Pressureless Reaction Sintering of AlON Using Aluminum Orthophosphate as a Transient Liquid Phase

Abstract

Use of aluminum oxynitride (AlON) in transparent armor systems has been difficult due to the expense and limitations of the processing methods currently necessary to achieve transparency. Development of a pressureless processing method based on direct reaction sintering of alumina and aluminum nitride powders would reduce costs and provide a more flexible and practical manufacturing method. It may be possible to develop such a processing method using liquid phase sintering; as long as the liquid phase does not remain in the final sample. AlPO 4 forms a liquid phase with Al 2 O 3 and AlN at the temperatures required to sinter AlON, and slowly decomposes into P 2 O 5 and alumina. Therefore, it was investigated as a possible transient liquid phase for reaction-sintered AlON. Small compacts of alumina and aluminum nitride with up to of 15.0 wt% AlPO 4 additive were pressed and sintered. It was found that AlPO 4 formed the requisite transient liquid phase, and it was possible to adjust the process to produce AlON samples with good transmission and densities of 3.66-3.67 g/cc. XRD confirmed the samples formed were AlON, with no trace of any remaining phosphate phases or excess alumina or aluminum nitride. Based on the results, it was concluded that AlPO 4 could be utilized as a transient liquid phase to improve the density and transmission of AlON produced by pressureless reaction sintering.

A thermodynamic approach to obtaining transparent spinel (MgAl2O4) by hot pressing

“…6 Several reviews have already been published, giving a thorough description of the extensive effort dedicated to this class of materials. [7][8][9][10] Magnesium aluminate (MgAl 2 O 4 ) spinel -simply called spinel from now on -has a cubic crystal structure and is therefore optically isotropic. In addition to a high melting point, it has excellent mechanical properties, outstanding hardness and a superb transparency from the near UV down to the mid IR * Corresponding author.…”

Reaction Sintering of Transparent Aluminum Oxynitride (AlON) Ceramics Using MgO and Y₂O₃ as Co-Additives

“…Previous work has discovered that certain additives can be beneficial to the sintering of AlON. Hartnett, Gentilemen, and Macquire have reported that additives such as yttrium and lanthanum improve the sintering and microstructure of AlON, and subsequent researchers have also reported on the beneficial nature of these additives [14]. Not only must the correct additive be identified, but numerous processing variables such as compositions, maximum temperature, and holding times must be defined if porosity or swelling is to be avoided.…”

Highly Transparent Mg0.27Al2.58O3.73N0.27 Ceramic Prepared by Pressureless Sintering

“…Transparent spinel‐type polycrystalline ceramics like MgAl 2 O 4 and γ‐AlON have been widely studied. They possess both optical transparency from ultraviolet to middle‐infrared wavelengths and excellent mechanical properties, which makes them as promising candidates instead of sapphire for transparent armors, missile domes, window materials, etc.…”