Hertzian crack propagation in graded glass/ceramic composites

In literature, the concept of material gradation is shown to inhibit surface crack initiation in glass/ceramic composites subjected to Hertzian indentation. However, surface cracks could yet initiate due to relatively higher loadings or in the presence of surface flaws/defects. Hence, characterization of graded composites concerning the resistance against Hertzian crack initiation and propagation manifests itself as a prominent matter. In this study, axisymmetric Hertzian cracks evolving in graded glass/ceramic composites propelled by a rigid cylindrical punch are investigated employing a novel recursive method, called the stacked-node propagation procedure. Crack trajectories and their propagation susceptibilities are predicted via the minimum strain energy density (MSED) criterion regarding the crack growth resistance (R-curve) of ceramics. The stress trajectory approach is also considered for a homogeneous glass to reveal the reliance and effectiveness of the MSED criterion in the present crack problems. The Mori–Tanaka relations are adopted to model the elastic modulus and Poisson's ratio variations through the composites, which are implemented on the simulations via the homogeneous finite element approach. Hertzian crack problem of a practically producible graded composite comprised of oxynitride glass and a fine-grained silicon nitride ceramics (Si₃N₄) is treated as a case study. The degree of material gradation is assessed for the mitigation of surface crack initiation and propagation risks.     

High thermal-conductivity rGO/ZrB2-SiC ceramics consolidated from ZrB2-SiC particles decorated GO hybrid foam with enhanced thermal shock resistance

Graphene derivative materials exhibit excellent mechanical and thermal properties, which have been extensively used to toughen ceramics and improve thermal shock resistance. To overcome the thermal agglomeration of graphene oxide (GO) during heating and drying process, ZrB₂-SiC particles decorated GO hybrid foam with uniformly anchored ceramic particles was synthesized by electrostatic self-assembly and liquid nitrogen-assisted freeze-drying process. Densified rGO/ZrB₂-SiC ceramics with varying microstructure, thermal physical and mechanical properties were obtained by adjusting the content of decorated ceramic particles. Although the flexural strength of rGO/ZrB₂-SiC ceramics have an attenuation compared with that of ZrB₂-SiC ceramic, the thermal conductivity, work of fracture and thermal shock resistance are greatly improved. rGO/ZrB₂-SiC ceramics exhibit delayed fracture and increasing R-curve behavior during the crack propagation. The novel preparation technology allows for the well dispersion of rGO in ZrB₂-SiC ceramics and can be easily extended to other ceramic or metal materials systems.     

Comparative evaluation of two different methods for thermal shock resistance of laminated ZrB2-SiCw/BN ceramics

The thermal shock resistance (TSR) of laminated ZrB₂–SiC_w/BN ceramic was evaluated through indentation-quench and quenching-strengthening methods. It was correspondingly compared to monolithic ZrB₂–SiC_w ceramic. In the indentation-quench method with consideration to crack propagation on the surface layer, the critical thermal shock temperature of laminated ZrB₂–SiC_w/BN ceramic with surface residual tensile stress was 550?°C, which was lower than monolithic ZrB₂–SiC_w ceramic (600?°C). Unlike the microscopic method of crack growth measurement through indentation-quench testing, the quenching-strengthening method, which was based on the macroscopic properties of the material, mainly characterizing the residual strength subsequently to thermal shock, the critical thermal shock temperatures of the laminates and monolithic were 609?°C and 452?°C, respectively. Compared to the brittle fracture of ZrB₂–SiC_w ceramics, the deflection, bifurcation and delamination of the cracks as the main TSR mechanisms of the laminated ceramics, were revealed through quenching-strengthening method, which was more suitable for the TSR characterization of laminated ceramics.     

Design,fabrication and mechanical properties of multidimensional graded ceramic tool materials

A type of multidimensional graded ceramic tool materials (MGTMs) was designed and fabricated by vacuum hot-pressing sintering technology. The microstructure and compositional distribution of tool simultaneously changed in two different directions. The tool-chip and tool-workpiece contact regions were designed to have high hardness, and metal phases Mo and Ni were added to produce a gradual increase in toughness from the outer layer to core layer. The effect of orientation angle, thickness ratio and sintering parameters on the mechanical properties and microstructure were investigated. The experimental results showed that the composites, sintered at 1700 °C for 15 min, with the orientation angle of 30° and a thickness ratio of 0.4, had the optimal comprehensive mechanical properties. In addition, the crack propagation paths were observed to analyze the toughening mechanisms for the multidimensional graded ceramic tool materials. It was found that there is a crack resistance behavior when the crack extended from the outer layer to transition layer. The effect of the angle between the crack and graded interface on crack propagation paths was also investigated and the results indicated that the small angle was favorable for the crack deflection when the cracks passed through the graded interface. The residual thermal stress contributed to the occurrence of the crack bridging and transgranular fracture inside the Al₂O₃ grains, while the intergranular fracture and crack deflection were observed around the TiC grains.     

Fractal characterization of ceramic crack patterns after thermal shocks

This work utilized a combination of experimental evidence and fractal geometric method to assess the effect of crack extension concerning the thermal shock on residual strength of ceramics. Sintered alumina (Al₂O₃) ceramic slabs were bundled and quenched in water under different thermal shock temperatures. The fractal dimension of thermal shock crack patterns on the interior surface and the cooled surface was calculated by the Box-counting method. Fracture energy of a fractal pattern of microcracks in quasi-brittle solids was employed to explain the relationship between crack length and fractal dimensions. The results show that if the crack propagation has the same crack length but a larger fractal dimension, it will absorb more fracture energy. The thermal shock crack patterns of Al₂O₃ ceramics with different grain sizes were analyzed, and the smaller grain size ceramic had a higher fractal dimension of crack patterns than the larger one.     

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.     

Effects of frequency on electrical fatigue behavior of ZnO-modified Pb(Mg1/3Nb2/3)0.65Ti0.35O3 ceramics

This work aims to study the effects of frequency on the electrical fatigue behavior of ZnO-modified Pb(Mg_1/3Nb_2/3)_0.65Ti_0.35O₃ (PMNT) ceramics. Changes in microstructures, ferroelectric and piezoelectric properties of the ceramics at bipolar electrical fatigue frequencies of 5, 10, 50 and 100 Hz were observed. The thickness of damaged surface of the ceramics decreased with increasing frequency. The degradation of properties of the ceramics fatigued at low frequency was greater than those fatigued at high frequency. The degradation by electrical fatigue at lower frequencies, 5 and 10 Hz, could be caused by the effects of both field screening and domain pinning, while at higher frequencies the fatigue was mainly a result of the field screening effect. The fatigue properties of ZnO-modified PMNT ceramics was compared to Pb-based and Pb-free ferroelectric ceramics. It was found that the fatigue endurance of ZnO-modified PMNT ceramic was greater than that of hard PZT ceramic but less than that of Pb-free ferroelectric ceramic.     

Piezoelectricity,thermal stability,and fatigue resistance in Nb and Ta-doped Bi4Ti3O12 high-temperature piezoceramics

The effect of Nb/Ta donor doping on the piezoelectricity, thermal stability, and fatigue resistance of bismuth titanate Bi₄Ti₃O₁₂ (BIT) ceramics was investigated in relation to their structural and oxygen vacancy-related electrical properties. As the Nb/Ta doping amount increased, the activation energy of oxygen vacancy conduction increased, indicating a reduction in the concentration of oxygen vacancies. The improved electrical insulating properties of the Nb/Ta-doped Bi₄Ti₃O₁₂ ceramics (BTNT) with fewer oxygen vacancies, contributed to their effective poling and strong piezoelectricity. Outstanding piezoelectric performance with high piezoelectric constant (39 pC/N) and Curie temperature (690 °C) could be achieved in the 0.005 mol Nb/Ta-doped BTNT ceramic with high density and anisotropic grain growth. The BTNT ceramics exhibited superior thermal aging stability and fatigue resistance compared to the BIT ceramic, suggesting that the reduction of oxygen vacancy defects plays a decisive role in enhancing elevated-temperature-induced and electric-field-induced degradation stabilities.     

Thermal shock resistance study of stereolithographic additive-manufactured Al2O3 ceramics by in situ digital radiography

Thermal shock resistance is critical to ensure the service safety of ceramic hot-end components. The thermal shock performance of stereolithographic additive-manufactured ceramics has not yet been studied. In this study, a series of thermal shock experiments with various temperature differences was conducted on stereolithographic additive-manufactured Al₂O₃ ceramics. The surface cracks were analysed based on photographs captured before and after the thermal shock experiments. Three-point bending tests with in situ X-ray digital radiography were conducted to determine the thermal shock resistance. Crack initiation, propagation, and coalescence were observed under flexural loads. The critical temperature difference of the stereolithographic additive-manufactured Al₂O₃ ceramics was determined to be 267.22 °C. The crack length increased and residual strength decreased with increasing temperature differences. The layered structure of the stereolithographic additive-manufactured ceramics slowed crack propagation. We expect that this study will serve as a reference for the performance of stereolithographic additive-manufactured Al₂O₃ ceramics in extreme environments.     

Influence of graphite on the low-frequency fatigue behavior of zirconium diboride ceramics

The fatigue behavior of a ZrB₂-based ceramic containing SiC and graphite was compared to a ZrB₂-SiC reference material based on bending testing, quantitative calculations as well as crack growth and fracture characterization. The addition of graphite flake makes ZrB₂-SiC-Graphite ceramics exhibit fatigue failure behavior at very high stress level (93% of the characteristic strength, σ₀), owing to the increased K_Ic promoted by crack deflection, bridging, bifurcation and pull-out of graphite, while the fatigue behavior of ZrB₂-SiC appears when the maximum stress is below ~86%σ₀. However, both the slow crack growth exponents of the graphite containing ceramic, n and nc values, which reflect the fatigue resistance in static and cyclic fatigue conditions, respectively, are only 1/4 as compared to the reference graphite-free ceramic. This may be due to the weak boride/graphite interfaces, which lead to the decrease of the initial critical stress intensity factor (K_c-initial) value from 2.6 to 2.0 MPa m^1/2.     

Structure and properties of clay ceramics for thermal energy storage

In this paper, the structure‐property relationships of a clay ceramic with organic additives (biomass and biochar) are investigated to develop an alternative material for thermal energy storage. The firing transformations were elucidated using X‐ray pair distribution function analysis, differential scanning calorimetry, and scanning electron microscopy. It was found that the biomass increased the porosity, which resulted in a decrease of the specific heat capacity. On the other hand, the biochar remained in the clay ceramic without any interaction with the clay matrix up to 950°C. The specific heat capacity of the clay ceramic increased from 1.20 to 1.49 kJ/kg·K for a 30 wt% addition of biochar. The clay ceramic with a 30 wt% addition of biochar also conserved a high flexural strength of 11.1 MPa compared to that of the clay ceramic without organic additives (i.e., 18.9 MPa). Furthermore, the flexural strength only decreased by 23% after 100 thermal cycles. The crack growth associated with the thermal fatigue was limited by crack bridging and crack trapping. Hence, the current results suggest that clay/biochar ceramics can be as efficient as molten salts in thermal energy storage with the added benefit of an ease of use in the physical form of bricks.     

Multiscale toughening of ZrB2-SiC-Graphene@ZrB2-SiC dual composite ceramics

The design of bioinspired architectures is effective for increasing the toughness of ceramic materials. Particularly, a dual composite equiaxial architecture is ideal for fabricating weak interface-toughened ZrB₂-SiC ceramics with isotropic performance. In this paper, ZrB₂-SiC-Graphene@ZrB₂-SiC dual composite ceramics were synthesized via an innovative processing technique of granulating-coating method. ZrB₂-20 vol.% SiC containing 30 vol.% Graphene was selected as weak interface to realize multiscale toughening and improve the thermal shock resistance of ZrB₂-SiC ceramic materials. The incorporation of ZrB₂-SiC-Graphene weak interface into the ZrB₂-SiC matrix improved the damage tolerance and critical thermal shock temperature difference. The design of equiaxial structures moderated the anisotropy of performance in different planes. The graphene sheets incorporated in the ZrB₂-SiC-Graphene interface phase played a key role in multiscale toughening, including macroscopic toughening of crack deflection and microcracks, and microscopic toughening of graphene bridging and pull-out.     

Thermal properties of polytetrafluoroethylene/Sr2Ce2Ti5O16 polymer/ceramic composites

Polytetrafluoroethylene (PTFE) composites filled with Sr₂Ce₂Ti₅O₁₆ ceramic were prepared by a powder processing technique. The structures and microstructures of the composites were investigated by X‐ray diffraction and scanning electron microscopy techniques. Differential scanning calorimetry showed that the ceramic filler had no effect on the melting point of the PTFE. The effect of the Sr₂Ce₂Ti₅O₁₆ ceramic content [0–0.6 volume fraction (vf)] on the thermal conductivity, coefficient of thermal expansion (CTE), specific heat capacity, and thermal diffusivity were investigated. As the vf of the Sr₂Ce₂Ti₅O₁₆ ceramic increased, the thermal conductivity of the specimen increased, and the CTE decreased. The thermal conductivity and thermal expansion of the PTFE/Sr₂Ce₂Ti₅O₁₆ composites were improved to 1.7 W m^?1 °C^?1 and 34 ppm/°C, respectively for 0.6 vf of the ceramics. The experimental thermal conductivity and CTE were compared with different theoretical models. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Thermal Expansion of Homogeneous and Gradient Solid Solutions in the System KZr2(PO4)3─KTi2(PO4)3

Low-thermal-expansion ceramics having arbitrary thermal expansion coefficients were synthesized from homogeneous solid solutions in the system KZr₂(PO₄)₃─KTi₂(PO₄)₃ (KZP–KTP). Dense and strong ceramics were fabricated by sintering at 1100° to 1200°C with 2 wt% MgO. The thermal expansion coefficient increased from 0 to +3 × 10⁻⁶/°C with increasing x in KZr_{2 − x}Ti_x (PO₄)₃ (KZTP). In addition, a functionally gradient material with respect to thermal expansion was prepared by forming a series of KZTP solid solutions in a single ceramic body. By heating a pile of KZP and KTP ceramics in contact with each other, KZP and KTP bonded together to form a KZTP gradient solid solution near the interface.     

Fabrication,thermal shock resistance,and dielectric property of α-Si3N4-based ceramic coating on porous Si3N4 ceramics

A dense α-Si₃N₄-based ceramic protective coating was successfully prepared on porous Si₃N₄ ceramics by a liquid infiltration and filling method. The coating composed of a primary α-Si₃N₄ phase and secondary O'-Sialon, β-Sialon, and Y–Si–Al–O–N glass phase. After thermal shock at ΔT = 1000°C for five times, cracks were produced, but the tip of crack stopped inside the coating; so the coated porous Si₃N₄ ceramics still had a good waterproof ability and its water absorption was only 7%. During thermal shock, toughening mechanisms involving needle-like O'-Sialon particle bridging, crack deflection, and rough fracture, occurred within the cracks, contributing to thermal shock resistance of the coating. The dielectric constant of the coated porous Si₃N₄ ceramics showed a slow increase trend with increasing temperature, and it reached the maximum value of 3.57 at 1100°C at the frequency of 11 GHz. The dielectric loss increased slowly as the temperature increased from room temperature to 900°C, but it started to increase evidently when the temperature was over 900°C.     

Stepwise-Graded Si3N4–SiC Ceramics with Improved Wear Properties

The processing of stepwise graded Si₃N₄/SiC ceramics by pressureless co-sintering is described. Here, SiC (high elastic modulus, high thermal expansion coefficient) forms the substrate and Si₃N₄ (low elastic modulus, low thermal expansion coefficient) forms the top contact surface, with a stepwise gradient in composition existing between the two over a depth of ∼1.7 mm. The resulting Si₃N₄ contact surface is fine-grained and dense, and it contains only 2 vol% yttrium aluminum garnet (YAG) additive. This graded ceramic shows resistance to cone-crack formation under Hertzian indentation, which is attributed to a combined effect of the elastic-modulus gradient and the compressive thermal-expansion-mismatch residual stress present at the contact surface. The presence of the residual stress is corroborated and quantified using Vickers indentation tests. The graded ceramic also possesses wear properties that are significantly improved compared with dense, monolithic Si₃N₄ containing 2 vol% YAG additive. The improved wear resistance is attributed solely to the large compressive stress present at the contact surface. A modification of the simple wear model by Lawn and co-workers is used to rationalize the wear results. Results from this work clearly show that the introduction of surface compressive residual stresses can significantly improve the wear resistance of polycrystalline ceramics, which may have important implications for the design of contact-damage-resistant ceramics.     

A new arrangement of SiC particles in reaction bonded Si3N4-SiC ceramic refractory and crack behaviors under thermal shock

High temperature protection brick lining is important for super-charged boilers. In practice, Si₃N₄ bonded SiC ceramics are usually chosen as the raw material of refractory bricks due to their excellent performance under high temperature. In the field of the ceramic refractory material, a main goal is to improve the resistance of ceramics under thermal shock because their inherent brittleness may cause failure under sudden change in temperature. In this paper, we fulfilled this goal by introducing a new particle arrangement called “double dispersion” for the SiC particle-reinforced ceramic refractory material. And we established the micro-structure models for both the original and the modified ceramic refractory material. To study the influence of the particle arrangement on the fracture toughness, we performed simulations of the crack initiation and propagation under the same thermal load for the original and the modified material. The results showed that the “double dispersion” method can improve the thermal shock resistance of the reaction-bonded Si₃N₄-SiC ceramic refractory.     

Fatigue Crack Growth Behavior of Silicon Nitride: Roles of Grain Aspect Ratio and Intergranular Film Composition

The role of microstructure in affecting the fatigue crack growth resistance of grain bridging silicon nitride ceramics doped with rare earth (RE = Y, La, Lu) oxide sintering additives was investigated. Three silicon nitride ceramics were prepared using MgO‐RE₂O₃ and results were compared with a commercial Al₂O₃‐Y₂O₃‐doped material. Decreasing stress intensity range (ΔK) fatigue tests were conducted using compact‐tension specimens to measure steady‐state fatigue crack growth rates. Specimens doped with MgO‐RE₂O₃ additives showed a significantly higher resistance to crack growth than those with Al₂O₃‐Y₂O₃ additives and this difference was attributed to the much higher grain aspect ratio for the MgO‐RE₂O₃‐doped ceramics. When the crack growth data were normalized with respect to the total contribution of toughening by bridging determined from the monotonically loaded R‐curves, the differences in fatigue resistance were greatly reduced with the data overlapping considerably. Finally, all of the MgO‐RE₂O₃‐doped silicon nitrides displayed similar steady‐state fatigue crack growth behavior suggesting that they are relatively insensitive to the intergranular film.     

Ice-templated assembly strategy to construct oriented porous Ti3SiC2 ceramics for thermal management and electromagnetic interference shielding in harsh thermal environments

Given the electromagnetic interference (EMI) and heat aggregation issue faced by electronic components, an urgent need exists to integrate EMI shielding and thermal conductivity in one material. Herein, a novel lightweight porous Ti₃SiC₂ ceramic with ordered structural arrangement was fabricated by using budget-friendly raw materials through ice template design and in-situ reaction synthesis. Leveraging the excellent conductivity and thermal conductivity of Ti₃SiC₂, a dual-functional advanced material with efficient EMI shielding and thermal management capabilities was obtained. At room temperature, porous Ti₃SiC₂ ceramics can achieve a shielding effectiveness of 35.44 dB and a thermal conductivity of 12.17 W/mK, with performance that can be tuned by porosity. In further, the porous Ti₃SiC₂ ceramic can work stably in thermal environments from room temperature to 700 °C or in corrosive environments rich in acid, alkali, and salts due to its excellent high temperature oxidation resistance and corrosion resistance. In view of the dual-functional characteristics and the stability of operation in harsh thermal environments, ordered porous Ti₃SiC₂ ceramics are promising for modern maritime and aerospace applications.     

Thermodynamic properties of aluminum titanate-mullite composite ceramics containing heat stabilizer

Using Al₂O₃ and TiO₂ as raw materials, adding MgO as heat stabilizer and mullite as enhancer, aluminum titanate-mullite multiphase ceramics were successfully prepared by solid phase synthesis. The effects of MgO and mullite were systematically studied on the phase composition, microstructure, thermal stability, sintering properties, and mechanical properties of aluminum titanate ceramics. The results showed that the introduction of Mg²⁺ can partially replace Al³⁺ to form Mg_xAl_2(1-x)Ti_(1+x)O₅ solid solution, improved the thermal stability of aluminum titanate ceramics, and promoted the formation and growth of grains, which reduced the sintering temperature. The crack deflections caused by mullite particles improved the mechanical properties. The filling effect of mullite particles and the formation of silica in mullite raw materials were conducive to ceramic densification. The statistics of Mg4M10 sample were as follows: the porosity was only 2.9%, the flexural strength was as high as 64.15 MPa, and the thermal expansion coefficient was 1.35 × 10⁻⁶ K⁻¹ (RT-700°C), encouraging the application of ceramics with high thermal mechanical properties.