Materials science

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Depiction of two "Fullerene Nano-gears" with multiple teeth.

Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates elements of applied physics and chemistry. With significant media attention focused on nanoscience and nanotechnology in recent years, materials science has been propelled to the forefront at many universities. It is also an important part of forensic engineering and failure analysis. Materials science also deals with fundamental properties and characteristics of materials.

Contents 1 History 2 Fundamentals 3 Classes of materials 4 Materials in industry 4.1 Ceramics and glasses 4.2 Composite materials 4.3 Polymers 4.4 Metal alloys 4.5 Digital materials 4.5.1 Isotropic DMs 4.5.2 Anisotropic DMs 4.5.3 Digital materials research 5 Sub-disciplines of materials science 5.1 Methods, processes, and related topics 6 Professional organizations 7 International conferences 8 See also 9 Notes and references 10 Bibliography 11 Further reading 12 External links

History

Main article: History of materials science

The material of choice of a given era is often a defining point. Phrases such as Stone Age, Bronze Age, and Steel Age are good examples. Originally deriving from the manufacture of ceramics and its putative derivative metallurgy, materials science is one of the oldest forms of engineering and applied science. Modern materials science evolved directly from metallurgy, which itself evolved from mining and (likely) ceramics and the use of fire. A major breakthrough in the understanding of materials occurred in the late 19th century, when the American scientist Josiah Willard Gibbs demonstrated that the thermodynamic properties related to atomic structure in various phases are related to the physical properties of a material. Important elements of modern materials science are a product of the space race: the understanding and engineering of the metallic alloys, and silica and carbon materials, used in the construction of space vehicles enabling the exploration of space. Materials science has driven, and been driven by, the development of revolutionary technologies such as plastics, semiconductors, and biomaterials.
Before the 1960s (and in some cases decades after), many materials science departments were named metallurgy departments, from a 19th and early 20th century emphasis on metals. The field has since broadened to include every class of materials, including ceramics, polymers, semiconductors, magnetic materials, medical implant materials and biological materials (materiomics).

Fundamentals

The basis of materials science involves relating the desired properties and relative performance of a material in a certain application to the structure of the atoms and phases in that material through characterization. The major determinants of the structure of a material and thus of its properties are its constituent chemical elements and the way in which it has been processed into its final form. These characteristics, taken together and related through the laws of thermodynamics, govern a material’s microstructure, and thus its properties.
The manufacture of a perfect crystal of a material is currently physically impossible. Instead materials scientists manipulate the defects in crystalline materials such as precipitates, grain boundaries (Hall–Petch relationship), interstitial atoms, vacancies or substitutional atoms, to create materials with the desired properties.
Not all materials have a regular crystal structure. Polymers display varying degrees of crystallinity, and many are completely non-crystalline. Glasses, some ceramics, and many natural materials are amorphous, not possessing any long-range order in their atomic arrangements. The study of polymers combines elements of chemical and statistical thermodynamics to give thermodynamic, as well as mechanical, descriptions of physical properties.
In addition to industrial interest, materials science has gradually developed into a field which provides tests for condensed matter or solid state theories. New physics emerge because of the diverse new material properties which need to be explained.

Classes of materials

Materials science encompasses various classes of materials, each of which may constitute a separate field. There are several ways to classify materials. For instance by the type of bonding between the atoms. The traditional groups are ceramics, metals and polymers based on atomic structure and chemical composition. New materials has resulted in more classes.^[1] One way of classifying materials is:

• Biomaterials
• Carbon
• Ceramics
• Composite materials
• Glass
• Metals
• Nanomaterials
• Polymers
• Refractory
• Semiconductors
• Thin Films
• Functionally Graded Materials

Materials in industry

Radical materials advances can drive the creation of new products or even new industries, but stable industries also employ materials scientists to make incremental improvements and troubleshoot issues with currently used materials. Industrial applications of materials science include materials design, cost-benefit tradeoffs in industrial production of materials, processing techniques (casting, rolling, welding, ion implantation, crystal growth, thin-film deposition, sintering, glassblowing, etc.), and analytical techniques (characterization techniques such as electron microscopy, x-ray diffraction, calorimetry, nuclear microscopy (HEFIB), Rutherford backscattering, neutron diffraction, small-angle X-ray scattering (SAXS), etc.).
Besides material characterization, the material scientist/engineer also deals with the extraction of materials and their conversion into useful forms. Thus ingot casting, foundry techniques, blast furnace extraction, and electrolytic extraction are all part of the required knowledge of a metallurgist/engineer. Often the presence, absence or variation of minute quantities of secondary elements and compounds in a bulk material will have a great impact on the final properties of the materials produced, for instance, steels are classified based on 1/10 and 1/100 weight percentages of the carbon and other alloying elements they contain. Thus, the extraction and purification techniques employed in the extraction of iron in the blast furnace will have an impact of the quality of steel that may be produced.
The overlap between physics and materials science has led to the offshoot field of materials physics, which is concerned with the physical properties of materials. The approach is generally more macroscopic and applied than in condensed matter physics. See important publications in materials physics for more details on this field of study.

Ceramics and glasses

Si₃N₄ ceramic bearing parts

Another application of the material sciences is the structures of glass and ceramics, typically associated with the most brittle materials. Bonding in ceramics and glasses are using covalent and ionic-covalent types with SiO₂ (silica or sand) as a fundamental building block. Ceramics are as soft as clay and as hard as stone and concrete. Usually, they are crystalline in form. Most glasses contain a metal oxide fused with silica. At high temperatures used to prepare glass, the material is a viscous liquid. The structure of glass forms into an amorphous state upon cooling. Windowpanes and eyeglasses are important examples. Fibers of glass are also available. Diamond and carbon in its graphite form are considered to be ceramics.
Engineering ceramics are known for their stiffness, high temperature, and stability under compression and electrical stress. Alumina, silicon carbide, and tungsten carbide are made from a fine powder of their constituents in a process of sintering with a binder. Hot pressing provides higher density material. Chemical vapor deposition can place a film of a ceramic on another material. Cermets are ceramic particles containing some metals. The wear resistance of tools is derived from cemented carbides with the metal phase of cobalt and nickel typically added to modify properties.

Composite materials

A 6 μm diameter carbon filament (running from bottom left to top right) compared to a human hair.

Filaments is commonly used for reinforcement in composite materials.
Another application of material science in industry is the making of composite materials. Composite materials are structured materials composed of two or more macroscopic phases. Applications range from structural elements such as steel-reinforced concrete, to the thermally insulative tiles which play a key and integral role in NASA's Space Shuttle thermal protection system which is used to protect the surface of the shuttle from the heat of re-entry into the Earth's atmosphere. One example is reinforced Carbon-Carbon (RCC), The light gray material which withstands re-entry temperatures up to 1510 °C (2750 °F) and protects the Space Shuttle's wing leading edges and nose cap. RCC is a laminated composite material made from graphite rayon cloth and impregnated with a phenolic resin. After curing at high temperature in an autoclave, the laminate is pyrolized to convert the resin to carbon, impregnated with furfural alcohol in a vacuum chamber, and cured/pyrolized to convert the furfural alcohol to carbon. In order to provide oxidation resistance for reuse capability, the outer layers of the RCC are converted to silicon carbide.
Other examples can be seen in the "plastic" casings of television sets, cell-phones and so on. These plastic casings are usually a composite material made up of a thermoplastic matrix such as acrylonitrile-butadiene-styrene (ABS) in which calcium carbonate chalk, talc, glass fibers or carbon fibers have been added for added strength, bulk, or electrostatic dispersion. These additions may be referred to as reinforcing fibers, or dispersants, depending on their purpose.

Polymers

Microstructure of part of a DNA double helix biopolymer.

Polymers are also an important part of materials science. Polymers are the raw materials (the resins) used to make what we commonly call plastics. Plastics are really the final product, created after one or more polymers or additives have been added to a resin during processing, which is then shaped into a final form. Polymers which have been around, and which are in current widespread use, include polyethylene, polypropylene, PVC, polystyrene, nylons, polyesters, acrylics, polyurethanes, and polycarbonates. Plastics are generally classified as "commodity", "specialty" and "engineering" plastics.
PVC (polyvinyl-chloride) is widely used, inexpensive, and annual production quantities are large. It lends itself to an incredible array of applications, from artificial leather to electrical insulation and cabling, packaging and containers. Its fabrication and processing are simple and well-established. The versatility of PVC is due to the wide range of plasticisers and other additives that it accepts. The term "additives" in polymer science refers to the chemicals and compounds added to the polymer base to modify its material properties.
Polycarbonate would be normally considered an engineering plastic (other examples include PEEK, ABS). Engineering plastics are valued for their superior strengths and other special material properties. They are usually not used for disposable applications, unlike commodity plastics.
Specialty plastics are materials with unique characteristics, such as ultra-high strength, electrical conductivity, electro-fluorescence, high thermal stability, etc.
The dividing lines between the various types of plastics is not based on material but rather on their properties and applications. For instance, polyethylene (PE) is a cheap, low friction polymer commonly used to make disposable shopping bags and trash bags, and is considered a commodity plastic, whereas medium-density polyethylene (MDPE) is used for underground gas and water pipes, and another variety called Ultra-high Molecular Weight Polyethylene UHMWPE is an engineering plastic which is used extensively as the glide rails for industrial equipment and the low-friction socket in implanted hip joints.

Metal alloys

The study of metal alloys is a significant part of materials science. Of all the metallic alloys in use today, the alloys of iron (steel, stainless steel, cast iron, tool steel, alloy steels) make up the largest proportion both by quantity and commercial value. Iron alloyed with various proportions of carbon gives low, mid and high carbon steels. An iron carbon alloy is only considered steel if the carbon level is between 0.01% and 2.00%. For the steels, the hardness and tensile strength of the steel is related to the amount of carbon present, with increasing carbon levels also leading to lower ductility and toughness. Heat treatment processes such as quenching and tempering can significantly change these properties however. Cast Iron is defined as an iron–carbon alloy with more than 2.00% but less than 6.67% carbon. Stainless steel is defined as a regular steel alloy with greater than 10% by weight alloying content of Chromium. Nickel and Molybdenum are typically also found in stainless steels.
Other significant metallic alloys are those of aluminium, titanium, copper and magnesium. Copper alloys have been known for a long time (since the Bronze Age), while the alloys of the other three metals have been relatively recently developed. Due to the chemical reactivity of these metals, the electrolytic extraction processes required were only developed relatively recently. The alloys of aluminium, titanium and magnesium are also known and valued for their high strength-to-weight ratios and, in the case of magnesium, their ability to provide electromagnetic shielding. These materials are ideal for situations where high strength-to-weight ratios are more important than bulk cost, such as in the aerospace industry and certain automotive engineering applications.

Digital materials

This section may be confusing or unclear to readers. Please help clarify the section; suggestions may be found on the talk page. (February 2012)

Digital Materials (DMs) are engineered materials manufactured from two or more different constituent materials, according to a digitally encoded three dimensional (3D) phase structure design (the DM code), and produced by an additive manufacturing (AM) process.^[2]

• 

  Figure 1: A schematic representation of a rasterized 3D object (stapler) showing voxels made of a single constituent material
  
• 

  Figure 2: A schematic representation of a rasterized 3D object (stapler) showing voxels made of two different constituent materials
  
• 

  Figure 3: Schematic representation of the cross section of an Isotropic DM
  

In DMs the constituent materials are combined together at a voxel level (figures 1-3) to create domains or phases with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or microscopic scale within the finished DM structure. Hierarchical structures in which macro voxels are created from more than one constituent material are also possible.
DMs can be divided into two main categories: Isotropic and Anisotropic DMs.

Isotropic DMs

In this type of DM, the constituent materials are combined homogeneously as for example, a non-continuous phase made of constituent material “a” randomly “dispersed” within a continuous phase made of constituent material “b” (figure 3). A uniform combination between constituent materials, or a mix of uniform and random combinations are also possible. The combination of constituent materials can be at the single voxel level as in figure 3, but may also be at higher level as for example a number of voxels as the minimum amount of one constituent material.

Anisotropic DMs

These types of DMs have an anisotropic 3D phase structure design, and therefore anisotropic properties along different axis within a single object (figure 4).

Figure 4: Example of anisotropic DM phase structure design

Anisotropic DMs may also be Geometry Dependent; in this type of DM there is a “dialog” between the object design and the DM code, which results in different constituent material arrangements in different object designs, or in different regions within a single object. In Geometry Dependent DMs the DM code is responsible for defining the rules that govern the DM composition as a function of a respective object geometry and size; analogous to the way the genetic code in living organisms is responsible for dictating the characteristics of a living organism. The DM code defining a Geometry Dependent DM comprises a set of rules or algorithms that when applied to the manufacturing of a specific object, permits allocating constituent materials in different object regions, according to the specific object design. Thus, in Geometry Dependent DMs, the DM is not be produced for subsequent use in the manufacture of a desired object; rather the DM production process and the object manufacturing process are one.
An example of a Geometry Dependent DM code algorithm is presented here:

1. Two constituent materials: A and B
2. The object is made of constituent material A in all object regions besides those defined as made of constituent material B
3. Constituent material B is used to form as 1mm deep region comprising the outer surface of the object

In this example, every object region having a thickness of 2mm or less, is produced solely from constituent material B, while object regions thicker than 2mm are constructed of two regions, an outermost region of 1mm in thickness made of constituent material B and a core made of constituent material A.

Figure 5: A schematic representation of the cross section of a Geometry Dependent DM comprising two different constituent materials; constituent material B in yellow and constituent material A in red.

Geometry Dependent DMs can be further divided into two subcategories: Step DMs and Graded DMs. While in Step DMs constituent materials are combined in substantially well-defined phases, as shown in figure 5, in Graded DMs the DM composition varies gradually along at least one defined trajectory or axis of an object, in a graded fashion. An example of a Graded DM is one in which the DM composition on one side of the object is substantially richer in one constituent material than another, while on other side of the object, the DM composition is substantially richer in the other constituent material, and where the content ratio between the constituent materials gradually varies from one side of the object to the other.
There is also the possibility of Geometry Dependent DMs having Step as well as Graded characteristics within a single object.

Digital materials research

Different aspects in the field of DMs have recently been the focus of intensive research, in the industrial sector as well as in the academic sector. According to Mary C. Boyce et al.,^[3] co-continuous glassy polymer/rubbery materials with sub-millimeter feature size, fabricated using a 3D printer, expose enhancements in stiffness, strength and energy dissipation. According to Mary C. Boyce, “geometric and topological arrangement of the constituent materials provides avenues to engineer the macroscale material properties”.
A data-driven process for designing and fabricating materials with desired deformation behavior has been reported.^[4] According to this report, “an optimization process that finds the best combination of stacked layers that meets a user’s criteria specified by example deformations” has been developed. In this study, and in order to demonstrate the optimization process validity, objects with complex heterogeneous materials were fabricated using a modern multi-material 3D printer.
In another study,^[5] “a complete pipeline for measuring, modeling, and fabricating objects with specified subsurface scattering behaviors” was proposed. According to the authors, the process was validated by producing homogeneous and heterogeneous materials using a multi-material 3D printer.
Neri Oxman ^[6] takes nature as a model, and proposes what she calls “Variable Property Design (VPD)”, as a method for design, in which “material assemblies are modeled, simulated and fabricated with varying properties”, in order to give an answer to functional constraints. Oxman's Variable Property Rapid Prototyping approach aims at integrating between material properties and environmental constraints within the computational modeling environment and as part of the form-generation and fabrication process.^[7]
Hod Lipson et al. ^[8] has reported the simulation of materials properties as a function of their “digital material” composition. According to this report, properties as stiffness, CTE, and failure modes were obtained by varying voxel manufacturing precision, the percentage of randomly distributed constituent materials, and the voxel microstructure. Furthermore, it has been stated by the authors that material properties may be tuned anywhere between the respective properties of two constituent materials, by simple randomly halftone a percentage constituent materials. In addition, properties such as stiffness or negative Poisson's ratio, has been reported to be obtained using relatively dense common materials, by the inclusion of a hierarchical voxel microstructure.

Sub-disciplines of materials science

Below is a list of discplines within or related to the materials science field. These range from biomaterials, to ceramics, to metals, to textile reinforced materials. Also note that these are linked to the respective main article.

• Biomaterials – materials that are derived from and/or used with biological systems.
• Ceramography – the study of the microstructures of high-temperature materials and refractories, including structural ceramics such as RCC, polycrystalline silicon carbide and transformation toughened ceramics
• Crystallography – the study of how atoms in a solid fill space, the defects associated with crystal structures such as grain boundaries and dislocations, and the characterization of these structures and their relation to physical properties.
• Electronic and magnetic materials – materials such as semiconductors used to create integrated circuits, storage media, sensors, and other devices.
• Forensic engineering – the study of how products fail, and the vital role of the materials of construction
• Forensic materials engineering – the study of material failure, and the light it sheds on how engineers specify materials in their product
• Glass Science – any non-crystalline material including inorganic glasses, vitreous metals and non-oxide glasses.
• Materials Characterization – such as diffraction with x-rays, electrons, or neutrons, and various forms of spectroscopy and chemical analysis such as Raman spectroscopy, energy-dispersive spectroscopy (EDS), chromatography, thermal analysis, electron microscope analysis, etc., in order to understand and define the properties of materials. See also List of surface analysis methods
• Metallography - Metallography is the study of the physical structure and components of metals, typically using microscopy.
• Metallurgy – the study of metals and their alloys, including their extraction, microstructure and processing.
• Microtechnology – study of materials and processes and their interaction, allowing microfabrication of structures of micrometric dimensions, such as MicroElectroMechanical Systems (MEMS).
• Nanotechnology – rigorously, the study of materials where the effects of quantum confinement, the Gibbs–Thomson effect, or any other effect only present at the nanoscale is the defining property of the material; but more commonly, it is the creation and study of materials whose defining structural properties are anywhere from less than a nanometer to one hundred nanometers in scale, such as molecularly engineered materials.
• Surface science/Catalysis – interactions and structures between solid-gas solid-liquid or solid-solid interfaces.
• Textile Reinforced Materials – materials in the form of ceramic or concrete are reinforced with a primarily woven or non-woven textile structure to impose high strength with comparatively more flexibility to withstand vibrations and sudden jerks.
• Some practitioners consider rheology a sub-field of materials science, because it can cover any material that flows. However, modern rheology typically deals with non-Newtonian fluid dynamics, so it is often considered a sub-field of continuum mechanics. See also granular material.
• Tribology – the study of the wear of materials due to friction and other factors.

Methods, processes, and related topics

Below are links to topics that explain methods, proceses and related topics in order to enhance understanding of materials science.

• Alloying, corrosion, and thermal or mechanical processing, for a specialized treatment of metallurgical materials—with applications ranging from aerospace and industrial equipment to the civil industries
• Biomaterials, physiology, biomechanics, biochemistry, for a specialized understanding of how materials integrate into biological systems, e.g., through materiomics
• Crystallography, quantum chemistry or quantum physics, for the structure (symmetry and defects) and bonding in materials (e.g., ionic, metallic, covalent, and van der Waals bonding)
• Diffraction and wave mechanics, for the science behind characterization systems, e.g., X-ray Diffraction (XRD) transmission electron microscopy (TEM)
• Electronic properties of materials, and solid-state physics, for the understanding of the electronic, thermal, magnetic, and optical properties of materials
• Mechanical behavior of materials, to understand the mechanical properties of materials, defects and their propagation, and their behavior under static, dynamic, and cyclic loads
• Phase transformation kinetics, for the kinetics of phase transformations (with particular emphasis on solid-solid phase transitions)
• Polymer properties, synthesis, and characterization, for a specialized understanding of how polymers behave, how they are made, and how they are characterized; exciting applications of polymers include liquid crystal displays (LCDs, the displays found in most cell phones, cameras, and iPods), novel photovoltaic devices based on semiconductor polymers (which, unlike the traditional silicon solar panels, are flexible and cheap to manufacture, albeit with lower efficiency), and membranes for room-temperature fuel cells (as proton exchange membranes) and filtration systems in the environmental and biomedical fields
• Semiconductor materials and semiconductor devices, for a specialized understanding of the advanced processes used in industry (e.g. crystal growth techniques, thin-film deposition, ion implantation, photolithography), their properties, and their integration in electronic devices
• Solid-state physics is the study of rigid matter, or solids, through methods such as quantum mechanics, crystallography, electromagnetism, and metallurgy. It is the largest branch of condensed matter physics. Solid-state physics studies how the large-scale properties of solid materials result from their atomic-scale properties. Thus, solid-state physics forms the theoretical basis of materials science. It also has direct applications, for example in the technology of transistors and semiconductors.
• Thermodynamics, statistical mechanics, and physical chemistry, for phase equilibrium conditions, phase diagrams of materials systems (multi-phase, multi-component, reacting and non-reacting systems)
• Transport phenomena for the transport of heat, mass, and momentum in materials processing.