- Spray coating/ thermal sealing
- High-velocity oxy-fuel spraying (HVOF)
- Laser welding / laser cladding
- Plasma-powder welding (PTA)
Thermal spray coating
— tried and tested technology. This technology has been used in almost all industries for over 100 years.
This method allows — by applying a protective coat on the metal surface — to prevent wear, corrosion and other combined aggressive product exposures. In all thermal spraying processes, fine particles of the applied coating are heated in the dispenser and are applied to the surface under pressure. If the temperature and velocity of the particles is sufficiently high, they form a layer with the desired characteristics and required strength of surface bond.
The advantage of thermal spraying lies in possibility to always find the ideal solution for protection against wear by combining base materials and big amount of different additives. Selection of the coating system also depends on the application method.
Oxy-fuel gas spattering and melting
Due to expansion of gases during combustion of acetylene and oxygen, powdery substance is sprayed on the prepared workpiece surface soldering thereto.
The powders are divided into self-fluxing and self-adhesive. As a rule, self-fluxing powders require heat treatment. Melting is usually carried out using an oxy-acetylene torch.
Advantages:
A wide variety of sprayed materials.
Gas and watertight covering.
No porous layers.
Good wear resistance at a linear, pin and specific load.
Very high adhesion strength.
Metallurgical bond with the base material.
Subsequently, the surface is easily processed during turning, milling, grinding and polishing operations.
Good corrosion resistance to alkalis, mild acids and aqueous solutions.
Disadvantages:
It cannot be applied in part.
Chrome-plated steel is not suitable for adhesion to fusion-bonded mixtures.
High thermal load of the base material.
Shape limitations of products.
Layer thickness: 0.3-0.8 mm.
Adhesion: very good (diffusion).
Thermal load: very high, up to 1050 ° C.
Basic materials: stainless/double steel- nickel alloyed.
Treatment: possible turning, grinding, polishing of layers.
High-velocity oxy-fuel spraying (HVOF)
Unlike plasma spraying, high-velocity oxy-fuel spraying occurs at almost supersonic speeds (> 2000 m/ s). Powder is supplied to the combustion chamber or afterwards. Gas is combusted in the combustion chamber continuously and at high pressure. High pressure produced by combustion of fuel gas and oxygen mixture into the combustion chamber, and an expansion nozzle develops high velocity of the gas jet, and therefore spray particles speed up. Particle velocity – up to 800 m/ s.
Advantages:
Low thermal load on the base material.
Possible high hardening of layers.
Good abrasion resistance at linear load.
High tensile strength of tungsten carbide and chromium carbide of > 70 MPa.
The coating does not depend on the material.
Partial coverage is possible.
High density of the applied layer.
Very low porosity (<2%).
Small metallurgical changes.
Thin layers with high dimensional accuracy.
Disadvantages:
Microporous layers.
Low pin load on a layer.
Low edge strength and low impact strength.
Reduction of the corrosion protection.
Layer thickness: 0.1-0.3 mm.
Thermal load: low 100-300 ° C.
Treatment: grinding and polishing of layers is possible.
Laser welding coating
When applying coating by laser welding, the laser beam is directed through optical components onto the workpiece surface, and heats the base material by the focusing lens. Via a coaxial powder feed nozzle, powdered material is supplied simultaneously with carrier gas or inert gas, and the melting process takes place. This creates a very good surface adhesion due to pyrometallurgical binding.
Advantages:
Very low part heating and, as a consequence, minimum deformation at a negligible heat affected zone.
No part modification is needed.
Very good adhesion due to pyrometallurgical binding (targeted welding by precisely controlled process).
High precision of complex parts contours.
High powder efficiency of > 70%, which provides an environmentally safe technology.
Partial coverage is possible.
Complex geometry objects are acceptable.
High edge strength and high impact strength.
After coating, the part can be easily turned, milled and ground.
Disadvantage:
Only weldable materials are acceptable.
Layer thickness — 2 mm, layering is possible.
Adhesion: good adhesion due to pyrometallurgical binding.
Thermal load: low 100-300 ° C
Base material: welded steel and alloys.
Treatment: turning, grinding, polishing of layers is possible.
Application: Coatings are used to improve wear, corrosion and heat resistance in such fields as valve industry, drilling and boring, engineering, engine technology, etc. in extreme conditions (high pressure, continuous operation and extreme wear).
Plasma-powder welding (PTA-plasma transferred arc)
During PTA, welding is performed on the item surface. A plasma arc with high density is used as a heat source. Metal powder is used as coating material. The electric arc is formed between the fixed electrode and the workpiece. Under the influence of the plasma gas, e.g., argon, helium or mixtures of argon and helium, plasma is formed in the electric arc between the central tungsten electrode (-) and a water-cooled anode sheet. Powder is fed with carrier gas to the burner, heated by the plasma jet and applied to the surface. Here, it is completely dissolved in the molten metal on the bottom.
Advantages:
Heat penetration zone is very small.
Insignificant mixing of the base material with the coating material.
Almost non-porous coating.
Strong adhesion is possible due to pyrometallurgical binding with the base material.
Complex geometry items are acceptable.
High edge strength and high impact strength.
Can be easily turned, milled and ground.
Thickness of the coating.
Disadvantage:
Only weldable materials and alloys are acceptable.
Layer thickness 0.5- 3 mm depending on the material used.
Adhesion: very high adhesion due to welding.
Thermal load: high 200-800 ° C.
Base material: welded steels and alloys.
Treatment: turning, grinding, and polishing.