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

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MATERIALS CLASSIFICATION

Metals

Definition and Physical Properties of Metals

In general, a metal is typically described as a chemical element, or

mixture of elements, with the following properties:

Metals are typically hard when they are in their solid state (although

there 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 remaining

elements are gases at normal temperatures, leaving only a handful of other

elementslike sulfur, carbon, phosphorus, and bromineto make up the

balance.

Metals are part of a formal, scientific classification system of all the

chemical elements. The basic unit of any element is the atom. The word

derives 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 charges

of the nucleus and electrons are equal, but opposite. Thus, the overall

electrical charge of an atom is neutral. The outermost electrons in the

atoms of metals are held loosely. They can travel easily from atom to

atom. The main characteristic that distinguishes a metal from a nonmetal

is the presence of these free electrons. They give metals their many unique

properties, such as their excellent heat and electrical conductivity.

As with any liquid, the atoms of a molten metal move freely around

each other. If the temperature of a molten metal is lowered to the point

IN THIS SECTION YOU

WILL LEARN:

differences between metals, alloys,

blends, and composites what the four basic classes of thermal

spray materials are

examples of widely used coatings for each

class and their properties

typical applications where coatings are

used

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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 an

orderly crystalline structure. Metal crystals form in fixed geometric pat-

terns, called space lattices. The three principal crystal patterns that metals

form (Figure 3.1) are body-centered cubic (bcc), face-centered cubic

(fcc), and hexagonal close-packed (hcp). These different crystal patterns

give metals their special properties. For example, the bcc type includessome of the stronger metals such as iron and chromium. The fcc types are

softer and more ductile. They include copper, aluminum, and gold. The

hcp metals tend to be more brittle and include zinc, magnesium, and

titanium.

Another important concept in the study of materials is bonding between

dissimilar atoms. If enough electrical energy is applied to an atom, one or

more 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 gain

an extra electron and become a negative ion. Ions of opposite charges can

attach to each other to become a neutral molecule, which is the basicbuilding block of a chemical compound. This kind of bonding of atoms is

known as ionic bonding. Another kind of bonding is known as covalent

bonding. In this case, molecules are held together by atoms that share

their 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.

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Materials Classification

Pure Metals, Alloys, Blends, and Composites

A pure metal consists entirely of atoms of only one element. It has its

own unique physical properties such as melting point, boiling point, and

thermal or heat conductivity. Examples of some important pure metals are

aluminum (Al), copper (Cu), and zinc (Zn). In the commercial world, pro-

ducing metals with purities near 100% is not usually a practical reality. Thepure metals in use are typically at least 98% pure, however. Higher levels

of refinement are sometimes necessary. In the semiconductor industry, for

example, even slight contaminants in metallic components can have a

major effect on performance, so the materials are more highly purified.

An alloy is the intimate combination of two or more pure metals that

have been dissolved, while molten, one into the other. Alloys usually have

properties different from those of their individual constituents. An alloy

could be harder, softer, stronger, less easily corroded, and so forth, than

the individual metals composing it.

When an alloy is prepared, consideration must be given to the atomic

interrelations between the constituent elements and their crystal struc-

tures. Alloys are usually manufactured by melting the metals together in a

special furnace to form a simple solution. Alloys are important because

they often have very useful properties that cannot be obtained from the

pure metals alone. The scientific study of alloys is a science in itself.

Some familiar examples of alloys are brass, NiChrome, Monel, and

Stellite.

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 or

fused together as in the case of alloys or composites. For this reason,

blends can be separated into their original components by mechanical

means, such as vibrating tables, to separate heavy from light materials.

Sieves can also be used to separate powder particles that are distinct in

size. When blends are thermally sprayed, the resulting coating is a fine

mixture of each of the individual ingredients. This is a useful way to cre-

ate new coating materials with unique properties. The components of a

blend may also be glued together using organic or water-soluble

binders, which burn off during spraying.

One example is aluminum powder blended with a polyester plastic

powder. This mixture, which is normally plasma sprayed, is used as an air

seal in almost every jet engine in use today. Since aluminum and polyester

are 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.

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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 or

more powder materials whose particles are held together with a glue-like

binder. Thus, each composite powder particle consists of an amalgam of

the individual components. The binder burns away during spraying and

does not participate in the coating. The composite method of powder man-

ufacture is an important way to create new materials. It eliminates the

problem 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 automotive

and aerospace parts.

Typical Metals Used in Thermal Spray

The most important and widely used pure metals, alloys, blends, and

composites in the thermal spray industry are shown in Table 3.1.

Examples of Application Areas

Pure Metals.Atmospheric Corrosion. Wire-sprayed zinc applied using

either flame or electric arc guns is a standard for protecting steel that is

exposed to industrial and rural environments. In fact, this is one of theoldest uses of the thermal spray process. Some examples of parts that are

coated 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 than

zinc in this kind of environment. Thermally sprayed aluminum is widely

used by the U.S. Navy for shipboard protection. Interestingly, however,

20 m

Figure 3.2 Micrograph showing a blend of two metals.

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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 4050 heat protectionCu (copper) Soft-medium All High conductivity and

RH 6085 machinability

Mo (molybdenum) Hard Wire, plasma Friction and scuff resistanceZn (zinc) Soft Wire Corrosion resistance

Alloys

AlSi Soft-medium Wire, combustion Excellent machinability

RH90 powder, plasmaCuAlFe Medium All Excellent bearing

(aluminum bronze) RB 5065

FeCrSiMnC Hard Wire Repair of steel parts(stainless steel) RC 4050

NiCr Medium Combustion powder, High-temperatureRB 9095 plasma, HVOF protection or bonding

NiCrAlY Medium Plasma, HVOF High-temperatureRB 8595 protection or bonding

Blends

AlSi polyester Soft Plasma Abradable seals

R15Y 6575Cr2C3-NiCr Hard Plasma, HVOF Wear resistance up to

(nickel RC 6570 815 C (1500 F)

chrome/chromecarbide)

Intermetallics

NiAl Medium All Bonding

RB 80NiAlMo Medium All Bonding

RB 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 surface

that resists scuffing or scoring on the cylinder walls of automobile

engines. The molybdenum (or moly) coating provides protection during

cold engine starts because of its low friction against cast iron, and its abil-

Figure 3.3 Piston rings with combustion wire-sprayed molybdenumcoatings.

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Materials

ity to absorb oil due to its surface porosity. Tests have proven that molyb-

denums resistance to scuffing is superior to chrome plate by an almost

two-to-one margin.

Alloys.Machinery Repair. Type 421, a hard stainless steel, is used in a

wide range of machine repair applications. These include repair of lathe

bedways, 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 for

machine repair and also to provide quality bearing surfaces. This alloy has

the necessary hardness to stand up to high loads and speeds, yet is soft

enough for use on bushings and wear rings. Some of the many applica-

tions for aluminum bronze include repair of pump impellers, automotive

transmission 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 thermal

barrier coatings (TBCs) that are used on jet-engine combustion cham-

bers and blades (Figure 3.4).

Blends. Abradable Clearance Control. A blend of aluminum-silicon

alloy and special high-temperature polyester powder is applied by plasma

spraying to most jet engines to reduce the spacing between the tips of the

compressor blades and the inner wall of the engine shroud. During start-

up and flight, the rotating blades make contact with the coating and cut a

groove 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 power

would 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.

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harmlessly through the engine. It does not interfere with the compressor

or 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 parts

that constantly run hot, are being blasted with fine dust, or are subjected to

constant, small vibrations (fretting). Some jet or turbine parts that are

treated this way are fuel nozzles, flanges, and stator blades.

STOP READING. GO TO VIDEO.VIDEO CLIP 2: Metallic

Thermal Spray Coatings

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Materials

Ceramics

Definition and Physical Properties of Ceramics

Ceramics used in thermal spray are usually chemical compounds of a

metal and a nonmetal such as oxygen (Table 3.2). These types of ceramics

are known as oxides. In general, ceramic materials can also include

cement, clays, and many other common materials. Characteristics of

ceramics include:

Ceramics are frequently hard (mechanically resistant), yet brittle

(crack easily).

QUIZ 2. Now take some time to check your knowledge

of 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 components

together.

(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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