Thermal Spray Coatings - Processes and Properties

Background

Interest in Thermal Spraying ranges from the extremely practical to the unusual and theoretical studies of the research workers. Somewhere in the middle is the July 2000 issue of Materials Research Bulletin entitled Thermal Spray Processing of Materials. The seven articles cover everything from the basics of tools and process selection, through modelling and the dynamics of deposit formation or the fabrication of fuel cells, barrier coatings for gas turbine engines or spraying of polymers. This summary touches on most areas.

The Process

During thermal spraying, particles of 1 to 50 micron are at least partially melted and accelerated to high velocities by a flame or an arc. The particles splatter onto a surface and build a layer whose quality is assessed by the oxide content, porosity and bond strength to the substrate. The sprayed material can be metal, ceramic or polymer. The temperature, velocity and typical features for 5 spray types are given in the table.

Table 1. Characteristics of some thermal spray processes.

Type

Temperature C

Velocity m/s

Features

Combustion

3000

40 -

100

Porosity and oxidation

High Velocity Oxy-Fuel

3000

400 -

800

Dense, good adhesion, compressive stress

Two wire arc

3000 - 6000

50 -

150

Dense, thick

Plasma

5000 - 25,000

80 -

300

Ceramics porous

Cold spray

Ambient

400 -

800

Dense, compressive stresses

Differences Between Thermal Spray Processes

The supersonic velocity of the HVOF spray gives a thin, dense and well bonded splat and variants of the gun are being developed for many applications. Other traditional methods melt two wires in an arc or feed powder into a high speed plasma gas stream and, for high performance applications, use a low pressure or inert gases. Developments now include radio-frequency induction plasma with its high temperatures or the opposite process with a high velocity, low temperature, localised spray which gives pure, dense material with high compressive stresses.

Coating Structure

Sprayed layers are anisotropic and look like a brick wall with interlocked splats. Their major defects are pores or microcracks in brittle materials while oxides and porosity characterise metal sprayed layers. Imperfections arise from unmelted particles, isolated large particles, low velocity impact, metal oxidation and cold induced fragmentation of splats. The assessment of quality requires a variety of techniques possibly including porosimetry, thermal conductivity or even neutron diffraction in research work. Automotive uses are an important area of application including piston rings and oxygen sensors. Although at present in a research phase, the high pressure shock (up to 30GPa) in splattering a particle allows the formation of high pressure phases which will remain stable in nanocrystalline form at room temperatures, e.g. 5 to 10 nm diamond phases have formed from a sprayed nickel-graphite powder. Other applications being developed include spray synthesis, formation of multi-layers for sensors and designing metal layers with resistivity two or more times the bulk resistivity.

Characterising Thermal Spray Processes

Modeling has demonstrated the complexity of the gas-particle interactions and, for example, the distribution of non-molten particles in a spray. It also shows how difficult it is to ensure that a new spray process remains in the permissible narrow window and illustrates why it takes some time to develop new processes. HVOF modeling offers challenges because of the supersonic properties of the gas stream and the effort in characterising the combustion process. However, particle vaporisation is not usually as much of a concern as in a plasma because the gas temperatures are lower, particles are delivered faster and the thermal conductivity of the particles tends to be higher in HVOF. Measurement of in-fight particle size, velocity and temperature to verify model calculations use techniques such as radiance comparisons, laser Doppler velocimetry and substantial computation effort. The methods must encompass temperature differences of several hundred degrees and velocity variations of more than 100m/s!

Residual Stresses in Thermal Spray Coatings

The dynamic formation of the coating covers the tens of microseconds for splattering up to the milliseconds to form a layer and the seconds until the next gun pass. The final result is observed by sectioning a coating but there is some difficulty in measuring the dynamic processes involved in a micron size particle splattering itself onto a cold surface. Models tend to deal with either the splat or the layer piling process in the first instance often with simplifications. Including surface roughness and temperature with particle size, temperature and shape, direction and composition determined mechanical properties is a real challenge. The work assists in understanding residual stresses which is a significant determinant of the fatigue, spallation and thermal cycling resistance performance of thermal spray coatings. Residual stresses arise from substrate blasting or quenching of the splat on a cool substrate although it is below 50MPa for ceramics because of microcracking although strong alloys could have stresses up to 300MPa. The mismatch in thermal expansion coefficients between surface and spray materials also causes stress which worsen with increased temperature difference between spray and ambient and when there are temperature gradients in the sprayed part.

Applications of Thermal Spray Coatings

Other papers described the spray fabrication of fuel cell components and the deposition of thermal barrier (ceramic) coatings on gas turbine engine components. These ceramic coating properties are determined by the complex interaction of ceramic sintering and creep, thermal conductivity and elastic modulus kinetics, thermal fatigue and bond coat oxidation.

Thermally Sprayed Polymers

The growing practice of polymer spraying was also reported. The materials include polyethylene, PE copolymers, polyester, nylon, fluoropolymers, ketones and liquid crystal polymers. Plasma, HVOF and combustion processes have all been used and the operating conditions must be selected to maintain the window between polymer degradation and poor coalescence of particles. The coating temperature must be chosen to optimise the required properties as density maximum will occur at a lower temperature than the maximum toughness and hardness. Since most polymer applications are in corrosive environments, it is important to ensure full coalescence and eliminate porosity and splat interfaces. Applications of coatings include magnetic or wear resisting polymer composite materials.

Source: Materials Australia, Vol. 32, no. 6, pg. 14, November/December 2000.

For more information on this source please visit The Institute of Materials Engineering Australasia.

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