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Silicon carbide (SiC) is arguably one of the most versatile compounds on Earth. In the years since commercial production began, it has become an invaluable asset in dozens of applications ranging from a protective reentry skin on the space shuttle to structural materials, and in the automotive, electronics, steel production, pharmaceutical, chemicals, nuclear and many other industries. It is even popular as a gemstone substitute for diamonds, as it has many of the latter’s characteristics, including extreme hardness and transparency. One of SiC’s most interesting developments is Hexoloy®, which – for three decades – has built upon SiC’s strengths while increasing its versatility.

Development of Hexoloy SiC began at the Carborundum Company (that would soon become part of Saint-Gobain) with the hope of making a kiln support beam that could increase kiln capacity. The results were considerably better than expected: Not only did the new beam increase capacity by 25 percent in one high temperature kiln application, it had twice the lifetime of traditional SiC beams and was much stronger, making possible kiln cars with thinner beams. This, in turn, allowed a fifth deck to be added to a typical four-deck car without increasing height or width.

Figure 1 (left to right—before and after): When Hexoloy SiC was introduced, the new beam increased capacity by 25 percent in one high temperature kiln application. (Source: Saint-Gobain Structural Ceramics)

Figure 2: Hexoloy has been used to solve the problem of leakage in vehicle water pumps. (Source: Adobe)

One of Hexoloy’s early achievements was solving the problem of leakage in vehicle water pumps, a classic failure mode for decades. Since water pump manufacturers began using Hexoloy SiC seals rather than graphite, carbon or alumina, the problem has essentially vanished. The material is also widely used to form thin and strong thermocouple protective tubes with a closed end surrounding thermocouples and other sensors exposed to high temperatures or hostile substances.

Since Hexoloy’s inception, Saint-Gobain has made dramatic advances in the material and is available in many shapes and sizes (Figure 3) achieved by bonding agents and other additives developed by Saint-Gobain that allow mass production of very complex shapes, some of which are unachievable with most ceramics. Hexoloy is now used in numerous applications such as those in Table 1 throughout many industries, with new ones frequently being discovered; for example, manufacturers of lithium ion powder for batteries for electric vehicles fire materials in roller hearth kilns whose rollers are made from SiC. By replacing their previous SiC material with Hexoloy, they have increased production throughput as well as efficiency and increased roller life.

Figure 3: Hexoloy is available in a variety shapes for specific applications. (Source: Saint-Gobain Structural Ceramics)

The particulate filters used by diesel engines have also joined Hexoloy’s list of applications, as some of the filters’ components are fired in kilns at very high temperatures using Hexoloy SiC kiln furniture, where it has a distinct advantage of smaller cross-section, strength, low creep, and longer life over competing materials. The use of Hexoloy is also being investigated for its ability to withstand exposure to additional acids and gases, which if successful, would add to its applications.

Table 1 – Typical Hexoloy Applications

What Makes It Special

Most benefits of Hexoloy are derived from its hardness, universal chemical and corrosion resistance, high-temperature strength, resistance to thermal shock and stability over temperature (low creep). It is second only to diamond in hardness; has a maximum working temperature of 1650° C (Figure 4) in air; resists corrosion, oxidation, and erosion far more effectively than tungsten carbide or coated stainless steel; has flexural strength of 55,000 lbs/in.²; has low wear rate and coefficient of friction; and also has sufficient thermal conductivity to make it a replacement for ductile metals in high-temperature applications.

Figure 4: When tested alongside competitive materials, only Hexoloy achieved an operating temperature of 1600° C. (Source: Saint-Gobain Structural Ceramics)

Hexoloy is also chemically inert. That is, it reacts with virtually no other substance, from the most corrosive chemicals, hot gases and liquids, and the strongest acids and bases, even at temperatures of 1650° C. The few exceptions to note are caustic chemicals and molten ferrous based metals, use here is not recommended. It also imparts nothing to the product to which it is applied, an enormous benefit possessed by no competing material.

Hexoloy has almost no porosity, achieving density of at least 98 percent of SiC’s theoretical maximum, higher than any other type of SiC, which translates into higher strength and stability. The surface of the material is very smooth owing to its fine-grained structure and high density, which allows intricate components to be made to precise tolerances. Its surface quality and tight dimensional control yields parts that typically require little or no additional machining or finish grinding. Hexoloy’s high temperature strength and high thermal conductivity help it withstand thermal-shock resistance far better than tungsten carbide, aluminum oxide and many other ceramic materials.

Table 2 – Differences between Hexoloy versions

There are five variants of Hexoloy as shown in Table 2, each one dedicated to the needs of specific applications by stressing certain of its inherent characteristics or adding a feature such as enhanced electrical conductivity (Hexoloy SG). Collectively, they cover all the scenarios where the material is most likely to be used. However, those scenarios grow continuously as Saint-Gobain expands the reach of Hexoloy into markets dominated by conventional materials for years or even decades. Visit www.hexoloy.com for details.