Gordon England Surface Engineering Forum

Thermal Spray Coatings

Contents: Bonding,  Coating structure,  Stress,  Properties, Porosity,  Oxide, Surface texture, Strength, NDT, Process factors, Links

Nature of Thermal Spray Coatings

What is a thermal ( flame ) spray coating? A coating produced by a process in which molten or softened particles are applied by impact onto a substrate.

A common feature of all thermal spray coatings is their lenticular or lamellar grain structure resulting from the rapid solidification of small globules, flattened from striking a cold surface at high velocities.

Schematic of a single thermal spray particle splat on a flat substrate

Fig.l Schematic diagram of thermally sprayed spherical particle impinged onto a flat substrate

Schematic Diagram of a Thermal Spray Metal Coating

Schematic Diagram of  Thermal Spray Metal Coating

Microstructure of a Nickel Chromium Thermally Sprayed Coating. The lamellar structure is interspersed with oxide inclusions and porosity

Fig.2. A typical microstructure of a metallic thermally sprayed coating. The lamellar structure is interspersed with oxide inclusions and porosity.

Links to other Photomicrographs: Plasma Sprayed Coatings / Plasma Sprayed Chromium Oxide Coatings 1 * Plasma Sprayed Chromium Oxide Coatings 2 *Combustion Thermal Sprayed Coatings / HVOF Thermal Spray Coatings / Plasma Spray WC/Co / WC/NiCrBSi / Rogues Gallery / Thermal spraying onto Composite Substrates / Effect of Metallographic Preparation Technique on Thermal Spray Coatings

BONDING.                                                                                                 back to contents)

The bonding mechanisms at the thermal spray coating/substrate interface and between the particles making up the thermal spray coating is an area which in many cases is still subject to speculation. It generally suffices to state that both mechanical interlocking and diffusion bonding occur.

Thermal Spray Coating Bonding Mechanisms:

Factors effecting bonding and subsequent build up of the coating:


Cleaning and grit blasting are important for substrate preparation. This provides a more chemically and physically active surface needed for good bonding. The surface area is increased which will increase the coating bond strength. The rough surface profile will promote mechanical keying.

Individual particle cooling rates on impact can be of the order of 1 million º C per second (106Ks-l). Thermal interaction is obviously very limited. Important with regard to diffusion bonding (temperature and time dependent).

Increase in thermal and kinetic energy increases chances of metallurgical bonding. (temperature, velocity, enthalpy, mass, density and specific heat content etc.. ). Thermal spray materials like Molybdenum, Tungsten, and Aluminium / metal composites produce so called "self bonding" coatings. These materials have comparatively high bond strengths (increased metallurgical or diffusion bonding ) and can bond to clean polished substrates

Molybdenum and other refractory metals have very high melting points thus the interaction between substrate and coating particles will be increased due to the higher temperatures involved and longer cooling cycles. Also molybdenum oxide volatilizes and does not get in the way of metallurgical bonding.

Aluminium / metal composites produce increased levels of exothermic reaction due to reactions of aluminium with metals like nickel to produce nickel aluminide and with oxygen producing aluminium oxide. The increased thermal action increases degree of diffusion bonding.

Higher preheat temperatures for the substrate increase diffusion bonding activities but will also increase oxidation of the substrate which could defeat the objective of higher bond strengths.

High kinetic energy thermal spraying using HEP, HVOF and cold spray produce high bond strengths due to the energy liberated from high velocity impacts. The high density tungsten carbide/cobalt and cold spray coatings are good examples.

Metallurgical or diffusion bonding occurs on a limited scale and to a very limited thickness (0.5 µm max. with heat effected zone @ 25µm) with the above type coatings.

Fused coatings are different. These are remelted and completely metallurgically bonded with the substrate and its self.

COATING STRUCTURE                                                                               (back to contents)

High cooling rates or super cooling (106 Ks-l) of particles can cause the formation of unusual amorphous (glassy metals) microcrystalline and metastable phases not normally found in wrought or cast materials.

A large proportion of thermal spraying is conducted in air or uses air for atomisation. Chemical interactions occur during spraying, notably oxidation. Metallic particles oxidise over their surface forming an oxide shell. This is evident in the coating microstructure as oxide inclusions outlining the grain or particle boundaries. Some materials (such as titanium) interact with or absorb other gases such as hydrogen and nitrogen.

Coatings show lamellar or flattened grains appearing to flow parallel to the substrate. The structure is not isotropic, with physical properties being different parallel to substrate (longitudinal) than across the coating thickness (transverse). Strength in the longitudinal direction can be 5 to 10 times that of the transverse direction.

The coating structure is heterogeneous relative to wrought and cast materials. This is due to variations in the condition of the individual particles on impact. It is virtually impossible to ensure that all particles are the exact same size and achieve the same temperature and velocity.

All conventionally thermally sprayed coatings contain some porosity (0.025% to 50% ). Porosity is caused by:

The above interactions can make the coatings very different from their starting materials chemically and physically.

STRESS                                                                                                          (back to contents)

Cooling and solidification of most materials is accompanied by contraction or shrinkage. As particles strike they rapidly cool and solidify. This generates a tensile stress within the particle and a compressive stress within the surface of the substrate. As the coating is built up, so are the tensile stresses in the coating. With a lot of coatings a thickness will be reached where the tensile stresses will exceed that of the bond strength or cohesive strength and coating failure will occur.

Thermal spray coatings and the effects of stress

High shrink materials like some austenitic stainless steels are prone to high levels of stress build up and thus have low thickness limitations. Look out for thickness limitation information on coating data sheets. Generally thin coatings are more durable than thick coatings.

Spraying method and coating microstructure influence the level of stress build up in coatings. Dense coatings are generally more stressed than porous coatings. Notice that Combustion powder sprayed coatings generally have greater thickness limitations than plasma coatings.

Contrary to that just mentioned, the systems using very high kinetic energy and low thermal energy (HVOF, HEP, cold spray) can produce relatively stress free coatings that are extremely dense. This is thought to be due to compressive stresses formed from mechanical deformation (similar to shot peening) during particle impact counteracting the tensile shrinkage stresses caused by solidification and cooling.

PROPERTIES                                                                             (back to contents)

Compare properties of unsuported coatings with wrought/cast bulk equivalents:

PROPERTY COATING WROUGHT/CAST
Strength low (5-30%) high (100%)
Ductility very low (1-10%) high (100%)
Impact strength low high
Porosity high low
Hardness higher particulate/micro-hardness       higher bulk/macro-hardness
Wear Resistance high low
Corrosion Resistance       low high
Machining poor good

This table shows a very general relative trend. There will always be exceptions.

This comparison generally shows coating properties in a bad light, and does not take into consideration that coatings are usually supported by a substrate. Coatings are generally only used to give surface properties such as wear resistance and not to add strength.

Remember, bulk strength supplied by the substrate (cheap, strong and ductile). Surface properties supplied by the coating ( wear and corrosion, etc..). Due to the small quantity of material required for a coating, more exotic materials can be used economically. The properties of some coatings cannot be fabricated by any other method.

Properties of coatings should be considered in their own right and not the properties of the original material prior to spraying as they can be very different physically and chemically.

Porosity                                                                                           (back to contents)

This is present in most thermally sprayed coatings (except VPS, post heat treated coatings or fused coatings). 1 to 25% porosity is normal but can be further manipulated by changes in process and materials.

Porosity can be detrimental in coatings with respect to:


Porosity can be important with respect to:


Oxide                                                                                                          (back to contents)

Most metallic coatings suffer oxidation during normal thermal spraying in air. The products of oxidation are usually included in the coating. Oxides are generally much harder than the parent metal. Coatings of high oxide content are usually harder and more wear resistant. Oxides in coatings can be detrimental towards corrosion, strength and machinability properties.

Surface Texture                                                                                          (back to contents)

Generally the as-sprayed surface is rough and textured. The rough and high bond strength coatings are ideal for bond coats for less strongly bonding coatings. Many coatings have high friction surfaces as-sprayed and this property is made use of in many applications (rolling road drum surfaces for MOT brake testing). Some plasma sprayed ceramic coatings produce smooth but textured coatings important in the textile industry. Other applications make use of the abrasive nature of some coating surfaces. Thermally sprayed coatings do not provide bright high finish coatings with out finishing like that of electroplated deposits.

Strength                                                                                                       (back to contents)

Coatings generally have poor strength, ductility and impact properties. These properties tend to be dictated by the "weakest link in the chain" which in coatings tends to be the particle or grain boundaries and coating/substrate interface. Coatings are limited to the load they can carry, and thus require a substrate for support, even then, coatings are poor when point loaded.

Internal tensile coating stresses generally adversely effect properties. Effective bond strength is reduced and can be destroyed by increasing levels of internal stress. This in turn effects coating thickness limits. Coatings on external diameters can be built up to greater thickness than that on internal diameters.

Surface properties such as wear resistance are usually good, but the properties are more specific to the material or materials used in the coating. The properties of a substrate need only to be strength, ease of fabrication and economic (like mild steel). The coating supplies the specific surface properties desired. For example, materials used for applications of thermal barrier and abradable clearance control by nature have poor strength and thus benefit from being applied as a coating onto a substrate which supplies the strength.

Some Properties Thermally Sprayed Coatings can Provide:


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NON-DESTRUCTIVE TESTING OF THERMALLY SPRAYED COATINGS.

There are very few reliable NDT methods available for thermally sprayed coatings. The majority of tests for coatings tends to be of a destructive nature, which, obviously can not be used on the actual coated part going into service and therefore, must be considered as a test for process control.

The main practical NDT methods used are:

Ultrasonic and magnetic particle flaw detection methods have proved to be poor with thermally sprayed coatings due to the very high number of particle boundaries giving flaw like responses and causing high levels of interference.

Hardness testing is generally considered a destructive test for coatings unless made in a non-working area.

Advanced techniques like thermography, Thermal wave interferometry and acoustic emission are presently being researched and are still laboratory set-ups with limited practical use for industry.

Destructive testing such as hardness, bend, bond strength, metallography etc.. are important to prove the process and coating integrity expected in the component.

The limited non-destructive testing available for thermally sprayed coatings should emphasise the need for a high standard of quality control over the process, to ensure a high level of confidence in the coated products.

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Factors Effecting The Thermal Spray Coating Process
Schematic of Thermal Spray Coating Process Factors

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