A non-aqueous processing route for phosphate-protection of AlN powder against hydrolysis

A commercial AlN powder was treated in an ethanol with H3PO4 and Al(H2PO4)3 to passivate its surface against hydrolysis. The treated powder was quite stable in water for >72 h as confirmed by pH measurement, X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and energy dispersive analysis with X-rays. The coating layer was stable up to 1200 °C as confirmed by thermogravimetry and differential thermal analysis. The aqueous processing ability of treated AlN powder was tested in aqueous gelcasting of β-SiAlON reaction precursor mixtures for fabricating dense ceramic bodies and 500 mL thin wall crucibles. The sintered gelcast β-SiAlON exhibited the following properties: bulk density of 3.07 g/cm3, apparent porosity of 0.13%, water absorption capacity of 0.04%, linear shrinkage of <16%, β-SiAlON content of >90%, hardness of 14 GPa and fracture toughness of 3.95 MPa m1/2.