Engineering Materials

Today's design engineer's "toolbox" includes carbon, ceramics, glass, and noise-control materials to complement traditional construction materials.

Remarkable developments in structural materials technologies have taken place in the last 30 years. Formerly brittle ceramics are being transformed into materials tough enough to withstand engine environments. Carbon bearings do the job in places where lubrication is difficult. These and other old-but-new materials are appearing in a variety of applications, ranging from cutting tools and engine pistons to tennis racquets.

Chapters on Engineering Materials:

Carbon Advanced Ceramics Glass Noise Control Materials Refractory Hard Metals

Manufactured carbon comprises a large family of materials, available in hundreds of grades and shapes including plates, rods, tubes, and rings. Many of the grades have been developed for specific applications.

The parts are made from mixtures of coke and graphite powder bonded with carbon. The proportions and types of these materials are selected to obtain the lubricity and wear resistance required in the finished component. The carbon binder is derived from coal-tar pitch or synthetic resins chosen for their high coking yield. The mixture is then formed by compression molding or warm extrusion.

The formed shape is fired at temperatures as high as 1,300°C in an oxygen-free environment, which converts the binder to carbon. The resulting baked carbon part contains interconnected pores, which are usually impregnated with resins, fused salts, glasses, or molten metals. Impregnation enhances physical properties and can also reduce rubbing friction and wear rates.

During the firing operation, carbon parts shrink by 5 to 15%. Shrinkage of a given grade is predictable, however, and small parts having dimensions within a 1% tolerance band can be produced without machining. Some shapes are graphitized by a second baking to at least 2,600°F.

Most manufactured carbon parts require some machining to produce required tolerances. Only the hardest carbide or diamond-tipped tools can be used for some grades. Because of this, the larger U.S. carbon producers offer machining service for producing parts to customer specifications.

Properties:

Manufactured carbon offers a number of desirable characteristics for engineered parts. It is

A good conductor of heat and electricity. Self-lubricating; it slides on metals without galling or welding. Corrosion resistant; it is unaffected by solvents, caustics, and most acids.

In addition, carbon does not soften or melt. It is stronger at 2,760°C (5,000°F) than at room temperature. Like ceramics, however, manufactured carbon is brittle. Its strain-to-failure ratio in tension ranges from 0.05 to 1.0%, depending on grade and temperature. Due to its texture, however, its strength is not sensitive to surface scratches or nicks. Its fatigue resistance exceeds that of most metals.

Manufactured carbon parts are unaffected by reducing gases but are slowly attacked by oxygen at elevated temperature. Weight loss rates in air reach 0.1%/hr at temperatures ranging from 315 to 700°C, depending on grade and surface area. Manufactured carbon can also be weakened by strong oxidizing liquids such as hot, concentrated nitric acid.

Properties of carbon-graphite composites are largely controlled by the fillers and binders used, forming methods, baking temperature, and impregnant, if used. Fillers such as coke can produce harder, more abrasive carbons than can graphite fillers. Very fine filler grains lead to stiffer, stronger carbons. Fine-grain mixes, however, usually require processing by compression molding rather than extrusion.

Applications:

The most common use of manufactured carbon is as sliding elements in mechanical devices. It is used as the primary rubbing face in most mechanical seals. Used as a brush, it transfers electrical current to the rotating commutator on small electric motors. Carbon vanes, piston rings, or cylinder liners are used in most small air pumps and drink-dispenser pumps. Carbon is also used for pistons in chemical-metering pumps and for metering valves in gasoline pumps. All these applications require the carbon to slide on metal with a coefficient of friction below 0.2 and a wear rate below 0.001 in. per million inches rubbed.

Bearings are another significant application area for manufactured carbon. In many cases, characteristics of the carbon bearing are tailored to satisfy a wide range of requirements. This is done by impregnating the porous, as-baked or as-graphitized carbon with various materials -- for example, resin, babbitt, copper, or glass -- or by combining impregnation with chemical conversion of the carbon surface (to hard silicon carbide).

These self-lubricating materials are particularly suited for environments containing dust or lint, repeated steam cleaning, solvents, or corrosive fluids, low or high temperatures, high static loads, or hard vacuums. Some of these conditions require impregnants in the carbon material. Other applications using manufactured-carbon bearings are those inaccessible for lubrication or where product contamination (from lubricants) cannot be tolerated.

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.

Glass is an amorphous solid made by fusing silica with a basic oxide. Although its atoms never arrange themselves in a crystalline order, atomic spacing in glass is tight. Glass is characterized by transparency, hardness at atmospheric temperatures, and excellent resistance to weathering and most chemicals except hydrofluoric acid.

Most glass is based on the silicate system and is made from three major constituents: silica (SiO), lime (CaCO³), and sodium carbonate (NaCO³). Various oxides are added to tailor the properties of glass to meet specific requirements.

Nonsilicate-system glasses, which account for only about 5% of all glass produced (excluding glass ceramics, described in the chapter, Engineering ceramics), include phosphate glasses (that resist HF acid), rare-earth borate glasses (for high refractive index), heat-absorbing glasses (made with FeO), and systems based on oxides of aluminum, vanadium, germanium, and other metals. Nearly all glasses can be categorized into one of six basic types, based on chemical composition. Within each type, except for fused silica, are several distinct compositions.

Soda-lime glass, the most common type, is the glass of bottles, windows, light bulbs and drinking glasses. Its composition is similar to that of the earliest man-made glass -- a mixture of the oxides of silicon, calcium, and sodium. Approximately 90% of all glass melted today is soda-lime (or simply "lime" as it is commonly called). This inexpensive glass is readily fabricated to a wide variety of shapes. Resistance to high temperatures and to sudden temperature changes is poor, and resistance to attack by chemicals is only fair.

Borosilicate glass the oldest type of glass to have appreciable resistance to thermal shock and higher temperatures, also has excellent resistance to chemical attack. In this glass structure, the first to carry the Pyrex trademark, some of the SiO² is replaced by boric oxide.

Borosilicate glass has a low coefficient of thermal expansion and is, thus, suited for telescope mirrors and other precision parts. Also, because this glass can withstand thermal shock, it is used for oven and laboratory ware, headlamp lenses, and boiler gage glasses. Most borosilicate glasses have better resistance to acids than do soda-lime glasses, but poor resistance to alkalis. Glass fibers used in reinforcing plastic compounds are a modified borosilicate glass.

Lead-alkali glass or lead glass, contains lead monoxide, PbO, to increase its index of refraction. This glass is a better electrical insulator than soda-lime or borosilicate glasses. Lead glass is used for optical applications such as prisms and lenses and as a shield against atomic radiation. It is easy to work and is well suited for slow, manual operations. Because of its natural luster, lead glass is used for fine crystal tableware. Like lime glass, lead glass has poor resistance to high temperatures and to thermal shock.

B>Aluminosilicate glass (in which some alumina, Al²O³, replaces silica) is another thermal-shock-resisting glass similar to borosilicate but able to withstand higher operating temperatures. These glasses also resist chemical attack and are good electrical insulators. Aluminosilicate glasses are suited for high-performance applications such as high-temperature thermometers, space-vehicle windows, and ignition tubes. Coated with an electrically conductive film, they are used as resistors in critical electronic circuitry. Aluminosilicates cost about three times more than borosilicates and are appreciably more difficult to fabricate.

96% silica glass is the only one of the six categories that contains a single composition. This glass consists simply of silica (silicon dioxide) in the noncrystalline, or amorphous, state. Fused silica, most expensive of all glasses, offers the maximum resistance to thermal shock as well as the highest permissible operating temperature (900°C for extended periods, to 1,200°C for short periods). It also has maximum transmission in the ultraviolet range and the highest resistance to chemical attack of any glass. Fused silica is used in applications where requirements are extremely strict, such as mirror blanks for astronomical telescopes, ultrasonic delay lines, optical communications waveguides, and crucibles for growing crystals. Fabrication of fused silica is difficult, and the number of available shapes is, therefore, sharply limited.

These six types of glass can be grouped in three pairs. Soda-lime and lead-alkali are termed soft glasses because they soften or fuse at relatively low temperatures. Borosilicate and aluminosilicate are called hard glasses because they soften or fuse at relatively higher temperatures. And 96% silica and fused silica are the hardest of all.

The oldest of the glasses is soda-lime, which was known some 4,000 years ago. Lead-alkali was developed in 1676, borosilicate in 1912, aluminosilicate in 1936, 96% silica in 1939, and fused silica in 1952.

Today, many glass products are made from composites, made up of several glasses of differing composition. High-strength tableware is made of a sandwich of a low-expansion glass and a high-expansion glass core. Optical communications fibers (waveguides) are drawn from a boule built up from glass having a controlled variation in composition. Aerospace-vehicle windows are composed of multiple panes of glass, each pane with a unique property; the outermost panes are heat resistant, the innermost panes are mechanically strong.

Light-sensitive glasses, although not considered a basic type, are available in three grades. Photochromic glass darkens when exposed to ultraviolet radiation and fades when the ultraviolet stimulus is removed or when the glass is heated. Some photochromic compositions remain darkened for a week or longer. Others fade within a few minutes after ultraviolet is removed. A chief use for the faster-fading compositions is in eyeglass lenses that automatically darken and fade when exposed to or removed from sunlight.

Photosensitive glass also responds to light, but in a different manner from photochromic glass. When exposed to ultraviolet energy and then heated, photosensitive glass changes from clear to opal. When the UV exposure is made through a mask, the pattern of the mask is reproduced in the glass. The image developed is permanent and will not fade, as would a similar image in a photochromic glass. The exposed, opalized photosensitive glass is much more soluble in hydrofluidic acid than the unexposed glass. Immersion in this acid produces shapes, depressions, or holes by etching away of those exposed and developed areas.

Photochromatic glasses are full-color photosensitive glasses. Developed in 1978 by Corning Glass Works laboratories, their characteristics imply applications such as information storage, decorative objects, windows, or other transparencies, and containers. Photochromic glasses have true color permanence.

In order to control noise, designers must first determine where the noise source or sources are how much each is contributing to the overall level, and their frequency signatures. This can be done with a combination of instruments: a Type I 1/3-octave band sound-level meter, a sound intensity analyzer, and if there may be significant structural-borne noise, by model analysis of various components.

Once the sources have been identified and quantified, they can be ranked by how each contributes to the overall noise level. This is most important because if lower-level noise contributors are silenced first, this will not reduce the overall level. For example, when the exhaust is louder than the air intake on a gas or diesel engine, reducing the air intake noise gains little noise reduction until a properly sized muffler is installed.

When trying to control noise in any type machinery, designers generally end up employing some, if not all, four elements of noise control. These are absorption, barrier, dampers, and gasketing materials.

Absorption material:

A good candidate will "soak up" airborne sound-energy waves by changing the wave energy into heat as it passes through the absorption medium. Absorption materials are generally either fibrous or cellular. Common fibrous material are fiberglass, mineral wool, and ceramic. These are applied as blanket or semirigid sheets which can be cut to shape. Generally, they are film faced or bagged to prevent the fibers from being dislodged and causing problems in air-handling systems or rotating machines when bearings or other components could prematurely wear out if they became contaminated.

Newly developed melamine and polyimide cellular materials offer significant advantages over traditional urethanes in many respects. The melamine required only 40% the weight of a comparable urethane, costs 30% more, and has no smoke or toxic by-product of combustion. The polyimide foam has similar characteristics to the melamine but costs 10 to 12 times as much, and is not as hydrolytically stable.

Film facing will reduce overall absorption at higher frequencies. This may be a problem if most of the noise energy generated is also at these frequencies.

Barriers:

There are two types of barriers -- those that already exist (walls, cabinets, enclosures, etc.) and supplemental barriers. Supplemental barriers are those which you add if the existing enclosure wall is not thick enough. In this case, mass is the key to controlling noise. The mass law for homogeneous materials gives a rough approximation of the amount of noise that can be reduced given a specific materials mass.

However, the same reduction cannot be achieved at all frequencies. There is less attenuation (noise reduction) in the lower frequencies than the highs for a given mass, in this case, 9.6 oz/ft², or lead. The mass law predicts only 6-dB additional attenuation if the weight is doubled. But separating the two sheets of lead with a ¼-in. decoupling layer of open-cell urethane foam produces a significantly greater increase particularly above 500 Hz. Bonding a single layer of 10 oz/ft² to a 20-gage steel panel would result in even greater noise reduction.

These figures all assume "perfect" walls in which there are no openings. This is not very practical in most everyday applications. When openings are created for pipes, wires, air, and products to enter and exit, noise is let out. The amount of noise reduction expected from a perfect wall or enclosure diminishes drastically in the real world. As the opening size increases as a percentage of the total enclosure area, the actual noise reduction decreases.

For example, with a transmission loss potential of 20 dB or greater and 10% opening, designers can never get more than 10-dB noise reduction. However, this can be drastically increased by sealing around wires, pipes, and air ducts. In addition, noise reduction is increased by providing absorption-lined tunnels for materials that have to be fed into and out of the enclosure. These "supplemented" barriers are easy to fabricate and install either mechanically or with preapplied pressure-sensitive adhesive.

Damping:

All materials have a natural frequency. When they are excited by some source at this natural frequency, they will vibrate. This causes the air surrounding the material to vibrate and produce noise. Sometimes referred to as "oil can" or "drum head" phenomenon, this type of noise can be controlled by damping. Properly applied damping materials will only work if the metal or plastic to which they are applied is vibrating at or near their resonant frequency. If a mechanically driven plate is vibrating, damping will not stop it. And it should be noted that all damping materials are temperature sensitive, so they must be selected both for their temperature range and the operating temperature of the material to which they are applied.

Damping materials work to reduce the vibration in the material to which they are applied by dissipating the vibrating energy as heat, rather than radiating it as acoustic energy or noise. Damping materials are termed "viscoelastic," having both elastic and viscous properties. Essentially, the material is stretched when it is bonded to a vibrating surface. There are two types of damping material, homogeneous or free-layer damping and constrained layer. Homogeneous/free-layer materials are generally vinyls which have platelet-type fillers in them. As the material to which they are applied vibrates, the platelets slide against one another, and this friction between platelets converts the vibration energy into heat.

Free-layer damping material, made from stable vinyl and other polymers, work over an extremely wide frequency (50 to 5,000 Hz) and are very stable over a long period of time (10 to 20 years or more).

The other type of damping is constrained layer. Here, the viscoelastic polymer is homogeneous (not filled) and is sandwiched between two plates. These are bonded together, usually with a structural epoxy adhesive. The ratio of the base thickness to the constraining plate thickness is between 1:1 and 4:1. Better damping is achieved at the 1:1 ratio.

Polymer thickness is determined by the frequencies to be attenuated. Generally, the thicker the polymer, the lower the frequency and conversely the thinner the polymer, the higher the frequency.

Polymer selection is a function of the operating temperature of the material to which it will be applied. Again, each formulation has a finite temperature range over which it will be effective. This temperature range is somewhere in the neighborhood of 80°F, for any given polymer, so that one that works are -20 to 60°F will not be suitable for the 80 to 160°F range. All manufacturers of damping materials give the specific temperature range, frequency, and thickness of the constraining layer.

Damping treatment can achieve considerable noise reduction. As much as 14 dB have been achieved by this method alone, but it must be properly designed.

Gasketing: Although this is a subject which generally receives little attention, it is essential to achieving the full potential of a cabinet enclosure. Gasketing materials are generally soft, pliable foamed vinyls, urethanes, or neoprenes, although other materials are used.

The most important characteristic they exhibit for noise control is sealability or conformability to the irregular surfaces between which they are placed. Closed-cell materials make better gaskets than open cell, but their design is more complicated because they are harder to compress due to the entrapped gas in each cell. This is overcome by extruding them with cross sections having profiles such as "H, X, and Z." Costs for gaskets are minimal compared to the noise reduction they can achieve. A good gasket material properly designed for the application will easily reduce noise by 6 dB or more, and cost about $0.25/linear ft.

Refractory hard metals (RHMs) are a ceramiclike class of materials made from metal-carbide particles bonded together by a metal matrix. Often classified as ceramics and sometimes called cemented or sintered carbides, these metals were developed for extreme hardness and wear resistance.

The RHMs are more ductile and have better thermal shock resistance and impact resistance than ceramics, but they have lower compressive strength at high temperatures and lower operating temperatures than most ceramics. Generally, properties of RHMs are between those of conventional metals and ceramics. Parts are made by conventional powder-metallurgy compacting and sintering methods.

Many metal carbides such as SiC and BC are not RHMs but are true ceramics. The fine distinction is in particle bonding: RHMs are always bonded together by a metal matrix, whereas ceramic particles are self-bonded. Some ceramics have a second metal phase, but the metal is not used primarily for bonding.

Four RHM systems are used for structural applications and, in most cases, several grades are available within each system.

Tungsten carbide with a 3 to 20% matrix of cobalt is the most common structural RHM. The low-cobalt grades are used for applications requiring wear resistance; the high-cobalt grades serve where impact resistance is required.

Tantalum carbide and tungsten carbide combined in a matrix of nickel, cobalt, and/or chromium provide an RHM formulation especially suited for a combination of corrosion and wear resistance. Some grades are almost as corrosion resistant as platinum. Nozzles, orifice plates, and valve components are typical uses.

Titanium carbide in a molybdenum and nickel matrix is formulated for high-temperature service. Tensile and compressive strengths, hardness, and oxidation resistance are high at 2,000°F. Critical parts for welding and thermal metalworking tools, valves, seals, and high-temperature gaging equipment are made from grades of this RHM.

Tungsten-titanium carbide (WTiC³) in cobalt is used primarily for metal-forming applications such as draw dies, tube-sizing mandrels, burnishing rolls, and flaring tools. The WTiC² is a gall-resistant phase in the RHM containing WC as well as Co.