Silicon nitride (Si3N4) ceramics are artificial synthetic strong covalent bond compounds. Since their large-scale production was achieved in the 1950s, they have attracted much attention due to their unique combination of properties [1]. Compared with metals and polymer materials, silicon nitride ceramics have a series of characteristics such as high hardness, high-temperature resistance (maintaining high flexural strength at high temperatures), low thermal expansion coefficient, and excellent acid and alkali corrosion resistance [1]. Their basic structural unit is the [SiN4] tetrahedron, with silicon atoms at the center and nitrogen atoms at the vertices, forming a continuous and strong network structure in three-dimensional space, which is the structural basis for their many excellent properties [2]. Over the years, to overcome the problems of low self-diffusion coefficient and difficulty in achieving dense sintering caused by their strong covalent bonds, researchers have developed various sintering processes (such as reaction sintering, hot-pressing sintering, and gas pressure sintering) and introduced sintering aids [1, 3]. With continuous breakthroughs in preparation technology, silicon nitride ceramics have gradually expanded from initial structural materials to functional materials with specific requirements for thermal, electrical, and biological properties, becoming one of the indispensable key materials in modern industry and national defense construction [3, 4].

The properties of silicon nitride ceramics are closely related to their microstructure, phase composition, and preparation process. Their main properties can be summarized as follows: Crystal structure and phase transformation: Silicon nitride mainly has two crystal forms: the low-temperature stable α-Si3N4 and the high-temperature stable β-Si3N4. The α phase is usually the main phase of the raw powder. During high-temperature liquid phase sintering, the α phase irreversibly transforms into the β phase through a dissolution-diffusion-precipitation mechanism [1, 4]. Mechanical properties: Silicon nitride ceramics have extremely high hardness (Vickers hardness can reach 18-21 GPa), high flexural strength (about 600-1400 MPa), and good fracture toughness (about 3-12 MPa·m1/2) [1, 2, 4]. Their density (about 3.10-3.26 g/cm3) is much lower than that of bearing steel, but the elastic modulus is higher [4]. Thermal properties: Silicon nitride ceramics are heat-resistant, with an oxidation onset temperature in air of 1300-1400°C, and their mechanical properties remain stable at high temperatures [1, 2]. They have excellent thermal shock resistance, a low coefficient of thermal expansion (about 3.2×10-6/K), which is close to that of silicon [1, 4]. Through process optimization, silicon nitride ceramics with high thermal conductivity can be obtained, with a theoretical thermal conductivity exceeding 200 W/(m·K), and actual products reaching 90-177 W/(m·K) [1, 5, 6]. Chemical stability: Silicon nitride ceramics are chemically stable and can resist the corrosion of most inorganic acids and alkali solutions. Functional characteristics: By regulating the composition and structure (such as preparing porous silicon nitride), specific functions can be imparted to the material, such as low dielectric constant, good wave transmission, high specific surface area, and excellent biocompatibility and osteoinductivity [4, 7, 8].

In the aerospace field, silicon nitride ceramics are widely used in the thermal protection systems and wave-transmitting components of high-speed aircraft due to their high-temperature resistance, anti-ablation, low density, and excellent dielectric properties. Thermal protection and structural components: Porous silicon nitride ceramics combine the high-temperature resistance of silicon nitride with the low thermal conductivity and low density of porous materials, making them highly suitable for use as thermal protection cover materials for aerospace vehicles [9]. Additionally, silicon nitride ceramics are also applied in key components such as turbine blades, combustion chamber linings, and missile radomes. The tail nozzle of the Japanese space probe Akatsuki, for instance, utilized silicon nitride ceramic materials, effectively enhancing its performance [5]. Wave-transmitting materials: Porous silicon nitride ceramics have a low dielectric constant and low tangent of loss, making them ideal wave-transmitting materials for the radomes of high-speed aircraft. By adjusting the porosity and pore size, they can be adapted to wide-band working environments, minimizing interference and loss to radar signals while ensuring structural strength [8, 9].

In the mechanical field, silicon nitride ceramics are mainly used as wear-resistant, corrosion-resistant, and high-temperature-resistant structural components, significantly enhancing the performance limits and service life of mechanical equipment. Bearing balls and grinding balls: This is one of the most classic and successful applications of silicon nitride ceramics. Compared with steel bearings, silicon nitride ceramic bearing balls have a lower density (reducing weight by 40%), which can significantly reduce the centrifugal force during high-speed rotation, extending the bearing life; they have a low coefficient of friction, enabling self-lubrication; high hardness and excellent wear resistance; and a low coefficient of thermal expansion, ensuring high operational stability [1, 2, 4]. They are widely used in high-speed, high-precision or corrosive environments such as precision machine tool spindles, aircraft engines, electric vehicles, and chemical pumps. In the ultrafine grinding industry, silicon nitride ceramic grinding balls, due to their high hardness, low wear, and low pollution characteristics, are gradually replacing traditional grinding media [3, 4]. Cutting tools: Silicon nitride ceramic cutting tools have high hardness and good red hardness, allowing for high-speed cutting at temperatures above 800°C. When processing difficult materials such as cast iron and high-temperature alloys, their efficiency and lifespan far exceed those of hard alloy cutting tools [3, 5]. Corrosion-resistant and wear-resistant components: In the chemical and metallurgical fields, silicon nitride ceramics are used to manufacture valves, sealing rings, nozzles, pipe linings, and thermocouple protection tubes, among others, taking advantage of their excellent resistance to acid and alkali corrosion and particle erosion, solving the problems of metal components being prone to corrosion and having short lifespans [3, 4].

With the development of electronic devices towards high power and high integration, heat dissipation has become a key bottleneck. Silicon nitride ceramics, due to their high theoretical thermal conductivity and excellent mechanical properties, have become ideal candidate materials for the next generation of high-performance electronic packaging substrates. High thermal conductivity packaging substrates: Compared with commonly used alumina (Al2O3) and aluminum nitride (AlN) substrates, silicon nitride substrates not only have good thermal conductivity (>90 W/(m·K)), but also possess higher fracture toughness (~6.5 MPa·m1/2) and flexural strength (>600 MPa), with excellent thermal shock resistance, meeting the requirements of repeated thermal cycling in harsh conditions such as electric vehicles [5, 6]. Research has successfully fabricated silicon nitride ceramics with thermal conductivities as high as 133-177 W/(m·K) by optimizing sintering aids (such as using MgSiN2 instead of MgO to reduce lattice oxygen) and sintering processes (such as reactive re-sintering), laying the foundation for high-end packaging [1, 6]. Semiconductor equipment components: In semiconductor manufacturing equipment, silicon nitride ceramics are also used to manufacture wafer carriers, heaters, etching machine components, etc., taking advantage of their high purity, resistance to plasma corrosion, and low pollution [1]. 3.4 Applications in the Medical Field

Silicon nitride ceramics possess excellent biocompatibility, antibacterial properties, osteoinductive ability, and mechanical properties comparable to human bones, making them promising for biomedical applications, particularly in orthopedic and dental implants. Orthopedic implants: The elastic modulus of silicon nitride closely matches that of human bones, which can mitigate the "stress shielding" effect. In physiological environments, its surface can release silicate ions and trace amounts of ammonia, stimulating osteoblast activity, inhibiting bacterial growth, and promoting bone integration [7]. Since the late 1980s, silicon nitride has been successfully employed as a spinal interbody fusion device. Long-term clinical follow-up (exceeding 20 years) has demonstrated that silicon nitride fusion devices effectively promote bone growth, achieve solid fusion, and exhibit no toxic reactions [2, 7]. Dental implants: Research into the use of silicon nitride as a dental implant material is growing. Its distinctive micro/nano-scale rough surface morphology is conducive to osteoblast adhesion and proliferation [7]. Furthermore, silicon nitride exhibits inhibitory effects on common oral pathogenic bacteria such as Porphyromonas gingivalis and Staphylococcus aureus, thereby reducing the risk of peri-implantitis [7]. Additionally, silicon nitride possesses semi-attenuating properties to X-rays, enabling clear visualization of its interface with bone tissue during imaging examinations with minimal artifacts, which facilitates postoperative assessment [7]. Bone tissue engineering scaffolds: Porous silicon nitride ceramics can serve as scaffold materials for bone defect repair. Their porous structure provides space for cell ingrowth, nutrient transport, and vascularization, while the material's inherent bioactivity can accelerate new bone formation [7, 8]. 3.5 Other Applications

Metallurgical industry: Utilizing its excellent resistance to metal melt erosion and thermal shock, silicon nitride ceramics are used to manufacture components such as aluminum liquid temperature measurement thermocouple sleeves, aluminum smelting furnace linings, crucibles, and channels, with lifespans far exceeding those of stainless steel and corundum materials [2]. Automotive industry: Silicon nitride ceramic turbocharger rotors, due to their low density and small inertia, can significantly reduce turbo lag, improve engine response speed and efficiency, and have been applied in high-end automobiles [2]. Chemical and environmental protection: Porous silicon nitride ceramics, with their high-temperature resistance, corrosion resistance, and high filtration accuracy, can be used as high-temperature gas filters, catalyst carriers, membrane separation components, etc. [8].

Silicon nitride ceramics, as an advanced ceramic material with outstanding performance, have been deeply integrated into strategic emerging industries such as aerospace, high-end manufacturing, semiconductor electronics, and biomedicine, beyond their initial application in structural components. In the aerospace sector, they serve as thermal protection and wave-transmitting materials, supporting the development of hypersonic vehicles. In the mechanical field, high-performance ceramic bearings and cutting tools enhance the ultimate performance and reliability of equipment. In the semiconductor industry, high-thermal-conductivity silicon nitride substrates offer a key solution to the heat dissipation bottleneck of next-generation power devices. In biomedicine, their excellent biocompatibility and antibacterial properties bring new hope to bone repair and dental implants. However, the further development of silicon nitride ceramics still faces challenges: Firstly, the preparation technology of high-performance and low-cost silicon nitride powders (especially the silicon imide pyrolysis method) needs to be advanced to reduce reliance on imported high-end powders [4, 6]. Secondly, the precise forming and low-cost manufacturing of complex-shaped components (such as personalized medical implants and complex channel structures) await the further maturation and application of near-net-shape technologies like additive manufacturing (3D printing) [10, 5, 7]. Lastly, a deeper understanding of the performance degradation mechanisms and life prediction under extreme conditions (such as ultra-high temperatures, strong corrosion, and long-term loading) is still required. In the future, with the advancement of material preparation science, the deepening of interdisciplinary integration, and the promotion of the "structure-function integration" design concept, silicon nitride ceramics will surely play their role as an "all-round champion" on a broader stage, making greater contributions to technological progress and industrial upgrading.