ENGINEERING MATERIALS

Engineering Materials consist of:-

1. Cementing and binding materials

2. Lime

3. Cement

4. Gypsum plasters

5. Ceramics

6. Glass

7. Clay Products

8. Refractories

9. Abrasives

10. Composite

11. Adhesive

12. Lubricants

13. Rocket fuels

14. insulators

The field of application of a particular engineering material are governed by the characteristics and various properties such as:-

1. Chemical properties: Reactivity, Solubility, chemical effects like corrosion, acidity

2. Electrical properties: Insulations, Dielectric strength

3. Mechanical properties: Elasticity, hardness.

4. Optical properties: Transmission, refractivity, reflectivity.

5. Physical Properties: Bulk density, durability, porosity, fire resistance.

6. Thermal properties: Thermal expansion, specific heat

7. Technological properties: Castability, Weldability

Cementing and Building materialsThese are inorganic materials.Having the tendency of setting and hardening

on mixing with water or air.Two types:

(i) Hydraulic cementing materials: Portland cement

(ii) Non-Hydraulic cementing materials: Lime

Lime

LIME:-

Lime is calcium oxide obtained by calcination (heating to red hot in the presence of air) of naturally occurring CaCO3 in the form of limestone, chalk, marble to about 900: C so that all the CO2 and moisture content removed from it.

CaCO3 calcination CaO + CO2

Types of Lime

Different types of lime obtained depending of the chemical composition of lime stone. Limestone usually contain magnesium carbonate, aluminium oxide, iron and silicon oxide.

Four Types:

1. Fat lime or high-calcium lime

2. Dolomitic or high-magnesia lime

3. Hydraulic lime

4. Lean or poor lime

Fat Lime

Contain 95% CaO and less than 2% oxides of iron, aluminium and silicon.

Non-hydraulic cementing material, setting occur only by drying.

During its slaking large amount of heat is evolved.

Uses: 1) Whitewashing and plastering the walls.

2) With sand form lime mortar to fill the joints.

3) Used in metallurgical processes

4) Used for water softening

5) Used in glass industry

Dolomitic Lime

Obtained from naturally occurring dolomite (CaCO3.MgCO3).

It contain more than 25% of MgO. It slakes very slowly with less heat

evolution. It yields plastic mortar with an easy

and smooth working. Uses: Used in the formation of basic

refractories and as a flux in metallurgical process.

Hydraulic lime

It is obtained from the lime stone containing 5-30% clay.

Due to hydraulic properties set to hard mass on immersing in water.

It slakes with difficulty Uses: Used as a substitute for

cementing the thick walls and marine works.

Lean or poor lime

It contain 70-80% CaO, >5% MgO and smaller proportion of silica and alumina.

It slakes slowly and shows a behavior in b/w fat lime and dolomitic lime.

Uses: It is used to make mortar for interior works and plaster finishing.

Manufacture of lime

It is manufactured by calcination of limestone in vertical kiln using coal or producer gas as fuel.

Properties of Lime

Slaking – Exothermic reaction. (280 kcal/kg) (volume increases to 2-21/2 times)

Plasticity – Easy spreading (MgO improves plasticity)

Sand carrying capacity – to reduce the shrinkage of lime (fat lime > dolomitic lime) (CaO has greater sand carrying capacity)

Setting and hardening – Involve hydration and carbonation

Hardness – High Mg content better hardness

Gypsum Plaster

Gypsum (CaSO4.2H2O) CaSO4.2H2O

CaSO4.1/2H2O

CaO + SO3 CaSO4

Hemi hydrate

Anhydrite

150 : C

600: C

800 : C

Applications of Gypsum

1. Plaster of Paris: It is CaSO4.1/2H2O form of gypsum. Used in making plaster casting moulds, indoor wall plastering. In plaster boards.

2. Keen’s plaster: It is CaSO4 form of gypsum. It is less soluble in water as compared to hemi-hydrate so set and harden slowly. Used for plastering exterior walls.

3. Estrich Plaster: It is obtained by heating gypsum above 800 : C . Set and hardened very slowly.

Cement: It is a lime based product having adhesive and cohesive properties.

Manufacturing of Cement

Chemical Composition of CementThe cement is usually expressed in terms of

CaO, SiO2, Al2O3, Fe2O3. These oxides exist in the form of four phases :-

1. Dicalcium silicate, 2CaO.SiO2 (abbreviated As C2S)

2. Tricalcium silicate, 3CaO.SiO2 (abbreviated As C3S)

3. Tricalcium aluminate, 3CaO.Al2O3 (abbreviated As C3A)

4. Tetracalcium aluminoferrite, 4CaO.Al2O3.Fe2O3 (abbreviated As C4AF)

Points to remember

The free oxides of CaO, MgO, K2O, Na2O, TiO, SO2, H2O, CO2 constitute the remaining 10% weight of cement.

Mg rich dolomitic limestone is not suitable for cement manufacturing since the MgO being less basic than CaO does not readily react with the acidic oxides of clay and remains as such in the clinker.

Setting and Hardening of cementThe process of setting occur in two

stages:-1. Initial set: It depends on temp.

and quantity of H2O. For portland cement it is 45 minutes

2. Final setting: It occur over a few hours (about 10 hours for portland cement).

Hardening a slow process takes a few days for the complete crystallization.

The setting and hardening process is a hydration process in which hydrated calcium silicate formed in the gel form called tobermolite gel.

This gel precipitate over the cement grains and slow down the initial fast hydration.

The setting time and properties of the cement depends on the alumina and ferric oxide content, a higher content accelerate the setting process.

Important points

The rapid hydration of alumina causes the quick setting, it is controlled by addition of gypsum ( 2 – 5 % ) to clinker to retard the setting time.

Large excess of gypsum leads to cracking of the set cement due to expansion.

Other substances such as POP, anhydrite, sugar decrease the setting time and alkali carbonates, chlorides, accelerates the setting process.

Types of cement

1. Natural cement: mixture of natural calcareous and argillaceous material.

2. White portland cement: free from iron oxide.

3. Pozzolona cement: 45-65% silica, 10-20% alumina, <10% ferric oxide, some minor oxides of Ca, Mg, Na, K, Ti.

4. Water proof cement: mixture of portland cement and calcium stearate or non-saponifiable oil.

5. Slag cement: Highly resistant to sea water and sulphate solution. It is the mixture of blast furnace slag, Ca(OH)2 and CaSO4.

6. Super sulphated cementand and High alumina cement: resistant to sea water and sulphate solution.

7. Portland blast furnace cement: decreased rate of hardening.

8. Barium and strontium cement: Here Ca is replaced by Ba and Sr. Used to form concrete act as radiation shield.

9. Slow setting cement: Mixture of portland cement with starch or cellulose as retarders.

Weathering of cement and Concrete Due to the environmental conditions the

deterioration of cement and concrete. There are two types of environmental conditions:-

1. Attack by acid solution of CO2, organic and inorganic acids:- Attack of these acids increases with decrease in pH. Acidic water damage the Lime structure and hydrolyse the aluminate and silicate.

Prevention: Coating the cement with protective agents such as drying oil, epoxy resin paint, paint with SiF4.

2. Attack of sulphate solution: Sulphate solution react with Ca(OH)2 to form

gypsum. The C3A phase of hardened cement react with CaSO4 form Calcium sulphoaluminate with a huge increase in volume (about 227%) . This volume expansion lead to the destruction of cement and concrete.

Prevention: 1) By increasing the amount of C4AF in cement

which form a protective layer of C4F.2) By curing with superheated steam which

converts the free Ca(OH)2 into more resistant hydrated mono calcium silicate.

Supersulphated steam and high alumina cement have greater resistance to sulphate attack.

Supersulphated steam is a mixture of granulated slag(80 – 85 %), anhydrite(10-15%) and portland cement (5%).

Alumina cement consists of equal proportions of alumina and lime, 20 % iron oxide and 4-7 % silica.

The hardened supersulphated and alumina cement cannot react further with sulphate as no free lime is available.

Admixtures of Concrete Chemical substances added to improve the

properties of cement. Admixture include:-

1. Air entraining agent: They form microscopic bubbles when added to concrete with water and get attached to hydrophobic part. Concrete has pores which retains water and during winter season they expand on freezing and leads to cracking and if it contains air entrainer it prevents its cracking. e.g. oleic acid, caprylic acid.

2. Water decreasing agent: These decrease the water content up to 10-20% without affecting the structure of cement. e. g. melamine formaldehyde resin and sulphonate.

3. Setting accelerators: e. g. CaCl2, Calcium formate accelerate the hydration of C2S anf C3S phases of cement.

Ceramics:Clay products are further classified as: White ware : White/ pale cream with a

refractory body and glaze. Raw materials are free from iron oxide. Porcelain process: Body and glaze in

single firing China process: Glaze is developed in

second firing. Bisque: Porous body of the article dried

and fired in a biscuit oven.

Stoneware: strong and hard Made from crushed pottery,, clays

and stones fired at high temperatures.

Used for making sanitary fixtures and drainage pipes.

They have high chemical and thermal resistance and low coefficient for thermal expansion.

Earthenware: They are made from same materials fired at low temperature and they are soft clay products.

Structural Clay products: They include bricks and tiles made

from clay and sand by moulding, drying and firing.

Glass: It is an amorphous solid , transparent, hard and brittle.

Manufacture of glass: raw materials are soda lime and silica.

Calcium carbonate, lead oxide, potassium carbonate, alumina are added to improve the quality and yield different types.

Broken glass called cullet is added to ease the melting and decrease the cost.

Three process:I. Melting : Raw materials along with cullet are

ground and fused in furnaces. Producer gas( CO +N2) and air provide temp.

of about 1800 °C. Acidic silica + Basic Oxides = Silicates (glass)II. Forming and shaping: Molten glass is formed

and shaped in desired shapes.

Irregular shapes are made by applying pressure to glass of high viscosity. Moulding is used for making regular shapes

III. Annealing: controlled slow cooling. Done to avoid strains and stress built due to differential rates of cooling of external and internal parts.

Longer the annealing period, better is the glass.

IV. Finishing : Involves cutting, polishing and cleaning.

Properties of glass: Poor conductor of heat and electricity,

Transparent to light

Homogeneous structure

High compressive strength

Resistant to acids except HF as it forms volatile SiF4 and silica.I t is used for etching glass.

Different types of glass Soda Lime glass/ Soft Glass: Made from

Sodium silicate, Made insoluble by adding lime. melts at low temperatures, RESISTANT to devitrification (Loss of Plasticity and hence to be shaped).

Used in making electric bulbs, wind panes and cheap table ware.

Borosilicate glass/ Pyrex/Jena Glass: Contains silica and boron with Al, Na and K oxides in minor amounts.

Hard glass with low thermal expansion, high thermal and chemical resistance, high melting point.

Used in manufacture of laboratory ware, electric insulators, kitchenware.

Flint glass/ Lead Glass: contains silica, Lead and potassium oxide.

Soft and easy to grind., high refractive index.

Used in making optical lenses, radiation shields and neon sign tubes.

Potash-Lime/ Hard Glass: contains silica, Calcium carbonate and potassium carbonate. High melting temp. More stable towards chemicals.

Alumina glass: contains 20% of alumina along with B, Mg and Ca oxide. High softening temperature.Used in discharge tubes and combustion tubes.

Vitreosil: 99.5% pure silica glass. Low coefficient of thermal expansion and highly transparent. Used in chemical plants and electric insulators.

Toughened Glass: obtained by prestressing, and tempering/thermal strengthening. Tempering involves heating to its annealing temp. and rapidly exposing to cold blast of air. The surface becomes dense and interior becomes plastic due to difference in cooling rate. Used in window panes of automobiles.

Machining of the glass should be done before tempering.

Safety/ Laminated glass: made by pressing a sheet of glass in alternate layers of synthetic rubber. This glass is tough and shatter proof. The glass pieces donot fly when glass breaks suddenly. Used in windshields of aircraft.

Insulating glass: made by hermetically sealing two glass plates separately by a gap of about 10 mm thickness filled with air. Used for thermal insulation against heat.

Optical glass: highly homogeneous. Consist of lead silicate, phosphorous and cerium oxide. Cerium oxide absorbs uv rays harmful for eyes. Used for making lenses.

Glass ceramic/ Pyroceram: polycrystalline vitrified glass formed by controlled crystallisation in nucleating agents such as TiO2, ZrO2, Cu etc. It has greater hardness and impact strength as compared to ordinary glass.

Refractories: are inorganic materials capable of withstanding high temperature. They resist corrosive and abrasive action. Used in construction of furnaces and kilns. Characteristics: High resistance to change in physical, chemical

and mechanical properties at high temperatures. Chemical inertness to corrosive metals. Good thermal strength or resistance to thermal

shocks. Resistance to abrasion and erosion by gases or

molten metals. Mechanical and structural strength to withstand

load. Low permeability or ability to contain heat

without loss to surroundings.

Classification of refractories:

Acidic •Eg: Silica or Alumina•Are attacked by basic environment

Basic •Eg: CaO Or MgO•Are attacked by acidic environment

neutral •Carbon, graphite, SiC, ZrO2•Can withstand slightly acidic and basic conditions

Properties of refractories: Chemical inertness: Acidic refractories should be

exposed to acidic and basic refractories should be exposed to basic environment.

Refractoriness: Ability to withstand deformation and is measured by fusion/softening temperature of the material. Softening temp. is detemined by Pyrometric cone equivalent(PCE) or seger Cone Test. Refractory material is ground in form of small, slim pyramid shaped cone of standard dimensions and heated at the rate of 10˚C/ min along with standard seger cones placed on the same plaque.The softening behaviour is compared with that of standard by noting the interval of temperature at which cone starts to bend and the final temperature at which tip of the cone touches the base.

PCE value of test refractory is taken as the no. of the standard Seger Cone which shows a similar behaviour. Thus, it should have a high softening temperature.

Refractoriness Under Load/ Strength of a refractory: They should have high mechanical strength to withstand the load applied, without breaking under operating temperatures. Fire clay and high alumina collapse while silica bricks have good load bearing capacity. Load bearing capacity is evaluated by RUL test by applying a constant load of 3.5 or 1.75 kg/cm2 on the refractory specimen of size 75 cm high and 5 cm2 and heating at a constant heating rate of 10 C /min monitoring the deformation. RUL is expressed as the temp at which 10% deformation takes place.RUL for a high temp refractory under a constant load of 3.5 kg/cm2 is 1350˚C and for a moderate temp refractory is 1100 ˚C.

Dimensional Stability: It should have high resistance to reversible and irreversible changes under operating conditions.Permanent changes may occur due to fusion of low temperature fusing constituent resulting in shrinkage of material as in case of fire clay bricks. Permanent changes occur due to transformation of one crystalline form into another and lead to contraction as in case of magnesite bricks, whereas silica bricks undergo irreversible expansion and convert into tridymite .

Lubricants: Are substances used to reduce friction

btwn two moving surfaces. The study of wear and tear, mechanisms

of friction and lubrication is called tribology.

Irregularities appearing on rough surfaces in the form of peaks are called asperities.

Siezure is the prevention of movement of surfaces due to strong adhesion.

Scuffing is the removal of metal from the surface due to forced movement under siezed conditions.

Mechanism of lubrication: Thin film/ Boundary Lubrication:involves a thin film

adsorbed on surfaces and held by weak forces. Coefficient of friction is reduced to 0.05-0.15.The thickness of lubricant is not enough to cover all the asperities and hence lead to wearing of the machinery. It occurs due to non availability of continous film of lubricant. Friction can be prevented by using lubricant of low shear strength and high oiliness( vegetable oils).

Fluid Film/Hydrodynamic Lubrication: involves a thick film of lubricant and friction is considerably reduced to a value of 0.001. Since no direct contact of surfaces is there, no wear takes place. Observed in sewing machines, guns and scientific instruments. Hydrocarbon oils with long polymers are useful.

Extreme Pressure Lubrication: involves chemical action on the part of lubricant. Under high load and speed resulting in high temp, lubricant film melts and breaks completely. Hence spcl additives called extreme pressure additives are added to lubricating oils. S, P and Cl containing cmpds are added. They donot reduce friction but prevent welding of surfaces. These additives are not suitable for inert metal surfaces such as Ag, Cr and Ti.

Classification of lubricants: Liquid lubricants: derived from petroleum

oils as well as from animal and vegetable 0ils. Theey provide continous film btwn moving surfaces. They also help in cooling, corrosion inhibition and sealing.

Characteristics of good lubricant: High Boiling pt Thermal stability Low freezing point Resistance to

corrosion Adequate viscosity resistance to

oxidation

Liquid lubricant consist of three classes:

Mineral/petroleum oil: Obtained from distillation of petroleum(Light, medium and heavy). Wax, asphalt are present as impurities and hence cannot be used as such as lubricants as wax has low pour point and hence interfere with lubricating action. Asphalt also deposit as carbon and deposit as sludge.

Purification is done by : solvent extraction,dewaxing and finishing operations.

It consist of mixture of parrafins(low viscosity and density and easily oxidised), napthalenes(low pour point) and aromatic hydrocarbons((stable to oxidation but form sludge).

Animal and vegetable oil: glycerides of high fatty acids and called as fixed oils as they decompose on heating. They are used as additives of petroleum oils because of high oiliness.

Blended oils: derived from petroleum oils.

Oiliness carriers: vegetable oils(castor or coconut oil) and fatty acids(palmitic and stearic acid).

Pour point depressants: enable the oil to remain fluid even at low temp. Eg: Phenol and polyesters.

Viscosity index improvers: are high mol. Wt. polymers. Eg: Polyisobutylenes and polymethacrylates.

Antioxidants: retard the oxidation of oil by getting themselves oxidised. Eg: 2-napthol, phenyl-1-napthylamine.

Corrosion inhibitors: alkyl succinic acids and organic phosphates.

Antiwear agents or extreme pressure additives: tricresyl phosphate(also used as abrasion inhibitor) and zinc dialkyldithiophosphate. Detergents(2-10%) are used to prevent deposits in internal combustion engines.

Antifoaming agents: glycol and glycerol.

Properties of Liquid lubricants:

Viscosity: Viscosity decreases with increase in temperature. A high value of V.I indicates that viscosity is only slightly affected by change in temp and vice versa. V.I can be improved by addition of linear polymers.

Flash and fire point:To determine the use of lubricating oil at high temp. Flash Point is defined as lowest temp. at which oil give enough vapours to give a flash when they come in contact with flame.

Fire point lowest temp. at which vapours of oil burn continously at least for 5 sec with test flame.

Oil should have above its service temp so as to avoid risk of fire.

Cloud and pour point: Used in determining the source of the lubricant.The temperature at which oil becomes cloudy and solidifies are called cloud and pour point respectively.Pour point signifies the min. temp. at whch oil can be transferred by pouring.

Oiliness: ability of oil to stick on the surface. Mineral oils are poor in oiliness, Hence vegetable oils are added to mineral oils.

Volatility: shoul have low volatility as they volatise leaving behind residual oil having different characteristics.

Emulsification: indicates the tendency of oil to get mixed with water. Water attracts dirt and other solids and hence it should not form stable emulsion with water.Steam emulsion temp(S.E.N) is the time required in seconds to separate water and oil in distinct layers. Good lubricant should have low S.E.N.

Neutralisation no: is the measure of acidic and basic impurities. Acid value is defined as no. of mg of KOH required to neutralise acids in 1 g of the oil.Generally this value is less than 0.1.

Saponification no: is the no. of mg of KOH required to saponify fatty acids present in 1 g of the oil.

Carbon Residue: this value is quite high and increases on heating of oils. Good lubricant should have low carbon content.

Aniline point: indicates possible deterioration of oil with rubber surfaces. Aromatic hydrocarbons tend to dissolve rubber and hence oil should have low aromatic content. The higher the aniline point, lower is the aromatic content.

Ash content: indicates the presence of materials that cause abrasion and wear.

Corrosion Stability: a good lubricant should not cause any corrosion to copper strip.

Decomposition stability: stability of oil towards oxidation, hydrolysis and pyrolysis.

Precipitation no: % of asphalt in oil. Specific gravity: used for identifying oil

from unknown source. Mechanical stability: used for judging

the lubricant under high pressure.

Greases:

Are defined as solid to semi solid dispersion of a thickening agent in a liquid lubricant. Thickening agent are metal soaps. Na, Li, Ca or Al soaps are used as gelling agents.The greases are nemed after the soap used in their manufacture.

Calcium /cup grease: prepared by saponifying fatty acid with Ca hydroxide. Insoluble in water , water resistant and most commonly used. Can be used upto 70°C.

Sodium based: higher dropping point. Slightlu soluble in water. And can not be used under wet conditions.Used upto 120°C.

Lithium based: water resistant and stable upto 80°C.

Al based: high wter resistance and high adhesive characteristics.Used for lubricating chains.

Axle greases: prepared by addition of lime to resin and fatty acids. Cheap and used for less delicate instruments working under high load and low speed.

Properties of Greases: Consistency/ yield value: distance in tenths of

millimeter that a standard cone penetrates vertically into the sample under a load of 150 g, temp. of 25°c, and time of 5 seconds.

Drop point: is the temp at which it becomes sufficiently fluid so as the drop of cup falls from a cup having a hole of specified diameter.

Solid lubricants:Both organic and Inorganic Characteristics: Strong adhesion to surfaces Low Shear strength Chemically inert Good Thermal conductivity Stability at operating temperature Graphite and Molybdenum disulphide are most widely

used as they have layered structure. Mica and boron nitride also have layered structure but are ineffective as lubricants becase of poor adherence to surfaces and strong interlayer binding.

Graphite is used as solid powder in oil less bearings.

Choice of Lubricants: Properties of Lubricant should not change under

adverse conditions such as fluctuations in temperature, load and in oxidising or corroding atmospheres.

Cutting fluids and emulsions: used where metal and alloys undergo cutting, machining and grinding operations. The chosen lubricant is required to lubricate as well as cool the tool and are called cutting fluids. They also prevent distortion and dimensional inaccuracies as they remove unwanted solid particles.

For heavy cutting, mineral oils of low viscosity blended with fatty oils and chlorinated compounds, called cutting oils are used because they remain attached to metal surface.

For light cutting, emulsions of lubricating oils in aqueous soap solutions are used.Oil act as lubricant while water acts as coolant.

Lubricants for internal combustion engines: Should have high thermal stability, oxidation stability as they are exposed to high temperature. Petroleum based lubricating oils are used for this.

Lubricants for gears: should have good oiliness, adhesion, resistance to oxidation and high load bearing capacity as high pressure and centrifugal forces prevail in engines. Mineral oils are used along with chlorine ,soaps, S or P compounds.

Lubricants in transformers: should have good dielectric and heat transfer properties as they have to dissipate heat. Highly refined mineral oils are used.

Lubricants for refrigeration: should have low pour point, low cloud point and low viscosity. Napthalenic based oils are used.