Wire Flame Spray (Combustion Wire Spray)
The Wire (or Rod) Flame spray process utilises a set of drive rolls powered by an air turbine or electric motor to draw a single metal/alloy wire through the Flame spray gun. At the gun nozzle, fuel gas of acetylene, propane, LPG or hydrogen is mixed with oxygen in specific volumetric proportions using a siphon plug and lit to create a flame, which is then shaped at the gun’s air cap by compressed air. The metal wire is fed concentrically into the flame, melted and atomized by the compressed air, and the molten droplets are propelled towards a prepared substrate where they quickly solidify and bond to the substrate to assimilate the coating.
Wire Flame spray is commonly used for Engineering and Anticorrosion coatings. Either hard (higher melt temperature) or soft (lower melt temperature) wires can be used. Rod Flame spray systems deposit economical ceramics from Al, Cr and Zn Oxide feedstock rods. Flame spray has been replaced by Arc spray due to greater bond strengths, ease of use, speed of deposition and running/maintenance costs. However certain in-situ applications (particularly Anticorrosion) favour Flame spray equipment because of its relative low weight and the fact that a power supply is not required.
Powder Flame Spray
Powder Flame spray (Combustion Spray) uses a siphon plug arrangement to mix fuel gas of acetylene, propane or hydrogen with oxygen in controlled proportions to provide consistent operation and inhibit backfiring. Powder is injected into the lit fuel gas-oxygen flame where it is melted by the heat of combustion, propelled towards the substrate surface where it quickly solidifies to assimilate the coating. Typical gas stream temperatures are in excess of 3,000°C. A cap at the flame exit that delivers compressed air and surrounds and appropriately shapes the flame according to the material being processed.
The Powder Flame spray process is similar to the Wire Flame spray process, except that it has the advantage of using powder materials as the coating feedstock, which permits a broader range of coating material options (in particular ceramics).
Plasma Spray is typically found in two variations, Conventional or APS (Air Plasma Spray) and the purer coating yielding VPS (VacuumPlasma Spray).
Plasma is often referred to as the fourth state of matter. Plasma, like the other three states of matter (Solid, Liquid and gas) has it's own unique properties. Just as most substrates will become solid if cooled enough, any substance will become a plasma if heated enough. In a plasma the electrons are stripped from the atoms creating a substance that resembles a gas but that conducts electricity. Plasmas occur naturally on the earth in lightning bolts, flames, electrical discharges and the Northern Lights (aurora borealis).
In all applications the Plasma is created by passing an electric current (typically 500 to 1000A) through a primary gas such as argon or nitrogen. The resultant exiting Plasma plume provides an energy heat source of around 15,000°C under high pressure which heats and propels the coating material onto the substrate. The powder feedstock is normally injected via an inert carrier gas into the arc (at a position and angle suited to the material being sprayed) externally, however there are internal injection arrangements. Plasma temperatures in the powder heating region range from about 6000 to 15,000 °C (11,000 to 27,000 °F), significantly above the melting point of any known material. Commercial plasma spray guns operate in the range of 20 to 200 kW.
High Velocity Oxy Fuel spraying (HVOF)
High Velocity Oxy Fuel spraying (HVOF) is basically a high velocity flame spray process and in principle a small rocket engine. HVOF powders are injected axially into the expanding hot gases where they are propelled forward, heated and accelerated onto a surface to form a coating. Gas velocities exceeding 7000 fps (4772 mph) have been reported with temperatures approaching 2,300°C (4,172°F) depending on the fuel gas used. The coupling of inertially driven/highly plasticized particles can achieve coatings approaching that of theoretical density. Disadvantages include low deposition rates and in-flight the oxidation of particles. Future efforts will focus on applying thick coatings and improvements in processes control including in-flight transit time and exposure to atmospheric oxygen.Variation of this process include Wire HVOF and High Velocity Air Fuel (HVAF), where Air is used in place of Oxygen.
In the Arc Spray process, two wires are fed into the spray gun and brought into contact with each other at the nozzle. These emerging wires of the material to be sprayed are then charged and an arc generated between them, which causes the tips of the wires to melt. The point of intersection of these wires is positioned directly in front of a carrier jet of compressed air or nitrogen which is used to atomise and propel the molten material onto the work-piece. A mechanical feed mechanism pushes both wires forward to maintain the arc and the flow of material. Typical feed mechanisms are either electric or pneumatic motor driven and are available in push, pull or push/pull arrangements. The latter being the most reliable and farthest reaching. The molten spray solidifies on the work-piece surface to form a dense, strongly adherent coating suitable for Corrosion Protection orComponent Reclamation. Sprayed coatings may also be used to provide Wear Resistance, Electrical and Thermal Conductivity or Free Standing Shapes. Arc spraying is relatively inexpensive, easy to learn, portable, and fairly simple to maintain. Low particle velocities enable high maximum coating thickness for a given material. Recent advancements in nozzle and torch configurations are providing greater control over coating quality and the spray pattern. With the right equipment, it's possible to produce an elongated spray pattern or to spray components with very small internal diameters. As far as its shortcomings, arc spraying is limited to electrically conductive solid wires and cored wires. The introduction of cored wires has enabled the deposition of complex alloys as well as carbide-containing metal alloys that were only attainable using powdered materials as feedstock. The temperature of the arc is controllable to a maximum of approximately 5,000°C (10,000°F). Arc spray equipment can spray any type of materials which have melting points below 5,000°C (10,000°F.)