Thermal Spraying - Synopsis on Current Research

Thermal spraying consists of depositing materials onto a variety of components in a

molten or semi-molten state to form a continuous and congruent coated surface. Thermal spray

coating applications include thermal insulation, wear, oxidation, corrosion resistance, sealing

systems, vibration and sound absorption, and dimensional repair. The powder material used for

coating can be metals, alloys, composites, cermets, amorphous alloys (compounds contain P, Si,

B favoring amorphization), etc.

Coating properties and its performance under service conditions are controlled by various

input material properties as well as process parameters. Various parameters affecting the process

are; (i) powder particle size (micron/submicron/agglomerated nanosize particles), (ii) size

distribution, (iii) morphology (spherical/irregular/blocky), (iv) specific mass (affected by void

content), (v) powder flowability, (vi) homogeneity of particle composition, (vii) oxidation levels

of particle, (viii) phase structure and crystallinity of powder, (ix) powder manufacturing route

(atomization/fused and crushed powders/milled and sintered

powders/milling/cryomilling/mechanical alloying and milling/spray-drying/cladding/sol-

gel/particle spheroidization), (x) injection of particle/suspension into energetic gas flow, (xi) in-

flight aerodynamic behavior of particles, and spray operating conditions such as; (xii) spray

distance, (xiii) substrate temperature control. The spray operating conditions have a bearing on

residual stress pattern and its relaxation which in turn decides the adhesion of coating and its

durability. Controlled segmentation resulting in through-thickness cracks or promotion of

microcracking and interspalt debonding, causes in-plane stresses to go to zero at the crack faces

and thus increases the lifetime of thermal sprayed coatings.

Broader particle size distribution causes different particle momentum imparting larger

particle trajectory distribution and hence different particle velocities and temperature distribution

at impact on the substrate. On contrary in suspension spraying, narrow size distribution is desired

for good coating.

Particle mass flow rate, particle morphology and resulting particle injection into gas jet

affects particle heating and acceleration. Equally important to consider is the chemical reactions

of the spray particle with the surrounding atmosphere and/or solid reactions while determining

spray conditions. In-flight oxidation of bulk metallic glasses is known to cause destabilization of

glass particle and hence their phase stability and formability are largely affected by the chemical

composition dependent critical cooling rate.

Recent research has been focused to understand the in-flight chemical reactions in

powder particles and their effect on resulting coatings. The in-flight chemical reactions of

particle are controlled by diffusion mechanism - partial pressure of reactive gases reaches critical

pressure, or convection mechanism - velocity difference between surrounding gases and molten

particle induce convective motion in particle or solid state reactions (SHS, self-propagating high-

temperature synthesis) - agglomerated/cladded particles react to produce intermetallic

compounds and finally deposited on substrate.

Wear-, corrosion-, and oxidation resistant coatings are produced by high velocity

combustion methods such as detonation gun (D-gun), high-velocity oxy-fuel (HVOF), high

velocity air fuel (HVAF), or lower velocity flame spray; whereas thermal barrier coatings

(TBCs) are deposited using air plasma spray (APS), or electron beam physical vapor deposition

(EB-PVD). Plasma or HVOF processes where suspension spraying of sub-micron or nano-sized

particles is preferred, spray conditions are more complex. The process is highly dependent on

penetration of particle suspension into hot gas jet, its fragmentation and resultant penetration of

individual droplets into gas jet. Very low spray distances owing to low inertia of particles implies

high thermal fluxes of the order of 25 MW/m2 to the substrate.

Contrary to the conventional thermal spraying processes, cold spraying is another

emerging technology. Cold spray process propels powder particles at supersonic speeds in order

to bond to the substrate and deposited layers, and hence gives much higher deposition rates.

Powder particles are not melted in this process and hence particles are free of oxidation as well

as absence of heat-affected zone in substrate material. Unlike conventional thermal spraying,

particle morphology plays an important role on account of its deformation properties on impact

and peening effect on previous deposited layers. Cold sprayed coatings exhibit bulk-like

properties with respect to the thermal and electrical properties and hence mostly used to repair

cracks, dimensional repair or to deposit dense coatings of filler materials.

Current research involves continuous development in materials with enhanced utility,

equipment, consumables, diagnostic tools in process equipment, process simulations studying

aerodynamic profile of in-flight particles and hot gas jets, surface preparation techniques, and

post-coating surface treatment. In last decade, higher voltage and low current guns have been

designed leading to improvement in process efficiency, reduced production cycle time,

feasibility of low helium or helium-free spraying. Recently a coating method, suspension or

solution precursor plasma spray (SPS or SPPS) process has been developed which produces

TBCs with novel columnar microstructures comparable to the EB-PVD and lowest reported

thermal conductivities.

Another research front deals with the development and continuous improvement in the

powder material to be coated. For example, to improve system efficiency in energy generation,

many studies are being conducted to significantly lower the thermal conductivities of the

coatings, achieve thinner coating systems and higher engine temperatures, maintain lower

substrate temperature, significant reduction in back-side cooling to improve overall turbine

efficiency. New coating material is to be developed for TBCs by inducing lattice imperfections

such as grain boundaries, solute cations, and oxygen vacancies which scatter lattice waves, as

well as larger imperfections such as porosity and inclusions to scatter photons, in order to reduce

heat conduction via lattice waves and photons. To achieve this, doping of YSZ with heavier ions

such as Nd, Gd, and Yb to create stable defect structure and introduction of interfaces/density

changes parallel to coat/bond coat interface is being studied. New affordable thermal spray

coatings are being developed for solid oxide fuel cells (SOFCs) which operate under severe

mechanical and chemical degradation conditions at high temperature. Another challenging

application is the plasma-sprayed nanostructured coatings with better conductivity in TBCs,

lower friction and better wear.

Aerospace and industrial gas turbine applications makes up for almost 60% of the overall

market, while remaining 40% is distributed over applications in oil and gas, biomedical, pulp and

paper, and electronics industries.

References

1) Canan U. Hardwicke, and Yuk-Chiu Lau, Advances in Thermal Spray Coatings for Gas

Turbines and Energy Generation: A Review, J. of Thermal Spray Tech., Vol. 22(5), June

2013, p 564-576.

2) P. Fauchais, G. Montavon, and G. Bertrans, From Powders to Thermally Sprayed Coatings,

J. of Thermal Spray Tech., Vol. 19(1-2), Jan 2010, p 56-80.

3) T. W. Clyne and S. C. Gill, Residual Stresses in a Thermal Spray Coatings and Their Effect

on Interfacial Adhesion: A Review of Recent Work, J. of Thermal Spray Tech., Vol. 5(4),

Dec 1996, p 401-418.