Mechanical stresses upon phase transitions

Mechanical stress studies were carried out on three different groups of functional coatings using a purpose-built system. Functional coatings have become increasingly important in recent years due to their interesting technological applications. In this work three different groups of coatings were studied. Transition metal oxides are used as optical coatings, hard coatings, etc., phase change films find application in optical data storage technology, while optically switchable coatings have been applied in switchable windows, rear-view mirrors etc. For all materials it applies that they must promise a certain lifetime, as a loss of functionality would lead to loss of function for the entire device. Hence, it is important to understand and control their mechanical stresses. The stress measurement system used to measure mechanical stresses is based on a laser which scans across the surface of film, which is deposited on thin Si or glass substrates. By monitoring the motion of the reflected beam during the scan, the curvature of the sample can be determined. The curvature can be used to calculate the mechanical stress in the film by use of the Stoney equation. Within the group of transition metal oxides (TiO2, ZrO2, HfO2, V2O5, Nb2O5, Ta2O5, MoO3 and WO3), which has its primary application as optical coatings, a systematic study of deposition stresses was carried out. The films were deposited using DC-magnetron sputtering. The obtained results were explained on the basis of models for stress build-up. It was found that the mechanical stress in these metal oxides could be correlated with the structure of the films, and both the structure formation and the mechanical stress were explained by the impact of energetic ions on the growing film. The mechanical stress build-up in switchable materials belonging to different material classes were compared. The investigated materials included tungsten oxide, magnesium and gadolinium. It was found that the optical switching of these materials was associated with compressive stresses of the order of 100, 650 and 2100 MPa, respectively. Additionally, it was found that only the switching of tungsten oxide was fully elastic. Studies of the mechanical stress build-up upon crystallization in phase change materials used for optical data storage were performed. It was found that the stress change upon crystallization is only a fraction of the stress that could be expected assuming an elastic behavior. Studies of the viscous flow in the amorphous phase yielded that the stress relaxation can be simulated using a bimolecular model, and the viscosity increase rate and the isoconfigurational activation energy characteristic of the studied alloys were determined. Considering the complete findings of these studies, it is obvious that they show the importance of control of mechanical stresses in functional coatings. A failure of the film through cracking and delamination is fatal for any application relying on the properties of such coatings