Im ersten Teil dieser Arbeit wird zunächst die TWB definiert, theoretische Überlegungen der Wärmespannung werden angefügt. Auch wird über die verschiedenen Methoden der Abschreckprüfung berichtet. Die Beurteilungskriterien, die in der Praxis herangezogen werden, werden aufgezählt. Anschließend wird eine spezielle Prüfeinrichtung beschrieben und auf die Resonanz- bzw. Schwingungsdauer-Messung eingegangen. Im 2. Teil der Arbeit werden Entwicklungsarbeiten an SiC-Massen behandelt.
Die Temperaturwechselbeständigkeit wurde durch die bestimmung des Resonanzfrequenzabfalls und durch die Bestimmung der Schwingungsdauer bei Stoßbeanspruchung ermittelt. Beide Verfahren werden beschrieben, verglichen und bewertet. Vorrangig wurde die Bestimmung des Resonanzfrequenzabfalls durchgeführt, da dieser Wert gut und schnell bestimmt werden kann und es sich hierbei um eine zerstörungsfreie Prüfmethode handelt. Beide Prüfmethoden führen zu etwa vergleichbaren Aussagen über die TWB. Hierbei ist der Schwingungsdauermessung mit dem Grindo-Sonic-Gerät der Vorzug zu geben, da diese Werte schneller und genauer angezeigt werden als mit der Resonanzfrequenzmessung. Außerdem wirden Risse mit einer Größeren Wahrscheinlichkeit früher angezeigt. Einer Einfluß auf die TWB haben die Porosität, die Rohdichte und der SiC-Gehalt. Eine gute TWB kann demnach erwartet werden, wenn die Porosität hoch liegt und Körper mit einer Rohdichte zwischen 2,38 g/cm3 und 2,44 g/cm3 hergestellt werden können.
Klang- oder Klopfproben sind im keramischen Bereich seit jeher bekannt. So erkennt man bei Anschlagen mit einem harten Gegenstand gegen einen keramischen Körper (Rohr, Gefäß, Fliese, etc.) am Klang, ob ein Körper porös oder dicht- schwach- oder hoch-gebrannt ist.
Hierbei ist die Aussage nicht quntitativ und ist abhängig von der Körperart, -Masse, -Dichte und nicht zuletzt von der subjektiven Beurteilung der Prüfperson.
Mit dem “Grindosonic-Gerät” wurde ein Frequenzmeßgerät entwickelt, um bei Stoßerregung auftretenden Grundschwingungen (Eigenfrequenzen) zu messen.
Hexagonal boron nitride (BN) is the material of choice for manufacturing break rings for the horinzontal continuous casting steel. Due to the criticality of this application, only a 100% acceptance level can be tolerated. A non-destructive testing method was successfully developed to accomplish this. Comparison of this method with more traditional non-destructive tests showed it to be comparable.
In this paper we present important physical, thermal, mechanical and optical properties of cubic silicon carbide produced via a bulk chemical vapor (CVD) process developed at CVD Incoporated. This CVD SiC has been identified as the leading mirror material for high energy synchrotron radiation because of its high thermal conductivity, low thermal expansion, high polishability, and high reflectance in the vacuum UV. However, it has been difficult to obtain high quality, monolithic. CVD SiC mirrors in large sizes i.e. greater than 10-20 cm. Recently, CVD Incorparated has been successful in scaling an SiC CVD process to produce large monolithic pieces of SiC up to 60 cm (24 in.) in diameter and plates up to 76 cm (30 in.) long by 46 cm (18 in.) wide with thicknesses up to 13 mm (0.5 in.). The properties of this material that can make it attractive for optical applications, such as synchrotron optics, will be discussed.
TiZrC and TiZrB2 solid solution materials were synthesized by conventional powder production methods. These materials exhibited non-linear, and generally superior, mechanical properties when compared to end member constituents.
The elastic properties of ceramic materials are important to their performance in severe abrasion and wear applications and also provide a useful, quantitative measure of quality. We have measured the Young’s modulus E, shear modulus G, and Poisson’s ratio n of several commercial polycrystalline cubic boron nitride and diamond products, in the form of free-standing disks, using the dynamic resonance method. The latter method is accurate and fast enough for routine quality control. Measurements on polycrystalline cBN yielded values of E, G, and n in the ranges of 630-770 GPa, 270-340 GPa, and 0.14-0.18, respectively, depending on the volume fraction of superabrasive, binder phase and microstructure. As a point of comparison, the orientation-averaged values of E, G, and n for pure, equiaxed, polycrystalline cBN are calculated as 909 GPa, 405 GPa, and 0.12, respectively. Measured values of E, G, and n for sintered diamond lay in the ranges of 915-990 GPa, 415-450 GPa, and 0.10-0.11, respectively. The modulus results are compared to selected additional material properties.
The partial substitution of alumina for feldspar in an electrical porcelain leads to an increase in Young’s modulus from 38.27 to 62 Gpa, reaching the maximum value when the firing temperature is 1350°C.
The elastic moduli of both oxidized and reduced MgO, CaO and SrO doped lanthanum chromites were measured by the dynamic Young’s Modulus method. The elastic modulus was also measured as a function of the CaO content in both the oxidized and reduced states. From Reitveld fitting of the X-ray diffraction data, corresponding cation to anion atomic distances were calculated and compared. Changes in the elastic modulus correspond to changes in the Cr+3 to O-2 average interatomic distance for the dopants used.
Structural properties such as flexural moduli and strength have been measured for a range of porous alumina specimens of different initial powder sizes and final porosities, sintered using the capsule-free hot isostatic pressing method. This processing method produces a porous body in which the closed porosity is negligible. The relationship of these structural properties to total porosity has been investigated. The results indicate that both a power and an exponential function could adequately describe the porosity dependence of flexural strength. The strength values obtained were test method dependent, and were significantly lower for specimens with sintering aids. A power law model based on a critical porosity, as proposed by Phani, gave the best fit for the modulus measurement data. No dependence of mechanical properties on particle size was observed. The strength measurement results did not appear to support suggestions that better strength could be obtained by the capsule-free hot isostatic pressing method than conventional sintering, as reported elsewhere.
A practice was demonstrated that could independently determine, with high resolution, bulk E, G and ν of a disk, square, hexagonal, and half-hexagonal ceramic tiles or plates. The method combines modal finite element analysis and the flexural and torsional resonance values (measured by impulse excitation of vibration) for a given geometry and material. The consideration of both resonances is important in this practice because ν is able to be explicitly determined as a consequence and its value does not need to be assumed to determine E and G (as would occur when only one of their resonant frequency values is known).
Si3N4/BN fibrous monoliths were prepared with 4 wt% Y2O3 added as a sintering aid to the Si3N4. Residual carbon, present in the billet before hot-pressing, was shown to influence the final microstructure. The sintering aid glass, known to migrate into the BN cell boundaries during hotpressing, was not sufficient in quantity to prevent premature shear failure when samples were tested in flexure. Increasing the hot-pressing temperature alleviated this problem. For flexure samples tested at 1400°C, fibrous monoliths fabricated with 4 wt% Y2O3 demonstrated linear-elastic loading behavior at a greater stress than fibrous monoliths fabricated with 6-wt%-Y2O3/2-wt%-Al2O3 sintering aids.
The Mode I fracture toughness (KIC) of a small-grained Si3N4 was determined as a function of hot-pressing orientation, temperature, testing atmosphere, and crack length using the single-edge precracked beam method. The diameter of the Si3N4 grains was <0.4 µm, with aspect ratios of 2–8. KIC at 25°C was 6.6 ± 0.2 and 5.9 ± 0.1 MPa.m1/2 for the T–S and T–L orientations, respectively. This difference was attributed to the amount of elongated grains in the plane of crack growth. For both orientations, a continual decrease in KIC was observed through 1200°C, to 4.1 MPa.m1/2, before increasing rapidly to 7.5–8 MPa.m1/2 at 1300°C. The decrease in KIC through 1200°C was a result of grain-boundary glassy phase softening. At 1300°C, reorientation of elongated grains in the direction of the applied load was suggested to explain the large increase in KIC. Crack healing was observed in specimens annealed in air. No R-curve behavior was observed for crack lengths as short as 300 µm at either 25° or 1000°C.
The relationship between microstructure and mechanical properties was studied in Al2O3–ZrO2 eutectic rods. The material, produced by directional solidification using the laser-heated float zone method, was formed mainly of colonies consisting of a fine interpenetrating or ordered network of ZrO2 and a-Al2O3 surrounded by a thick boundary region that contained pores and other defects. The flexure strength of the eutectic rods was excellent (>1.1 GPa) owing to the small critical defect size and the high toughness (7.8 MPa.m1/2). No microstructural changes were observed after about 1 h of exposure at 1700 K, and the eutectic oxide maintained a very high strength up to this temperature. The nature of the critical defects that led to fracture, the toughening micromechanisms, and the differences between the longitudinal and transverse strength are discussed in the light of the microstructural features of the material.
The paper reviews recent work on the fracture mechanics of ring crack formation, from pre-existing small surface cracks, when a hard sphere is pressed elastically against a hard surface. It summarises the ways in which data from these “Hertzian” tests may be used to determine the extent of surface cracking damage, the materials fracture toughness, and the strength of any residual stress in the surface. Examples are given of experiments applying these test methods.
Crack deflection and the subsequent growth of delamination cracks can be a potent source of energy dissipation during the fracture of layered ceramics. In this study, multilayered ceramics that consist of silicon nitride (Si3N4) layers separated by boron nitride/silicon nitride (BN/Si3N4) interphases have been manufactured and tested. Flexural tests reveal that the crack path is dependent on the composition of the interphase between the Si3N4 layers. Experimental measurements of interfacial fracture resistance and frictional sliding resistance show that both quantities increase as the Si3N4 content in the interphase increases. However, contrary to existing theories, high energy absorption capacity has not been realized in materials that exhibit crack deflection but also have moderately high interfacial fracture resistance. Significant energy absorption has been measured only in materials with very low interfacial fracture resistance values. A method of predicting the critical value of the interfacial fracture resistance necessary to ensure a high energy-absorption capacity is presented.
The microstructure and interfacial fracture energy of silicon nitride/boron nitride fibrous monoliths, GBN, were determined as a function of starting silicon nitride composition and temperature using the method described by Charalambides. The glassy phase created by the sintering aids added to the silicon nitride cells was shown to migrate into the boron nitride cell boundaries during hot-pressing. The amount of glassy phase in the boron nitride cell boundaries was shown to strongly influence GBN at room temperature, increasing the fracture energy with increasing amounts of glass. Similar trends in the interfacial fracture energy as a function of temperature were demonstrated by both compositions of fibrous monoliths, with a large peak in GBN observed over a narrow temperature range. For silicon nitride cells densified with 6 wt% yttria and 2 wt% alumina, the room-temperature interfacial fracture energy was 37 J/m2, remaining constant through 950°C. A sharp increase in GBN, to 60 J/m2, was observed between 1000° and 1050°C. This increase was attributed to interactions of the crack tip with the glassy phase in the boron nitride cell boundary. Measurements at 1075°C indicated a marked decrease in GBN to 39 J/m2. The interfacial fracture energy decreased with increasing temperature in the 1200° to 1300°C regime, plateauing between 17 to 20 J/m2. A crack propagation model based on linkup of existing microcracks and peeling/cleaving boron nitride has been proposed.
Despite the considerable improvement in the understanding of transformation toughening accomplished in the last three decades, it remains an important challenge to be able to control and tune the tetragonal ZrO2 phase transformability and associated toughness of polycrystalline tetragonal zirconia (Y-TZP) ceramics. The problem of controlling the toughness of Y-TZP is investigated in the present paper by comparing the mechanical behaviour of a number of ceramics derived from commercial co-precipitated and yttria-coated zirconia starting powders as well as a range of experimental powder mixtures, obtained by mixing monoclinic and 3 mol % yttriastabilised powders. Based on the experimental results, a simple route to tailor the toughness of YTZP ceramics is reported. The effectiveness of this approach was investigated and the microstructural origin influencing transformation toughening is elucidated. The difference in toughness is explained in terms of the tetragonal grain size and the overall amount and distribution of yttria in the sintered ceramics. The overall yttria stabiliser content is of primary importance, the yttria distribution however was found to be an additional important microstructural variable influencing the transformation toughness of the investigated Y-TZP ceramics.
Young’s modulus experiments have been carried out to study the kinetics of ab-plane crack propagation in single domain YBa2Cu3Ox (YBCO) during a prolonged oxygen heat treatment at 400ºC up to 188 h. It has been found that the modulus value experiences a rapid fall between the annealing time 48 and 96 h. X-ray difraction (XRD) experiments have been carried out to investigate the structural phase transition during the oxygenation process. Consistently, a difraction peak shift has been observed in this region that indicates a massive tetragonal-to-orthorhombic (T-to-O) transition. SQUID magnetization measurements of annealed samples have shown clear oxygen inhomogeneity in this regime suggesting a T-to-O phase boundary in the crystal. A physical model is provided to describe the operating mechanism of crack propagation in single domain YBCO.
The mechanical properties of Bi2Sr2CaCu2O8+d fibers produced via laser-induced directional solidification at different growth rates were determined through longitudinal and transverse tension tests, as well as flexure tests. In addition, polished sections of as-received fibers and the fracture surfaces of the broken samples were examined using scanning electron microscopy to elucidate the relationship between the microstructure and the mechanical properties. The fibers were anisotropic, and the transverse fiber strength was very low, because of early failure via cleavage of the grains perpendicular to the c-axis. The longitudinal strength and the degree of anisotropy increased as the fiber growth rate decreased, whereas the transverse strength followed the opposite trend. This behavior was due to changes in the porosity and the alignment of the crystals along the fiber axis.