Influence of thin coatings deposited by a dynamic ion mixing technique on the fatigue life of titanium alloys

The technique of dynamic ion mixing involving a physical vapour deposition method and a simultaneous ion implantation has been used in order to improv e the fatigue resistance of two titanium alloys. This process allows the de position of adherent NiTi and SIC amorphous coatings of the order of 1 mu m thick. Both treated substrates have been tested at room temperature in the low cycle fatigue range, revealing significant fatigue life improvement.. NiTi and SiC films modify the surface deformation mechanisms of fatigued ma terials and largely suppress or delay crack initiation. These effects depen d, however, on the nature! of the film, the microstructure of the substrate and the stress amplitude applied during the fatigue tests. To explain the fatigue results, the mechanical properties of these thin films have been ch aracterized by different techniques: scratch-tests, micro-Vickers indentati ons, Young's modulus measurements by a resonant frequency method and "fract ure stress" determination by in situ tensile tests. The results have shown that their mechanical properties are very different to those of the corresp onding classically deposited solid materials and are influenced by the film thickness. The results are discussed according to the mechanical propertie s of the coatings and the substrate deformation and damage modes associated with their microstructure. (C) 1999 Kluwer Academic Publishers.