Silicon nitride (Si3N4) and silicon carbide (SiC) systems have been used in industry for many years, and both systems have a range of properties that make them excellent choices in a number of demanding environments. Whether it is handling molten non-ferrous metals, resisting wear, preventing chemical attack or providing strength at elevated temperatures, both systems have proven to be excellent performers.
However, fully dense materials in either compound require high-temperature manufacturing (above 1700°C for Si3N4 and as high as 2100°C for SiC). Unless a significant amount of a liquid-forming sintering aid is used, the process also requires applied pressure in order to form fully dense bodies. This increases the cost of manufacture and also limits the size and shapes of components that can be manufactured.
While some applications do require the ultimate properties that fully dense, single-compound materials can provide, this comes at a significant cost that is often prohibitive and prevents the uptake by industry of advanced ceramic components. In many industrial applications, the required performance can be achieved by using ceramic materials that, while still having excellent properties, can be manufactured using lower cost materials and manufacturing processes.
Furthermore, in applications where thermal shock is encountered, the fully dense, strong and hard materials are not necessarily the best choice, with better performance being achieved by materials with some porosity present and that have lower modulus, strength and hardness as a result. For high-temperature applications, the presence of free silicon (such as in reaction-bonded SiC) is a limiting factor beyond the melting point of silicon and materials that have a lower density and are free of silicon metal demonstrate better high-temperature properties.
Over recent decades, the use of materials that combine the best properties of both Si3N4 and SiC have been developed and manufactured in such a way that the manufacturing route minimizes costs and industry has taken up these materials in a number of applications. The development of silicon nitride-bonded silicon carbide is one example where, in general, the hard and refractory SiC phase is in the majority while the fine-grained silicon nitride phase acts as a bond phase between the SiC grains.
The material takes advantage of the lower temperature required to form and sinter Si3N4, with the end result being a partly porous body (nominally around 8-10% porosity) but one that still shows good strength, abrasion resistance, oxidation resistance, chemical resistance and hot strength up to about 1550°C. A notable development was the use of the in-situ nitriding of silicon metal powder in a nitrogen atmosphere to form the Si3N4, rather than starting with more expensive alpha Si3N4 powder. Furthermore, the starting materials are compatible with water, thus allowing aqueous-based slip casting methods to be used and allowing large and complex shaped components to be produced.
Nitride-bonded silicon carbides are available in a number of variants with a wide range of properties. For example, strengths (modulus of rupture, MOR) can range between less than 30 MPa to about 170 MPa, depending on the formulation of the bond phase and the method of production.
In the late 1970s, silicon nitride materials were further developed through the discovery of the alloys that were possible with alumina (Al2O3) known as SiAlONs; the name comes from the combination of Si, Al, O and N. The phase that has found the most use in industry is beta sialon, which has the formula Si6-zAlzOzN8-z, where z ranges from 0 (pure beta Si3N4) to 4.
The development allowed Si3N4 type materials to be sintered far more readily due to the ease with which a liquid phase could form at the sintering temperature, particularly in the presence of a sintering aid such as a rare earth oxide. As a result, pressureless sintering could be used to form dense materials in large and complex near-net-shape form. As the value of z increases (more Al2O3), so the properties change, with the major advantages lying in the enhanced chemical resistance and thermal shock resistance. The development of elongated beta grains in the liquid phase can also enhance the materials’ fracture toughness.
Enhanced sialon-bonded silicon carbide has a low porosity, high strength (both at ambient temperature and at elevated temperatures), good thermal shock resistance, and high resistance to abrasion while remaining chemically stable to most acids and alkalis. It also has good oxidation resistance in air, where a protective refractory glaze is formed that prevents further oxidation. The enhanced sialon bonding system has been developed through the use of proprietary sintering aids and results in a body that is stronger than conventional silicon or sialon nitride-bonded grades by at least 20%, has an excellent resistance to oxidation, is chemically stable, and is resistant to most acids and alkalis.
These materials can be processed in a number of ways that allow near-net-shape components to be produced. By careful choice of raw materials, sialon-bonded SiC can deliver excellent green strength without the use of binders means casting yields are high and noxious kiln emissions are eliminated. Since the material exhibits very low shrinkage on firing, the minimal of fettling or machining is required to produce engineering components. Aqueous slip casting and pressure casting are both currently used, while dry pressing of powders is also possible for small components. Hot pressing or spark plasma sintering have also been shown to produce very low-porosity grades of enhanced sialon bonded SiC that exhibit higher densities and thus high strength, hardness, and wear resistance.
The beneficial properties of enhanced sialon-bonded silicon carbide make it a viable option for multiple applications. Saggars have been manufactured for use in high-temperature, fast-fire conditions that would conventionally require the use of heavy and thick sections of cordierite-type materials with limited cyclic lifetimes. Following trials in a range of thicknesses, the ideal thickness of enhanced sialon-bonded SiC can be identified that provides both sufficient strength and the right level of thermal shock resistance for many challenging applications.
Gas burner nozzles and thermocouple sheaths have also been produced for a variety of uses, particularly in the heavy clay and brick industries. In these applications, a resistance to thermal shock, good oxidation resistance, resistance to gas erosion and an ability to withstand large differences in temperature across the body are all important.
Major applications in kiln furniture are for firing porcelain, bone china, sanitaryware, building products and technical ceramics. The high strength, good thermal properties and oxidation resistance make enhanced sialon-bonded SiC an attractive product for thin, lightweight kiln furniture, enabling fast firing, reduced firing costs and an increase in capacity, as well as allowing automatic handling and loading of kilns. The enhanced strength and high reliability (high Weibull modulus for this class of material) mean they can offer significant advantages over oxide ceramics or conventional nitride-bonded SiC.
In fast-firing environments (typically less than 7 hours cold-to-cold) for vitreous china and porcelain, enhanced sialon-bonded SiC has been used as plate setters that are thinner than traditional refractory materials, such as cordierite and other silicon carbides. This is proving an advantage in enabling manufacturers to not only reduce the weight of refractory in the furnace but also to increase the capacity for finished products. The oxidation resistance of enhanced sialon-bonded SiC results from a protective refractory oxide layer either formed during manufacture or during the first-firing cycle in service. This skin is also self-healing if the tile, setter or beam is damaged in service.
Enhanced sialon-bonded SiC is being used in wear-resistant applications where excellent abrasion resistance is required, along with resistance to acidic mediums. Compared to other materials in its class, enhanced sialon-bonded SiC has shown excellent resistance to abrasion by fine silica with an almost four-fold reduction in the worn volume of material compared with aluminum oxide grades.
Molten metals such as copper, zinc and aluminum, as well as some other non-ferrous metals, do not wet or react with enhanced sialon-bonded SiC, making it viable for pump parts, pouring spouts, launders and crucibles for handling these metals.