What is Thermal Spray - MECPL

1 Introduction ..2 Methods ..2 Molton Metal FlameSpray ..2 Powder Flame Spraying ..3 Wire Flame Spraying ..3 Ceramic Rod Flame Spraying ..3 Detonation Flame Spraying ..4 High Velocity Oxy/Fuel Spraying (HVOF) ..4 Cold Spray ..5 Nontransferred Plasma Arc Spraying ..6 Electric Arc Spraying ..7RF Plasma Spraying ..7 References ..7 Acknowledgements ..8 International Thermal Spray Association Information ..8 Thermal Spray Processes used by Various Industrial Segments Chart ..9 Thermal Spray Coating Applications According to Industry Served Chart ..10 Industrial Use of Gas Metallic Materials Chart ..11 Mission:To strengthen the level of awareness in general industry andgovernment on the increasing capabilities and advantages of thermalspray technology for surface engineering through business opportunities,technical support, and a social network that contributes to the growthand education of the Thermal Spray is Thermal Spray ?

2 Graphic courtesy of Westaim Ambeonthermal spraying, a group of coating processes in which finely divided metallic or nonmetallicmaterials are deposited in a molten or semimolten condition to form a coating. The coating materialmay be in the form of powder, ceramic-rod, wire, or molten materials. (1)International Thermal Spray Association208 Third Street Fairport Harbor, Ohio 44077 USAvoice: fax: Spray Coating ProcessTHERMAL SPRAYING: what it Was and what it Has Becomeby Frank J. HermanekOn the eve of celebrating the one hundred anniversary ofits discovery, Thermal spraying looks back to its roots - earlyexperiments in which liquids were broken up into fineparticles by a stream of high-pressure gas. Efforts moredirected at producing powders rather than constructingcoatings.

3 It fell to one Dr. Max Ulrick Schoop of Zurich whorecognized the possibility that a stream of molten particlesimpinging upon themselves could create a coating. His work,and that of his collaborators, resulted in the establishmentof the Thermal Spray process. This process has fostered aworldwide industry serving over thirty technology sectorsand generating sales of over two billion dollars per article traces the history and development of theprincipal flame and electrical Thermal Spray spraying, a group of coating processes in whichfinely divided metallic or nonmetallic materials are depositedin a molten or semimolten condition to form a coating. Thecoating material may be in the form of powder, ceramic-rod,wire, or molten materials.

4 (1)In the early 1900s Dr. M. U. Schoop and his associatesdeveloped equipment and techniques for producing coatingsusing molten and powder metals. Several years later, inabout 1912, their efforts produced the first instrument forthe spraying of solid metal in wire form. This simple devicewas based on the principle that if a wire rod were fed intoan intense, concentrated flame, (the burning of a fuel gaswith oxygen), it would melt and, if the flame weresurrounded by a stream of compressed gas, the molten metalwould become atomized and readily propelled onto a surfaceto create a coating. This process was initially referred to asmetallizing. Currently the technique is known as oxy-fuel orflame spraying. Other oxy-fuel methods include wire, powder(metallic and ceramic), molten metal, ceramic-rod,detonation and high velocity oxy-fuel (HVOF).

5 In addition to using chemical means to plasticize the inputconsumables electrical currents are also used. Typically,electrical energy is used to create a heat source into whichpowder, and more recently wires, are fed, melted/plasticizedand conveyed onto the surface to be coated. Major,commercially employed electrical methods, used to constructcoatings include non-transferred arc plasma, RF plasma, andwire arc. Based upon the two (2) heat sources a "family tree"of Thermal Spray methods can be constructed, Figure Spray METHODSM olten Metal FlameSpraymolten metal flame spraying, a Thermal spraying processvariation in which the metallic material to be sprayed is in themolten condition(1)It has long been recognized that fluids may be broken upinto very fine particles by a stream of high velocity gasemanating from a nozzle.

6 Early experiments using thisatomizing approach appear to have been directed atproducing metallic powders rather than coatings. It was leftto Schoop to appreciate the possibility that a stream ofmetallic particles, formed from a molten source, could producea coating. Myth has it that Schoop developed the conceptwhen playing "soldiers" with his son and observing thedeformation of lead pellets being fired from a toy cannonagainst a brick wall. Whatever the rationale, it can be statedthat the pioneer work of Schoop resulted in the discovery anddevelopment of metal spraying and subsequently the "ThermalSpray Process".The first Spray technique developed by Schoop was theoutcome of experiments in which molten metal was pouredinto a stream of high velocity gases.

7 Schoop's apparatusconsisted of a compressor supplying air to a heated helicaltube. The heated air was used to pressurize a crucible filledwith molten metal and eject it out as a fine Spray that wouldadhere to a suitable surface. This system was bulky, primitiveand inefficient; however, the concept did lead to thedevelopment of portable and user friendly are no further accounts of molten metal spraying bySchoop, it appears that hisefforts were directed atdeveloping and improvingpowder and wire flamespraying. However, work byothers continued as a 1924 Dutch patent, describingequipment for spraying lowmelting point metals, wasgranted to Jung andVersteeg(2). Mellowes Ltdcommercialized the processin the UK. Their systemconsisted of a gun, afurnace, an air compressorand a fuel supply.

8 The gunhad many air and gasvalves, a heating chamber2 Figure 1. Thermal sprayapplication methods byheat source schematic. (burner), nozzle, handle and a melting pot. The pot wasbulky having the ability to store kg (4 lb) of moltenlead. The pot sat atop the heating chamber, which wassimilar in construction to a Bunsen burner. Compressed air,fed to the burner, intensified the flame. The handle juttedout and downward from the pot; it was insulated using woodand asbestos. Metal exited the pot through a front orificewhere it was directed into a nozzle. Compressed airsurrounded the nozzle, atomizing the molten metaland propelling it to the surface to be coated. The molten metal process has advantages anddisadvantages.

9 Advantages include: cheap rawmaterials; use of inexpensive gases; and, gun designis very basic. Noteworthy disadvantages are: gun iscumbersome to use in the manual mode, can only beheld in a horizontal plane; high maintenance due tohigh temperature oxidation and molten metalcorrosion; and, useful only with low meltingtemperature for the molten metal Thermal Spray processinclude the fabrication of molds, masks and formsfor the plastics industry, using low melting pointbismuth based alloys (the Cerro family of alloys);the deposition of solder alloys to joints that would becoalesced using torches or ovens; and, the production ofmetal Flame Sprayingpowder flame spraying, a Thermal Spray process in which thematerial to be sprayed is in powder form.

10 (1)Powder flame spraying is probably the simplest of all thespray processes to describe - feed a powder through thecenter bore of a nozzle where it melts and is carried by theescaping oxy-fuel gases to the work piece. Unfortunately,this approach yields coatings high in oxides and with voidcontents approaching 20 volume percent (v/o). However,coating quality can be improved by feeding air to the nozzlethrough a small jet, which reduces the pressure in a chamberbehind the nozzle. This chamber is connected to the powderfeed hopper. In this way a gentle stream of gas is suckedinto the gun and carries powder with it. A typical gun isillustrated in Figure 2. This concept was developed by Fritz Schori(2)in the early1930s. However, the amount of powder that can besupported by a gas stream depends on many factorsincluding powder characteristics.