Thermal Spray Coatings

Thermal spray coating involves spraying molten or semi-molten coating materials onto a surface to improve or modify its properties, making it more corrosion resistant, wear resistant, or durable. This can be achieved, for example, through metal spraying a substrate with a coating of stainless steels. The process uses thermal energy, like a flame, plasma, or arc, to heat the coating material, which is then propelled onto the substrate where it cools to form a thin layer on the base material.

Thermal spraying can be used to deposit metals, cermets (such as WC-Co), ceramics, or polymers in layers typically 60 μm to 10 mm thick, for a wide range of engineering applications. Almost any material can be deposited, so long as it melts or becomes plastic during the spraying process. Blends of different materials to produce composite or graded thermal spray coatings are possible. At the substrate surface, the particles form 'splats' or 'platelets' that interlock and build up to form the coating.

The deposit does not fuse with the substrate or form a solid solution. Instead, the bond is primarily mechanical, based on the melting and acceleration of the coating material. Hence, adhesion depends on the molten particle velocities at impact as well as the surface condition, which must be clean and roughened by grit blasting or machining prior to spraying. Bond strengths between ca. 10 MPa and >80 MPa can be obtained, depending on the thermal spray coating process and spray parameters used.

Conventional thermal spray techniques include flame spraying and electrical arc spraying. These have been used since the mid-1900s and continue to be developed. More advanced thermal spray processes include detonation gun, high velocity oxy-fuel (HVOF) (for tungsten carbide), plasma, and suspension/precursor spraying. Each process gives different coating characteristics for the same coating material, which determines the coating performance.

Thermal Spray Process

The thermal spray process works as follows:

A coating material, often in powder / wire form, is fed into a spray gun
A thermal energy source, such as a flame, arc, or plasma, melts or vaporises the material
The molten material is then propelled at high velocity onto the substrate
As the particles strike the surface, they flatten and cool, forming a strong, bonded coating
The thickness of the coating can be controlled by repeating the process

Insights

Find out more about thermal spray coatings at TWI:

Depositing Silicon Carbide Coatings by Thermal Spraying

TWI expertise led to a breakthrough in the deposition of silicon carbide (SiC)-based thermal spray powders.

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Corrosion Behaviour of Thermal Sprayed Aluminium Coating

Research into the characteristics and electrochemical corrosion behaviour of TSA Coatings.

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Damage Tolerance of Thermal Spray Aluminium (TSA) Coatings

Assessing the effect of damage on the performance of TSA-coated steel in simulated marine immersion conditions.

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Core Research Programme (CRP) and Joint Industry Projects (JIP)

Core Research

Each year the TWI Core Research Programme (CRP) addresses challenges on behalf of our Industrial Members as well as developing specific technologies and processes. Each of the projects under the CRP is focussed on engineering, materials or manufacturing technologies.

Find out more here

Joint Industry Projects

TWI also conducts Joint Industry Projects (JIPs) that bring together groups of Industrial Members to share the cost of research activities in areas of mutual industrial interest, gaining exclusive access to the outcomes. These projects cover a broad range of topics.

Find out more here

Thermal Spray Applications

Common applications for thermal spraying include the repair of rotating and moving parts for vehicles, pumps and other machinery, protective coatings to prevent wear and corrosion, and specific material coatings for electronic components, parts for power generation, and other industry requirements.

Thermal spraying processes can offer a technically superior and commercially competitive solution to many industrial challenges, including:

Replacement of chrome or cadmium plating
Wear, corrosion and thermal protection, e.g. biomass and waste-to-energy plants, offshore structures, subsea pump internals, impeller wear rings, valve plugs/balls/discs and seats, rock drill internals, compressor blades and vanes, bearing housings, glass plungers, printing rolls, textile guides
Biocompatible coatings on orthopaedic and dental implants
Dielectric coatings for electrical insulation in power hybrid circuits and heating elements
Spray forming, e.g. manufacture of thin walled hollow cylindrical objects from pre-shaped cores
Component surface restoration after wear or damage

Benefits

Thermal spraying can improve the life of machinery, structures and components, reducing the costs associated with maintenance and unexpected downtimes. Thermal spraying can prevent the need to replace a part and is applicable to a wide range of industries.

Conductive coatings use materials such as copper, aluminum, or silver to create a pathway for electrons, while insulating (dielectric) coatings use materials like aluminium oxide or yttrium to create a non-conductive, thermal barrier layer for applications such as shielding. The final electrical properties depend on the intrinsic properties of the sprayed material and the microstructure of the resulting coating, including electrical conductivity.

Thermal Spray Coating at TWI

TWI has decades of experience with thermal spray coatings – providing support and advice to industry on all aspects of thermal spraying. The services we provide for our Industrial Members include:

Applications development
Coatings research and development
Consultancy services and materials selection
Coating failure investigations
Coating characterisation and testing

We use a wide range of commercially-available thermal spraying equipment, so that technology developments are easily transferable to industry. The available process technologies are:

Latest generation cold spray system
Three different HVOF systems
Plasma spraying (APS)
Twin wire arc spray
Flame spray

In addition, we have undertaken several research projects related to thermal spraying, including the development of improved sacrificial coatings for seawater corrosion resistance of steel structures, corrosion mitigation of boiler tubes in biomass or waste-to-energy plants, and the development of antifouling coatings for renewable energy applications.

To find out more about thermal spray coatings and how we can help you solve your industry challenges, please email contactus@twi.co.uk.

For more information please email:

contactus@twi.co.uk