Thermal annealing behaviour of alloyed DLC films on steel: Determination and modelling of mechanical properties

A shortcoming of diamond-like carbon (DLC) films is the poor stability of t heir microstructure and properties at elevated temperatures. In this study, the effect of annealing on the stability of DLC films alloyed with silicon and deposited on steel is investigated. A comprehensive study of the mecha nical properties is carried out by a novel method combining normal indentat ions with micro- and macroindentors assisted by finite element calculations of the indentation. The mechanical properties of the layers are correlated to structural changes in the film and to interface reactions. While it has become a common practice to determine hardness and the Young's modulus of thin films by nanoindentation and to calculate residual stresse s from the bending of the film/substrate system, evaluation of the interfac e toughness, which is a measure of adhesion, and of the film rupture streng th is less straightforward. Here, Hertzian-type ring cracks are generated i n the film by nanoindentation of the film/substrate system with spherical d iamond tips. From the critical load for crack generation the film rupture s trength is deduced using finite element calculations. Similarly, Rockwell C hardness tests in combination with calculations are performed to measure t he interface toughness. Applying these methods to DLC films on steel, it has been found that the Yo ung's modulus decreases with increasing silicon content and the residual st ress drops below 1 GPa. The rupture strength approaches its theoretical lim it of E/10. Annealing at 500 degrees C reduces the adhesion energy signific antly. The variation of mechanical properties can be attributed to structur al changes in the film as investigated by Raman spectroscopy. (C) 1999 Else vier Science S.A. All rights reserved.