Thermal Sprayed Coatings
Thermal spray is defined as "...applying these coatings takes place by means of special devices / systems through which melted or molten spray material is propelled at high speed onto a cleaned and prepared component surface..." This definition does not sufficiently describe the thermal spray process.
The coating feedstock material is melted by a heat source. This liquid or molten material is then propelled by process gases and sprayed onto a base material, where it solidifies and forms a solid layer. The individual aspects of a thermal sprayed coating follows.
Substrate Materials
After the removal of surface impurities by chemical or mechanical methods, the surface is usually roughened using a blasting procedure. This activates the surface by increasing the free surface energy and also offers the benefit of increased surface area for bonding of the sprayed particles. Heat from the hot particles is transferred to the cooler base material. As the particles shrink and solidify, they bond to the roughened base material. Adhesion of the coating is therefore based on mechanical "hooking". The amount of metallurgical bond caused by diffusion between the coating particles and base material is small and can be neglected for discussions about bonding mechanisms (exception: Molybdenum). Surface roughening usually takes place via grit blasting with dry corundum. In addition, other media, such as chilled iron, steel grit or SiC are used for some applications. Besides the type of grit, other important factors include particle size, particle shape, blast angle, pressure and purity of the grit media. In principle, any material that does not decompose as it is melted can be used as a thermal spray coating material. Depending on the thermal spray process, the coating material can be in wire or powder form.
The liquid or molten coating particles impact the surface at high speed. This causes the particles to deform and spread like "pancakes" on the substrate.Coating Material
In the table, some of the most frequently used classes of materials are listed, along with a typical example, characteristics and sample applications. Choosing a coating material that is suitable for a specific application requires special knowledge about the service environment as well as knowledge about the materials.
Apart from the physical characteristics, such as coefficient of expansion, density, heat conductivity and melting point, additional factors, such as particle shape, particle size distribution and manufacturing process of powder material (i.e., agglomerated, sintered, composited) will influence coating performance. As most spraying materials are available as alloys or blends, this leads to a nearly unlimited number of combination options, and only through many years of experience and broad know-how can a proper selection be made.
Material Class
Typical Alloy
Characteristics
Example Application
Pure metals
Zn
Corrosion protection
Bridge construction
Self-fluxing alloys
FeNiBSi
High hardness, fused
minimal porosityShafts, bearings
Steel
Fe 13Cr
Economical,
wear resistanceRepair
MCrAlY
NiCrAlY
High temperature
corrosion resistanceGas turbine blades
Nickel-graphite
Ni 25C
Anti-fretting
Compressor inlet ducts
Oxides
Al2O3
Oxidation resistance,
high hardnessTextile industry
Carbides
WC 12Co
Wear resistance
Shafts
Thermal Spray Coating Processes
There are several different processes used to apply a thermal sprayed coating. They are:
Wire Flame Spray
With the wire flame spray process, the wire spray material is melted in a gaseous oxygen-fuel flame.
The fuel gas can be acetylene, propane or hydrogen.
The wire is fed concentrically into the flame, where it is melted and atomized by the addition of compressed air that also directs the melted material towards the workpiece surface.
Powder Flame Spray
This coating process is based on the same operational principle as the wire flame spray process, with the difference that the coating material is a spray powder. Thus, a larger selection of spray materials is available, as not all spray materials can be manufactured in wire form.
The wire is fed concentrically into the flame, where it is melted and atomized by the addition of compressed air that also directs the melted material towards the workpiece surface.
Process Comparison
The processes previously discussed differ fundamentally by the thermal and kinetic energy imparted to the spray particles by each process. The thermal energy is determined by the attainable flame temperature and the kinetic energy of the spray particle is a function of gas velocity. An energy comparison of the spray processes is represented in Figure 8. The high temperature of plasma spraying is particularly suitable for materials with a high melting point, such as ceramics.
The HVOF process, having high kinetic energy and comparatively low thermal energy, results in a positive effect on the coating characteristics and is favorable for spray materials such as tungsten carbide coatings. The comparison of the processes is largely of interest in relation to the coatings that result. Table 3 lists some important coating characteristics, organized by material class.
| Characteristics | Coating Type | Powder Flame Spray | HVOF Spray | Electric Arc Wire Spray | Plasma Spray |
|---|---|---|---|---|---|
| Gas temperature [oC] [oF] |
3000 5400 |
2600 - 3000 4700 - 5400 |
4000 (Arc) 7200 (Arc) |
12000 - 16000 21500 - 29000 |
|
| Spray rate [kg/h] [lb/h] |
2 - 6 4.5 - 13 |
1 - 9 2 - 20 |
10 - 25 22 - 55 |
2 - 10 4.5 - 22 |
|
| Particle velocity [m/s] [ft/s] |
up to 50 up to 160 |
up to 700 up to 2300 |
approx. 150 approx. 500 |
up to 450 up to 1500 |
|
| Bond strength [MPa] [psi] [MPa] [psi] [MPa] [psi] [MPa] [psi] [MPa] [psi] |
Ferrous alloys Non-ferrous alloys Self-fluxing alloys Ceramics Carbides |
14 - 21 2000 - 3000 7 - 34 2000 - 5000 83+ (fused) 12000+ (fused) 14 - 34 4000 - 5000 34 - 48 5000 - 7000 |
48 - 62 7000 - 9000 48 - 62 7000 - 9000 70 - 80 10000 - 11500 --- --- 83+ 12000+ |
28 - 41 4000 - 6000 14 - 48 4000 - 7000 15 - 50 2200 - 7200 --- --- --- --- |
21 - 34 --- 14 - 48 4000 - 7000 --- --- 21 - 41 3000 - 6000 55 - 69 8000 - 10000 |
| Coating thickness [mm] [in] [mm] [in] [mm] [in] [mm] [in] [mm] [in] |
Ferrous alloys Non-ferrous alloys Self-fluxing alloys Ceramics Carbides |
0.05 - 2.0 0.002 - 0.080 0.05 - 5.0 0.002 - 0.200 0.15 - 2.5 0.006 - 0.100 0.25 - 2.0 0.010 - 0.075 0.15 - 0.8 0.006 - 0.030 |
0.05 - 2.5 0.002 - 0.100 0.05 - 2.5 0.002 - 0.100 0.05 - 2.5 0.002 - 0.100 --- --- 0.05 - 5.0 0.002 - 0.200 |
0.1 - 2.5 0.004 - 0.100 0.1 - 5.0 0.004 - 0.200 --- --- --- --- --- --- |
0.4 - 2.5 0.015 - 0.100 0.05 - 5.0 0.002 - 0.200 --- --- 0.1 - 2.0 0.004 - 0.080 0.15 - 0.8 0.006 - 0.030 |
| Hardness [HRC] | Ferrous alloys Non-ferrous alloys Self-fluxing alloys Ceramics Carbides |
35 20 30 - 60 40 - 65 45 - 55 |
45 55 30 - 60 --- 55 - 72 |
40 35 --- --- --- |
40 50 30 - 60 45 - 65 50 - 65 |
| Porosity [%] | Ferrous alloys Non-ferrous alloys Self-fluxing alloys Ceramics Carbides |
3 - 10 3 - 10 < 2 (fused) 5 - 15 5 - 15 |
< 2 < 2 < 2 --- < 1 |
3 - 10 3 - 10 --- --- --- |
2 - 5 2 - 5 --- 1 - 2 2 - 3 |