In powder coating, the powdered paint may be applied by either of two techniques.
The item is lowered into a fluidised bed of the powder, which may or may not be electrostatically charged, or
The powdered paint is electrostatically charged and sprayed onto the part.
The part is then placed in an oven and the powder particles melt and coalesce to form a continuous film.
Types of powder coatings
There are two main categories of powder coatings: thermosets and thermoplastics. The thermosetting variety incorporates a cross-linker into the formulation. When the powder is baked, it reacts with other chemical groups in the powder to polymerize, improving the performance properties. The thermoplastic variety does not undergo any additional reactions during the baking process, but rather only flows out into the final coating.
The most common polymers used are polyester, polyurethane, polyester-epoxy (known as hybrid), straight epoxy (fusion bonded epoxy) and acrylics.
Production:
The polymer granules are mixed with hardener, pigments and other powder ingredients in a mixer
The mixture is heated in an extruder
The extruded mixture is rolled flat, cooled and broken into small chips
The chips are milled and sieved to make a fine powder
The powder coating process
The powder coating process involves three basic steps:
Part preparation or the pre-treatment
The powder application
Curing
During the curing process (in the oven) a chemical cross-linking reaction is triggered at the curing temperature and it is this chemical reaction which gives the powder coating many of its desirable properties.
Preparation
The basis of any good coating is preparation. The vast majority of powder coating failures can be traced to a lack of a suitable preparation.
Removal of oil, soil, lubrication greases, metal oxides, welding scales etc. is essential prior to the powder coating process. It can be done by a variety of chemical and mechanical methods. The selection of the method depends on the size and the material of the part to be powder coated, the type of soil to be removed and the performance requirement of the finished product.
Chemical pre-treatments involve the use of phosphates or chromates in submersion or spray application. These often occur in multiple stages and consist of degreasing, etching, de-smutting, various rinses and the final phosphating or chromating of the substrate. The pre-treatment process both cleans and improves bonding of the powder to the metal. Recent additional processes have been developed that avoid the use of chromates, as these can be toxic to the environment. Titanium zirconium and silanes offer similar performance against corrosion and adhesion of the powder.
Another method of preparing the surface prior to coating is known as abrasive blasting or sandblasting and shot blasting. Blast media and blasting abrasives are used to provide surface texturing and preparation, etching, finishing, and degreasing for products made of wood, plastic, or glass. The most important properties to consider are chemical composition and density; particle shape and size; and impact resistance.
Silicon carbide grit blast medium is brittle, sharp, and suitable for grinding metals and low-tensile strength, non-metallic materials. Plastic media blast equipment uses plastic abrasives that are sensitive to substrates such as aluminum, but still suitable for de-coating and surface finishing. Sand blast medium uses high-purity crystals that have low-metal content. Glass bead blast medium contains glass beads of various sizes.
Cast steel shot or steel grit is used to clean and prepare the surface before coating. Shot blasting recycles the media and is environmentally friendly. This method of preparation is highly efficient on steel parts such as I-beams, angles, pipes, tubes and large fabricated pieces.
Different powder coating applications can require alternative methods of preparation such as abrasive blasting prior to coating. The online consumer market typically offers media blasting services coupled with their coating services at additional costs.
The preparation treatment is different for different materials.
In general, for all applications the preparation treatment for aluminium is as follows:
Clean | Or | Clean |
Rinse | Rinse | |
Etch | Etch | |
Rinse | Rinse | |
Chromate | Phosphate | |
Rinse | Rinse | |
Demin Rinse | Demin Rinse |
Oils and greases are removed in weak alkali or neutral detergent solutions and the surface is etched to remove heavy oxides. After rinsing, the aluminium is dipped into a chromate or phosphate solution to form a conversion coating on the aluminium. This film is chemically attached to the aluminium. After rinsing the aluminium is finally rinsed in demineralised water. Some non-chrome, dried in place pretreatment is beginning to come onto the market; currently, these are not recommended for exterior applications.
The conversion coating has two functions:
It presents a surface to the powder which favours adhesion more than the oxides that form very readily on aluminium surfaces, and
It reduces the incidence of under film corrosion, which may occur at holidays in the coating.
The use of demineralised water reduces the presence of chemical salts on the aluminium surface. These salts have been found to cause filiform corrosion in humid conditions.
For steel the preparation for interior applications may be:
Clean
Rinse
Derust
Rinse
Iron Phosphate
Rinse
Acidulated Rinse
For exterior applications:
Clean
Rinse
Etch
Rinse
Grain Refine
Zinc Phosphate
Rinse
Acidulated Rinse
The grain refiner is used after acid cleaning of steel surfaces and before zinc phosphating, otherwise the zinc phosphate coatings produced will be very coarse with low adhesion. The powder coating applied to a coarse phosphate will produce rough coatings (a little like "sandpaper") and possess low adhesion.
For hot dipped galvanized coatings, which have been stored for more than about 4 hours before powder coating, the following process is necessary for exterior applications.
Clean
Rinse
Etch
Rinse
Grain Refiner
Rinse
Zinc Phosphate
Acidulated Rinse
The etch is required to remove the zinc corrosion products which begin to form almost immediately the zinc is removed from the galvanizing kettle. The grain refiner ensures a fine phosphate is produced.
Powder application processes
The most common way of applying the powder coating to metal objects is to spray the powder using an electrostatic gun, or corona gun. The gun imparts a positive electric charge on the powder, which is then sprayed towards the grounded object by mechanical or compressed air spraying and then accelerated toward the workpiece by the powerful electrostatic charge. There is a wide variety of spray nozzles available for use in electrostatic coating. The type of nozzle used will depend on the shape of the workpiece to be painted and the consistency of the paint. The object is then heated, and the powder melts into a uniform film, and is then cooled to form a hard coating. It is also common to heat the metal first and then spray the powder onto the hot substrate. Preheating can help to achieve a more uniform finish but can also create other problems, such as runs caused by excess powder.
How is it done - electrostatic spray?
The powder is applied with an electrostatic spray gun to a part that is at earth (or ground) potential.
Before the powder is sent to the gun it is fluidised:
to separate the individual grains of powder and so improve the electrostatic charge that can be applied to the powder and
so that the powder flows more easily to the gun.
Because the powder particles are electrostatically charged, the powder wraps around to the back of the part as it passes by towards the air offtake system. By collecting the powder, which passes by the job, and filtering it, the efficiency of the process can be increased to 95% material usage.
The powder will remain attached to the part as long as some of the electrostatic charge remains on the powder. To obtain the final solid, tough, abrasion resistant coating the powder coated items are placed in an oven and heated to temperatures that range from 160 to 210 degrees C (depending on the powder).
Under the influence of heat a thermosetting powder goes through 4 stages to full cure: Melt, Flow, Gel, Cure. The final coating is continuous and will vary from high gloss to flat matt depending on the design of the powder by the supplier.
Powder coating guns
There are at least three types of electrostatic guns in use:
Corona charging guns where electric power is used to generate the electrostatic charge. Corona guns are either internal or external charging.
Tribo charging guns where the electrostatic charge is generated by friction between the powder and the gun barrel. In this case, the powder picks up a positive charge while rubbing along the wall of a Teflon tube inside the barrel of the gun. These charged powder particles then adhere to the grounded substrate. Using a tribo gun requires a different formulation of powder than the more common corona guns. Tribo guns are not subject to some of the problems associated with corona guns, however, such as back ionization and the Faraday cage effect.
"Bell" charging guns where the powder is charged by being "flung" from the perimeter of the "bell"
Not all powder is applied using guns. One system makes use of electrostatic tunnels. Powder can also be applied using specifically adapted electrostatic discs.
Another method of applying powder coating, called the fluidized bed method, is by heating the substrate and then dipping it into an aerated, powder-filled bed. The powder sticks and melts to the hot object. Further heating is usually required to finish curing the coating. This method is generally used when the desired thickness of coating is to exceed 300 micrometres. This is how most dishwasher racks are coated.
Electrostatic fluidized bed coating: Electrostatic fluidized bed application uses the same fluidizing technique and the conventional fluidized bed dip process but with much less powder depth in the bed. An electrostatic charging medium is placed inside the bed so that the powder material becomes charged as the fluidizing air lifts it up. Charged particles of powder move upward and form a cloud of charged powder above the fluid bed. When a grounded part is passed through the charged cloud the particles will be attracted to its surface. The parts are not preheated as they are for the conventional fluidized bed dip process.
Electrostatic magnetic brush (EMB) coating: an innovative coating method for flat materials that applies powder coating with roller technique, enabling relative high speeds and a very accurate layer thickness between 5 and 100 micrometre. The base for this process is conventional copier technology. Currently in use in some high- tech coating applications and very promising for commercial powder coating on flat substrates (steel, aluminium, MDF, paper, board) as well in sheet to sheet and/or roll to roll processes. This process can potentially be integrated in any existing coating line.
Curing
When a thermoset powder is exposed to elevated temperature, it begins to melt, flows out, and then chemically reacts to form a higher molecular weight polymer in a network-like structure. This cure process, called crosslinking, requires a certain temperature for a certain length of time in order to reach full cure and establish the full film properties for which the material was designed. Normally the powders cure at 200°C (390°F) for 10 minutes. The curing schedule could vary according to the manufacturer's specifications.
The application of energy to the product to be cured can be accomplished by convection cure ovens or infrared cure ovens.
How is colour introduced?
Colour is added to powder coatings during the manufacturing process, i.e. before the powder reaches the powder coater. There is little that can be done to change the colour consistently, once the powder leaves the manufacturing plant.
Removing powder coating
Methylene chloride is generally effective at removing powder coating, however most other organic solvents (acetone, thinners, etc.) are completely ineffective. Most recently the suspected human carcinogen methylene chloride is being replaced by benzyl alcohol with great success. Powder coating can also be removed with abrasive blasting. 98% sulfuric acid commercial grade also removes powder coating film. Certain low grade powder coats can be removed with steel wool, though this might be a more labor-intensive process than desired.
Why powder coat?
Powder coating produces a high specification coating which is relatively hard, abrasion resistant (depending on the specification) and tough. Thin powder coatings can be bent but this is not recommended for exterior applications.
The choice of colours and finishes is almost limitless, if you have the time and money to have the powder produced by the powder manufacturer.
Powder coatings can be applied over a wide range of thickness. The new Australian Standard, "AS/NZS 4506 - Thermoset powder coatings", will recommend 25 micron minimum for mild interior applications and up to 60 micron minimum for exterior applications. Care must be exercised when quoting minimum thickness because some powder will not give "coverage" below 60 or even 80 micron. "Coverage" is the ability to cover the colour of the metal with the powder. Some of the white colours require about 75 micron to give full "coverage". One of the orange colours must be applied at 80 micron.
Colour matching is quite acceptable batch to batch.
Installations and maintenance
During installations, the powder coating should be protected from damage due to abrasion and materials of construction such as mortar and brick cleaning chemicals.
Once installed, maintaining the initial appearance of a powder coating is a simple matter. The soot and grime which builds up on surfaces from time to time contains moisture and salts which will adversely affect the powder coating and must be removed. Powder coatings should be washed down regularly (at least once each 6 months in less severe applications and more often in marine and industrial environments). The coating should be washed down with soapy water -- use a neutral detergent - and rinsed off with clean water.
When powder coated items are installed without damage to the powder coating and they are maintained regularly, they should be relatively permanent. The correctly applied coating, although not metallurgically bonded to the metal will not crack, chip or peel as with conventional paint films.
Current market
In 2010, the global demand on powder coatings amounts approximately US$5.8 billion. Driven by the development of new material, new formulations and advancement of equipment and application processes, the powder coating market presents a rapid annual growth of around 6% through 2012 to 2018. Currently, the industrial uses are the largest application market of powder coatings. Automotive industry experiences the most dynamic growth. Steady and strong growth is also expected by furniture and appliance markets. Furthermore, the application of powder coatings in IT & Telecommunication is also being widely explored.
UNIT 4 | CERMET |
A cermet is a composite material composed of ceramic (cer) and metallic (met) materials. A cermet is ideally designed to have the optimal properties of both a ceramic, such as high temperature resistance and hardness, and those of a metal, such as the ability to undergo plastic deformation. The metal is used as a binder for an oxide, boride, or carbide. Generally, the metallic elements used are nickel, molybdenum, and cobalt. Depending on the physical structure of the material, cermets can also be metal matrix composites, but cermets are usually less than 20% metal by volume.
Cermets are used in the manufacture of resistors (especially potentiometers), capacitors, and other electronic components which may experience high temperatures.
Cermets are being used instead of tungsten carbide in saws and other brazed tools due to their superior wear and corrosion properties. Titanium nitride (TiN), Titanium carbonitride (TiCN), titanium carbide (TiC) and similar can be brazed like tungsten carbide if properly prepared however they require special handling during grinding.
More complex materials, known as Cermet II, are being utilized since they give considerably longer life in cutting tools while both brazing and grinding like tungsten carbide.
Some types of cermets are also being considered for use as spacecraft shielding as they resist the high velocity impacts of micrometeoroids and orbital debris much more effectively than more traditional spacecraft materials such as aluminum and other metals.
After World War II, the need to develop high temperature and high stress-resistant materials in the US became clear. During the war, German scientists developed oxide base cermets as substitutes for alloys. They saw a use for this for the high-temperature sections of new jet engines as well as high temperature turbine blades. Today ceramics are routinely implemented in the combuster part of jet engines because it provides a heat resistant chamber. Ceramic turbine blades have also been developed. These blades are lighter than steel and allow for greater acceleration of the blade assemblies.
The United States Air Force saw potential in the material technology and became one of the principal sponsors for various research programs in the US. Some of the first universities to research were Ohio State University, University of Illinois, and Rutgers University.
The word cermet was actually coined by the United States Air Force, the idea being that they are a combination of two materials, a metal and a ceramic. Basic physical properties of metals include ductility, high strength, and high thermal conductivity. Ceramics possess basic physical properties such as a high melting point, chemical stability, and especially oxidation resistance.
The first ceramic metal material developed used magnesium oxide (MgO), Beryllium oxide (BeO), and aluminum oxide (Al2O3) for the ceramic part. Emphasis on high stress rupture strengths was around 980C. Ohio State University was the first to develop Al2O3 based cermets with high stress rupture strengths around 1200C. Kennametal, a metal-working and tool company based in Latrobe, PA, developed the first titanium carbide cermet with a 2800 psi and 100 hour stress-to-rupture strength at 980C. Jet engines operate at this temperature and further research was invested on using these materials for components.
Quality control in manufacturing these ceramic metal composites was hard to standardize. Production had to be kept to small batches and within these batches, the properties varied greatly. Failure of the material was usually a result of undetected flaws usually nucleated during processing.
The existing technology in the 1950s reached a limit for jet engines where little more could be improved. Subsequently, engine manufactures were reluctant to develop ceramic metal engines.
Interest was renewed in the 1960s when silicon nitride and silicon carbide were looked at more closely. Both materials possessed better thermal shock resistance, high strength, and moderate thermal conductivity.
Ceramic-To-Metal Joints And Seals
Cermets were first used extensively in ceramic-to-metal joint applications. Construction of vacuum tubes was one of the first critical systems, with the electronics industry employing and developing such seals. German scientists recognized that vacuum tubes with improved performance and reliability could be produced by substituting ceramics for glass. Ceramic tubes can be outgassed at higher temperatures. Because of the high-temperature seal, ceramic tubes withstand higher temperatures than glass tubes. Ceramic tubes are also mechanically stronger and less sensitive to thermal shock than glass tubes. Today, cermet vacuum tube coatings have proved to be key to solar hot water systems.
Ceramic-to-metal mechanical seals have also been used. Traditionally they have been used in fuel cells and other devices that convert chemical, nuclear, or thermionic energy to electricity. The ceramic-to-metal seal is required to isolate the electrical sections of turbine-driven generators designed to operate in corrosive liquid-metal vapors.
Bioceramics
Bioceramics play an extensive role in biomedical materials. The development of these materials and diversity of manufacturing techniques has broadened the applications that can be used in the human body. They can be in the form of thin layers on metallic implants, composites with a polymer component, or even just porous networks. These materials work well within the human body for several reasons. They are inert, and because they are resorbable and active, the materials can remain in the body unchanged. They can also dissolve and actively take part in physiological processes, for example, when hydroxylapatite, a material chemically similar to bone structure, can integrate and help bone grow into it. Common materials used for bioceramics include alumina, zirconia, calcium phosphate, glass ceramics, and pyrolytic carbons.
One important use of bioceramics is in hip replacement surgery. A hip joint essentially is a multiaxial ball and socket. The materials used for the replacement hip joints were usually metals such as titanium with the hip socket usually lined with plastic. The multiaxial ball was tough metal ball but was eventually replaced with a longer lasting ceramic ball. This reduced the roughening associated with the metal wall against the plastic lining of the artificial hip socket. The use of ceramic implants extended the life of the hip replacement parts.
Cermets are also used in dentistry as a material for fillings and prostheses.
Transportation
Ceramic parts have been used in conjunction with metal parts as friction materials for brakes and clutches.
Other applications
The United States Army and British Army have had extensive research in the development of cermets. These include the development of lightweight ceramic projectile proof armor for soldiers and also Chobham armor.
Cermets are also used in machining on cutting tools.
A cermet of depleted fissiable material (e.g. uranium, plutonium) and sodalite has been researched for its benefits in the storage of nuclear waste. Similar composites have also been researched for use as a fuel source.
AG/PD Powders
Cermet has the capability of producing coprecipitated silver palladium powders in any desired alloy ratio from 100% Ag to 100% Pd and in a wide range of surface areas. These powders are produced by using Cermet's proprietary chemical precipitation techniques and yield spherical, monosized, highly dispersed powders. By carefully controlling our processes, Cermet can produce these powders as:
Solid solution alloys
Physical mixtures
Crystalline Atomic Scale Mixtures
Layered (Pd core, Ag shell)
Layered (Ag core, Pd shell)
Oxidation Resistant
Controlled Sintering
Controlled Wetting Characteristics
Powders can also be manufactured in a wide variety of particle sizes, surface areas and tap densities.
Normal practice at Cermet is to produce highly alloyed powders in typical lot sizes of up to 2000 troy ounces (60 Kg) per production batch.
As the trend towards lower Pd content Ag/Pd alloys in MLCC inner electrodes continues, Cermet has found that true alloy powders with Ag contents in excess of 90% do not disperse as easily as their higher Pd content counterparts. In response to customer needs, Cermet manufactures the FG, A, D, and PA Series AgPd powders.
The FG series powders offer many of the characteristics of the higher Pd content powders in that the FG materials have a Pd rich exterior.
The A series powders are coated during processing. They will mill into extremely smooth, high quality inks with a minimum of effort.
The D series and PA series powders are pre-milled and coated. These are the ultimate materials for ease of milling into smooth electrode inks.
All versions of these powders can be manufactured with organic or inorganic coatings to control dispersion, oxidation behavior, sintering, and metal-dielectric interfacial properties.
100% Silver Products
Cermet's unique processing technologies allow the precipitation of monosized, highly spherical powders which are particularly useful as multilayer capacitor electrodes, especially the new generation of ULF dielectrics being introduced into the marketplace. The smooth, dense laydowns which can be obtained make these products ideal for hybrid applications as well.
Besides our spherical materials, Cermet also manufactures a line of silver powders, flakes and powder/flake combinations. These materials are well suited to the manufacture of terminations for numerous types of passive components.
100% Palladium Powders
Cermet has the capability of producing palladium powders in a wide variety of surface areas. These powders are produced using Cermet's proprietary chemical precipitation techniques and yield spherical, monosized, highly dispersed powders. By carefully controlling our processes, Cermet can produce these powders as:
· Crystalline Powder
· Oxidation Resistant (B series)
· Controlled Sintering
· Controlled Wetting Characteristics
Cermet is committed to the environment, safety, and quality. Through our ISO 9001:2008 Quality Management System, Cermet meets all the requirements of this important standard.
Cermet provides:
1) High quality products
2) On-time and in-full delivery
3) Quick responses to our customers’ requests.
Our ISO 14001:2004 Environmental Management System provides a healthy and safe workplace through proper treatment of our employees and the environment.
Cermet:
1) Adheres to all environmental regulations
2) Recycles all forms of usable waste
3) Trains employees in safe practices and environmental awareness
4) Continually strives to improve our environmental stewardship and safety record.
UNIT 5 | CARBIDES |
Hardmetal is the term used to signify a group of sintered, hard, wear-resisting materials based on the carbides of one or more of the elements tungsten, tantalum, titanium, molybdenum, niobium and vanadium, bonded with a metal of lower melting point usually cobalt. Tungsten carbide is the most widely used.
In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by chemical bonding type as follows: salt-like, covalent compounds, interstitial compounds, and “intermediate” transition metal carbides. Examples include calcium carbide, silicon carbide, tungsten carbide (often called simply carbide), and cementite, each used in key industrial applications.
Salt-like carbides. These carbides are composed of highly electropositive elements such as the alkali metals, alkaline earths, and group 3 metals including scandium, yttrium and lanthanum. Aluminium from group 13 forms carbides, but gallium, indium and thallium do not. These materials feature isolated carbon centers, often described as "C4−", in the methanides or methides; two atom units, "C22−" in the acetylides; and three atom units "C34−" in the sesquicarbides. The naming of ionic carbides is not consistent and can be quite confusing.
Methanides. Carbides of this class decompose in water producing methane. Two such examples are aluminium carbide Al4C3 and beryllium carbide Be2C.
Дата добавления: 2015-11-04; просмотров: 21 | Нарушение авторских прав
| <== предыдущая лекция | | | следующая лекция ==> |