Overview of Hot Isostatic Pressing Technology [1,2]
Hot Isostatic Pressing (HIP) is a process in which an inert gas medium (such as argon or nitrogen) is used to apply equal pressure in all directions to a material under high temperature (900–2000°C) and high pressure (50–200 MPa), thereby achieving densification.
HIP research began in 1955, when Battelle Institute in the United States carried out work for the development of nuclear reactor materials. It was first used for diffusion bonding of fuel elements in atomic energy reactions; based on this application, it was then called "gas pressure bonding." In 1963, the technology was introduced to Europe. The Swedish company ASEA used a prestressed steel-wire wound structure to manufacture high-pressure vessels. Its compact, safe, and reliable structure laid a solid foundation for the vigorous development of HIP technology. Later, through continuous improvements by institutions such as Kennametal, the Beijing Central Iron and Steel Research Institute (hereinafter referred to as CISRI), and AE Company of the United States, the development and application of HIP equipment were greatly expanded.
The demand for HIP processes in production applications such as powder metallurgy material preparation, casting densification, aerospace and industrial gas turbine components, rocket engines, satellites and spacecraft body castings, composite joining, and near-net shaping has been increasing day by day. In the preparation of high-performance special materials, it shows advantages unmatched by other technologies.
Development of Hot Isostatic Pressing Technology at Home and Abroad
2.1 Domestic Development of Hot Isostatic Pressing Technology [3,4]
CISRI began researching China's first hot isostatic press in 1972. Through many years of research and development, CISRI mastered the key technologies of hot isostatic presses: high-temperature and high-pressure technology. In terms of equipment safety, reliability, and stability, it reached the U.S. ASEM standard, and the main technical indicators of the hot isostatic presses it produced reached the level of existing hot isostatic presses from the Swedish company AVURE.
In July 2008, the largest domestic hot isostatic press at that time (Φ1250 mm × 2500 mm, 1350°C, 150 MPa), manufactured by CISRI for its holding company Advanced Technology & Materials Co., Ltd., was successfully installed, debugged, and officially put into operation. The commissioning of this equipment filled the gap in large-scale hot isostatic pressing technology in China and created conditions for the development and production of high-temperature, high-strength alloys, functional ceramics, composite materials, superhard materials, and other high-tech material products for aerospace, the nuclear industry, electronics and metallurgy, ships, and other fields.
In July 2024, China Iron & Steel Research Institute Group Co., Ltd. officially released China's first set of ultra-large hot isostatic pressing equipment, HIPEX1850. This is currently the largest hot isostatic pressing equipment in China and the second largest in the world in terms of size, and it can be used for the processing of large core components in fields such as China's aerospace industry. The release of this equipment marks that China's hot isostatic pressing equipment manufacturing technology has reached a world-leading level in equipment size, processing parameters, modular design, intelligent control, equipment stability, and other aspects. The equipment adopts a prestressed steel-wire wound structure, with a total steel wire weight of 350 t and a total length of about 6,000 km. It has a material load capacity of 30 t per furnace and can perform efficient and precise hot isostatic pressing processing on core components such as ultra-large aero-engine casings, engine blades, fuel nozzles, and turbine disks.
2.2 Foreign Development of Hot Isostatic Pressing Technology
In 1965, the first hot isostatic press developed by Battelle Institute in the United States came into being, marking the birth of hot isostatic pressing equipment [5]. In the earliest hot isostatic pressing equipment, the connection between the end cover and the cylinder body of the high-pressure vessel was threaded, and safety and operability could not be guaranteed. After many years of research and design, today's advanced hot isostatic presses have a prestressed steel-wire wound frame structure, and the connection between the end cover and the cylinder body of the high-pressure vessel is threadless.
At present, the main manufacturers of foreign hot isostatic pressing equipment include Quintus of Sweden, EPSI of Belgium, AIP of the United States, Bodycote of the United Kingdom, and Kobe Steel of Japan. Among them, Quintus is now a wholly owned subsidiary of Kobe Steel, Japan. Quintus's hot isostatic pressing equipment adopts a prestressed steel-wire wound structure and is in a leading position in the ultra-high-pressure field. Since manufacturing its first QUINTUS® isostatic press in 1965, it has delivered nearly 1,000 prestressed steel-wire wound cylinders and components worldwide, supplied about 60% of hot isostatic pressing equipment, and about 80% of that equipment has an effective diameter greater than 1 m. It has also developed the world's largest hot isostatic pressing equipment (effective hot zone: Ø2050 mm × 4200 mm; maximum working temperature and pressure: 1350°C/118 MPa), which was successfully delivered to MTC Company in Japan [6].
Application of Hot Isostatic Pressing Technology in Advanced Ceramics
In its early period, because the overall cost of the equipment was relatively high, the development of hot isostatic pressing technology was slow, and its applications were concentrated only in a few fields such as military industry and nuclear reactions. In recent years, with the continuous advancement of science and technology, various fields have imposed increasingly stringent requirements on the use of materials. Hot isostatic pressing technology occupies an advantageous position in preparing materials with excellent comprehensive properties such as high density, high purity, high uniformity, and high toughness. This technology has become a practical technology in the production of many high-performance materials today and is also an indispensable new technology for the development of new materials [3].
3.1 Improving Density
Advanced ceramics are high-performance inorganic non-metallic materials prepared through precise design and engineering. They possess unique properties such as high-temperature resistance, high insulation, wear and corrosion resistance, and biocompatibility. They are hailed as "the ultimate materials for extreme environments" and are also core strategic materials for semiconductors, AI computing power, new energy vehicles, aerospace, and medical dentistry. However, advanced ceramic products generally contain 5%–10% gaseous pores, which reduce the toughness and strength of the products. Therefore, reducing gaseous pores and improving density can significantly improve the performance of ceramic products. At present, hot isostatic pressing densification treatment is the best process for improving the density of ceramic products.
The hot isostatic pressing densification process places the product in a sealed vessel. A compressor feeds high-pressure argon into the vessel, while a heating furnace inside the vessel heats it, so that the product is densified under the combined action of high temperature and high pressure. The process can be divided into two stages. In the first stage, internal defects and pores undergo plastic deformation under the condition that the external pressure is greater than the high-temperature strength of the material. The defects and pores are destroyed, causing their internal surfaces to come into contact with each other. In the second stage, when the external pressure is lower than the high-temperature strength of the material, the material undergoes high-temperature creep, and at the same time the contacting alloy surfaces mutually infiltrate and diffusion-bond, thoroughly eliminating defects and pores. Through this treatment, the material density basically approaches the theoretical density [7].
Zhang Yong et al. from the University of Science and Technology Beijing [8] studied silicon nitride foamed ceramics. Using a hot isostatic pressing densification process at a temperature of 1600–1750°C, a pressure of 134–188 MPa, and a holding time of 1–2 h, the density increased from 2.674 g/cm³ to 3.332 g/cm³, and the compressive strength increased by nearly three times.
3.2 Eliminating Porosity and Defects
The chemical bonds of transparent ceramics are mostly covalent or ionic bonds. Without applying auxiliary pressure, it is difficult to sinter them to full density, and residual pores will affect the optical transmittance of the ceramics. HIP technology is a powerful tool for developing and producing transparent ceramics with high optical quality [9]. This technology uses an inert gas as the medium; high temperature and high pressure reduce the yield strength of the material and improve plastic deformation and atomic diffusion ability. It is an effective technology for eliminating pores, homogenizing composition, and stabilizing material properties.
The characteristic of hot isostatic pressing being able to eliminate closed pores in polycrystalline materials exactly meets the requirements of transparent ceramics for as few pores as possible and ideal optical transmittance. Because the sintering temperature of transparent ceramics is generally higher than 1300°C, it is difficult to directly sinter ceramic powder loaded into a metal sheath. Instead, ceramic green bodies can only be pre-sintered until the pores become closed, and then hot isostatic pressing densification is carried out [10]. The greatest role of hot isostatic pressing post-treatment for transparent ceramics is that, under high temperature and high pressure, atomic diffusion ability is enhanced, prompting pores in the ceramic to migrate to grain boundaries or the workpiece surface, thereby reducing and eliminating pores.
Toci et al. [11] used vacuum sintering and hot isostatic pressing to prepare 3 at.% Yb:Sc2O3 laser transparent ceramics. Comparing the two samples, the grain size of the sample after hot isostatic pressing increased slightly to 2.8 μm, almost no pores could be seen inside the grains, and the transmittance of all samples at 1100 nm exceeded 70%. This indicates that hot isostatic pressing can eliminate pores produced by vacuum sintering without significant grain growth. In order to obtain fine-grained ceramics, the vacuum sintering temperature should not be too high. Vacuum pre-sintering at 1650°C combined with hot isostatic pressing at 1800°C is the optimal process for this transparent ceramic material.
3.3 Refining Grains
Through the synergistic effect of high temperature and high pressure, HIP achieves densification of ceramics at a relatively lower temperature, while inhibiting abnormal grain growth in the ceramic and reducing opportunities for the existence of twin boundaries and low-angle grain boundaries [12]. Therefore, by using ultrafine or nano-sized raw materials and adopting relatively low pre-sintering and hot isostatic pressing temperatures, fine grains and grain boundaries with a narrower width can be obtained.
3.4 Near-Net Shaping of Large-Sized and Irregular Ceramics
Hot isostatic pressing technology simultaneously applies high-pressure inert gas and high temperature to the product. It can effectively remove internal voids and form strong metallurgical bonding throughout the material, greatly solving the difficulties in preparing ceramics or ceramic-metal composite materials, and it has significant advantages especially in preparing large-sized and complex-shaped ceramic materials. In addition, a very prominent feature of hot isostatic pressing sintering is that its furnace chamber is made very large. Combined with its isostatic pressing characteristics, it can complete the sintering of large-sized complex irregular parts, and the sintered products do not require subsequent mechanical machining.
In addition to the above functions, another typical application of hot isostatic pressing post-treatment is ceramic coatings. Coatings produced by thermal spraying are prone to two defects: first, poor bonding strength to the substrate; second, pores are likely to appear in the coating, resulting in low density, which causes the coating to easily peel off. Through hot isostatic pressing post-treatment, the density of the coating can be improved, while the bonding force with the substrate increases, delamination disappears, and the coating and substrate become an integrated whole.