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RESEARCH PRODUCT

Rhéologie et déformation du verre dans des conditions extrêmes

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Hardness measurements performed at room temperature have proven that glass can flow under elevated pressure. During industrial glass production processes, the actual distribution of stress components in the glass during scribing remains, to date, poorly quantified, and thus continues to be challenging to model numerically. To better quantify the viscous contribution to the rheology of glass, the effect of pressure on the viscosity and structural relaxation of glass, needs to be quantified experimentally by in situ deformation measurements.In this thesis, I performed experiments and models to study: (i) the volumetric relaxation of glass after high-pressure treatment above the glass transition region in Piston-cylinder apparatus and volume recovery measurements; (ii) the uniaxial deformation of glass under high pressure in Paterson press; (iii) the deviatoric torsional deformation of glass in high-pressure torsion apparatus. In Chapter 3, I developed a simplified and effective pressure cell together with an experimental procedure to compress samples of SCHOTT N-BK7® glass under static high pressures in a piston-cylinder apparatus. Results from the density and volume recovery measurements show that the glass samples were effectively densified in piston-cylinder apparatus with the density at room temperature increases linearly with frozen-in pressure. To explain the experimental data, a mathematical model was developed based on a suggestion by Gupta (1988) with two internal parameters, named fictive temperature (T_f) and fictive pressure (P_f), which fits the experimental data well.In Chapter 4, I experimentally quantified the effect of pressure and temperature on the viscosity of SCHOTT N-BK7® glass, by performing in situ deformation experiments at temperatures between 550 and 595 °C and confining pressures between 100 MPa and 300 MPa. Experiments were performed at constant displacement rates to produce almost constant strain rates between 9.70 × 10−6 s-1 and 4.98 × 10-5 s-1. The resulting net axial stresses range from 81 MPa to 802 MPa, and the finite strains range from 1.4 % to 8.9 %. The mechanical results show that the SCHOTT N-BK7® glass is viscoelastic near the glass transition temperature at 300 MPa of confining pressure. To elucidate the data, we incorporated both 1-element and 2-element generalized Maxwell viscoelastic models in an inversion approach, for which we provide MATLAB scrips. Results show that the 2-element Maxwell model fits the experimental data well. The stress decreases with increasing temperature at 300 MPa and the temperature dependence yields a similar activation energy (601±10 kJ·mol-1 or ∆H/R= 7.2×104 K) to a previously reported value at 1-atm (615 kJ·mol-1 or ∆H/R= 7.4×104 K). The SCHOTT N-BK7® glass shows a limited linear increase of viscosity with increasing pressure of ~0.1 log10(Pa·s)/100 MPa, which is in agreement with the results from Chapter 3. Finally, in Chapter 5, I applied a high-pressure torsion (HPT) apparatus to deform SCHOTT SF6® glass and attempted to quantify the effect of pressure and temperature on the shear deformation of glass subjected to pressures from 0.3 GPa to 7 GPa and temperatures from 25 ℃ to 496 ℃. Results show that the plastic yield deformation was occurring during the HPT experiments on the SF6 glass at elevated temperature from 350 ℃ to 496 ℃. The yield stress of SF6 glass decreases with increasing temperature and decreasing pressure. An extended Arrhenius model with one set of parameters, namely infinite yield stress Y0=0.17±0.1 GPa, activation energy Ea=4.8±0.5 kJ·mol-1 and activation volume Va=1.4±0.2 cm3·mol-1, can explain the experimental results well. Overall, this work demonstrates the significance of studying the pressure dependence of the rheology of glass. The experiments and models presented can benefit to the understanding on the rheology of glass under extreme conditions. Yet, linking yield stress and viscosity remains an open question.