Micromechanics of Creep-Crack Growth in a Glass-Ceramic

The near-tip strain and opening behavior of creep cracks in a glass-ceramic were studied at temperatures for which grain-boundary sliding is the dominant deformation mechanism. Using the stereoimaging displacement measurement technique, near-tip creep strain and crack-opening displacement (COD) increments were obtained as functions of distance from the crack tip and time of creep. Both the time and radial dependence of the strain and COD were observed to be consistent with the Riedel and Rice (RR) field when the time of creep and crack extension were small. Increasing creep time and crack extension led to the formation of a localized shear zone located directly ahead of the crack tip. Further crack extension tended to follow this shear zone within which creep damage accumulated. Neither the rate of COD nor the strain-rate distribution within this shear zone could be described by the RR field or the Hui and Riedel (HR) field for growing cracks. Steady-state, stable crack growth exhibiting both Mode I and II components was observed in the glass-ceramic despite a creep exponent of ≅2. Varying the test temperature and applied stress intensity influenced both the development of the crack-tip shear zone and the crack growth process. These observations are discussed on the basis of the interrelationships between the near-tip field, creep damage accumulation, and grain-boundary sliding.     

Origin of the Creep-Crack Growth Threshold in a Glass-Ceramic

The growth threshold, K _th, of large creep cracks in a glass-ceramic has been investigated to determine the causes of the arrest and nonpropagating behaviors below a characteristic stress intensity, K , value. Using replication techniques, the dominant creep demage mechanisms during creep-crack growth, which included grain-boundary cavity and microcrack formation, were identified as a function of K levels and crack extension rates. Quantitative measurements of cavity density, D ( r ), and microcrack density, Ψ( r ), revealed that the values of D ( r ) and Ψ( r ) both decrease with the K level and with increasing distance, r , from the crack tip at a given K level. At K values below K _th, the cavity density became zero as preexisting grain-boundary cavities were sintered, suggesting that the growth threshold originated from sintering of creep cavities. At K levels above K _th, microcracks located in the wake of the main crack were observed to be sintered also. Possible mechanisms responsible for the apparent self-healing of microcracks and creep cavities in the glass-ceramic are provided, together with theoretical and experimental support.     

Role of Internal Friction in Indentation Damage in a Mica-Containing Glass-Ceramic

The indentation response of a mica-containing glass-ceramic that exhibits shear-driven yield in an indentation test is interpreted in terms of events occurring on the microstructural scale. It is proposed that shear-driven damage within the specimen occurs via internal sliding along cleavage planes within the mica platelets. The sliding surfaces in this case are considered to be atomically smooth so the real and apparent areas of contact coincide. The frictional shear stress is thus independent of the normal forces arising from thermal mismatch stresses and only depends on the work of adhesion of the interface and the scale of the contacts. The scale of contacts for these materials lies within an intermediate zone in which the frictional shear stress arises from the stress required to nucleate dislocation-like discontinuities within the material. This leads to a size effect similar to that experienced by a crack in Mode II loading and is in accordance with previous work in which a connection between such a size effect and the macroscopic response of the material was identified. This work has particular relevance to the design and manufacturing of ceramics in machining, wear, bearings, and coatings applications.     

Dynamic Fatigue of a Lithia-Alumina-Silica Glass-Ceramic

A dynamic fatigue study was performed on a Li₂O-Al₂O₃-SiO₂ glass-ceramic in order to assess its susceptibility to delayed failure. Fracture mechanics techniques were used to analyze the results for the purpose of making lifetime predictions for optical elements made from this material. The material has reasonably good resistance ( N = 20) to stress corrosion in ambient conditions. Analysis also indicated the elements should survive applied stresses incurred during grinding and polishing operations.     

建筑装饰微晶玻璃板中应力产生与形变关系分析

CaO-Al2O3-SiO2系统微晶玻璃作为微晶玻璃中的一个新品种,具有非常鲜明的特点,并得到广泛的应用.在微晶玻璃的生产中,微晶玻璃板在由玻璃颗粒烧结、晶化以后会出现板面变形的现象.大多数的板面变形与微晶玻璃装饰板内的结构应力有关.本文讨论了CaO-Al2O3-SiO2系统微晶玻璃中形变与内应力产生的原因,这将对微晶玻璃的生产提供一定的参考.     

Heavy Metals Bioremediation from Polluted Water by Glass-Ceramic Materials

The objective of this research was to study the removal of cadmium and lead from an aqueous solution through a biological treatment. For this purpose a glass-ceramic material was manufactured from industrial and urban wastes. Biofilms of microorganisms found in wastewater were developed on its surface, and continuous tests were conducted in the presence and absence of the biofilm to analyze the glass-ceramic's ability to remove the heavy metals from an aqueous environment. The results suggest that this bioremediation process, developed on an industrial scale, could represent an alternative to the chemical processes currently used.     

Subcritical Crack Growth in Sintered Silicon Nitride Exhibiting a Rising R-Curve

Subscritical crack growth of sintered silicon nitride was analyzed in terms of the R -curve. Provided that the stress intensity at the crack tip governs the subcritical crack-growth velocity, the K _I– V relationship of sintered silicon nitride exhibiting a rising R -curve is shown to shift to the high- K _I region as the crack advances.     

Effect of Interfacial Layers on Wear Behavior of a Dental Glass-Ceramic

Wear studies on a glass-ceramic dental restorative material were performed under two types of lubrication conditions with distilled water. In one set of experiments, the contact interface was continuously flushed with fresh water to remove the wear debris. In a second set of experiments, the water was not replaced during the testing period. Flushing resulted in a lower friction coefficient and more than twice the wear volume compared with the nonflushing condition. Examination of the worn surfaces in SEM confirmed the presence of an interfacial layer at the sliding interface for the nonflushing experiments. These results suggest that the specific conditions at the sliding interfaces and particularly the nature of interfacial layers must be considered when evaluating the tribological performance of ceramics and especially when the wear data from different studies are compared.     

HK40炉管焊缝抗蠕变裂纹扩展能力的试验

在全面分析已服役１０ａ的ＨＫ４０炉管损伤状况的基础上,研究了ＨＫ４０炉管母材和焊接接头的蠕变裂纹扩展速率,评价了这两种不同部位抗蠕变裂纹扩展的能力。结果表明：在长期正常的运行中,特别是服役后期,下部炉管焊缝中的蠕变裂纹扩展较母材快,微观组织损伤和焊缝中的应力集中是致使焊缝蠕变裂纹扩展较快的两个重要因素。     

Measurement of Very Slow Crack Growth in Glass

The rate of very slow crack growth in glass is measured by inducing small, controllable changes in the direction of propagation of Hertzian cone cracks at known times. After completion of a growth sequence, the sample is sectioned to reveal the fracture surface. The stress intensity factor at each stage of crack growth is calculated by using finite-element modeling of the stresses near the crack tip. Data are presented for crack growth velocities as low as 10⁻¹⁴ m/s in soda–lime glass. These data provide strong evidence for the existence of a subcritical limit for crack growth in this material.     

Spherulitic Growth from a Phase-Separated Vitreous Matrix in a Cordierite–Y-Stabilized Zirconia Glass-Ceramic

Crystallization behavior, especially spherulitic growth of a cordierite–3 mol% Y₂O₃–ZrO₂ glass composition annealed at 950° to 1370°C was studied by optical and electron microscopy. Microstructural analysis revealed that glass-inglass phase separation occurred prior to the crystallization of β-quartz solid solution ( ss ). ZrO₂ was involved in promoting the nucleation of β-quartz ss ; the crystallization evolved from a cellular to a spherulitic morphology, and β-quartz transformed to α-cordierite as the temperature was raised. ZrO₂ was eventually expelled intergranularly and intragranularly to form slightly misoriented domains upon annealing, and the dendritic or clustered tetragonal t -ZrO₂ crystals transformed to monoclinic ( m ) in symmetry. A possible dragging force exerted between α-cordierite and a glassy droplet phase is proposed to explain the corrugated cordierite/glass interface.     

Crystallization of a Glass-Ceramic by Epitaxial Growth

The crystallization mechanism in a modified Li₂O-Al₂O₃-SiO₂glass containing P₂O₅ nucleating agent was investigated by transmission electron microscopy, X-ray diffraction, and differential thermal analysis. During an initial 1000°C treatment P₂0₅ and Li₂O react to precipitate Li₃PO₄ crystallites. At lower temperatures cristobalite, lithium metasilicate, and lithium disilicate crystallize by epitaxial growth on those Li₃PO₄crystallites. Crystallographic orientation relations for epitaxy were determined by electron diffraction, and lattice misfits were found to be in the range –5.3 to +3.8%. These results provide the first direct proof that glass-ceramics can crystallize by epitaxial growth on heterogeneous nuclei formed from intentionally added nucleating agents.     

Ceramic and Glass-Ceramic Packaging in the 1990s

A broad overview of packaging involving interconnecting, powering, protecting, and cooling semiconductor chips to meet a variety of computer system needs is presented. The general requirements for ceramics in terms of their thermal, mechanical, electrical, and dimensional control requirements are presented, both for high-performance and low-performance applications. Glass-ceramics are identified as the best candidates for high-performance systems, and aluminum nitride, alumina, or mullite are identified for low-performance systems. Glass-ceramic/copper substrate technology is discussed as an example of high-performance ceramic packaging for use in 1990s. Lower-dielectric-constant ceramics such as composites of silica, borosilicate, and cordierite, with or without polymers and porosity, are projected as potential ceramic substrate materials by the year 2000.     

Preparation of Zirconia-Toughened Bioactive Glass-Ceramic Composite by Sinter–Hot Isostatic Pressing

High-strength bioactive ceramics, MgO-CaO-SiO₂-P₂O₅ glass-ceramic composites toughened by zirconia, were prepared by sinter-hot isostatic pressing (sinter-HIPing) to achieve easier mass production and higher reliability. Raw materials for preparing a dense presintered body were investigated to obtain almost complete densification by sinter-HIPing. It was found that the densification of a presintered body was influenced by residual glassy phases in the crystallized glass particles. By using a controlled crystallized glass powder and a zirconia powder as raw materials, a presintered body with a relative density higher than 94% was prepared, and then it was densified to near its theoretical density by sinter-HIPing. This bioceramic exhibited an extremely high bending strength of 400 to 1000 MPa and fracture toughness of 3 to 5 MPa.m^1/2 for 30 to 80 vol% of zirconia.     

Microstructural Evolution in Fast-Heated Cordierite-Based Glass-Ceramic Glazes for Ceramic Tile

The crystallization mechanism of α-cordierite from a B₂O₃- and TiO₂-containing glass submitted to fast heating in the cordierite primary phase field of the CaO–MgO–Al₂O₃–SiO₂ quaternary system was investigated. Addition of B₂O₃ to a SiO₂-rich glass suppressed the formation of μ-cordierite. This suppression facilitated densification by viscous flow before crystallization. Powder X-ray diffractometry, field-emission electron scanning microscopy, and energy-dispersive X-ray analysis revealed that α-cordierite nucleated directly from glass on the boundaries of original particles and was probably favored by TiO₂, which acted as a nucleant. The growth kinetics of α-cordierite crystals was fast, and the crystals seemed to grow by consuming glass directly from the growing interphase. The estimated amount of α-cordierite in the glass heated at 1160°C was 69.5 wt%, as determined using the Alegre method. The final microstructure consisted of an arrangement of well-shaped hexagonal prisms, with sizes <3 μm, immersed in a residual glassy phase. The feasibility to develop new glass-ceramic glazes using the present and previous works is considered.     

Carbon Interfacial Layers Formed by Oxidation of SiC in SiC/Ba-Stuffed Cordierite Glass-Ceramic Reaction Couples

Reaction couples between α-SiC and cordierite (2MgO·2Al₂O₃·5SiO₂═ Mg₂Al₄Si₅O₁₈) were prepared by sandwiching (and enclosing) SiC single crystals between plates of Ba-stuffed magnesium aluminosilicate (Ba-MAS) glass and hot-pressing; the Ba-MAS was subsequently crystallized at 1000° to 1200°C in argon or air. No reaction occurred at the SiC/Ba-MAS interfaces during hot-pressing, but crystallization heat treatments caused formation of amorphous carbon reaction layers at the SiC/cordierite interfaces, due to concurrent oxidation via the reaction SiC + O₂→ SiO₂+ C. The thickness of the carbon of the carbon layer was variable. These results suggest that formation of C layers at SiC/silicate interfaces in other composites (containing Nicalon fibers, for example) depends more on thermochemistry and less on the details of SiC nonstoichiometry than has heretofore been supposed.     

用C形试样研究HK——40炉管蠕变裂纹扩展

高温下工作的炉管因蠕变裂纹的扩展导致了炉管的失效.本文采用C形试样进蠕变裂纹扩展试验,并用弹性应力强度因子K,净截面应力σ_(net)以及蠕变条件下的能量积分C~*,对获得的不同温度和不同载荷下的裂纹扩展数据进行处理,结果表明:C~*能够较好地描述蠕变裂纹扩展的整个过程.把材料的蠕变性能与裂纹扩展性能联系在一起,进一步讨论了温度的影响.对炉管受力情况进行简化,把断裂力学的方法应用到炉管的残余寿命估计中去,获得了初步的结果.     

Effect of Ribbon Orientation on the Fracture Toughness of a Metallic-Glass-Ribbon-Reinforced Glass-Ceramic Matrix Composite

A commercially available amorphous metal ribbon incorporated into a glass-ceramic matrix was assessed from the standpoint of fracture toughness. The ribbon orientation (with respect to the long and short transverse sides) relative to the opening crack significantly affected the fracture toughness of such composites. The strong dependence of the fracture toughness on the ribbon orientation was correlated with the differences in the bending contributions of the ribbons in the different orientations. The effect of the ribbon orientation on the crack growth rate in the matrix was also investigated.     

Effect of pH variation on the subcritical crack growth parameters of glassy matrix ceramics

The goals of our study were to calculate the subcritical crack growth (SCG) parameters of two veneering ceramics stored in water or Streptococcus mutans (S. mutans) biofilm and remineralizing medium, with indentation flaws. Feldspar (VM7) and leucite-reinforced (VM13) glass ceramic disks (Vita Zahnfabrik, Bad Säckingen, Germany) were made according to ISO 6872. Some specimens were indented with a Vickers diamond and the crack dimensions were measured. The specimens were fractured for a calculation of inert strength or further stored in water or submitted to pH variation, under preloading tension. Finally, the SCG parameters were calculated after the specimens were fractured under four stressing rates (MPa/s). Weibull analysis was conducted on non-indented specimens. XPS was performed as qualitative analysis. The subcritical crack of leucite ceramic did not vary with the media storage, but the glass-ceramic experienced a retarded growth after pH variation. The materials presented low Weibull modulus. Qualitative elemental analyses showed chemical modification on both ceramics. Therefore, the crack growth of leucite-reinforced ceramic was less affected by the environment pH than glass-ceramic.     

PE100管及其热熔接头的蠕变裂纹扩展行为

聚乙烯(PE)管性能优异,广泛应用于城市水及燃气供应系统。PE管的主要破坏形式是长期静压载荷下的慢速裂纹扩展失效。在蠕变条件下,采用光滑试样和裂纹圆棒试样对PE100管及其热熔接头进行了测试,得到了基于蠕变断裂参数C*的蠕变裂纹扩展动力学关系式。利用扫描电子显微镜(SEM)分析了裂纹圆棒试样的断面形貌,对比分析结果发现,蠕变裂纹扩展失效模式对应的最大应力为15.05 MPa,热熔接头熔合面分布有约11个/mm^2、直径范围为1~5μm的微气孔,热熔接头断裂微纤平均长度比母材约小20%~45%。当热熔对接时,熔合面存在的微气孔以及系带分子的浅渗透是导致PE100热熔接头蠕变裂纹扩展抗力降低的主要原因。