Protection of Titanium Alloy Surfaces by Thermal Spraying and Laser Treatment

Titanium alloys are used in several fields due to their outstanding high specific strength, low density and excellent corrosion resistance, but the low wear resistance confines their applicability. The performance of CVD and PVD coatings is limited by the low hardness of the substrate, which cannot supply sufficient support, and conventional thermal spray coating materials do not provide the excellent corrosion properties. Laser alloying also often results in a decreased corrosion resistance and additionally in embrittlement. The use of boron for laser alloying or dispersing of diborides permits the incorporation of extremely hard boride phases without significant decrease of the matrix materials corrosion resistance and ductility, as there is no solubility of boron in titanium. Laser alloying with boron paste and dispersing of TiB₂ in Ti6Al4V surfaces is carried out with CO₂ lasers and an adapted inert gas shower apparatus. Typically a melt pool depth of 200 - 300 µm is achieved and the boride precipitates permit an increase of the surface hardness from 350 HV0.05 in the initial state to about 800 HV0.05. The surface area is characterized by means of optical microscopy, SEM and EDXS. Vacuum plasma spraying is used to provide a technology for deposition of TiB₂ layers with defined thickness prior to laser treatment on surfaces with complex shape and in order to evaluate the direct applicability of thermally sprayed TiB₂ coatings.