- Solution precursor plasma spray
Solution Precursor Plasma Spray (SPPS) is a
thermal sprayprocess where a feedstock solution is heated and then deposited onto a substrate. Basic properties of the process are fundamentally similar to other plasma sprayingprocesses. 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 nanometersized microstructures without the injection feed problems normally associated with powder systems and flexible, rapid exploration of novel precursor compositions. [Eric H. Jordan, L. Xie, C. Ma M. Gell, N. Padture, B. Cetegen, J. Roth, T. D. Xiao and P. E. C. Bryant, "Superior Thermal Barrier Coatings Using Solution Precursor Plasma Spray", "Journal of Thermal Spray", 13(1), 2004, p 57-65] [L. Xie, X. Ma, E.H. Jordan, N. P. Padture, T.D. Xiao and M. Gell, "Deposition of Thermal Barrier Coatings Using Solution Precursor Plasma Spray Process", "Journal of Material Science", 39, 2004 p1639-1636]
The use of a solution precursor was first reported as a coating technology by Karthikeyan, et al. [Karthikeyan J.,Berndt C.C., Tikkanen J., Wang J.Y., King A.H., Herman H, "Preparation of Nanophase Materials by Thermal Spray Processing of Liquid Precursors", "Nanostructured Materials", 9(1), 1997, p 137-140] . [Karthikeyan J.,Berndt C.C., Tikkanen J., Wang J.Y., King A.H., Herman H, "Nanomaterial Powders and Deposits Prepared by Flame Spray Processing of Liquid Precursors", 8(1), 1997, p 61-74] [Jeganathan Karthikeyan, Christopher C. Berndt, Sri Reddy, Jenn-Yue Wang, Alexander H. King, and Herbert Herman, "Nanomaterial Deposits Formed by DC Plasma Spraying of Liquid Feedstocks", "Journal of the American Ceramic Society", 81, 1998, p 121-128] 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, [N. P. Padture, K. W. Schlichting, T. Bhatia, A. Ozturk, B. Cetegen, E. H. Jordan, M. Gell, S. Jiang, T. D. Xiao, P. R. Strutt, E. Garcia , P.Miranzo and M. I. Osendi, "Towards Durable Thermal Barrier Coatings with Novel Microstructures Deposited by Solution Precursor Plasma Spray", "Acta Materialia", 49, 2001, p 2251–2257] YAGfilms, [Sujatha D. Parukuttyamma, Joshua Margolis, Haiming Liu, Clare P. Grey, Sanjay Sampath, Herbert Herman, and John B. Parise, "Yttrium Aluminum Garnet (YAG) Films through a Precursor Plasma Spraying Technique", "Journal of the American Ceramic Society", 84(8), 2001, p 1906–908] and silicon ceramic coatings. [E. Bouyer, G. Schiller, M. Muller, and R. H. Heane, "Thermal Plasma Chemical Vapor Deposition of Si-Based Ceramic Coatings from Liquid Precursors", "Plasma Chemistry and Plasma Processing", 21(4), 2001, p 523-546] Since then, extensive research on the technology has been explored in large part by the [http://www.uconn.edu University of Connecticut] and [http://www.inframat.com Inframat Corporation] .
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 plasma plume or High Velocity Oxygen Fuel (HVOF) combustion flame. Once the solution is injected, the droplets go through several chemical and physical changes [Ozturk, A. and Cetegen B. M., "Modeling of Axially and Transversely Injected Precursor Droplets into a Plasma Environment", "International Journal of Heat and Mass Transfer", 48(21-22), 2005, p 4367-4383] 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. [L. Xie, X. Ma, E.H. Jordan, N. P. Padture, T.D. Xiao and M. Gell, "Deposition of Thermal Barrier Coatings Using Solution Precursor Plasma Spray Process", "Journal of Material Science", 39, 2004 p1639-1636] [L. Xie, X. Ma, E.H. Jordan, N. P. Padture, T.D. Xiao and M. Gell, "Identification of Coating Deposition Mechanisms in the Solution-Precursor Plasma-Spray Process using Model Spray Experiments", "Materials Science and Engineering" A, 362, 2003, p 204-212]
Thermal Barrier Coatings
Most current research on SPPS has examined is application to create
thermal barrier coatings (TBCs). These complex ceramic/ metallicmaterial systems are used to protect components in hot sections of gas turbine and diesel engines. [Padtre, Nitin P., Gell, Maurice, Jordan, Eric H., "Thermal Barrier Coatings for Gas-Turbine Engine Applications", "Science", 296, 2002, p 280-285] 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. [L. Xie, X. Ma, E.H. Jordan, N. P. Padture, T.D. Xiao and M. Gell, "Highly Durable Thermal Barrier Coatings Made by the Solution Precursor Plasma Spray Process", "Surface and Coatings Technology", 177-178, 2004, p 97-102] [Amol Jadhav, Nitin Padture, Fang Wu, Eric Jordan , Maurice Gell, "Thick ceramic thermal barrier coatings with high durability deposited using solution-precursor plasma spray", "Materials Science and Engineering" A, 405, 2005 p 313-320] [Liangde Xie, Eric H. Jordan and Maurice Gell, "Phase and Microstructural Stability of Precursor Plasma Sprayed Thermal Barrier Coatings", "Material Science and Engineering" A, 381, 2004, p 189-195] 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 unpryolized material in the coating. [Liangde Xie, Dianying Chen, Eric H. Jordan, Alper Ozturk, Fang Wu, Xinqing Ma, Baki M. Cetegen and Maurice Bell, "Formation of Vertical Cracks in Solution- Precursor Plasma- Sprayed Thermal Barrier Coatings", "Surface Coatings and Technology", 201, 2006, p 1058-1064] . 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 conductivityto some of the lowest reported values for TBCs. [Xinqing Ma, Fang Wu, Jeff Roth, Maurice Gell, Eric Jordan, "Low Thermal Conductivity Thermal Barrier Coating Deposited by the Solution Plasma Spray Process", "Surface and Coatings Technology", 201, 2006, p 3343-3349] . [X. Q. Ma, T. D. Xiao, J. Roth, L. D. Xie, E. H. Jordan, N. P. Padture, M. Gell, X. Q. Chen, J. R. Price, "Thick Thermal Barrier Coatings with Controlled Microstructures Using Solution Precursor Plasma Spray Process", "Thermal Spray 2004: Advances in Technology and Application", ASM International, May 10-12, 2004 (Osaka, Japan), ASM International, 2004] 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.
The SPPS process is adapted to existing thermal spray systems. Application costs are significantly less than EB-PVD coatings and slightly higher than Air Plasma Spray coatings. [Maurice Gell, Fang Wu, Eric H. Jordan, Nitin P. Padture, Baki M. Cetegen, Liangde Zie, Alper Ozturk, Eric Cao, Amol Jadhav, Dianying Chen, and Xinqin Ma, The Solution Precursor Plasma Spray Process for Making Highly Durable Thermal Barrier Coatings, Proceedings of GT2005, ASME Turbo Expo 2005]
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