Thermal Spraying

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What is Thermal spraying ?

Thermal Spraying metal coatings are depositions of metal which has been melted immediately prior to projection onto the substrate. The metals used and the application systems used vary but most applications result in thin coatings applied to surfaces requiring improvement to their corrosion or abrasion resistance properties.
Thermal Spraying covers a wide range of techniques in which material is heated rapidly an a hot gaseous medium and simultaneously projected at high velocity onto a surface, to produce a coating.
In other words, this method in its most basic form is the propelling of a powdered metal alloy via a heat source onto a component to form a metallurgical bond with the components base material.
The range of alloys that can be sprayed is immense based mainly on Nickel, Cobalt, Aluminum and Iron but containing elements as diverse as boron, molybdenum and tungsten in fact it would be true to say that an alloy can be provided for almost any application.
Sprayed metal coatings have been used for a number of years and exposure tests have proved them to be superior to conventional paint coatings.



The different processes for thermal spraying can be grouped into two categories:

? There are lower energy processes often referred as metallising, or ?cold? processes in which the alloy is sprayed onto the base material either directly or onto a pre-sprayed bondcoat of pure nickel. An exothermic reaction takes place bonding the alloy to the surface.
In this category? arc spraying and flame spraying processes are included. These are frequently used for spraying metals for corrosion resistance, such as zinc and aluminium.

? The second category covers higher energy processes sush as plasma spraying, detonation gun and high velocity combustion spraying. A technique recently attracting considerable interest is the High Velocity Oxy-Fuel (HVOF) process.
The processes of the second category are basically the same as the first but when the alloy has been deposited it is then fused either in a vacuum furnace or with the aid of a gas pre-heating torch to form a dense homogenous coating metallurgical bonded to the base material, this second process is used where a point loading on the component is expected. Unfortunately a great deal of heat is generated with this second process, therefore it is not recommended for finished or slender components that could be susceptible to distortion.
In all cases the types of structures differ considerably from those produced by either gaseous or solution-state processes.


How is the alloy sprayed ?

The alloy is sprayed via a torch that is connected to a gas source such as oxygen and acetylene.
The delivery system of the torch is dependant on its manufacturer, however two main methods are employed:

? micro-pulverised alloy powder purchased in containers that fits onto the torch

? alloy in wire form that is either ground by the torch and fed into the flame or simply atomised in the?
flame itself



What materials can be coated ?

Almost all metals can be coated sometimes with a thickness' of as much as 25mm { ? " ] although cost would prohibit thick coatings on large components. Here is a list of some of the materials that can be coated.

?Carbon Steels including tool steels such as D3,D2,O1 etc.
?Stainless Steels
?Cast Irons
?Cast Steels
?Certain Bronzes and Brasses
?Certain Magnesium Alloys


What alloys can be sprayed ?

As mentioned above the range of alloys that can be sprayed is immense, alloy selection is based upon the application to which it is being put, for example it is possible to spray phosphor bronze onto an aluminium component to produce a bearing surface, Tungsten Carbide can be sprayed onto a mild steel base to give superb abrasion resistance, Stainless Steel can be spayed onto medium carbon steel for resistance to rust and a nickel chrome iron material may be used to repair a worn shaft, putting it back into service quickly and almost certainly making it last longer than the original.

?Nickel Alloys - for corrosion resistance to acids and alkalis - provide good cutting edges on blades etc ?Cobalt Alloys - good resistance to wear, abrasion and corrosion at elevated temperatures
?Aluminum Alloys ? for corrosion resistance
?Iron based Alloys - Cutting edges, good self lubricating properties
?Copper - gaskets, conductance etc
?Phosphor Bronze - Excellent bearing material
?Ceramics - Resistance to thermal shock and wear - High thermal and electrical resistance
?Plastics ? Resistance to corrosion from water and mild acid and alkali's



? Enhance wear and/or corrosion resistance
? Provide specific frictional characteristics to the surface
? Use for dimensional restoration
? Uses as thermal barrier, thermal conductor, electrical conductor or resistor
? Uses as electromagnetical shielding, enhance or retard of radiation



? Extremely wide variety of materials that can be used to make a coating
? Ability of most of the thermal spray processes to apply a coating substrate without significantly???????
? Ability to strip or recoat worn or damaged coatings without changing the properties or dimensions of?
the part
? Very cost effective especially on large components with small wear areas
? Ideal in breakdown situations where spare parts are unavailable
? Excellent for reclaiming obsolete parts - Classic cars/bikes etc
? Inexpensive base materials can be used and coated in certain areas for a particular resistance
? Specialised alloys for a specialised component - corrosion -RF shielding - thermal barriers etc
? Coating where making a component from the alloy is unfeasible - Valve parts, offshore applications
? Making moulds by spraying onto a component coated with release agent




? Line-of-sight nature of these processes. They can only coat what the torch or gun can ?see?
? Size limitations prohibiting the coating of small, deep cavities into which a torch or gun will not fit



Arc Spraying


In this process, the raw material in the form of a pair of metallic wires (with electrically opposed charge), comprising the spray material, are fed together in such a manner that a controlled arc occurs at the intersection heating and melting them. This molten material is atomised by a cone of compressed air and propelled towards the workpiece. The molten spray solidifies on the component surface to form a dense, strongly adherent coating.
This process differs from the other thermal spray processes in that there is no external heat source such as a gas fleme or electrically induced plasma.
Major advantages of the Arc Spraying process are that the coatings are available for almost instant use with no drying or curing times and there is no risk of damaging the component.
In addition, the deposits possess a higher degree of bond strength than most other thermally sprayed deposits and the use of compressed air and electricity alone mean more economic coatings.
Another advantages are:

? high deposition rates
? low substrate heating
? less expensive to operate

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