Wire arc spray is a form of thermal spraying where two consumable metal wires are fed independently into the spray gun. These wires are then charged and an arc is generated between them.
The heat from this arc melts the incoming wire, which is then entrained in an air jet from the gun. This entrained molten feedstock is then deposited onto a substrate. This process is commonly used for metallic, heavy coatings
SPRAY & FUSION
In this process, a combustion powder spray gun is utilized to deposit a wide variety of materials onto a substrate. The powders used for spray and fuse hardfacing are typically compositions of Ni, Cr, Co, Bo, Fe, W and WC in varying blends.
After the coating has been sprayed to a pre-determined thickness, an oxygen-acetylene torch, or a furnace, is used to heat the part to approximately 2000 degrees Fahrenheit, fusing the coating within itself and to the substrate, thereby achieving a true metallurgical bond. With this process, coatings can be applied with a hardness up to 80Rc.
High-velocity, oxy-fuel, (HVOF) devices are a subset of flame spray. There are two distinct differences between conventional flame spray and HVOF. HVOF utilizes confined combustion and an extended nozzle to heat and accelerate the powdered coating material.
Typical HVOF devices operate at hypersonic gas velocities, i.e. greater than MACH 5. The extreme velocities provide kinetic energy which help produce coatings that are very dense and very well adhered in the as-sprayed condition.
HVOF is most commonly used to produce very wear resistant coatings such as cermets (ceramic and metal mixes) like tungsten-carbide cobalt, see figure above. Coatings of this type have wear resistance similar to sintered carbide materials.
Because HVOF produces very dense coatings (porosity levels typically less than 0.5%), it can be used to produce very good corrosion resistant coatings made from materials such as Inconel®, Stellite®, stainless steel, and ceramics. Some unique coatings produced by TST using HVOF technology are ultra-high density ceramics that provide excellent dielectric strength.
Flame spray is divided into three subcategories, based on the form of the feedstock material, either powder-, wire-, or rod-flame spray. Flame spray coating utilizes combustible gasses to create the energy necessary to melt the coating material.
Combustion is essentially unconfined, in that there is no extension nozzle in which acceleration can occur. Common fuel gases include hydrogen, acetylene, propane, natural gas, etc. The lower temperatures and velocities associated with conventional flame spraying typically result in higher oxides, porosity, and inclusions in coatings.
Plasma spray is the most versatile of the thermal spray processes. Plasma is capable of spraying all materialsthat are considered sprayable.
In plasma spray devices, an arc is formed in between two electrodes in a plasma forming gas, which usually consists of either argon/hydrogen or argon/helium. . As the plasma gas is heated by the arc, it expands and is accelerated through a shaped nozzle, creating velocities up to MACH 2. Temperatures in the arc zone approach 36,000°F (20,000°K). Temperatures in the plasma jet are still 18,000°F (10,000°K) several centimeters form the exit of the nozzle.