alumina, the water lubrication reduced the wear rate.     

Triboelectrochemical investigation of the friction and wear behaviour of TiN coatings in a neutral solution

Triboelectrochemical techniques use an electrochemical set-up (mainly of the three-electrode type) for controlling the potential of the surface of a conducting material subjected to rubbing in a tribometer. In this way it is possible to carry out friction and wear tests in electrolytic solutions under well-defined chemical conditions determined by the applied electrode potential. In addition, triboelectrochemical techniques offer the possibility of following in-situ and in real time the kinetics of electrochemical oxidation reactions (corrosion) by the simple measure of an electrical current. In the present study triboelectrochemical experiments were carried out on sputter deposited TiN coatings sliding against alumina in a borate solution (pH 8.4). The surface of selected worn coatings was characterised by X-ray photoelectron spectroscopy (XPS) and the topography by scanning electron microscopy (SEM). Results show that the rate of wear critically depends on the prevailing (electro)chemical conditions which determine the chemical surface state of the TiN coating. The behaviour is attributed to the lubricating properties of surface oxide films having a thickness in the nanometre range.     

Effect of porosity on dry sliding wear of Al-Si alloys

Porosity adversely affects the pressure tightness (ability of a system to hold pressure), mechanical properties and wear resistance of components. The amount of porosity and the size and shape of pores have a great impact on material removal during wear. Al-Si alloy with different porosity levels was prepared using powder metallurgy technique. Dry sliding wear behavior was investigated against AISI 52100 bearing steel ball. It was found that the wear rate gradually rises with surface porosity, then drops as porosity and pore size reach critical levels. A model was developed to explain the relationship between porosity and pore size.     

Coating thickness effect on initial wear of nitrogen-doped amorphous carbon in nano-scale sliding contact: Part II—theoretical modeling

This is the second paper of a two-part report. In the first paper, empirical data on the wear particle generation in carbon nitride coatings subjected to repeated sliding contact with a spherical diamond counter-face is reported. The effect of coating thickness on the wear particle generation is also discussed in the first paper. In this paper, a simplified theoretical expression, combining the Coffin-Manson equation with the analytical solution of a proposed elastic perfectly-plastic indentation model, is introduced. The expression successfully correlates critical number of friction cycles for wear particle generation N_c to coating thickness h, contact pressure P and radius of spherical asperity on the tip of the diamond pin R. With this expression, the lifespan of sliding components can be predicted.The theoretical results computed for diamond pin with a specific asperity radius value of 250 nm were compared with the experimental results reported in the first paper. The theoretical model successfully predicts the maximum lifespan of a component, N_c, in repeated sliding applications. The influences of various contact pressures and asperity radii on the maximum lifespan were also assessed using the model.     

An investigation on sliding wear behavior of PVD coatings

Ti-6Al-4V alloy rubbing against aluminum-bronze 630 was evaluated in this work. High velocity oxygen fuel (HVOF) WC-10%Co-4%Cr thermal sprayed and TiN, CrN and DLC physical vapor deposition (PVD) coatings were applied to increase titanium substrate wear resistance. Pin-on-disk tests were performed with a normal force of 5 N and at a speed of 0.5 m/s, with a quantitative comparison between the five conditions studied. Results showed higher wear resistance for Ti-6Al-4V alloy DLC coated and aluminum-bronze 630 tribological pair and that the presence of graphite carbon structure acting as solid lubricant was the main wear preventing mechanism.     

Friction and dry sliding wear of bismaleimide filled with carbon nanotubes

Three types of bismaleimide–carbon nanotubes (CNTs) nanocomposites were fabricated using two types of original multiwalled CNTs with different diameters and one amide functionalized CNTs. The influence of diameter, content and functionalization of CNTs on the flexural and dry sliding wear behaviour were measured with universal testing machine and pin-on-disc wear apparatus. The experimental results indicated that at 1.5 wt-%, the bismaleimide-functionalized MWCNTs exhibited highest flexural strength of 156 MPa which is increased by 164% as compared to the neat matrix, and lowest specific wear rate of 1.8 × 10^?4 mm³ N^?1 m^?1 which is decreased by 90% as compared to the neat matrix. This was attributed to the dispersion of CNTs in the matrix and the filler-matrix adhesion and internal strength of the composite.     

Coating thickness effects on initial wear of nitrogen-doped amorphous carbon in nano-scale sliding contact: Part I—in situ examination

This paper, the first of a two-part series, presents the empirical data obtained from in situ examination on the generation of wear particles on carbon nitride coatings by a spherical diamond counter-face during repeated sliding contacts. In particular, the effect of coating thickness, varying from 1 to 500 nm, on the generation of wear particles was examined.Based on the in situ examination, the shape transition maps for generated wear particles were obtained for carbon nitride coatings of various thickness. The results show that the critical number of friction cycles, Nc, for the transition from “no observable wear particles” to “wear particle generation” generally increased with increasing coating thickness. It was noted that up to 20 friction cycles, the maximum Hertzian contact pressure, P_max, for “no observable wear particles” regime can be increased from 1.39Y to 1.53Y if silicon was coated with carbon nitride coating thicker than 10 nm, where Y is defined as the yield strength of silicon.     

The wear of wrought aluminium alloys under dry sliding conditions

The sliding wear response of several wrought aluminium alloys (2124, 3004, 5056 and 6092) against a high purity alumina (99.9%) counterface was investigated, at a fixed sliding speed of 1 m/s and a load range of 23–140 N. The counterface was chosen so as to minimise the chemically driven aspects of adhesive wear. Severe wear was observed at all loads, with specific wear rates ranging from 0.37×10⁻⁴ to 2.37×10⁻⁴ mm³/N m. In all cases a mechanically mixed layer (MML) was formed, principally from severely work hardened aluminium alloy, but also including fine alumina particles. The thickness and morphology of the layer depended strongly on alloy composition, but the specific wear rate did not depend on the MML properties in a simple manner. The surface work hardening characteristics differed between alloys, but as with the MML, there was no simple relationship between surface work hardening characteristics and specific wear rate. The main correlation was found between the normalised wear rate and normalised pressure, which implies that the hardness of the starting aluminium alloy is the critical variable.     

Dry sliding wear response of some industrial powder coatings

This investigation analyses the sliding wear response of polyester and polyphthalamide powder coatings deposited by electrostatic spraying and ‘hot dipping’ fluidised bed. Tribological tests were conducted under dry conditions in a pin-on-disc arrangement, using a spherical counterpart. The experimental results showed that the deposition technology, the coating material and the thickness of the coating play key roles in determining the wear response of powder coatings. In particular, polyester coatings are deposited by a fluidised bed offer superior wear endurance, along with a low abrasion volume and a low wear rate. Conversely, polyphthalamide coatings are susceptible to faster wear by local cutting and plastic fatigue mechanisms.     

Analysis of wear behaviour of titanium carbonitride coatings

The kinetics of chemical reactions occurring during the coating process and the wear behaviour of Ti(C,N) coatings, including variations in wear displacement with the temperature of the lower specimen, were investigated in this work. The variations in the wear displacement and the temperature of the lower specimen are regressed by an eight-order polynomial function. The lower specimens were coated by a titanium film as underlayer and three kinds of coating material including TiN, Ti(C,N) or TiC were deposited as the top layer. The Ti(C,N) coatings were prepared by varying the gas flow rates of nitrogen and acetylene to form eight kinds of specimen. The tribological behaviour demonstrated by these eight specimens is discussed. The experimental data for the atomic ratios of [C] and [N] can be well expressed using the theory of diffusion rate and the theory of reaction rate for the deposition of ceramic coatings. The variations in the wear displacement gradient with the temperature of the lower specimen can give information on the adhesive behaviour arising before and after three-body wear. The wear rates of the upper and lower specimens due to adhesive wear are dependent on the operating conditions. The specimen with a higher final wear displacement was likely to produce on the upper specimen a higher wear rate when operating at 0.705 m s⁻¹. The thicker the adhesive layer, the lower the wear rate of the lower specimen produced. When the sliding speed was elevated to 1.41 m s⁻¹, the specimen with a higher final wear displacement often produced a lower wear rate on the upper specimen, and also caused higher wear rates on the lower specimens.     

Investigations on the influence of graphite filler on dry sliding wear and abrasive wear behaviour of carbon fabric reinforced epoxy composites

In this experimental study, the dry sliding wear and two-body abrasive wear behaviour of graphite filled carbon fabric reinforced epoxy composites were investigated. Carbon fabric reinforced epoxy composite was used as a reference material. Sliding wear experiments were conducted using a pin-on-disc wear tester under dry contact condition. Mass loss was determined as a function of sliding velocity for loads of 25, 50, 75, and 100 N at a constant sliding distance of 6000 m. Two-body abrasive wear experiments were performed under multi-pass condition using silicon carbide (SiC) of 150 and 320 grit abrasive papers. The effects of abrading distance and different loads have been studied. Abrasive wear volume and specific wear rate as a function of applied normal load and abrading distance were also determined.The results show that in dry sliding wear situations, for increased load and sliding velocity, higher wear loss was recorded. The excellent wear characteristics were obtained with carbon-epoxy containing graphite as filler. Especially, 10 wt.% of graphite in carbon-epoxy gave a low wear rate. A graphite surface film formed on the counterface was confirmed to be effective in improving the wear characteristics of graphite filled carbon-epoxy composites. In case of two-body abrasive wear, the wear volume increases with increasing load/abrading distance. Experimental results showed the type of counterface (hardened steel disc and SiC paper) material greatly influences the wear behaviour of the composites. Wear mechanisms of the composites were investigated using scanning electron microscopy. Wear of carbon-epoxy composite was found to be mainly due to a microcracking and fiber fracture mechanisms. It was found that the microcracking mechanism had been caused by progressive surface damage. Further, it was also noticed that carbon-epoxy composite wear is reduced to a greater extent by addition of the graphite filler, in which wear was dominated by microplowing/microcutting mechanisms instead of microcracking.     

Wear and friction of filled polyamide coatings

The aim of this article is to report wear and friction studies of cylindrical test steel pins coated with polyamide (PA6) incorporating fillers (iron, copper, aluminium, tin, lead) and solid lubricants (graphite, molybdenum disulphide (MoS₂)) to improve the mechanical, thermal, and frictional properties of polyamide (PA6). The coated testpins were loaded against a rotating cylinder of bearing steel in dry, lubricated, abrasive conditions at constant load and sliding speed. The test results show a significant reduction in wear and friction for both copper-filled polyamides and lead-filled polyamides.