*2.2. Materials, Devices and Spraying Parameters*

The additional material for flame-spraying was obtained by mixing in the ball mill appropriate proportions of aluminum powder (EN AW 1000 series) with carbon nanotubes and aluminum powder in the form of filter dust carburite (Table 1). Carbon nanotubes that were used in the test are commercially available multi-walled carbon nanotubes MWCNTs, produced in the Catalytic Chemical Vapor Deposition (CCVD) process, NANOCYLTM NC7000 (Belgium Nanocyl SA, Sambreville, Belgium) (Table 2).


**Table 2.** Structure and specification of Nanocyl NC 7000 carbon nanotubes and of filter dust carburite. *Materials* **2019**, *12*, x FOR PEER REVIEW 4 of 18

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Structure of Nanocyl NC 7000 carbon nanotubes

Surface area m<sup>2</sup> /g 250–300 BET Note: 1) pyrolytically deposited carbon on the surface of the NC 7000.

Structure of filter dust carburite


Granulation [mm] % >1 mm up to 10%; <0,06 mm up to 70%

Moisture content % 1 Carburite content % 94 Granulation [mm] % >1 mm up to 10%; <0,06 mm up to 70% The subsonic flame spraying process was carried out cold in accordance with the standard EN 13507:2018 [35] on workstation, equipped with hand-guided modern oxyacetylene system (CastoDyn DS 8000 (Messer Eutectic Castolin, Gliwice, Poland). Final surface preparation was done by shot blasting sheets prior to spraying with sharp-edged cast iron of 0.5–1.5 mm shot grain size in accordance with standard ISO 2063-1:2017 [36]. Final surface roughness of the steel substrate after shot blasting was R<sup>a</sup> = 12 μm, R<sup>z</sup> = 85 μm. Before the spraying process, steel plates with dimensions of 150 × 150 × 5 mm<sup>3</sup> were preheated with a gas burner up to a temperature of 40 °C (the temperature of preheating was measured using pyrometer). The standard modular nozzles regulating the flame outlet SSM 40 (Messer Eutectic Castolin, Gliwice, Poland) and the neutral flame (ratio O2/C2H<sup>2</sup> = 1,2) were used. This allowed to obtain the proper spray jet [37,38]. The flame jet burner was guided in a The subsonic flame spraying process was carried out cold in accordance with the standard EN 13507:2018 [35] on workstation, equipped with hand-guided modern oxyacetylene system (CastoDyn DS 8000 (Messer Eutectic Castolin, Gliwice, Poland). Final surface preparation was done by shot blasting sheets prior to spraying with sharp-edged cast iron of 0.5–1.5 mm shot grain size in accordance with standard ISO 2063-1:2017 [36]. Final surface roughness of the steel substrate after shot blasting was R<sup>a</sup> = 12 μm, R<sup>z</sup> = 85 μm. Before the spraying process, steel plates with dimensions of 150 × 150 × 5 mm<sup>3</sup> were preheated with a gas burner up to a temperature of 40 °C (the temperature of preheating was measured using pyrometer). The standard modular nozzles regulating the flame outlet SSM 40 (Messer Eutectic Castolin, Gliwice, Poland) and the neutral flame (ratio O2/C2H<sup>2</sup> = 1,2) were used. This allowed to obtain the proper spray jet [37,38]. The flame jet burner was guided in a horizontal position covering the whole surface of the sheet. During the process the spraying direction was changed several times by 90°, until obtained thickness of coating was about 1,0 mm. Distance between the torch and the sprayed surface was 200 mm. The parameters and flame type were The subsonic flame spraying process was carried out cold in accordance with the standard EN 13507:2018 [35] on workstation, equipped with hand-guided modern oxyacetylene system (CastoDyn DS 8000 (Messer Eutectic Castolin, Gliwice, Poland). Final surface preparation was done by shot blasting sheets prior to spraying with sharp-edged cast iron of 0.5–1.5 mm shot grain size in accordance with standard ISO 2063-1:2017 [36]. Final surface roughness of the steel substrate after shot blasting was <sup>R</sup><sup>a</sup> <sup>=</sup> <sup>12</sup> <sup>µ</sup>m, R<sup>z</sup> <sup>=</sup> <sup>85</sup> <sup>µ</sup>m. Before the spraying process, steel plates with dimensions of <sup>150</sup> <sup>×</sup> <sup>150</sup> <sup>×</sup> 5 mm<sup>3</sup> were preheated with a gas burner up to a temperature of 40 ◦C (the temperature of preheating was measured using pyrometer). The standard modular nozzles regulating the flame outlet SSM 40 (Messer Eutectic Castolin, Gliwice, Poland) and the neutral flame (ratio O2/C2H<sup>2</sup> = 1,2) were used. This allowed to obtain the proper spray jet [37,38]. The flame jet burner was guided in a horizontal position covering the whole surface of the sheet. During the process the spraying direction was changed several times by 90◦ , until obtained thickness of coating was about 1,0 mm. Distance between the torch and the sprayed surface was 200 mm. The parameters and flame type were constant for each test (Figure 1).

constant for each test (Figure 1).

constant for each test (Figure 1).

horizontal position covering the whole surface of the sheet. During the process the spraying direction was changed several times by 90°, until obtained thickness of coating was about 1,0 mm. Distance between the torch and the sprayed surface was 200 mm. The parameters and flame type were **Sample** 

The criterion for visual assessment of the powder coatings' quality, was to make the surface layers characterized by the appropriate thickness, good adhesion to the substrate, low porosity, continuity and uniformity of obtained coatings [39]. Optimal parameters of flame-spraying of aluminum, aluminum with carbon nanotube reinforcement and aluminum with filter dust carburite reinforcement coatings have been determined on the basis of preliminary technological tests (Table 3). The view of representative samples with flame-sprayed coatings on the aluminum matrix are shown in Figure 2. *Materials* **2019**, *12*, x FOR PEER REVIEW 5 of 17 reinforcement coatings have been determined on the basis of preliminary technological tests (Table 3). The view of representative samples with flame-sprayed coatings on the aluminum matrix are shown in Figure 2.

*Materials* **2019**, *12*, x FOR PEER REVIEW 5 of 18

The criterion for visual assessment of the powder coatings' quality, was to make the surface layers characterized by the appropriate thickness, good adhesion to the substrate, low porosity, continuity and uniformity of obtained coatings [39]. Optimal parameters of flame-spraying of aluminum, aluminum with carbon nanotube reinforcement and aluminum with filter dust carburite

**Figure 1.** Flame-spraying process: (**a**) a diagram of handheld flame jet burner; (**b**) a photo from trials of flame-spraying aluminum coatings with CastoDyn DS 8000 burner**. Figure 1.** Flame-spraying process: (**a**) a diagram of handheld flame jet burner; (**b**) a photo from trials of flame-spraying aluminum coatings with CastoDyn DS 8000 burner. materials using CastoDyn DS 8000 torch**. Assist. Gas Number of Mass of** 

**Table 3.** Parameters of flame-sprayed aluminum and aluminum coatings reinforced with carbon materials using CastoDyn DS 8000 torch**. Table 3.** Parameters of flame-sprayed aluminum and aluminum coatings reinforced with carbon materials using CastoDyn DS 8000 torch. **Number Type of Powder Oxygen Pressure Acetylene Pressure (Compressed Air) Pressure the Orifice for the Used Powder** 


3 Al + 1% CNT 4 0.7 3 2 100.8 57.0 Note: 1) CNT—carbon nanotubes Nanocyl NC 7000 wt.%; 2) C—carburite wt.%. Note: 1) CNT—carbon nanotubes Nanocyl NC 7000 wt.%; 2) C—carburite wt.%.

4 Al + 0.5% C2) 4 0.7 3 2 99.7 59.4 Note: 1) CNT—carbon nanotubes Nanocyl NC 7000 wt.%; 2) C—carburite wt.%.

**Powder Yield [%]**

(a) (b) **Figure 2.** *Cont*.

**Figure 2.** View of test samples with flame-sprayed coatings on the aluminum matrix: (**a**) aluminum powder of the EN AW 1000 series; (**b**) aluminum powder EN AW 1000 series with addition of 0.5 wt.% carbon nanotubes (Nanocyl NC 7000); **(c**) aluminum powder of the EN AW 1000 series with the addition of 1 wt.% carbon nanotubes (Nanocyl NC 7000); (**d**) aluminum powder of the EN AW 1000 series with the addition of 1 wt.% carburite**. Figure 2.** View of test samples with flame-sprayed coatings on the aluminum matrix: (**a**) aluminum powder of the EN AW 1000 series; (**b**) aluminum powder EN AW 1000 series with addition of 0.5 wt.% carbon nanotubes (Nanocyl NC 7000); **(c**) aluminum powder of the EN AW 1000 series with the addition of 1 wt.% carbon nanotubes (Nanocyl NC 7000); (**d**) aluminum powder of the EN AW 1000 series with the addition of 1 wt.% carburite.

#### *2.3. Visual and Metallographic Examination of Coatings 2.3. Visual and Metallographic Examination of Coatings*

In each case, the entire surface of the sample was subjected to visual tests to assess the quality and identify any imperfections in the form of cracks, discontinuities, unevenness, porosity or lack of coating adhesion. Macro and microscopic examinations were performed on Olypmus GX 71 optical microscope (Olympus Corporation, Tokyo, Japan). The observations were made on the cross-section of metallographic samples cut from the centre of element. Samples were polished and etched in Aqua Regia. Selected areas of flame-sprayed coatings (aluminum and aluminum with addition of carbon materials) have been subjected to chemical composition analysis on JEOL 5800LV SEM scanning microscope and also EDX (Jeol Ltd., Tokyo, Japan). In each case, the entire surface of the sample was subjected to visual tests to assess the quality and identify any imperfections in the form of cracks, discontinuities, unevenness, porosity or lack of coating adhesion. Macro and microscopic examinations were performed on Olypmus GX 71 optical microscope (Olympus Corporation, Tokyo, Japan). The observations were made on the cross-section of metallographic samples cut from the centre of element. Samples were polished and etched in Aqua Regia. Selected areas of flame-sprayed coatings (aluminum and aluminum with addition of carbon materials) have been subjected to chemical composition analysis on JEOL 5800LV SEM scanning microscope and also EDX (Jeol Ltd., Tokyo, Japan).

#### *2.4. Hardness Measurements of Coatings 2.4. Hardness Measurements of Coatings*

The coating-hardness measurement was made with the Vickers method using Microhardness Tester 401MVD™ (Wilson Instruments An Instron Company, MA, USA). The examinations were carried out in conformity to ISO 6507-1:2018 standard [41]. The load applied during the hardness measurement was 0.98 N. The hardness measurement was made at the polished cross-section of the samples with flame-sprayed coatings. Ten hardness measuring points were made on the cross-section each sprayed coating. The coating-hardness measurement was made with the Vickers method using Microhardness Tester 401MVD™ (Wilson Instruments An Instron Company, Norwood, MA, USA). The examinations were carried out in conformity to ISO 6507-1:2018 standard [40]. The load applied during the hardness measurement was 0.98 N. The hardness measurement was made at the polished cross-section of the samples with flame-sprayed coatings. Ten hardness measuring points were made on the cross-section each sprayed coating.

### *2.5. Erosive Wear Resistance of Coatings*

*2.5. Erosive Wear Resistance of Coatings* The erosive wear tests of flame-sprayed coatings were carried out in accordance with ASTM G76-95 [42], as shown in Figure 3. Aluminum oxide powder (Al2O3) with particle diameter of 71 µm was used as the erodent material. Particle velocity was set at 70 ± 2 m/s, the erodent expenditure was 2.0 ± 0.5 g/min and the nozzle distance from the sample surface was 10 mm. The tests were carried out at 90° and 30° erodent impact angle. The average weight loss was determined based on three tests. The erosion rate was determined according to the Equation (1), The erosive wear tests of flame-sprayed coatings were carried out in accordance with ASTM G76-95 [41], as shown in Figure 3. Aluminum oxide powder (Al2O3) with particle diameter of 71 µm was used as the erodent material. Particle velocity was set at 70 ± 2 m/s, the erodent expenditure was 2.0 ± 0.5 g/min and the nozzle distance from the sample surface was 10 mm. The tests were carried out at 90◦ and 30◦ erodent impact angle. The average weight loss was determined based on three tests. The erosion rate was determined according to the Equation (1),

$$\text{erosion rate [g/min]} = \text{mass loss of sample [g]} \text{exposure time [min]} \tag{1}$$

However, the erosive wear resistance using Equation (2): However, the erosive wear resistance using Equation (2):

$$\begin{aligned} \text{erosive wear resistance } [0.001 \text{mm}^3/\text{g}] &= \text{volume loss of the sample } [\text{mm}^3] \text{:}\\ \text{total mass of the centroid used in the test } [\text{g}] \end{aligned} \tag{2}$$

*Materials* **2019**, *12*, x FOR PEER REVIEW 7 of 18

**Figure 3.** Erosion resistance testing according to ASTM G 76-95 [42]: (**a**) a schematic view, (**b**) the interior view of the erosion measuring chamber**. Figure 3.** Erosion resistance testing according to ASTM G 76-95: (**a**) a schematic view, (**b**) the interior view of the erosion measuring chamber. *2.6. Abrasive Wear Resistance of Coatings*

#### *2.6. Abrasive Wear Resistance of Coatings 2.6. Abrasive Wear Resistance of Coatings* Metal-mineral wear resistance tests of aluminum matrix coatings were provided in accordance

Metal-mineral wear resistance tests of aluminum matrix coatings were provided in accordance with ASTM G65-00, Procedure E [43]. During the test, the rubber-wheel made 1000 revolutions and the abrasive flow rate of A.F.S. Testing Sand 50–70 was 335 g/min. The force applied pressing the test coupon against the wheel was TL = 130 N (test load-TL). After the abrasive wear resistance test, the test specimen was weighed at weight sensitivity 0,0001 [g]. Converting mass loss to volume loss was as follows: Metal-mineral wear resistance tests of aluminum matrix coatings were provided in accordance with ASTM G65-00, Procedure E [42]. During the test, the rubber-wheel made 1000 revolutions and the abrasive flow rate of A.F.S. Testing Sand 50–70 was 335 g/min. The force applied pressing the test coupon against the wheel was TL = 130 N (test load-TL). After the abrasive wear resistance test, the test specimen was weighed at weight sensitivity 0.0001 [g]. Converting mass loss to volume loss was as follows: with ASTM G65-00, Procedure E [43]. During the test, the rubber-wheel made 1000 revolutions and the abrasive flow rate of A.F.S. Testing Sand 50–70 was 335 g/min. The force applied pressing the test coupon against the wheel was TL = 130 N (test load-TL). After the abrasive wear resistance test, the test specimen was weighed at weight sensitivity 0,0001 [g]. Converting mass loss to volume loss was as follows: volume loss [mm<sup>3</sup> ] = mass loss [g]:density [g/cm<sup>3</sup> ] x 1000 (3)

] = mass loss [g]:density [g/cm<sup>3</sup>

$$\text{volume loss [mm}^3\text{]} = \text{mass loss [g]:density [g/cm}^3\text{]} \times 1000\tag{3}$$

] x 1000 (3)

The tests were carried out on abrasion tester shown in Figure 4. The tests were carried out on abrasion tester shown in Figure 4.

volume loss [mm<sup>3</sup>

The tests were carried out on abrasion tester shown in Figure 4.

[43] (**a**) a schematic view, (**b**) picture of the device used**.**

**3. Results**

**3. Results**

(**a**) (**b**) **Figure 4.** Metal-mineral wear resistance test stand–according to ASTM G65–00, Procedure E standard [43] (**a**) a schematic view, (**b**) picture of the device used**. Figure 4.** Metal-mineral wear resistance test stand–according to ASTM G65–00, Procedure E standard (**a**) a schematic view, (**b**) picture of the device used.

**Figure 4.** Metal-mineral wear resistance test stand–according to ASTM G65–00, Procedure E standard

#### **3. Results** *Materials* **2019**, *12*, x FOR PEER REVIEW 8 of 18

#### *3.1. Metallographic Test Results 3.1. Metallographic Test Results*

The structure of each tested flame-sprayed coating cross-section is presented in Figure 5. The SEM structures of tested aluminum matrix coatings with chemical composition are presented in Figures 6–9. The structure of each tested flame-sprayed coating cross-section is presented in Figure 5. The SEM structures of tested aluminum matrix coatings with chemical composition are presented in Figures 6–9.

**Figure 5.** The macro and microstructure of the flame-sprayed pure aluminum and aluminum matrix with carbon nanotubes and carburite reinforcement coatings; etching: HCl + HNO3*.* **Figure 5.** The macro and microstructure of the flame-sprayed pure aluminum and aluminum matrix with carbon nanotubes and carburite reinforcement coatings; etching: HCl + HNO3.

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**Figure 6***.* The structure of EN AW 1000 aluminum powder flame-sprayed coating with marked chemical composition tested areas on SEM*.* **Figure 6.** The structure of EN AW 1000 aluminum powder flame-sprayed coating with marked chemical composition tested areas on SEM. **Figure 6***.* The structure of EN AW 1000 aluminum powder flame-sprayed coating with marked chemical composition tested areas on SEM*.*

flame-sprayed coating with marked chemical composition tested areas on SEM*.* **Figure 7***.* The structure of aluminum powder with 0.5 wt.% Nanocyl NC 7000 carbon nanotubes flame-sprayed coating with marked chemical composition tested areas on SEM*.* **Figure 7.** The structure of aluminum powder with 0.5 wt.% Nanocyl NC 7000 carbon nanotubes flame-sprayed coating with marked chemical composition tested areas on SEM.

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*Materials* **2019**, *12*, x FOR PEER REVIEW 10 of 18

**Figure 8***.* The structure of aluminum powder with 1 wt.% Nanocyl NC 7000 carbon nanotubes flamesprayed coating with marked chemical composition tested areas on SEM*.* **Figure 8.** The structure of aluminum powder with 1 wt.% Nanocyl NC 7000 carbon nanotubes flame-sprayed coating with marked chemical composition tested areas on SEM. **Figure** *.* The structure of aluminum with 1 wt.% Nanocyl 7000 nanotubes flamesprayed coating with marked chemical composition tested areas on SEM*.*

**Figure 9***.* The structure of aluminum powder with 0.5 wt.% carburite carbon nanotubes flame-sprayed coating with marked chemical composition tested areas on SEM*.* **Figure 9***.*The structure of aluminum powder with 0.5 wt.% carburite carbon nanotubes flame-sprayed coating with marked chemical composition tested areas on SEM*.* **Figure 9.** The structure of aluminum powder with 0.5 wt.% carburite carbon nanotubes flame-sprayed coating with marked chemical composition tested areas on SEM.

*3.2. Hardness Measurements*

*3.2. Hardness Measurements*

#### *3.2. Hardness Measurements Materials* **2019**, *12*, x FOR PEER REVIEW 11 of 18

The hardness measurements on flame-sprayed aluminum and aluminum matrix reinforced with carbon material coatings, were carried out on the surface at 5 points along one measuring line (Table 4) and on cross-section of the samples (Figures 10 and 11), according to the scheme showed on Figure 10. The hardness measurements on flame-sprayed aluminum and aluminum matrix reinforced with carbon material coatings, were carried out on the surface at 5 points along one measuring line (Table 4) and on cross-section of the samples (Figures 10 and 11), according to the scheme showed on Figure


**Table 4.** Surface hardness results of the flame-sprayed aluminum and aluminum reinforced with carbon material coatings. 10. **Table 4***.* Surface hardness results of the flame-sprayed aluminum and aluminum reinforced with

**Figure 10.** Hardness measurements scheme and HV 0.1 results of the flame-sprayed aluminum and aluminum reinforced with carbon materials coatings*.* **Figure 10.** Hardness measurements scheme and HV 0.1 results of the flame-sprayed aluminum and aluminum reinforced with carbon materials coatings.

30

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**Figure 11.** Comparison of average cross-sectional hardness and standard deviation for each of the investigated coatings*.* **Figure 11.** Comparison of average cross-sectional hardness and standard deviation for each of the investigated coatings.

#### *3.3. Tests Results of the Coatings Erosive Wear Resistance 3.3. Tests Results of the Coatings Erosive Wear Resistance*

Notes: density of aluminum spray coating 2.72 [g/cm<sup>3</sup>

*3.4. Tests Results of the Coatings' Wear Resistance*

flame-sprayed pure aluminum coating.

The relative erosive wear resistance test results of the flame-sprayed aluminum, aluminum with carbon nanotube reinforcement and aluminum with filter dust carburite reinforcement coatings are presented in Table 5 and Figure 12. The relative erosive wear resistance test results of the flame-sprayed aluminum, aluminum with carbon nanotube reinforcement and aluminum with filter dust carburite reinforcement coatings are presented in Table 5 and Figure 12.


**Table 5.** Summary of results obtained during the erosion wear test ASTM G76−95 [42]. **Table 5.** Summary of results obtained during the erosion wear test ASTM G76−95 [41].

Al + 1% CNT 0.0045 1.654 0.00056 0.10212 Notes: density of aluminum spray coating 2.72 [g/cm<sup>3</sup> ], mass of erodent used 16.2 [g], test time 8 [min].

The wear resistance test results of the flame-sprayed aluminum, aluminum with carbon nanotube reinforcement and aluminum with filter dust carburite reinforcement coatings are presented in Table 6 and Figure 13. The metal-mineral type wear resistance of the flame-sprayed aluminum with carbon nanotubes and aluminum with carburite coatings were compared to the

Al + 0.5% C 0.0039 1.434 0.00049 0.08851

], mass of erodent used 16.2 [g], test time 8 [min].

**Figure 12.** The surfaces of flame-sprayed aluminum and aluminum matrix reinforced with carbon material coatings after erosive wear resistance tests; comparison the erosion effect on samples surfaces for each tested angle of erodent particles incidence. **Figure 12.** The surfaces of flame-sprayed aluminum and aluminum matrix reinforced with carbon material coatings after erosive wear resistance tests; comparison the erosion effect on samples surfaces for each tested angle of erodent particles incidence.

#### *3.4. Tests Results of the Coatings' Wear Resistance*

**Table 6.** Summary of results obtained during the abrasive wear test ASTM G65 [43]. **Specimen Designation Number of Specimen Weight Before Test [g] Weight after Test [g] Mass Loss [g] Average Mass Loss [g] Average Volume Loss [mm3] Relative 1) Abrasion Resistance** Al S1-1 43.9675 43.8413 0.1262 0.1418 52.1324 1.00 S1-2 42.3855 42.2281 0.1574 The wear resistance test results of the flame-sprayed aluminum, aluminum with carbon nanotube reinforcement and aluminum with filter dust carburite reinforcement coatings are presented in Table 6 and Figure 13. The metal-mineral type wear resistance of the flame-sprayed aluminum with carbon nanotubes and aluminum with carburite coatings were compared to the flame-sprayed pure aluminum coating.

Al <sup>+</sup> 0.5% CNT S2-1 56.5170 56.3924 0.1246 0.1286 47.26103 1.10 S2-2 53.8604 53.7279 0.1325

**4. Discussion**


**Table 6.** Summary of results obtained during the abrasive wear test ASTM G65 [42].

Notes: density of aluminum spray coating 2.72 [g/cm<sup>3</sup> ]; 1) relative to sprayed coatings of the aluminum without carbon materials. without carbon materials.

#### Specimen designation / Number of specimen

**Figure 13.** The surfaces of flame-sprayed aluminum and aluminum with carbon material coatings after wear resistance metal-mineral tests. **Figure 13.** The surfaces of flame-sprayed aluminum and aluminum with carbon material coatings after wear resistance metal-mineral tests.

slight roughness, lack of porosity and cracks (Figure 2). During the flame-spraying process, carbon material particles added to aluminum powder did not oxidize completely in the oxyacetylene flame.

Visual and metallographic tests of the flame-sprayed aluminum and aluminum with carbon material reinforcement (0.5 wt.% and 1 wt.% of carbon nanotubes Nanocyl NC 7000 and 0.5 wt.% of

#### **4. Discussion**

Visual and metallographic tests of the flame-sprayed aluminum and aluminum with carbon material reinforcement (0.5 wt.% and 1 wt.% of carbon nanotubes Nanocyl NC 7000 and 0.5 wt.% of carburite) have shown that by using proper parameters of the process it is possible to receive coatings with acceptable quality level, characterized by proper adhesion to the substrate, lack of delamination and even thickness over the entire surface. The outer surface of the coatings was characterized by a slight roughness, lack of porosity and cracks (Figure 2). During the flame-spraying process, carbon material particles added to aluminum powder did not oxidize completely in the oxyacetylene flame. Carbon nanotubes (melting point 4526 ◦C [43]) and carburite (melting point 3550 ◦C [44]) in the oxyacetylene flame has formed with aluminum Al-Cx type agglomerates, which due to the large volume and lower heat source temperature than other thermal spraying methods (oxyacetylene flame temperature 3160 ◦C [45]) migrated in large quantities to the coatings. Partially melted and partially only plasticized in a gas flame, Al-Cx agglomerates collided with the substrate at high speed, (Figure 1b) and in this way formed a fine-grained coating structure. Powder flame spraying process (PFS) in comparison with, for example, plasma spraying, increases the probability of stopping carburite and carbon nanotubes (CNT) in flame sprayed composite coating with aluminum matrix. Presence of carbon materials in aluminum powder flame-sprayed coatings is initially confirmed by metallographic microscopic tests, which revealed areas carburite and carbon nanotubes on specimens (Figure 5, Al + 0.5% CNT). Presence of carbon materials can be observed on the entire cross-section of the coating, also at the outer surface. Inside the Al-Cx composite coatings, no cracks were found, only the presence of individual cavities. The tests made using scanning electron microscope have shown presence of some areas consisting small carbon materials inclusions. These were observed in the all-aluminum coatings with carbon material reinforcement. For the coating with 0.5 wt.% CNT, inclusion areas consisted of 33.05 wt.% C; for coating with 1 wt.% CNT, the carbon content was lower in tested area (20.25 wt.% C), while for the coating with 0.5 wt.% of carburite, carbon content was almost two times higher than in the coating with same content of CNTs and amounted to 59.76 wt.% C. In aluminum coating without carbon material addition, carbon and oxygen were found, (Figure 6). A small amount of carbon in the aluminum coating may be caused by the ease of thermal decomposition of acetylene in the gas flame and the physicochemical properties of unsaturated hydrocarbons [46]. Acetylene is dissociated into active carbon atoms (acetylene black, characterized by high purity) and hydrogen molecule. The oxygen content in the aluminum coating is the result of oxidation of the aluminum particles in the gas flame and the atmosphere. The addition of carbon materials to the aluminum powder causes the carbon to bind oxygen as a strong deoxidizer; that is why its presence was not found in composite coatings with aluminum matrix and carbon material (carburite and carbon nanotubes) reinforcement. These results should still be confirmed using more advanced research methods, e.g. XRD X-ray diffraction or Raman spectroscopy. These studies will be completed and presented in another publication.

The hardness measurements of tested coatings were proceeded using standard ISO 6507 [40]. The measurements were done both on the external surface and the cross-section of the sprayed coatings. These tests showed that using addition of 0.5 wt.% and 1 wt.% carbon nanotubes to aluminum coating (34.1 HV 0.1) caused an increase in its hardness of 8.2 HV 0.1 for the 0.5 wt.% of carbon nanotube reinforcement and by 9.5 HV 0.1 for 1 wt.% of carbon nanotube reinforcement. Addition of carburite to aluminum had not significant influence on the coating hardness (Figure 11).

Erosive wear resistance test results have shown that the addition of carbon materials to aluminum powder does not increase the erosive wear resistance of flame-sprayed coatings. During these tests, the aluminum coatings with carbon nanotubes had worn out by erosion with large and small angles of erodent incidence more than aluminum coatings with carburite and much more than aluminum coatings without carbon materials reinforcement. It was observed that for all tested coatings erosive wear resistance was better during using smaller angle of erodent incidence (Table 4).

The best metal-mineral type wear resistance had the aluminum coating with carburite. The wear resistance of this coating was 19% higher than pure aluminum coating. The aluminum coatings with addition of 0.5 wt.% and 1 wt.% of carbon nanotubes in comparison to pure aluminum coating had better relative wear resistance by 10% and 11% (Table 5). The cause of decreasing wear using aluminum coatings with carbon material addition was increased glide of ceramic abrasive particles on metal.

Based on the conducted study and the obtained results, it can be concluded that it is possible to introduce carbon particles in the form of carbon nanotubes (CNT) and also carburite into the aluminum matrix by means of flame spraying method. The flame spraying is an effective and cheaper alternative to the technology of laser surface treatment of metals. Properly selected parameters of the flame spraying process allow to preserve the properties of particles of carbon materials, their even distribution in the coating, proper bonding with the matrix and prevent the effects of their thermal degradation. The produced composite is characterized by a low friction coefficient. The tribological characteristics of produced test coating of aluminum reinforced by carbon particles in the form of carbon nanotubes and carburite show that the coatings can be classified as sliding materials. Additionally, the coatings are characterized by high wear resistance. The obtained result should be considered as a preliminary information on a new group of materials, which can find application in the automotive industry. They are the basis for the design and optimization of friction materials operated at elevated temperatures (e.g. pistons, engine blocks), systems subject to intensive wear (e.g. brake discs, cylinders), as well as in propulsion systems (e.g. bearings), providing low friction coefficient and also high ability for absorption of vibration. Further research should be focused on the investigation of the effect of doping the aluminum matrix with carbon nanotubes (CNT) on the wear mechanisms, change of microstructure of the counter-specimen and also tests which will allow to determine the tribological characteristics of the materials at elevated temperatures.

### **5. Conclusions**

The analysis carried out comparing properties of flame-sprayed EN AW 1000 aluminum coatings and aluminum matrix coatings with carbon materials reinforcement (0.5 wt.% and 1 wt.% of Nanocyl NC 7000 carbon nanotubes and 0.5 wt.% of carburite) resulted in the following conclusions:


**Funding:** This research was financed from the own resources of the Silesian University of Technology. **Conflicts of Interest:** The author declare no conflict of interest.
