Key Engineering Materials Vols. 317-318

Abstract: The feasibility of an internal heating method for the fabrication of large-scale nanostructured TiO2 samples by high pressure/low temperature sintering is studied. The primary apparatus used is a piston-cylinder type. The arrangement inside the die cavity consists of a circular cylindrical green compact, a graphite furnace to heat the sample, an aluminum container to protect the sample and an aluminum silicate or lava tube for electrical insulation. When the sample inside the mold achieved the desired sintering temperature, the temperature of the thermally insulated mold stayed low enough not to damage the mold itself. An obtained cylindrical sample has dimensions of 12.7 mm (1/2 inch) in diameter by 28.6 mm (1 1/8 inch) long. Its average grain size is 64 nm and the Vicker’s hardness is 5.3 GPa.

Abstract: The growth of carbon nanotubes (CNTs) from surface of carbon fibers rods (CFRs) as used supports is under investigation using the Fe-catalytic chemical vapor deposition method. TEM studies indicate that under the experimental conditions Fe catalysts on CFRs produces mostly bundles of multiwall carbon nanotubes (MWCNTs). SEM results show the best presence of bundles of MWCNTs for Fe catalysts of 6.5 mol % at 800°C.

Abstract: Transitional metals, such as V, Nb, Mo and Ti, were used to control the growth of ultrafine WC particles. Based on a study of the microstructures these metals were effective in inhibiting the growth of WC. The interaction parameter measured between those metals and W in a Co melt was a positive quantity. This indicates that the presence of V, Nb, Mo or Ti would tend to reduce the solubility of W in a Co melt, thus inhibiting the dissolution and growth of WC.

Abstract: Two α-Al2O3/YAG composite powders have been prepared by reverse-strike precipitation, starting from chlorides aqueous solutions, the former containing 50 vol% of the two phases (labelled as AY50) and the latter made of 90 vol% of alumina and 10 vol% of YAG (AY90). The as-prepared powders were characterised by DTA/TG simultaneous analysis as well as by XRD analysis performed after calcination at different temperatures. A systematic TEM analysis was performed on AY50 powders pre-treated at different temperatures, in order to investigate the crystallites size evolution as a function of the temperature. After that, samples were compacted by uniaxial pressing and sintered at 1600°C for 3h. SEM observations revealed a homogeneous microstructure made of micronic alpha-alumina and YAG grains. For limiting grain growth through the decreasing of the maximum sintering temperature, an innovative activation procedure by coupling suitable thermal and mechanical treatments of the powders was performed. After that, high densification (>95% of the theoretical density) was easily achieved by performing a free sintering in the temperature range between 1320° and 1420°C, with different soaking times at the maximum temperature. The resulting sintered bodies showed an effective retention of the nano-size of the primary particles. By SEM, highly-homogeneous nanostructures, with an average grains size of about 200 and 300 nm for AY50 and AY90, respectively, were observed.

Abstract: The aim of this paper is to study the effect of relaxing the assumption in the Peierls analysis that the dislocation must be wide compared to the atom spacing. To do this the use of the continuum description of the in-plane strains caused by the presence of an edge dislocation is replaced by an atomistic interaction taken to be linear elastic. It is found that in this case the inplane interactions give a contribution to the overall misfit energy changes that are not present in the Peierls analysis because of the use of a continuum approach. This contribution modifies these energy changes so that the total misfit energy is a minimum at the conventional low energy positions (whereas in the Peierls analysis it is a maximum) and gives values of the Peierls stress in reasonable agreement with those measured.

Abstract: Shot peening is commonly used to modify material surface layers and improve the strength of metal components. As it occurred the same technique can be applied for brittle ceramics. High compressive stresses up to 2.4 GPa were introduced into near surface region of alumina and zirconia ceramics by ultrasonic shot peening maintaining its surface integrity. Dependence between diameter of tungsten balls, treatment time (at constant mass of balls in the housing and vibration amplitude) and level of compressive stress introduced was determined for both ceramics with nano and micrograins. Coarser grained ceramics was found to be more sensible to structural changes responsible for stress creation than the smaller one. High microplastic deformation in shot peened surface layer of ceramics was observed by X-ray diffraction. An increase of hardness and surface resistance to fracture with increasing level of compressive stress was found.

Abstract: Although porous ceramics are materials with high potential for helping conserve the environment, the characteristics of pore-related mechanical properties have not yet been examined sufficiently. The R-curve behavior of porous ceramics was estimated using the crack stabilizer technique developed by Nojima et al. Also, the critical frontal process zone (CFPZ) size for porous ceramics was estimated from the strength and fracture toughness of the materials used. The results revealed that the R-curve behavior was almost flat in porous ceramics, in contrast with a steeply rising R-curve behavior for porous silicon carbide observed previously, and that the CFPZ size of porous ceramics was larger than that of dense ceramics. A schematic explanation for the crack extension in porous materials was presented to discuss the R-curve behavior of porous ceramics.

Abstract: Machining damage and its influence on strength was examined for alumina ceramics with various grain sizes, which were prepared by dry-pressing method with spray-dried granules, followed by sintering at 1350-1550°C for 2-27 hours. Grain sizes of the sintered bodies were 1-6μm. After machining at the same grinding force, specimens were placed in an alcohol solution of fluorescent dye and dried. The fracture strength was measured by 4 point bending. The machining flaws in the specimens were observed with a confocal scanning laser fluorescence microscope. Machining flaws appeared continuous, wide and deep in the specimen with small grain size, and discontinuous with large size. On ground surfaces, intra-grain fracture appeared on the surface, whereas inter-grain fracture dominates. The continuous, wide and deep flaws were attributed to the dissipation of stored energy associated with the cracks propagation. The result suggested that the stored energy on the machining process increased with decreasing grain size. The strength of the specimen with 1 μm grain size reduced from 500MPa to 250 MPa with machining damage. The strength depended remarkably on the depth of the machining damage.

Abstract: We attempted to characterize by Raman piezo-spectroscopy residual stresses as they develop in chemical vapor deposition (CVD) Al2O3 coatings on Si3N4 ceramic substrates. According to a piezo-spectroscopic procedure coupled with a confocal configuration of the optical probe used, two-dimensional stress maps could be collected at various depths along the thickness of the coating. By comparing Al2O3 coatings produced on Si3N4 substrates at different CVD temperatures, a tensile residual stress field has been detected in the coating, whose magnitude increased with increasing the CVD temperature. As for the three-dimensional distribution of tensile residual stress within the Al2O3 coating, it was found that the stress value was minimum at the coating external surface, while it gradually increased to reach a maximum near the coating/substrate interface. Similarly, the compressive stress within the Si3N4 substrate was maximized near the coating/substrate interface and decreased with proceeding towards the substrate material bulk. It could be concluded that confocal Raman piezo-spectroscopy is a very suitable tool for three-dimensional stress characterization of ceramic coating materials.

Abstract: The present paper is based on the contribution by Niihara and his co-workers devoted to indentation testing of ceramic materials, while it provides new observations of peculiarities registered during nanoindentation of sapphire, GaAs and InGaNAs deposited by MBE-technique. Exploiting previous studies of the spherical indentation in sapphire, the present authors recognized different causes that result in the apparently similar pop-in phenomenon for sapphire and GaAs-based semiconductors. The finite element modeling of the quasi-plastic nanoindentation of the ( 1 1 20) plane of sapphire with the elastically deformable tip confirmed that the deformation of sapphire is governed by twinning which causes pop-in phenomenon, as suggested earlier by Niihara et al. The singularities registered for GaAs-based crystals are associated with dislocation movement within {111} slip bands, which is in contrast to the case of sapphire.