Sintering and mechanical properties of β‐SiC powder obtained from waste tires

Plasma synthesized SiC powder obtained from quartz and carbonaceous residue of waste tires was successfully sintered at 1925 ℃ by pressureless liquid-phase method using yttria and alumina as sintering aids (T-SiC). Comparison with sintered SiC obtained from commercial powder (C-SiC) put in evidence of similar sintered density (98%T.D.), but much finer microstructure of T-SiC than that of C-SiC. T-SiC also showed higher flexural strength than C-SiC both at room temperature (508 vs. 458 MPa) and at 1500 ℃ (280 vs. 171 MPa). Difference in liquid phase was responsible for the differences in hardness and fracture toughness. The high value of the Young's modulus of T-SiC (427 MPa) confirmed the high degree of sinterability of this powder and that it can be a promising candidate for structural applications with high added value. © 2016, The Author(s)

Liquid phase sintered SiC ceramics from starting materials of different grade Cerâmicas à base de SiC sinterizadas via fase líquida a partir de matérias-primas de diferentes purezas

Possibility of high performance ceramics manufactured from commercial SiC powder of technical grade has been shown. Sintering behavior and microstructure formation under conditions of liquid phase sintering (LPS) with oxynitride sintering aids (AlN-Y2O3) of three SiC-based compositions have been investigated. Two of the compositions were based on Alcoa 1000 SiC powder of technical grade, and the third one, which was used as a reference, was based on H.C. Starck UF-15 fine grade commercial powder. Milling process used for Alcoa 1000 SiC powder granulometry improvement has been investigated in detail, while chemical treatment of milled SiC powders has been used for pick-up impurities removal. Dilatometric experiments showed that SiC powder of technical grade after appropriate treatment exhibits sinterability comparable with the fine grade SiC. Microstructural investigations performed on sintered samples showed that the final microstructure of the Alcoa 1000 SiC based materials was practically identical with the H.C. Starck SiC based reference ones. Preliminary investigations of hardness and fracture toughness were carried out revealing excellent results for the materials produced from cheaper, nationally produced starting powder.<br>Neste trabalho é apresentada a possibilidade de obtenção de cerâmicas de SiC de alto desempenho a partir de matéria-prima comercial de grau técnico. Foi realizado o estudo de sinterização via fase líquida e desenvolvimento microestrutural de três composições à base de SiC tendo como aditivos de sinterização AlN e Y2O3 . Duas destas composições são à base de SiC-1000 da Alcoa, grau técnico, e a terceira, utilizada como referência, à base do UF-15 da H.C. Starck - Alemanha, pó comercial de granulometria fina. O processo de moagem do pó SiC-1000 da Alcoa foi acompanhado por medidas de distribuição granulométrica e posterior ataque químico, para remoção de impurezas. Os pós de grau técnico, após tratamento de moagem apropriado, apresentam sinterabilidade comparável à do pó de referência, em experimentos em dilatômetro. Os materiais sinterizados com matéria-prima à base de SiC da Alcoa apresentam microestrutura bastante similar aos obtidos com o pó de referência, da H.C. Starck. Foi feita análise preliminar das propriedades de dureza e tenacidade à fratura das cerâmicas desenvolvidas e os resultados obtidos para os materiais produzidos com o pó nacional são excelentes

Experimental and analytical ways of finding the function of the maximum accumulated damage under operating modes with overloads

The problem of overloading was characterized as a factor of load history in the modern resource assessment methodology. The signs by which a loading cycle can be considered an overload were defined. A correlation was obtained between the failure mechanics approach and the damage accumulation approach to survivability prediction. An experimental and analytical method of its adjustment has been developed based on the regularities of the impact of loads on the accumulated damage. Its use in obtaining models of damage accumulation in 40Cr and 35CrMnSi steels was shown. New experimental data were obtained on the behavior of the accumulated damage function in the stress localization zones during bending, and an explanation of its non-monotonicity under the influence of operational factors was found

Development of the microstructure of the silicon nitride based ceramics

Basic regularities of silicon nitride based materials microstructure formation and development in interrelation with processing conditions, type of sintering additives, and starting powders properties are discussed. Models of abnormal or exaggerated grain growth are critically reassessed. Results of several model experiments conducted in order to determine the most important factors directing the microstructure formation processes in RE-fluxed Si3N4 ceramics are reviewed. Existing data on the mechanisms governing the microstructure development of Si3N4-based ceramics are analyzed and several principles of microstructure tailoring are formulated

Estudo de sinterização do compósito Si3N4/TiC Sintering studies of the Si3N4/TiC composite

A busca por melhores propriedades mecânicas e maior confiabilidade de cerâmicas à base de Si3N4 faz com que seus compósitos sejam desenvolvidos e intensivamente estudados. A maioria das pesquisas realizadas foca as interações e propriedades mecânicas obtidas, mas interações químicas entre as fases foram pouco estudadas. O objetivo deste trabalho é estudar as interações químicas que ocorrem durante a sinterização do compósito Si3N4-20%vol. TiC. A cinética de sinterização foi estudada por dilatometria, com patamar a 1750 &deg;C/1 h em atmosfera dinâmica de N2. Os resultados de dilatometria indicam alterações na cinética de sinterização, quando comparado ao Si3N4 monolítico. Para avaliar as fases que se formam em temperaturas intermediárias, os compósitos foram tratados termicamente entre 1400 &deg;C e 1850 &deg;C. As amostras foram caracterizadas por difração de raios X. Foi observada a formação da solução sólida TiCN e da fase SiC.<br>During the last years much attention was given towards the better properties and reliability of Si3N4 based composites. Most of the research focus on the improvement of the mechanical properties but not on the chemical interaction between the phases of the composite. The purpose of this work is to study the chemical interactions between Si3N4 and TiC during the sintering process. The kinetics of sintering was studied by means of dilatometry, at 1750 &deg;C/1 h in a dynamic atmosphere of N2. Thermal treatments were carried out in the temperature range 1400 - 1850 &deg;C to analyze the reactions. The phases were determined by X-ray diffraction. Alterations on the sintering kinetics due to the presence of the second phase were observed. These alterations must be related to the formation of the TiCN solid solution and SiC, observed in X-ray diffraction

Estudo da viabilidade de obtenção de cerâmicas de SiC por infiltração espontânea de misturas eutéticas de Y2O3-AlN, Y2O3-Al2O3, R2O3-AlN Study of the viability to produce SiC ceramics by Y2O3-AlN, Y2O3-Al2O3, R2O3-AlN spontaneous infiltration

As cerâmicas de carbeto de silício, SiC, apresentam excelentes propriedades quando obtidas por infiltração de determinados líquidos. Na infiltração o tempo de contato entre o líquido e o SiC a temperaturas elevadas é muito curto, diminuindo a probabilidade de formação dos produtos gasosos que interferem negativamente na resistência da peça final, como ocorre na sinterização via fase líquida. O objetivo deste trabalho é mostrar uma correlação entre molhabilidade e capacidade de infiltração de alguns aditivos em compactos de SiC. Foram preparados compactos de SiC por prensagem isostática a frio e posterior pré-sinterização via fase sólida. Nesses compactos foram infiltradas misturas de Y2O3-AlN, Y2O3-Al2O3, R2O3-AlN, nas composições eutéticas, 10 ºC acima da temperatura de fusão das respectivas misturas por 4, 8 e 12 min. Após infiltração, as amostras foram analisadas quanto à densidade aparente e real, fases cristalinas, microestrutura e grau de infiltração, sendo que as amostras infiltradas com Y2O3-AlN apresentaram melhores resultados.<br>Silicon carbide ceramics, SiC, obtained by liquid infiltration have shown excellent properties. In infiltration process the contact time of the liquid with SiC at elevated temperature is short, decreasing the probability to form gaseous products that contribute negatively in the final product properties. This phenomenon occurs during SiC liquid phase sintering. The purpose of the present study was to investigate the correlation between wettability and infiltration tendency of some additives in SiC compacts. SiC compacts were prepared by cold isostatic pressing followed by solid phase pre-sintering. Into the compacts were introduced Y2O3-AlN, Y2O3-Al2O3, R2O3-AlN liquids with eutectic compositions at a temperature 10 ºC higher than the melting point of each mixture for 4, 8 and 12 min. Before infiltration, the samples were analyzed by determining densities, crystalline phases, and degree of infiltration. The better results were presented by sample with infiltration with the Y2O3-AlN mixture