In parallel, new powder milling, spray drying and sintering techniques resulted in improved hardmetal properties and performance. Notably, the continuous improvement of vacuum sintering technology and, starting from the late 1980’s, hot isostatic pressure sintering (SinterHIP) led to new standards in hardmetal quality.
The history of tungsten powder metallurgy, and especially that of the hardmetal industry, is characterized by a steadily widening range of available grain sizes for processing in the industry; while, at the same time, the grain size distribution for each grade of WC powder became narrower and narrower.
The most important reason for this widening of the spectrum of available WC grades is that, besides those variations achieved by cobalt contents and some carbide additives, the properties of WC-Co hardmetals such as hardness, toughness, strength, modulus of elasticity, abrasion resistance and thermal conductivity can be widely varied by means of the WC grain size. While the spectrum of available WC grain sizes ranged from 2.0 to 5.0 µm in the early days of the hardmetal industry in the mid 1920’s, the grain sizes of WC powders now used in hardmetals range from 0.15 µm to 50 µm, or even 150 µm for some very special applications.
Based on the wide range of grain sizes now available, not only very hard and abrasion resistant, but also very tough, hardmetals can be produced for widespread applications in high tech tools, wear parts and mining tools as well as for many sectors of the engineering industry.
There has been a rapid development in mining and stone cutting tools, with improved performance which has led to the increasing substitution of steel tools by cemented carbide tools, in particular in the oil industry. Notably, the use of very coarse grained hardmetals is growing in this application area.
A large portion of the tungsten volume in cemented carbide is today used in wear part applications, where there is a wide range of products from the very small (such as balls for ball-point pens) to large and heavy products, such as punches, dies or hot rolls for rolling mills in the steel industry.
Most of these wear parts and the mining tools are made of straight WC-Co hardmetals without any addition of other carbides.
Fine and ultrafine grained WC hardmetals have become more and more important today in the field of wear parts, tools for chipless forming and cutting tools for cast iron, non ferrous alloys and wood.
Application range of straight grade cemented carbides
The first submicron hardmetals were launched on the market in the late 1970s and, since this time, the microstructures of such hardmetals have become finer and finer. The main interest in hardmetals with such finer grain sizes derives from the understanding that hardness and wear resistance increase with decreasing WC grain size.
A special application for these fine or ultrafine WC hardmetals, involving large quantities of cemented carbide, is in drills for the drilling of the very fine holes in printed circuit boards for the computer and electronic industries. For this purpose, new cemented carbide compositions, based on extremely fine-grained carbide, have been introduced.
Basic data for different WC-Co and WC-(W,Ti,Ta,Nb)C-Co hardmetal grades | ||||||
Grade (wt%) | Hardness HV30 | Compressive strength (N × mm -2) | Transverse rupture strength (N × mm -2) | Young’s modulus (kN × mm -2) | Fracture toughness (MPa × m -1/2) | Mean thermal expansion coefficient (10 -6 × K -1) |
WC-4Co | 8.5 | 5.0 | ||||
WC-6Co/S* | 10.8 | 6.2 | ||||
WC-6Co/M** | 9.6 | 5.5 | ||||
WC-6Co/C*** | 12.8 | 5.5 | ||||
WC-25Co/M | 14.5 | 7.5 | ||||
WC-6Co-9.5 (Ti,Ta,Nb)C | 9.0 | 6.0 | ||||
WC-9Co-31 (Ti,Ta,Nb)C | 8.1 | 7.2 |
S* = submicron; M** = fine/medium; C*** = coarse
UNIT 8 | METAL CLAY |
Metals such as silver, bronze, copper, gold, and steel have been made into materials known as metal clay. These materials are used by artists and jewelers to make art objects in a home or studio setting. When fired the organic binder burns off and the microscopic metal powder in the clay is sintered. Silver and gold metal clays can be fired in a normal kiln environment while most base metal clays must be fired in a reduced atmosphere using activated carbon to prevent oxidation from inhibiting proper sintering.
What is Metal Clay?
Metal clay first came out in Japan in 1990[1] to allow craft jewelry makers to make sophisticated looking jewelry without the years of study needed to make fine jewelry.
Metal clay is a crafting medium consisting of very small particles of metal such as silver, gold, platinum, or copper mixed with an organic binder and water for use in making jewelry, beads and small sculptures. Originating in Japan in 1990, metal clay can be shaped just like any soft clay, by hand or using molds. After drying, the clay can be fired in a variety of ways such as in a kiln, with a handheld gas torch, or on a gas stove. The binder burns away, leaving the pure sintered metal. Shrinkage of between 8% and 30% occurs (depending on the product used). Alloys such as bronze or steel also are available.
Silver and gold metal clays have been available to the public since the mid-1990's. Mitsubishi and Aida Chemicals make Precious Metal Clay (PMC) and Art Clay Silver respectively. The ingredients are just fine silver particles, an organic binder and water. They can be worked in the same way as ceramic clays - rolled, formed, textured and cut. When the water is allowed to evaporate and the dry piece is fired, the result is a 99.9% fine silver item which is hallmark quality. There is also a gold variety of metal clay which works the same way. To find out about the history of PMC, click here.
Bill Struve was the first person to develop bronze and copper clays which come as ready mixed lumps of clay and have the brand name Bronzclay™ and Copprclay™. These were released in mid 2008.
Hadar Jacobson has produced bronze and copper clays that come as powders which you mix with water. Both these products work in a similar way to the silver and gold clays although the firing is more complicated. The original bronze and copper clays require an oxygen free environment to fire so the dried pieces are buried in activated carbon in a stainless steel pan and fired in a kiln. They cannot be torch fired and require a very long firing in the kiln.
The makers of Art Clay, Aida Chemicals launched their own copper clay, called Art Clay Copper, in September 2009. This has a much simpler firing schedule than the other copper clays available and can be torch fired or kiln fired. Find out more about Art Clay Copper here.
Prometheus Copper Clay is also now available. Prometheus Copper Clay fires in 30 minutes at 850 degrees C and requires no special firing pan. And it comes as syringe clay as well as lump clay.
Hadar Jacobson launched a copper clay powder and a bronze clay powder with a simpler firing schedule in December 2009. They are called Quick-Fire Copper and Quick-Fire Bronze.
Prometheus™ bronze clay is also available. Developed in Europe, this bronze clay has a shorter, lower temperature firing schedule without the need for activated carbon. It still needs a kiln to fire it. Available as lump clay and in syringe form.
Hadar Jacobson has also developed Stainless Steel Clay which became available in December 2009. It comes in two forms, traditional and Quick-Fire. Hadar launched White Bronze Clay in early 2010. In June 2010, Pearl Grey Steel clay powder became available. In July 2011, Hadar launched Rose Bronze Clay.
Bill Struve launched Fast Fire Bronzclay™ in July 2010. Mitsubishi launched PMC Pro in July 2010. Both these products were launched at the PMC Conference.
In December 2010, Meteor bronze clay was launched by a French developer and then Meteor copper clay was released in January 2011. Meteor white bronze clay was launched in July 2011.
Metal Clay Mania Clay has developed bronze, copper and brass clays that come in powder form.
In early 2011, Lisa Cain developed hallmark quality Sterling silver metal clay by combining the existing commercial clays. She shared her method of mixing and firing this clay with the metal clay community through the March 2011 edition of Metal Clay Artist Magazine.
Check back often to find out what's new in the world of metal clay, send us your questions and we'll add them to the FAQ page.
If you are a complete beginner, check out our metal clay beginners page. You'll find basic information, links to more resources and a useful article about how to work with metal clay without spending lots of money on tools and equipment.
Silver Metal Clay
Silver metal clay results in objects containing 99.9% pure silver, which is suitable for enameling. Although gold metal clay is more expensive, it provides richer color. Lump metal clay is sold in sealed packets to keep it moist and workable. The silver versions are also available as a softer paste in a pre-filled syringe which can be used to produce extruded forms, in small jars of slip and as paper-like sheets, from which most of the moisture has been removed. Common brands of silver metal clay include Precious Metal Clay (PMC) and Art Clay Silver (ACS).
Precious Metal Clay (PMC)
PMC was developed in the early 1990s in Japan by metallurgist Masaki Morikawa.[3] As a solid-phase sintered product of a precious metal powder used to form a precious metal article,[1] the material consists of microscopic particles of pure silver or fine gold powder and a water-soluble, non-toxic, organic binder that burns off during firing. Success was first achieved with gold and later duplicated with silver.
The PMC brand includes the following products:
The original formula of PMC, now called "standard": fired at 900 °C (1,650 °F) for 2 hours, shrinks by 30% during firing.
PMC+: fired at 900 °C (1,650 °F) for 10 minutes or 800 °C (1,470 °F) for 30 minutes; shrinks 15%, due to a particle size reduction. PMC+ is also available in sheet form which can be worked like paper; for example, for origami.
PMC3: fired at 599 °C (1,110 °F) for 45 minutes or 699 °C (1,290 °F) for 10 minutes; shrinks by 10%. It can also be fired using a butane torch by heating it to orange heat for at least 2 minutes. It has a longer working life than the older formulations. It is also available in slip and paste forms which can be painted onto the surface of an object to be used as a mould.
Aura 22: a 22 k gilding material, a gold paste intended to be painted onto the surface of silver PMC pieces, or ready-made silver objects.
PMC Pro: a harder product which is only 0.900 silver, hence it canot be hallmarked as sterling silver. It also requires kiln firing in a tub of activated carbon for 1 hours at 760 °C (1,400 °F).
Art Clay Silver (ACS)
ACS was developed by AIDA Chemical Industries, also a Japanese company. ACS followed PMC Standard with their Art Clay Original clay (more like PMC+ than PMC Standard), which allows the user to fire with a handheld torch or on a gas hob. Owing to subtle differences in the binder and suggested firing times, this clay shrinks less than the PMC versions, approximately 8–10%.
Further developments introduced the Art Clay Slow Dry, a clay with a longer working time. Art Clay 650 and Art Clay 650 Slow Dry soon followed; both clays can be fired at 650 °C (1,202 °F), allowing the user to combine the clay with glass and sterling silver, which are affected negatively by the higher temperatures needed to fire the first generation clays. AIDA also manufacturers Oil Paste, a product used only on fired metal clay or milled fine silver, and Overlay Paste, which is designed for drawing designs on glass and porcelain.
In 2006 AIDA introduced the Art Clay Gold Paste, a more economical way to work with gold. The paste is painted onto the fired silver clay, then refired in a kiln, or with a torch or gas stove. When fired, it bonds with the silver, giving a 22ct gold accent. The same year also saw Art Clay Slow Tarnish introduced, a clay that tarnishes less rapidly than the other metal clays.
Lump Metal Clays
Lump metal clay in bronze was introduced in 2008 by Metal Adventures Inc. and in 2009 by Prometheus. Lump metal clays in copper were introduced in 2009 by Metal Adventures Inc. and Aida. Because of the lower cost, the bronze and copper metal clays are used by artists[5] more often than the gold and silver metal clays in the American market place. Due to Hallmarking requirements laid out in the UK Bronze and Copper are not regarded as highly. Furthermore, due to the complex firing process and the fact that during the firing process acidic vapour is emitted and can result in extreme wear and tear on your kiln eventually leading to the requirements of a replacement kiln, it is often regarded as not as economically logical to use these cheaper metals. The actual creation time of a PMC Bronze or Copper piece is also far greater than that of its PMC3 counterpart.
Base Metal Clays
Base metal clays, such as bronze, copper, and steel metal clays are best fired in the absence of oxygen to eliminate the oxidation of copper by atmospheric oxygen. A simple means to accomplish this (place the pieces in activated carbon inside a container) was developed by Bill Struve.
MIM PRODUCTS
Metal injection moulding (MIM)has over the past decade established itself as a competitive manufacturing process for small precision components which would be costly to produce by alternative methods
It is capable of producing
· in both large and small volumes
· complex shapes
· from almost all types of materials including metals, ceramics, intermetallic compounds, and composites.
Components made by MIM technology are finding new applicationsin industry sectors such as automotive, chemical, aerospace, business equipment, computer hardware, bio-medical and armaments.
MIM and Powder Metallurgy
Metal injection moulding (MIM) is a development of the traditional powder metallurgy (PM) process and is rightly regarded as a branch of that technology.
The standard PM process is to compact a lubricated powder mix in a rigid die by uniaxial pressure, eject the compact from the die, and sinter it.
Quite complicated shapes can be and are regularly being produced by the million, but there is one significant limitation as regards shape.
After compaction in the die the part must be ejected, i.e. pushed out of the die cavity. It will be obvious, therefore, that parts with undercuts or projections at right angles to the pressing direction cannot be made directly.
That limitation is substantially removed by the metal injection moulding process developed during the last decade and now expanding rapidly.
Plastic Material
The use of injection moulding for the production of quite intricate parts in a number of plastic materials has been known for many years, and most of us come into contact with them in some form or other every day.
One important feature of such parts is that they are relatively cheap.
However, for engineering applications these thermo-plastic materials have quite inadequate mechanical properties.
Metal and Ceramic Material
Some improvement is made possible by the use of solid fillers - ceramic or metal powders – but the real breakthrough occurred when it was found possible to incorporate a very high volume fraction of metal powder in a mix so that, instead of a filled plastic part, a plastic-bonded metal or ceramic part is produced.
Careful removal of the plastic binder leaves a skeleton of metal or ceramic which, although fragile, can be handled safely and sintered in much the same way as traditional die compacted parts.
After sintering densities of 95% or more are reached and the mechanical properties are, for that reason, generally superior to those of traditional PM parts.
UNIT 10` | NANOTECHNOLOGY |
Nanotechnology may be defined as the ability to work at the molecular level, atom by atom, to create large structure with fundamentally new molecular organization. Since this technology is developed from the scale range of 10-9 m or 10 A, its interaction by intermolecular and inter-atomic forces will lead to the next industrial revolution. Thus, providing the ability to control and manipulate the physical, biological, and chemical properties of a system. In other words, materials will be constructed from the bottom-up instead of conventional methods, top-down methods. The ability to largely influence the properties and structure of materials has made nanotechnology a quickly developing field that has been gaining interests among the public due in part to the possibilities that this technology provides.
Many scientists suggest that nanotechnology will fuel enormous growth within the areas of Biotechnology and Bio-medical Chemistry, and Atomic Positioning. Here the possibilities are endless. Using nanotechnology will play a major role in the field of Bio-medical Chemistry. Many pharmaceutical companies are performing research to decline the particle size. If drugs were able to have smaller particle size they would be better absorbed by digestive tract lining therefore the amount necessary would be reduced making medicines more affordable. The ability to deliver antibiotics in aerosol form to the lungs would mean easier ways of treating infections like tuberculosis. By decreasing particle size, the bioavailability increases. This would lead to faster-reacting drugs and many other applications in the health industry.
“Nanorobots,” are imagined molecules that could be made to seek out certain cells and perform a task in that particular area of the body, for example, the seeking out of liver cells and facilitating the production of a certain proteins. By developing manmade nano-robots, it would be possible to manipulate and self-assemble cells that cause diseases and cancers and eliminate current health problems. The potential of nano-robots in the field of Biotechnology reaches beyond healing living cells.
Nanotechnology experts contend that human life can be extended by nanorobots’ ability to self-copy and keep human cell replicating in normal ways, which reduce aging. The ability to build materials from the smallest particles means that atomic properties can be adapted for certain purposes for example; the electronics field. The ability to manipulate properties means that these materials can be made to be better conductors leading to circuits that are smaller and faster. The circuits will in turn be able to achieve more complicated functions. The potential for this technology is endless. For example, the National Nanotechnology Initiative have set these challenges for research scientists to accomplish: reducing entire content of the library of Congress to a sugar cube, constructing new plastics and other polymers with the strength of steel, improving computers speed by a factor of millions, and etc. Improving the performance of a catalyst is very important for economical and social aspects of society.
In an article discussing the economic contributions of catalysis noted that “one-third of material gross national product in the U.S. involves a catalytic process somewhere in the production chain”. An example is the petroleum industry. Many of their processes heavenly depend on catalysts to produce oil, gasoline, and other petroleum products. If a catalyst’s efficiency improves by 3 %, this may lead to an increase of millions, possible billions of dollars for the petroleum industry. The social impact would be the price reduction of different petroleum products for the consumer.
The environment also benefits since harmful bi-products will subside as a result of increasing the yield of a process. In order to improve catalyst, scientists are investigating the nanocatalyst as an avenue. The advantages of using a nanocatalyst are profound. By nanocatalyst being very small in size, the particle size creates a very high surface to volume ratio. This increase the performance of the catalyst since there is more surface to react with the reactants. Another advantage is that nanocatalysts are able to place where traditional catalysts will not fit.
Nanopowders
The basis of nanotechnology is the ability to form nano-sized particles, for example nanopowders, which are solid particles that measure on the nanoscale, usually comprised of three to five molecules together. Nanopowders can be used in most of the aforementioned applications; therefore, they have been a field of great interest. Nanopowders have been of extreme interest in the pharmaceutical field. Drug delivery has been impacted in several ways due to the advances in nanopowder technology. Smaller particles are able to be delivered in new ways to patients, through solutions, oral or injected, and aerosol, inhaler or respirator. New production processes allow for encapsulation of pharmaceuticals which allow for drug delivery where needed within the body. Dosing of pharmaceuticals had also improved. Smaller particles mean better absorption by the body therefore less drug is needed. Because of a combination of these, side effects are lessened due to better use of pharmaceuticals.
Production of Nanopowders
Although there are many uses for nanopowders within the pharmaceutical field, the production of these powders has been of great interests. For this reason, at this point in time the research around nanopowders has been focused mostly on methods of production.
Conventional Methods
Conventional methods of particle size reduction include milling, grinding, jetmilling, crushing, and air micronization. There are several drawbacks to these methods. First, they might not accomplish the desired amount of particle size reduction. The second drawback is associated with the physical and chemical properties of the materials undergoing size reduction.
Certain compounds are chemically sensitive or thermo-liable, such as explosives, chemical intermediates, or pharmaceuticals which cannot be processed using conventional methods due to the physical effects of these methods. Other compounds such as, polymers, pigments or dyes, etc. maybe difficult to process by conventional methods due to physical properties such as physical degradation under high pressures or temperatures, “softness”, or waxy texture.
Nanopowder Production
The most common methods of nanopowder production involve the rapid depressurization of saturated solutions. This causes the saturate to precipitate out of the solution in the form of nanopowder.
There are several methods for production of nanopowders. One of them is through a chemical route. Using the point of zero charge concept, single molecules or small packets of molecules may be synthesized, usually on a support.
· Homogeneous nucleation occurs at fast saturation and temperature drops.
· Certain dye, chemical intermediates, pharmaceutical and biological compounds, polymers and explosives, which are chemically sensitive, thermo-labile, waxy or soft, are difficult to powderize by conventional techniques.
· Most new techniques for nanopowder production use CO2 non-toxic, non-flammable, cheap and easy recyclable.
· beta=(actual concentration/saturation concentration)
· Size, size distribution, morphology, polymorphic nature all depend on evolution of beta
· Where large levels of super saturation are quickly attained small particle size distribution occurs and smaller crystals are formed due to nucleation phenomena being enhanced over crystal growth, meaning smaller particles.
· The efficiency of a crystallization process is highly related to the compound solubility and solute concentration evolution, which determines the super saturation during the crystallization process.
Rapid Expansion of Supercritical Fluids
Rapid Expansion of Supercritical Solutions (RESS) is a crystallization technique that uses the properties of a supercritical fluid, typically CO2, as a solvent to facilitate nanopowder production.
The RESS process is described in two steps: solubilization and particle formation. The driving force for this process is caused by the rapid depressurization of the supercritical fluid dissolved with the solute of interest through a nozzle to cause fast nucleation and fine particle generation.
RESS process operation As showing in Figure 1, the RESS process begins by subjecting liquid CO2 to high pressure and temperature in order to reach the supercritical region of the fluid. The supercritical fluid is then mixed in the extraction autoclave with the solute. At the extraction step, the flow rate plays a role since the thermodynamic equilibrium may not have been obtained. This may affect the solvating power of the solvent since temperature fluctuations occur outside the region of thermodynamic equilibrium. The next step involves the depressurization of the mixture from high pressure to atmospheric pressure by a nozzle. This rapid decrease in pressure causes nucleation by lowering the solvating power of solvent. Since CO2 is in the gaseous state at ambient conditions, the solute precipitates and is gather in the collecting device. The CO2 is then purges out of the container through a valve.
Advantages and Disdvantages
There are many advantages to the RESS process. Although the process takes place at high pressures, the temperatures required are fairly low. This lowers energy costs. Another advantage is the lack of significant environmental hazards. Since CO2 is not a hazardous material when released in the quantities used in this process and it is almost pure CO2, meaning that there is little of the solute left after the process is completed, there are almost no environmental considerations that have to be taken into account for this process. The greatest advantage of all is the ability to make extremely small particles in the micro or nano scale. For all the advantages though there are some disadvantages to this process. The major disadvantage is the cost of operating at the high pressures required for the process. High pressures are costly due to pumping expenses as well as the equipment costs. Besides the high costs of being a high pressure process, the other major disadvantage is that the RESS process only works for solutes that are soluble in CO2. If the solute is not soluble in CO2 then another process must be considered.
Supercritical Anti-Solvent The Supercritical Anti-Solvent process (SAS) uses solvent/anti-solvent binary systems to induce the formation of nano and micro-size particles. The supercritical fluid (i.e. CO2) acts as an anti-solvent that causes the crystallization of the solute. The main driving force for this process is the droplet formation, which is caused by the solvent/anti-solvent interaction. Since many results are influenced by the process arrangement, an investigation of the different SAS-related processes will be discussed.
Batch operation The precipitation vessel or precipitator is loaded with a specific quantity of liquid solution. The supercritical fluid CO2 is injected into the vessel, typically from the bottom to achieve a better solvent and anti-solvent mixture. Once injected and due to the dissolution of the compressed gas, the expanded solvent has a lower solvating power than the pure solvent. This mixture becomes supersaturated and solute precipitates to form micro-size particles. After a holding time, the expanded solution is drained under isobaric conditions to wash and clean the precipitated particles.
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