Abstract

Superelastic shape memory materials are of special interest in medical applications due to the large obtainable strains, the constant stress level and their biocompatibility. TiNi sputtered tubes have high potential for application as vascular implants, e.g. stents, whereas superelastic TiNi-polymer-composites could be used for novel applications in orthodontics and medical instrumentation as well as in certain areas of mechanical engineering. In orthodontic applications, lowering the forces which are applied on the teeth during archwire treatment is of special importance due to tooth root resorption, caused by the application of oversized forces. Furthermore, the use of superelastic materials or composites enables the application of constant forces independent of diminutive tooth movements during the therapy due to the superelastic plateau. Superelastic TiNi thin films have been fabricated by magnetron sputtering using extremely pure cast melted targets. Special heat treatments were performed for the adjustment of the superelastic properties and the transformation temperatures. A superelastic strain exceeding 6% at 36 °C was obtained.

Introduction

TiNi based shape memory effects are related to a reversible phase transformation between a high temperature phase (austenite, B2) and a low temperature phase (martensite, B190) showing a different crystal structure. Phase transformation can be achieved by temperature (one-way or two-way effect) and/or stress (superelasticity). In Ni-rich TiNi alloys, an intermediate phase can appear after a heat treatment below 500 °C, this phase is called the R phase (rhombohedral) and the existence of this R phase supports the formation of stress induced martensite. Superelastic shape memory materials are of special interest in medical applications due to the large obtainable strains (up to 8 %), the constant stress level and their biocompatibility.