The Structure of Metals Mechanical Behavior, Testing and Manufacturing Properties of Materials

1. The Structure of Metals • Mechanical Behavior, Testing and Manufacturing Properties of Materials • Physical Properties of Materials • Metal Alloys: Structure and Strengthening by Heat Treatment • Ferrous Metals and Alloys: Production, General Properties and Applications

2. 6. Nonferrous Metals and Alloys: Production, General Properties and Applications 7. Polymers: Structure, General Properties and Applications 8. Ceramics, Graphite and Diamond: Structure, General Properties and Applications 9. Composite Materials: Structure, General Properties, and Applications

3. Chapter Objectives • Ceramics: their structure and properties. • Types of ceramics and their typical applications. • Glasses: their structure, properties, and applications. • Diamond: its properties and applications in abrasive machining and micromachining. • Graphite: its uses as a mold material and as fibers in reinforced plastics.

4. Chapter Outline • Introduction • The Structure of Ceramics • General Properties and Applications of Ceramics • Glasses • Glass Ceramics • Graphite • Diamond

5. 8.1 Introduction • This chapter describes the general characteristics and applications of those ceramics, glasses, and glass ceramics that are of importance in engineering applications and in manufacturing. • Because of their unique characteristics, the properties and uses of two forms of carbon, namely graphite and diamond, are also discussed here.

6. 8.2 The Structure of Ceramics • Ceramics are compounds of metallic and nonmetallic elements. • Ceramics can be divided into two general categories: 1. Traditional ceramics—such as whiteware, tiles, brick, sewer pipe, pottery, and abrasive wheels 2. Industrial ceramics (also called engineering, high-tech, or fine ceramics)—such as turbine, automotive, and aerospace components (Fig. 8.1); heat exchangers; semiconductors; seals; prosthetics; and cutting tools.

7. 8.2 The Structure of Ceramics

8. 8.2 The Structure of Ceramics • The structure of ceramic crystals (containing various atoms of different sizes) is among the most complex of all material structures. The bonding between these atoms is generally covalent (electron sharing, hence strong bonds) or ionic (primary bonding between oppositely charged ions, thus strong bonds).

9. 8.2 The Structure of Ceramics • Ceramics are available in single crystal or in polycrystalline form. • Grain size has a major influence on the strength and properties of ceramics; the finer the grain size (hence the term fine ceramics), the higher the strength and toughness.

10. 8.2.1 Raw Materials • Raw materials used for making ceramics is clay, which has a fine-grained sheet-like structure. • Other major raw materials for ceramics that are found in nature are flint (a rock composed of very fine-grained silica, ) and feldspar (a group of crystalline minerals consisting of aluminum silicates plus potassium, calcium, or sodium). • Porcelain is a white ceramic composed of kaolin, quartz, and feldspar; its largest use is in appliances and sanitary ware.

11. 8.2.2 Oxide Ceramics • There are two major types of oxide ceramics: alumina and zirconia (Table 8.1).

12. 8.2.2 Oxide Ceramics Alumina • Also called corundum or emery, alumina (aluminum oxide, ) is the most widely used oxide ceramic—either in pure form or as a raw material to be blended with other oxides. • It has high hardness and moderate strength. • Structures containing alumina and various other oxides are known as mullite and spinel; they are used as refractory materials for high-temperature applications. • The mechanical and physical properties of alumina are suitable particularly in applications such as electrical and thermal insulation and in cutting tools and abrasives.

13. 8.2.2 Oxide Ceramics Zirconia • Zirconia has good toughness; good resistance to thermal shock, wear, and corrosion; low thermal conductivity; and a low friction coefficient. • Partially stabilized zirconia (PSZ) has high strength and toughness and better reliability in performance than does zirconia. • Typical applications include dies for the hot extrusion of metals and zirconia beads used as grinding and dispersion media for aerospace coatings, for automotive primers and topcoats, and for fine glossy print on flexible food packaging.

14. 8.2.2 Oxide Ceramics Zirconia • Two important characteristics of PSZ are its coefficient of thermal expansion (which is only about 20% lower than that of cast iron) and its thermal conductivity. • Transformation-toughened zirconia (TTZ) has higher toughness because of dispersed tough phases in the ceramic matrix.

15. Example 8.1 Ceramic Knives The use of ceramics now is being extended to ceramics knives generally made o zirconium oxide. Ceramic knives are produced by a process that starts with a ceramic powder mixed with various binders and compacted (molded) into blanks under high pressure. It then is fired (sintered) a temperatures above 1000°C for several days. The blanks then are ground an polished on a diamond wheel to form a sharp edge, and the handle is attached The Mohs hardness of the zirconium oxide ceramic is 8.2, a compared with 6 for hardened steel and a maximum of 10 for diamond.

16. Example 8.1 Ceramic Knives Among the advantages of ceramics knives over steel knives: (a) Because o their very high hardness and wear resistance, ceramic knives can last months and even years before sharpening. (b) They are chemically inert. Consequently, the do not stain, food does not stick (hence are easy to clean), and they leave n metallic taste or smell. (c) Because they are lightweight, they are easier to use. The knives should be stored in wooden knife blocks, and handled carefully Sharp impact against other objects (such as dishes or dropping it on its edge on hard surface) should be avoided, as the sharp edges of the knife can chip. Also, they should be used only for cutting (not for prying) and in cutting meat, contact with bones is not advisable. Knives have to be sharpened at the factory to precise edge shape using diamond grinding wheels. Ceramic knives are more expensive than steel knives, typically ranging from $60 for a 75-mm paring knife to $250 for a 150-mm serrated knife.

17. 8.2.3 Other Ceramics • Major ceramics of other types may be classified as follows: Carbides • Typical carbides are those made of tungsten and titanium and of silicon. • Some examples of carbides are: • Tungsten carbide (WC) • Titanium carbide (TiC) • Silicon carbide (SiC)

18. 8.2.3 Other Ceramics Nitrides • Another class of ceramics is the nitrides, examples of which are: • Cubic boron nitride (cBN) • Titanium nitride (TiN) • Silicon nitride (Si3N4)

19. 8.2.3 Other Ceramics Sialon • Sialon consists of silicon nitride with various additions of aluminum oxide, yttrium oxide, and titanium carbide. • It has higher strength and thermal-shock resistance than silicon nitride.

20. 8.2.3 Other Ceramics Cermets • Cermets are combinations of a ceramic phase bonded with a metallic phase. • There is a combine of high-temperature oxidation resistance of ceramics with the toughness, thermal-shock resistance, and ductility of metals. • A common application of cermets is in cutting tools.

21. 8.2.4 Silica • Abundant in nature, silica is a polymorphic material—that is, it can have different crystal structures. • The most common form of silica is quartz, which is a hard, abrasive hexagonal crystal used extensively in communications applications as an oscillating crystal of fixed frequency because it exhibits the piezoelectric effect. • Silicates are products of the reaction of silica with oxides of aluminum, magnesium, calcium, potassium, sodium, and iron.

22. 8.2.4 Silica • Lithium aluminum silicate has a very low thermal expansion and thermal conductivity and good thermal-shock resistance. • It also has a very low strength and fatigue life, thus it is suitable only for nonstructural applications.

23. 8.2.5 Nanoceramics and composites • It is called nanoceramics or nanophase ceramics, these materials consist of atomic clusters containing a few thousand atoms. • Nanocrystalline second-phase particles (on the order of 100 nm or less) and fibers also are used as reinforcements in composites. • These composites have enhanced properties, such as tensile strength and creep resistance.

24. 8.3 General Properties and Applications of Ceramics • Compared to metals, ceramics typically have the following relative characteristics: brittleness; high strength and hardness at elevated temperatures; high elastic modulus; low toughness, density, thermal expansion; and low thermal and electrical conductivity. • Because of their sensitivity to flaws, defects, and surface or internal cracks; the presence of different types and levels of impurities; and different methods of manufacturing, ceramics can have a wide range of properties.

25. 8.3.1 Mechanical Properties • The mechanical properties of several engineering ceramics are presented in Table 8.2.

26. 8.3.1 Mechanical Properties • The tensile strength of a polycrystalline ceramic increases with decreasing grain size and porosity. • This relationship is represented approximately by the expression where P is the volume fraction of pores in the solid

27. 8.3.1 Mechanical Properties • The modulus of elasticity of ceramics is related approximately to its porosity by the expression where is the modulus at zero porosity.

28. 8.3.1 Mechanical Properties • Ceramic components that are to be subjected to tensile stresses may be prestressed in much the same way that concrete is prestressed • Prestressing the shaped ceramic components subjects them to compressive stresses. The methods used include: • Heat treatment and chemical tempering • Laser treatment of surfaces • Coating with ceramics having different thermal-expansion coefficients • Surface-finishing operations (such as grinding) in which compressive residual stresses are induced on the surfaces

29. 8.3.1 Mechanical Properties • Major advances have been made in improving the toughness and other properties of ceramics, including the development of machinable and grindable ceramics.

30. 8.3.2 Physical Properties • Most ceramics have a relatively low specific gravity, ranging from about 3 to 5.8 for oxide ceramics as compared to 7.86 for iron. • Thermal conductivity in ceramics varies by as much as three orders of magnitude (depending on their composition). • The thermal conductivity k is related to porosity by where is the thermal conductivity at zero porosity and P is the porosity as a fraction of the total volume.

31. 8.3.2 Physical Properties • The tendency toward thermal cracking (called spalling when a small piece or a layer from the surface breaks off) is lower with the combination of low thermal expansion and high thermal conductivity. • Another characteristic is the anisotropy of thermal expansion of oxide ceramics, when the thermal expansion varies with differing direction through the ceramic. This behavior causes thermal stresses that can lead to cracking of the ceramic component.

32. 8.3.3 Applications • Ceramics have numerous consumer and industrial applications. • The capability of ceramics to maintain their strength and stiffness at elevated temperatures makes them very attractive for high-temperature applications. • Coating metal with ceramics is another application; it may be done to reduce wear, prevent corrosion, or provide a thermal barrier. • Other attractive properties of ceramics are their low density and high elastic modulus.

33. 8.3.3 Applications • Their high resistance to wear makes them suitable for applications such as cylinder liners, bushings, seals, bearings, and liners for cylinders and gun barrels.

34. Example 8.2 Ceramic gun barrels The wear resistance and low density of ceramics have lead to investigation regarding their use as liners for gun barrels. Their limited success has lead to more recent developments in making composite ceramic gun barrels, which have improved performance over traditional steel barrels. The 50 calibre zirconia ceramic barrel is formed in several separate segments, each 150–200 mm long and with a wall thickness of 3.75 mm by the shaping and sintering processes

35. Example 8.2 Ceramic gun barrels The segments subsequently are machined to the required dimensions and surface finish. Zirconia has been chosen for its high toughness, flexural strength, specific heat, operating temperature, and very low thermal conductivity. The thermal properties are important to the performance of the barrel and the bullet. The separate ceramic segments are then joined, and the barrel is wrapped with a carbon-fiber/polymer-matrix composite which subjects the ceramic barrel to a compressive stress of 690 MPa, thus greatly improving its capacity to withstand tensile stresses developed during firing. The inside of the barrel is then rifled (cutting of internal spiral grooves to give rotation to the exiting bullet for gyroscopic stability) and fitted to a breech.

36. Example 8.3 Ceramic ball and roller bearings Silicon-nitride ceramic ball and roller bearings are used when high temperature, high speed, or marginally lubricated conditions occur. The bearings can be made entirely from ceramics or just the ball and rollers are ceramic and the races are metal, in which case they are referred to as hybrid bearings (Fig. 8.2). Examples of machines utilizing ceramic and hybrid bearings include high-performance machine

37. Example 8.3 Ceramic ball and roller bearings tool spindles, metal-can seaming heads, high-speed flow meters and the Space Shuttle’s main booster rocket’s liquid oxygen and hydrogen pumps. The ceramic spheres have a diameter tolerance of 0.13 mm and a surface roughness of 0.02 mm. They have high wear resistance, high fracture toughness, perform well with little or no lubrication, and have low density. The balls have a coefficient of thermal expansion one-fourth that of steel, and they can withstand temperatures of up to 1400°C. Produced from titanium and carbon nitride by using powder-metallurgy techniques, the full-density titanium carbonitride or silicon nitride bearing-grade material can be twice as hard as chromium steel and 40% lighter. Components up to 300 mm in diameter can be produced.

38. 8.3.3 Applications Bioceramics • Because of their strength and inertness, ceramics are used as biomaterials (bioceramics) to replace joints in the human body, as prosthetic devices, and in dental work.

39. 8.4 Glasses • Glass is an amorphous solid with the structure of a liquid. It has been supercooled (cooled at a rate too high to allow crystals to form). • All glasses contain at least 50% silica, which is known as a glass former. • Depending on their function, these oxides are known as intermediates (or modifiers).

40. 8.4 Glasses • Almost all commercial glasses are categorized by type (Table 8.3). • Soda-lime glass (the most common type) • Lead-alkali glass • Borosilicate glass • Aluminosilicate glass • 96%-silica glass • Fused silica glass

41. 8.4 Glasses

42. 8.4 Glasses • Glasses also can be referred to as hard or soft, usually in the sense of a thermal rather than mechanical property.

43. 8.4.2 Mechanical Properties • The behavior of glass, like that of most ceramics, generally is regarded as perfectly elastic and brittle. • Glass in bulk form generally has a strength of less than 140 MPa. • The strength of glass usually is measured by bending it. • The phenomenon of static fatigue, observed in ceramics, also is exhibited by glasses.

44. 8.4.3 Physical Properties • Glasses have low thermal conductivity, high electrical resistivity, and dielectric strength. • Their thermal expansion coefficients are lower than those for metals and plastics, and may even approach zero.

45. 8.5 Glass Ceramics • Although glasses are amorphous, glass ceramics have a high crystalline component to their microstructure. • Most glass ceramics are stronger than glass. • These products first are shaped and then heat treated, with devitrification (recrystallization) of the glass occurring. • They have a near-zero coefficient of thermal expansion; as a result, they have high thermal-shock resistance.

46. 8.6 Graphite • Graphite is a crystalline form of carbon having a layered structure with basal planes or sheets of close-packed carbon atoms. • Consequently, graphite is weak when sheared along the layers. • Amorphous graphite is known as lampblack (black soot) and is used as a pigment. • Graphite has high electrical and thermal conductivity and good resistance to thermal shock and to high temperature.

47. 8.6 Graphite Graphite Fibers • An important use of graphite is as fibers in reinforced plastics and composite materials. Carbon and Graphite Foams • These foams have important properties of high service temperatures, chemical inertness, low thermal expansion, and thermal and electrical properties that can be tailored for specific applications. • Carbon foams are available either in graphitic or nongraphitic structures.

48. 8.6 Graphite Buckyballs • A more recent development is the production of carbon molecules (usually C60) in the shape of a soccer ball, called buckyballs. • Also called fullerenes, these chemically-inert spherical molecules are produced from soot and act much like solid lubricant particles.

49. 8.7 Diamond • Diamond-like carbon (DLC) also has been developed and is used as a diamond film coating. • Diamond particles also can be coated with nickel, copper, or titanium for improved performance in grinding operations. • Because of its favorable characteristics, diamond has many important applications, such as: • Cutting-tool materials, as a single crystal or in polycrystalline form • Abrasives in grinding wheels, for grinding hard materials • Dressing of grinding wheels (i.e., sharpening of the abrasive grains) • Dies for drawing wire less than 0.06 mm in diameter • Coatings for cutting tools and dies

50. Concept Summary • Several nonmetallic materials are of major importance both in engineering applications and in manufacturing processes. Ceramics, which are compounds of metallic and nonmetallic elements, generally are characterized by high hardness, high compressive strength, high elastic modulus, low thermal expansion, high temperature resistance, good chemical inertness, low density, and low thermal and electrical conductivity. On the other hand, they are brittle and have low toughness. Nanophase ceramics have better properties than common ceramics. Porosity in ceramics has important effects on their properties.