Ceramic materials experiments

Figure 1.1a is a typical stress-strain curve for a brittle ceramic having only elastic deformation up to the point of fracture. As indicated in the introduction, ceramics fail in a typically brittle manner, due to the ionic nature of the bonds, which prevent slip via dislocation motion. The fact that brittle catastrophic failure in ceramics is likely is an indication that very little energy is absorbed in the process of fracmre. Pure aluminum oxide behaves as indicated in Fig. 1.1b. The fracture strain of a ceramic is 0.0008-0.001. One can state that ceramics at room temperature are Hookean until fracture. In general, ceramic materials experience very little or no plastic deformation prior to fracture. Slip is difficult due to the structure and the strong local electrostatic potentials (a consequence of the ionic or covalent bonds).

Often, ceramic materials experience creep deformation as a result of exposure to stresses (usually compressive) at elevated temperatures. In general, the time-deformation creep behavior of ceramics is similar to that of metals (Section 8.12) however, creep occurs at higher temperatures in ceramics. High-temperature compressive creep tests are conducted on ceramic materials to ascertain creep deformation as a fimction of temperature and stress level. 

The same type of experiment has been carried out on electrodes of different superconducting materials. As deducible from Figure 16, not all of these ceramic materials undergo severe degradation in the presence of water. 

At this point, experiments must be performed. Experimental results for the erosive wear of the selected candidate ceramic materials in coal slurries are presented in Table 8.4. Notice that the wear rate has a very rough inverse correlation with which is consistent with some of the descriptions of erosive wear from the previous section. Any of these ceramic materials is suitable for the piping and pump components based solely on wear rate, with the lowest wear rate for SiC being the most attractive. Formability and economic criteria can be applied to assist in the final material selection. 

In testing the abrasiveness of floor finish of man-made materials, special variations of Shore s scleroscope are commonest. For ceramic and rock floor tiles, the commonest testers are Bohme s disc and the Teledyne Abra-der. For sewage and chemical stoneware, the standards now in force recommend abrasiveness tests with the use of a sandblasting jet blower. For technical reasons, a more precise Mackensen blower method for stoneware testing has been in use in Poland for several years. From the experience gained over years, a standard recommending Mackensen s method has been laid down for stoneware abrasiveness tests. Research results proved the usefulness of this method also for other ceramic materials. 

All our preliminary CVD experiments were carried out under an atmosphere of N2 gas in order to determine the intrinsic ability of the studied molecules to serve as precursors to ceramic materials. Further studies could be run under a reactive medium such as H2 in order to reduce the C and O content of the films. This was done in the case of compound 19. Cold-wall CVD experiments on this molecule were performed at 973 K and normal pressure under H2 carrier gas. These experiments resulted in the formation of highly pure VC films.38 XPS and EPMA-WDS analyses of these films showed both the free carbon and oxygen contents to be lower than the limits of detection of the techniques. Various factors can account for these results diminution of the C content induced by H2, stabilization of the Cp ligand in the gas phase due to the presence of f-butyl groups, and decomposition mechanism involving a methyl activation leading to the formation of V = CH2 species.38... 

Until 1986 many experiments were carried out using metals and alloys. However, in 1986 Georg Bednorz and Alex Muller, two researchers of the IBM laboratory in Ruschlikon near Zurich, published an article in the Zeitschrift fur Physik in which they announced that they had made a superconducting ceramic material. It turned out to be a compound made of barium, copper, lanthanum and oxygen, which became superconducting at 35 K. They were awarded the Nobel prize for this discovery. After this, superconductors developed rapidly, at least as far as critical temperature is concerned. 

The book is aimed at ceramics in the year 2000 and the subject is introduced via two routes, i.e. the period of approximately 25,000 BC until about 1850 AD in which clay and related natural raw materials prevailed, and the period from 1850 until 2000 with clay and other, mainly synthetic raw materials. Despite the length of the period in which only clay was used, this book will pay relatively little attention to it. The reason for this is the fact that the number of application of ceramic materials increased considerably with the introduction of synthetic raw materials. Moreover, natural sciences have boomed enormously after World War II and consequently so did the education in ceramics. The main part of this book is therefore devoted to ceramics after 1945. Because one of the applications of this book is in the field of education, the text is occasionally illustrated with simple experiments. 

Wotting G, Hennicke J, Feuer H, Thiemann KH, Vollmer D, Fechter E, Sticher F, Geyer A (2001) Reliability and Reproducibility of Silicon Nitride Valves Experiences of a Field Test. In Heinrich JG, Aldinger F (eds) Ceramic Materials and Components for Engines. Wiley VCH, Weinheim, p 181... 

The manufactured 90 mm x 90 mm x 10 mm three-layered B4C/B4C-30wt%SiC tiles were tested as armor [67], The photographs of the experiment set-up of the ballistic test as well as a residual impression in the clay box that was used as one of the criteria in the ballistic performance of laminates are shown in Fig. 7.17. The ballistic penetration tests were performed to evaluate the ballistic performance of the laminates. Depth of penetration tests were used to evaluate the ballistic performance of the composite laminates. In addition, pure B4C monolithic ceramics were used as a standard for the test. Test panels were made using the three-layered B4C/B4C-SiC laminate and B4C monolithic ceramic material as the hard face. While the B4C monolithic tile had 100% of its theoretical density, the three-layered B4C/B4C-30wt%SiC laminates had about 3-4% of porosity. A commonly used Spectra fiber-reinforced polymer composite was used as backing plates. The targets were mounted on clay and the projectile was shot at the target at a specific velocity. 

During the previous year, efforts were made to apply SLM with ceramics, and it proved to be successful. This is a new technology available now for ceramic materials. The principle of this technique has been described earlier. First preliminary experiments show promising results [18]. 

Use of ceramic materials in high temperature structural applications is often limited by creep resistance. However, several recent studies have shown that composite reinforcement can drastically reduce the creep rates compared to the unreinforced ceramic matrix.12,20,27-31 Most of these studies have been conducted in air, which is a strong oxidizing environment. In a few cases, investigators have attempted to isolate the effects of oxidation from creep by conducting parallel experiments in both air and inert atmospheres.27,28 The following is a description of the salient points made in these investigations. 

First electrochemical studies on structuring and modification of different high superconductor surfaces have recently been started [6.190]. One of the main problems is the instability of oxide ceramic material in neutral and acidic aqueous electrolyte solutions at room temperature [6.196-6.198]. HTSC surfaces corrode, and superconductivity was found to decrease within the topmost layers of IfTSC samples after water contact. This aging effect decreases in alkaline media [6.197]. However, sufficient long term stability of HTSC samples was only found in aprotic solvents such as acetonitrile. Therefore, experiments were carried out in acetonitrile-containing... 

Data from crack velocity experiments are used to Improve the reliability of ceramic materials. It is particularly useful for the application of proof-testing to structural ceramics, because it leads to an estimate of lifetime under load after the proof test has been used to weed out mechanically defective components. For this purpose, the data in the low crack growth velocity range are of prime importance, because it is, by and large, slow... 

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