Advanced Engineering Materials

Advanced engineering materials covers a variety of materials and related manufacturing techniques, such as composites; ceramics; intermetallics; coatings; and high-temperature, cellular, and biomedical materials.

Intermetallics, or intermetallic compounds, are any class of substances composed of definite proportions of two or more elemental metals, rather than continuously variable proportions as found in solid solutions. Intermetallics offer crystal structures with properties markedly different from their constituents. In these materials, the electrons holding them together are highly localized in the form of valence or ionic bonds. This helps intermetallics, such as nickel aluminide, to act as if the atoms are connected by rigid rods. These materials hold promise for scenarios in which high temperatures can increase dislocations in regular metals. Intermetallics have higher temperature tolerances, higher stress tolerances, and greater resistance to chemical attacks. The structure is brittle, and while they do not flow, when subject to high stress they are prone to failure by cracking.

This class of compounds was initially found or developed by William Hume-Rothery, a British founder of scientific metallurgy, known internationally for his work on the formation of alloys and intermetallic compounds.

These materials have other metallic characteristics, including luster and electrical and thermal conductivity. These include Ni-Al, Fe-Al, and Ti-Al intermetallics. Further, intermetallics have potentials and corrosion resistance differing from that of the matrix. Noble and inert metallics may induce galvanic corrosion of the matrix, whereas active intermetallics may go into dissolution and leave fissures or crevices. These compounds also frequently play a role in catalysts, since their formation on conventionally supported catalysts can result from modification or promoting the catalytic materials. The variety of crystal and electronic structures also allows for different structure-reactivity relationships of in situ stable intermetallic compounds and can enable the knowledge-based development of new catalytic systems circumventing a trial-and-error approach and resulting in new materials like noble metal-free hydrogenation catalysts.

Successful applications of intermetallics do not result from the knowledge of their composition, crystal structure, and intrinsic properties, but require a skillful, understanding control of their processing. These compounds achieved practical use before they were recognized as chemical objects owing to their relatively easy accessibility—with the simplest intermetallic compounds and alloys based on those that can be obtained by direct reaction of the compounds by melting, sintering, or reduction from oxides. The compounds have been found to have unique physical properties such as superconductivity, hydrogen storage capability, high saturization, and magnetization, with an increased understanding offering further opportunities.

A metal foam is a cellular structure consisting of a solid metal, which is frequently aluminum, with gas-filled pores comprising a large portion of the volume. The pores can be sealed (also known as closed-cell foam) or interconnected (also known as open-cell foam). The defining characteristic of metal foams is a high porosity: typically only 5 to 25 percent of the volume is the base metal. While the strength of the material is due to the square-cube law.

Microscopic images of different cellular structures of metal sponges or foams.

Metallic foam, overall, is a popular term used for most kinds of metallic materials that contain voids. The various expressions, which are not mutually exclusive and often used interchangeably, include the following:

Metal foams have attracted interest because of the attractive property combinations they can offer, particularly in terms of specific stiffness and specific energy absorption. These materials are often inspired by nature. Wood, bones, and sea sponges are well-known examples of these types of structures. In fact, solid metallic foams are the conserved image of the corresponding liquid metallic foam. Attempts to improve the foam quality of any type of foam concentrate on the foam physics, or the bubble formation, foam nucleation, growth, stability, development, or gas diffusion in the liquid state, where the foam structure is evolving.

The manufacturing process for metallic foams is classified based on the state of matter in which the metal is processed: solid, liquid, gaseous, or ionised. Liquid metal can be foamed by injecting gas or gas-releasing blowing agents or by producing supersaturated metal-gas solutions. Indirect methods involve investment casting, the use of space-holding filler materials, or the melting of powder compacts that contain a blowing agent. If inert gas is trapped in powder compacts, a subsequent heat treatment can produce cellular metals even in the solid state. The same is true for sintering methods, metal powder slurry foaming, or extrusion and sintering of polymer or powder mixtures. Electro-deposition or metal vapor deposition also allows for the production of highly porous metallic structures. There are non-destructive and destructive methods for developing metal foams.

The applications of metal foams are linked to the properties that such kinds of materials can offer and especially to those that are unique. Some properties relate to those of the matrix, such as elasticity, temperature, or corrosion resistance, while others appear in combination with the cellular structure, such as low density, large surface area, or damping. These properties depend largely on density, but are also influenced by the quality of the cellular structure in the sense of cell connectivity, cell roundness, and diameter distribution, fraction of the solid contained in the cell nodes, edges, or the cell faces.

Important properties for metal foam include the weight and density and whether mass-related properties can offer the needed properties for an application, such as damping or energy absorption. As well, for structural applications, the stiffness of the foam becomes an important property. When considering density, there is often a correction for the contribution to the density of the entrapped gas in the closed cell foams; for the tensile strength, a weak property of foams in general, as the resistance against a crack depends on the weakest link of the material, which is a thin cell wall which can break easily; and for the bending stiffness of a flat panel of metal foam of a given weight is inversely proportional to its density which offers a special loading case where foams show its potential.

One of the better properties of metal foams is the energy absorption potential. Foams can absorb a maximum of mechanical energy without exceeding a certain stress limit due to plastic, irreversible deformation over a long range. This property, usually isotropic, makes foams almost ideal crash barriers. As well, the mechanical properties of metal foams can be improved, similar as for bulk metals, by different hardening mechanisms, such as alloy composition, grain refinements, and heat treatments. Further, structural design and property optimizations are possible depending on the application and practical aspects. And heat or hardening treatments need to be compatible with the foaming procedure of foam stability.

The applications for metal foam can be broken down into structural applications; functional applications; architectural applications; and design, art, and decoration.

Biomaterials, or biomedical materials, play an integral role in medicine, in that they can restore function and facilitate healing for people after injury or disease. These can be natural or synthetic materials and are used in medical applications to support, enhance, or replace damaged tissue or biological function. The first use of biomaterials dates back to antiquity, when ancient Egyptians used sutures made from animal sinew.

The modern field of biomaterials combines medicine, biology, physics, and chemistry, with more influences from tissue engineering and materials science. Metals, ceramics, glass, and living cells and tissue can be used in creating a biomaterial. They can be reengineered into molded or machined parts, coatings, fibers, films, foams, and fabrics for use in biomedical products and devices, which may include heart valves, hip joint replacements, dental implants, or contact lenses. These are often biodegradable, and are sometimes bio-absorbable, meaning they can be gradually eliminated from the body after fulfilling a function.

Biomaterials are manufactured or processed to be suitable for use as medical devices, or components thereof, and are usually intended to be in long-term contact with biological materials. And research on biomaterials, for an example, has been connected with tissue engineering, which works to generate artificial tissues through the combination of cells with synthetic materials. Part of this requires the development of nano-structured scaffolds for tissue engineering originated from either natural or synthetic polymers. This can include the incorporation of bioactive and soluble glass particles or fibers to generate biocomposites.

Powder metallurgy is a metal-forming process performed by heating compacted metal powder to below their melting points. Although the process has been around for a long time, more recently it has been recognized as a better way of producing high-quality parts for a variety of applications. This success is due to the advantages in material utilization, shape complexity, near-net-shape, and dimensional control. These can contribute to sustainability, making powder metallurgy a recognized green technology.

The different technologies for powder metallurgy and for fabricating semi-dense and fully dense components include the conventional powder metallurgy process (referred to as press-and-sinter), metal injection molding, hot isostatic pressing, powder forging, and metal additive manufacturing. The materials made in these processes are not made of only metals and metallic alloys and often incorporate ceramics, ceramic fibers, and intermetallic compounds, including:

• Cermets
• Intermetallic compounds
• Metal matrix composites
• Nanostructured materials
• High-speed steels

Modern powders have been produced in more sophisticated processes that have enhanced finished part density, strength, and other properties. They have been used to mass-produce complex parts and designs for aerospace, power, medical, and automotive industries in order to validate the process overall. Since the 1970s, powders have been used as basic as grinding scrap metals into powder. This produced a powder with varying particle size, with a low quality that led to parts that were not fully dense. Which translated into high porosity and accelerated wear or surface fatigue failure. However, powder metallurgy was still used by engineers because of the difficult or impossible shapes capable when compared to other manufacturing processes.

Generally, powder metallurgy is competitive with casting, machining, and forging and often associated with the aerospace, medical, and power industries. And while these industries are low volume in nature, the powder metallurgy process and related parts have grown in part due to the automotive industry, with the production of gears and parts for mass production beginning by the 1980s. And in the medical field, biomaterials have been manufactured for the orthopedic applications using powder metallurgical techniques. The advantage of powder metallurgy-produced parts is that they contain the necessary porosity that comprises their mechanical properties closer to those of human bone, allowing the transport of bodily fluid and the growth of new tissue through the implant, which enhances the healing process. These pores can also be impregnated by drugs or growth factors, which can be eluted during healing to support the healing process.

Laser material processing uses laser energy to modify the shape or appearance of a material. The method of material modification provides a number of advantages, such as the ability to quickly change designs, produce products without the need for retooling, and improve the quality of finished products. Another advantage of laser material processing is compatibility with a multitude of materials. Compatible materials range from non-metals such as ceramics, composites, plastics and polymers, and adhesives, to metals, including aluminum, iron, stainless steel, and titanium.

As well, laser processing has reached a greater maturity, especially in the cases of laser cutting, welding, marking, and drilling processes. Developments in additive manufacturing and micro and nano manufacturing have enabled new capabilities that lasers can bring to the manufacturing industry. With the availability of high brightness lasers such as fiber and disk lasers, as well as ultra-fast lasers such as femto- and pico- second lasers, new beam material interaction phenomena appear.

And, with the development in laser material processing with hybrid laser arc welding, which combines a laser beam with gas metal arc welding to produce the hybrid process, allows for high processing speeds and penetration associated with laser beam welding, while allowing the accommodation of joint gaps, and efficient filler addition through the gas metal arc welding process.

The effects produced by laser energy interacting with a material strongly depend upon the wavelength and power level of the laser and absorption characteristics and chemical composition of material. Common wavelengths for laser material processing are 10.6 to 9.3 micron produced by CO₂ lasers, and 1.06 micron produced by fiber lasers. A range of power levels is available for each laser type to optimize the laser energy-material interaction. The effects of laser energy-material interaction are material ablation or material modification.

Material ablation is a physical process that removes material. Material is removed completely from top to the bottom surface or partially from the top of the material down to a specified depth. Material ablation is used for laser cutting, laser engraving, and laser drilling.

Surface material modification is a physical process that alters the properties or appearance of a material. Material modification is used for laser marking on the surface of a material by changing the appearance or properties of the material. Terms for laser cutting, laser engraving, and laser marking are commonly referred to as laser processes.

Examples of different laser processing for materials and the interactions.

There are various applications across industries where laser processing can offer new manufacturing opportunities. For example, in solar cells, ultrafast laser techniques have been developed to overcome the obstacles inherent in higher solar cell production. Challenges such as improved efficiency and high throughput have been met by realizing the potential of ultrafast lasers for surface modification and high-speed manufacturing. The nontraditional materials in these cells require different processing methods to be developed, and ultrafast lasers provide a fast, precise, non-contact method that can texture silicon or foil surfaces for improved light trapping.

Laser processing has also found use in biomedical applications, where ultrafast lasers have been used for microstructuring, nanoparticle formation, and other biomedical applications. The goal of these techniques is to influence the interaction between biological cells and artificial surfaces. This can offer a chance to increase the lifetime of implants, and in turn offer more biosensors and artificial tissues in biomedicine. The surface modifications necessary in a number of biomedical devices can be successful when micro-texturing is necessary and can be done with ultrafast lasers. For example, micro-texturing of metallic or ceramic substrates for bone implants improves biocompatibility.

As well, with the possibility of ultrafast laser processing, there is a suggestion that 3D objects can be fabricated with a size comparable to a living cell and comprising finer details, and could further suggest the realization of remotely controllable micro-bots to perform in vivo healing missions or the creation of all-optica information processors integrated on a single 3D microchip and robust non-erasable optical memory structures.

High temperature materials are any polyimides, metal alloys, and carbon-polymer composites resist damage, increase product lifetimes, and save energy. They can benefit industries such as coatings, adhesives, foams, thin films, composites, and resins. As thermodynamics indicates that the higher the temperature, the greater the efficiency of the conversion of heat to work; therefore, the development of materials for combustion chambers, piston, valves, rotors, and turbines blades that can function at ever-higher temperatures is of importance. As an example, the original steam engine had an efficiency loss of less than 1 percent, while modern turbines achieve efficiencies of 35 percent or more.

Part of this improvement has come from improved design and metalworking accuracy, but a large portion is the result of using improved high-temperature materials. These materials used in later designs including high-temperature alloys containing nickel, molybdenum, chromium, and silicon that were developed to not melt or fail at temperatures above 540 degrees Celsius. And, with new combustion processes nearing useful limits achievable with metals, new materials are being explored that can function at higher temperatures, particularly intermetallic compounds and ceramics.

The structural features that limit the use of metals at high temperatures are both atomic and electronic. All materials contain dislocations. The simplest of these are the result of planes of atoms that do not extend all through the crystal, so there is a line where the plane ends that has fewer atoms than normal. In metals, the outer electrons are free to move. And this gives a delocalized cohesion that, when stress is applied, dislocations can move to relieve the stress. The result is that metals are ductile: they can be easily worked into desired shapes, but when stress they yield plastically rather than breaking immediately. This is a desirable feature, but the higher the temperature, the less useful the material becomes. Ceramics such as silicon carbide and intermetallics such as nickel aluminide hold promise because the electrons that hold them together are localized in the form of valence or ionic bonds. And because of these bonds, the materials can operate at far higher temperatures. As well, when these materials fail under stress, they break rather than gradually yield plastically.

For metal alloys, there are three categories of heat resistant alloys: alloys that are subjected to small amounts of heat stress, alloys that are subjected to moderate amounts of heat stress, and alloys that are subjected to heat stress for long periods of time. These include the following alloys:

The usage of molecular or polymer precursors provide a tool for controlling the composition, microstructure, and resulting properties of particular ceramics. The properties of these ceramics are tailored by the choice of the diversity of precursor substances leading to advantages in processing of layers, infiltration processes, or preparation of matrix materials for composites as well as bulk materials. The production of high-purity ceramic materials from low-molecular weight, inorganic or organic element precursors can allow for the precise tailoring of the ceramic material which enables new high-temperature or electronic applications.

The properties of these materials include high hardness, modulus, and strength; stability in extreme environments with good thermostability, high oxidation, and corrosion resistance; offers high adhesive attraction and low surface tension; durability with wear resistance, anti-fouling, and anti-biofilm formation properties; and low toxicity and higher biocompatibility. Overall, polymer-derived ceramics are intended to offer mechanical and chemical properties stable up to temperatures slightly below 2000 degrees Celsius.

Since the 1990s, there has been an increase in interest and research into polymer-derived ceramics for high temperature applications. This is because polymer-derived ceramics are unique because amorphous ceramics are not achievable through other ceramic synthesis techniques. Polymeric network structure become amorphous microstructures during pyrolysis and form nanodomains of 1 to 3 nm, which provides good creep and oxidation resistance up to 1500 degrees Celsius. Other applications include fibers, matrices, microelectromechanical systems, semiconductors, membranes, coatings and adhesives.

For processing of polymer-derived ceramics, the conversion of polymeric precursors under an atmosphere of inert gas at moderate temperatures has shown to be a versatile route for non-oxidic ceramic materials, especially as compared to the more energy consuming conventional sintering of similar materials. As well, generally, the starting materials are inexpensive and can easily be processed using well-established shaping methods used for the processing of polymers. By varying the precursor composition or the temperature regime of pyrolysis or by addition of filler particles hybrid materials and advanced ceramics with interesting properties can be obtained. Porous or dense microstructures, high temperature, oxidation, and chemical resistance or magnetic and electroconducting behavior can be provided and engineered for different applications.