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Ultra-resistant glasses produced by chemical tempering: towards a better understanding of the Na+/K+ interdiffusion mechanisms and stress relaxation processes

par Ragoen, Céline
Président du jury Delplancke, Marie-Paule
Promoteur Godet, Stéphane
Publication Non publié, 2017-11-24

Thèse de doctorat

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Résumé :

Chemical strengthening via ion exchange is commonly used in the industry to produce ultra-thin and resistant glasses used for high-tech applications. This process results in the formation of an ion-exchanged region near the glass surface that is under significant compressive stress, which tremendously improves the mechanical strength and the scratch resistance of the glass. The aim of this thesis is to deepen our fundamental understanding of the ion-exchange mechanisms. Since the mechanical strength against brittle fracture of ion-exchanged glasses depends on the compressive stress at the surface and the depth of interdiffusion, special attention was paid to the two main parameters affecting those properties: the parent glass composition and stress relaxation processes.First, the effect of the parent glass composition on interdiffusion properties was analyzed. We focused our study on the effect of the alkali oxide:alumina ratio and the ionic potential (charge:radius) of divalent cations on the interdiffusional properties. An increase of the Al2O3 content in Na+/K+ ion-exchanged silicate glasses leads to a reduction of the depth of interdiffusion layer and an enhancement of the surface compressive stress. These trends are shown to be consistent with the compositional variation of the glass connectivity and hence, the stress relaxation occurring during the ion-exchange. It is also found that the depth of interdiffusion is higher for glasses containing divalent cations with a higher ionic potential such as Mg and Zn. This is attributed to the larger free volume available and the weaker Na-O bonds in those glasses. Second, the structural origin of stress relaxation is investigated. In the Na+/K+ ion exchanged glasses, the K+-foreign cations are introduced in the cages of the Na+-host cations. Due to stress relaxation, the size of the K-O coordination shell in ion-exchanged glasses increases. This increase is achieved by two structural adaptation mechanisms: a contraction of the Na-O, Ca-O and Mg-O coordination shell and both a shortening of the Si-NBO distances and an opening of the Q4 Si-O-Si angles. The greater knowledge developed in this thesis on the effect of parent glass composition on interdiffusion mechanisms and stress relaxation processes provides further insight into practical ways to optimize the mechanical strength of ion-exchanged glasses.