Phase transformation in carbon-coated nitinol, with application to the design of a prosthesis for the reconstruction of the anterior cruciate ligament

Abstract

Ultra-low temperature isotropic carbon was vapour-deposited on a near equiatomic Ti-Ni (Nitinol) alloy (49.9 at % Ti-50.1 at % Ni). The objective of the carbon coating was the induction of growth of collagenous tissue, as part of a preliminary design of a prosthesis for the reconstruction of the anterior cruciate ligament. Differential scanning calorimetry was used to study the phase transformation of carbon-coated Nitinol. X-ray diffraction and Auger electron spectroscopy were used in the study of the carbon/Nitinol interface. The results show that unannealed coatings do not inhibit the Nitinol phase transformation regardless of the thickness of the coatings. However, on heat treating the coated samples, a TiC layer forms at the carbon/Nitinol interface. The thickness of that layer increases with increasing the time of heat treatment. Surface constraint of the Nitinol by the TiC results in a marked drop in the austenitic start (A _s) and martensitic finish (M _f) temperatures of thin samples. The inhibition of both the premartensitic and martensitic transformations increases with the increase in thickness of the TiC interface.