A.T. Alpas

Effect of SiC particulate reinforcement on the dry sliding wear of aluminium-silicon alloys (A356)

Abstract The dry sliding wear behaviour of cast aluminium 7% silicon alloys (A356) reinforced with SiC particles was investigated by means of a block-on-ring (52100 bearing steel) type wear rig. Wear rates of the composites with 10–20 vol.% SiC were measured over a load range of 1–150 N at sliding velocities of 0.16 and 0.8 m s −1 . Detailed electron metallography and X-ray diffraction analyses were undertaken to clarify the effect of SiC particles on the wear mechanisms. Observations indicated the following. (1) At low loads, corresponding to stresses lower than the particle fracture strength, SiC particles acted as load-bearing elements and their abrasive action on the steel counterface caused transfer of iron-rich layers onto the contact surfaces. In this regime, SiC reinforced composites exhibited wear rates about an order of magnitude lower than those of the unreinforced alloys in which wear occurred by subsurface crack nucleation, around the silicon particles, and growth. (2) Above a critical load determined by the size and volume fraction of SiC particles, carbide particles at the contact surfaces were fractured. A subsurface delamination process by the decohesion of SiC-matrix interfaces tended to control the wear, resulting in wear rates similar to those in the unreinforced matrix alloy. (3) An abrupt increase in the wear rates (by a factor of 10 2 ) occurred in the unreinforced aluminium-silicon alloy at 95 N. SiC reinforcement was proved to be effective in suppressing the transition to severe wear rate regime.

Effect of microstructure (particulate size and volume fraction) and counterface material on the sliding wear resistance of particulate-reinforced aluminum matrix composites

The effects of microstructure (namely, particulate volume fraction and particulate size) and the counterface materials on the dry-sliding wear resistance of the aluminum matrix composites 2014A1-SiC and 6061Al-Al2O3 were studied. Experiments were performed within a load range of 0.9 to 350 N at a constant sliding velocity of 0.2 ms-1. Two types of counterface materials, SAE 52100 bearing steel and mullite, were used. At low loads, where particles act as loadbearing constituents, the wear resistance of the 2014A1 reinforced with 15.8 µm diameter SiC was superior to that of the alloy with the same volume fraction of SiC but with 2.4 µm diameter. The wear rates of the composites worn against a steel slider were lower compared with those worn against a mullite slider because of the formation of iron-rich layers that act asin situ solid lubricants in the former case. With increasing the applied load, SiC and A12O3 particles fractured and the wear rates of the composites increased to levels comparable to those of unreinforced matrix alloys. The transition to this regime was delayed to higher loads in the composites with a higher volume percentage of particles. Concurrent with particle fracture, large strains and strain gradients were generated within the aluminum layers adjacent to contact surfaces. This led to the subsurface crack growth and delamination. Because the particles and interfaces provided preferential sites for subsurface crack initiation and growth and because of the propensity of the broken particles to act as third-body abrasive elements at the contact surfaces, no improvement of the wear resistance was observed in the composites in this regime relative to unreinforced aluminum alloys. A second transition, to severe wear, occurred at higher loads when the contact surface temperature exceeded a critical value. The transition loads (and temperatures) were higher in the composites. The alloys with higher volume fraction of reinforcement provided better resistance to severe wear. Wearing the materials against a mullite counterface, which has a smaller thermal conductivity than a counterface made of steel, led to the occurrence of severe wear at lower loads.

Transition between mild and severe wear in aluminium alloys

Abstract Mild and severe wear behaviours of a wrought aluminium alloy (6061 Al) were studied as a function of applied load and sliding velocity. Experiments were performed under unlubricated conditions using a block-on-ring (SAE 52100 steel) configuration within a load range of 1–450 N and a sliding velocity range of 0.1–5.0 m s −1 . Three different forms of transition between mild and severe wear, namely, load-, sliding velocity- and sliding distance-induced transitions have been observed. An empirical wear transition map has been constructed to delineate the conditions under which severe wear initiated. The role of contact surface temperature on wear transitions was also studied. It was observed that the transition to severe wear occurred when the bulk surface temperature exceeded a critical temperature. The dominant wear mechanisms in each wear regime were identified and classified in a wear mechanism map.

Wear mechanism maps for metal matrix composites

Empirical wear transition maps have been constructed to delineate the load velocity conditions under which wear transitions occurred in an A356 AI alloy and an A356 Al20%SiC composite. Experiments were performed in dry sliding conditions, using a block-on-ring (SAE 52100) configuration, within a load range of 0.2–400 N and a sliding velocity range of 0.2–5.0 m s−1. Both materials displayed transitions from mild to severe wear at specific load and sliding velocity combinations. The mild wear regime for the composite was expanded to a higher range of sliding speeds and loads by comparison with unreinforced A356 Al. Within the mild regime, two sub-regimes existed for both materials, namely where mixing/oxidation occurred at low sliding speeds to produce a thermally insulating surface layer, and another at higher speeds where this layer was removed. In the composite an additional wear transition, i.e. from mild to ultra-mild regime, occurred at low loads and velocities where the wear rates of the composite were at least two orders of magnitude lower than unreinforced A356 Al, due to the load supporting effect of the particles at the contact surfaces. In both materials, the transition from mild to severe wear occurred at conditions where the surface (bulk) temperature exceeded a critical value of approximately 125 °C in the A356 Al and 338 °C for the composite. The temperature data were summarised in the form of surface temperature maps on log load versus log velocity axes. Wear mechanisms in each regime were determined using scanning electron microscopy, and energy dispersive spectroscopy techniques that were used to analyse morphologies, microstructures and chemical compositions of worm surfaces and wear debris. The dominant mechanisms in each regime were identified and correlated with wear rate and temperature map data to summarise the effects of reinforcement on wear rates and transitions.

Wear regimes and transitions in Al2O3 particulate-reinforced aluminum alloys

The effect of the applied load on the unlubricated sliding wear behaviour of a 6061 Al alloy reinforced with 20 vol.% Al2O3 particles was studied. Experiments were performed using a block-on-ring type wear rig. Wear of the control alloy, i.e. the unreinforced 6061 Al, and the wear of the counterface (AISI 52100 steel) were also studied. Three wear rate regimes were observed in the composite: in region I, i.e. at low loads (less than 10 N) the wear rates were less than 2 × 10−5 mm3 m−1. This was followed by a transition region where the rates increased by a factor of 102. In region II that covered mid-range loads the wear rates raised steadily, 10−3 to 10−2 mm3 m−1, up to 230 N where a second transition took place to a severe wear regime (region III). In the unreinforced 6061 Al, only regions II and III were observed. At low loads the wear resistance of the composite (region I) was two orders of magnitude higher than that of the unreinforced alloy. In region II there was no significant difference between the wear rates of the unreinforced and the Al2O3-reinforced alloys. However, the transition from region II to III occurred at a lower load in the unreinforced 6061 Al (60 N). Metallographic studies performed to delineate the rate controlling wear mechanisms revealed that low wear rates in region I resulted from the load-bearing capacity of Al2O3 particles and the formation of transfer layers on the contact surfaces of the composites. When the applied load exceeded the fracture strength of the particles, particles at the surface were fractured and wear occurred by a process of subsurface crack growth. Al2O3 particles promoted crack nucleation and growth and acted as third-body abrasives resulting in wear rates similar to those in the unreinforced 6061 Al. The transition to severe wear rate regime is shown to be controlled by frictional heating to a critical temperature. This temperature was higher for the composite material for which the transition was postponed to higher loads.

Effect of grain size on friction and wear of nanocrystalline aluminum

Abstract The friction and wear characteristics of nanocrystalline aluminum were investigated as a function of grain size. Nanocrystalline aluminum samples with an average diameter of 16.4 nm were produced using an r.f. magnetron sputtering technique. The grain size was increased (up to 98.0 nm) by an isothermal annealing treatment at 573 K. Hardness measurements were performed using an ultra-microhardness indentation system and it was observed that within the grain size range of 15–100 nm the hardness-grain size data could be well represented by the Hall-Petch relationship. Friction and wear measurements were made using a miniature pin-on-disk type tribometer under unlubricated conditions both in air and in vacuum. The coefficient of friction of aluminum tested against a stainless steel pin varied with the sliding distance. At the early stages of sliding the coefficient of friction rose to a peak value, and this was followed by a decrease to a steady-state value. The transition on the friction curve corresponded to a similar transition from a severe wear regime to a mild wear above a characteristic sliding distance on the cumulative volume loss versus sliding distance curve. The value of the peak coefficient of friction decreased from μp = 1.4 for aluminum with a coarse grain size (106 nm) to μp = 0.6 for the nanocrystalline aluminum with a grain size of 16.4 nm. The coefficient of friction of nanocrystalline aluminum showed a 30% increase when tested in vacuum. In the nanocrystalline grain range, the wear rates were found to be linearly dependent on the square root of the grain size. An empirical equation based on the Archards Law is proposed to describe the effect of grain refinement on the wear resistance under unlubricated sliding conditions. A qualitative understanding of wear processes is developed in terms of the variation of the surface morphology and subsurface strength with sliding distance.

Delamination wear in ductile materials containing second phase particles

Abstract Plastic deformation and damage accumulation below the contact surfaces play an important role in sliding wear of ductile materials. In this study, metallographic techniques have been developed and used to determine the magnitude of the shear strains and the microhardness gradients in near surface regions in an aluminum-7% silicon alloy. Under dry sliding wear conditions, both the magnitude of plastic strains and the depth of heavily deformed zones increased with sliding distance and applied load. The flow stress and the plastic strains in the deformed zones are shown to obey a work hardening law which can be expressed in the form of a Voce type constitutive equation. Scanning electron microscopy (SEM) studies on the longitudinal sections below the worn surfaces indicated that thin flake-shaped debris particles were generated by a process of subsurface delamination occurred via cracks which originated from silicon particles within the deformed zones (but not at the contact surface) and propagated parallel to the surface. A model based on the hypothesis that delamination cracks are formed by the coalescence of voids at a critical depth below the worn surfaces has been proposed. It is shown that the critical depth for maximum rate of damage accumulation is determined by a competition beetween the plastic strain which enhances void growth and the hydrostatic pressure which suppresses it.

Effect of temperature on the sliding wear performance of Al alloys and Al matrix composites

Abstract The effect of ceramic particulate and graphite additions on the high temperature dry sliding wear resistance of two Al alloys was studied. The experiments were performed using a ring-on-flat sliding contact configuration against hardened SAE 52100 bearing steel counterfaces on an apparatus built for testing at controlled temperatures. Conditions were selected such that the materials in contact were kept in an isothermal atmosphere and the generation of frictional heat was minimised by the use of a low load (11.55 N) and sliding speed (0.1 m s −1 ). For unreinforced 6061 Al and A356 Al alloys a transition from mild to severe wear occurred in the ranges 175–190 °C and 225–230 °C respectively. With the addition of 20 vol.% Al 2 O 3 to 6061 Al, the mild to severe wear transition was raised to a range between 310–350 °C. Likewise, an addition of 20 vol.% SiC to the A356 Al increased this transition to 440–450 °C. A hybrid A356 Al composite containing 20 vol.% SiC and 10 vol.% graphite remained in a mild wear regime at the highest test temperature of 460 °C. All the reinforced alloys were able to withstand considerable thermal softening effects while remaining in a mild sliding wear regime. This is attributable to the formation of protective transfer layers of comminuted reinforcing particulates and transferred steel debris from slider counterfaces. Graphite in the hybrid composite introduced greater mild wear losses compared with the other composites due to increased friability and contact surface extrusion effects. The absence of severe wear phenomena in this composite contributes to the inhibition of comminution and fracture by graphite entrained in the surface tribolayer.

Tribo-layer formation during sliding wear of TiN coatings

Abstract The effects of atmospheric humidity and microstructure of counterface materials on the formation of tribo-layers on TiN coatings have been investigated. Pin-on-disc sliding wear experiments were conducted on physical vapour deposition (PVD) TiN-coated high speed steel (HSS) discs against HSS, mild steel and A356 Al–15% SiC pin materials under conditions of low and high relative humidity (RH). The TiN coatings undergo rapid wear by tribochemical oxidation and polishing at low sliding speeds and contact loads. This effect is reversed when contact loads and sliding speeds are raised and tribochemical wear is diminished. Increasing the humidity raises TiN wear rates and tribochemical wear seen at low loads and speeds, extending these phenomena to higher loads and sliding speeds. Hard abrasive inclusions or particles (e.g. SiC) in the counterface material prevent the formation of stable tribo-layers on the TiN surface and accelerate tribochemical wear effects by microabrasion of the TiN, particularly at high RHs.

Plastic deformation and damage accumulation below the worn surfaces

Abstract Delamination of material layers adjacent to the worn surfaces is a commonly observed form of wear in unlubricated or poorly lubricated surfaces. In ductile materials, the delamination process usually involves large plastic deformation and subsurface damage. In this study, metallographic techniques have been used to determine the extent of plastic deformation and strain localization events during the sliding wear of annealed OFHC copper samples. Tests were performed using a block-on-ring type wear machine under constant load and constant velocity conditions. Subsurface displacement and microhardness gradients were measured as a function of sliding distance. It was observed that both the magnitude of plastic strain (and stress) gradients and the depth of highly deformed layers vincreased with the sliding distance. The flow stress and strains at the subsurface regions are shown to obey a Voce type constitutive equation. Wear proceeded mainly by a mechanism of delamination via subsurface crack growth. It is proposed that the competition between the plastic strain which enhances void growth and the hydrostatic pressure which suppresses it is responsible for the generation of a damage gradient so that the delamination takes place at a certain depth where the damage accumulation rate is maximum. A model based on the Rice and Tracey analysis of ductile void growth is developed and used to determine the location of subsurface crack propagation.