Preparation and temperature-resistance characteristics of novel dense SiAlCN ceramics

The compact green bodies, prepared via a novel solid-liquid mixing method of precursors, were successfully pyrolyzed to obtain the dense bulk SiAlCN ceramics at 1000 °C. It can be seen from their SEM that they have uniform and dense microstructure, indicating that this method can be used to prepare bulk ceramics. In order to verify that they can be used as sensor heads, their temperature-resistance characteristics and repeatability were tested. The results show that the conductive mechanism belongs to Arrhenius's Tailed-State and Extended-State in the temperature range of 500–650 °C and 650–930 °C, respectively. And it shows that SiAlCN ceramics can be used as the sensor heads for high-temperature sensors.     

Accurate measurement of fracture toughness in structural ceramics

The measured values of fracture toughness for ceramics are closely correlated with the sharpness of notch tips, which in turn influences the accurate measurement of fracture toughness. Here, typical structural ceramics, i.e., 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP), ZrB₂, ZrB₂-SiC and ZrB₂-SiC-Grapite, were used for the measurement of fracture toughness, and the effect of notch tip radius on the fracture toughness values of these typical structural ceramics was investigated. Ultra-sharp notches with a tip radius less than 1 μm can be fabricated by laser, lower than the critical notch tip radius in ceramics below which the fracture toughness value almost remains constant, and improved accuracy and consistency of fracture toughness measurement can be obtained by this method compared with traditional method.     

Residual stress and fracture toughness of sintered body of ZrO2-GO composite ceramics material

Zirconia ceramics and carbon-based materials are widely adopted in medical and dental applications due to their excellent biocompatibility and aesthetics. However, fracture toughness of ceramic materials limits their application in clinical dentistry because of the existence of residual stress. In this study, zirconia/graphene oxide (ZrO₂-GO) composite ceramics were fabricated by hot-press sintering. Residual stresses developed on the surface of ZrO₂-GO composite ceramics were evaluated by X-ray residual stress analysis and indentation techniques. The variation of surface residual stress with GO content was evaluated, and found to be consistent with that of fracture toughness. The generation of residual stress was found to be directly related to fracture toughness. Residual stress calculated by theoretical formula of indentation method was consistent with that measured by X-ray diffraction in line with the content of GO. Based on above results, it is concluded that 0.1–0.15 wt% GO composite ceramics possessed better mechanical properties.     

Determining specimen thickness limit in fracture toughness tests applied to dental ceramics and metal alloy interfaces

The aims of the current study are to present computational simulations based on the finite element method in order to determine the lowest viable thickness of materials used to make ceramic samples based on the interface of groups of materials (Ni–Cr/ceramics, alumina/ceramics), and samples made of each of these materials, alone; as well as to report the effect of variation in the stress intensity factor of these interfaces (β). Test conditions were simulated in Abaqus CAE software and mesh features focused the crack tip. Fracture toughness analysis was carried out by simulating the three-point bending test (SEVNB specimen of the Fracture Mechanic standards). Based on the present results, the lowest material thickness necessary to meet the stress intensity requirement at the crack tip was 8 mm of each material, under symmetry condition, for sandwich composition. Such a thickness included fracture process zone (FPZ) size and plastic zone (PZ). Sandwich thickness was enough to maintain the test conditions established for the standards. Samples produced in the present study met the requirements of the fracture toughness test, based on the ASTM E?399 standard - specimens with the lowest possible sandwich thickness were obtained and they met the stress intensity requirements of the crack tip. This finding pointed towards a sample based on using a reduced amount of test materials - such as those used for dental restoration. The lowest material thickness in the sandwich was 8 mm, for all groups of materials, under symmetry conditions. This thickness includes fracture process zone size and plastic zone size. The herein proposed sandwich thickness was enough to maintain the test conditions established by fracture mechanic standards.     

Boron carbide/graphene platelet ceramics with improved fracture toughness and electrical conductivity

Boron carbide/graphene platelet (B₄C/GPLs) composites have been prepared with a different weight percent of GPLs as sintering additive and reinforcing phase, hot pressed at 2100 °C in argon. The influence of the GPLs addition on fracture toughness (K_IC) and electrical conductivity was investigated. Single Edge V-Notched Beam (SEVNB) method was used for fracture toughness measurements and the four-point Van der Pauw method for electrical conductivity measurements. With increasing amount of GPLs additives, the fracture toughness increased due to the activated toughening mechanisms in the form of crack deflection, crack bridging, crack branching and graphene sheet pull-out. The highest fracture toughness of 4.48 MPa.m^1/2 was achieved at 10 wt.% of GPLs addition, which was ∼50% higher than the K_IC value of the reference material. The electrical conductivity increased with GPLs addition and reached the maximum value at 8 wt.% of GPLs, 1.526 × 10³ S/m in the perpendicular and 8.72 × 10² S/m in the parallel direction to the hot press direction, respectively.     

Improved fracture toughness of laminated ZrB2-SiC-MoSi2 ceramics using SiC whisker

In this study, SiC whisker (SiC_w) was introduced to ZrB₂ matrix layer of laminated ZrB₂/BN ceramics to improve fracture toughness. Laminated ZrB₂-SiC_w/BN ceramics were prepared by tape casting and spark plasma sintering. For comparison, monolithic ZrB₂-SiC_w and laminated ZrB₂-SiC_p/BN ceramics were also prepared using the same method. The introduction of SiC whiskers increased fracture toughness of laminated ZrB₂-SiC_w/BN ceramics to 13.31?±?0.33?MPa?m^1/2 for all samples. This was related to the multi-scale toughening mechanism, including delamination and crack deflection issued from the laminate structure at the macroscopic level, as well as whiskers bridging and pullout at the microscopic view. The R-curve behaviors of all samples revealed improved resistance to crack propagation of laminated ZrB₂-SiC_w/BN when compared to ZrB₂-SiC_p/BN and ZrB₂-SiC_w issued from multi-scale toughening design.     

Determination of fracture toughness of Y-TZP ceramics

The goal of this study is to determine the fracture toughness of 2Y-TZP and 2.5Y-TZP ceramics by single-edge V-notched beam (SEVNB) method and single-edge notched beam (SENB) method. The errors of fracture toughness values tested by SENB are also evaluated. The actual fracture toughness values obtained by SEVNB method are 6.4 ± 0.1 and 5.3 ± 0.1 MPa m^1/2 for 2Y-TZP and 2.5Y-TZP, respectively. After SENB method testing, the phase transformability (t-ZrO₂ → m-ZrO₂) on fractured surface is higher than that of SEVNB method testing. The relationship of fracture toughness values between by SEVNB and by SENB method is established.     

Micro-scale fracture toughness of textured alumina ceramics

Enhanced fracture resistance of textured alumina is ascribed to crack deflection along grain boundaries. In this work, we quantify and compare the micro-scale fracture toughness of textured alumina grains and grain boundaries by micro-bending tests. Notched micro-cantilevers were milled from single alumina textured grains (perpendicular to the [0001] direction) and across several textured grains (along the [0001] direction), using a focused ion beam technique. Bending tests were performed with a nanoindenter. A shape function for notched pentagonal-shaped cantilevers was developed using finite element analysis. The critical stress intensity factor at the notch tip was determined based on the measured fracture loads. The micro-scale fracture toughness of the textured alumina grain boundaries (2.3 ± 0.2 MPa m^1/2) was about 30% lower than that of the grains (3.3 ± 0.2 MPa m^1/2). These findings at the micro-scale are paramount for understanding the macroscopic fracture behaviour of textured alumina ceramics.     

Microstructure and mechanical properties of hot-pressed SiC nanofiber reinforced SiC composites

This study reports on the preparation and mechanical properties of a novel SiC_nf/SiC composite. The single crystal SiC nanofiber(SiC_nf) reinforced SiC ceramic matrix composites (CMC) were successfully fabricated by hot pressing the mixture of β-SiC powders, SiC_nf and Al–B–C powder. The effects of SiC_nf mass fraction as well as the hot-pressing temperature on the microstructure and mechanical properties of SiC_nf/SiC CMC were systematically investigated. The results demonstrated that the 15 wt% SiC_nf/SiC CMC obtained by hot pressing (HP) at 1850 °C with 30 MPa for 60 min possessed the maximum flexural strength and fracture toughness of 678.2 MPa and 8.33 MPa m^1/2, respectively. The nanofibers pull out, nanofibers bridging and cracks deflection were found by scanning electron microscopy, which are believed can strengthen and toughen the SiC_nf/SiC CMC via consuming plenty of the fracture energy. Besides, although the relative density of the prepared SiC_nf/SiC CMC further increased with the sintering temperature rose to 1900 °C, the further coarsend composites grains results in the deterioration of the mechanical properties for the obtained composites compared to 1850 °C.     

Notch radius effect on fracture toughness of ceramics pertinent to grain size

Unlike fracture toughness, the notch fracture toughness of a ceramic is not a constant; rather, it increases with the notch-root radius ρ in a notched specimen. In this study, by analyzing the fracture measurements of eight different notched ceramics with an average grain size G of 3–40 μm, a simple model describing the relation between the notch fracture toughness and fracture toughness is proposed as a function of the relative notch-root radius ρ/G. The normal distribution is incorporated to consider the inevitable scatter in measurements where fracture mechanisms and errors are present. The results demonstrate that the model can effectively predict the quasi-brittle fracture variation trend for ceramics, including the upper and lower bounds, with 96% reliability, from a normal distribution; thus, it can address virtually all of the experimental data. We also determined that the notch fracture toughness approximates the fracture toughness if ρ ≤ G.     

Influence of graphene nanoplatelets (GNPs) on mode I fracture toughness of an epoxy adhesive under thermal fatigue

Abstract

The contribution of graphene nanoplatelets (GNPs) for enhancing the fracture toughness of a commonly used room-cured epoxy, used to bond E-glass/epoxy composite adherends, is evaluated. A comprehensive experimental investigation is conducted to examine the performance and degradation of adhesively bonded joints subject to cyclic thermal loading using the standard double cantilever beam (DCB) specimens. Several groups of DCB specimens were fabricated using the adhesive reinforced with four different GNPs weight-percentages (i.e. 0.0, 0.25, 0.5 and 1%). The specimens are subsequently subjected to various numbers of thermal cycles (to a maximum of 1000 heating/cooling cycles), and then tested, and the resulting mode I fracture toughness values are evaluated and compared. The extent and modes of damage captured through microscopy and scanning electron microscopy images are presented and discussed. In addition, a computational framework, using the cohesive zone modeling technique, is developed for predicting the response of the adhesives and their damage evolution.     

High hardness and high fracture toughness B4C-diamond ceramics obtained by high-pressure sintering

The B₄C-diamond composite with high hardness and toughness was first prepared by high-pressure sintering of B₄C and diamond powders at 5 GPa and 1600 °C. The effect of the diamond fraction on the densification, microstructure and mechanical properties of B₄C-diamond composite were investigated. The results indicated that the hardness of the as-prepared composite ceramics increased gradually with the increase in diamond content. The composite having 40 vol% diamond exhibited excellent comprehensive mechanical properties with a relative density of 98.3%, a density of 2.86 g/cm³, Vickers hardness of 39.8 GPa and fracture toughness of 8.1 MPa·m^1/2. The use of superhard diamond enhanced the fracture toughness of the B₄C while maintaining its lightweight and high hardness. The main toughening mechanisms were crack bridging, crack deflection and pull-out of homogeneously dispersed diamond grains. Superhard second phase dispersion high-pressure sintering provides a new technical route to improve the properties of advanced composites.     

ZrB2-based composites toughened by as-received and heat-treated short carbon fibers

In order to improve the fracture toughness of ZrB₂ ceramics, as-received and heat treated short carbon fiber reinforced ZrB₂-based composites were fabricated by hot pressing. The toughening effects of the fibers were studied by investigating the relative density, phase composition, microstructure and mechanical properties of the composites. It was found that the densification behavior, microstructure and mechanical properties of the composites were influenced by the fibers’ surface condition. The heat treated fiber was more appropriate to toughen the ZrB₂-based composites, due to the high graphitization degree, low surface activity and weak interfacial bonding. As a result, the fracture toughness of the composites with heat-treated fiber is 7.62 ± 0.12 MPa m^1/2, which increased by 10% as compared to the composites with as-received fiber (6.89 ± 0.16 MPa m^1/2).     

Microstructure control of SiCw/SiC composites based on SLS technology

The preparation of SiC_w/SiC materials was realized by SLS technology. The effect of SiC powder size on the number and size of SiC whisker formation was investigated. The tortuosity and diameter of open pores are introduced to modify the classical molecular collision model, and the relationship model between the growth rate of SiC whisker and porosity was established. The influence mechanism of powder size on the number of whisker growth was revealed. According to the model, the number of in-situ whisker growth in powder can be calculated, and the calculated results by using this model agreed with the test results. So it is suitable for in-situ whisker microstructure control under SLS technology, and also suitable for other 3D printed whisker in-situ reinforced ceramic material systems. This is of great significance to expand the application of 3D printing ceramic matrix composites.     

Effect of molten zone ablated by femtosecond laser on fracture toughness of oxide ceramics

In this paper, an ultra-sharp V-notch was produced by femtosecond laser on the green bodies of 3Y-TZP, Al₂O₃ and 8Y-FSZ, respectively. After sintering, the fracture toughness of those ceramic samples was tested by single-edge V-notched beam (SEVNB) method. For comparison, the fracture toughness of ceramic samples with an ultra-sharp V-notch ablated by femtosecond laser directly on the sintered test bars was also determined as a regular testing route. The results reveal that the two different preparation methods of ultra-sharp V-notches can obtain the actual fracture toughness of 3Y-TZP, Al₂O₃ and 8Y-FSZ. It proves that the influence of thermal effect of ablation process caused by femtosecond laser in front of the notch tip on the fracture toughness can be negligible.     

A detailed analysis of the determination of fracture toughness by nanoindentation induced radial cracks

The correlation between the choice of reference material and model utilized in determining fracture toughness from indentation-induced radial cracks was critically investigated with six commonly used reference materials. Initially, the empirical constants in Anstis’s and Laugier’s equations were calculated, compared and analyzed. According to the values of the constants, the reference materials were categorized into three groups. This classification was further verified by evaluating the corresponding constants of 13 additional equations. To account for the classification, FIB technique was employed to examine the crack morphology in the reference materials – Si (100) features an almost Half-penny-shaped crack while the cracks in SiC (0001) display nearly as a rectangle, both far from the assumptions employed by the models. Subsequently, an improved and more reliable procedure to determine fracture toughness is proposed. Finally, with this procedure, the fracture toughness of a diamond/SiC composite film is determined to be 11.1 ± 1.1 MPa m^1/2.     

Microstructure and fracture toughness of in-situ nanocomposite coating by thermal spraying of Ti3AlC2/Cu powder

The low fracture toughness of ceramic coatings has always hindered their wide application. In this study, an in-situ nanocomposite coating was prepared by the atmospheric plasma spraying of a 50 wt% Ti₃AlC₂-50 wt% Cu mixed powder. The in-situ nanocomposite coating was found to have an unusual microstructure with a nano-micrometre phase synergistic enhancement, which consisted of submicrometre-thick layers of Cu and nanoparticles of Cu(Al), Ti₄O₅, TiO₂, and Al₂TiO₅. Thus, in the spraying process, Al was delinked out of Ti₃AlC₂, forming a large amount of plastic Cu(Al) with Cu. The delinked channel provided a path for Cu to diffuse into Ti₃AlC₂, which a spatial Cu network structure was formed in the coating. The in-situ nanocomposite coating has high fracture toughness and crack growth resistance by a three-point bending test. This paper reports a new method to prepare a high-fracture-toughness composite ceramic coating.     

Facile synthesis of a carbon-rich SiAlCN precursor and investigation of its structural evolution during the polymer-ceramic conversion process

A liquid carbon-rich SiAlCN precursor is facilely synthetized by hydrosilylation between liquid polyaluminocarbosilane (LPACS) and 1,3,5,7-tetravinyl- 1,3,5,7-tetramethylcyclotetrasilazane {[CH₃(CH₂CH₂)SiNH]₄} (TeVSZ). The structural evolution during the polymer-ceramic conversion process is investigated by various methods. The results show that the main cured mechanism is β-addition on hydrosilylation, although α-addition on hydrosilylation, polymerization of vinyl groups and dehydrocoupling reaction between N–H bonds also occur during the cured process. During the pyrolysis process, dehydrogenation and dehydrocarbonation condensation reactions, transamination reactions occur, leading to formation of a three-dimensional network inorganic structure at 400–800 °C, where part of Al–O bonds convert to Al–N bonds. Then the network inorganic structure undergoes demixing and separation into amorphous SiAlCN(O) phase, where the amorphous turbostratic free carbon phase also form at 800–1200 °C. With demixing and decomposition of the amorphous carbon-rich SiAlCN(O) phase, the crystalline β-SiC and graphitic carbon start to form at about 1400 °C, the crystalline sizes of them both enlarge with increasing temperature. However, the crystal growth of β-SiC is distinctly inhibited due to existence of the rich carbon phase, tiny amounts of Al₂O₃ and AlN. In addition, a small amount of AlN can promote the formation of α-SiC at 1800 °C.     

Reliable evaluation of fracture toughness in ceramics via nanosecond laser notching method

As a new type of notching approach, laser notching method always neglecting the significant impact of equivalent notch angle (θ) on fracture toughness results, although it has successfully solved the problem of introducing sharp enough V-notches in ceramics. Here, porous ZrB₂ and dense ZrB₂-SiC ceramics are taken as the experimental targets, and the effect of θ on the fracture toughness values of these typical ceramics is investigated. To measure the fracture toughness accurately, the θ should be less than 60°, namely, the laser notch depth should be larger than the traditional U-groove root radius. Nanosecond laser notching method shows great advantages in high notch depth (easy to reach 300 μm) and sufficient accuracy (tip radius less than 1 μm), and is deemed to have a good prospect of standardization.