Solution precursor plasma spray
Encyclopedia
Solution Precursor Plasma Spray (SPPS) is a thermal spray process where a feedstock solution is heated and then deposited onto a substrate. Basic properties of the process are fundamentally similar to other plasma spraying processes. However, instead of injecting a powder into the plasma plume, a liquid precursor is used. The benefits of utilizing the SPPS process include: the ability to create unique nanometer sized microstructure
s without the injection feed problems normally associated with powder systems and flexible, rapid exploration of novel precursor compositions.
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plume or High Velocity Oxygen Fuel (HVOF) combustion flame. Once the solution is injected, the droplets go through several chemical and physical changes and can arrive at the substrate in a several different states, from fully melted to unpyrolized. The deposition state can be manipulated through spray parameters and can be used to significantly control coating properties, such as density and strength.
/metallic material systems are used to protect components in hot sections of gas turbine and diesel engines. The SPPS process lends itself particularly well to the creation of these TBCs. Studies report the generation of coatings demonstrating superior durability and mechanical properties. Superior durability is imparted by the creation of controlled through thickness vertical cracks. These cracks only slightly increase coating conductivity while allowing for strain
relief of stress
generated by the CTE mismatch between the coating and the substrate during cyclic heating. The generation of these through thickness cracks was systematically explored and found to be caused by the depositing a controlled portion of unpyrolized material in the coating. Superior mechanical properties such as bond strength and in-plane toughness result from the nanometer sized microstructure that are created by the SPPS process.
Other studies have shown that engineered coatings can reduce thermal conductivity
to some of the lowest reported values for TBCs. These low thermal conductivities were achieved through the generation of an alternating high-porosity, low-porosity microstructure or the synthesis of a low conductivity precursor composition with rare earth
dopants.
coatings and slightly higher than Air Plasma Spray coatings.
Microstructure
Microstructure is defined as the structure of a prepared surface or thin foil of material as revealed by a microscope above 25× magnification...
s without the injection feed problems normally associated with powder systems and flexible, rapid exploration of novel precursor compositions.
Background
The use of a solution precursor was first reported as a coating technology by Karthikeyan, et al. In that work, Karthikeyan showed that the use of a solution precursor was in fact feasible, however, well adhered coatings could not be generated. Further work was reported in 2001 which refined the process to produce thermal barrier coatings, YAG films, and silicon ceramic coatings. Since then, extensive research on the technology has been explored in large part by the University of Connecticut and http://www.inframat.comInframat CorporationInframat Corporation
Inframat Corporation is a nanotechnology company founded in 1996 with headquarters and a pilot plant / R&D facility in Manchester, CT. Product focus is in the areas of medical and industrial nanocoatings, high surface area nanofibrous media for water treatment, and magnetic nanocomposites...
].
The process
The precursor solution is formulated by dissolving salts (commonly zirconium and yttrium when used to formulate thermal barrier coatings) in a solvent. Once dissolved, the solution is then injected via a pressurized feed system. As with other thermal spray processes, feedstock material is melted and then deposited onto a substrate. Typically, the SPPS process sees material injected into a plasmaPlasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
plume or High Velocity Oxygen Fuel (HVOF) combustion flame. Once the solution is injected, the droplets go through several chemical and physical changes and can arrive at the substrate in a several different states, from fully melted to unpyrolized. The deposition state can be manipulated through spray parameters and can be used to significantly control coating properties, such as density and strength.
Thermal Barrier Coatings
Most current research on SPPS has examined is application to create thermal barrier coatings (TBCs). These complex ceramicCeramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
/metallic material systems are used to protect components in hot sections of gas turbine and diesel engines. The SPPS process lends itself particularly well to the creation of these TBCs. Studies report the generation of coatings demonstrating superior durability and mechanical properties. Superior durability is imparted by the creation of controlled through thickness vertical cracks. These cracks only slightly increase coating conductivity while allowing for strain
Strain (materials science)
In continuum mechanics, the infinitesimal strain theory, sometimes called small deformation theory, small displacement theory, or small displacement-gradient theory, deals with infinitesimal deformations of a continuum body...
relief of stress
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...
generated by the CTE mismatch between the coating and the substrate during cyclic heating. The generation of these through thickness cracks was systematically explored and found to be caused by the depositing a controlled portion of unpyrolized material in the coating. Superior mechanical properties such as bond strength and in-plane toughness result from the nanometer sized microstructure that are created by the SPPS process.
Other studies have shown that engineered coatings can reduce thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
to some of the lowest reported values for TBCs. These low thermal conductivities were achieved through the generation of an alternating high-porosity, low-porosity microstructure or the synthesis of a low conductivity precursor composition with rare earth
Rare earth element
As defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...
dopants.
Costs
The SPPS process is adapted to existing thermal spray systems. Application costs are significantly less than EB-PVDElectron Beam Physical Vapor Deposition
Electron Beam Physical Vapor Deposition or EBPVD is a form of physical vapor deposition in which a target anode is bombarded with an electron beam given off by a charged tungsten filament under high vacuum. The electron beam causes atoms from the target to transform into the gaseous phase...
coatings and slightly higher than Air Plasma Spray coatings.