A comparison of the properties of AlON and LiAlON transparent ceramics prepared by reaction sintering and hot isostatic pressing

Abstract

The synthesis of LiAlON and AlON ceramics was investigated using reaction sintering of LiAl5O8–Al2O3–AlN and Al2O3–AlN raw powders at 1400–1800 °C. The phase, microstructural evolution, optical transmittance, and lattice structure were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet visible (UV-VIS) and Fourier transform infrared spectra (FTIR), and transmission electron microscopy (TEM), respectively. Transparent LiAlON and AlON ceramics were fabricated by reaction sintering (1750 °C for 20 h) and post-hot isostatic pressing (HIP) in an Ar atmosphere (180 MPa at 1840 °C for 5 h). The results showed that LiAl5O8 promoted the formation of a single oxynitride phase, and single-phase LiAlON and AlON were obtained at 1700 °C and 1750 °C, respectively. LiAl5O8 depressed the volume expansion of the sintered bodies at higher temperatures (≥1600 °C) caused by the chemical reaction between AlN and Al2O3. Volume expansion was evident in the Al2O3–AlN system, but this expansion was eliminated in the LiAl5O8–Al2O3–AlN system. The obtained LiAlON sample had higher in-line transmittance (86.8% at 3.3 μm) and higher flexural strength (approximately 332 MPa). In contrast, the AlON ceramic was less transparent (<4%) and had lower flexural strength (approximately 240 MPa), which was mainly caused by the many residual pores and carbon impurities, as identified by SEM, Ranman spectroscopy, and carbon-sulfur analysis. The Vickers hardness at 0.49–9.8 N loads of oxynitrides exhibited minimal differences.