Spray Coatings

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The Spraying Process

The spraying process and microstructure of the coating increase the voltage in the coatings. Please note that spray enamels generally have greater thickness restrictions than plasma coatings. In contrast to the one just mentioned, systems that use very high kinetic energy and low thermal energy (High-Velocity Oxygen Fuel (HVOF) coating, HEP, cold spray) can produce relatively stress-free coatings that are extremely dense. First, models must deal with the layering or layering process, usually with simplifications. The challenge is to determine mechanical properties such as surface roughness, particle size, direction, shape and composition at higher temperatures. Research helps gain insight into residual stresses, which are an important determinant of chipping, fatigue and thermal shock resistance in spray coatings. Residual stresses arise as a result of sandblasting or extinguishing the sprayer on a cool surface.

Thermal Spray Coatings

Thermal spraying is a popular method of applying a metal, ceramic, plastic or composite coating to a device or material. This process is used in a wide variety of industries, including medicine, aviation, manufacturing and energy. Coatings are mainly used to improve some of the material properties in a device or component. 

Coating materials available for thermal spraying include metals, alloys, ceramics, plastics and composites. Combustion or arc discharge is usually used as a source of energy for thermal spraying. The surface may not heat up considerably, which allows the coating of flammable substances. 

Thermal spray coatings have been used for several decades to improve protection against wear, corrosion and heat in various industrial sectors. To achieve the best coating functionality, the relationship between the coating mechanisms and their mechanical properties must be known. Although the mechanisms for creating such coatings have been fairly well studied, it is still difficult to obtain detailed information about the mechanical properties due to the heterogeneity of the coating. Until now, macroscopic methods such as four-point bending or microhardness at relatively high loads have been used to measure mechanical properties. Recently, attempts have been made to determine the adhesion of thermal spray coatings by scratch tests in the same way as in the area of ​​thin layers, i.e. by scratching the top surface of the coating with increasing stress. However, due to the much greater thickness and roughness of the surface of the thermal spray coatings, the process proved unsatisfactory.

Rough and Self-Adhesive Coatings

Rough and self-adhesive coatings are ideal for adhesive coatings for less adhesive coatings. Some plasma sprayed ceramic coatings produce smooth but structural coatings that are important in the textile industry. Other applications use the abrasive nature of some coating surfaces. Thermally sprayed coatings do not provide glossy high gloss coatings that are not finished like galvanised deposits.

Thermal Metallisation Processes

Most thermal metallisation processes with spraying use similar technologies such as spray gun, spray materials and carrier gas. When the microparticles hit the surface, the particles solidify and form a coating. For thermal spraying materials that are not heat resistant, cold spraying is an alternative in which powder mixed with a carrier gas is used to create coatings on the substrate.

HVOF

HVOF (High-Velocity Oxygen Fuel) coating is a process that uses a burner that allows the flame to spread with each nozzle use.  The end result is an extremely thin coating applied evenly. Its corrosion resistance is better than that of plasma coatings, but it is not suitable for high temperatures.

As with all thermal spray coating processes, the HVOF coating material is heated and accelerated by the gas flow to the surface of the element for better properties. In the HVOF coating process, the gas flow is created by mixing and igniting oxygen and fuel (gas or liquid) in the combustion chamber and accelerating the gas at high pressure through the nozzle. The powder is introduced into this stream, where it is heated and accelerated to the surface of the element. The resulting thermal spray coating consists of thin overlapping platelets. 

Statistics

  • All conventionally thermally sprayed coatings have some porosity (0.025% to 50%). 
  • Porosity from 1 to 25% is normal but can be further manipulated by process and material changes. 
  • Corrosion rates in the range from 10 to 20 µm/year were obtained for coatings with a 5% retention.