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.     

Automated classification of wear particles based on their surface texture and shape features

In this study, the automated classification system, developed previously by the authors, was used to classify wear particles. Three kinds of wear particles, fatigue, abrasive and adhesive, were classified. The fatigue wear particles were generated using an FZG back-to-back gear test rig. A pin-on-disk tribometer was used to generate the abrasive and adhesive wear particles. Scanning electron microscope (SEM) images of wear particles were acquired, forming a database for further analysis. The particle images were divided into three groups or classes, each class representing a different wear mechanism. Each particle class was first examined visually. Next, area, perimeter, convexity and elongation parameters were determined for each class using image analysis software and the parameters were statistically analysed. Each particle class was then assessed using the automated classification system, based on particle surface texture. The results of the automated particle classification were compared to both the visual assessment of particle morphology and the numerical parameter values. The results showed that the texture-based classification system was a more efficient and accurate way of distinguishing between various wear particles than classification based on size and shape of wear particles. It seems that the texture-based classification method developed has great potential to become a very useful tool in the machine condition monitoring industry.     

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.     

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.     

An electron microscopy study of worn ceramic surfaces

An electron microscopy investigation of worn surfaces of monolithic alumina, whisker-reinforced alumina, monolithic silicon carbide and silicon-silicon carbide has been performed. The emphasis has been to explore the mechanisms of wear debris generation and tribofilm formation and their influence on the tribological behaviour in closed contacts of the flat-on-flat category.     

On the wear debris of polyetheretherketone: fractal dimensions in relation to wear mechanisms

It has been recognized that wear debris contains extensive information about wear and friction of materials. Investigation of wear debris is important for tribological research. In order to find out an effective way that is able to diagnose and predict the wear state of polymers, the authors investigated the relationship between the wear debris morphology and the wear behaviour of the bulk material. Polyetheretherketone (PEEK) was employed as the model material. Its sliding wear and friction properties were measured by means of a pin-on-disc apparatus. At a constant sliding velocity of 1 m s⁻¹, the specific wear rate was independent of load under lower loading conditions (1–4 MPa) but increased with a rise in load under higher loading conditions (4–8 MPa). The coefficient of friction was insensitive to the variation of contact pressure. The possible mechanisms involved were analysed on the basis of the wear debris morphology as well as the wear performance. Fractal geometry, which describes non-Euclidean objects, was applied to the quantitative analysis of the boundary texture of the wear debris due to the fact that the qualitative assessment of the wear debris morphology was not effective enough to reflect the geometrical variation of the fragmental shapes. The experimental results demonstrated that the wear debris were fractals, and could be characterized with the fractal dimensions which were determined by the slit island method. In addition, it was found that the fractal dimension of the wear debris was closely related to the wear behaviour of PEEK, and can be regarded as a measure of wear rate.     

Vibration monitoring in sliding wear of plasma sprayed ceramics

Dry frictional contact between two surfaces, one made of plasma sprayed ceramic coatings of Al₂O₃ and Al₂O₃---TiO₂ combination and the other made of steel, is analyzed. The experiments were conducted using a pin-on-disc set-up in the load range of 5–35 N and for sliding distances up to 14 km. The interactions between friction, wear and vibrations due to influence of normal load, sliding speed and system dynamics are investigated in the present paper. Two vibration parameters of pin in the load direction (vertical) are monitored, namely the r.m.s. acceleration and the kurtosis, which seem to be influenced considerably by the wear process and indicate correlation with wear mechanisms taking place such as stick-slip and grain pull-out, as evidenced by scanning electron microscopy of worn surfaces. The study shows that a range of frequency is to be utilized for vibration monitoring to include natural frequencis of the system consisting of pin in contact with disc. This could be estimated by a standard impulse hammer test. The pin acceleration decreases with increase in load and sliding distance, but with respect to sliding speed, the vibration level intially decreases but increases beyond the sliding speed of 1.5 m s⁻¹. Among the three ceramic coatings, TiO₂ is found to be most wear resistant, exhibiting the lowest friction coefficient and a low vibration level. Variation in kurtosis with run-in wear indicates smoothing of Al₂O₃ due to grain pull out.     

Fretting wear and friction behaviour of engineering ceramics

Friction and wear characteristics of partially stabilized zirconia (PSZ) fretted against itself and against high carbon steel were investigated. The results for the transformation toughened PSZ ceramics are compared with the behaviour of more brittle alumina ceramic under the same test conditions. Fretting tests in air were carried out on a high frequency wear test rig at room temperature using a cross-cylinder configuration. It was found that both the oxide ceramics were more resistant to fretting wear than the steel. Surface cracking was observed on the alumina wear scars while microfracture and delamination dominated on the PSZ wear scars. When metallic samples were fretted against ceramics, metallic film transfer to the ceramic surfaces occurred.     

The lubricated wear of ceramics : Part II: The wear and friction of silicon nitride and steel in the presence of mineral oil or an ester based lubricant

The friction and wear characteristics of combinations of silicon nitride, alumina and AISI 52100 steel in the presence of mineral oil containing anti-wear, dispersant and detergent additives have been investigated in a tri-pin-on-disc machine. The tests were carried out at a nominal temperature of 100°C for a range of sliding speeds, loads and total sliding distances. In Part II of this two-part paper a comparison will be made between the tribological performance of these sliding pairs of materials in mineral oil and ester based lubricant environments. The results of the investigation showed that the alumina performed relatively poorly under these test conditions, whereas silicon nitride showed good potential as an improved wear resisting material compared with 52100 steel. Wear factors of the order of 10⁻¹⁰ mm³/Nm were deduced for the alumina, while values as low as 10⁻¹¹ mm³/Nm were typical of the silicon nitride sliding against 52100 steel discs. The alumina pins wore by a process of brittle fracture at the surface, whereas the silicon nitride pins wore primarily by a tribochemical polishing mechanism. The rate of tribo-chemical wear was found to be proportional to the nominal contact area.     

Modeling and simulation of wear in a pin on disc tribometer

A very efficient, incremental implementation of Archard’s wear model on the global scale for pin wear and disc wear in a pin-on-disc tribometer is presented. The results from the model are in good agreement with experimental results. The identified wear model is implemented in a finite element based tool (Wear-Processor) for 3D wear simulations and the results compare favorably with that from the global wear modeling scheme.     

The effects of iron particles on the friction and wear mechanisms of ceramics in ethanol

For the combinations of an Si₃N₄ pin and five kinds of ceramic disk (SiC, Si₃N₄, Al₂O₃, ZrO₂, TiC), a friction and wear test was carried out in ethanol and in ethanol containing iron particles (1 wt.%, average diameter d = 200 nm, D = 12 μm under cohered condition) under a load in the range 5.88–11.50 N, at a sliding velocity of 0.138–0.196 m s⁻¹. A topographical analysis was also performed on the microasperities of the wear surfaces to estimate the behavior of the iron particles, and the degree of surface damage. As a result, the following facts were found. (1) The addition of iron particles in ethanol decreased both the wear rates of SiC and TiC disks and the mating pins, and also decreased the wear rate of the Al₂O₃ disk but increased that of the mating pin. The addition increased the wear rates of both ZrO₂ and Si₃N₄ disks and the mating pins. (2) The average coefficients of friction with the addition of iron particles were greater than those without iron particles. (3) The wear rates of pin and disk depended on the topographies of wear surfaces and the wear index Γ.     

Summarized characteristics for determination of the service life by the wear of industrial ceramicss

The calculation dependence developed based on the theoretical methods and experimental results includes the summarized characteristics to determine the wear rate of industrial ceramics. It takes into account the ceramics’ mechanical and thermophysical properties, the resistance to cracking, the stress-strain state, the thermal condition of the rubbing bodies, and the effect of the environment. The proposed dependence is compared with the available semiempirical dependencies of the ceramics’ wear rate.     

Severity of wear: A new index

A Time Dependent Particle Quantifier (TDPQ) system can measure the concentration of ferrous wear debris suspended in a lubricant, and the severity of wear associated with the particle size of this suspended debris. A coding system is proposed covering: PQ index (total wear); initial TDPQ slope (large particles); and final TDPQ slope (small particles). Correlation with existing measurement systems is also given.     

Animation of the evolution of surface finish with wear

In recent years one of the most significant developments in surface metrology has been the three-dimensional characterization of engineering surfaces. The impetus for this development was primarily motivated by a need for a more realistic visualization of surfaces than the traditional two-dimensional surface profile could provide. There have been several approaches to three-dimensional characterization of engineering surfaces including using time series and fractals. A significant application of these mathematical models would be to use them as the basis for the animation of the evolutionary changes of the surface with wear. By considering simple wear models, this paper demonstrates how computer animation illustrating the evolution of a three-dimensional surface with wear may be realized. Non-Gaussian fractals and time series are used as the basis for three-dimensional characterization and the animation is achieved using the standard morphing techniques provided by Autodesk 3D Studio.     

Sliding metallic wear test with in-process wear measurement: A new approach to collecting and applying wear data

A wear test is described in which the edge of a hard wedge is loaded against the periphery of a rotating disc of softer specimen material. The applied normal load is kept approximately constant during a test. As the test progresses and the disc diameter is reduced by the wear taking place the wedge moves radially inward. By measuring this inward movement during a test it is shown how the wear can be continually monitored. Results are given and it is shown how these might be applied in practice, taking into account the influence of both surface roughness and lubrication.     

A study of mirror-like grinding of fine ceramics with in-process electrolytic dressing

Fine ceramics have the properties of high hardness, chemical inertness, high thermal resistance and low electrical conductivity, but, because of high hardness and brittleness, they are very difficult to machine. Therefore, a superabrasive diamond wheel is used for mirror-like grinding of this material. In this study, an in-process electrolytic dressing system for carrying out mirror-like surface grinding was constructed. Using this system the grinding force for fine ceramics was reduced. This work shows that the application of electrolytic dressing is beneficial in obtaining a mirror-like surface when grinding fine ceramics.     

The development of the modified Hurst orientation transform for the characterization of surface topography of wear particles

A modified Hurst orientation transform (HOT) method for characterization of wear particle surfaces is proposed and described in this paper. The method involves the calculation of self-affine Hurst coefficients in all directions and displays the calculated coefficient values in a form of rose plot. The calculation of individual Hurst coefficients, H, is based on the rescale range (r/s) analysis (r(d)/s∼ d ^H ). The rose plot is then used to obtain three texture surface parameters, i.e.: texture aspect ratio, texture minor axis and texture direction. The effectiveness of this modified HOT and resulting surface texture parameters was evaluated. The method was first applied to computer-generated images of isotropic and anisotropic particle fractal surfaces and then to field emission scanning electron microscope images of wear particles found in synovial joints. The ability of the surface parameters to reveal surface isotropy or anisotropy, measure roughness and determine the dominant direction of surface texture was assessed. The effects of measurement conditions such as noise, gain variations and focusing on the surface parameters were also investigated. The results demonstrate that the HOT and surface texture parameters developed can successfully be used in the characterization of wear particle surface topography. This revised version was published online in August 2006 with corrections to the Cover Date.     

Characterization of wear scar surfaces using combined three-dimensional topographic analysis and contact resistance measurements

In this paper, a technique for the quantitative characterization of wear scar surfaces, using combined three-dimensional topographical analysis and contact resistance measurements, is introduced. Parameters for the characterization of wear surfaces, developed during sliding of pin-on-disk specimens in oxygen at high temperature, such as wear volume, roughness, average wear depth on the disk specimen, surface coverage by wear-protective oxide layers and their distributions over the wear surface, are presented and calculated. Such analyses provide more effective data for the analysis of wear processes and wear mechanisms.This method has been applied to the analysis of dry reciprocating sliding wear of a nickel-base alloy, N80A, at temperatures to 600°C. It was found that there was usually a difference between the wear rates of the pin and the disk. This difference increased with increase in temperature, the wear of the pin being much less than that of the disk at the higher temperatures. Although the total wear of both the pin and the disk decreased considerably with increase in temperature, the damage to the disk, judged by the wear depth of the scar, was much higher at elevated temperatures than at low temperatures. The roughnesses of the wear surfaces generally increased with increase in temperature. Less than 50% coverage of the scar surfaces by wear-protective oxide layers was sufficient for the severe-to-mild wear transition. However, the distribution of the wear-protective layers over the wear surfaces was non-uniform. Most of them were concentrated near the centre of the scar, along the sliding direction, under the present conditions. These features of the wear scar surfaces were mainly related to the adhesion and compaction of wear debris particles onto the wear surfaces, leading to development of the wear-protective layers at the various temperatures.     

A study of contact-start-stop wear tracks by TOF SIMS

In magnetic hard disk drive system, an ultra thin layer of lubricant is coated to the thin film media surface to prevent wear. Under the condition of relative motion, the displacement and replenishment of the lubricant at the head and media contact area are the factors that control the friction and wear behavior of the system. In this study, we investigate the sliding wear disk surface prepared by contact-start-stop (CSS) test using TOF SIMS (Time of Flight Secondary Ion Mass Spectrometry). TOF SIMS is a power tool for surface analysis with both high spatial and high mass resolution. Our investigations show that the lubricant thickness variation of the disk media at the contact area can be captured by sharp ion map images of TOF SIMS, and the thickness can be inferred based on the relative ion fragment intensity. In addition, the composition variation of the slider material and the magnetic layer materials can also be monitored. Finally the sliding effect is analyzed.