Constrained sintering of alumina micro-ring films on stiff and compliant substrates: Constriction or dilation?

The sintering behavior of ring-shaped alumina films on different types of substrates (polished sapphire and platinum coated sapphire) has been investigated. Variation of substrate material, layer thickness and heating schedule led to different interface properties which were quantified using the interfacial friction parameter as a measure of the slip distance of the free edge. This interfacial friction implies the compliance of the film-substrate interface, thus allowing the quantification of the ability of the free edges to slide along the substrate. Ring structures were made with outer radii of 100 –500 µm and the ratios of inner to outer radii were 0.1 (broad ring segment with small inner hole) and 0.4 (narrow ring segment with big inner hole). During sintering, the outer film edge was found to recede in all systems, whereas the behavior of the free edge located at the inner hole was strongly dependent on the interfacial friction. When sintering of alumina micro-rings were carried out on sapphire substrate, a high interfacial friction resulted in the positive displacement of the inner edge, causing the inner hole to open. Sapphire substrates coated with platinum offered a lower interfacial friction resulting in the opening of the hole in a more distinct manner as compared to the uncoated sapphire substrates. As the thickness of the film also affects the interfacial friction, a thicker film (27 µm) on platinum coated substrate had very low friction. This resulted in a negative displacement of the inner ring leading to closing of the hole. These effects are in qualitative agreement with the predicted analytical model for constrained sintering of annular ceramic films. A combination of film thickness, substrate material and aspect ratio of the ring either leading to constriction or dilation of the annular alumina film were carefully investigated.