Materials Science Forum Vols. 591-593

Abstract: SiC fiber-reinforced SiC matrix composite (SiCf/SiC) is one of the leading candidates in ceramic materials for engineering applications due to its unique combination of properties such as high thermal conductivity, high resistance to corrosion and working conditions. Fiber-reinforced composites are materials which exhibit a significant improvement in properties like ductility in comparison to the monolithic SiC ceramic. The SiCf/SiC composite was obtained from a C/C composite precursor using convertion reaction under high temperature and controlled atmosphere. In this work, SiC phase presented the stacking faults in the structure, being not possible to calculate the unit cell size, symmetry and bond lengths but it seem equal card number 29-1129 of JCPDS.

Abstract: Boron Carbide is a ceramic material of technological application due to its extreme hardness and high chemical and thermal stability. The effect of synthesized boron carbide addition on pressureless sintering and hot-pressing of a commercial B4C was investigated. B4C synthesized by carbothermal reduction using carbon black as carbon source was mixtured in 10, 30 and 50 wt% to a commercial B4C. Powder mixtures were compacted into pellets and sintered by pressureless sintering at 2050 °C/30min Samples were compared to a pure commercial B4C and characterization results have not showed great differences. Relative densities of as-sintered materials exceed 93% of theoretical for all compositions and microhardness Hv of ∼ 32 GPa was obtained.

Abstract: In this work, the mechanical properties of SiC samples with (5 wt%) alumina and (5 wt%) rare earth oxides (REO) additions were investigated at room and high temperatures. The possibility of using REO instead of pure yttria could bring cost reductions to obtain sintered SiC ceramics. The powders were mixed, milled, sieved and hot-pressed at 1800°C in argon atmosphere. Hardness and fracture toughness were evaluated at room temperature. The compressive creep behavior was carried out under stress range from 150 to 300MPa and temperatures of 1300, 1350 and 1400°C. After creep tests, the surface analysis of the samples by XRD and SEM showed indications of oxidation reactions, cavity and cracking. The possible creep mechanisms activated in this system were diffusion, grain boundary sliding and cavitation

Abstract: During gray cast iron cutting, the great rate of mechanical energy from cutting forces is converted into heat. Considerable heat is generated, principally in three areas: the shear zone, rake face and at the clearance side of the cutting edge. Excessive heat will cause undesirable high temperature in the tool which leads to softening of the tool and its accelerated wear and breakage. Nowadays the advanced ceramics are widely used in cutting tools. In this paper a composition special of Si3N4 was sintering, characterized, cut and ground to make SNGN120408 and applyed in machining gray cast iron with hardness equal 205 HB in dry cutting conditions by using digital controlled computer lathe. The tool performance was analysed in function of cutting forces, flank wear, temperature and roughness. Therefore metal removing process is carried out for three different cutting speeds (300 m/min, 600 m/min, and 800 m/min), while a cutting depth of 1 mm and a feed rate of 0.33 mm/rev are kept constant. As a result of the experiments, the lowest main cutting force, which depends on cutting speed, is obtained as 264 N at 600 m/min while the highest main cutting force is recorded as 294 N at 300 m/min.

Abstract: Due to their high hardness and wear resistance Si3N4 based ceramics are one of the most suitable cutting tool materials for machining hardened materials. Therefore, their high degree of brittleness usually leads to inconsistent results and sudden catastrophic failures. Improvement of the functional properties these tools and reduction of the ecological threats may be accomplished by employing the technology of putting down hard coatings on tools in the state-of-the-art PVD processes, mostly by improvement of the tribological contact conditions in the cutting zone and by eliminating the cutting fluids. However in this paper was used a Si3N4 based cutting tool commercial with a layer TiN coating. In this investigation, the performance of TiN coating was assessed on turning used to machine an automotive grade compacted graphite iron. As part of the study were used to characterise the performance of cutting tool, flank wear, temperature and roughness. The results showed that the layer TiN coating failed to dry compacted graphite iron under aggressive machining conditions. However, using the measurement of flank wear technique, the average tool life of was increased by Vc=160 m/min.The latter was also observed using a toolmakers microscope and scanning electron microscopy (SEM).

Abstract: The control of the heating curve to manipulate microstructure during sintering is a way that has being studied and it presents advantages such as simplicity and economy. In this work, it was studied the sintering in two-steps of a commercial ultrafine alumina. For this, the alumina power was deagglomerated in milling ball and the specimens for sintering were pressed. Sintering was performed in a dilatometer, with constant heating rate of 15°C/min up to 1500°C. By these results, heat treatment temperatures for two-step sintering were defined. The sintering specimens were characterized through the apparent density measures using Archimedes method, the grain size measures using image analysis program and microstructural analysis using a scanning electron microscope. The results showed that the two-step sintering influence in the development of the final microstructure and permit the control of the grain size and density.

Abstract: In this work, silicon carbide ceramics were developed by liquid phase sintering using AlN-Y2O3 as additive. Two compositions were obtained using SiC powders and different contents of AlN-Y2O3. The powders were mixed/homogeneizated and subsequently dried and deagglomerated. Powder mixtures were compacted by cold isostatic pressing. Samples were sintered at 2080oC, for 1h, under 0.2 MPa-N2 atmosphere. Sintered samples were characterized by X-Ray diffraction and density. The oxidation behavior was investigated and related to the densification and additive-content. Samples were submitted to the tests at 1200, 1300 or 1400oC, in air for 120 hours. Weight gain of the samples is plotted as function of the exposure time, obtaining the evolution of the oxidation on the surface of the samples. Based on the results, the parabolic oxidation content (kp) and activation energy were determined. The results indicate that the samples present parabolic behavior in all conditions. The activation energy results indicate that the phenomena of diffusion of oxygen ions into the oxide layer and interfacial reactions between oxide layer and intergranular phase are the responsible for oxidation mechanism.

Abstract: Many processes in ceramic manufacturing require handling of fine powders with particle sizes down to sub-micron range. Problems that are often experienced with these powders, such as stoppages and/or surges, can be predicted and prevented by first measuring relevant flow properties of these powders, and then using these properties to design a handling system. In this paper we will review common problems with handling such powders and the relevant flow properties tests, such as permeability, compressibility, cohesive strength and friction, as well as how these properties can be used to prevent and solve problems. Issues related to handling titanium dioxide (TiO2) will be used as an illustration.

Abstract: In this work the cyclic fatigue life of 3mol.%Y2O3-stabilized zirconia polycrystalline ceramics, doped with 5%wt 3CaO.P2O5,-SiO2-MgO, has been investigated. Samples with 5 and 10%wt were cold uniaxial pressed (80MPa) and sintered in air at 1200 and 1300oC for 120 minutes. Sintered samples were characterized by X-Ray diffraction and Scanning Electronic Microscopy. Hardness and fracture toughness were determined using Vicker’s indentation method, and Modulus of Rupture was determined by four-point bending testing. Furthermore, the cyclic fatigue tests were also realized by four-point bending tests, under frequency of 25 Hz and stress ratio, R, of 0.1, for the best condition. In this condition, highly dense samples were obtained and presented values of hardness, fracture toughness and bending strength of 11.3 ±0.1GPa, 6.1±0.4MPa.m1/2 and 320±55MPa, respectively. The increasing of stress level leads to decreasing of the number of cycles and the number of run-out specimens. The stress induced tetragonal-monoclinic (t-m)-ZrO2 transformation, observed by X-Ray diffraction, contributes to the increasing of the fatigue life. Samples 3Y-TZP presents clearly a range of loading conditions where cyclic fatigue can be detected.

Abstract: Many researchers became interested in the discovery of Bi2Sr2CaCu2O8+δ oxides with critical temperature of around 80 K. It is known that the critical temperature is related to the CuO2 planes of the material. For this reason, the study of the interstitial oxygen in these oxides is of great relevance. The samples were prepared by means of conventional solid state reactions, through the stoichiometric mixture of precursory powders. After the sinterization, the samples were submitted to measurements of density, electrical resistivity, x-ray diffraction, scanning electron microscopy and energy dispersion spectroscopy, with the objective of performing their characterization. The measurements of mechanical spectroscopy were performed by a torsion pendulum. The results show three relaxation processes in the temperature range of 200 and 700 K, with activation energy of approximately 0.9 eV, which has been attributed to the dynamics of the interstitial oxygen present in the material.