Thermal-spray in any form requires specialized equipment and consumables, besides special learning and training , so that it may not be readily available. Many different coating materials were developed to cover most different requirements of diverse applications. The suppliers are ready to provide general information on application and properties of Thermal-spray equipment and products , but would not take responsibility as to the suitability of a certain product for a given application.
The responsibility remains of the developer or of the engineering function, and should be based mainly on previous experience with similar applications and on field tests conducted on prototypes. The surface designated to receive Thermal-spray should be prepared, first by thorough cleaning, and then by roughening by a process that will not contaminate it with oil or other objectionable substances. One most used preparation process is grit blasting by rough mesh sand or aluminum oxide or other proprietary products. Therefore it is imperative to reserve a facility only for preparation to Thermal-spray and to use special oil filters for compressed air.
One of the most important functions of roughening , besides cleaning and providing new metal surfaces to ensure the best Thermal-spray bond, is to relieve or limit the tensile stresses and the shrinkage that develop in the metallized layer upon cooling on the base metal. They display substantial bond strength even on surfaces not previously roughened, and are used as intermediate layer to promote Thermal-spray bonding of successive layers.
AWS TSS - AWS TSS THERMAL SPRAYING: PRACTICE THEORY. AND APPLICATION
Different processes for ever more exacting requirements. One should remark that in the past the low energy applications of Thermal-spray were also known under the generic name of Metallizing. In the following, two low power and three high power Thermal-spray processes are presented. The Thermal-spray wire type gun, normally manually operated but mounted in the tool post of a lathe, consists of a unit which feeds the wire, and of a gas head which controls the flows of fuel gas, oxygen and compressed air.
The wire is fed by knurled rolls , rotated either by an electric motor or by a compressed air turbine. The gas head provides the flow regulation of the gases through valves, the control of the combustion flame for the wire and of the propellant for the atomized molten droplets. These are picked up in the gas and compressed air stream and projected with force against the workpiece which typically is not heated much.
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It is important to maintain constantly the proper ratio of fuel and oxygen to develop a consistent flame, and to control the wire feed to obtain correct atomization and metal spray. This Thermal-spray process is carried out with a special torch that feeds two electrode wires, with opposite electrical charges, meeting at their tips where an arc is struck between them, in operation. The electrode metal is atomized and sprayed by the propelling gas, usually compressed air, unto the substrate.
This process has been used for spraying aluminum or zinc on steel structures to protect them in marine environment. This Thermal-spray process was a development which had some popularity when it first appeared. The equipment itself is specialized and cannot be used for other processes , and the properties of the sprayed layers obtained by the following processes are improved. The first of the Thermal-spray processes characterized by higher energy imposed upon the sprayed particles in comparison with the previous processes is Plasma spray.
The hotter flame permits spraying refractory materials that could not be processed at lower energies. The bond properties of the sprayed layers are higher and the level of porosity and internal defects is lower than with the previous ones, so that there are definite applications for which high energy processes are a must.
As already remarked in previous pages, a Plasma is a very hot gas in a highly ionized form, that is one deprived of some of its electrons by the passage through a powerful electric arc. A special torch or gun is designed to generate the plasma flame by passing high pressure gas through a constricted or confined arc between water cooled non consumable electrodes, a cathode and a hollow anode nozzle.
The plasma flame meets and carries along the powder particles, fed through the side of the nozzle. To provide for proper and continuous Thermal-spray operation, all of the main parameters must be constantly controlled and adjusted. That is done by automatic provisions once the flame temperature has been selected by changing the ratio of electric current to plasma gas. With a special arrangement one can spray a buffer layer with one material and then move gradually to a different material.
The proportions of the two change continuously along the thickness of the sprayed overlay: such a development has very useful and interesting applications. It helps to control and limit stresses due to differences in thermal expansion between the materials. A robot holding the plasma gun points it against the workpiece fastened to a rotating positioner. The robot is instructed to provide the plasma layers where needed, in the most accurate and repeatable process. Plasma spray can be applied in air or in a vacuum chamber, and in this case some of the properties are improved.
The D-Gun or detonation gun is a piece of Thermal-spray equipment developed to produce superior properties coating, and its name is identified with the process it supports. It is a water cooled barrel where oxygen, fuel mostly acetylene and powder are admitted through valve controlled ports.
The explosive mixture is ignited by a spark at every cycle to propel the heated particles of the powder at supersonic speed.
The succession of explosions performing the coating work, produces and elevate level of noise. Explosions noise must be controlled by locating the equipment in properly insulated facilities. By accelerating the coating particles to supersonic speed , the process achieves a remarkably high degree of bond strength at the substrate interface, and a very limited level of porosity.
The process is essentially a continuous one: depending on the parts to be coated they may end up hotter then with other processes reviewed here.
The equipment is similar to the plasma spray gun with the essential modifications needed to sustain higher temperatures and gas speed. It was developed in the search for better properties of the deposited layers at a time when the D-Gun was proprietary and enjoyed a monopolistic market position. AWS C2. To see the article click on PWL In the same issue two references are made to a new promising development called Cold Spray that achieves similar or even better results without heating the sprayed particles.
A new website page on this subject was recently added. Click on Cold Spray to see it. Click on PWL to read it. Click on PWL to see it. It is particularly beneficial for the application of PEEK composite material to inside radii and diameters, such as the inside diameter surface of diffuser wall 98 illustrated in FIG.
The preferred process for creating and applying PEEK composite layer 78 or involves initial preparation of substrate layer 72 , 96 or 98 for receipt of a polymer layer via a thermal spray process. In the preferred embodiment, the substrate is a metallic material, such as Ni Resist or stainless steel but other metallic materials may be appropriate depending on the specific application. A first step in the process is preparation of the substrate material. The substrate material preferably is cleaned by removing dirt, moisture, oil and other contaminants from the surface to be coated.
To facilitate adherence, it is also desirable to roughen the surface to be coated. It is preferred that the surface be roughened by grit blasting. For example, the substrate may be grit blasted with aluminum oxide having a grit mesh size In another step of the process, the polymeric material is prepared for use in coating the substrate, e.
It is preferred that the polymeric material have a high melting temperature, i. In the most preferred embodiment, a PEEK material is used to prepare a composite material in powdered form. These materials enhance the low coefficient of friction and excellent wear properties of PEEK.
Additionally, the selection of appropriate particle size can be critical to the HVOF process. It has been determined that optimal particle sizes for the various components of the PEEK composite are approximately 70 microns for the PEEK; approximately 53 microns for the PTFE; and approximately 6 microns for the carbon particles.
Spraying specialist according to DVS®-Guideline
Although specific mixture percentages and particle sizes have been provided, other mixture ratios, particle sizes, and mixture components may be amenable to the process of the present invention. After cleaning and grit blasting of the substrate material, e.
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The bonding layer preferably is a metallic material having sufficient surface asperities to facilitate the mechanical bonding of the PEEK composite layer to the substrate. Preferably, a single layer of metallic material, such as nickel aluminum alloy, is applied.
This material has desired characteristics at high temperature and provides excellent bonding to a stainless steel substrate. Other bonding layer materials may work better with substrates formed of materials other than stainless steel. In the preferred embodiment, the nickel aluminum alloy is arc sprayed against the substrate. Arc spraying, as is generally known to those of ordinary skill in the art, uses a high energy electric arc generated by bringing two electrically energized wires into contact with each other.
The arc energy melts the wires, and compressed air atomizes the molten material and propels it onto the substrate, leaving a bonding layer.
Preferably, the bond layer has good thermal conductivity to help dissipate heat from the PEEK layer, particularly when the PEEK material is used as a bearing surface. It has been determined that an optimal thickness for the bond coat is in the range of approximately 0. Following preparation of the substrate, application of the bonding layer, and preparation of the PEEK composite material, the PEEK composite material is applied to the substrate over the bonding layer by a thermal spray.
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An optimum window of spray parameters has been established to ensure low porosity and great bond strength to permit the PEEK composite layer to be used in load bearing environments as well as protective coating environments. The Thermal Spray Gun is equipped with a 12 mm combustion chamber, and the fuel gas, preferably hydrogen, to oxygen ratio is 3.
Additionally, a carrier gas, preferably nitrogen, is flowed through the thermal spray gun at a flow rate of 30 scfh to feed powder into the combustion chamber. The powderized PEEK composite mixture is fed to the thermal spray gun via an electronically controlled, pressurized hopper unit, as is well known to those of ordinary skill in the art. The powder particles of the PEEK composite are partially or preferably fully melted and propelled towards the substrate and bonding layer. This creates a stream of semi-molten or molten particles or platelets that hit the substrate to form a continuous coating typically having a lamellar structure.
In the preferred embodiment, the PEEK composite powder is fed at a rate of 11 grams per minute and the thermal spray gun is moved at a traverse speed of millimeters per second with a standoff of 7 inches.
What Does A Thermal Spray Operator Do?
The standoff refers to the distance between the substrate and the outlet tip of the thermal spray gun. The PEEK composite coating is built up in multiple passes to a thickness between approximately 0. Typically, there is one preheat cycle and 30 passes, following which, the coating is allowed to cool by a natural slow cool. PEEK composite layer 78 or , it may be advantageous to adopt a post-deposition annealing process.
The post-deposition annealing process provides a more durable coating. It facilitates the removal of the thermal history and residual stress. It also increases the level of crystallinity of the PEEK composite coating. The above-described method provides a PEEK composite coating that is easily applied and has low porosity, typically on the order of less than one percent porosity.
The PEEK composite layer is particularly amenable to use as a bearing surface because of its low coefficient of friction, excellent wear properties and low porosity achieved with this process. It will be understood that the foregoing description is of preferred exemplary embodiments of this invention, and that the invention is not limited to the specific forms shown.