Introduction: Classification and Properties of Materials

Transcription

1 CHAPTER 1 Introduction: Classification and Properties of Materials 1.1 INTRODUCTION Materials have always been an integral part of human civilization and social development, e.g. we designate periods in the past as the stone, Bronze and Iron ages. Recent advances in technologies rely on sophisticated materials all of them used devices, products, and systems that consist of materials. With the rapid advances in computer technology, design engineering have become quite sophisticated. Nowadays, mathematical models have been developed to study stress and process kinetics; computers are used to make detailed drawings and machines are controlled by minicomputers. Materials that are used by us are in one form or another some in their pure elemental form, e.g. copper, silver, nickel, etc.; some in the form of alloys and compounds, e.g. steel, brass etc. and some as composites, e.g. fibreglass, wood, etc. The selection of materials and most appropriate manufacturing process depends on several factors, but the most important considerations are shape complexity and properties of material. However, the properties of materials are ultimately linked with the microstructure and processing. Many times, a materials problem is one of selecting the right material from many thousands that are available. There are several criteria on which the final decision is normally based. First of all, the inservice conditions must be characterized, for these will dictate the properties required of the material. However, rarely does a material possess the maximum or ideal combination of properties. Clearly, it may be necessary to trade off one characteristic for another. For example, a material having a high strength will have only a limited ductility. This means, one will have to make a reasonable compromise between two or more properties may be necessary. The second consideration in the selection of a material is any deterioration of material properties that may occur during service operation, e.g. significant reductions in mechanical strength may result from exposure to elevated temperatures or corrosive environments. Finally, probably the overriding consideration is that of cost. A material may have the ideal set of properties but is prohibitively expensive. Obviously, some compromise will have to be made. An engineer must have a good knowledge of the various characteristics and structure property relationships, as well as processing techniques of materials. This helps him to make judicious materials choices based on these criteria.

2 2 Engineering Materials 1.2 CLASSIFICATION OF MATERIALS Based on chemical make up and atomic structure, solid materials have been conveniently grouped into three basic categories: metals, ceramics and polymers. Most materials fall into one distinct grouping or another, although there are also some intermediates. In addition to these, there are also three other groups of important engineering materials: composites, semiconductors and biomaterials. There are also advanced materials utilized in high-technology (or high-tech). Recently, a group of new and stateof-the art materials called as smart (or intelligent) materials being developed. Very recently, scientists have developed nano-engineering materials. A brief description of the material types and representative characteristics are: (i) Metals Normally metallic materials are combinations of metallic elements. Metallic materials have large number of nonlocalized electrons, i.e. electrons are not bound to particular atoms. Many properties of metals are directly attributable to these electrons. All metals are characterized by metallic properties, e.g. luster, opacity, malleability, ductility and electrical conductivity. Although metals compose about three fourth of the known elements but few find service in their pure form. The desired properties for engineering purposes are often found in alloys. Typical examples of metallic materials are iron, aluminium, copper, zinc, etc. and their alloys. They can be used either in bulk or powder form. Metals are extremely good conductors of electricity and heat are not transparent to visible light; a polished metal surface has a lustrous appearance. Moreover, metals are quite strong, yet deformable, which accounts for their extensive use in structural applications. Metallic materials are always crystalline in nature. Scientists have developed amorphous (non-crystalline) alloys by very rapid cooling of a melt or by very high-energy mechanical miling. Recently, scientists have developed materials through rapid solidification called as quasicrystals. These are neither crystalline nor amorphous, but form an ordered structure somewhere between two known structures. These materials are expected to exhibit far reaching electrical properties. (ii) Ceramics These are crystalline compounds between metallic and non-metallic elements. They are most frequently oxides, nitrides and carbides. Nowadays graphite is also categorized in ceramics. The wide range of materials which falls within this classification include ceramics that are composed of clay minerals, cement and glass. Glass is grouped with this class because it has similar properties but most glasses are amorphous. Ceramics are characterised by high hardness, abrasion resistance, brittleness and chemical inertness. Ceramics are typically insulative to the passage of electricity and heat, and are more resistant to high temperatures and harsh environments than metals and polymers. With regard to mechanical behaviour, these materials are hard but very brittle. These materials are widely categorized into oxide and non-oxide ceramics. (iii) Polymers Many of these are organic substances and derivatives of carbon and hydrogen. Polymers include the familiar plastic and rubber materials. Usually polymers are classified into three categories: thermoplastic polymers, thermosetting polymers and elastomers, better called as rubbers. Polymers have very large molecular structures. Most plastic polymers are light in weight and are soft in comparison to metals. Polymer materials have typically low densities and may be extremely flexible and widely used as insulators, both thermal and electrical. Few examples of polymers are polyesters, phenolics, polyethylene,

3 Introduction: Classification and Properties of Materials 3 nylon and rubber. The overriding consideration of the selection of a given polymer is whether or not the material can be processed into the required article easily and economically. (iv) Composites A composite is a composition of two or more materials in the first three categories, e.g. metals, ceramics and polymers, that has properties from its constituents. Large number of composite materials have been engineered. Few typical examples of composite materials are wood, clad metals, fibre glass, reinforced plastics, cemented carbides, etc. Fibre glass is a most familiar composite material, in which glass fibres are embedded within a polymeric material. A composite is designed to display a combination of the best characteristics of each of the component materials. Fibre glass acquires strength from the glass and the flexibility from the polymer. A true composite structure should show matrix material completely surrounding its reinforcing material in which the two phases act together to exhibit desired characteristics. These materials as a class of engineering material provide almost an unlimited potential for higher strength, stiffness, and corrosion resistance over the pure material systems of metals, ceramics and polymers. Many of the recent developments of materials have involved composite materials. Probably, the composites will be the steels of this century. Nowadays, the rapidly expanding field of nano composites is generating many exciting new materials with novel properties. The general class of nano composite organic or inorganic material is a fast growing field of research. Significant efforts are going on to obtain control of nano composite materials depend not only on the properties of their individual parents but also on their morphology and interfacial characteristics. The lamellar class of intercalated organic/inorganic nano composites and namely those systems that exhibit electronic properties in atleast one of the composites offers the possibility of obtaining well ordered systems some of which may lead to unusual electrical and mechanical properties. Polymer-based nano composites are also being developed for electronic applications such as thin-film capacitors in integrated circuits and solid polymer electrolytes for batteries. No doubt, the field of nano composites is of broad scientific interest with extremely impressive technological promise. The properties of these four varieties of materials have been contrasted in Table 1.1. Figure 1.1 shows the evolution of various types of composites along with the classification of the engineering materials. Table 1.1 Four varieties of materials and the contrast between their properties Properties Metals Ceramics Polymers Strength Moderately high High (hard) Low Resistance to loss of Moderate High Poor strength with rise in temperature Ease of fabrication Good (ductile) Poor (brittle) Very good (ductile) Conduction (Thermal and electrical) Good conductor Insulator Insulator Resistance to chemical attack Poor Inert Inert Dimensional stability (stiffness) High High Poor Specific gravity Very high Low (light) Low (light) Lusture Excellent Poor Poor Composites

4 4 Engineering Materials Metals Steel reinforced concrete Steel-cord tyres Cements: (ceramic particles bonded together by metals) Composites Polymers Carbon fibres reinforced polymer (CFRP) glass fibre reinforced polymer (GFRP) Ceramics and glasses Fig. 1.1 Evolution of various types of composites. Figure also shows the classification of engineering materials. (v) Semiconductors These materials have electrical properties that are intermediate between electrical conductors and insulators. Moreover, the electrical characteristics of semiconducting materials are extremely sensitive to the presence of minute concentrations of impurity atoms; these concentrations may be controlled over very small spatial regions. Silicon, Germanium and compounds listed in Table 1.2 form the vast majority of semiconducting crystals. Table 1.2 Semiconducting compounds Column in periodic table IV IV III V II VI I VII Compounds SiC AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb ZnO, ZnS, CdS, CdSe, CdTe CuCl, AgI These semiconducting materials are used in a number of solid state devices, e.g. diodes, transistors, photoelectric devices, solar batteries, radiation detectors, thermistors and lasers. The semiconductors have made possible the advent of integrated circuitary that has completely revolutionized the electronics and computer industries. (vi) Biomaterials These materials are employed in components implanted into the human body for replacement of diseased or damaged body parts. Biomaterials should not produce toxic substances and must be compatible with body tissues, i.e. should not cause adverse biological reactions. We may note all of the above materials,

5 Introduction: Classification and Properties of Materials 5 i.e. metals, ceramics, polymers, composites, semiconductors, etc. may be used as biomaterials. Some of the biomaterials are utilized in artificial hip replacements. (vii) Advanced Materials These are new engineering materials which exhibit high strength, great hardness, and superior thermal, electrical, optical and chemical properties. Advanced materials have dramatically altered communication technologies, reshaped data analysis, restructured medical devices, advanced space travel and transformed industrial production process. These materials are often synthesized from the biproducts of conventional commodity materials and often possess following characteristics: These materials are created for specific purposes, These materials are highly processed and possess a high value-to weight ratio, These materials are developed and replaced with high frequency, and These materials are frequently combined into new composites. Nowadays, there is considerable interest in making advanced materials that are usually graded by chemical composition, density or coefficient of thermal expansion of material or based on microstructural features, e.g. a particular arrangement of second-phase particles or fibres in a matrix. Such materials are referred as functionally graded materials. Instead of having a step function, one may strive to achieve a gradual change. Such gradual change will reduce the chances of mechanical and thermal stresses, generally present otherwise. We may note that the concept of a functionally graded material is applicable to any material metal, polymer or ceramic. A lot of research work is going on these materials. (viii) Materials of the Future (a) Smart Materials Smart or intelligent materials form a group of new and state of art materials now being developed that will have a significant influence on many of present-day technologies. The adjective smart implies that these materials are able to sense changes in their environments and then respond to these changes in predetermined manners traits that are also found in living organisms. In addition, the concept of smart materials is being extended to rather sophisticated systems that consist of both smart and traditional materials. The field of smart materials attempts to combine the sensor (that detects an input signal), actuator (that performs a responsive and adaptive function) and the control circuit or as one integrated unit. Acutators may be called upon to change shape, position, natural frequency, or mechanical characteristics in response to changes in temperature, electric fields, and or magnetic fields. Usually, four types of materials are commonly used for actuators: shape memory alloys, piezoelectric ceramics, magnetostrictive materials, and electrorheological/magnetorheological fluids. Shape memory alloys are metals that, after having been deformed, revert back to their original shapes when temperature is changed. Piezoelectric ceramics expand and contract in response to an applied electric field (or voltage); conversely these materials also generate an electric field when their dimensions are altered. The behaviour of magnetostrictive materials is analogous to that of the piezoelectric ceramic materials, except that they are responsive to magnetic fields. Also, electrorheological and magnetorheological fluids are liquids that experience dramatic changes in viscocity upon application of electric and magnetic fields, respectively. The combined system of sensor, actuator and control circuit or as one IC unit, emulates a biological system (Fig. 1.2).

6 6 Engineering Materials These are known as smart sensors, microsystem technology (MST) or micromechanical systems (MEMS). Materials/devices employed as sensors include optical fibres, piezoelectric materials (including some polymers) and MEMS. Sensor Control and computation Brain Actuators and motors Muscles Fig. 1.2 Integrated sensor actuator systems with controller are analogous to biological systems. For example, one type of smart system is used in helicopters to reduce aero-dynamic cockpit noise that is created by the rotating rotor blades. Piezoelectric sensors inserted into the blades, monitor blade stresses and deformations; feedback signals from these sensors are fed into a computer controlled adaptive device, which generates noise cancelling antidose. MEMS devices are small in size, light weight, low cost, reliable with large batch fabrication technology. They generally consist of sensors that gather environmental information such as pressure, temperature, acceleration, etc., integrated electronics to process the data collected and actuators to influence and control the environment in the desired manner. The MEMS technology involves a large number of materials. Silicon forms the backbone of these systems also due to its excellent mechanical properties as well as mature micro-fabrication technology including lithography, etching, and bonding. Other materials having piezoelectric, piezoresistive, ferroelectric and other properties are widely used for sensing and actuating functions in conjunction with silicon. (b) Nano Materials Nano-structured (NS) materials are defined as solids having microstructural features in the range of nm (= (1 100) 10 9 m) in at least in one dimension. These materials have outstanding mechanical and physical properties due to their extremely fine grain size and high grain boundary volume fraction. Usually, the clusters of atoms consisting of typically hundreds to thousands on the nanometer scale are called as nanoclusters. These small group of atoms, in general, go by different names such as nano particles, nanocrystals, quantum dots and quantum boxes. Significant work in being carried out in the domain of nano-structured materials and nano tubes since they were found to have potential for high technology engineering applications. Nano-structured materials exhibit properties which are quite different from their bulk properties. These materials contain a controlled morphology with atleast one nano scale dimension. Nano crystals, nano wires and nano tubes of a large number of inorganic materials have been synthesized and characterized in the last few years. Some of the nano materials exhibit properties of potential technological value. This is particularly true for nano-structures of semiconducting materials such as metal chalcogenides and nitrides. The mixing of nano-particles with polymers to form composite materials has been practiced for decades. For example, the clay reinforced resin known as Bakelite is the first mass-produced polymer-nanoparticle composites and fundamentally transformed the nature of practical household materials. Even before bakelite, nano composites were finding applications in the form of nano particle-toughened automobile tires prepared by blending carbon black, zinc oxide, and/or magnesium sulfate particles with vulcanized rubber. Despite these early successes, the broad scientific community was not galvanized by nano composites until the early 1990s, when reports revealed that adding mica to nylon produced a five-fold increase in the yield and tensile strength of the material. Subsequent developments have further contributed to the surging interest in polymer nano particle composites.

7 Introduction: Classification and Properties of Materials 7 Significant progress has been made in various aspects of synthesis of nano-structured materials. The explosion of both academic and industrial interest in these materials over the past decade arises from the remarkable variations in fundamental electrical, optical and magnetic properties that occur as one progresses from an infinitely extended solid to a particle of material consisting of a countable number of atoms. The focus is now shifting from synthesis to manufacture of useful structures and coatings having greater wear and corrosion resistance. Materials produced out of nano particles have some special features, e.g. (i) very high ductility (ii) very high hardness ~4 to 5 times more than usual conventional materials (iii) transparent ceramics achievable (iv) manipulation of colour (v) extremely high coercivity magnets (vi) developing conducting inks and polymers. 1.3 MODERN MATERIALS NEEDS Material science has expanded from the traditional metallurgy and ceramics into new areas such as electronic polymers, complex fluids, intelligent materials, organic composites, structural composites, biomedical materials (for implants and other applications), biomimetics, artificial tissues, biocompatible materials, auxetic materials (which grow fatter when stretched), elastomers, dielectric ceramics (which yield thinner dielectric layers for more compact electronics), ferroelectric films (for non-volatile memories), more efficient photovoltaic converters, ceramic superconductors, improved battery technologies, self-assembling materials, fuel cell materials, optoelectronics, artificial diamonds, improved sensors (based on metal oxides, or conducting polymers), grated light values, ceramic coatings in air (by plasma deposition), electrostrictive polymers, chemical mechanical polishing, alkali-metal thermoelectric converters, luminescent silicon, planar optical displays without phosphors, MEMS, and super molecular materials. However, there still remain technological challenges, including the development of even more sophisticated and specialized materials, as well as consideration of the environmental impact of materials production. Material scientists are interested in green approaches, by entering the field of environmental biological science, by developing environmentally friendly processing techniques and by inventing more recyclable materials. Nano materials is another emerging field called as Nano materials by severe plastic deformation (SPD) which involves the application of very high strains and flow stresses to work pieces. The respective new processes yields microstructural features and properties in materials (notably metals and alloys) that differ features and properties in materials (notably metals and alloys) that differ from those known for conventional cold worked materials. Specifically, pore-free grain refinements down to nanometer dimensions, and dislocation accumulations upto the limiting density of m 2 are observed. SPD yields an increase in tensile ductility without a substantial loss in strength and fatigue behaviour. Furthermore, unusual phase transformations leading to highly metastable states have been reported and are associated with a formation of supersaturated solid solutions, disordering, amorphization, and a high thermal stability. Moreover, superplastic elongations in alloys that are generally not superplastic can be achieved. This affords a superplastic flow at strain rates significantly faster than in conventional alloys, enabling the rapid fabrication of complex parts. Finally, the magnetic properties of severely plastic deformed materials are different from their conventional counterparts. In particular, one observes an enhanced zemanence in hard magnetic materials, a decrease of coercivity, (i.e. energy loss) in soft magnetic materials, and an induced magnetic anisotropy. Clearly, the fields of materials is extending into new territory, and this trend is expected to continue. Figure 1.3 displays the development of essential material properties during the 20th century.

8 8 Engineering Materials (BH) max (MGOe) Strength of permanent magnets Nd Fe B 2 14 Sm (Co, Fe, Cu) 2 17 AlNiCo Steel CoSm Year (a) 1600 T ( C) 1200 Operating temperature of engines Turbojet Air-cooled engine Steam engine Year (b) T (K) Critical temperature (T C) for superconductors Year (c) Components per chip Components per chip Bipolar MOS Three dimensional Year (d) Steel Metal matrix composites 40 Weight Nickel Titanium Aluminium Ceramic matrix composites Resin based composites Tyne Spey RB211 RB E4 Year (e) Trends in jet engine material usage Fig. 1.3 Improvement of materials properties during the 20th century.

9 Introduction: Classification and Properties of Materials 9 One may only speculate what kinds of discoveries might be made by the next generation of scientists and engineers if they would let their imagination roam freely into yet unexplored realms. Among these discoveries may be (i) A completely different family of materials which are not derived from already existing substances but are instead newly created by modification of genes, i.e. by gene technology. These biologically generated materials could possibly be custom-designed with respect to their physical properties, stability, and their recyclability. They may be created from renewable, inexhaustible resources or by bacteriologic transformations of already existing products. (ii) The energy of the future may not be generated by burning wood, coal, or oil, or by involving fissionable or fusionable elements, but by exploiting hitherto unknown disturbances that are neither electromagnetic nor of particle nature. This energy source may be tapped, should it exist, and it is hoped that mankind will have developed at that point a high degree of morality so that it may not be misused for destructive purposes. (iii) Presently, mankind is quite fixated on the concept that matter consists exclusively of atoms built from protons, neutrons, electrons and a handful of other particles. It is not possible that another type of matter does exist which is built of different particles beyond our imagination. May be this alternate form of matter will be discovered in this century, should it exist. (iv) The abundance of radioactive waste produced from reactors, etc. surely will be a challenge to future generations. New techniques will have to be found which are capable of manipulating the ratio of protons and neutrons in radioactive elements which will transform them into nonradioactive isotopes. (v) The transition temperature (T c ) at which superconduction commences may again be substantially raised by employing new materials which have a striking similarity to fibres spun by animals or which are otherwise created in the body of animals or humans such as in nerve cells. (vi) Smart acoustic materials may be discovered which compensate incoming sound with a complementary sound, thus eliminating any noise. In particular, the acoustical properties of materials will probably be studied more intensely in the future than they have been in the past. (vii) The storage of energy in batteries, etc. is at present most inefficient and requires bulky devices. New techniques will probably be discovered that raise the energy to mass ratio and increase the efficiency by involving a plasma technology, which is harnessed in containers consisting of hightemperature resistant materials. (viii) Materials may be found which, when weakened by fatigue or cracking, will activate a self-healing mechanism that returns the material, while in use, to its originally intended properties without external intervention. (ix) New types of trees or plant species may be genetically engineered which can be harvested in about 7-8 years rather than in the present 30-year time interval. (x) The process of photosynthesis may be copied and used to create new materials and energy. These few instances may serve as stimuli for research scientists and engineers and thus may lead materials science into new dimensions for the betterment of mankind. We hope that future developments will definitely be linked to societal issues. 1.4 THE PROPERTY SPECTRUM OF ENGINEERING MATERIALS Properties of materials play an important role in their selection for specific purposes. The engineer has to decide the properties required of a material for a part under design and then he has to weigh the

10 10 Engineering Materials properties of candidate materials. There are literally speaking hundreds of properties that are measured in laboratories for the purpose of comparing various materials, but one has to concentrate on the more important ones. The major properties to be considered in material selection can be put under four categories: chemical, physical, mechanical and dimensional properties. Figure 1.4 gives a brief account of these properties. Chemical Physical Mechanical Dimensional Composition Structure Colour Density Strength (Tensile, compressive, Shear, Yield, Bend Fatigue, Creep) Size Shape Corrosion resistance Melting point Surface smoothness Ductility Conductivity (Electrical, Thermal) Toughness Tolerence Heat capacity Fatigue toughness Reflectivity Rigidity Dielectric strength Wear resistance Micrography Magnetic permeability Opacity Service life Fig. 1.4 Major categories of the properties of materials. (a) Chemical properties: These properties are characteristics of a material which relate to the structure of a material and its formation from the elements. Composition, structure and corrosion resistance are the important chemical properties of a material. One can determine the composition of a material by analytical chemistry in metals, the composition usually refers to the percentage of various elements present and make up the metal. The composition is a fundamental consideration while selecting a material. The engineer should have atleast some idea about the composition of the material. The chemical properties describe the combining tendencies, corrosion characteristics, reactivity, solubilities, etc., of a material. Few of the chemical properties are: