Chemical aspects of wear of alumina ceramics

In our investigation, the effects of the tribochemically-induced dissolution of alumina ceramics and modulation of near-surface forces (surface charge) within the tribocontact were studied. The wear and friction behaviors of alumina were investigated using a reciprocating sliding test in different chemical environments. The samples for the tests were hemispherical pins and plates of polished alumina, both prepared by a near-net-shaping method.

The sliding tests were conducted in water-based liquids with different pH values or with the addition of a polyelectrolyte to control the surface charge at solid surfaces. The coefficient of friction was continuously recorded during the tests and the wear-loss was subsequently determined for all samples. The results show a significant effect of the chemical agents on the coefficient of friction as well as on the material-removal rate in different aqueous media. The results are discussed in terms of the chemical and electrochemical properties of the materials in the tribocontact.     

An AFM study of single-contact abrasive wear: The Rabinowicz wear equation revisited

A nanoscale study of the abrasive wear behaviour of a ductile monophasic metallic alloy (the stainless steel AISI 316L) is presented. By using atomic force microscopy (AFM) based techniques, particularly a diamond tip mounted on a stiff steel cantilever, the contact of a single abrasive asperity was simulated, and it was possible to determine accurately the load threshold below which no measurable wear occurs. It was observed that, once this nanoscale threshold for wear is overcome, the worn volume increases linearly with the load, as predicted by the Rabinowicz model. However, it was found that, although this critical threshold for measurable wear is most certainly related with the yield-onset of plastic deformation, it cannot be predicted by using directly a criterion based on the bulk microhardness. Hence, the results presented in this paper strongly indicate that indentation size effects play a crucial role on the response to abrasive wear at the asperity contact level.     

The correlation of abrasive wear tests

Previous attempts to correlate theoretically the results obtained from some simple abrasive wear tests are reviewed briefly. While these theories all predict the behaviour of pure metals quite well, they fail with heat-treated alloys. It has been suggested that elastic deflections in these alloys may change significantly the shapes and depths of the scratches formed when they are abraded, but surprisingly no attempt has yet been made to include them in a model of abrasion. In this paper a simple plastic model of abrasion is modified to allow for elastic effects. An expression is deduced relating the abrasion resistance of a metal to its hardness and Young's modulus, and the values it predicts are found to agree well with a wide range of published experimental results.     

Importance of the triboemission process for tribochemical reaction

Chemical reactions under boundary lubrication conditions are distinct from those of thermochemical ones. Flash temperature is very short and holding in esteem the mechanically triggered chemistry and thermally triggered chemistry at the contact of asperities caused by the flash temperature effect, the following question arises. ‘Can this reaction initiation process be considered in terms of overcoming the activation energy by heat or by another form of energy’? The purpose of this paper is to discuss that question in terms of the hypothesis: The common denominator of tribochemical reactions is that they might be initiated by low-energy electrons. This is consistent with the negative-ion-radical action mechanism (NIRAM) approach assuming that tribochemical reactions are initiated by electrons in the energy-range of 1–4 eV. The hypothesis is also relevant to the next author's hypothesis saying that the intermediate reactive species of both tribochemical reactions and some heterogeneous catalytic reactions are produced by the same mechanism, governed by the NIRAM approach. The catalyst effect on a chemical reaction is to increase its rate. The reaction rate is further increased when the catalyst is under friction (tribocatysis). The primary objective of this work is to better understand mechanisms of tribochemical reactions and catalytic processes. To initiate thermochemical reactions, heat should be supplied. The same is due to heterogeneous catalytic reactions, however, the catalyst lowers the reaction activation energy. Even a very high calculated flash temperature is short lived, thus, it rather cannot initiate tribochemical reactions by heat. The present author assumes that flash temperature can be expressed in the form of the thermionic emission. Bearing this in mind, it is possible to hypothesize that heterogeneous catalytic reactions are also initiated by thermal electrons. Accordingly, the tribocatalytic process might be initiated and/or enhanced by triboelectrons. Important objective of this work is to better understand both tribochemical reactions and catalytic/tribocatalytic processes. Tribochemistry of simple environment friendly lubricant compounds and examples of water synthesis catalytic and tribocatalytic processes are presented and discussed.     

Three-body abrasive wear of TiC–NiMo cermets

The mechanism of three-body abrasive wear of TiC-base cermets was studied. The wear rate of a series of cermets with different percentage of NiMo binder phase (20–60 wt%) was studied. Silica sand was used as an abrasive. The wear rate of the cermets decreases with the increase of TiC and Mo content, which corresponds to the increase in the bulk hardness. The post-run wear tracks of the worn blocks were analyzed with SEM. The material is removed by several processes such as extrusion and removal of the binder and also fractures of the carbide grains and the carbide network.     

Cermets surface transformation under erosive and abrasive wear

The subsurface developed in ceramic-metal composites WC-Co, TiC-NiMo and Cr₃C₂-Ni during tribological testing (abrasion, sliding, erosion) under different conditions (impact angle, velocity, pressure, temperature) is the primary concern of the study. Mechanisms responsible for mechanically mixed layer (MML) development and wear resistance of materials are discussed in details. Instrumented indentation combined with consecutive polishing-testing procedure was used for mechanical characterisation. Microstructural features of the worn surface and subsurface region were studied with the help of optical microscope and scanning electron microscope equipped with energy dispersive spectroscopy analyser to evaluate difference between properties of the bulk and modified subsurface layers of materials.Formation of subsurface layer is found to be an essential feature of materials response to applied loading. Modified layer consists of highly deformed binder metal; cracked and decohesed large grains of carbides; embedded and/or fused debris of erosive/abrasive particles; and products of oxidation.     

Diamond particle shape: Its measurement and influence in abrasive wear

The classification of diamond particles in terms of their abrasive characteristics is addressed in this work. Specifically, diamond particles of different grades have been studied in terms of their shape to identify useful trends and correlations with experimental wear rate. Ten diamond types, typically used by the abrasives industry and exhibiting varied shape, were selected. They included highly geometric single crystals, crushed single crystals, and polycrystalline diamond particles, with nominal diameters of between 65 and 197 μm. Electronic boundary projections were obtained using a digital-camera-equipped optical microscope, which were then processed using proprietary software. The parameters calculated include: diameter (minimum, minor and maximum), aspect ratio, convexity and sharpness. Interesting correlations were found between convexity and sharpness that engendered both these parameters to be considered as useful measures of wear rate. This was reinforced by experimental wear tests, using grinding wheels manufactured from six of the 10 diamond types, which demonstrated excellent correlations of sharpness (0.991 correlation coefficient), and convexity (0.987 correlation coefficient), with the wear rate of a polyurethane workpiece.     

Using parallel scratches to simulate abrasive wear

Abrasive wear is currently classified according to different particle dynamics: (a) the sliding of active particles on the sample surface and (b) the rolling of abrasive particles between the surfaces. In this paper, instrumented laboratory tests are used to present a new methodology for the simulation of abrasive wear. The rolling of the abrasives is represented by a sequence of indentations, and the sliding of the active particle by a sequence of scratches. A new piece of equipment was especially developed to reproduce the action of an abrasive particle. Two high resolution sliders drive the sample horizontally while the indenter is moved vertically by another slider. Besides this, a high resolution piezoelectric translator is used to control the indenter movement while a 3D load cell controls the intensity of the process. A worn surface produced in a rubber wheel abrasive wear test was used as the reference for the simulation. Its topography was assessed by using laser interferometry and scanning electron microscopy and showed that the prevailing wear mechanism was parallel scratches. The results showed that the superimposition of scratches is the basis which makes it possible to correlate topographical parameters of the reference to the controlling variables used in the simulation. A special method to describe the average depth of the scratches in function of the distance between them (superimposition) was developed. Wear occurs when superimposition is greater than 80%. The average depth of the scratches increased according to an elevation in the degree of superimposition and to the augmentation of normal load. This simulation methodology produced a surface topographically and morphologically similar to that of the reference.     

Wear behaviour of alumina based ceramic cutting tools on machining steels

The advanced ceramic cutting tools have very good wear resistance, high refractoriness, good mechanical strength and hot hardness. Alumina based ceramic cutting tools have very high abrasion resistance and hot hardness. Chemically they are more stable than high-speed steels and carbides, thus having less tendency to adhere to metals during machining and less tendency to form built-up edge. This results in good surface finish and dimensional accuracy in machining steels. In this paper wear behaviour of alumina based ceramic cutting tools is investigated. The machining tests were conducted using SiC whisker reinforced alumina ceramic cutting tool and Ti[C,N] mixed alumina ceramic cutting tool on martensitic stainless steel-grade 410 and EN 24 steel work pieces. Flank wear in Ti[C,N] mixed alumina ceramic cutting tool is lower than that of the SiC whisker reinforced alumina cutting tool. SiC whisker reinforced alumina cutting tool exhibits poor crater wear resistance while machining. Notch wear in SiC whisker reinforced alumina cutting tool is lower than that of the Ti[C,N] mixed alumina ceramic cutting tool. The flank wear, crater wear and notch wear are higher on machining martensitic stainless steel than on machining hardened steel. In summary Ti[C,N] mixed alumina cutting tool performs better than SiC whisker reinforced alumina cutting tool on machining martensitic stainless steel.     

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.     

Scratch and wear performance of prosthetic femoral head components against crosslinked UHMWPE sockets

Total hip arthroplasty is a highly successful procedure where the hip joint is replaced by an artificial ball and socket joint. Bearing wear continues to be a contributing factor to implant failure. Prosthetic femoral heads roughen in vivo which leads to increased wear. Along with the introduction of improved polyethylene which reduces wear by up to 99%, improved femoral head materials have been introduced to improve resistance to abrasion. The abrasion resistance of two of these improved femoral heads was assessed in this study and compared to a cobalt chromium (CoCr) femoral head. The resulting wear performance against a polyethylene acetabular component was assessed. The bulk ceramic (zirconia toughened alumina) femoral head exhibited superior abrasion resistance compared to CoCr (97% reduction in damage) as well as reduced wear after abrasion (97% reduction in wear). The oxide coated zirconium niobium femoral head showed inferior abrasion resistance compared to CoCr (99% increase in damage) as well as increased wear after abrasion (161% increase in wear). Both femoral head surfaces utilize hard ceramic materials, however, the thin ceramic coating on top of a softer metallic substrate of the oxide coated bearing was unable to withstand aggressive abrasion.     

Influence of local soil conditions on mouldboard ploughshare abrasive wear

A major problem related to use of tillage equipment is ploughshare wear due to abrasion by soil hard particles, as it seriously affects tillage quality and agricultural production economy. In the present study it is attempted to relate ploughshare cutting edge wear to soil conditions, such as hardness of the soil particles, moisture content and particle granular composition, plough conditions and share surface hardness. Soil moisture exerts a stronger effect upon wear than soil type characterized by its granular composition and an increase in the former decreases wear, with sandy soil as the exception. Wear increases, as plough area or plough time increases. A strong correlation was detected between soil type and share wear for different values of share hardness. Useful predictive regression models were developed, when possible, towards wear reduction by selection of appropriate tillage factors resulting to prolonged effective use of shares for the aforementioned cases.     

Characterization of abrasive grain’s behavior and wear mechanisms

Grinding is a finishing process largely used in motor industry, aeronautics, space industry and precision cutting tool manufacturers. The grinding process can be summarized by the action of a grinding wheel on a workpiece. The wheel is constituted by abrasive grains. Thus grinding is in fact the action of grains on the workpiece. The grain behavior changes according to numerous parameters (geometry, mechanical characteristics, wear mechanisms). In some cases abrasive wear is observed while micro-cutting is obtained in some other cases.In this paper two useful and complementary experimental approaches for the interface physics understanding is presented. The study of the cutting power is carried out using a high-speed scratch test device in order to understand the grain behavior and the wear mechanisms for several wheel surface speeds. In this paper an approach for the specific abrasion energy computation is also presented.     

High-temperature abrasive wear testing of potential tool materials for thixoforming of steels

High temperature abrasive wear performance of Inconel 617, Stellite 6 alloys and X32CrMoV33 hot work tool steel was investigated. The wear resistance of the latter is degraded at 750 °C due to its inferior oxidation resistance. Extensive oxidation co-occuring with abrasive wear at 750 °C leads to substantial material loss due to the lack of a protective oxide scale, sufficiently ductile to sustain the abrasion without extensive spalling. The wear resistance of the Inconel 617 and Stellite 6 alloys, on the other hand, improves at 750 °C owing to protective oxides that sustain the abrasion without spalling.     

Study of abrasive wear phenomena in dry and slurry 3-body conditions

Machinery and equipment used in abrasive environments, such as the mining industry, suffer from severe wear. In order to understand wear and to prolong the life time of the machinery, it is important to understand how materials respond to wear depending on the environmental and tribological conditions imposed.This paper exposes a comparative study between the influence of two abrasive environments (dry and slurry) on hard particle coatings and steels. To study this, the 3-body wear behaviour was evaluated in a dry environment using a continuous abrasion test (CAT) and in a slurry environment using a slurry steel wheel abrasion test (SSWAT) method. Both tests are capable of experimentally modelling the high stress wear at 45 N and 216 N, using quartz sand as an abrasive. The tests were performed on two types of coatings processed by sintering and hardfacing and martensitic steel was used as a reference. The wear was indicated as volume loss by measuring the samples before and after the tests. Furthermore, the specific wear energy was calculated in order to have a fundamental understanding about the material's response to wear. A correlation between the wear rate and the particle brakeage index (PBI) was done for the dry conditions using different loads, in order to explain the interdependence between the two parameters and the change in the wear mechanism between the two loads. The influence of load on the wear of the materials showed different wear mechanisms on coatings compared to the steel in the same environmental conditions. However, a change in wear mechanism at different load levels was observed, which might be directly dependent on the change of the particle's motion from sliding to rolling combined with the change in their shape and size. The results showed that the need to study the influence of different abrasive conditions on the material wear is crucial in order to improve the lifetime and the cost efficiency of the machinery used in such environments. The hard-particle coatings showed comparatively low wear rates promising a great potential in improving the lifetime of industrial equipments in different environments.     

Enhancement of abrasive wear resistance in consolidated Al matrix composites via extrusion process

Abstract

The present study addresses the dry wear behaviour of aluminium matrix composites under different sliding speeds and applied loads. Values of the friction coefficient of the matrix alloy and composite materials were in expected range for light metals in dry sliding conditions. The higher coefficient of friction was the consequence of established contact between hard SiC particles and the counter body material. The rough and smooth regions are distinguished on the worn surface of the composites similar to the unreinforced Al alloy. Plastic deformation occurred when the applied specific load was higher than the critical value. The high shear stresses on the sliding surface cause initiation and propagation of the cracks in the subsurface, leading to the loss of material from the worn surface in the form of flakes. The debrises of the composites at low wear rate comprise a mixture of the fine particles and small shiny metallic plate-like flakes and are associated with the formation of more iron rich layers on the contact surfaces.     

Review of oxidational wear: Part I: The origins of oxidational wear

This is the first part of a two-part in-depth review of the oxidational wear of metals. It discusses the parallelism between the formation of an oxide film for dry contact conditions and of other surface films for lubricated contacct. Wear modes are unified into two major classes of mild and severe wear, including both lubricated both dry and conditions. Oxidational wear is a mechanism of mild wear in which protective oxide films are formed at the real areas of contact (during the time of a contact) at the contact temperature, T_cc. When the oxide reaches a critical thickness ξ, usually 1 to 3 μm, the oxide breaks up and eventually appears as wear particles. These oxides are preferentially formed on plateaux, which alternately carry the load - as they reach their critical thickness - and are removed. Temperature is important in determining the structure of the oxide film present, which in turn affects the wear properties of the sliding interfaces. Hence, this part of the review concludes with a thorough treatment of the thermal aspects involved during the sliding of a typical laboratory simulation of the oxidational wear of steel specimens without lubrication. This treatment shows how the general surface temperature (T_s) and the division of heat (?)_at the interface can be calculated and used, in conjunction with the measured wear rate (w), to give information about a possible surface model consisting of N contracts on the (thermally expanded) operative plateau, the height of the plateau being identical to the critical oxide film thickness (ξ) mentioned above.Part II outlines recent research to determine the oxidational constant, ie the activation energy and the Arrhenius constant, relevant to oxidational wear. It is found that the Arrhenius constant for oxidational wear is different from that for static oxidation tests. Some typical values of N, ξ and T_c are calculated from oxidational wear experiments. A new oxidational wear theory designed to take into account the oxide growth which occurs at the general surface temperature, T_s (where T_s < T_c) , whilst operation plateau is out-of-contact. This theory is most relevant to weat at elevated temperatures, where it is not permissible to assume that out-of-contact oxidation is negligible. After a brief review of the small amount of work done on the effects of partial oxygen pressures on oxidational wear, Part II concludes with a discussion of the possible connection between the general oxidational wear theory for dry contacts and the wear which occurs at lubricated contacts.     

Preliminary study of micro-scale abrasive wear of a NiTi shape memory alloy

Nitinol (NiTi), a nearly equiatomic nickel-titanium alloy is known for its unique shape memory and superelastic properties, which result from martensitic transformations. It is the material of choice for numerous biomedical applications such as endovascular stents, vena cava filters, dental files and guidewires for non-invasive surgery, etc. Micro-scale abrasion tests (MSATs) have been performed on the NiTi shape memory alloy, so as to evaluate the influence of different commercial abrasives such as silicon carbide, alumina and glass on the wear behaviour. The aim of the work was the selection of the most effective abrasive for cleaning the inner surface of laser-cut cardiovascular stents. Abrasive particles have been characterised by X-ray diffraction, SEM and EDS before and after MSATs. Worn surfaces have been studied by stylus profilometry, SEM and atomic force microscopy. The effect of abrasive particle hardness, size and angularity on the wear behaviour has been evaluated and discussed.     

The abrasive wear of metals

A quite general relationship exists between abrasive wear and the ploughing contribution to friction. Tensile stresses are developed in the material displaced by the ploughing indenter and these give rise to wear particles.     

Correlation between the surface quality and the abrasive grains wear in optical glass lapping

The subject of this study is to determine the relation between the optical glass surface quality and the wear of abrasive grains used in finishing process. The glass surface quality was characterized by the roughness (rms,CLA and peak to valley). Alumina abrasive grains (Al₂O₃) are used with average sizes (80, 40, 20, 7 μm) respectively. After 2 min lapping for each fraction of grains the following RMS are obtained 1.39, 0.57, 0.51, 0.33 μm. The corresponding peak to valley are respectively 7.5, 3.66, 2.88, 2.10 μm. The grains wear was characterized by the grains edges wear (roundness) and by their fractures. An optical microscope (CMM Scope Check) and a SEM are used for their observation. The alumina grains size distribution was also studied using a laser diffraction particle size analyzer (Shimadzu Sald-2001).