Effects of AlN content on mechanical and optical properties of AlON transparent ceramics

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Abstract

In this study, AlON powders with different AlN contents were synthesized by the carbothermal reduction nitridation method and used to prepare transparent AlON ceramics. Lattice parameters of AlON powders were calculated from XRD patterns to evaluate the effects on microstructure. The influences of the AlN content on the properties of the resulting AlON ceramics was investigated in aspect of density, mechanical properties and optical properties. With an increase in the AlN content, the density of the AlON ceramics increased, the grain size decreased, and the hardness and fracture toughness increased. Transparent AlON ceramics were obtained at the AlN content of 21.6–29.7 mol%. The refractive index of the AlON ceramics increased from 1.688 to 1.702 at 4000 nm with an increase of AlN content. The correlation between the composition and properties of the AlON ceramics obtained in this study will provide both theoretical and experimental reference for industrial AlON ceramic manufacturing.

Introduction

Aluminum oxynitride (AlON), which was first synthesized by Yamaguchi and Yanagida in 1959 [1], is a spinel-type solid-solution formed in pseudobinary AlN–Al2O3 systems. Owing to its excellent mechanical performance, good chemical resistance, and high transparency over the ultraviolet-mid-infrared region, transparent AlON ceramics are considered as promising materials for transparent armors, visible-infrared windows, and domes [[2], [3], [4], [5]]. Therefore, various methods have been developed to synthesize AlON powders as well as to manufacture transparent ceramics.

In 1979, Corbin et al. [6] investigated the phase diagram of the pseudobinary Al2O3–AlN system at 1850–2150 °C and examined the cubic AlON region at high temperatures. Willems et al. [7] examined the cubic AlON region in the pseudobinary Al2O3–AlN system at 1500–1850 °C by studying the variation of phase and lattice parameters with temperature and composition. According to the phase diagram proposed by Willems, AlON exists at temperatures above 1650 °C, and the composition range increases with temperature, for example, the composition of 19–34 mol% AlN is observed at 1850 °C. This indicates that the composition of AlON is variable.

As expected, the composition affect the properties of AlON ceramics. Hartnett et al. [8] and Surmet Company [9] reported that the refractive index of AlON ceramics increases with an increase in the AlN content. Therefore, it is particularly important to control and regulate the AlN content to prepare AlON transparent ceramics with different properties. AlON powders with different compositions have been prepared using the solid-state reaction method by changing the AlN/Al2O3 ratio of the raw materials [6,7,[10], [11], [12]]. Aldinger et al. [11] reported that AlON powders with different compositions can be synthesized at 1850 °C for 2.5 h when the AlN content in raw materials was in the range of 28–39 mol%. Park et al. [12] synthesized AlON powders at 1750 °C for 2 h with 20–25 mol% AlN in raw materials. McCauley et al. [13] sorted out the results of different researchers and reported that the lattice parameter of AlON increases with an increase in the AlN content. However, they found there were large deviations in the correspondence between the AlN contents and lattice parameters of AlON by different researchers. McCauley believed that it is because all these researchers regarded the composition of raw materials as the composition of the final AlON products. However, the composition of the final products was different from that of the raw materials. McCauley suggested that the difference in the composition is due to the vaporization of Al2O3 during the reaction or because of the incorporation of N2 into AlON. As proposed by Forslund et al. [14] Al2O3 transformed into gas species of AlOx at the temperature over 1500 °C during carbothermal reduction. The influence of vaporization is sensitive for AlON because of much higher synthesis temperature. In order to resolve the uncertainty in AlON composition, Chen et al. [13] determined the composition of AlON ceramics instead of raw materials by using energy-dispersive spectroscopy (EDS). Hartnett et al. [8] analyzed the nitrogen content of AlON ceramics using the inert gas fusion method, which showed higher accuracy than EDS, and obtained the relationship between the nitrogen content and lattice parameter of the ceramics.

Although the solid-state reaction method is a simple and feasible method to synthesize AlON ceramics, the ceramics obtained using this method do not exhibit adequate optical transmittance for practical applications [15,16]. Carbothermal reduction nitridation (CRN) is another method to synthesize AlON powder. These powders are in turn used to prepare high-quality AlON transparent ceramics [[17], [18], [19], [20], [21]].

In theory, when AlON is prepared by the CRN method, the carbon black content of the raw materials affects the AlN content of AlON, which in turn affects the properties of the corresponding AlON ceramics. Wang et al. [22] reported that AlON powders can be synthesized by carrying out CRN at 1750 °C for 2 h at the carbon black contents of 5.6–6.0 wt%. Shi et al. [23] synthesized AlON powders by carrying out CRN at 1730 °C for 2 h by using 5.6 wt% nanosized carbon black as the carbon source. However, the effects of the carbon black content in raw materials on the AlN content of AlON and the properties of AlON ceramics is not clear.

In this study, AlON powders were prepared by the CRN method. The effects of the carbon black content on the AlN content and the synthesizing temperature of the AlON powders were investigated. The effects of the AlN content on the microstructure and optical and mechanical properties of the AlON ceramics were investigated.

Section snippets

Material synthesis

High-purity γ-Al2O3 (>99.97%, Dalian Hiland Photoelectric Material Co., Ltd., Dalian, China) and carbon black (>97.5%, Sinopharm Chemical Reagent Co. Ltd, China) were used as the raw materials with the carbon black proportion of 4.2–7.0 wt%.

The raw materials: γ-Al2O3 and carbon black were dispersed in alcohol and ball-milled at 200 rpm for 20 h. The obtained slurry was dried at 60 °C for 24 h. The dried mixture was screened through a 100-mesh nylon sieve, and then heated to 1620 °C for 5 h

Results and discussion

Fig. 1 shows the FESEM images of the γ-Al2O3/C mixture after ball milling and its C, Al, and O EDS mappings. The images in Fig. 1(b)–(d) indicate that γ-Al2O3 and carbon black formed a homogeneous mixture.

After calculating the residual carbon black content left after the reaction, the effective carbon black content of the raw materials was calculated (Table 1). The theoretical nitrogen contents (in the form of AlN) of the product powders synthesized at 1780 °C were calculated using equation (2).

Conclusions

The effect of the AlN content, which is tailored by carbon black addition, on the properties of the AlON powders and ceramics were investigated. The lattice parameter of the AlON ceramics increased with an increase in the AlN content. When the AlN content is higher than 24.9 mol%, the lattice parameters of the powders obtained at 1830 °C were larger than those of the powders obtained at 1780 °C. The hardness, fracture toughness, and refractive index of the ceramics increased with an increase in

Declaration of competing interest

There are no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

References (33)

  • Y.Y. Wang et al.

    Preparation and properties of AlON powders

    Ceram. Int.

    (2018)
  • M. Ding et al.

    Formation of twins in AlON material and its effects on the Vickers hardness and fracture toughness

    Ceram. Int.

    (2019)
  • Y. Yang et al.

    Composition dependence of physical and optical properties in Ge-As-S chalcogenide glasses

    J. Non-Cryst. Solids

    (2016)
  • X.D. Wang et al.

    Thermodynamic study and synthesis of gamma-aluminum oxynitride

    Scand. J. Metall.

    (2002)
  • Y.C. Shan et al.

    A fast pressureless sintering method for transparent AlON ceramics by using a bimodal particle size distribution powder

    Ceram. Int.

    (2015)
  • G. Yamaguchi et al.

    Study on the reductive spinel - a new spinel formula AlN-Al2O3 instead of the previous one Al3O4

    Bull. Chem. Soc. Jpn.

    (1959)
  • Cited by (0)

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