Alterations of surface mechanics of nickel titanium shape memory alloys after plasma surface treatment

Abstract

Orthopaedic implant constructs usually involve the assembling of metallic screws, plates and rods in which fretting is always expected at implant junctions. Metallic debris is suspected to be associated with post-operative infections. Nickel-titanium (NiTi) shape memory alloy has potential as an orthopedic implant. However, nickel particulate debris released from these materials remains a concern. We have therefore enhanced its surface mechanical properties by using plasma immersion ion implantation (PIII). The current paper presents the surface mechanical properties of the nitrogen PIII treated NiTi materials as compared with that of untreated NiTi, medical grade stainless steel and titanium alloy samples. The surface chemistry of the untreated and implanted samples was examined by X-ray photoelectron spectroscopy (XPS) survey scan. Nano-indentation and scratching tests were conducted to identify the surface hardness and E modulus of all the samples. The XPS survey-scan results revealed that TiN and TiO2 and small amounts of NiO were detected on the nitrogen-implanted surfaces. However, the depth profile of the nitrogen treated sample suggested that the NiO concentration was very low as compared with that on the untreated NiTi. Nano-indentation results suggested that the hardness of the nitrogen treated surface was generally higher than that of the untreated NiTi substrate at superficial layer. Compared to the stainless steel, the hardness of the nitrogen treated layer in the layer of 25 nm was higher. However, it was lower than that of the Ti sample except at the topmost region. In summary, the surface hardness of the nitrogen treated NiTi was more superior than that of the untreated NiTi and SS samples, but inferior to that of Ti. SUMMARY: Orthopaedic implant constructs usually involve the assembling of metallic screws, plates and rods in which fretting is always expected at implant junctions. Metallic debris is suspected to be associated with post-operative infections. The current paper presents the surface mechanical properties of the nitrogen PIII treated NiTi materials as compared with that of untreated NiTi, medical grade stainless steel and titanium alloy samples.