Thermal Resistance of Grain Boundaries in Alumina Ceramics and Refractories

The influence of grain boundaries on heat transfer through polycrystalline alumina has been investigated between 20° and 500°C. The thermal conductivities of small-grained porous ceramics and large-grained dense ceramics have been measured using the laser-flash technique. Two methods have been developed to assess the average thermal resistance of a grain boundary. The first method is based on the comparison of room-temperature thermal conductivities for dense ceramics that have various average grain sizes. This method yields a value of 0.9 × 10⁻⁸ m²·K·W⁻¹. The second method, particularly suitable for porous ceramics, is based on the extrapolation of the inverse of the thermal conductivity versus temperature to give an intercept with the axis at T = 0 K. This value is attributed to the thermal resistance of grain boundaries. By taking into account the influence of the pore content using an effective medium theory, the average thermal resistance of a grain boundary has been evaluated to be 1.3 × 10⁻⁸ m²·K·W⁻¹ in dense alumina and 2.2 × 10⁻⁸ m²·K·W⁻¹ in alumina containing a pore volume fraction of 0.3.     

提高高铝陶瓷耐磨性能研究

实验以工业用Al2O3为主要原料,在MgO-CaO-SiO2系统添加稀土氧化物La2O.和Y2O3,优化配方组成,制备出高性能的研磨介质,其氧化铝含量为98％.实验对陶瓷试样的吸水率和磨损率进行测试表征.采用XRD粉末衍射测试仪和SEM分析了试样的物相组成和显微结构.研究讨论了成形压力和烧结温度对氧化铝陶瓷性能的影响.     

Mixed-Mode Fracture of Ceramics in Asymmetric Four-Point Bending: Effect of Crack-Face Grain Interlocking/Bridging

The mixed-mode fracture of a large-grain-size alumina ceramic and a soda-lime glass is investigated. These ceramics are tested using straight-through precracked or notched specimens. The straight-through precrack is introduced by the single-edge-precracked beam method. Precracked or notched specimens are subjected to combined mode I/II or pure mode II fracture, under asymmetric four-point bending, and pure mode I fracture, under symmetric four-point bending. A pure mode II fracture is never achieved in the precracked polycrystalline alumina by the crack-face friction inevitably induced by grain interlocking/bridging. The crack-face friction in sliding mode reduces the local mode II stress intensity factor in the crack-tip region and produces a sizable amount of mode I deformation. Accounting for the contribution of the crack-face friction to the crack-tip local stress intensity factors, K _I and K _II, in mixed-mode fracture tests, the experimental results of the K _I/ K _{I c} versus K _II/ K _{I c} envelope and the initial angle of noncoplanar crack extension are in good agreement with the theoretical predictions of the maximum hoop-stress theory.     

Experimental Study of Fracture of Glass:I, The Fracture Process

A concept of fracture is developed from experimental data. Fractures are found to originate at flaws or cracks of finite size, most of which are at the surface. The mechanism is one of crack propagation which begins when the local stress at the crack exceeds a minimum value. The rate of propagation increases with crack growth until a critical stress is reached at the crack tip which coincides with a limiting crack velocity. This limiting condition is identified with the boundary of the mirror surface of the fracture. From calculations to be presented in Part 11, the critical stress is estimated to be several million pounds per square inch.     

Experimental Observations of High Fracture Resistance in Silicon Carbide/Alumina Laminated Composites

Model laminated composites were fabricated with porous-Al₂O₃ interfaces between SiC bars. The porous Al₂O₃ was deposited using an aerosol spray deposition technique, and the sandwich specimen was fabricated by hot pressing. Residual thermal stresses were present in the interface because of the difference in the coefficients of thermal expansion of SiC and Al₂O₃. Crack deflection was observed with measured interfacial fracture resistances that were considerably higher than the deflection threshold predicted by the He–Hutchinson criterion. Examination of the fracture surface revealed a tortuous crack path and significant crack–flaw interaction.     

Analytical Modeling for Bridging Stress Function Involving Grain Size Distribution in a Polycrystalline Alumina

In order to investigate the microstructural effect on the R -curve behavior in a polycrystalline alumina, an analytical model has been proposed based on the relationship between bridging stress and crack opening displacement. The crack opening displacement was measured using an in situ SEM fracture method, and then used for a fitting procedure to obtain the bridging stress distribution. The results indicated that the bridging stress function and the R -curve computed by the current model were consistent with those computed by the power-law relation, and that the grain size distribution was closely related to the bridging stress. Thus, the current model explained well the correlation between the bridging stress distribution and the local-fracture-controlling microstructural parameter to interpret the microfracture mechanism, including the R -curve behavior.     

Grain-Size Dependence of Fracture Energy in Ceramics: I, Experiment

Evaluation of literature data combined with new experimental measurements shows that crystal structure is a basic factor in the grain-size dependence of the critical fracture energy of ceramics. Notch beam (NB), double cantilever beam (DCB), and work-of-fracture (WOF) tests of materials having a cubic crystal structure generally agree and show, at most, a limited variation of fracture energy with grain size. However, maxima of the order of 25% in γ at intermediate grain size and 25% decreases in γ at larger grain size may occur. Materials of noncubic crystal structure show broader variations in γ with G . Both DCB and WOF tests show γ passing through maxima that are typically 100 to >400% of values at fine or large grain size. Limited DT data are consistent with this trend but various results of NB tests are mixed. Some NB test data agree with the DCB and WOF trends, whereas others taken from the same materials show no change or only a limited continuous decrease of γ as G increases.     

Strength-Processing Defects Relationship Based on Micrographic Analysis and Fracture Mechanics in Alumina Ceramics

Characterization of processing defects in structural ceramics is very important for the investigation of mechanical property, because processing defects are origin of fracture and govern the strength of products. The relationship between defects and strength is represented by simple equation which contains factor of defect size. In this study, size of defects in alumina ceramics was measured by optical microscopy with sintered body thinned to about 150 μm. The size distribution was used for strength calculation based on linear fracture mechanics. Average and Weibull modulus of calculated strength were very agreed with experimentally measured them. The result shows that defects in sintered body govern the strength and are quantitatively related to strength by fracture mechanics.     

High-Temperature Bridging in a Vitreous Bonded Alumina

The load supported by crack wake bridges was studied in glassy alumina at 1240°C using short double cantilever beam specimens with a rear notch wedge. Direct observation of fracture surfaces revealed the sizes and shapes of the high-temperature bridges. The bridged area was stochastic in nature but there was a clear trend toward decreased area bridged with increasing crack opening displacement consistent with existing models. Fracture surface observations on specimens without a rear notch were compared to bridge distributions in wedged specimens. Because of the complex bridge morphology the load supported by the bridges cannot be modeled as viscous uniaxial columns.     

The Utility of R-Curves for Understanding Fracture Toughness-Strength Relations in Bridging Ceramics

The mechanical behavior of four rare earth (RE)-Mg-doped Si₃N₄ ceramics (RE=La, Lu, Y, Yb) with varying grain-boundary adhesion has been examined with emphasis on materials containing La and Lu (which represent the extremes of RE ionic radius). Fracture-resistance curves ( R -curves) for all ceramics rose very steeply initially, giving them exceptional strength and relative insensitivity to flaw size. The highest strength was seen in the Lu-doped material, which may be explained by its steeper initial R -curve; the highest "apparent" toughness (for fracture from millimeter-scale micronotches) was seen in the lowest strength La-doped material, which may be explained by its slowly rising R -curve at longer crack lengths. Excellent agreement was found between the predicted strengths from R -curves and the actual strengths for failures originating from natural flaws, a result attributed to careful estimation of the early part of the R -curve by deducing the intrinsic toughness, K ₀, and the fact that this portion of the R -curve is relatively insensitive to sample geometry. Finally, it was found that RE elements with relatively large ionic radius (e.g., La) tended to result in lower grain-boundary adhesion. This implies that there is a small window of optimal grain-boundary adhesion which can lead to the fastest rising R -curves (for short cracks) and the highest strengths. The importance of this work is that it reinforces the notion that factors which contribute to the early part of the R -curve are critical for the design of ceramic microstructures with both high strength and high toughness.     

Model for Toughness Curves in Two-Phase Ceramics: I, Basic Fracture Mechanics

A fracture mechanics model is presented for the toughening of ceramics by bridging from second-phase particles, resulting in toughness curve ( T -curve) behavior. It is assumed that the second-phase particles are in a state of residual thermal expansion dilatational mismatch relative to the matrix. In the long-crack region, these stresses augment frictional sliding stresses at the interphase boundaries, enhancing the crack resistance; in the short-crack region, the same stresses drive the crack, diminishing the crack resistance. The principal manifestation of these countervailing influences is a reduced sensitivity of strength to initial flaw size, i.e., an increased flaw tolerance. In seeking to incorporate these key physical elements, our model opts for mathematical simplicity by assuming uniformly distributed stresses in two bridging domains: in the first, at small crack-wall separations, a constant opening stress; in the second, at larger separations, a constant closing stress. The uniform crack-plane distributions allow for simple closed-form solutions of the crack K -field equations, and thence an analytical formulation for the T -curve. Indentation-strength data on a "reference" Al₂O₃/Al₂TiO₅ ceramic composite are used to demonstrate the main theoretical predictions and to calibrate essential parameters in the T -curve formulation. The utility of the model as a route to microstructural design is addressed in Part II.     

Microstructure of 96% Alumina Ceramics: I, Characterization of the As-Sintered Materials

Characterization of the microstructure and microchemistry of a group of commercial 96% Al₂O₃ ceramics, using analytical and conventional electron microscopy techniques, was conducted. A continuous glassy grain-boundary phase was found in all samples, in addition to a number of intragranular and intergranular crystalline second phases; the phases present depended on the original boundary-glass composition. The faceted nature of many of the Al₂O₃-glass interfaces was studied and is thought to be an equilibrium structure.     

Some Effects of Cavities on the Fracture of Ceramics: I, Cylindrical Cavities

The problem of fracture from cylindrical cavities in brittle materials was analyzed. The analysis is based on the conditions required to extend cracks that preexist at the cavity surface and involves the combined use of fracture mechanics and statistics solutions for crack extension in the concentrated stress field around the cavity. The pertinence of the approach is substantiated by experimental studies of fracture from cavities in polycrystalline alumina.     

Piezoresistivity in PTCR Barium Titanate Ceramics: I, Experimental Findings

Positive temperature coefficient of resistance (PTCR) barium titanate is the active material in a ceramic sensor which employs piezoresistivity to detect changes in applied stress. High-purity, chemically prepared barium titanate is donor-doped with 0.30 at.% lanthanum, and <0.10 at.% of a transition-metal counterdopant may be added to enhance the PTCR effect. Tape-cast sheets of undoped and PTCR BaTiO₃ are laminated to produce a three-layer trilaminate—a sintered structure which has two semiconducting PTCR layers separated by an insulating layer. The trilaminate is stressed in a four-point bend configuration (placing one semiconducting layer completely in tension, the other in compression), and the resistivities for both stress states are measured concurrently as functions of applied stress and temperature. Results are presented for various semiconducting layer compositions and sintering conditions.     

耐磨抗冲击氧化铝陶瓷异型件的研制

主要研究了高铝快速磨罐、弧形衬板、球磨机磨口、研钵等异型件的配方及注浆成型工艺。试验配制了高铝泥浆用复合稀释荆及复合添加剂，使制品的显微结构均匀，密度得到了显著提高。     

低阻PTC陶瓷材料的研究

本研究采用传统的固相反应工艺，以国产工业级化工原料，掺杂施主元素Nb、La、Y、Tn以及显微结构调节剂BN和ASTL相，通过实验探讨了低阻PTC陶瓷材料的配方组成、工艺制度对材料电性能的影响，成功制备出性能良好的低阻PTC陶瓷材料。     

Knoop and Vickers Indentation in Ceramics Analyzed as a Three-Dimensional Fracture

A three-dimensional fracture analysis is applied to the Knoop and Vickers indentation fracture of ceramics. A brief discussion of the accuracy of the analysis applied to model the step load on the crack face caused by the residual stresses is given. A study is made of the effect of the elongated plastic zone in Knoop indentation on the unloaded radial fracture. It is shown that for small indentation loads the published experimental data can be verified by varying the depth reached by the semielliptical plastic zone with given surface length. An analysis and interpretation of the interaction between the two halfpenny-shaped radial cracks induced by Vickers indentation is also given.     

Mode I Fracture Resistance of a Laminated Fiber-Reinforced Ceramic

The mode I fracture resistance of a ceramic matrix composite has been measured. Simultaneous observations have revealed that the resistance is dominated by frictional dissipation upon the pullout of fibers that fracture in the crack wake off the crack plane. Numerical and analytical crack growth simulations have been compared with the experimental results. One important feature in this comparison concerns the occurrence of large-scale bridging. With these effects taken into account, the simulations and the experiments are found to be in good correspondence for acceptable magnitudes of the interface sliding stress.