Efeitos de um campo elétrico na viscosidade e no mecanismo de riscamento do vidro soda-cal-sílica
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Data
2018-03-02
Autores
Ingraci Neto, Rubens Roberto
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Editor
Universidade Estadual Paulista (Unesp)
Resumo
Nesta tese se investigou a redução da viscosidade do vidro soda-cal-sílica induzida pela ação de um campo elétrico e o aproveitamento desse efeito na inibição da nucleação de trincas durante o riscamento, que introduz danos similares aqueles característicos da usinagem por abrasão; assim, se introduziu um novo caminho para usinagem em modo dúctil de materiais frágeis. Os danos que são inseridos nos materiais frágeis como vidros e cerâmicas avançadas pelas etapas tradicionais de usinagem prejudicam a aplicação destes em componentes de alta confiabilidade com exatidão dimensional e geométrica. Uma vez que o regime plástico destes materiais pode ser acentuado por um campo elétrico, estudou-se a hipótese de que um método de usinagem assistido por campo elétrico pode vencer as fronteiras dos processos convencionais. Examinaram-se as relações entre o campo elétrico e a redução na viscosidade do vidro por meio de ensaios de compressão. Amostras cilíndricas aquecidas a uma taxa de 10 ºC/min e comprimidas com uma tensão inicial de 3 MPa revelaram que a temperatura de amolecimento decresceu de maneira exponencial com o aumento da intensidade do campo elétrico aplicado, caindo de 543 ºC para 418 ºC quando sob um campo elétrico de 1 000 V/cm. Os ensaios de compressão isotérmicos com a temperatura do forno a 400 ºC, 500 ºC e 550 ºC e campo elétrico de 1 000 V/cm, evidenciaram que após o início da condução da corrente elétrica pelo vidro sua viscosidade diminuiu em até 1 000 vezes e se estabilizou, tal como a potência elétrica que era dissipada. Isso levou a conjectura de um equilíbrio térmico como consequência da cooperação entre o aquecimento Joule e a relaxação elétrica do vidro sob a ação do campo elétrico, o que permitiu o movimento dos íons de sódio e potássio e também dos pares de elétrons e buracos, resultando na relaxação estrutural e redução da viscosidade. Não obstante, este comportamento foi reversível nos testes realizados com campo elétrico pulsado, o que é essencial para um processo de usinagem, pois permite que o material seja deformado ou removido facilmente, mas, uma vez cessada a aplicação do campo elétrico, o produto final retorna às propriedades mecânicas iniciais desejadas. De fato, os ensaios de riscamento, que guardam analogias com os processos de usinagem, demonstraram que a aplicação de um campo elétrico de 1 200 V/cm inibiu a nucleação de trincas em anel e radiais ao longo dos riscos produzidos com carregamentos verticais de 14 N, 24 N e 64 N por um riscador de diamante cônico de raio de ponta 0,5 mm no vidro aquecido a 484 ºC. A menor densidade espacial de trincas, ou seja, o maior espaçamento entre elas, condisse com o previsto para redução da viscosidade. Trincas mais afastadas representam volumes maiores da estrutura do vidro que se deformam plasticamente, antes que as tensões mecânicas nas bandas de cisalhamento aumentem e resultem na ruptura das ligações do material. Estes resultados corroboram a hipótese que o amolecimento induzido por campo elétrico em materiais frágeis pode possibilitar o emprego desses em novos produtos de alta confiabilidade.
This thesis investigated the decrease of the viscosity of the soda-lime-silica glass induced by an electric field and the use of this effect in the inhibition of crack nucleation during a scratching test, which introduces damages similar to those of abrasive machining; thus presenting a new path for ductile machining of brittle materials. The damage inserted into fragile materials such as glass and advanced ceramics by traditional machining processes hamper the application of these in high reliability products with dimensional and geometric accuracy. Since the plastic regime of these materials can be accentuated by an electric field, the hypothesis that an electric field assisted machining method can overcome the boundaries of conventional processes had been studied. The relationships between the electric field and the reduction in the viscosity of the glass were examined by means of compression tests. Cylindrical samples compressed with an initial stress of 3 MPa under a constant heating rate, 10 ºC/min, revealed that the softening temperature decreased exponentially with the increase of the electric field, dropping from 543 ºC to 418 ºC when under an electric field of 1 000 V/cm. The isothermal compression tests, with furnace temperature at 400 ºC, 500 ºC and 550 ºC and electric field of 1 000 V/cm, showed that after the electric current began to flow through the glass, its viscosity decreased by up to 1 000 times and then stabilized, such as the electrical power being dissipated. This led to the conjecture of a thermal equilibrium as a consequence of the cooperation between the Joule heating and the electric relaxation of the glass under the action of the electric field, which allowed the movement of the sodium and potassium ions and also of the pairs of electrons and holes, resulting on structural relaxation and viscosity reduction. Withal, this behavior was reversible in tests carried out with pulsed electric field, which is essential for a machining process, since it allows the material to be deformed or easily removed, but once the electric field application has ceased, the final product returns to the desired initial mechanical properties. In fact, scratching tests, which have analogies with the machining processes, have shown that the application of an electric field of 1 200 V/cm inhibited the nucleation of ring and radial cracks along the scratches produced with vertical loads of 14 N, 24 N and 64 N by a conical diamond tool with a tip radius of 0.5 mm on glass heated at 484 ºC. The lower spatial density of cracks, that is, the greater spacing between them, was in agreement with the expected reduction in viscosity. More distant cracks represent larger volumes of the glass structure that deform plastically before the mechanical stresses in the shear bands increase and result in the rupture of the material bonds. These results corroborate the hypothesis that the softening induced by electric field in fragile materials can make possible the use of these in new products of high reliability.
This thesis investigated the decrease of the viscosity of the soda-lime-silica glass induced by an electric field and the use of this effect in the inhibition of crack nucleation during a scratching test, which introduces damages similar to those of abrasive machining; thus presenting a new path for ductile machining of brittle materials. The damage inserted into fragile materials such as glass and advanced ceramics by traditional machining processes hamper the application of these in high reliability products with dimensional and geometric accuracy. Since the plastic regime of these materials can be accentuated by an electric field, the hypothesis that an electric field assisted machining method can overcome the boundaries of conventional processes had been studied. The relationships between the electric field and the reduction in the viscosity of the glass were examined by means of compression tests. Cylindrical samples compressed with an initial stress of 3 MPa under a constant heating rate, 10 ºC/min, revealed that the softening temperature decreased exponentially with the increase of the electric field, dropping from 543 ºC to 418 ºC when under an electric field of 1 000 V/cm. The isothermal compression tests, with furnace temperature at 400 ºC, 500 ºC and 550 ºC and electric field of 1 000 V/cm, showed that after the electric current began to flow through the glass, its viscosity decreased by up to 1 000 times and then stabilized, such as the electrical power being dissipated. This led to the conjecture of a thermal equilibrium as a consequence of the cooperation between the Joule heating and the electric relaxation of the glass under the action of the electric field, which allowed the movement of the sodium and potassium ions and also of the pairs of electrons and holes, resulting on structural relaxation and viscosity reduction. Withal, this behavior was reversible in tests carried out with pulsed electric field, which is essential for a machining process, since it allows the material to be deformed or easily removed, but once the electric field application has ceased, the final product returns to the desired initial mechanical properties. In fact, scratching tests, which have analogies with the machining processes, have shown that the application of an electric field of 1 200 V/cm inhibited the nucleation of ring and radial cracks along the scratches produced with vertical loads of 14 N, 24 N and 64 N by a conical diamond tool with a tip radius of 0.5 mm on glass heated at 484 ºC. The lower spatial density of cracks, that is, the greater spacing between them, was in agreement with the expected reduction in viscosity. More distant cracks represent larger volumes of the glass structure that deform plastically before the mechanical stresses in the shear bands increase and result in the rupture of the material bonds. These results corroborate the hypothesis that the softening induced by electric field in fragile materials can make possible the use of these in new products of high reliability.
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Palavras-chave
Vidro soda-cal-sílica, Campo elétrico, Viscosidade, Ensaio de riscamento, Transição dúctil-frágil, Soda-lime-silica glass, Electric field, Viscosity, Scratching test, Ductile to brittle transition