Wear of particulate metal matrix composites

Particulate reinforcing phases in metal-matrix composites (MMCs) can provide resistance to both sliding wear and abrasion by protecting the softer metallic matrix. In this review, the wear behavior of particulate MMCs in dry and lubricated sliding, and also under conditions of abrasive wear, is discussed. Stable tribological performance can be achieved under conditions of mild sliding wear. While wear rates normally reach an equilibrium (or steady state) in the mild wear regime, accelerated wear rates (with sliding distance and load) are observed in the severe wear regime due to large scale plastic deformation, and in some instances, local melting of the surface layers. MMCs can show significantly lower wear rates compared to unreinforced alloys over a substantially wider range of pressure and sliding speed. Wear mechanism maps are effective ways of delineating the regimes of dominant sliding wear mechanism (oxidation, delamination, seizure etc.) and show the effect of frictional temperature on wear transitions. The very small wear rates observed in lubricated sliding define a new wear regime different from mild and severe wear not only in terms of the infinitesimal rate of material removal but the mechanisms of material removal involved are also different. Tribolayers that form during the running in period are responsible for the very low wear rates observed in this ultra-mild wear regime. Reinforcement can also provide excellent improvements in resistance to low-stress abrasive wear. However, major enhancement of wear resistance in the severe sliding wear regime, or to high-stress abrasion, cannot be expected from these composites

Investigation of the role of tribolayer formation in improving drilling performance of Ti-6Al-4V using minimum quantity of lubrication

Relationships between tribological properties of Ti-6Al-4V and the machining behaviour of the same alloy drilled by a WC-Co cutting tool using minimum quantity lubrication (MQL) were studied. The tribological behaviour of Ti-6Al-4V sliding against WC-Co was evaluated by determining the changes in the coefficient of friction (COF) using cutting fluids employed in MQL and flooded drilling tests. When pin-on-disk tests were carried out under the boundary lubricated condition, the MQL fluid (MQLF) provided a low COF of 0.16 at 25 ℃ and 0.24 at 250 ℃. The observed low COFs were attributed to formation of a tribolayer on the WC-Co counterfaces. A high COF of 0.45 observed at 300 ℃ indicated that the tribolayer was no longer stable. Application of conventional water-based cutting fluid (CF) also resulted in a low COF of 0.26 at 25 ℃ but the COF rose to 0.32 at 100 ℃. During drilling tests, MQLF was supplied at a rate of 80 ml/h using an external MQL system. The average drilling torque was reduced by 35% compared to CF as a result of the formation of tribolayer on the cutting edge of the tool as well as on the drilled hole surfaces, as the cutting temperature remained &lt;250 ℃. According to Raman and X-ray photoelectron spectroscopy (XPS) analyses, the tribolayer on the drilled hole surface was formed as a result of degradation of additives in the MQL. The presence of phosphate- and sulfate-based anti-wear components in the tribolayer proved to be beneficial in preventing metal-metal contacts during drilling and provided a low COF. The low COF between tool-workpiece contact resulted in a low and uniform torque response throughout the drilling cycles and prevention of built-up edge (BUE) formation, leading to an improved surface finish. </jats:p