Smoothed particle hydrodynamics simulation and experimental analysis of SiC ceramic grinding mechanism

Crack induced surface/subsurface damage in SiC ceramic grinding limits the industrial application. A single-grain scratching simulation based on the smoothed particle hydrodynamics (SPH) has been used to analyze the SiC grinding mechanism, including the material removal process, scratching speed effect on crack propagation, ground surface roughness, and scratching force. The simulation results showed that the material removal process went through the pure ductile mode, brittle assisted ductile mode, and brittle mode with the increase of the depth of cut. The critical depth of cut for ductile-brittle transition was about 0.35?µm based on the change of ground surface crack condition, surface roughness, and maximum scratching force. Increasing the scratching speed promoted the transformation of deep and narrow longitudinal crack in the subsurface into the shallow and wide transverse crack on the surface, which improved the surface quality. The SPH simulation results were indirectly validated by the cylindrical grinding experiments in terms of the critical single grain depth of cut for ductile-brittle transition, and the trend of ground surface roughness and grinding forces.     

Analysis of SiC/SiC composites for energy applications at ambient conditions

Plain weave planar and biaxially braided tubular SiC/SiC CMCs are evaluated in tension and four-point bending, respectively, at ambient conditions. Custom-designed fixtures for CMC testing are developed for each loading mode and are coupled with three-dimensional digital image correlation. Stereoscopic image correlation analysis reveals crack initiation and failure sites to provide insight into stress redistribution mechanisms. Scanning electron microscopy is performed postmortem to determine the influence of microstructural features on crack initiation and failure. Crack spacing is measured in situ by stereoscopic image correlation and confirmed by SEM measurements to relate to underlying tow-tow crossing points. Triangulated surface heights of plain weave tow architecture are used to determine that subtle differences in neighboring transverse tow angle, which vary within a range of ±4° from horizontal, have no significant effect on final fracture location. The results presented reaffirm the state of current SiC/SiC CMCs developed for energy applications and will help to further improve SiC/SiC and other CMCs.     

Cracking behavior of ZrB2-SiC-Graphite sharp leading edges during thermal shock

Crack initiation and propagation of ZrB₂-SiC-Graphite (ZSG) sharp leading edges (SLEs) subjected to thermal shock were systematically evaluated by the water spraying method followed by a crack dyeing treatment. Distinct differences in the crack patterns among different test conditions were observed, and the cracking behavior of ZSG SLEs (including crack initiation time, crack number and critical failure temperature) was revealed to be strongly dependent on both the cooling rate and the microstructure. The crack propagation during thermal shock could be considered as a quasistatic process (crack speed was lower than 1?cm/s) that needed to be driven by continuous cooling.     

Discrete element simulation of SiC ceramic containing a single pre-existing flaw under uniaxial compression

A model of a SiC ceramic containing a single pre-existing flaw was established based on the discrete element method. The effects of the flaw inclination angles, which ranged from 0° to 75°, on the mechanical properties of the specimen under uniaxial compression were studied. The evolution of the force-chain field, displacement field and stress field around the pre-existing flaw in the process from the load to failure was also analysed. The results showed that the flaw inclination angle affected the mechanical properties of the specimen as well as the initiation and propagation of the first crack. Based on the investigation of the force chain field, it was found that the distribution curve of the normal force carried by the parallel bond in the specimen with the corresponding angles under compression is similar to the “peanut” rose diagram, while the shear force distribution curve is similar to the "butterfly wings" rose diagram. In addition, in the analysis of the displacement field and the stress field, the displacement field around the flaw can be divided into four types in the process from specimen loading to its failure. Meanwhile, it was found that initiation of the first crack was affected by tensile stress. With the propagation of the first crack, the tensile stress concentration region at the flaw tip moved and dissipated correspondingly.     

Structural design and energy absorption mechanism of laminated SiC/BN ceramics

The laminated silicon carbide/boron nitride (SiC/BN) ceramics with different structural designs were fabricated by pressureless sintering at 1900?°C for 1?h in argon flow. The alumina (Al₂O₃)-and yttrium(III) oxide (Y₂O₃)-doped SiC ceramic exhibited a significant intergranular fracture behavior, which could be attributed to the yttrium aluminum garnet (YAG) phase located at the grains boundaries. The bending strength and fracture toughness were used to characterize the crack propagation including the delamination cracking, crack kinking, and crack deflection. The energy absorption in the process of crack propagation was characterized by the work of fracture (WOF) and damping capacity. The mode of crack propagation changed with the change in the structure and variation of BN content in the BN layer. The delamination cracks occurred inside the BN layer or at the interface between SiC and BN layers. The sample with a gradient structure exhibited the combination of delamination cracks occurring at the interface and inside the BN layer, which showed the maximum WOF of 2.43?KJ?m^?2, bending strength of 300?MPa, and fracture toughness of 8.5?MPa?m^1/2. The damping capacity varied with the change of the structure and the amplitude. The sample with a gradient structure exhibited the damping capacity of 0.088 and the maximum loss modulus of 9.758?GPa.     

Creep and Fatigue Behavior in an Enhanced SiC/SiC Composite at High Temperature

The monotonic tension, creep, and fatigue behavior of an enhanced SiC/SiC composite was investigated at a temperature of 1300°C in air and argon. The improved creep and fatigue resistances were determined and compared to those of the standard SiC/SiC composite. The effects of additives (glass-forming, boron-based particulates) in the matrix on the creep and environmental resistance of the enhanced SiC/SiC composite were discussed. Crack propagation in the matrix of the enhanced SiC/SiC composite was different from that in the standard SiC/SiC composite. The filling of the glassy phases in the cracks prohibited the diffusion of oxygen from the environment. As a result, creep and fatigue properties in the enhanced SiC/SiC composite in air at high temperatures was improved.     

Strengthening/toughening of 2D C/SiC composites by coating

SiC and SiC_w/SiC coatings were prepared on two-dimensional carbon fiber reinforced silicon carbide ceramic matrix composites (2D C/SiC), and strengthening/toughening of the composite by the coatings was investigated. After coating, the density of the C/SiC composites was increased effectively and the mechanical properties were improved significantly. Compared with SiC coating, SiC_w/SiC coating showed the more significant effect on strength/toughness of the composites. Coatings had two effects: surface strengthening and matrix strengthening. The latter was the dominant effect. The surface strengthening can increase the crack initiation stress, while the matrix strengthening can enhance the crack propagation resistance. The former effect increased the strength and the latter effect increased the toughness.     

Study of Crack Propagation in PMMA Using Ultrasonic Technique

Attenuation of ultrasonic waves was measured in different notched specimens of Poly(methylmethacrylate) (PMMA) at different points of crack propagation path. Crack propagation takes place when specimens, with initial single edge notch produced using sharp milling cutter, are subjected to different crosshead speeds of a tensile testing machine. These measurements were performed on specimens having different thicknesses by using pulse echo technique at different frequencies. Attenuation measurements results were confirmed by microscopic images.

The present study showed that attenuation increases when crack propagation speed increases as a result of an increment of crosshead speed of the applied load. Such relation is not affected by the specimens' thickness. The resulting attenuation curves elucidated the neck zone of the crack.     

Effects of SiC on the microstructures and mechanical properties of B4C–SiC–rGO composites prepared using spark plasma sintering

In this study, B₄C–SiC–rGO composites with different SiC contents were prepared by spark plasma sintering at 1800 °C for 5 min under a uniaxial pressure of 50 MPa. The effects of SiC on the microstructures and mechanical properties of the B₄C–SiC–rGO composites were investigated. The optimal values for flexural strength (545.25 ± 23 MPa) and fracture toughness (5.72 ± 0.13 MPa·m^1/2) were obtained simultaneously when 15 wt.% SiC was added to 5 wt.%–GO reinforced B₄C composites (BS15G5). It was found that SiC and rGO inhibited the grain growth of B₄C and improved the mechanical properties of the B₄C–SiC–rGO composites. The clear and narrow grain boundaries of rGO–B₄C and rGO–SiC, as well as the semi-coherent B₄C–SiC interface, indicated strong interface compatibility. The twin structures of SiC and B₄C observed in the composites improved their fracture toughness. Crack deflection and crack bridging caused by the SiC grains as well as rGO bridging and rGO pull-out were observed on the crack propagation path.     

Microstructure and tensile behavior of (BN/SiC)n coated SiC fibers and SiC/SiC minicomposites

Interphase plays an important role in the mechanical behavior of SiC/SiC ceramic-matrix composites (CMCs). In this paper, the microstructure and tensile behavior of multilayered (BN/SiC)_n coated SiC fiber and SiC/SiC minicomposites were investigated. The surface roughness of the original SiC fiber and SiC fiber deposited with multilayered (BN/SiC), (BN/SiC)₂, and (BN/SiC)₄ (BN/SiC)₈ interphase was analyzed through the scanning electronic microscope (SEM) and atomic force microscope (AFM) and X-ray diffraction (XRD) analysis. Monotonic tensile experiments were conducted for original SiC fiber, SiC fiber with different multilayered (BN/SiC)_n interfaces, and SiC/SiC minicomposites. Considering multiple damage mechanisms, e.g., matrix cracking, interface debonding, and fibers failure, a damage-based micromechanical constitutive model was developed to predict the tensile stress-strain response curves. Multiple damage parameters (e.g., matrix cracking stress, saturation matrix crack stress, tensile strength and failure strain, and composite’s tangent modulus) were used to characterize the tensile damage behavior in SiC/SiC minicomposites. Effects of multilayered interphase on the interface shear stress, fiber characteristic strength, tensile damage and fracture behavior, and strength distribution in SiC/SiC minicomposites were analyzed. The deposited multilayered (BN/SiC)_n interphase protected the SiC fiber and increased the interface shear stress, fiber characteristic strength, leading to the higher matrix cracking stress, saturation matrix cracking stress, tensile strength and fracture strain.     

High-temperature fatigue crack propagation mechanism of orthogonal 3-D woven amorphous SiC Fiber/SiC/YSi2-Si matrix composites

To elucidate degradation mechanisms attributable to high-temperature fatigue crack propagation, a study was conducted of 3-D woven SiC_f/SiC CMC in which amorphous SiC fiber was used as a reinforcement material and in which a matrix was formed through low-temperature melt infiltration. From a high-temperature fatigue test conducted at 1373 K in the atmosphere with stress of 142 MPa or more, the fracture lifetime of newly developed SiC_f/SiC CMC was found to be longer than that of SiC_f/SiC CMC, which uses crystalline SiC fiber. Furthermore, repeatedly applying high temperatures during high-temperature fatigue tests and using X-ray computed tomography, fatigue cracks were found to propagate in a direction across 0-degree fiber bundles that undergo stress. Electron mapping of regions with crack propagation revealed that oxidation eliminates boron nitride (BN), which has a crack deflection effect. The SiC fibers and matrix are fixed through the formation of oxides. Cracks propagate because of the consequent decrease in toughness of the SiC_f/SiC CMC. In regions without crack propagation, fracture surfaces were not covered with oxides. These regions underwent forcible fracture in the final stage of the high-temperature fatigue tests. From the test results presented above, SiC_f/SiC CMC is considered to undergo fracture when the effective cross-sectional area is reduced because of crack propagation accompanying oxidation and when the test load exceeds the tensile strength of the residual cross-sectional area. However, some cracks in the matrix produced by a low-temperature melt infiltration process were closed by oxides derived from YSi₂. Because of crack closing, crack propagation is presumed to be avoided. Also, LMI-CMC showed excellent high-temperature fatigue properties at pressures higher than 150 MPa, which exceeds the proportional limit.     

In-situ fabrication of laminated SiC/TiSi2 and SiC/Ti3SiC2 ceramics by liquid silicon infiltration

The laminated silicon carbide/titanium silicon (SiC/TiSi₂) and silicon carbide/titanium silicon carbide (SiC/Ti₃SiC₂) ceramics were successfully designed and fabricated by liquid silicon (Si) infiltration. When the thickness of TiC layer was 150 and 450?µm, the TiSi₂ and Ti₃SiC₂ phases were the main products in the TiC layer, respectively. The as-fabricated structural unit of laminated SiC/Ti₃SiC₂ ceramics consisted of five layers of functionally graded materials, which has multiscale layered structure containing macro-layered structure and nano layered structure. The generation of hierarchical structure was attributed to the diffusion of Ti elements and in-situ formation of TiSi₂ and Ti₃SiC₂. The growth direction of Ti₃SiC₂ was anisotropic, thus providing more paths for the crack propagation via deflection, branching, and delamination during fracture process. However, the crack propagation inside the Ti₃SiC₂ phase included the pull out, bridging, lamination, deflection, and fracture of the single layer, which are the energy absorption and damage tolerance mechanisms of the Ti₃SiC₂ phase.     

Effects of gradual matrix crack closure on the constitutive behavior of SiC/SiC composites upon unloading

Gradual matrix closure and its effects on the constitutive behavior of SiC/SiC composites are examined in the present study. Real-time matrix crack detection and a macroscopic loading–unloading tensile test are performed on SiC/SiC minicomposites. To verify the effects of matrix crack closure, stress-strain responses under loading and unloading with and without crack closure are discussed. The experimental and numerical results show that matrix cracks close gradually upon unloading, and gradual matrix closure greatly reduces the unloading stiffness.     

3D microstructure model and thermal shock failure mechanism of a Si3N4-bonded SiC ceramic refractory with SiC high volume ratio particles

In this study, an efficient method was proposed to establish 3D microstructure model of a Si₃N₄-bonded SiC ceramic refractory with SiC high volume ratio particles and its failure mechanism under thermal shock was studied based on the established microstructure model. The proposed modeling method based on modified 3D Voronoi tessellation method and “precise shrinkage ratio method” was able to establish 3D geometric model of a SiC ceramic refractory with SiC high volume fraction particles more quickly than usual methods. The modified 3D Voronoi tessellation method generated Voronoi polyhedrons (VPs) limited in finite space perfectly. The proposed “precise shrinkage ratio method” achieved a precise volume fraction of SiC particles in the established microstructure model. The crack initiation and propagation under thermal shock were calculated by employing the extended finite element method (XFEM) on the established microstructure model. The results showed the failure mode on micro-scale clearly and efforts of interface strength on the failure mode were also explored. The proposed modeling method was especially suitable for establishing 3D microstructure models of ceramic composites or isotropic metal-ceramic particle composites with high volume fraction particles and extended the use of VPs.     

Toughening Behavior of a Two-Dimensional SiC/SiC Woven Composite at Ambient Temperature: I, Damage Initiation and R-Curve Behavior

The damage initiation and R -curve behavior for a two-dimensional (2-D) SiC/SiC woven composite are characterized at ambient temperature and related to in situ microscopic observations of damage accumulation and crack advance. Matrix cracking and crack deflection/branching are observed and dominate fracture behavior in the early loading stage such that primary crack extension occurs at apparent stress intensity values as high as 12 MPam^1/2. Linear elastic fracture mechanics (LEFM), though questionable, was assumed to be valid in the early stages of damage initiation prior to primary crack advance, but was clearly invalid once primary crack extension had occurred. Such a high primary crack extension toughness value is confirmed by a renotch technique whereby the crack wake is removed and the fracture resistance drops close to the initial value. Based on microstructural observations, multiple matrix cracks are found to be arrested at fiber bundles. The key to toughening appears to be associated with the mechanics of crack arrest at fiber bundles in the woven architecture. Toughening mechanisms include multiple matrix cracking (similar to microcracking), crack branching, and crack deflection in the crack frontal zone. Application of models to evaluate toughening based on these mechanisms results in values comparable to experimental data. In the regime of primary crack extension, a J -integral technique was applied to investigate the R -curve behavior and results showed a rising J_R -curve which started at 1500 J/m² and reached 6150 J/m² after about 13 mm of primary crack extension. There was evidence of substantial crack bridging by fiber tows and fibrous pull-out in this regime of crack advance.     

In-situ characterization on crack propagation behavior of SiCf/SiC composites during monotonic tensile loading

The microstructure and crack propagation path of 2.5D SiC_f/SiC composites were observed by synchrotron radiation x-ray computed micro tomography (SR-μCT) equipped with in-situ tensile device. The results showed that the pore morphologies of the SiC_f/SiC composites are mainly divided into three types in three-dimension space: interconnected pores, isolated pores and micro pores in fiber bundles. The crack initiation occurred at the root of the defects under in-situ tensile load and the crack was perpendicular, parallel to the stress axis or mixed mode to propagate. At the interface scale between fiber and matrix, the crack deflection will be controlled by physical parameters such as fracture energy release rate and the modulus of elasticity. At the fiber bundle scale, the crack is easy to shear propagate along the interface between weft and warp fiber bundles due to the existence of the mechanical bonding and residual tensile stress.     

Effect of the incorporation of SiC nanowire with double protective layers on SiC coating for C/C composites

To maintain the thermal stability of SiC nanowires during SiC coating fabrication process, carbon and SiC double protective layers were covered on the surface of nanowires. And SiC nanowires with double protective layers toughened SiC coating were prepared by pack cementation. The results showed that after introducing the SiC nanowires with double protective layers, the fracture toughness of the SiC coating was increased by 88.4 %. The coating protected C/C for 175 h with a mass loss of 3.67 %, and after 51 thermal shock cycles, the mass losses of the oxidized coating were 3.96 %. The double protective layers are beneficial to improve the thermal stability of nanowires, leading to good fracture toughness and thermal shock resistance of SiC coating. SiC nanowires consume the energy of crack propagation by fracture, pullout and bridging, leading to an increase in fracture toughness.     

Analysis of crack initiation and propagation in Thermal Barrier Coatings using SEM-Based geometrical model with extended finite element method

Thermal Barrier Coatings (TBC) are widely used to protect the metallic components that operate at harsh conditions of elevated temperatures and oxidizing environments. Thermally grown oxide (TGO) causes cracks formation in the top coat (TC) that may lead to spallation failure of TBC. This work investigates effect of pores and TGO thickness on crack initiation and propagation due to thermal mismatch between TBC layers. Image processing is used to convert an SEM image, including pores, into a finite element (FE) model. An FE model using XFEM implemented in ABAQUS was developed to investigate crack initiation and propagation for various TGO thicknesses considering the effect of plastic deformation of BC, TGO and substrate. Results show that presence of pores changes the critical sites for crack initiation from the TC/TGO interface to be around the pores within the TC. Crack initiation temperatures and crack lengths were found to be affected with both TGO thickness and pores.     

Interpreting acoustic energy emission in SiC/SiC minicomposites through modeling of fracture surface areas

The relationship between acoustic emission (AE) and damage source areas in SiC/SiC minicomposites was modeled using insights from tensile testing in-scanning electron microscope (SEM). Damage up to matrix crack saturation was bounded by: (1) AE generated by matrix cracking (lower bound) and (2) AE generated by matrix cracking, and fiber debonding and sliding in crack wakes (upper bound). While fiber debonding and sliding exhibit lower strain energy release rates than matrix cracking and fiber breakage, they contribute significant damage area and likely produce AE. Fiber breaks beyond matrix crack saturation were modeled by two conditions: (i) only fiber breaks generated AE; and (ii) fiber breaks occurred simultaneously with fiber sliding to generate AE. While fiber breaks are considered the dominant late-stage mechanism, our modeling indicates that other mechanisms are active, a finding that is supported by experimental in-SEM observations of matrix cracking in conjunction with fiber failure at rupture.     

Preparation of SiC/SiC composites with high toughness by reaction of the SiCP+C double-cladding layer with liquid silicon

SiC/SiC composites prepared by liquid silicon infiltration (LSI) have the advantages of high densification, matrix cracking stress and ultimate tensile strength, but the toughness is usually insufficient. Relieving the residual microstress in fiber and interphase, dissipating crack propagation energy, and improving the crystallization degree of interphase can effectively increase the toughness of the composites. In this work, a special SiC particles and C (SiC_P +C) double-cladding layer is designed and prepared via the infiltration of SiC_P slurry and chemical vapor infiltration (CVI) of C in the porous SiC/SiC composites prepared by CVI. After LSI, the SiC generated by the reaction of C with molten Si combines with the SiC_P to form a layered structure matrix, which can effectually relieve residual microstress in fiber and interphase and dissipate crack propagation energy. The crystallization degree of BN interphase is increased under the effects of C-Si reaction exotherm. The as-received SiC/SiC composites possess a density of 2.64 g/cm³ and a porosity of 6.1%. The flexural strength of the SiC/SiC composites with layered structure matrix and highly crystalline BN interphase is 577 MPa, and the fracture toughness reaches up to 37 MPa·m^1/2. The microstructure and properties of four groups of SiC/SiC composites prepared by different processes are also investigated and compared to demonstrate the effectiveness of the SiC_P +C double-cladding layer design, which offers a strategy for developing the SiC/SiC composites with high performance.