I N D U S T R I A L R E S E A R C H

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PRINCIPAL PUBLICATION AND AUTHORS

Influence of growth process and crystal defects on sapphire brittleness, J. Muzy (a), M. Fivel (b), S. Labor (a), D. Guignier (a), J. Baruchel (c), T.N. Tran Thi Caliste (c), V. Nagirnyi (d), I. Romet (d,e), T. Duffar (b), J. Cryst. Growth 618, 127327 (2023); https:/doi.org/10.1016/j.jcrysgro.2023.127327 (a) R.S.A. Le Rubis SA, Jarrie (France) (b) Univ. Grenoble Alpes, CNRS, Grenoble INP, SIMAP, Grenoble (France) (c) ESRF (d) Institute of Physics, University of Tartu, Tartu (Estonia) (e) Institute of Physics PAS, Warsaw (Poland)

X-rays show influence of crystal defects on sapphire watch glass brittleness

Sapphire manufacturer R.S.A. Le Rubis used the ESRF s X-rays to characterise structural defects in sapphire used for watch glass manufacturing. The results explain why some manufacturing processes produce stronger, scratch-resistant glass than others.

Due to its very high strength, synthetic colourless sapphire is used as scratch-resistant glass for luxury watches. In order to minimise shard occurrence, R.S.A. Le Rubis, a French sapphire manufacturer, wanted to understand how fracture strength is related to the crystal production conditions and to structural defects. Clock-making sapphires are grown at R.S.A. by the Verneuil or by the edge-defined fed-film growth (EFG) processes. Bending tests show a higher fracture strength for Verneuil crystals compared to EFG ones.

X-ray characterisations were performed at beamline BM05. White beam topography and rocking curve imaging (RCI) revealed structural defects in the crystals (Figure 130). It appears that the high fracture strength of the Verneuil crystals is due to higher dislocation densities, about one hundred times more than in EFG crystals. In fact, higher heat fluctuations in the Verneuil process lead to higher dislocation densities.

This work concluded that dislocations are crystallographic defects beneficial for increasing the fracture strength of single crystals, as their motion absorbs a part of the mechanical stresses experienced by the watch glass.

Fig. 130: a) RCI full width at half maximum map of the (11 20) diffraction on a 1 cm2 area of an EFG-grown sapphire single-crystal

watch glass. Dislocations, in red, are aligned on the (0001) glide planes. Zoom shows dislocation loops. b) RCI intensity map of the (0006) diffraction on 21 perpendicular sections of the same sample. Zoom shows Kato interference fringes characteristic of the very good crystallinity of the sample.