July/August_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 1 8 5 0 iTSSe TSS iTSSe TSS challenges need to be addressed and resolved before SPS be- comes an economic and capable industrial process, especially for complex turbine components (Fig. 2). HIGHER TEMPERATURE THERMAL BARRIER COATINGS WITH THE COMBINED USE OF YTTRIUM ALUMINUM GARNET AND THE SOLUTION PRECURSOR PLASMA SPRAY PROCESS Maurice Gell, Jiwen Wang, Rishi Kumar, Jeffery Roth, Chen Jiang, and Eric H. Jordan Gas turbine engines are widely used in the transporta- tion, energy, and defense industries. Increasing demand for more efficient gas turbines requires higher turbine operating temperatures. For more than 40 years, yttria-stabilized zir- conia (YSZ) has been the dominant thermal barrier coating (TBC) due to its outstandingmaterial 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 TBCwas 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 mate- rials (superalloys and TBCs) has been at the rate of about 50°C per decade over the past 50 years. In contrast, SPPS YAG TBCs offer near-term potential of a >200°C improvement in tem- perature capability (Fig. 3). COMPARISON OF SINGLE-PHASE AND TWO-PHASE COMPOSITE THERMAL BARRIER COATINGS WITH EQUAL TOTAL RARE-EARTH CONTENT Amarendra K. Rai, Michael P. Schmitt, Mitchell R. Dorfman, Dongming Zhu, and Douglas E. Wolfe Rare-earth zirconates have been the focus of advanced thermal barrier coating research for nearly two decades. How- ever, their lack of toughness prevents wide-scale adoption due to lack of erosion and thermal cyclic durability. There are generally two methods of improving toughness: intrinsic modification of the coating chemistry and extrinsic modifica- tion of the coating structure. This study compares the efficacy of these two methods for a similar overall rare-earth content via the air plasma spray process. The extrinsically toughened coatings were comprised of a two-phase composite containing 30 wt% Gd 2 Zr 2 O 7 (GZO) combined with 70 wt% of a tough- er t′ low-k material (ZrO 2 -2Y 2 O 3 -1Gd 2 O 3 - 1Yb 2 O 3 ; mol%), while a single-phase fluorite with the overall rare-earth content equivalent to the two-phase composite (13 mol% rare-earth) was utilized to ex- plore the intrinsically toughened concept. Coatings were then characterized via x-ray diffraction, energy-dispersive spectros- copy, and scanning electron microscopy. Performance was evaluated via erosion, thermal conductivity, thermal annealing (500 h), and thermal cycling. It was shown that the extrinsic method provides im- proved erosion and thermal conductivity response over the single phase, but at the expense of high-temperature stability and cyclic life (Fig. 4). Fig. 4 — SEMmicrographs of spray-dried GZO powder. Fig. 3 — Microstructure of C f /Al composites strengthened by carbon fiber bundles of 750 filaments. 16