New Development Trends of Advanced Ceramics Abroad

1. Introduction

Special ceramics can be formed by various methods including hot injection molding, hot pressing, isostatic pressing and vapor deposition. Because their chemical composition, microstructure and properties differ from ordinary ceramics, they are called special ceramics or high-tech ceramics, and are referred to as fine ceramics in Japan. Different chemical compositions and structures determine different special properties and functions of special ceramics, such as high strength, high hardness, high toughness, corrosion resistance, electrical conductivity, insulation, magnetism, light transmission, semiconductivity, as well as piezoelectric, photoelectric, electro-optical, acousto-optic and magneto-optical effects.

Due to their special properties, such ceramics can serve as engineering structural materials and functional materials in machinery, electronics, chemical processing, metallurgy, energy, medicine, laser, nuclear reaction, aerospace and other fields. Some economically developed countries, especially Japan, the United States and Western European countries, have invested substantial manpower, material and financial resources in R&D of special ceramics to accelerate the new technological revolution and lay a material foundation for emerging industries. As a result, special ceramics have developed rapidly with major technological breakthroughs. Special ceramics occupy an increasingly important position in modern industrial technology, particularly in high-tech and new-technology fields. By early this century, the international market size of special ceramics was projected to reach 50 billion US dollars. Therefore, many scientists predict that special ceramics will certainly occupy a very important position in the scientific and technological development of the 21st century.

2. New Progress in Production and Processing Technologies

(1) Powder Preparation

In powder preparation, the most remarkable development is ultra-high-temperature technology. Using ultra-high-temperature technology, special ceramics can be developed at low cost, as can new types of glass such as optical fibers, magnetic glass, hybrid integrated circuit boards, zero-expansion crystallized glass, high-strength glass, artificial bones and dental components. In addition, ultra-high-temperature technology enables the development of tantalum, molybdenum, tungsten, vanadium-iron alloys, titanium and other materials applicable to cutting-edge fields including space flight, ocean exploration and nuclear fusion. For example, Japan manufactures diamonds at 4000-15000℃ under one atmosphere, with efficiency 120 times higher than the commonly adopted low-temperature low-pressure plasma technology.

Ultra-high-temperature technology offers the following advantages: it can produce substances that cannot be produced by previous methods; it obtains materials of extremely high purity; production efficiency is greatly improved; and operating procedures are simplified and easier to implement. At present, Japan leads in ultra-high-temperature technology. According to statistics, Japan's special ceramic market size driven by ultra-high-temperature technology was expected to exceed 20 trillion yen in 2000. In addition, dissolution-method powder preparation, chemical vapor deposition of ceramic powder, sol-gel production of mullite ultrafine powder and plasma gas-phase reaction methods have also attracted attention. Most of these methods were developed or perfected in recent years.

(2) Forming and Sintering

In forming and sintering, hot isostatic pressing (HIP) attracts the most attention. Compared with hot pressing, HIP subjects materials to omnidirectional uniform pressure, resulting in homogeneous ceramic bodies. Moreover, HIP can apply pressures of several thousand atmospheres, enabling materials to complete sintering at lower temperatures. Currently, conventional commercial HIP equipment offers maximum operating temperature and pressure of 2000℃ and 2000 atmospheres.

(3) Precision Machining of Special Ceramics

In precision machining of special ceramics, vacuum diffusion bonding is one of the most promising methods. Vacuum diffusion bonding not only produces metal-ceramic products with high strength, high density and high geometric dimensional precision (leak rate no greater than 5×10⁻¹¹ m³·Pa/s), but also eliminates the need for precious rare solders. It can be used to manufacture metal-ceramic products of various shapes and sizes, especially large-scale ones.

In addition, machining ceramics with cutting tools has aroused great interest. At present, this work remains at the research and experimental stage. By using ultra-high-precision lathes and single-crystal diamond turning tools with micrometer-scale depth of cut and fine feed rates, machining accuracy of approximately 0.1 micron can be achieved. Therefore, many countries are developing and researching this machining technology as a branch of ultra-precision machining.

3. New Developments in Applications

Thanks to numerous excellent properties, special ceramics have wide applications. Classified by material performance:

• Special ceramics with excellent heat resistance can serve as ultra-high-temperature materials for high-temperature structural materials in atomic energy applications and high-temperature electrode materials;
• Special ceramics with excellent thermal insulation can serve as new high-temperature insulation materials for high-temperature furnaces, heat-treatment furnaces, high-temperature reaction vessels and nuclear reactors;
• Special ceramics with excellent thermal conductivity hold great promise as heat sinks for large-scale and very-large-scale integrated circuit electronic devices;
• Hard special ceramics with excellent wear resistance have broad applications, with current R&D focusing on bearings and cutting tools;
• High-strength ceramics can be used for gas turbine combustors, blades, turbines and casings, as well as machine tool beds, bearings, combustion nozzles and other mechanical components. The United States, Japan and Germany have invested substantial resources, with representative materials including silicon nitride, silicon carbide, sialon, aluminum nitride and zirconia;
• Hexagonal boron nitride ceramics with self-lubricating properties attract much attention, and overseas R&D is being accelerated;
• In the bioceramics field, research on alumina and apatite materials for artificial teeth, artificial bones and artificial joints continues to advance.

4. Future R&D Priorities

• Basic technology research on special ceramics: sintering mechanisms, inspection technologies, powder preparation technologies, etc.;
• Research on superconducting ceramics;
• Thin-film and amorphization technologies of special ceramics are important directions for enhancing ceramic functions;
• Ceramic fiber technology, focusing on silicon carbide and silicon nitride ceramic fibers and whisker-reinforced metal matrix composites;
• Development of porous ceramic materials;
• Ceramic matrix composite R&D: ceramic-reinforced ceramics and ceramic fiber-reinforced metals;
• Non-nitride ceramic direction: ceramic engines, high-pressure heat exchangers and ceramic cutting tools;
• Bioceramics: development of new materials adapted to biomedical scenarios.