Thermal spraying is an established surface technology process that is used in numerous branches of industry. By applying functional coatings, the properties of component surfaces can be improved or restored in a targeted manner – whether in terms of wear resistance, corrosion protection or functional properties such as electrical insulation and thermal insulation. A wide range of spraying processes and materials are available, which can be individually combined depending on requirements.  

Basics and advantages of thermal spray processes

Thermal spraying refers to a group of coating processes in which an additional material is melted by external heat and applied to a prepared surface at high speed. The resulting layers adhere primarily mechanically and make it possible to change or optimize the properties of component surfaces in a targeted manner – without subjecting the base material to high thermal stress.  

A characteristic feature of all thermal spraying processes is the low heat input, which allows even temperature-sensitive materials to be coated. Unlike welding, the base material is not melted, which prevents distortion and structural changes. In addition, thermally sprayed coatings can be applied with high dimensional accuracy and variable thickness, from thin functional layers to coating systems several millimeters thick.  

A key advantage is the variety of materials: from pure metal and alloys to ceramics and composites, a wide range of materials can be processed. This diversity opens up a wide range of applications – from wear and corrosion protection to electrical insulation and thermal barriers. Partial or full-surface application is also possible, allowing targeted adaptation to component-specific requirements.  

In addition, thermal spraying enables the reconditioning of worn or damaged components. Instead of manufacturing new parts, existing components can be refurbished with suitable coatings and continue to be used. The process is therefore not only technically efficient, but also helps to conserve resources. 

Overview of thermal spraying processes 

Various thermal spraying processes are available depending on the requirements for coating properties, substrate material and component geometry. These differ in the energy source used, the degree of particle acceleration and the thermal influence on the substrate. The selection of the appropriate process is decisive for the quality and functionality of the coating. 

Flame spraying and variants

In classic flame spraying, the spray material – usually in powder or wire form – is fed into a fuel gas-oxygen flame, where it is melted and applied to the component by compressed air or a gas jet. The flame temperatures are below the melting temperatures of many ceramics, which is why this process is particularly suitable for metallic and low-melting materials.  

Variants such as wire flame spraying or powder flame spraying with melting are often used for repair work or dimensional corrections. Melting after spraying produces gas- and liquid-tight layers with high mechanical strength.  

Metal flame spraying, for example with aluminum wire, is also widespread and is used for corrosion protection – especially in maritime environments, as aluminum offers cathodic protection against the base material. 

High Velocity Oxy Fuel (HVOF)

High Velocity Oxy Fuel (HVOF) uses controlled combustion of gas or kerosene to generate extremely high jet velocities. The spray material is not completely melted, but is applied to the substrate in a plastic state with high kinetic energy.  

HVOF coatings are characterized by very dense, adhesive layers with high hardness and low pore content. They are particularly wear-resistant and offer an excellent alternative to hard chrome – especially with regard to environmental and occupational safety requirements. The high process energy also enables improved adhesion to metallic base materials.

Plasma spraying – also known as atmospheric plasma spraying

Plasma spraying uses an electric arc that ionizes a process gas flowing through it, generating a plasma with temperatures of up to 20,000 °C. This means that even high-melting point materials can be sprayed. This means that even refractory materials such as oxide ceramics or hard metals can be processed.  

In atmospheric plasma spraying (APS), the process takes place under ambient pressure and is often used to produce functional ceramic coatings. Typical applications are electrical insulation layers, thermal insulation layers or wear protection layers. Internal burners are also used here, for example for coating internal surfaces or hollow cylinders. 

Wire arc spraying, wire flame spraying and powder flame spraying

Arc spraying is based on an electric arc between two wire electrodes. The molten metal is atomized by compressed air and applied to the substrate. This process offers a high material application rate and is often used for large-area corrosion protection coatings – for example in steel or bridge construction.  

Powder flame spraying is used when powdered materials are to be processed. It is economical and versatile and is suitable for the rapid restoration of surface functions.  

Wire flame spraying, a variant of classic flame spraying, uses metallic wires as the starting material. The process is comparatively inexpensive and is well suited to the application of simple protective and repair coatings.  

Materials for thermal coating

The choice of coating material is a key factor in the performance of thermally sprayed coatings. Depending on the desired function – whether wear protection, corrosion resistance or thermal insulation – metals, alloys, hard metals or ceramics are used. Combinations of different materials, so-called pseudo-alloys, are also possible. 

Metallic coatings: Pure metals, alloys and pseudo-alloys

Metallic coatings are among the most frequently used materials in thermal spraying. Pure metals such as aluminum or zinc are primarily used for cathodic corrosion protection – for example in shipbuilding or offshore applications. Aluminium is particularly suitable for flame spraying as it is characterized by its resistance to oxidation and good emergency running properties.  

Steels and similar alloys are mainly used for repair coatings. They are built up in layers, whereby the mechanical properties of the original material are retained. Self-flowing alloys – for example nickel-based alloys – allow the applied layer to be melted down, resulting in a particularly dense and wear-resistant surface.  

So-called pseudo-alloys consist of a combination of several materials with complementary properties. These include, for example, Ni-graphite coatings with self-lubricating properties or ceramic-metal composite coatings that offer particularly high wear resistance. Combinations of metal and plastic to create non-stick properties are also possible. 

Ceramic materials: Thermal spraying with ceramics

Ceramics are mainly processed in oxide form in thermal spraying, for example as aluminum oxide, chromium oxide or zirconium oxide. Due to their high melting points, they are mainly used in the plasma spraying process.  

Ceramic coatings are characterized by excellent corrosion and wear resistance. They are used as protective coatings in abrasive or chemically aggressive environments, as electrical insulators or as thermal barrier coatings – for example in turbine technology. They are also used in the lining of crucibles, as they are barely wetted by molten metal.  

Thermal spraying with ceramics makes it possible to apply functional layers with precisely defined properties without the underlying substrate being altered by heat. This means that even temperature-sensitive materials can be reliably coated. 

Hard metal coatings and alternatives to hard chrome

Hard metals such as tungsten carbide (WC) or chromium carbide (Cr₃C₂) offer extremely high wear resistance to mechanical loads – particularly in the event of abrasive or erosive attack. In combination with metallic binders (e.g. cobalt), dense, hard coatings are created that can withstand even high temperatures and corrosive media. 

Hard metal coatings are often used as an environmentally friendly and high-performance alternative to galvanic hard chrome plating. Applied using the HVOF process, they impress with their low porosity, high adhesive strength and excellent service life – even in demanding industrial applications, such as in the paper, printing or oil and gas industries.

Typical applications of thermal spraying

Thermal sprayed coatings are used across all industries when components have to withstand particular stresses or require specific functional properties:

Wear protection: 

• Use of hard metals, ceramics and composite materials.  
• Protection against abrasion, erosion and adhesive wear.  
• Typical components: Rollers, shafts, bearing points, pistons.

Corrosion protection:

• Metallic coatings made of aluminum, zinc or nickel alloys.  
• Suitable for aggressive media (e.g. seawater, chemicals).  
• Applications in offshore technology, chemical industry, power plant construction.  

Repair coatings:

• Restoration of worn or damaged components.  
• Resource-saving alternative to new production.  
• Particularly relevant for high-priced components or components that are difficult to replace.  

Electrical and thermal functions:

• Ceramic insulation layers or conductive metal layers.  
• Thermal barrier coatings in high-temperature environments. 

Areas of application:

• Mechanical and plant engineering, paper and printing industry  
• Aerospace, automotive engineering, energy technology  

Standards and quality assurance for thermal spraying

The European standard DIN EN 657 and the international standard ISO 14917 form the basis for the classification of thermal spraying processes. They differentiate the processes according to the energy source used – such as arc, fuel gas or plasma – and define basic terms and classifications.  

In addition to the standard-compliant execution of the coating process, quality assurance is also an essential component. This includes testing the coating thickness, adhesive strength, porosity, hardness and surface roughness. This is supplemented by non-destructive testing methods, for example to detect cracks or inclusions.  

In addition, proper pre-treatment of the substrate – in particular cleaning and roughening the surface – is crucial for the adhesion of the coating. Reproducible results that meet the required technical specifications can only be achieved under controlled conditions.  

Compliance with these standards not only ensures the technical function of the coatings, but also enables their economical and safety-relevant use in industrial processes.