Transcript

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

reduced production cycle time. whereas thermal barrier coatings (TBCs) are deposited using air plasma spray (APS). 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. and oxidation resistant coatings are produced by high velocity combustion methods such as detonation gun (D-gun). high-velocity oxy-fuel (HVOF). dimensional repair or to deposit dense coatings of filler materials. surface preparation techniques. Plasma or HVOF processes where suspension spraying of sub-micron or nano-sized particles is preferred. To achieve this. Cold spray process propels powder particles at supersonic speeds in order to bond to the substrate and deposited layers. consumables. significant reduction in back-side cooling to improve overall turbine efficiency. The process is highly dependent on penetration of particle suspension into hot gas jet. corrosion-.temperature synthesis) . Another research front deals with the development and continuous improvement in the powder material to be coated. high velocity air fuel (HVAF). particle morphology plays an important role on account of its deformation properties on impact and peening effect on previous deposited layers. achieve thinner coating systems and higher engine temperatures. Recently a coating method. Cold sprayed coatings exhibit bulk-like properties with respect to the thermal and electrical properties and hence mostly used to repair cracks. and hence gives much higher deposition rates. For example. 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. in order to reduce heat conduction via lattice waves and photons. feasibility of low helium or helium-free spraying. to improve system efficiency in energy generation. diagnostic tools in process equipment. and oxygen vacancies which scatter lattice waves. equipment. doping of YSZ with heavier ions . In last decade. spray conditions are more complex. Contrary to the conventional thermal spraying processes. maintain lower substrate temperature. as well as larger imperfections such as porosity and inclusions to scatter photons. and post-coating surface treatment. Wear-. higher voltage and low current guns have been designed leading to improvement in process efficiency. Unlike conventional thermal spraying.agglomerated/cladded particles react to produce intermetallic compounds and finally deposited on substrate. its fragmentation and resultant penetration of individual droplets into gas jet. many studies are being conducted to significantly lower the thermal conductivities of the coatings. or lower velocity flame spray. Current research involves continuous development in materials with enhanced utility. process simulations studying aerodynamic profile of in-flight particles and hot gas jets. New coating material is to be developed for TBCs by inducing lattice imperfections such as grain boundaries. solute cations. Very low spray distances owing to low inertia of particles implies high thermal fluxes of the order of 25 MW/m2 to the substrate. or electron beam physical vapor deposition (EB-PVD). cold spraying is another emerging technology.

p 564-576. June 2013. 5(4). and Yb to create stable defect structure and introduction of interfaces/density changes parallel to coat/bond coat interface is being studied. while remaining 40% is distributed over applications in oil and gas. Clyne and S. J. of Thermal Spray Tech. Vol. J. Aerospace and industrial gas turbine applications makes up for almost 60% of the overall market. Vol. J. Hardwicke. Another challenging application is the plasma-sprayed nanostructured coatings with better conductivity in TBCs.. . Dec 1996. 22(5). G. lower friction and better wear.such as Nd. and G. of Thermal Spray Tech. and Yuk-Chiu Lau. p 401-418. 19(1-2). pulp and paper.. From Powders to Thermally Sprayed Coatings. and electronics industries. Montavon. Advances in Thermal Spray Coatings for Gas Turbines and Energy Generation: A Review. Bertrans. p 56-80. Residual Stresses in a Thermal Spray Coatings and Their Effect on Interfacial Adhesion: A Review of Recent Work. biomedical. References 1) Canan U. 3) T. C. W. 2) P. Gd. Vol. Gill.. of Thermal Spray Tech. Fauchais. 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. Jan 2010.