OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 3 2023 Results of microhardness measurements of the fabricated materials The evaluation of the mechanical properties of the coatings was carried out by measuring the microhardness according to the Vickers method. Fig. 8 shows the mean microhardness values of the surfaced layers. The average microhardness of the coating obtained without the CrB powder appending was 192 ± 5 HV0.1, which corresponds to the base material hardness. The addition of 5 % CrB led to an increase in hardness to 263 ± 15 HV0.1. An increase in the amount of CrB powder in the surfacing mixture to 10 %, 20 %, and 30 % led to an increase in the average microhardness values to 543 ± 59 HV0.1, 762 ± 43 HV0.1, and 1,141 ± 91 HV0.1, respectively. A significant increase in hardness is due to the formation of large amount of borides of various compositions and morphologies in the material structure. Wear resistance of the coatings The aforementioned changes in the structure and hardness of the coatings should affect its tribological characteristics. Fig. 9 shows typical diagrams that illustrate the dependence of coefficient of friction on the duration of the test. At the initial stage of testing, a wear-in process is observed accompanied by a gradual increase in the coefficient of friction. After the wear-in stage, the values of the friction coefficient stabilize. It has been established that the structure of the coatings affects the duration of the wear-in process. In materials obtained by surfacing a mixture containing up to 10 % CrB, the duration of this stage is about 800 s. Increasing the content of CrB in the surfacing mixture to 20 % and 30 % leads to a 4-fold reduction in wear-in time. The average values of the coefficient of friction are shown in Table 4. It was found that appending of 5 % and 10 % CrB into the powder mixture did not affect significantly the value of the friction coefficient. At the same time, the appending of 20 % and 30 % CrB promoted the reduction of the coefficient of friction from 0.71 to 0.62 and 0.57, respectively. The wear resistance of the surfaced layers was evaluated by the volume of the wear crater (Table 4). The volume of the worn material of the coatings obtained by adding 5 % and 10 % CrB to the surfacing mixture was greater than that of the reference material. An increase in the amount of CrB particles to 20 % and 30 % favorably affected the wear resistance of the materials under investigation. It has been established that the volume of the worn material of the coatings obtained by surfacing the mixtures with 20 % and 30 % CrB was 0.17 ± 0.04 mm3 and 0.1 ± 0.04 mm3, respectively, which was 3 and 6 times lower than the value obtained for the specimen without CrB appending (0.61 ± 0.1 mm3). Fig. 8. Average microhardness of the coatings Fig. 9. Graphical representation of the change in the value of the friction coefficient during testing
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