Cubic BN was synthesized from hexagonal BN in the presence of AlN as a catalyst at high temperatures above 1000°C and pressures above 5.5GPa under controlled atmosphere. In order to protect AlN from oxidation during the high pressure and temperature treatment, the addition of benzene to the mixture of hex. BN and AlN was found to be more efficient than that of toluene. Fully dense cubic BN-AlN sintered compacts were fabricated simultaneously with the conversion of hex. BN at 6.5GPa and 1600°C. The Vickers microhardness of the sintered compact of 20mol% AlN-80mol% cubic BN was about 4500kg/mm2 at room temperature. Direct bonding of cubic BN grains was observed in the sintered compact.
Powder mixtures of AlN and ZrO2(PSZ), in which the amounts of ZrO2 content were 6∼70mol%, were hot-pressed at 1200∼2000°C for 30 minutes under a pressure of 20MPa in reducing atmosphere. When the amount of ZrO2 in the powder mixture was 33mol% (ZrO2/AlN=1/2), all raw powders of AlN and ZrO2 completely reacted, giving a good sintered composite material with brilliant golden colour and finely dispersed reacted products of AlON Spinel, α-Al2O3 and ZrN. For the systems above 33mol% ZrO2, excess ZrO2 (cubic phase) remained as an unreacted material, while below 33mol% ZrO2 unreacted AlN remained. In the case of the ratio of ZrO2/AlN=1/2, a good sintered composite material was obtained and its bending strength σf, toughness KIc, Vickers hardness HV and Youngs modulus E were 448MPa, 2.72 MN/m3/2, 15.3GPa, and 354GPa, respectively. ZrN, one of the reaction products, was oxidized by the heat-treatment at 750°C in air and changed to ZrO2. This oxidation reaction of ZrN to ZrO2 gave volume expansion of about 27%. The effect of the surface compressive stress and microcracks arising from this volume expansion on the mechanical properties of the sintered body were discussed.
An attempt was made to fabricate ZrN1-xCx and TaN1-xCx (0≤x≤1.0) ceramics without additives in order to obtain highly pure and dense polycrystalline anion-substituted materials by high pressure reaction hot-pressing. Compositional and temperature dependences of Vickers microhardness, fracture toughness and thermal conductivity of ZrN-ZrC and TaN-TaC systems were examined. The compositional dependence of lattice constant of ZrN1-xCx obeyed Vegard's law. The full solid solution of TaN1-xCx could not be synthesized, but a partial solid solution system with the composition range of 0.3≤x≤1.0 was prepared. The microhardness of ZrN1-xCx decreased with increasing nitrogen content, but the microhardness of TaN1-xCx increased with nitrogen content. The fracture toughness of ZrN1-xCx decreased linearly from 4.6 to 3.3MN/m3/2 with increasing mole fraction X. Little change in KIc of TaN1-xCx was observed at the range of 0.3≤x≤1.0 and the value of KIc was 4.5MN/m3/2. The thermal conductivity of ZrN1-xCx at room temperature decreased continuously with increasing X. But the thermal conductivity of TaN1-xCx decreased rapidly with increasing carbon content and reached the minimum value at TaN0.5C0.5, and then increased with increasing amount of carbon.
Hot isostatic pressing of Si3N4 powders with and without additives was performed using a glass container, and also various kinds of pressure-less sintered Si3N4 were HIP'ed without a container. The effects of HIP treatment on density, microstructure, flexural strength, microhardness and fracture toughness of Si3N4 ceramics were studied. By HIPing of Si3N4 ceramics using a glass container, it was difficult to reach the theoretical density. The microhardness of HIP'ed Si3N4 without additives was low, and the fracture toughness of HIP'ed Si3N4 with and without additives was about 5MN/m3/2. Thermal conductivity of HIP'ed Si3N4 without additives was 22∼25W/m·K, and it decreased with increasing the amount of additives. The density, flexural strength and microhardness of pressure-less sintered Si3N4 which contained Al2O3 and Y2O3 as oxide additives were remarkably improved by HIP treatment using nitrogen as a pressure transmitting gas. It is very important to select the sintering conditions for fabricating the pre-sintered body of Si3N4 in order to improve the mechanical properties of Si3N4 ceramics by HIPing treatment.
As a part of the study to establish a suitable method for representing the variations of tensile and bending strengths of brittle materials, computer-simulated fracture experiments were carried out on a number of specimens having a fixed number of penny-shaped cracks of different sizes at randomly distributed positions, by using the fracture mechanics approach, and the values of tensile and bending strengths were plotted on the Weibull distribution diagrams. The main results obtained were as follows: (1) When the fracture origin is a deep-inner crack, the tensile strength varies depending upon maximum size of cracks and thus the variation of tensile strength is representable by one Weibull parameter. When the number of existing cracks is large, a surface or near-surface crack becomes the fracture origin and the strength drops considerably depending upon the crack size distribution as well as the position of the critical crack. Thus, more than one Weibull parameter are required to represent the variation of tensile strength. (2) As for bending tests, the strength varies with the crack size as well as the position of the critical crack, and thus, it is difficult to represent the variation of bending strength with one Weibull parameter. By choosing both the numbers of surface and inner cracks and the positions of critical cracks, it seems possible to reproduce a Weibull distribution similar to the experimental one.
It has been found that both the Vickers hardness and the elastic constants of PbO-B2O3 glasses increase with increasing PbO content up to their maximum values at about 42 mole% PbO and then decrease. It has also been shown that, unlike the Vickers hardness and elastic modulus, the fracture toughness and the related fracture energy of the same glasses appear to decrease monotonically with increasing PbO content. In the present study, indentation and fracture behaviours of PbO-B2O3 glasses containing 20.3 to 60.0 mole% PbO were examined on the basis of plastic flow theories for glass. The flow stress was calculated from the hardness and Young's modulus data according to Marsh's theory of indentation. The calculated results indicated that, although the flow stress itself varied with PbO content in a similar manner as the hardness and elastic modulus, the ratio σy/E, where σy is the flow stress and E is the Young's modulus, was found to decrease monotonically with PbO content. This variation of σy/E could be explained well by the plastic flow theory for oxide glasses proposed by Argon, which predicts a relationship between theoretical flow stress and glass transition temperature. The fracture energy variation with PbO content was interpreted in terms of plastic deformation which may occur at the crack tip. By assuming that glass behaves as a non-work-hardening solid and that failure occurs when the plastic zone at the crack tip reaches a critical size, fracture energy behaviour of the present glasses could be reasonably interpreted.
New ceramics such as Si3N4, SiC, PSZ, etc. are expected to be widely utilized as the wear resistant materials for bearing, mechanical seal or other sliding contact applications. For this purpose, it is essential that their tribological properties are thoroughly understood and the effects of load, sliding velocity, temperature, atmosphere and other sliding conditions on friction and wear are clarified. Since ceramics are usually fabricated through sintering process, they have a wide range of microstructures and physical properties, which should affect their tribological properties. In this paper, some mechanical properties, friction and wear of Si3N4 ceramics, fabricated mostly by pressure sintering, were measured, and their interrelations were investigated. The sliding tests were conducted between the same materials with a pin-on-disk friction and wear tester. The friction force was monitored by a load cell, and the wear volume was obtained by measuring the weight loss of the pin sample. The coefficient of friction decreased as the hardness and the fracture toughness of the sample increased. This fact may suggest that the friction force is proportional to the real contact area and is related to the fracture process at the contact point. Any relationship was hardly recognized between the specific wear rate and the hardness or the fracture toughness as a whole, but a negative correlation may probably exist for Si3N4 ceramics with MgO additives. The wear mechanism of Si3N4 ceramics was also investigated by observing the wear surfaces and wear particles with a scanning electron microscope.
Enhancement of ionic conductivity in the LiBr·H2O-Al2O3 composite solid electrolyte was studied by means of the 7Li spin-lattice relaxation time T1. In α-LiBr·H2O, T1 was determined by the Li diffusion with an activation energy of 52kJ mol-1. In the composite solid electrolyte, two values of T1 were observed at each temperature. One of them was attributed to LiBr·H2O, because it had the same value as T1 of LiBr·H2O, while the other had a very low activation energy of 18kJ mol-1. This is the first time that the presence of a highly conductive phase in the composite solid electrolyte has been confirmed experimentally. Magnetization showed that the volumes of these two phases were about equal. The thickness of the highly conductive layer on an alumina particle was estimated to be 0.07∼0.2μm.
The influence of cathodic overprotection at E=-1.2V vs. SCE (Saturated Calomel Electrode) on the corrosion fatigue crack growth behavior of a high-tensil steel HT80 has been invesitgated at R=0.1-0.8 and f=0.1-0.01Hz in 3.5%NaCl solution. The major acceleration factors dominating corrosion fatigue crack growth behavior under cathodic overprotection have been discussed. At the free corrosion potential (E=Ec), an acidified solution within cracks (pH=2) made the crack wall dissolve and the crack blunting, which causes a decrease in Kop from that in dry air. At E=-1.2V vs. SCE the pH value within cracks was 10, causing an increase in Kop due to the caustic corrosion products (caustic corrosion-products induced wedge-effect). The effect of load ratio on da/dN at E=Ec was negligible in terms of ΔKeff. At E=-1.2V vs. SCE da/dN at R=0.8 accelerated from those at R<0.8 at high ΔKeff, which was brought about by static SCC being dominating crack growth behavior. On the other hand, da/dN became greater as the frequency decreased both at E=Ec and E=-1.2V vs. SCE. The major acceleration factor was hydrogen embrittlement at E=Ec, whilst they were hydrogen embrittlement and stress-assisted caustic dissolution at low ΔKeff, and hydrogen embrittlement at high ΔKeff at E=-1.2V vs. SCE.
It can be regarded that the corrosion fatigue life consists of three processes; the growth of corrosion pits, the initiation of fatigue cracks from these pits and the propagation of fatigue cracks. Especially, the fatigue crack propagation life for a member subjected to considerably fast stress repetition, such as a turbine blade, is considered to be extremely short and sometimes neglected compared to other lives. The corrosion fatigue life in such a case may be estimated by predicting the corrosion pit growth behavior and the critical pit depth of fatigue crack, initiation with a help of the fracture mechanics technique. It is made clear in this paper that the life predicted, regarding the corrosion pit as a microcrack, is in good agreement with the empirical results as well as that predicted by means of the conventional method proposed by the authors.
Effectiveness of polymer electrolytes containing PO43- (+Zn2+)-type inhibitors on prevention of pitting of carbon steels, both acid pickled and mill scaled, in a variety of concentrated cooling water was investigated. The pitting rate during a month of the test period could be reduced to less than 0.05mm depth by optimizing the concentrations of the polymer electrolyte scale inhibitor and corrosion inhibitor and the quality of cooling water to be used.
Erosion corrosion of an usual carbon steel in a variety of aqueous slurries of 10% Al2O3 powder composed of 0.1M NaOH, concentrated cooling water (synthetic rather than natural), 3% NaCl, and 0.01M HCl was investigated from the electrochemical point of view. The total weight loss, Wt, was experimentally divided into the following three terms: the mechanical loss, Wm, the corrosion loss, Wc, and the loss due to erosion corrosion, Wec. Wm was measured under catholic polarization whereby no corrosion occurred. On the other hand, Wc was obtained in the respective solutions free of alumina powder. Therefore, Wec was evaluated as the difference between Wt and the sum of Wm and Wc. Of these, Wm was relatively small compared to other factors under the experimental conditions employed. The weight loss due to corrosion associating with erosion, i.e., Wc+Wec, was discussed from the electrochemical concept.
Two-dimensional quantitative analysis of SEM (Scanning Electron Microscope) images of fracture and corrosion damaged surfaces was made by using the computer image processing and pattern recognition technique. This SEM image analysis was found to be successful with enough accuracy for automatic measurement of fatigue striation spacing, identification of corrosion pits and their statistical distribution, and classification of intergranular and transgranular cracking portions. The quantitative and statistical analysis of SEM images, which have required specialists's experience, a great deal of labor and much time, can be carried out now quickly and accurately by using a digital computer with fast-processing capability.