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Solution precursor plasma-sprayed (SPPS) yttrium aluminum garnet (YAG) thermal barrier coatings (TBCs) have previously been shown to have higher temperature capability and reduced thermal conductivity compared to state-of-the- art TBCs. This previous work was conducted using a relatively low enthalpy plasma gun (Metco 9 MB) and TBCs were deposited on laboratory specimens. The primary goal of this work was to advance the state of technology readiness of SPPS YAG TBC coatings by using a high enthalpy cascaded arc gun (Sinplex Pro) to produce varied microstructures optimized for specific engine components: a fuel nozzle tip, an annular combustor liner, and turbine ceramic outer air seals. The microstructure and properties of these TBCs have been characterized and shown to be superior to those obtained previously. Based on these favorable results, the processing technology was transferred to solar turbines incorporated. Their process optimization of coatings for the three engine components and the rig and engine testing of the coated components will be described in Part II of this paper. [ABSTRACT FROM AUTHOR]

Gas turbine efficiency can be improved by increasing the operating temperature and by reducing the need for cooling air, both of which are enabled by higher temperature materials. Silicon carbide based ceramic matrix composites (CMCs) have shown promise for achieving higher temperatures. However, to survive in the high-temperature high-humidity combustion environment, environmental barrier coatings (EBCs) are essential to limit silicon loss of CMCs and its associated damage. Atmospheric plasma spray (APS) is a popular process for making such coatings. In this work, an alternative deposition process is explored, specifically the solution precursor plasma spray (SPPS), where it will be shown for ytterbium monosilicate EBCs that such process directly deposits thin (< 50 μm), dense and crystalline EBC topcoats without annealing of amorphous phases as is needed in the APS process. In addition, the SPPS process allows for quick compensation of system-specific element loss observed in both SPPS and APS processes. Finally, the secondary phase present in EBCs are much smaller in size for the SPPS process and hence less likely to produce cracking compared to APS EBCs. [ABSTRACT FROM AUTHOR]

Low thermal conductivity in yttrium aluminum garnet (YAG)-based thermal barrier coatings (TBCs) made by solution precursor plasma spray (SPPS) can be achieved by creating planar arrays of porosity called inter-pass boundaries (IPBs) as shown in Part I. In the current work, the mechanism of IPBs formation is studied through analysis of precursor entrainment and collection of single/raster step deposition patterns. It is concluded that the IPBs are formed by trapping precursor that under/over penetrates the plasma jet. CMAS interaction tests on SPPS YAG TBCs with heavy IPBs show an improvement of 123X and 15X over APS YSZ and SPPS YAG-light IPBs TBCs, respectively. It is demonstrated that the exceptional coating performance is because of the engineered heavy IPBs which branch out from the vertical cracks and run parallel to the surface. The CMAS melt gets drawn in the IPBs due to the capillary forces, leading to a shallow infiltration depth. The IPBs have a porosity of 70%, thus act as reservoirs for CMAS. Based on the favorable results, an alternate CMAS mitigation strategy is proposed that solely relies on microstructural features instead of the conventional approach where a vigorous reaction between CMAS-TBCs is desirable to form secondary phases. [ABSTRACT FROM AUTHOR]

Yttrium aluminum garnet thermal barrier coatings (TBCs) deposited by the solution precursor plasma spray process (SPPS) have been shown to have a 200 °C temperature advantage compared to air plasma spray yttria-stabilized zirconia TBCs (Gell et al. in J Therm Spray Technol 27:1-13, 2018). These properties were generated on specimens and included phase stability to 1900 °C, improved sinter resistance, reduced thermal conductivity, and enhanced CMAS resistance. Part I of this paper describes the development of the SPPS processing parameters with different microstructures for three solar engine components (fuel nozzle injectors, annular combustor, and turbine ceramic outer air seals). This Part II paper describes the technology transfer to Solar Turbines Incorporated, coating of the three components and the successful rig and engine tests. [ABSTRACT FROM AUTHOR]

Solution precursor plasma spray (SPPS) is a coating deposition process that uses conventional plasma spray equipment, and solution precursors, rather than ceramic or metal powders, as starting materials. Because the process is exposed to oxygen at high temperatures, nearly all coatings, to date, are oxide ceramics. In this review, both the advantages and the disadvantages of the SPPS process and some comparisons made to the suspension plasma spray (SPS) process will be discussed. The advantages of the SPPS process include rapid exploration of compositions and fabrication of advanced coatings with unique microstructural features. Examples presented span densities from porous thermal barrier coatings (TBCs) to dense TiO coatings. Two TBCs are in an advanced development stage: (1) a low thermal conductivity YSZ TBC and (2) a high-temperature yttrium aluminum garnet TBC. As for disadvantages, there are (1) the additional development work for each new precursor and (2) a lower standoff distance and deposition rate than the APS process, related to the evaporation of the solvent. The SPS process shares the same disadvantages. In developing new coatings, a number of factors should be considered and understood, which would help to shorten future development efforts. Future directions of the SPPS process will also be discussed. [ABSTRACT FROM AUTHOR]

Advanced thermal barrier coatings (TBCs) with lower thermal conductivity, increased resistance to calcium-magnesium-aluminosilicate (CMAS), and improved high-temperature capability, compared to traditional yttria-stabilized zirconia (YSZ) TBCs, are essential to higher efficiency in next generation gas turbine engines. Double-layer rare-earth zirconate/YSZ TBCs are a promising solution. From a processing perspective, solution precursor plasma spray (SPPS) process with its unique and beneficial microstructural features can be an effective approach to obtaining the double-layer microstructure. Previously durable low-thermal-conductivity YSZ TBCs with optimized layered porosity, called the inter-pass boundaries (IPBs) were produced using the SPPS process. In this study, an SPPS gadolinium zirconate (GZO) protective surface layer was successfully added. These SPPS double-layer TBCs not only retained good cyclic durability and low thermal conductivity, but also demonstrated favorable phase stability and increased surface temperature capabilities. The CMAS resistance was evaluated with both accumulative and single applications of simulated CMAS in isothermal furnaces. The double-layer YSZ/GZO exhibited dramatic improvement in the single application, but not in the continuous one. In addition, to explore their potential application in integrated gasification combined cycle environments, double-layer TBCs were tested under high-temperature humidity and encouraging performance was recorded. [ABSTRACT FROM AUTHOR]

Critical properties of TBCs include low thermal conductivity (TC) and high cyclic durability. In recent years, the solution precursor plasma spray (SPPS) process has been effectively used for deposition of TBCs with enhanced cyclic durability for yttria-stabilized zirconia (YSZ) and the higher temperature yttrium aluminum garnet (YAG) TBCs. Improvements in deposition efficiency (DE) and deposition rate (DR) were identified as needed for cost-effective commercialization. In this work, SPPS YAG TBCs with low TC and high DE and DR have been produced with microstructures containing a high density of planar arrays of increased porosity called inter-pass boundaries (IPBs). The IPBs have been shown to reduce the TC of SPPS YAG TBCs by 39% to a value of 0.58 W/mK at 1300 °C. Increasing the precursor feed rates and decreasing the raster scan step heights generate high-density IPB microstructures, while simultaneously increasing the DE and DR by 53 and 190%, respectively. The SPPS YAG microstructures with high-density IPBs also exhibit improved thermal cycling and sinter resistance compared to APS YSZ. [ABSTRACT FROM AUTHOR]

Gas-turbine engines are widely used in transportation, energy and defense industries. The increasing demand for more efficient gas turbines requires higher turbine operating temperatures. For more than 40 years, yttria-stabilized zirconia (YSZ) has been the dominant thermal barrier coating (TBC) due to its outstanding material properties. However, the practical use of YSZ-based TBCs is limited to approximately 1200 °C. Developing new, higher temperature TBCs has proven challenging to satisfy the multiple property requirements of a durable TBC. In this study, an advanced TBC has been developed by using the solution precursor plasma spray (SPPS) process that generates unique engineered microstructures with the higher temperature yttrium aluminum garnet (YAG) to produce a TBC that can meet and exceed the major performance standards of state-of-the-art air plasma sprayed YSZ, including: phase stability, sintering resistance, CMAS resistance, thermal cycle durability, thermal conductivity and erosion resistance. The temperature improvement for hot section gas turbine materials (superalloys & TBCs) has been at the rate of about 50 °C per decade over the last 50 years. In contrast, SPPS YAG TBCs offer the near-term potential of a > 200 °C improvement in temperature capability. [ABSTRACT FROM AUTHOR]