Dry sliding wear behaviour at ambient and elevated temperatures of plasma transferred arc deposited aluminium composite coatings

Abstract

Plasma transferred arc (PTA) surfacing is a surface engineering process in which a coating is deposited on the substrate by the injection of metal powders and/or ceramic particles into the weld pool created by the formation of a plasma plume. The present work involved the tribological evaluation of metal matrix composite (MMC) coatings deposited onto an aluminium alloy using the PTA technique. Coatings were fabricated by the deposition of an Al–Ni powder containing either Al₂O₃ or SiC particles. Dry sliding wear behaviour of the coatings was evaluated at ambient and elevated temperatures. Under sliding conditions of low applied stress and ambient temperature, reinforcement properties such as interfacial structure and fracture toughness have a significant influence on wear resistance. The SiC particles, which exhibit high interfacial bonding and toughness, support the matrix by acting as load bearing elements, thereby delaying the transition in wear mechanism as applied stress increases. As applied stresses exceed the fracture strength of the SiC and Al₂O₃ particles, these particles suffer fragmentation and/or debonding and no longer support the matrix. At higher stresses and elevated temperature, matrix properties such as flow stress and the tribolayer formation play more important roles in determining wear resistance.     

Evaluation on tribological design coatings of Al2O3, Ni–P–PTFE and MoS2 on aluminium alloy 7075 under oil lubrication

The current work evaluated the friction and wear properties of tribological design surface coatings on aluminium alloy 7075 under various speed and nominal contact pressure. Hard-anodized Aluminium Oxide (Al₂O₃), burnished Refractory Metal Sulfide (MoS₂) and composite electroless nickel coatings with polytetrafluoroethylene (Ni–P–PTFE) particles were subjected to pin-on-disc sliding test against grey cast iron (GCI) under Mach 5 SL SAE 10 W-30 lubrication. The results indicated that Ni–P–PTFE composite coating possessed excellent friction–reduction capability but limited wear resistance due to low mechanical strength. Al₂O₃ coated sample showed outstanding wear resistance with high friction characteristic leading to high surface contact temperature. Furthermore, MoS₂ coating improved the wear resistance of the aluminium alloy.     

Processing,microstructure and properties of ultrasonically processed in situ MgO–Al2O3–MgAl2O4 dispersed magnesium alloy composites

AZ91 alloy matrix composites are synthesized by in situ reactive formation of hard MgO and Al₂O₃ particles from the addition of magnesium nitrate to the molten alloy. The evolved oxygen from decomposition of magnesium nitrate reacts with molten magnesium to form magnesium oxide and with aluminium to form aluminium oxide. Additionally, these newly formed oxides react with each other to form MgAl₂O₄ spinel. Application of ultrasonic vibrations to the melt increased the uniformity of particle distribution, avoided agglomeration, and decreased porosity in the castings. Ultrasound induced physical phenomena such as cavitation and melt streaming promoted the in situ chemical reactions. Well dispersed, reactively formed hard oxides increased the hardness, ultimate strength, and strain-hardening exponent of the composites. Presence of well-dispersed hard oxide particles and stronger interface resulting from cavitation-enhanced wetting of reactively formed particles in the AZ91 alloy matrix improved the sliding wear resistance of the composites.     

The tribological characteristics of the Al-20Si-3Cu-1Mg alloy reinforced with Al2O3 particles in relation to the hardness of a mating steel

The application of aluminium in automotive engines requires the material to be strong, stiff and, more importantly, wear resistant, which calls for reinforcement with hard ceramic particles. The resultant wear resistance of an aluminium matrix composite is affected not only by the intrinsic properties of the material but also by extrinsic factors involved in the wear process. Few studies have been conducted on the influence of mating material on the wear resistance of aluminium matrix composites and that of the whole friction couple as a system. This paper presents the results of the pin-on-disk wear tests of a potential piston material, the Al-20Si-3Cu-1Mg alloy reinforced with 10 vol.% Al₂O₃ particles, with the variation of the hardness of a steel counterface from 28 to 58 HRC. The work shows that the wear rate of the composite is significantly affected by the hardness of the counter-specimen. For a higher wear resistance of the composite, the mating steel should also be harder. A soft steel counterface would result in increased wear of both the composite and the steel, and thus increased total wear of the friction couple. The observed change in wear rate with the hardness of the counter-specimen is associated with the predominant wear mechanism. The work also shows that the friction coefficient of the composite specimen is also affected by the hardness of the counter-specimen, in addition to the pressure applied in the wear tests.     

Interface analysis and wear behavior of Ni particulate reinforced aluminum–silicon composites produced by PM

This study is concerned with the influence of Nickel, as reinforcement, in an aluminum–silicon (AlSi) alloy when regarding wear behavior. For these composites, the effect of Ni content, in the tribopair performance, was evaluated. For this purpose, the pin but also the counterface wear behavior was analyzed.Nickel particulate reinforced aluminum–silicon (AlSi) composites, with 5, 12.5 and 20 wt.% Ni were produced by a hot-pressing route. Microstructural characterization showed a uniform distribution of the Ni particulates in the AlSi matrix. EDS and XRD analyses revealed that the particle/matrix interface was formed by Al₃Ni intermetallic. Reciprocating pin-on-plate wear tests were performed with AlSi and AlSi–Ni pins against a gray cast iron (GCI) counterface. It was observed that the wear behavior of the AlSi–Ni/GCI tribopair is improved when compared with the AlSi/GCI system.     

Effect of high energy ball milling on solid state reactions in Al–25 at.-%Ni powders

Abstract

The effect of high energy ball milling on the solid state reactions between aluminium and nickel in Al–25 at.-%Ni powders has been investigated using scanning electron microscopy, thermal analysis techniques, and X-ray diffractometry. It has been observed that the microstructure of the powder particles evolves in three stages: stage I is the formation of entrapped nickel particles in the aluminium matrix structure; stage II is the formation of an Al–Ni multilayered structure; and stage III is the formation of Al₃Ni single phase. The temperature required to activate the reaction between aluminium and nickel during heating decreases by more than 200 K as the powder particle microstructure evolves from the entrapped particle structure to the multilayered structure, and then it decreases gradually with decreasing nickel layer thickness. The nucleation and lateral growth of Al₃Ni phase at the Al/Ni interfaces occurs at much lower temperatures than those required for the transverse growth of Al₃Ni. The fraction of Al₃Ni formed through nucleation and lateral growth at the interface is almost linearly proportional to the interfacial area. The activation energy for nucleation and lateral growth of Al₃Ni at the Al/Ni interfaces is independent of nickel layer thickness, but the activation energy for transverse growth of Al₃Ni decreases substantially with decreasing nickel layer thickness. The latter is attributed to the observation that the nickel layers are thinned by plastic deformation and thus contain an increasingly higher density of dislocations.     

Abrasive wear of FeCr (M7C3–M23C6) reinforced iron based metal matrix composites

Abstract

The effects of volume fraction, particle size, and sintered porosity of FeCr (M₇C₃–M₂₃C₆) particulates on the abrasive wear resistance of powder metallurgy (PM) Fe alloy metal matrix composites have been studied under different abrasive conditions. It was seen that the abrasive wear rate of the composites increased with an increase in the FeCr volume fraction in tests performed with 80 grade SiC abrasive paper, but it decreased for tests conducted with 220 grade SiC abrasive paper. Furthermore, the wear rates decreased with an increase in FeCr size for composites containing the same amount of FeCr. Hence it is deduced that Fe alloy composites reinforced with larger size FeCr particles are more effective against abrasive wear than those reinforced with smaller ones. At the same time the results show that the beneficial effects of hard FeCr particulates on wear resistance far outweighed the detrimental effects of sintered porosity in the PM metal matrix composites. In addition, the fabrication of composites containing soft particles such as graphite or copper favours a reduction in the coefficient of friction, and increases the matrix hardness of the composite. For this reason graphite and copper were used in the matrix in different amounts to test their effect on the wear resistance. Increase in graphite and copper volume fraction allowed the formation of additional phases, which had high hardness and wear resistance. It was also found that the wear rate of the composites decreased considerably with graphite and copper addition.     

Preparation and properties of cast aluminium alloy-granite particle composites

With a view to develop light weight, low cost and abrasion resistant material cast aluminium alloy composites dispersed with granite particles were prepared and their properties were evaluated. Natural mineral granite was crushed and treated prior to its incorporation in the aluminium alloy. Liquid metallurgy techniques was used to prepare composites involving the following steps: melting of aluminium alloy in graphite crucible, stirring of the melt, addition of granite particles and reactive metal in the melt and pouring the composite melt into permanent moulds. Physical, mechanical, tribological and metallographic properties of composites were studied. It was observed that there was reasonably uniform dispersion of granite particles in the matrix. Hardness and tribological (abrasive wear) properties of the base alloy improved considerably due to addition of the granite particles into it. This clearly indicates that these cast aluminium alloy based composites can be used as wear resistant materials.     

Effect of heat treatment on morphology of Fe-rich intermetallics in Al–Cu alloys

The iron-rich intermetallics are considered to be stable in aluminium alloys and usually hard to dissolve into the matrix during solution heat treatment (SHT). Nevertheless, solid-state transformations between Fe-rich intermetallics may happen and will play a great role in breaking-up of the large Fe-rich intermetallics particle during hot deformation process. In this study, the solid-state phase transformation of the iron-rich intermetallics from Al_mFe to β-Fe (Al₇Cu₂Fe) has been studied using SEM, XRD and deep-etched technique in a 2xxx Al–Cu cast alloy during SHT. The mechanism of solid-state transformation for the Fe-rich intermetallics was discussed, including the solid-state reaction, nucleation and growth of iron-rich intermetallics and Al phases.     

Effect of texture evolution on tensile properties of Al–Ni eutectic alloy fabricated by equal channel angular pressing

Abstract

The present study investigated in detail the effect of texture evolution on the mechanical properties of an Al–5·7 wt-%Ni eutectic alloy, which was subjected to severe plastic deformation by the equal channel angular pressing (ECAP) technique. The ECAP procedure was carried out using two strain introduction methods, route B_C and route A, at a temperature of 298 K and a pressing rate of 0·33 mm s^?1. The as pressed microstructures were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results indicated that the Al–Ni eutectic alloy specimens after ECAP processing by route B_C and route A methods had very different microstructures, which strongly affected the tensile properties of the specimens. It was demonstrated that after ECAP processing by route B_C, fine Al₃Ni particles of ～300 nm were homogeneously dispersed in the aluminium matrix, and the specimens showed no clear anisotropy in tensile properties. After ECAP processing by route A, however, eutectic textures containing α-Al and Al₃Ni fibrous dispersoids had a highly anisotropic distribution and were demonstrated to have significantly anisotropic tensile properties. Based on the experimental results, the fracture mechanism during tensile testing of the Al–Ni eutectic alloy using different strain induction methods is discussed.     

Microstructure evolution and tribological properties for new AlSi9Cu3/5%GrCu composite

A metallic matrix composite, with AlSi9Cu3 matrix reinforced with 5% copper coated graphite (Gr_Cu) was processed in semi solid state by centrifugal casting. This technique allows the uniform controlled distribution of the reinforcing material to provide improved tribological properties in certain area. The graphite particles were copper coated for a better embedding in the matrix. The microstructure evolution revealed compounds containing Mn, Si and Mg in the matrix alloy and the final composite and controlled distribution of the copper coated graphite as reinforcing material. The hardness measurement showed 48% improvement towards the matrix alloy and the Young modulus showed 27% improvement. The friction coefficient and wear rate obtained revealed a very good and promising behavior of the composite processed in semi solid state for ball bearings ring application.     

Effect of metal coating of reinforcements on the microstructure and mechanical properties of Al-Al2O3 nanocomposites

The effects of various methods of reinforcement modification on the microstructure and mechanical properties of Al–Al₂O₃ nanocomposites were investigated. Alumina nanoparticles were modified by electroless deposition of Cu, Ni and Co. Subsequently, aluminium matrix nanocomposites reinforced with uncoated and coated nanoparticles were produced by the stir casting method. The results of microstructural analysis showed improved wettability of coated nanoparticles in the molten aluminium alloy. Furthermore, coated nanoparticles exhibited a more desirable interface with the matrix and were homogenously distributed within it. The mechanical properties of the nanocomposites were improving significantly when coated nanoparticles were used as reinforcements. Among the reinforcement modification methods, Ni-coating was recognised as being more effective for improving the mechanical properties of Al–Al₂O₃ nanocomposites.     

Effect of nickel on the microstructure and mechanical property of die-cast Al–Mg–Si–Mn alloy

The effect of nickel on the microstructure and mechanical properties of a die-cast Al–Mg–Si–Mn alloy has been investigated. The results show that the presence of Ni in the alloy promotes the formation of Ni-rich intermetallics. These occur consistently during solidification in the die-cast Al–Mg–Si–Mn alloy across different levels of Ni content. The Ni-rich intermetallics exhibit dendritic morphology during the primary solidification and lamellar morphology during the eutectic solidification stage. Ni was found to be always associated with iron forming AlFeMnSiNi intermetallics, and no Al₃Ni intermetallic was observed when Ni concentrations were up to 2.06 wt% in the alloy. Although with different morphologies, the Ni-rich intermetallics were identified as the same AlFeMnSiNi phase bearing a typical composition of Al_[100–140](Fe,Mn)_[2–7]SiNi_[4–9]. With increasing Ni content, the spacing of the α-Al–Mg₂Si eutectic phase was enlarged in the Al–Mg–Si–Mn alloy. The addition of Ni to the alloy resulted in a slight increase in the yield strength, but a significant decrease in the elongation. The ultimate tensile strength (UTS) increased slightly from 300 to 320 MPa when a small amount (e.g. 0.16 wt%) of Ni was added to the alloy, but further increase of the Ni content resulted in a decrease of the UTS.     

Friction and wear of P/M Al-20Si-Al2O3 composites in kerosene

The results of friction and wear of powder metallurgy (P/M) Al-20 wt% Si-3 wt% Cu-1 wt% Mg-(2.5–10) vol% Al₂O₃ particulate-reinforced composites have been compared with those of the P/M aluminium alloy matrix and A-390 cast piston aluminium alloy. It was found that Al₂O₃ reinforcement reduces wear by five to eight times when mating with cast iron in kerosene: The higher the reinforcement volume, the lower was the wear. With increased volume of reinforcement the wear mechanism of composites changed from the adhesive to the fatigue/delaminating one. The wear of the cast-iron counter sample was several times higher than that for P/M composites. Considering the life of the piston-piston ring couple, the piston composite with 10 vol% Al₂O₃ appears to be the best. The rate of clearance development for this couple is twice as low as that for the conventional piston alloys.     

Continuous observation of microstructural degradation during tensile loading of particle reinforced aluminium matrix composites

Abstract

The microstructural degradation of aluminium alloy composites by external tensile loading was continuously observed by in situ scanning electron microscopy tensile testing. The composites, which contained spherical Al₂O₃ and angular SiC particles, were prepared by the powder extrusion method. Some microcracks were initiated at small plastic strains after yielding in both composites by inteliace debonding and particle cracking. Angular particles generate microvoids at smaller strains than spherical particles. The microcracks do not propagate with increasing external loading because of the ductility of the matrix, but a number of new microcracks developed just before failure. Most microcracks are due to interface debonding rather than particle cracking. Many dimples on the matrix aluminium alloy are observed on some particles seen in the fracture surface, which proves that there is relatively strong bonding between the matrix and the particles in the composite produced by powder extrusion. However, part of the original surface observed on the debonded particles indicates that an incompletely bonded interface also exists in the composites.

MST /3111     

Microstructural studies of aluminium-silicon alloy reinforced with alumina fibres

The microstructure of an alumina fibre reinforced Al-7wt% Si alloy has been investigated. It was shown that the Al-Si eutectic structure which characterized this alloy was markedly changed by the presence of the fibres, with coarsening of silicon particles and a reduction in primary aluminium grain size. The coarse silicon particles exhibited twinning but no orientation relationship with the aluminium. Fine silicon precipitates were also present and these had a cube-cube orientation relationship with the aluminium lattice. Lath-like intermetallics, FeSiAl₅ and FeSi₂Al₄ with monoclinic and tetragonal structures, were identified which existed in equilibrium and had the epitaxial relationship (001)_mono//(001)_tet and [100]_mono//[100]_tet. The iron was a contaminant introduced in the course of composite fabrication.Dislocations were a common feature of the aluminium matrix, with a typical density of 4×10⁷mm^–2. Nevertheless, dislocation hardening of the metal matrix was not detected. No evidence of Mg₂Si precipitates in the metal matrix was found, but the small addition (0.2wt%) of magnesium to the alloy was discovered to segregate at the fibre-aluminium interface. This segregation was believed to result in improved wettability of the two constituents, encouraging the formation of a strong fibre-matrix bond, and producing desirable properties of the composite in the transverse direction.     

Effect of SiO2 content in AI2O3 on interfacial bonding strength of aiuminium/AI2)3 composite

Abstract

In this study, the interfacial bonding strength between pure aluminium or an aluminium alloy (Al-12Si-1Cu-1Mg-2Ni) and Al₂O₃ containing 0-20 wt-%SiO₂ was measured using a B scale Rockwell hardness indenter. The interface was analysed using scanning electron microscopy, Auger electron spectroscopy, and X-ray diffraction. The results obtained were as follows. In the pure aluminium/Al₂O₃ composite the interfacial bonding strength was not strongly affected by the SiO₂ additions, but the interfacial strength of the aluminium alloy/Al₂O₃ composite could be significantly improved by small additions (2-5 wt-%) of SiO₂ in the Al₂O₃. Magnesium was the only alloying element to segregate at the interface. At the interface of the pure aluminium/Al₂O₃–20SiO₂ specimen, Al₂O₃ and silicon were detected, and MgAl₂O₄ and Al₂O₃ were detected in the aluminium alloy/Al₂O₃–20SiO₂ specimen. The interfacial chemical reaction involving magnesium is believed to be important in increasing the interfacial bonding strength.     

Experimental investigation on tribological behaviours of PA6, PA6-reinforced Al<Subscript>2</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>3</Subscript> and PA6-reinforced graphite polymer composites

This article reports on the preparation, characterization and experimental investigation of polyamide 6 (PA6) reinforced with alumina oxide (Al₂O₃) and graphite composites. The test specimens were prepared in an injection-moulding machine by varying the weight proportions of Al₂O₃ and graphite particles blended with PA6. The tribological properties of the composites were observed by using pin-on-disc wear test rig under dry sliding conditions. The worn surfaces of the composites were examined using scanning electron microscope. The addition of Al₂O₃ and graphite significantly enhanced the tribological properties of PA6. The PA6 containing 30 wt% Al₂O₃ and 20 wt% graphite revealed the best tribological behaviours due to the stronger interfacial bonding characteristics with improved wear resistance. Further, the thermal stability of Al₂O₃ and graphite particles was studied through thermogravimetric analysis test. It was also found that further addition of Al₂O₃ and graphite in PA6 had no significant improvement in wear resistance, the co-efficient of friction and heat generation.     

Composites of aluminium alloys: fabrication and wear behaviour

In this paper processes for fabrication of aluminium-alloy composites containing paniculate non-metals, the net shape forming of these composites, their microstructures, their friction and wear behaviours and their mechanical properties are described. Composites of two wrought (2014 and 2024) and one cast (201) aluminium alloys containing 2 to 30 wt% of Al₂O₃ and SiC particles in the size range of 1 to 142m were prepared. The non-metallic particles were added to a partially-solid vigorously-agitated matrix alloy. The particles were then retained in the matrix until interface interaction, for example, the formation of MgAl₂O₄ spinel in the case of Al₂O₃ particles, were faciliated. These composites were solidified and subsequently reheated to above their liquidus temperature and formed under high pressure in a closed-die forging type of apparatus. Composites with particulate additions of size larger than 5m possessed homogeneous structures; particles of size 1m, however, tended to cluster. The wear behaviour of the composites was studied using a pin-on-disc type machine. It was shown that composites containing large amounts of non-metals, 20 wt%, exhibit excellent wear resistance whilst those with small to moderate amounts of non-metals possess tensile properties comparable to the matrix alloy. Increasing the amount of particulate additions results in reduced ductility. Finally, a method was investigated of producing components with high weight-fractions of non-metals near their surface.     

Wear behaviour of aluminium matrix TiB2 composite prepared by in situ processing

Abstract

The wear behaviour and microstructure of aluminium and Al-12Si alloy (A413) matrix composites containing 1 and 5 vol.-%TiB₂ particles have been investigated. The composites were prepared by an in situ reactive slag technique. The wear surfaces and wear products were studied after reciprocating and rolling - sliding tests. Wear resistance increased with increasing particle content, and the Al-12Si composites were more wear resistant than those with Al matrixes. The wear mechanisms are briefly discussed.