*2.2. Manufacturing of Samples and Surface Machining*

The wax patterns were fabricated using basic wax "Stoma" (Kharkiv Oblast, Ukraine). The cobalt chromium metal alloy "I-bond NF" (Interdent d.o.o., Slovenia) (Co 63%, Cr 24%, W 8%, Mo 3%, So 1%) was used to cast a metal framework using Bego Fornax T (BEGO Bremer Goldschlägerei Wilh. Herbst GmbH & Co. KG, Germany). The geometry of samples is shown in Figure 1a.

Then the mating/contact surfaces were processed (the blue area in Figure 1a). The metal frame was veneered with IPS Inline ceramic mass (Ivoclar vivadent, Germany), firing was carried out in a Programat E5000 furnace (Ivoclar vivadent, Germany) by following instructions of the manufacturer [37].

At the first stage of manufacturing PFM restoration, an opaque layer was applied to two separate samples (see Figure 1), which was then sintered in an oven under firing mode I in Table 1. After firing, the samples were cooled and treated with an Emmevi (Hansgrohe-Axor, Germany) steam jet in a pressure mode of 0.6 MPa. The second layer was re-applied with an opaque layer and the same treatment was carried out. The next step was to apply a layer of porcelain to connect the two samples according to the scheme shown in Figure 1b, and baked in a connected form in the oven in mode II (Table 1). The thickness of the opaque layer and ceramics are shown in Figure 1c. Two connected samples were cooled after sintering and subjected to steam jet treatment. At the last stage, a layer of dentin was applied along the edge of the junction of the two samples and baked in mode III (Table 1). After cooling, the sample was cleaned using a steam jet machine.

**Figure 1.** Sketch of the sample (**a**)—the sample, contacts surface is highlighted by blue; (**b**) the scheme of joint the samples: 1, 2—samples, 3—opaque layer, 4—ceramic layer; (**c**) the scheme of the joint layer: opaque and ceramic layers thickness. **Figure 1.** Sketch of the sample (**a**)—the sample, contacts surface is highlighted by blue; (**b**) the scheme of joint the samples: 1, 2—samples, 3—opaque layer, 4—ceramic layer; (**c**) the scheme of the joint layer: opaque and ceramic layers thickness.

At the first stage of manufacturing PFM restoration, an opaque layer was applied to **Table 1.** Firing parameters for ceramic layer.


step was to apply a layer of porcelain to connect the two samples according to the scheme shown in Figure 1b, and baked in a connected form in the oven in mode II (Table 1). The where T—firing temperature, B—stand-by temperature, S—closing time, t—heating rate, H—holding time, V1—vacuum on temperature, V2—vacuum off temperature.

### thickness of the opaque layer and ceramics are shown in Figure 1c. Two connected samples were cooled after sintering and subjected to steam jet treatment. At the last stage, a *2.3. The Technique of Plasma-Electrolyte Formation of the Microrelief of the Surface of Cobalt-Chromium Alloys*

layer of dentin was applied along the edge of the junction of the two samples and baked in mode III (Table 1). After cooling, the sample was cleaned using a steam jet machine. **Table 1.** Firing parameters for ceramic layer. **Mode T, °C B, °C S, min t, °C/min H, min V1, °C V2, °C**  I 930 403 6 100 2 450 929 The method of surface microrelief formation of metal samples by plasma-electrolyte treatment consists of the use of gas discharges with liquid electrodes. The treatment process occurs as a result of the gas discharge combustion on the surface of a metal electrode dipped in an electrolyte solution. Depending on the polarity of the electrode and the type of material, various processes can occur: surface polishing, oxide ceramic coating formation, changes in surface roughness, application of metallic nanostructured coatings etc. To achieve our goal—control changes in roughness—it is necessary to use the cathode polarity of the active processed electrode.

II 910 403 4 60 1 450 909 III 900 403 4 60 1 450 899 where T—firing temperature, B—stand-by temperature, S—closing time, t—heating rate, The flat plate of the sample (9 in Figure 2) was fixed and immersed to a certain depth by the electrode system (3 in Figure 2). The work used a DC power supply (1 in Figure 2) with a continuously adjustable voltage, consisting of a diode bridge (diodes SD 246) and

H—holding time, V1—vacuum on temperature, V2—vacuum off temperature.

a laboratory autotransformer 1 M with a voltage range from 1 to 400 V (depending on the experimental conditions, a smoothing capacitor filter (C = 1560 µF) is connected to the power supply). Additional resistance (5 in Figure 2) also was added. The voltage and discharge current were measured using two digital universal measuring devices APPA 305 (6 in Figure 2) and APPA 109N (7 in Figure 2), the relative measurement error is 0.8%. An oscilloscope FLUKE scopemeter 190-062 (4 in Figure 2) was used to monitor the system. Electrolytic bath (2 in Figure 2) was filled with electrolyte—sodium chloride aqueous solution of concentration: 1%, 3% and 5% by weight. For each concentration of solution was carried out removal of the current-voltage characteristics of the plasma-electrolyte treatment. Temperature measurements were carried by thermocouple (8 in Figure 2). experimental conditions, a smoothing capacitor filter (C = 1560 μF) is connected to the power supply). Additional resistance (5 in Figure 2) also was added. The voltage and discharge current were measured using two digital universal measuring devices APPA 305 (6 in Figure 2) and APPA 109N (7 in Figure 2), the relative measurement error is 0.8%. An oscilloscope FLUKE scopemeter 190-062 (4 in Figure 2) was used to monitor the system. Electrolytic bath (2 in Figure 2) was filled with electrolyte—sodium chloride aqueous solution of concentration: 1%, 3% and 5% by weight. For each concentration of solution was carried out removal of the current-voltage characteristics of the plasma-electrolyte treatment. Temperature measurements were carried by thermocouple (8 in Figure 2).

*Materials* **2022**, *15*, x FOR PEER REVIEW 5 of 20

ode polarity of the active processed electrode.

*Cobalt-Chromium Alloys* 

*2.3. The Technique of Plasma-Electrolyte Formation of the Microrelief of the Surface of* 

The method of surface microrelief formation of metal samples by plasma-electrolyte treatment consists of the use of gas discharges with liquid electrodes. The treatment process occurs as a result of the gas discharge combustion on the surface of a metal electrode dipped in an electrolyte solution. Depending on the polarity of the electrode and the type of material, various processes can occur: surface polishing, oxide ceramic coating formation, changes in surface roughness, application of metallic nanostructured coatings etc. To achieve our goal—control changes in roughness—it is necessary to use the cath-

The flat plate of the sample (9 in Figure 2) was fixed and immersed to a certain depth by the electrode system (3 in Figure 2). The work used a DC power supply (1 in Figure 2) with a continuously adjustable voltage, consisting of a diode bridge (diodes SD 246) and a laboratory autotransformer 1 M with a voltage range from 1 to 400 V (depending on the

**Figure 2.** Scheme of the experimental installation of plasma-electrolyte formation of the microrelief of the surface: 1—electric power supply, 2—an electrolytic bath, 3—an electrode system, 4—an oscilloscope, 5—an additional resistance, 6—a voltmeter, 7—an ammeter, 8—a thermocouple and 9—a fixed sample. **Figure 2.** Scheme of the experimental installation of plasma-electrolyte formation of the microrelief of the surface: 1—electric power supply, 2—an electrolytic bath, 3—an electrode system, 4—an oscilloscope, 5—an additional resistance, 6—a voltmeter, 7—an ammeter, 8—a thermocouple and 9—a fixed sample.

The main parameters affecting the treatment process are the magnitude and shape of the applied voltage, the current strength of the discharge circuit and the temperature The main parameters affecting the treatment process are the magnitude and shape of the applied voltage, the current strength of the discharge circuit and the temperature of the electrolyte. In the experiments, a smoothed voltage form obtained by using a capacitive filter was used, and by adjusting it, we changed the processing mode. The use of an active processed cathode polarity electrode leads to local melting of its surface under the action of randomly occurring single microdischarges. Depending on the discharge power and the temperature of the electrode itself, the formation of various microholes is observed, which in turn collectively form the overall surface roughness. The formation of microholes occurs as a result of melting of the surface and partial release of the electrode material into the electrolyte. Different treatment regimen forms different surface roughness. So a large number of samples for the case of plasma-electrolyte treatment is explained by the variety of choice of combustion regimes for discharges with liquid electrodes.
