Cosme Roberto Moreira Silva

Mechanical properties of ceramic composites

Abstract Silicon-nitride-based ceramic materials are strong candidates for structural applications, such as cutting tools, considering their high thermal and mechanical properties. However, such materials can suffer catastrophic failures, because they are very sensitive to flaws. In this work, silicon-nitride-based matrix was reinforced with silicon carbide whiskers, aiming to increase fracture toughness. Monolithic silicon nitride samples were also produced. All compositions were pressureless and hot uniaxial pressed, followed by physical and mechanical properties determination. Fracture toughness of composites showed higher fracture toughness values than those from monolithic materials, probably by the activation of crack and crack-bridging mechanisms.

Evaluation of the reliability of Si3N4-Al2O3 -CTR2O3 ceramics through Weibull analysis

The objective of this work has been to compare the reliability of two Si3N4 ceramics, with Y2O3/Al2O3 or CTR2O3/Al2O3 mixtures as additives, in regard to their 4-point bending strength and to confirm the potential of the rare earth oxide mixture, CTR2O3, produced at FAENQUIL, as an alternative, low cost sinter additive for pure Y2O3 in the sintering of Si3N4 ceramics. The oxide mixture CTR2O3 is a solid solution formed mainly by Y2O3, Er2O3, Yb2O3 and Dy2O3 with other minor constituents and is obtained at a cost of only 20% of pure Y2O3. Samples were sintered by a gas pressure sintering process at 1900 °C under a nitrogen pressure of 1.5 MPa and an isothermal holding time of 2 h. The obtained materials were characterized by their relative density, phase composition and bending strength. The Weibull analysis was used to describe the reliability of these materials. Both materials produced presented relative densities higher than 99.5%t.d., b-Si3N4 and Y3Al5O12 (YAG) as cristalline phases and bending strengths higher than 650 MPa, thus demonstrating similar behaviors regarding their physical, chemical and mechanical characteristics. The statistical analysis of their strength also showed similar results for both materials, with Weibull moduli m of about 15 and characteristic stress values so of about 700 MPa. These results confirmed the possibility of using the rare earth oxide mixture, CTR2O3, as sinter additive for high performance Si3N4 ceramics, without prejudice of the mechanical properties when compared to Si3N4 ceramics sintered with pure Y2O3.

Mechanical properties of sialon

Abstract Silicon nitride based ceramics have several important applications in the fabrication of hard materials, such as ceramic cutting tools. In this work, aluminum nitride, yttrium oxide and cerium oxide were used as sintering aids on pressureless sintered Si 3 N 4 . The relationships between the type and quantities of these additives with micro hardness (HV), fracture toughness and formed crystalline phases, such as Sialon, have been evaluated. The correlation between microstructure and the mechanical properties has been studied, using the scanning electron microscopy.

A contribution of X ray diffraction analysis in the determination of creep of Si3N4 ceramics

The understanding of the creep behavior of silicon nitride (Si3N4) is extremely complex because of a large number of parameters influencing simultaneously the creep deformation of the materials. In general, the main creep mechanisms acting in these materials are grain boundary sliding or materials transport by solution-precipitation process. In this work, the creep behavior has been monitored by X ray diffraction analysis, determining the peak intensity ratio of the (101) and (210) planes of b-Si3N4. This characterization technical, allied the microstructural analysis can contribute to determination of creep mechanisms acting in material. The b-Si3N4 grains are highly elongated with aspect ratios ranging between 3 and 11. Therefore, the intensity of the peaks related to the basal plane (101) tends to be higher when compared to the lateral planes (210). During creep deformation occurs alignment of the elongated b-Si3N4 grains in the plane parallel to the direction of the applied load, reflecting in the peak intensity ratio. Crept samples presented variations in the (101)/(210) peak intensity ratio of b-Si3N4 indicating that grain rotation can to be contributing with creep deformation. In this way, the use of X ray diffractometry as a mean to characterize microstructural changes during creep has been shown successfully.

Microstructural Evolution of Ti-13Nb-13Zr Alloy during Sintering

With the prolonged average duration of life, there is an increase concern for repair of bone, joints and teeth which deteriorated and lose their functions. Thus, research of artificial materials for implants has assumed an important role in the implants development. The trend of the current research in orthopedic implants is based in the development of titanium alloys with low modulus of elasticity, next to the bone, and toxic elements free. In this work, results of the Ti-13Zr- 13Nb alloy sintering are presented. This alloy due its high biocompatibility and lower modulus of elasticity is a promising candidate for implants fabrication. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering between 800 at 1500 °C, in vacuum. Sintering behavior was studied by means of dilatometry. Sintered samples were characterized for phase composition, microstructure and microhardness by X-ray diffraction, scanning electron microscopy and Vickers indentation, respectively. Density was measured by Archimedes method. It was shown that the samples were sintered to high densities and presented homogeneous microstructure from the elements dissolution. Processing parameters were optimized in order to reduce the interstitial pick-up (O, C, N and H) and to minimize grain growth during sintering.

Characterization of Ti-35Nb-7Zr-5Ta Alloy Produced by Powder Metallurgy

Abstract: Titanium and titanium alloys present the highest biocompatibility among metallic biomaterials. The ideal titanium alloy for orthopedic applications should have low modulus of elasticity (near the bone), excellent mechanical strength, high corrosion resistance, formability and no potential toxic elements. Among titanium alloys, the Ti-35Nb-7Zr-5Ta alloy, due its high biocompatibility and lower Young’s modulus is a promising candidate for implants material. The titanium alloys production by powder metallurgy, starting from the elementary powders, is a viable route due at the smaller costs and larger operational facilities. The Ti-35Nb-7Zr-5Ta samples were manufactured by blended elemental method from a sequence of uniaxial and cold isostatic pressing with subsequent densification by sintering between 900 at 1700 °C, in vacuum, under a heating rate of 20 °C×min-1 for 1h. The objective of this work is the analysis of alloy microstructural evolution from the powders dissolution under the increase of the sintering temperature. For the alloy microstructural characterization, scanning electron microscopy and Vickers microhardness measurements, were used. Density was measured by Archimedes method. The samples presented high densification, an homogeneous microstructural development, with complete dissolution of alloying elements in the titanium matrix with the temperature increase.

Synthesis of Gadolinium Doped Ceria Ceramic Powder by Polymeric Precursor Method (Pechini)

The synthesis by polymeric precursors method (Pechini) was used to acquire gadolinium doped ceria forming Ce0,8Gd0,2O1,9 system, reaching high stoichiometric control features and nanosized particles to form dense solid electrolyte of high ionic conductivity. The synthesis was performed with cerium and gadolinium nitrates hexahydrates, citric acid and ethylene glycol. After the pre-calcination at 250°C/18h a resin was obtained like an expanded foam (puff). According to the iterature, this fact indicates that there is a reduction of agglomerates amount in a ceramic powder. A thermogravimetry-differential thermal analysis evaluated the thermal behavior of the resin. Infrared spectroscopy determined the organic matter and nitrates presence, before and after the calcination process. The X-ray diffraction identified the fluorite-type structure and was determined the crystallite size by the Scherrer equation in 22 and 46 nm for the powder calcined respectively at 600 and 800°C. The scanning electron microscopy evaluated the agglomeration degree and the morphology of the powders.

Beta Ti-45Nb and Ti-50Nb Alloys Produced by Powder Metallurgy for Aerospace Application

Beta titanium alloys parts are used on advanced aerospace systems because of their high strength to weight ratio and excellent corrosion resistance. Production of powder metallurgy titanium alloys components may lead to a substantial reduction in the cost, compared to those produced by conventional cast and wrought processes, because additional working operations and material waste can be avoided. In this work, beta Ti-45Nb and Ti-50Nb were produced by the blended elemental technique, followed by uniaxial and cold isostatic pressing with subsequent densification by sintering. Sintered samples were characterized for phase composition by XRD, microstructure by SEM, hardness by Vickers indentation, specific mass by the Archimedes method and elastic modulus by resonance ultrasound. The sintered samples presented only the beta phase, higher hardness and lower elastic modulus when compared to Ti6Al4V alloy and experimental specific mass value near theoretical specific mass. These characteristics are adequate for application on several aerospace parts.

Turning of CFRC Composites Using Si3N4 and Thin CVD Diamond Coated Si3N4 Tools

Carbon fibre reinforced carbon composites were turned using uncoated and CVD diamond coated Si3N4 cutting tools. Si3N4 inserts were fabricated by pressureless sintering and some of them were coated with a 2.5μm thick diamond film produced by hot filament chemical vapour deposition. The cutting forces and the tool wear behaviour were investigated as a function of the cutting speed (v = from 2.5 to 10 m·s -1 ). The cutting forces were recorded and the final tool damage was evaluated by optical and scanning electron microscopy observations. The turning tests performed with the uncoated tips show that the cutting forces notably increase with the cutting speed and the cutting length. Severe wear of the tool flank face takes place due to rubbing by the abrasive carbon powder generated during the turning operation, the tool wear increasing with the cutting speed. Conversely, CVD diamond coated Si3N4 tips exhibit low and constant cutting forces at the different cutting parameters, due to the formation of a carbon lubricant layer. The existence of this layer and the outstanding diamond hardness anticipates the use of CVD diamond coated materials for the high speed machining of CFRC composites.

Densification and Microstructural Behaviour on the Sintering of Blended Elemental Ti-35Nb-7Zr-5Ta Alloy

Beta titanium alloys, e.g., are now the main target for medical materials. Ti-35Nb-7Zr- 5Ta alloy were manufactured by blended elemental (BE) powder method, which appears to be one of the most promising technique for titanium parts production at reduced cost. The process employs hydrided powders as raw materials with low production costs and oxygen content. Among the titanium alloys recently developed, Ti-35Nb-7Zr-5Ta is distinguished for presenting low modulus of elasticity, high mechanical resistance and superior biocompatibility. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering among 800 at 1500 °C, in vacuum. Sintering behavior was studied by means of dilatometry. Sintered samples were characterized for phase composition, microstructure and microhardness by X-ray diffraction, scanning electron microscopy and Vickers indentation, respectively. Density was measured by Archimedes method. In this work, an alternative blending technique (with planetary mill) was used. The samples presented a good densification and a totally β-type microstructure, with complete dissolution of alloying elements in the titanium matrix with the temperature increase with low pore content.