PSI- Issue 9

Alexandre Chmel et al. / Procedia Structural Integrity 9 (2018) 3–8 Chmel et al. / Structural Integrity Procedia 00 (2018) 000–000

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Fig. 2. FL time series excited by a striker in ZnS ceramics produced by the HP (a), PVD (b), and CVD (c) methods.

3. Discussion The earlier studies of the particle / rain damaging of ZnS optical elements used in mobile devices had shown that the intensity of the mass loss under abrasive treatment depends significantly on the grain size of tested ceramics, which, in turn, affects the material hardness, Chang et al. (2003), Coad et al. (1998) and Yoder (2015). All cited above experiments were performed with ZnS compounds synthesized by the CVD method, which provides the best optical homogeneity and the lowest content of impurities. Our spectroscopic study confirmed the excellent damage resistance of ZnS–CVD in comparison with ZnS ceramics obtained by the HP and PVD methods. In distinct from either mechanical or optical determination of the damage tolerance, the PL spectroscopy specified the material degradation in terms of the structural perfectness of crystallites that compose ceramics. The PL band at 370 nm manifested itself in all spectra thus indicating the closeness of monocrystalline particles to their stoichiometric composition to one extent or another. The decrease of the 370-nm band intensity in ground ZnS–HP and ZnS–PVD signalized the degradation (cracking or/and deformation) of crystallites embedded in intergranular substance. However, in the spectrum of damaged ZnS–CVD, the 370-nm band intensity was the same as before grinding. Consequently, the cracking occurred through removal of whole crystallites with unharmed crystal lattice. This conclusion was supported indirectly by the hardness data, which were obtained at various indentation load (Table 2). At low load (when the indentation was determined mainly by the plastic process with the lacking or minimum cracks), the value of ZnS–CVD hardness was the mean between that of ZnS–HP and ZnS–PVD. However, at higher load (≥ 50 g) when the indentation was accompanied by cracking around an impression, ZnS–CVD hardness