High-temperature mechanical properties and oxidation resistance of SiCf/SiC ceramic matrix composites with multi-layer environmental barrier coatings for turbine applications

Continuous silicon carbide fiber reinforced silicon carbide (SiC_f/SiC) ceramic matrix composites are considered promising materials as high-temperature components of advanced aero-engines. However, due to their susceptibility to oxidation and corrosion at high temperature, environmental barrier coatings (EBCs) must be applied on the surface of SiC_f/SiC. In this study, Si/Y₂SiO₅/LaMgAl₁₁O₁₉ (LMA) multi-layer EBCs were fabricated to protect SiC_f/SiC by using atmospheric plasma spraying (APS). The high-temperature tensile fatigue performance of SiC_f/SiC with and without EBCs was evaluated. The results indicated that EBCs significantly improved the tensile fatigue properties of SiC_f/SiC at high temperature in air atmosphere. Meanwhile the bending strength of specimens after isothermal aging or not was also tested. The multi-layer EBCs in this study may be a promising EBCs system for SiC_f/SiC after some improvements.     

Characterisation of joined surface modified SiCf/SiC composites

This work has focused on surface engineering coupled with brazing to join SiC_f/SiC composites and to improve the joint strength. The surface engineering was carried out through the Selective Thermal Removal (STR) of SiC fibres from a SiC_f/SiC composite to obtain “brush-like” surfaces; the modified composites were then joined by means of an AgCuTi braze alloy. In order to investigate whether a “brush-like” interface could enhance the mechanical strength of the joint by increasing mechanical interlocking with the brazing alloy at the micron scale, the joints were tested with and without surface engineering by means of single lap offset under compression.The treatment to form a roughened surface suitable for mechanical keying of the braze metal led to an improved ductility of the joint; the fracture surfaces demonstrated that the proposed method is promising, even though the treatment damages locally the composite.     

Residual stress measurements in melt infiltrated SiC/SiC ceramic matrix composites using Raman spectroscopy

Raman spectroscopy was utilized to characterize the chemical composition and residual stresses formed in melt infiltrated SiC/SiC CMCs during processing. Stresses in SiC fibers, in SiC chemical vapor (CVI) infiltrated matrix, in SiC melt infiltrated matrix, and in free silicon were measured for two different plates of CMCs. Stresses in the free silicon averaged around 2?GPa in compression, while stresses in the matrix SiC were 1.45?GPa in tension. The SiC CVI phase had stresses ranging between 0.9?GPa and 1.2?GPa in tension and the SiC fibers experienced stresses of .05–0.7?GPa in tension. These results were validated with the proposed model of the system. While the mismatch in the coefficients of thermal expansion between the constituents contributes to the overall residual stress state, the silicon expansion upon solidification was found to be the major contributor to residual stresses within the composite.     

Preparation and characterization of YSZ abradable sealing coating through mixed solution precursor plasma spraying

Yttria-stabilized zirconia (YSZ) ceramic matrix abradable sealing coatings were prepared by plasma spraying of a blend of YSZ solution precursor with YSZ nano-particles. The microstructure and phase compositions of the prepared abradable sealing coatings were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). In addition, the mechanical, high-temperature oxidation, and tribological properties of the coatings were systematically investigated. The results show that addition of YSZ nano-particles increased porosity and bond strength and decreased the hardness of the coating. The optimum performance value was achieved by addition of 5?g nano-particles into the coating. The coatings maintained excellent thermal stability through a ten-cycle thermal shock test at 1150?°C. The 8YSZ-5 coating had an improved oxidation constant of 5.540?×?10^?4 and exhibited remarkable oxidation kinetics at 1150?°C. The friction coefficient of the mixed solution precursor coating was remarkably decreased compared with a traditional ceramic matrix abradable sealing coating. The results indicate that mixed solution precursor plasma spraying increased abradable sealing coating application performance.     

Modeling of fatigue failure for SiC/SiC ceramic matrix composites at elevated temperatures and multi-scale experimental validation

The fatigue failure of ceramic matrix composites at elevated temperatures was predicted using the micromechanics method. Multiple micro-damage models were developed to describe the evolution processes of matrix cracking, interface wear, and fiber fracture during fatigue loading. On this basis, the fatigue life was calculated. To validate the fatigue failure model, multi-scale experiments were conducted. In the macroscale, the S-N curve was obtained by the fatigue test. In the microscale, multiple in-situ measuring methods were developed through which the matrix crack density, the interfacial shear stress, and the percentage of fracture fibers were obtained. Both the macroscale and microscale experimental results were in good agreement with the predicted results. Therefore, the fatigue failure model developed in the present work is accurate.     

Interaction of fiber matrix bonding in SiC/SiC ceramic matrix composites

Non-oxide ceramic matrix composites (CMC) based on SiC fibers with SiC matrix were fabricated by polymer infiltration and pyrolysis (PIP) and characterized regarding their microstructural features and their mechanical properties. The fiber preform was made using winding technology. During the winding process, the SiC fiber roving was impregnated by a slurry containing SiC powder and sintering additives (Y₂O_3, Al₂O₃ and SiO₂). This already helped to achieve a partial matrix formation during the preform fabrication. In this way, the number of PIP cycles to achieve composites with less than 10% open porosity could be reduced significantly. Additionally, damage-tolerant properties of the composites were obtained by an optimal design of the matrix properties although only uncoated fibers were used. Finally, composites with a strength level of about 500 MPa and a damage-tolerant fracture behavior with about 0.4% strain to failure were obtained.     

Air plasma-sprayed Y2O3 coatings for Al2O3/Al2O3 ceramic matrix composites

Al₂O₃/Al₂O₃ ceramic matrix composites (CMC) are candidate materials for hot-gas leading components of gas turbines. Since Al₂O₃/Al₂O₃ CMC are prone to hot-corrosion in combustion environments, the development of environmental barrier coatings (EBC) is mandatory. Owing to its favorable chemical stability and thermal properties, Y₂O₃ is considered a candidate EBC material for Al₂O₃/Al₂O₃ CMC. Up to 1 mm thick Y₂O₃ coatings were deposited by means of air plasma spraying (APS) on Al₂O₃/Al₂O₃ CMC with a reaction-bonded Al₂O₃ bond-coat (RBAO). APS Y₂O₃ coatings exhibit a good adherence in the as-deposited state as well as upon isothermal annealing up to 1400 °C. Moreover, furnace cyclic testing performed at 1200 °C revealed an excellent durability. This is explained by the formation of a continuous, approximately 1 μm thick reaction zone at the APS Y₂O₃/RBAO interface. The reaction zone between Y₂O₃ and Al₂O₃ comprises three layers of thermodynamically stable yttrium-aluminates exhibiting strong bonding, respectively.     

Volumetric assessment of fatigue damage in a SiCf/SiC ceramic matrix composite via in situ X-ray computed tomography

To enhance the understanding of matrix cracking and damage progression on the macroscopic scale, within a 0/90° fibre reinforced SiC_f/SiC ceramic matrix composite (CMC), X-ray computed tomography (XCT) imaging and analysis have been performed in conjunction with a commercially available in-situ mechanical loading device. CMC test coupons were subjected to tensile cyclic loads and inspected using XCT without removal from the tensile loading device. Attempts to measure and quantify the resulting damage using volumetric image analysis techniques are presented, by characterising the crack network from XCT images acquired at both the maximum and minimum load condition during selected fatigue cycles. The XCT detection of significant crack development within the first loading half-cycle shows good agreement with cumulative acoustic emission energy data recorded under similar test conditions. The results are seen as an important step towards correlating the damage behaviour detected via different NDE and health monitoring techniques.     

Machinability improvement in milling of SiCf/SiC composites based on laser controllable ablation pretreatment

The poor machinability of SiC_f/SiC composites greatly limits its application and promotion. The laser-induced ablation products of SiC_f/SiC composites are powdery, loose and porous. Milling of laser ablated samples demonstrated that the force and heat were almost negligible when milling ablation products. Accordingly, a laser ablation pretreatment milling (LAPM) process of SiC_f/SiC composites was proposed. Under the LAPM process, after the laser ablation treatment with controllable depth, the cutting allowance could be achieved in only one pass, which greatly improved the machining efficiency compared with the conventional milling process. The material removal rate was greatly improved on the premise of ensuring the machining quality. Taking the milling of tensile specimens as an example, compared with conventional milling, the total processing time of the specimen was reduced by 31.29 % by LAPM process. Therefore, LAPM provides a potential feasible process scheme for greatly improving the machinability and machining efficiency of SiC_f/SiC composites.     

Tension-tension fatigue behavior of a melt-infiltrated SiC/SiC ceramic matrix composites in a combustion environment

An experimental thermo-mechanical facility was developed with conditions towards that of the combustion environments experienced by the hot section components of a jet engine. Two different melt-infiltrated (MI) ceramic matrix composites (CMCs) were evaluated, one containing Hi-Nicalon Type S fibers and the other Tyranno SA fibers. Specimens considered in this study were subjected to fatigue loading with a stress ratio of 0.1, frequency of 1 Hz and a specimen surface temperature of 1200 ± 20 °C. Results indicate that fatigue life in the combustion environment was an order of magnitude lower compared to the furnace environment and is attributed to the hostile environment present with the burner rig. Post-test microscopy was conducted in order to understand the damage mechanisms and oxidation behavior. Polished longitudinal sections of the burner rig specimens revealed longitudinal cracking which could be attributed to the presence of thermal gradient stress. Electrical resistance (ER) was implemented to monitor the damage.     

Low temperature mullite forming pre-ceramic resins of high ceramic yield for oxide matrix composites

Monophasic mullite precursors, namely aluminosiloxanes were synthesized by a novel synthetic route, in which co-hydrolysis and condensation of aluminium tri secbutoxide and tetraethoxy silane was achieved in presence of con. HCl in a non-polar medium. Aluminosiloxanes were characterized by FT-IR, NMR, and elemental analyses. Spectral analysis confirms the presence of Si-O-Al bonds in all the samples and also validates the incorporation of more aluminium via Si-O-Al bonds with increasing Al/Si mole ratio in the precursor. These FT-IR and NMR data also attest the precursor level homogeneity in all the samples. The aluminosiloxanes are obtained as low viscous resins and are capable of giving high ceramic residue qualifying them as ceramic matrix precursors for CMCs. The effect of Al/Si ratio on the ceramic conversion was studied. All the precursors showed the formation of mullite at 1000 °C. This low temperature mullite formation is a key factor in developing oxide CMCs without fiber damage. The results obtained from the study show that the composition of the ceramic can be controlled between a silica rich mullite phase and near-stoichiometric mullite phase by suitably selecting the Al/Si monomer feed ratio of the precursors. This aspect provides a greater scope for designing application-specific ceramic matrices for space applications.     

Investigations on continuous-wave laser and pulsed laser induced controllable ablation of SiCf/SiC composites

To lay a foundation for the feasibility exploration of laser-induced ablation-assisted machining for SiC_f/SiC composites, combined with numerical simulation and experiments, the laser-induced ablation mechanism of SiC_f/SiC composites was studied, and the relationship between laser parameters and ablation depth was analyzed. The laser-induced ablation products of SiC_f/SiC composites mainly consisted of recrystallized 3C-SiC and amorphous SiO₂, which were powdery and porous. According to the stratification characteristic, the ablation products were divided into attached smoke dust layer, sublimate recrystallization layer, heat-affected layer, and unaffected layer from the surface to the inside of the material. By adjusting the laser parameters (significant factors were the scanning speed and the scanning spacing), the depth of laser-induced ablation was adjustable and controllable. The ablation depth was greater in continuous-wave (CW) mode due to the continuous energy input. Therefore, CW laser is more suitable for generating larger and various ablation depths to match various cutting allowances.     

Application of the pack cementation process on SiC/SiC ceramic matrix composites

The potentials and limitations of a halide-activated pack cementation process on SiC/SiC Ceramic Matrix Composites for the development of bond coats as part of environmental barrier coating (EBCs) systems were investigated. Different pack compositions using chromium, aluminum and alloys of these elements were tested and the kinetics of coating formation were examined in addition to their microstructure. The results and their analogy to diffusion couples were discussed and it was shown that coating elements which form silicides and carbides are promising candidates for coatings deposited on SiC/SiC via pack cementation. Based on such considerations a two-step pack cementation was proposed, which used chromium, one of the suitable elements, in a first step, to finally achieve an alumina-forming coating. The oxidation resistance of the developed coating was tested via thermogravimetric analysis and compared to the uncoated material. The coating protected the fiber-matrix interface of the SiC/SiC Ceramic Matrix Composites from oxidation.     

The design of zirconium and hafnium germanate interphase in SiCf/SiC composites

Unidirectional SiC_f/SiC minicomposites with SiC matrix derived by polymer-impregnation pyrolysis (PIP), reinforced with SiC fibers coated with zirconium or hafnium germanate were fabricated. Microdebonding indentation tests for SiC_f/SiC composites with one- and multilayered germanate interphase were performed. Interfacial shear stress depending on the number of germanate interfacial layers and morphology was determined. The microstructure of the minicomposites and indented fracture surfaces were studied by scanning electron microscopy (SEM). It was stated that an increase in the number of interfacial coatings leads to a decrease in the interfacial frictional stress in SiC_f/SiC minicomposites with germanate interphases. The key factor of interphase weakening is the formation of a weak interlayer bonding within the interphase but not germanate layered crystal structure itself. Thus, bonding at the fiber/matrix boundary could be regulated by the number of layers of ZrGeO₄ or HfGeO₄ in the interphase zone.     

Damage and failure analysis of a SiCf/SiC ceramic matrix composite using digital image correlation and acoustic emission

The analysis of failure behaviors of continuous fiber-reinforced ceramic matrix composites (CMCs) requires the characterization of the damage evolution process. In service environments, CMCs exhibit complex damage mechanisms and failure modes, which are affected by constituent materials, meso architecture, inherent defects, and loading conditions. In this paper, the in-plane tensile mechanical behavior of a plain woven SiC_f/SiC CMC was investigated, and damage evolution and failure process were studied in detail by digital image correlation (DIC) and acoustic emission (AE) methods. The results show that: the initiation of macro-matrix cracks have obvious local characteristic, and the propagation paths are periodically distributed on the material surface; different damage modes (matrix cracking and fiber fracture) would affect the AE energy signal and can be observed in real-time; the significant increase of AE accumulated energy indicates that serious damage occurs inside the material, and the macroscopic mechanical behavior exhibits nonlinear characteristic, which corresponds to the proportional limit stress (PLS) of the material.     

Modeling creep behavior in ceramic matrix composites

In this work, a three-dimensional viscoplasticity formulation with progressive damage is developed and used to investigate the complex time-dependent constituent load transfer and progressive damage behavior in ceramic matrix composites (CMCs) subjected to creep. The viscoplasticity formulation is based on Hill's orthotropic plastic potential, an associative flow rule, and the Norton-Bailey creep power law with Arrhenius temperature dependence. A fracture mechanics-informed isotropic matrix damage model is used to account for CMC brittle matrix damage initiation and propagation, in which two scalar damage variables capture the effects of matrix porosity as well as matrix property degradation due to matrix crack initiation and propagation. The Curtin progressive fiber damage model is utilized to simulate progressive fiber failure. The creep-damage formulation is subsequently implemented as a constitutive model in the generalized method of cells (GMC) micromechanics formulation to simulate time-dependent deformation and material damage under creep loading conditions. The developed framework is used to simulate creep of single fiber SiC/SiC microcomposites. Simulation results are in excellent agreement with experimental and numerical data available in the literature.     

Fabrication of tough SiCf/SiC composites by electrophoretic deposition using a fabric coated with FeO-catalyzed phenolic resin

A hybrid processing route based on vacuum infiltration, electrophoretic deposition, and hot-pressing was adopted to fabricate dense and tough SiC_f/SiC composites. The as-received Tyranno SiC fabric preform was infiltrated with phenolic resin containing 5 wt.% FeO and SiC powders followed by pyrolysis at 1700 °C for 4 h to form an interphase. Electrophoretic deposition was performed to infiltrate the SiC-based matrix into the SiC preforms. Finally, SiC green tapes were sandwiched between the SiC fabrics to control the volume fraction of the matrix. Densification close to 95% ρ_theo was achieved by incorporating 10 wt.% Al₂O₃-Sc₂O₃ sintering additive to facilitate liquid phase sintering at 1750 °C and 20 MPa for 2 h. X-ray diffraction and Raman analyses confirmed the catalytic utility of FeO by the formation of a pyrolytic carbon phase. The flexural response was explained in terms of the extensive fractography results and observed energy dissipating modes.     

Development and evaluation of sub-element testing of SiC/SiC ceramic matrix composites at elevated temperatures

SiC/SiC ceramic matrix composites (CMCs) are being developed for use in aero-engines to replace nickel superalloy components. Sub-element testing acts as the key stepping stone in bridging understanding derived from basic coupon testing and more complex component testing. This study presents the development of high temperature C-shape sub-element testing with the use of digital image correlation to study damage progression. The specimen is designed with a bias towards a mixed mode-stress state more similar to what a CMC component may see in service. Both monotonic and fatigue tests were completed on C specimens and compared with predicted behaviour from modelling. Test data from both test types suggested that specimens were failing once they reached a critical radial stress level. However evidence from fractography of specimens showed that in both monotonic and fatigue tests radial cracks (driven by hoop stresses) are initiating prior to circumferential cracks.     

Damage analysis of a SiCf/SiC ceramic matrix composite under stepwise fatigue loading with acoustic emission

Continuous fiber-reinforced ceramic matrix composites (CMCs) exhibit different damage mechanisms at multiple scales under cyclic loading. In this paper, the tension-tension fatigue behavior of a plain woven SiC_f/SiC CMC was investigated, and damage accumulation and evolution process were studied in detail via acoustic emission (AE) method. With the increase of cycles, the material exhibits obvious hysteresis behavior affected by interfacial slip and wear mechanisms. Most of the fibers with radial fracture characteristic have relatively high strength, showing excellent toughening property. In the stepwise cyclic loading process, the Kaiser effect of AE determines the initiation of AE activities at each initial loading moment, which shows obvious nonlinear damage accumulation behavior of the material. High-energy events are related to significant matrix cracking and fiber fracture, and the evolution process of material damage initiation and propagation is monitored in real time.