Float glass fracture toughness determination by Hertzian contact: experiments and analysis

In this work, we use a Hertzian indentation test to estimate the fracture toughness of float glass. Reference is made to two models for fracture toughness determination, that are based on two different physical approaches, the first one on the crack appearing at the surface of the sample and the second one on the cone crack developed under the surface. Our experimental fracture toughness values were higher than the one expected from the literature. By analysing the reasons for errors and limitations of the models, it has been possible, by adapting the latter to our special case of indentation, to derive a toughness value close to the one generally assumed for glass and obtained by other techniques. When using indenters with mechanical properties different from those of glass, we show that the friction between indenter and sample has an important influence on the fracture toughness determination. But other aspects, such as residual stresses or mechanical properties changes under loading, can also to some extent yield faulty estimations of the fracture toughness of glass in the Hertzian indentation test.     

热屏间距对泡生法蓝宝石单晶生长影响研究

太空、军事和科研等高科技领域的持续发展极大促进了对蓝宝石晶体的需求,泡生法是蓝宝石晶体的主要制造方法之一;热场结构对所得蓝宝石晶体的质量具有重要影响.本文对采用泡生法工艺制造蓝宝石单晶过程中,具有内置7层氧化锆外置8层钼金属的新型热屏结构间距进行研究.通过数值模拟考察热屏间距对单晶炉功率、固-液界面形状和晶体热应力的影响确定了合理的热场结构;并与试验生产结果进行对比验证.结果表明:热屏间距增大使得单晶炉功率明显提升,并引起固-液界面凸度增大;而蓝宝石晶体热应力出现减小.综合考察三个影响因素的影响,最后确定热屏间距为5 mm时单晶炉能耗较低,可用于制造高质量的蓝宝石晶体.     

Mechanical properties and adhesion of hydrated glass surface layers

This paper reviews results for interfacial adhesion and fracture of silicate glasses that demonstrate the effect of hydrated glass surface layers on the mechanical properties of glass. First, it is shown how the generation of hydrated surface layers formed on alkali borosilicate glasses can control crack propagation rates. Crack growth data, solution analysis and surface stress measurements are used to support a fracture model that involves the generation of surface stress on the crack walls behind the crack tip. A fracture mechanics based model is used to show that stressed layers can contribute to the crack tip stress intensity in a way that either increases or decreases the rate of crack propagation. In the case of alkali containing silicate glasses, tensile stresses formed on the crack walls increase the crack tip stress and contribute to the formation of a low velocity plateau in the stress intensity vs. crack velocity curve. Second, fracture mechanics test techniques are used to examine the adhesive bond formed between hydrated surface layers and bulk silicate glass. The adhesive bonds formed by sol-gel precursors composed of colloidal silica, hydrolyzed organosilanes and alkali silicate solutions are compared to determine the mechanism of interfacial bonding to dense silica substrates. The formation of siloxane bonds across the interface depends upon the nature of the silicate polyanions in solution. For the case of soluble alkali silicate derived films, heat treatments at temperatures as low as 200°C can result interferfacial adhesion energies as large as the fracture energy of silica glass. These results have important implications to the aging and repair of surface damage in glass as well as the adhesion of sol-gel derived thin films.     

Specific features of the formation of terrace-step nanostructures on the (0001) surface of sapphire crystals

The terrace-step structures on atomically smooth vicinal surfaces of sapphire crystals subjected to thermal annealing after mechanical and chemical treatments have been investigated by atomic-force microscopy. The influence of the vicinal angles and conditions of thermal treatment (including annealings in air and vacuum) of sapphire plates on the metric parameters of terrace-step structures has been analyzed. It is shown that steps with a height from monoatomic (0.22 nm) to 3 nm or more can be formed on the sapphire surface by changing the vicinal angle and thermal-treatment conditions. The results are interpreted in terms of the minimum surface energy and surface diffusion processes.     

The role of surface and volume defects in the fracture of glass under quasi-static and dynamic loadings

In this paper, the role of surface and volume defects on fracture in soda-lime glass is analyzed when samples are submitted to quasi-static or dynamic loadings. To investigate fracture, different experiments are carried out, namely, quasi-static compression of glass spheres and edge-on-impacts. The first test series aims at studying crack initiation. Different surface treatments are performed to study their influence on the failure load, from which it is concluded that initiation of cracks occurs in the vicinity of the contact surface. The second series is concerned with the examination of crack patterns under dynamic loadings with two different strikers (i.e., soft/flat and hard/perforating projectiles). Crack initiation under dynamic load histories is investigated near and far from the impact zone and it is concluded that it cannot take place within the volume, except in a very small zone close to the impact point. Conversely, initiation of damage from the surface, at a location far from the impact point, is possible and clearly present with a soft projectile.     

红外窗口和整流罩材料研究现状与发展趋势

介绍红外窗口的三个综合性能评价因子:热破裂品质因子、光畸变品质因子、抗热冲击品质因子;并用它们评价了氟化镁、尖晶石、氮氧化铝、蓝宝石、硫化锌和硒化锌、金刚石等主要的红外窗口和整流罩材料;综述其国内外的研究现状,并对其未来的发展趋势发表了自己的观点.     

Crack growth threshold in soda lime silicate glass: role of hold-time

Atomic force microscopy was used to examine the shapes of cracks and residual features left behind on the fracture surface after holding cracks at a stress-intensity factor below the fatigue threshold for soda lime silicate glass. After propagating a crack at a stress-intensity factor of K_I=0.37 MPa m^1/2, the crack was held either at K_I=0.24 MPa m^1/2, or K_I=0.1 MPa m^1/2 for periods ranging from 1 h to approximately 200 h. Cracks held at the higher K_I left featureless lines on the fracture surface. Becoming more pronounced as the hold-time increased, these lines marked the position of a corrosive notch that formed during the hold period. At about 30 nm from the crack tip, the crack surface displacement decreased reaching zero at the crack tip. The crack tip shape was that of an ogee arch. At the lower hold valued of K_I, crack bifurcation occurred during which the crack became wavy, part of the crack propagating into the crack plane, part out of the crack plane when the crack was repropagated. A smaller crack tip displacement was observed for these cracks. Results of this study are believed to be a consequence of corrosion of the fracture surface caused by a basic solution formed when alkali ions (Na⁺ and K⁺) at the crack tip exchange with hydrogen ions in solution.     

MOVPE growth of InN buffer layers on sapphire

We report on the MOCVD growth of InN buffer layers on sapphire substrate for InN growth. The approach used assumes that an optimized InN buffer layer has to exhibit at least the same crystalline quality and sapphire surface coverage than the GaN buffer layers allowing to grow high crystalline quality GaN on sapphire. The buffer layers were characterized by AFM and GID measurements. Sapphire nitridation was investigated: it has a strong influence on in-plane crystalline quality. Two kinds of buffer layers were optimized according to the GaN buffer layer specifications: one of them only presented In droplets at its surface. It was shown that the small amount of In droplets increases the adatoms mobility of the main layer overgrown, leading to a 25% decrease of its in-plane mosaicity, compared to InN films directly grown on sapphire. To achieve a same improvement on InN buffer layer free of In droplets, the InN main layer growth temperature had to be increased from 550 °C. to 600 °C.     

On the parameters affecting the fracture modes in glass-ceramic materials

The influence of the composition, shape and mean size of the crystallites on the mechanical performance of glass-ceramic materials that contain solid wastes originating from petrochemical distillery and steel scrap recycling facilities has been investigated. It was found that the fracture modes depend on the composition and the shape of the separated crystallites. Additionally, it was found that the increase in their mean size reduces crack propagation, while the fracture mode shifts from transgranular to intergranular. The above shift in the mode of fracture is accompanied by a change in the type of the radial cracks, namely from primary radial in the former to secondary radial in the latter case. Taking into account that the fracture mode determines the strength and toughness, by manipulating the functional properties of glass-ceramic materials, their mechanical performance can be tailored according to the requirements of the potential applications. The results of the present study will be used to determine the composition and morphology of the glass-ceramic materials, in order to achieve the desired morphology that will result in the proper mechanical performance.     

Strain-reduced GaN thick-film grown by hydride vapor phase epitaxy utilizing dot air-bridged structure

A 300 μm GaN thick-film, in diameter 1.5 in, was demonstrated without any crack by hydride vapor phase epitaxy (HVPE) growth. The technique used in relaxing the residual stress caused by differences of thermal expansion coefficients (TEC) and lattice constants between GaN and sapphire substrate to prevent GaN film from crack is called a dot air-bridged structure. After the laser lift-off process, 300-μm-thick freestanding GaN wafer, in diameter 1.5 in, could be fabricated. The compressive stress in the dot air-bridged structure was measured by micro-Raman spectroscopy with the E₂(high) phonon mode. The compressive stress could be reduced to as small as 0.04 GPa, which could prevent the crack during the epitaxial process for GaN growth by HVPE. It is important to obtain a large-area crack-free GaN thick-film, which can be used for fabricating freestanding GaN wafer.     

直流等离子体喷射制备无裂纹自支撑金刚石膜体的晶体组织设计

在自支撑金刚石膜体中发现网状、河流状和环状三种裂纹形式,这几种裂纹形式依沉积温度不同而不同.首次采用了原子力显微镜分析了金刚石自支撑膜体裂纹断裂机制,发现了穿晶断裂和沿晶断裂两种机制,其中,在网状裂纹中,穿晶断裂机制占主要地位;环状裂纹中,沿晶断裂机制占主要地位,而河流状裂纹是两种机制的混合.对应X射线衍射结果,(111)晶面占优的膜体易于开启穿晶断裂机制,(220)晶面占优的膜体易于开启沿晶断裂机制.使用Raman 谱测试的膜体中的本征应力在几十到几百MPa之间,且在膜体中存在应力剖面分布.Raman谱的结果还显示低缺陷的膜体组织有利于阻挡裂纹扩展.通过建立简单力学分析模型,推测了膜体组织对断裂强度的作用.根据实验结果和力学模型,制备了最厚2mm、最大直径120mm的无裂纹自支撑金刚石膜.     

改进热交换法生长蓝宝石晶体的气泡研究

采用改进的热交换法生长的蓝宝石晶体,气泡是其主要缺陷之一.本文采用数值模拟研究了晶体生长过程中氦气流量对坩埚内温场、固液界面形状的影响.并结合晶体生长实验结果,分析了在实际的晶体生长过程中,氦气流量的线性增加对晶体内气泡的尺寸、形态和分布的影响.     

c面白宝石衬底表面形貌对气相传输平衡法制备的γ-LiAlO2层质量的影响

采用气相传输平衡(VTE)技术,在(0001)面白宝石衬底表面上成功地制备出单相γ-LiAlO2层.研究了白宝石衬底表面形貌对γ-LiAlO2层质量的影响,发现白宝石衬底的表面粗糙度和退火处理是两个影响γ-LiAlO2层质量的重要因素.要制备高质量的γ-LiAlO2层,适度的表面粗糙度是恰当的.对白宝石衬底进行退火处理,γ-LiAlO2层的择优取向变差.并对其中可能的机理进行了探讨.     

Fracture in precursor-derived Si–C–N ceramics – Analysis of crack roughness and damage mechanisms

Roughness analysis of fracture in precursor-derived amorphous and phase segregated Si–C–N ceramics using fractal methods is reported, towards examining the possible correlations between fractal scaling of roughness and fracture properties as well as fracture damage mechanisms. Topography of the fracture surfaces created at a crack velocity of ～10^?4 m/s was recorded using atomic force microscopy, and analyzed using RMS roughness and second order height–height correlation functions. The evolution of roughness was well correlated with the evolution of structural and compositional inhomogeneities in the amorphous materials, and the formation of second phases in the phase segregated materials. All the investigated fracture surfaces displayed self-affine scaling with a correlation length of ～50–100 nm and a roughness exponent of 0.8 ± 0.1, commensurate with the universal exponent conjectured by Bouchaud et al. corresponding to dynamic damage regime. No correlation was observed between the roughness exponents and the fracture toughness of the corresponding materials. Examination of the crack opening near the tip region revealed no persistent damage cavities assignable to ‘plastic deformation’ preceding fracture, suggesting that the fracture in the Si–C–N ceramics proceeds in a brittle manner at the employed crack velocities.     

Influence of nitrogen-containing reducing media on the optical and luminescence characteristics of sapphire

The optical and luminescence characteristics of sapphire crystals grown in nitrogen-containing reducing media based on CO have been investigated. It is shown that the nitrogen dissolution in the oxygen sublattice of sapphire under reducing conditions is accompanied by the formation of F ⁺ centers and aggregate F ₂ centers (defects that significantly influence the optical absorption and X-ray-, photo-, and thermoluminescence in sapphire). It is established that the formation of Al₂O₃:N occurs at fairly low nitrogen concentrations in the growth medium (10?C20 vol % when a crystal is grown at P ?? 0.2 Torr and 1 vol % when growth is carried out at P = 800?C950 Torr).     

热屏结构对泡生法蓝宝石单晶炉功率影响研究

蓝宝石晶体因优异的综合性能被广泛用于航空航天等高性能要求领域.泡生法是目前生产大直径蓝宝石单晶的主要方法,热场对生产工艺、产品质量和单晶炉功率具有重要影响;并将影响生产成本.本文对氧化锆及钼金属组合式热屏中氧化锆材料内置、外置及材料不同组合方式对泡生法蓝宝石单晶炉功率的影响进行研究,得到合理的热场结构;并与实际生产结果进行验证.结果表明:相比于传统的15层钼保温结构,加入氧化锆保温层会明显降低单晶炉能耗,其中氧化锆内置的热场结构对单晶炉能耗降低影响更为明显;随着氧化锆层由0增加至15层,单晶炉能耗显著降低,相比传统15层钼保温结构,15层氧化锆保温结构炉体功率降低了38;.     

白宝石晶体的高温强度及强化技术研究进展

白宝石高温强度的降低主要是由于c轴方向受到压应力时产生r面双晶所引起的.高温强度的下降导致其抗热震性能降低,从而限制了白宝石晶体作为红外窗口和头罩材料的应用.白宝石晶体的强化主要以消除双晶核以及控制双晶的扩展来实现.本文分析了影响白宝石强度的主要因素,在此基础上对目前白宝石晶体强化的方法进行了总结和分析.     

Elektronenmikroskopische Untersuchungen zur Rißausbreitung bei der Zugverformung gewalzter Molybdän-Einkristalle

The effect of dislocation structure on mechanical properties, crack propagation and fracture was investigated in molybdenum single crystals predeformed by rolling to produce a homogeneously distributed dislocation structure or a cell structure. The rolled crystals were notched and deformed by tension at various temperatures (493, 293 and 193 K). It is shown, that the crystals with cell structure ((111) [112 ] orientation) have the higher strength, but the crystals with homogeneous structure ((001) [110] orientation) are more ductile. In cell structure cracks propagate catastrophically without pronounced necking. The homogeneous structure becomes unstable during tensile loading above the transition temperature, but crack propagation occurs slowly involving complete necking of the crystal. At low temperatures (193 K) cleavage fracture on {100} planes takes place both in the crystals with homogeneous and cell structure.     

Crystal cracking analysis and three‐dimensional effects during Kyropoulos sapphire growth

Kyropoulos (Ky) method is the most suitable technique for the growth of high‐quality sapphire single crystal for substrate applications. Cracks are often observed in the grown ingot that significantly reduces industrial productivity. In the paper, cracking causes are analyzed by examining crystal shape, thermal stress and three‐dimensional effects during the stable growth of sapphire crystal. It is found that locally induced thermal stress around the shoulder of the crystal is the largest. However, thermal stress is not fully responsible for the cracks, since the predicted stress level is lower than a critical value regarding crystal cracking. Polycrystalline growth or/and other crystal defects must be another factor that degrades the critical value and makes the crystal more fragile. Simulation results further show that crystal shape has less effect on the thermal stress level, although experiments have shown that crack‐free crystals usually have smooth surfaces. The initial cracking position in the ingot predicted in simulation agrees well with experimental observation after considering crystal defects in a qualitative discussion. From the view of three‐dimensional simulation, the variation of heating condition during growth may result in high thermal stress locally that leads to the cracks at one side of the crystal. Additionally, three‐dimensionally unexpected temperature drop of the heater may be responsible for the sticking‐to‐crucible phenomenon at the shoulder region of the grown crystal.     

Novel UV devices on high-quality AlGaN using grooved underlying layer

A grooved Al_0.25Ga_0.75N underlying layer on an AlN-coated sapphire substrate was used to grow crack free and low dislocation density Al_0.25Ga_0.75N to successfully realize high-performance UV A light emitters. A light-emitting diode grown on a grooved AlGaN underlying layer exhibited an output power of 12 mW at a DC current of 50 mA for a peak emission wavelength of 345 nm with an external quantum efficiency of 6.7%, which is the highest to date in this wavelength region. We also fabricated UV A laser diodes with an emission wavelength of 356 nm at a pulsed injection current of 414 mA.