Microwave reactive sintering to fully transparent aluminum oxynitride (ALON) ceramics

Aluminum oxynitride (ALON) has an approximate composition of Al23O27N5(9Al2O3·5AlN). ALON can be sintered to fully transparent ceramic material having mechanical and optical properties similar to those of sapphire with the advantages of an isotropic cubic crystal structure. The transmission range of ALON can extend from 0.2 μm in the UV through the visible to 6.0 μm in the infrared, which makes it a very useful material for many optical applications in wide range. Combined with the high strength and high hardness, ALON is an ideal material for transparent armor product [1, 2]. The conventional fabrication of transparent ALON ceramics involves using pre-synthesized ALON powder to form a green body, followed by sintering in a nitrogen atmosphere at high temperatures (>1850 ◦C) for extended period (20–100 h) [3]. A single-step preparation method was also tried to make transparent ALON ceramics, by mixing Al2O3 and AlN powders and subsequent reactive sintering at 1850 ◦C for 1 h at 3 bar nitrogen atmosphere. But the sintered body in the case was translucent [4]. Microwave sintering is a novel sintering process that is fundamentally different from the conventional sintering process. In conventional sintering, the sintering driven force, temperature, is generated by external heating elements (in resistance heating) and then is transferred to the samples via radiation, conduction and convection. In microwave process, the processing materials themselves absorb microwave power and then convert microwave energy to heat within the sample volume itself, and hence the heating is very rapid. The microwave processing of materials has major advantages of higher energy efficiency, enhanced reaction and sintering rate, cycle time and cost savings [5]. In the last four years, in this laboratory we have successfully sintered various ceramics, composites, and even powdered metals to full density using microwave processing [6, 7]. Some highly transparent ceramic samples, such as alumina, spinel, and aluminum nitride, have been successfully prepared by microwave sintering process in our lab. Compared to the conventional sintering process, the microwave sintering to highly transparent ceramic samples can be conducted at lower sintering temperatures and much shorter sintering times [8]. In our earlier work, it was demonstrated that synthesis of single phase ALON could be achieved by microwave heating at 1650 ◦C for 1 h [9]. But the density of the fired samples was only 82% of the theoretical density of ALON and they were opaque. This means that the synthesized ALON samples did not achieve full densification, even though a single phase ALON was formed. To prepare fully dense and transparent ALON samples, it needs further sintering at higher temperatures. The results in the present work showed that fully dense and highly transparent ALON ceramic samples can be made by microwave sintering at 1800 ◦C with a soaking time of 60 min at that temperature. The ALON green samples in this work were prepared by mixing high purity α-Al2O3 powder (SM8, Baikalox, Baikoski International, NC, USA) and AlN powder (Grade C, H.C. Starck, Laufenburg, Germany). The properties of the starting powders are shown in Table I. It was found that the addition of a small amount of Y2O3 increased the densification and improved the transparency of the sintered bodies during microwave sintering. Therefore the starting mixture contained 67.5 mole percent of Al2O3, 33.5 mole percent of AlN, to which 0.5% (by weight) Y2O3 in form of Y(NO3)3·6H2O was added. The powders with 3 wt.% of binder (Acryloid) were ball-milled in acetone for 24 h. After drying, the mixture was compacted uniaxially into pellets of diameter 12.7 mm and height 3 mm at a pressure of 30 MPa. Finally, the pellets were isostatically pressed at 250 MPa for 5 min. Microwave sintering was carried out by using a home-made 1.5 Kw, 2.45 GHz single mode microwave applicator in flowing pressure nitrogen at ambient pressure. The heating rate was kept around 100 ◦C/min by controlling the incident microwave power. The phase composition of the samples was examined using X-ray diffraction (XRD). The densities of the sintered ALON samples were measured by the Archimedes method. The optical microscope (Olympus) was used to study the microstructures, and the Varian spectrophotometer (CARY 2300) was used to measure the transmittance of the sintered samples. Fig. 1 shows the densification behaviors of the ALON samples during microwave sintering process. All microwave sintered samples exhibited only a pure ALON phase composition which was confirmed by XRD.