3.3.2. EDX Analysis

In order to observe the presence of the deposited ceramic layers, the line EDX analysis was performed together with the SEM analysis of the P3–143–9 passes, P6–6420–9 passes and P9–136–9 passes samples. This analysis series had as the point of study the edge area of the samples in order to highlight as accurately as possible the presence, distribution and concentration of the chemical elements which form the ceramic microlayers and also the biopolymeric support. 3.3.2. EDX AnalysisIn order to observe the presence of the deposited ceramic layers, the line EDX analysis was performed together with the SEM analysis of the P3–143–9 passes, P6–6420–9 passes and P9–136–9 passes samples. This analysis series had as the point of study the edge area of the samples in order to highlight as accurately as possible the presence, distribution and concentration of the chemical elements which form the ceramic microlayers and also the biopolymeric support.

The EDX in-line analysis (Figure 6—yellow arrow) of all samples reflects the abundant presence of two chemical elements, carbon and oxygen. Their existence is closely related to the chemical composition of the biopolymer matrix which presents, according to the previous determinations, C and O in similar mass proportions, 48 ± 0.02% and 52 ± 0.02%, respectively [35]. The acquisition of data is a difficult one because the polymer matrix is not an electrical conductor, so the electrons coming out of the material are of much lower intensity than, for example, in the case of an analyzed metallic material. The EDX in-line analysis (Figure 6—yellow arrow) of all samples reflects the abundant presence of two chemical elements, carbon and oxygen. Their existence is closely related to the chemical composition of the biopolymer matrix which presents, according to the previous determinations, C and O in similar mass proportions, 48 ± 0.02% and 52 ± 0.02%, respectively [35]. The acquisition of data is a difficult one because the polymer matrix is not an electrical conductor, so the electrons coming out of the material are of much lower intensity than, for example, in the case of an analyzed metallic material.

For the sample covered with Zirconia–Titania–Yttria Composite Powder (Figure 6a), the presence of ceramic microparticles in the first 20 µm is observed, an area that we consider to coincide with the thickness of the deposited ±0.02 ceramic layer (blue arrow). Titanium is the chemical element found in the most significant amount in the area of the

that came off of the brittle ceramic layer in the moment of EDX preparation. The higher amount of carbon than oxygen is attributed to the carbides that form during the embedding of molten microparticles in the solid (cold) polymeric matrix, carbides that provide

Chromium being a hard metallic element during the preparation of the P6–6420–9 passes sample, this one is visible over the entire analyzed surface; however, it can be observed that in the first part of the analyzed distance (first 20 µm), the amount of chromium is higher (Figure 6b). As in the case of the P3–143–9 passes sample, the amount of carbon

surface hardness. Oxides are also formed but in a much smaller amount.

lighted by the analyzed sample.

**Figure 6.** In line EDX analysis of the coated samples. (**a**) P3–143–9 passes, (**b**) P6–6420–9 passes, (**c**) P9–136–9 passes **Figure 6.** In line EDX analysis of the coated samples. (**a**) P3–143–9 passes, (**b**) P6–6420–9 passes, (**c**) P9–136–9 passes.

3.3.3. XRD Analysis The main purpose of the XRD analysis was to determine the structure of the samples made of Arboblend V2 Nature and coated with ceramic micropowders, Amdry 6420 (Cr2O3), Metco 143 (ZrO2 18TiO2 10Y2O3) and Metco 136F (Cr2O3-xSiO2-yTiO2), but also to identify possible crystal phases. Figure 7 shows the phase diffractograms for the three samples with distinct ceramic coatings: P3–143–9 passes, P6–6420–9 passes and P9–136–9 passes. It can be seen that two For the sample covered with Zirconia–Titania–Yttria Composite Powder (Figure 6a), the presence of ceramic microparticles in the first 20 µm is observed, an area that we consider to coincide with the thickness of the deposited ±0.02 ceramic layer (blue arrow). Titanium is the chemical element found in the most significant amount in the area of the ceramic layer, but the other two elements are highlighted by the graphic. The presence of a small amount of Zr and Ti during the entire test distance is due to the microparticles that came off of the brittle ceramic layer in the moment of EDX preparation. The higher amount of carbon than oxygen is attributed to the carbides that form during the embedding of molten microparticles in the solid (cold) polymeric matrix, carbides that provide surface hardness. Oxides are also formed but in a much smaller amount.

is higher than that of oxygen. In the 35–75 µm interval, the graph highlights a steep decrease in the amount of elements reflected by the sample, but most likely this can be attributed to the transition between the deposited layer and the biopolymer support.

For the P9–136–9 passes sample (Figure 6c), the in–line analysis reveals the presence of the ceramic layer on the right side of the SEM image (last 15 µm), and the graph reflects the existence of the micropowder chemical elements. Silicon being a semiconductor chemical element reflects its presence in the structure of the ceramic layer much more than the than the other constituents. The Cr, Ti and O of the composite micropowders were high-

Chromium being a hard metallic element during the preparation of the P6–6420– 9 passes sample, this one is visible over the entire analyzed surface; however, it can be observed that in the first part of the analyzed distance (first 20 µm), the amount of chromium is higher (Figure 6b). As in the case of the P3–143–9 passes sample, the amount of carbon is higher than that of oxygen. In the 35–75 µm interval, the graph highlights a steep decrease in the amount of elements reflected by the sample, but most likely this can be attributed to the transition between the deposited layer and the biopolymer support.

(red).

For the P9–136–9 passes sample (Figure 6c), the in–line analysis reveals the presence of the ceramic layer on the right side of the SEM image (last 15 µm), and the graph reflects the existence of the micropowder chemical elements. Silicon being a semiconductor chemical element reflects its presence in the structure of the ceramic layer much more than the than the other constituents. The Cr, Ti and O of the composite micropowders were highlighted by the analyzed sample. chemical composition ((C3H4O2)n) [50,51]. The presence of this compound is not accidental because it is due to the fact that the thickness of the deposited layer is very thin (small microparticles), thus, the equipment is detecting one of the basic material constituents. In the case of the other two coatings, the presence of predominant peaks is observed, which correspond to the crystallization of certain compounds as follows: • The P3–143–9 passes sample (blue diffractogram) has a strong crystalline aspect due to the large amount of zirconium dioxide particles present in the structure of the ce-

of the three samples have a crystalline structure (P3–143–9 passes, P9–136–9 passes) highlighted by specific peaks, and the third (P6–6420–9 passes green diffractogram) has a semicrystalline structure with the presence of small peaks of chromium oxide at four distinct 2θ angles, 24.25 °, 33.39°, 35.88° and 54.61°, respectively, of low intensity, 1036 to 356 [47– 49]. The major peak registered at 16.73°, with a diffraction intensity of 2643, which, according to the literature, may be associated with the presence of polylactic acid in its the

*Polymers* **2021**, *13*, x FOR PEER REVIEW 14 of 22

#### 3.3.3. XRD Analysis ramic powder. Thus, to the ZrO2 compound can be assigned the peaks from 2θ =

The main purpose of the XRD analysis was to determine the structure of the samples made of Arboblend V2 Nature and coated with ceramic micropowders, Amdry 6420 (Cr2O3), Metco 143 (ZrO<sup>2</sup> 18TiO<sup>2</sup> 10Y2O3) and Metco 136F (Cr2O3-xSiO2-yTiO2), but also to identify possible crystal phases. 31.14°, 38.43°, 60°, 82° and 84.78° [46,52,53]. Titanium dioxide, as in the case of the P9–136–9 passes sample, is found at angles of 27.43°, 28.34°, 63.02° and 74.41° [54,55]. However, the intensity of the diffraction peaks is quite low, the highest being registered in the case of the 2θ = 28.34° angle. The low angle from 2-theta = 43°, was iden-

Figure 7 shows the phase diffractograms for the three samples with distinct ceramic coatings: P3–143–9 passes, P6–6420–9 passes and P9–136–9 passes. It can be seen that two of the three samples have a crystalline structure (P3–143–9 passes, P9–136–9 passes) highlighted by specific peaks, and the third (P6–6420–9 passes green diffractogram) has a semi- crystalline structure with the presence of small peaks of chromium oxide at four distinct 2θ angles, 24.25◦ , 33.39◦ , 35.88◦ and 54.61◦ , respectively, of low intensity, 1036 to 1356 [47–49]. The major peak registered at 16.73◦ , with a diffraction intensity of 2643, which, according to the literature, may be associated with the presence of polylactic acid in its the chemical composition ((C3H4O2)n) [50,51]. The presence of this compound is not accidental because it is due to the fact that the thickness of the deposited layer is very thin (small microparticles), thus, the equipment is detecting one of the basic material constituents. tified as the specific angle of Y2O3 crystallization [56]. • The P9–136–9 passes sample (red diffractogram) shows diffraction maxima associated with the presence of the polymer matrix, which has in its structure polylactic acid (16.73°) and lignin or natural fibers (19.04°) [53, 57]. Diffraction angles corresponding to the coating with ceramic micropowder are also visible: The specific peaks to Cr2O3 crystallization at 2-theta angles of low intensity are 30.35°, 31.70 °, 35.16 °, 50.48° and 54.12° [47–49, 58]. For SiO2 microspheres, a peak located at about 2θ = 22.5 is observed [59]. No other diffraction peaks can be detected for this compound. According to the literature [54,55], the diffraction angles that can be attributed to the titanium dioxide (TiO2) present are at 27.33° and 32.13° in the case of Metco 136F micropowder.

**Figure 7.** XRD analysis for ceramic coated samples: P3–143–9 passes (blue), P6–6420–9 passes (green), P9–136–9 passes **Figure 7.** XRD analysis for ceramic coated samples: P3–143–9 passes (blue), P6–6420–9 passes (green), P9–136–9 passes (red).

In the case of the other two coatings, the presence of predominant peaks is observed, which correspond to the crystallization of certain compounds as follows:

• The P3–143–9 passes sample (blue diffractogram) has a strong crystalline aspect due to the large amount of zirconium dioxide particles present in the structure of the ceramic powder. Thus, to the ZrO<sup>2</sup> compound can be assigned the peaks from 2θ = 31.14◦ , 38.43◦ , 60◦ , 82◦ and 84.78◦ [46,52,53]. Titanium dioxide, as in the case of the P9–136–9 passes sample, is found at angles of 27.43◦ , 28.34◦ , 63.02◦ and 74.41◦ [54,55]. However, the intensity of the diffraction peaks is quite low, the highest being registered in the case of the 2θ = 28.34◦ angle. The low angle from 2-theta = 43◦ , was identified as the specific angle of Y2O<sup>3</sup> crystallization [56].

• The P9–136–9 passes sample (red diffractogram) shows diffraction maxima associated with the presence of the polymer matrix, which has in its structure polylactic acid (16.73◦ ) and lignin or natural fibers (19.04◦ ) [53,57]. Diffraction angles corresponding to the coating with ceramic micropowder are also visible: The specific peaks to Cr2O<sup>3</sup> crystallization at 2-theta angles of low intensity are 30.35◦ , 31.70◦ , 35.16◦ , 50.48◦ and 54.12◦ [47–49,58]. For SiO<sup>2</sup> microspheres, a peak located at about 2θ = 22.5 is observed [59]. No other diffraction peaks can be detected for this compound. According to the literature [54,55], the diffraction angles that can be attributed to the titanium dioxide (TiO2) present are at 27.33◦ and 32.13◦ in the case of Metco 136F micropowder. *Polymers* **2021**, *13*, x FOR PEER REVIEW 15 of 22 *3.4. Scratch Analysis*  The scratch test was performed in order to evaluate the adhesion of the hard (ceramic) coatings made on the surface of the Arboblend V2 Nature biopolymeric material. Analyzing the curves presented in Figure 8, it is observed that one of them, the green curve (P6–6420–9 passes), shows a sudden and gradual transition of the apparent friction

#### *3.4. Scratch Analysis* the polymeric material is better than in the case of the other two coatings due to the pres-

The scratch test was performed in order to evaluate the adhesion of the hard (ceramic) coatings made on the surface of the Arboblend V2 Nature biopolymeric material. ence of chromium oxide. The other two tests have a good scratching behavior, but the P3– 143–9 passes test (blue curve), recorded higher A-COF values than in the case of the P9–

coefficient (A-COF), which means that the adhesion between the deposited thin layer and

Analyzing the curves presented in Figure 8, it is observed that one of them, the green curve (P6–6420–9 passes), shows a sudden and gradual transition of the apparent friction coefficient (A-COF), which means that the adhesion between the deposited thin layer and the polymeric material is better than in the case of the other two coatings due to the presence of chromium oxide. The other two tests have a good scratching behavior, but the P3–143–9 passes test (blue curve), recorded higher A-COF values than in the case of the P9–136–9 passes test. 136–9 passes test. The more peaks that appear in the variation of the apparent friction coefficient, the better the adhesion between the deposited layer and the polymeric material is. For the P6–6420–9 passes sample, a high amplitude peak of A-COF is registered at the beginning of the test; the explanation for this would be related to the deposition granulation. It is very possible that the tip of the cutting tool (pin) has hung an area of deposited material with a larger granulation.

**Figure 8.** Results regarding the scratching behavior of the samples coated with ceramic layers: blue curve—P3–143–9 passes; green curve—P6–6420–9 passes; red curve—P9–136–9 passes. **Figure 8.** Results regarding the scratching behavior of the samples coated with ceramic layers: blue curve—P3–143–9 passes; green curve—P6–6420–9 passes; red curve—P9–136–9 passes.

The samples injected from Arboblend V2 Nature and coated with ceramic micropowders showed the following behavior during the 60 s of testing (Figure 9): For sample P3– The more peaks that appear in the variation of the apparent friction coefficient, the better the adhesion between the deposited layer and the polymeric material is.

143–9 passes, blue curve, an increase in A-COF is observed in the first 4 s, after which its value begins to decrease sharply until the 16 s when it registers an increase followed quickly by a decrease. Starting with at 19 s, the average value of A-COF increases and begins to stabilize, reaching a maximum value of 0.53 at 50 s. The mean value of A-COF For the P6–6420–9 passes sample, a high amplitude peak of A-COF is registered at the beginning of the test; the explanation for this would be related to the deposition granulation. It is very possible that the tip of the cutting tool (pin) has hung an area of deposited material with a larger granulation.

was 0.29 ± 0.16. The behavior of the sample coated with Zirconia–Titania–Yttria composite powder is a typical one, and the coatings in the first part of the test register variations of A-COF in order to later stabilize. Sample P6–6420–9 passes, green curve, reflects a completely different behavior from the first test. In the first part of scratching, the first 3 s, it The samples injected from Arboblend V2 Nature and coated with ceramic micropowders showed the following behavior during the 60 s of testing (Figure 9): For sample P3–143–9 passes, blue curve, an increase in A-COF is observed in the first 4 s, after which

reaches the maximum value of A-COF at 1.62, followed by a sudden decrease until 6 s.

attributed to the variable dimensions (9–30 µm) of the microparticles that constitute the ceramic powder. The progressive increase recorded in the last 20 s of testing reflects the

its value begins to decrease sharply until the 16 s when it registers an increase followed quickly by a decrease. Starting with at 19 s, the average value of A-COF increases and begins to stabilize, reaching a maximum value of 0.53 at 50 s. The mean value of A-COF was 0.29 ± 0.16. The behavior of the sample coated with Zirconia–Titania–Yttria composite powder is a typical one, and the coatings in the first part of the test register variations of A-COF in order to later stabilize. Sample P6–6420–9 passes, green curve, reflects a completely different behavior from the first test. In the first part of scratching, the first 3 s, it reaches the maximum value of A-COF at 1.62, followed by a sudden decrease until 6 s. Next, the sample registers two fluctuations, and starting at 40 s, the A-COF value begins to increase, at the end of the 60 s reaching the value of 1.37. These fluctuations can be attributed to the variable dimensions (9–30 µm) of the microparticles that constitute the ceramic powder. The progressive increase recorded in the last 20 s of testing reflects the fact that the test pin detached ceramic microparticles from the sample surface, thus gradually becoming more and more rough. The mean value of A-COF for this sample, 0.56 ± 0.42, is the highest compared to the other two tested samples. The last test subjected to tribological determination, P9–136–9 passes, red curve, as well as the previous test, records fluctuations throughout the test with the A-COF value at the end of the test reaching a maximum of 0.37. The average value of A-COF for this sample is the lowest, 0.18 ± 0.08. The value registered is very close to that of the injected samples and not covered with ceramic layer, 0.16 for rotational determinations and 0.13 for oscillation ones [60]. This similarity comes from the fact that the coverage of the sample was not uniform, the adhesion and incorporation of Chromia–Silica composite powder, as it could be observed in the SEM images being very low, had a very high percentage both in the P6–6420–9 passes test and in the P9–136–9 passes test. This lack of deposition is due to the thermal behavior of chromium oxide, which is found in a very high percentage both in the P6–6420–9 passes and P9–136–9 passes samples. *Polymers* **2021**, *13*, x FOR PEER REVIEW 16 of 22 fact that the test pin detached ceramic microparticles from the sample surface, thus gradually becoming more and more rough. The mean value of A-COF for this sample, 0.56 ± 0.42, is the highest compared to the other two tested samples. The last test subjected to tribological determination, P9–136–9 passes, red curve, as well as the previous test, records fluctuations throughout the test with the A-COF value at the end of the test reaching a maximum of 0.37. The average value of A-COF for this sample is the lowest, 0.18 ± 0.08. The value registered is very close to that of the injected samples and not covered with ceramic layer, 0.16 for rotational determinations and 0.13 for oscillation ones [60]. This similarity comes from the fact that the coverage of the sample was not uniform, the adhesion and incorporation of Chromia–Silica composite powder, as it could be observed in the SEM images being very low, had a very high percentage both in the P6–6420–9 passes test and in the P9–136–9 passes test. This lack of deposition is due to the thermal behavior of chromium oxide, which is found in a very high percentage both in the P6–6420–9 passes and P9–136–9 passes samples.

**Figure 9.** A-COF variation with test time for samples coated with ceramic micropowders: (blue) P3–143–9 passes; (green) P6–6420–9 passes; (red) P9–136–9 passes. **Figure 9.** A-COF variation with test time for samples coated with ceramic micropowders: (blue) P3–143–9 passes; (green) P6–6420–9 passes; (red) P9–136–9 passes.

**Table 5.** Values of A-COF recorded by samples covered with ceramic layers.

**Medium Value** 

P3–143–9 passes 0.29 ± 0.16 0.53 50 P6–6420–9 passes 0.56 ± 0.42 1.62/1.37 3.0/60

Table 5 shows the results of scratch testing in the case of the three samples covered with ceramic micropowders. Table 5 shows the results of scratch testing in the case of the three samples covered with ceramic micropowders.

To conduct the microindentation test, three samples were tested for each type of ceramic powder used to coat the Arboblend V2 Nature biopolymeric material. Repeated testing was to confirm the experimental stability. Figure 10 shows the evolution of the force as a function of the depth of penetration for all three samples subjected to analysis. The software package used (UMT Test Viewer, 2.16) allowed the reading of both the microhardness values and the Young's modulus. These values are presented in Table 6.

**A-COF Maximum**  **Time of A-COF Maximum[s]** 

**Sample A-COF** 

*3.5. Microindentation Test* 


**Table 6.** Results obtained by microindenting the samples coated with ceramic micropowders.

**Max Depth (µm)** 

Average 8.98 ± 0.01 73.12 ± 0.4 1.56 ± 0.11 0.11 ± 0.00

Average 8.99 ± 0.01 69.79 ± 2.75 2.04 ± 0.44 0.12 ± 0.01

1 8.99 73.55 1.52 0.11 2 8.7 72.78 1.69 0.11 3 8.98 73.02 1.48 0.11

1 8.99 66.64 1.69 0.13 2 8.98 71.70 1.91 0.11 3 8.98 71.02 2.53 0.10

**Young's Modulus (GPa)** 

**Micro Hardness (GPa)** 

**Table 5.** Values of A-COF recorded by samples covered with ceramic layers. 1 8.99 53.77 2.92 0.16

**(N)** 

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**Sample Test Max Load** 

#### *3.5. Microindentation Test* the zirconium-based ceramic coating (P3–143–9 passes), the chromium oxide coatings have a higher hardness (P6–6420–9 passes, P9–136–9 passes) [35]. Another justification of

P3–143–9 passes

P6–6420–9 passes

To conduct the microindentation test, three samples were tested for each type of ceramic powder used to coat the Arboblend V2 Nature biopolymeric material. Repeated testing was to confirm the experimental stability. Figure 10 shows the evolution of the force as a function of the depth of penetration for all three samples subjected to analysis. The software package used (UMT Test Viewer, 2.16) allowed the reading of both the microhardness values and the Young's modulus. These values are presented in Table 6. the results comes from the reduced layer thickness due to the small dimension of microparticles (P3–143–9 passes) than in the case of the other two types of ceramic micropowders. The lowest dispersion of the results was obtained in the case of the P3–143–9 passes sample, most likely due to the fact that the deposited ceramic layer was uniform. In addition, the other samples tested did not show large differences.

**Figure 10.** Results of microindentation tests for samples coated with ceramic micropowders: (**a**) P3–143–9 passes; (**b**) P6– 6420–9 passes; (**c**) P9–136–9 passes. **Figure 10.** Results of microindentation tests for samples coated with ceramic micropowders: (**a**) P3–143–9 passes; (**b**) P6– 6420–9 passes; (**c**) P9–136–9 passes.

**5. Conclusions**  The coating of bio-based polymers has gained the attention of researchers worldwide. The purpose of coatings with various layers, be they ceramic, metal, etc., as well as in the case of reinforcements, is to increase the characteristics of the base material so that The force applied to the indenter increases constantly during the charging phase and is maintained at the maximum value of 10 N. This phase is called creep, after which there is a decrease to zero in the discharge phase.

it responds better to certain industrial applications and can even replace a certain material, such as metal. The coatings have been realized in order to increase the mechanical, tribological and thermal characteristics of the samples (wear, hardness and increase in thermal resistance), thus becoming suitable in applications that require harsh working conditions,





attributed in this situation to the size of the ceramic microparticles.

value of the apparent friction coefficient (A-COF) being 0.18 ± 0.08.

to the following results:

coated material is.

the melting point (2435 ˚C).


**Table 6.** Results obtained by microindenting the samples coated with ceramic micropowders.

According to the data obtained from the micro-indentation, the P9–136–9 passes sample, although it does not have a uniform coating, presents the best values of microdurity, (0.17 ± 0.01 GPa), the maximum indentation depth being 52.42 ± 1.77 µm. Compared to the zirconium-based ceramic coating (P3–143–9 passes), the chromium oxide coatings have a higher hardness (P6–6420–9 passes, P9–136–9 passes) [35]. Another justification of the results comes from the reduced layer thickness due to the small dimension of microparticles (P3–143–9 passes) than in the case of the other two types of ceramic micropowders.

The lowest dispersion of the results was obtained in the case of the P3–143–9 passes sample, most likely due to the fact that the deposited ceramic layer was uniform. In addition, the other samples tested did not show large differences.

#### **4. Conclusions**

The coating of bio-based polymers has gained the attention of researchers worldwide. The purpose of coatings with various layers, be they ceramic, metal, etc., as well as in the case of reinforcements, is to increase the characteristics of the base material so that it responds better to certain industrial applications and can even replace a certain material, such as metal. The coatings have been realized in order to increase the mechanical, tribological and thermal characteristics of the samples (wear, hardness and increase in thermal resistance), thus becoming suitable in applications that require harsh working conditions, especially in the automotive industry. The coating of sample with ceramic microlayers led to the following results:


9) formed the hardest layer (0.17 ± 0.01 GPa), which was demonstrated during the microindentation test. Analyzing the obtained results from the SEM and scratch analyses point of view, it can be concluded that the deposition was not uniform due to the fact that the adhesion between the microparticles of chromium oxide, silicon oxide and titanium oxide is lower than in the case of the other two samples, the average value of the apparent friction coefficient (A-COF) being 0.18 ± 0.08.

According to the obtained results regarding the adhesion of the ceramic layers on the polymer surface, it can be stated that the samples showed strong chemical bonds at the interface between the thin layers and Arboblend V2 Nature bio-based polymer. Thus, these coated materials can be used in industrial applications that require high surface hardness and thermal resistance. They can also successfully replace various non-biodegradable polymeric materials used in various applications such as those in the automotive and electronics industry (telephone covers, housings, worm wheels, car wiper system, etc.).

**Author Contributions:** Conceptualization, D.N.; data curation, B.I. and M.B. (Marcelin Benchea); formal analysis, S.-N.M. and A.M.; methodology, M.B. (Mihai Boca); supervision, D.N. and B.I.; validation, D.N.; writing—original draft, S.-N.M. and A.M.; writing—review and editing, S.-N.M.; funding acquisition, D.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by a publication grant of the TUIASI, project number GI/P21/2021.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** This work was supported by a publication grant of the TUIASI, project number GI/P21/2021.

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

#### **Nomenclature**

