THERMAL SPRAYING
GOVERNING SPECIFICATION(S): MIL-STD-869
Thermal spraying is an overlay process and materials used for spraying are principally in the form of powder, rod or wire. Thermal spraying usually consists of pre spray treatments and sometimes post spray treatments. The pre-spray treatments include degreasing, and micro or macro roughening of the surface to be treated. Grit blasting is the standard technique of surface roughening. Usually ceramic grits such as silica or aluminum oxide are chosen as the blasting media.
Virtually any material that can either be drawn into wire or formed into a powder can be applied so long as it has a true melting point. Some plastics do not meet these criteria.
Some applications include:
- Dimensional restoration of worn or mis-machined parts.
- Hard facing/Wear resistance.
- Corrosion resistance.
- Thermal Barriers, ceramics.
- Conductivity.
- Dielectric.
- Clearance control, abrasive/abradable.
- Decorative/Cosmetics.
Depending upon the application, many Thermal Spray coatings can be applied on site at customer locations.
Technical information on the three basic forms of Thermal Spraying performed by CTT are include in this packet.
FLAME SPRAY:
This is also called combustion flame spraying. Here, the flame is generated by the combustion of fuel gas, usually acetylene and oxygen. In this case, the chemical energy of the combustion of the fuel gas and oxygen is used to heat and melt the material. The flame temperature is the order of 5,000° F. – 7,000° F. There are basically two types, one is wire spray and the other is powder spray. When material is fed through the flame either in the form of wire or powder, the flame melts the material, which is then atomized by a stream of compressed air to form droplets. These droplets are successively propelled against the substrate. The approximate velocity of impingement is around 200 – 300 ft./sec. The spray distance, angle of spraying, wire/powder feed rate play a significant role in determining the coating microstructure and are controlled by proven process parameters.
TWIN WIRE ARC SPRAY:
Two wires which are consumable electrodes are drawn from spools to form a liquid droplet due to arc heating and are accelerated by compressed air. The jet of air acts to atomize the molten wire and to propel the finely atomized particle to the substrate. If wires are of different materials, pseudo-alloy coatings can be produced. Arc current and voltage are usually in the range of 300 amps and 20 – 40 volts.
PLASMA SPRAY:
The word Plasma in the physical sense describes a condition of matter where electrons are separated from their atoms (i.e.) the matter is said to be ionized. During plasma spraying one is working with partial ionization of the gases and with temperatures in the range of 30,000K. The different stages of plasma spraying include plasma generation, plasma jet formation, particle injection, particle heat and momentum transfer and particle deposition.
A powerful arc is initiated between a thoriated tungsten cathode and water cooled copper anode nozzle. Plasma gas (usually argon) is introduced along the cathode and is heated by the arc to plasma temperatures, leaving the anode nozzle as a plasma jet. Fine powder suspended in a carrier gas is injected into the plasma jet, where the powder particles are accelerated and heated. The molten powder particles impinge with high velocities on to the substrate.
The sprayed coatings are built up in layers. Although the desired thickness of the coating may vary depending upon the intended application, the coatings are typically .005" - .050" thick.
