Ceramic coatings processed by spraying of siloxane precursors (polymer-spraying)

Si-based inorganic polymers (siloxanes, silazanes, carbosilanes, etc.) dissolved in an organic solution exhibit a sufficiently low viscosity to be processed by spraying. For this work methyl-silesquisiloxanes precursors have been used aiming for an Si–O–C composition. A two-flux nozzle has been designed and build-up which enables a full beam of about 12° opening angle. The primary particle size of the sprayed droplet was in the range of 5–10 μm. If the substrate is sufficiently near to the nozzle the droplets could wet and coat the surface. High temperature treatment under flowing nitrogen initiated a polymerisation (200 °C) followed by ceramisation at higher temperatures (up to 1000 °C). In spite of the weight loss, a good bonding of the coating on the substrate without delamination or segmentation cracks was obtained, which could be demonstrated by SEM and TEM investigations. The shrinkage was anisotropic and occurred predominantly toward the surface rather than parallel. This type of thin film effect limited to a thickness to 3 μm/per coating cycle. The microstructure consisted out of an amorphous Si–O–C matrix with very small C precipitates. The interface is basically straight with an atomic bonding showing very good adhesion but also some adaptation dislocations on the substrate side.