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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to mechanically polished discs of 20-30 mm in diameter and of 1-2 mm in thickness. One face of each disk was treated by dry grinding with the average grain size of grinding agent of ~ 10 2 µm.

Table 2. Vickers hardness (H V ) of ZnS ceramics at various indentation loads.

Indentation load (g)

HP

CVD

PVD

H V (GPa) 4.2 ± 0.5 3.6 ± 0.2 2.8 ± 0.1

Note

H V (GPa) 5.3 ± 0.6 3.4 ± 0.4 2.7 ± 0.3

Note

H V (GPa) 6.8 ± 1.2 5.7 ± 0.5 3.7 ± 0.1

Note

10 20 50

No cracks No cracks

No cracks

No cracks

Rare cracks (if any) Crack both from IC* and IE** Long cracks both from IC* and IE** Multiple cracking

Rare cracks (if any) Cracks both from IC* and IF*** Cracks both from IC* and IF*** Multiple cracking

Small cracks in IC*

100

2.89 ± 0.07

Cracks in IC*

2.1 ± 0.2

3.3 ± 0.3

200

2.80 ± 0.06

Cracks from IC* and IE**

2.0 ± 0.2

3.0 ± 0.3

*Indentation corners; **Indentation edges; ***Indentation faces

2.2 Equipment The PL emission from the samples was excited with a UVTOP280TO39HS LED at wavelength of 285 nm. The emitted light was transmitted to fiber-optic spectrometer AvaSpec-ULSi2048L-USB2 OEM characterized by the ultra low light scattering. The PL spectra were recorded in the range of structure-sensitive bands of ZnS (350-650 nm) with the resolution of 4 nm. Our consideration was focused on the behavior of the feature at 370-380 nm corresponding with the excitonic band gap of 3.48 eV, Morozova et al. (2001). The near band edge peaks are indicative of stoichiometric material, which emerge only in sufficiently perfect, not distorted crystals, Harris (1999). Since ZnS exhibits the strong absorbance at the length of the exiting 285 nm light, the light penetration depth was less than one micron. In other words, the recorded PL spectra referred to the very thin surface layer. The FL was excited by a weight falling onto a pointed harden striker established on the sample face. The emitted light was collected with a quartz lens and directed onto a photomultiplier FEU136. The single-electron pulse duration of the FEU136 was 8 ns. The characteristic time of the resistance-capacitance network (cables + photomultiplier) was 5 ns. An analogue-to-digital convertor ASK-3106 provided the dynamic range 2 mV to 10 V (70 dB) in the time range 10 ns to 100 c. The duration of recorded FL time series was 500 µs. 2.3 Results The PL spectra of polished and ground samples of ZnS ceramics produced by the HP, PVD, and CVD methods are depicted in Fig. 1. The 370-nm band was well-pronounced in the spectra of all polished surfaces but its intensity varied noticeably in dependence of the technological pre-history of a sample. Its intensity in the spectrum of the HP prepared ceramic (Fig. 1a) decreased about two times after grinding the sample. The intensities of the bands corresponding to centers emitting at 445 nm, 486 nm, and 520 nm remained the same. According to Kole and Kumbahar (2012), the peak at 445 nm is associated with the interstitial zinc ( I zn ) lattice defect; the poor resolved peaks at 486 nm at 520 nm are attributed to sulfur vacancy ( V s ) and zinc vacancy ( V zn ) states, respectively. This PL outcome shows the noticeable degradation of the energy-band structure in the abrasive-treated material with conserving, in general, the amount of local defect centers. The spectrum of the polished PVD-produced sample (Fig. 1b) shows the intensive emission at 370 nm emerging from large crystallites (Table 1). Its intensity was considerably reduced in the spectrum of ground sample. A rather small longwave shift of the 370-nm peak evidenced some expanding of the energy gap of the crystal. The effect may be explained by the prevalent outcome of the disordered intercrystallite substance during grinding treatment. The intensities of the concomitant bands in both polished and ground PVD samples exhibited very low intensity, what seems to be caused by especial features of the applied technology.