Open Access is an initiative that aims to make scientific research freely available to all. To date our community has made over 100 million downloads. It’s based on principles of collaboration, unobstructed discovery, and, most importantly, scientific progression. As PhD students, we found it difficult to access the research we needed, so we decided to create a new Open Access publisher that levels the playing field for scientists across the world. How? By making research easy to access, and puts the academic needs of the researchers before the business interests of publishers.
We are a community of more than 103,000 authors and editors from 3,291 institutions spanning 160 countries, including Nobel Prize winners and some of the world’s most-cited researchers. Publishing on IntechOpen allows authors to earn citations and find new collaborators, meaning more people see your work not only from your own field of study, but from other related fields too.
Hot isostatic pressing (HIP) is pivotal in advancing ceramic materials by consolidating and densifying them through high temperature and pressure. This technique significantly improves mechanical, thermal, and electrical properties, resulting in ceramics with enhanced structural integrity and reduced porosity. HIP addresses challenges of traditional fabrication methods by achieving near-theoretical density while minimizing residual pores and defects. The outcome is ceramics with superior mechanical strength, wear resistance, and thermal stability, making them ideal for demanding applications such as cutting tools, biomedical implants, and high-performance engine parts. Additionally, HIP facilitates the production of complex-shaped ceramic components, enabling intricate designs and the integration of multiple functionalities within a single part. It can also consolidate ceramic-based composites with reinforcing fibers or particles, providing tailored properties for specific applications. Overall, HIP is essential in the development of high-performance, reliable, and cost-effective ceramic components necessary for various industries, including aerospace, energy, and healthcare.
Hot isostatic pressing (HIP) is a manufacturing process designed to decrease the porosity and increase the density of various ceramic materials. This technique improves the mechanical properties and performance of these materials by subjecting components to high temperatures and isostatic gas pressure in a high-pressure chamber. Argon is commonly used as a pressurizing gas due to its inert nature, which prevents chemical reactions with the materials. As the chamber is heated, the internal pressure rises, often aided by continuous gas pumping to maintain the desired pressure. The uniform application of pressure from all directions is why this process is called “isostatic.” Currently, HIP is extensively used in the production of ceramic components. The advantages of the HIP process have led to its widespread adoption in shaping structural ceramics. Non-oxide ceramics can be converted into high-density components using this method. Additionally, components produced through HIP usually have small grain sizes and often do not require additives. The combination of high-density and fine grain structure results in components with outstanding properties. In the HIP forming process, temperatures typically range from 500°C to 1900°C, and pressures range from 500 to 2000 kgf/cm2. Key applications of these high temperatures and pressures include: (1) producing components capable of withstanding high operational pressures, (2) enhancing physical properties through phase transformations under pressure, (3) accelerating solid-gas chemical reactions, (4) significantly reducing the need for sintering additives in ceramics, (5) enabling the manufacture of parts that closely match their final shapes, minimizing the need for complex machining, and (6) preventing excessive grain growth There are generally two main methods for shaping parts using hot isostatic pressing: (a) pre-shaping through techniques such as slip casting, cold isostatic pressing (CIP), additive manufacturing (e.g., 3D printing), and plastic forming methods (e.g., injection molding). After pre-shaping, the products are sealed in chambers or glass capsules to undergo the HIP process. (b) The pre-shaped components initially undergo pressureless sintering, achieving a density of 90–95% with minimal porosity. To reach near-theoretical maximum density, the components are then subjected to a post-HIP process [1, 2, 3].
The encapsulation method is a widely utilized technique for high-density sintering of powders. This process involves placing the powder into a mold, sealing the inlet, and applying isostatic pressing to the capsule. By using hot gas to exert pressure, this technique enables the production of parts with enhanced density [4].
The applications of encapsulation are diverse, encompassing the sintering of powders that are difficult to process by conventional methods, the creation of composites, and the impregnation of porous materials with a molten phase. A crucial element of the encapsulation process is the effective sealing of the capsule, which ensures it remains gas-tight. The capsule’s function is to uniformly deliver gas pressure to the powder, facilitating compaction and the subsequent formation of the component [5].
Various materials and techniques can be used for encapsulation, including metals, glasses, and coatings. Metal encapsulation typically employs materials with high softening points and adequate plasticity. Glass encapsulation is particularly beneficial for producing ceramic parts, as it minimizes surface contamination and allows for easier removal. Coated capsules may utilize a chemical vapor deposition (CVD) process to apply a micrometer-thick coating on the component’s surface [4].
Distinct encapsulation methods each offer unique advantages:
They enable high-density fabrication of materials that are challenging to sinter with traditional techniques, such as spherical powders, superalloys, and nitride ceramics like boron nitride and silicon nitride,
They provide some control over the capsule atmosphere, allowing for processes such as vacuum conditions when purity is essential,
They incorporate preheating systems tailored for specific heat treatment needs,
They facilitate the creation of hollow cylinders using the hard-core technique, employing a material with superior temperature resistance and mechanical strength compared to the original powder.
Nevertheless, the encapsulation method has its limitations:
It requires a complex preprocessing phase, including the shaping and sealing of capsules, to successfully perform the hot isostatic pressing (HIP) process.
There are restrictions in choosing encapsulation materials due to differences in the thermal expansion coefficients between the encapsulating material and the part.
2.1 Metal encapsulation
The powder or component formed from the powder is sealed in a metal capsule and subjected to the hot isostatic pressing (HIP) process. The choice of material for encapsulation depends on factors such as the heat treatment temperature, the reactivity of the powder with other materials (type of capsule), and cost considerations. Typically, the maximum recrystallization temperature for metals and superalloys is below 1500°C. In heat treatment processes involving alloys and metals, stainless steel capsules are commonly employed due to their ease of fabrication and welding. A primary reason for encapsulation is to achieve precise geometrical shapes; thus, metals with good malleability facilitate the shaping of the metal powder for sintering. However, encapsulation is generally limited to simple geometries, such as cylindrical shapes. The composition, impurity levels, and average particle size of the powder significantly influence the mechanical properties of the final sintered component. When employing the HIP process for encapsulation to produce high-density parts, the bulk density of the components is crucial. Consequently, the use of spherical particles with high compressibility is a key aspect of this process. For powders that cannot be compressed to 60% density, preforming techniques—such as cold isostatic pressing (CIP)—are recommended. The dimensions of the capsule required for the encapsulation process are determined by the relative density of the molded or sintered powder or component [6].
As illustrated in Figure 1, metal capsules typically consist of a thin welded metal body, with the top and bottom sealed using tubes that serve as powder and vacuum inlets. Once the powder is packed into the capsule, air, moisture, and gases are evacuated via a vacuum process. The heat treatment process, initiated by HIP, begins at lower temperatures. Due to the softening of the steel at elevated temperatures, heating continues until the ductility temperature of the steel is reached. Subsequently, gas pressure is incrementally increased until both pressure and temperature reach their maximum values. During the HIP process, the capsule experiences uniform pressure from all directions, allowing the materials undergoing heat treatment to approach their final properties [6]. After the hot isostatic pressing process, the capsule must be removed. Its shape and structure significantly differ from those of the initial capsule and powder, and in many cases, the capsule may have reacted with the formed component, leading to penetration. Consequently, the removal of the capsule can be a time-consuming and costly endeavor. Mechanical methods, such as turning, and chemical dissolution are potential techniques for this removal process [4].
Figure 1.
Typical mild steel capsule.
2.2 Glass encapsulation method
Another encapsulation method involves the use of glass capsules, which are then sealed under vacuum. This process is akin to metal encapsulation; however, it becomes challenging to apply when the components exhibit irregular shapes or high porosity. Glass capsule technology is particularly effective for fabricating engineering ceramic parts that require high sintering temperatures, such as silicon nitride and silicon carbide. The mechanisms and loading conditions for the powder within the sample are largely consistent with those used in metal capsules. The advantages of utilizing glass capsules include their availability, cost-effectiveness, and reduced preparation time [6].
2.2.1 Glass encapsulation material
To address the challenges associated with the metal encapsulation process—such as weld points and the brittleness of steel—while accommodating the higher temperatures required for sintering high-temperature ceramics like silicon nitride, a glass encapsulation process has been developed. This method offers several advantages, including the production of components that are closer to their final shape and improved cost-effectiveness due to the easier removal of glass material. It is important to note that the glass encapsulation technique is still in the developmental stages and has shown promising results at the laboratory scale, though further advancements will require time. The glasses used in encapsulation must exhibit high-temperature tolerance, typically ranging from 1600°C to 2000°C, making them unsuitable for metal capsules. Additionally, the softening point and viscosity of the glass are critical for effectively sintering engineering ceramic components, as these properties ensure efficient performance and optimal pressure transfer. During the initial heating stage, glass capsules surrounding the raw material powders act as a barrier to neutral gas flow, and given the potential reactions between the encapsulating material and the glass, it is essential for the glass to have a softening point lower than the sintering temperature of the components. As the temperature increases, the layers of material gradually migrate toward the component, and the lack of gas penetration within these layers leads to pressure buildup and compaction of the powder. A table detailing various types of glasses and their characteristics is provided below [7, 8]. The properties needed for encapsulation glasses are highly dependent on the glass composition, and all the composition constraints mentioned earlier must be followed to ensure that all the necessary properties are integrated into a single glass.
The properties required for encapsulation glasses depend critically upon the composition of the glass, and all of the composition limitations hereinabove set forth must be observed if all of the required properties are to be combined in a single glass. The coefficient of thermal expansion (CTE) of glass significantly influences the glass encapsulation process. A CTE value that closely matches that of the material being processed under hot isostatic pressing (HIP) helps mitigate the formation of microcracks. Furthermore, the glass utilized as a capsule must be precisely bonded to the bulk material. The strain point refers to the temperature at which internal stresses in the glass are relaxed over a few hours. In contrast, the annealing point indicates the temperature where these internal stresses are relieved within a few minutes [9].
Key performance indicators (KPIs) in the glass encapsulation method include strain points, annealing points, softening points, and working temperatures, as outlined in Table 1. Additionally, Figure 2 illustrates the relationship between temperature and viscosity in graphical form. Once the temperature reaches the annealing point, deformation of the glass occurs, driven by applied forces. It is crucial to achieve the glass’s annealing temperature; if stress is applied before this temperature is reached, small cracks and breakage may develop in the glass capsule [10].
Properties of glasses employed in the glass encapsulation process.
Figure 2.
The correlation between glass temperature and viscosity [10].
In a research study, an intermediate layer is used, which can be created by different methods, such as spraying or deep coating, and has a thickness ranging from 0.5 to 1 mm. After coating, the ceramic piece is degassed at room temperature and then placed into the hot isostatic pressing (HIP) system, where the temperature is raised until the outer layer is completely melted. The powder between the part and the glass layer prevents the penetration of the glass into the part. Applying pressure after changing the initial state of the outer layer of glass takes place in the temperature range of 600–1100°C, while the inner layer of glass powder also changes state at 1300–1600°C. Then, the hot isostatic pressing process is carried out in the temperature range of 1600°C with a pressure of 100–200 MPa. After the glass processing is finished, the glass is removed from the piece using the sandblasting process. Additionally, BN powder is used as a spacer layer between the part and the glass to facilitate the separation of the capsule from the part, and B2O3 can also be used as a protective layer by mixing it with Si3N4 at a ratio of 50:50 by weight, where the B2O3 reacts with the surface layer of Si3N4 at a temperature of approximately 1000°C, forming a boron nitride layer that prevents the penetration of glass into the silicon nitride ceramic. The pre-sintered body of silicon nitride is placed inside a capsule made of SiO2 or borosilicate glass, which is then kept in an oven at 100 with a vacuum of 0.1 Pa for 8 hours before being sealed and heated to a temperature of 1250°C, where the softening point of the glass is reached, and the glass changes its shape. The capsule is then placed in the HIP chamber, and a pressure between 200 and 300 MPa and a temperature between 1700°C and 1800°C are applied to the parts for at least 2 hours. Using this method, more complex shapes can be produced, and compression of Si3N4 using HIP in SiO2 powder is another method, where silicon nitride ceramic is placed in a bed of SiO2 powder inside a capsule of borosilicate glass, usually Pyrex [11, 12].
2.2.2 Sealing of glass capsules
Solder glasses can be classified as either vitreous or devitrifying. Vitreous glasses are thermoplastic materials that melt and flow at consistent temperatures each time they are heated. On the other hand, devitrifying solder glasses, often referred to as frits, are surface-nucleating thermosetting materials. When heated, they crystallize, resulting in their unique expansion properties [13]. The proximity of the glass to the main body raises the potential for chemical interaction due to temperature and pressure variations during the hot isostatic pressing (HIP) process. Therefore, it is advisable to select materials that exhibit minimal reactivity. Additionally, it is essential to ensure that the sintering temperature remains below the glass’s softening point. Several techniques can be employed for sealing components intended for encapsulation with glass during the HIP process, including Ref. [2]:
Sealing using the glass ampoule method
Sealing under applied pressure
Sealing by immersion in powder
Sealing via the sintered glass method
Sealing through a coating method
2.2.2.1 Glass ampoule method
In this method, the article is placed inside a glass ampoule and surrounded by glass powder, which is sealed using oxyacetylene flames. During sealing, a vacuum is simultaneously applied to ensure the interior of the parts is free of gases. Subsequently, the ampoule is heated in a hot isostatic pressing (HIP) unit to reach the glass softening temperature under reduced pressure before applying gas pressure. However, this technique has several disadvantages. First, the vacuum sealing process is sensitive; any leaks can cause the glass capsule to rupture when gas pressure is increased, potentially damaging heating elements. Second, the method is limited to samples with simple geometries. Lastly, the choice of glass composition is critical, as the capsule must soften at a temperature slightly below the sintering temperature while maintaining high viscosity throughout the densification process. A schematic representation of this process is shown in Figure 3 [14].
Figure 3.
Sealing process using the glass ampoule method [14].
2.2.2.2 Pressurize method
In this method, the component is initially shaped using powder metallurgy techniques and then surrounded by glass powder. Following this, heat treatment is performed until the glass powder softens. A uniaxial press is then employed to apply pressure, facilitating the welding of the powders and ensuring they bond together [14, 15].
2.2.2.3 Sintered glass method
In this process, glass powder is initially mixed with an appropriate solution to create a slurry, which is then applied to the surface of the preformed component. Following the application, the slurry is dried and sintered, resulting in a sealing coating that prevents gas penetration. If this coating process is performed in a single step, there is a risk of cracking and delamination. To mitigate this, the coating procedure is repeated multiple times until the desired thickness is achieved [14, 15, 16].
2.2.2.4 The glass bath method
The method described in Ref. [6] involves processing a porous body by immersing it in powdered glass with a low softening temperature. This approach is similar to the encapsulation method but without the need for the complex encapsulation technique. However, this method has some drawbacks—the low-density green body tends to float on the molten glass, requiring a means to keep the workpiece submerged below the glass surface. Additionally, the low-softening-temperature glass often penetrates the pores of the workpiece, necessitating a specialized coating to be applied over the entire surface of the workpiece. The shaped components are initially encased in glass powder and then subjected to heat treatment, followed by the hot isostatic pressing (HIP) process. It is crucial that the sintering temperature exceeds the softening temperature of the glass, with the process beginning at the glass’s softening point, at which point argon gas is injected to apply pressure, and the temperature is further elevated to reach the sintering point of the components during the immersion in glass powder [15, 17].
2.2.2.5 Coating method
The rationale for this method dates back to the proposal of coating the pores of a preformed raw material to prevent argon gas penetration. Instead of employing various types of capsules and sealing methods, the idea was to utilize a thin coating to obstruct argon gas during the hot isostatic pressing (HIP) process. Coating techniques considered for this purpose include plasma spraying, which creates a protective layer, as well as Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). However, a major drawback of this approach is its inability to effectively fill all pores and create a uniform barrier against argon gas penetration. Conversely, one of the significant advantages of this method lies in its simplicity and cost-effectiveness, as well as the rapid removal of the coated layer using acids. Table 2 presents the advantages and disadvantages of each method for sealing glass capsules [15].
Method
Advantages
Disadvantages
Glass ampoule
Simple method for forming cylindrical components
Needs a conduit to create a vacuum
Exact regulation of the piece’s environment
Shaping intricate components using this method is challenging
Sealing through the application of pressure
The sealing process can occur at a reduced temperature relative to alternative methods
The risk of microcracks and deformation in the component is markedly elevated
It is a straightforward method for sealing rectangular and square components
It requires special sealing devices
Precise control of the atmosphere of the piece
Glass immersion method
No specialized equipment is necessary
It necessitates a significant quantity of glass powder
Minimizing the steps involved in the sealing and encapsulation process
It requires a higher temperature for sealing
Capability to utilize various types of glass, including powder, flat, and cylindrical forms
Sintered glass method
It needs only a small quantity of glass
The likelihood of microcracks and deformation in the component is notably increased
The ability to form parts with complex shapes
Table 2.
Analysis of glass capsule sealing techniques [15].
During the design, simulation, and manufacturing of capsules for the hot isostatic pressing process, several critical factors warrant careful consideration [18]:
Assessment of variables: It is essential to analyze various parameters influencing the rheological behavior of capsule materials and granules, including geometric configurations and the preparation conditions employed during compression.
Material research: Conducting thorough research to select the appropriate materials for the capsules, as well as determining the optimal wall thickness, is crucial for ensuring effectiveness and durability.
Thermal and pressure optimization: Investigating and optimizing the thermal and pressure profiles that affect both the capsule and the granulated materials is necessary for successful outcomes.
Softening point analysis: Evaluating the softening points of the capsule materials under pressure is important, particularly in the formation of powder granules in geometrically complex regions.
Advancements in technology: The development and implementation of innovative technologies in capsule design and fabrication can significantly enhance their physical, mechanical, and weldability characteristics.
Oxide and non-oxide ceramics represent two distinct categories of advanced ceramics with numerous industrial and technological applications. Within the oxide ceramics, notable examples include alumina and zirconia which are stabilized with magnesia, yttria, or ceria. In contrast, advanced non-oxide ceramics encompass silicon-based materials, such as silicon nitride and silicon carbide. Hot isostatic pressing (HIP) is a widely utilized technique for the fabrication of both ceramics and metals, including components featuring intricate geometries, high-density ceramics, and ceramic matrix composites (CMCs). This method’s primary advantage lies in its ability to produce and densify advanced ceramic components characterized by complex shapes. The sintering process employed for the fabrication of ceramics and ceramic matrix composites can focus on either shaping and densification or solely on densification. Noteworthy benefits of HIP include a substantial enhancement of the mechanical properties of the resultant materials [4, 19].
3.1 Silicon nitride (Si3N4)
There are various processes for shaping silicon nitride ceramics, with one of the most prevalent and established methods being cold isostatic pressing (CIP) combined with sintering. In one study, silicon alpha-nitride powder was utilized alongside alumina and yttria as sintering aids, applying weight percentages of 3% and 5%, respectively. The reaction between the SiO2 formed on the surface of the silicon nitride powder and the additives enhanced the mechanical strength of the silicon nitride components [19].
Typically, sintering processes differ from hot isostatic pressing (HIP). Commonly employed nonpressure sintering techniques include vacuum sintering and the use of inert gases like argon. Additionally, gas pressure sintering of silicon nitride (GPSSN) is an effective method for the initial production of high-density silicon nitride parts, achieving significant density improvements. Silicon nitride is recognized as a distinctive material suitable for high-temperature applications and the ability to withstand substantial mechanical loads. Dense silicon nitride components are generally sintered utilizing oxide sintering and liquid phase formation. However, the presence of an intergranular glass phase in sintered parts can notably reduce their mechanical strength. To address this, research into simultaneous temperature and pressure sintering technologies, such as HP-GPS-HIP, has been undertaken. Hot pressing, another technique in this field, has notable limitations, particularly regarding the complexity of the parts that can be produced; it is often impractical to fabricate complex shapes using this method. The GPS process, known as GPSSN, employs simultaneous gas pressure and temperature during the sintering process. This approach facilitates the sintering of complex geometries due to the isostatic pressure from the gas. However, a significant drawback of this method is the relatively low mechanical pressure used, which may prevent the final components from achieving theoretical density [2].
When considering non-oxide ceramics, it is crucial to employ processes that minimize the formation of oxide phases while remaining cost-effective. Recently, the generation of oxide phases has garnered significant attention. In research conducted by Zehui Du, silicon nitride ceramics were produced using a 3D printing process to create cellular structures referred to as “honeycomb.” This innovative approach replaces traditional shaping methods, such as extrusion. To achieve densification, the samples underwent hot isostatic pressing at 1700°C for 2 hours [20].
In a 2023 study by Gizowska and colleagues, silicon nitride ceramics and silicon nitride reinforced with silicon carbide nanoparticles (Si3N4-nSiC) were examined, with weight percentages ranging from 1% to 10%. The samples were produced using two sintering methods: under ambient conditions and high isostatic pressure. The researchers investigated the influence of sintering conditions and the concentration of nano silicon carbide particles on the thermal and mechanical properties, as summarized in Table 3. The inclusion of highly conductive silicon carbide particles enhanced thermal conductivity, particularly in composites containing 1% by weight of the carbide phase (115.6 W m−1 K−1) compared to the thermal conductivity of silicon nitride ceramics (111.4 W m−1 K−1) under identical conditions. However, an increase in the carbide phase led to a reduction in densification efficiency during sintering, which negatively affected both thermal and mechanical performance. Sintering performed via hot isostatic pressing demonstrated advantages in terms of mechanical properties.
Materials
Sintering condition
Relative green density
Relative density
Open porosity
Thermal diffusivity
Thermal conductivity
%
%
%
mm2 s−1
W m−1 K−1
Si3N4
1800°C HIP 1800°C
60.5
98.5
1.6
14.0
11.4
Si3N4 + 1 wt.% nSiC
1800°C HIP 1800°C
62.3
98.2
0.3
13.9
15.6
Si3N4 + 5 wt.% nSiC
1800°C HIP 1800°C
61.3
94.7
3.7
13.4
13.2
Si3N4 + 10 wt.% nSiC
1800°C HIP 1800°C
62.1
94.6
17.5
13.2
11.7
Table 3.
Impact of sintering conditions and the concentration of nanosilicon carbide particles on the thermal and mechanical properties of silicon nitride [21].
3.2 SiAlON
Sialon ceramics are among the most widely utilized ceramics. Their densification occurs through a liquid-phase sintering process, where a liquid phase is initially created from molten SiO2 and Al2O3 and subsequently removed in the final stage. Given the constraints of the Sialon sintering process, employing a high-speed sintering method can significantly aid in densification and achieving the desired properties of this ceramic. To this end, hot isostatic pressing and encapsulation techniques are utilized to achieve near-theoretical density in Sialon ceramics. A Vycor glass capsule is used in this process. During heat treatment, the temperature is initially raised to 1300°C (the glass softening temperature), followed by an increase in pressure to 200 MPa. The sintering occurs at a maximum temperature of 1780°C, with a dwell time of 2 hours at this temperature. The results indicate that the combination of high temperature and short sintering times facilitates the complete formation of Sialon and reduces the particle size to 500 nm. Additionally, employing hot isostatic pressing as a supplemental process can further enhance the density of the sintered components [22, 23, 24].
3.3 Alumina
Alumina is one of the most widely utilized materials in the densification of ceramics, recognized for its exceptional properties as a ceramic oxide, particularly when subjected to the hot isostatic pressing (HIP) process. Its superior mechanical characteristics make it a cost-effective choice for applications such as foot joints. The hot isostatic pressing process enhances the mechanical properties of alumina by uniformly applying temperature and pressure, which effectively reduces the open pores on the surface of the closed alumina ceramic and increases the bulk density of the components [25].
Various methods exist for the performing of alumina ceramics, with the most significant techniques including cold isostatic pressing, slurry casting, and plastic forming methods such as injection molding and extrusion. For instance, in a research study aimed at shaping alumina components, submicron powder was subjected to a pre-sintering process at 1240°C for 2 hours. This was followed by hot isostatic pressing at a temperature of 1400°C and a pressure of 150 MPa, conducted in an argon atmosphere [26].
Typically, preformed alumina parts are coated with boron nitride powder to minimize reaction between the glass and the bulk material during the sintering process. To encapsulate alumina parts at a sintering temperature of 1400°C, Vycor glass capsules are employed [27].
3.4 Transparent ceramics
Transparent ceramics have numerous potential applications, including armor, windows, and various optical elements. Key challenges in the development of transparent ceramics include microporosity, the presence of disruptive secondary phases, and wide grain boundaries. These factors adversely affect the refractive index between air and the material, leading to significant light scattering and a reduction in the transparency of the final products. Extensive research has been conducted to achieve transparent ceramics, focusing on the synthesis of high-purity powders, the selection of appropriate additives and sintering aids, and the enhancement of shaping and sintering techniques. Generally, raw ceramic components do not attain densities close to theoretical values. Consequently, employing methods that apply pressure and temperature simultaneously is crucial for achieving high optical transparency, specifically targeting porosity levels below 0.01%. These methods are particularly effective for producing transparent ceramics in a timely manner [27, 28].
3.5 AlON
Hot isostatic pressing (HIP) is an effective thermal treatment method for achieving partial densification of transparent ceramics. For instance, the incorporation of LiAl5O8 as a sintering aid, combined with HIP treatment at 2000°C and a pressure of 2000 bar, has enabled AlON ceramics to achieve a light transmission of 65% in the visible spectrum. Similarly, the addition of Y2O3-La2O3 with the HIP technique has resulted in achieving 78% transparency in AlON ceramics within the visible light range. Furthermore, employing a two-step sintering process along with HIP and nanoscale additives has increased transparency to 86% under visible light conditions [28, 29].
A critical factor in the performance of optical ceramics is the level of residual stress remaining in transparent ceramic components. To mitigate the effects of residual stress, post-baking treatments such as hot isostatic pressing are employed. Due to the high pressures and temperatures involved in HIP, its effectiveness in reducing residual stresses has been extensively studied. Observations indicate that the HIP process significantly enhances the removal of residual stresses and improves the coefficient of visible light transmission [28].
3.6 MgAl2O4
Transparent spinel ceramics are widely utilized in military applications due to their excellent thermo-mechanical properties at elevated temperatures, cost-effectiveness, and capability to form complex shapes. As a result, they are among the most commonly used ceramics in the optical industry. Various techniques are employed to fabricate raw spinel components, including slurry casting, cold isostatic pressing, and the sol-gel method. The sintering process varies based on advancements in the field. Some studies have employed single-step sintering techniques under pressure and temperature, such as spark plasma sintering and hot pressing. Conversely, other approaches utilize two-step sintering, involving pre-sintering followed by sintering under pressure and temperature, using methods like gas pressure sintering (GPS) and hot isostatic pressing (HIP). Ultimately, selecting different processing methods is essential to achieve optimal mechanical strength, thermal resistance, and high optical transparency in the visible light spectrum, underscoring the importance of HIP in the production of these ceramics. A variety of additives are used in the manufacturing of spinel ceramics, including LiF, MgF2, CaO, B2O3, Y2O3, and SiO2. However, using these additives in excessive quantities can lead to the formation of a liquid phase and significant grain growth, thereby compromising mechanical strength. In conclusion, the use of the hot isostatic pressing process at temperatures ranging from 1500°C to 1700°C, with pressures between 1500 and 2000 bar and a dwell time of 3 hours, can effectively enhance both the optical transparency and mechanical strength of transparent spinel ceramics [21, 30, 31, 32, 33, 34, 35].
3.7 Transparent alumina
In a study, fine-grained aluminum oxide particles with sizes ranging from 0.8 to 2 micrometers were employed. The results indicated that at a sintering temperature of 1400°C, preformed components of various grain sizes achieved a density approaching theoretical density. This was accomplished by maintaining a holding time of 4 hours at the maximum temperature under isostatic pressure of 2000 bar. Transparent polycrystalline alumina (PGA) is subjected to heat treatment at 1800°C in a hydrogen atmosphere. These transparent alumina ceramics have applications in the military, particularly in optical lenses and dielectric ceramics. The PGA production process consists of multiple heat treatment stages, with the final stage of the hot isostatic pressing (HIP) process occurring at 1700°C and a pressure of 2000 bar, maintained for up to 3 hours for optimal clarification. Typically, additives such as MgO are incorporated to facilitate the production of PGA [27, 36, 37].
In conclusion, the production of engineering ceramic parts using hot isostatic pressing (HIP) technology offers significant advantages, including enhanced structural integrity through the reduction of porosity and defects, which leads to superior mechanical strength and thermal stability. HIP enables the fabrication of complex geometries and the consolidation of ceramic-based composites, allowing for tailored properties that meet specific application needs. Additionally, the consistent quality and high-performance of HIP-processed ceramics contribute to their cost-effectiveness and reliability across various industries. Overall, HIP technology is crucial for advancing the capabilities and applications of engineering ceramics, making it an invaluable method for producing high-performance components.
This research was conducted with financial support from Imam Hossein University, and the authors would like to extend their gratitude to the university’s president, Dr. Hasani Ahangar.
References
1.Carlsson R. The shaping of engineering ceramics. Materials and Design. 1989;10(1):10-14. DOI: 10.1016/0261-3069(89)90029-0
2.Koizumi M. Hot Isostatic Pressing—Theory and Applications. Dordrecht: Springer Netherlands; 1992. DOI: 10.1007/978-94-011-2900-8
3.Loh NL, Sia KY. An overview of hot isostatic pressing. Journal of Materials Processing Technology. 1992;30(1):45-65. DOI: 10.1016/0924-0136(92)90038-T
4.Atkinson HV, Davies S. Fundamental aspects of hot isostatic pressing: An overview. Metallurgical and Materials Transactions A. 2000;31(12):2981-3000. DOI: 10.1007/s11661-000-0078-2
5.Wright JM. Encapsulation method for hot isostatic pressing. US4960550A. 1989
6.Bocanegra-Bernal MH. Hot Isostatic Pressing (HIP) technology and its applications to metals and ceramics. Journal of Materials Science. 2004;39(21):6399-6420. DOI: 10.1023/B:JMSC.0000044878.11441.90
7.ElRakayby H, Kim K. Effect of glass container encapsulation on deformation and densification behavior of metal powders during hot isostatic pressing. International Journal of Material Forming. 2018;11(4):517-525. DOI: 10.1007/s12289-017-1361-8
8.Richards K. Process for encapsulating a shaped body for hot isostatic pressing by sol-gel method. US5613993A. 1995
9.Adlerborn J. Method for hot isostatic pressing powder bodies. US4081272A. 1975
10.Callister WD. Materials Science and Engineering: An Introduction. 4th ed. New York: John Wiley & Sons; 1997
11.Westman A-K, Larker HT. Interaction of encapsulation glass and silicon nitride ceramic during HIPing. Journal of the European Ceramic Society. 1999;19(16):2739-2746. DOI: 10.1016/S0955-2219(99)00059-X
12.Huusmann O. Glass encapsulation in hip chamber description of methods used in practice. In: Hot Isostatic Pressing—Theory and Applications. Dordrecht: Springer Netherlands; 1992. pp. 517-525. DOI: 10.1007/978-94-011-2900-8_70
13.Alpha JW. Glass sealing technology for displays. Optics and Laser Technology. 1976;8(6):259-264. DOI: 10.1016/0030-3992(76)90039-6
14.Mouzon J, Maitre A, Frisk L, Lehto N, Odén M. Fabrication of transparent yttria by HIP and the glass-encapsulation method. Journal of the European Ceramic Society. 2009;29(2):311-316. DOI: 10.1016/j.jeurceramsoc.2008.03.022
15.Leriche A, Aleksandrowicz P, Thierry B. Study of glass ceramic diffusion during hot isostatic pressing of encapsulated PZT ceramics. MRS Proceedings. 1991;251:265. DOI: 10.1557/PROC-251-265
16.Kwon O, Messing GL. Gas diffusion during containerless hot isostatic pressing of liquid-phase sintered ceramics. Journal of the American Ceramic Society. 1989;72(6):1011-1015. DOI: 10.1111/j.1151-2916.1989.tb06260.x
17.Ekström T, Käll PO, Nygren M, Olsson PO. Dense single-phase β-sialon ceramics by glass-encapsulated hot isostatic pressing. Journal of Materials Science. 1989;24(5):1853-1861. DOI: 10.1007/BF01105715
18.Zhilin PL, Gavrilov GN, Ryabtsev AD, Bazhenov EO. The problem of capsule manufacturing for hot isostatic pressing. IOP Conference Series: Materials Science and Engineering. 2020;971(2):022049. DOI: 10.1088/1757-899X/971/2/022049
19.Ekbom R. Application of HIP/PM technique for gas and steam turbines. Metal Powder Report. 1990;45(4):284-289. DOI: 10.1016/S0026-0657(10)80056-6
20.Furong N et al. Fused deposition modeling of Si3N4 ceramics: A cost-effective 3D-printing route for dense and high performance non-oxide ceramic materials. Journal of the European Ceramic Society. 2022;42(15):7369-7376. DOI: 10.1016/j.jeurceramsoc.2022.08.041
21.Kim S-Y et al. Optimization of Ca additive and HIP condition in fabrication of the transparent MgAl2O4 ceramics. International Journal of Refractory Metals and Hard Materials. 2024;120:106577. DOI: 10.1016/j.ijrmhm.2024.106577
22.Huang Z et al. Stereolithography 3D printing of Si3N4 cellular ceramics with ultrahigh strength by using highly viscous paste. Ceramics International. 2023;49(4):6984-6995. DOI: 10.1016/j.ceramint.2022.10.137
23.Bartek A, Ekström T, Herbertsson H, Johansson T. Yttrium α-sialon ceramics by hot isostatic pressing and post-hot isostatic pressing. Journal of the American Ceramic Society. 1992;75(2):432-439. DOI: 10.1111/j.1151-2916.1992.tb08198.x
24.Ekström T, Olsson P-O, Holmström M. O′-sialon ceramics prepared by hot isostatic pressing. Journal of the European Ceramic Society. 1993;12(3):165-176. DOI: 10.1016/0955-2219(93)90118-B
25.Ariff TF et al. Optimizing the synthesis of alumina inserts using hot isostatic pressing (HIP). IOP Conference Series: Materials Science and Engineering. 2018;290:012044. DOI: 10.1088/1757-899X/290/1/012044
26.Bhandhubanyong P, Akhadejdamrong T. Forming of silicon nitride by the HIP process. Journal of Materials Processing Technology. 1997;63(1-3):277-280. DOI: 10.1016/S0924-0136(96)02635-0
27.Yin J, Li X, Lai Y, Zhang X, Yu S, Luo Y. Ultra-high quality factor of transparent Al2O3 ceramics fabricated by vacuum sintering and post hot isostatic pressing. Ceramics International. 2023;49(22):36879-36884. DOI: 10.1016/j.ceramint.2023.09.018
28.Jiang N et al. Fabrication of highly transparent AlON ceramics by hot isostatic pressing post-treatment. Journal of the European Ceramic Society. 2017;37(13):4213-4216. DOI: 10.1016/j.jeurceramsoc.2017.04.028
29.Tanaka I, Pezzotti G, Okamoto T, Miyamoto Y, Koizumi M. Hot isostatic press sintering and properties of silicon nitride without additives. Journal of the American Ceramic Society. 1989;72(9):1656-1660. DOI: 10.1111/j.1151-2916.1989.tb06298.x
30.Li J, Zhang B, Tian R, Mao X, Zhang J, Wang S. Hot isostatic pressing of transparent AlON ceramics assisted by dissolution of gas inclusions. Journal of the European Ceramic Society. 2021;41(7):4327-4336. DOI: 10.1016/j.jeurceramsoc.2021.02.035
31.Drdlikova K, Klement R, Rychnovsky D, Maca K, Drdlik D. Optical properties of Tb3+- and Cr3+-doped MgAl2O4 ceramics prepared by capsule- and carbon-free hot isostatic pressing. Journal of the European Ceramic Society. 2024;44(9):5440-5448. DOI: 10.1016/j.jeurceramsoc.2023.11.038
32.Liu Y, Zhu J, Dai B. Transparent MgAl2O4 ceramics prepared by microwave sintering and hot isostatic pressing. Ceramics International. 2020;46(16):25738-25740. DOI: 10.1016/j.ceramint.2020.07.051
33.Dunaev AA et al. Hot isostatic pressing-induced structural changes in MgAl2O4 ceramics. Inorganic Materials. 2023;59(5):526-529. DOI: 10.1134/S0020168523050035
34.Kim J-M, Kim H-N, Park Y-J, Ko J-W, Lee J-W, Kim H-D. Microstructure and optical properties of transparent MgAl2O4 prepared by Ca-infiltrated slip-casting and sinter-HIP process. Journal of the European Ceramic Society. 2016;36(8):2027-2034. DOI: 10.1016/j.jeurceramsoc.2016.02.018
35.Liu Q et al. Microstructure and properties of MgAl2O4 transparent ceramics fabricated by hot isostatic pressing. Optical Materials. 2020;104:109938. DOI: 10.1016/j.optmat.2020.109938
36.Uematsu K, Itakura K, Uchida N, Saito K, Miyamoto A, Miyashita T. Hot isostatic pressing of alumina and examination of the hot isostatic pressing map. Journal of the American Ceramic Society. 1990;73(1):74-78. DOI: 10.1111/j.1151-2916.1990.tb05093.x
37.Barinov SM, Ponomarev VF, Shevchenko VY. Effect of hot isostatic pressing on the mechanical properties of aluminum oxide ceramics. Refractories and Industrial Ceramics. 1997;38(1):9-12. DOI: 10.1007/BF02768225
Written By
Pouria Dehghani, Seyed Salman Seyed Afghahi and Farshad Soleimani
Submitted: 21 August 2024Reviewed: 10 September 2024Published: 05 March 2025
Open access peer-reviewed chapter
