CMAS challenges to CMC-T/EBC systems

Gas turbine technology is undergoing a major transition with the recent implementation of SiC based ceramic composites (CMCs) in aircraft engines. While the potential improvement in temperature capability (≥1500°C) is unprecedented, there are a number of issues that limit the full exploitation of such potential. In addition to the longstanding concern for low temperature oxidative embrittlement and the limited temperature capability of current bond coats and matrices, the susceptibility of the protective SiO2 to volatilization in the combustion environment requires the use of environmental barrier coatings (EBCs) to achieve durability targets. Most EBC concepts, however, are based on silicates and are thus susceptible to degradation by molten silicate deposits generically known as CMAS originating from mineral debris ingested into engines with the intake air. This presentation will discuss the thermodynamic and mechanistic foundation of the degradation of EBCs by CMAS, recent progress in establishing the relevant phase equilibria for these systems, and the role of the CMAS composition on the extent of degradation, as well as perspective on mitigation. (Research supported by ONR, AFOSR and the P&W Center of Excellence in Composites at UCSB.

Ductile-Phase Toughening of Brazed Joints

A heat treatment is presented that uses ductile-phase toughening to mitigate the effect of brittle intermetallics in a Ni-based braze alloy. The fracture resistance has been enhanced by creating a microstructure containing elongated ductile γ-(Ni) domains that align, preferentially, across the joint. The development of this beneficial microstructure is based on an understanding of the transient dissolution, isothermal solidification, and coarsening phenomena. Due to slow kinetics, the elimination of intermetallics by diffusion is avoided in favor of ductile domain formation through solidification control. The toughening has been attributed to a combination of bridging and process zone dissipation, enabled by the ductile phase

Matrix crack spacing in brittle matrix composites

A model describing the evolution of matrix cracks in undirectional continuous fiber, brittle matrix composites is developed. The approach involves calculation off the steady state strain energy release rate available for crack extension in terms of the constituent properties, the applied stress and the distances to the neighboring cracks. Interactions between cracks are found to occur when the crack spacing falls below twice the slip length. The model provides an analytical solution to the crack spacing for periodic arrays of cracks. Comparisons are conducted with predictions derived from computer simulations of random cracking. The effects of the matrix flaw density are briefly considered

Stochastic aspects of matrix cracking in brittle matrix composites

A computer simulation of multiple cracking in fiber-reinforced brittle matrix composites has been conducted, with emphasis on the role of the matrix flaw distribution. The simulations incorporate the effect of bridging fibers on the stress required for cracking. Both short and long (steady-state) flaws are considered. Furthermore, the effects of crack interactions (through the overlap of interface slip lengths) are incorporated. The influence of the crack distribution on the tensile response of such composites is also examined

Metal-Matrix Composites and Thermal Spray Coatings for Earth Moving Machines

In the fifth quarter, tooling for the steel MMC effort was redesigned based on the findings from the pressure casting trials of the previous quarter. While awaiting the arrival of that tooling, gravity casting trials were performed to assess modified performing technology and new hard particle systems. Steel-boride composite systems demonstrated good wetting and infiltration behavior, and fully infiltrated steel-boride composites were obtained under certain conditions. However, preform floating and particle dissolution are challenges which must be overcome. Ceramic oxide composites successfully pressure cast in a hot isostatic press at UC Santa Barbara were characterized and subject to fracture toughness testing. Resulting differences in fracture toughness are believed to be due to differences in matrix hardness, potentially imparted through reaction of the molten steel with the particles. Some evidence of bonding between the steel and oxide particles was noted on fracture surfaces. Arc lamp processing trials at Oak Ridge National Laboratory demonstrated that thermal spray coatings were successfully designed to facilitate fusion. All coatings investigated developed some degree of metallurgical bond after lamp fusion and for most coatings lamp fusion also further increased coating hardness. An overview of the progress during the 1st quarter of this project is given below. Research details are provided in the limited rights appendix to this report

Post-buckling and dynamic response of angled struts in elastic lattices

This paper presents analytical and reduced-order numerical solutions describing the non-linear response of slender, elastic struts (or plates) inclined relative to the loading direction. The solutions provide a highly efficient framework to predict post-buckling behaviors in cellular structures, including: stability regimes, peak strains during and after buckling, the work dissipated via cyclic loading, the impact of biaxial loading, and the role of geometric imperfections in the struts. Regime maps are presented that illustrate configurations that lead to snap-through, permanent deformation after unloading, strut failure, and enhanced hysteresis during cyclic loading. The maps illustrate that reversible snap-through events only occur within a very specific range of relative density (e.g. ̴0.25-0.4 for rhombic lattices). A highly efficient non-linear single degree-of-freedom dynamics model is derived from the statics solution, and is shown to be in excellent agreement with fully explicit, non-linear, dynamic finite element simulations for inclined struts. This simplified dynamics model is used to quantify the relationships between quasi-static responses, loading frequencies and energy dissipation during cycling loading. A key finding is that effective damping during cyclic loading is dramatically increased by non-linear behavior, even when the corresponding quasi-static result exhibits zero hysteresis. The implications for structured foams and the design of lightweight structural dampers is briefly discussed

Powder Processing of Ceramic matrix Composites

Powder processing of ceramic matrix composites is reviewed with emphasis on (1) forming powder compacts containing reinforcements, (2) the effect of the reinforcement network on the shrinkage and strengthening of the powder matrix during a heat treatment, (3) a novel method for producing a metal-reinforced ceramic composite and (4) a novel method for producing a laminar ceramic composite containing brittle fibers. Preliminary properties for the two new composite-processing methods are given. © 1991