Effects of Load and Speed on the Friction and Wear Properties of Copper Alloys

With the gradual increase in the power of diesel engines in China, higher requirements were put forward for the friction and wear performance of the sliding bearing materials of key parts in its harsh working environment. The dry friction and wear properties of three typical sliding bearing copper alloy materials QSn7-0.2, CuZn31Si1, Cu9Ni6Sn, which are currently and widely used in high-power diesel engine connecting rod heads, were compared and studied, and their wear mechanisms were further discussed and analyzed. By using the MMW-1A friction and wear testing machine, the pin-on-disk experiment were carried out at room temperature and without lubrication. The test duration was 60 min (including a 10-minute running-in period). The load were 18 N, 36 N, 72 N, 108 N and speed were 0.12 m/ s, 0.24 m/s, 0.36 m/s, 0.48 m/s. The effects of load and speed on the dry friction and wear properties were studied. Dry friction and wear properties of the sliding bearing copper alloy materials were discussed on three aspects, i.e. average friction coefficient, wear rate, and wear surface morphology and composition analysis. By increasing the load, the average friction coefficient of QSn7-0.2 first increased and then decreased, the average friction coefficient of CuZn31Si1 gradual increased, while the average friction coefficient of Cu9Ni6Sn first decreased, then increased and then decreased. The maximum average friction coefficients were 0.67 at 72 N for QSn7-0.2.. CuZn31Si1 had an average friction coefficient of 0.55 at 72 N, and a maximum value of 0.61 at 108 N. Cu9Ni6Sn had a maximum average of 0.53 at 72 N. By increasing the rotation speed, the average friction coefficient of QSn7-0.2, CuZn31Si1, Cu9Ni6Sn consistent increased first and then decreased. At a speed of 0.36 m/s, the average friction coefficients of QSn7-0.2, CuZn31Si1, Cu9Ni6Sn were the largest respectively 0.67, 0.55, 0.53. By increasing the load, the wear rate of QSn7-0.2 first decreased, then increased, and then decreased. The wear rate of CuZn31Si1 gradual increased. The wear rate of Cu9Ni6Sn increased and then decreased. At a load of 72 N, the wear rate of QSn7-0.2 was 9.1115×10⁻⁴ mm³/(N·m), which was 2.4 times and 2.6 times the wear rate of CuZn31Si1 and Cu9Ni6Sn. At 108 N, the wear rate of CuZn31Si1 was 3.710×10⁻⁴ mm³/(N·m). The average wear rate of QSn7-0.2, CuZn31Si1 and Cu9Ni6Sn increased first and then decreased with the increase of rotating speed. The maximum wear rates of CuZn31Si1 and Cu9Ni6Sn were 7.7774×10⁻⁴ mm³/(N·m) and 5.2321×10⁻⁴ mm³/(N·m) at a rotation speed of 0.24 m/s. At a rotation speed of 0.36 m/s, the maximum wear rate of QSn7-0.2 was9.1115×10⁻⁴ mm³/(N·m). At a speed of 0.36 m/s and a test load of 72 N, the worn surface of QSn7-0.2 showed large areas of adhesive wear pits and massively abrasive particles, suggesting adhesive wear. When the test load was 108 N, thin and deep furrowed scratches and large area of adhesive spalling pits appeared on the worn surface of CuZn31Si1, indicating abrasive wear and adhesive wear. Cu9Ni6Sn's worn surface had several adjacent spalling pits and many pits, which was evidence of fatigue wear. Under the condition of a load of 72 N and a rotation speed of 0.36 m/s, the worn surface of QSn7-0.2 had furrowed scratches and adhesive spalling pits, and the surface damage was more severe. This was a typically adhesive wear phenomenon accompanied by mild abrasive wear. The worn surface of CuZn31Si1 had a large area of spalling pits, and a plastic deformation in the form of pushing and corrugated, which belonged to abrasive wear and mild plastic deformation wear. The worn surface of Cu9Ni6Sn showed shallow furrowed scratches and corrugated plastic deformation, which belonged to mild plastic deformation wear accompanied by mild abrasive wear. At the same time, the wear condition of the friction surface was also affected by elements such as carbon and oxygen on the worn surface. The adsorbed carbon species on the wear surface played a role in reducing wear, thereby reducing the friction coefficient and reducing the wear. Under the action of frictional heat, the oxidation of metal elements had a bearing and lubricating effect. By incxreasing the load and speed, the values of average friction coefficient and wear rate were Cu9Ni6Sn<CuZn31Si1<QSn7-0.2. When the load increaseds, the wear mechanism of QSn7-0.2 varied from abrasive wear to adhesive wear; The wear mechanism of CuZn31Si1 changed from plastically deformed wear with mild abrasive wear to abrasive and adhesive wear. The wear mechanism of Cu9Ni6Sn changed from mild abrasive and plastically deformed wear to fatigue wear. When the rotational speed increased, the wear mechanism of QSn7-0.2 changed from plastically deformed wear to adhesive wear, and the whole process was accompanied by mild abrasive wear. The wear mechanism of CuZn31Si1 changed from abrasive wear to plastically deformed wear. The wear mechanism of Cu9Ni6Sn changed from slightly abrasive wear to coexistence of abrasive wear and plastically deformed wear.