PSI - Issue 28

Nina Ogoreltceva et al. / Procedia Structural Integrity 28 (2020) 1340–1346 Nina Ogoreltceva et al. / Structural Integrity Procedia 00 (2019) 000–000

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It was shown that the optimal content of the water-soluble binder in the composition based on the TD1, TD2, TD3 powders is 35 wt. % (with the water/binder ratio 2:1). That content exhibits good uniformity, workability, fluidity of the composition, and reasonable degree of dimensional yield after drying and backing. The dilation/contraction was recorded continuously as a function of temperature during heating and cooling stages. Dilatometry curves showed that the carbonization is complete at about 800  C. A weight of stable carbonaceous residue (a coke/char yield) formed by the thermal decomposition of unit mass of the binder was not less than 30 wt. %. The overall expansion between 20  C and 950  C is +0.52%. The overall contraction is -0.58%. The average value of the coefficient of linear thermal expansion of the coating samples (5.7  10 -6 K -1 ) is close to the value of CTE of the cathode block (5.3  10 -6 K -1 ). The laboratory experiments showed that the coating obtained on the base of the finest powder TD2 is characterised by the highest value of compressive strength and low porosity (Tabl.2). However, that coating material was characterised with poor adhesion and low resistance to cracks initiation. After the drying and baking stages, the coating exhibited debonding due to shrinkage at the outside edges of the substrate surface with deep lateral cracks. For the coating obtained on the base of the coarse TD3 powder the low workability of the composition during application and low physical and mechanical properties were observed. The use of the mixture of the TD2 and TD3 powders allows overcoming the problems of each powder and increasing the properties of the coating. The coating demonstrates the expected retention improvement, lower open porosity and electrical resistivity and higher bulk density and compressive strength than those for TD3. It can be noticed that the coating on the base of TD1 is of required properties without any supplementary technological stages and that is the reason for its selection as the main TiB 2 -C coating composition. Table 2. Physical and mechanical properties of laboratory specimens of TiB 2 -based coating with a constant amount of binder 35 wt.% at RT Powder  , (g/cm 3 )  , (%)  c , (MPa) ER, (µ  m)  adh , (MPa) TD1 2.2 ± 0.1 20 ± 2 36 ± 2 50 ± 4 3.0 ± 0.2 TD2 2.4 ± 0.1 19 ± 2 45 ± 2 46 ± 3 - TD3 1.8 ± 0.1 29 ± 2 20 ± 3 68 ± 4 - TD2 + TD3 (1:1) 2.0 ± 0.1 24 ± 2 28 ± 1 52 ± 6 1.5 ± 0.1 For the selected TD1-based coating other physical and mechanical properties such as hardness (274±10 HB) and electrical resistivity at 950º C (34 µ  m) were measured. The wettability by molten aluminium (contact angle <90) and linear wear rate (3.1 cm/year) were monitored using a laboratory electrolysis cell. To characterize the microstructure by SEM-EDS, the obtained samples of coatings were subjected to a typical metallographic preparation procedure (mounting, grounding and polishing). Representative cross-sections of the TiB 2 C coating based on the TD1 powder are shown in Fig 3.

Fig. 3. SEM-micrographs of TiB 2 -C coating based on TD1 powder: (a) coating/carbon cathode block interface; (b) typical microstructure.