Significant Improvement in Tribological Properties of CoCrNi Medium-Entropy Alloy by Silicon Addition

<p dir="ltr">The development of medium-entropy alloys (MEAs) has attracted significant attention due to their unique combination of superior mechanical properties and potential for high-temperature applications. Among these, CoCrNi-based MEAs have emerged as a promising class of materials known for their excellent strength and stability. The enhancement of these properties through the addition of Si has been reported but not thoroughly explored. This study investigated the effect of varying Si content on the microstructure, hardness, and wear resistance of CoCrNiSix MEAs.</p><p dir="ltr">In this work, CoCrNiSix (x = 0.1, 0.2, 0.3) MEAs were fabricated by casting. The ingots were homogenized, cold-rolled, and subsequently annealed at two different temperatures, namely 500°C and 800°C for 30 minutes to study the effect of thermal treatment on microstructure and mechanical properties. Wear tests were carried out under dry sliding condition using a ball-on-disc tribometer. The tests were conducted at room temperature (RT) and high temperatures (HT) of 400°C, 600°C, and 800°C to evaluate the coefficient of friction (CoF) and wear resistance of the alloys. The microstructure, hardness, and wear resistance of the CoCrNiSix MEAs were studied in detail.</p><p dir="ltr">The research found that increasing Si content significantly refined the microstructure and enhanced the phase stability, particularly in the CoCrNiSi0.3 alloy. Annealing at 500°C improved the hardness of the alloys, leading to superior wear resistance, while annealing at 800°C induced extensive recrystallization and grain growth, resulting in a reduction in hardness. However, this reduction was less pronounced in alloys with higher Si content.</p><p dir="ltr">Wear testing from RT to 800°C revealed that higher Si content consistently reduced the CoF and improved wear resistance. The CoCrNiSi0.3 alloy demonstrated the best overall performance, especially after 500°C annealing. At HT, the wear mechanisms varied with temperature, with 800°C showing the most pronounced plastic deformation and oxidation. Despite the reduced mechanical strength at this temperature, the formation of protective oxide layers led to lower wear rates, particularly in the CoCrNiSi0.3 alloy.</p><p dir="ltr">This study highlighted the critical role of Si in enhancing the microstructure, hardness, and wear resistance of CoCrNiSix MEAs, making these alloys promising candidates for applications requiring high durability and performance at elevated temperatures.</p>