PSI - Issue 20

Sharin P.P. et al. / Procedia Structural Integrity 20 (2019) 236–241 Sharin P.P. et al. / Structural Integrity Procedia 00 (2019) 000–000

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3

The sintered contact pairs were separated at room temperature during shear test using a special device (Fig.1c). In this case, the force required to destroy the diamond-metal contact was recorded. Contact strength was determined according to the formula:

F

cont σ =

.

(1)

S

Each of the surfaces obtained after separation of the diamond-metal tablet contact pairs was studied by scanning electron microscopy (SEM), X-ray diffraction and Raman spectroscopy methods.

b

c

a

Fig. 1. (a) Scheme of diamond-metal contact pairs position; (b) temperature-time mode of the vacuum furnace operating; (c) shear test device scheme (c): where 1 – diamond, 2 – a metal tablet, 3 – a graphite rod, 4 – a ceramic substrate, 5 – knife, 6 – grip for metal tablet.

It should be noted that the use of a polished working surface of natural diamond in experiments eliminated the influence of its natural roughness, various surface defects such as of inclusions, shells and cracks on the formation of the diamond-metal interfacial structure. In addition, it provided the observation and study of the interfacial zone microstructure, chemical and phase composition over the entire cross-sectional area of the contact interaction of diamond with metals surface after separation of samples as pointed by Sharin (2016). 3. Results and discussion Fig. 2 shows a SEM-images of the diamond (a-d) and metal tablets (e-h) surfaces after their separation. The structure of the diamond-metal tablet interface has a mirror symmetry, that is, the structure of the diamond surface is a mirror reflection of the surface structure of the metal tablet. It is generally known that the metal atoms when heated in vacuum, reaching the diamond surface due to various transport mechanisms (solid-phase diffusion or transfer through the gas phase), enter with the carbon atoms of the diamond into a thermochemical interaction, forming a metal-carbide coating on the diamond surface. The obtained SEM images in the contact interaction of diamond with carbide-forming metals with the same temperature and pressure ranges specified in the experiment show a significant difference in their structures. In Fig. 1 it is shown that almost the entire surface of the diamond surface is metallized (light areas) when interacting with chromium (a) and iron (c). A metallized coating with inclusion of a continuous graphite layer forms during sintering of diamond with cobalt (Fig. 1b). The formed metal-carbide coating almost completely passes to the surface of the metal tablet when the diamond titanium contact pair separates (Fig. 1d, h). The phase composition of each of the separated surfaces is presented in Table 2. The values of the diamond-metal contact strength after the shear test are given in Table 3. The metal carbide coating formed by the interaction of diamond with iron was so firmly attached to the diamond that the separation occurred along the metal carbide coating-iron tablet boundary, as well as through the diamond crystal body (dark areas in Fig. 1c, g).