**5. Conclusions**

The high prevalence of dental diseases and untimely dental treatment leads to the loss of natural teeth. Missing or damaged teeth could be replaced by dental prosthesis, including fixed metal-ceramic restorations. The functional duration of dental prosthesis depends on many factors and one of them is the preservation of the integrity of the dental prosthesis by itself. Despite using PFM restorations various complications are possible such as decementation of crowns, the development of gingiva inflammation and chipping of the porcelaine layer. The reasons for the complications are patients' individual physiology, incorrect assessment of the clinical case, violation of clinical and laboratory stages, etc. These reasons include also technological imperfection of manufacturing of dental prosthesis and as a result the adhesion quality of metal frame veneering. Sandblasting is a widespread technology in dental laboratories, but it leads to impurities on the contact surfaces. And the impurities can negatively influence the adhesion. The formation of metal surface morphology and roughness affects adhesion, but nowadays there is no certain way to provide it.

A feature of the plasma-electrolyte treatment is the uniform microdischarges combustion over the entire surface of the crown. With intense combustion melting of the entire structure can occur. To avoid this, regimes were used at lower voltages and currents, which made it possible to implement microlocal melting of the surface without overheating the entire structure. In this regard, the purpose of the study was to determine the optimal parameters of the surface treatment of the metal frame to increase the adhesion of metal and ceramics and to develop a method for forming a microrelief of the surface.

To achieve the result, experimental samples were made of cobalt-chromium alloy. Metal frames were processed by 4 different methods: polishing, milling, sandblasting and plasma-electrolyte treatment. Ceramic layer was applied to the treated surface according to the manufacturer's recommendations. Morphological and profilometric studies of

the samples allowed estimating the dependence between shear strength and the surface roughness. The developed method of microrelief formation by plasma-electrolyte treatment allows obtaining the required surface roughness of metal frame by changing the parameters of gas discharge combustion. At the same time, the geometry of the hollows and protrusions has a spherical shape that can reduce the local stress concentration. It was found that during sandblasting, abrasive sand particles remain on the metal surface and the geometry of the cavities and loads are sharp. According to the results of determining the CVC, a mode (159–178 V, 70–74 ◦C) was selected to obtain a given value of the roughness parameter Ra 1.45 ± 0.16 µm, which allowed increasing the shear strength of ceramics to the metal samples surfaces up to 8.35 ± 0.21 MPa.

Further research requires studying the effect of plasma-electrolyte microrelief formation on the resulting surface structure in terms of phase composition, microhardness, and Poisson's ratio. It is very important, since during the combustion of single microdischarges, the point of melting is rapidly cooled in the electrolyte. This cooling leads to an increase in microhardness. And most likely, in the study of microhardness from the surface into the depth of the sample, its gradient decrease will be observed. This will make it possible to create a buffer transition layer from ductile metal to brittle ceramic, thereby reducing the likelihood of ceramic chipping. Therefore, the logical continuation of this work is the study of changes in roughness parameters for different geometries and types of metal frames of dental crowns.

In the same time, the received results make it possible to state an optimistic forecast usage of the plasma-electrolyte processing for improving the adhesion. Of course pull-off strength should be measured for the same roughness values. The evaluation of dependence between normal strength and roughness is planned for future research. Additionally, cyclic tests are mapped out. It is also planned to carry out modeling of a dental prosthesis in the process of chewing by the finite element method.

The proposed method of surfacing allows increasing the shear strength between cobaltchromium alloy and ceramics. Consequently such an approach increases the vitality of mounted dental construction. The usage of low voltages and currents allows the implementation of the developed method in dental labs and clinics. The plasma-electrolyte processing duration (about 1 min), allows to produce the surfacing during dental's appointment.

**Author Contributions:** Conceptualization, R.S. and G.S.; methodology, R.S., O.S. and R.K.; investigation, L.S., R.K., F.S., E.K., L.K. and N.K.; data curation, V.S. and O.S.; writing—original draft preparation, L.S., F.S., R.K. and O.S.; writing—review and editing, L.S., F.S., L.K., R.K. and O.S.; supervision, G.S. and R.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** The results in Sections 2.1, 3.1 and 3.2 are obtained under support from the Russian Science Foundation (project No. 21-79-30062). The results in Sections 2.4, 2.6 and 3.2 are obtained under support from the Kazan Federal University Strategic Academic Leadership Program ("PRIORITY-2030").

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest to this work.
