Advanced ceramics

Nov. 15, 2002
The major attraction of structural ceramics has always been the capability of operating at temperatures far above those of metals.

The major attraction of structural ceramics has always been the capability of operating at temperatures far above those of metals. Structural applications now include engine components, cutting tools, valves, bearings, and chemical-process equipment. Electronic applications for ceramics with low coefficient of thermal expansion and high thermal conductivity include superconductors, substrates, magnets, capacitors, and transducers.

Advanced ceramics are differentiated from traditional ceramics such as brick and porcelain by their higher strength, higher operating temperatures, improved toughness, and tailorable properties. Also known as engineered ceramics, these materials are replacing metals in applications where reduced density and higher melting points can increase efficiency and speed of operation. The nature of the bond between ceramic particles helps differentiate engineering ceramics from conventional ceramics. Most particles within an engineering ceramic are self-bonded; that is, joined at grain boundaries by the same energy-equilibrium mechanism that bonds metal grains together. In contrast, most nonengineering ceramic particles are joined by a so-called ceramic bond, which is a weaker, mechanical linking or interlocking of particles. Generally, impurities in nonengineering ceramics prevent the particles from self-bonding.

The modulus of rupture (MOR), also called flexural strength, measures the strength of ceramics for critical, high-strength applications. In the MOR test, the sample -- usually a rectangular plate -- is supported near the ends, and a bending load is applied at its center. The load is increased until the sample ruptures. Two loading conditions are commonly used: In a three-point test, the load is applied at one point midway between the two supports; a more uniform four-point version calls for a load applied at two points equidistant from the supports.

Published property values for ceramic materials can be misleading. While the data may be scientifically valid, they only represent a particular measurement method on a particular piece of material at a particular time. Without a complete material characterization and the use of standard measurement techniques, the values may have little applicability. Thermal-conductivity values, for example, are highly dependent on microscopic and macroscopic characteristics, such as crystal structure, orientation, and other properties. The nature and magnitude of porosity in a ceramic specimen, for instance, can affect thermal conductivity by a factor of two or three.

The inherent brittleness of ceramics makes special considerations necessary in designing with these materials. In ductile metals, localized stresses that exceed the yield point are usually relieved by local plastic deformation that redistributes the stress into a wider area, preventing fracture. Ceramics, however, have no such yield point; they fail when localized stresses exceed material strength. Typically, elastic behavior is linear right up to the fracture point. Moreover, they usually have high moduli of elasticity, which results in fracture at relatively small strains.

Several companies are conducting research programs aimed at increasing the ductility, or toughness, of ceramic materials. The directions that appear most promising involve transformation toughening and reinforcing the matrix with a dispersed phase such as fibers or whiskers -- for example, silicon-nitride fibers in a silicon-carbide structure.

Sensitivity to process-related defects, in combination with a lack of ductility, intensifies the need for dependable nondestructive evaluation (NDE) methods for engineering ceramics. Successful use of these materials for demanding applications requires accurate, reliable information to improve processing technology, eliminate critical defects, and increase yield. Another requirement is a solid understanding of appropriate NDE signals, leading to realistic accept/reject criteria for ceramic components. New evaluation methods now being used promise both improved flaw-detection capability and the reliable detection of smaller defects.

Metal oxide ceramics: Although most metals form at least one chemical compound with oxygen, only a few oxides are useful as the principal constituent of a ceramic. And of these, only three are used in their fairly pure form as engineering ceramics: alumina, beryllia, and zirconia.

The natural alloying element in the alumina system is silica. However, aluminas can be alloyed with chromium (which forms a second phase with the alumina and strengthens the ceramic) or with various oxides of silicon, magnesium, or calcium.

Aluminas serve well at temperatures as high as 3,500°F provided they are not exposed to thermal shock, impact, or highly corrosive atmospheres. Above 3,700°F, strength of alumina drops. Consequently, many applications are in steady-state, high-temperature environments, but not where abrupt temperature changes would cause failure from thermal shock. Aluminas have good creep resistance up to about 1,500°F, above which other ceramics perform better. In addition, aluminas are susceptible to corrosion from strong acids, steam, and sodium.

Beryllia ceramics are efficient heat dissipaters and excellent electrical insulators. They are used in electrical and electronics applications, such as microelectric substrates, transistor bases, and resistor cores. Beryllia has excellent thermal shock resistance (some grades can withstand 1,500°F/sec changes), a very low coefficient of thermal expansion, and a high thermal conductivity. It is expensive, however, and is an allergen to which some persons are sensitive.

Zirconia is used primarily for its extreme inertness to most metals. Zirconia ceramics retain strength nearly up to their melting point -- well over 4,000°F, the highest of all ceramics. Applications for fused or sintered zirconia include crucibles and furnace bricks.

Transformation-toughened zirconia ceramics are among the strongest and toughest ceramics made. These materials are of three main types: Mg-PSZ (zirconia partially stabilized with magnesium oxide, Y-TZP (Yttria stabilized tetragonal zirconia polycrystals), and ZTA (zirconia-toughened alumina).

Applications of Mg-PSZ ceramics are principally in low and moderate-temperature abrasive and corrosive environments -- pump and valve parts, seals, bushings, impellers, and knife blades. Y-TZP ceramics (stronger than Mg-PSZ but less flaw tolerant) are used for pump and valve components requiring wear and corrosion resistance in room-temperature service. ZTA ceramics, which have lower density, better thermal shock resistance, and lower cost than the other two, are used in transportation equipment where they need to withstand corrosion, erosion, abrasion, and thermal shock.

Many engineering ceramics have multioxide crystalline phases. An especially useful one is cordierite (magnesia-alumina-silicate), which is used in cellular ceramic form as a support for a washcoat and catalyst in catalytic converters in automobile emissions systems. Its low coefficient of thermal expansion is a necessary property for resistance to thermal fracture.

Glass ceramics: Glass ceramics are formed from molten glass and subsequently crystallized by heat treatment. They are composed of several oxides that form complex, multiphase microstructures. Glass ceramics do not have the strength-limiting porosity of conventional sintered ceramics. Properties can be tailored by control of the crystalline structure in the host glass matrix. Major applications are cooking vessels, tableware, smooth cooktops, and various technical products such as radomes.

The three common glass ceramics, lithium-aluminum-silicate (LAS, or beta spodumene), magnesium-aluminum-silicate (MAS, or cordierite), and aluminum-silicate (AS, or aluminous keatite), are stable at high temperatures, have near-zero coefficients of thermal expansion, and resist various forms of high-temperature corrosion, especially oxidation. LAS and AS have essentially no measurable thermal expansion up to 800°F. The high silica content of LAS is responsible for the low thermal expansion, but the silica also decreases strength. LAS is attacked by sulfur and sodium.

MAS is stronger and more corrosion resistant than LAS. A multiphased version of this material, MAS with aluminum titanate, has good corrosion resistance up to 2,000°F.

AS, produced by leaching lithium out of LAS particles prior to forming, is both strong and corrosion resistant. It has been used, for example, in an experimental rotating regenerator for a turbine engine.

A proprietary ceramic (Macor, of Corning Glass Works), called machinable glass ceramic (MGC), is about as strong as alumina. It also has many of the high-temperature and electrical properties of the glass ceramics. The main virtue of this material is that it can be machined with conventional tools. It is available in bars, or it can be rough formed, then finish machined. Machined parts do not require firing.

A similar glass ceramic is based on chemically machinable glass which, in its initial state, is photosensitive. After the glass is sensitized by light to create a pattern, it is chemically machined (etched) to form the desired article. The part can then be used in its glassy state, or it can be fired to convert it to a glass ceramic. This material/process combination is used where precision tolerances are required and where a close match to thermal expansion characteristics of metals is needed. Typical applications are sliders for disk-memory read/write heads, wire guides for dot-matrix printers, cell sheets for gas-discharge displays, and substrates for thick-film and thin-film metallization.

Another ceramiclike material, glass-bonded mica, the moldable/machinable ceramic, is also called a "ceramoplastic" because its properties are similar to those of ceramics, but it can be machined and molded like a plastic material. A glass/mica mixture is pressed into a preform, heated to make the glass flow, then transfer or compression molded to the desired shape. The material is also formed into sheets and rods that can be machined with conventional carbide tooling. No firing is required after machining.

The thermal-expansion coefficient of glass-bonded mica is close to that of many metals. This property, along with its extremely low shrinkage during molding, allows metal inserts to be molded into the material and also ensures close dimensional tolerances. Molding tolerances as close as ±0.0005 in. can be held. Continuous service temperatures for glass-bonded mica range from cryogenic to 700 or 1,300°F depending upon material grade.

Carbides and nitrides: Several metal carbides and nitrides qualify as engineering ceramics. Most commonly used are boron carbide and nitride, silicon carbide and nitride, and aluminum nitride.

Boron carbide is noted for its very high hardness and low density -- unusual qualities for a brittle ceramic -- which qualify this ceramic for lightweight, bulletproof armor plate. The material has the best abrasion resistance of any ceramic, so it is also specified for pressure-blasting nozzles and similar high-wear applications. A limitation of boron carbide is its low strength at high temperatures.

Despite their higher cost, silicon carbide (SiC), aluminum nitride (AlN), and boron nitride (BN) are challenging alumina, particularly for the more critical applications. BN, for example, has a high dielectric strength and near-zero thermal expansion in some ranges.

Silicon carbide and silicon nitride are the high-temperature, high-strength "superstars" of the engineering ceramics. These are the strongest structural ceramics for high-temperature oxidation-resistant service. However, SiN and SiC do not easily self-bond. Consequently, many processing variations have been devised to fabricate parts from these materials, creating a number of trade-offs in cost, fabricability, and properties. Either ceramic can be consolidated by hot pressing. Under the combination of high temperature and pressure -- with, in some cases, additives that act as bond-forming catalysts -- fully dense material can be formed.

The hot-pressed ceramic is extremely strong and tough at high temperatures, but the manufacturing process is limited to simple shapes, bars, or billets. Complex parts made by hot pressing must be machined to shape -- a slow and costly process of ultrasonic machining, EDM (if possible), or diamond grinding.

On the other hand, SiN and SiC particles can be bonded without pressure by a number of processes, variously called reaction bonding, recrystallization (for silicon carbide), or reaction sintering. With these processes, "green" parts can be dry or isostatically pressed, extruded, slip-cast, or, in some cases, formed by conventional plastic molding techniques such as injection molding, then sintered. Complex shapes, close to finished size, can be produced by these techniques, but the ceramic is only about 80% as dense as the hot-pressed counterpart and has lower strength and poorer thermal shock resistance.

Silicon carbide -- either hot pressed or reaction bonded -- is not as strong as silicon nitride up to about 2,600°F, silicon-nitride grain boundaries soften, or creep, and strength drops. Above 2,600°F, silicon carbide is the stronger ceramic. At 2,400°F, however, strength of hot-pressed ceramics nearly equals that of reaction-bonded ceramics.

Hot-pressed SiC is harder and more difficult to EDM that SiN, which has lower thermal expansion and better thermal shock resistance than SiC. Electrical resistivity of silicon carbide is low at low frequencies and high at high frequencies -- an unusual characteristic that qualifies this material as a semiconductor.

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