On wear as a mechanism of granule attrition

Mechanisms of granule or agglomerate attrition are studied in the light of the principles of fracture mechanics. Fracture properties are measured for agglomerated systems including both glass beads bound by polymeric binders and pesticide products. In the present work, the sizes of typical granules are less than the critical specimen sizes capable of gross fracture, as calculated from fracture theory. This implies that the mode of granule attrition is primarily one of erosion of abrasive wear and not gross fracture. Previous abrasive wear research is reviewed, with the aim of establishing the dependence of agglomerate bar wear on material properties. Idealized bar agglomerates are studied, as they allow convenient characterization of the dependence of granule erosion on material properties. Bar wear rates are foud to parallel results from the ceramics wear literature. In particular, wear rate is found to have a similar but somewhat different dependence on fracture toughness than the work of Mullier et al.⁴, where both fluid-bed granule erosion rate and wear of bar agglomerates were found proportional to 1/K_c.     

Friction of some commercial polymer-based bearing materials against steel

Cylindrical test pins of some commercial polymer-based bearing materials (comprising two nylons 6, a filled nylon 6/6, a filled ultra-high molecular weight polyethene (uhmwpe) and three polyurethanes) were rotated, in dry conditions and at constant load and sliding speed, on circular tracks on stationary discs of steel gauze and abrasive paper.Wear against run-in steel gauze was proportional to the sliding time (distance), with the specific wear rate, v_sp, (wear volume per unit area per unit sliding distance) varying with the nominal pressure, p, according to v_sp = Kp^α. Values of K and α are presented enabling comparison of the fatigue wear of the materials at various loads against steel (or a counterface with rounded asperities) in non-transfer film conditions. Nylon 6 showed the least wear and the polyurethanes showed the greatest wear, up to pressures of 3.43 MN m⁻² (500 lbf in⁻²).With abrasive paper, the circular path became progressively clogged with transfer films and wear debris, and the wear volume, ΔW, diminished with time, t, throughout the test duration, following the relationship ΔW = Dt^c, where both c and D are functions of the wear path diameter. c appears to be related to the film transfer capability of the polymer. The best overall abrasive wear resistance (in transfer film conditions) was exhibited by the filled uhmwpe, followed by two polyurethanes. Nylon 6 showed relatively poor abrasion resistance under these conditions. The mechanical properties indicate, with one exception, a similar ranking order for non-transfer film conditions     

Abrasive wear of some commercial polymers

Cylindrical test pins of some commercial polymer-based bearing materials (comprising two nylons 6, a filled nylon 6/6, a filled ultra-high molecular weight polyethene (uhmwpe) and three polyurethanes) were rotated, in dry conditions and at constant load and sliding speed, on circular tracks on stationary discs of steel gauze and abrasive paper.Wear against run-in steel gauze was proportional to the sliding time (distance), with the specific wear rate, v_sp, (wear volume per unit area per unit sliding distance) varying with the nominal pressure, p, according to v_sp = Kp^α. Values of K and α are presented enabling comparison of the fatigue wear of the materials at various loads against steel (or a counterface with rounded asperities) in non-transfer film conditions. Nylon 6 showed the least wear and the polyurethanes showed the greatest wear, up to pressures of 3.43 MN m^?2 (500 lbf in^?2).With abrasive paper, the circular path became progressively clogged with transfer films and wear debris, and the wear volume, ΔW, diminished with time, t, throughout the test duration, following the relationship ΔW = Dt^c, where both c and D are functions of the wear path diameter. c appears to be related to the film transfer capability of the polymer. The best overall abrasive wear resistance (in transfer film conditions) was exhibited by the filled uhmwpe, followed by two polyurethanes. Nylon 6 showed relatively poor abrasion resistance under these conditions. The mechanical properties indicate, with one exception, a similar ranking order for non-transfer film conditions     

The effect of abrasive grain size on the transition between abrasive and adhesive wear

Experiments were carried out in which SiC abrasives with a grain size range of 3–150 μm were inserted between sliding metals. The metals were pure aluminium, copper, iron, nickel and zinc. The test geometry was a tube end against a flat surface. The effect of grain size can be classified into three regimes. In the first, where abrasive grains are larger than a critical size d_c (about 50 μm), the wear rate is independent of grain size. In the second regime the wear rate decreases as abrasive grains become smaller than d_c to a limit at a transition grain size d_t (about 10 μm). In the third regime the wear rate is high and independent of abrasive grain size. The wear debris consists of large metal flakes with abrasive particles mixed in. Although abrasive particles are present, the wear is primarily adhesive, and the action of the abrasive particles is to promote the removal of metallic wear debris from the contact region.     

Abrasive wear behaviour of B4C particle reinforced Al2024 MMCs

In this study, the effects of volume fraction and particle size of boron carbide on the abrasive wear properties of B₄C particle reinforced aluminium alloy composites have been studied. For this purpose, a block-on-disc abrasion test apparatus was utilized where the samples slid against the abrasive suspension mixture at room conditions. The volume loss, specific wear rate and roughness of the samples have been evaluated. The effects of sliding time, particle content and particle size of B₄C particles on the abrasive wear properties of the composites have been investigated. The dominant wear mechanisms were identified using scanning electron microscopy. The results showed that the specific wear rate of composites decreased with increasing particle volume fraction. Furthermore, the specific wear rate decreased with increasing the size of particle for the composites containing the same amount of B₄C. Hence, it is deduced that aluminium alloy composites reinforced with larger B₄C particles are more effective against the abrasive suspension mixture than those reinforced with smaller B₄C particles.     

Effect of brittleness index and sliding speed on the morphology of surface scratching in abrasive or erosive processes

In static identation of brittle surfaces, fracture and deformation operate on a comparable scale. K_c/H governs the ductile or brittle nature of the contact, hence H/K_c is a convenient brittleness index. A pendular scratching test was developed which extends to dynamic contact analytical results pertaining to static identation: the objective was to establish investigative means necessary for the selection of semi-brittle, hard coatings subject to abrasive or erosive wear. Different semi-brittle surfaces were tested. A transition scratching depth from ductile to brittle abrasion was observed. the static brittleness index, K_c/H, is connected directly to this scratching transition depth. The pendular scratching test permits determination of K_c/H, and thereby the toughness, K_c, of hard coatings. Prediction of the morphology of surface scratching is possible using this brittleness index. Brittle scratch susceptibility can be related directly to poorer wear resistance.     

Assessing the corrosion–wear behaviours of Hastelloy C276 alloy in seawater

A systematic investigation has been carried out in the present work to study the electrochemical and corrosion–wear behaviours of Hastelloy C276 alloy sliding against Al₂O₃ pin in artificial seawater, using a pin‐on‐disc tribometer integrated with a potentiostat for electrochemical control. It can be observed that the cathodic shift of open circuit potential and three order of magnitude increase of current density formed due to sliding. The total corrosion–wear loss increases with increasing applied potential. Interestingly, the total material loss at the applied potential of 0.5 and 0.9 V is more than two times of that of pure mechanical wear, confirming the synergy between wear and corrosion. And, the contributions of wear‐induced corrosion (ΔK_c) and corrosion‐induced wear (ΔK_w) are dominant, especially at higher applied potentials. Copyright © 2015 John Wiley & Sons, Ltd.     

Computational investigation of testing parameter effects on abrasive wear behaviour of AL2O3 particle-reinforced MMCS using statistical analysis

The wear rate model of 7.3?vol.% Al₂O₃ particle-reinforced aluminium alloy composites with 16 and 66???m particle sizes fabricated by molten metal mixing method was developed in terms of applied load, particle size of reinforcement, abrasive grain size and sliding distance based on the Taguchi method. The two-body abrasive wear behaviour of the specimens was investigated using a pin-on-disc abrasion test apparatus where the sample slid against different SiC abrasives under the loads of 2 and 5?N at the room conditions. The orthogonal array, signal-to-noise ratio and analysis of variance were employed to find out the optimal testing parameters. The test results showed that particle size of reinforcement was found to be the most effective factor among the other control parameters on abrasive wear, followed by abrasive grain size. Moreover, the optimal combination of the testing parameters was determined and predicted. The predicted wear rate results were compared with experimental results and found to be quite reliable.     

Two-body abrasion of some polymers against 6–50 μm SiC abrasives

Single- and multiple-pass two-body abrasion tests were run on Nylon $\text{6}\text{6}\text{+ 20\%}$ polytetrafluoroethylene (PTFE) and polycarbonate + 10% PTFE sliding dry against 6–50 μm SiC abrasives. A functional relationship was developed between the single-pass wear rate and the abrasive particle size for abrasive particle sizes less than or equal to 10.4 μm. The single-pass abrasive wear rate was 20–40 times greater than the multiple-pass wear rate for each material when it was slid against abrasive grains with a mean size not exceeding 10.4 μm. This was due to the formation of loose polymer fibril wear debris in single-pass sliding and of transferred plateaux of polymer in multiple-pass sliding. The rate of increase in wear with particle size was about 20 times greater for single-pass sliding than for multiple-pass sliding. Above a mean abrasive particle size of 10.4 μm the type of mechanism in both single-pass and multiple-pass sliding was that of ploughing.     

Abrasive wear performance of carbon fabric reinforced polyetherimide composites: Influence of content and orientation of fabric

Dry abrasive wear performance of five plain weave carbon fabric (CF) reinforced Polyetherimide (PEI) composites, developed with increasing CF contents (in the step of 10 vol%) is reported in this paper. It was observed that composite reinforced with 65 vol% CF (IP₆₅) exhibited the best tensile and shear strength and closely followed the leader (IP₇₅) in flexural strength. IP₆₅ when abraded against silicon carbide paper showed highest wear resistance (W_R) and lowest friction coefficient (μ) among all composites. The composites IP₈₅ and IP₄₀ containing highest and lowest amount of CF respectively showed least enhancement in strength properties and poorest wear performance. Parallel studies on the influence of fabric orientation with respect to the sliding plane and direction, on W_R showed that when CF was oriented parallel to the sliding plane, it had poorest wear resistance. The performance improved for the composites when fabric was oriented normal to the plane. The parallel or anti-parallel orientation of warp or weft fibers with respect to sliding direction showed marginal changes in friction and wear performance. Wear mechanisms were suggested and strongly supported by worn surface analysis using SEM.Efforts were also done to investigate the wear-property correlation. It was observed that the W_R was directly proportional to the product of interlaminar shear strength (I_s) and elastic modulus (E). Fairly good linearity was shown for specific wear rate (K₀) as a function of factor (μP/I_sE) where μ is coefficient of friction and P is the normal pressure (N/mm²).     

Tribological Behavior of an Aluminum Matrix Composite with Al4SiC4 Reinforcement under Dry Sliding Condition

In the present research work, an aluminum-based metal matrix composite with in situ Al₄SiC₄ particles has been developed by the incorporation of TiC particles in commercial aluminum melt through a stir-casting method. Microstructure evaluation in correlation to developed hardness and mechanical properties was performed. Furthermore, the dry sliding wear behavior of commercial aluminum and commercial aluminum–5 vol% Al₄SiC₄ composite was investigated at low sliding speed (1 ms^?1) against a hardened EN 31 disk at different loads. The wear mechanism involved adhesion and microcutting–abrasion at lower loads. On the other hand, at higher loads, abrasive wear involving microcutting along with adherent oxide formation was observed. The overall wear rate increased with load in the alloy as well as in the composite. Moreover, the overall wear rate of the composite was lower than that of the commercial aluminum at all applied loads.` The severe wear region at 39.2 N load in the case of the commercial aluminum–5 vol% Al₄SiC₄ composite was found to be delayed up to a longer sliding distance compared to commercial aluminum. The in situ Al₄SiC₄ particles offered resistance to adhesive wear. Accordingly, the commercial aluminum–5 vol% Al₄SiC₄ composite exhibited superior wear resistance compared to the commercial aluminum.     

Effect of applied potential on the tribocorrosion behaviors of Monel K500 alloy in artificial seawater

A systematic investigation has been carried out in the present work to study the electrochemical and tribocorrosion behaviors of Monel K500 alloy sliding against Al₂O₃ pin in artificial seawater. It can be observed that the cathodic shift of open circuit potential and two order of magnitude increase of current density formed due to sliding. The total tribocorrosion volume loss increases with increasing applied potential. Interestingly, the total material loss at applied potential of 0.5 V and 0.9 V is more than three times of that under pure mechanical wear, confirming the synergy between wear and corrosion. The contribution of wear-induced-corrosion (ΔK_c) and corrosion-induced-wear (ΔK_w) are dominant especially at high applied potential.     

Prediction of abrasive wear rate of in situ Cu–Al2O3 nanocomposite using artificial neural networks

In this work, artificial neural networks (ANNs) technique was used in the prediction of abrasive wear rate of Cu–Al₂O₃ nanocomposite materials. The abrasive wear rates obtained from series of wear tests were used in the formation of the data sets of the ANN. The inputs to the network are load, sliding speed, and alumina volume fraction. Correlation coefficients between the experimental data and outputs from the ANN confirmed the feasibility of the ANNs for effectively model and predict the abrasive wear rate. The comparison between the ANNs and the multi-variable regression analysis results showed that using ANNs technique is more effective than multi-variable regression analysis for the prediction of abrasive wear rate of Cu–Al₂O₃ nanocomposite materials. Optimization of the training process of the ANN using genetic algorithm (GA) is performed and the results are compared with the ANN trained without GA. Sensitivity analysis is also done to find the relative influence of factors on the wear rate. It is observed that load and alumina volume fraction effectively influence the wear rate.     

Microstructure and Tribological Properties of a HfB<Subscript>2</Subscript>-Containing Ni-Based Composite Coating Produced on a Pure Ti Substrate by Laser Cladding

A HfB₂-containing Ni-based composite coating was fabricated on Ti substrates by laser cladding, and its microstructure and tribological properties were evaluated during sliding against an AISI-52100 steel ball at different normal loads and sliding speeds. The morphologies of the worn surfaces were analyzed by scanning electron microscopy (SEM) and three-dimensional non-contact surface mapping. The results show that wear resistance of the pure Ti substrate and NiCrBSi coating greatly increased after laser cladding of the HfB₂-containing composite coating due to the formation of hard phases in the composite coating. The pure Ti substrate sliding against the AISI-52100 counterpart ball at room temperature displayed predominantly adhesive wear, abrasive wear, and severe plastic deformation, while the HfB₂-containing composite coating showed only mild abrasive wear and adhesive wear under the same conditions.     

Improvement of the Oxidation and Wear Resistance of Pure Ti by Laser-Cladding Ti<Subscript>3</Subscript>Al Coating at Elevated Temperature

Ti₃Al coating was in situ synthesized successfully on pure Ti substrate by laser-cladding technology using aluminum powder as the precursor. The composition and microstructure of the prepared coating were analyzed by transmission electron microscopy, scanning electron microscopy (SEM), and X-ray diffraction technique. Thermal gravimetric analysis was used to evaluate the high-temperature oxidation resistance of the Ti₃Al coating. The friction and wear behavior was tested through sliding against Si₃N₄ ball at elevated temperature of 20, 100, 300, and 500°C. The morphologies of the worn surfaces and wear debris were also analyzed by SEM and three-dimensional non-contact surface mapping. The results show that the Ti₃Al coating with high microhardness, high-temperature oxidation resistance, and high temperature wear resistance. The pure Ti substrate is dominated by severe adhesion wear, abrasive wear, fracture, and severe plastic deformation at lower temperature, and severe adhesion wear, abrasive wear, plastic deformation, oxidation, and nitriding wear at higher temperature, whereas the Ti₃Al coating experiences only moderate abrasive and adhesive wear when sliding against the Si₃N₄ ceramic ball counterpart. In addition, the wear debris of the laser-cladding Ti₃Al coating sliding and Si₃N₄ friction pairs are much smaller than that of pure Ti substrate and Si₃N₄ friction pairs at elevated temperature.     

Effect of friction cycles on wear particle generation of carbon nitride coating against a spherical diamond

The in-situ observations of wear particle generation of carbon nitride coating on silicon repeatedly sliding against a spherical diamond have been studied in terms of the critical friction cycles and normal loads. An environmental scanning electron microscope (E-SEM), in which a pin-on-disk tribotester was installed, has in-situ provided direct evidence of when and how the wear particle generation do occur during the repeated sliding of carbon nitride coating against a spherical diamond. The in-situ observations of non-conductive carbon nitride coating are therefore available free from surface charging with controllable relative humidity. The repeated sliding tests at a sliding speed of 50 μm/s have been carried out with the purpose of observing the ‘No wear particle generation’ region when varying normal load from 10 to 250 mN. It appears that until 20 friction cycles, the maximum Hertzian contact pressure P_max for ‘No wear particle generation’ can be improved from 1.39 Y to 1.53 Y if silicon is coated by carbon nitride with a thickness of 10 nm, where Y is defined as the yield strength of silicon. The applicable enlargement of the ‘No wear particle generation’ region of carbon nitride coating has therefore been comparatively discussed with the silicon substrate from the view points of the friction coefficient and the specific wear rate. The mode transition maps have also been summarized for the repeated sliding of carbon nitride coating in terms of ‘No wear particle generation’, ‘Wear particle generation by microcutting’ and ‘Wear particle generation by microcutting and microfracturing’ three typical modes.     

Dimensional analysis for abrasive wear behaviour of various polyamides

Abrasive wear behaviour of a series of polyamides (PAs) with different methylene to amide ratio (CH₂/CONH) was analysed using Buckinghams dimensional analysis method and efforts for quantifying the contribution of the material properties towards the abrasive wear performance were also made. In order to calculate the wear coefficient (K), the data based on the experimental wear volume, operating parameters and the material properties were fitted into the non-linear wear equation. The non-linear wear equation was derived based on pi theorem using the dimensional analysis technique. The wear coefficient K decreased as load and abrasive grit size were increased. The theoretical and experimental wear volume correlated well in most of the cases. Among the selected material properties, the fracture stress (_*) and the critical crack length (C_*) were found to be the most important parameters, which controlled the abrasive wear behaviour of PAs.     

Embeddability behaviour of some Pb-free engine bearing materials in the presence of abrasive particles in engine oil

One of the tribological requirements on engine bearing material is its ability to safely embed contaminant particles onto its surface and minimise damage to both the bearing and crankshaft surfaces. In this work, a journal bearing test rig that operates under constant load has been employed to investigate the embeddability behaviour of selected multi-layered Pb-free engine bearing materials at three different rotational speeds using engine oil contaminated with SiC particles. Experimental results have shown that third-body abrasive wear is influenced by the lubricant film thickness. There was also difference in embeddability of the different materials. Bismuth-based overlay and MoS₂ containing polyamide-imide-based overlay-coated materials show higher wear compared to tin-based overlay and a polyamide-imide-based composite overlay-coated material. Steel counter surfaces sliding against bismuth-based overlay and MoS₂ containing polyamide-imide-based overlay exhibited higher wear than those sliding against tin-based overlay and polyamide-imide-based composite overlay.     

Wear study of Ni–WC composite coating modified with CeO2

In the present investigation, Ni–WC composite powder was modified with the addition of CeO₂ in order to form a new composition of Ni–WC–CeO₂. The Ni–WC and Ni–WC–CeO₂ compositions were used for coating deposition by high-velocity oxy-fuel (HVOF) spraying process so as to study the effect of CeO₂ addition on microstructure, distribution of various elements, hardness, formation of new phases, and abrasive wear behavior. Further, the effect of load, abrasive size, sliding distance, and temperature on abrasive wear behavior of these HVOF-sprayed coatings was investigated by response surface methodology. To investigate the abrasive wear behavior of HVOF-sprayed coatings four factors such as load, abrasive size (size in micrometers), sliding distance (meters), and temperature (°C) with three levels of each factor were investigated. Analysis of variance was carried out to determine the significant factors and interactions. Investigation showed that the load, abrasive size, and sliding distance were the main significant factors while load and abrasive size, load and sliding distance, abrasive size and sliding distance were the main significant interactions. Thus an abrasive wear model was developed in terms of main factors and their significant interactions. The validity of the model was evaluated by conducting experiments under different wear conditions. A comparison of modeled and experimental results showed 4–9% error. The abrasive wear resistance of coatings increases with the addition of CeO₂. This is due to increase in hardness with the addition of CeO₂ in Ni–WC coatings.     

Two-Body Abrasive Wear of the Outside Shell Surfaces of Mollusc <Emphasis Type="Italic">Lamprotula fibrosa</Emphasis> Heude, <Emphasis Type="Italic">Rapana venosa</Emphasis> Valenciennes and <Emphasis Type="Italic">Dosinia anus</Emphasis> Philippi

Natural biological surfaces and biomaterials have some distinguishing properties for adapting themselves to natural surroundings. The outside shell surfaces of mollusc species often undergo the abrasive wear action from the sand particles in water sand slurry in natural conditions. The two-body abrasive wear behavior of the outside shell surfaces of three mollusc species Lamprotula fibrosa Heude, Rapana venosa Valenciennes and Dosinia anus Philippi was examined. Abrasive material used for tests were quartz sand (96.5 wt.%) with three different size ranges and powdered bentonite (3.5 wt.%). The two-body abrasive wear tests were run on a rotary disc type abrasive wear testing machine. The results showed that the abrasion resistance of the outside shell surfaces of the three mollusc species was higher when the corrugations on the shell surfaces were perpendicular to the sliding direction of the abrasive material than that when the corrugations on the shell surfaces were parallel to the sliding direction of the abrasive material. Basically, the shell of Lamprotula fibrosa Heude possessed the highest abrasion resistance among the three species of shell; the abrasion resistance of the shell of Rapana venosa Valenciennes was the lowest; and the abraded depth of the three species of shell increased with an increased abrasive particle size and relative sliding velocity. The abraded surfaces were observed with scanning electron microscope.