Ti and Ti‐6Al‐4V Coatings by Cold Spraying and Microstructure Modification by Heat Treatment

Abstract

Titanium and its alloys has been extensively used in implants as biomaterials due to their excellent biocompatibility, especially with bioactive ceramics as coatings on their surfaces, such as hydroxyapatite (HA) or its composites. Thermal spraying techniques have been widely used to fabricate these biocoatings. However, owing to the adoption of high temperature plasma, arc or flame in the thermal spraying processes, it will be inevitable to cause the oxidation of metallic spray materials or phase transformation of many feedstocks. Cold spraying (also termed cold gas dynamic spraying), as a new emerging coating technique, has been widely investigated because of its high deposition efficiency and capability of mass production of many metallic, composite and nanostructured coatings. In this process, spray particles are injected into a high speed gas jet in a converging-diverging de Laval type nozzle and accelerated to a high velocity (typically 300–1200 m/s). The deposition of the particles takes place through the intensive plastic deformation upon impact in a solid state at a temperature well below the melting point of the sprayed material. Consequently, the deleterious effects, such as oxidation, phase transformation, grain growth and other problems inherent to the conventional thermal spraying processes can be minimized or eliminated. The actual bonding mechanism of cold sprayed particles is still not well understood. The most prevailing bonding hypothesis is that plastic deformation of the particles upon impact may disrupt thin surface oxide films, and provides intimate conformal contact under high local pressure, thus permitting bonding to occur. It has been widely accepted that for a given spray material, there exists a critical velocity resulting in a transition from erosion of the substrate to deposition of the particles. Only those particles achieving a velocity higher than the critical value can be deposited. When the particle velocity is higher than the critical value, the deposition efficiency increases with increasing the particle velocity. Generally, the deformability of the sprayed metallic particles accounts mainly for the microstructure of the assprayed coatings. As reported in literature, it is easy to produce the dense coatings of pure Cu, Zn, etc. owing to their good deformability, whereas it is difficult to form the dense coatings of some high strength alloys, such as stainless steel and MCrAlY, except at a relatively high particle velocity, such as with helium as accelerating gas. However, for Ti powder, although it has a relativley low strength, it is difficult to form a dense Ti coating. It its more difficult to obtain a dense Ti-6Al-4V coating by cold spraying taking its high strength into account. In the previous studies, the porosity of the cold sprayed Ti or its alloy coatings was reported as 5 % ∼ 25 % depending on the spray conditions, while with a relatively high depostion efficiency (usually > 50 %). Their bond strength was usally less than 15 MPa, which is even worse for Ti-6Al-4V coating. Therefore, as structural coatings, it is necessary to improve their mechanical properties. Blose tried to heat-treat these porous deposits by hot isostatic pressing (HIP) and significantly improved their density and strengh. On the other hand, it was reported that post-spray heat treatment of cold sprayed coatings can remarkably modify their microstructure and properties. Taking into accout the potential applications of these porous Ti, its alloy and/or composite coatings as biomaterials, this paper presents the microstructure of cold sprayed Ti and Ti-6Al-4V coatings and the effect of heat treatment on coating microstructure modification. C O M M U N IC A TI O N S