No abstract

The tribological properties of a centrifugally cast lead-free copper alloy (C90300), containing an average of 13 vol pct graphite particles (5 m), have been studied. Friction tests were carried out at three different loads of 44, 88, and 176 N using a pin-on-disk testing method for the base copper alloy and the copper-graphite composite against a 1045 steel disk counterface. The friction coefficient, temperature rise, and weight loss of the pin and disk were measured. To understand the wear mechanism, the wear debris and the surfaces of the pin and the disk were analyzed before and after the tests, using scanning electron microscope (SEM) and energy-dispersive X-ray (EDX) analysis. The friction coefficient of the copper-graphite pins was lower than that of the base-alloy pins for all applied loads, which was attributed to the presence of the graphite in the matrix. It was also observed that the presence of graphite in the matrix reduces the transfer of iron from the counterface to the pins, but enhances the transfer of materials from the pins to the counterface. The temperature rise in the counterface running against the base-alloy pins was larger than the temperature rise in the counterface running against the copper-graphite pins, both tested under similar conditions. In addition, the effect of element transfer on the friction coefficient, variations in the weight of the pins and the counterface, as well as the surface roughness, are attributed to the formation of a graphitic tribolayer on the surface of the copper-graphite pins. An isostrain model predicting the friction coefficient of the composites is proposed, which agrees well with the measurements in the present article as well as with measurements made by other investigators. [10]

Centrifugal casting technique was used to impart better tribological properties to the inner periphery of centrifugal castings of a C90300 copper alloy originally containing 13 vol pct graphite particles. Microstructural observation of centrifugally cast copper alloy containing graphite particles shows that a graphite-free zone and a graphite-rich zone (25 vol pct) with a unique microstructure are formed near the outer and the inner periphery of the centrifugally cast cylinders, respectively. Wear tests were conducted using a pin-on-disc apparatus running against a cast-iron counterface under dry conditions at applied loads between 44.5 and 267 N and at a sliding speed of 1 m/s. The worn surfaces of pin and counterface were analyzed using scanning electron microscopy (SEM) and energy dispersive Xray (EDX) analysis. The wear rate (1.89 ϫ 10 Ϫ13 to 7.59 ϫ 10 Ϫ13 m 3 /m) and the temperature (50 ЊC to 170 ЊC) at the counterface for the pins from the graphite-rich zone of the centrifugal castings were found to be lower than the friction coefficient (0.52 to 0.75), the wear rate (6.32 ϫ 10 Ϫ12 to 3.16 ϫ 10 Ϫ11 m 3 /m), and the temperature (70 ЊC to 200 ЊC) at the counterface for the pins from the graphitefree zone (of the same centrifugal casting under similar conditions). A greater transfer of the copper phases from the pin to the cast-iron counterface was observed visually from the pin of the graphitefree zone than from the pin of the graphite-rich zone, which was also confirmed by EDX analysis. This leads to an increase in the weight of the counterface running against the pin from the graphitefree zone with an increase in the applied load. Despite the presence of graphite in cast iron, the presence of graphite in the matrix of mating copper alloys lead to improved tribological properties. The effect of graphite particles on tribological properties of the composites was discussed in terms of the transfer of iron and copper phases, the interparticle distance between graphite in cast-iron and copper-graphite alloys, and the deformability of the matrix containing graphite.