Joseph E. Grady

Durability Modeling of Environmental Barrier Coating (EBC) Using Finite Element Based Progressive Failure Analysis

The necessity for a protecting guard for the popular ceramic matrix composites (CMCs) is getting a lot of attention from engine manufacturers and aerospace companies. The CMC has a weight advantage over standard metallic materials and more performance benefits. However, these materials undergo degradation that typically includes coating interface oxidation as opposed to moisture induced matrix which is generally seen at a higher temperature. Additionally, other factors such as residual stresses, coating process related flaws, and casting conditions may influence the degradation of their mechanical properties. These durability considerations are being addressed by introducing highly specialized form of environmental barrier coating (EBC) that is being developed and explored in particular for high temperature applications greater than 1100°C. As a result, a novel computational simulation approach is presented to predict life for EBC/CMC specimen using the finite element method augmented with progressive failure analysis (PFA) that included durability, damage tracking, and material degradation model. The life assessment is carried out using both micromechanics and macromechanics properties. The macromechanics properties yielded a more conservative life for the CMC specimen as compared to that obtained from the micromechanics with fiber and matrix properties as input.

Environmental barrier coating (EBC) durability modeling using a progressive failure analysis approach

Ceramic matrix composites (CMCs) are getting the attention of most engine manufacturers and aerospace firms for turbine engine and other related applications. This is because of their potential weight advantage and performance benefits. As a protecting guard for these materials, a highly specialized form of environmental barrier coating (EBC) is being developed and explored for high temperature applications that are greater than 1100 °C1,2. The EBCs are typically a multilayer of coatings and are on the order of hundreds of microns thick. CMCs are generally porous materials and this feature is somewhat beneficial since it allows some desirable infiltration of the EBC. Their degradation usually includes coating interface oxidation as opposed to moisture induced matrix degradation which is generally seen at a higher temperature. A variety of factors such as residual stresses, coating process related flaws, and casting conditions may influence the strength of degradation. The cause of such defects which cause cracking and other damage is that not much energy is absorbed during fracture of these materials. Therefore, an understanding of the issues that control crack deflection and propagation along interfaces is needed to maximize the energy dissipation capabilities of layered ceramics. Thus, evaluating components and subcomponents made out of CMCs under gas turbine engine conditions is suggested to demonstrate that these material will perform as expected and required under these aggressive environmental circumstances. Progressive failure analysis (PFA) is applied to assess the damage growth of the coating under combined thermal and mechanical loading conditions. The PFA evaluation is carried out using a full-scale finite element model to account for the average material failure at the microscopic or macroscopic levels. The PFA life prediction evaluation identified the root cause for damage initiation and propagation. It indicated that delamination type damage initiated mainly in the bond and intermediate coating materials then propagated to the substrate. Results related to damage initiation and propagation; behavior and life assessment of the coating at the interface of the EBC/CMC are presented and discussed.

Criteria for Crack Deflection-Penetration in EBC Coated Ceramics: A Parametric Study.

This article discusses results obtained from a parametric study to analytically evaluate the impingement of a crack at the interface of an environmental barrier coating (EBC) and a monolithic Silicon nitride (Si3N4) layered ceramics substrate. The study establishes a correlation that leads to determine if the crack is arrested or advanced by either penetrating or deflecting along the EBC/substrate interface. A finite-element-based fracture mechanics methodology is utilized to perform these calculations. Critical parameters determining penetration-deflection conditions in relation to EBCs physical characteristics, such as porosity level, voids, and mini cracks, are determined for a single layer and multi-layered coating system coordinating the interactions between the EBCs (Mullite, Mullite mixture, Silicon nitride, etc.) and the substrate structure. Results showing thermo-mechanical stresses and stress/strain energy release relations with respect to crack penetration-deflection are presented and discussed as the crack is advanced.

Rapid Chemical Vapor Infiltration of Silicon Carbide Minicomposites at Atmospheric Pressure

The chemical vapor infiltration technique is one of the most popular for the fabrication of the matrix portion of a ceramic matrix composite. This work focuses on tailoring an atmospheric pressure deposition of silicon carbide onto carbon fiber tows using the methyltrichlorosilane (CH3SiCl3) and H2 deposition system at atmospheric pressure to create minicomposites faster than low pressure systems. Adjustment of the flow rate of H2 bubbled through CH3SiCl3 will improve the uniformity of the deposition as well as infiltrate the substrate more completely as the flow rate is decreased. Low pressure depositions conducted at 50 Torr deposit SiC at a rate of approximately 200 nm*h-1, while the atmospheric pressure system presented has a deposition rate ranging from 750 nm*h-1 to 3.88 μm*h-1. The minicomposites fabricated in this study had approximate total porosities of 3 and 6% for 10 and 25 SCCM infiltrations, respectively.

Test Validation of Environmental Barrier Coating (EBC) Durability and Damage Tolerance Modeling Approach

Protection of Ceramic Matrix Composites (CMCs) is rather an important element for the engine manufacturers and aerospace companies to help improve the durability of their hot engine components. The CMC’s are typically porous materials which permits some desirable infiltration that lead to strength enhancements. However, they experience various durability issues such as degradation due to coating oxidation. These concerns are being addressed by introducing a high temperature protective system, Environmental Barrier Coating (EBC) that can operate at temperature applications1, 3 In this paper, linear elastic progressive failure analyses are performed to evaluate conditions that would cause crack initiation in the EBC. The analysis is to determine the overall failure sequence under tensile loading conditions on different layers of material including the EBC and CMC in an attempt to develop a life/failure model. A 3D finite element model of a dogbone specimen is constructed for the analyses. Damage initiation, propagation and final failure is captured using a progressive failure model considering tensile loading conditions at room temperature. It is expected that this study will establish a process for using a computational approach, validated at a specimen level, to predict reliably in the future component level performance without resorting to extensive testing.