Relationship between elastic and mechanical properties of dental ceramics and their index of brittleness

The purpose of the study was to verify the effects of a number of materials’ parameters (crystalline content; Young's modulus, E; biaxial flexure strength, σ_i; Vickers hardness, VH; fracture toughness, K_Ic; fracture surface energy, γ_f; and index of brittleness, B) on the brittleness of dental ceramics. Five commercial dental ceramics with different contents of glass phase and crystalline particles were studied: a vitreous porcelain (VM7/V), a porcelain with 16 vol% leucite particles (d.Sign/D), a glass-ceramic with 29 vol% leucite particles (Empress/E1), a glass-ceramic with 58 vol% lithium-disilicate needle-like particles (Empress 2/E2), and a glass-infiltrated alumina composite with 65 vol% crystals (In-Ceram Alumina/IC). Discs were constructed according to manufacturers’ instructions, ground and polished to final dimensions (12 mm × 1.1 mm). Elastic constants were determined by ultrasonic pulse-echo method. σ_i was determined by piston-on-3-balls method in inert condition. VH was determined using 19.6 N load and K_Ic was determined by indentation strength method. γ_f was calculated from the Griffith–Irwin relation and B by the ratio of HV to K_Ic. IC and E2 showed higher values of σ_i, E, K_Ic and γ_f, and lower values of B compared to leucite-based glass-ceramic and porcelains. Positive correlations were observed for σ_i versus K_Ic, and K_Ic versus E^1/2, however, E did not show relationship with HV and B. The increase of crystalline phase content is beneficial to decrease the brittleness of dental ceramics by means of both an increase in fracture surface energy and a lowering in index of brittleness.     

Effect of second phase addition of zirconia on the mechanical response of textured alumina ceramics

The effect of second phase addition of zirconia on the mechanical response of textured alumina was analysed. Highly textured monolithic tape-casted alumina was obtained through templated grain growth. Compositions containing 1, 2, 5 and 10 vol% of (i) non-stabilised and (ii) 3 mol% yttria-stabilised zirconia, respectively, were investigated. XRD analyses revealed that the texture degree decreased with increasing second phase content. Microstructural analysis showed zirconia grains inside the textured alumina grains for contents ≤ 5 vol%, affecting the mode of fracture. Fracture toughness of textured alumina significantly decreased with the addition of a second phase. In the case of non-stabilised zirconia, the constraint of the alumina matrix and the small grain size led to a lower fracture toughness in comparison to monolithic textured alumina (K_Ic = 5.1 MPa m^1/2). The fracture toughness of textured alumina with 3 mol% yttria-stabilised zirconia was comparable to equiaxed alumina, independent of the content ratio (K_Ic = 3.5 MPa m^1/2).     

Fracture Toughness of Advanced Structural Ceramics: Applying ASTM C1421

The three methods of determining the quasi‐static Mode I fracture toughness (K_Ic) (surface crack in flexure—SC, single‐edge precracked beam—PB, and chevron‐notched beam—VB) found in ASTM C1421 were applied to a variety of advanced ceramic materials. All three methods produced valid and comparable K_Ic values for the Al₂O₃, SiC, Si₃N₄, and SiAlON ceramics examined. However, not all methods could successfully be applied to B₄C, ZrO₂, and WC ceramics due to a variety of material factors. The coarse‐grained microstructure of one B₄C hindered the ability to observe and measure the precracks generated in the SC and PB methods while the transformation toughening in the ZrO₂ prevented the formation of the SC and PB precracks and thus made it impossible to use either method on this ceramic. The high strength and elastic modulus of the WC made it impossible to achieve stable crack growth using the VB method because the specimen stored a tremendous amount of energy prior to fracture. Even though these methods have passed the rigors of the standardization process there are still some issues to be resolved when the methods are applied to certain classes of ceramics. It is recommended that, when appropriate, at least two of these methods be employed to determine the K_Ic, especially when a new or unfamiliar ceramic is being evaluated.     

Enhanced mechanical properties in ceramic multilayer composites through integrating crystallographic texture and second-phase toughening

Inherent brittleness and low mechanical reliability usually inhibit the application of ceramic materials in many structural applications. In this work, we demonstrate that integrating crystallographic texture and second-phase toughening strategies can effectively improve fracture resistance and mechanical reliability in alumina multilayer composites. Composites consisted of equiaxed (1-x)Al₂O₃-xZrO₂ and highly [0001]-textured Al₂O₃ layers were fabricated, and effects of ZrO₂ amount on fracture behavior and mechanical properties of the composites were studied. Increasing ZrO₂ amount x results in larger thermal expansion difference between equiaxed and textured layers. The composites with equiaxed layers containing 30 vol% ZrO₂ exhibit high apparent fracture toughness K_{apt, c} ~11.7 MPa·m^1/2 and work of fracture γ_WOF ~1540 J/m², which correspond respectively to about 260% and 410% enhancements relative to those without ZrO₂ addition. Moreover, adding ZrO₂ remarkably reduces sensitivity of failure stress to flaw size in the multilayer composites, and the failure stress substantially increases with increasing ZrO₂ content. The greatly enhanced mechanical performance achieved here can be mainly attributed to higher magnitude of compressive stresses, more crack bifurcations and longer crack deflection paths within the textured layers. This work can provide important guidelines for developing novel “bio-inspired” materials with improved fracture resistance and flaw tolerance behavior.     

Effect of ZrO2 content on the microstructure and flexural strength of Al2O3–ZrO2 composites with the stored failure energy-fragmentation relations

High flexural strength is an important mechanical property for a ceramic armor component to withstand high tensile stresses and protect its structural integrity against multiple hits. Also, larger fragments are required in fragmentation as larger fractured parts are harder to leave of the way for the penetrator and cause more abrasion and higher penetration resistance. In this study, the effect of different ZrO₂ content (0, 0.5, 1, 3, 5, 10, 20 vol%) on the flexural strength of Al₂O₃–ZrO₂ composites was investigated with relationship of the stored failure energy-crack length to evaluate the fragmentation behavior under possible impact conditions. Monotonic equibiaxial flexural strength test was used to measure the fracture strength. The highest strength was obtained for 20 vol% ZrO₂ containing composite as 435 ± 78 MPa, ~ 24% increase in comparison with the pure Al₂O₃. The transformation of tetragonal to monoclinic phase occurred during the strength test in the 10 and 20 vol% ZrO₂ content composites. 20 vol% ZrO₂ containing composite had the smallest total crack length accompanying the largest fragment size for a given fracture energy among all the composites due to the stress-induced transformation of ZrO₂ consumes energy that results in decreasing effective crack driving energy required for the crack branching.     

Bioinspired nacre-like 2024Al/B4C composites with high damage tolerance

The bio-inspired 2024Al/B₄C composites with a laminate-reticular hierarchical architecture were constructed by squeeze casting of 2024Al into loose freeze-cast ceramic scaffolds. This pressurized infiltration process provided a clean and well-bonded interface without physical gaps. By regulating the initial suspension concentration (20, 25, 30 and 35 vol%), the effects of different ceramic content on the microstructure, damage-tolerance behavior and toughening mechanisms of the composites parallel and perpendicular to the ice-growth direction were investigated. The strength and toughness in the longitudinal direction were greater than that in the transverse direction. The 2024Al/20 vol% B₄C composite in the longitudinal direction yielded the highest flexural strength of 658 MPa, crack-initiation toughness (K_Ic) of 18.4 MPa m^1/2 and crack-growth toughness (K_Jc) of 27.5 MPa m^1/2. The unique damage-tolerant properties were attributed to multiple toughening mechanisms, including crack deflection, branching and blunting, ductile-ligament bridging and multiple-crack propagation, as evidenced by the stable crack growth and rising R-curve behavior during fracture. The significantly decreased damage tolerance in the transverse direction was mainly due to inadequate toughening tools. On the other hand, both the flexural strength and fracture toughness reduced remarkably as the ceramic content increased. The 2024Al/35 vol% B₄C composite fractured in a single-crack mode and the crack growth path was almost straight, showing a relatively low ?exural strength (502 MPa) and crack-initiation toughness (9.1 MPa m^1/2). The toughening mechanism was discussed in terms of the relationship between structural characteristics and cracking mode.     

The fracture toughness of natural fibre- and glass fibre-reinforced SMC

The fracture toughness properties, in terms of stress intensity factor K_Ic and strain energy release rate G_Ic, of hemp fibre mat-reinforced sheet moulding compound (H-SMC) are measured using the compact tension (CT) method and compared with those of glass fibre-reinforced SMC (G-SMC). Three material parameters were considered for composite optimisation: fibre volume fraction, CaCO₃ filler content and hemp fibre surface treatments using either alkaline, silane or a combination of these two treatments. The highest fracture toughness for H-SMC composites was obtained at a fibre loading of around 30?vol.-%, while it was also shown that the fracture toughness properties of H-SMC are sensitive to mineral filler content. Surface treatment of the hemp fibres using a combined alkaline-silane treatment resulted in a significant improvement in fracture toughness of H-SMC composites. Optimised H-SMC composites exhibited fracture toughness properties similar to those of G-SMC at fibre contents of 20?vol.-%, with K_Ic values of around 6?MPa.m^?1/2.     

Reactivity between rare‐earth oxides based thermal barrier coatings and a silicate melt

The reactivity between rare‐earth (RE‐) oxide stabilized ZrO₂ or HfO₂ thermal barrier coatings (TBCs) and a calcium‐magnesium‐aluminum‐silicate (CMAS) melt was studied at 1310°C. These reactions are representative of the ingestion of siliceous materials by the intake air of gas turbines (e.g., in aircraft engines) at high temperatures (>1200°C). These materials can melt and react with coated components in the hot section, resulting in premature failure. The goal of this work was to probe the effect of various RE (RE = Y, Yb, Dy, Gd, Nd, and Sm) oxides in the melt phase equilibrium and stability of the top‐coating system. Thermodynamic calculations of the phase assemblage of the (1?x) ZrO₂‐xY₂O₃ coating materials and CMAS melt are compared with the experimental findings. CMAS was found to penetrate the samples at the grain boundaries and dissolve the coating materials to form silicate phases containing the RE elements. Furthermore, apatite and garnet crystalline phases formed in the samples with total RE‐oxide content higher than 16 mol% in the reaction zone for the ZrO₂ system. In general, samples with nominal compositions ZrO₂‐9Dy₂O₃, HfO₂‐7Dy₂O₃, ZrO₂‐8Y₂O₃, HfO₂‐6Er₂O₃, ZrO₂‐9.5Y₂O₃‐2.25Gd₂O₃‐2.25Yb₂O₃, and ZrO₂‐30Y₂O₃ exhibited lower reactivity, or more resistance, to CMAS than the other coating compositions.     

Effects of yttrium on wettability and interactions between molten superalloy and SiO2-based ceramic core

In order to obtain high-quality superalloy castings, the wettability and interactions between superalloy melts with various Y contents and SiO₂-based ceramic cores were investigated at 1823 K. The results indicated that the wettability and interface reactions were affected by the content of Y in the alloy. For the alloys with Y content less than 0.011 wt%, no Y-oxide was found at the interface, but HfO₂, Al₂O₃ and ZrO₂ phases were formed, and the wetting angle dropped slightly. However, different Y-oxides precipitated at the alloy-ceramic interface for the alloys with Y content more than 0.017 wt%, and the wetting angle dropped sharply. When the content of Y was 0.017 and 0.025 wt%, Al₂O₃, Y₃Al₂(AlO₄)₃, HfO₂ and ZrO₂ phases were formed at the interface. When the content of Y was 0.1 wt%, YAlO₃, Y₃Al₅O₁₂, Y₄Al₂O₉, HfO₂ and ZrO₂ phases were formed. The formation of different reaction products was mainly caused by the change of Y activity (a_Y) in the alloy. The reaction between Y and SiO₂ could improve the wettability of the system apparently.     

The fracture toughness of inorganic glasses

Measuring the fracture toughness (K_Ic) of glasses still remains a difficult task, raising experimental and theoretical problems as well. The available methods to estimate K_Ic are reviewed, with emphasis on their respective advantages and drawbacks. In view of our current understanding, this analysis gives precedence to the SEPB method. The ultimate glass strength, the critical flaw size, and the indentation load for the onset of crack initiation are discussed, in the light of the fundamentals of fracture mechanics and classical background regarding the mechanics of brittle materials. Analytical expressions were further proposed to predict the fracture energy and fracture toughness of glasses from different chemical systems from their nominal compositions. The theoretical values were compared with the experimental ones, as obtained by self‐consistent methods when available. The agreement observed in most cases suggests that measured K_Ic values correspond to the crack propagation regime (as opposed to the crack initiation threshold), and supports previous investigations in glasses and ceramics, which showed that a crack tip is nearly atomically sharp in these materials (but for metallic glasses). Some ideas to design tougher glasses are finally presented.     

Exploration of the oxidation behavior and doping ratio of the Si–HfO2 bond layer used in environmental barrier coatings

Hafnia doping is expected to improve the performance of the silicon-bond layer of environmental barrier coatings (EBCs) for SiC-based ceramic matrix composites. The optimal doping ratio, distribution of HfO₂, and oxidation mechanism of the bond layer have not yet been fully addressed. A prototype Si–HfO₂ bond layer with a designed HfO₂-rich area was used to examine its oxidation behavior. A random dispersion model was developed to calculate the optimal HfO₂ doping ratio and its appropriate distribution state. The simulation results recommended that 20–30 vol% is the optimal doping ratio, where HfO₂ is well dispersed inside Si without forming networks. This enables HfO₂ to react with and consume SiO₂ without accelerating oxygen diffusion inside the bond layer. This was confirmed by oxidation experiments on Si–xHfO₂ tablets, in which the thinnest thermally grown oxide was achieved for the 20 vol% HfO₂-doped Si tablet. Both the microstructure design and material composition selection are highly important to further boost the performance of the EBCs.     

Synthesis of pure rod-like α-Si3N4 powder with in situ C/SBA-15 composite

Well crystallized pure rod-like α-Si₃N₄ powder (PRSN) without β-Si₃N₄ was successfully synthesized by carbothermal reduction-nitridation (CRN). In situ carbon/mesoporous silica composite (C/SBA-15) was used as a new kind of raw material. Due to in situ composited carbon, the CRN temperature was decreased and the phase transition from α to β-Si₃N₄ was hindered. The sintering temperature was lowered to 1380 °C and the soaking time of the optimal synthesis condition was reduced to 6 h. Moreover, the as-synthesized rod-like α-Si₃N₄, which is induced by SBA-15, was used to enhance the fracture toughness (K_Ic) of α-Si₃N₄ based ceramics, which was sintered by spark plasma sintering (SPS). Compared with the undoped ceramics (2.9 MPa m^1/2), α-Si₃N₄ ceramics doped with 10 vol% PRSN exhibited a higher K_Ic value (4.9 MPa m^1/2), and lower dielectric loss in MHz frequency range. The results demonstrated that the PRSN powder would be promising for toughening α-Si₃N₄ based ceramics.     

Relationships between fracture toughness,Y2O3 fraction and phases content in modern dental Yttria-doped zirconias

The relationship between fracture toughness and Yttria content in modern zirconia ceramics was revised. For that purpose, we evaluated here 10 modern Y₂O₃-stabilized zirconia (YSZ) materials currently used in biomedical applications, namely prosthetic and implant dentistry. The most relevant range between 2-5 mol% Y₂O₃ was addressed by selecting from conventional opaque 3 mol% YSZ up to more translucent compositions (4−5 mol% YSZs). A technical 2YSZ was used to extend the range of our evaluation. The bulk mol% Y₂O₃ concentration was measured by X-Ray Fluorescence Spectroscopy. Phase quantification by Rietveld refinement considered two tetragonal phases or an additional cubic phase. A first-account of the fracture toughness (K_Ic) of the pre-sintered blocks is given, which amounted to 0.4 – 0.7 MPa√m. In the fully-densified state, an inverse power-law behavior was obtained between K_Ic and bulk mol% Y₂O₃ content, whether using only our measurements or including literature data, challenging some established relationships. A linear relationship between K_Ic and the fraction of the transformable t-phase was established within the range of 30–70 vol%.     

Size driven thermodynamic crossovers in phase stability in zirconia and hafnia

Hafnia (HfO₂) and zirconia (ZrO₂) are of great interest in the quest for replacing silicon oxide in semiconductor field effect transistors because of their high permittivity. Both exhibit extensive polymorphism and understanding the energetics of their transitions is of major fundamental and practical importance. In this study, we present a systematic thermodynamic summary of the influence of particle size on thermodynamic phase stability in hafnia and zirconia using recently measured enthalpy data from the literature. The amorphous phase is found to be the most energetically stable above 165 and 363 m²/g of surface area for HfO₂ and ZrO₂, respectively. Below 16 and 20.3 m²/g of surface area, respectively, the monoclinic phase is the most energetically stable for HfO₂ and ZrO₂. At intermediate sizes there are closely balanced energetics among monoclinic, tetragonal, and cubic phases. The energy crossovers reflect decreasing surface enthalpy in the order monoclinic, tetragonal, cubic and amorphous for both hafnia and zirconia.     

Fracture toughness of glass fiber-reinforced acetal polymer

The critical stress field intensity factor for crack propagation, K_Ic, was determined for a large number of glass fiber-reinforced acetal copolymer compositions and for the unfilled resin. The results were interpreted in terms of a model previously proposed for the tensile behavior of these materials. The K_Ic could be regarded as a linear function of the contribution of the fiber reinforcement to the tensile strength, but was otherwise substantially independent of the amount and length of the fibers and the nature of the fiber finish. From this relationship it was estimated that the inherent flaw size of these materials was of the order of magnitude of the fiber length. The observed variation of K_Ic with loading rate was also consistent with the model. The notched Izod impact strength of these same materials was shown to be roughly equivalent to G_Ic, the critical strain energy release rate, or fracture energy per unit area, which can be computed from K_Ic by the methods of fracture mechanics. The behavior of these crack propagation parameters is consistent with the previous hypothesis that failure is initiated by loss of adhesion between the matrix and those fibers which lie transverse to the applied load.     

Effect of ZrB2 on promoting the phase transformation mechanism of Si3N4-based ceramics at low temperature

In this study, the mechanism of the effect of ZrB₂ on phase transformation of Si₃N₄ at a low temperature and the influence of its content on Si₃N₄-based ceramics were investigated. Previous study has shown that oxide impurities, i.e., B₂O₃ and ZrO₂ on ZrB₂ particles, alone cannot contribute to phase transformation of Si₃N₄ at a low temperature. But, the introduction of 0.5 vol% ZrB₂ into Si₃N₄ ceramics can promote the α-β phase transformation of Si₃N₄, which is confirmed to be the role of boron by comparison of the experimental results obtained from the addition of 0.5 vol% Zr and 0.5 vol% B. Increasing the ZrB₂ content from 0 vol% to 2.5 vol% intensifies the α-β phase transformation while decreasing the α phase content of Si₃N₄-based ceramics, accompanied by a slight grain growth, leading to a decrease in hardness. At the same time, aspect ratio and the quantities of elongated grains per square micron increase, and thus the fracture toughness increases significantly. However, when the content of ZrB₂ increases to 5 vol%, the Si₃N₄-based ceramics not only have a substantial decrease in hardness, but also the fracture toughness fails to be effectively improved due to high porosity and the decrease in aspect ratio and the quantity of elongated grains per square micron. The current study demonstrates that the dense Si₃N₄-based ceramics with high hardness and toughness (hardness ∼19.9 ± 0.2 GPa, toughness ∼6.27 ± 0.19 MPa m^1/2) can be prepared successfully at 1600 °C by introducing 0.5 vol% ZrB₂.     

Fabrication of structure-controlled hydroxyapatite/zirconia composite

The structure-controlled hydroxyapatite/zirconia (HAp/ZrO₂) composites were fabricated. At first, cylindrical hydroxyapatite (HAp) samples were prepared by the extrusion process, and then the extruded HAp cylindrical samples were coated with 3 mol% of Y₂O₃ partially stabilized ZrO₂ slurry, dried and aligned unidirectionally to form a composite bulk. The volume fraction of ZrO₂ in the HAp/ZrO₂ composite was estimated to be about 23 vol%. Bulk density and bending strength of the composites increased with sintering temperature. Fracture energy of HAp/ZrO₂ composite sintered at 1350 °C was approximately 1.6 times higher than that of monolithic HAp. Although the bending strength of HAp/ZrO₂ composite prepared in this study was relatively low, it exhibited high fracture energy than HAp monolithic and a non-brittle fracture behavior was obtained without using fiber as the reinforcement.     

The effect of amine/epoxy ratio on the fracture toughness of tetrafunctional epoxy resin

Summary. The effect of amine/epoxy ratio on the fracture toughness (K_Ic) of tetrafunctional epoxy resin was investigated. K_Ic value was measured by single-edge notch-bend test. The K_Ic value of the tetrafunctional epoxy resin increased with increasing the amount of amine curing agent. This result was explained with the structural viewpoint of the epoxy network. The network structure of the tetrafunctional epoxy was analyzed with dynamic thermomechanical measurement and in-situ near IR technique. Received: 19 June 1997/Accepted: 17 Juli 1997     

Increasing fracture toughness of zirconia-based composites as a synergistic effect of the introducing different inclusions

The effect of multi-walled carbon nanotubes (MWCNTs) and hexagonal boron nitride (h-BN) inclusions on the fracture toughness of yttria-stabilized zirconia (YSZ) ceramics has been studied. It is shown that an increase in the MWCNTs and h-BN content has a positive effect on the K_1C of zirconia ceramics. The greatest increase in the fracture toughness of YSZ ceramics was observed with the introduction of hexagonal boron nitride particles. For YSZ ceramics, the K_1C value was ≈6.1 MPa m^1/2, for ceramics with a 5 wt % of h-BN K_1C ≈ 9.2 MPa m^1/2. It was shown that an increase of the YSZ ceramics fracture toughness with the introduction of MWCNTs and h-BN, both and separately was provided by the combined action of several mechanisms of increasing the work of crack propagation. In addition, in all composites obtained in this work, the transformation of tetragonal ZrO₂ into monoclinic was observed.     

Synthesis and mechanical properties of (Mo0.94Nb0.06)(Si0.97Al0.03)2−x vol% SiC composites fabricated via SHS-HP

(Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂–SiC composites were consolidated via vacuum HP sintering from the mixture of (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂ powder and SiC nanoparticles. The (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂ powder was prepared by self propagating synthesis (SHS). The effects of SiC content on the ambient temperature mechanical properties and microstructure of the SHS-HP composites were investigated. The results show that the indentation fracture toughness, bending strength and Vickers hardness of (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂−x vol% SiC composites increase gradually with the increase of SiC content. The grain size and relative density of the composites decrease gradually with increase of SiC content. (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂−15 vol% SiC composite exhibits excellent mechanical properties: Vickers hardness 12.62 GPa, fracture toughness 5.01 MPa m^1/2, bending strength 472 MPa. With the addition of SiC, the fracture mode transforms from a mixed mode of transcrystalline and intercrystalline fracture to mainly transcrystalline fracture.