Editorial Feature

Non-Oxide Advanced Ceramics Widen their Applications

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Advanced ceramics, which are also described as engineered, fine or technical ceramics, have been used as solutions to many severe material problems over the past 30 to 40 years. These problems include those involving corrosion, erosion, wear, temperature and electrical insulation of the material, as well as any combinations of these issues. Many of these issues have been overcome by users selecting from a wide range of materials that are loosely categorized as an oxide or non-oxide ceramics. In fact, the users who have found and utilized these alternative materials have been able to carry out procedures and processes hitherto impossible, resulting in improved yields, life, and manufacturing efficiencies of many industrial processes.

Silicon Carbide and Silicon Nitride      

The use of non-oxide ceramics has resolved some of the most extreme wear and corrosion problems, regardless of whether high temperature and/or severe thermal shock is also involved. Among the numerous advanced ceramic materials that currently exist today include silicon nitride and silicon carbide, both of which offer features that can benefit specific applications.  

As an excellent abrasive material, silicon carbide has historically been used to produce and create grinding wheels and other products that must perform abrasive procedures. Today, silicon carbide is often favored as a technical ceramic when there is a need to conduct heat away from the material, as it has exhibits high thermal conductivity, strength, hardness, elastic modulus, and thermal shock resistance. Some of the most common applications of silicon carbide ceramics can be found in abrasives, refractories, ceramics, and other high-performance applications.  
Silicon nitrides, which are near as light as silicon carbide, are commonly employed if thermal shock or other mechanical demands are such that a high thermal shock resistance and fracture toughness are desirable. The precise microstructure of silicon nitrides provides excellent thermal shock resistance and resilience against fractures, thereby making these materials particularly resistant to both physical impact and thermal shock. Some of the most common products made from silicon nitride ceramics include advanced ceramic tubes, cutting ceramics, components for pumps, and much more.

Reaction Bonded Silicon Nitride

Silicon nitride exists in two distinct forms, one of which includes reaction bonded silicon nitride, which is typically an intermediate strength material, as it has 15% to 25% porosity within the structure. The nature of its formation, which is achieved by the direct nitridation of parts performed from silicon powder, provides it’s characteristic porosity that is extremely fine and tortuous. Under normal conditions, most liquids and gases will not pass through the pores.

The pores are a legacy of the need for nitrogen to penetrate the structure during the reaction. This material is densified by the reaction, rather than contraction and, as such, unlike other ceramics, exhibits no dimensional changes during the firing process. This eases the burden on the manufacturer and can result in avoidance of, or minimal, expensive diamond grinding.

As a result, these properties allow reaction bonded silicon nitride to be suitable for a wide range of jigs and fixtures for heat treatment processes, products for handling molten non-ferrous alloys, chemical engineering processes, welding and cutting processes and many other applications. The material typically has a density of around 2.4g/cm3 (75% of theoretical) with a strength of 240MPa. Thermal conductivity and expansion are low with high electrical resistivity.

Reaction bonded silicon nitride can fulfill a wide range of demands. While this may be true, it is important to note that this grade of material cannot cover the entire spectrum of requirements alone; therefore, a technical ceramic that exhibits higher strength and resistance properties is often sought-after. However, a recent mining project led to the creation of a fully reacted and bonded silicon nitride material that exhibited predictable wear performance. This novel material was found to exhibit more predictable wear throughout its life, as well as reduce unscheduled downtime when in use. Furthermore, the researchers that developed this material found that it exhibited an excellent performance rate in thermal cycling conditions, thereby indicating the potential for this transformed silicon nitride material to be particularly useful for future refractory purposes.

Sintered Reaction Bonded Silicon Nitride

The second type of silicon nitride technical ceramic is sintered reaction bonded silicon nitride, which is a fully dense material that exhibits a high strength of 800 MPa, no porosity, and high fracture toughness of up to 7.5. The company Ceradyne has achieved these precise properties in its own sintered reaction bonded silicon nitride material through the use of its unique silicon nitride processing route. In their procedure’s design, the researchers here are capable of producing components that exhibit complex shapes and in a wide range of sizes.

Applications of Sintered Reaction Bonded Silicon Nitride

Successful usage expands into components which experience high compressive stresses, such as cam rollers in diesel engines and other automotive applications, wear segments on machines for producing paper, chemical, food, pharmaceutical, oil, and gas industry valves, seals, rotating parts and wear plates, location pins for projection welding, cutting tooltips, abrasive powder blast nozzles, metal forming tooling, wire and tube drawing and wire guides. Many of these applications have a customer base that uses the parts in high volume and for several years without experiencing any problems throughout the material’s lifetime.

Advanced Ceramics as Alternative Materials

Design engineers, production engineers, and mechanical engineers have often compromised on the materials that they have chosen for an application because ceramic solutions appeared to be too novel and posed a high risk. However, as the range of successful ceramic solutions grows, the risk element has suppressed. Today, it is often possible to contact ceramic manufacturers with a new idea for a solution to a problem and to expect that one of a portfolio of materials will have been used successfully in a similar situation.

Ceradyne’s Advanced Ceramic Operations has concentrated its market growth around the development expertise of its engineering and production personnel and their outstanding knowledge of nonoxide ceramics.

Other Non-Oxide Advanced Ceramics

Along with silicon nitride, products are manufactured in silicon carbide, and aluminum nitride, in which the thermal conductivity is 150 Wm-1K-1 challenges beryllium oxide for substrate applications.

Semiconductor Manufacturing

Silicon nitride, silicon carbide and aluminum nitride can be produced in high purity, thereby offering semiconductor manufacturers with products that can survive the harsh environments often experienced in semiconductor wafer manufacture.

Ballistic Armour

Meanwhile, silicon carbide/boron carbide and titanium diboride now offer complete ballistic resistance to a very wide range of threats for personnel and ground, sea and air-based transport.

Producing Non-Oxide Ceramics by 3D Printing

Just as every other industry has incorporated additive manufacturing, or three-dimensional (3D) printing into the development of their products, so has the technical ceramics industry. Researchers have been able to produce dense structural ceramic parts by 3D printing through the use of aqueous Si3N4 ink. These materials exhibited comparable mechanical properties as compared to those that were produced by traditional dry-pressing processes.

The Outlook

The scale of current advanced ceramic production has reduced manufacturing costs dramatically, while also improving the critical material properties of these ceramics. By working together with the ceramic manufacturer, many new applications can be brought to fruition, while also facilitating the development of further efficiency and quality improvements, cost reduction, and faster production rates.

Sources and Further Reading

This article was updated on the 15th March, 2019.

 

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