Tribological behavior of metal matrix composites
Abstract
The wear and friction behavior of continuous graphite fiber reinforced metal matrix composites was investigated. Composite materials were tested against 4620 steel at 54 m s−1 at room temperature in air without lubricant. The graphite fibers studied included rayon-, pitch- and polyacrilonitrile (PAN)-based fibers. Both high modulus and high strength PAN-based fibers were examined. The fibers were incorporated into copper- and silver-based alloys by means of a liquid metal infiltration technique. The results of this study indicate that the type of graphite fiber in the composite is the most significant factor in the wear and friction behavior of metal matrix composites. In some high modulus fiber tin-bronze composites the fiber fraction influences the wear rate but not the coefficient of friction. Neither the matrix alloy nor the composite tensile strength per se correlate with the friction and wear properties; however, there are specific trends for the various matrix alloys.
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Cited by (22)
The wear behaviour of δ-alumina fibre-reinforced aluminium–zinc–copper alloy composites produced by squeeze casting was investigated using a pin-on-disc wear machine. Samples of metal matrix composites were tested in the normal and planar-random orientations sliding against a smooth steel counter-surface disc at a fixed speed under different loads without lubrication. Wear surfaces of the MMCs and of their matrix alloy were also examined in a scanning electron microscope. The experimental results showed that the composite exhibited a low wear rate compared to the unreinforced matrix material. The wear rate decreased with an increase in volume fraction of fibre, and increased with increasing load. The optimum wear resistance occurred when most of the fibres were oriented normal to the sliding surface. Scanning electron microscopy of the worn surfaces showed that microfracture of the fibre, and reattachment of a fragmented surface layer in the normal orientation, and large scale microfracture of the fibre, and subsurface delamination were the principal mechanisms of wear in the planar-random orientation composites. Oxidation and partial delamination were predominant in the unreinforced matrix alloy.
The effect of sliding speed and microstructure on the dry wear properties of metal-matrix composites
1998, WearThe effects of sliding speed and microstructure on the dry wear and friction properties of unreinforced 2014 aluminium alloy matrix and its unidirectional boron fibre-reinforced composites were investigated. Tests were conducted on the composites with fibres oriented normal and parallel to the sliding direction, rubbing against a rotating steel disc at speeds of 0.6, 1.0 and 1.6 m s−1 under different loads. Wear surfaces and subsurface sections of the wear samples of the matrix and composites were examined by scanning electron microscopy after wear testing. The metal-matrix composites showed excellent wear resistance compared with the unreinforced matrix, but the orientation of the fibres with respect to the sliding direction was found to affect the wear rates. The normal orientation (N) displayed a better wear resistance than the parallel orientation (P) when the tests were conducted at speeds of 0.6 and 1.0 m s−1, but there was little difference at the highest speed of 1.6 m s−1. The presence of fibres reduced the amount of surface damage and subsurface plastic deformation for both sliding directions. The fibres in the surfaces of the N-oriented samples were chipped at the ends during wear tests, and the detached fragments of fibre became embedded into the soft aluminium alloy matrix, giving a low wear rate, especially at low sliding speed. At higher speeds more rapid wear occurred, but the onset of oxidative wear at the highest speed of 1.6 m s−1 gave a very low wear rate. In the P-oriented samples, at low speed, many segments of fibres were pulled out from the wear surfaces owing to friction against the disc, and surface ploughing occurred, giving a high wear rate which mostly peaked at 1.0 m s−1. At the highest speed fibre fragmentation replaced pull-out and the small fibre fragments remained embedded in the surface, which with the development of surface oxides, reduced the wear rate to its minimum value. In general the friction coefficient of the matrix and composites decreased with increased sliding speed, but many tests showed a peak at a speed of 1.0 m s1. The matrix had a lower friction coefficient than the composites, and at the lower load the friction coefficient increased with fibre content for both fibre orientations, but at the higher load behaviour was erratic owing to the opposing effects of fibre fracture and matrix oxidation. For the P-orientation the friction coefficients of the composites were lower than those of the N-orientation, and most peaked at the intermediate speed.
The mechanical and tribological properties of Cu-Nb in situ composites were studied. The composites were prepared by consumable arc melting of Cu and Nb electrodes, by casting, and by successive deformation of the cast ingot. Dry sliding friction and wear tests were performed in a pin-on-disk set-up, with the composite pin rubbing under atmospheric conditions against a hardened tool steel disk surface. The effect of Nb proportion on the tribological behavior of Cu-Nb composites was studied. It was found that the coefficient of friction decreased with increasing Nb proportion in the composite and that Cu-20vol.%Nb composite had the best wear resistance. The effect of prior deformation strain on the coefficient of friction and wear of Cu-15vol.%Nb composites was also investigated. The micromechanisms of wear were studied by scanning electron microscopy. It was concluded that cracking in the severely work hardened regions led to the generation of wear particles.
The friction and wear behavior of three uniaxial metal-matrix composites (graphite/Al, stainless steel/Al and Al2O3/Al-Li) were investigated. Unidirectional sliding tests were conducted in the three principal orientations (longitudinal, transverse, and normal). The experimental results suggest that graphite/Al is a low-friction, low-wear composite. In contrast, stainless steel/Al is a high-friction, high-wear and highly anisotropic composite. Scanning electron microscopic observations of the worn surfaces revealed that in all three composites fiber pull-out was the predominant mechanism of wear in transverse sliding, and is the principal reason for wear anisotropy.
Room temperature wear characteristics of Al<inf>2</inf>O<inf>3</inf>-particle-reinforced aluminum alloy composite
1991, Materials Science and Engineering ARoom temperature dry wear and friction properties of a metal matrix composite sliding against cast iron have been studied in a pin-on-plate reciprocating wear tester. The composite consists of Al2O3 particles in an aluminum alloy matrix. Wear is caused by abrasion, interfacial adhesion and ploughing of the softer surface with the asperities of the harder surface and the debris. With increasing load the friction coefficient and wear initially increase and then ecrease. The decreases in wear and friction are caused by increased hardness resulting from re-embedding of the fractured Al2O3 particles near the surface of the composite plate at high loads. The wear characteristics of the composite have been studied over the range of normal stress from 0.31 to 4.72 MPa.
Sliding wear
1987, Tribology SeriesThis chapter provides an overview of sliding wear. Sliding wear can be characterized as a relative motion between two smooth solid surfaces in contact under load, where surface damage during the translational sliding does not occur by deep surface grooving because of penetration by asperities or foreign particles. The surfaces may be of metallic or nonmetallic nature, and lubricated or unlubricated. Many different parameters of a tribosystem are involved to some extent in the friction and wear of sliding pairs. In sliding contact, wear can occur because of adhesion, surface fatigue, tribochemical reaction and/or abrasion. Many factors influence the prevailing wear mechanism. The type of contact—namely, elastic or plastic—is a function of the tangential traction on the surface, the contact area and material properties such as the yield strength. The formation of adhesive junctions among asperities on two clean solid surfaces can be described by the atomic structure, briefly highlighted in the chapter.