Effect of nitriding voltage on the impact load fatigue and fracture toughness behaviour of CoCrMo alloy nitrided utilising a HIPIMS discharge

Abstract CoCrMo alloy specimens were plasma nitrided using a High Power Impulse Magnetron Sputtering (HIPIMS) discharge. In this work the effect of nitriding voltage (−700 V to −1100 V) on the microstructure, surface hardness, impact load fatigue resistance and fracture toughness (KIc) of the alloy has been investigated. Results revealed that the specimens treated at lower nitriding voltages (−700 V and −900 V) develop a nitrided layer consisting a mixture of Co4N + Co2-3N phases. As the nitriding voltage increased (−1000 V and −1100 V), this transformed into a thick layer consisting mainly of Co2-3N with a minor contribution from CrN/Cr2N phases. Accordingly, surface hardness tests after nitriding showed a significant improvement in hardness value (H = 23 GPa) as compared to the untreated specimen, (H = 7.9 GPa). The impact resistance of the alloy also increased with the nitriding voltage. Impact crater profiling of the specimens subjected to impact load tests showed that the depth of the crater decreased significantly, especially at higher nitriding voltages. At the end of the impact load test (one million impacts), the crater depth for an untreated alloy (12.78 μm) was found to be twice to the crater depth measured for the specimen nitrided at −1100 V (7.1 μm). Impact testing results indicate that the fatigue endurance limit of the CoCrMo alloy increased steadily and considerably with the increase of the nitriding voltage. HIPIMS plasma nitriding resulted in a layer material with improved plain strain fracture toughness (KIc), with higher values (KIc) = 1011 MPamm1/2 (−700 V specimen) were calculated as compared to KIc = 908 MPamm1/2 for the untreated specimens. Critical material parameter ratios such as H/E (elastic index or elastic strain to failure) and H3/E2 (plastic index) of the nitrided layers were calculated using surface hardness (H) and elastic modulus (E) values obtained with the help of nanoindentation tests. Systematic improvement in the values of H/E and H3/E2 ratios calculated for all nitrided specimens validated the increase in fracture toughness and impact load fatigue resistance of the nitrided specimens as compared to the corresponding properties of the untreated CoCrMo base alloy.