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Abstract: A characteristic of almost any type of dry or boundary lubricated tribological contact is that the composition and tribological properties of the original interface will change during use. The modifications include topography changes (smoothening or roughing), formation of micro-cracks, phase transformations, deformation hardening, formation of oxides, formation of solid films by reactions with lubricant additives, transfer of material from the counter surface, and so on. The thickness of these layers and films range from atomic monolayers (e.g., hydrogen termination of diamond surfaces) to tens of μm (e.g., plastic deformation of metals). Since the tribological properties of tools, wear parts and mechanical components then are determined by these modified surfaces rather than by the original, they deserve attention and careful assessment. Without the knowledge about how these surface layers are formed and how they modify the tribosystem, it is not possible to predict the friction and wear properties of a material in a given tribological situation. Thus, post-test evaluation including microscopy and analysis of both surfaces and cross-sections is required. A number of surface modifications selected from a wide variety of tribosystems is briefly presented. The corresponding surface modifications range from the most well known and simple to the more complex and relatively unexplored. Finally, the potential of designing materials and lubricants for improved surface modification is discussed and examples are given of current research projects on promoting formation of protecting low-friction tribofilms in boundary-lubricated systems. [Copyright &y& Elsevier]

Electro-deposition of Ni has been used in an attempt to seal the open porosity of plasma sprayed alumina coatings. By means of scanning electron microscopy, the open porosity was found to be completely sealed. The as deposited and sealed coatings have been evaluated and compared by scratching, abrasion, particle erosion and sliding wear tests to determine the effects of the sealing treatment on the tribological properties. Also, studies of Vickers indentations were performed to assess the effect of the sealing on crack propagation. The mechanisms of damage including wear have been identified and discussed.It is concluded that electro-deposition of Ni is a promising sealing treatment for sprayed alumina coatings intended for tribological applications. The hardness increased and the wear resistance was found to increase markedly in all wear tests as a result of the sealing. [Copyright &y& Elsevier]

An electrolytic deposition procedure has been used to completely seal the open porosity of plasma sprayed alumina with Cu. Scanning electron microscopy (SEM) showed that the open porosity was sealed, leaving only closed pores in the coating. Both sealed and unsealed materials have been evaluated and compared using standard tribological testing, including scratch testing, abrasion, particle erosion and wear in dry sliding contact. As a result of the sealing procedure the wear rate was found to decrease markedly in erosion and abrasion while the wear rate increased in dry and lubricated sliding. The dominant mechanisms responsible for the wear are discussed, as well as the advantages and limitations of electro-deposition as an alternative sealing procedure for sprayed coatings. It is concluded that electro-deposition is a promising sealing technique for selected tribological applications. [Copyright &y& Elsevier]

Abstract: In percussive and rotary percussive rock drilling, the rock is crushed into small fragments by the repeated hard impact of the drill bit, and subsequently removed by flushing water or air. To avoid excessive wear, the steel drill bit is equipped with a set of cemented carbide buttons that protrude from the bit to take the actual impact. The severe contact against the rock results in some wear of the button, but also in formation of surface layers of rock material and penetration and impregnation of rock material into the cemented carbide structure. This situation, with serious implications for the wear and fracture of the buttons, have previously not been reported. The present findings represent a significantly new understanding of the wear of the rock button material. The deterioration mechanisms are described in detail, using examples from a range of real drilling applications in different rock types. During operation, material in the surface layer of the drill button shifts from that of the original cemented carbide into an uncontrolled composite. This composite is formed by the WC carbide hard phase and a binder consisting of a mixture of cobalt and rock. This new material should be expected to exhibit properties significantly different from the original cemented carbide. [Copyright &y& Elsevier]

Low-friction coatings are used more and more frequently, particularly in situations and applications with insufficient or no lubrication. A good example of such coatings is amorphous carbon, which is produced both in pure form (a-C:H) and doped with metal (Me-C:H). The knowledge of what actually occurs when one metal in a Me-C:H coating is exchanged with another has so far been rather limited. Also, when producing these films hydrogen is incorporated in the substrate as well as in the film, which can be detrimental to the overall properties.Here, a newly adopted co-sputtering technique, utilizing a carbon target partly covered by metal-foil strips, was used to deposit non-hydrogenated carbon coatings alloyed with Ta, W and Zr on ball-bearing steel (BBS) substrates. The metal content varied between 0 and 41 at.%, and the resulting films were analyzed with respect to phase composition and textures, chemical composition, microstructural morphology, as well as mechanical and tribological properties. All alloyed coatings displayed a nanocomposite microstructure, with 3–6 nm metal-carbide crystallites embedded in a matrix of amorphous carbon. The amount of metal-carbide phase increased with increasing amounts of metal which led to a large increase in hardness and elastic modulus. An increased metal content did however not affect the carbide size to any notable extent. Ball-on-disk tests show that metal additions cause a sharp drop in friction coefficient from 0.21 to about 0.05, depending on the metal used. This is however accompanied by an increase in wear rate. The coating best combining low friction and low wear rate was alloyed with 20 at.% Ta. Best possible protection of the counter surface was offered by coatings containing 30 at.% Ta or more. [Copyright &y& Elsevier]