*Article* **Improvement of the Tribological Properties and Corrosion Resistance of Epoxy–PTFE Composite Coating by Nanoparticle Modification**

**Lixia Ying 1, Yunlong Wu 1, Chongyang Nie 1,\*, Chunxi Wu <sup>1</sup> and Guixiang Wang <sup>2</sup>**


**Abstract:** In order to meet the requirements of high corrosion resistance, wear resistance, and selflubrication of composite coatings for marine applications, epoxy matrix composite coatings containing PTFE and TiO2 nanoparticles were prepared on the steel substrate. With silane coupling agent KH570 (CH2=C(CH3)COOC3H6Si(OCH3)3), titanium dioxide nanoparticles were modified, and organic functional groups were grafted on their surface to improve their dispersion and interface compatibility in the epoxy matrix. Then, the section morphology, tribological, and anticorrosion properties of prepared coatings, including pure epoxy, epoxy–PTFE, and the composite coating with unmodified and modified TiO2, respectively, were fully characterized by scanning electron microscopy, friction–abrasion testing machine, and an electrochemical workstation. The analytical results show that the modified TiO2 nanoparticles are able to improve the epoxy–PTFE composite coating's mechanical properties of epoxy–PTFE composite coating including section toughness, hardness, and binding force. With the synergistic action of the friction reduction of PTFE and dispersion enhancement of TiO2 nanoparticles, the dry friction coefficient decreases by more than 73%. Simultaneously, modified titanium dioxide will not have much influence on the water contact angles of the coating. A larger water contact angle and uniform and compact microstructure make the composite coating incorporated modified TiO2 nanoparticles show excellent anti-corrosion ability, which has the minimum corrosion current density of 1.688 <sup>×</sup> <sup>10</sup>−<sup>7</sup> <sup>A</sup>·cm<sup>−</sup>2.

**Keywords:** composite coating; epoxy–PTFE; modified TiO2; tribological properties; corrosion resistance

#### **1. Introduction**

For the advantages of excellent friction, stable chemical property, and low cost [1–3], epoxy resin is one of the excellent polymer coating materials, which is widely applied in the metal protection, electronics, and medical equipment [4,5]. Especially, epoxy–PTFE composite coatings have low friction coefficient and anticorrosion ability as well as high temperature resistance [6]. Such coatings could increase the anticorrosion capacity of metals and the self-lubricity of bearings as well as modify the hydrophobic and ice-phobic properties for wind turbine blades [7–9].

However, due to the curing shrinkage and warpage deformation of epoxy, there are always a lot of micro-pores and cracks in the composite coating. Moreover, the low hardness of PTFE will also lead to the poor wear resistance of the epoxy–PTFE composite coating [10,11]. A viable solution is to add hard or inorganic particles, such as TiO2, CuO, CuF2, CuS, and Al2O3, which will improve the tribological properties of the coating [12–15].

Larsen et al. [14] obtained a composite coating containing CuO and PTFE by mixing CuO and PTFE with epoxy solution. The evidence shows that both CuO and PTFE particles are well dispersed in the epoxy. The incorporation of PTFE and CuO has a positive synergistic effect on the friction and wear properties when the content of CuO is in the range

**Citation:** Ying, L.; Wu, Y.; Nie, C.; Wu, C.; Wang, G. Improvement of the Tribological Properties and Corrosion Resistance of Epoxy–PTFE Composite Coating by Nanoparticle Modification. *Coatings* **2021**, *11* , 10 . https://dx.doi.org/10.3390/ coatings11010010

Received: 1 December 2020 Accepted: 22 December 2020 Published: 24 December 2020

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2020 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/).

of 0.1–0.4 vol.%. Hamad et al. [15] investigated the mechanical properties of toughened epoxy by utilizing two kinds of nanoparticles sizes of TiO2 (17 and 50 nm) at different weight fractions (1%, 3%, 5%, and 10%). The results indicate that the addition of a small fraction of TiO2 nanoparticles can bring an improvement in the mechanical properties of epoxy composite. Researchers also expect that the addition of some lubricating substances will have a good effect on the performance of the epoxy resin composite coating [16,17]. The tribological properties of epoxy composites were also studied by Chang et al. [18]. With different proportions of graphite, PTFE, short carbon fiber, and TiO2 and their combinations as additions, the frictional coefficient, wear resistance, and contact temperature of composite coatings were tested in a dry sliding condition with different sliding velocities and contact pressures. Results show that TiO2 works best in improving the wear resistance of epoxy.

On the other hand, in order to meet the requirements of a corrosive environment, the corrosion resistance of polymer coatings has received much interest recently. Shi et al. [19] reported two methods to improve the dispersing of nanoparticles in epoxy coatings, including silane treatment and preparing nano-oxide paste. Compared with nano-TiO2 paste and silane-treated nano-SiO2, the latter is better than the former in improving the corrosion resistance and hardness of epoxy. Fadl et al. [20] fabricated TiO2 nanoparticles by a simple template-free sol-gel method and mixed them with poly-dimethylamino siloxane (PDMAS) to form a PDMAS/TiO2 nanocomposite as a modifier for polyamine-cured epoxy coating. The corrosion resistance of PDMAS/TiO2 epoxy coating versus unmodified epoxy was investigated by a salt spray accelerated corrosion test. The PDMAS/TiO2 epoxy coating has better corrosion mitigation and a self-healing effect. Radoman et al. [21] obtained epoxy/TiO2 nanocomposites by the incorporation of modified TiO2 nanoparticles with gallic acid esters in epoxy. Due to the deoxidizing effect of modified TiO2, the nanocomposites have better corrosion resistance than that of the pure epoxy. Therefore, it is possible to obtain better corrosion resistance of epoxy composite coating by incorporating and modifying nanoparticles.

From the perspective of comprehensive performance, TiO2 nanoparticles were selected as the additive in this study. SiO2 nanoparticles are mainly used to improve the mechanical properties and heat resistance of epoxy composites. Nano-zinc oxide has the characteristics of high activity, large specific surface area, easy agglomeration, and long dispersion time before preparation. Therefore, it is a difficult point to prepare nano-zinc oxide-modified epoxy resin. The surface of Al2O3 contains a large number of hydroxyl groups, which makes it difficult to evenly disperse in epoxy materials. On the contrary, TiO2 nanoparticles not only have good chemical stability but also have excellent heat resistance and UV protection, so it is widely used in the fields of UV-resistant materials, packaging materials, and coatings. At the same time as rigid nanoparticles and its strong adhesion, TiO2 is often used as a modified filler to improve the bending strength, tensile strength, and impact strength of epoxy resin. In the wear resistance test, TiO2 nanoparticles can significantly improve the wear resistance of epoxy resin, because TiO2 nanoparticles have a large specific surface area and a large contact surface with the substrate, which requires more external energy when sliding [22,23].

Overall, the current research shows that pure epoxy resin coating has good corrosion resistance, but the high friction coefficient and poor wear resistance could not meet the requirements for use under friction conditions. Adding PTFE can reduce the friction coefficient, but the hardness and wear resistance are still low. Generally, incorporating soft phase PTFE and hard phase TiO2 is an effective way to improve the tribological performance of a coating. Nevertheless, due to agglomeration and the poor compatibility of nanoparticles in the coating, composite coatings that involve nanoparticles often show low corrosion resistance. In the paper, the research focused on the improvements of both the tribological property and corrosion resistance of epoxy composite coating. TiO2 nanoparticles were modified by a silane-coupling agent to reduce their surface tension, avoid agglomeration, and improve the interfacial compatibility with epoxy firstly. Then, the influence of modified

TiO2 on the tribological properties and corrosion resistance of an epoxy–PTFE composite coating were analyzed in detail.

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

#### *2.1. Materials*

The carbon steel SK85 was chosen as the substrate with dimensions of 30 × <sup>12</sup> × 1 mm3 and with the nominal composition as follows: 0.80%–0.90% C, ≤0.35% Si, ≤0.50% Mn, ≤0.03% P, ≤0.03% S, ≤0.20% Cr, ≤0.25% Ni, and ≤0.30% Cu. Epoxy resin (E44) with viscosity range from 40,000 to 45,000 mPa·s at 25 ◦C was purchased from Nanjing Star Synthetic Materials Co., LTD., Nanjing, China. TiO2 nanoparticles with an average diameter of 40 nm were purchased from Shanghai Macklin Biochemical Co., LTD., Shanghai, China. The silane coupling agent KH570 (CH2=C(CH3) COOC3H6Si(OCH3)3, ≥99%, Silica Co., LTD., Nanjing, China) was used as a modifier. The ethanol (CH3CH2OH, ≥99.7%, Fuyu Fine Chemical Co., LTD., Tianjin, China) was used as a dispersant. The acetic acid (CH3COOH, ≥99.8%, Sinopharm Group Chemical Reagent Co., LTD., Beijing China) was used to adjust the pH value of the solution. Additionally, all reagents were used without further purification.

#### *2.2. TiO2 Modification*

In this study, titanium dioxide nanoparticles were selected as the strengthening particles, the average diameter of which are 40 nm. They were modified by the silane coupling agent KH570 (CH2=C(CH3) COOC3H6Si(OCH3)3) to graft organic functional groups on the surface of TiO2 nanoparticles. The unique method is described as follows.

Ethanol was used as the dispersant, and the pH value was adjusted by acetic acid to 6. The silane coupling agent KH570 and TiO2 nanoparticles were added into a certain volume of dispersed solutions at a mass ratio of 15:100. Firstly, the configured modified liquid was sonicated by ultrasonic liquid processors for 10 min. Then, it was disposed with a magnetic stirrer at 50 ◦C for 240 min with a speed of 300 r/min. Subsequently, the modified powder was washed with ethyl alcohol and deionized water three times to remove excess organosilane. Finally, collected precipitates were dried at 80 ◦C in a drying oven and stored in vials for further testing. The presence of organic phase on the modified TiO2 nanoparticles surface was tested with the FT-IR spectrum (PerkinElmer Co., LTD., Shanghai, China).

#### *2.3. Coating Preparation*

Four kinds of coatings were prepared, including epoxy, epoxy–PTFE, epoxy–PTFE/TiO2 (unmodified), and epoxy–PTFE/TiO2 (modified). Carbon tool steel (SK85) and *n*-butanol are chosen for the matrix material and diluent, respectively, and phenolic amine resin (T13) was the curing agent for epoxy resin (E44). The mass ratio of T13/*n*-butanol/E44 was 1:2:4. The mass content of PTFE and TiO2 was controlled at 15% and 2%, respectively. During the preparation process, after stirring the mixture thoroughly, it was ultrasonicated for 10 min to allow complete defoaming. Subsequently, the as-cleaned steel panels were immersed in the mixture solution adequately for 20 min, and we used a small mold to ensure that the thickness of the samples were uniform at 52 ± 3 μm. Then, the drying was carried out in an oven at 90 ◦C for 180 min. Finally, samples were successfully prepared for evaluating their surface morphology as well as mechanical and corrosion properties.

#### *2.4. Coating Performance Testing*

The performance test of coatings mainly includes the surface morphology, hardness, bonding force, tribological properties, and corrosion resistance. The section morphology of the coatings was characterized by scanning electron microscopy (SEM, FEI, Quanta200, OR, USA). Shore durometers (Petey Testing Instrument Co., LTD., Guangzhou, China) were used for testing the micro-hardness. The average micro-hardness value was acquired by averaging the results of six measurements. The binding force of the coatings—the average

results of two sets of measurements—was tested by pull-off tester (DeFeLsko Co., LTD., Ogdensburg, NY, USA). The tribological properties of the coatings were examined by a friction-abrasion testing machine (SFT-2M, ZhongKe-kaihua, Lanzhou, China) at a load of 5.0 N and a stage rotated speed of 200 r/min. The friction counterparts were GCr15 steel balls with a diameter of 5.0 mm; the radius of the friction circle was set to 2 mm, the duration of each wear test was 10 min. Subsequently, the friction–wear behavior of the coatings was estimated by SEM. The corrosion resistance of the composite coatings was measured by an electrochemical workstation (CHI660B, Chenhua, Shanghai, China) after being soaked in 3.5% NaCl solution for 72 h. The samples were used as the working electrode, while a platinum sheet and a saturated calomel electrode (SCE) were the counter and the reference electrodes, respectively. The test frequency of electrochemical impedance spectroscopy (EIS) ranged from 1000 Hz down to 10 Hz, and the scanning voltage was 10 mV. The corrosion potential (*E*corr) and corrosion current density (*i*corr) were obtained from the potentiodynamic polarization curves. All corrosion tests were performed at room temperature.

#### **3. Results and Discussions**

#### *3.1. The Modification of TiO2 Nanoparticles*

The transmission FT-IR spectra of TiO2 nanoparticles with and without KH570 modification were compared, as shown in Figure 1. For unmodified TiO2 nanoparticles, the absorption band between 3400 and 3500 cm−<sup>1</sup> corresponds to the hydroxyl group (–OH) due to the partial electron–hole pairs migration on the surface of TiO2 nanoparticles. The vibration absorption peak is situated in the wavenumber interval of 500–750 cm−1, which demonstrates the presence of Ti–O–Ti groups. However, the new absorption bands appear in the FT-IR spectrum of modified TiO2 nanoparticles in curve *b*, and there are some characteristic absorption peaks of KH570 at 2917 cm−<sup>1</sup> (–CH3 and –CH2), 1717 cm−<sup>1</sup> (C=O), 1620 cm−<sup>1</sup> (C=C), 500–750 cm−<sup>1</sup> (Ti–O–Si), so it could be inferred that the organic functional group has been grafted onto the surface of TiO2 nanoparticles successfully.

**Figure 1.** FT-IR results of unmodified TiO2 and modified TiO2.

Figure 2 is the energy-dispersive spectrometer (EDS) pattern of the TiO2 nanoparticles before and after modification. The content of Si and C elements in Figure 2b is much higher than that in Figure 2a. The silane coupling agent molecule can be adsorbed on the surface of TiO2 nanoparticles by its hydrophilic end and can react with the surface –OH groups on TiO2 nanoparticles; therefore, the modified TiO2 nanoparticles contain Si and C elements from KH570(CH2=C(CH3)COOC3H6Si(OCH3)3). All these experimental results demonstrate that the organic functional group of KH570 was successfully grafted onto the surface of TiO2 nanoparticles.

**Figure 2.** The energy-dispersive spectrometer (EDS). (**a**) TiO2 (unmodify), (**b**) TiO2 (modify).

The fractured surface of four kinds of coatings was exhibited by scanning electron microscopy (SEM), as shown in Figure 3. It can be seen that the fractured surface of epoxy is very smooth (Figure 3a) even at 5000 times magnification. With the addition of PTFE and TiO2 nanoparticles, the fractured surface of the composite coatings becomes much rougher than that of the pure epoxy coating. Furthermore, by comparing Figure 3b,c, it can be observed that there are still some flat areas and aggregated TiO2 nanoparticles on the cross-section of the coating with unmodified TiO2 nanoparticles (Figure 3c). However, with modified TiO2 nanoparticles, the composite coating fracture surfaces are much rougher (Figure 3d), which can be attributed to the dispersive distribution of TiO2 preventing crack propagation, and the excellent interfacial compatibility reducing the crack source. Generally, an increased section roughness means that the path at the crack tip is distorted, and the coating might absorb more section energy during the fracture process and possess better fracture toughness. Therefore, due to the grafting of organic functional groups adsorbed on the modified TiO2 nanoparticles surface, the toughness of the composite coating is improved [24,25].

(**c**) (**d**) **Figure 3.** SEM images of section morphology of different coatings. (**a**) Unmixed epoxy, (**b**) epoxy-PTFE, (**c**) epoxy-PTFE/TiO2(unmodify), (**d**) epoxy-PTFE/TiO2(modify).

#### *3.2. Hardness*

Figure 4 shows the hardness values of different coatings. It indicates that the hardness of epoxy–PTFE–TiO2 (modify), epoxy, epoxy–PTFE–TiO2 (unmodify), and epoxy–PTFE coatings are arranged from high to low. The soft particles of PTFE improve the lubricity of the coating but also reduce its hardness. In contrast, modified hard TiO2 nanoparticles can enhance the hardness of epoxy–PTFE composite coating better than unmodified hard TiO2 nanoparticles. This is due to the modified TiO2 nanoparticles dispersing more evenly in the coating, which has a dispersion strengthening effect in the coating.

**Figure 4.** Hardness of different coatings.

#### *3.3. Interfacial Adhesion*

The interface bonding adhesion strength of different coatings is given in Figure 5. As Figure 5 shows, PTFE enhances the interfacial adhesion of epoxy coatings from 1.43 to 2.30 MPa. On the other hand, with the addition of modified TiO2 nanoparticles, the bonding strength value of the composite coating reaches 2.70 MPa, which is higher than that of the coating containing unmodified TiO2 nanoparticles. We interpreted that the introduction of organic functional groups on the surface of TiO2 nanoparticles resulted in the formation of hydrogen bonds and Si–O bonds between the TiO2 nanoparticles and the coating. The modified TiO2 nanoparticles have a strong interfacial adhesion with the surrounding particles, and the van der Waals force attraction becomes stronger; therefore, the epoxy–PTFE–TiO2 (modify) composite coating has the highest interfacial adhesion value. This corresponds to the improvement in section roughness and the hardness of the coating.

**Figure 5.** The results of interfacial adhesion.

#### *3.4. Tribological Properties*

Figure 6 shows the friction coefficient curves of different coatings versus the sliding time. As shown in Figure 6, with the addition of PTFE to epoxy, the friction coefficient decreases from 0.6 to about 0.16. Furthermore, compared to curve *C* of adding unmodified TiO2 nanoparticles, the friction coefficient of the composite coating is less than 0.10 after adding modified TiO2 nanoparticles, as shown in curve *D*.

**Figure 6.** Friction coefficients of different coatings.

The SEM images of the worn surfaces are shown in Figure 7. After adding PTFE, the wear track width of the epoxy coating decreases from 634.21 (Figure 7a) to 321.56 μm (Figure 7b). Furthermore, with the addition of unmodified TiO2 nanoparticles (Figure 7c), the width of the wear track narrows to 274.51 μm. With modified TiO2 nanoparticles, the wear track becomes much smoother and narrower (Figure 7d). On the other hand, there are many micropores on the worn surface of epoxy, which indicates that the epoxy coating itself is not densification. When PTFE is incorporated, there are no obvious micropores. The PTFE with self-lubricating property spreads on the wear surface (as shown in curve A and curve B), reduces the friction coefficient, and brings a much smoother worn track (Figure 7b). When unmodified TiO2 nanoparticles were added into the coating, particle agglomeration results in some micropores emerging on the worn surface (Figure 7c). However, there are seldom obvious micropores on the worn surface with modified TiO2 nanoparticles (Figure 7d). Obviously, when modified TiO2 nanoparticles coexist in the composite coating, uniformly dispersed nanoparticles and excellent interfacial compatibility lead to the narrowest and smoothest wear track. The composite coating D has the most excellent wear resistance.

Summing up, the friction coefficient of the composite coatings with PTFE is significantly improved because of the formation of PTFE lubricating transfer film on the wear surface. However, the wear resistance of a soft epoxy–PTFE composite coating is still weakened. With PTFE and TiO2 coexisting in the epoxy–PTFE/TiO2 (unmodify) composite coating, the combination of soft phase particles and hard phase particles leads to a higher wear resistance. In contrast, a relatively good interfacial compatibility brought by the modified TiO2 nanoparticles decreases the furrow and adhesive effects, which leads to a lower friction coefficient, much smoother worn surface, and much higher wear resistance. Meanwhile, the addition of PTFE and TiO2 nanoparticles may affect the mechanical properties, and the good interfacial compatibility of modified TiO2 nanoparticles may improve the mechanical stability of the coating, which may also affect the tribological properties. As described by Homaeigohar et al., the uniform distribution of functionalized graphite nanofilaments can improve the mechanical stability of nanocomposite hydrogels, and the addition of tricalcium phosphate can affect the mechanical properties of

the polythylene [26,27]. All in all, the synergistic action of the friction reduction of PTFE and dispersion enhancement of modified TiO2 nanoparticles of PTFE makes the epoxy– PTFE/TiO2 (modify) composite coating show excellent friction reduction and anti-wear performance.

Wear scar profile Magnification of wear marks (**d**)

#### *3.5. Anti-Corrosion Properties*

#### 3.5.1. Hydrophobic Performance

The water contact angle of coatings was measured for evaluating their hydrophobic property, as shown in Figure 8. The water contact angles of the pure epoxy coating and epoxy/PTFE composite coating are 74.81◦ and 100.90◦, respectively, which indicates that PTFE can improve the hydrophobic property of the coating. With the further addition of TiO2 nanoparticles to the epoxy–PTFE coating, due to the presence of hydroxyl groups on the surface of TiO2 nanoparticles, the water contact angle of the composite coating is reduced greatly to 56.84◦, which is manifested as hydrophilic. The small contact angle of coating C will result in the worst corrosion resistance. In contrast, with adding the modified TiO2 nanoparticles instead of unmodified TiO2, the water contact angle of the composite coating approaches that of the pure epoxy coating, and the hydrophilicity of the coating decreased greatly. However, due to the steric hindrance effect, the hydroxyl groups on the surface of TiO2 are not fully replaced by organic functional groups and still have a certain hydrophilicity.

When PTFE as hydrophobic particles were added to the epoxy coating, the water contact angle rose to 100.90◦, showing hydrophobicity. Due to the presence of hydrophilic hydroxyl groups on the surface of TiO2, the extremely poor interfacial compatibility between TiO2 and epoxy coatings, resulting in easy agglomeration of TiO2. Therefore, as shown in Figure 3c, there are still some flat areas on the cross-section of the coating, and the water contact angle of epoxy–PTFE/TiO2 (modified) drops to 56.84◦ and shows hydrophilicity. Due to the influence of steric hindrance, part of the hydroxyl groups on the modified TiO2 surface is replaced with organic functional groups. The interface compatibility between TiO2 nanoparticles and the epoxy coating is enhanced, and the coating surface becomes rougher, as shown in Figure 3d. The water contact angle of the composite coating approaches that of the pure epoxy coating, while the hydrophilicity of the coating decreases greatly. All in all, the increase of the contact angle is beneficial to improve the corrosion resistance of the coating.The extremely poor interfacial compatibility between TiO2 and epoxy coatings, resulting in easy agglomeration of TiO2.

#### 3.5.2. Potentiodynamic Polarization

The potentiodynamic polarization curves of different coatings are shown in Figure 9, and Table 1 exhibits the corrosion current density (*i*corr) and corrosion potential (*E*corr). Experimental results indicate that the corrosion current density decreases and the corrosion potential of the coating increases with the addition of PTFE.

**Figure 9.** Potentiodynamic polarization curves of different coatings.


**Table 1.** Potentiodynamic polarization parameters of different coatings.

Due to the high hydrophilicity of TiO2 nanoparticles, the containing corrosive medium of water molecules is more likely to penetrate the coating, get in the substrate, and reduce the corrosion resistance of the coating thereby. As shown in Figure 9, after adding unmodified TiO2 nanoparticles into the epoxy–PTFE coating, the corrosion current density of the coating increases, and the corrosion potential decreases. In contrast, after adding the modified TiO2 nanoparticles, the composite coating of epoxy–PTFE–TiO2 (modified) has the minimum corrosion current density of 1.688 × <sup>10</sup>−<sup>7</sup> <sup>A</sup>·cm−2, and the corrosion potential increases to some extent (−0.503 V). In comparison, the corrosion resistance of the coating before the titanium dioxide modification is the worst. It can be speculated that the modified TiO2 nanoparticles have better interfacial compatibility with epoxy, which prevents the immersion of corrosive media and thus improves the corrosion resistance of the epoxy–PTFE coating.

#### 3.5.3. Electrochemical Impedance Spectroscopy

Figure 10 shows the test data of the electrochemical impedance spectroscopy (EIS). As can be seen from the results in Figure 10a,b, with PTFE and unmodified TiO2 nanoparticles involved in the coating, the arc radius in the Nyquist figure and the impedance value of the low-frequency region in the Bode figure are very small; therefore, the addition of unmodified TiO2 nanoparticles will reduce the corrosion resistance of the epoxy–PTFE coating. On the contrary, after adding modified TiO2 nanoparticles into the epoxy–PTFE coating, both the arc radius in the Nyquist figure and the impedance value of low frequency in the Bode figure are larger, and the corrosion resistance of the epoxy–PTFE coating is greatly improved. The analysis results are consistent with the Tafel curve test results. Obviously, the poor corrosion resistance of coating C has been effectively addressed by the modification treatment of TiO2 nanoparticles.

**Figure 10.** Electrochemical impedance of the coatings. (**a**) Nyquist, (**b**) Bode.

#### **4. Conclusions**

In order to meet the requirements of high corrosion resistance, wear resistance, and self-lubricating property of composite coatings for marine applications, the epoxy composite coatings containing PTFE and TiO2 nanoparticles were prepared in this research. Through the modification treatment of TiO2, the compactness of the coating is increased and the hydrophilicity is decreased, which leads to excellent tribological properties and corrosion resistance. The specific conclusions are as follows:


**Author Contributions:** Conceptualization, L.Y. and G.W.; methodology, L.Y. and G.W.; validation, L.Y.; formal analysis, L.Y. and C.W.; investigation, L.Y. and Y.W.; writing—original draft preparation, L.Y., C.N., and Y.W.; writing—review and editing, L.Y., C.N., and Y.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors acknowledge the financial support from the Natural Science Foundation of Heilongjiang Province (JJ2019LH1520) and the Fundamental Research Funds for the Central Universities (3072020CF0705).

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

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

#### **References**


### *Article* **Structural Features and Tribological Properties of Detonation Gun Sprayed Ti–Si–C Coating**

**Bauyrzhan Rakhadilov 1,2, Dastan Buitkenov 1,\*, Zhuldyz Sagdoldina 1, Bekbolat Seitov 3, Sherzod Kurbanbekov <sup>3</sup> and Meruyert Adilkanova <sup>4</sup>**

<sup>1</sup> Research Center Surface Engineering and Tribology, Sarsen Amanzholov East Kazakhstan University,


**Abstract:** The paper considers the research results of structural-phase state and tribological characteristics of detonation coatings based on Ti–Si–C, obtained at different filling volumes of the explosive gas mixture barrel of a detonation gun. The results analysis indicates that the phase composition and properties of detonation coatings strongly depend on the technological parameters of spraying. With an increase of the explosive mixture in the filling volume of the detonation barrel up to 70% of the coatings consist mainly of the TiC phase, because high temperature leads to a strong decomposition of Ti3SiC2 powders. Thus, the XRD results confirm that at 70% of the explosive gas mixture's filling volume, partial decomposition and disintegration of the powders occurs after detonation spraying. We established that detonation coatings based on titanium carbosilicide obtained at the explosive gas mixture's filling volume at 60% are characterized by high wear resistance and adhesive strength. Thermal annealing was performed after spraying in the temperature range of 700–900 ◦C for 1 h to reduce microstructural defects and improve the Ti–Si–C coating characteristics. As a result of the heat treatment in the Ti–Si–C system at 800 ◦C, we observed that an increase in the volume fraction of the Ti3SiC2 and TiO2 phases led to a 2-fold increase in microhardness. This means that the after-heat-treatment can provide a sufficient reaction time for the incomplete reaction of the Ti–Si–C (TSC) coating during the detonation gun spraying. Thus, annealing can provide an equal distribution of elements in the coatings.

**Keywords:** detonation gun spray; structure; carbolized titanium; hardness; wear resistance; phase; adhesion; heat treatment

#### **1. Introduction**

Currently, carbides, silicides, and transition metals have aroused considerable interest due to increasing demand. In particular, the most frequently mentioned phases in the Ti–Si–C system are TiC, Ti5Si3, and Ti3SiC2. They attract considerable interest due to their unique metallic combination and ceramic properties. As metals, they have good electrical and thermal conductivity, high plasticity, good machinability and excellent thermal shock resistance. As ceramics, they have low density, high stiffness, high melting points, and good resistance to oxidation and corrosion [1–3]. Such exceptional properties result from the coexistence of strong covalent ionic MX bonds and weak metallic MA bonds within a layered hexagonal structure (space group P63/mmc) of MAX materials, which are created by repeating a three-layer structure (consisting of two Mn + 1xN layers intercalated by a single atomic layer A) [4]. A unique distinctive feature of these materials is the layered structure of their crystal lattice—the regular arrangement of layers of M and A atoms

**Citation:** Rakhadilov, B.; Buitkenov, D.; Sagdoldina, Z.; Seitov, B.; Kurbanbekov, S.; Adilkanova, M. Structural Features and Tribological Properties of Detonation Gun Sprayed Ti–Si–C Coating. *Coatings* **2021**, *11* , 141 . https://doi.org/ 10.3390/coatings11020141

Received: 26 December 2020 Accepted: 19 January 2021 Published: 28 January 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**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/).

Ust-Kamenogorsk 070000, Kazakhstan; rakhadilovb@mail.ru (B.R.); sagdoldina@mail.ru (Z.S.)

of elements that have reduced binding energy between them. Tightly packed layers of titanium atoms alternate with layers of silicon atoms, and carbon atoms occupy octahedral interstices between titanium atoms [5,6]. These properties make the phase material Ti3SiC2 MAX ideal for extreme condition applications. In addition, system Ti–Si–C has good characteristics under conditions of abrasive wear and corrosion. However, production coatings based on Ti–Si–C production by traditional methods are associated with a high temperature and duration of their obtaining process. Sprayed coatings based on Ti3SiC2 are usually accompanied by phases TiC and Ti–Si. The short reaction time of powder mixtures and the decomposition of Ti–Si–C at high temperature are the main problems for the phase's purity. An analogous problem occurs with plasma spraying of coatings [7]. In addition, using heat treatment to improve the characteristics of the resulting coatings is not well studied. Considering their tribological application, the question of how to deposit Ti3SiC2-based coatings with high wear resistance is still undecided.

Among the coating methods, detonation spraying and high-velocity oxy-fuel (HVOF) spraying [8–11] have an obvious advantage in comparison to plasma spraying and other spraying methods because of high particle flight speed and lower operating temperature. Detonation spraying coatings from powder are based on the use of explosion energy of a fuel–oxygen mixture and are known as a promising method for obtaining coatings from various materials with good adhesion [12]. The higher velocity flow of particles allows providing higher density and adhesive strength of the detonation coating. The essential advantage of the detonation spraying method is the ability to accurately control the amount of explosive gas mixture used for each shot of the detonation gun, which allows changing the degree of thermal and chemical effects of detonation products on the particles of spraying powder [13]. Depending on the composition of the acetylene–oxygen explosive mixture, from a O2/C2H2 ratio and from the nature of the gas carrier, chemical interactions may occur between the individual phases of the composite particles [14,15]. In this regard, the detonation method of coating is of considerable interest. Therefore, much attention is directed at obtaining detonation coatings from binary and ternary phases relevant to the Ti–Si–C system.

In connection with the above, the aim of this work is to study the structural features and tribological properties of coatings based on Ti–Si–C, which are obtained by the detonation method under various deposition modes, as well as to study the effect of thermal annealing on the structural and phase states of coatings based on Ti–Si–C.

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

By detonation spraying on the surface of steel U9 (with 0.94 wt.% C) Ti3SiC2 coatings were obtained. The chemical composition of the powder was Ti: 74 wt.%; SiC: 20 wt.%; C: 6.0 wt.%, and powder particle size was between 20 and 40 μm. Before spraying, the substrate was sandblasted to improve the adhesive strength of the coatings. The value of the surface roughness parameter after sandblasting was on average (Ra) 3.2 microns. The distance between the treated surface of the sample and the detonation barrel was 200 mm. The straight barrel diameter was 20 mm.

The CCDS2000 (LIH SB RAS, Novosibirsk, Russia) detonation set-up was used to obtain the coatings, which has a system of electromagnetic gas valves that regulate the supply of fuel and oxygen and control the system purging (Figure 1). The acetylene–oxygen mixture was used as a fuel gas, which is the most used fuel during detonation spraying of powder materials. Spraying was carried out at the ratio of the acetylene–oxygen mixture O2/C2H2 = 1.856. The volume of the explosive gas mixture of the detonation gun barrel was varied from 50% to 70%. The average shot frequency of working gases at 4 Hz was: acetylene 4–7; propane–butane mixture 2 ... 3, 5; oxygen 10 ... 12; nitrogen 10 ... 15 m3/h. Nitrogen was used as a carrier gas.

**Figure 1.** Schematic diagram of the CCDS2000 detonation complex: 1—control computer; 2—gas distributor; 3—mixing-ignition chamber; 4—spark plug; 5—barrel valve; 6—fuel line; 7—oxygen line; 8—gas valves; 9—gas supply unit; 10—indicated part of the barrel; 11—powder feeder; 12 workpiece; 13—manipulator; 14—the muzzle of the barrel; F1—acetylene; F2—propane–butane; O2—oxygen; N2—nitrogen.

The research phase composition of the samples was studied by X-ray diffractometer X'PertPro (Philips Corporation, The Nederlands) using CuKα radiation. The shooting was carried out in the following modes: tube voltage U = 40 kV; tube current I = 20 mA; exposure time 1s; shooting step Δ2θ ~ 0.02◦ and 2θ = 10–90◦. The surface morphology was studied by scanning electron microscopy using secondary (SEs) and backscattered electrons (BSEs) on a Vega3 (Tescan, Brno, Czech Republic) scanning electron microscope. Tribological tests for sliding friction were performed on a high-temperature tribometer TRB3 (Anton Paar Srl, Peseux, Switzerland) using the standard "ball-disc" technique (international standards ASTM G 133-95 and ASTM G 99) [16,17]. A counterbody was used comprising a ball with a diameter of 3.0 mm made of SiC-coated steel. The tests were carried out at a load of 10 N and a linear velocity of 3 cm/s, a radius of wear curvature of 4 mm, the friction path was 41 m. Wear tracks were studied using a non-contact 3D profilometer MICROMEASURE 3D (STIL, France) station. The CSEM Micro Scratch Tester (Neuchatel, Switzerland) was used to study the adhesive characteristics of coatings by the "scratching" method. Scratch testing was performed at a maximum load of 30 N; the rate of change of normal loading on the sample was 29.99 N/min, the speed of movement of the indenter was 9.63 mm/min, the length of the scratch was 10 mm, the radius of tip curvature was 100 microns. To obtain reliable results, three scratches were applied to the surface of each coated sample. The obtained coatings with mechanical properties (Young's modulus, hardness) were studied by a NanoScan-4D Compact (FSBI TISNCM, Russia) nanohardometer. The tests were carried out at a load of 200 mN. Loading time, unloading time, and the time of supporting the maximum load were each 5 s. The dependence of the penetration depth on the applied force at the loading and unloading stages was determined by the Oliver–Pharr method. At least 10 measurements were carried out on each sample, the results of which were averaged. Sample tests for abrasive wear were carried out on an experimental stand (Figure 2a) against soft fixed abrasive particles according to the "rotating roller–flat surface" scheme in accordance with GOST 23.208-79, which conforms to the American standard ASTM C 6568 [18,19]. Sample tests for impact and abrasive wear were carried out on an experimental stand in accordance with GOST 23.207-79 (Figure 2b) [20]. For a comparative assessment of the wear resistance of detonation coatings, tests were carried out in the following modes: impact energy E = 3.3 J, impact velocity *υ* = 1 m/s, and impact frequency n = 200 min−1. The samples tested for abrasive and impact-abrasive wear underwent 8–10 tests. After each test, the mass loss of the samples was determined and was given the average value with the standard deviation. The microhardness of the samples was measured by a diamond indenter on a PMT-3M (LOMO, Russia) device in accordance with GOST 9450-76 [21], at a load of 200 g and an exposure time of 10 s [22]. Thermal annealing of coated samples was carried out in a laboratory tubular electric furnace of resistance SUOL-0.4.4/12 (0.4.4—dimensions of working space, 40 × 400 mm; 12—the nominal temperature of the working space, 1200 ◦C) in a vacuum of 10−<sup>2</sup> Pa at temperatures of 700 ◦C, 800 ◦C, and 900 ◦C for 1 h, followed by cooling in the furnace. The temperature was measured and regulated by a precision high-precision temperature controller HTC-2 [23].

**Figure 2.** The experimental test stand for testing of samples: (**a**) abrasive wear according to the "rotating roller–flat surface" scheme; (**b**) impact and abrasive wear.

#### **3. Results and Discussions**

Figure 3 shows the diffractograms of Ti3SiC2 powder and Ti–Si–C coating obtained at the barrel filling volume of an explosive gas mixture from 50% to 70%. The results of powder XRD analysis showed that the powder consisted of Ti3SiC2 as the main phase and TiC as the secondary phase. The diffractograms of Ti–Si–C coatings showed a decrease in the intensity of Ti3SiC2 diffraction lines and an increase in TiC intensity, which indicated a partial decomposition of the Ti–Si–C system and agreed with the data [24–31]. A decrease in the intensity of diffraction lines in the Ti–Si–C system after detonation spraying was due to the deintercalation of silicon from Ti–Si–C lattice layers [25,26] since the silicon flatness had weak connections with Ti–C flatness. This occurred due to detonation spraying when the Ti–Si–C system lost a certain amount of silicon due to its high "fugacity" [27]. XRD analysis showed that when the barrel was filled with explosive mixtures to 50% and 60%, a low degree of Ti3SiC2 decomposition was achieved, and also after spraying the appearance of reflexes (100) and (106) of the Ti3SiC2 phase was detected. With an increase in the filling volume of the detonation barrel to 70%, a decrease in the intensity of the diffraction peaks of Ti3SiC2 was observed because of the decomposition of the powder into TiC. In the detonation wave flow, the Ti3SiC2 powder decomposed due to high-speed shock interaction heated to high temperatures. Thus, the XRD results confirmed that at 70% of the explosive mixture's filling volume, partial decomposition and disintegration of the powders occurred after detonation spraying.

**Figure 3.** Diffractogram of Ti3SiC2 powder and Ti–Si–C coatings obtained at different filling volumes of the explosive gas mixture of the detonation gun barrel.

The SEM image shows that the cross section of the sprayed coating is not flat and continuous. From the image's analysis, it follows that the coating structure has an inhomogeneous structure with pores, a typical layered, wave-like arrangement of structural components. Significant pores (the dark area highlighted by a circle) can be seen in the image of the coating cross section (Figure 4b,d). The border between the coating and the base has a characteristic zigzag shape. The structure consists of small particles and several large flat spots with periodic observations of the morphology of terraces in certain areas (Figure 4a,c), the light gray area indicates mainly the Ti3SiC2 phase, highlighted by a rectangle, and the friable and dark gray area indicates the TiC phase. Dark areas have large volume fractions in the coatings. The surface roughness of coatings Ti–Si–C (Ra) comprises 2.5–2.65 μm.

**Figure 4.** SEM images of the morphology of coating cross section: (**a**) Ti–Si–C 60% while 100 μm; (**b**) Ti–Si–C 60% while 20 μm; (**c**) Ti–Si–C 70% while 100 μm; (**d**) Ti–Si–C 70% while 20 μm.

One of the main factors determining the coating quality was adhesion. Figure 5 shows the adhesive strength testing results of scratch testing. The moment of coating adhesive or cohesive failure was fixed visually after testing using an optical microscope with a digital camera, also by changing two parameters: acoustic emission and friction force. It should be noted that not all recorded events associated with the destruction of the coating describe the actual adhesion of the coating to the substrate. Various registration parameters during testing processes allowed fixing different coating failure stages; so, Lc1 means the moment of appearance of the first crack, Lc2 the peeling of coating areas, and Lc3 the plastic abrasion of the coating to the substrate [32]. According to the type of change in the acoustic emission (AE) amplitude, it was possible to judge the crack formation intensities and their development in the sample during scratching. The Ti–Si–C system coatings that occurred during the explosive gas mixture filling volumes of 50% and 60% were visible, the first crack was formed at a load of Lc1 = 12 N. Then the process continued in a certain cycle. A corresponding peak of acoustic emission accompanied each crack formation (Figure 5a,b). Partial abrasion of the coating to the substrate was judged by a sharp change in the friction force growth intensity. This occurred at a load of Lc3 = 29 N, which was also confirmed by visual observation, noting a change in the color of the sample material at the bottom of the scratch (Figure 5a,b). The Lc3 value indicated a high adhesive strength of coating adhesion to the substrate.

**Figure 5.** Results of the scratch test of Ti–Si–C coatings obtained at different filling volumes of the explosive gas mixture of the detonation gun barrel: (**a**) 50%, (**b**) 60%, and (**c**) 70%.

The Ti–Si–C system coating obtained at the explosive gas mixture filling volume of 70% shows appearance cracks (Figure 4c) observed at Lc1 = 8 N load. According to adhesion tests, it can be argued that the cohesive destruction of the sample coating occurred at 8 N, and its adhesive destruction at 29 N. The Ti–C system has a higher stiffness, so it is natural to expect minimal elastic and intense plastic deformation during the adhesion test [33].

For coatings of this functional purpose, wear resistance is one of the most important exploitation properties, which is reflected both in the capacity of structures as a whole and in the conservation of the geometric dimensions of individual parts. The results of coating tribological tests showed that the filling volume of the explosive mixture and the coating structures had a significant impact on the value of the friction coefficient of the coating surface and wear resistance. Therefore, in the case of composite coatings Ti–Si–C obtained with the filling volumes of the explosive mixture at 50% and 60% in the detonation gun barrel, the friction coefficient at the initial stage of testing to 18.40 and 25.10, respectively, was 0.15–0.20 μ and slightly increased; subsequently the friction coefficient increased monotonously from 0.25 to 0.60 μ (Figure 6). The obtained coating Ti–Si–C system's friction coefficient at a filling volume of 70% was 0.60 μ. According to the XRD analysis results, the increasing wear resistance of the Ti–Si–C system coatings with detonation barrel filling volumes of 50% and 60% was related to a larger proportion of the Ti3SiC2 phase.

With the profilometer were taken images of the wear track of the studied samples (Figure 6). Assessing the samples' wear resistance based on the wear tracks' geometric parameters illustrated that the depth of the sample track at 70% of the explosive mixture's filling volume was significantly greater when compared with others. The detonation coatings based on titanium carbolized were characterized by high wear resistance.

**Figure 6.** Tribological tests results of Ti–Si–C coatings obtained at different filling volumes of the explosive gas mixture of the detonation gun barrel and track profiles: (**a**) 50%; (**b**) 60%; (**c**) 70%

The study results of the mechanical characteristics of the obtained coatings were carried out by the Oliver–Pharr method, and typical dynamic loading–unloading diagrams are shown in Figure 7. From the analysis of the loading and unloading curves, it can be seen that the penetration depth of the nano-indenter in the cases of explosive gas mixture filling volumes of 50% and 60% was less than in the case of 70% filling volume. According to the analysis of the indentation curves, it can be concluded that the elastic stiffness of the coatings during 70% filling was higher (Figure 7b) when compared to the rest (Figure 7a,b). According to the XRD analysis results, when the filling volume of the explosive mixture in the detonation gun barrel increased to 70% a coating was formed with a high content of the TiC phase. Thus, the results of nano-indentation and scratch testing were in good agreement and confirmed the formation of TiC, which had a higher stiffness compared to Ti3SiC2.

**Figure 7.** Loading–unloading curves for Ti–Si–C coatings obtained at a different explosive gas mixture filling volumes of the detonation gun barrel: (**a**) 50%; (**b**) 60%; (**c**) 70%.

The values of hardness and modulus of elasticity of the research samples obtained from the loading–unloading curve analysis are shown in Table 1. As visible from Table 1, coatings with a high content of Ti3SiC2 had higher hardness values compared to the coating with the prevailing TiC phase.


**Table 1.** Results of nano-indentation.

The detonation spraying process is characterized by a significant number of technological parameters. The complexity and insufficient study of the phenomena underlying it makes it very difficult to trace the relationship of individual parameters to determine the process optimal modes, using one-factor experiments. Therefore, experimental and statistical methods of regressive analysis and the theory of experiment planning are used when optimizing the process. Abrasive and impact-abrasive wear are two of the main factors that limit working parts, machines, and equipment components for various purposes. To assess the resistance of Ti–Si–C coatings to abrasion and impact-abrasive wear, tests were carried out on special stands. Table 2 shows the test results of the abrasive and impact-abrasive wear. The coated samples' mass loss was less than that of the original sample, which indicates an increased resistance to impact and abrasive wear. This is due to the presence of a larger proportion of the hardening carbide phase TiC in the Ti3SiC2 coating. This is due to an increase of the strengthening phase TiC in the composition of the protective coating. There is a significant increase in internal stresses and a decrease in the amount of the more plastic phase, which ultimately decreases toughness.

**Table 2.** Abrasive and impact-abrasive wear test results.


According to the determination results of the samples' mass loss after testing wear on the fixed abrasive (Figure 2), the greatest strength was found in a coating obtained by the 60% (0.0122 g) filling volume of the detonation gun barrel and the smallest was in the filling volume of 70% (0.0203 g), the wear resistance of all coatings was higher than the initial sample (0.0265 g). The detonation coatings based on titanium carbolized were characterized by high wear resistance.

Thus, in this work, we made the effort to obtain composite coatings based on Ti–Si–C with the Ti3SiC2 phase by detonation spraying. The analysis of the obtained experimental results indicated that the phase composition and properties of detonation coatings strongly depend on the technological parameters of spraying. With an increase of explosive gas mixture in the filling volume of the detonation barrel up to 70% of the coatings consisted mainly of TiC phases, since high temperature leads to a strong decomposition of Ti3SiC2 powders [34]. Consequently, the successful production of high-purity MAX-phase coatings by detonation spraying was not reported. Based on the analysis of literature sources [35,36] and preliminary studies, it was suggested that if gas–thermal deposition of substances of the Ti–Si–C system is carried out, it would be possible to obtain a multiphase coating containing such phases as carbides, silicides, and carbosilicides of titanium, and during subsequent heat treatment–regulation of its phase composition. Thermal annealing was performed in the temperature range 700–900 ◦C for 1 h.

The Vickers hardness of Ti–Si–C composite coatings before and after annealing is shown in Figure 8. The hardness of the composite coating increased significantly with increasing the annealing temperature: at T = 700 ◦C, the microhardness was 1150 HV; at T = 800 ◦C, the microhardness was 1400 HV; and at T = 900 ◦C, the microhardness decreased to 850 HV.

Aiming to identify the cause of the change in microhardness, we performed XRD analysis of coatings before and after annealing. The results of XRD analysis of coatings showed (Figure 9) that the coating before annealing consisted of TiC and Ti3SiC2 phases.

**Figure 9.** Diffractograms of Ti–Si–C coatings at different annealing temperatures.

After annealing, the formation of TiO2 phases and an increase in the reflex intensities (103) and (108) of Ti3SiC2 phases were observed. Compared to the after-sprayed coatings, the phase fraction of the Ti3SiC2 phase in the after-annealing coatings increased significantly. A change in the fraction of phases indicated a solid-phase transformation during thermal activation. An increase in the TiO2 line intensity was observed after annealing at T = 900 ◦C, which indicated an increase in the thickness of the oxide layer. The increase in microhardness after annealing was associated with an increase in the content of Ti3SiC2 phases in coatings. At the same time, after annealing at T = 900 ◦C, the decrease in microhardness was insignificant due to an increase in the oxide layer's thickness. After annealing at 800 ◦C, an increase in the Ti3SiC2 line intensity indicated an increase in the symmetric movements of C atoms in the Ti3SiC2 molecule. This means that subsequent heat treatment

provided sufficient reaction time for an incomplete reaction of the Ti–Si–C (TSC) coating during detonation spraying.

The microstructure of Ti–Si–C800 consisted of a titanium-rich region (light region) and a TiCx region diluted by Si (light gray regions). Heat treatment can lead to the diffusion of C and Si atoms [37]. Thus, annealing can provide a more homogeneous distribution of atoms in coatings after annealing. This can be verified by displaying the Ti–Si–C and Ti–Si–C800 elements in Figure 10. As shown in Figure 10, the Ti, C, and Si maps show separate rich and scarce areas. Moreover, the C and Si atoms have a similar distribution in most regions of the element map. The red color on the Ti map corresponds to the dark zones of C deficiency and Si deficiency on the C and Si maps, respectively, identified as the Ti phase (C, Si). After annealing at 800 ◦C, the three elements showed a more homogeneous distribution (Figure 11). This means that the high temperature provided a more intense diffusion of C and Si atoms.

**Figure 10.** The microstructure of the cross section of coatings with a color images of energy-dispersive spectrometer (EDS) mapping and the result of analysis after annealing at 800 ◦C.

**Figure 11.** The coatings' surface morphology before (**a**) and after (**b**) annealing at 800 ◦C.

The results of tribological testing of coatings showed that the temperature of thermal annealing and the structure of the coatings themselves had a significant impact on the coefficient of friction of the coating surface and wear resistance. So, in the case of composite Ti–Si–C coatings, the friction coefficient was 0.65–0.70 before annealing. After thermal exposure at temperatures up to 800 ◦C, the friction coefficient at the initial stage of testing (up to 12.40 m) was 0.30–0.35 and a slight increase occurred, at which the friction coefficient increased monotonically from 0.35 to 0.70 as in the case before annealing (Figure 12). According to the results of the X-ray phase analysis, an increase in the wear resistance of the surface layers of the Ti–Si–C composite material after 800 ◦C was associated with the formation of TiO2 and the presence of a larger fraction of the TiC carbide hardening phase (Figure 9). In [38,39], it was shown that an oxide compound based on TiO2 increases

the wear resistance and strength of materials. Detonation coatings based on titanium carbosilicide are characterized by high wear resistance.

**Figure 12.** Results of tribological experiments of Ti–Si–C coatings before and after annealing at 800 ◦C.

#### **4. Conclusions**

The paper described an experimental study of the effect of the detonation gas filling mode on the phase composition and strength characteristics of the Ti–Si–C coatings system. It was shown that the phase composition of detonation coatings can be significantly changed relative to the phase composition of the initial powders, depending on the filling volume of the detonation barrel with an explosive acetylene–oxygen mixture. When the filling volume of the detonation barrel with an explosive mixture increases to 70%, the coatings consist mainly of TiC phases. The Ti3SiC2 powder partially decomposes into TiC due to the high-speed shock interaction of high temperatures in the detonation wave flow. The X-ray phase analysis results showed that when filling the barrel with explosive mixture to 50% and 60%, a low degree of Ti3SiC2 decomposition can be achieved, the coatings consist mainly of Ti3SiC2 phases with small TiC content. With an increase in the detonation barrel's filling volume of the explosive acetylene–oxygen mixture, the heating temperature of the sprayed powder increases. High temperature contributes to the decomposition of Ti3SiC2 powder into TiC. Thus, the XRD results confirmed that when the explosive gas mixture filling volume is 70%, partial decomposition and disintegration of the powders occur after detonation spraying. It is established that detonation coatings based on titanium carbosilicide obtained at the explosive gas mixture filling volume of 60% are characterized by high wear resistance and adhesive strength.

Thermal annealing was performed after spraying in the temperature range of 700–900 ◦C for 1 h to reduce microstructural defects and improve the Ti–Si–C coating characteristics. As a result of heat treatment in the Ti–Si–C system at 800 ◦C, an increase in the volume fraction of the Ti3SiC2 and TiO2 phases leading to a 2-fold increase in microhardness was observed. This means that the after-heat-treatment can provide a sufficient reaction time for the incomplete reaction of the Ti–Si–C (TSC) coating during the detonation gun spraying. Thus, annealing can provide an equal distribution of elements in the coatings.

**Author Contributions:** B.R., Z.S., M.A. and D.B. designed the experiments; B.S. and D.B. performed the experiments; B.R., Z.S. and S.K. analyzed the data; B.R., Z.S. and D.B. wrote, reviewed and edited the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** The article uses the results obtained with the support of grant funding of the Ministry of Education and Science of the Republic of Kazakhstan, grant AR08957719.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available in the article.

**Conflicts of Interest:** The authors declare that there is no conflict of interest regarding the publication of this manuscript.

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