Advanced ceramics, also known as special ceramics, refer to ceramic materials prepared on the basis of traditional ceramics using advanced chemical and physical technologies, with excellent properties in mechanics, thermal, optical, acoustic, and electrical fields. Compared with metals and polymer materials, advanced ceramic materials possess incomparable structural properties such as high hardness, high modulus, high-temperature resistance, and corrosion resistance, as well as excellent functional properties such as electrical insulation, light transmission, and wave transparency. Therefore, they are increasingly used in aerospace, information technology, national defense and military industry, biomedical, and new energy fields.
The preparation process of advanced ceramics mainly includes powder preparation, green body forming, and sintering. Among these, forming and sintering are the direct factors determining the properties of ceramic products. Forming refers to making powder into a green body with specified dimensions and shape and certain mechanical strength. Sintering refers to the process in which the green body, at a high temperature below its melting point, eliminates pores, densifies, and forms a sintered body with certain strength.
01 Powder Preparation
The preparation methods of advanced ceramic powders can mainly be summarized into three categories: solid-phase methods, liquid-phase methods, and gas-phase methods.
The solid-phase method uses solid substances as starting raw materials to prepare ultrafine ceramic powders. Examples include high-temperature solid-state reaction, carbothermal reduction, solid-gas reaction, salt thermal decomposition, and self-propagating high-temperature combustion synthesis. It is characterized by being suitable for mass production and relatively low cost, but sometimes impurities are present.
The liquid-phase method is characterized by easy control of chemical composition, uniform mixing at the ionic or molecular scale, and the ability to prepare nano- and submicron-scale ceramic powders with high purity and good sintering activity. It is especially suitable for synthesizing single-oxide or composite-oxide powders for structural ceramics and functional ceramics, and therefore is widely used. Common liquid-phase methods include chemical precipitation, hydrothermal methods, and sol-gel methods.
The gas-phase method synthesizes the desired ceramic powder through chemical reactions of reactant gases in a locally high-temperature environment. It includes chemical vapor deposition, plasma gas-phase synthesis, and others. Powders synthesized by gas-phase methods have a large specific surface area and readily adsorb moisture or agglomerate, making them unsuitable for sintering high-density structural ceramics and electronic ceramic materials. In addition, the raw materials and reaction synthesis equipment used in gas-phase methods are expensive and have very small production capacity, resulting in high powder preparation costs.
02 Forming Processes
According to the flow and rheological properties of the body, forming methods can be divided into three categories: dry body press forming, plastic body forming, and slurry forming.
Press forming, also called compression molding, involves adding a small amount of binder to the powder (moisture content controlled at 4%–7%, or even as low as 1%–4%) for granulation, then placing the granulated powder in a mold and pressing it under a press to form a green body of a certain shape. Press forming is divided into dry pressing and isostatic pressing.
Plastic forming uses a plastic body as the starting material and utilizes pressure generated by mold movement to plastically deform the body into a green body. Plastic materials mainly consist of ceramic powder, binder, plasticizer, and solvent, and their raw material preparation process is demanding. The plastic body contains a relatively large amount of forming agent, but generally no more than 30%. Plastic forming methods include extrusion, tape calendering, injection molding, and others.
Slurry forming uses a flowable ceramic suspension as the raw material. In addition to powder particles, the slurry also contains water and dispersant. Slurry forming mainly includes slip casting, gelcasting, tape casting, and others.
Both plastic forming and slurry forming belong to wet forming processes. Compared with dry forming, wet processes can more easily control powder agglomeration and reduce impurity content, thereby producing ceramic products with complex shapes.
As the application fields of advanced ceramics continue to expand, new technologies such as centrifugal deposition, electrophoretic deposition, direct coagulation casting, and solid freeform fabrication have emerged one after another.
03 Sintering Processes
Ceramic green bodies need to be sintered to obtain a dense microstructure. Essentially, sintering is a process of mutual diffusion among different powder components. By strengthening bonding between particles, it gives ceramic materials a uniform microstructure, stable shape, and relatively high mechanical strength. Common sintering processes include pressureless sintering, hot isostatic pressing sintering, spark plasma sintering, and others.
Pressureless sintering is sintering materials under atmospheric pressure without applying pressure. This method is generally carried out under oxygen or a special gas atmosphere. During pressureless sintering, the formed green body is not subjected to external pressure; the aggregate of powder particles is simply heated under atmospheric pressure and transformed into a bonded grain body.
Gas pressure sintering refers to applying a certain gas pressure during high-temperature sintering of ceramics, in the range of 1–10 MPa (i.e., 10–100 atm), to suppress decomposition and weight loss of ceramic materials at high temperatures. This can increase the sintering temperature, further promote densification, and yield high-density ceramic products.
Hot-press sintering is a densification process combining mechanical pressure and high temperature. By simultaneously applying axial pressure and heating in a mold, it significantly enhances the sintering driving force. This process has three core advantages: achieving rapid densification, shortening the sintering cycle, and greatly improving the mechanical properties of materials. It is especially suitable for preparing high-performance ceramics.
Hot isostatic pressing sintering uses an inert gas as the medium and achieves ceramic densification under high temperature and constant high pressure. For some ceramic materials with low self-diffusion coefficients that are difficult to densify by conventional sintering, hot isostatic pressing sintering can be used to increase the driving force for diffusion. Hot isostatic pressing sintering meets the high-pressure and high-temperature conditions required for sintering transparent ceramics and has unique advantages in sintering transparent ceramic green bodies.
Spark plasma sintering (SPS) utilizes direct Joule heating generated by a pulsed direct current, assisted by mechanical pressure. Due to its sintering characteristics, it can be used to rapidly prepare ultra-high-temperature ceramics. For ultra-high-temperature ceramics such as Ta2AlC, TaC, and ZrB2-SiC, the SPS process can achieve short sintering times, obvious fine-grain strengthening effects, significantly improved mechanical properties, and enhanced oxidation resistance.
The cold sintering process, under high pressure, adds a transient solvent to the powder to promote particle rearrangement and diffusion, thereby promoting densification of ceramic powder at a relatively low temperature and in a relatively short time. This process creates the possibility of low-temperature sintering to prepare high-performance structural and functional ceramics. At present, this process has been successfully applied to the preparation and sintering of various materials, including typical oxide ceramics, ceramic/ceramic composites, microwave dielectric ceramics, ferroelectric and piezoelectric ceramics, solid electrolytes, thermoelectric materials, and ceramic/2D nanomaterial composites.
Oscillatory pressure sintering technology is based on hot-press sintering and changes constant pressure into dynamic pressure. Through particle rearrangement caused by continuous oscillatory pressure, it increases the packing density of the powder before sintering. At the same time, oscillatory pressure provides a greater sintering driving force for powder sintering, helping to promote grain rotation and sliding and plastic flow within the sintered body, thereby accelerating densification of the green body. This process has been used in the preparation of various structural ceramics, and high-performance ceramic materials such as ZrO2, Al2O3, and Si3N4 have been successfully developed.