Showing papers by "Tanguy Rouxel published in 2022"

Review: Elaboration, structure and mechanical properties of oxynitride glasses

Abstract: Oxynitride glasses are glasses where threefold coordinated nitrogen atoms substitute for twofold oxygen ones, hence resulting in a larger interatomic cross-linking degree. Such glasses were first observed at the grain boundary in silicon nitride ceramics, where they govern the high-temperature behavior. Later, they were prepared as bulk materials and motivated numerous researches, thanks to their large viscosity, glass transition range, elastic moduli, hardness, and fracture toughness among inorganic and non-metallic glasses. In different chemical systems that were investigated, the synthesis routes and the sources for these exceptional mechanical properties are reviewed. Oxynitride glasses are not easy to process and suffer from the loss of transparency as nitrogen is incorporated over some critical content. Nevertheless, they are attractive “specialty” glasses in various niche areas, thanks to their large refractive index and dielectric constant, improved chemical durability, high softening point, etc., and majorly to their exceptional mechanical properties.

What we can learn from crystals about the mechanical properties of glass

Abstract: Glasses and crystals from the same chemical system mostly share the same interatomic bond strength. Nevertheless, they differ by the arrangement of bonds in space, which gives birth to different atomic packing efficiencies. We show in this review that as far as the elastic moduli and hardness are concerned, the atomic packing density predominates over the bond strength. The shear modulus of a glass is usually much smaller than the one of the crystallized polymorphs, thanks to a more efficient packing of atoms in the latter. In contrast, the increase in hardness is quite limited, likely because of the additional contribution of dislocation activity to the deformation processes beneath the indenter in the case of crystals (shear plasticity). We also show that the occurrence of chemical heterogeneities (weak channels) at the mesoscopic scale in glasses, which is often associated with the lack of long range atomic ordering, promotes easy fracture paths and is responsible for the low toughness and fracture surface energy.

Mechanical properties of Oxynitride glasses

Abstract: Oxynitride glasses combine a high refractoriness, with Tg typically >850°C, and remarkable mechanical properties in comparison with their parent oxide glasses. Their Young's modulus and fracture toughness reach 170 GPa and 1.4 MPa m.5, respectively. Most reports show good linear relationships between glass property values and nitrogen content. There is a clear linear dependence of Young's modulus and microhardness on fractional glass compactness (atomic packing density). They also have a better resistance to surface damage induced by indentation or scratch loading. The improvements stem from the increase of the atomic network cross-linking—because of three-fold coordinated nitrogen—and of the atomic packing density, despite nitrogen being lighter than oxygen and the Si–N bond being weaker than the Si–O bond. For constant cation composition, viscosity increases by ∼3 orders of magnitude as ∼17 eq.% oxygen is replaced by nitrogen. For rare earth oxynitride glasses with constant N content, viscosity, Young's modulus, Tg, and other properties increase with increasing cation field strength (decreasing ionic radius). Research continues to find lighter, stiffer materials, including glasses, with superior mechanical properties. With higher elastic moduli, hardness, fracture toughness, strength, surface damage resistance, increased high temperature properties, oxynitride glasses offer advantages over their oxide counterparts.