Design of damage tolerant and crack-free layered ceramics with textured microstructure

This work demonstrates damage tolerant behavior of ceramic laminates designed with residual stresses and free of surface edge cracks. Non-periodic architectures were designed by embedding 2 textured alumina (TA) layers between 3 equiaxed alumina-zirconia (AZ) layers. Compressive residual stresses of ∼ 250 MPa were induced in the textured layers. Indentation strength tests showed that textured compressive layers arrested the propagation of cracks. Results were compared to periodic architectures with the same volume ratio of TA and AZ materials. Crack propagation was arrested in both periodic and non-periodic designs; the minimum threshold-strength being higher in the latter. Non-periodic architectures with compressive layers as thin as ∼ 200 μm showed no evidence of surface edge cracks, yet still reached minimum threshold strength values of ∼ 300 MPa. In addition, the textured microstructure promoted crack bifurcation in the thin compressive layers and thus enhanced the damage tolerance of the material.     

High-temperature fracture behaviour of layered alumina ceramics with textured microstructure

Mimicking the damage tolerance of biological materials such as nacre has been realised in textured layered alumina ceramics, showing improved reliability as well as fracture resistance at room temperature. In this work, the fracture behaviour of alumina ceramics with textured microstructure and laminates with embedded textured layers are investigated under uniaxial bending tests at elevated temperatures (up to 1200 °C). At temperatures higher than 800 °C monolithic textured alumina favours crack deflection along the basal grain boundaries, corresponding to the transition from brittle to more ductile behaviour. In the case of laminates, the loss of compressive residual stresses is counterbalanced by the textured microstructure, effective up to 1200 °C. This study demonstrates the potential of tailoring microstructure and architecture in ceramics to enhance damage tolerance within a wide range of temperatures.     

Ballistic evaluation and damage characterization of 3-D printed,alumina-based ceramics for light armor applications

The goal of this work was to present results of mechanical characterization and ballistic investigation of 3-D printed alumina (Al₂O₃)-based armor plates, manufactured using two additive manufacturing-based methods: pressurized spray deposition (PSD) and direct ink write (DIW), to determine the maturity of these additive-based processes against the industry standard process. The DIW plates exhibited superior hardness, flexural strength, and density compared to the PSD plates, and in many respects, even eclipsed some of the properties of the commercial isopressed (IP) material. Plates (90 mm × 90 mm × 8 mm) of these composition were manufactured for ballistic analysis in accordance with established ballistic characterization procedures, using a 50.8-mm-thick Aluminum 6061 plate as backing and witness plates in the case of penetration or deformation. Six alumina plates were examined ballisitically (one shot per plate) against the 12.7 mm APM2 projectile (45.9 g) at an impact velocity of 840 m/second. The plates that were manufactured using the DIW method provided a higher impact resistance than the PSD method, however, both did not perform as well as the traditionally processed IP material, due to the presence of defects introduced due to the printing processes.     

Processing and characterisation of three-layer alumina-based composites with enhanced damage tolerance

Three-layer alumina-based composites reinforced with iron in the inner layer and with chromium in the outer layers were fabricated by first uniaxially pressing the three-layer assembly, followed by cold isostatic pressing at 300 MPa and sintering in a graphite vacuum furnace at 1500 °C for 1 h.

The residual compressive stresses in the outer Cr–Al/Al₂O₃ layers and the residual tensile stresses in the Fe–Al/Al₂O₃ inner layer were predicted as a function of composition and the thickness ratio of the outer and inner layers. Theoretical calculations showed that the compressive stresses in the outer layers increased while the tensile stresses in the inner layer decreased with decreasing outer layer thickness. The existence of compressive stresses was verified by microscopic evidence, which showed that propagation of cracks perpendicular to the interface is suppressed in the outer layer, but promoted in the inner layer.

Indentation and subsequent strength testing showed that these layered composites exhibited improved damage tolerance. Three-layer composites showed four-point bend strengths exceeding the bend strength of unindented monolithic Al₂O₃ even after indentation at 300 N.     

Intrinsic microstructures of silica-bonded porous nano-SiC ceramics

Silica-bonded porous SiC ceramics were fabricated using nano-β-SiC powder-carbon black template compacts by sintering in air at 600°C-1200°C. The intrinsic microstructures of the porous ceramics were characterized by high-resolution transmission electron microscopy, which led to the following observations: (a) a core (SiC)-shell (SiO₂) structure was formed, owing to the partial oxidation of nano-SiC particles during sintering; (b) a low-temperature (800°C) β-to-α polytypic phase transformation was observed, owing to the oxidation-induced residual thermal stresses; and (c) non-graphitic carbons were precipitated inside the SiC core, owing to the segregation of C atoms emitted at the strained SiC-SiO₂ interface.     

Role of Microstructure in Dynamic Fatigue of Glass-Ceramics after Contact with Spheres

Dynamic fatigue data are reported for fine- and coarse-grained micaceous glass-ceramics after contact damage with spheres. The strengths of indented specimens are measured at stressing rates from ∼10⁻² to 10⁴ MPa·s⁻¹ in water. The strength degradation is substantially faster in the coarse-grained structure, and is accelerated further by multicycle contact loading. Failures originate from contact sites in all cases but undergo a progressive transition from classical cone cracks to quasi-plastic microcrack zones with increases in the grain size and the number of contact cycles. The results highlight the particularly deleterious effect of quasi-plastic damage accumulation on lifetime.     

Improved tensile strength and toughness of dense C/SiC-SiBC with tailored PyC interphase

In this study, in order to improve the tensile strength and toughness of dense C/SiC-SiBC, the thickness of PyC interphase (e_PyC) was increased from 200 nm to 400 nm. C/SiC (e_PyC≈200 nm) was also fabricated as a benchmark comparison. C/SiC-SiBC (e_PyC≈200 nm) exhibited higher axial thermal residual stress (TRS) than C/SiC (e_PyC≈200 nm). Increased axial TRS resulted in increased interfacial shear strength (τ) through fiber bending, which caused lowered mechanical properties of C/SiC-SiBC (e_PyC≈200 nm). By increasing the thickness of PyC interphase from 200 nm to 400 nm, the axial TRS in C/SiC-SiBC decreased. Accordingly, the radial stress induced by fiber bending decreased, leading to a reduced τ in C/SiC-SiBC (e_PyC≈400 nm). Decreased axial TRS and τ are beneficial to the effective loading of fiber bundles and pull out of fibers. Therefore, C/SiC-SiBC (e_PyC≈400 nm) performed excellent tensile strength (250 ± 11 MPa) and fracture toughness (23.7 ± 0.5 MPa·m^1/2).     

Off-axis damage tolerance of fiber-reinforced composites for aerospace systems

Off-axis strength retention of continuous carbon fiber-reinforced dense ZrB₂-based ceramics (C_f/ZrB₂) after thermal or indentation damage was evaluated. Thermal damage was in-situ induced and characterized by cyclic dilatometric analysis. Indentation damage was induced through Vickers indentation and then characterized by digital microscopy. The investigation of Vickers imprints suggested that residual stresses promoted the material pileup onto the fibers’ plane and the appearance of out-of-plane freed fibers (OFF). On the other hand, thermal damage reduced the residual stresses and left inner freed fibers (IFF) that enhanced the elastic response. Finally, the flexural tests on damaged specimens unexpectedly revealed that C_f/ZrB₂ kept its load bearing capability either after thermal or indentation damage (in both cases) and showed damage insensitivity although tested in fully matrix-dominated loading configuration (off-axis configuration).     

Quantification of Grain Boundary-Pore Contact During Sintering

A stereological method has been used to determine the degree of grain boundary-pore contact during sintering of Al₂O₃. Al₂O₃ doped with 200 ppm MgO exhibits a degree of contact of 5.7 times that expected from random intersections with pores, while pure Al₂O₃ shows a pore contact factor of 4.8. These data are larger than the values of 2.8 for sintered or hot-pressed UO₂, computed from published data, and values of 1.7 and 1.8 for sintered W and Cu powders, respectively. The degree of grain boundary-pore contact for each material remains constant throughout densification from pressed powder to near full density.     

92黄色氧化铝陶瓷的研制及其显微结构分析

将ZrO2＋Y2O3复合添加剂加入到92白色氧化铝陶瓷基料中,经常压烧结制备了黄色氧化铝陶瓷。实验结果表明：当添加0．6％（质量分数,下同）ZrO2和0．3％Y2O3时,可以使氧化铝陶瓷呈现均匀的黄色。通过SEM检测对氧化铝陶瓷的显微结构进行了分析。     

Micro- and macroscopic design of alumina ceramics by robocasting

The manufacturing of alumina ceramics with auxetic and macro-cellular lattice structure and with textured microstructure was achieved by using robocasting and TGG-technique (Templated Grain Growth). After sintering at different temperatures a grain growth effect was observed in robocast ceramic filaments, which showed a microstructural gradient in particle orientation and porosity from the edge to the middle. The particle orientation during robocasting was analytically and numerically investigated by calculating the inhomogeneous shear rate distribution in the extrusion nozzle.     

Processing and properties of low cost macroporous alumina ceramics with tailored porosity and pore size fabricated using rice husk and sucrose

The present study demonstrates a cost effective way to fabricate porous ceramics with tailored microstructures using rice husk (RH) of various range of particle sizes as a pore former and sucrose as a binder as well as a pore former. Sample microstructures reveal randomly oriented elongated coarse pores and fine pores (avg. size 4 μm) created due to burnout of RH and sucrose, respectively. Porous alumina ceramics with 20–66 vol% porosity and 50–516 μm avg. pore size (length), having isolated and/or interconnected pores, were fabricated using this process. Mechanical properties of the porous samples were determined as a function of porosity and pore microstructure. The obtained porous ceramics exhibited flexural strength of 207.6–22.3 MPa, compressive strength of 180–9.18 MPa, elastic modulus of 250–18 GPa and hardness of 149–18 HRD. Suggested application area includes thermal, filtration, gas purging etc.     

Development of ultrahigh-entropy ceramics with tailored oxidation behavior

In the last decade, single-phase high-entropy materials have attracted considerable research interest owing to their unexpected existence and unique combinations of properties. Recent development of 5-cation high-entropy carbides (HECs) has demonstrated alluring properties compared to the rule of mixtures and binary carbides. Proposed here is the development of ultrahigh-entropy carbides (UHECs) containing 6+ principal elements with greater combinatorial possibilities. The use of 6+ multi-cation compositions allows for the design of ceramics with further tunable properties, while likely possessing higher orders of entropic stabilization. There are 133 possible carbide compositions containing 6, 7, 8, or 9 refractory metal cations in equiatomic ratios. Candidate selection for fabrication and material testing was accelerated using a machine learning model that was originally trained to predict the synthesizability of five cation disordered metal carbides. Two compositions from each category of six through eight cations, one containing Cr and one without, plus the one possible nine cation carbide were fabricated and characterized. The potential for these 6+ cation UHECs as improved materials for oxidative environments is demonstrated by comparing the oxidation performance of a 5- and 7-cation system after 10 min at 1973 K in air. The oxidation behavior is correlated with Ellingham diagrams, and it is demonstrated that the 7-cation carbide has the ability to form a transitional stable 5+ cation HEC layer as elements preferentially form oxides, which results in significantly improved oxidation resistance.     

Properties and microstructural aspects of TiO2‐doped sintered Alumina‐Zirconia composite ceramics

The effect of titania content on the densification, the phase transformation, the microstructures, and mechanical properties of 50 wt% Al₂O₃‐50 wt% ZrO₂ (12 mol% CeO₂) was evaluated. Ceramic composites with different TiO₂ content (0.27, 5, 10 wt%) were prepared by pressureless sintering at low temperature (1400°C) for 2 hours in air. Dense ceramic was obtained by adding 5 wt% of TiO₂ loading to improved mechanical properties. The microstructure analysis provided lots of information about solid‐state reactivity in alumina‐zirconia‐titania ternary system. The content of TiO₂ strongly affected the phases evolution and the grain growth during sintering. Furthermore, a significant effect on mechanical properties and fracture behavior was also observed.     

氟化物及制备工艺对高纯氧化铝陶瓷组织形貌的影响

以高纯氢氧化铝为主要原料,利用扫描电子显微镜、X射线衍射、X射线荧光光谱及X射线光电子能谱等技术,研究了引入NH₄F或AlF₃对α-Al₂O₃转变特征、氧化铝粉体及陶瓷显微结构的影响。结果表明：引入NH₄F或AlF₃促进α-Al₂O₃晶体形貌由颗粒状向片状转变。但是经高温煅烧,形成的片状α-Al₂O₃晶体又转变为颗粒状晶粒。粉体经不同压力成型后的块状样品,在1300℃煅烧后,α-Al₂O₃晶体仍保留片状形态。但是进一步升温到1600℃烧结的氧化铝陶瓷,晶体形貌由片状逐渐转变为多面体。热压条件下,氧化铝陶瓷晶体仍能够保持片状,没有出现多面体形态。随着烧结温度的提高,样品烧结致密化程度不断增大,相对密度从77.6%增加到92.2%。相比以片状粉体为原料烧结的α-Al₂O₃陶瓷,以颗粒状粉体为原料烧结的陶瓷相对密度要大,在1600℃烧结的α-Al₂O₃陶瓷密度从92.2%提高到93.0%。     

Restraining of calcium contamination in near‐net shape alumina ceramics during slip casting

The traditional method for shaping ceramics is by slip casting on gypsum molds; however, its application for near‐net shaping of ceramic components is limited due to contamination by calcium ions. The focus of this study is the modification of the mold to limit Ca²⁺ contamination and to maintain favorable sucking properties. Cement was added to a standard gypsum mold to suppress its erosion, and a decrease in the sucking rate was observed due to its reduced macroporosity. The highest values of green densities were obtained at gypsum/cement weight ratios of 90/10 and 70/30. The microstructure analysis showed that alumina blocks prepared from the molds containing higher quantities of cement (30 or 50 wt%) were resistant to abnormal grain growth caused by Ca²⁺ contamination from the gypsum. The gypsum/cement mixtures for making molds for slip casting significantly limit mold erosion due to a lower sucking rate and abnormal grain growth of the slip cast samples because of the decreased diffusivity of Ca²⁺ ions. Therefore, the present modification of the mold renders the slip casting method more suitable for the near‐net shaping of ceramics.     

Mechanical characterization of sol-gel alumina-based ceramics with intragranular reinforcement of multiwalled carbon nanotubes

Multiwalled carbon nanotubes (MWCNTs) have been widely considered for mechanical reinforcement of ceramic matrix composites. Nevertheless, the efficiency of this reinforcement strategy is under debate due to fabrication issues, such as a good homogenization or the location of the MWCNTs inside the matrix composite. Regarding this, the intragranular location of the MWCNTs has been deemed a crucial feature for optimizing the reinforcement compared to the typical intergranular placement achieved by conventional procedures. Recently, the sol-gel method has been reconsidered, as it promotes the intragranular placement of the MWCNTs. This work presents the mechanical characterization of these composites synthesized by the sol-gel method, where crack-bridging has been revealed as toughening mechanism. Finally, the conventional use of the bibliographical Young's modulus of pure alumina for the estimation of the fracture toughness is discussed, obtaining significant improvements of the fracture toughness when indentation measurements are treated by considering elastic moduli obtained by nanoindentation.     

Prediction of edge and tunnelling crack formation in layered ceramics using a stress-energy fracture criterion

A coupled stress-energy criterion is utilized to predict initiation of both edge and tunnelling cracks in layered ceramics containing thermal residual stresses. Edge (surface) cracks may originate in layers having high compressive in-plane stresses while tunnelling (internal) cracks may form in layers with high tensile in-plane stresses. This work investigates the influence of both the residual stresses magnitude and layer thickness on the formation of surface cracks and provides a design map defining safe regions where no cracks will be present in the sintered multilayer architecture upon reaching the room temperature. Necessary stress and energy inputs to evaluate the coupled criterion are calculated using the finite element method. Simulation results are validated with experimental observations on sample architectures fabricated with layers of various thicknesses and in-plane thermal residual stresses. The good agreement demonstrates the potential of the stress-energy coupled criterion for designing crack-free multi-layered ceramic architectures.     

High damage tolerant Al2O3 composite ceramics constructed with short Al2O3 fibers

The catastrophic fracture characteristics of ceramic materials have become one of the most serious factors limiting their application in critical areas, as a result, it is urgent to overcome the brittleness and improve the damage tolerance of ceramic materials. Herein, a series of Al₂O₃ composite ceramics developed with short Al₂O₃ fibers and a compound interface phase composed of Al₂O₃ and h-BN powders, followed by investigating their fracture behaviors and damage tolerance. Results show that these composites present progressive fracture manners with the rising resistance curve (R-curve) behaviors, and the maximum crack growth toughness of the sample with 15% compound interface phase reaches above 10 MPa·m^1/2 (135% increase with respect to the reference alumina). Meanwhile, the composite ceramic exhibits an excellent ability to resist catastrophic failure with a large critical crack size (105.47 ± 19.11 μm) and high damage tolerance parameter (0.71 ± 0.06 m^1/2), which are close to 14.57 times and 5.92 times higher than those of the reference alumina. The superior performances are mainly attributed to the precise combination of compound interface phase for inducing crack and interlocking Al₂O₃ fibers for load capacity.     

Tailored pore structures and mechanical properties of porous alumina ceramics prepared with corn cob pore-forming agent

In this work, the effects of porosity and different particle sizes of pore-forming agent on the mechanical properties of porous alumina ceramics have been reported. Different grades of porous alumina ceramics were developed using corn cob (CC) of different weight contents (5, 10, 15, and 20 wt%) and particle sizes (<63 µm, 63-125 µm and 125-250 µm) as the pore-forming agent. Experimental results showed that total porosity and pore cavity size of the porous alumina ceramics increased with rising addition of CC pore former. Total porosity increased with increasing particle size of CC with the Al₂O₃-^<63CC₅ sample exhibiting the lowest total porosity of 41.3 vol% while the highest total porosity of 68.1 vol% was exhibited by the Al₂O₃-^125-250CC₂₀. The particle size effect of CC on the mechanical properties revealed that diametral tensile strength and hardness of the porous alumina ceramics deteriorated with increasing particle size of CC pore former. The Al₂O₃-^<63CC₅ sample exhibited the highest diametral tensile strength and hardness of 25.1 MPa and 768.2 HV, respectively, while Al₂O₃-^125-250CC₂₀ exhibited the lowest values of 1.1 MPa and 35.9 HV. Overall, porous alumina ceramics with the smallest pore sizes under each particle size category exhibited superior mechanical properties in their respective categories.