Cyclic thermal shock behaviors of carbon fibers reinforced SiCN ceramics with bio-inspired Bouligand-like architectures

Ceramic matrix composites (CMCs) are commonly used for high temperature components in aircrafts. However, thermal shock, as a typical loading case, will cause high thermal stresses in CMCs resulting in brittle fracture failure, and material cracking caused by thermal shock can further reduce the effectiveness of thermal protection function. In the present paper, we propose a bionic hierarchical fiber preform design method to improve the thermal shock resistance of ceramics. The effect of architectures of fiber preforms of continuous carbon fiber-reinforced CMCs on the thermal shock resistance was investigated to understand its importance and the related mechanical mechanisms. Thermal shock (cycling) tests were performed with continuous carbon fibers reinforced SiCN ceramic matrix composites (C_f/SiCN) prepared by PIP. 3D micro-CT scan and three-point bending tests were also conducted to evaluated the resultant damage. The results showed that smaller internal damage and higher thermal shock resistance can be obtained in comparison to pure SiCN ceramics, and the underlying mechanism can be explained by the fact that smaller pitch angle can resist the through-thickness crack propagation via promoting diffused in-plane damage. The present study offers a possibility in developing biomimetic C_f/SiCN ceramics with excellent thermal shock behavior.     

Thermal Shock Resistance of Ceramic Foams

Thermal shock behavior of a variety of open-cell ceramic foams was evaluated using infrared heating and forced air cooling. The extent of damage after thermal shock was determined by a nondestructive, dynamic resonance technique. The damage in foams was found to be strongly dependent on cell size and weakly dependent on density. In zirconia-based foams, damage was found to increase with an increase in zirconia content. A thermal stress resistance parameter R '_f was derived to predict the effect of cell size and density on the damage incurred in foams. The experimental results were found to corroborate the predictions fairly well but a better approach was to compare the maximum applied thermal strains with the degree of damage.     

Processing and Thermal Shock Resistance of a Polymer-Derived MoSi2/SiCO Ceramic Composite

In this paper, we report a study on the thermal shock resistance (TSR) of MoSi₂/SiCO ceramic composites obtained through controlled pyrolysis of a gel-derived precursor. MoSi₂-filled gel is prepared by casting a sol obtained from MoSi₂ powder dispersed in methyltriethoxysilane. The pyrolysis product can be described as a porous ceramic composite formed by a SiCO matrix with a dispersion of MoSi₂ particles. Mechanical characterization is performed on bar samples by four-point bending. The TSR is investigated either by evaluating the R parameter (associated with strength, elastic modulus, and thermal expansion coefficient), or with the conventional water quenching technique. In both cases, the results suggest that the studied ceramic material displays a good TSR, which makes it a candidate for high-temperature application.     

Does a True Fatigue Limit Exist for Continuous Fiber-Reinforced Ceramic Matrix Composites?

The high-cycle high-frequency fatigue behavior of a Nicalon-fiber-reinforced calcium aluminosilicate ceramic composite was investigated. A key goal of the room-temperature fatigue experiments was to determine if a true fatigue limit or endurance limit existed for this ceramic matrix composite. Although no fatigue failures occurred beyond 10⁷ cycles, the stress–strain hysteresis modulus and frictional heating continued to change up to 10⁸ cycles, at which point the 200 Hz experiments were terminated. This suggests that fatigue damage continued to evolve and that a true fatigue limit may not exist in ceramic matrix composites that have undergone interfacial frictional sliding.     

Fiber Debonding in Residually Stressed Brittle Matrix Composites

The competition between initial fiber debonding versus fiber failure marks a crucial event of the microstructural failure process in fiber-reinforced brittle matrix composites. In this study, the role of a thermal residual stress field on the debonding conditions is examined theoretically and analytically. The analysis is based on two critical observations, the first being that the mechanics at the tip of a kink crack are driven only by the singularity at the main crack tip. Following from the first is the second observation that any thermal stress effects on the debonding criteria should enter only through the phase angle ψ ^T of the total stress intensity factor at the main crack tip. In general, this stress intensity factor has a thermal as well as a mechanical load contribution. It is shown that when the thermal and mechanical stress intensities, K ^R and K ^t , respectively, are in phase , i.e., ψ ^R =ψ ^t , the existing debonding conditions are universal and can be used even in the presence of thermal loads. On the contrary, when K ^R and K ^t are out of phase , i.e., ψ ^R ≠ψ ^t , events such as the delamination of thick films or debonding of inclined aligned fibers in brittle matrix composites become sensitive to the presence of the thermal stresses.     

Delamination Resistance of Two Hybrid Ceramic-Composite Laminates

Hybrid laminar composites that are comprised of alternating layers of ceramic sheets and fiber-reinforced ceramic-matrix composite (CMC) layers exhibit attractive mechanical properties, including a high first cracking stress and a high strain to failure. To achieve these properties, a strong bond must exist between the ceramic and CMC layers; otherwise, delamination will occur readily between the layers. The present study focuses on the delamination resistance of such laminates at ambient and elevated temperatures. The delamination resistance of interfaces that have been subjected to mixed-mode loading has been measured for two different hybrid composites by using edge-notched flexure specimens. At low temperatures, delamination occurs by a process that involves multiple matrix cracking within the CMC layers normal to the fibers, followed by cracking of the matrix parallel to the fibers at or near the ceramic/CMC interface. The corresponding fracture energies are typically in the range of ∼100-300 J/m²—comparable to the delamination resistance of the CMC itself. At elevated temperatures, delamination occurs via cavitation and rupture of the matrix within the CMC layers at or near the ceramic/CMC interface, with an attendant loss in toughness (to ∼10-30 J/m²). The loss in toughness occurs most rapidly at temperatures that are close to the strain point of the matrix phase; this represents the life-limiting temperature for this class of composites.     

Mechanical Properties, Thermal Shock Resistance, and Thermal Stability of Zirconia-Toughened Alumina-10 vol% Silicon Carbide Whisker Ceramic Matrix Composite

Zirconia-toughened alumina (Al₂O₃–15 vol% Y-PSZ (3 mol% Y₂O₃)) reinforced with 10 vol% silicon carbide whiskers (ZTA-10SiC _w ) ceramic matrix composite has been characterized with respect to its room-temperature mechanical properties, thermal shock resistance, and thermal stability at temperatures above 1073 K. The current ceramic composite has a flexural strength of ∽550 to 610 MPa and a fracture toughness, K _IC , of ∽5.6 to 5.9 MPa·m^1/2 at room temperature. Increases in surface fracture toughness, ∽30%, of thermally shocked samples were observed because of thermal-stress-induced tetragonal-to-monoclinic phase transformation of tetragonal ZrO₂ grains dispersed in the matrix. The residual flexural strength of ZTA–10 SiC _w ceramic composite, after single thermal shock quenches from 1373–1573 to 373 K, was ∽10% higher than that of the unshocked material. The composite retained ∽80% of its original flexural strength after 10 thermal shock quenches from 1373–1573 to 373K. Surface degradation was observed after thermal shock and isothermal heat treatments as a result of SiC whisker oxidation and surface blistering and swelling due to the release of CO gas bubbles. The oxidation rate of SiC whiskers in ZTA-10SiC _w composite was found to increase with temperature, with calculated rates of ∽8.3×10⁻⁸ and ∽3.3×10⁻⁷ kg/(m²·s) at 1373 and 1573 K, respectively. It is concluded that this ZTA-10SiC _w composite is not suitable for high-temperature applications above 1300 K in oxidizing atmosphere because of severe surface degradation.     

Cellular Molybdenum Silicide/Silicon Carbide Composites from Stems of Maize

Biomimicking of woods has been conducted extensively in the past. In this work, the unique microstructural feature of maize stems has been explored. These stems appear to be promising precursor materials for producing fiber-reinforced composites with unique anisotropic properties. A unique melt infiltration technique has been adopted to mineralize this structure into ceramic composites. The cellular morphology of the precursor stem was retained in the final composite. SiC–Si and MoSi₂–SiC cellular composites were obtained by infiltration with Si and Si–MoSi₂–Al mixture, respectively. It has been found that a small (2 wt%) addition of aluminum in the infiltrating mixture can help in attaining a homogeneous distribution of MoSi₂ in the SiC cellular matrix.     

Properties and Microstructure of Molybdenum Disilicide–β';-SiAlON Particulate Ceramic Composites

Particulate ceramic composites that were composed of a combustion-synthesized β';-SiAlON matrix and dispersed MoSi₂ particles were hot pressed at 1600°C in a nitrogen atmosphere. The physical and mechanical properties of the composites that contained 15, 30, and 45 vol% MoSi₂ were evaluated. The average four-point bend strength, fracture toughness, and Vickers hardness of the composites were in the ranges of 500-600 MPa, 3-4 MP·am^1/2, and 11-13 GPa, respectively. The measured mechanical strength and hardness were very similar to the values that were predicted from the rule of mixtures. The fracture toughness of the combustion-synthesized β';-SiAlON (2.5 MPa·m^1/2) was apparently enhanced by the MoSi₂ particles that were added. The increase in the fracture toughness was predominately attributed to the residual thermal stress that was induced by the thermal expansion mismatch between the MoSi₂ particles and the β';-SiAlON matrix. The composites showed improved electrical conductivity and oxidation resistance over monolithic β';-SiAlON. High-resolution transmission electron microscopy examination of the composites indicated that the MoSi₂ was chemically well compatible with the β';-SiAlON.     

Progress in Processing and Performance of Porous-Matrix Oxide/Oxide Composites

All-oxide continuous fiber-reinforced ceramic matrix composites are enabling materials for high-temperature structural applications in oxidizing environments. However, their industrial use requires further improvements in material performance as well as a significant reduction of the processing costs. This article gives some insight into a novel colloidal processing route. A porous mullite matrix was designed to obtain damage-tolerant behavior as well as high-temperature long-term stability. Laminated composites were formed with conventional techniques similar to the manufacture of polymer matrix composites. This simple and low-cost process leads to homogeneous microstructures with improved material properties compared with the state of the art in continuous fiber-reinforced oxide/oxide composites. The developed composites in the present study exhibit favorable mechanical properties both at room temperature and after thermal aging for 1000 h at temperatures up to 1300°C.     

Effects of Damage on Thermal Shock Strength Behavior of Ceramics

When subjected to severe thermal shock, ceramics suffer strength degradation due to the damage caused by the shock. A fracture-damage analysis is presented to study the effects of damage on the thermal shock behavior of ceramics. It is assumed that a narrow strip damage zone is developed at the tip of a preexisting crack after a critical thermal shock and the damage behavior can be described by a linear strain-softening constitutive relation. Damage growth and strength degradation are determined based on fracture and damage mechanics. Numerical calculations are carried out for two ceramic materials, and the strength degradation agrees quite well with experimental results. The effects of bridging/damage stress, the fracture energy of the bridging/damage zone, and specimen size on thermal shock strength behavior are studied. A higher fracture energy can enhance the residual strength of thermally shocked ceramics and, for a given fracture energy, a higher bridging stress is needed to reduce the strength degradation. It is also shown that the thermal shock strength behavior is size-dependent, and a high value of ( K _IC/O_b)², where K _IC is the intrinsic fracture toughness and O_b is the bending strength, can improve significantly the residual strength.     

Boron Nitride–Aluminum Nitride Ceramic Composites Fabricated by Transient Plastic Phase Processing

BN–AlN ceramic composites have been successfully fabricated by a novel process referred to as transient plastic phase processing (TPPP). The process used BN as both the reactant phase and the matrix and Al as the transient plastic phase. The products AlN and AlB₁₂ were regarded as the reinforcing phases. With the addition of Al powder in BN, both the mechanical and thermal properties were improved. Relatively high green density (2.03 g/cm³, 82.0% of theoretical density (TD)) and as-sintered density (2.18 g/cm³, 92.6%TD), high bending strength (106 MPa), and high thermal conductivity (72 W/(m·K)) were attained for one kind of BN–AlN composite. A low thermal expansion coefficient of 2.0 × 10⁻⁶/K was also achieved.     

Physical and Mechanical Properties of Bulk Ta4AlC3 Ceramic Prepared by an In Situ Reaction Synthesis/Hot-Pressing Method

Bulk Ta₄AlC₃ ceramic was prepared by an in situ reaction synthesis/hot-pressing method using Ta, Al, and C powders as the starting materials. The lattice parameter and a new set of X-ray diffraction data were obtained. The physical and mechanical properties of Ta₄AlC₃ ceramic were investigated. Ta₄AlC₃ is a good electrical and thermal conductor. The flexural strength and fracture toughness are 372 MPa and 7.7 MPa·m^1/2, respectively. Typically, plate-like layered grains contribute to the damage tolerance of Ta₄AlC₃. After indentation up to a 200 N load, no obvious degradation of the residual flexural strength of Ta₄AlC₃ was observed, demonstrating the damage tolerance of this ceramic. Even at above 1200°C in air, Ta₄AlC₃ still retains a high strength and shows excellent thermal shock resistance, which renders it a promising high-temperature structural material.     

Al2TiO5-ZrTiO4-ZrO2 Composites: A New Family of Low-Thermal-Expansion Ceramics

The characterization and properties of ceramic composites containing the phases Al₂TiO₅, ZrTiO₄, and ZrO₂ are described. The range of compositions investigated gives very low average thermal expansions (α_24–1000°C as low as −2.0 × 10⁻⁶°C⁻¹) and excellent high-temperature stability. The low thermal expansions are apparently due to a combination of microcracking by the titanate phases and a contractive phase transformation by the ZrO₂. The crystal chemistry and microstructure of the product are processing dependent. Although the composites represent a complex microcracking system, the low thermal expansions and high-temperature stability make them potential candidates for commercial applications requiring thermal shock resistance.     

Thermal shock resistance of refractory composites with Zirconia and Silicon-Carbide inclusions and alumina binder

With the goal of designing a castable refractory for an aerospace application with optimum resistance to thermal shock, three different particle-reinforced ceramic composites are designed and compared. Different volume fractions of Silicon Carbide (SiC) particles, Zirconia (ZrO₂) bubbles, and Zirconia solid particles dispersed in an alumina (Al₂O₃) matrix are used in the fabrication of these castables. Destructive and nondestructive testing procedures are implemented to evaluate their thermomechanical properties, both before and after a custom designed thermal shock cycle. After demonstrating the applicability of thermal shock indices, the variation of these indices due to thermal shock is measured experimentally and utilized as a design tool. Multiple micro-scale damage mechanisms were observed, all of which are various forms of structural deformation.     

Thermal Conductivity of Platelet-Filled Polymer Composites

Platelet-shaped particles of similar size and shape were investigated as fillers for improving the thermal conductivity of polymer–ceramic composite materials. The conductivities of composites filled with hard, stiff ceramic particles exceeded 3.5 W·(m·K)⁻¹, or >20 times the conductivity of the polymer matrix, and were shown to be almost independent of the intrinsic filler conductivity range of 33–300 W·(m·K)⁻¹. In contrast, the thermal conductivity of composites filled with soft, platelet-shaped BN fillers reached over 13 W·(m·K)⁻¹. A mechanism is proposed whereby deformation of the soft filler particles provides improved particle-to-particle connectivity and allows greater packing density, resulting in the ability to achieve much higher conductivity than is possible for hard and stiff particles of similar initial morphology. Experimental results are discussed in light of various thermal conductivity prediction models in the literature.     

Mechanical and thermal shock properties of Cf/SiBCN composite: Effect of sintering densification and fiber coating

In this work, resin-derived carbon coating was prepared on carbon fibers by polymer impregnation pyrolysis method, then silicoboron carbonitride powder was prepared by mechanical alloying, and finally carbon fiber-reinforced silicoboron carbonitride composites were prepared by hot-pressing process. The effects of sintering densification and fiber coating on microstructure, mechanical properties, thermal shock resistance, and failure mechanisms of the composites were studied. Fiber bridging hinders the sintering densification, causing more defects in fiber-dense area and lower strength. However, higher sintering temperature (1800–2000°C) can improve mechanical properties significantly, including bending strength, vickers hardness, and elastic module, because further sintering densification enhances matrix strength and fiber/matrix bonding strength, while the change of fracture toughness is not obvious (2.24–2.38 MPa·m^1/2) due to counteraction of higher debonding resistance and less pull-out length. However, fiber coating improves fracture toughness greatly via protecting carbon fibers from chemical corrosion and damage of thermal stress and external stress. Due to lower coefficient of thermal expansion, lower fiber loading ratio, less stress concentration at the fiber/matrix interface, and better defect healing effect, lower sintering temperature favors thermal shock resistance of composites, and thermal shock recession mechanisms are the damage of interface.     

Cyclic Thermal Shock Resistance of Several Advanced Ceramics and Ceramic Composites

The cyclic thermal shock behavior of two Si₃N₄ ceramics, two SiC-whisker-reinforced alumina composites (Al₂O₃/SiC_w), a SiC-particulate-reinforced alumina (Al₂O₃/SiC_p), and an alumina continuously reinforced with SiC fibers (Al₂O₃/SiC_f) composite has been studied. Specimens were repeatedly quenched from 1473 K into a fluidized bed with a heat transfer coefficient of 1400 W/(Km²) [250 Btu/(hft²F)]. The thermal shock damage was assessed by room-temperature flexure strength measurements. Si₃N₄ and Al₂O₃/SiC_p showed no noticeable damage after 100 cycles, whereas Al₂O₃/SiC_w and Al₂O₃/SiC_f degraded substantially. The experimental results are discussed and rationalized in terms of finite element simulations and microstructural observations. Our analysis suggests that the thermal shock performance of other materials may be estimated from comparisons with the present work.     

Effects of Interfacial Films on Thermal Stresses in Whisker-Reinforced Ceramics

Toughening of whisker-reinforced (or fiber-reinforced) ceramics by whisker pullout requires debonding at the whisker/matrix interface. Compressive clamping stresses, which would inhibit interface debonding and/or pullout, are expected in composites where the matrix has a higher thermal expansion coefficient than the whisker. Because such mismatch in thermomechanical properties can result in brittle composites, it is important to explore approaches to modify the thermal stresses in composites. As a result, the effects of a film at the whisker/matrix interface on the stresses due to thermal contraction mismatch upon cooling are considered in this study. Analysis of various properties of the film are considered for the whisker/matrix systems, in particular for SiC/Al₂O₃, SiC/cordierite, and SiC/mullite composites. Reduction of thermomechanical stresses is shown to occur when the interfacial film has a low Young's modulus. Also, when the whisker has a lower thermal expansion coefficient than the matrix (e.g., SiC/Al₂O₃), the interfacial stresses generated during cooling decrease as the thermal expansion coefficient of the film increases.     

Thermal Shock Resistance of Sintered Alumina/Silicon Carbide Nanocomposites Evaluated by Indentation Techniques

The thermal shock resistance of sintered Al₂O₃/1, 2.5, and 5 vol% SiC nanocomposites was studied using two indentation techniques. In the first technique, "indentation thermal shock" measurements were made of the extension of median/radial cracks around Vickers indentations after quenching from various temperatures (up to 480°C) into a bath of boiling water. This technique allowed a critical thermal shock temperature, Δ T _C^Ind, to be quantitatively evaluated. In the second technique, "indentation fatigue" tests were conducted on the thermally shocked specimens; repeated indentations were made at the same site, and the number of load cycles needed to initiate lateral fracture was measured. The results showed that nanocomposites with an addition of SiC nanophase as low as 1 vol% had a thermal shock resistance superior to that of pure Al₂O₃.