Developments in Ceramic Materials Research

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T. T. Basiev, V. A. Demidenko, K. V. Dykel’skii et al.
It is estimated based on the approved formula taking into account the elastic and hardness
characteristics of the material under study [32]. For the Vickers indenter, we have K1C =
0.016(E/H) 1/2 P/C 3/2 , where E is Young's modulus. For fluorite, E = 8.73 10 11 dyne/cm 2 [32].
The experiments demonstrate that the microhardness of ceramic samples is only slightly
higher (by 10 - 12%) than that of CaF2 single crystals, but their crack resistance increases
significantly: the fracture toughness K1C is higher by a factor of approximately 3 - 6 (see
Table 1).
Table 1. The values of microhardness H and fracture toughness K1C
Samples microhardness H, GPa fracture toughness K1C, MPa⋅m -0,5
Composition Designation at Р=0,5 Н at Р =1 Н at Р = 0,5 Н at Р = 1 Н
CaF2 Single crystal - 200± 7 - 1,5 ± 0.15
CaF2 Natural ceramics - 225 ± 5 - 6,3 ± 0.6
CaF2 Artificial ceramics - 260±10 - 4,7±0.3
Ca0.98Er0.02F2.02 Artificial ceramics
sample B(2)4-06
305 250 9,9 9,4
The scratch-hardness tests of the ceramic samples (scratches are made from the centre of
a circle along the radius with a step of 15 [degree]) does not reveal any hardness anisotropy,
whereas for CaF2 single crystals the directions are characterized by the highest
hardness.
The thermal conductivity k is an important technological parameter. It is due to the high
thermal conductivity of calcium fluoride single crystals this material is attractive for the
development of powerful laser systems. One of the purposes of this work is to compare the
thermal conductivity of a fluorite single crystal and samples of natural and artificial optically
transparent CaF2 ceramic. The thermal conductivity is determined experimentally by the
absolute stationary method of longitudinal thermal flux [28]. The objects of research are three
samples of CaF2 with almost the same geometry dimensions 5 x 5 x 40 mm 3 . One of the
samples is cut from a single-crystal boule grown by the method of vertical directed
crystallization. Another sample is cut from a piece of natural optical ceramic from the Suran
deposit. The third sample is cut from ceramic prepared by the hot pressing method. The
distance between the temperature sensors is 20 mm. The temperature range of 50 - 300 K is
provided by evacuation of nitrogen vapor from the chamber. The results of measurements are
presented in Figure 10. The values of thermal conductivity for all samples coincide within
experimental error of 5%. At T = 300 K the thermal conductivity is k = 10.3 ± 0.1 W/m K;
and it strongly increases with decreasing temperature to k = 245 ± 7 W/m K at T = 50 K.
Analysis of transmission spectra of CaF2 single crystals and ceramic samples in wide
spectral range is provided (Figure 11). In the IR spectral range the transmission of the single
crystal and ceramics is identical (Figure 11a), but in UV the difference is significant (Figure
11b).
The optical losses in the ceramics are determined quantitatively by calorimetric method
of absorption coefficients measurement in highly transparent solids described elsewhere [33].
In doing so rectangular parallelepiped samples 8.8 x 14 x 45 mm 3 are cut and polished. The
loss factors are found to be 2.05 10 -2 cm -1 and 6.3 10 -3 cm -1 for the ceramics and the CaF2
single crystal, respectively.