PSI- Issue 9

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

4

2

properties of products and could be easily shaped. Therefore, the damage resistance of the front optics made of this soft semiconductor material remains the important problem over a protracted period of time, Coad et al. (1998), Rozenburg and Urruti (2013), Johnson et al. (2013) and Yoder (2015). The assessment of the surface damage resistance is usually proceeds by the conventional method lying in measuring the mass loss after the scratching a piece of material by silica particles, Chang et al. (2003) or performed with the help of the transmission loss measured by optical microscopy in ZnS products subjected to sand or rain erosion, Jilbert and Field (2000) and Telling et al. (1997). Laboratory model experiments on the damaging process showed that the damage resistance of ZnS ceramic depends on its grain size, Jilbert and Field (2000), Chang et al. (2003) and Rozenburg and Urruti (2013). Chang et al. (2003) reported the monotonous increase of the mass loss with the grain size decrease. Townsend and Field (1990) argued for the mean grain size of 8 µm as providing the optimum particle and rain erosion resistance. To our best knowledge, any perturbations in the fine structure of the ZnS crystal lattice affected by mechanical forcing were not considered up to now in the context of material aging of this type. In this communication, the cumulative action of the harsh environment was simulated by the abrasive treatment of the polished surface of ZnS-made plates. Impacts of isolated hard particles were modelled with shocks produced by a pointed striker. Harris (1999) reported the similar morphology of damages produced by either scratching or solid particle impact. A response of crystal lattice on mechanical forcing was studied by both the photoluminescence (PL) and fractoluminescence (FL) methods, Chandra et al. (2016). The PL spectrum presents a complicated pattern of the electronic defects in ZnS, McCloy and Potter (2013). In addition, the intensity and positions of the PL bands are very sensitive to the impurities and thermal prehistory of material. The FL lighting from deformed A II B VI semiconductors is caused by motion and multiplication of charged dislocations, which produce localized electric fields and the corresponded band bending, Bredikhin and Shmurak (1979). The tunneling of electrons from electron-trapping centers to the conduction band causes the electron-hole recombination with light emission, Bredikhin and Shmurak (1979) and Chandra (1998). The main method of manufacturing the IR high-transparent ZnS ceramics is the chemical vapor deposition (CVD) process. The A II B IV ceramics of various compositions produced by the CVD method exhibit the most optical homogeneity and the lowest content of impurities as compared to other methods. The range of grain size variation in the ZnS–CVD ceramics is restricted by 2 to 20 µm. In order to assess spectroscopically the damage resistance of ZnS ceramics in a broader range of grain sizes, the samples obtained by hot pressing (HP) and by physical vapor deposition (PVD), which differed in grain size in more than two orders of magnitude, were tested alongside with the CVD ceramics. Despite of limited use of HP and PVD processes in demanding applications of A II B VI ceramics, a comparison of responses of these chemically equivalent compounds on erosive damaging with the behavior of ZnS–CVD ceramics accentuated some important properties of the latter material.

Table 1. Some characteristics of the ZnS ceramics.

Method

Max processing temperature (ºC)

Average grain size, (µm)

Growth rate, (µm/h)

HP

950 750

~1

800

CVD PVD

~10

70

1000

>200

400

2. Experimental 2.1 Samples

The ZnS products obtained by three above-mentioned methods differed significantly in their physical and mechanical properties including the average size of crystallites (Table 1). In order to assess the role of grain size in the mechanical behavior of ceramics under consideration, the Vickers hardness (V H ) tests were carried out with varying the indentation load (Table 2). The samples grown by CVD and PVD methods were cut normally to the growth direction. Specimens were shaped