REN Zhe 1, ZHOU Fen 1, WU Xi 2, SHANG Weixiang 2, 3, BAO Jinxiao 1
(1. School of Materials Science and Engineering, Inner Mongolia University of Science and Technology, Baotou 014000, Inner Mongolia, China; 2. School of Rare Earth Industry, Inner Mongolia University of Science and Technology, Baotou 014000, Inner Mongolia, China; 3. Key Laboratory of Green Extraction & Efficient Utilization of Light Rare-Earth Resources, Inner Mongolia University of Science and Technology, Baotou 014000, Inner Mongolia, China)
Extended Abstract:[Background and purpose] Due to their outstanding properties, including high-temperature resistance, superior strength, excellent toughness, high hardness and oxidation resistance, silicon nitride (Si3N4) ceramics have been widely employed in various fields, such as metallurgy, mechanical engineering, medicine and aerospace. However, addressing the coloration issue of Si3N4 ceramics is a significant challenge. Most rare-earth and transition metal oxides decompose at elevated temperatures, making them unsuitable for color modification of Si3N4 ceramics. Furthermore, transition metal ions, due to their limited solubility in the β-Si3N4 lattice, tend to dissolve into the glass phase at grain boundary or interact with rare-earth ions and sintering aids, leading to the formation of new crystalline phases. Consequently, by identifying the electronic transitions and oxidation states of rare-earth metal ions within the Si3N4 lattice, the color of Si3N4 ceramics can be effectively tailored. As a low-valence niobium oxide, NbO exhibits significant potential as a direct black pigment, simplifying the fabrication process of colored ceramics while reducing production costs. Moreover, black is one of the most commonly used colors for smartphone backplates and is highly favored by consumers. Based on this consideration, NbO was selected as a pigment in this study to directly synthesize black Si3N4 ceramics.[Methods] Commercially available Si3N4 powder was utilized as the raw material, with Al2O3 and Y2O3 serving as sintering aids. The ceramic samples were fabricated via hot-press sintering. Prior to sintering, the Si3N4 powder was subjected to sand milling, followed by mixing with NbO at a predetermined mass ratio. The mixture was then ball-milled in a silicon nitride jar to ensure homogeneous dispersion and enhance the sintering properties. The resulting slurry was dried in an oven, ground and sieved to obtain fine powder. The powder was initially compacted using uniaxial dry pressing with 20 mm diameter molds at a pressure of 8 MPa, followed by cold isostatic pressing at 200 MPa to form green bodies. The samples were subsequently sintered in a vacuum hot-press furnace in Ar at 1600 ℃ for 1 h, at an applied pressure of 30 MPa, for, ultimately yielding black Si3N4 ceramic samples. Phase composition of the sintered ceramics was analyzed by using X-ray diffraction (XRD) at a scan rate of 10 (°)·min−1. Microstructure and elemental composition were characterized by using an ultra-high-resolution field emission scanning electron microscope (FE-SEM) equipped with an energy-dispersive spectrometer (EDS). Optical absorption properties were examined using a solid-state UV-Vis spectrophotometer, at a scanning rate of 2 nm·s−1, within wavelength range of 200–800 nm. Additionally, a colorimeter was employed to analyze the color characteristics of the samples. Mechanical properties, including Vickers hardness and fracture toughness, were evaluated using a Vickers hardness tester.[Results] XRD patterns indicate that the diffraction peaks correspond to the β-Si3N4 and Si2N2O. Fracture surface microstructure analysis results reveal that α-Si3N4 grains gradually dissolve into the liquid phase and subsequently precipitate as β-Si3N4 rods, accompanied by the formation of Si2N2O. The transformation alters the grains from a rod-like morphology to a lath-like structure, thereby enhancing the fracture toughness of the silicon nitride ceramics. At a doping level of 5 wt.% NbO, the Vickers hardness reaches its maximum value of 21.51 GPa, while the fracture toughness stabilizes at 10.14 MPa·m1/2. As the NbO content is increased, the reflectance of the ceramic samples decreases, leading to continuous darkening. When the NbO doping level reaches 9 wt.%, the samples exhibit the lowest reflectance and the highest blackness.[Conclusions] To overcome the challenges of monochromaticity and uneven coloration in traditional silicon nitride ceramics, this study was aimed to utilize hot-press sintering and optimized NbO doping to fabricate high-performance black silicon nitride ceramics with stable phase composition, uniform coloration and excellent mechanical properties. With 5 wt.% NbO, the samples exhibited optimal overall performance, with Vickers hardness of (21.51±0.17) GPa and fracture toughness of (9.98±0.11) MPa·m1/2. During hot-press sintering process, α-Si3N4 transformed to β-Si3N4, contributing to self-toughening. Meanwhile, NbO doping enhanced the crystal structure of β-Si3N4 and facilitated the formation of the tougher secondary phase Si2N2O, thereby improving mechanical properties of the ceramics. The black coloration of the silicon nitride ceramics originated from the d-d electronic transitions of Nb2⁺ in the visible light spectrum, which effectively absorbed all visible wavelengths, resulting in the characteristic black appearance of the samples.
Key words: silicon nitride ceramics; HPS; color