REVIEW Tribological properties of ultrafine-grained materials processed by severe plastic deformation Nong Gao • Chuan Ting Wang • Robert J. K. Wood • Terence G. Langdon Received: 20 October 2011 / Accepted: 23 December 2011 / Published online: 26 January 2012 Ó Springer Science+Business Media, LLC 2012 Abstract 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 lim- ited 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 tor- sion (HPT) and accumulative roll-bonding (ARB). Introduction Interest in the processing of bulk ultrafine-grained (UFG) materials through the application of severe plastic deforma- tion (SPD) has grown significantly over the last decade [1]. Processing by SPD refers to various experimental procedures of metal forming that can impose very high strains to materials thereby introducing a large volume of dislocations and pro- ducing exceptional grain refinement. A unique feature of SPD processing is that the high strain is imposed without any sig- nificant change in the overall dimensions of the workpiece. Another feature is that the shape is retained by using special tool geometries that prevent free flow of the material and thus introduce a significant hydrostatic pressure. Several SPD processing techniques are now available including equal-channel angular pressing (ECAP) [2], high-pressure torsion (HPT) [3], accumulative roll-bonding (ARB) [4], multi-directional forging (MDF) [5] and twist extrusion (TE) [6]. The application of SPD processing permits the relatively easy fabrication of bulk materials having typically more than *1000 grains in any direction within the sample volume. Thus, processing by SPD leads to very small grain sizes and the processed metals are usually termed UFG materials where the average grain sizes are typically less than 1 lm. To date, extensive studies have been performed to evaluate the microstruc- tural evolution, tensile properties and thermal stability of SPD-processed UFG materials. These investigations have established that UFG materials possess superior mechani- cal properties to their coarse-grained (CG) counterparts including a combination of high strength and good ductility at room temperature and a potential for exhibiting high strain rate and low temperature superplasticity at elevated temperatures. By contrast, the tribological properties of UFG materials have received only limited attention. N. Gao (&)  C. T. Wang  T. G. Langdon Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK e-mail: n.gao@soton.ac.uk C. T. Wang  R. J. K. Wood National Centre for Advanced Tribology, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK T. G. Langdon Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA 123 J Mater Sci (2012) 47:4779–4797 DOI 10.1007/s10853-011-6231-z