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Item-typ:Veröffentlichung, Fatigue behavior and damage analysis of PIP C/SiC composite(Elsevier Science, 2022-10-01); ; ; ; In this work, we study the fatigue behavior of a C/SiC composite produced by several cycles of polymer infil- tration and pyrolysis (PIP). Fatigue tests were performed with maximum stresses corresponding to 60–90% of the tensile strength of the composite. During the fatigue tests, acoustic emission (AE) monitoring was performed and the measured AE energy was utilized to quantify the damage and distinguish possible damage mechanisms. Most of the fatigue damage in the form of matrix cracking, interface damage and fiber breakage occurs in the first cycle. As loading cycles proceeded, damage in form of matrix crack re-opening and interfacial friction constantly accumulates. Nevertheless, all samples survived the run-out of 1,000,000 cycles. After the fatigue tests, an in- crease of the tensile strength is observed. This phenomenon is associated with the relief of process-induced in- ternal thermal stresses and the weakening of the fiber-matrix interface. In general, the studied material shows very high relative fatigue limit of 90% of its tensile strengthWissenschaftlicher ArtikelBand:42Heft:1370 74 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Evaluation of mechanical properties of a dense SiC/SiCN composite produced via PIP process(2022-03-01); ; ; ; The material behavior of Polymer Infiltration and Pyrolysis based SiC/SiCN composites is studied and the characteristic thermal and mechanical properties in on- (0/90 °) and off-axis (±45 °) direction are summarized. The tensile properties are determined at room temperature and 1300 °C. Based on the ratio of Young’s modulus and strength between on- and off-axis loading, a new approach for the classification of Weak Matrix Composites (WMC) and Weak Interface Composites (WIC) is proposed, which seems to be reasonable for various CMCs. Even without fibre coating mechanical behavior of SiC/SiCN is similar to that of WIC. In order to explain this, a microstructure model is developed and confirmed by analysis of fracture surface. The effect of temperature on the tensile properties is investigated through analysis of residual thermal stresses. Even though at 1300 °C the strength is slightly lower, the fracture strain increased significantly from RT to 1300 °C.Wissenschaftlicher Artikel110 121 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Thermal exposure effects on the long‐term behavior of a mullite fiber at high temperatureThe behavior of an oxide fiber at elevated temperatures was analyzed before and after thermal exposures. The material studied was a mullite fiber developed for high-temperature applications, CeraFib 75. Heat treatments were performed at temperatures ranging from 1200°C to 1400°C for 25 hours. Quantitative high-temperature X-ray analysis and creep tests at 1200°C were carried out to analyze the effect of previous heat treatment on the thermal stability of the fibers. The as-received fibers presented a metastable microstructure of mullite grains with traces of alumina. Starting at 1200°C, grain growth and phase transformations occurred, including the initial formation of mullite, followed by the dissociation of the previous alumina-rich mullite phase. The observed transformations are continuous and occur until the mullite phase reaches a state near the stoichiometric 3/2 mullite. Only the fibers previously heat treated at 1400°C did not show further changes when exposed again to 1200°C. Overall, the heat treatments increased the fiber stability and creep resistance but reduced the tensile strength. Changes observed in the creep strain vs. time curves of the fibers were related to the observed microstructural transformations. Based on these results, the chemical composition of the stable mullite fiber is suggested.Wissenschaftlicher ArtikelBand:100Heft:9110 108 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Phase-field simulation of abnormal anisotropic grain growth in polycrystalline ceramic fibers(Elsevier {BV}, 2020-08-05); ; ; ; The present work proposes a phase-field approach to realistically simulate abnormal grain growth in polycrystalline ceramic fibers at temperatures above 1000 °C. Under these conditions, grain growth is characterized by the formation of large elongated rectangular grains in a matrix of normal grains. The multi-phase-field model is extended by mechanisms which are responsible for the abnormal anisotropic growth: anisotropic interface energy, anisotropic interface mobility and a recrystallization driving force. Two types of the mobility anisotropy are considered: anisotropy due to the misorientation between grains and anisotropy due to the dependency on inclination angles. The experimental data from heat treatments of the ceramic fiber Nextel 610 at 1200–1300 °C with different dwell times are examined by the proposed model. The comparison of the experiment and the simulation results at various times shows that the extended multi-phase-field model is able to simulate the microstructure realistically. In most model cases, abnormal grains have a rectangular shape, similar to the experiment. The best fit of the experimental grain size distribution and the grain shape is achieved by the simulation with the misotientation dependency of the interface mobility. Furthermore, it was shown that a strong decrease on the grain growth rate with time, observed in the experiment, can be reproduced by the simulations taking into account the segregation of impurities on grain boundaries that results in the decreasing grain boundary mobility.Wissenschaftlicher ArtikelBand:185147 174 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Obtaining complex‐shaped oxide ceramic composites via ionotropic gelation(Wiley, 2018-08-23); ; ; In this communication, we present a new processing method for obtaining oxide ceramic composites based on the ionotropic gelation technique. Nextel 610 fiber fabrics were infiltrated by an alumina-zirconia suspension with a total solid content of 50 vol.% and alginate as the binder. Subsequently, the suspension was slowly cross-linked by adding Al3+ cations and transferred to a gel state. The gelled fabric layers could be easily cut, stacked and shaped, as well as joined to other ceramic materials and composites. Furthermore, the fiber content could be adjusted by pressing the layers together. In summary, the composites produced with this technique presented a very good fiber bundle infiltration, matrix with fine porosity and excellent mechanical properties. Nextel 610/alumina-zirconia composites sintered at 1200°C for 1 h showed bending strength of 306 MPa, interlaminar shear strength of 9.8 MPa and nominal fracture toughness of 13.6 MPa m0.5.Wissenschaftlicher ArtikelBand:102Heft:1122 112 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Enhancing thermal stability of oxide ceramic matrix composites via matrix doping(2022-02-23); ; ; To overcome the main limitation of oxide ceramic matrix composites (Ox-CMCs) regarding thermal degradation, the use of matrix doping is analyzed. Minicomposites containing Nextel 610 fibers and alumina matrices with and without MgO doping were produced. The thermal stability of the minicomposites was evaluated considering their microstructure and mechanical behavior before and after thermal exposures to 1300 C and 1400 C for 2 h. Before heat treatment, both composite types showed very similar microstructure and tensile strength. After heat treatment, densification, grain growth and strength loss are observed. Furthermore, the MgO dopant from the matrix diffuses into the fibers. As a result, abnormal fiber grain growth is partially suppressed and MgO-doped composites show smaller fiber grains than non-doped composites. This more refined microstructure leads to higher strength retention after the heat treatments. In summary, doping the matrix can increase the overall thermal stability without impairing the room-temperature properties of Ox-CMCs.Wissenschaftlicher ArtikelBand:42Heft:7181 424 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Damage analysis of 2.5D C/C-SiC composites subjected to fatigue loadings(Elsevier {BV}, 2019-03-11); ; ; ; Damage analyses of a ceramic matrix composite during fatigue and quasi-static loads were performed by acoustic emission (A.E.) monitoring. The material studied was a 2.5D C/C-SiC composite produced by chemical vapor infiltration followed by liquid silicon infiltration. The analysis done during the first 200 cycles of a fatigue test showed that the number of A.E. hits is a good parameter for the quantification of damage. Furthermore, the A.E. hit energy was associated with the type of damage. In this sense, the damage developed during the fatigue loading was related to matrix crack initiation, propagation and re-opening, as well as fiber-matrix friction. Quasi-static tests on post-fatigue samples showed that the previous fatigue loadings increased the material`s damage threshold and hindered the development of new damage. Particular attention was given to the sample after 2,000,000 cycles as this sample showed distinct A.E. signals that could be related to fiber debonding.Wissenschaftlicher ArtikelBand:39Heft:7110 103 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Characterization and modeling of tensile properties of continuous fiber reinforced C/C-SiC composite at high temperaturesThe material behavior of Polymer Infiltration and Pyrolysis based SiC/SiCN composites is studied and the characteristic thermal and mechanical properties in on- (0/90 ◦) and off-axis (±45 ◦) direction are summarized. The tensile properties are determined at room temperature and 1300 ◦C. Based on the ratio of Young’s modulus and strength between on- and off-axis loading, a new approach for the classification of Weak Matrix Composites (WMC) and Weak Interface Composites (WIC) is proposed, which seems to be reasonable for various CMCs. Even without fibre coating mechanical behavior of SiC/SiCN is similar to that of WIC. In order to explain this, a microstructure model is developed and confirmed by analysis of fracture surface. The effect of temperature on the tensile properties is investigated through analysis of residual thermal stresses. Even though at 1300 ◦C the strength is slightly lower, the fracture strain increased significantly from RT to 1300 ◦C.Wissenschaftlicher ArtikelBand:41Heft:5127 143 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, On the dimensional analysis for the creep rate prediction of ceramic fibers(Elsevier {BV}, 2018-07-20); ; ; A novel semi-empirical equation for the steady-state creep rate description of ceramic fibers is here proposed. Dimensional analysis was applied to identify the relationship between the variables related to the creep rate. Besides the variables known from the Arrhenius creep equation, a temperature factor that accounts for changes with the temperature was also taken into consideration. The resultant equation shows that the creep rate is proportional to the temperature, applied stress and two material constants, as well as inversely proportional to the fiber grain size. The two material constants k and C described in the equation can be easily determined by creep tests at different temperatures under the same applied stress. Therefore, this equation can be more efficiently applied to predict the creep rate of ceramic fibers under different applied stresses and initial grain sizes. To confirm this, the proposed equation was used to fit the creep rate data set of two ceramic fibers, Nextel 720 and CeraFib 75. In summary, the equation provides a good fit (adjusted R-squared up to 0.97) and prediction for the experimental data of tests with different temperatures, applied stresses and initial grain sizes.Wissenschaftlicher ArtikelBand:44Heft:13103 89 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Phase-field modeling of grain growth in presence of grain boundary diffusion and segregation in ceramic matrix mini-composites(Elsevier {BV}, 2021-01-29); ; ; ; The grain boundary diffusion and segregation can influence the grain growth kinetics, the grain size distribution, and therefore the mechanical properties of the ceramic matrix composites. The present paper proposes a phasefield modeling approach to simulate the grain growth in polycrystalline alumina fibers embedded in alumina matrix at temperatures above 1000 ◦C in presence of the grain boundary diffusion of dopants from the matrix to the fiber and vice versa. The multi-phase-field model for grain growth [I. Steinbach and F. Pezzolla, Phys. D, 134 (1999) 385] is extended by the incorporation of the grain boundary diffusion, grain boundary segregation model, and the dependence of the interface mobility on the segregation concentration. The kinetic parameters of the model which allow describing the real microstructure evolution were estimated by the comparison to the experimental measurements. The simulation and experimental results of the grain growth with the diffusion of dopants in Nextel 610 fibers show the significant effect of the grain boundary diffusion on the grain size distribution. The results of numerical tests were used to adjust the values of the grain boundary diffusion coefficients by the experimental data at different temperatures by means of an inverse method. From the simulations, the diffusion coefficient of Mg was estimated to be 6–7 times higher than that of Si.Wissenschaftlicher ArtikelBand:190148 111