Design of Si<Subscript>3</Subscript>N<Subscript>4</Subscript>-based ceramic laminates by the residual stresses

Ceramic laminates with strong interfaces between layers are considered a very promising material for different engineering applications because of the potential for increasing fracture toughness by designing high residual compressive and low residual tensile stresses in separate layers. In this work, Si₃N₄/Si₃N₄-TiN ceramic laminates with strong interfaces were manufactured by rolling and hot pressing techniques. The investigation of their mechanical properties has shown that the increase in apparent fracture toughness can be achieved for the Si₃N₄/Si₃N₄-20 wt.%TiN composite, while further increase of TiN content in the layers with residual tensile stresses lead to a formation of multiple cracks, and as a result, a significant decrease in the mechanical performance of the composites. Micro-Raman spectroscopy was used to measure the frequency shift across the Si₃N₄/Si₃N₄-20 wt.%TiN laminate. These preliminary Raman results can be useful for further analysis of residual stress distribution in the laminate.     

Surface strengthening and toughening of Si3N4/TiC layered composite by slip casting

Layered composite using monolithic Si₃N₄ as outer layers and Si₃N₄-15v/o TiC as inner core was fabricated by slip casting and pressureless sintering. As the composite is cooled from high sintering temperature, the difference in coefficients of thermal expansion between the constrained inner core and outer layer is expected to establish a compressive surface stress. The existence of this residual stress was verified by theoretical analysis and Vicker's indentation for the samples with various outer layer thickness. The layered composites exhibited greater strength, apparent fracture toughness and damage resistance due to the presence of compressive surface stresses in the layer.     

Reaction Bonding of Three-Layer Alumina-Based Composites

Three-layer alumina-based composites with Al₂O₃-containing mullite as the outer layer and ZrO₂-containing alumina as the inner layer were fabricated by die-pressing and reaction bonding. The effects of the outer-layer compositions on the densification behavior of three-layer composites were investigated. The existence of residual stresses in the layers was verified using indentation methods. Compared with single-layer ceramics, three-layer composites exhibit an improved fracture toughness and an excellent damage resistance due to the presence of the compressive stresses in the outer layers.     

Estimation of apparent fracture toughness of ceramic laminates

The paper presented deals with the fracture behaviour of ceramic laminates. The residual stresses in individual layers of Al₂O₃/5vol.%t-ZrO₂ (ATZ) and Al₂O₃/30vol.%m-ZrO₂ (AMZ) are determined. Assumptions concerning linear elastic fracture mechanics and small scale yielding are considered. In this frame the procedure based on a generalization of Sih’s strain energy density factor to the case of a crack touching the interfaces between two dissimilar materials is used for determination of effective values of the stress intensity factor on material interfaces. An important increase of fracture toughness at the AMZ/ATZ interface was predicted in comparison to the fracture toughness of individual material components. Predicted values were compared with data available in the literature and mutual good agreement was found. The procedure suggested can be used for estimation of resistance to crack propagation through multilayered structures and its design. The procedure can contribute to enhancing the reliability and safety of structural ceramics or, more generally, of layered composites with strong interfaces.     

化学气相浸渗2D Cf/(SiC-C)复合材料的微观结构与强韧性

采用等温等压化学气相浸渗法(ICVI)制备了二维碳纤维增韧碳化硅碳二元基复合材料(2D C_f/(SiC-C)).利用扫描电镜(SEM)和背散射电子成像(BSE)研究了其基体的微观结构, 并与二维碳纤维增韧碳化硅陶瓷基复合材料(2D C_f/SiC)比较了室温力学性能和断口形貌.结果表明：2D C_f/(SiC-C)复合材料的基体是由SiC与热解碳(PyC)组成的多层结构, PyC基体层分布均匀而连续, 且与SiC基体层结合紧密.纤维束内部PyC基体层较厚的2D C_f/(SiC-C)复合材料具有较高的强韧性, 其拉伸强度、断裂应变、断裂韧性和断裂功分别比2D C_f/SiC复合材料的提高了3%、142%、22%和58%.SiC与PyC组成的多层基体使2D C_f/(SiC-C)复合材料的纤维在拔出过程中发生了两次集中拔出, 且第一次集中拔出的纤维对复合材料的强韧性起主要作用.     

Residual stresses in particle-reinforced ceramic composites using synchrotron radiation

Residual stresses were determined in particle-reinforced ceramic composites using synchrotron based x-ray diffraction. The baseline Si₃N₄ and the Si₃N₄-TiN composites were processed by turbomilling, pressure casting, and isopressing. They were then continuously sintered to full density, under a pressureless, flowing nitrogen atmosphere. The flexural strength, fracture toughness, and residual stress were measured for as-machined samples and following quenching in water from 1000°C, 1100°C, and 1200°C. The residual stresses for both the baseline Si₃N₄ and the Si₃N₄-TiN composites were determined from the (441) and (531) reflections, by applying the 2-sin² method. The measured residual stresses were compared with the flexural strength and fracture toughness results to determine the effects of residual stress and thermal shocking on the mechanical properties of each material. In both the baseline Si₃N₄ and Si₃N₄-TiN composites, after thermal shocking, the compressive residual stresses were developed in directions both parallel and perpendicular to the sample surface. The residual compressive stresses for the Si₃N₄-TiN composites were much higher than the baseline Si₃N₄. As a result, both fracture toughness and flexural strength of the Si₃N₄-TiN composites were improved. In addition, the addition of the TiN appears to improve both the strength and toughness of the baseline Si₃N₄.     

Intrinsic stress effect on fracture toughness of plasma enhanced chemical vapor deposited SiNx:H films

The apparent fracture toughness for a series of plasma enhanced chemical vapor deposition SiN_x:H films with intrinsic film stress ranging from 300 MPa tensile to 1 GPa compressive was measured using nanoindentation. The nanoindentation results show the measured fracture toughness for these films can vary from as high as > 8 MPa⋅√m for films in compression to as low as < 0.5 MPa⋅√m for the films in tension. Other film properties such as density, Young's modulus, and hydrogen content were also measured and not observed to correlate as strongly with the measured fracture toughness values. Various theoretical corrections proposed to account for the presence of intrinsic or residual stresses in nanoindent fracture toughness measurements were evaluated and found to severely underestimate the impact of intrinsic stresses at thicknesses ≤ 3 μm. However, regression analysis indicated a simple linear correlation between the apparent fracture toughness and intrinsic film stress. Based on this linear trend, a stress free/intrinsic fracture toughness of 1.8 ± 0.7 MPa⋅√m was determined for the SiN_x:H films.     

Damage evolution of angle-ply SCS-6/Ti composites under static and fatigue loading

The effect of fibre orientation and laminate stacking sequence on the tensile and fatigue behaviour of SCS-6/Ti 15-3 composites were investigated. The laminates used in this study were: (90)₆, (0/ ± 45)_s, (0/90)_s, and (90/ +-45)_s. The initiation and progression of microstructural damage at various stress levels was thoroughly characterized. It was found that fatigue life at high applied stresses were controlled by fibre fracture; progressive damage involving fibre fracture, interfacial debonding and matrix cracking became dominant at low applied stresses. Observation of the damage mechanisms in the angle-ply laminates under cyclic loading suggests that increasing the fibre-matrix bonding strength may improve the load carrying capability and fatigue life of laminates containing off-axis plies.     

Effects of rolling and hot pressing on mechanical properties of boron carbide-based ceramics

A study of hot pressed B₄C-based laminates, after rolling and without rolling, has been performed to elucidate the existence of fracture resistance/crack length anisotropy induced by this processing technique. While the crack lengths/fracture resistance was affected significantly by the presence of the residual stresses in B₄C/B₄C–ZrB₂ laminates, no differences in Vickers crack lengths were observed in B₄C/B₄C laminates prepared by rolling and hot pressing, as compared to the crack lengths seen in pure B₄C ceramics prepared by hot pressing without rolling. X-ray diffraction analysis confirmed that no texture has been formed during the rolling and hot pressing of B₄C ceramics.     

Microstructure and mechanical properties of a three-layer B4C/Al–B4C/TiB2–B4C composite

A three-layer structure material, consisting of B₄C/Al, B₄C/TiB₂ and B₄C composites, was obtained using a two-step method for both hot pressing and aluminum infiltration in vacuum. The three-layer B₄C/Al–B₄C/TiB₂–B₄C composite showed good interfacial bonding. Before aluminum infiltration the B₄C porous layer in the three-layer preform looked like a three-dimensional network of interconnected capillaries. The microstructures of both B₄C/TiB₂ and B₄C layers showed no apparent changes before and/or after aluminum infiltration. The three-layer composite showed improved fracture toughness than that of B₄C material and higher comprehensive hardness than that of B₄C/Al material.     

Mechanical property and R-curve behavior of the B4C/BN ceramics composites

The B₄C/BN ceramics composites were fabricated by the hot-pressing process. In this paper, the mechanical property and R-curves behavior of the B₄C/BN composites were investigated. The fracture strength and fracture toughness of the B₄C/BN microcomposites and the B₄C/BN nanocomposites decreased gradually with the increasing content of h-BN. The fracture strength and fracture toughness of the B₄C/BN nanocomposites were significantly improved in comparison with the B₄C/BN microcomposites. The damage resistance and R-curves behavior of the B₄C monolith and the B₄C/BN composites were evaluated by the indentation-strength in bending technique (ISB). The fracture strength of the B₄C monolith, the B₄C/BN microcomposites and the B₄C/BN nanocomposites decreased gradually with the increase of the indentation load. The B₄C/BN nanocomposites retained relative higher fracture strength in comparison with the B₄C monolith and the B₄C/BN microcomposites under the equivalent indentation load. The B₄C monolith, the B₄C/BN microcomposites and the B₄C/BN nanocomposites all exhibited the rising R-curves behavior. The B₄C/BN nanocomposites exhibited the higher rising R-curve behavior than that of the B₄C monolith and the B₄C/BN microcomposites. The toughness mechanisms of the composites were investigated. The B₄C/BN composites with the h-BN content more than 20 wt.% exhibited excellent machinability. The slowly rising R-curves behavior remarkably improved the machinability of the composites.     

Microstructures and fracture toughness of Ti–(43–48)Al–2Cr–2Nb prepared by electromagnetic cold crucible directional solidification

In the present paper, the microstructures of directionally solidified Ti–(43–48)Al–2Cr–2Nb alloys prepared by electromagnetic cold crucible directional solidification technique were studied in detail. The results show that with the decrease of Al content, the interlamellar spacing of α₂/γ decreases, but the volume fraction of B₂ phase increases. In addition, the fracture toughness of these alloys with different lamellar orientation was investigated by single edge notched beam three-point bending test (SENB). It is revealed that B₂ phase has greatly impacted on the fracture toughness. When the laminates are perpendicular to the loading stress, the increasing of B₂ phase leads to a lower fracture toughness value. On the contrary, B₂ phase becomes barriers for crack propagations and increases fracture toughness value when the laminates are parallel to the loading stress.     

Preparation of B<Subscript>4</Subscript>C-ZrB<Subscript>2</Subscript>-SiC eutectic ceramics by arc melting method

The B₄C-ZrB₂-SiC ternary composites with super hard and high toughness were obtained by arc melting in argon atmosphere. Microstructures were observed by SEM, and phase compositions were analyzed by XRD. The hardness and fracture toughness of ternary composites are 28 GPa and 4.5 MPa·m^1/2. The eutectic mole composition is 0.39B₄C-0.25ZrB₂-0.36SiC, and the eutectic lamellar microstructure is composed of B₄C matrix with the lamellar ZrB₂ and SiC grains.     

Preparation of B4C-ZrB2-SiC eutectic ceramics by arc melting method

The B₄C-ZrB₂-SiC ternary composites with super hard and high toughness were obtained by arc melting in argon atmosphere. Microstructures were observed by SEM, and phase compositions were analyzed by XRD. The hardness and fracture toughness of ternary composites are 28 GPa and 4.5 MPa·m^1/2. The eutectic mole composition is 0.39B₄C-0.25ZrB₂-0.36SiC, and the eutectic lamellar microstructure is composed of B₄C matrix with the lamellar ZrB₂ and SiC grains.     

Preparation and tensile properties of amorphous Fe78Si9B13/nano-Ni laminated composite

Amorphous Fe₇₈Si₉B₁₃/nano-Ni laminated composite was prepared by electrodeposition method. The tensile properties of laminated composite at room temperature were examined. The laminated composite exhibits a very high tensile strength and reasonable tensile elongation. This is attributed to a good bonding between amorphous Fe₇₈Si₉B₁₃ layer and nano-Ni layers. The amorphous layer can deform in conformity with Ni layers and be significantly stretched without fracture. The apparent surface energy γ_f of amorphous Fe₇₈Si₉B₁₃ ribbon in the laminated composite is 7 times larger than γ_f of amorphous ribbon in monolithic form. The isostrain model may be insufficient to explain the tensile behavior of the laminated composite.     

Improvements in mechanical properties of TiB2 by the dispersion of B4C particles

Densities up to 99% of the theoretical value were achieved by hot-pressing of TiB₂-B₄C composites at 1700° C for 1 h using 1 vol % Fe as a sintering aid. The microstructure consists of dispersed B₄C particles in a fine-grained TiB₂ matrix. Addition of B₄C particles increases the fracture toughness of TiB₂ (to 7.6 MPa m^1/2 at 20 vol % B₄C) and yields high fracture strength (to 700 MPa at 10 vol % B₄C). Microstructural observations indicate that the improved strength is a result of a higher density, smaller grain size and intergranular fracture, and the toughness increase is a result of crack deflection around the B₄C particles.     

Microstructure and mechanical behavior of AA6082-T6/SiC/B4C-based aluminum hybrid composites

The microstructural and mechanical behavior of hybrid metal matrix composite based on aluminum alloy 6082-T6 reinforced with silicon carbide (SiC) and boron carbide (B₄C) particles was investigated. For this purpose, the hybrid composites were fabricated using conventional stir casting process by varying weight percentages of 5, 10, 15, and 20?wt% of (SiC?+?B₄C) mixture. Dispersion of the reinforced particles was studied with x-ray diffraction and scanning electron microscopy analyses. Mechanical properties such as micro-hardness, impact strength, ultimate tensile strength, percentage elongation, density, and porosity were investigated on hybrid composites at room temperature. The results revealed that the increase in weight percentage of (SiC?+?B₄C) mixture gives superior hardness and tensile strength with slight decrease in percentage elongation. However, some reduction in both hardness and tensile strength was observed in hybrid composites with 20?wt% of (SiC?+?B₄C) mixture. As compared to the un-reinforced alloy, the improvement in hardness and tensile strength for hybrid composites was found to be 10% and 21%, respectively. Reduction in impact strength and density with increase in porosity was also reported with the addition of reinforcement.     

Response of aluminium-infiltrated boron carbide cermets to shock wave loading

Shock-recovery and shock-spallation experiments were performed on two compositions of aluminium-infiltrated B₄C cermets as a function of shock pressure. Sixty-five per cent volume B₄C-Al cermets were recovered largely intact after shock loading up to pressures of ca. 12 GPa which permitted a critical study of the microstructural changes produced by the shock. Significantly, shock loading to between 12 and 13 GPa produced a combination of dislocation debris, stacking faults and deformation twins in a small fraction of the B₄C grains. Fragmentation of shock-loaded 80% B₄C-Al samples prevented meaningful microstructural investigation. Spall-strength testing also provided indirect evidence for the Hugoniot elastic limits (HEL) of these composites. Spall-strength calculations based on an elastic equation of state for 65% B₄C-Al indicated that the elastic regime extended up to shock pressures of ca. 10 GPa, or approximately 65% of the HEL of polycrystalline B₄C. A complete loss of spall strength was then observed at the transition to a plastic equation of state at a pressure of 12 GPa which coincided with observations of plasticity within the B₄C-substructure. This study demonstrated that composites containing a highly ductile phase combined with a high compressive strength ceramic phase could support high dynamic tensile stresses by resisting the propagation of catastrophic cracks through the brittle ceramic substructure.     

Preparation and characterization of aluminium alloy sheet Aramid fibre-laminated composites

Laminated composites consisting of alternate layers of aluminium alloy sheets and unidirectional Kevlar-49 fibre epoxy composites were prepared using two different aluminium alloys DTD 687 and aluminium-lithium alloy. Tensile, compressive and interlaminar shear strengths of the laminates were measured. The residual stresses in the aluminium alloy sheets arising out of thermal mismatch between aluminium alloys and aramid fibres were also measured. It is found that the laminates have lower density, higher tensile strength and marginally lower Young’s modulus as compared with monolithic alloy sheets.