The role of residual stresses in layered composites of Y–ZrO2 and Al2O3

Laminar composites, containing layers of Y–TZP and either Al₂O₃ or a mixture of Al₂O₃ and Y–ZrO₂ have been fabricated using a sequential centrifuging technique of water solutions containing suspended particles. Controlled crack growth experiments with notched beams of composites were done and showed the significant effect of barrier layer thickness and composition on crack propagation path during fracture. Distinct crack deflection in alumina layers was observed. The increase of crack deflection angle with the alumina layer thickness was also found. In the case of the barrier layer made of a mixture, crack deflection did not occur independently on layer thickness. The observed changes have been correlated with the radial distribution of residual stresses in barrier layers created during cooling of sintered composites from fabrication temperature. The stresses found were the result of the difference in the thermal expansion and sintering shrinkage of alumina and zirconia and the crystallographically anisotropic thermal expansion of the alumina. The residual stress distribution has been measured by piezo-spectroscopy based on the optical fluorescence of Cr⁺³ dopants in alumina.     

Characterisation of mechanical and fracture behaviour of Al2O3/ZrO2/BaTiO3 laminate by indentation

The paper describes the preparation of laminate piezo-ceramic composite consisting of Al₂O₃, ZrO₂ and BaTiO₃ layers and proves the idea of residual stresses utilization for crack deflection and handling with the brittleness of BaTiO₃. The laminate was prepared by alternate electrophoretic deposition. Although the laminate was sintered at 1300 °C and consisted of layers having a density between 57 % (ZrO₂) and 73 % (BaTiO₃), the hardness and elastic modulus of layers corresponded to those of free sintered monolithic ceramics at a comparable level of porosity. The crack deflection at the interface between individual layers was observed having the same effect and magnitude as deflection observed in the case of fully dense Al₂O₃/ZrO₂ laminates. An interlayer developed on the interface between Al₂O₃ and BaTiO₃ had no negative impact on crack propagation.     

Microstructure and mechanical behavior of TiO2-MnO-doped alumina/alumina laminates

Tapes of TiO₂-MnO-doped alumina (d-Al₂O₃) and pure alumina (Al₂O₃) were shaped via tape casting. Laminates with three different layer numbers and respective thicknesses were produced and sintered at 1200°C. The microstructure and mechanical behavior of laminates were investigated and compared to the respective monolithic references (d-Al₂O₃ and Al₂O₃). The use of dopants in alumina decreased the initial sintering temperature, leading to higher densification at 1200°C (~98% theoretical density (TD)) when compared to Al₂O₃ (~73% TD). The higher density was reflected in a higher Young's modulus and hardness for doped alumina. A region of diffusion of dopants in pure alumina layers was observed along the interface with doped layers. The mechanical strength of d-Al₂O₃ samples sintered at 1200°C was not statistically different from Al₂O₃ samples sintered at 1350°C. The strength of laminates composed of doped layers with undoped, porous interlayers did not change. Nevertheless, as the thickness of these porous interlayers increases, a loss of strength was observed. Monolithic references showed constant values of fracture toughness (K_IC), ~2 MPa·m^1/2, and linear crack path. On the other hand, K_IC of laminates increases when the crack propagates from weak Al₂O₃ layers to dense d-Al₂O₃ layers.     

Design and processing of Al2O3–Al2TiO5 layered structures

Al₂O₃–Al₂TiO₅ layered composites were manufactured by a colloidal route from aqueous Al₂O₃ and TiO₂ suspensions with 50 vol.% solids. The mechanical behaviours of individual monolithic composite materials were combined and taken as basis for the design of the layered structures. Residual stresses which are likely to occur due to processing and thermally introduced misfits were calculated and considered for the manufacture of the laminates.Monoliths with 10, 30 and 40 vol.% of second phase showed that increasing proportions of aluminium titanate decrease strength and increase the non-linear behaviour.In order to obtain the desired combination of mechanical behaviours of the layers, two laminate designs with external and central layers of one composition and the alternating internal layer of the other composition were chosen taking into account chemical compatibility and development of residual stresses. In the system AA10, external and central layers of monophase Al₂O₃ with high strength were combined with intermediate layers of Al₂O₃ with 10 vol.% of Al₂TiO₅. The system A10A40 was selected to combine low strength and energy absorbing intermediate layers of Al₂O₃ with 40 vol.% of Al₂TiO₅ and sufficient strength provided by external layers of Al₂O₃ with 10 vol.% of Al₂TiO₅.The stress–strain behaviour of the laminates was linear up to their failure stresses, with apparent strain for zero load after fracture larger than that corresponding to the monoliths of the same composition as that of the external layers. Moreover, the stress drop of the laminate samples occurred in step-like form thus suggesting the occurrence of additional energy consuming processes during fracture.     

Crack propagation and residual stress in laminated Si3N4/BN composite structures

The level of residual stress and crack propagation in a new generation of laminates, based on silicon nitride (Si₃N₄) layer and a mixture of boron nitride (BN) and alumina (Al₂O₃) interlayer, was presented. The structure consists of alternated concentric rings of Si₃N₄ separated by the weak BN interlayer possessing no planes of easy crack propagation and fracture resistance much larger than that of any classical planar laminates. The results on direction of crack propagation and residual stress in relation to inter-layer composition, the number of layers, and their thickness are investigated and reported. The effect of residual stress on crack propagation was studied by using Vicksrs intentation. The highest compressive residual stress of ∼170 MPa was found in samples with five layers possessing an average layer thickness of ∼310 × 10⁻⁶ m.     

Lateral cracks during sliding indentation on various optical materials

A series of static and sliding indentation (ie, scratching) was performed and characterized on a wide range of optical workpiece materials [single crystals of Al₂O₃ (sapphire), SiC, Y₃Al₅O₁₂ (YAG), CaF₂, and LiB₃O₅ (LBO); a SiO₂–Al₂O₃–P₂O₅–Li₂O glass ceramic (Zerodur); and glasses of SiO₂:TiO₂ (ULE), SiO₂ (fused silica), and P₂O₅–Al₂O₃–K₂O–BaO (Phosphate)] at various applied loads using various indenters (Vickers, 10 µm conical, and 200 µm conical). Despite having different load dependencies, the lateral crack depth formed during sliding indentation quantitatively scales with that formed during static indentation, explaining why static indentation has been historically effective in describing various grinding parameters. Depending on the indenter geometry, the amount of residual trench damage (plastic deformation and local fracturing) during sliding indentation was often enhanced by more than an order of magnitude compared with static indentation. A simple ploughing scratch model, which considers both tangential and normal stresses (where the tangential stress is amplified by relatively small tangential contact area), explains this enhancement and other observed trends. Accounting for the high correlation between residual trench depth and volumetric fracturing, the model is extended to estimate the amount of fracture damage as a function of the material properties of the workpiece, indenter geometry, and applied load. Such a model has utility in the design of optimized grinding processes, particularly the abrasive geometry. Finally, at higher loads (>1 N), lateral cracks were often observed to preferentially propagate in the forward scratching direction, as opposed to perpendicular to the scratch as typically observed. High-speed imaging of the scratch process confirms that these cracks propagate ahead of the sliding indenter during the scratching event. Finite element stress analysis suggests the ploughing frictional forces increase the mode I tensile stresses at the leading edge of the sliding indenter explaining the direction of crack propagation of such cracks.     

Fracture behavior of Al2O3–SiO2 castables by the wedge splitting test and digital image correlation technique

The fracture mechanisms are helpful for the optimization and design of toughness and microstructure of refractories. Fracture behavior of ultra-low cement bonded Al₂O₃–SiO₂ castables was researched using the wedge splitting test coupled with digital image correlation technique (WST-DIC). Flexibility of Al₂O₃–SiO₂ castables is improved by introducing andalusite aggregates into the castables. The characteristic length L_CH, a parameter used to assess flexiblity of materials, was observed to reach 287.2 mm in andalusite-containing Al₂O₃–SiO₂ castables, more than 5 times that of reference castables. Microcracks toughening is the main toughening mechanisms for flexibility improvement of the Al₂O₃–SiO₂ castables containing andalusite. Microcrack network in the Al₂O₃–SiO₂ castables could be designed by exploiting the volume expansion caused by mullitization of andalusite and the coefficient of thermal expansion (CTE) mismatch between the andalusite aggregate and the matrix. Unlike andalusite-free castables, castables containing andalusite possess a distinct fracture process zone (FPZ), the crack branching and deflection can be observed around the main crack during the fracture process, which leads to the prolong of the crack propagation path, the increase of the dissipation energy during the fracture, and the enhancement of resistance to crack propagation.     

Structural design and energy absorption mechanism of laminated SiC/BN ceramics

The laminated silicon carbide/boron nitride (SiC/BN) ceramics with different structural designs were fabricated by pressureless sintering at 1900?°C for 1?h in argon flow. The alumina (Al₂O₃)-and yttrium(III) oxide (Y₂O₃)-doped SiC ceramic exhibited a significant intergranular fracture behavior, which could be attributed to the yttrium aluminum garnet (YAG) phase located at the grains boundaries. The bending strength and fracture toughness were used to characterize the crack propagation including the delamination cracking, crack kinking, and crack deflection. The energy absorption in the process of crack propagation was characterized by the work of fracture (WOF) and damping capacity. The mode of crack propagation changed with the change in the structure and variation of BN content in the BN layer. The delamination cracks occurred inside the BN layer or at the interface between SiC and BN layers. The sample with a gradient structure exhibited the combination of delamination cracks occurring at the interface and inside the BN layer, which showed the maximum WOF of 2.43?KJ?m^?2, bending strength of 300?MPa, and fracture toughness of 8.5?MPa?m^1/2. The damping capacity varied with the change of the structure and the amplitude. The sample with a gradient structure exhibited the damping capacity of 0.088 and the maximum loss modulus of 9.758?GPa.     

Mechanical properties of graphene platelet-reinforced alumina ceramic composites

Alumina (Al₂O₃) ceramic composites reinforced with graphene platelets (GPLs) were prepared using Spark Plasma Sintering. The effects of GPLs on the microstructure and mechanical properties of the Al₂O₃ based ceramic composites were investigated. The results show that GPLs are well dispersed in the ceramic matrix. However, overlapping of GPLs and porosity within ceramics are observed. The flexural strength and fracture toughness of the GPL-reinforced Al₂O₃ ceramic composites are significantly higher than that of monolithic Al₂O₃ samples. A 30.75% increase in flexural strength and a 27.20% increase in fracture toughness for the Al₂O₃ceramic composites have been achieved by adding GPLs. The toughening mechanisms, such as pull-out and crack deflection induced by GPLs are observed and discussed.     

TiN/Al2O3 composites and graded laminates thereof consolidated by spark plasma sintering

A rapid consolidation process named Spark Plasma Sintering (SPS) has been applied to compact duplex TiN/Al₂O₃ composites and graded laminates thereof. Fully or nearly fully compacted samples were prepared at 1500 °C with a holding time of 3 min under a pressure of 75 MPa. The unusually high grain-growth rate of Al₂O₃ occurring in monolithic Al₂O₃ at this sintering temperature is prevented by the addition of TiN particles. Crack-free graded laminates with distinct interfaces between layers were prepared by loading samples inside the die, both symmetrically and asymmetrically. The observed mechanical properties, e.g. hardness and fracture toughness, are related to the microstructural features of the compacted samples.     

Toughening of Layered Ceramic Composites with Residual Surface Compression

Effects of macroscopic residual stresses on fracture toughness of multilayered ceramic laminates were studied analytically and experimentally. Stress intensities for edge cracks in three-layer, single-edge-notch-bend (SENB) specimens with stepwise varying residual stresses in the absence of the crack and superimposed bending were calculated as a function of the crack length by the method of weight function. The selected weight function and the method of calculation were validated by calculating stress intensities for edge cracks in SENB specimens without the residual stresses and obtaining agreement with the stress-intensity equation recommended in ASTM Standard E-399. The stress-intensity calculations for the three-layer laminates with the macroscopic residual stresses were used to define an apparent fracture toughness. The theoretical predictions of the apparent fracture toughness were verified by experiments on three-layer SENB specimens of polycrystalline alumina with 15 vol% of unstabilized zirconia dispersed in the outer layers and 15 vol% of fully stabilized zirconia dispersed in the inner layer. A residual compression of ∼400 MPa developed in the outer layers by the constrained transformation of the unstabilized zirconia from the tetragonal to the monoclinic phase enhanced the apparent fracture toughness to values of 30 MPa^.m^1/2 in a system where the intrinsic fracture toughness was only 5 to 7 MPa^.m^1/2.     

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.     

Influence of residual and bridging stresses on the R-curve behavior of Mo- and FeAl-toughened alumina

Alumina matrix was toughened using either metal molybdenum or intermetallic FeAl particles. Mo and FeAl dispersoids were chosen because they have different thermomechanical properties (i.e. Young's modulus, Poisson ratio, as well as thermal expansion coefficient), giving rise to different residual stresses in the matrix. The R-curve behavior of these composites was first studied by stable-crack propagation experiments as a function of the volume fraction of dispersoid. The optimum fraction for toughening was different in the two composites: 25 and 15 vol% addition led to maximum toughness in the Mo- and FeAl added composite, respectively. This difference was ascribed to residual stresses. Microscopic observation of the crack path revealed, in both composites, the systematic presence of dispersoids acting as bridging sites in the crack wake, but only a few of them were plastically stretched. Residual stresses in the Al₂O₃ matrix, after sintering and microscopic bridging tractions during crack propagation, were quantitatively assessed using microprobe fluorescence spectroscopy. Bridging microstresses were assessed in situ by a linear map along the crack profile, at the critical condition for fracture propagation. Experimentally collected residual stresses and bridging stresses were discussed to explain the different fracture behavior of the composites.     

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.     

Characterization of residual compressive stresses in layered ceramics by positron annihilation spectroscopy

Microstructural characterization of stress states of alumina layers subjected to residual compressive stresses was performed by positron annihilation spectroscopy (PAS). Al₂O₃–t-ZrO₂ three layered samples were appropriately designed and processed by sequential slip casting with symmetric structure. The zirconia contents inside the core layer were selected in order to generate compressive stresses of variable intensity in the outer Al₂O₃ layers. PAS results highlighted a correlation between positron annihilation parameters and microstructural changes related to the generation of residual stresses.     

Alumina Toughened Zirconia Nanocomposite Incorporating Al2O3 Whiskers

We investigated the Vickers hardness and fracture toughness of an Al₂O_3(n) + 70 wt% ZrO₂ (TZ‐3Y)_n nanocomposite with addition of 2.5 wt% Al₂O₃ whiskers. Densities greater than 95% were reached after conventional sintering at 1500°C. The fracture toughness was increased 62% over pure Al₂O₃. Microcracking and crack deflection can be the mechanisms responsible to improve the fracture toughness. The use of ATZ composites with a low percent of whiskers can be a promising biomedical material for medical and dental applications given its large increase in fracture toughness over pure alumina and the observed relief from aging issues of zirconia.     

Crack propagation and stress distribution in binary and ternary directionally solidified eutectic ceramics

The development of directionally solidified eutectic (DSE) ceramics for gas turbine applications necessitates improving their strength and toughness. The early stage of crack propagation is investigated in either binary (Al₂O₃/Y₃Al₅O₁₂, Al₂O₃/GdAlO₃ and Al₂O₃/Er₃Al₅O₁₂) or ternary (Al₂O₃/Y₃Al₅O₁₂/ZrO₂, Al₂O₃/GdAlO₃/ZrO₂ and Al₂O₃/Er₃Al₅O₁₂/ZrO₂) DSE ceramics. Post-mortem scanning electron microscopy (SEM) examination of biaxial flexure induced cracks revealed crack deflection and branching in the various phases and in the phase boundaries. These observations are correlated to analytical and finite element (FE) internal stress calculations, FE determination of the axial shear stress component in the interfaces in the vicinity of the specimen surface (free-edge effect) and FE calculations of the stress distribution resulting from an applied loading. Results from ruby (Cr³⁺) fluorescence piezo-spectroscopy measurements are analyzed, taking into account the hydrostatic and plane stress hypotheses. Moreover, transmission electron microscopy (TEM) examinations have confirmed the role of interfaces in the crack nucleation and propagation modes.     

Fabrication and mechanical properties of Al2O3-SiCw-TiCnp ceramic tool material

Whiskers and nanoparticles are usually used as reinforcing additives of ceramic composite materials due to the synergistically toughening and strengthening mechanisms. In this paper, the effects of TiC nanoparticle content, particle size and preparation process on the mechanical properties of hot pressed Al₂O₃-SiC_w ceramic tool materials were investigated. The results showed that the Vickers hardness and fracture toughness of the materials increased with the increasing of TiC content. The optimized flexural strength was obtained with TiC content of 4 vol% and particle size of 40 nm. The particle size has been found to have a great influence on flexural strength and small influence on hardness and fracture toughness. It was concluded that the flexural strength increased remarkably with the decreasing of the TiC particle size, which was resulted from the improved density and refined grain size of the composite material due to the dispersion of the smaller TiC particle size. SEM micrographs of fracture surface showed the whiskers to be mainly distributed along the direction perpendicular to the hot-pressing direction. The fracture toughness was improved by whisker crack bridging, crack deflection and whisker pullout; the TiC nanoparticles in Al₂O₃ grains caused transgranular fracture and crack deflection, which improved the flexural strength and fracture toughness with whiskers synergistically. Uniaxial hot-pressing of SiC whisker reinforced Al₂O₃ ceramic composites resulted in the anisotropy of whiskers’ distribution, which led to crack propagation differences between lateral crack and radical crack.     

The effects of microstructure on mechanical and electrical properties of W dispersed Al2O3 ceramics

This work aims to enhance the fracture toughness of brittle Al₂O₃ ceramics and apply insulated Al₂O₃ ceramics with electrical conductivity by dispersing second tungsten (W) metal particles. In order to investigate the effects of W dispersion on mechanical and electrical properties, Al₂O₃–W composites with various amounts of W (ranging from 5 vol% to 20 vol%) were fabricated by the hot-press sintering method at various sintering temperatures. Microstructure analysis revealed submicron Al₂O₃ matrix grains and W particles. The existence of three phases of Al₂O₃, W, and AlWO₄ was confirmed by X-ray diffraction patterns. All Al₂O₃–W composites showed higher fracture toughness than monolithic Al₂O₃. The toughening mechanism was attributed to crack deflection and crack bridging. Transgranular fracture was visible in all composites. Electrical resistivity dramatically lowered from 2.9 × 10¹² Ω cm of monolithic Al₂O₃ to 4.1 × 10² Ω cm of the composite with 20 vol% W addition. The percolation threshold is calculated as 18.5%. With the increase in sintering temperature, the amount of W particles was decreased and Al₂O₃ grains became large, leading to the reduced number of conductive pathways formed by the dispersed W particles. As a result, electrical conductivity was decreased.     

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.