可加工陶瓷的弱界面结构特征及其研究进展

评述了具有弱界面结构的可加工陶瓷的研究进展.弱界面通过层状结构相或两相间的弱结合引入材料中,在材料加工过程赋予材料一定的“塑性”,或使裂纹产生偏转、分支和桥联,改变陶瓷的断裂行为和加工去除形式.分析了陶瓷可加工性与弱界面结构设计的关系,指出了显微结构优化设计与制备高性能可加工陶瓷的途径.     

一种新型可加工陶瓷材料: CeZrO2/CePO4

对可加工Ce-ZrO2／CePO4陶瓷材料的设计制备和性能进行了研究。结果表明：在载荷作用下弱界面处易形成微裂纹，并发生裂纹联接，使材料在加工过程中以晶粒去除形式为主，实现了该材料的金属刀具加工，且加工损伤变小。弱界面处微裂纹的形成、偏转和桥联等形式，耗散了主裂纹扩展的能量，增加了材料的断裂功，可在一定程度上改善材料的强度等力学性能。随着CePO4加入量的增加，弱界面增多，Ce-ZrO2／CePO4陶瓷材料的可加工性变好但力学性能却变差。烧成温度等工艺因素对Ce-ZrO2／25％CePO4(质量分数)陶瓷的强度等力学性能影响较大，在1550℃下加热2h，颗粒尺寸与发生裂纹桥联等形式相适应，可充分发挥弱界面的作用。     

梯度功能陶瓷刀具材料的残余应力设计及制备

根据陶瓷刀具切割时刀楔内的应力分布规律及刀具损坏机理的分析,首次提出以陶瓷刀具材料的组成分布进行设计以及梯度的模型,通过对不同组成分布的刀具材料制备过程中的残余应力及刀具切削过程中的热应力,机械应力进行模拟,以残余应力与外加应力部分抵消为目标优化设计了梯度组成分布,按设计结果,采用粉末铺填－热压烧结工艺研制成功Ａｌ２Ｏ３／ＴｉＣ系梯度功能陶瓷刀具材料。     

新型陶瓷刀具的机械应力分析

计算了梯度功能陶瓷刀具和普通陶瓷刀具内的最大Mises应力，最大剪应力和最大拉应力。在低速切削时梯度功能陶瓷和普通陶瓷刀具内机械应力场基本相同，前刀面的破损形式基本一样。     

液相烧结SiC陶瓷

采用Ｙ２Ｏ３,Ａｌ２Ｏ３为烧结助剂,研究了烧结助剂的含量及组成对ＳｉＣ陶瓷的烧结性和力学性能的影响。结果表明,Ｙ２Ｏ３,Ａｌ２Ｏ３原位形成了ＹＡＧ,材料以液相烧结机制致密化,与传统的固相烧结相比,液相烧结使ＳｉＣ陶瓷性能显著提高,在较佳条件下,材料强度和韧性分别达到７０７ＭＰａ,１０．７ＭＰａ·ｍ＾１／２,显微结构观察表明,裂纹偏转和微裂纹为主要的增韧机理,这与其弱的界面结合有关,而强度的提高则得     

陶瓷刀具材料的现状与发展

本文阐述了陶瓷刀具材料的发展现状 ,着重论述了氧化铝系陶瓷和氮化硅系陶瓷材料技术。对陶瓷刀具材料的发展趋势进行了综述。指出超微粉刀具、复相陶瓷刀具、涂层刀具及金属陶瓷是陶瓷刀具材料的研究方向。     

钇补强颗粒弥散陶瓷复合材料增韧机制的微观结构表征

在Al2O3/（W,Ti）C陶瓷复合材料中适量添加稀土元素钇能显著提高其断裂韧性。本文运用SEM与TEM技术,从微观结构的角度探讨了其增韧机制。表明,由于稀土钇的添加,使材料内部形成不同程度的强弱界面,它们与扩展中的裂纹相互作用,使得裂纹桥联、裂纹分支、裂纹偏转以及微裂纹增韧机制得到明显增加和加强,从而以多种增韧机制及其协同作用共同提高稀土补强Al2O3/（W,Ti）C陶瓷复合材料的断裂韧性。     

压痕法研究ZrO2层状复合陶瓷的抗热震性

用压痕法测试了单层和层状两种ZrO2陶瓷材料的抗热震性能.研究结果表明ZrO2层状复合陶瓷的临界热震温差△Tc=400℃,比ZrO2单层陶瓷高出150℃左右,同时还表现出其△Tc与陶瓷厚度无关的优异性质.现场试验结果也证明,ZrO2层状复合陶瓷抵抗1500℃冷热骤变的能力优于ZrO2单层陶瓷.研究认为,界面压应力作用部分或全部抵消了热冲击应力,使裂纹在界面处发生偏转,提高了材料的断裂能和断裂功,导致材料的抗热震性提高.     

新型梯度功能陶瓷刀具材料的设计模型

本文首次提出对陶瓷刀具材料的组成分布，微观结构进行设计以形成梯度模型，通过不同组成分布的刀具工作过程中的热应力，机械应力及材料制备过程中的残余热应力进行了模拟，以使刀具材料内部形成与外载应力部分抵消的机制，从而提高陶瓷刀具抗磨损破损的性能。     

多孔Si3N4–SiO2复相陶瓷及其性能

采用近净尺寸成型制备工艺–氧化烧结结合溶胶浸渍再烧结法,制备了多孔Si3N4–SiO2复相陶瓷。讨论了制备工艺对材料的成分、微结构和性能的影响规律。研究表明：随着硅溶胶浸渍量的增加,材料的抗弯强度、硬度、断裂韧性、密度和介电常数均增大。分别采用压痕法和单边切口梁法对材料的断裂韧性进行了测定和比较。结果表明：采用压痕法测定断裂韧性时,多孔Si3N4–SiO2复相陶瓷的增韧机理有裂纹偏转、裂纹分叉、裂纹桥接以及孔的钝化。采用单边切口梁法测定断裂韧性时,多孔Si3N4–SiO2复相陶瓷的增韧机理只有裂纹偏转。     

Crack development behavior in thermally sprayed anti-oxidation coating under repeated thermal-oxygen coupling environment

The crack development behavior in thermally sprayed anti-oxidation coating was investigated after long-term and short-term oxidation with repeated thermal cycles from 1500 °C to room temperature. According to the distribution characteristics, the formed cracks can be divided into three types: type-A cracks with multi-directional features, type-B cracks originated from the inner interface bulges and type-C cracks initiating at surface oxide layer. Based on the analytical math models (blunt crack model and interface roughness model), the maximum stress at different positions was evaluated from the perspective of inner interface roughness, uneven oxide film, original microcracks and gathering micropores. The original vertical type-A cracks are most dangerous due to the highest crack tip stress. However, the micropore distribution or appropriate interface may promote transformation of vertical type-A cracks to less dangerous horizontal type-A cracks. This study on crack development behavior provides a fundamental insight and further avenues to optimize the composition and structure of thermally sprayed ceramic coating.     

Cracks at adhesive interfaces

The application of Griffith's energy balance argument to cracks at adhesive interfaces is studied. Adhesive interfaces are generally brittle, representing the simplest form of fracture mechanics geometry because cracks are constrained to travel along the interface, giving a defined crack path which eases analysis. Experimentally, such cracks may be propagated along the interface between optically smooth rubber pieces, and measured through the transparent material. The development of adhesive fracture test-pieces since Griffith's time reveals difficulties in his reasoning, and allows improved understanding of the energy balance method. The most important conclusion is that stress does not normally enter the cracking criterion. It is demonstrated experimentally that stress may remain constant while the crack criterion changes. The strength of an adhesive interface is shown to be a meaningless parameter; instead, the work of adhesion, or adhesive energy, which is the work of adhesion together with energy losses, should be used to define the behaviour of cracks at interfaces.     

Numerical study of residual stress and crack nucleation in thermal barrier coating system with plane model

The residual stresses could cause extensive damage to thermal barrier coatings and even failure. A finite element model of thermal barrier coating system had been designed to simulate the residual stresses and then to analyze the crack nucleation behavior. The distribution of normal and tangential stress components along top coat (TC) / thermally grown oxide (TGO) and TGO / bond coat (BC) interfaces are shown in this work. It is found that the maximum tensile stress along TC/TGO interface occurs in the peak region during heating-up, and that along TGO/BC interface is also located in the peak region, but during the process of cooling-down. A parameter correlating the normal stress component with corresponding tangential one was used to evaluate the interfacial cracks, indicating that cracks will initiate at the peak-off region of TC/TGO interface in the heating-up phase, but for TGO/BC interface, cracks will initiate at the peak position in the cooling-down phase.     

Cracking during pyrolysis of preceramic polymers within glass microtubes

Cracking of preceramic polymers during pyrolysis under highly-constrained conditions is examined by X-ray computed tomography of fine glass microtubes containing the pyrolyzing material. The microtubes represent model geometries that mimic the long channels between fibers during production of ceramic composites by precursor impregnation and pyrolysis. Complementary fracture mechanics analyses of interface cracking and crack kinking are used to glean insights into the conditions under which periodic alternating cracks form. A key finding is that alternating cracks are an inherent feature of constrained pyrolysis. This feature is attributable in large part to the high energy release rates for interface cracks to kink into the pyrolyzing material under the hydrostatic tension developed during pyrolysis. It also requires interfaces with toughness comparable to that of the pyrolyzing material, to prevent large-scale interface separation. The results further indicate the need for small uniform spaces for pyrolysis within fiber preforms in order to produce networks of fine periodic pyrolysis cracks; these networks in turn facilitate impregnation and pyrolysis in subsequent processing cycles.     

Effects of Matrix Cracks on the Thermal Diffusivity of a Fiber-Reinforced Ceramic Composite

Effects of matrix cracks and the attendant interface debonding and sliding on both the longitudinal and the transverse thermal diffusivities of a unidirectional Nicalon/MAS composite are investigated. The diffusivity measurements are made in situ during tensile testing using a phase-sensitive photothermal technique. The contribution to the longitudinal thermal resistance from each of the cracks is determined from the longitudinal diffusivity along with measurements of crack density. By combining the transverse measurements with the predictions of an effective medium model, the thermal conductance of the interface (characterized by a Biot number) is determined and found to decrease with increasing crack opening displacement, from an initial value of ∼1 to ∼0.3. This degradation is attributed to the deleterious effects of interface sliding on the thermal conductance. Corroborating evidence of degradation in the interface conductance is obtained from the inferred crack conductances coupled with a unit cell model for a fiber composite containing a periodic array of matrix cracks. Additional notable features of the material behavior include: ( i ) reductions of ∼20% in both the longitudinal and the transverse diffusivities at stresses near the ultimate strength, ( ii ) almost complete recovery of the longitudinal diffusivity following unloading, and ( iii ) essentially no change in the transverse diffusivity following unloading. The recovery of the longitudinal diffusivity is attributed to closure of the matrix cracks. By contrast, the degradation in the interface conductance is permanent, as manifest in the lack of recovery of the transverse diffusivity.     

Comprehensive dynamic failure mechanism of thermal barrier coatings based on a novel crack propagation and TGO growth coupling model

Comprehensive understanding of failure mechanism of thermal barrier coatings (TBCs) is essential to develop the next generation advanced TBCs with longer lifetime. In this study, a novel numerical model coupling crack propagation and thermally grown oxide (TGO) growth is developed. The residual stresses induced in the top coat (TC) and in the TGO are calculated during thermal cycling. The stresses in the TC are used to calculate strain energy release rates (SERRs) for in-plane cracking above the valley of undulation. The overall dynamic failure process, including successive crack propagation, coalescence and spalling, is examined using extended finite element method (XFEM). The results show that the tensile stress in the TC increases continuously with an increase in an undulation amplitude. The SERRs for TC cracks accumulate with cycling, resulting in the propagation of crack toward the TC/TGO interface. The TGO cracks nucleate at the peak of the TGO/bond coat (BC) interface and propagate toward the flank region of the TC/TGO interface. Both TC cracks and TGO cracks successively propagate and finally linkup leading to coating spallation. The propagation and coalescence behavior of cracks predicted by this model are in accordance with the experiment observations. Therefore, this study proposed coating optimization methods towards advanced TBCs with prolonged thermal cyclic lifetime.     

On crack evolution with texturization of bonding layer in thermal barrier coating

The interface morphology of the bonding layer has a considerable effect on the damage and failure of sandwich-structured thermal barrier coatings. This work investigated the comprehensive effects of a grooved texture produced using laser ablation on the local surface strain, interfacial stress and strain, and crack behavior of the bonding layer in a thermal barrier coating system. The distribution and evolution of the local surface strain was obtained using the digital image correlation method. The interfacial stress, and the strain between the ceramic and bonding layers, were determined through a simulation of the plane-strain model, and the morphology and propagation of cracks were observed in thermal barrier coatings under an external tensile load. The results indicated that the local surface strain of the thermal barrier coating increased with the texturization of the bonding layer, whereas the fluctuation decreased. There were two inflection points in the local surface strain–time curves, corresponding to the initiation of surface cracks and that of interfacial transverse cracks. The surface cracks were initiated earlier than those without the texturization of the bonding layer. However, the behavior of the interfacial cracks was more complicated. If the roughness of the texture, defined as Rc, was small, the surface cracks propagated vertically to the interface between the ceramic and bonding layers, and turned into transverse cracks, leading to a separation of the ceramic layer. If Rc was greater than 22 μm, the surface cracks went further down to the interface between the bonding layer and substrate, and propagated horizontally, resulting in the separation of both the ceramic and bonding layers. Meanwhile, interfacial cracking and separation were deferred. A large roughness resulted in good cohesion between the ceramic and bonding layers, and a high stiffness for the coating, which improved the damage resistance and extended the life of the coating.     

Forming Single-Phase Laminates via the Gelcasting Technique

Single–phase laminates of iron titanate were formed by gelcasting in both the presence and absence of a magnetic field to produce alternating layers of textured and nontextured microstructure, respectively. X–ray analysis was performed on each lamina verifying that alignment was maintained throughout processing. Tunnel cracks were found in trilayer laminates (nontextured/textured/nontextured) when the alignment direction was parallel to the interface between layers. The cracks are consistent with a stress profile of residual tension parallel to the interface in the textured layer.     

Prevention of Cracks in Functionally Graded SiO2‐Mo Materials Fabricated by Uniaxial Compression Casting and Pressureless Sintering

In industrial high‐intensity discharge lamps, cracks and delaminations occasionally develop at the interface between SiO₂ and the Mo foil in the seal. Here, functionally graded SiO₂‐Mo materials for use in these lamps were fabricated by uniaxial compression casting and pressureless sintering. Consequently, vertical cracks developed across the sintered body layers, and interfacial cracks developed between the 100 wt% SiO₂ and 90 wt% SiO₂‐Mo layers. Therefore, the effects of residual stress, difference in the coefficient of thermal expansion (CTE), and difference in the volume shrinkage on these cracks were investigated. Vertical cracks were suppressed when residual stress was relaxed by annealing near the annealing point of silica glass during the cooling step in the sintering process. Interfacial cracks were suppressed when the difference in the CTE of the interface between the 100 wt% SiO₂ and 90 wt% SiO₂‐Mo layers was relaxed by inserting layers of 95 wt% SiO₂‐Mo between them. Furthermore, the suppression effect became stronger when the difference in the volume shrinkage of the layers was relaxed by sintering to join the separately sintered monolayers. Thus, the development of these cracks was influenced by the residual stress, CTE, and volume shrinkage. Therefore, these cracks can be prevented by optimizing these factors.     

Contact-Induced Transverse Fractures in Brittle Layers on Soft Substrates: A Study on Silicon Nitride Bilayers

An analysis of transverse cracks induced in brittle coatings on soft substrates by spherical indenters is developed. The transverse cracks are essentially axisymmetric and geometrically conelike, with variant forms dependent on the location of initiation: outer cracks that initiate at the top surface outside the contact and propagate downward; inner cracks that initiate at the coating/substrate interface beneath the contact and propagate upward; intermediate cracks that initiate within the coating and propagate in both directions. Bilayers consisting of hard silicon nitride (coating) on a composite underlayer of silicon nitride with boron nitride platelets (substrate), with strong interfacial bonding to minimize delamination, are used as a model test system for Hertzian testing. Test variables investigated are contact load, coating/substrate elastic-plastic mismatch (controlled by substrate boron nitride content), and coating thickness. Initiation of the transverse coating cracks occurs at lower critical loads, and shifts from the surface to the interface, with increasing elastic-plastic mismatch and decreasing coating thickness. This shift is accompanied by increasing quasi-plasticity in the substrate. Once initiated, the cracks pop in and arrest within the coating, becoming highly stabilized and insensitive to further increases in contact load, or even to coating toughness. A finite element analysis of the stress fields in the loaded layer systems enables a direct correlation between the damage patterns and the stress distributions: between the transverse cracks and the tensile (and compressive) stresses; and between the subsurface yield zones and the shear stresses. Implications of these conclusions concerning the design of coating systems for damage tolerance are discussed.