International audienceThe authors regret that a typographic error has slipped into one of the num... more International audienceThe authors regret that a typographic error has slipped into one of the numerical values of the coefficients needed to get a sigmoidal curve fitting using Eq. (11). In sub-section 3.2 on page 5958 of the original paper, the βR coefficient should read 0.0008 instead of 0.008
Mechanoluminescence (ML) is the phenomenon of light emission during a mechanical loading. The phe... more Mechanoluminescence (ML) is the phenomenon of light emission during a mechanical loading. The phenomenon is known since a long time (in sugar, since the 17 th century at least). Many materials have been developed to produce elastico-luminescence (light emission during pure elastic deformation). We have developed glass composites with elastico-luminescent particles, namely SrAl2O4:Eu,Dy crystal, in order to obtain “smart glasses”, able to provide information on the stress they undergo, before they break.
A glass containing mechanoluminescent crystalline particles behaves as a photonic sponge: that is... more A glass containing mechanoluminescent crystalline particles behaves as a photonic sponge: that is to say it fills up with trapped electrons when exposed to UV light, and it emits light when submitted to a mechanical loading, similar to a sponge soaked with water that is wringed under mechanical action! A major finding of the present study is that the elasto-mechanoluminescence effect showing up on unloading is governed by the deviatoric part of the applied stress (no effect under hydrostatic pressure). Furthermore, the structural source for this phenomenon was elucidated by a detailed density functional theory analysis of the e− energetics at the possible oxygen vacancy sites within the crystalline phase. Both the e− trapping and detrapping processes under load could be explained. An analogy with hydraulic circuits and the rheology of viscoelastic media was successfully introduced to pave the way to a constitutive law for the mechano-optical coupling phenomenon.
The effect of spherulitic crystallization on the elastic moduli and fracture toughness of a bariu... more The effect of spherulitic crystallization on the elastic moduli and fracture toughness of a barium aluminum silicate glass was investigated. The crystallization process results in Ba2Si3O8 phase and is initiated from Ba rich nuclei. Nucleation is optimal in the 690-720 °C interval. Young's modulus is increased by 12.5% when the glass-ceramic conversion is nearly complete. Nevertheless, as the size and the volume fraction of crystals are increased, some microcracking shows up upon cooling from the crystallization temperature. An optimal improvement of the fracture toughness (SEPB method) by 27 % is observed for a 49 % volume fraction of 5 to 10 m large spherulites.
The actual displacement field in a glass during an in-situ Vickers indentation experiment was det... more The actual displacement field in a glass during an in-situ Vickers indentation experiment was determined by means of X-ray tomography, thanks to the addition of 4 vol % of X-ray absorbing particles, which acted as a speckle to further proceed through digital volume correlation. This displacement was found to agree well with the occurrence of densification beneath the contact area. The intensity of the densification contribution (Blister field proposed by Yoffe) was characterized and provides evidence for the significant contribution of densification to the mechanical fields. Densification accounts for 27% of the volume of the imprint for the studied glass, that is expected to be less sensitive to densification than amorphous silica or window glass. A major consequence is that indentation cracking methods for the evaluation of the fracture toughness, when they are based on volume conservation, as in the case of Hill-Eshelby plastic inclusion theory, are not suitable to glass. The onset for the formation of the subsurface lateral crack was also detected. The corresponding stress is z 14 GPa and is in agreement with the intrinsic glass strength.
A transparent composite material consisting of a soda-lime-silicate glass matrix containing up to... more A transparent composite material consisting of a soda-lime-silicate glass matrix containing up to 0.5 % w gold nanoparticles (10 to 20 nm) was successfully obtained by coating glass grains and a subsequent sintering using a spark plasma sintering (SPS) furnace. Coating was achieved thanks to the reduction of gold(III) chloride trihydrate by sodium borohydride in an ethanol solution. The sintered composites were dense and transparent, but for the largest gold content where the sample exhibits a deep red color. When submitted to laser irradiation (532 nm, up to 100 W/cm²), the composite experiences a fast, localized and much more significant increase of its temperature in comparison with the pristine glass material. A temperature as high as 550 °C was reached, allowing the healing of surface crack.
Glass-ceramics (GC) generally possess enhanced mechanical properties compared to their parent gla... more Glass-ceramics (GC) generally possess enhanced mechanical properties compared to their parent glasses. The knowledge of how crystallization evolves and affects the mechanical properties with increasing temperature is essential to optimize the design of the crystallization cycle. In this study, we crystallized a glass of the MgO-Al 2 O 3-SiO 2 system with nucleating agents TiO 2 and ZrO 2. The crystallization cycle comprised a 48 hour nucleation treatment at the glass-transition temperature followed by a 10 hour growth step at a higher temperature. During this cycle, the evolution of crystalline phases was followed by high-temperature X-ray diffraction (HTXRD), which revealed the presence of karooite (MgOÁ2TiO 2), spinel (MgOÁAl 2 O 3), rutile (TiO 2), sillimanite (Al 2 O 3 ÁSiO 2), and sapphirine (4MgOÁ5Al 2 O 3 Á2SiO 2). The same heat treatment was applied for in situ measurement of elastic properties: elastic modulus, E, shear modulus, G, and Poisson's ratio, m. The evolution of these parameters during the heating path from room temperature to the final crystallization temperature and during the nucleation and the crystallization plateaus is discussed. E and G evolve significantly in the first two hours of the growth step. At the end of the crystallization process, the elastic and shear moduli of the GC were approximately 20% larger than those of the parent glass. K E Y W O R D S crystallization, elastic constants, glass-ceramics, mechanical properties 1 | INTRODUCTION Glass-ceramics (GC) are high-tech polycrystalline materials obtained by controlled crystallization of certain glasses. GC can be used in a wide range of applications, 1 such as bioactive materials for bone replacement, 2,3 optical materials, 4 dental components, 5 and mechanically resistant devices. 6-9 Crystallization may occur in a homogeneous or heterogeneous manner. 10 Heterogeneous nucleation is more likely to occur than homogeneous nucleation because the corresponding surface energy is minimized at certain nucleation sites. Volumetric heterogeneous nucleation is normally favored with the aid of nucleating agents. Different microstructures obtained through controlled crystallization may lead to distinct mechanical properties. For instance, Serbena et al. 11 have shown that different volume fractions of lithium disilicate crystals embedded in an isochemical glass matrix lead to widely different values of fracture toughness, K IC. Also, Peitl et al. 2 have shown that the fracture strength of a bioactive
An observed minimum in the critical crack initiation load at Poisson's ratio (ν) of 0.21-0.22 in ... more An observed minimum in the critical crack initiation load at Poisson's ratio (ν) of 0.21-0.22 in Na2O-TiO2-SiO2 glasses was investigated. Vickers indentation was used to examine hardness and average cracking length, fracture toughness was measured through the single-edge pre-cracked beam method, and volumes of densification and shear flow around indents were measured using atomic force microscopy. Relations between the critical crack initiation load and hardness, average crack length, fracture toughness, and the volume fractions of densification and shear flow were studied. No correlations were observed between hardness, average crack length, or fracture toughness with the critical crack initiation load. A link between the minimum in crack initiation load and a change in deformation mechanisms (densification versus shear flow) was observed.
The effects of composition on indentation deformation and cracking behavior of Na 2 O-TiO 2-SiO 2... more The effects of composition on indentation deformation and cracking behavior of Na 2 O-TiO 2-SiO 2 glasses were studied in the light of structural considerations and parameters such as the atomic packing density (C g) and the network energy, using a combination of elastic measurements and micro-hardness indentation experiments. Na 2 O-TiO 2-SiO 2 glasses with titania contents of 4-10 mol% and sodium oxide contents of 10-25 mol% were prepared through a traditional melt-quench process. Indentation experiments were conducted using a Vickers indenter with loads ranging from 10 mN to 49 N. Critical loads for crack initiation and cracking patterns were systematically investigated and correlated with the elastic properties of glass. In this ternary system concerning a relatively large range of Poisson's ratio (ν), a minimum in critical crack initiation load was observed at a ν of 0.21-0.22. This study brings to light the unusual role of titanium in the glass network, that give birth to peculiar trends in the structural and mechanical properties.
Because of a relatively low atomic packing density, (C g) glasses experience significant densific... more Because of a relatively low atomic packing density, (C g) glasses experience significant densification under high hydrostatic pressure. Poisson's ratio () is correlated to C g and typically varies from 0.15 for glasses with low C g such as amorphous silica to 0.38 for close-packed atomic networks such as in bulk metallic glasses. Pressure experiments were conducted up to 25 GPa at 293 K on silica, soda-lime-silica, chalcogenide, and bulk metallic glasses. We show from these high-pressure data that there is a direct correlation between and the maximum post-decompression density change.
The crack initiation load and fracture toughness were characterized as a function of diamond part... more The crack initiation load and fracture toughness were characterized as a function of diamond particle content, up to 25 vol%, in silicon oxycarbide glass matrix by means of Vickers indentation and single edge notch beam (SENB) technique, respectively. The larger fracture toughness value of 3.21 ± 0.3 MPa m 1/2 was reached for 20 vol% diamond content composites and the value was 4 times higher than that of the unreinforced glass. The addition of diamond particles greatly influenced the crack initiation load, which increased from 2.9 to 49.0 N. The enhancement in the fracture toughness and crack initiation load can be explained by both the intrinsic mechanical properties of diamond (especially the elastic properties; E ∼ 1100 GPa) and the diamond/SiOC glass interfacial bonding. A clear correlation was found between the fracture energy, the reinforced interparticle spacing and the residual stress arising upon cooling due to thermal expansion mismatch between the matrix and the diamond particles.
Besson, J.-L., Billieres, D., Rouxel, T., Goursat, P., Flynn, R. and Hampshire, S.(1993), Crystal... more Besson, J.-L., Billieres, D., Rouxel, T., Goursat, P., Flynn, R. and Hampshire, S.(1993), Crystallization and Properties of a Si-Y-Al-ON Glass-Ceramic. Journal of the American Ceramic Society, 76: 21032105. doi: 10.1111/j. 1151-2916.1993. tb08340. x
Postsintering treatment of pressureless-sintered silicon nitride at 1,250 C (i.e., above the glas... more Postsintering treatment of pressureless-sintered silicon nitride at 1,250 C (i.e., above the glass transition temperature of the amorphous phase) by hot isostatic pressing was performed to diminish the quantity of residual intergranular amorphous phase that results from densification aids. The samples were first embedded in a powder (SiC or Al[sub 2]O[sub 3]), which worked as a diffusion barrier and a pressure transmitter; then, both sample and surrounding powder were encapsulated in evacuated tubes of borosilicate glass or stainless steel. Energy dispersive X-ray spectrometry proved that, during the treatment, a noticeable amount of intergranular vitreous phase exuded out of the sample into powder pores. As a consequence, the creep resistance was enhanced by a factor of 3. When the HIP treatment was associated with a subsequent crystallization treatment, a further improvement in creep resistance was obtained.
In France, fission products are being vitrified for a possible final geological disposal. Under d... more In France, fission products are being vitrified for a possible final geological disposal. Under disposal conditions, corrosion of the glass by groundwater as well as stress corrosion because of stresses occurring at surface flaws cannot be excluded. Within this framework, the mechanical behavior of the French simulated nuclear waste glass SON68 was studied by Vickers indentation and fracture experiments in air and in a corrosive solution. The glass was corroded at 901C in a solution enriched with Si, B, and Na. The results showed that the glass corrosion enhances the cracks propagation relative to experiments in air. The indentation fracture toughness (K IC) obtained using a four-point bending test showed that the K IC of the glass decreased with increasing corrosion time.
To estimate the ratio of densification to Vickers indentation volume, three-dimensional images of... more To estimate the ratio of densification to Vickers indentation volume, three-dimensional images of Vickers indentations on several glasses, including silicate glasses and bulk metallic glass (BMG), were obtained before and after annealing using an atomic force microscope. Large volume recovery of Vickers indentation by annealing was observed for all glasses but BMG. Following previous studies, this recovered volume almost corresponded to the densified volume under a Vickers indenter, and the compositional dependence of densification was discussed. The ratios of densification to the total indentation volume for silica and soda-lime glasses were 92% and 61%, respectively. It was concluded that densification was a general property for silicate glasses and that the ratios of densification to the total indentation volume for all the glasses correlated well with Poisson’s ratios of the glasses.
Three oxynitride glasses from the Y-Si-AI-ON system and differing in their N/O ratio were studied... more Three oxynitride glasses from the Y-Si-AI-ON system and differing in their N/O ratio were studied in the 800-1000 ~ temperature range. Their viscosities were measured using a threepoint bending test through the glass transition domain. For a given temperature, 4.8 wt% N 2 enhances the viscosity by three orders of magnitude in comparison with the corresponding oxide glass. Nitrogen also improves creep resistance. The activation enthalpy for creep, above Tg, is of the same order as those measured for silicon nitride ceramic (-~900 kJ mol-~). The elastic moduli were determined by ultrasonic techniques, from room temperature up to 1200 ~ which allowed calculation of the free activation enthalpy for viscous flow. Owing to the sharp decrease of shear modulus in the glass transition domain, the free activation enthalpy (~500 kJ mo1-1) greatly differs from the activation enthalpy usually measured in creep studies.
The temperature dependence of Young's modulus has been investigated by ultrasonic-echography in t... more The temperature dependence of Young's modulus has been investigated by ultrasonic-echography in the 20-1400°C temperature range for different oxynitride glasses, silicon nitride ceramics, and SiC/Si 3 N 4 particulate composites. Glasses exhibit a transition between a slow softening (elastic) regime and a rapid softening one which almost coincides with the glass transition range, and is located between 840 and 920°C. The transition observed in ceramics is located between 1080 and 1150°C and accounts for the behavior of the intergranular amorphous films. The higher the Y/Al ratio or the SiC content, the higher the transition temperature, and the smaller the softening rate above the transition range. Following the theories of thermally activated flow phenomena and of hierarchically constrained dynamics for glass relaxation, an expression for the correlation factor (b) was derived from the softening rate above the transition temperature. The estimated values for b range between 0.5 and 0.7 and are consistent with previously reported values, as obtained by stress relaxation or by mechanical spectroscopy techniques.
In spite of the potential applications of glasses in the BaO-TiO2-SiO2 (BTS) system, the mechanic... more In spite of the potential applications of glasses in the BaO-TiO2-SiO2 (BTS) system, the mechanical properties of these glasses were little studied so far. Several centimeters large batches of glasses along the 0.3BaO-xTiO2-(0.7-x)SiO2 composition line were prepared. Their creep behavior and elastic moduli were investigated as a function of x (from x = 0 to x = 0.3). Titanium increases the atomic packing density as well as the volume density of energy and so do the elastic moduli. An excellent agreement is found between the elastic softening index () and the liquid fragility (m) (according to Angell's concept [1]). Nevertheless, as TiO2 is added the glass-forming liquid becomes more "fragile". The explanation might lie in the structural similarity between the fresnoite crystal and the glass structure consisting of island-like Si-Ti rich units surrounded by barium-rich channels. These islands would contribute to the increase in rigidity whereas the Ba-rich channels, acting as a lubricant, would make the viscous flow more temperature sensitive.
International audienceThe authors regret that a typographic error has slipped into one of the num... more International audienceThe authors regret that a typographic error has slipped into one of the numerical values of the coefficients needed to get a sigmoidal curve fitting using Eq. (11). In sub-section 3.2 on page 5958 of the original paper, the βR coefficient should read 0.0008 instead of 0.008
Mechanoluminescence (ML) is the phenomenon of light emission during a mechanical loading. The phe... more Mechanoluminescence (ML) is the phenomenon of light emission during a mechanical loading. The phenomenon is known since a long time (in sugar, since the 17 th century at least). Many materials have been developed to produce elastico-luminescence (light emission during pure elastic deformation). We have developed glass composites with elastico-luminescent particles, namely SrAl2O4:Eu,Dy crystal, in order to obtain “smart glasses”, able to provide information on the stress they undergo, before they break.
A glass containing mechanoluminescent crystalline particles behaves as a photonic sponge: that is... more A glass containing mechanoluminescent crystalline particles behaves as a photonic sponge: that is to say it fills up with trapped electrons when exposed to UV light, and it emits light when submitted to a mechanical loading, similar to a sponge soaked with water that is wringed under mechanical action! A major finding of the present study is that the elasto-mechanoluminescence effect showing up on unloading is governed by the deviatoric part of the applied stress (no effect under hydrostatic pressure). Furthermore, the structural source for this phenomenon was elucidated by a detailed density functional theory analysis of the e− energetics at the possible oxygen vacancy sites within the crystalline phase. Both the e− trapping and detrapping processes under load could be explained. An analogy with hydraulic circuits and the rheology of viscoelastic media was successfully introduced to pave the way to a constitutive law for the mechano-optical coupling phenomenon.
The effect of spherulitic crystallization on the elastic moduli and fracture toughness of a bariu... more The effect of spherulitic crystallization on the elastic moduli and fracture toughness of a barium aluminum silicate glass was investigated. The crystallization process results in Ba2Si3O8 phase and is initiated from Ba rich nuclei. Nucleation is optimal in the 690-720 °C interval. Young's modulus is increased by 12.5% when the glass-ceramic conversion is nearly complete. Nevertheless, as the size and the volume fraction of crystals are increased, some microcracking shows up upon cooling from the crystallization temperature. An optimal improvement of the fracture toughness (SEPB method) by 27 % is observed for a 49 % volume fraction of 5 to 10 m large spherulites.
The actual displacement field in a glass during an in-situ Vickers indentation experiment was det... more The actual displacement field in a glass during an in-situ Vickers indentation experiment was determined by means of X-ray tomography, thanks to the addition of 4 vol % of X-ray absorbing particles, which acted as a speckle to further proceed through digital volume correlation. This displacement was found to agree well with the occurrence of densification beneath the contact area. The intensity of the densification contribution (Blister field proposed by Yoffe) was characterized and provides evidence for the significant contribution of densification to the mechanical fields. Densification accounts for 27% of the volume of the imprint for the studied glass, that is expected to be less sensitive to densification than amorphous silica or window glass. A major consequence is that indentation cracking methods for the evaluation of the fracture toughness, when they are based on volume conservation, as in the case of Hill-Eshelby plastic inclusion theory, are not suitable to glass. The onset for the formation of the subsurface lateral crack was also detected. The corresponding stress is z 14 GPa and is in agreement with the intrinsic glass strength.
A transparent composite material consisting of a soda-lime-silicate glass matrix containing up to... more A transparent composite material consisting of a soda-lime-silicate glass matrix containing up to 0.5 % w gold nanoparticles (10 to 20 nm) was successfully obtained by coating glass grains and a subsequent sintering using a spark plasma sintering (SPS) furnace. Coating was achieved thanks to the reduction of gold(III) chloride trihydrate by sodium borohydride in an ethanol solution. The sintered composites were dense and transparent, but for the largest gold content where the sample exhibits a deep red color. When submitted to laser irradiation (532 nm, up to 100 W/cm²), the composite experiences a fast, localized and much more significant increase of its temperature in comparison with the pristine glass material. A temperature as high as 550 °C was reached, allowing the healing of surface crack.
Glass-ceramics (GC) generally possess enhanced mechanical properties compared to their parent gla... more Glass-ceramics (GC) generally possess enhanced mechanical properties compared to their parent glasses. The knowledge of how crystallization evolves and affects the mechanical properties with increasing temperature is essential to optimize the design of the crystallization cycle. In this study, we crystallized a glass of the MgO-Al 2 O 3-SiO 2 system with nucleating agents TiO 2 and ZrO 2. The crystallization cycle comprised a 48 hour nucleation treatment at the glass-transition temperature followed by a 10 hour growth step at a higher temperature. During this cycle, the evolution of crystalline phases was followed by high-temperature X-ray diffraction (HTXRD), which revealed the presence of karooite (MgOÁ2TiO 2), spinel (MgOÁAl 2 O 3), rutile (TiO 2), sillimanite (Al 2 O 3 ÁSiO 2), and sapphirine (4MgOÁ5Al 2 O 3 Á2SiO 2). The same heat treatment was applied for in situ measurement of elastic properties: elastic modulus, E, shear modulus, G, and Poisson's ratio, m. The evolution of these parameters during the heating path from room temperature to the final crystallization temperature and during the nucleation and the crystallization plateaus is discussed. E and G evolve significantly in the first two hours of the growth step. At the end of the crystallization process, the elastic and shear moduli of the GC were approximately 20% larger than those of the parent glass. K E Y W O R D S crystallization, elastic constants, glass-ceramics, mechanical properties 1 | INTRODUCTION Glass-ceramics (GC) are high-tech polycrystalline materials obtained by controlled crystallization of certain glasses. GC can be used in a wide range of applications, 1 such as bioactive materials for bone replacement, 2,3 optical materials, 4 dental components, 5 and mechanically resistant devices. 6-9 Crystallization may occur in a homogeneous or heterogeneous manner. 10 Heterogeneous nucleation is more likely to occur than homogeneous nucleation because the corresponding surface energy is minimized at certain nucleation sites. Volumetric heterogeneous nucleation is normally favored with the aid of nucleating agents. Different microstructures obtained through controlled crystallization may lead to distinct mechanical properties. For instance, Serbena et al. 11 have shown that different volume fractions of lithium disilicate crystals embedded in an isochemical glass matrix lead to widely different values of fracture toughness, K IC. Also, Peitl et al. 2 have shown that the fracture strength of a bioactive
An observed minimum in the critical crack initiation load at Poisson's ratio (ν) of 0.21-0.22 in ... more An observed minimum in the critical crack initiation load at Poisson's ratio (ν) of 0.21-0.22 in Na2O-TiO2-SiO2 glasses was investigated. Vickers indentation was used to examine hardness and average cracking length, fracture toughness was measured through the single-edge pre-cracked beam method, and volumes of densification and shear flow around indents were measured using atomic force microscopy. Relations between the critical crack initiation load and hardness, average crack length, fracture toughness, and the volume fractions of densification and shear flow were studied. No correlations were observed between hardness, average crack length, or fracture toughness with the critical crack initiation load. A link between the minimum in crack initiation load and a change in deformation mechanisms (densification versus shear flow) was observed.
The effects of composition on indentation deformation and cracking behavior of Na 2 O-TiO 2-SiO 2... more The effects of composition on indentation deformation and cracking behavior of Na 2 O-TiO 2-SiO 2 glasses were studied in the light of structural considerations and parameters such as the atomic packing density (C g) and the network energy, using a combination of elastic measurements and micro-hardness indentation experiments. Na 2 O-TiO 2-SiO 2 glasses with titania contents of 4-10 mol% and sodium oxide contents of 10-25 mol% were prepared through a traditional melt-quench process. Indentation experiments were conducted using a Vickers indenter with loads ranging from 10 mN to 49 N. Critical loads for crack initiation and cracking patterns were systematically investigated and correlated with the elastic properties of glass. In this ternary system concerning a relatively large range of Poisson's ratio (ν), a minimum in critical crack initiation load was observed at a ν of 0.21-0.22. This study brings to light the unusual role of titanium in the glass network, that give birth to peculiar trends in the structural and mechanical properties.
Because of a relatively low atomic packing density, (C g) glasses experience significant densific... more Because of a relatively low atomic packing density, (C g) glasses experience significant densification under high hydrostatic pressure. Poisson's ratio () is correlated to C g and typically varies from 0.15 for glasses with low C g such as amorphous silica to 0.38 for close-packed atomic networks such as in bulk metallic glasses. Pressure experiments were conducted up to 25 GPa at 293 K on silica, soda-lime-silica, chalcogenide, and bulk metallic glasses. We show from these high-pressure data that there is a direct correlation between and the maximum post-decompression density change.
The crack initiation load and fracture toughness were characterized as a function of diamond part... more The crack initiation load and fracture toughness were characterized as a function of diamond particle content, up to 25 vol%, in silicon oxycarbide glass matrix by means of Vickers indentation and single edge notch beam (SENB) technique, respectively. The larger fracture toughness value of 3.21 ± 0.3 MPa m 1/2 was reached for 20 vol% diamond content composites and the value was 4 times higher than that of the unreinforced glass. The addition of diamond particles greatly influenced the crack initiation load, which increased from 2.9 to 49.0 N. The enhancement in the fracture toughness and crack initiation load can be explained by both the intrinsic mechanical properties of diamond (especially the elastic properties; E ∼ 1100 GPa) and the diamond/SiOC glass interfacial bonding. A clear correlation was found between the fracture energy, the reinforced interparticle spacing and the residual stress arising upon cooling due to thermal expansion mismatch between the matrix and the diamond particles.
Besson, J.-L., Billieres, D., Rouxel, T., Goursat, P., Flynn, R. and Hampshire, S.(1993), Crystal... more Besson, J.-L., Billieres, D., Rouxel, T., Goursat, P., Flynn, R. and Hampshire, S.(1993), Crystallization and Properties of a Si-Y-Al-ON Glass-Ceramic. Journal of the American Ceramic Society, 76: 21032105. doi: 10.1111/j. 1151-2916.1993. tb08340. x
Postsintering treatment of pressureless-sintered silicon nitride at 1,250 C (i.e., above the glas... more Postsintering treatment of pressureless-sintered silicon nitride at 1,250 C (i.e., above the glass transition temperature of the amorphous phase) by hot isostatic pressing was performed to diminish the quantity of residual intergranular amorphous phase that results from densification aids. The samples were first embedded in a powder (SiC or Al[sub 2]O[sub 3]), which worked as a diffusion barrier and a pressure transmitter; then, both sample and surrounding powder were encapsulated in evacuated tubes of borosilicate glass or stainless steel. Energy dispersive X-ray spectrometry proved that, during the treatment, a noticeable amount of intergranular vitreous phase exuded out of the sample into powder pores. As a consequence, the creep resistance was enhanced by a factor of 3. When the HIP treatment was associated with a subsequent crystallization treatment, a further improvement in creep resistance was obtained.
In France, fission products are being vitrified for a possible final geological disposal. Under d... more In France, fission products are being vitrified for a possible final geological disposal. Under disposal conditions, corrosion of the glass by groundwater as well as stress corrosion because of stresses occurring at surface flaws cannot be excluded. Within this framework, the mechanical behavior of the French simulated nuclear waste glass SON68 was studied by Vickers indentation and fracture experiments in air and in a corrosive solution. The glass was corroded at 901C in a solution enriched with Si, B, and Na. The results showed that the glass corrosion enhances the cracks propagation relative to experiments in air. The indentation fracture toughness (K IC) obtained using a four-point bending test showed that the K IC of the glass decreased with increasing corrosion time.
To estimate the ratio of densification to Vickers indentation volume, three-dimensional images of... more To estimate the ratio of densification to Vickers indentation volume, three-dimensional images of Vickers indentations on several glasses, including silicate glasses and bulk metallic glass (BMG), were obtained before and after annealing using an atomic force microscope. Large volume recovery of Vickers indentation by annealing was observed for all glasses but BMG. Following previous studies, this recovered volume almost corresponded to the densified volume under a Vickers indenter, and the compositional dependence of densification was discussed. The ratios of densification to the total indentation volume for silica and soda-lime glasses were 92% and 61%, respectively. It was concluded that densification was a general property for silicate glasses and that the ratios of densification to the total indentation volume for all the glasses correlated well with Poisson’s ratios of the glasses.
Three oxynitride glasses from the Y-Si-AI-ON system and differing in their N/O ratio were studied... more Three oxynitride glasses from the Y-Si-AI-ON system and differing in their N/O ratio were studied in the 800-1000 ~ temperature range. Their viscosities were measured using a threepoint bending test through the glass transition domain. For a given temperature, 4.8 wt% N 2 enhances the viscosity by three orders of magnitude in comparison with the corresponding oxide glass. Nitrogen also improves creep resistance. The activation enthalpy for creep, above Tg, is of the same order as those measured for silicon nitride ceramic (-~900 kJ mol-~). The elastic moduli were determined by ultrasonic techniques, from room temperature up to 1200 ~ which allowed calculation of the free activation enthalpy for viscous flow. Owing to the sharp decrease of shear modulus in the glass transition domain, the free activation enthalpy (~500 kJ mo1-1) greatly differs from the activation enthalpy usually measured in creep studies.
The temperature dependence of Young's modulus has been investigated by ultrasonic-echography in t... more The temperature dependence of Young's modulus has been investigated by ultrasonic-echography in the 20-1400°C temperature range for different oxynitride glasses, silicon nitride ceramics, and SiC/Si 3 N 4 particulate composites. Glasses exhibit a transition between a slow softening (elastic) regime and a rapid softening one which almost coincides with the glass transition range, and is located between 840 and 920°C. The transition observed in ceramics is located between 1080 and 1150°C and accounts for the behavior of the intergranular amorphous films. The higher the Y/Al ratio or the SiC content, the higher the transition temperature, and the smaller the softening rate above the transition range. Following the theories of thermally activated flow phenomena and of hierarchically constrained dynamics for glass relaxation, an expression for the correlation factor (b) was derived from the softening rate above the transition temperature. The estimated values for b range between 0.5 and 0.7 and are consistent with previously reported values, as obtained by stress relaxation or by mechanical spectroscopy techniques.
In spite of the potential applications of glasses in the BaO-TiO2-SiO2 (BTS) system, the mechanic... more In spite of the potential applications of glasses in the BaO-TiO2-SiO2 (BTS) system, the mechanical properties of these glasses were little studied so far. Several centimeters large batches of glasses along the 0.3BaO-xTiO2-(0.7-x)SiO2 composition line were prepared. Their creep behavior and elastic moduli were investigated as a function of x (from x = 0 to x = 0.3). Titanium increases the atomic packing density as well as the volume density of energy and so do the elastic moduli. An excellent agreement is found between the elastic softening index () and the liquid fragility (m) (according to Angell's concept [1]). Nevertheless, as TiO2 is added the glass-forming liquid becomes more "fragile". The explanation might lie in the structural similarity between the fresnoite crystal and the glass structure consisting of island-like Si-Ti rich units surrounded by barium-rich channels. These islands would contribute to the increase in rigidity whereas the Ba-rich channels, acting as a lubricant, would make the viscous flow more temperature sensitive.
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Papers by Tanguy Rouxel