Magnetic layer formation on plasma nitrided CoCrMo alloy

MFM imaging of expanded austenite formed on 304 SS and CoCrMo alloys

“…The ferromagnetic state for the γ N phase/layers observed in this study is mainly linked to large lattice expansions (~9%) due to high N contents (~35 at.%) [27]. As an interstitial impurity, nitrogen expands the host lattices of the stainless steel and CoCrMo alloy.…”

“…The result is quite new and significant since, to the authors' knowledge, this is the second study to date reporting ferromagnetism for the expanded phase in this alloy system. The original study [27], which explored plasma nitriding of medical grade CoCrMo alloy with compositions similar to that used in the present work, found that the expanded phase/layer formed at 400°C on this alloy has ferromagnetic properties as evidenced through the observation of stripe domains via MFM imaging and through hysteresis curves via MOKE. In comparison, there are quite a few studies reporting ferromagnetism for the γ N phase/layer formed on austenitic stainless steels by various ion beam as well as gas nitriding processes [19].…”

“…While, according to the literature of the last twenty years, there has been considerable research related to the expanded austenite phase in CoCr based alloys (the expanded phase in CoCrMo alloy was first reported by Wei et al in 2004 [11]), there is only one study related to the magnetic nature of the γ N phase in this alloy system [27]. This study involving low temperature nitriding of CoCrMo alloy at 400°C provided strong evidence for the ferromagnetic nature of the γ N phase in this alloy through MFM observation of stripe domain structures as well as the hysteresis loops obtained through MOKE analysis.…”

Internal stress assisted nitrogen diffusion in plasma nitrided medical CoCr alloys

“…For CoCr class alloys the unique nitrogen mass transport was noticed [4,8,9,13,15,16] but not further investigated. So, according to the both class similarities, the objective of this work is to apply the same methodic for nitrogen behavior in CoCr alloys explanation, as it was used for ASS.…”

“…The incorporation of nitrogen into the original lattice on interstitial sites is associated with the expansion of the alloy lattice and an anomalous diffusion of nitrogen in CoCr base alloys [13,14]. Nitrogen diffusion profiles deviate from the classical diffusion-limited incorporation erfc shape [15,16]. This anomalous diffusion is similar to ASS, where different models are applied to describe this particular shape (plateau type) of the nitrogen depth distribution profiles [17e24].…”