clasificacion de materiales

© 2001 ASM International® • Materials Park, OH 44073-0002 • Phone: (440) 338-5151 • Fax: (440) 338-4634 9

3MATERIALS CLASSIFICATION

Metals

Definition and Physical Properties of Metals

In general, a metal is typically described as a chemical element, ormixture of elements, with the following properties:

• Metals are typically hard when they are in their solid state (althoughthere are exceptions, such as lead).

• They are usually shiny or lustrous.

• Metals are typically heavy; that is, they have relatively high density.

• They are malleable (able to be formed and shaped) and ductile (eas-ily drawn or bent).

• They are good conductors of both heat and electricity.

Of all the known elements, about 75% are metals. Many of the remainingelements are gases at normal temperatures, leaving only a handful of otherelements—like sulfur, carbon, phosphorus, and bromine—to make up thebalance.

Metals are part of a formal, scientific classification system of all thechemical elements. The basic unit of any element is the atom. The wordderives its name from the Greek word atomos, which means indivisible.Atoms are the basic building blocks of all materials. A single atom con-sists of a positively charged nucleus, surrounded by a cloud of negativelycharged particles called electrons. In a normal atom, the electrical chargesof the nucleus and electrons are equal, but opposite. Thus, the overallelectrical charge of an atom is neutral. The outermost electrons in theatoms of metals are held loosely. They can travel easily from atom toatom. The main characteristic that distinguishes a metal from a nonmetalis the presence of these free electrons. They give metals their many uniqueproperties, such as their excellent heat and electrical conductivity.

As with any liquid, the atoms of a molten metal move freely aroundeach other. If the temperature of a molten metal is lowered to the point

IN THIS SECTION YOUWILL LEARN:

• differences between metals, alloys,blends, and composites

• what the four basic classes of thermalspray materials are

• examples of widely used coatings for eachclass and their properties

• typical applications where coatings areused

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10 © 2001 ASM International® • Materials Park, OH 44073-0002 • Phone: (440) 338-5151 • Fax: (440) 338-4634

Materials

where it solidifies, its atoms lose energy. At that point, the force of attrac-tion between atoms becomes strong, and they arrange themselves into anorderly crystalline structure. Metal crystals form in fixed geometric pat-terns, called space lattices. The three principal crystal patterns that metalsform (Figure 3.1) are body-centered cubic (bcc), face-centered cubic(fcc), and hexagonal close-packed (hcp). These different crystal patternsgive metals their special properties. For example, the bcc type includessome of the stronger metals such as iron and chromium. The fcc types aresofter and more ductile. They include copper, aluminum, and gold. Thehcp metals tend to be more brittle and include zinc, magnesium, andtitanium.

Another important concept in the study of materials is bonding betweendissimilar atoms. If enough electrical energy is applied to an atom, one ormore of the outer electrons can be removed, and the atom becomes posi-tive charged. The atom is now called a positive ion. An atom can also gainan extra electron and become a negative ion. Ions of opposite charges canattach to each other to become a neutral molecule, which is the basicbuilding block of a chemical compound. This kind of bonding of atoms isknown as ionic bonding. Another kind of bonding is known as covalentbonding. In this case, molecules are held together by atoms that sharetheir outer electrons with each other. Common gases such as oxygen (O2)and hydrogen (H2) are bonded in this manner. Ceramics, discussed later,are important examples of covalent compounds.

Body-centered cubic (bcc)

Face-centered cubic (fcc)

Hexagonal close-packed (hcp)

Figure 3.1 The three principal crystal patterns that metals form.



Materials Classification

Pure Metals, Alloys, Blends, and Composites

A pure metal consists entirely of atoms of only one element. It has itsown unique physical properties such as melting point, boiling point, andthermal or heat conductivity. Examples of some important pure metals arealuminum (Al), copper (Cu), and zinc (Zn). In the commercial world, pro-ducing metals with purities near 100% is not usually a practical reality. The“pure” metals in use are typically at least 98% pure, however. Higher levelsof refinement are sometimes necessary. In the semiconductor industry, forexample, even slight contaminants in metallic components can have amajor effect on performance, so the materials are more highly purified.

An alloy is the intimate combination of two or more pure metals thathave been dissolved, while molten, one into the other. Alloys usually haveproperties different from those of their individual constituents. An alloycould be harder, softer, stronger, less easily corroded, and so forth, thanthe individual metals composing it.

When an alloy is prepared, consideration must be given to the atomicinterrelations between the constituent elements and their crystal struc-tures. Alloys are usually manufactured by melting the metals together in aspecial furnace to form a simple solution. Alloys are important becausethey often have very useful properties that cannot be obtained from thepure metals alone. The scientific study of alloys is a science in itself.Some familiar examples of alloys are brass, NiChrome, Monel, andStellite®.

Blends are a formulation of materials that are frequently used as ther-mal spray powders. Blends are a simple, physical mixture of two or moremetallic or nonmetallic powders (Figure 3.2). They are not melted orfused together as in the case of alloys or composites. For this reason,blends can be separated into their original components by mechanicalmeans, such as vibrating tables, to separate heavy from light materials.Sieves can also be used to separate powder particles that are distinct insize. When blends are thermally sprayed, the resulting coating is a finemixture of each of the individual ingredients. This is a useful way to cre-ate new coating materials with unique properties. The components of ablend may also be “glued” together using organic or water-solublebinders, which burn off during spraying.

One example is aluminum powder blended with a polyester plasticpowder. This mixture, which is normally plasma sprayed, is used as an airseal in almost every jet engine in use today. Since aluminum and polyesterare both low-density materials, the mixture remains uniform during han-dling or spraying. This is important, because blends of powdered materi-als should be similar in particle size and density to be successful;otherwise, the particles will tend to separate.

The metal blends used commercially are commonly blends with ceram-ics or plastics rather than with other metals.



Materials

Composite powders are unique to the thermal spray industry. A com-posite implies something made up of diverse parts, and so it is with com-posite thermal spray particles. A composite powder consists of two ormore powder materials whose particles are held together with a glue-likebinder. Thus, each composite powder particle consists of an amalgam ofthe individual components. The binder burns away during spraying anddoes not participate in the coating. The composite method of powder man-ufacture is an important way to create new materials. It eliminates theproblem of powder separation that can occur with blends.

One well-established use of thermal spray composites is the fiberglass/polyester materials that are used to manufacture lightweight automotiveand aerospace parts.

Typical Metals Used in Thermal Spray

The most important and widely used pure metals, alloys, blends, andcomposites in the thermal spray industry are shown in Table 3.1.

Examples of Application Areas

Pure Metals. Atmospheric Corrosion. Wire-sprayed zinc applied usingeither flame or electric arc guns is a standard for protecting steel that isexposed to industrial and rural environments. In fact, this is one of theoldest uses of the thermal spray process. Some examples of parts that arecoated for corrosion protection are bridges, guard rails, transformer cases,and lamp-posts. The rough texture of sprayed zinc also provides an excel-lent base for the application of paint.

Marine Corrosion. For protection against salt spray or saltwater immer-sion, pure aluminum is the recommended choice. It will last longer thanzinc in this kind of environment. Thermally sprayed aluminum is widelyused by the U.S. Navy for shipboard protection. Interestingly, however,

20 µm

Figure 3.2 Micrograph showing a blend of two metals.




Table 3.1 Typical Metals, Alloys, Blends, and Composites Used in Thermal Spray

Coating Hardness Spray Method Application

Pure Metals

Al (aluminum) Soft All Corrosion resistance,RH 40–50 heat protection

Cu (copper) Soft-medium All High conductivity and RH 60–85 machinability

Mo (molybdenum) Hard Wire, plasma Friction and scuff resistance

Zn (zinc) Soft Wire Corrosion resistance

Alloys

AlSi Soft-medium Wire, combustion Excellent machinabilityRH90 powder, plasma

CuAlFe Medium All Excellent bearing(aluminum bronze) RB 50–65

FeCrSiMnC Hard Wire Repair of steel parts(stainless steel) RC 40–50

NiCr Medium Combustion powder, High-temperature RB 90–95 plasma, HVOF protection or bonding

NiCrAlY Medium Plasma, HVOF High-temperature RB 85–95 protection or bonding

Blends

AlSi � polyester Soft Plasma Abradable sealsR15Y 65–75

Cr2C3-NiCr Hard Plasma, HVOF Wear resistance up to (nickel RC 65–70 815 °C (1500 °F)chrome/chrome carbide)

Intermetallics

NiAl Medium All BondingRB 80

NiAlMo Medium All BondingRB 80

zinc provides better performance than aluminum in fresh water. Today,AlZn alloys, such as 85/15, are also being used successfully.

Piston Rings. The outer diameters of piston rings are coated with wire-sprayed molybdenum (Figure 3.3). This produces a hard bearing surfacethat resists scuffing or scoring on the cylinder walls of automobileengines. The molybdenum (or “moly”) coating provides protection duringcold engine starts because of its low friction against cast iron, and its abil-

Figure 3.3 Piston rings with combustion wire-sprayed molybdenumcoatings.



Materials

ity to absorb oil due to its surface porosity. Tests have proven that molyb-denum’s resistance to scuffing is superior to chrome plate by an almosttwo-to-one margin.

Alloys. Machinery Repair. Type 421, a hard stainless steel, is used in awide range of machine repair applications. These include repair of lathebedways, engine crankshafts, pump shafts, and engine pulleys on farmtractors. Previously, these were typically replaced with new parts.

Aluminum bronze, an alloy of copper, aluminum, and iron, is used formachine repair and also to provide quality bearing surfaces. This alloy hasthe necessary hardness to stand up to high loads and speeds, yet is softenough for use on bushings and wear rings. Some of the many applica-tions for aluminum bronze include repair of pump impellers, automotivetransmission housings, and marine pump shafts.

High-Temperature Resistance. Thermally sprayed alloys of nickel �chromium � aluminum � yttrium (NiCrAlYs) are used to protect jet-engine turbine blades from high temperatures and corrosion. These coat-ings are also used as high-temperature bond coats for ceramic thermalbarrier coatings (TBCs) that are used on jet-engine combustion cham-bers and blades (Figure 3.4).

Blends. Abradable Clearance Control. A blend of aluminum-siliconalloy and special high-temperature polyester powder is applied by plasmaspraying to most jet engines to reduce the spacing between the tips of thecompressor blades and the inner wall of the engine shroud. During start-up and flight, the rotating blades make contact with the coating and cut agroove into the coating (due to abrasion), forming a tight air seal. Thesekinds of coatings are therefore called abradables. Without these coatings,the engines would consume much more fuel and their overall powerwould be reduced. The coating that is removed by the abrasion travels

Figure 3.4 Jet-engine combustion chambers and other components withhigh-temperature thermal barrier coatings.




harmlessly through the engine. It does not interfere with the compressoror turbine blades.

Jet Engine Repair. Jet or gas turbine engines are routinely repaired dur-ing maintenance overhauls with many different thermal spray coatings.One widely used blended material is chromium carbide and nickel-chrome alloy. This is a high-temperature coating that survives tempera-tures approaching 982 °C (1800 °F). This is an excellent choice for partsthat constantly run hot, are being blasted with fine dust, or are subjected toconstant, small vibrations (fretting). Some jet or turbine parts that aretreated this way are fuel nozzles, flanges, and stator blades.

STOP READING. GO TO VIDEO.VIDEO CLIP 2: MetallicThermal Spray Coatings



Materials

Ceramics

Definition and Physical Properties of Ceramics

Ceramics used in thermal spray are usually chemical compounds of ametal and a nonmetal such as oxygen (Table 3.2). These types of ceramicsare known as oxides. In general, ceramic materials can also includecement, clays, and many other common materials. Characteristics ofceramics include:

• Ceramics are frequently hard (mechanically resistant), yet brittle(crack easily).

QUIZ 2. Now take some time to check your knowledgeof metals, metal alloys, composites, and blends. Check

your answers against the answer key in the back of this workbook.

1. Metals are good conductors of heat and electricity because of:

(A) free electrons that move between the atoms.(B) their crystal structure.(C) ionic bonds.

2. A pure metal contains:

(A) two or more elements.(B) alloys.(C) only one element.

3. Metal alloys are:

(A) blends of powders.(B) two or more types of metal atoms combined together.(C) easily separated into individual components.

4. In thermal spray, composites are referred to as:

(A) powder particles that contain one or more finer powder com-ponents.

(B) powder particles that contain a binder to hold the componentstogether.

(C) both (A) and (B).

5. An important application of thermal spray zinc is:

(A) high-temperature resistance.(B) piston rings.(C) atmospheric corrosion.


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