Effects of plasma nitriding ion beam flux density and time on the properties of CoCrMo alloy

• We identify the reactive species in the nitrogen plasma.
• The effect of deposition time on nitriding behavior of CoCrMo alloy is investigated.
• The effect of ion beam flux density on nitriding behavior is investigated.
• Plasma nitriding can obviously improve the surface of CoCrMo alloy.

The effects of plasma nitriding ion beam flux density and deposition time on structural and tribological properties of medical forged CoCrMo alloy were investigated. Plasma nitrided processes under pure nitrogen were conducted at various ion beam flux densities for 2, 4, 6 and 8 h. The active species included in the nitrogen plasma, phase compositions, microstructures and surface microhardness were characterized by means of OES, XRD, SEM and microhardness test techniques. A milliammeter was used to detect the ion beam flux density. Dry wear tests were performed using a ball-on-disc tribotester. The OES data suggested that excited neutral molecules N2∗ and N2+ played important roles in DC plasma nitriding. The experimental analyses confirmed that plasma nitriding process has shown promise in producing thicker, harder, better hydrophilic and more wear resistance layers than the conventional CoCrMo alloys. As compared with untreated CoCrMo alloy, the nitrided CoCrMo alloy showed lower wear rates and wear scar widths, and the specimen nitrided at 5.8 mA/cm2-8 h exhibited the lowest wear rate and better dry-sliding wear resistance. The adhesive wear was the main mechanism for untreated CoCrMo alloy while the wear mechanisms of nitrided specimens were fatigue wear, abrasive wear and slight adhesive wear.