Refractories are important supporting materials for high-temperature industries. During high-temperature service, refractories are subjected to severe thermal shock, complex chemical corrosion, and mechanical wear, and must balance superior properties like thermal shock resistance, corrosion resistance, and high strength. However, the key service properties of refractories are often interdependent, such as superior corrosion resistance requiring a dense structure, which sacrifices the thermal shock resistance of the material. The superior thermal shock resistance requires a certain amount of pores to relieve thermal stress, but this reduces the strength and corrosion resistance of the material. Developing novel refractories with gradient composition and structure based on the different service micro-environments of refractory products in different regions, such as cold and hot surfaces, to enhance key service performance in different zones is an important research direction. In this paper, a gradient functional material with “21R-SiAlON dense layer AlN–SiC solid solution reinforced corundum porous structure” was developed based on the sensitivity of metal Al and Si to oxygen partial pressure at high temperatures.

Tabular alumina (3–1 mm, 1–0 mm; w(Al₂O₃) > 99.31%; Anmai Aluminum Co., China), α–Al₂O₃ powder (≤5 μm; w(Al₂O₃) > 99.28%), active α–Al₂O₃ powder (≤ 5 μm; w(Al₂O₃)>99.28%; Anmai Aluminum Co., China), metal aluminum powder (≤ 45 μm; w(Al)>99.77%, Henan Yuayang Co., China) and silicon powder (≤ 45 μm; w(Si)>99.52%; Yingkou Xianren Island Taihe Silicon Co., China) were used as raw materials, with phenolic resin (residual carbon of about 56%, Shengquan Group, China) as a binder. In the preparation, the weighted raw materials were mixed in a planetary mixer for 40 min to obtain uniform mixed slip. Afterwards, Al–Si–Al₂O₃ composite green brick samples (with the sizes of 230 mm × 113 mm × 65 mm) were obtained by compacting the mixed slip under 200 MPa by a model 630 t friction press. After natural dehydration for 24 h, the sample bricks were dried in a tunnel kiln at 200 ℃ for 12 h. The dried samples were nitriding sintered in a high-temperature nitriding furnace at 1550 ℃ for 3 h.

The phase composition of the sintered samples was analyzed by X-ray diffraction (XRD; Ultima IV, Rigaku Co., Japan). The microstructures of the samples were analyzed by scanning electron microscopy (SEM) in a model NOVA NANOSEM 450 electron microscope (FEI Co., USA). The pore structure of the samples was determined by a model FF35 CT microfocus industrial computered tomography (CT, YXLON International GmbH, Germany).

Based on the sensitivity of metal Al and Si to oxygen partial pressure at high temperatures, Al–Si–Al₂O₃ composites are sintered in a nitrogen atmosphere at 1550 ℃for 3 h to prepare a gradient functional material, which is composed of “21R–SiAlON dense layer-AlN–SiC solid solution reinforced corundum with a porous structure”. The synthesis mechanism is analyzed by CT, SEM, and XRD combined with thermodynamic analysis. The results indicate that P(O₂) on the surface of the sample is higher, where the “active oxidation to indirect nitridation” mechanism of Al and Si is predominated. Al and Si on the surface layer preferentially undergo active oxidation, generating gaseous sub oxides such as Al₂O and SiO, and reducing the local P(O₂). 21R–SiAlON is formed via a reaction of metal Al, Si, active α–Al₂O₃ powder and N₂. Under the action of concentration gradient, the preformed gaseous sub-oxides diffuse into the surface layer and react with nitrogen to form 21R–SiAlON, thus promoting the staggered growth of sheet-like 21R–SiAlON and ultimately forming a dense layer (with porosity<4%), which is constructive to the corrosion resistance of the material. As the active oxidation of outer Al and Si consumes O₂ layer by layer, P(O₂) inside the brick becomes extremely low, and the “direct nitriding/carbonization” reaction mechanism of Al and Si becomes dominant. Al and Si inside the brick react directly with nitrogen and residual carbon of phenolic resin, generating granular AlN–SiC solid solution reinforcement phase in-situ, forming a high-strength porous structure (with a porosity of 15%), which can alleviate a stress concentration caused by temperature gradient and improve thermal shock resistance.

A gradient functional material with a “21R–SiAlON reinforced denselayer—AlN–SiC solid solution reinforced porous structure” was prepared via nitriding Al–Si–Al₂O₃ composite in a nitrogen atmosphere at 1550 ℃ for 3 h. During sintering, there were two reaction mechanisms for metal Al and Si in different regions of the outer and inner layers of the sample, i.e., the “active oxidation to indirect nitridation” mechanism and the “direct nitridation/carbonization” mechanism in the outer and inner layers resulting in a gradient change in the composition and structure of the material. In the outer layer of the sample, the oxygen partial pressure was higher, and metal Al and Si preferentially underwent an active oxidation, generating gaseous sub oxides (i.e., Al₂O, AlO, SiO, etc.), which reduced the oxygen partial pressure. As the oxygen partial pressure decreased, Al powder, Si powder, and α–Al₂O₃ micro powder reacted with nitrogen to form 21R–SiAlON. Gaseous metastable phases such as Al₂O and SiO diffused to the outer layer of the sample to deposition, further promoting the staggered growth and development of sheet-like 21R–SiAlON, forming a dense layer. The porosity of this dense layer was less than 4%, and it was mainly composed of closed micropores. With the consumption of O₂ by the outer metal Al and Si of the sample, the oxygen partial pressure inside the sample was reduced to an extremely low level. Metal Al and Si directly reacted with N₂ and residual carbon of phenolic resin, generating AlN–SiC solid solution reinforcement phase in-situ. The porosity inside the sample was approximately 15%.