Chemistry of Engineering Materials PDF

Chemistry of Engineering Materials What is Engineering Materials? refers to the group of materials that are used in the construction of manmade structures and components. The primary function of an engineering material is to withstand applied loading without breaking and without exhibiting exc...

Chemistry of Engineering Materials What is Engineering Materials? refers to the group of materials that are used in the construction of manmade structures and components. The primary function of an engineering material is to withstand applied loading without breaking and without exhibiting excessive deflection. The major classifications of engineering materials include metals, polymers, ceramics, and composites. Why Material Science & Engineering is important to technologists? 1. Mechanical engineers search for high temp material so that gas turbines, jet engines etc. can operate more efficiently and wear resistance materials to manufacture bearing materials. Why Material Science & Engineering is important to technologists? 2. Electrical engineers search for materials by which electrical devices or machines can be operated at a faster rate with minimum power losses. 3. Aerospace & automobile engineers search for materials having high strength-to weight ratio. Why Material Science & Engineering is important to technologists? 4. Electronic engineers search for material that are useful in the fabrication & miniaturization of electronic devices 5. Chemical engineers search for highly corrosion-resistant materials Why Material Science & Engineering is important to technologists? 4. To a civil engineer the performance of materials in structures and their ability to resist various stresses are of prime importance. Selection of Materials for Engineering Purposes The best material is one which serve the desired objective at the minimum cost. The following factors should be considered while selecting the material: 1. Availability of the materials. 2. Suitability of the materials for the working conditions in service. 3. The cost of the materials. Structure of Metals Atoms An atom is the smallest unit of matter that retains all of the chemical properties of an element. Atoms combine to form molecules, which then interact to form solids, gases, or liquids. Structure of an Atom Atoms consist of three basic particles: protons, electrons, and neutrons. The nucleus (center) of the atom contains the protons (positively charged) and the neutrons (no charge). The outermost regions of the atom are called electron shells and contain the electrons (negatively charged). Atoms have different properties based on the arrangement and number of their basic particles. Atomic Number and Mass Number The atomic number is the number of protons in an element, while the mass number is the number of protons plus the number of neutrons No. of protons = atomic number No. of electrons (neutral atom) = no. of protons Mass number = no. of protons + no. of neutron No. of Neutrons = mass number – atomic number Quantum Numbers Quantum numbers is used describe the distribution of electrons in the atom. The three coordinates that come from Schrodinger's wave equations are the principal (n), angular (l), and magnetic (m) quantum numbers. These quantum numbers describe the size, shape, and orientation in space of the orbitals on an atom. Quantum Numbers The principal quantum number (n) designates the principal electron shell. Because n describes the most probable distance of the electrons from the nucleus, the larger the number n is, the farther the electron is from the nucleus, the larger the size of the orbital, and the larger the atom is. n can be any positive integer starting at 1, as n=1 designates the first principal shell (the innermost shell). The first principal shell is also called the ground state, or lowest energy state. This explains why n cannot be 0 or any negative integer, because there exist no atoms with zero or a negative amount of energy levels/principal shells The angular quantum number (l) describes the shape of the orbital. Orbitals have shapes that are best described as spherical (l = 0), polar (l = 1), or cloverleaf (l = 2). They can even take on more complex shapes as the value of the angular quantum number becomes larger. Quantum Numbers The angular quantum number (l) describes the shape of the orbital. Orbitals have shapes that are best described as spherical (l = 0), polar (l = 1), or cloverleaf (l = 2). They can even take on more complex shapes as the value of the angular quantum number becomes larger. Quantum Numbers The magnetic quantum number (ml) determines the number of orbitals and their orientation within a subshell. Consequently, its value depends on the orbital angular momentum quantum number l. Given a certain l, ml is an interval ranging from –l to +l , so it can be zero, a negative integer, or a positive integer. ml = -l, (-l + 1), (-l + 2), …, -2, -1, 0, 1, 2,…, (l -1), (l -2), +l Example If n=3, and l =2, then what are the possible values of ml ? Since ml must range from –l to +l, then ml can be: -2, -1, 0, 1, or 2. Electron Configurations The electron configuration of an atom describes the orbitals occupied by electrons on the atom. One rule governing electron configuration is the Aufbau Principle, which states that each successive electron occupies the lowest energy orbital available. Aufbau principle The Aufbau principle dictates the manner in which electrons are filled in the atomic orbitals of an atom in its ground state. It states that electrons are filled into atomic orbitals in the increasing order of orbital energy level. According to the Aufbau principle, the available atomic orbitals with the lowest energy levels are occupied before those with higher energy levels. Example 3. Write the electron configuration of boron The order of increasing energy of the orbitals is then read off by following these arrows, starting at the top of the first line and then proceeding on to the second, third, fourth lines, and so on. This diagram predicts the following order of increasing energy for atomic orbitals. Pauli Exclusion Principle The Pauli Exclusion Principle states that, in an atom or molecule, no two electrons can have the same four electronic quantum numbers. As an orbital can contain a maximum of only two electrons, the two electrons must have opposing spins. This means if one is assigned an up-spin (+ 1/ 2 ), the other must be downspin (- 1 /2 ). Hund's Rule states that: Every orbital in a sublevel is singly occupied before any orbital is doubly occupied. All of the electrons in singly occupied orbitals have the same spin (to maximize total spin). Chemical Bonding Chemical compounds are formed by the joining of two or more atoms. A chemical bond is the physical phenomenon of chemical substances being held together by attraction of atoms to each other through sharing, as well as exchanging, of electrons -or electrostatic forces. Covalent Bonds Covalent chemical bonds involve the sharing of a pair of valence electrons by two atoms. Such bonds lead to stable molecules if they share electrons in such a way as to create a noble gas configuration for each atom. Covalent Bonds Ionic Bonds For atoms with the largest electronegativity differences (such as metals bonding with nonmetals), the bonding interaction is called ionic, and the valence electrons are typically represented as being transferred from the metal atom to the nonmetal. Once the electrons have been transferred to the non-metal, both the metal and the non-metal are considered to be ions. The two oppositely charged ions attract each other to form an ionic compound. Metallic bond A metallic bond is the sharing of many detached electrons between many positive ions, where the electrons act as a "glue" giving the substance a definite structure. Metals have low ionization energy. Therefore, the valence electrons can be delocalized throughout the metals. Delocalized electrons are not associated with a particular nucleus of a metal, instead, they are free to move throughout the whole crystalline structure forming a "sea" of electrons. Metallic bond Metallic bonds are the chemical bonds that hold atoms together in metals. They differ from covalent and ionic bonds because the electrons in metallic bonding are delocalized, that is, they are not shared between only two atoms. Instead, the electrons in metallic bonds float freely through the lattice of metal nuclei. This type of bonding gives metals many unique material properties, including excellent thermal and electrical conductivity, high melting points, and malleability. Metallic bond These are formed when the valence electrons of metal atoms are shared by more than one neighbouring atom. The metal atoms are held together by a “sea” of electrons floating around. Metals consist of a lattice of positive ions through which a cloud of electrons moves. The positive ions will tend to repel one another, but are held together by the negatively charged electron cloud. The mobile electrons, known as conduction electrons, can transfer thermal vibration from one part of the structure to another i.e., metals can conduct heat. They are good conductors of electricity also. Metallic bond Conductivity of Metals Metals conduct heat well because of the sea of delocalized electrons. When you heat the metal, the atoms vibrate. Because the electrons are not bound to a certain atom, they can vibrate more freely, cause more repercussions, and travel, more quickly through the metal. The vibrations are passed from one atom to another very rapidly. Cl Cl Conductivity of Metals The electrons drift slowly through the structure as the metal is heated. As the metal heats up, the electrons move faster; they travel colliding with both atoms and other electrons. Thus, the heat is passed quickly through the metal. Electrical Conductivity of Metals Electricity is energy created by the free or controlled movement of charged particles such as electrons. In other words, electricity is energy created by electrons in motion. Because the valence electrons in metals are relatively free to move about, when you apply a negative charge to the end of a piece of metal and a positive charge to the other end, the free (delocalized) electrons move away from the negative charge and toward the positive charge. Electrical Conductivity of Metals Malleability and Ductility Ductility is a solid material's ability to change shape under tensile stress. Tensile stress is the stress on an object that results from pulling or stretching (think of the word “tension”). This property is often characterized by the ability of the material to be stretched into a wire. Malleability is the material's capacity to change shapes under compressive stress. Malleability is often characterized by the capacity of the material to form a thin sheet when it is hammered or rolled. Malleability and Ductility The delocalized electrons of the metallic bond in the 'sea' of electrons allow the metal atoms to roll over each other when a stress is applied. Because of this ability, the metal can be hammered into sheets (malleable) or pulled into wires (ductility), depending on the type of stress. Physical Properties of Metals The physical properties of the metals include appearance, luster, color, size and shape, weight, density, melting point, boiling point and freezing point, glass transition temperature and permeability. Mechanical Properties of Metals The mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical forces and load. These mechanical properties of the metal include strength, stiffness, elasticity, plasticity, ductility, brittleness, malleability, toughness, resilience, creep and hardness. 1. Strength. It is the ability of a material to resist the externally applied forces without breaking or yielding. The internal resistance offered by a part to an externally applied force is called stress. 2.Stiffness. It is the ability of a material to resist deformation under stress. The modulus of elasticity is the measure of stiffness. 3.Elasticity. It is the property of a material to regain its original shape after deformation when the external forces are removed. This property is desirable for materials used in tool 4.Plasticity. It is property of a material which retains the deformation produced under load permanently. This property of the material is necessary for forgings, in stamping images on coins and in ornamental work. Mechanical Properties of Metals 5. Ductility. It is the property of a material enabling it to be drawn into wire with the application of a tensile force. A ductile material must be both strong and plastic. The ductility is usually measured by the terms, percentage elongation and percentage reduction in area. The ductile material commonly used in engineering practice (in order of diminishing ductility) are mild steel, copper, aluminium, nickel, zinc, tin and lead 6. Brittleness. It is the property of a material opposite to ductility. It is the property of breaking of a material with little permanent distortion. Brittle materials when subjected to tensile loads, snap off without giving any sensible elongation. Cast iron is a brittle material. 7. Malleability. It is a special case of ductility which permits materials to be rolled or hammered into thin sheets. A malleable material should be plastic but it is not essential to be so strong. The malleable materials commonly used in engineering practice (in order of diminishing malleability) are lead, soft steel, wrought iron, copper and aluminium. Mechanical Properties of Metals 8. Toughness. It is the property of a material to resist fracture due to high impact loads like hammer blows. The toughness of the material decreases when it is heated. It is measured by the amount of energy that a unit volume of the material has absorbed after being stressed up to the point of fracture. This property is desirable in parts subjected to shock and impact loads. 9. Machinability. It is the property of a material which refers to a relative case with which a material can be cut. The machinability of a material can be measured in a number of ways such as comparing the tool life for cutting different materials or thrust required to remove the material at some given rate or the energy required to remove a unit volume of the material. It may be noted that brass can be easily machined than steel 10. Resilience. It is the property of a material to absorb energy and to resist shock and impact loads. It is measured by the amount of energy absorbed per unit volume within elastic limit. This property is essential for spring materials. Mechanical Properties of Metals 11. Creep. When a part is subjected to a constant stress at high temperature for a long period of time, it will undergo a slow and permanent deformation called creep. This property is conside 12.Fatigue. When a material is subjected to repeated stresses, it fails at stresses below the yield point stresses. Such type of failure of a material is known as fatigue. The failure is caused by means of a progressive crack formation which are usually fine and of microscopic size. This property is considered in designing shafts, connecting rods, springs, gears, etc 11. Hardness. It is a very important property of the metals and has a wide variety of meanings. It embraces many different properties such as resistance to wear, scratching, deformation and machinability etc. It also means the ability of a metal to cut another metal. The hardness is usually expressed in numbers which are dependent on the method of making the test. The hardness of a metal may be determined by the following tests : (a) Brinell hardness test, (b) Rockwell hardness test, (c) Vickers hardness (also called Diamond Pyramid) test, and (d) Shore scleroscope Thermal Properties of Metals The thermal properties include thermal conductivity, expansion coefficient, resistivity, thermal shock resistance, thermal diffusivity. Electrical Properties of Metals The electrical properties include conductivity, resistivity, dielectric strength, thermoelectricity, superconductivity, electric hysteresis. Magnetic Properties of Metals The magnetic properties include ferromagnetism, paramagnetism, diamagnetism, magnetic permeability, coercive force, curie temperature, magnetic hysteresis Chemical Properties of Metals The chemical properties include reactivity, corrosion resistance, polymerization, composition, acidity, alkalinity. Optical Properties of Metals The optical properties include reflectivity, refractivity, absorptivity, transparency, opaqueness, color, luster. Metallurgical Properties of Metals The metallurgical properties include grain size, heat treatment done / required, anisotropy, hardenability. Classification of Engineering Materials Engineering materials are classified into the following broad groups: It is the systematic arrangement or division of materials into groups on the basis of some common characteristic According to General Properties According to Nature of Materials According to Applications Metals Metals are the most commonly used class of engineering material. Metal alloys are especially common, and they are formed by combining a metal with one or more other metallic and/or non-metallic materials. The combination usually occurs through a process of melting, mixing, and cooling. The goal of alloying is to improve the properties of the base material in some desirable way. Metal alloy compositions are described in terms of the percentages of the various elements in the alloy, where the percentages are measured by weight. Ferrous metals These are metals and alloys containing a high proportion of the element iron They are the strongest materials available and are used for applications where high strength is required at relatively low cost and where weight is not of primary importance. As an example of ferrous metals such as: bridge building, the structure of large buildings, railway lines, locomotives and rolling stock and the bodies and highly stressed engine parts of road vehicles. Ferrous Alloys Ferrous alloys have iron as the base element. These alloys and include steels and cast irons. Ferrous alloys are the most common metal alloys in use due to the abundance of iron, ease of production, and high versatility of the material. The biggest disadvantage of many ferrous alloys is low corrosion resistance Carbon Steel Carbon is an important alloying element in all ferrous alloys. In general, higher levels of carbon increase strength and hardness, and decrease ductility and weldability. Carbon steels are basically just mixtures of iron and carbon. They may contain small amounts of other elements, but carbon is the primary alloying ingredient. The effect of adding carbon is an increase in strength and hardness. Most carbon steels are plain carbon steels, of which there are several types Low-Carbon Steel Low-carbon steel has less than about 0.30% carbon. It is characterized by low strength but high ductility. Some strengthening can be achieved through cold working, but it does not respond well to heat treatment. Low-carbon steel is very weldable and is inexpensive to produce. Common uses for low-carbon steel include wire, structural shapes, machine parts, and sheet metal. Medium-Carbon Steel Medium-carbon steel contains between about 0.30% to 0.70% carbon. It can be heat treated to increase strength, especially with the higher carbon contents. Medium-carbon steel is frequently used for axles, gears, shafts, and machine parts. High-Carbon Steel High-carbon steel contains between about 0.70% to 1.40% carbon. It has high strength but low ductility. Common uses include drills, cutting tools, knives, and springs. Low-Alloy Steel Low-alloy steels, also commonly called alloy steels, contain less than about 8% total alloying ingredients. Low-alloy steels are typically stronger than carbon steels and have better corrosion resistance Tool steels are primarily used to make tooling for use in manufacturing, for example cutting tools, drill bits, punches, dies, and chisels. Alloying elements are typically chosen to optimize hardness, wear resistance, and toughness. Stainless Steel Stainless steels have good corrosion resistance, mostly due to the addition of chromium as an alloying ingredient. Stainless steels have a chromium composition of at least 11%. Passivation occurs with chromium content at or above 12%, in which case a protective inert film of chromic oxide forms over the material and prevents oxidation. The corrosion resistance of stainless steel is a result of this passivation. Cast Iron Cast iron is a ferrous alloy containing high levels of carbon, generally greater than 2%. The carbon present in the cast iron can take the form of graphite or carbide. Cast irons have a low melting temperature which makes them well suited to casting. They also have better flow characteristics when molten helping them to fill the mould more easily. Cast iron has been used for many applications as engine blocks and gears. Advantages Better corrosion resistance than steels in most environments., Very high strength in compression, Very easy to cast Disadvantages Very brittle, Poor weldability Gray Cast Iron Gray cast iron is the most common type. The carbon is in the form of graphite flakes. Gray cast iron is a brittle material and its compressive strength is much higher than its tensile strength. The fracture surface of gray cast iron has a gray color, which is how it got its name. It has a low tensile strength, high compressive strength and no ductility. A very good property of grey cast iron is that the free graphite in its structure acts as a lubricant. Due to this reason, it is very suitable for those parts where sliding action is desired. It can be machined reasonably but cannot be welded easily Ductile Cast Iron (Nodular Cast Iron) The addition of magnesium to gray cast iron improves the ductility of the material. The resulting material is called nodular cast iron because the magnesium causes the graphite flakes to form into spherical nodules. It is also called ductile cast iron. Nodular cast iron has good strength, ductility, and machinability. Common uses include crankshafts, gears, pump bodies, valves, and machine parts. White Cast Iron White cast iron has carbon in the form of carbide (known as cementite), which is the hardest constituent of iron. It has a high tensile strength and low compressive strength. Since it is hard, therefore, it cannot be machined with ordinary cutting tools but requires grinding as shaping process. White cast iron is primarily used for wear-resisting components as well as for the production of malleable cast iron. Malleable Cast Iron Malleable cast iron is produced by heat treating white cast iron. The heat treatment improves the ductility of the material while maintaining its high strength. It is used for machine parts for which the steel forgings would be too expensive and in which the metal should have a fair degree of accuracy, e.g. hubs of wagon wheels, small fittings for railway rolling stock, brake supports, parts of agricultural machinery, pipe fittings, door hinges, locks etc. Wrought iron It is the purest iron which contains at least 99.5% iron but may contain up to 99.9% iron. The wrought iron is a tough, malleable and ductile material. It cannot stand sudden and excessive shocks. It can be easily forged or welded. It is used for chains, crane hooks, railway couplings, water and steam pipes. Non – ferrous metals These materials refer to the remaining metals known to mankind. The pure metals are rarely used as structural materials as they lack mechanical strength. They are mainly used with other metals to improve their strength. They are used where their special properties such as corrosion resistance, electrical conductivity and thermal conductivity are required. The non-ferrous metals are usually employed in industry due to the following characteristics: Ease of fabrication (casting, rolling, forging, welding and machining), Resistance to corrosion, Electrical and thermal conductivity Weight Classification of non-ferrous metals and alloys Aluminium It is white metal produced by electrical processes from its oxide (alumina), which is prepared from a clayey mineral called bauxite. It is a light metal having specific gravity 2.7 and melting point 658°C. The tensile strength of the metal varies from 90 MPa to 150 MPa. Aluminium is a strong silver metal. It turns easily being softer than steel. In its pure state, the metal would be weak and soft for most purposes, but when mixed with small amounts of other alloys, it becomes hard and rigid. So, it may be blanked, formed, drawn, turned, cast, forged and die cast. Its good electrical conductivity is an important property and is widely used for overhead cables. The high resistance to corrosion and its non-toxicity makes it a useful metal for cooking utensils under ordinary condition and thin foils are used for wrapping food items. It is extensively used in aircraft and automobile components where saving of weight is an advantage. Aluminium has a low melting point and it is the metal used for sand casting. Aluminum Alloys The aluminium may be alloyed with one or more other elements like copper, magnesium, manganese, silicon and nickel. The addition of small quantities of alloying elements converts the soft and weak metal into hard and strong metal, while still retaining its light weight The main aluminium alloys are: Duralumin. It is light strong alloy of aluminium, copper, manganese and magnesium. It is composed of 95% aluminium, 4% copper, 0.5% manganese and 0.5% magnesium. This alloy possesses maximum tensile strength (up to 400 MPa) after heat treatment and age hardening. After working, if the metal is allowed to age for 3 or 4 days, it will be hardened. This phenomenon is known as age hardening. Y-alloy. It is also called copper-aluminium alloy. The addition of 4% copper to pure aluminium increases its strength and machinability. It is mainly used for cast purposes and used in aircraft engines for cylinder heads and pistons. Magnalium. It is made by melting the aluminium with 2 to 10% magnesium in a vacuum and then cooling it in a vacuum or under a pressure of 100 to 200 atmospheres. It also contains about 1.75% copper. Due to its light weight and good mechanical properties, it is mainly used for aircraft and automobile components. 4. Hindalium. It is an alloy of aluminium and magnesium with a small quantity of chromium. It is the trade name of aluminium alloy produced by Hindustan Aluminium Corporation Ltd, Renukoot (U.P.). It is produced as a rolled product in 16 gauge, mainly for anodized utensil manufacture. Nickel Alloys Nickel alloys have high temperature and corrosion resistance. Common alloying ingredients include copper, chromium, and iron. Common nickel alloys include Monel, K-Monel, Inconel, and Hastelloy Copper It is one of the most widely used non-ferrous metals in industry. It is a soft, malleable and ductile material with a reddish-brown appearance. Its specific gravity is 8.9 and melting point is 1083°C. The tensile strength varies from 150 MPa to 400 MPa under different conditions. It is a good conductor of electricity. It is largely used in making electric cables and wires for electric machinery and appliances, in electrotyping and electroplating, in making coins and household utensils. It may be cast, forged, rolled and drawn into wires. It is non-corrosive under ordinary conditions and resists weather very effectively. Copper in the form of tubes is used widely in mechanical engineering. It is also used for making ammunitions. It is used for making useful alloys with tin, zinc, nickel and aluminium. Copper Alloys The copper alloys are broadly classified into the following two groups: Copper-zinc alloys (Brass). The most widely used copper-zinc alloy is brass. There are various types of brasses, depending upon the proportions of copper and zinc. Yellow or gold in color. This is fundamentally a binary alloy of copper with zinc each 50%. Brasses are very resistant to atmospheric corrosion and can be easily soldered. They can be easily fabricated by processes like spinning and can also be electroplated with metals like nickel and chromium. Brass are used for decorative effect and used to produce bushes as it is resistant to wear. Bronze (copper and tin) is also used for bushes. Brass spelter and wire is used when brazing. Copper-tin alloys (Bronze). The alloys of copper and tin are usually termed as bronzes. The useful range of composition is 75 to 95% copper and 5 to 25% tin. The metal is comparatively hard, resists surface wear and can be shaped or rolled into wires, rods and sheets very easily. In corrosion resistant properties, bronzes are superior to brasses. Gun Metal It is an alloy of copper, tin and zinc. It usually contains 88% copper, 10% tin and 2% zinc. This metal is also known as Admiralty gun metal. The zinc is added to clean the metal and to increase its fluidity. The metal is very strong and resistant to corrosion by water and atmosphere. Originally, it was made for casting guns. It is extensively used for casting boiler fittings, bushes, bearings, glands, etc. Lead It is a bluish grey metal having specific gravity 11.36 and melting point 326°C. It is so soft that it can be cut with a knife. It has no tenacity. It is extensively used for making solders, as a lining for acid tanks, cisterns, water pipes, and as coating for electrical cables. The lead base alloys are employed where a cheap and corrosion resistant material is required. Tin It is brightly shining white metal. It is soft, malleable and ductile. It can be rolled into very thin sheets. It is used for making important alloys, fine solder, as a protective coating for iron and steel sheets and for making tin foil used as moisture proof packing. Zinc Base Alloys The most of the die castings are produced from zinc base alloys. These alloys can be casted easily with a good finish at fairly low temperatures. They have also considerable strength and are low in cost. The usual alloying elements for zinc are aluminium, copper and magnesium and they are all held in close limits. These alloys are widely used in the automotive industry and for other high production markets such as washing machines, oil burners, refrigerators, radios, photographs, television, business machines, etc. Nickel Base Alloys The nickel base alloys are widely used in engineering industry on account of their high mechanical strength properties, corrosion resistance, etc. 1. Monel metal. It is an important alloy of nickel and copper. It has a tensile strength from 390 MPa to 460 MPa. It resembles nickel in appearance and is strong, ductile and tough. It is superior to brass and bronze in corrosion resisting properties. It is used for making propellers, pump fittings, condenser tubes, steam turbine blades, sea water exposed parts, tanks and chemical and food handling plants 2. Inconel. It consists of 80% nickel, 14% chromium, and 6% iron. Its specific gravity is 8.55 and melting point 1395°C. This alloy has excellent mechanical properties at ordinary and elevated temperatures. It can be cast, rolled and cold drawn. It is used for making springs which have to withstand high temperatures and are exposed to corrosive action. It is also used for exhaust manifolds of aircraft engines. Nickel Base Alloys 3. Nichrome. It consists of 65% nickel, 15% chromium and 20% iron. It has high heat and oxidation resistance. It is used in making electrical resistance wire for electric furnaces and heating elements. 4. Nimonic. It consists of 80% nickel and 20% chromium. It has high strength and ability to operate under intermittent heating and cooling conditions. It is widely used in gas turbine engines. Non – metallic material Synthetic materials These are non – metallic materials that do not exist in nature, although they are manufactured from natural substances such as oil, coal and clay. They combine good corrosion resistance with ease of manufacture by moulding to shape and relatively low cost. Synthetic adhesives are also being used for the joining of metallic components even in highly stressed applications. Nickel Base Alloys Wood: This is naturally occurring fibrous composite material used for the manufacture of casting patterns. Rubber: This is used for hydraulic and compressed air hoses and oil seals. Naturally occurring latex is too soft for most engineering uses but it is used widely for vehicle tyres when it is compounded with carbon black. Glass: This is a hardwearing, abrasion-resistant material with excellent weathering properties. It is used for electrical insulators, laboratory equipment, optical components in measuring instruments etc and, in the form of fibers, is used to reinforce plastics. It is made by melting together the naturally occurring materials: silica (sand), limestone (calcium carbonate) and soda (sodium carbonate) Emery: This is a widely used abrasive and is a naturally occurring aluminum oxide. Nowadays it is produced synthetically to maintain uniform quality and performance. Ceramic: These are produced by baking naturally occurring clays at high temperatures after moulding to shape. They are used for high – voltage insulators and high – temperature – resistant cutting tool tips. Diamonds: These can be used for cutting tools for operation at high speeds for metal finishing where surface finish is greater importance. For example, internal combustion engine pistons and bearings. They are also used for dressing grinding wheels. Oils: Used as bearing lubricants, cutting fluids and fuels. Silicon: This is used as an alloying element and also for the manufacture of semiconductor devices. Ceramics Ceramics are solid compounds that may consist of metallic or nonmetallic elements. The primary classifications of ceramics include glasses, cements, clay products, refractories, and abrasives. Characteristic of ceramics: Brittleness High thermal and electrical resistance High resistance to corrosion Opaque High temperature stability Uses of Ceramics Food storage Roofing tiles Bricks Sewer pipes Insulators in electric equipment and light fixtures Oven walls Space shuttle insulation Food storage Roofing tiles Bricks Sewer pipes Insulators in electric equipment and light fixtures Oven walls Space shuttle insulation Glass Glasses are common materials and are seen in applications including windows, lenses, and containers. Glasses are amorphous, whereas the other ceramics are mainly crystalline. Primary advantages of glasses include transparency and ease of fabrication. The base element of most glasses is silica, and other components can be added to modify its properties. Common processes used to form glass include: heating until melting, then pouring into mould to cast into useful shapes heating until soft, then rolling heating until soft, then blowing into desired shapes Cements Cements are materials that, after mixing with water, form a paste that then hardens. Because of this characteristic, cements can be formed into useful shapes while in paste form before they harden into rigid structures. Plaster of Paris is one common cement. The most common cement is called Portland cement, which is made by mixing clay and limestone and then firing at high temperature. Portland cement is used to form concrete, which is made by mixing it with sand, gravel, and water. It can also be mixed with sand and water to form mortar. Like other ceramics, cements are weak in tension but strong in compression. Cement is very inexpensive to produce, and it used widely in the construction of buildings, bridges, and other large structures. Clay Products Clay is a very common ceramic material. It can be mixed with water, shaped, and then hardened through firing at high temperature. The two primary classifications of clay products include structural clay products and whitewares. Structural clay products include bricks, tiles, and piping. Whitewares include pottery and plumbing fixtures. Refractories Refractory ceramics can withstand high temperatures and extreme environments. They can also provide thermal insulation. Brick is the most common refractory ceramic. Abrasives Abrasive ceramics are hard materials that are used to cut, grind, and wear away other softer materials. Typical properties of abrasives include high hardness, wear resistance, and temperature resistance. Abrasives can either be bonded to a surface (e.g. grinding wheels and sand paper), or can be used as loose grains (e.g. sand blasting). Common abrasives include cemented carbide, silicon carbide, tungsten carbide, aluminum oxide, and silica sand. Diamond is also an excellent abrasive, but it is expensive Factors affecting materials properties: 1. Heat treatment. This is the controlled heating and cooling of metals to change their properties to improve their performance or to facilitate processing. 2. Processing. Metal is hot worked or cold worked depending upon the temperature at which it is flow formed to shape. These temperatures are not easy to define - for instance, lead hot works at room temperature and can be beaten into complex shapes without cracking 3. Environmental reactions. The properties of materials can also be affected by reaction with environment in which they are used. Resting of steel Unless steel structures are regularly maintained by rest neutralization and painting process, resting will occur. The rest will eat into the steel, reduce its thickness and, therefore, its strength. In extreme cases an entire structure made from steel may be eaten away. Dezincification of brass Brass is an alloy of copper and zinc and when brass is exposed to a marine environment for a long time, the salt in the sea water pray react with the zinc content of the brass so as remove it and leave it behind on spongy, porous mass of copper. This obviously weakness the material which fails under normal working conditions. Degradation of plastic Many plastics degrade and become weak and brittle when exposed to the ultraviolet content of sunlight. Special dyestuffs have to be incorporated into the plastic to filter out these harmful rays. Polymers Polymers are materials that consist of molecules formed by long chains of repeating units. They may be natural or synthetic. Many useful engineering materials are polymers, such as plastics, rubbers, fibers, adhesives, and coatings. Polymers are classified as thermoplastic polymers, thermosetting polymers (thermosets), and elastomers. Thermoplastic Polymers The classification of thermoplastics and thermosets is based on their response to heat. If heat is applied to a thermoplastic, it will soften and melt. Once it is cooled, it will return to solid form. Thermoplastics do not experience any chemical change through repeated heating and cooling (unless the temperature is high enough to break the molecular bonds). They are therefore very well suited to injection moulding. Thermosetting Polymers Thermosets are typically heated during initial processing, after which they become permanently hard. Thermosets will not melt upon reheating. If the applied heat becomes extreme however, the thermoset will degrade due to breaking of the molecular bonds. Thermosets typically have greater hardness and strength than thermoplastics. They also typically have better dimensional stability than thermoplastics, meaning that they are better at maintaining their original dimensions when subjected to temperature and moisture changes. Elastomers Elastomers are highly elastic polymers with mechanical properties similar to rubber. Elastomers are commonly used for seals, adhesives, hoses, belts, and other flexible parts. The strength and stiffness of rubber can be increased through a process called vulcanization, which involves adding sulfur and subjecting the material to high temperature and pressure. This process causes cross-links to form between the polymer chains