**4. Discussion**

The developed method of microrelief formation by plasma-electrolyte treatment allows to obtain the required surface roughness of metal frame by changing the parameters of gas discharge combustion. The microrelief arises due to combustion of individual microdischarges that melt the surface and create roughness. Size of microholes can be changed by controlling microdischarge parameters. This method of processing allows to obtain a clean surface, devoid of foreign impurities that may occur during abrasive processing. This method gives an opportunity to achieve the same results on the same parameters of plasma-electrolyte treatment. Also plasma-electrolyte treatment leads to the formation of a microrelief with spherical shape hollows and protrusions that can reduce the local stress concentration [40–42].

The main application of the plasma-electrolyte treatment is realized for surface polishing [43], heat treatment of products [44] and the formation of functional coatings [45]. In the presented work, the combustion of gas discharges with liquid electrodes was used to form a microrelief or controlled change in surface roughness. This application of the plasma-electrolyte treatment is achieved due to the possibility of local melting of the surface by single microdischarges. It should be mentioned that the study of the influence of the plasma-electrolyte treatment on the surface of the CoCr alloy was carried out on flat samples. Actually a dental crown has a complex geometry with different wall thicknesses. In the case of uniform burning of individual discharges on the surface of the dental crown, uneven heating of different parts of the crown is expected. But since the process takes place in an aqueous electrolyte solution, overheating of the CoCr alloy doesn't occur. However, there may be a change in the power of individual microdischarges depending on the geometry of the crown, which can lead to different roughness parameters, such as Ra. So, changes in roughness parameters for different geometries and types of metal frames of dental crowns should be investigated additionally.

For the proposed treatment method, as well as for classical methods, shear strength was measured. It was shown a nonlinear relationship between roughness parameters Ra and the shear strength value, such results correlate with other authors [34–36]. It was proposed to describe the obtained dependence by exponential interpolation. Such interpolation was received excluding the influence of the chemical component and microrelief over shear strength. Unlike plasma-electrolyte treatment, traditional methods (polishing, milling, and sandblasting) provide sharp geometry of hollows and protrusions.

As the result shear strength nonlinearly depends on the value of the roughness parameters. Similar trend of shear strength with respect to surface roughness was found by

Budhe et al. [34], Ghumatkar et al. [35] and Sekercioglu et al. [46]. Thus, the maximum shear strength appears near 1.5 µm Ra value for aluminum AA6061 [34] and steel S235JR— EN 10,025 [46]. Ghumatkar showed that the maximum shear strength appears near 2 µm Ra value for aluminum AA6063 and steel AISI1045 [35]. Comparing the dependence between roughness and shear strength for AA6061 and AA6063 it can be noted that maximum value remains about 5 MPa. Meanwhile the roughness (in terms of Ra) was about 1.75 µm for AA6061 and 2.25 µm for AA6063. Similar results obtained for AISI1045 and S235JR—EN 10,025—maximum shear strength localized at Ra values equal to 1.8 µm and 1.5 µm respectively. The influence of chemical adhesion with the frame microrelief is observed indirectly. It reflects on the absolute value of adhesion during the shear strength. So, the fluctuation of Ra value for maximum shear strength can be explained by chemical adhesion and the number of roughness points in the experiments.

A shear adhesion study showed that with standard metal surface treatments, the dispersion of test results was not big and it was sufficient to carry out tests on 3 samples. In contrast, plasma treated samples gave a large dispersion; so 16 samples were carried out. Subsequently, it turned out that the dispersion of results is greatly affected by temperature and current intensity.

So, the certain shear strength can be reached according to the received interpolation between mode and roughness and between roughness and adhesion. This increasing the value of the shear strength by 183% compared to the traditional method. The obtained results allow us to estimate the localization of the maximum shear strength depending on the roughness. Despite it, the determination of shear strength magnitude according to roughness and surface machining is still open, since it is influenced by other characteristics of the microrelief, chemical adhesion either.

The proposed method obtained samples with a value of Ra equal to 1.45 ± 0.16 µm and shear strength equal to 8.35 ± 0.21 MPa. The shear strength exceeds the same indicator for sandblasting by almost two times.
