Introduction g-aluminum oxynitride (g-AlON) is widely used for optical lens, transparent arms, and infrared windows due to its high transparency from the ultraviolet to mid-infrared range. To obtain a high-performance AlON transparent ceramic, each stage of its manufacture process should be controlled with cares, i.e., powder synthesis, forming, sintering and annealing. Sintering aid (i.e., Y2O3, CaCO3, Y2O3-La2O3, etc.) is also used to improve the property of AlON transparent ceramic. However, the sintering temperature is still rather high, resulting in a high production cost. Recently, SiO2 is used in AlON system, leading to a decreased temperature of hot isostatic pressing (HIP) (i.e., 1 810 ℃). Unfortunately, however, the evaporation of SiO decomposed from SiO2 could increase the micro-pores in pre-sintered bodies, leading to the density variations and then optical inhomogeneity after HIP. It is thus necessary to explore new Si-contained additive that is stable at a high temperature for the preparation of high-performance AlON transparent ceramics. Mullite (3Al2O3·2SiO2) as a main phase of AlON has a superior high-temperature thermal stability, which is a promising application in AlON system. In this paper, AlON transparent ceramics were fabricated via pressureless sintering and hot isostatic pressing. In addition, the effect of mullite content on the microstructure of pre-sintered and hot isostatic pressed AlON ceramics and their corresponding optical property was also investigated. Materials and method An γ-AlON powder with 0.02%-0.20% mullite in alcohol was ground by a planetary mill with high-purity alumina balls. After drying, sieving and then calcining, the mixed powder was pressed into pellets and then cold isostatic pressed to obtain AlON green bodies. The green bodies were pressureless-sintered in a flowing N2 atmosphere at 1 880 ℃ for 4 h, and further hot isostatic pressed in a flowing Ar atmosphere at 1 800 ℃ and 200 MPa for 6 h to obtain AlON transparent ceramics.The phase analysis of specimens was identified by a model D8 Advance X-ray diffractometer (XRD, Bruker Co., Germany) using nickel-filtered Cu Kα radiation (λ =1.540 6 ?). The bulk density of AlON ceramics was determined by a water immersion method based on Archimedes’ principle. The chemically-etched and fracture surfaces of AlON ceramics were characterized by a model TM3000 scanning electron microscope (SEM, Hitachi Co., Japan). The average grain size was determined by a linear intercept line method with a correction factor of 1.56 via a software named Nano Measurer. The Raman spectra were recorded by a model XploRA One-532 Raman spectrometer (Horiba Co., Japan) using a 532 nm Ar+ laser. The optical in-line transmittance spectra of HIPed AlON ceramics were measured by a model V-770 UV-Vis-NIR spectrophotometer (JASCO Co., Japan) and a model FT/IR-4600 Fourier transform infrared spectrometer (FTIR, JASCO Co., Japan). Results and discussion The ground AlON powder mixed with mullite has a homogeneous distribution and the particle sizes are in the range of 0.2-1.6 μm with an average value of 0.7 μm. After pressureless sintering, all the XRD patterns of AlON ceramics with 0.02%-0.20% mullite match well with the standard cubic AlON phase (JCPDS 48-0686), and no extra peaks appear, indicating the dissolution of mullite in AlON lattice. The Raman spectraindicate the effect of the mullite content on the structure of the pre-sintered AlON ceramics. The phonon modes located at 316, 372, 628, 764 cm-1 and 920 cm-1 appear in all the samples, which are corresponded to the 3T2g, Eg and A1g modes of spinel. No shift of the Raman bands occurs and new bands do not appear, indicating the same structure and phases in AlON with different mullite contents.All the pre-sintered AlON ceramics have a milk-white color and are opaque. The microstructure of fracture surfaces indicates that pores easily appear in all the samples. Large pores appear and density decreases in AlON ceramics with 0.10%-0.20% mullite. The grain size increases steadily with increasing mullite content. For the low content and high-temperature stability of mullite, a reaction of solid-state sintering is proposed as 3Al2O3·2SiO2+8/3AlAl→2SiAl+VAl+13/3Al2O3 Furthermore, for an oxygen-rich AlON phase in the formation of AlON, the covalent bond strength of Al-N is higher than that of Al-O, and the diffusivity of oxygen is faster than that of nitrogen. AlON with a higher Al2O3 content is easy to sintering, leading to the grain growth.Al2O3 content increases and grains grow with increasing mullite content. The grains grow fast at a high mullite content, resulting in some trapped pores within the grains. All the hot isostatic pressed AlON ceramics have a density of > 99.2% of the theoretical value and a high transparency. The density, grain size and transmittance of hot isostatic pressed AlON ceramic firstly increase and then decrease with increasing mullite content. AlON transparent ceramic doped with 0.05% mullite exhibits the maximum transmittance of 81.9% (4 mm thickness) at 2 000 nm, with a density of 3.67 g/cm3 and an average grain size of 80.4 μm. Some pores in the grains appear in pre-sintered bodies at a higher mullite content, inhibiting the grain growth during the HIP process and desceasing the grain size. Also, some pores in the grains cannot eliminate and remain after HIP, degrading the transmittance. Conclusions AlON transparent ceramics were fabricated via pressureless sintering at 1 880 ℃ and hot isostatic pressing (HIP) at 1?800 ℃, respectively, with AlON powder and mullite., The same structure of AlON with different mullite contents of 0.02%-0.20% could be obtained. AlON transparent ceramic with 0.05% mullite exhibited the maximum transmittance of 81.9% (4 mm thickness) at 2 000 nm. At a mullite content of > 0.10%, some pores in grains appeared in pre-sintered bodies, inhibiting the grain growth in the HIP process.