Chuan Ting Wang

Tribological properties of ultrafine-grained materials processed by severe plastic deformation

Processing by severe plastic deformation (SPD) has been developed extensively over the last two decades in order to produce ultrafine-grained (UFG) materials having submicrometre or nanometre grain sizes. An important material property for UFG materials is good wear resistance so that they may be used in a range of structural applications. An examination of the published data shows that only limited reports are available to date on the wear behaviour of SPD-processed materials and, furthermore, many of these results appear to be conflicting. The correlation of hardness and wear is limited because the wear property is a system property that in practice is influenced by a range of factors. Accordingly, this review is designed to examine recent reports related to the wear resistance of materials processed by SPD with particular emphasis on alloys processed using equal-channel angular pressing (ECAP), high-pressure torsion (HPT) and accumulative roll-bonding (ARB).

Processing of an ultrafine-grained titanium by high-pressure torsion: an evaluation of the wear properties with and without a TiN coating.

A commercial purity (CP) Grade 2 Ti was processed by high-pressure torsion (HPT) using an imposed pressure of 3.0GPa at room temperature. The HPT processing reduced the grain size from ∼8.6 μm in the as-received state to ultra-fine grains (UFG) of ∼130 nm after HPT. Tensile testing showed the HPT-processed Ti exhibited a good combination of high ultimate tensile strength (∼940 MPa) and a reasonable elongation to failure (∼23%). Physical vapour deposition was used to deposit TiN coatings, with a thickness of 2.5 μm, on Ti samples both with and without HPT processing. Scratch tests showed the TiN coating on UFG Ti had a critical failure load of ∼22.5 N whereas the load was only ∼12.7 N for the coarse-grained Ti. The difference is explained using a simple composite hardness model. Wear tests demonstrated an improved wear resistance of TiN coating when using UFG Ti as the substrate. The results suggest that CP Ti processed by HPT and subsequently coated with TiN provides a potentially important material for use in bio-implants.

Tribology testing of ultrafine-grained Ti processed by high-pressure torsion with subsequent coating

A grade 2 pure Ti was processed by high-pressure torsion (HPT) under 3.0xa0GPa for 10 revolutions to achieve an improved strength. Wear tests revealed that HPT only slightly improved the wear resistance of pure Ti. Subsequently, a TiN coating with a thickness of 2.5xa0μm was deposited on different Ti substrates to improve the wear resistance. Both indentation and scratch testing demonstrated a much improved load-bearing capacity when ultrafine-grained Ti was chosen as the substrate compared with coarse-grained Ti. All results indicate that pure Ti processed by HPT, when combined with a subsequent coating, represents a good candidate material for bio-implant applications.

Wear Behaviour of Al-1050 Alloy Processed by Severe Plastic Deformation

An Al-1050 alloy was processed by ECAP and HPT, respectively. Dry sliding wear tests were conducted on the as-received and SPD-processed samples under various sliding conditions. A comparison of wear rate indicated that SPD processing decreased the wear resistance. Two main wear mechanisms were observed. The initial wear stage was dominated by severe platelet wear mechanism and later wear was dominated by an oxidation wear mechanism. The results show the severe wear stage of SPD-processed Al-1050 is much longer than that of the as-received Al-1050, which is attributed to the loss of work hardening capacity after SPD processing.

Wear resistance of SPD-processed alloys

Various different severe plastic deformation processes (SPD) have been developed to produce ultra-fine grained (UFG) materials during the last two decades. One very important material property that the UFG materials should have for structural materials application is good wear resistance. This review paper presents some recent work related to the wear resistance of materials processed by SPD, in particular for alloys processed by using equal-channel angular pressing (ECAP) and accumulative roll-bonding (ARB).

Microstructural evolution and microhardness variations in a Cu–36Zn–2Pb alloy processed by high-pressure torsion

A coarse-grained Cu–36Zn–2Pb alloy with an initial grain size of ~54xa0μm was processed by high-pressure torsion (HPT) at room temperature under an applied pressure of 6.0xa0GPa through 1–10 turns, and the evolution of microstructure and microhardness was investigated. Analysis by X-ray diffraction (XRD) showed that in HPT processing the β′-phase transforms to an α-phase and a {111} texture is formed. Microscopic examination showed that dislocations were first formed at equivalent strains of not more than ~25 and when the equivalent strain increased to ~40 there was evidence for twins and secondary twinning. Fine grains were formed with an increase in equivalent strain to ~100 and with further straining these refined grains acted as precursors for additional grain refinement. The refined equiaxed grain size was ~250xa0nm after HPT through an equivalent strain of ~100 and the results show the microhardness reached a saturation value of ~220 Hv.