**1. Introduction**

The thermal coating process finds its applicability in areas such as the automotive, aerospace and naval industries in order to improve the corrosion resistance, wear resistance and lifetime of the equipment part as follows: thermal barrier coatings for components that operate in severe thermal conditions (turbine blades, fuel parts, vanes) with the role of increasing their life by improving resistance to oxidation, heat and corrosion [1]; hard metal coatings such as tungsten and chromium carbide are commonly used to increase the wear and corrosion resistance of the parts surfaces [2]; corrosion resistance after heat treatment of a Cr3C2–NiCr coating showed significant improvement due to both the microstructural changes and the presence of a metallurgical bond at the coating–substrate interface [3];

**Citation:** Mazurchevici, S.-N.; Marguta, A.; Istrate, B.; Benchea, M.; Boca, M.; Nedelcu, D. Improvements of Arboblend V2 Nature Characteristics through Depositing Thin Ceramic Layers. *Polymers* **2021**, *13*, 3765. https://doi.org/10.3390/ polym13213765

Academic Editors: Domenico Acierno and Antonella Patti

Received: 7 October 2021 Accepted: 27 October 2021 Published: 30 October 2021

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**Copyright:** © 2021 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/).

titanium plasma spraying (TPS) used on polymers as polyethylene/polyamide highlighted good mechanical properties and increased hardness [4].

One of the most common thermal deposition techniques is Atmospheric Plasma Spray (APS), and the coatings obtained by zirconium and chromium ceramic layer depositions are widely spread materials in mechanical applications [5]. The main drawbacks of these coatings are given by the micro-cracks, isolation inhomogeneity and residual stresses (appears during cooling process) [6].

The Al2O<sup>3</sup> (aluminium oxide (alumina)) powder with high purity used for coating, usually applied by the plasma spray method, presents electrical insulation in terms of thermal conductivity and dielectric characteristics. Furthermore, the coatings are wearresistant, have higher hardness, higher temperature stability and are chemically inert. The ceramic coating is suitable for electrical, electronic and semiconductor manufacturing tool applications, electrostatic chucks and capacitors, vacuum chamber lines, etc. The high purity of the material ensures that it will not contaminate the semi-conductor components [7]. Nickel, zirconium, aluminium, molybdenum and chromium, as powder elements, are a good choice for applications that require high toughness with moderate resistance to fretting, erosion and scuffing. Zinc-based coatings are widespread in applications that require increased corrosion resistance, but if this chemical element is not alloyed with other elements such as tungsten carbide or nickel, it has low mechanical characteristics [8]. The degradation of the Zn–polymer interface has been studied by the authors of the paper [9], and they point out that the delamination can be inhibited by CO<sup>2</sup> gas in a humid environment. Inhibition depends very much on the polymer matrix's affinity for carbon dioxide. Other researchers such as [10] have used nano-zinc-oxide (5 wt %) and epoxy acrylate in order to obtain a corrosion protection coating for mild steel panels.

It is well known that the addition of ceramic particles, metals particles and biopolymers as a substrate provides a combination of properties of all three types of materials: biopolymer matrices and metallic and ceramic reinforcement components. This may result in the improvement of the physical and mechanical properties of the composite [11].

In the literature, the applications of function coatings refer mainly to corrosion protection characteristic, and there is a limited emphasis on equally important properties—for example, mechanical robustness, which is significant for determining the applicability of developed coatings. However, there are discussions on the principal working theories, procedures for preparation, performance investigations and applications of superhydrophobic coatings, for instance: a viable preventative method for controlling metal corrosion due to their mechanical stability and durability, and the shortcoming consists in the ability of maintaining this characteristic for a prolonged period of time [12]; corrosion inhibition of metallic materials with the help of smart coatings involves difficulties in achieving some features as thermal stability, resistances to scratching and strong chemical acids, high optical transmission, in situ healing, etc. [13]; coating of stainless steel and titanium bipolar plates in order to improve the corrosion resistance and electrical conductivity in PEMFC (protonexchange membrane fuel cells), which has, for the moment as the main impediment in the large-scale spread of the product, a high manufacturing cost [14]; excellent anticorrosion durability obtained by coating the polyaniline–graphene oxide composite with zinc-based waterborne [15]; anticorrosive coatings in the marine field face the degradation, loss of adhesion and failure of coating systems [16]; metallic substrates (aluminum alloy) coated with active corrosion protection systems (with self-healing ability) such as silica−zirconia nanoparticles highlighted long-term corrosion protection and the ability to self-heal defects, and these characteristics are obtained by rigorous control of the inhibitor regular release at the moment when the corrosion process begins to arise [17]. A comparative review that accounts for all factors, including the durability and other mechanical properties, is essential for understanding the applicability of an advanced coating at a practical level.

In addition, another area targeted by coating researchers is related to composites based on polymer matrices with the inclusion of magnetic nano-sized particles: Polydimethylsiloxane (PDMS) coated with different concentrations of nanosized Ni@C core-shell [18];

Ni-silicone elastomagnetic composites [19]; polyacrylamide-based hydrogels coated with Ni ferrite [20]; polyetherurethane (TFX) and a biodegradable multiblock copolymer (PDC) with poly(p-dioxanone) as hard segment and poly(ε-caprolactone) as soft segment were investigated as matrix component, coated with iron oxide particles [21]; and oligo(εcaprolactone)dimethacrylate/butyl acrylate, coated with Fe3O<sup>4</sup> [22]; carbon-fiber–epoxy composites coated with two thermal ceramic particles (lass flakes and aluminum titanate) in order to create a thermal barrier for the substrate [23]. The interest of researchers was focused mainly on elastomagnetic effects [19] and the wide prospects of applications as follows: Ni ferrite with a highly organized structure as humidity sensors [20]; magnetic nanoparticles for practical applications which involve sensors and biosensors [24]; magnetoresistive sensors for applications where the ultimate field detection limits are required or as readers in hard disk drives [25]; Mg substitution on Ni-ferrite ceramics with applications in biomedicine, gas detection, heterogeneous catalysis, adsorption, etc. [26]; shape-memory materials through the inductive heating of magnetic nanoparticles in thermoplastic polymers [21]; and the incorporation of surface-modified superparamagnetic nanoparticles into a polymer matrix [22]. These materials also demonstrate prospects for biomedicine: drug delivery, hyperthermia, magnetic resonance imaging contrast enhancement [27] and the manipulation of cell membranes [28]; recording media and high-frequency applications electromagnetic-wave-absorption materials [29], microwave absorption [30] and gigahertz microwave absorption [31].

In this paper, the authors have analysed three types of plasma jet coatings for intermediate layers (micrometallic powder) and one type of ceramic coating (aluminium oxide). The coatings were made on Arboblend V2 Nature substrate materials. The samples were obtained using injection moulding, and the method used for coating was atmospheric plasma spraying.

The registered trademark Arboblend®, developed by scientists and engineers from the German company Tecnaro in collaboration with those from the Fraunhofer Institute for Chemical Technology, is a 100% biodegradable biopolymer [32], and a part of the current research group has investigated the behavior of samples covered with metallic intermediate layer and ceramic final layer in the past but not at such a deep level and not following an experimental plan [33].

The aim of the manuscript was to obtain a new material with improved properties that can then be used successfully as a substitute for synthetic plastics in the automotive industry. Given this objective, the biodegradable material Arboblend V2 Naure was chosen, for which the research team previously studied the properties. The next step was to realize the coating with ceramic microparticles by using the APS method, after which the same characteristics were studied. The present study is not found in the research activity of other authors, so the proposed research through both technology and experimental results is constituted as a novelty element.

The introduction section provides a coatings overview. This chapter is followed by a description of the materials and methods used in the analysis of the coated samples. The results of the experimental research are presented and commented on in Section 3.4, and the conclusions part suggestively presents general comments on the main obtained results.

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

The thermoplastic material selected to be coated with ceramic microparticles was Arboblend V2 Nature. According to the information provided by the producers but also to some studies from the specialized literature [32,34,35], the basic matrix of the polymer is lignin, this being extracted from annual vegetable plants, so it is not necessary to use wood raw materials that require dozens of years or even longer to reach maturity and be used in the forestry and paper industry. This is important to mention because another source of lignin used for this material comes from its extraction from the paper industry waste. In addition, the Arboblend V2 Nature structure can contain a significant amount of polylactic acid (PLA—also biodegradable polyester) and other constituents such as bio-polyamides (bio-PA), cellulose, natural vegetable fibers and, for processing in good conditions, contains a small amount of natural additives (resins, waxes, shellac) [33,36,37].

Obtaining the necessary samples for coating with ceramic layers was realized by injection in the mold using the SZ-600H equipment (SHEN ZHOU, Zhangjiagang, China). The dimensions of the samples were (70 <sup>×</sup> <sup>50</sup> <sup>×</sup> 10) mm<sup>3</sup> . The following technological parameters were used for injection: material melting temperature—165 ◦C; injection pressure—100 MPa; injection speed—80 m/min; cooling time—30 s.

The preparation of the rectangular biopolymer samples consisted in the fixed adhesion between metal strips, followed by the blasting and removal of impurities in order to obtain a surface roughness that was as low as possible. The final stage of preparation was degreasing with ethyl alcohol.

Atmospheric plasma spray (APS) technology (SPRAYWIZARD-9MCE, Sultzer-Metco, Westbury, New Yorkxz, USA/9MBspraying gun) was used to cover the injected samples. The technological coverage parameters used are shown in Table 1.

The deposition rate of the microparticles was constant. The thickness of the deposited ceramic layer was of the micrometers order and for the thermal control of the samples' melting temperature, a laser pyrometer was used throughout the process.

Three ceramic powders were used for the coating: Amdry 6420 (Cr2O3), Metco 143 (Cr2O3-xSiO2-yTiO2) and Metco 136F (ZrO<sup>2</sup> 18TiO<sup>2</sup> 10Y2O3). The three micropowders were deposited on three samples injected from Arboblend V2 Nature. On each sample, a distinct number of passes was made, namely, 5, 7 and 9 passes, in order to study the improvement or not of the mechanical characteristics with the increase in the deposited ceramic layer. However, this manuscript will only present the results for the samples obtained by performing 9 passes, because the objective of the paper is to highlight the uniformity and homogeneity of the deposited layers, which is not entirely revealed by the samples obtained with 3 and 5 passes. The experimental plan used to cover the samples with ceramic micropowders is highlighted in Table 2.

**Table 1.** Technological parameters used during the counting process.


**Table 2.** Experimental plan used to cover the samples with ceramic layer.


The ceramic micro powders were purchased from the Oerlikon Metco manufacturer (Bella Vista, New South Wales, Australia). The microparticles dimensions and shapes are varied as follows [38]:


In order to determine the thermal, structural and morphological behavior, only the samples with 9 successive passes were selected for this study: sample 3 coated with Zirconia–Titania–Yttria Composite Powder, Metco™ 143, further noted with P3–143–9 passes; sample 6 coated with Chromium Oxide powder, Amdry 6420, noted with P6–6420– 9 passes; sample 9 coated with Chromia–Silica composite powder, Metco 136F, noted with P9–136–9 passes. The used equipments for these analyzes were as follows:

Differential scanning calorimetry *(DSC)* was performed on a DSC 200 F3 Maia differential scanning calorimeter (NETZSCH-Gerätebau GmbH, Selb, Germany). The calibration of the device was realised in accordance with mercury (Hg), zinc (Zn), Indium (In), Tin (Sn) and Bismuth (Bi) standards. The mass of the analyzed samples was less than 30 mg. The experiments were analyzed in the atmosphere of inert gas (Ar). In this experiment, a sample and a reference (an empty crucible) were subjected to the same temperature program. The temperature program consisted of heating from room temperature (RT ≈ 20 ◦C) to 200 ◦C, then cooling from this temperature to RT. The heating and cooling speed used was 10 K/min. During the experiment, the reference and sample temperatures were measured and the temperature difference recorded between the two was converted into heat flux. The recorded thermograms were then evaluated using Proteus software (provided by NETZSCH). The tangent method was used to determine the transition temperatures. The area was determined by using a rectilinear baseline. Temperature of the transformation beginning (Tonset), temperature assigned to the peak (Tpeak), temperature at the end of the transformation (Tend) and the amount of absorbed or dissipated heat were determined.

Thermogravimetric curves (*TG*), derived thermogravimetric curves (*DTG*) and differential thermal analyzes (*DTA*), were determined using Mettler Toledo TGA/SDTA 851

equipment. The mass of samples subjected to thermal decomposition was between 2.9 and 3.9 mg. It was worked in an air atmosphere with a flow rate of 20 cm3/min. The study was realized in the temperature range of 25–700 ◦C using a heating rate of 10 ◦C/min. The processing of thermogravimetric curves was performed with the STARe SW 9.10 software from Mettler Toledo (Columbus, OH, USA). As in the case of DSC analysis, the beginning (Tonset), maximum (Tpeak) and end (Tend) temperatures of each thermal degradation stage were determined. For each identified stage, it is indicated the loss mass percentage residue (W%).

SEM structural analysis (Scanning Electron Microscopy) was performed on the QUANTA 200 3D electron microscope (FEI Company, Fremont, CA, USA). Micrographic maps of the samples coated with ceramic micropowders were made on their surface to observe mainly the uniformity of the deposition. The main parameters considered were the following: the pressure inside the microscope chamber—60 Pa; detector—(Large Field Detector) for the analysis of non-conductive samples such as polymers, textile fibers, powders, etc.; tilt angle—0◦ ; secondary electron acceleration voltage—20 Kv; working distance—15 mm; magnification power—500×–2000×.

To determine the chemical elements that appeared with the deposition of the ceramic layers, an Energy-dispersive X-ray spectroscopy (EDX) together with a SEM was performed. The SEM equipment was VegaTescan LMHII (TESCAN ORSAY HOLDING, Kohoutovice, Czech Republic) with EDX detector X Flash 6I10 from Bruker, Germany, using Esprit 2.2 software. The type of EDX analysis was in-line in order to capture as accurately as possible the difference in chemical composition between the resulted microceramic layer and substrate material.

X-ray diffraction analysis (XRD) was performed with the X'Pert Pro MRD X-ray diffractometer, which has a Cu kα anode X-ray tube, λ–1.54 Å, Panalytical equipment (PANalytical, Almelo, the Netherlands), on which a voltage of 45 kV was applied, the variation of the diffraction angle (2θ) being between 10 and 90◦ . Two X'Pert Data Collector programs were used to process the data and make the diagrams, namely X'Pert High Score Plus version number 3 and X'Pert Data Viewer version number 2.2 g (Malvern Panalytical, Malvern, UK). This analysis aimed to identify the existence of crystallization phases specific to ceramic micropowders deposited on the surface of the samples from Arboblend V2 Nature. The identification of the crystallization phases was performed by comparing the obtained data with those from the scientific literature.

The chemical composition analyses were performed in five distinct points, and an overall average composition was made with the help of Minitab software.

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

#### *3.1. DSC Analyse*

In order to establish the physical transformations that take place during the gradual heating of the Arboblend V2 Nature samples coated with ceramic micropowders, a DSC analysis was performed. Three distinct samples were used, one from each type of powder: P3–143–9 passes; P6–6420–9 passes; and P9–136–9 passes. The sample size was less than 5 mm; their mass was less than 20 mg.

During the heating of the three selected samples, three transformations were highlighted, two of them endothermic (Ist and IIIrd) and one exothermic (IInd), the same thermal behavior being highlighted for the sample injected from Arboblend V2 Nature but not covered with a ceramic layer [34,35].

The variation in the heat flow in relation to the recorded temperature for the three phase transformations of each analyzed sample is shown in Figure 1. In order to highlight the possible different thermal compartments of the coated samples, the three signals were overlapped.

**3. Results and Discussion** 

mm; their mass was less than 20 mg.

covered with a ceramic layer [34,35].

*3.1. DSC Analyse* 

overlapped.

**Figure 1.** Highlighting the thermal behaviors of the tested samples: Ist—first transformation; IInd—second transformation; IIIrd—third transformation; P3–143–9 passes (green), P6–6420–9 passes (blue), P9–136–9 passes (red). **Figure 1.** Highlighting the thermal behaviors of the tested samples: Ist—first transformation; IInd—second transformation; IIIrd—third transformation; P3–143–9 passes (green), P6–6420–9 passes (blue), P9–136–9 passes (red).

Both Figure 1 and Table 3 reveal changes in the temperatures at which the phase transformations take place. This aspect can be attributed to the small but still existing mass difference between the three analyzed samples (P3–143–9 passes 12.6 mg, P6–6420–9 passes 16.7 mg, P9–136–9 14.5 mg). At the same time, the amount of heat absorbed or dissipated is slightly different [39]. Another reason that could have generated this difference is the thickness of the deposited layer with the completion of the nine passes. As the dimensions of the microparticles differ in the case of the three ceramic powders, it is expected that the powder, whose microparticles are larger, will form a thicker layer, and thus the phase transitions will take place at slightly higher temperatures and the amount of absorbed or dissipated heat will be lower. Both Figure 1 and Table 3 reveal changes in the temperatures at which the phase transformations take place. This aspect can be attributed to the small but still existing mass difference between the three analyzed samples (P3–143–9 passes 12.6 mg, P6–6420–9 passes 16.7 mg, P9–136–9 14.5 mg). At the same time, the amount of heat absorbed or dissipated is slightly different [39]. Another reason that could have generated this difference is the thickness of the deposited layer with the completion of the nine passes. As the dimensions of the microparticles differ in the case of the three ceramic powders, it is expected that the powder, whose microparticles are larger, will form a thicker layer, and thus the phase transitions will take place at slightly higher temperatures and the amount of absorbed or dissipated heat will be lower.

In order to establish the physical transformations that take place during the gradual heating of the Arboblend V2 Nature samples coated with ceramic micropowders, a DSC analysis was performed. Three distinct samples were used, one from each type of powder: P3–143–9 passes; P6–6420–9 passes; and P9–136–9 passes. The sample size was less than 5

During the heating of the three selected samples, three transformations were highlighted, two of them endothermic (Ist and IIIrd) and one exothermic (IInd), the same thermal behavior being highlighted for the sample injected from Arboblend V2 Nature but not

The variation in the heat flow in relation to the recorded temperature for the three phase transformations of each analyzed sample is shown in Figure 1. In order to highlight the possible different thermal compartments of the coated samples, the three signals were

The critical temperatures of the three transformations are as follows: Tonset is the starting temperature; Tpeak is the middle temperature; Tend is the finishing temperature (determined using the tangent method); and ∆H/m is the amount of dissipated/absorbed heat (using a rectilinear baseline).

Analyzing the first phase transformation, it is observed that the three samples register an endothermic maximum around 65 ◦C, a slightly lower transition temperature (64.7 ◦C) that the P3–143–9 passes sample (Zirconia–Titania–Yttria Composite Powder), whose powder has a smaller granulation. The first peak can be associated with a slow monotropic transformation of the solid-solid type and of some metastable crystals [40], which takes place with reduced heat absorption, −8.81 kJ/kg for P3–143–9 passes, −4.29 kJ/kg for the P6–6420–9 passes and −5.57 kJ/kg in the case of the P9–136–9 passes. The variation in the absorbed heat can be attributed, first of all, to the thickness of the deposited layers but also to the mass difference of the analyzed samples.

**Table 3.** Calorimetric characterization of samples coated with ceramic layers.


The second peak takes place around the temperature of 86 ◦C, the powder with the highest granulation, the P9–136–9 passes, registering an increase in the transformation with 1.5 ◦C higher than the other two covered samples. The exothermic peak can be associated with the base biopolymer crystallization or with the reticular reorganization of lignin, the basic matrix of the biopolymer [39].

The third peak occurs with considerable heat absorption, in the case of all analyzed samples: −40.1 kJ/kg for the sample with smaller microparticles (P3–143–9 passes) and −48.38 kJ/kg for the sample with higher granulation (P9–136–9 passes). The endothermic transformation is attributed to the melting of the Arboblend V2 Nature biopolymer around the temperature of 170 ◦C, with small variations depending on the size of the ceramic microparticles.

#### *3.2. TG Analyses*

Knowing the thermal stability of Arboblend V2 Nature samples coated with ceramic powders is essential because their use in applications that require operation in severe working conditions, whether it is wear resistance or thermal resistance, requires the study of thermogravimetric behavior. It is desirable that the coating with ceramic microparticles increases the mechanical characteristics but also the thermal stability. Figure 2 compares the thermogravimetric (TG), derived thermogravimetric (DTG) and differential thermal (DTA) curves for the three samples coated with ceramic layers made from nine successive passes.

The main thermogravimetric characteristics of the P3–143–9 passes, P6–6420–9 passes and P9–136–9 passes samples are presented in Table 4.


**Table 4.** Thermogravimetric characteristics of the samples covered with ceramic micropowders.

Tonset, the temperature at which thermal degradation begins at each stage; Tend, the temperature at which the thermal degradation ends at each stage; Tpeak, the temperature at which the degradation rate at each stage is maximum; W%, percentage mass loss at each stage; residue, the amount of degraded sample remaining at a temperature above 700 ◦C.

> The three coated samples with different ceramic layers highlight two decomposition stages, the first recorded being around a temperature of 345 ◦C, with a significant mass loss of over 85%: this decomposition is attributed to the structural degradation of the basic constituent of the material, lignin. This stage consists of the formation of aromatic hydrocarbons, guaiacyl-/syringyl-type and hydroxy-phenolic compounds and more [40]. According to the manufacturer [32], another constituent contained by the analyzed biopolymer is PLA, which decomposes in considerable proportions in this temperature range [41,42]. According to the literature [43,44], PLA and pure lignin degrade completely up to a temperature of 500 ◦C.

**Figure 2.** Thermogravimetric curves of the coated samples: (a) TG, (b) DTG and (c) DTA. **Figure 2.** Thermogravimetric curves of the coated samples: (**a**) TG, (**b**) DTG and (**c**) DTA.

The main thermogravimetric characteristics of the P3–143–9 passes, P6–6420–9 passes and P9–136–9 passes samples are presented in Table 4. The curves obtained from the TG analysis were overlapped to highlight their difference in behavior, and the thermal stability of the coated samples is similar (Figure 2).

**Table 4.** Thermogravimetric characteristics of the samples covered with ceramic micropowders. **Sample Stage Tonset [°C] Tpeak [°C] Tend [°C] W [%] DTA characteristic Residue [%]**  P3–143–9 passes I 289 341 369 84.98 exo 3.81 II 413 423 436 11.21 exo P6–6420–9 passes I 282 346 373 84.61 exo 6.60 II 413 426 438 8.79 exo P9–136–9 passes I 281 347 367 88.06 exo 1.68 II 415 426 438 10.26 exo Tonset, the temperature at which thermal degradation begins at each stage; Tend, the temperature at which the thermal degradation ends at each stage; Tpeak, the temperature at which the degradation rate at each stage is maximum; W%, percentage mass loss at each stage; residue, the amount of degraded sample remaining at a temperature above 700 °C. In the second stage with a Tpeak around 425 ◦C, there is a mass loss in a percentage much lower than 10%, attributed to the thermal oxidation of the carbonic residue that appeared from the pyrolysis of lignin and/or PLA but also of another biodegradable constituent of the biopolymer that was introduced by the manufacturer as a binder (resin, wax, shellac, etc.) [33]. At a temperature of 700 ◦C, depending on the used ceramic powder type, a certain amount of residual mass is found. It is observed that the sample P6–6420–9 passes has the highest percentage of residue, 6.6%, most likely due to the higher amount of microparticles than, for example, in the case of the P3–143–9 passes sample, where the amount of ceramic powder deposited is much lower. The ceramic powders at the end of the analysis temperature have not yet reached the melting point of approximately 2500 ◦ C, their working temperature varying from 540 ◦C (P6–6420–9, P9–136–9 passes) to 980 ◦C (P3–143–9 passes) [38]. In addition, inorganic substances that are found in the composition of the biopolymer are very likely to be part of the residual mass [45].

The curves obtained from the TG analysis were overlapped to highlight their difference in behavior, and the thermal stability of the coated samples is similar (Figure 2). Figure 2c shows the DTA curves where the melting temperature of Arboblend V2 Nature can be observed, 169 ◦C, very close to the values obtained by calorimetric analysis, especially in the case of the P3–143–9 passes sample.
