New Roles For Ceramic Parts

Anders F. Henriksen
Mary T. Spohn
Assn. of American Ceramic Component Manufacturers
Westerville, Ohio

Engineered ceramics routinely handle tough applications that would quickly destroy other materials. They at providing long life and maintaining tight tolerances despite high temperatures and corrosive or abrasive environments.

For instance, the electronics industry relies on ceramics to serve as high-temperature insulators and stable substrates under thick and thin-film deposited circuits. Steelmakers use zirconia-ceramic orifices to handle 2,800°F molten steel as it is cast into billets. And alumina — one of the hardest materials known — is well suited for sandblast nozzles, water-jet cutting tools, and high-temperature crucibles. Few nonce-ramic materials come close to meeting such rigorous demands.

The defense and aerospace industries were among the first to use engineered ceramics widely, but new materials and processing methods offer today's designers a wide range of cost-effective options to suit many critical needs. As a result, ceramics now play a key role in chemical and metal processing, semiconductor fabrication, power generation, and many other industries.

CERAMIC BASICS
Ceramics are loosely defined as inorganic, nonmetallic materials subjected to high temperatures during manufacture or use. Engineered ceramics consist of both oxides, such as alumina and silica, and nonoxides such as carbides, nitrides, and borides. Designers usually turn to ceramics when they need parts with physical properties or performance beyond the scope of metals or plastics. These capabilities include:

High-temperature resistance. By definition, ceramics tolerate temperatures of at least 1,000°F.

High stiffness. Ceramics are among the stiffest engineered materials. Cemented carbides, for example, are three times stiffer than steel. Other carbide ceramics have nearly twice steel's stiffness.

Compressive strength. Ceramics often have excellent compressive strength. Long-term properties such as fatigue resistance and creep are usually good under compressive loads and ceramics do not deform plastically or exhibit permanent elongation and reduction in area under load, even at elevated temperatures.

Chemical resistance. Ceramics do not degrade when exposed to most chemicals. One reason is that ceramics are often compounds readily found in nature. Thus there is little driving force for corrosion, which returns materials to their natural state. Alumina (Al2O3), for instance, is the most stable oxide known, inert to all chemicals except fluorides and HF. This makes it well suited for use in severely corrosive environments that would dissolve metals and plastics.

Electrical insulation. Most ceramic materials are excellent electrical insulators. Alumina, for instance, with a room-temperature volume resistivity of more than 10 ¹⁴ ohm-cm and a melting point of 2,090°C, is ideal for spark-plug insulators.

Ceramics are usually not a designer's first option because they can be a bit pricey compared with components made of metal and plastic, due to the more elaborate processing steps necessary to make them. But in the long run they can prove to be a lot more economical. When high performance and long life in extreme conditions are factored in, ceramics often produce savings that far outweigh the material's initial cost.

Ceramics are also generally brittle and fail catastrophically when stressed beyond their limit, although the observed fracture strength (modulus-of-rupture) for ceramics can be as high as 10 ⁶ psi for reinforcement materials such as silica fibers and whiskers of other crystalline ceramics.

And ceramics lack fracture toughness, making them a poor match for components in tension. Once a crack starts, it propagates rapidly and the part is likely to fail. This is particularly true if the component has surface flaws, scratches or nicks from grinding, or internal flaws such as inclusions, pores, or microcracks.

CUSTOMIZED MATERIALS
Researchers are constantly working to develop new ceramic chemistries as well as improve forming and processing methods. Such efforts broaden the range of available materials and the types of applications they can handle. By adjusting factors such as porosity, grain size, and heat treatment, suppliers can tailor individual components to handle specific needs.

Ceramists also routinely add minor amounts of other compounds to change crystal structure and, with it, physical properties. For example, pure zirconia (ZrO2) is not a particularly useful engineered material. The crystal structure transforms from cubic to tetragonal when cooled, which produces a nonisotropic volume change in each crystal. This means parts made from pure zirconia powder will disintegrate while cooling.

However, alloying zirconia with yttria, calcia, ceria, or magnesia forces the high-temperature cubic form to remain stable at room temperature. The resulting material is fully stabilized cubic zirconia for gem stones or partially stabilized zirconia (PSZ) suited for industrial applications.

PSZ's bending strength (>110 ksi) exceeds that of hardened tool steel. Manufacturers use it in applications such as industrial and domestic cutting knives and surgical scalpels because the hard cutting edge does not wear nearly as fast as ones made of steel or carbide. Though not ductile, high strength permits PSZ to flex under load, making it useful for high-temperature springs.

Heat-treating PSZs produces an intragranular tetragonal-zirconia precipitate which compressively stresses the entire structure and considerably increases mechanical bend strength. Called transformation-toughened zirconia (TTZ), this is analogous to austenitic hardening of steel. Similarly, zirconia is sometimes alloyed with alumina (AZT) or zirconia is added to alumina (ZTA), creating very strong prestressed materials which can be used in load-bearing applications.

When structural properties are a top concern, suppliers often suggest Al2O3 and move to higher or lower performing materials as needed. Alumina has a Young's modulus of 5.3 10 ⁴ ksi and a compressive strength on the order of 380 ksi. Thus, it is stiffer than steel and has better compressive strength than many hardened tool steels. Moreover it resists all oxidizing and reducing agents. Ceramics such as zirconia (PSZ), silicon carbide, hotpressed silicon nitride, or aluminum nitride may fit the bill when higher strength is needed.

Ceramics are often considered electrical and thermal insulators, but there are exceptions. For instance, aluminum nitride has high thermal conductivity while silicon carbide has high electrical conductivity.

Adding carefully selected compounds also lets designers tailor electrical properties of certain ceramics. For example, adding lead oxide to zirconium titanate creates lead-zirconium-titanate (PZT), a piezoceramic. Piezoceramics change shape in response to electrical signals and, conversely, generate an electric charge when squeezed. They are widely used as transducers in microwave ovens and ultrasound and medical imaging devices, as flow-control sensors, and as automotive airbag and safety sensors.

Partially stabilized zirconia is unusual in that it becomes an ionic electrical conductor above 600°C. It conducts electricity by atomic transport of oxygen through its crystal lattice. This makes it useful for oxygen sensors in automotive engines, as semipermeable membranes in high-temperature sodium batteries, and as probes for measuring carbon content in molten steel.

PROCESSING CONSIDERATIONS
Ceramic materials do not deform plastically so they cannot be stamped, drawn, or bent like metals and plastics. Instead, they are usually produced in a three-step process. First, the parts are shaped through techniques such as extrusion, dry pressing, isostatic pressing, slip casting, tape casting, and injection molding. Next follows a drying or debindering step. Finally, the parts are fired and the raw-material powders shrink to form a dense, polycrystalline ceramic body as close as possible to the intended shape.

It is important to realize that firing always distorts the part to some degree — the shrinkage alone can be as much as 25 linear percent. Because they are so hard, ceramic parts must be ground with diamond-coated tools to correct for distortion after firing, which increases costs. Therefore, it is prudent to consult with an experienced manufacturer who can recommend design techniques that allow for distortion and minimize the need for machining. The resulting cost savings can be considerable. It is also critical to consider all dimensions with respect to tolerances, preferably in the "as fired" state. General guidelines for tolerances after firing are ±1% or ±0.005 in., whichever is greater, for parts up to 6 in.; and ±2%. for parts over 6 in.

In addition to heading AACCM, Dr. Anders Henriksen is president of Ceramco Inc., Center Conway, N.H.