Above: tortuous crack path that emerges in anisotropic, heterogeneous microstructure.

1. Understanding Microstructural Effects on TBC ToughnessCarlos G. Levi, University of California-Santa Barbara, DMR 1105672 Outcome:New toughening mechanism conclusively demonstrated in dense-vertically cracked (DVC) air plasma sprayed (APS) thermal barrier coatings (TBCs). Impact: Findings can guide microstructural strategies for improving the fracture resistance of high temperature coatings with evolving structure. • Explanation:Tests on 8YSZ and 20YSZ using the DCB test developed earlier in this program showed that the toughness enhancement results from the combination of the DVC structure with the ferroelasticity of the t’ YSC ceramic. Absent DVC (APS 8YSZ) or ferroelasticity (DVC 20YSZ) the measured toughness is considerably lower. Fracture surfaces corresponding to the ERR curves on the left graph. Note the much flatter fracture path in the APS 8YSZ material. Representative ERR curves for tests of two compositions , with equivalent microstrucure (20YSZ and 8YSZ) as well as 8YSZ with two different microstructures.

2. Understanding Phase Stability in YSZ from First PrinciplesCarlos G. Levi, University of California-Santa Barbara, DMR 1105672 Outcome:New fundamental insight on the interplay between aliovalent stabilizers and anion vacancies has emerged from first principles calculations of the stability of ZrO2 solid solutions. Impact: Insight helps explain the stabilizing effects of different cations enabling new stabilizing strategies for high temperature coating materials. • Explanation:Energetic calculations show that a vacancy surrounded by two host and two dopant cations is subject to a charge field that drives the vacancy to exchange places with an anion in the next coordination sphere. Free energy difference for cubic and tetragonal ZrO2, as computed via the harmonic approximation and thermodynamic integration on the basis of DFT-LDA first-principles calculations. The inclusion of anharmonic effects yields a phase transition temperature of ~2550 K, in excellent agreement with experiment. Dipole-moment driven relaxation of the oxygen vacancy (viz. next nearest neighbor oxygen) from the first to the second coordination shell of the yttrium cation as simulated by DFT-LDA.

3. An Integrated Approach to System DynamicsCarlos G. Levi, University of California-Santa Barbara, DMR 1105672 • Outcome: Highly parallelized simulation tools have been developed to predict the role of microstructure on cracking and damage in elastic materials. • Impact: Simulations with explicit representations of heterogeneity enable new connections between intrinsic toughness, microstructural characteristics and macroscopic properties, so that: (i) simulations guide processing to target favorable microstructures and improve macroscopic performance, and (ii) the effect of changing microstructure can be simulated. • Explanation: Discrete element methods have been incorporated in an highly efficient parallelized architecture, which allows for simulations with millions of elements. These elements can be arranged to describe individual microstructural features, while the cohesive laws connecting elements (and regions of elements) represent intrinsic fracture parameters (e.g. toughness). Cracking patterns – including branching, microcracking, the emergence of a dominant flaw – can then predicted as a natural outcome of loading. Future work will incorporate microstructural evolution to determine the impact of dynamically evolving structure and properties. Above: tortuous crack path that emerges in anisotropic, heterogeneous microstructure. Below: crack length as a function of applied curvature (bending) used to calculate macroscopic toughness as a function of intrinsic toughness and microstructural properties.

4. A New Femtosecond Laser-Based Interfacial Toughness ProbeCarlos G. Levi, University of California-Santa Barbara, DMR 1105672 Outcome: An ultrafast laser pulse applied to the TGO-Bond coat interface initiates local detachment and subsequent propagation permits assessment of the local toughness of said interface. Impact: This new technique permits quick assessment of local interfacial toughness and its spatial variation across a specimen and/or through varying service conditions, providing a new tool to guide bond coat development. Explanation: The thermally grown oxide (TGO) on a bond coat is under intense biaxial compression upon cooling due to the thermal expansion mismatch between the layers. A mechanics analysis shows that a delamination of suitable size will buckle out of plane in order to relieve strainenergy. The resulting behavior of the buckled film (no growth, growth, or spall) is used to determineproperties of the film and the interface. a) b) c) d) Schematic showing the buckling test. a) fs laser pulse is incident on the TGO-bond coat interface creating a local delamination. b) TGO is under biaxial compression and buckles out of plane to relieve strain energy. c) Interface cracks propagate and buckle grows. d) Interface cracks kink into the TGO and buckle spalls off. Students involved in this project: D.J. Jorgensen (GS), R. W. Jackson (PD).

5. First Principles Prediction of Transport Coefficients in Ni-Al AlloysCarlos G. Levi, University of California-Santa Barbara, DMR 1105672 • Outcome:Mechanisms and diffusion parameters for A1 and L12 phases in Ni-Al alloys as a function of Al content. • Impact: Research establishes foundation for modeling ternary diffusion, as needed to improve stability of environmental barriers on Ni base superalloys. • Explanation: Methods developed for B2-NiAl were extended to the (Ni) and Ni3Al phases. First-principles calculations of migrationbarriers show strong composition dependence. Cluster expansion of migration barriers are combined with kinetic Monte Carlo simulations to predict Onsager transport coefficients.

6. Broader ImpactsCarlos G. Levi, UC Santa Barbara, DMR 1105672 Industrial Interactions An international workshop on High Temperature Coatings was held at UCSB in January, 2012, with the attendance of academic, industrial and government agency representatives. A Goali meeting was held in conjunction with this workshop. A summer workshop was also held at GE Global Research in August 2012, with participation of PI’s, students, post-docs and all industrial collaborators. GS Chandra Macauley (left) was a summer intern at GE Global Research, mentored by co-PI Lipkin, working on processing and characterization of novel plasma sprayed Zr(Ce,Ti)O2identified in previous work under the Goali project. She is continuing work on this topic at UCSB. Collaboration with the Goali partners remains stimulating and effective. The project continues to benefit from collaborations with other companies, including PWA, Honeywell, Rolls-Royce and Alcoa Howmet. International and Educational Experiences GS David Poerschke (left) did an internship at the Karlsruhe institute of Technology in Germany, mentored by Prof. Hans Seifert and Dr. Damian Cupid. His focus was on thermodynamic property measurements and developing a thermodynamic database for high temperature coatings. UG student Damian Leon continued working as an intern during 2012, mentored by E. Donohue and J. Krogstad. C. Carbogno, a post-doctoral researcher from FHI-Berlin, and Luis Gutierrez, a research scientist from CINVESTAV (Mexico) spent much of 2012 at UCSB collaborating with this project. GS Michael Wang, from University of Melbourne, visited UCSB during this period to work with Pollock and Begley on characterization and modeling of complex microstructures.