|M.Sc Thesis||Department of Materials Science and Engineering|
|Supervisor:||Prof. Kaplan Wayne D.|
Aluminum oxynitride (AlON) is a polycrystalline ceramic material with potential use in applications requiring high strength combined with optical transparency. Due to its cubic spinel structure, polycrystalline AlON has isotropic optical and thermal properties, making it a candidate material to replace single crystal forms of oxides currently in use for optical applications, such as transparent armor and missile domes.
In order to achieve optical transparency full density is required, and as a result the sintering process for AlON usually includes elevated temperatures combined with pressure and/or long sintering durations. To overcome this difficulty and for controlling microstructural evolution, dopants are often introduced. The solubility of these elements is very low (assumed to be in the order of tens of ppm), which results in their enrichment to grain boundaries even at very low doping levels. Until recently, the precise measurement of solubility limits in ceramics has not been performed directly.
This study began with developing a new method for producing AlON green bodies with relatively high density and improved mechanical properties, based on hydrolysis of AlN in water. Following this, a direct method was applied to determine the solubility limits of La and Y in AlON. This method was first optimized by determining the solubility limit of Mg in a-Al2O3. The solubility limits were measured using wavelength dispersive spectroscopy (WDS) mounted on a scanning electron microscope (SEM). Measurements were conducted on rapidly-cooled polished samples without thermal or chemical etching. The results indicated the solubility limits of La and Y in AlON at 1870°C to be 498±82 and 1775±128 ppm, respectively. Using this data, doped AlON samples (both below and above the solubility limit) were prepared, and the influence of La and Y on sintering and microstructural evolution was studied.