Metal ion release from nitrogen ion implanted CoCrMo orthopedic implant material

Abstract

CoCrMo alloys are used as orthopedic implant materials because of their excellent mechanical and corrosion properties. However, when placed in vivo, these alloys release Co, Cr, Mo ions to host tissues, which may give rise to significant health concerns over time. Nitrogen ion implantation can be used to form protective layers on the surface of CoCrMo orthopedic alloys by modifying the near surface layers of these materials. In this study, medical grade CoCrMo alloy (IS0 5832-12) was ion implanted with 60 keV nitrogen ions to a high dose of 1.9 × 10 18 ions/ cm 2 at substrate temperatures of 100, 200 and 400°C. The N implanted layer microstructures, implanted layer phases, and thicknesses were studied by a combination of Bragg-Brentano (θ/2θ) and grazing incidence (Seeman-Bohlin) X-ray diffraction (XRD and GIXRD) and crosssectional scanning electron microscopy (SEM). Atomic force microscopy (AFM) was used for roughness analysis of N implanted as well as aspolished surfaces. Static immersion tests were performed to investigate metal ion release into simulated body fluid (SBF) by electrothermal atomic absorption spectrometry (ETAAS) and inductively coupled plasma optical emission spectrometry (ICP-OES). XRD and SEM analyses indicated that the N implanted layers were ∼ 150-450 nm thick and composed of the (Co,Cr,Mo) 2+x N nitride phase and a high N concentration Co-based FCC phase, γ N depending on the substrate temperature. ETAAS analysis results showed that in vitro exposure of the N implanted surfaces resulted in higher levels of cobalt ion release into the simulated body fluid compared to the untreated, polished alloy. The higher Co release from the N implanted specimens is attributed to the nature of the implanted layer phases as well as to the rougher surfaces associated with the N implanted specimens compared to the relatively smooth surface of the untreated material. SEM analysis of N implanted and untreated specimens after immersion tests clearly indicated calcium phosphate formation on the as-polished CoCrMo alloy, indicating a degree of bioactivity of the untreated metal surface which is absent in the N implanted specimens.

CoCrMo alloy treated by floating potential plasma assisted nitriding and plasma based ion implantation: Influence of the hydrogen content and of the ion energy on the nitrogen incorporation

“…The virgin D30-CoCrMo presents well-defined peaks (111) and (200) of diffraction corresponding to the (111) and (200) of the face centred cubic structure, called γ phase [7] or α phase [10]. As presented in Fig.…”

“…γ N structure, that is the original f.c.c. structure swelled by the insertion of nitrogen in the octahedral sites [7,10]. Compared to the original D30-CoCrMo γ peaks, the (111) peak Table 1 CoCrMo alloy compositions; treatment parameters, mean surface roughness R a of the surface after treatment.…”

“…This modification of the cell parameters is often linked to the nitrogen content by means of Vegard's law for instance. Following this argument, the (200) oriented grains would then have a higher nitrogen content than the (111) ones [7,10]. However, the residual compressive stress induced by the incorporation of nitrogen have to be taken into account and they don't have the same influence on the different grains due to the orientation dependence of the elastic constants [24].…”

“…Few studies report the treatments of CoCrMo alloys by means of ion implantation [6][7][8], plasma nitriding [9][10][11][12][13] or plasma based ion implantation (PBII) [13][14][15][16]. In order to conserve the intrinsic good corrosion resistance, the nitriding has to avoid CrN precipitates or layer formation.…”

“…γ phase, similar to the one obtained with stainless steel nitriding. Although these surface treatment methods are found to be profitable for wear resistance [9,11], the cobalt ion release seems to be increased by the treatments compared to the initial substrate [7]. By increasing the temperature (600 to 800°C), CrN and Cr 2 N are detected by X-ray diffraction.…”