**1. Introduction**

Today, aluminum alloys are widely used in the automotive, aerospace, and radio electronics industries, nuclear engineering, mining, oil and gas production, as well as in construction and other areas of modern engineering [1–3]. Relatively high ratios between strength characteristics and specific weight, high thermal conductivity, good machinability

**Citation:** Ropyak, L.; Shihab, T.; Velychkovych, A.; Bilinskyi, V.; Malinin, V.; Romaniv, M. Optimization of Plasma Electrolytic Oxidation Technological Parameters of Deformed Aluminum Alloy D16T in Flowing Electrolyte. *Ceramics* **2023**, *6*, 146–167. https://doi.org/10.3390/ ceramics6010010

Academic Editors: Amirhossein Pakseresht and Kamalan Kirubaharan Amirtharaj Mosas

Received: 6 November 2022 Revised: 29 December 2022 Accepted: 5 January 2023 Published: 10 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

by cutting and plastic deformation, and high corrosion resistance are the most important properties that make aluminum alloys technologically attractive and cost-effective structural materials [4–6].

However, in many cases, insufficient wear resistance, heat resistance, and vulnerability to thermal shock remain the restraining factors that significantly limit the scope of application of aluminum alloys [7–9]. Aluminum parts are usually operated at temperatures up to 230 ◦C because, at higher temperatures (T > 0.4 Tm, where T<sup>m</sup> is the melting temperature of the alloy), diffusion processes occur, especially grain boundary diffusion, which begins to play an important role, which leads to extensive growth grains during deformation [10,11]. Attention should also be paid to the problem of hydrogen embrittlement of aluminum alloys, which occurs in modern tanks with high-pressure hydrogen [12], the danger of sulfide corrosion cracking of critical parts of drilling, oil and gas industrial and pumping equipment [13], as well as problems regarding the interaction of biological environments with metal implants [14–16].

In order to take advantage of the significant advantages and eliminate certain disadvantages of aluminum alloys for specific practical applications, engineers use constructive, operational, and technological methods.

Design methods include selection of rational forms of parts from aluminum alloys [17], rational choice of the alloy grade [18,19], as well as conducting stress state studies [20], temperature calculations of layered compositions [21,22], and use of monitoring of damage to coatings [23–25].

Some customers are skeptical about protective coatings of working surfaces of products because, in practice, application of coatings sometimes faces the problem of their premature destruction [26,27]. Abnormal conditions during operation can cause accelerated reduction in the service life of machine components with protective coatings [28,29]. Therefore, "base material–coating" compositions should always be required to combine special properties (for example, heat resistance and wear resistance) with a sufficient margin of strength [30–32]. Here, first, it is necessary to evaluate the strength of the "base material– coating" composition as a two-layer deformable body under the action of operational loads [33,34].

Thus far, significant progress has been made in the field of mechanics of thin films, coatings, and overlays in the presence of singular stress fields caused by sharp defects or localized loads. The influence of the curvature and shape of the damaged surface on the strengthening effect of the applied coating was studied in works [35,36] using the theory of thin shells. Stress concentration near crack-like defects in coating itself was the subject of studies described in publications [37,38]. 1D [39–41] and 2D [42,43] models are used to develop analytical methods for assessing the stress state of layered coatings under local loading. An example of such an analysis of a ceramic–aluminum coating under an arbitrarily oriented load concentrated along a line is [44].

Among the technological methods, the choice of methods of mechanical processing of parts to reduce manufacturing errors, including considering technological heredity [45] to ensure long-term operation throughout the life cycle of products [46] with coatings, deserves attention. To protect aluminum alloys from wear corrosion at elevated temperatures, various methods of surface strengthening are used [47,48], namely titanium modification during ultrasonic shock treatment [49], silicon carbide during laser surface treatment [50], formed coating by electric spark alloying method [51], high-speed oxygen-fuel HVOF coating [52], electrochemical chromium plating [53], hard anodizing [54], and plasma electrolytic oxidation (PEO), which is also known as micro-arc oxidation (MAO) [55–58].

It is known that, among metal coatings, the most widespread are chrome, and, among non-metallic coatings, oxide. PEO can be a promising alternative [59] to replace use of environmentally harmful chrome plating processes for both aluminum [53] and other metals [60–62].

Among several strengthening methods, PEO should be singled out, which is intensively developed and is currently one of the most popular, environmentally friendly, and

fairly cost-effective technologies for forming oxide layers on aluminum and its alloys [55–58], as well as other metals, for example, titanium [63,64], magnesium [65], and steels [66]. In addition, PEO is one of the fundamental ways to improve the operational properties of products by modifying the working surfaces of aluminum parts in order to transform the surface layer into a hard wear-resistant and heat-resistant oxide ceramic [55–58]. PEO is a technological process of forming ceramic layers on the surface of aluminum alloys in an electrolyte under high voltages in the mode of spark and micro-arc discharges, which enables obtaining higher-quality oxide coatings that are firmly attached to the base of the part, comparable to hard anodizing and plasma spray ceramic [67]. The PEO process is accompanied by discharges that develop under the influence of a strong electric field in a system consisting of a part (substrate), an oxide layer, a vapor–gas envelope, an electrolyte, and an electrode (usually a galvanic bath made of stainless steel) [58]. Electrical breakdown in this system leads to emergence of a plasma state in the discharge channel, in which, during plasma–chemical reactions, the substrate material is transformed into chemical compounds consisting of the substrate material itself (including alloying elements), oxygen, and electrolyte components [56,57]. When applying rational technological parameters of the PEO process, we obtain a multifunctional wear-resistant and heat-resistant oxide ceramic coating that has a reliable chemical and metallurgical connection with the main material of the part [55–58,63–65,67].

Aluminum deformed alloy D16T has fairly high mechanical properties compared to other aluminum alloys, and its products are widely used in industry and everyday life. This alloy belongs to the aluminum alloys of the Al–Cu–Mg system, and it is subject to hardening and natural aging. Intermetallics formed in the microstructure are the main factor in the strengthening mechanism of this alloy. However, these intermetallics lead to microelectrochemical inhomogeneity of the alloy, which leads to pitting corrosion, intergranular corrosion, or delamination [68–70]. Therefore, deformed alloys of such a system are used clad with a layer of aluminum and/or subjected to PEO [71]. Predicting the phase composition of multicomponent Al-based alloys during PEO within certain thermodynamic approaches [72–74] or first-principles calculations [75,76] is complicated due to formation of metastable gradient structures, whose redistribution requires detailed statistical analysis.

Microstructural properties, part surface morphology, wear resistance, heat resistance, frictional heat resistance, and other operational properties of oxide ceramic coatings formed by the PEO method depend on the component composition of the electrolyte [56–58,77] and technological process parameters [56–58,78–80]. In particular, in [81], the influence of technological parameters of PEO on corrosion properties of oxide coatings was investigated. Researchers [82–84] studied the tribological characteristics of coatings formed by PEO under various types of lubrication and established high wear resistance during friction in a pair with carbon steel and revealed the effect of hydrogen on the wear resistance of steels in contact with PEO layers synthesized on aluminum alloys during tests in mineral lubricant [85]. A number of studies are devoted to study of thermal conductivity, thermal protective properties [56,86–89], and thermal shock resistance [90] of oxide ceramic coatings on various materials, the results of which demonstrate low thermal conductivity and high protective properties of these coatings, including taking into account the thermal conductivity of the substrate [91]. Papers [92,93] report on use of aluminum oxide to reinforce composite coatings to improve their protective properties.

Thus, depending on the selected material of the substrate, electrical parameters of the PEO process (current density, ratio of anode and cathode currents, voltage, frequency, time), chemical composition, concentration, and temperature of the electrolyte, it is possible to obtain oxide ceramic coatings with a complex of physical and mechanical properties that most fully satisfy the specific operational requirements of consumers [55–58]. The complexity of choosing rational technological parameters of PEO is due to the diversity of the chemical composition of aluminum alloys, as well as the component composition of electrolytes, temperature, and electrical regimes of coating formation. However, there is

practically no information in the literature about the tribological and thermal characteristics of parts with oxide coatings and the processes of formation of PEO coatings in the flowing electrolyte. Therefore, development of PEO strengthening processes for formation of oxide ceramic coatings on the surface of parts made of heterogeneous alloys is an urgent problem. Use of the experiment planning methodology during development of various technologies enables significantly reducing the number of experiments and establishing optimal process modes [94–100]. The technological process of PEO should be optimized specifically for the selected brand of deformed aluminum alloy and depending on the field of application and the desired operational properties of the parts covered with enveloping ceramic coatings.

The study aims to determine the optimal technological parameters of the PEO process and the component composition of the electrolyte to ensure formation of a wear-resistant oxide ceramic coating on aluminum deformed alloy D16T in the flowing electrolyte.

To achieve the goal, the following tasks should be addressed:


#### **2. Materials and Methods**

#### *2.1. Research Materials and Equipment*

The authors studied the strengthening process of PEO samples made of aluminum deformed alloy D16T (State Standart GOST 4784–2019 Aluminum and aluminum alloys are deformable. Marks (ISO 209:2007, NEQ)) of the Al–Cu–Mg system, hardened and naturally aged). Chemical composition and properties are presented in Tables 1 and 2, respectively.


**Table 1.** Chemical composition of the aluminum deformed alloy D16T, mass.%.

**Table 2.** Physical and mechanical properties of the aluminum deformed alloy D16T.


Manufacturing of parts with operational surfaces strengthened with PEO coatings was carried out by the developed technological process (Figure 1).

Based on a review of scientific and technical literature, patents, and the results of our own preliminary studies of the PEO process, we chose a water-based silicate–alkaline electrolyte for experiments. For preparation of electrolytes, chemically pure components were used: potassium hydroxide (KOH), State Standart GOST 9285−78 and sodium silicate (Na2SiO3), and State Standart GOST 13078−81. The electrolyte was prepared by simply mixing the components in distilled water, which was obtained using water distiller DE-4-2.

**Figure 1.** Structural diagram of manufacturing of parts with operational surfaces strengthened with PEO coatings. **Figure 1.** Structural diagram of manufacturing of parts with operational surfaces strengthened with PEO coatings.

Based on a review of scientific and technical literature, patents, and the results of our own preliminary studies of the PEO process, we chose a water-based silicate–alkaline electrolyte for experiments. For preparation of electrolytes, chemically pure components were used: potassium hydroxide (KOH), State Standart GOST 9285−78 and sodium silicate (Na2SiO3), and State Standart GOST 13078−81. The electrolyte was prepared by simply mixing the components in distilled water, which was obtained using water distiller DE-4-2. PEO of aluminum deformed alloy D16T was carried out on a device developed by us, which enables formation of oxide coatings on parts in a flowing electrolyte. The installation includes an alternating current power source (voltage up to 1000 V), a system for measuring voltage and current values in the cathodic and anodic periods, an electrolyte circulation system, variable electrochemical cells with stainless steel electrodes, an PEO of aluminum deformed alloy D16T was carried out on a device developed by us, which enables formation of oxide coatings on parts in a flowing electrolyte. The installation includes an alternating current power source (voltage up to 1000 V), a system for measuring voltage and current values in the cathodic and anodic periods, an electrolyte circulation system, variable electrochemical cells with stainless steel electrodes, an electrolyte cooling system, an exhaust ventilation system, and a damage blocking system electric current. Before forming the PEO coating, the aluminum alloy samples were subjected to mechanical processing to obtain a surface roughness of *Ra* = 0.8–1.1 µm. In the studies, the average value of *Ra* = 0.9 µm was taken. During formation of the oxide coating, the cylindrical sample was placed vertically, coaxially with the axis of the electrochemical cell. Technological modes of coating formation are shown in Table 3. The thickness of the formed working layer of the oxide coating was approximately 300 µm.

electrolyte cooling system, an exhaust ventilation system, and a damage blocking system electric current. Before forming the PEO coating, the aluminum alloy samples were sub-


**Table 3.** Coding of factors and levels of their variation during PEO experiments.

#### *2.2. Microstructure Observation and Mechanical Properties Measurement*

The microhardness of coatings and the base was measured on transverse microsands using a PMT-3 hardness tester. The load on the diamond Vickers pyramid was 0.5 N and 1.0 N with the holding time of 12 s to 15 s.

The cone-likeness of the coated part is the deviation of the profile of the longitudinal section for which the components are rectilinear, but not parallel, determined from the dependence:

$$\mathcal{Y}\_{\mathbb{C}} = (d\_{\max} - d\_{\min}) / \mathcal{Z}\_{\prime} \tag{1}$$

where *d*max and *d*min are the maximum and minimum diameter of the coated part, respectively. Measurements were carried out using a clock-type indicator with a division price of 1 µm.

The total thickness of oxide coatings was determined on cross-sectional microsections with an optical microscope MIM-7 and PMT-3 hardness tester using an eyepiece micrometer with a resolution of 1 µm.

We conducted the microscopic studies of oxide coatings, PEO-formed on aluminum alloy D16T, at the Center for collective use of scientific instruments "Center for Electron Microscopy and X-ray Microanalysis" of the Karpenko Physico-mechanical Institute of the National Academy of Sciences of Ukraine, Lviv. We used a ZEISS Stemi 2000-C (Germany) optical microscope, and ZEISS EVO 40XVP (ZEISS Group, Jena, Germany) scanning electron microscope with a micro X-ray spectral analysis system and an INCA ENERGY 350 (Oxford Instruments, Abingdon, UK) energy dispersive X-ray spectrometer. Before conducting electron microscopic studies, we applied a monolayer of gold to the surface of the samples to increase their conductivity.

#### *2.3. Wear Test*

Wear tests of samples with PEO coatings were carried out on a bench for researching friction pairs during reciprocating motion, which simulates the real operating conditions of elements of metal–rubber friction pairs of piston pumps: cylinder sleeve–piston seal; piston rod–seal; plunger–seal [101]. The amount of weight loss due to wear samples was determined by the gravimetric method: weighing the test specimens on the analytical axis of VLA-200g-M (weight accuracy of 0.1 mg) before and after the test. Before each weighing, the samples were thoroughly wiped with ethyl alcohol and dried. The arithmetic mean value from three tests for wear of the PEO ceramic coatings was chosen as the resulting value for determining the wear resistance.

Evaluation of the frictional heat resistance of oxide coatings formed by PEO was carried out according to recommendations (State Standart GOST 23.210−80 Ensuring the wear resistance of products. The method of evaluating the frictional heat resistance of materials). Experiments were carried out on the machine for testing materials for friction UMT-1 (PA "Tochprilad", Ivanovo, USSR) during dry friction of samples according to the "ring–ring" face scheme. Dimensions of the samples: outer diameter D = 28 mm, inner diameter d = 20 mm, h = 15 mm, area of the nominal friction surface of the stationary sample A = 2.9 cm<sup>2</sup> . The rotation frequency of the moving sample is 1000 min–1, with an accuracy of ±5%. The load on the sample is 200 N, with an accuracy of ±4%. The temperature in the friction contact zone was measured using an automatic electronic potentiometer of accuracy class 0.5 using a chromel-copel (E) thermocouple of the ChK type, the measurement range of which is from −50 to +600 ◦C. The essence of the method is that the rotating and stationary ring samples of the studied pair of materials are installed coaxially, pressed against each other by end working surfaces with a given axial force, and the temperature of frictional heating is controlled. A stationary ring sample is mounted in a hinge assembly to ensure self-alignment. For comparative tests, structural steel alloyed 40ChN (State Standart GOST 4543−2016), λ = 44 W/(m·degree) was used. The frictional heat resistance of materials (coatings) is judged by the dependence of the temperature change on the test time. The duration of the tests, after the samples were run-in, was monitored with a stopwatch.

#### *2.4. Methods of Planning Experimental Research*

Experiment planning was used to optimize the technological parameters of the PEO process in the flowing electrolyte.

To conduct experimental studies, the results of which allow to calculate the coefficients of the regression equations, that is, the dependences for microhardness *YH*, wear *YW*, and the cone-likeness *YC*:

$$\mathcal{Y}\_H = f(\mathbb{C}, \, i, \, v, T); \mathcal{Y}\_W = f(\mathbb{C}, \, i, \, v, T); \mathcal{Y}\_\mathbb{C} = f(\mathbb{C}, \, i, \, v, T) \tag{2}$$

of PEO coatings on D16T aluminum alloy from the technological parameters of the process: the mass ratio of the concentration of the components of the electrolyte Na2SiO3/KOH sodium silicate and potassium hydroxide (*C*), current density *(i*, A/dm<sup>2</sup> ), electrolyte flow rate (*v*, cm/s), and electrolyte temperature (*T*, ◦C) chose an orthogonal central composite plan of the second order.

In our case, for four experimental factors *k* = 4 and two experiments in the center of plan *<sup>n</sup>*<sup>0</sup> = 2, the total number of experiments was *<sup>N</sup>* = 2<sup>4</sup> + 2 <sup>×</sup> 4 + 2 = 26.

Orthogonal central composite planning of PEO experiments was carried out on five coded levels with a step equal to α = 1.4826: −α; −1; 0; +1; +α.

The selected factors meet all the requirements for them. The accuracy of maintaining the technological parameters of the PEO process was 3−5%. The intervals of variation are given in Table 3.

The order of experiments was chosen from a series of random numbers to exclude the influence of unregulated and uncontrollable factors.

To simplify the calculations, when determining the coefficients of the regression equations and their evaluation, a transition was made from the natural values of the factors to coded values (Table 3). Each experiment was repeated three times at the same level of factors and the arithmetic mean value of the initial parameter in each experiment was calculated (Table 4).

A second-order polynomial was used to describe the response function (optimization parameter) of the investigated technological process of forming PEO coatings in the flowing electrolyte:

$$Y = b\_0 + \sum\_{i=1}^{k} b\_i \mathbf{x}\_i + \sum\_{i \neq j}^{k} b\_{ij} \mathbf{x}\_i \mathbf{x}\_j + \sum\_{i=1}^{k} b\_{ii} \mathbf{x}\_i^2 \tag{3}$$

where *b*0, *b<sup>i</sup>* , *bij*, *bii*¯regression coefficients, *x<sup>i</sup>* ¯factors of the experiment, *k* = 4 .

The coefficients of the regression equation were calculated in matrix form according to known formulas.


**Table 4.** Plan-matrix of experimental results of the PEO process.

The significance of the obtained coefficients of the regression equations was checked by Student's *t*-test. The coefficient was considered statistically significant under the condition that |*bs*|> *t*·*S*(*bi*), where the *t*-test, selected from the reference table for the significance level of 0.05 and degrees of freedom *fE*; *S*(*bi*)—coefficient error (here, *b<sup>i</sup>* is considered *b*0, *b<sup>i</sup>* , *bij*, *bii*). Statistically insignificant coefficients were discarded and regression equations were refined. The resulting regression equations were tested for adequacy using Fisher's *F*-test.

The calculated value *F<sup>c</sup>* was compared to that selected from the reference table (*Ftabl*) for the *fad* and *f<sup>E</sup>* degrees of freedom using 0.05 significance level.

If the condition of regression equation *F<sup>c</sup>* < *Ftabl* is fulfilled, it can be considered adequate at the accepted level of significance 0.05.

On the basis of the obtained regression equations, response surfaces were constructed and the levels of the same output of parameter *Y* on the horizontal plane were calculated. To determine the optimal values of the technological parameters of the PEO process in the flowing electrolyte, we took the partial derivatives from the regression equations, equated them to zero, and solved the system of equations. Note that more general systems of equations with partial derivatives are considered in the article [102]. Based on the obtained regression equations, the response surfaces of the optimization parameters (microhardness *YH*, wear *YW*, and cone-likeness *YC*) were constructed from two variable technological parameters, with the other two fixed at the basic level *(C*, *i*, *v*, and *T).*

### **3. Results and Discussion**

#### *3.1. Optimization of the PEO Process Technological Parameters*

Ensuring the quality of the working surface of cylindrical parts and improving the operational properties of products is achieved by using PEO in the flow electrolyte. In order to substantiate the technological parameters of the PEO process in the flowing electrolyte and to establish the laws of their influence on the physical and mechanical properties of the coating, parameters of the shape of the part and laboratory experimental studies of samples with coatings formed according to the developed PEO technology were conducted. We studied the change in microhardness, wear, and cone-likeness of the cylindrical surface of the part with PEO coating depending on the mass ratio of the concentrations of the electrolyte components *C*, the current density *i*, the electrolyte flow rate *v*, and the temperature of the electrolyte *T*. In order to evaluate the quality indicators of the PEO coatings, the microhardness of the coating was measured, tested on wear during reciprocating motion, and the cone-likeness of cylindrical parts was determined.

The unknown coefficients of the regression equation were determined by the matrix method in coded form. The significance of the coefficients of the regression equation was checked at the 0.05 level using Student's *t*-test. Statistically insignificant coefficients were discarded. After that, they were recoded according to a known method into natural values.

Response functions (optimization parameter) reflecting the dependence of the microhardness of the oxide coating *YH*, wear *YW*, and cone-likeness *Y<sup>C</sup>* of the cylindrical surface depending on the mass ratio of the concentrations of the electrolyte components *C*, the current density *i*, and the electrolyte flow rate *v*, the temperature of the electrolyte *T*, based on the results of the orthogonal of the central composite planning of the experiment in the natural values of the variable factors, the regression equation of the second order takes the following form for:

• microhardness *YH*:

$$\begin{aligned} Y\_H &= -33.1213 + 12.2770 \text{C} + 2.2724i + 0.5159 \text{T} - 1.2249 \text{C}^2 \\ -0.3538i^2 - 0.0037 \text{T}^2 + 0.2905 \text{Ci} - 0.0411 \text{CT} + 0.0174i \text{Ti} \end{aligned} \tag{4}$$

• wear *YW*:

$$\begin{array}{c} \Upsilon\_W = 46.4174 - 7.1806 \text{C} - 3.0236i - 0.1534v \\ + 0.6687 \text{C}^2 + 0.2752i^2 + 0.0007v^2; \end{array} \tag{5}$$

• cone-likeness *YC*:

$$Y\_{\mathbb{C}} = 0.7054 - 0.1151\mathbb{C} - 0.0470i - 0.0002i T + 0.00115\mathbb{C}^2 + 0.0041i^2. \tag{6}$$

After checking the adequacy of the obtained second-order models for this PEO technological process, the Fisher test was applied. Therefore, as *F<sup>c</sup>* < *Ftabl*, it satisfies the requirements for the hypothesis about the adequacy of the regression equations, which was accepted.

The obtained second-order regression equations in natural values can be directly used to calculate microhardness *YH*, wear *YW*, and cone-likeness *Y<sup>C</sup>* of parts with an oxide coating depending on the change in the mass ratio of concentrations of electrolyte components *C*, current density *i*, A/dm<sup>2</sup> , electrolyte flow rate *v*, cm/s, and electrolyte temperature *T*, ◦C, which are, respectively, within the following limits: used to calculate microhardness ு, wear ௐ, and cone-likeness of parts with an oxide coating depending on the change in the mass ratio of concentrations of electrolyte components *C*, current density *i*, A/dm2, electrolyte flow rate *v*, cm/s, and electrolyte temperature *T*, °C, which are, respectively, within the following limits: 2.6 ≤≤ 6.3; 2 ≤≤ 8; 8.3 ≤≤ 141.7; 8 ≤≤ 72.2.

= 0.7054 − 0.1151 − 0.0470 − 0.0002 + 0.00115<sup>ଶ</sup> + 0.0041ଶ. (6)

After checking the adequacy of the obtained second-order models for this PEO technological process, the Fisher test was applied. Therefore, as < ௧, it satisfies the requirements for the hypothesis about the adequacy of the regression equations, which

The obtained second-order regression equations in natural values can be directly

*Ceramics* **2023**, *6*, FOR PEER REVIEW 10

• cone-likeness :

was accepted.

2.6 ≤ *C* ≤ 6.3;2 ≤ *i* ≤ 8;8.3 ≤ *v* ≤ 141.7;8 ≤ *T* ≤ 72.2 To analyze the influence of the technological parameters of the PEO process in the

To analyze the influence of the technological parameters of the PEO process in the flow electrolyte (variable factors) on the optimization parameters, response surfaces and their two-dimensional sections were constructed depending on two variable factors (the other two factors were at a constant basic level) (Figures 2–4). flow electrolyte (variable factors) on the optimization parameters, response surfaces and their two-dimensional sections were constructed depending on two variable factors (the other two factors were at a constant basic level) (Figures 2–4).

**Figure 2.** *Cont*.

**Figure 2.** Response surfaces of dependence of the microhardness ு coating on the technological parameters of the PEO process in the flowing electrolyte: (**a**)—*C*, *i*, *v =* 75 cm/s, *T =* 50 °C; (**b**)—*C*, *v*, *i =* 5 A/dm2, *T =* 50 °C; (**c**)—*C*, *T*, *i =* 5 A/dm2, *v =* 75 cm/s; (**d**)—*i*, *v*, *C =* 4.4, *T =* 50 *°*C; (**e**)—*i*, *T*, *C =* 4.4, *v =* 75 cm/s; (**f**)—*v*, *T*, *C =* 4,4, *i* = 5 A/dm2*.* **Figure 2.** Response surfaces of dependence of the microhardness *Y<sup>H</sup>* coating on the technological parameters of the PEO process in the flowing electrolyte: (**a**)—*C*, *i*, *v =* 75 cm/s, *T =* 50 ◦C; (**b**)—*C*, *v*, *i =* 5 A/dm<sup>2</sup> , *T =* 50 ◦C; (**c**)—*C*, *T*, *i =* 5 A/dm<sup>2</sup> , *v =* 75 cm/s; (**d**)—*i*, *v*, *C =* 4.4, *T =* 50 ◦C; (**e**)—*i*, *T*, *C =* 4.4, *v =* 75 cm/s; (**f**)—*v*, *T*, *C =* 4.4, *i* = 5 A/dm<sup>2</sup> .

Analysis of the obtained second-order regression Equations (4)–(6) and constructed response surfaces (Figures 2–4) shows that the values of the optimization parameters microhardness, wear, and cone-likeness for the oxide coating depend on almost all the technological parameters of the PEO process *C*, *i*, *v*, *T*.

In addition, the optimal values of the technological parameters of the PEO process in the flow electrolyte for the D16T alloy were determined to ensure the maximum microhardness, minimal wear of the coating, and minimal cone-likeness of the cylindrical surface of the PEO-coated part, which are presented in Table 5.


**Table 5.** Optimal values of technological parameters of the PEO process.

 (**e**) (**f**)

*v =* 75 cm/s; (**f**)—*v*, *T*, *C =* 4,4, *i* = 5 A/dm2*.*

**Figure 2.** Response surfaces of dependence of the microhardness ு coating on the technological parameters of the PEO process in the flowing electrolyte: (**a**)—*C*, *i*, *v =* 75 cm/s, *T =* 50 °C; (**b**)—*C*, *v*, *i =* 5 A/dm2, *T =* 50 °C; (**c**)—*C*, *T*, *i =* 5 A/dm2, *v =* 75 cm/s; (**d**)—*i*, *v*, *C =* 4.4, *T =* 50 *°*C; (**e**)—*i*, *T*, *C =* 4.4,

**Figure 3.** Response surfaces of dependence of the wear ௐ coating on the technological parameters of the PEO process in the flowing electrolyte: (**a**)—*C*, *i*, *v* = 75 cm/s, *T* = 50 °C; (**b**)—*C*, *v*, *i* = 5 A/dm2, *T* = 50 °C; (**c**)—*C*, *T*, *i* = 5 A/dm2, *v* = 75 cm/s; (**d**)—*i*, *v*, *C* = 4.4, *T* = 50 °C; (**e**)—*i*, *T*, *C* = 4.4, *v* = 75 cm/s; (**f**)—*v*, *T*, *C* = 4,4, *i* = 5 A/dm2. **Figure 3.** Response surfaces of dependence of the wear *Y<sup>W</sup>* coating on the technological parameters of the PEO process in the flowing electrolyte: (**a**)—*C*, *i*, *v* = 75 cm/s, *T* = 50 ◦C; (**b**)—*C*, *v*, *i* = 5 A/dm<sup>2</sup> , *T* = 50 ◦C; (**c**)—*C*, *T*, *i* = 5 A/dm<sup>2</sup> , *v* = 75 cm/s; (**d**)—*i*, *v*, *C* = 4.4, *T* = 50 ◦C; (**e**)—*i*, *T*, *C* = 4.4, *v* = 75 cm/s; (**f**)—*v*, *T*, *C* = 4.4, *i* = 5 A/dm<sup>2</sup> .

**Figure 4.** Response surfaces of dependence of the cone-likeness surface coating on the technological parameters of the PEO process in the flowing electrolyte: (**a**)—*C*, *i*, *v* = 75 cm/s, *T* = 50 °C; **Figure 4.** Response surfaces of dependence of the cone-likeness *Y<sup>C</sup>* surface coating on the technological parameters of the PEO process in the flowing electrolyte: (**a**)—*C*, *i*, *v* = 75 cm/s, *T* = 50 ◦C; (**b**)—*C*, *v*, *i* = 5 A/dm<sup>2</sup> , *T* = 50 ◦C; (**c**)—*C*, *T*, *i* = 5 A/dm<sup>2</sup> , *v* = 75 cm/s; (**d**)—*i*, *v*, *C* = 4.4, *T* = 50 ◦C; (**e**)—*i*, *T*, *C* = 4.4, *v* = 75 cm/s; (**f**)—*v*, *T*, *C* = 4.4, *i* = 5 A/dm<sup>2</sup> .

It should be noted that, at the maximum value of microhardness of the formed oxide coating, its minimal wear is ensured, and the optimal values of the technological parameters of the PEO process are within the factor space (Table 5). For the optimization parameters of cone-likeness, the optimal values of technological parameters of the process are also within the factor space.

Analysis of the results of planning the experiment provided in Table 5 shows that the optimal values of the technological parameters of PEO in the flow electrolyte for the optimization parameters microhardness of the oxide coating *YH*, wear *YW*, and cone-likeness *Y<sup>C</sup>* differ from each other. For the first two parameters of optimization–microhardness and wear, the optimal values of the technological parameters of the process practically coincide and are within the range of variation of the factors. That is, with maximum microhardness, minimal wear of the oxide ceramic coating is ensured. The optimal values of the technological parameters to ensure the minimum cone-likeness are slightly different from the optimal technological parameters to ensure the maximum microhardness and minimal wear. At the same time, in order to obtain the minimum value of *YC*, the optimal value of the electrolyte flow rate is beyond the range of variation of the factors, even taking into account the value of the star arm.

Premchand, C. et al. [103] studied the influence of the ratio of electrolyte components on the corrosion resistance of PEO coatings and found that excellent corrosion resistance is achieved when the coating is formed in an aqueous electrolyte with the same concentration ratio of the components (Na2SiO3/KOH) = 1.

Since wear resistance is an important operational characteristic for products that are operated in abrasive environments, the optimal values of the technological parameters of the PEO process in the flowing electrolyte were taken to be those that ensure the minimum amount of wear of parts *Y<sup>W</sup>* = 0.185 g: *C* = 4.98; and *i* = 6.61 A/dm<sup>2</sup> ; *v* = 103.96 cm/s; *T* = 53.59 ◦C. By substituting these values of the optimal technological parameters of PEO into the Formulas (4)–(6), we obtained a calculated value of microhardness *Y<sup>H</sup>* = 17.96 GPa; i.e., its value is slightly smaller than that obtained during the optimization *Y<sup>H</sup>* = 18.56 GPa and cone-likeness *Y<sup>C</sup>* = 10.9 µm; i.e., its value is slightly greater than that obtained during the optimization *Y<sup>C</sup>* = 10.6 µm (Tables 5 and 6). The necessary cone-likeness of the cylindrical surface in accordance with the technical requirements for production of parts, depending on their functional purpose, is expediently obtained during further machining operations– diamond grinding.


**Table 6.** Calculated values of the optimization parameters: *YH*, *Y<sup>C</sup>* according to the optimal values of the technological parameters of the PEO process, which ensure minimal wear *YW*.

It should be noted that, during PEO in the flowing electrolyte of cylindrical parts in a vertical position, the maximum increase in diameter for the outer surfaces of the shafts and a decrease for the inner surfaces of the bushings was observed in the lower part of the parts. To eliminate cone-likeness and increase the accuracy of cylindrical surfaces, considering technological heredity, during strengthening with oxide ceramic coatings, the

parts must be measured and mounted in a vertical position in the electrochemical cell before starting PEO in such a way that the minimum diameter of the shaft is from below, and, for the sleeve during application coating on the inner surface, its maximum diameter was from below.

Analysis of the results presented in Table 6 shows that the calculated values of the optimization parameters for microhardness and cone-likeness, obtained for the optimal values that ensure the minimum amount of wear, differ from their optimal values by 3.35% and 2.91%, respectively.

Thus, the obtained results of the study show the possibility of targeted control of the technological parameters of the PEO process in the flowing electrolyte by changing the technological parameters considering the technological heredity of manufacturing blanks of aluminum deformed alloy parts.

The obtained research results prove the possibility of practical application of the optimal values of the technological parameters of the PEO process in the flow electrolyte for serial production of parts, such as bodies of rotation, for example, cylinder sleeves and pump plungers at machine-building enterprises.

#### *3.2. Microscopic Studies*

We formed PEO coating (Figure 5) according to the obtained optimal values of technological parameters. According to Figure 5, PEO coatings have a uniform structure and do not have visible defects, such as pores, microcracks, and others. *Ceramics* **2023**, *6*, FOR PEER REVIEW 16

**Figure 5.** Optical micrograph of the PEO coating after finishing mechanical processing. **Figure 5.** Optical micrograph of the PEO coating after finishing mechanical processing.

Figure 6 shows the results of studying the morphology and composition of PEO coatings by SEM/EDS methods. Figure 6 shows the results of studying the morphology and composition of PEO coatings by SEM/EDS methods.

The interface between the base metal (D16T) and the PEO coating is shown in Figure 6a. As can be seen from the figure, the interface zone is characterized by integrity and the absence of significant delamination, indicating a high level of chemical bonding. It is caused by direct synthesis of the oxide phase from the base metal due to electrolyte elemental composition during the PEO process.

As can be seen from Figure 6b, the coating has the layered pancake-shaped structure typical of PEO coatings (Sikdar, S. et al.) [58]. The chemical composition analysis indicates the presence of key components for oxide formation (Al and O) and the components of the electrolyte (K, Na, Si) (Figure 6c). At the grain boundaries, there are many evenly distributed small pores formed as a result of gas migration during plasma discharge. The thickness of the PEO coatings was 180−210 µm, which determines the heat-protective properties of oxide ceramic coatings. The mechanism of formation of PEO coatings on aluminum alloys is described in publications [55–58].

**Figure 6.** SEM micrograph of the interface between the PEO coating and the base alloy D16T (**a**) and the EDS analysis of the PEO coating: (**b**)—area of EDS analysis; (**c**)—resulting spectrum and

The interface between the base metal (D16T) and the PEO coating is shown in Figure 6a. As can be seen from the figure, the interface zone is characterized by integrity and the

(**c**)

chemical composition at the analyzed area.

(**a**) (**b**)

coatings by SEM/EDS methods.

*Ceramics* **2023**, *6*, FOR PEER REVIEW 16

**Figure 5.** Optical micrograph of the PEO coating after finishing mechanical processing.

Figure 6 shows the results of studying the morphology and composition of PEO

**Figure 6.** SEM micrograph of the interface between the PEO coating and the base alloy D16T (**a**) and the EDS analysis of the PEO coating: (**b**)—area of EDS analysis; (**c**)—resulting spectrum and chemical composition at the analyzed area. The interface between the base metal (D16T) and the PEO coating is shown in Figure **Figure 6.** SEM micrograph of the interface between the PEO coating and the base alloy D16T (**a**) and the EDS analysis of the PEO coating: (**b**)—area of EDS analysis; (**c**)—resulting spectrum and chemical composition at the analyzed area. distributed small pores formed as a result of gas migration during plasma discharge. The thickness of the PEO coatings was 180−210 μm, which determines the heat-protective properties of oxide ceramic coatings. The mechanism of formation of PEO coatings on aluminum alloys is described in publications [55–58].

#### 6a. As can be seen from the figure, the interface zone is characterized by integrity and the *3.3. Frictional Heat Resistance 3.3. Frictional Heat Resistance*

temperature effects and wear.

thermal stresses in PEO ceramic coatings.

The results of evaluation of the frictional heat resistance of PEO coatings during the tests of the samples using the "ring–ring" end friction scheme are presented in Figure 7. The results of evaluation of the frictional heat resistance of PEO coatings during the tests of the samples using the "ring–ring" end friction scheme are presented in Figure 7.

**Figure 7.** Dependence of the temperature in the friction zone according to the "ring–ring" scheme on the duration of the tests: 1—D16T alloy with PEO coating; 2—steel 40ChN. **Figure 7.** Dependence of the temperature in the friction zone according to the "ring–ring" scheme on the duration of the tests: 1—D16T alloy with PEO coating; 2—steel 40ChN.

From the graphic dependences (Figure 7) of temperature changes in the friction zone on the duration of the tests, the temperature in the contact zone of the samples with the PEO coating stabilizes approximately 9–10 min after the start of the tests, reaches values

Further research is planned to study the effect of heating on the occurrence of

min). During testing of samples made of 40ChN steel, a higher temperature was established in the contact zone, which reached rather high values from 345 to 362 °C, in a less short period of time from 4 min to 6 min after the start of the tests. This attests to the high heat-shielding properties of the oxide ceramic coating and is consistent with the results of research by Curran J. et al. [88], where a low coefficient of thermal conductivity of PEO coatings is indicated, the value of which is from 0.5 to 1.5 W·m−1·K−1. Oxides in the composition of the ceramic coating formed by PEO provide high heat-shielding properties of the composition and protection of the base made of aluminum deformed alloy D16T from

From the graphic dependences (Figure 7) of temperature changes in the friction zone on the duration of the tests, the temperature in the contact zone of the samples with the PEO coating stabilizes approximately 9–10 min after the start of the tests, reaches values from 192 to 197 ◦C, and practically does not change during further wear oxide coating (30 min). During testing of samples made of 40ChN steel, a higher temperature was established in the contact zone, which reached rather high values from 345 to 362 ◦C, in a less short period of time from 4 min to 6 min after the start of the tests. This attests to the high heat-shielding properties of the oxide ceramic coating and is consistent with the results of research by Curran J. et al. [88], where a low coefficient of thermal conductivity of PEO coatings is indicated, the value of which is from 0.5 to 1.5 W·m−<sup>1</sup> ·K−<sup>1</sup> . Oxides in the composition of the ceramic coating formed by PEO provide high heat-shielding properties of the composition and protection of the base made of aluminum deformed alloy D16T from temperature effects and wear.

Further research is planned to study the effect of heating on the occurrence of thermal stresses in PEO ceramic coatings.

#### **4. Conclusions**

As a result of studies of the PEO technological process PEO in the flowing electrolyte of aluminum deformed alloy D16T:


**Author Contributions:** Conceptualization, L.R., A.V. and V.B.; methodology, L.R., T.S., A.V. and V.B.; software, L.R., A.V. and V.B.; validation, L.R., T.S., A.V., V.B. and M.R.; formal analysis, L.R., A.V., V.M. and M.R.; investigation, L.R., A.V., V.M., V.B. and M.R.; resources, L.R., T.S. and V.B.; data curation, L.R. and A.V.; writing—original draft preparation, L.R., A.V. and V.B.; writing—review and editing, L.R. and A.V.; visualization, L.R., T.S., A.V. and V.B.; supervision, L.R. and A.V.; project administration, L.R. and A.V.; funding acquisition, L.R. and A.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Ministry of Science and Education of Ukraine for the grant to implement projects 0121U109591 and 0122U002082.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are contained within the article.

**Acknowledgments:** The authors are grateful for help in statistical data processing hab. inz. Michał ˙ Bembenek, AGH, Department of Manufacturing Systems, Faculty of Mechanical Engineering and Robotics, AGH University of Science and Technology, Krakow, Poland. The authors are also grateful for thorough consultations with Vasyl Vytvytskyi and Tetiana Pryhorovska, Department of Engineering and Computer Graphics, Ivano-Frankivsk National Technical University of Oil and Gas. The authors thank the team of the Center for collective use of scientific instruments "Center for Electron Microscopy and X-ray Microanalysis" of the Karpenko Physico-mechanical Institute, National Academy of Sciences of Ukraine, Lviv for promptly conducting microscopic studies of PEO ceramic coatings on aluminum. The team of authors express their gratitude to the reviewers for valuable recommendations that have been taken into account to significantly improve the quality of this paper.

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

#### **References**


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