Thermal change in hot-pressed ZrS2 was investigated in an argon atmosphere up to 1500°C. During hot-pressing, ZrS2 released some sulfur atoms and the S/Zr ratio decreased. The crystal phase changed from ZrS2 to Zr3S4 at 1300°C-1500°C. In order to investigate this crystal phase change due to the variation in sulfur content, several Zr-S compounds in the range of S/Zr ratio 1.0-2.2 were synthesized. The lattice parameter measurements by the X-ray powder diffraction method revealed that the non-stoichiometric range of ZrS2 is narrow, while that of Zr3S4 is wide. The bulk density and compressive strength of the sintered body increased with increasing hot-pressing temperature, but the crystal phase change from ZrS2 to Zr3S4 during hot-pressing gave a bad effect to densification and strength of the sintered body. SEM photographs of the fractured surface of the sintered body showed grain growth along the direction parallel to the basal plane of ZrS2. It is supposed that the diffusion of Zr and S atoms to the direction parallel to van der Waals' layers is easier than that to the perpendicular direction in ZrS2. In Zr3S4, the partially occupied Zr layers extend in many directions and so the directional difference in the mobility of atoms is small compared with the case in ZrS2.
As a primary condition for vitrification procedure of molten slag in the SiO2-MgO-CaO-Al2O3-TiO2 system, the vitrification temperature TV and the devitrification temperature TC were determined at three different cooling rates as a function of slag composition. Both TV and TC were found to decrease with increasing cooling rate. DTA analysis on heating and cooling runs of the slag specimen showed that the TV was well above the liquidus temperature and the TC was just above it. In order to explain this phenomenon, a new type of liquid model was proposed by which a perceptible amount of nuclei are preserved even above the liquidus. Systematic analysis on the compositional dependences of TV and TC revealed that constituents SiO2, Al2O3 and TiO2 lower both TV and TC, while MgO and CaO raise them. This is considered to mean that Al2O3 and TiO2 would produce AlO4 and TiO4 tetrahedra in the presence of MgO and CaO, and these tetrahedra will work as glass network formers as will do SiO4 tetrahedra. Excess MgO or CaO, on the other hand, would produce non-bridging oxygens in the AlO4, TiO4 or SiO4 tetrahedra and will weaken vitrification tendency.
In order to examine the possibility of hot-forming of ceramics by utilizing their plastic property and to elucidate the mechanism of the process, a hot-extrusion processing was applied to polycrystalline Ba-ferrite (BaO·6Fe2O3). The effects of the stem speed and area contraction ratio on the microstructure and mechanical properties of the extruded Ba-ferrite were studied. Sintered Ba-ferrite samples were capsulated into stainless steel (SUS 304) to prevent the fracture of the ceramic, and heated from 1100°C to 1390°C. The heated capsule was immediately transferred into the container and then extruded at a stem speed of 3.5mm/sec to 17.5mm/sec with varying area contraction ratios from 48% to 84%. The extruded Ba-ferrite ceramic was found to exhibit finer and denser microstructure than the non-extruded. These effects were accelerated with larger deformation and lower stem speed. The Vickers hardness (Hv) and fracture toughness (Kc) of the extruded Ba-ferrite increased remarkably and their values were comparable to those observed for a hot-pressed body. As for Kc, anisotropy was observed: Kc perpendicular to the extruding direction was higher than that of the parallel direction. In the case of extrusion of the billet at large contraction ratios, smooth extrusion of the ceramic could not be realized due to the difference in deformation resistance between the capusule and Ba-ferrite.
The crack growth stability condition of brittle materials was considered using an equilibrium Gibbs free energy diagram, where the universal relationship of the dimensionless load and dimensionless displacement was utilized to calculate the Gibbs free energy of elastic fracture systems. The free energy diagram was then applied to a crack stabilizer which is specially designed for measuring the R-curve behavior of brittle ceramics showing unstable fracture. Some experimental R-curves of polycrystalline ceramics were measured by the use of the stabilizer.
Thermal stress fracture in 96% alumina-kovar bonded joints was studied by thermal elasto-plastic analysis. Thermal stress fracture of this joint occured at the center of the alumina plate at about 873K during post bonded heating and the fracture was detected by AE measurements. This phenomena was able to be simulated analytically by thermal elasto-plastic FEM and the stress normal to the direction of the bonded line was found to be a good parameter for thermal stress fracture. As the result of this analysis, it is predicted that the sensitivity of alumina for thermal fracture becomes higher with increasing the plate thickness of Alumina or Ag metallized layer.
Tensile tests were carried out on sintered Si3N4 at room temperature under the ambient condition, and the influence of defects upon the fracture stress was discussed. As the surface of specimens was prepared carefully, fracture of 57 specimens out of 60 originated from the defects such as voids or inclusions which resulted from the sintering process. As a parameter to describe the size of the defect, √area was used which is the square root of the area of a 3-dimensional defect projected to the direction of the maximum principal stress. The results obtained are as follows. (1) √area of defects which caused fracture nearly conforms to the first asymptotic distribution of the largest value and the fracture strength conforms to Weibull distribution. (2) √area is a good parameter to describe the size of the defect from the standpoint of fracture. (3) Defects such as voids and inclusions act exactly as cracks of the same size. (4) The fracture strength can be estimated by using the distribution of √area, the fracture toughness and the mean grain size of Si3N4.
Si3N4-based ceramics such as hot isostatically pressed Si3N4, hot pressed Si3N4, and hot pressed sialons containing 0, 30, 60 and 100% of α phase were corroded by K2SO4 and K2CO3 melts at 1150-1300°C and 925-1150°C, respectively. The surface reaction-controlled shrinking core model adequately described the relationship between the weight loss of the specimen and the time for the corrosion reactions in both K2SO4 and K2CO3 melts. The corrosion rate in K2CO3 melts decreased with increasing the content of Al and Y ions in the specimens. The fracture strength of the specimens corroded by K2SO4 and K2CO3 melts degraded to 2/3-2/5 of the original values before lossing 2% of its weight, and then became almost constant up to 30% of weight loss.
A cyclic fatigue testing device by using a piezo-electric bimorph actuator has been investigated to evaluate cyclic fatigue phenomena of fine ceramics under cyclic loading at high frequencies. The bimorph was constructed as a cantilever. A large deflection induced by input voltage at the resonant frequency is applied as a stress amplitude to a ceramic testpiece placed on the fixture of a three point bending apparatus. This device has the following features. (1) Frequency could be changed between 100 and 580Hz by changing a frequency adjusting weight. (2) Stress amplitude could be changed 0 to 65kgf by controlling input voltage.
Preferential attack behavior of welded parts of carbon steel pipes used for piping of industrial water was investigated by several kinds of corrosion tests and electrochemical measurements in chloride solutions. The attack under consideration was classified into two types: the grooving corrosion that occurs along the welded seam of electric resistance welded (ERW) steel pipes and selective corrosion at the weld metal of girth welded joints of steel pipes. The attack occurred preferentially at the portion whose corrosion potential is least noble than any other parts of the weld of carbon steel pipes. The potential of the welded seam was 20 to 60mV lower than that of the base metal of the ERW steel pipe. Difference in the polarization behavior between them was large in an ordinary carbon steel pipe, while the difference is small in a grooving corrosion resistant 0.2% Cu-0.08% Ni steel one. The evaluation of the susceptibility of welded steel pipes to selective corrosion can be done by the accelerated test, in which the specimen was polarized potentiostatically at -0.5V vs. SCE for 48hrs in 3% NaCl solution at 25°C. The test result was in good agreement with those of a loop test in synthetic sea water and a long term piping test in a chloride containing solution. The evaluation of the corrosion resistance at the girth welded parts as well as grooving corrosion resistance was carried out by the accelerated corrosion test. The use of a 0.5% Cu-0.3% Ni steel as a welding material was effective to improve the localized corrosion resistance of the welded joint of carbon steel pipes.
To determine a suitable material to assure the long life of fuel injection nozzles equipped in marine diesel engines, the hot corrosion test in accordance with the method prescribed by Japan Society for the Promotion of Science and other accompanying tests were carried out on several candidate materials. The materials selected by these laboratory tests were examined in the field by making the nozzeles for the engines in service. In these field tests, the high Ni-Cr-Al alloy nozzle tips had 28 times the life of the original nozzle tips made of material SKH51. It can be expected that the high Ni-Cr-Al alloy nozzle tips will be free from any maintenance for more than two years. These field test results generally agreed with the laboratory test results and proved that a suitable material could be successfully selected by the laboratory tests. In future, to obtain the laboratory test results closer to the field test results, it will be necessary that laboratory tests be so carried out as to simulate the service condition of the nozzles in the field or that bench tests be performed in addition to verify the laboratory test results. Finally, these tests have proved that for the present the high Ni-Cr-Al alloy is most effectively applied as the material for the fuel injection nozzle tips in the marine diesel engines on which low grade fuel oil is being often used recently.