The performance of zirconia ceramics is closely related to their microstructure, and the choice of sintering technology plays a key role in shaping their microstructure.
First, the traditional pressureless sintering technology is the basic method for sintering zirconia ceramics. During the pressureless sintering process, the zirconium oxide powder particles diffuse, rearrange and grow necks by slowly increasing the temperature. Since the sintering temperature and time are relatively long, the grains have enough time to grow, and it is easy to form a larger grain structure. This larger grain structure gradually increases the density of zirconia ceramics, but it also leads to certain limitations in its mechanical properties. For example, a larger grain size will reduce the fracture toughness of the material. When subjected to external forces, cracks are more likely to extend along the grain boundaries, thereby affecting the overall strength and service life of the ceramic. However, pressureless sintering is simple to operate and low in cost, and is suitable for the production of general zirconia ceramics products that do not require extremely high performance.
Second, hot pressing sintering technology significantly improves the microstructure and performance of zirconia ceramics. This technology applies a certain amount of pressure while heating. The presence of pressure promotes contact and diffusion between particles and accelerates the rate of material transfer. Compared with pressureless sintering, hot pressing sintering can achieve densification at a relatively low temperature and in a shorter time, effectively inhibiting the excessive growth of grains, and obtaining a smaller and more uniform grain size. The fine grain structure significantly improves the strength and toughness of zirconia ceramics, because cracks will encounter more grain boundary obstacles during the expansion process, consuming more energy, thereby improving the material's ability to resist crack expansion. In addition, zirconia ceramics prepared by hot pressing sintering have higher density and significantly reduced porosity, which also has a positive effect on its electrical and thermal properties. However, the hot pressing sintering equipment is complex and the cost is high, which limits its large-scale application.
Third, isostatic pressing sintering technology brings unique structural and performance characteristics to zirconia ceramics. Isostatic pressing sintering is to place the green body in a high-pressure container, apply pressure uniformly through a liquid medium, and sinter the green body in all directions under the same pressure. This method can make the pressure distribution inside the zirconia green body more uniform and avoid local stress concentration. During the isostatic pressing sintering process, the contact between particles is more sufficient, which is conducive to the migration and densification of materials. The prepared zirconia ceramics have a uniform microstructure and a higher density. Compared with other sintering methods, the performance consistency of isostatically pressed zirconia ceramics in different directions is better, the anisotropy is reduced, and it is suitable for the preparation of products with high requirements for performance uniformity, such as precision ceramic parts.
Fourth, microwave sintering technology, as a new sintering method, has a special impact on zirconia ceramics. Microwave sintering uses the interaction between microwaves and materials to produce a thermal effect inside zirconia ceramics, achieving rapid heating and sintering. Due to the non-contact heating and selective heating characteristics of microwaves, the temperature inside the ceramic can be uniformly increased, avoiding the temperature gradient problem in traditional sintering. During the microwave sintering process, the grain growth rate is relatively slow, and a fine and uniform grain structure can be obtained. At the same time, microwave sintering can significantly shorten the sintering time and improve production efficiency. In addition, since rapid sintering inhibits impurity diffusion and grain coarsening, the purity and performance of zirconia ceramics are improved, especially their mechanical and electrical properties are excellent. However, microwave sintering equipment is expensive and process control is difficult, and there are still certain limitations in large-scale application.
Fifth, spark plasma sintering (SPS) technology provides a new way to prepare zirconia ceramics. While applying pressure, SPS technology uses the discharge plasma generated by pulsed current to activate the surface of powder particles and promote rapid diffusion and sintering between particles. This technology can achieve densification of zirconia ceramics in a very short time, greatly shortening the sintering time. Due to the short sintering time, grain growth is effectively inhibited, thereby obtaining an ultrafine grain structure. Ultrafine-grained zirconia ceramics have extremely high strength, hardness and toughness, and their performance far exceeds that of ceramics prepared by traditional sintering methods. In addition, SPS technology can also accurately control parameters such as temperature, pressure and time during the sintering process, and achieve precise control of the ceramic microstructure and performance, which is suitable for the preparation of high-performance zirconia ceramics materials and functional devices.
Sixth, reaction sintering technology is also unique in the preparation of zirconia ceramics. Reaction sintering is to mix raw materials containing zirconium oxide with substances that can undergo chemical reactions, and generate zirconia ceramics through chemical reactions during the sintering process. This method can achieve the preparation of ceramics at a lower temperature and can form a unique microstructure during the reaction process. For example, by controlling the reaction conditions, the generated zirconium oxide grains can form a composite structure with other reaction products, thereby improving the performance of the ceramic. Zirconia ceramics prepared by reaction sintering have good dimensional stability, but due to the complexity of the reaction process, their density and performance uniformity may fluctuate to a certain extent, and the reaction process needs to be precisely controlled.
Seventh, different sintering technologies have their own advantages and disadvantages in affecting the microstructure and performance of zirconia ceramics. In practical applications, it is necessary to select appropriate sintering technology based on specific product requirements and performance indicators, taking into account factors such as cost, production efficiency, and equipment conditions. At the same time, with the continuous development of material science and sintering technology, it is expected that through technological innovation and optimization, the performance of zirconia ceramics will be further improved and its application areas will be expanded.
