**1. Introduction**

The modern development of dental science and practice makes it possible to maintain the dental health of the population at a high level. Despite effective preventive measures, new technologies of treatment and dental prosthetics, dental morbidity remains at a high level worldwide [1,2]. In most cases, dental rehabilitation requires the use of dentures to restore the integrity of the dentition [3–6].

One of the most common and traditionally used dental prosthetic restorations is nonremovable fixed dentures, in particular porcelain fused to metal (PFM) dentures. Many years of experience in the use of these prostheses shows their effectiveness, high survival over time and improves the aesthetics and quality of life of patients [7–10]. Despite this, in clinical practice there are often failures and complications associated with both errors in complex clinical planning and violations of laboratory stages during their manufacture.

There are enough publications in medical literature devoted to the issues of preserving the vitality of teeth selected as supports for metal-ceramic crowns, decementation of

**Citation:** Saleeva, L.; Kashapov, R.; Shakirzyanov, F.; Kuznetsov, E.; Kashapov, L.; Smirnova, V.; Kashapov, N.; Saleeva, G.; Sachenkov, O.; Saleev, R. The Effect of Surface Processing on the Shear Strength of Cobalt-Chromium Dental Alloy and Ceramics. *Materials* **2022**, *15*, 2987. https://doi.org/10.3390/ ma15092987

Academic Editor: Bruno Chrcanovic

Received: 9 March 2022 Accepted: 14 April 2022 Published: 20 April 2022

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prostheses, inflammatory phenomena of the dentoalveolar papillae and others [11–13]. All of these complications have a human factor (dentist—dental technician) and can be caused by the individual characteristics of the patient. Also the manufacturing technologies of dentures are not quite perfect and require constant improvement, the development of new high-quality and biologically inert materials.

As materials used in the manufacture of prostheses is porcelain which is applied to the metal surface. Glass ceramics are obtained by melting glass using the method of directed crystallization. It consists of silicon oxide, commonly known as quartz (SiO2) with negligible aluminum content. The basic principle of obtaining a solid ceramic material is its molding when the molten glass cools. At the same time, during its sequential heating, a controlled crystallization process occurs, as a result of which crystals appear and grow. The process of transformation from pure glass to partially crystalline is called "ceramization".

Noble metal alloys are widely used in dentistry [14]. However, base-metal alloy is an economical alternative to expensive gold alloy [15]. Nickel-chromium (Ni-Cr) and cobalt-chromium (Co-Cr) alloys are most widely used when cost and rigidity are taken into account [16,17]. However, the immediate advantage of the Co-Cr alloy is comparable performance to other base metal alloys, but without an allergenic nickel component [18], while titanium has low quality of metal-ceramic bond [19]. In prosthetic dentistry special grades of stainless steels are used, the so-called alloy steels: for stamping AISI 321 or AISI 321, for casting 20Cr18N9S2. The composition of stainless steels includes: 72% iron, 0.12% carbon, 18% chromium, 9–10% nickel, 1% titanium, 2% silicon. Alloy steels contain a minimum amount of carbon (its increase leads to an increase in hardness and a decrease in ductility of steel) and an increased content of specially introduced elements that ensure the production of alloys with the desired properties. Chromium gives resistance to oxidation. Nickel is added to the alloy to increase ductility and viscosity. Titanium reduces brittleness and prevents intercrystalline corrosion of steel. Silicon is present only in injection-molded steel and improves its fluidity. Stainless steel has good malleability and poor casting qualities.

The main problem in the manufacture of PFM restorations is the problem of poor adhesion of the ceramic coating to the metal frame [20–22]. Increasing the adhesion of materials will significantly increase the service lifetime of dentures, reduce costs, which in turn will improve the quality of life of patients.

One of the criteria for good adhesion is the absence of impurities on the two surfaces that will be compared. With regard to ceramic coating, the issue is at a high stage of solution and the dental technician only needs to observe the technology of applying the mass to the metal frame by excluding the ingress of impurities from the ambient air of the dental laboratory, then with regard to the preliminary preparation of the metal frame itself, a number of issues arise that require more in-depth study [23,24]. In particular, this concerns the use of sandblasting technology of the metal frame at the stages of cleaning from the molding mass after casting, preliminary mechanical grinding with milling cutters and finishing with an abrasive sand with a size of 50 µm. Unfortunately, the last stage does not exclude the possibility of introducing abrasive sand particles into the metal frame, which in the future may cause poor local adhesion and cause chipping of the ceramic veneering.

In addition, the shape, sharpness of the peaks and cavities of the microrelief, size, depth of surface defects and internal defects determine the strength of the material [25,26]. A review of the literature has shown that common clinical problems with ceramic materials are chips, marginal fractures and fractures of the main mass [27,28]. Volumetric fractures are still one of the main causes of failures in the use of PFM prostheses, but literary sources also suggest the long-term survival of various ceramic prostheses [29–31].

Due to the presence of surface roughness, developed cracks may not spread randomly, but occur at points with a higher stress concentration. The theory that crack nucleation begins at points of stress concentration caused by surface roughness was proposed by Mecholsky et al. [32], who loaded samples with grooves and furrows both perpendicular and parallel to the direction of loading.

Separately, a number of works devoted to the study of the processing of mating surfaces can be noted. Thus, Imbriglio et al. [33] investigated aluminum oxide and silicon carbide. A negative correlation was shown between the reduced powder size and adhesion. At the same time, the adhesion for silicon carbide did not statistically depend on the reduced powder size, and, as a consequence, roughness (the range of roughness 0.2–0.4 µm was studied).

Budhe et al. [34] investigated the adhesion to shear strength between aluminum and wooden samples during various surface processing. Thus, for aluminum samples, the maximum adhesion was achieved with a roughness of about 2 µm, but with an increase or decrease in roughness, the amount of adhesion decreased. The relationship between shear strength and surface roughness for wooden samples was nonlinear, the maximum adhesion was achieved at a roughness of about 1.5 µm, and then with a decrease in roughness, adhesion decreased. Similar results for shear strength of aluminum and steel were obtained by Ghumatkar et al. [35]. Murat et al. [36] investigated the pull-off adhesion testing for a medium-density fiberboard lined with polyvinyl chloride, and a nonlinear relationship between the amount of adhesion and roughness was also shown.

The purpose of the study is to increase the adhesion of cobalt-chromium alloy and ceramics by surface machining technology. Regimes for plasma-electrolyte treatment were investigated to find optimal surface microgeometry to increase the adhesion.
