SYNTHESIS AND PROCESSING
2.03 Synthesis/Processing of Silicon Nitride Ceramics
2.03.2 Types of Silicon Nitride
There are many means to prepare a silicon nitride component, and with these different techniques come different properties and behaviors. The fabrication technique utilized has lent its name to the material, and there is quite a litany of abbreviations derived therefrom.
2.03.2.1 Monolithics
2.03.2.1.1 RBSN and Offshoots
For instance, if one begins by forming a rough component shape from silicon powder, then nitrides that shape, one gets what is called reaction-bonded silicon nitride or RBSN. RBSN is, then, basically pure silicon and nitrogen combined. However, it was found early on that adding a catalyst such as Fe would increase the rate and extent of the reaction, so there may be impurities deliberately added (or they may be there from the original powders). Recently research and development efforts have included investigation of other metal catalysts, Synthesis/Processing of Silicon Nitride Ceramics 75
including the rare earths (Pavarajarn, Wongthavorn, & Praserthdam, 2007). This type of silicon nitride has relatively low strength, toughness, and oxidation resistance, primarily because it has residual porosity (up to ca 15%) and the microstructure has not developed the toughening high-aspect-ratio silicon nitride crystals that are required. Its major virtues were its strength retention to elevated temperatures, and the lack of major dimen- sional changes on densification (nitridation). A variant of this process is called sintered reaction-bonded silicon nitride (SRBSN), wherein a sintering aid such as yttrium oxide is added to the silicon powder prior to shaping.
Upon heating in nitrogen, silicon nitride is formed, and a liquid as well, from the reaction between the silicon and nitrogen with the oxygen and cation (e.g. Y) in the sintering aid. The liquid causes shrinkage and pore reduction, and allows development of a tougher crystal structure. Reduction of pore volume also increases oxidation resistance. This additional densification, however, introduces shrinkage and dimensional changes. A paper from 1960 gives an excellent summary of RBSN development up to that time (Parr, Martin, & May, 1960).
Efforts to further develop the SRBSN process have been continuing, with a number of papers being published between 2006 and the present.Zhu, Zhou, Hirao, and Lences (2006)found improved thermal conductivity in SRBSN through control of the purity of the initial powders, and maintaining low levels of oxygen and aluminum. The same team of authors then studied the sintering additives, including yttria and magnesia, as well as magnesium silicon nitride, again attempting to increase the thermal conductivity. They found that MgO and MgSiN2were equally effective (Zhu, Zhou, Hirao, & Lences, 2007).Zhu, Sakka, Zhou, and Hirao (2007)have further studied SRBSN, looking at the effects of addition of, for example, Li2O, along with yttria and magnesium silicon nitride.
Müller, Bauer, and Knitter (2009), Müller, Rögner, Okolo, Bauer, and Knitter (2010), andMüller, Bauer, Knitter, and Rögner (2010) have also investigated SRBSN recently, evaluating the reaction-bonding process itself, the densification behavior, and the resultant mechanical behavior and properties. They utilized a low- pressure injection-molding process to fabricate small components, and found the strength to be limited by the presence of secondary phases at the surface of bending-test specimens, and that these phases must be avoided to obtain good mechanical reliability.
Schematic diagrams of the processing of RBSN and SRBSN components are presented inFigures 3and4.
2.03.2.1.2 SSN/HPSN/HIPSN
The second major family of silicon nitrides is the hot-pressed silicon nitride(HPSN)/sintered silicon nitride (SSN)/hot isostatically pressed silicon nitride (HIPSN) series. In all of these, a densification aid, chosen from among any number of oxides, is added to silicon nitride powder. These additions started initially with MgO but rapidly progressed to yttrium oxide and then all of the rare earths, as well as mixtures of oxides (Cinibulk, Thomas, & Johnson, 1992). A huge amount of research was required on the development of the phase equilibria amongst these cation–oxygen–nitrogen–silicon pseudoquaternary compounds in order to truly understand what was happening. Many compounds new to the ceramic world were discovered, including a series of yttrium–silicon–oxygen–nitrogen compounds and their analogs among the rare earths.
Figure 3 Schematicflow diagram for the processing of reaction-bonded silicon nitride components.
76 Synthesis/Processing of Silicon Nitride Ceramics
Elucidation of the mechanism(s) of densification and microstructural development required a huge effort among many investigators. It was quickly determined that a liquid-phase sintering mechanism was occurring, with reaction between the silicon, oxygen, nitrogen, and other cations from additive oxides resulting in a viscous liquid at elevated temperatures.Hampshire and Jack (1981)studied the kinetics and mechanisms.Tsuge and Nishida (1978)showed how yttria and alumina additions to Si3N4resulted in high-strength materials if the glassy grain boundary phase could be at least partially crystallized. Studies of the grain boundary, and other accessory phases, also required expenditure of much effort, and this continues to this day (Bonnell, Tien, &
Ruhle, 1987; Cinibulk, Thomas, & Johnson, 1990; Shibata et al., 2004; Yang, Ohji, & Niihara, 2000; Ziegler, Idrobo, Cinibulk, Browning, & Ritchie, 2004).
Studies on the properties, especially the strength both at low temperatures and at elevated temperatures, resulted in a great deal of investigation into microstructures. Hirao et al. showed how to generate high- toughness and -strength silicon nitrides by controlling grain size and shape during processing, while others studied not only the microstructure but also the influence of the sintering aid and its resultant phase distri- bution (Becher et al., 1998; Hirao, Ohashi, Brito, & Kanzaki, 1995; Kleebe, Pezzotti, & Ziegler, 1999; Sajgalik, Dusza, & Hoffmann, 1995; Shen, Zhao, Peng, & Nygren, 2002; Sun et al., 1998).
Hot pressing is actually quite a simple process, involving pouring a mixture of silicon nitride powder and a densification aid into a graphite or boron nitride (BN)-lined mold and heating it while applying pressure to two punches. The atmosphere is typically vacuum or nitrogen. The pressure from the punches assists the driving force for pore closure, and a nearly 100% dense product can be attained. The uniaxial nature of this process causes the silicon nitride grains within the compact to grow in the direction normal to the pressure, with the result being that there exists a differential strength and toughness along the directions normal to versus parallel to the pressing direction.
HIPing obviates this problem of differential properties in varying direction, by applying a uniform pressure all around the component. Of course, this process cannot be utilized until the internal pores in the structure are closed, so one can either do a sinter-HIP process, where sintering is performed until closed porosity is attained, or one can utilize an enclosed HIP process, wherein the burned-out component is encapsulated in a metal container prior to heating/pressurization. A Swedish company named ASEA developed and patented a process called glass-encapsulated HIPing, and it or a variant of it was widely utilized in the 1980–2000 timeframe by various manufacturers (Adlerborn, Burnström, Hermansson, & Larker, 1987; Westman & Larker, 1999).
A schematic processflow diagram for the preparation of sintered or an HIPed silicon nitride component is presented inFigure 5. Note that sintering can be accomplished via simple furnace heating, but a large effort was made in the 1980s–1990s, and it has continued onward, to develop microwave heating (Sreekumar & Earl, 2009; Sreekumar & Traver, 2010). However, in retrospect it appears that little economic impact ever resulted Figure 4 Schematicflow diagram for the processing of sintered reaction-bonded silicon nitride components.
Synthesis/Processing of Silicon Nitride Ceramics 77
from this effort. More recently, a process called“spark plasma sintering”has been in vogue, and, as with mi- crowave sintering, it appears to have some benefits (Peng, Shen, & Nygren, 2005).
2.03.2.1.3 Chemically Derived
In the 1980s, a number of papers appeared in the literature concerning the preparation of silicon nitride ceramics via chemical processes: that is, a chemical compound containing a backbone of Si instead of C, and with accessory nitrogen atoms (typically a polysilazane), was prepared and pyrolized. This process was very appealing, but had a number of problems. Most of the molecules had carbon and hydrogen attached, and it was difficult to totally eliminate the carbon, so that a carbonitride compound was the result. The pyrolysis process also incorporated a huge weight loss and resultant shrinkage, so that making a shape and then maintaining it during heating was extremely difficult, if not improbable. Despite all the R&D to date, no structural ceramic product ever made it to the mass market from this approach. It is useful for the preparation of thinfilms, however, where the gas evolution and three-dimensional shrinkage can be accommodated by the thin dimension (Dixmier, Bellissent, Bahloul, & Goursat, 1994; Kroll, 2005; Schwab & Page, 1995).
2.03.2.1.4 SiAlONs
In the early days of silicon nitride development, it was discovered that the silicon nitride lattice could accommodate a certain amount of Al substitution for Si, if it was counterbalanced by the charge-balancing amount of O for N (Hampshire, Park, Thompson, & Jack, 1978). The resulting material exhibited a slightly expanded lattice compared to pure Si3N4, and could be fabricated into shapes with the same types and amounts of sintering aids as for SSN. As for pure silicon nitride, there exist both the alpha and the beta phase. However, alpha Si3N4tends to dissolve in the liquid phase present during sintering and precipitate as the beta phase, whereas in SiAlONs, one can retain either or both phases in thefinal product. A recent review of silicon nitride and sialon ceramics by Hampshire describes these materials and their phase relations and behaviors in depth (Hampshire, 2009).
2.03.2.2 Composites
The potential for, and application of, ceramic composites based on silicon nitride was reviewed in 1987 (Buljan
& Sarin, 1987). The following paragraphs elucidate the main points and update the work described.
2.03.2.2.1 Particulate
Much of the structural materials research in the 1980s involved composite theory, with researchers attempting to understand the principles of strength and toughness, including the main topic of fracture (crack origina- tion, crack propagation, slow crack growth at high temperatures, etc.). At the same time, experimenters were adding any number of different materials to silicon nitride bodies in an attempt to strengthen and/or toughen Figure 5 Schematic processflow diagram for the preparation of sintered or HIPed silicon nitride ceramic components.
78 Synthesis/Processing of Silicon Nitride Ceramics
(or harden) them. SiC was an obvious choice since it also had an Si basis and was known to exhibit a high hardness. TiC and others were also evaluated. Particulates had the advantage over whiskers andfibers of being readily incorporated into a body without having to worry about any preferred orientation affects. That is, composite additions with a high aspect ratio would tend to orient themselves during processing, whereas the more equidimensional particulates would not. On the other hand, one had to be concerned with maintaining the particulate additions equally well dispersed throughout the product: great differences in particle size, or worse, density could cause the particulate and the silicon nitride to separate during forming.
Because of the potential health problems that could be caused by the processing of ceramic or metal whiskers (next section), efforts on particulate-toughened silicon nitride focused on less problematic, but still highly anisotropic materials, such as microfibrils (not as small and inhalable as whiskers) and nanomaterials, include nanotubes. The potential adverse health effects of nanomaterials are as yet not fully explored, allowing materials processing research and development to proceed apace. Carbon nanotube production processes have been ramped up to such a level that they could possibly be economically feasible as additions to silicon nitride, and efforts have been made to develop such ceramics (Balázsi et al., 2005).
Microfibrils, or short fibers, have been introduced into Si3N4 ceramics, including chopped carbon fibers (Herzog, Woetting, & Vogt, 2007). These authors investigated use of a novel preceramic chemical as a precursor to the silicon nitride (RBSN), and studied the composite’s strength properties.
2.03.2.2.2 Whisker
In the 1980s, silicon carbide whiskers were made available at a relatively low cost, as a byproduct of the pyrolysis of, for example, rice hulls. Additional research determined the mechanism for the formation of these whiskers, and several processes arose by which one could make very pure, high-aspect ratio whiskers in large quantities. These became the basis for the aluminum oxide–SiC whisker cutting tools, sold by the Greenleaf, for instance. However, the apparent health problems caused by the asbestosfiber scare put the use of whiskers into great doubt, and the whisker technology basically disappeared (Das Chowdhury, Carpenter,
& Braue, 1992).
2.03.2.2.3 Fiber
Fiber composite technology, typically involving the wrapping of a mandrel with fibers, such as graphite or amorphous carbon, and then infiltration by a chemical precursor to a ceramic, was well known by the time silicon nitride ceramics became a hot topic. Researchers experimented with use of carbonfibers, then with the more recently available SiCfibers (and/or silicon carbide/nitride mixedfibers) as a basis for composites. Newer work has even evaluated silicafibers. Infiltration was performed with either liquid- or gas-phase precursors to the silicon nitride matrix. These were called ceramic matrix composites (CMCs). Enormous strides were made in understanding the nature of toughening, vis-à-vis the amount of bonding between the fiber and the matrix (Hyuga, Jones, Hirao, & Yamauchi, 2004; Lundberg, Pompe, Carlsson, & Goursat, 1990; Qi, Zhang, & Hu, 2007;
Saigal et al., 1993).
For turbine engine application, overcoating processes and compositions were developed, to protect the fiber–matrix bond interaction and to prevent complete oxidation of the structure (Ramasamy, Tewari, Lee, Bhatt, & Fox, 2010).