Characterisation of nitrogen-bearing surface layers on Ni-base superalloys

Nitrogen plasma immersion ion implantation (PI cubed) of a commercial Ni-base alloy (Inconel 601) was performed to assess the effects of such treatments on the alloy's wear and corrosion resistance. After 5 h treatment at 350 and 400 degrees celsius a super-saturated fcc phase was formed. At 450 and 525 degrees celsius CrN precipitates were formed in the nitrided layer. The hardness initially increased with increasing temperature from 6.83 ± 1.43 GPa at 350 degrees celsius to 15.05 ± 0.53 GPa at 450 degrees celsius. At 525 degrees celsius however a lower hardness of 9.40 ± 0.69 GPa was recorded. The hardness of the untreated alloy was 2.94 ± 0.16 GPa. All of the nitrided samples showed superior corrosion resistance to the untreated alloy in 3.56 wt.% NaCl but corrosion of the untreated core occurred at grain boundaries and especially at triple points on the nitrided surfaces. Unlike austenitic stainless steels, the presence of CrN in the nitrided layers did not cause a dramatic reduction in corrosion resistance. Unlubricated reciprocating pin-on-disc wear tests against an alumina ball showed that the PI-cubed treatment in all cases reduced wear (by as much as 3 orders of magnitude) and friction compared to the untreated alloy. Despite having the second lowest hardness the sample nitrided at 525 showed the lowest wear and friction