Effect of grain refinement and modification on the dry sliding wear behaviour of eutectic Al-Si alloys

The present study deals with an investigation of dry sliding wear behaviour of grain refined and or modified eutectic (Al-12Si) Al-Si alloy by using a Pin-On-Disc machine. The indigenously developed Al-1Ti-3B and Al-10Sr master alloys were used as grain refiner and modifier for the grain refinement of α-Al dendrites and modification of eutectic Si, respectively. Various parameters have been studied such as alloy composition, sliding speed, sliding distance and normal pressure. The cast alloys, master alloys and worn surfaces were characterized by SEM/EDX microanalysis. Results suggest that, the wear resistance of eutectic Al-Si alloys increases with the addition of grain refiner (Al-1Ti-3B) and or modifier (Al-10Sr). Further, the worn surface studies show that adhesive wear was observed in Al-12Si alloy in the absence of grain refiner and modifier. However, an abrasive and oxidative wear was observed when the grain refiner and modifier are added to the same alloy. Commercially available LM-6 (12.5%Si) alloy was used for comparison.     

Friction and wear of carbon steel near T1-transition under dry sliding

The wear phenomenon of metals under dry sliding is, generally, divided into two modes of severe and mild wear. A discontinuous transition between the wear modes often takes place in a certain load range. The T₁-transition is usually observed at lower levels of load or sliding velocity. There is a great difference in wear rate between severe and mild wear. This indicates that the occurrence of severe wear should be avoided, especially in the field of machine design to prevent energy loss, occurrence of noise and vibration, and life reduction of machines and their components. Therefore, it is important for machine designers to know the relationship between friction and wear and the difference in properties of the wear surfaces in the two wear modes. In this study, wear tests of 0.35% C steel in contact with itself under constant load were conducted in moist air at various contact loads under dry sliding. The friction and wear were measured continuously throughout each test. After the tests, the relationship between friction and wear and the difference in properties of the wear surfaces were investigated in each wear mode. From the results, the upper and lower critical loads (P_acr and P_Acr) appeared between severe and mild wear. The phenomenon of zero wear has been newly found in the early period at very low loads. The zero wear continued for a long sliding distance and then changes to mild wear. The critical load between zero wear and mild wear is defined as P_zerowear. The load was changed once in a step-wise manner from low to high levels in process of test. Since the rubbing history under mild wear condition at the low load in the first stage affected the properties of wear surface, the wear mode at the high load in the second stage changed from ‘mild wear’ to ‘quasi-mild wear’ having a low rate. From the relationship between sliding distance necessary for the appearance of quasi-mild wear and contact load in the first stage, the boundary curve between severe wear and quasi-mild wear in the second stage is hyperbolic. This curve gradually approaches P_zerowear with decreasing contact load. Thus, P_zerowear is one of the important critical loads for elucidating the test results under varying load.     

Effect of surface etching on the lubricated sliding wear of an eutectic aluminium–silicon alloy

The paper presents a study of the formation of wear grooves on near-eutectic aluminium–silicon alloy flats, by sliding a steel ball. The formation of the grooves are tracked on etched and unetched flats as functions of normal load and sliding distance. The groove is initially formed by plastic flow, and then expanded by micro-abrasion as the ball continues to slide on the groove. Etching causes surface hardening of the alloy, but, more importantly creates a surface topology that reduces the peak contact pressure, which discourages further plastic flow in the subsurface. This effect is rationalised using an existing contact mechanical model of indentation of rough surfaces.     

A study on friction and wear characteristics of nanometer Al2O3 /PEEK composites under the dry sliding condition

The friction and wear properties of the polyetheretherketone (PEEK) based composites filled with 5 mass% nanometer or micron Al₂O₃ with or without 10 mass% polytetrafluroethylene (PTFE) against the medium carbon steel (AISI 1045 steel) ring under the dry sliding condition at Amsler wear tester were examined. A constant sliding velocity of 0.42 m s⁻¹ and a load of 196 N were used in all experiments. The average diameter 250 μm PEEK powders, the 15 or 90 nm Al₂O₃ nano-particles or 500 nm Al₂O₃ particles and/or the PTFE fine powders of diameter 50 μm were mechanically mixed in alcohol, and then the block composite specimens were prepared by the heat compression moulding. The homogeneously dispersion of the Al₂O₃ nano-particles in PEEK matrix of the prepared composites was analyzed by the atomic force microscopy (AFM). The wear testing results showed that nanometer and micron Al₂O₃ reduced the wear coefficient of PEEK composites without PTFE effectively, but not reduced the friction coefficient. The filling of 10 mass% PTFE into pure PEEK resulted in a decrease of the friction coefficient and the wear coefficient of the filled composite simultaneously. However, when 10 mass% PTFE was filled into Al₂O₃/ PEEK composites, the friction coefficient was decreased and the wear coefficient increased. The worn scars on the tested composite specimen surfaces and steel ring surfaces were observed by scanning electron microscopy (SEM). A thin, uniform, and tenacious transferred film on the surface of the steel rings against the PEEK composites filled with 5 mass% 15 nm Al₂O₃ particles but without PTFE was formed. The components of the transferred films were detected by energy dispersive spectrometry (EDS). The results indicated that the nanometer Al₂O₃ as the filler, together with PEEK matrix, transferred to the counterpart ring surface during the sliding friction and wear. Therefore, the ability of Al₂O₃ to improve the wear resistant behaviors is closely related to the ability to improve the characteristics of the transfer film.     

Friction and dry sliding wear behaviour of cermets

The friction and wear behaviour of cermets/steel rubbing pairs were investigated. Friction and wear tests were carried out using three different crèmets on the base of tungsten, titanium and chromium carbides under dry sliding conditions against steel disk (0.45% C). Sliding wear tests were carried out using modified block-on-ring equipment at a sliding speed of 2.2 m/s and normal load 40 N.It is shown that wear resistance and coefficient of friction depend on the type and chemical composition of the cermets. The WC–Co cermets have the highest wear resistance. The wear rate of WC–Co and TiC–NiMo cermets increased with increasing binder content in the cermets. The wear of Cr₃C₂–Ni cermets is more complicated and depends on the composition of cermets. The wear of WC–Co cermets is caused mainly by preferential removal of the cobalt binder, followed by fracture of the intergranular boundaries and fragmentation of the carbide grains. The main wear mechanism in the TiC–NiMo cermets is polishing (micro-abrasion) and adhesion, resulting in a low wear rate. The main wear mechanism of Cr₃C₂–Ni cermets involves thermal cracking and fatigue-related crushing of large carbide grains and carbide framework and also adhesion.     

The role of triboparticulates in dry sliding wear

In this paper, wear processes and mechanisms for wear transitions with sliding time and temperature during sliding of a nickel-based alloy, N80A, in oxygen at temperatures to 250°C are discussed. Transitions in wear from high rates to low rates with sliding time were always observed at all the temperatures investigated. The transitions in wear were usually accompanied by transitions in contact resistance between the rubbing surfaces from nearly zero to positive high values. It was found that wear debris particles were heavily involved in the wear processes. The transitions in wear and contact resistance with sliding time mainly resulted from the development of wear-protective layers following the compaction of wear debris particles on the rubbing surfaces. The adhesion of triboparticulates to each other and to the rubbing surfaces played an important role in the rapid decrease in wear rate with sliding time and with increase in temperature. Processes involved in the development of the wear-protective particle layers and mechanisms for the wear transitions have been described on the basis of experimental observations. The importance of triboparticulates in wear and its implications for wear protection are discussed.     

Impact of Cr3C2/VC addition on the dry sliding friction and wear response of WC–Co cemented carbides

Two grades of WC–10 wt.%Co cemented carbide with or without addition of Cr₃C₂/VC grain growth inhibitor during liquid phase sintering were produced with the goal to investigate their reciprocating sliding friction and wear behaviour against WC–6 wt.%Co cemented carbide under unlubricated conditions. The tribological characteristics were obtained on a Plint TE77 tribometer using distinctive normal contact loads. The generated wear tracks were analyzed by scanning electron microscopy and quantified topographically using surface scanning equipment. The post-mortem obtained wear volumes were compared to the online assessed wear. Correlations between wear volume, wear rate and coefficient of friction on the one hand and sliding distance and microstructural properties on the other hand were determined, revealing a significant influence of Cr₃C₂/VC on the friction characteristics and wear performance.     

Toughening and dry sliding wear resistance of Co3Mo2Si alloys

The MgZn₂ type Laves phase of Co₃Mo₂Si has very attractive super high hardness and covalent dominated bonding nature for wear resistance applications. To make it feasible for real application, its inherent brittleness was effectively balanced by toughening it with the ductile cobalt solid solution (Co_ss) by the laser melting process. The toughening effect was evaluated qualitatively by the observation of the indentation behaviors and the micro-hardness variations of the alloys. The wear properties of the Co_ss toughened Co₃Mo₂Si alloys were evaluated in the dry sliding wear test condition at room temperature. Results indicated that the Co_ss toughened Co₃Mo₂Si alloys have excellent combination of strength and ductility, and display exceptional wear property.     

The role of dynamic recrystallization in dry sliding wear

In dry sliding wear dynamic recrystallization gives a better understanding than grain boundary sliding and fatigue of the process near the contact zone. The process explains the low value of the deformation measured by grain thickness reduction and the absence of visible deformation marks in the case of low melting materials. Dynamic recrystallization explains the almost total consumption of the friction energy by the plastic process. It is also possible to compute the geometry of the worn material in a wear test. This phenomenon is proved experimentally using a copper pin sliding against an SAE 1045 steel ring. Pins of both single crystal and polycrystalline copper were used. The structure was observed by examining thin foils of the material taken from near the contact zone in an electron microscope.     

Friction and wear of self-mated SiC and Si3N4 sliding in water

The friction and wear of self-mated SiC and Si₃N₄ with different initial roughness sliding in water were investigated with pin-on-disk apparatus at normal load of 5 N and sliding speed of 120 mm/s in ambient condition. It was found that, for self-mated Si₃N₄, the wear mechanism for surface smoothening to obtain low friction was tribochemical wear, but for self-mated SiC, it changed from mechanical wear into tribochemical wear with increasing sliding cycles. After running-in in water, self-mated Si₃N₄ exhibited lower steady-state friction coefficient than self-mated SiC did. For these two ceramics, initial and steady-state friction coefficients were hardly dependent on initial roughness. Initial roughness mainly affected the running-in period. The larger the initial roughness, the longer the running-in period, but the running-in period was much shorter for self-mated Si₃N₄ at each initial roughness than that for self-mated SiC.     

The influence of alumina particle dispersion and test parameters on dry sliding wear behaviour of zinc-based alloy

The present investigation deals with dry sliding wear characteristics of a zinc-based alloy (ZA 37) with and without Al₂O₃ particle dispersion over a range of sliding speeds and applied pressures. The matrix alloy has been examined under identical test conditions in order to examine the role played by the second phase alumina particles on wear behaviour. The observed wear behaviour of the samples has been explained in terms of specific characteristics like cracking tendency, lubricating, load bearing and deformability characteristics, and thermal stability of various microconstituents. The nature of predominance of one set of parameters (causing higher wear rate) over the other (producing a reverse effect) was thought to actually control the wear behaviour. Examinations of the characteristic of wear surfaces and subsurface regions also enabled to understand the operating wear mechanism and to substantiate the wear behaviour.At low sliding speed, significantly lower wear rate of the matrix alloy over that of the composite was noticed. This has been attributed to increased microcracking tendency of the composite than the matrix alloy. Reduced wear rate and higher seizure pressure experienced by the composite over that of the matrix alloy at the higher sliding speeds could be explained to be due to enhanced compatibility of matrix alloy with dispersoid phase and greater thermal stability of the composite in view of the presence of the dispersoid. The maximum temperature rise due to frictional heating has been observed to be low in the case of matrix alloy than composite at low speed while the trend reversed at higher speeds. In general, the wear rate and temperature increased with applied pressure and speed. Seizure pressure reduced with increasing speed while the seizure resistance (pressure) of the matrix alloy was more adversely affected by speed than that of the composite.     

Observation of delamination wear of lubricious tribofilm formed on Si3N4 during sliding against WC-Co in humidity air

Hot pressed silicon nitride that was exposed to high (90%) and low (32%) relative humidity was examined in ball-on-disc geometry against cemented carbide ball at various normal loads. The study indicated that Si₃N₄ tested at high R.H. gave less specific wear rate compared with Si₃N₄ at low R.H. The friction coefficient of Si₃N₄−WC-6% Co tribopairs was found in the range of 0.32–0.39 and 0.05–0.17 at low humidity and high humidity respectively. It is suggested that adsorbed moisture markedly affected the wear and friction properties of silicon nitride.Following the tests, SEM was used to elucidate the wear mechanism and particularly to delineate the effects of relative humidity on the wear and friction. SEM micrographs showed that the main wear mechanism at low relative humidity (32%) was caused by mechanical wear including abrasive grooves, large holes and polishing, whereas at high relative humidity (90%) the main mechanism was highly influenced by a tribochemical reaction related to the moisture adsorption from the environment. It is concluded that the removal of lubricious tribolayer was occurred by delamination induced crack propagation.     

Investigation into sliding wear performance of zinc-based alloy reinforced with SiC particles in dry and lubricated conditions

The objective of the present investigation was to assess the influence of SiC particle dispersion in the alloy matrix, applied load, and the presence of oil and oil plus graphite lubricants on the wear behaviour of a zinc-based alloy. Sliding wear performance of the zinc-based alloy and its composite containing SiC particles has been investigated in dry and lubricated conditions. Base oil or mixtures of the base oil with different percentages of graphite were used for creating the lubricated conditions. Results show a large improvement in wear resistance of the zinc-based alloy after reinforcement with SiC particles. The lubrication improved the wear resistance and friction behaviour of both the reinforced and base alloys. It was also observed that there exists an optimum concentration of graphite particles in the lubricant mixture that leads to the best wear performance. The composite experienced higher frictional heating and friction coefficient than the matrix alloy in all the cases except oil lubricated conditions; a mixed trend was noticed in the latter case. The wear rate and frictional heating increased with load while friction coefficient was affected in an opposite manner. Test duration influenced the frictional heating and friction coefficient of the samples in a mixed manner.Examination of worn surfaces revealed a change of predominating wear mechanisms from severe ploughing and/or abrasive wear for base alloy to delamination wear for the reinforced material under dry sliding conditions. The presence of the lubricant increased the contribution of adhesive wear component while reducing the severity of abrasion. This was attributed to the generation of more stable lubricant films on the contacting surfaces. Cross-sections of worn surfaces indicated substantial wear-induced plastic deformation, thereby suggesting adhesive wear to be a predominant wear mechanism in this study. The debris particles revealed deformed flakes and machining chips signifying the involvement of adhesion and abrasion modes of wear respectively.     

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.     

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.     

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.     

On dry sliding friction and wear behaviour of PEEK and PEEK/SiC-composite coatings

In this work, polyetheretherketone (PEEK) and PEEK/SiC-composite coatings were deposited on Al substrates using a printing technique to improve their surfaces performance. The objective of this work was to investigate coatings friction and wear behaviour. Especially, the effect of sliding velocity and applied load on coatings friction coefficient and wear rate was evaluated in range of 0.2-1.4 m/s and 1-9 N, respectively. Compared to Al substrate, the coated samples exhibit excellent friction coefficient and wear rate. For PEEK coating, under an applied load of 1 N, the increase in sliding velocity can result in decreasing of friction coefficient at a cost of wear resistance. Under a load of 9 N, however, PEEK coating exhibits the highest friction coefficient and wear rate at an intermediate velocity. These influences appear to be mainly ascribed to the influence of contact temperature of the two relative sliding parts. In most test conditions, the composite coating exhibits better wear resistance and a little higher friction coefficient. SiC reinforcement in composite coating plays a combined role. First of all, it might lead to energy dissipation for activation of fracture occurred on the interface of PEEK and the powders. Moreover, it can reduce coating ploughs and the adhesion between the two relative sliding parts.     

Sliding speed-temperature wear transition maps for Si3N4/iron alloy couples

The superior high temperature resistance of silicon nitride (Si₃N₄) based ceramics makes them suitable for tribological applications above room temperature or in high speed unlubricated sliding. There are some published works on the wear behaviour of Si₃N₄/metal alloys. However, experimental data are shown in a form that is not of direct use for engineers involved in materials selection. In the present work, Si₃N₄ pins were tested against tool steel and grey cast iron on a pin-on-disc tribometer. Ceramics were produced by hot-pressing and tested without lubrication at variable temperature and sliding speed. SEM/EDS and XRD analysis were used for chemical and microstructural characterisation of worn surfaces and wear debris. At low speeds (0.05–0.5 m s⁻¹) and room temperature, Si₃N₄ surfaces are polished-like due to a combination of humidity-assisted tribo-oxidation and abrasive action of very fine wear debris. At high sliding speeds (2–3.5 m s⁻¹), as well as for temperatures in the range 400–600°C, an extensive coherent tribolayer mainly composed by iron oxides spreads over the ceramic surfaces. Polishing and protection by adherent tribolayers are the mechanisms responsible for observed severe and mild wear regimes, respectively. Wear maps are constructed showing the transition of wear regimes in Si₃N₄/iron alloys contacts determined by constant flash temperature curves. Equations for calculation of bulk and flash contact temperatures in tribocontacts between dissimilar materials are deduced.     

Effect of quartzite and granite in wear surfaces on dry sliding

Abstract

The wear surfaces abraded with quartzite and granite were subjected to scratch tests. Sharp and blunt indenters were used with various constant loads to produce controlled abrasive wear tracks. The characteristics of deformation mechanisms and material removal were further studied using a scanning electron microscope to determine the differences in the tribological behaviour between the quartzite and granite wear surfaces.

The results indicate that quartzite residues are more uniformly distributed as individual particles on the wear surfaces and therefore provide more stable frictional forces. In the case of granite the abrasive residues are rather non-uniformly collected into piles of abrasives.     

Microstructural and sliding wear behaviour of a heat-treated zinc-based alloy

The effect of heat treatment on microstructure, hardness, tensile properties, fracture mode and wear behaviour during lubricated and dry sliding of the zinc-based alloy with 25 wt.%Al was studied. Microstructural investigation and chemical analysis of as-cast and heat-treated specimens, the fracture and worn surfaces, as well as wear debris were performed by scanning electron microscopy and energy dispersive spectroscopy. Wear tests were carried out using a disc-on block-type wear machine. By a relatively simple heat treating consisting of a short-term annealing in the single-phase region followed by water-quenching, the elongation has been markedly improved, while the strength was maintained high. The results indicate that the wear rate strongly depends on the microstructure, applied load and sliding conditions. The wear rate increases with load, and under dry sliding conditions the wear rate is approximately two orders of a magnitude higher than under lubricated conditions. During dry sliding the best wear behaviour was displayed by the water-quenched specimens, whereas slowly cooled specimens showed the higher wear rate. Lubrication strongly affects the wear behaviour. Contrary to dry sliding, slowly cooled specimens exhibit the best wear properties under lubricated conditions. The wear mechanisms were proposed for dry and lubricated sliding. An erratum to this article can be found at