Tribology - Materials,Surfaces & Interfaces

During electrical discharge machining (EDM) process, electrical energy is used for the machining of the components. Energy distribution in electrical discharge machining process is the distribution of input energy supplied during machining to various components. In order to improve the technological performance during EDM process, it is essential to understand the distribution of input energy in the entire system. An experimental study on the effect of EDM energy distribution parameter for tungsten carbide is presented. The copper tungsten electrode has been used for the study. Experiments have been performed in specially designed dielectric insulated tank. To minimise the energy wastage, workpiece as well as the electrode was covered with Teflon. Current and pulse duration have been selected as variable parameters. The objective of this study is to analyse the amount of electrical energy used for machining effectively. The detail of this study has been presented in this paper.

The fretting corrosion of a Ti–6Al–4V flat in contact with a poly(methyl methacrylate) (PMMA) ball in 0·9 wt-% NaCl solution was investigated using a fretting rig operating under electrochemical control. The effect of potential and of normal load on friction, wear and electrochemical response was studied under gross slip regime. No noticeable mechanical deterioration of the Ti–6Al–4V surface could be observed. At anodic potential, alloy corrosion was only slightly enhanced by fretting. Wear of PMMA was large and controlled by third body formation. A correlation between PMMA wear coefficient and thickness of third body was observed.

Low temperature plasma surface alloying with carbon (i.e. plasma carburising) of Stellite 21 Co–Cr alloy was conducted at temperatures from 400 to 500°C for 15 h in a gas mixture of 98 vol.-%H₂ and 2 vol.-%CH₄. The surface treated layers were characterised by XRD, SEM and microhardness tests. The corrosion and corrosive wear behaviour of the plasma carburised Stellite 21 Co–Cr alloy were studied respectively using electrochemical tests and well designed reciprocating wear tests in 3·5% NaCl solution. The results show that low temperature (≤460°C) plasma carburising can improve the corrosion resistance of Stellite 21 alloy; the corrosive wear resistance of Stellite 21 can be enhanced by up to three times; and the best corrosive wear resistance is achieved at the highest treating temperature (500°C). The detailed studies on the wear tracks indicate that the corrosive wear process was dependent on the individual wear and corrosion, as well as the synergetic effect.

WC–Co cemented carbides, including small angular tungsten carbides particles, are used extensively to improve wear resistance. Some additives can affect mechanical and wear properties of these materials. In this study, the effect of VC and (Ta, Nb)C content on wear of WC–10Co were considered. The tests were performed at normal load of 230 N and sliding distance of 800 m up to 3200 m. Wear tests were carried out using dry sand/rubber wheel apparatus. Wear rate, standard and modified wear coefficients were calculated. The microstructures of prepared specimens were examined by optical microscopy. The morphological analysis of the worn surfaces was made by SEM. The results show that VC content has more effect than (Ta, Nb)C content on wear behaviour. Wear mechanism is different in the specimens, but removal of cobalt rich phase and fracture of carbide grains is clear in all of specimens. Abrasive wear is prevailing in all specimens.

The effects of the speed sequence and SiO₂ content of Cu–SiO₂, sintered by powder metallurgy method, on friction and wear properties have been investigated at fixed speeds. The results indicate that the sequence of speeds employed in the tests plays great roles in the friction and wear properties. When the tests are executed from a lower speed to a higher speed, friction coefficients decrease and oscillate dramatically as the speed goes up, resulting in a severe wear. On the contrary, as the speed starts from a higher value, the friction coefficients are stable and wear is small. These phenomena can be explained by states of third bodies formed in the friction. The third body formed at lower friction speeds is usually granular, which is responsible for the coefficient oscillations and larger wear loss. At higher speeds, the third body formed is rather dense, leading to stable friction coefficients and lower wear loss.

Investigations were undertaken to determine the erosion corrosion resistance of nanostructured titanium dioxide coatings in 5 vol.-%–3·5 wt-% NaCl slurry at velocities ranging from 1 to 4 m s^–1 in a recirculating loop. Two types of nanopowders, spray dried and densified (AE 9342) and chemically precipitated and spray dried (AE 9303) were used. The results were compared with a conventional TiO₂ coated samples (SM 102). Specimen AE 9342 showed a higher resistance compared to AE 9303. No localised corrosion on the above specimen was observed. The erosion corrosion was caused by etching of intersplat boundaries. The erosion corrosion is dependent on surface topography. A homogeneous distribution of nanoagglomerates of unmelted, partially melted nanoparticles embedded in coatings, a large area of melted zone and porosity less than 1% enhances the erosion corrosion resistance of nanostructured titanium dioxide coatings.