Competing Indentation Deformation Mechanisms in Glass Using Different Strengthening Methods

Abstract

Chemical strengthening via ion exchange, thermal tempering, and lamination are proven techniques for the strengthening of oxide glasses. For each of these techniques, the strengthening mechanism is conventionally ascribed to the linear superposition of the compressive stress (CS) profile on the glass surface. However, in this work, we use molecular dynamics simulations to reveal the underlying indentation deformation mechanism beyond the simple linear superposition of compressive and indentation stresses. In particular, the plastic zone can be dramatically different from the commonly assumed hemispherical shape, which leads to a completely different stress field and resulting crack system. We show that the indentation-induced fracture is controlled by two competing mechanisms: the CS itself and a potential reduction in free volume that can increase the driving force for crack formation. Chemical strengthening via ion exchange tends to escalate the competition between these two effects, while thermal tempering tends to reduce it. Lamination of glasses with differential thermal expansion falls in between. The crack system also depends on the indenter geometry and the loading stage, i.e., loading versus after unloading. It is observed that combining thermal tempering or high free volume content with ion exchange or lamination can impart a relatively high CS and reduce the driving force for crack formation. Therefore, such a combined approach might offer the best overall crack resistance for oxide glasses.

Elasto-plastic behavior of amorphous materials: a brief review

“…While some authors claim the existence of universal mechanisms for plastic flow in amorphous materials [103], it seems that these universal features, apart from the yielding transition, result in fact from specific conditions between energy barrier fluctuations and elastic energy. For example, the case of shear banding in metallic glasses under a nanoindenter was extensively discussed [123][124][125]. However, the universal features observed in [103] are restricted to a small part of the sample.…”

Impact of pressure on the structure of glass and its material properties

“…Here, an ion exchange (IOX) process involving a molten salt is used to replace, e.g., the smaller Na + cations in the surface region of a sodium aluminosilicate glass by larger K + cations. [15][16][17] In this way, the surface is placed under a compressive stress that can be ≥0.5 GPa. It is also desirable for glass to have a high fracture toughness, i.e., an ability to resist fracturing once a crack is formed via the suppression of crack propagation.…”

Materials under pressure

“…As in other materials, glasses can be affected by both mechanical and chemical pressure, thereby changing the structure and properties of these materials. 44,45 High pressures in the GPa regime are produced under the tip of an indenter, which is used to simulate the scratching or deformation of the surface of the glass. 46,47 Glasses can have inherently long relaxation times.…”