Wear Fragments Research Articles

XPS study of transfer films of MoS2 filled nylon-46 composites sliding against steel counterfaces

When polymers and their composites are slid against metal counterfaces, they are worn off, and a part of the wear fragments deposits onto the counteface as a polymer transfer film [1]. In repeated sliding of polymeric materials on the same sliding track, their tribological behavior is affected by the transfer films [1–3]. Accordingly, for a comprehensive understanding about the tribological behavior of polymeric materials, it is very important to clarify the physical and chemical nature of the transfer films, such as their thickness, coherency and chemical composition. The effects of the transfer films on the tribological properties have been investigated in various polymer-metal sliding combinations [2–5]. Nylon is one of the engineering plastics used for many engineering parts and components. Among the various kinds of nylon, such as nylon-6, 66, 610 and 46, nylon-46 is a new engineering plastic which is expected to be superior to the other nylons because of its higher modulus and strength as a result of being highly crystalline. In our preliminary experiments on the tribological properties of nylon-46, it was observed that nylon-46 exhibited a relatively high sliding friction as well as the other nylons. Expecting to reduce the friction of nylon-46, a solid lubricant of MoS2 was examined as a filler. However, the incorporated MoS2 fillers had little effect in lowering the friction. It is well-known that the lubrication performance of MoS2 itself is controlled by many physical and chemical factors, such as crystallographic imperfection, water vapor, temperature and environmental atmosphere [6, 7]. Moreover, it was also reported that MoS2 could oxidize during sliding contact [8]. More recently, with MoS2 filled nylon 6 composites sliding against steel counterfaces, it was reported on the basis of X-ray photoelectron spectroscopy (XPS) that MoS2 fillers in the transfer films oxidized into MoO3, and that such a oxide of MoS2 may deteriorate the wear resistance of nylon 6 composites [9]. In the present study, with nylon-46 filled with various size of MoS2 particles, the characteristics of the transfer films produced in the sliding contact were investigated by means of XPS, especially the effect of filler size on the chemical change of MoS2 filler in the transfer films. As shown in Table I, nylon-46 and four kinds of its composites filled with various size MoS2 fillers of which the mean particle diameters of 0.5, 1.5, 10 and 100 μm were prepared. The filler content was 20wt% for all composite samples. A pin-on-disk type wear testing apparatus was employed in the sliding experiments. In order to investigate the characteristics of the polymer transfer films, the disk specimens for XPS analysis and SEM observation were prepared by rubbing one end surface of a cylindrical pin 3 mm in diameter against a 25 mm square steel disk 2 mm in thickness. The frictional surfaces of the cylindrical pin were finished with a #800 emery paper. The disk surfaces were finished with #1500 and #100 abrasive papers, and their surface roughnesses were 0.03 and 0.5 μm Ra, respectively. After the surface finishing, they were cleaned ultrasonically in acetone for 15 min before sliding testing. The pin was slid repeatedly against a circular track 16 mm in diameter with load of 10 N and velocity of 0.1 m/s, and in the ambient atmosphere at room temperature and at about 60% relative humidity. After 20 000 disk revolutions, the steel disk was immediately introduced into the analysis chamber. XPS spectra were recorded on a Model-255 ESCA/AES spectrometer (Ulvac-Phi Co.) using MgKα radiation. The pass energy of the analyzer was set at 25 eV. The analysis area of the XPS was estimated to be a circular one about 5 mm in diameter. Fig. 1a and b shows typical examples of SEM observations of transferred polymer materials adhered on the disks. Frictional tracks on the steel disks appear to be covered with relatively coherent transfer films, since the original random scratches finished with abrasive papers were not recognizable on the track. Fig. 2a, b and c shows XPS spectra of Mo3d, S2p and N1s for the transfer films of nylon-46 composites filled with MoS2 filler of 1.5 μm particle size in comparison with the spectra for the bulk composite. As shown in Fig. 2a, Mo3d spectrum of bulk composite exhibits a spin doublet composed of Mo3d5/2 and Mo3d3/2, named as Mo(1) and Mo(2). On the other hand, with those of the transfer films, a new additional Mo3d peak, named as Mo(3), at the binding energy of 238 eV that is about 3 eV higher than those of Mo3d3/2 spectrum of the bulk polymer Mo(2). This observation suggests that

Tribochemical interactions between molybdenum dithiophosphate and succinimide additives

Tribochemical interactions between antiwear/antifriction molybdenum dithiophosphate (Modtp) and dispersant amine additives (polyisobutenyl succinimide (PIBSI) and polyisobutenylacetylamine (PIB-Amine)) have been investigated by a coupling friction experiment in a Cameron Plint tribometer and chemical analysis by transmission electron microscopy (TEM), XPS and AES. Antagonist effects have been observed on friction data in the case of the Modtp/PIB-Amine combination in oil. In contrast, friction properties of the Modtp have not been modified in a significant way by adding PIBSI. Both XPS and AES suggest that the addition of PIB-Amine to Modtp has inhibited the formation of the tribofilm, whereas in the case of the addition of PIBSI to Modtp no noticeable change in the film composition was observed. Observation by TEM of wear fragments confirms an inhibition of the formation of the polyphosphate/MoS2 tribofilm when PIB-Amine is used as a detergent-dispersant additive. All the results suggested that the amine plays a fundamental role in inhibiting the Modtp tribofilm formation. The results with PIBSI allow us to conclude that the role of the amine depends either on its ability to form coordination complexes with Mo or on its adsorption properties on metallic surfaces. Copyright © 2000 John Wiley & Sons, Ltd.

Role of nitrogen in tribochemical interaction between Zndtp and succinimide in boundary lubrication

Tribochemical interactions between zinc dithiophosphate (Zndtp) and polyisobutene succinimide (PIBSI) were studied in the mild wear regime by a dual analysis approach. Both TEM analyses of wear fragments and inside wear scar XPS on tribofilms were obtained in the same location of the wear track. In presence of the succinimide, there are notable modifications in the composition of the zinc polyphosphate tribofilm which is classically formed in presence of Zndtp alone. Apart from amorphous zinc polyphosphate glass, the film contains also large amounts of oxidised species (oxides, sulphates and nitrates) together with residual succinimide function groups. The atomic ratio N/P is about 0.3. Moreover, compared to Zndtp, less tribofilm is produced due to some competition in adsorption process. The results highlight the role of the tribochemical reaction to explain the antagonism between the two additives. The presence of some iron oxide in the tribofilm material indicates that the anti-abrasive wear mechanism of Zndtp is hindered by the presence of the succinimide. The lower content of the tribofilm in sulphide species also suggests a lower efficiency to control the anti-seizure properties of the mixture of the two additives.

Tribochemical interactions between Zndtp, Modtc and calcium borate

Tribochemical interactions between antiwear zinc dithiophosphate (Zndtp), friction modifier molybdenum dithiocarbamate (Modtc) and overbased detergent calcium borate (OCB) lubricant additives have been investigated. Friction tests were performed in mild wear conditions under boundary lubrication, in order to enhance tribochemical surface effects. The nature of tribofilms formed was studied by coupling high‐resolution TEM on wear fragments and inside‐wear‐scar, micro‐spot XPS in the same location of the wear track (so‐called dual analysis). The performance of the Modtc/Zndtp mixture is mainly due to the generation of MoS2 single sheets and the digestion of MoO3 in the zinc polyphosphate glass formed. The final result of the tribochemical reaction is a two‐phase tribofilm composed of (i) non‐oriented MoS2 sheets (friction modifier) embedded in a carbon‐rich phase and (ii) a mixed Zn/Mo polyphosphate glass (antiwear). The Modtc/OCB mixture has a similar antiwear mechanism except that the oxide is not completely eliminated, due to the softer action of borate anion compared with phosphate one. Compared to the data obtained with binary combinations (Modtc/Zndtp, Modtc/OCB and Zndtp/OCB), we show here that the ternary system Modtc/Zndtp/OCB provides both a low wear rate and an ultralow friction value, while adding detergent and anti‐corrosive properties to the formulation. Our analytical data indicate that the synergistic effect can be attributed to an outstanding nanostructure of the tribofilm formed. It is composed of a single‐phase material containing perfectly oriented MoS2 single sheets embedded in a calcium and zinc borophosphate glass. The ternary system produces a smart material in the interface, because both functions (antiwear and friction reduction) are correlated. Compared to phosphate alone, the mechanism by which MoS2 sheets have been oriented in the borophosphate could be related to aligned molecules of the glassy polymer in the direction of sliding.

Frictional characteristics of Al–SiC composite brake discs

The use of light materials such as aluminium matrix composites reinforced with silicon carbide (SiC) in railway braking devices is considered. Four quarter‐scale discs were produced by the vortex method with two distinct matrices and a SiC reinforcement with two different shapes and rates. Continuous braking tests (120 s) were run with organic pads in a dry environment. The 390.0 matrix discs exhibited a higher wear resistance than one produced from a 514.0 matrix. The use of a spherical SiC, instead of an angular one, very markedly improved the wear resistance of the antagonist materials. During the braking tests, the wear fragments become oxidized and their presence in tribocontact increases friction and pad wear but decreases the disc wear.

Tribochemical interactions between molybdenum dithiophosphate and succinimide additives

Tribochemical interactions between antiwear/antifriction molybdenum dithiophosphate (Modtp) and dispersant amine additives (polyisobutenyl succinimide (PIBSI) and polyisobutenylacetylamine (PIB-Amine)) have been investigated by a coupling friction experiment in a Cameron Plint tribometer and chemical analysis by transmission electron microscopy (TEM), XPS and AES. Antagonist effects have been observed on friction data in the case of the Modtp/PIB-Amine combination in oil. In contrast, friction properties of the Modtp have not been modified in a significant way by adding PIBSI. Both XPS and AES suggest that the addition of PIB-Amine to Modtp has inhibited the formation of the tribofilm, whereas in the case of the addition of PIBSI to Modtp no noticeable change in the film composition was observed. Observation by TEM of wear fragments confirms an inhibition of the formation of the polyphosphate/MoS2 tribofilm when PIB-Amine is used as a detergent-dispersant additive. All the results suggested that the amine plays a fundamental role in inhibiting the Modtp tribofilm formation. The results with PIBSI allow us to conclude that the role of the amine depends either on its ability to form coordination complexes with Mo or on its adsorption properties on metallic surfaces. Copyright © 2000 John Wiley & Sons, Ltd.

A dual‐analysis approach in tribochemistry: application to ZDDP/calcium borate additive interactions

Tribochemical interactions between zinc dithiophosphate (ZDDP) and micellar calcium borate (CB) under boundary lubrication were investigated by coupling, in the same location of the wear track, both analytical TEM analysis of collected wear fragments from the tribofilm and XPS surface analysis of the tribofilm directly underneath. This is the so‐called dual‐analysis approach, which improves the interpretation of tribochemical reactions. The elemental composition inside the wear scars was analysed by micro‐spot XPS. By depth profiling, the film thickness could also be determined. In particular, the efficiency of the additive combination could be proven by quantification of iron oxide. The nature of wear particles was investigated in the TEM by using EELS and EDX simultaneously, with the result that phosphorus, boron and sulphur contributions have been carefully distinguished. The technique is very powerful for determining the composition of the material through quantification of both EELS and EDX spectra on the same specimen. The main result, when ZDDP and CB additives are used together, is the formation of a calcium and zinc borophosphate glass tribofilm. The overall data confirm the general friction‐induced glass model as being a unifying concept that explains the mechanisms of antiwear additives under boundary lubrication. Moreover, the analytical results strongly suggest the role of viscous flow of the magma state glass tribofilm above its glass transition temperature to be a main contribution to the antiwear mechanism under mild wear conditions.

MoS 2 single sheet lubrication by molybdenum dithiocarbamate

The mechanisms by which Modtc reduces friction in the centirange under boundary lubrication have been investigated using analytical tribometry. First, the SRV friction test was coupled with energy-filtering TEM on wear fragments and spatially-resolved XPS inside the wear scars. Second, we performed UHV friction tests on Modtc tribofilms previously created on a large area. The overall data demonstrate that the mechanisms of friction-reduction by Modtc is attributed to the effect of sliding between single layers of MoS 2 only, and not to intra-sliding in MoS 2 3-D crystal. Highly-dispersed MoS 2 sheets are present in a carbon matrix in the tribofilm material. The growth of the 2-D MoS 2 single sheets is thought to be formed by degradation of the Modtc molecule by electron transfer mechanisms activated by the friction process. The lubrication of the uncoated, stationary counterface is attributed to successive transfer of individual sheets towards the friction surface. Practically, in these conditions only a few per cent of dispersed MoS 2 is sufficient to lubricate at the same level as pure MoS 2.

Micellar calcium borate as an antiwear additive

The mechanisms of action of a new generation of antiwear additives is studied here by means of energy‐filtering transmission electron microscopy (EFTEM) carried out on the wear particles generated during friction tests between two ferrous surfaces (under boundary lubrication conditions). This paper deals with the structural and physico‐chemical changes that colloidal particles, calcium carbonate (CC) and calcium borate (CB) overbased salicylates detergents, have undergone during the build‐up of the interfacial antiwear tribofilm. EFTEM allowed us to investigate the nature of wear fragments originating from the film, stemming from CC and CB micelles, and to make a comparison regarding the tribofilm formation mechanisms. It appears that the CC wear debris are mainly crystalline and contain a high concentration of iron (as abrasive iron oxide Fe2O3), limiting their antiwear action. Consequently, CC micelles do not lead to an effective protective tribofilm. On the other hand, CB micelles do have an antiwear action, which we explained by the formation of a glassy iron borate tribofilm during the friction tests. Many of the CB wear fragments are composed of this amorphous material containing very small crystallites of residual calcite. Boron (contained in the CB micelles) is responsible for the presence of amorphous zones of the film and acts as a glass former, in a comparable way to phosphorus in zinc dithiophosphate.

Combined influence of an inert gas environment and a mechanical action on a graphite surface

The adsorption and reactivity properties of graphite wear fragments produced in a tribocontact depend on the gaseous environment surrounding the contact which conditions the type of wear (fatigue or abrasive wear) and the surface reactivity. The determination of the specific area of the wear fragments, using a nitrogen adsorption method (Brunauer, Emett and Teller method), permits to reach a description of the surface structure. We carried out on a pin on disc tribometer located in a vacuum chamber wear experiments on two samples of polycrystalline graphites, under different inert gaseous environments. We observed that the size of wear fragments (as well as the friction coefficient) mainly depends on the orientation of the crystallites. This mechanical phenomenon principally depends on the nature of the environment and on the normal load. Depending on these conditions, the powder can have different colorations, and its specific area shows variations from 140 m 2 g −1 to 285 m 2 g −1. Wear particles produced under inert environment present many active sites (edge sites) but their reactivity depends on their size and specific area. The most reactive can exhibit pyrophoric properties, when exposed to the ambient air.

Superlubricity of MoS 2: crystal orientation mechanisms

We have investigated the origin of the extraordinary low friction coefficient (in the 10 -3 range or even less) of pure and stoichiometric sputtered MoS 2 coatings, in ultrahigh vacuum. In these conditions, shear strengths of the interface as low as 1 MPa were measured. Importantly, the tribometer was operating in macroscopic contact conditions, typically at a long time-length scale. Friction-induced orientation of (0001) basal planes of MoS 2 grains parallel to the sliding direction was first verified by means of electron diffraction. Friction-induced rotation of these crystals around the c axis, during intercrystallite slip in the contact, was investigated by high resolution transmission electron microscopy performed on selected wear fragments. Atomic force microscopy at atomic resolution was also carried out on the surface inside and outside the wear scar. The data indicated that the vanishing of the friction force was due to frictional anisotropy in the interface between nanometre-scale domains in rotational disorder (intercrystallite slippage of incommensurate sulphur-rich hexagonal lattices). The term superlubricity was used here because of the zero friction state that could be theoretically predicted in these conditions. Finally, the mechanisms of MoS 2 superlubricity are thought to depend on the proper combination of the grain size, the two crystal orientation effects and the absence of contaminants.

Tribological characterization of alumina and silicon carbide under lubricated sliding

The aim of this study was to investigate the tribological properties of four different advanced ceramics - monolithic alumina, SiC whisker-reinforced alumina, monolithic silicon carbide and SiSiC-during lubricated sliding. Advanced techniques of electron microscopy and spectroscopy have been used to characterize the materials before and after testing. Tests have been performed where two flat discs were rotated against each other under closed contact in an environment of oil and water. The main wear mechanisms of the four ceramics are identified and discussed. What clearly emerges from these studies is the much more reliable performance of the silicon carbides than the aluminas. The silicon carbides have a low wear rate where microfracture and oxidation are the main deteriorating mechanisms. The capability to maintain smooth surfaces and thus also a high degree of hydrodynamic lubrication is largely due to the potential of the water to dissolve formed wear debris. The main wear mechanism of the aluminas is surface fracture. The rough fracture surfaces and the fact that the wear fragments form a discontinuous surface film will reduce the effect of the lubrication, thus accelerating the wear. Furthermore, a deformation layer with microcracks develops in the contact which decreases wear resistance.

Wear behaviour of uncoated and WC-C-coated alumina under closed sliding

The tribological properties of uncoated and WC-C-coated alumina when simulating mechanical face seal applications are reported. Silicon carbide was also included in the tests, mainly as a reference material, since it has shown good performance in these applications. Scanning electron microscopy, energy-dispersive X-ray spectroscopy and Auger electron spectroscopy were utilized to characterize the worn surfaces. The results show that coating alumina face seals with WC-C increases the wear resistance by one order of magnitude. Further, when used at low contact pressures (below 0.050 MPa), the WC-C coating also reduces the friction significantly. When used at high contact pressures (above 0.050 MPa), however, there is no significant difference in the friction characteristics. At high contact pressures (above 0.050 MPa) all investigated material combinations generate a tribo-film between the mating surfaces. Uncoated alumina forms a non-homogeneous surface film built up by very fine wear fragments, while the WC-C-coated alumina forms a tribo-film which consists mainly of a (W, Cr) oxide compound. In contrast, the silicon carbide forms a more homogeneous silica-based tribo-film. It is concluded that the mechanical and tribological properties of the tribo-films are decisive for the wear behaviour of ceramics run under dry sliding conditions. Further, if a surface coating is to be used in order to improve the tribological behaviour of journal bearings or face seals, it should have good adhesion to the underlying substrate and a high propensity to form a soft tribo-film with sufficiently good cohesion.

Role of Iron in the Amorphization Process in Friction-Induced Phosphate Glasses

Wear debris from ZDDP lubricated tests run under mild wear conditions have been investigated by X-ray Absorption Fine Structure Spectroscopy (EXAFS). Special attention has been paid to the state of iron in the amorphous wear fragments. Results indicate the presence of Fe3+ species as a modifier of the phosphate network formed during tribo-reaction. As the anti-wear regime of ZDDP is characterized by a decrease of the abrasive wear, it is suggested that the anti-wear action of ZDDP is reliable to the diffusion of iron oxides in the phosphate, this latter being gently delaminated. Presented at the 43rd Annual Meeting in Cleveland, Ohio May 9–12, 1988

A case of wear particle formation through shearing-off at contact spots interlocked through microroughness in adhesive wear

The shape and size distribution of wear particles and the interface morphology enables different wear modes to be distinguished, and hence allows comparison with corresponding wear models. The particles must be collected as soon as possible after their formation, to minimize deformation and chemical attack between the generation and analysis of the particles, and the concurrent damage to the interface. To this end, a correlated study of surface morphology and of wear particles was performed on bundles of copper wires of various diameters subject to unlubricated sliding on a copper substrate in adhesive wear. Different sliding conditions were used. In some tests the contact spot temperature was artificially increased through the passage of current to generate Joule heat; other tests were performed in argon rather than in the unregulated atmosphere. The results strongly support the “shearing-off” model of wear, operating at contact spots mechanically interlocked through microroughness, as indicated by the following. 1. (1) There was no evidence of any clinging of wear particles or fragments thereof to the harder side, as expected if adhesive forces had played a significant role in the primary wear event. 2. (2) Numerous partially formed but not yet fully detached wear particles were observed whose shape conformed exactly to that predicted by the shearing-off model. 3. (3) Evidence of microroughness was discovered on the surfaces of partly formed particles, the corresponding surfaces of wear particles and those of the contact spots on the other side. 4. (4) By contrast, none of these surfaces showed evidence of tearing or puckering that would indicate breaking of adhesive bonds. 5. (5) For all wear conditions, the size of the wear particles matched the size of the contact spots on the opposing side. 6. (6) The wear particles exhibited “similitude”, i.e. their ratio of length to width to thickness was virtually independent of the wear conditions, as predicted by the shearing-off model. 7. (7) The ratio of length to width to thickness of the particles was about 1:0.5:0.3, which is in close conformity with expectations from the shearingoff model without assistance from adhesion, whereas strong adhesive forces are expected to generate equiaxed wear particles.

ROLE OF IRON IN THE AMORPHIZATION PROCESS IN FRICTION-INDUCED PHOSPHATE GLASSES

Wear debris from ZDDP lubricated tests run under mild wear conditions have been investigated by X-ray Absorption Fine Structure Spectroscopy (EXAFS). Special attention has been paid to the state of iron in the amorphous wear fragments. Results indicate the presence of Fe3+ species as a modifier of the phosphate network formed during tribo-reaction. As the anti-wear regime of ZDDP is characterized by a decrease of the abrasive wear, it is suggested that the anti-wear action of ZDDP is reliable to the diffusion of iron oxides in the phosphate, this latter being gently delaminated. Presented at the 43rd Annual Meeting in Cleveland, Ohio May 9–12, 1988

The impact fracture wear test: A novel method of tablet evaluation

Tablet strength is usually determined by the diametral compression test or by the indentation hardness test. However, the limited data available from such tests make the development of a new test desirable. For this purpose, this report describes the development of a modified friability test, named the ‘impact fracture wear test’. It provides data which are not solely dependent upon the greatest flaw in the tablet and it stresses the tablet in a manner similar to subsequent processing. The relationship between tablet half-life ( t 50), a strength parameter, and relative density, and the effects of dwell time and particle size, were found to be similar to those described in the literature. The wear of castings of Plaster of Paris fits a first-order kinetic model well. The wear of compacts of sodium chloride deviates increasingly from first-order kinetics with increasing grain size and porosity. These deviations may be correlated with the non-homogeneity of tablet strength, the effects of tablet geometry, and of changes in process variables such as humidity, lubrication, preconsolidation, and storage time. Examination of the tablet and its wear fragments during the test provides information about the deformation and fracture characteristics of the material.

Frictional behavior of sputtered TiN

In this research, reciprocating sliding experiments were conducted on films of sputtered TiN. The primary emphasis was to evaluate the effect of the applied film thickness on the friction coefficient. Other parameters namely, surface roughness, substrate hardness and applied load were also varied and their effects were studied. From the friction data and the results of the microscopy studies, the dominant mechanism responsible for the transition and steady state friction behavior were evaluated. The results show that the gradual accumulation of minute wear fragments followed by the formation of a rigid transferred layer are responsible for the transition of the friction coefficient. During the steady state, sliding is occurring between the slider and a small portion of the transferred layer. The steady state friction coefficient varies with the applied coating thickness under the light load. Under the heavy load the coating thickness has no effect on the friction coefficient, except that the friction coefficient tends to be lower for the minimum film thickness.

Microcrystalline structure of milled dolomite and its constituent carbonates

The prolonged mechanical treatment of a small quantity of dolomite in a stainless-steel ball mill was insufficiently severe to cause disintegration into the component carbonate phases. Instead the dolomite was structurally degraded with the development of severe apparent lattice strain of about 4%, a unit cell dilated by about 0.6%, and locally misoriented carbonate ions. Contamination of the dolomite by wear fragments was slight, being at the rate of 10 ppm of Fe per hour. Similarly ground calcite and magnesite, in a 1:1 mole ratio, underwent mechanochemical transformation to dolomite. Magnesite was gradually incorporated into microregions of the calcite at a rate that was first order with respect to the unconverted calcite. The apparent lattice strain in the mixed carbonate varied in a cyclic manner; the lattice strain normal to (104) planes was a maximum of about 8% when the magnesite content of the mixed carbonate was about 30 mole%. The d-spacing was then 2.973 Å, which is about halfway between that of unstrained calcite, 3.035 Å, and dolomite, 2.904 Å. Eventually all the calcite that had not transformed to aragonite was converted into dolomite. The microstructure then closely resembled that of dolomite which had been milled for a similar time. Ground calcite is soft enough to give nonporous aggregates of welded crystals. The crystals of the harder dolomite, and magnesite, fracture to produce less strongly welded aggregates with pores spanning the micro, meso, and macro size ranges. In doing so, the specific surfaces of the latter each pass through a maximum between 11 and 12 m 2/g after 20 hr of grinding.

Wear characteristics, friction and surface topography observed in the dry sliding of as-cast and agehardening Al-Si alloys

Binary as-cast Al-Si alloys and two age-hardening commercial hypoeutectic and hypereutectic Al-Si alloys were studied under dry sliding conditions using a pin-bush machine. The bush was usually a hard steel. A detailed scanning electron microscopy study of worn surfaces and subsurfaces was carried out and machined surfaces were examined for comparative studies. Various surface topographies were noted depending on the contact stress and composition. Aluminium alloy deposits on the bush and subsequent back transfer of some of this deposit to the bush material clearly take place. There is an equilibrium state in this two-way transfer process. The criterion of the plasticity index is used to show that the interaction is plastic. Some elastic encounters are also possible because of work hardening of a sublayer when cracks may originate below the surface at points where the hertzian shear stress is maximum. Cracks may also propagate at the surface. It is suggested that the wear fragments are produced from the transferred material and a high modulus of elasticity of the hypereutectic alloys probably increases their propensity to wear.