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Thermal-sprayfor high performance components.SOLUTIONS with Effective, Powerful Advice
Metal spraying, ceramic spraying, flame spray, metallizing, plasma spray, plasma coating, HVOF, detonation gun, spray wire, spray powders, spray electrodes, tips, links, questions needing answers: these are some of the items presented here for the benefit of interested readers. What is in here for me? A comprehensive introduction to the subject of Thermal-spray, explaining the advantages and benefits to use this technique where appropriate, to gain in performance, reduced downtime and increased productivity, by economically producing long life implements. Thermal-spray is a generic term used for defining a class of processes designed to deposit a layer of finely divided particles sprayed upon a substrate, with the purpose of building up the worn out material or of providing different and enhanced properties on a new surface. The particles of metals, intermetallics, ceramics or metallic oxides are heated to elevated temperature and projected at high speed by a special gun, in a heated and semi-molten condition, on the base metal, so that they form an adherent coating presenting mechanical interlocking and metallurgical bond, without fusion of the base metal.
Thermal-spray coatings are used in a wide range of applications:
Although Thermal-spray processes involve the use of high temperature and kinetic energy, the effect in heating the workpiece is remarkably limited, so that in general one can obtain acceptable bond to deposited layers, without unduly heating the substrate. The different nature of bonding between Thermal-spray and Hardfacing, consists in the fact that in the latter there is proper fusion of the base metal and dilution of the coating, while in Thermal-spray there is no fusion and limited heating of the substrate. In certain cases, applications may overlap, in the sense that both Hardfacing and Thermal-spray would present acceptable solutions to the need of providing resistant surfaces. As usual the availability and economics of implementation would decide for the most appropriate way of realizing the overlay coating. Tip!: Many different and useful tests were devised for quality control of Thermal-spray processes, generally on specially prepared test pieces. However one of the best for examination of metallized parts is to submit them to grinding, if required by engineering drawings. If the coating survives the grinding process without being removed, then the bonding is generally good and adequate for most applications. Tip!: It should be appreciated that while Hardfacing is mostly applied using existing and generic welding equipment, Thermal-spray in any form requires specialized equipment and consumables, besides special learning and training, so that it may not be readily available, except by subcontracting, and anyhow it is almost unsuited to field application. Materials can be fed to the heat source either as wire or rod or as powders. The Thermal-spray equipment though is different in the two cases. 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 a prototype. An Article on "Filler Metals for Thermal Spray" was published in Practical Welding Letter issue No. 22 for June 2005. To see the article click on PWL#022. 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 short Contribution (Section 11) on Advancements in Thermal Spray was published in the Issue No. 37 of Practical Welding Letter for September 2006. To see the article click on PWL#037. An Article on Vacuum Plasma Spray was published in section (2) of Issue 44 of Practical Welding Letter for April 2007. To subscribe at no cost and receive each month our publication Practical Welding Letter, please click on Subscription. Surface preparation. 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). Other accepted practices involve rough turning or threading and knurling. Tip!: One must understand that even traces of oil in the compressed air used for sand blasting, may ruin the whole Thermal-spray process. Therefore it is imperative to reserve a facility only for preparation to Thermal-spray and to use special oil filters for compressed air. Check the cleanliness by air spraying on white paper. 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 diminish the tensile stresses and the shrinkage that develop in the metallized layer upon cooling on the base metal. Some materials like molybdenum and unreacted nickel aluminide are called "self-bonding" in that 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. 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.
Flame Spray process. 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. 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. Arc Spray process. 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. For best performance the coating is usually sealed with organic compounds. This Thermal-spray process was a development which had some popularity when it first appeared, but in recent times it has largely been supplanted by more modern and effective processes. 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. Plasma spraying. 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 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 is called a Plasma. 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 fed through the side of the nozzle and heats the particles to its very high temperature to a molten or plastic state, and then propels them with high velocity toward the surface to metallize. The gases used for making the plasma are normally nitrogen with about 10% hydrogen mixed to it. To provide for proper and continuous Thermal-spray operation, all of the main parameters must be constantly controlled and adjusted 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 so that the proportions of the two change continuously along the thickness of the sprayed overlay: such a development has very useful and interesting applications. In modern setups all the functions are computer controlled and a robot holding the plasma gun against the workpiece fastened to a rotating positioner, 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. Detonation gun. 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, and where the explosive mixture is ignited by a spark at every cycle to propel the heated particles of the powder at supersonic speed upon the substrate to be coated, located at a certain distance from its mouth. Cycles are repeated with frequency of a few to at most a few tens a second. The succession of explosions performing the coating work, produces and elevate level of noise which must be controlled by locating the equipment in properly insulated facilities. HVOF. The High Velocity Oxygen-Fuel (HVOF) Thermal-spray process belongs to those characterized by High Energy. 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, so that special provisions may be required to prevent overheating. The equipment is similar to the plasma spray gun with the required 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. The following Documents and Specifications provide important information.
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