A Thermal barrier coatings (TBCs) widely used to insulate metallic components in gas turbines are known for their ability to improve turbine durability and efficiency and repair frequency in gas turbine engines. TBCs remain the most effective thermal insulation approach, and their development has centered on partially stabilized zirconia (typically, 7-8YSZ). However, state-of-the-art atmospheric plasma spray (APS) or electron beam PVD (EB-PVD) 7-8YSZ TBCs are approaching their durability and temperature limits, which remains a challenge to overcome in the near future.

To address this challenge, Xinqing Ma and his colleagues have discovered rare earth oxide multidoped zirconium oxide-yttrium oxide (ZrO2-Y2O3) to be used as new TBCs. The main innovation of the solution plasma spray process (called SPPS process or SPS, Inframat trademark) is to directly form a solid deposit from a liquid precursor (not small particle suspension), and the resultant deposit to have nanostructure and other unique microstructural features, such as 3D nano and submicrometer-sized porosity, and superfine-sized splats. In conventional thermal spray technology, large particles or agglomerates (20 micrometers to 100 micrometers) are used to make coatings having defective microstructures.

In case of other similar work, the coating is made using the conventional plasma spray or EB-PVD. In this work, the researchers have made low-K (K--conductivity) TBCs using the SPPS technique. The SPPS-formed low-K TBC has a superior thermal cyclic resistance compared to those made by conventional APS and EB-PVD processes. The SPPS low-K TBC has demonstrated much lower thermal conductivity compared to the regular Y2O3 partially stabilized ZrO2 (YSZ) coating. The SPPS TBC also has a relatively higher porosity than APS-TBCs and EB-PVD TBCs, and the level of porosity is adjustable. Taking advantage of the liquid solution precursor, the SPPS is ready to change coating chemistry, and to obtain a multilayered or graded coating structure via an Inframat-designed and patent-issued liquid delivery system.

Figure 1 shows the scanning electron microscopy (SEM) micrographs of SPS-formed low-K TBC coatings at a low (above) and high magnitude (below). (a) Coating porosity 20%; (b) Porosity 24%; (c) Porosity 26%.

The low-K TBC has wide applications as it is required for advanced turbine aero-engines for next generation fighters that increase their inlet temperatures for higher efficiency. The low-K TBC also has the capability for higher temperature applications and better sintering resistance. Meanwhile, it provides 1/2- to 1/3-fold thermal conductivity of conventional YSZ TBC material. Other applications include land-based turbine engines for powder generation, ship power system, and high-performance race car engine components (such as pistons and exhaust valves). SPPS technology has been demonstrated for making porous, dense, and far-from-equilibrium coatings and structural preforms.
Figure 2 shows thermal conductivity measurement of the SPS low-K TBC simultaneously under laser high-heat flux cycling test.

Inframat has collaborated with Maurice Gell's and Eric Jordan's teams at the University of Connecticut on developing precursor chemistry. The SPPS process and coating evaluation was mainly funded by the Office of Naval Research (ONR). Inframat has been awarded a Department of Defense Navair Small Business Innovation Research project in developing low-K TBCs, in collaboration with UConn's team and D. Zhu and R. Miller at NASA Glenn Research Center. Inframat has focused on the process development and optimization. The low-K composition was provided by D. Zhu and Miller's team and they have also performed thermal conductivity measurement and thermal cycling tests under laser high-heat flux conditions. Inframat is working with Nitin Padture at Ohio State University to develop a new SPPS TBC system, funded by ONR.

So far there are three related patents on SPPS technology. They are as follows:
1. Patent issued (US Navy and Inframat): concept and process
2. Patent issued (Inframat and UConn): process and equipment
3. Patent pending (UConn and Inframat): process, material, and coating structure

Now, the researchers are widening the applications of the SPPS process for making dense thin film, coating, thick layer (>2 mm), and even preforms. "We also utilize the SPPS process for making functional films, such as for magnetic film and porous anode in solid oxide fuel cell," says Xinqing Ma, VP R&D, Inframat Corporation. He adds, "The work will overcome the challenges of making nanostructured film or coating at [a] one-step process, high efficiency, and deposition rate. The process can be adapted to most commercial thermal spray equipment and produce high performance and functional coatings on large dimension components at an affordable cost and high rate. It makes it possible to produce nanostructured industrial coatings."

Details:

Xinqing Ma, PhD,

VP R&D, Inframat Corporation,

74 Batterson Park Road, Farmington, CT 06032. Pilot plant,

156-J, River Road Willington, CT 06279.

Phone: 860-486-2358.

E-mail: xma@inframat.com.

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