Thermal analysis of the wear debris of polyetheretherketone

Differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA) were used to study the thermal performance of the wear debris and the worn pin tops of polyetheretherketone (PEEK) after unlubricated sliding wear tests at a constant sliding speed of 1 m s⁻¹. It was found that the friction interfacial temperature might have ranged from 300°C to 345°C, hence favouring plastic flow mechanisms to occur and resulting in lubricating effects. In addition, thermal oxidative cross-linking reactions took place in PEEK as the wear testing proceeded, causing a reduction in the crystallizability. The most significant changes in thermal behaviour were observed from the wear debris produced under lower load ngs (i.e. 1 and 3 MPa). The possible structure-property relationships that might have affected the wear mode and the morphological feature of the wear debris are discussed.     

Modelling sliding wear: From dry to wet environments

Corrosive species in various forms exist widely in the environment and can significantly affect wear behaviour of materials, usually accelerating wear. Under conditions where the environments are seemingly non-deleterious in terms of corrosivity, some species from the environment can still affect the tribological behaviour of materials. It is thus extremely important to recognise the roles of reactive species in affecting the tribological processes and to understand the processes of tribo-corrosion interactions. In this paper, the mechanisms of wear debris generation and the roles of reactive species in the generation of wear debris during sliding wear in gaseous or aqueous environments are discussed. The effect of environment on the development of wear-protective layers is described. Based on the proposed mechanisms, mathematical models for sliding wear in both dry and aqueous environments are outlined, and the validity of the models is assessed against experimental data in sliding conditions.     

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.     

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.     

A study on the formation of wear debris during abrasion

A set of five material specimens have been tested on five abrasives, some of which are harder, some softer than the materials, using the dynamic abrasive wear tester. Characteristics of selected wear debris have been observed by sem and wear debris of 9Cr2Mo steel analysed by Mossbauer spectroscopy. The test results show three wear mechanisms operating during abrasion: microcutting, plougging deformation and brittle fragmentation. Different abrasives formed different constituents of wear debris due to dissimilar wear conditions. Softer abrasive tended to form more ploughing debris, although some typical microcutting chips were produced. Crushing strength of abrasive may be an important factor in addition to hardness of abrasive. The microstructure of 9Cr2Mo steel wear debris has been changed by abrasion heat; this temperature could be estimated by Mossbauer spectroscopy.     

Morphologies of the wear debris of polyetheretherketone produced under dry sliding conditions: Correlation with wear mechanisms

Wear debris contains extensive information on the tribological behaviours of materials [M.Q. Zhang, Z.P. Lu, K. Friedrich, On the wear debris of polyetheretherketone: fractal dimensions in relation to wear mechanisms, Tribol. Int. 30 (1997) 87–102]. Investigations on wear debris morphology will be helpful for understanding materials’ friction and wear processes. In this work, the wear debris obtained from block-on-ring (BOR) tests of three polyetheretherketones (PEEKs) with different molecular weights was studied. The mechanical properties of the three PEEKs were characterized in a previous work [G. Zhang, A.K. Schlarb, Correlation of the tribological behaviors with the mechanical properties of poly-ether-ether-ketones (PEEKs) with different molecular weights and their fiber filled composites, Wear, 2008, in press]. In this work, the influences of the mechanical properties of PEEKs and apparent pressure on wear debris morphology were studied. Based on analyzing wear debris morphologies, possibly involved tribological mechanisms were discussed. The results indicate that the tribological mechanisms have a close relationship with the morphology of the wear debris. Under low pressures, particle-like wear debris suggests that the micro-cutting effect exerted by the protruding regions of the counterpart dominates the tribological behaviour. Under high pressures, rod-like, bamboo-raft-like and film-like debris were noticed. This fact suggests that the transferring of PEEK to the counterpart and the plastic flow occurring in the PEEK surface layer play important roles on material loss.     

Examination of wear debris produced using a four-ball machine

The original aim of this work was to identify characteristic wear debris from systems which scuff. The wear debris was identified but could not be considered characteristic as similar debris has been observed in delamination wear. During the metallographic examination of scuffed surfaces, a featureless white layer both on and below the surface was observed. This white layer was similar to adiabatic shear bands. It is suggested that the white layer is a form of microscopic friction weld between asperities. This implies that scuffing is a result of hot welding rather than cold welding, as accepted at present. The tests showed that the trapping and subsequent adhesion of wear debris onto the contact surfaces is a negligible factor in the overall wear process in this system.     

Quantitative estimation of wear amounts by real time measurement of wear debris in lubricating oil

A diagnostic technique that can estimate quantitatively wear amounts under lubricated condition was developed using our developed on-line particle counter. Wear tests were carried out by rubbing a bearing metal against a carbon steel in paraffin oil. The size and number of wear debris in the circulating oil could be measured in real time. The volume of each debris was calculated and, additionally, the total wear amount during a given duration was calculated by accumulating all debris volume. The wear amounts obtained by the quantitative estimation were fairly similar to the measured values of mass loss of the specimen.     

Evaluation of various methods to detect metallic wear particles in lube-oil filter debris

Studies were conducted to evaluate methods of detecting metallic wear particles in lube-oil filter debris. The methods studied were: low-temperature oxidation of the organic constituents in the debris, separation of ferromagnetic particles by passage through a magnetic field, and counting/sizing of metallic particles in a fluid stream using a commercially available detector. Laboratory-prepared simulations of wear debris and actual filter debris samples from operational military aircraft systems were used in the evaluation. The relative ability of each method to detect metallic wear particles is discussed, along with the effects of each method on subsequent debris examination.     

电感式磨粒传感器中铁磁质磨粒特性仿真研究

针对机械装置的在线监测传感器,模拟了铁磁质磨粒通过传感器过程中传感器线圈的磁场和感应线圈的感应电压瞬态变化特性.考虑了线圈与铁磨粒的材料、线圈匝数和激励线圈的输入电压等因素,应用Jmag Designer I0.4软件建立了传感器的二维有限元模型.仿真结果揭示了磨粒运动过程中线圈磁场与感应线圈中感应电压的变化规律,获得了感应电压与球形磨粒的直径大小的立方成正比,与磨粒运行速度成正比.研究结果对于电感式磨粒传感器的开发具有重要的指导价值.     

一种液压油磨粒检测新方法研究

液压油中的磨粒含有大量有关液压零部件磨损的重要信息,对磨粒的检测可有效预防液压系统的故障。针对电感式磨粒传感器对非铁磁性金属磨粒的检测能力较弱的缺点,本文提出了一种集成式磨粒检测装置,包括一个电容式传感器和一个电感式传感器。电容式传感器可对液压油中的气泡和金属颗粒实现区分检测,电感传感器可对液压油中的铁磁性和非铁磁性磨粒进行区分检测。通过结合比较电感和电容两个传感单元的检测结果,可实现对液压油中的气泡,铁磁性和非铁磁性金属颗粒的高精度检测。所设计的传感装置能够检测并区分80μm气泡,30μm铁颗粒和45μm铜颗粒。电容式传感器和电感式传感器的集成有效地弥补了两种检测方法各自的不足,这种多种传感器融合的方式对提高磨粒检测装置的检测精度具有重要的意义。     

Surface roughness evolutions in sliding wear process

Wear debris analysis is a technique for machine condition monitoring and fault diagnosis. One key issue that affects the application of wear debris analysis for machine condition monitoring is whether the morphology of the wear particles accurately depicts their original states and the surface morphology of the components from which the particles separate. This study aimed to investigate the evolution of the surface morphology of wear debris in relation to change in the surface morphology of wear components in sliding wear process. Sliding wear tests were conducted using a ball-on-disc tester under proper lubrication and improper lubrication conditions. The study of the particle size distribution and the surfaces of both the wear debris and the tested samples in relation to the wear condition and the wear rates of the wear components were carried out in this study. The evolutions of the surface topographies of both the wear debris and the wear components as wear progressed were investigated. This study has provided insight to the progress of material degradation through the study of wear debris. The results of this research have clearly demonstrated that: (a) there is a good correlation of the surface morphology of wear debris and that of the wear components, and (b) the surface morphology of wear debris contains valuable information for machine condition monitoring.     

The friction and wear properties of nanometre SiO2 filled polyetheretherketone

Nanometre SiO₂ filled-polyetheretherketone (PEEK) composite blocks with different filler proportions were prepared by compression moulding. Their friction and wear properties were investigated on a block-on-ring machine by running a plain carbon steel (AISI 1045 steel) ring against the composite block. The morphologies of the wear traces and the transfer film were observed by scanning electron microscopy (SEM). It was found that nanometre SiO₂ filled-PEEK exhibited considerably lower friction coefficient and wear rate in comparison with pure PEEK. The lowest wear rate was obtained with the composite containing 7.5 wt.% SiO₂. The SEM pictures of the wear traces indicated that with the frictional couple of carbon steel ring/composite block (fillec with 7.5 wt.% filler), a thin, uniform, and tenacious transfer film was formed on the ring surface. It was inferred that the transfer film contributed largely to the decreased friction coefficient and wear rate of the filled PEEK composites.     

Quantitative correlation of wear debris morphology: grouping and classification

Considerable interest has been expressed in the quantitative analysis of wear debris to minimize the subjectivity of visual assessment of debris morphology. Recent works involve the quantification of wear debris morphology using numerical parameters. In this paper, more comprehensive quantitative analysis of wear debris is performed: wear debris morphology is quantified with numerical parameters and, furthermore, quantitative correlation is performed to demonstrate how specific statistical data analysis techniques can be applied to carry out grouping and classification of debris. Grouping and classification are multivariate statistical techniques that can be used to find out morphological groups of wear debris and to classify wear debris into the predefined wear conditions, respectively. It is shown that statistical data analysis can provide systematic quantitative correlation of wear debris without subjective individual judgment.     

Wear mechanisms of laser-hardened martensitic high-nitrogen-steels under sliding wear

High-nitrogen tool steels (Fe, 15% Cr, 1% Mo, 0.3% C, 0.3% N) are applied, e.g. in bearings and gears in aeronautics and space technology. Their advantage compared to conventional, nitrogen-free tool steels is a superior corrosion resistance, which can be attributed to Cr, Mo, and N dissolved within the solid solution. In order to gain a sufficient toughness for application, these steels are tempered above 600°C bringing about precipitated carbides and nitrides, which bind Cr and N and, therefore, deteriorate the chemical properties. Within a DFG (German Research Council)-funded research project the authors show, that by means of laser hardening it is possible to dissolve a part of these precipitates — mainly nitrides resulting in improved properties under fatigue, wear and corrosion. This is brought about by a newly generated martensite with compressive residual stresses (fatigue, sliding wear), dissolution of Cr and N (corrosion) and a higher mechanical stability of the surfaces (sliding wear). This contribution focuses on the acting wear mechanisms under dry sliding wear. The investigations are carried out with pin-on-disk tests, with the disk as the actual specimen and a pin made of conventionally hardened 52100 bearing steel (100Cr6). It can be shown, that the wear properties of the high-nitrogen-steel are better than those of comparable conventional tool steels and that a laser treatment leads to a further improvement. Due to the fact that there is a tempered zone between overlapping laser-hardened areas, there is a change of acting mechanisms and, thus a distinct difference in wear rates. For the conventional corrosion resistant martensitic tool steel the difference between the tempered and the hardened zone is not as marked. Neither the wear mechanisms nor the wear rates differ distinctly. These effects and their influence on the wear behaviour is correlated with the microstructure of both steels before and after laser-hardening.     

Effect of testing conditions on friction and wear of a polyetheretherketone-based composite

Friction and wear characteristics of a type of polyetheretherketone (PEEK)- based composite were evaluated under two different loading pressures and sliding speeds (P = 1.0 MPa, V = 1.0 m/s and P=2.0 Mpa, V=3.3 m/s). The material was in contact with steel surfaces of two different roughnesses (Ra=0.15 μm and Ra=0.33 μm). Interface temperature, coefficient offriction, depth wear rate, and specific wear rate of the polymer composite changed considerably with the PV value and the counterface roughness. The interface temperature increased with increasing PV value, whereas the friction coefficient decreased. The depth wear rate at the higher PV value was much higher than that at lower PV. In addition, the rougher counterface resulted in a higher friction coefficient, depth wear rate, and specific wear rate, when the PV value was fixed. The effect of counterface roughness on the specific wear rate at the higher PV value was smaller than that at the lower PV. Further variations in friction and wear with testing conditions are discussed along with the corresponding microscopic observations of the worn polymer surfaces and the polymer transferred counterfaces.     

On the mechanisms of various fretting wear modes

According to relative motion directions for a ball-on-flat contact, there are four fundamental fretting wear modes, e.g., tangential, radial, torsional and rotational modes. In this paper, the mechanisms of these four fundamental fretting wear modes, particularly for the later three modes, have been reviewed from results obtained by the authors in combination with results from literature. Some general features have been reported. Differences both in running and degradation behavior have been discussed in detail. Results showed that some similar laws for three fretting regimes (partial slip regime, mixed regime and slip regime), fretting maps (running condition fretting map and material response fretting map), wear and cracking mechanisms obtained from the classic mode (i.e. tangential fretting) were also identified and useful to characterize the other modes. Nevertheless, the occurrence of relative slip for the radial fretting, the formation of mixed regime for the torsional fretting, the evolution of surface morphology for the rotational fretting were quite different compared to that of the classical fretting mode.     

基于人工神经网络预测磨合磨损的最佳分形维数

基于BP人工神经网络的L-M算法,建立了磨合磨损的分形参数预测模型。将该模型用于销一盘磨合磨损试验。对最佳分形维数进行了准确预测。该模型收敛速度快、误差小,输出结果与实验结果有极好的吻合性。     

Relationship of surface slope distributions to the wear mechanisms of ceramics

Many ceramics wear by more than a single wear mechanism. To understand how different wear mechanisms contribute to the wear properties of ceramics, quantitative measures of those mechanisms are required. Surface slope distribution analysis was investigated as a method of quantitatively analysing wear mechanism signatures, based on a simple model of the variation of slope with wear mechanism. Slope distribution was shown to be similar for materials within the same class of ceramics, e.g. silicon carbides. However, the range of slope parameters for a mechanism overlapped with that of other mechanisms, so that a mechanism was not found to yield a unique combination of slope parameters. A comparison of wear rate and slope distribution showed that wear could be correlated with specific sections of the distribution, and that this correlation was dependent upon the ‘dominant’ wear mechanism. Secondary effects on the slope distribution, e.g. porosity, invalidated the data for that material for use in slope analysis.     

On the Effect of Counterface Material and Aqueous Environment on the Sliding Wear of Carbon Fibre Reinforced Polyetheretherketone (PEEK)

Carbon fibre reinforced Polyetheretherketone (PEEK/CF) was exposed to unidirectional sliding at a speed of 28 mm/s against various counterparts. Some of these tests were repeated in water. The bearing steel produced the worst results. In this case, the carbon fibre reinforcement even increased the wear compared to unreinforced PEEK. Most other counterparts led to wear rates almost a factor 100 lower. Chemically inert hard counterparts performed best. The lowest wear rates were found against DLC. An aqueous environment usually accelerated the wear process. Only in case of alumina and DLC counterparts, the water lubrication reduced the wear rate. The addition of graphite plus Polytetraflourethylene (PTFE) to PEEK/CF reduced the sensitivity to the operation conditions.