Highly infrared transparent spark plasma sintered AlON ceramics
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Xialu Wei College of Engineering, San Diego State University, San Diego, CA 92182, USA
Xiannian Sun and Jiujun Xu Department of Materials Science and Engineering, Dalian Maritime University, Dalian 116026, China
Qinghua Qin Research School of Engineering, Australian National University, Acton, ACT 2601, Australia
Eugene A. Olevskyb) College of Engineering, San Diego State University, San Diego, CA 92182, USA (Received 25 December 2016; accepted 3 March 2017)
Spark plasma sintering (SPS) is adopted to fabricate transparent AlON ceramics at 1350–1500 °C under 40 MPa, using a bimodal c-AlON powder synthesized by the carbothermal reduction and nitridation method. After holding 10 min, high density samples are obtained, and their optical transmittance is investigated over the wavelength range of 1330–6000 nm. Despite the samples SPS-processed at 1350 °C indicate the presence of three-phases: c-AlON, a-Al2O3, and h-AlN, they show high infrared transparency, i.e., the maximum transmittance for 1.2 mm thick specimens is up to 77.3% at ;3900 nm. Also, the processed samples exhibit high hardness of 17.81 GPa. The high infrared transmittance should be mainly attributed to high density and rationally controlled grain size distribution, and the high hardness is apparently caused by a small grain size.
I. INTRODUCTION
Spinel-structured aluminum oxynitride (c-AlON) is an important single phase and stable solid solution ceramic within the Al2O3–AlN system.1 Transparent AlON ceramics have attracted a growing interest as infrared/visible window material, transparent armor, and other extremely durable optics due to its high strength and high hardness, high resistance to rain and sand damage, and excellent optical transparency properties.2–8 Transparent AlON can be fabricated by conventional pressureless sintering, hot pressing, or hot isostatic pressing.2,9–12 However, conventional fabrication of AlON ceramics usually requires sintering of a green body at high temperature ($1850 °C) for a long holding period ($6 h).13 Unfortunately, high sintering temperature and long holding time can easily lead to excessive grain growth and coarse microstructure, which, in turn, tend to decrease the hardness of the processed materials.14 Recently, our work has confirmed that AlON powder having a bimodal particle size distribution (PSD) can be more efficiently pressureless sintered under fast heating Contributing Editor: Eugene Medvedovski Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2017.96
of 40 °C/min and a lower sintering temperature of 1820–1880 °C. More importantly, the holding time of pressureless sintering used to obtain transparent AlON utilizing this bimodal powder can be remarkably shortened down to 1.5–2.5 h.15 It was revealed that the bimodal powder can keep its bimodal PSD throughout the whole sintering process and therefore it can be rapidly densified. At the early or middle stage of the pressureless sintering (before the formation of AlON, ,1700 °
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