Mechanical Characterization of Composite Coatings Formed by Reactive Detonation Spraying of Titanium

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Article Mechanical Characterization of Composite Coatings Formed by Reactive Detonation Spraying of Titanium

Sergey Panin 1,2,3,* ID , Ilya Vlasov 2,3, Dina Dudina 4,5 ID , Vladimir Ulianitsky 4, Roman Stankevich 3, Igor Batraev 4 and Filippo Berto 6

1 College of Physics, Jilin University, Changchun 130022, China 2 Institute of Strength Physics and Materials Science SB RAS, Tomsk 634055, Russia; [email protected] 3 Department of Materials Science in Mechanical Engineering, National Research Tomsk Polytechnic University, Tomsk 634050, Russia; [email protected] 4 Laboratory of Detonation Flows, Lavrentyev Institute of Hydrodynamics SB RAS, Novosibirsk 630090, Russia; [email protected] (D.D.); [email protected] (V.U.); [email protected] (I.B.) 5 Department of Mechanical Engineering and Technologies, Novosibirsk State Technical University, Novosibirsk 630073, Russia 6 Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway; ﬁ[email protected] * Correspondence: [email protected]; Tel.: +7-3822-286-904

Received: 18 August 2017; Accepted: 5 September 2017; Published: 8 September 2017

Abstract: The structure and mechanical properties of the coatings formed by reactive detonation spraying of titanium in a wide range of spraying conditions were studied. The variable deposition parameters were the nature of the carrier gas, the spraying distance, the O2/C2H2 ratio, and the volume of the explosive mixture. The phase composition of the coatings and the inﬂuence of the spraying parameters on the mechanical properties of the coatings were investigated. In addition, nanohardness of the individual phases contained in the coatings was evaluated. It was found that the composition of the strengthening phases in the coatings depends on the O2/C2H2 ratio and the nature of the carrier gas. Detonation spraying conditions ensuring the formation of composite coatings with a set of improved mechanical properties are discussed. The strength of the coatings was determined through the microhardness measurements and local characterization of the phases via nanoindentation. Three-point bending tests were employed in order to evaluate the crack resistance of the coatings. The strengthening mechanisms of the coatings by oxide or carbonitride phases were discussed.

Keywords: detonation spraying; three-point bending; electron microscopy; micromechanics; composites; titanium alloys; powder methods; phase transformation

1. Introduction At present, several coating deposition methods are known and widely used in the industry. The choice of a particular method is dictated by a combination of factors, such as the level of loading and operating conditions, the shape, size and geometry of machine parts to be coated, the required thickness of the coating, the necessity for mechanical post-processing, the requirements for adhesive and cohesive strength and the cost of the final product. Thermal spraying is widely used for surface strengthening and restoration. The variations of thermal spraying are plasma spraying, flame spraying, high-velocity oxy-fuel spraying (HVOF), cold gas-dynamic spraying and detonation spraying. Thermal spraying is used to deposit high-strength materials [1] and multicomponent coatings reinforced with finely dispersed inclusions, which ensure

Metals 2017, 7, 355; doi:10.3390/met7090355 www.mdpi.com/journal/metals Metals 2017, 7, 355 2 of 16 high coating performance [2,3]. In order to form coatings containing intermetallic phases with high adhesion strength and corrosion resistance, vacuum spraying [1,2,4] is employed or deposition is carried out in an inert gas atmosphere [5]. The sprayed metallic particles can chemically react with gases contained in the spraying atmosphere, which leads to the formation of finely dispersed ceramic phases in the coatings [4,6,7]. A widespread technique for producing dispersion-strengthened coatings is the formation of oxide inclusions in metal matrices. This is achieved through oxidation of the molten particles when spraying is carried out in air. In this case, atmospheric plasma spraying can be efficiently employed [8]. A substantial drawback of this method is the porosity of the coatings and the non-uniformity of their structure. One of the efficient technologies for restoration, repair and strengthening of machine parts is cold gas-dynamic spraying [9,10]. It is based on the layer-by-layer formation of coatings at supersonic flow velocities of the carrier gas and is associated with minimal heating of the sprayed material. The cohesive strength is, therefore, provided thanks to the high kinetic energy of the particles upon impact. This substantially reduces the probability of oxidation and eliminates the need for dynamic vacuum or an inert gas atmosphere. By selecting the optimal gas flow rate and the temperature of the gas, multicomponent coatings with a set of required physical and mechanical properties can be fabricated [10]. In addition to spraying of the powder mixtures, a unique combination of coating properties can be achieved through the deposition of alternating layers of different compositions [11]. In the framework of this classification, a technique of coating deposition from powder mixtures using concentrated pulsed energy flows has to be mentioned. This technique allows forming layers that possess low porosity, high adhesion to the substrate, and submicro- or nanocrystalline structures [12]. The drawback of this method is poor control of the structure and geometry of the coatings. Currently, detonation spraying and HVOF are the most efficient methods of depositing coatings that can protect machine parts from erosion, corrosion and different types of wear [13,14]. Low porosity of the coatings is achieved due to high velocities of the particles [5]. The method can also be applied for metallization of ceramics and periodic restoration of the worn machine parts [15–17]. In Ref. [18], the application of the HVOF technique to deposit WC-Co coatings is compared with the ecologically unfriendly chrome plating, which has been used in the industry for a long time. A comparative analysis of the structure and properties of Cu-Ni-In coatings formed by plasma and detonation sputtering onto Ti-4Al-6V substrate was performed in Ref. [19]. It was shown that the detonation-sprayed coatings show a higher adhesion, a reduced porosity and a more uniform structure than the plasma sprayed ones. The improved structural characteristics increase the fatigue life and resistance to fretting corrosion of the coatings. The surface quality of the thermally sprayed coatings depends on the surface finish and can be improved by droplet elimination [20]. Also, machining of the thermally sprayed coatings is needed to achieve dimensional tolerances [21]. Therefore, the mechanical strength and integrity of the coatings are important not only for their functional performance, but also as guaranteeing the capability of the coatings to withstand the machining operations. Chemical reactions involving the sprayed materials and phase transitions in the sprayed material can exert both positive and negative effects on the coating performance. TiO2 coatings formed by the HVOF technique were studied in Ref. [22]. The powder was introduced by both internal and external injection. It was shown that by varying the composition of the sprayed powder and the way of its introduction into the gas flow, it is possible to control the amount of anatase that affects the structure and photocatalytic properties of the coating. In this paper, the structure and mechanical properties of deposits formed on titanium substrates by reactive detonation spraying of a titanium powder were investigated. The aim of the present work was to determine the effect of the coating structure on the mechanical properties of the coatings. Metals 2017, 7, 355 3 of 16

2. Materials and Methods The deposition of the coatings onto titanium substrates was carried out using a computer controlled detonation spraying (CCDS2000) facility (Lavrentyev Institute of Hydrodynamics SB RAS, Novosibirsk, Russia) [23]. The substrates were sand-blasted before depositing the coating layers. Titanium (99% purity, average particle size 15 µm, “PTOM-2”, Russia) was used as a feedstock powder. The spraying distance was 10 mm or 100 mm. Air or nitrogen was used as a carrier gas. Spraying was performed using different compositions of acetylene-oxygen mixtures corresponding to O2/C2H2 molar ratios of 0.7, 1.1 and 2.5 (Table1). The use of nitrogen as a carrier gas reduces the concentration of oxygen in the spraying atmosphere and the oxidation degree of the material. It should be kept in mind that air, when used as a carrier gas, is not the only source of oxidizing agents, as the detonation products themselves contain oxidizing species. The concentrations of oxidizing species in the detonation products depend on the O2/C2H2 ratio. The spraying frequency was 4–5 shots per second. On average, in order to obtain a coating 300 µm thick on a spot 20 mm in diameter, 60–70 shots are needed. The volume fractions of the barrel ﬁlled with an explosive mixture (explosive charge) was 30–60% of the total barrel volume. Conditions of the enhanced nitride formation were created by introducing nitrogen into the explosive mixture.

Table 1. Parameters of detonation spraying and the coating thickness.

Spraying Spraying Explosive Carrier Coating Thickness O2/C2H2 Regime Distance, mm Charge, % Gas (hcoat), µm 1 1.1 100 30 air 270 2 2.5 100 30 air 275 3 1.1 10 40 nitrogen 160 4 0.7 10 40 nitrogen 291 5 0.7 10 50 nitrogen 190 6 0.7 100 50 nitrogen 750 7 1.1 + added 33 vol % N2 10 60 nitrogen 235 8 1.1 + added 33 vol % N2 100 60 nitrogen 285

Elemental microanalysis of the coatings was carried out by Energy Dispersive Spectroscopy (EDS) using an INCA unit (Oxford instruments, Abingdon, UK) attached to a LEO EVO 50 scanning electron microscope (Zeiss, Oberkochen, Germany). The X-ray diffraction (XRD) patterns of the coatings were recorded using a D8 ADVANCE powder diffractometer (Bruker AXS, Karlsruhe, Germany). Three test samples of the coatings were obtained for each selected set of the detonation spraying parameters (spraying regime). The detonation spraying experiments showed good reproducibility in terms of the phase composition and structure of the deposited layers. The three samples obtained as a result of three runs under the constant conditions were used for the preparation of samples for mechanical testing. Accordingly, for a mechanical characteristic, an average of three measurements is reported for each spraying regime. In order to study the correlation between the structure and the mechanical properties of the coatings, three-point bending tests were carried out using an electromechanical testing machine Instron 5582. This loading scheme was chosen as under applied external force, the surface layer (the detonation coating) plays a major role in providing resistance to deformation and fracture. The three-point bending tests in combination with photographing the specimen lateral surface allow gaining detailed information on the deformation behavior and estimating the cracking stress σcr [24]. The stress under three-point bending was calculated with the help of Equation (1) [25]:

M σ = max , (1) W Metals 2017, 7, 355 4 of 16

where Mmax is the largest value of bending moment Equation (2) (P is bending load; l is the distance between the supports (span). P × l M = , (2) max 4 W is the resistance momentum, which for rectangular-section specimens can be calculated by Equation (3) (b is the width and h is the gauge height of the specimens).

b × h2 W = . (3) 6 The shear stress was calculated by Equation (4) [26]:

h × σ τ = coat delam , (4) Lc where hcoat is the coating thickness, σdelam is the stress, at which the coating starts to delaminate (delamination stress) and Lc is the distance between the cracks in the coating. The interface fracture toughness (“coating shear resistance”) was determined by Equation (5) [26]:

1/2 Kc = τ × (π × Lc) . (5)

For mechanical testing, rectangular specimens 20 × 10 × 2.5 mm3 in size were prepared using electroerosion cutting. The span (the distance between the supports) was 19 mm. Since the titanium substrates possess high ductility, it was nearly impossible to fracture the specimen using the selected loading scheme. For this reason, the evaluation of the mechanical properties was carried out at a bending deﬂection of 2.5 mm. Scratch tests were carried out to evaluate the adhesion strength. A Revetest-RST scratcher (CSM-Instruments, Peseux, Switzerland) was used for these measurements. The velocity of the indenter was 2 mm/min. The load was increased from 0.6 N to 200 N. The friction coefﬁcient was calculated as an average over the entire scratch length. The depth of the scratch tracks was estimated by a white light optical interferometer NewView 6200 (Zygo, Berwyn, PA, USA). Microhardness of the coatings Hµ was measured on the polished surface parallel to the coating/substrate interface using a PMT-3 testing machine (LOMO, St. Petersburg, Russia). The load onto the Vickers pyramid was 100 g. Local characterization of the mechanical properties of the coating phases through nanohardness measurement was carried out by nanoindentation with the help of a Nano Indenter G200 (MTS Systems Corporation, Eden Prairie, MN, USA). Nanohardness was measured on the polished cross-sections of the coatings.

3. Results and Discussion

3.1. Three-Point Bending Test

The results of three-point bending tests (Figure1a, Table2) show that the value of the σ2.5 parameter in the specimens varies within a wide range (σ2.5 = 820–1055 MPa). However, the value of this parameter weakly correlates with the strength characteristics of the coatings, partially due to signiﬁcant differences in the coating thickness. It was concluded that the shape of the three-point bending diagram does not unambiguously show the contribution of the coating to the deformation resistance of the entire coated specimen. Thus, it is necessary to measure the mechanical properties of the coatings locally. Metals 2017, 7, 355 5 of 16 Metals 2017, 7, 355 5 of 16

(a) (b) (c)

Figure 1. ((a)) three-pointthree-point bendingbending diagram diagram for for coated coated titanium titanium specimens; specimens; (b (,cb),c scratching) scratching diagrams diagrams of theof the coatings. coatings.

Table 2.2. MechanicalMechanical propertiesproperties of thethe coatingscoatings and coatedcoated specimens measured by scratch tests and 3-point bending.

σ2.5, σcr, σdelam Kc, Friction Maximum Depth No. σ , σ , σ , K , τ, MPaτ, Hμ, GPa Friction Maximum Depth No.MPa 2.5 MPa cr MPadelam MPa·m1/2c H , GPa Coefficient, μ0 of the Scratch, μm MPa MPa MPa MPa·m1/2 MPa µ Coefﬁcient, µ of the Scratch, µm 1 955 98.4 540 10.4 237.3 3.70 ± 0.03 0.406 0 66.6 ± 2 1 850 955 140.5 98.4 630 540 12.7 10.4 295.3 237.3 3.70 3.84 ±0.03 0.07 0.406 0.412 66.6 63.4 2 850 140.5 630 12.7 295.3 3.84 ± 0.07 0.412 63.4 3 3825 82587.9 87.9 620 620 4.8 4.8 73.1 73.1 2.682.68± ±0.07 0.07 0.637 0.637 83.3 83.3 4 4 1055 1055 44.9 44.9 70 70 0.9 0.9 10.9 10.9 2.45 2.45± ±0.11 0.11 0.661 0.661 140.5 140.5 5 5 945 945 94.2 94.2 730 730 7.6 7.6 132.9 132.9 2.72 2.72± ±0.09 0.09 0.580 0.580 50.7 50.7 6 6 870 870 10 10 650 650 - - - - 2.02 2.02± ±0.01 0.01 0.594 0.594 102.2 102.2 7 945 372.1 800 11.6 228.7 2.74 ± 0.09 0.675 103.5 7 945 372.1 800 11.6 228.7 2.74 ± 0.09 0.675 103.5 8 875 33.5 560 6.8 93.1 2.29 ± 0.02 0.585 80.9 8 875 33.5 560 6.8 93.1 2.29 ± 0.02 0.585 80.9

3.2. Scratch Tests 3.2. Scratch Tests With the help of scratch tests, a number of mechanical characteristics of the coatings were With the help of scratch tests, a number of mechanical characteristics of the coatings were measured (Figure1, Table2). The values of the friction coefficient and the penetration depth of the measured (Figure 1, Table 2). The values of the friction coefficient and the penetration depth of the indenter at the maximum load were determined. The obtained results correlate with the coating indenter at the maximum load were determined. The obtained results correlate with the coating strength. The values of microhardness of specimens produced in regime No. 1 and No. 2 are the strength. The values of microhardness of specimens produced in regime No. 1 and No. 2 are the highest among the studied specimens (H100 = 3.7–3.84 GPa), while the friction coefficients are the highest among the studied specimens (H100 = 3.7–3.84 GPa), while the friction coefficients are the lowest (µ0 ~0.41). lowest (μ0 ~0.41). 3.2.1. Spraying Regime 1 3.2.1. Spraying Regime 1 During the scratch test, the indenter initially quickly penetrated into the coating at 55 µm, the track During the scratch test, the indenter initially quickly penetrated into the coating at 55 μm, the length reaching 1 mm. Upon further scratching, the penetration depth increased to 67 µm. track length reaching 1 mm. Upon further scratching, the penetration depth increased to 67 μm. This was probably related to an imperfect structure of the surface layer. The scratch profile This was probably related to an imperfect structure of the surface layer. The scratch profile oscillated due to a non-uniform structure of the coating (Figure1b, curve 1). The coating possesses oscillated due to a non-uniform structure of the coating (Figure 1b, curve 1). The coating possesses a a sufficiently high bending cracking resistance (σcr = 98.4 MPa), a high bending delamination stress sufficiently high bending cracking resistance (σcr = 98.4 MPa), a high bending1/2 delamination stress (σdelam = 540 MPa) and a high interface fracture toughness (Kc = 10.4 MPa·m ). (σdelam = 540 MPa) and a high interface fracture toughness (Kc = 10.4 MPa·m1/2). 3.2.2. Spraying Regime 2 3.2.2. Spraying Regime 2 With increasing load, the penetration depth of the pyramid during the scratch test changed almost linearlyWith (Figure increasing1c, curve load, 2). the At penetration the end of the depth test, of the the indenter pyramid reached during a depththe scratch of 63.4 testµm, changed which isalmost close linearly to the value (Figure observed 1c, curve during 2). At testingthe end of of specimen the test, the produced indenter in reached regime a No. depth 1. Thisof 63.4 agrees μm, withwhich a is higher close microhardnessto the value observed of specimen during produced testing of inspecimen regime No.produced 2 as compared in regime to No. specimen 1. This agrees No. 1. Inwith addition, a higher compared microhardness with specimen of specimen produced produced in regime in regime No. No. 1, specimen 2 as compared produced to specimen in regime No. No. 1. 2 possessesIn addition, a highercompared fracture with toughness,specimen produced a higher in interface regime strength,No. 1, specimen a higher produced bending in delamination regime No. 2 possesses a higher fracture toughness, a higher interface strength, a higher bending1/2 delamination stress σdelam = 630 MPa and a higher interface fracture toughness (Kc = 12.7 MPa·m , which is the 1/2 maximumstress σdelam value = 630 among MPa and the specimensa higher interface studied). fracture toughness (Kc = 12.7 MPa·m , which is the maximum value among the specimens studied).

Metals 2017, 7, 355 6 of 16

3.2.3. Spraying Regime 3 The pattern of the scratching diagram is similar to that of specimen produced in regime No. 2, while the indentation depth at the end of the profile is 83.3 µm (Figure1b, curve 3). This correlates well with a decrease in the microhardness down to 2.68 GPa. However, this coating has a higher bending 1/2 delamination stress (σdelam = 620 MPa). The interface fracture toughness is moderate (Kc = 4.8 MPa·m ).

3.2.4. Spraying Regime 4 The maximum indentation depth was 140.5 µm (Figure1c, curve 4). Such a penetration depth of the indentor should be attributed to structural heterogeneity conﬁrmed by the scratch proﬁle at distances of 3–5 mm from the onset of scratching. The cracking stress, the bending delamination stress and interface fracture toughness are low.

3.2.5. Spraying Regime 5 The indenter penetrated smoothly into the material without any signiﬁcant jumps. The maximum penetration depth was 50.7 µm (Figure1b, curve 5). The diagram shows several peaks and zones with different rates of indenter penetration, which indicate a heterogeneous structure of the coating. This correlates well with mechanical properties—high values of cracking stresses, bending delamination, interface toughness and microhardness.

3.2.6. Spraying Regime 6 The maximum depth of the indenter penetration was 102.2 µm, while the indentation proﬁle showed oscillations, which indicate a noticeable fraction of elastic recovery (Figure1c, curve 6). The depth of the track increased gradually with the applied load. At a scratching distance of 4 mm, the oscillation frequency increased, which may be attributed to structural heterogeneity at the given depth. This coating possesses the lowest hardness.

3.2.7. Spraying Regime 7 The proﬁle of the scratching diagram was close to linear shape (without pronounced oscillations), while the maximum penetration depth at the end of the track was 103.5 µm (Figure1b, curve 7). This specimen shows the maximum cracking stress among the compositions studied (σcr = 372 MPa). The stress delamination is also very high (σdelam = 800 MPa). The interface fracture toughness is 1/2 slightly higher than that of specimen No. 2 (Kc = 11.6 MPa·m ).

3.2.8. Spraying Regime 8 In terms of the shape of the indentation diagram and parameters of the test, this composition is similar to specimen produced in regime No. 7 (Figure1c, curve 8). However, the bending cracking stress is substantially lower (σcr = 33 MPa).

3.3. Coating Structure

3.3.1. Spraying Regimes No. 1 and No. 2. The analysis of the scanning electron microscope (SEM) images (Figure2) shows that the coating consists of a set of densely packed splats, which is typical of coatings obtained by thermal spraying. SEM and EDS data of coating produced in regime No. 1 allow detecting titanium and its oxides. The formation of oxynitrides was conﬁrmed by EDS; however, due to overlapping of titanium and nitrogen peaks in the EDS, the quantitative analysis is difﬁcult to conduct. The formation of the oxynitrides is related to the presence of nitrogen in the air (carrier gas). Metallic titanium in the micrographs is seen as bright regions against the background of a darker matrix of titanium oxides/oxynitrides. The oxygen content in such regions is ~35%. With the help of Metals 2017, 7, 355 7 of 16

XRD, the phase composition of the coating and the size of the crystallites were determined (Table3). It was found that the nanohardness of the regions corresponding to titanium oxides is 6 times higher thanMetals that of2017 metallic, 7, 355 titanium-rich regions (Figure3, Table3). 7 of 16

(a) (b)

Figure 2. Scanning electron microscope (SEM)-micrographs of the cross section of specimens Figure 2. Scanning electron microscope (SEM)-micrographs of the cross section of specimens produced produced in regime No. 1 (a,b) and No. 2 (c,d). in regime No. 1 (a,b) and No. 2 (c,d). The formation of the oxynitrides is related to the presence of nitrogen in the air (carrier gas). TableMetallic 3. Microstructuretitanium in the characteristicsmicrographs is and seen nanohardness as bright regions of the against coatings the detonation background sprayed of a darker under variousmatrix of regimes. titanium oxides/oxynitrides. The oxygen content in such regions is ~35%. With the help of XRD, the phase composition of the coating and the size of the crystallites were determined (Table 3). It Characteristic Phases was found that the nanohardnessChemical of the regions Composition corresponding to titanium oxidesCrystallite is 6 timesNanohard- higher Regime No. Regions on the Detected by than that of metallic titanium-richDetermined regions (Figure by EDS, 3, at. Table % 3). Size, nm Ness, GPa Cross-Section the XRD

Table 3. MicrostructureBright areas characteristics Ti ~99%, and Onanohardne ~1%ss of the coatings - detonation -sprayed under 2.7 ± 1 1various regimes. TiNxOy 20 Dark areas Ti ~65%, O ~35% TiO 25 16.3 ± 3 Characteristic Phases Regime Chemical Composition Ti2O3 Crystallite30 Nanohard- Regions on the Detected by No. Bright areasDetermined Ti ~92%, by OEDS, ~8% at. % -Size, nm - Ness, 7.8 GPa± 2 Cross-Section the XRD Bright areas Ti ~99%, O ~1% - TiO - 2.7 ± 1 2 Ti2O3 Dark areas Ti ~55%, O ~45% TiNxOy 20 - 9.7 ± 2 1 Ti O Dark areas Ti ~65%, O ~35% TiO3 5 25 16.3 ± 3 TiO2 Ti2O3 30 ± BrightBright areas areas Ti Ti ~92%, ~94%, O O ~8% ~6% - - - - 7.8 3.9 ± 2 0.6 3 TiOTiN 20 Dark areas Ti ~50%, O ~15%, C ~35% 7.7 ± 1.4 2 TiTiN2O3 0.3 20 Dark areas Ti ~55%, O ~45% - 9.7 ± 2 Bright areas Ti ~94%, O ~6%Ti3O -5 - 1.8 ± 0.8 4 TiO2 Dark areas Ti ~70%, O ~5%, C ~25% TiNvCw 20 3.6 ± 1.5 Bright areas Ti ~94%, O ~6% - - 3.9 ± 0.6 ± 3 Bright areas Ti ~93%, O ~7%TiN -20 - 4.6 0.8 5 Dark areas Ti ~50%, O ~15%, C ~35% 7.7 ± 1.4 Dark areas Ti ~70%, O ~10%, C ~20%TiN TiN0.3v Cw 20 20 7.6 ± 1.7 Bright areas Ti ~94%, O ~6% - - 1.8 ± 0.8 4 Bright areas Ti ~97%, O ~3% - - 1.8 ± 0.6 Dark areas Ti ~70%, O ~5%, C ~25% TiNvCw 20 3.6 ± 1.5 TiNvCw 30 6 Bright areas Ti ~93%, O ~7% - TiC - 80 4.6 ± 0.8 5 Dark areas Ti ~60%, O ~15%, C ~25% 6.8 ± 1.6 Dark areas Ti ~70%, O ~10%, C ~20% TiNvTiNCw 20 25 7.6 ± 1.7 Bright areas Ti ~97%, O ~3% TiN - 0.3 - 10 1.8 ± 0.6 TiNvCw 30 6 Dark areas Ti ~60%, O ~15%, C ~25% TiC 80 6.8 ± 1.6 TiN 25

Metals 2017, 7, 355 8 of 16

Table 3. Cont.

Characteristic Phases Chemical Composition Crystallite Nanohard- Regime No. Regions on the Detected by Determined by EDS, at. % Size, nm Ness, GPa Cross-Section the XRD Metals 2017, 7, 355 Bright areas Ti ~95%, O ~5% - - 2.5 8± of0.4 16 7 TiN 35 Dark areas Ti ~64%, O ~30%, C ~6% TiN0.3 10 4.5 ± 1 TiN0.3 25 Bright areas Ti ~95%, O ~5% - - 2.5 ± 0.4 Bright areas Ti ~95%, O ~5% - - 4.6 ± 0.5 7 TiN 35 8 Dark areas Ti ~64%, O ~30%, C ~6% TiN 4.5 ± 1 Dark areas Ti ~77%, O ~15%, C ~8% TiN0.3 25 - 6.8 ± 1.8 Bright areas Ti ~95%, O ~5% TiN - 0.3 - 4.6 ± 0.5 8 TiN Dark areas Ti ~77%, O ~15%, C ~8% - 6.8 ± 1.8 So, detonation spraying under the selected regime allowsTiN depositing0.3 a coating with a composite structure consisting of titanium and its oxides. The coating shows a low porosity and a uniform So, detonation spraying under the selected regime allows depositing a coating with a composite microstructure,structure consisting which can of betitanium attributed and toits aoxides uniform. The phase coating distribution shows a low over porosity the entire andcoating a uniform volume (Figuremicrostructure,2). Coating producedwhich can inbe regimeattributed No. to 2a has unifo a similarrm phase structure; distribution however, over the a largerentire coating number of splat-shapedvolume (Figure inclusions 2). Coating of smaller produced sizes arein regime observed. No. 2 A has noticeable a similar number structure; of however, micropores a larger of round shapenumber are a characteristic of splat-shaped feature inclusions of the of smaller coating. size Thes are oxygen observed. content A noticeable in the oxide-rich number of regionsmicropores is about 45%,of which round is shape higher are thana characteristic in specimen feature produced of the coating. in regime The oxygen No. 1 content (Figure in3, the Table oxide-rich3). In the regions regions consistingis about mostly 45%, which of metallic is higher titanium, than in thespecimen presence produced of oxygen in regime is also No. detected 1 (Figure (less 3, Table than 3). 8%). In the Theregions key consisting mechanical mostly characteristics of metallic titanium, of coating the pr producedesence of oxygen in regime is also No. detected 2 are improved(less than 8%). relative The key mechanical characteristics of coating produced in regime No. 2 are improved relative to to those of coating produced in regime No. 1: the fracture initiation stress is increased by 43%, those of coating produced in regime No. 1: the fracture initiation stress is increased by 43%, delamination stress—by 17%, and the interface fracture toughness—by 22%, (Table2). These changes delamination stress—by 17%, and the interface fracture toughness—by 22%, (Table 2). These changes are associatedare associated with with the the formation formation of of a a high high strength strength titanium oxide-based oxide-based matrix. matrix.

(a) (b)

(e) (f)

Figure 3. Cont.

Metals 2017, 7, 355 9 of 16 Metals 2017, 7, 355 9 of 16

Metals 2017, 7, 355 9 of 16

(g) (h)

Figure 3. Micrographs of the coating cross-section and Energy Dispersive Spectroscopy (EDS) results: Figure 3. Micrographs of the coating cross-section and Energy Dispersive Spectroscopy (EDS) results: specimen produced in regime No. 1 (a–d) and No. 2 (e–h). A circle within red rectangle shows the specimen produced in regime(g) No. 1 (a–d) and No. 2 (e–h). A circle within(h) red rectangle shows the location where EDS analysis was carried out. locationFigure where 3. Micrographs EDS analysis of the was coating carried cross-section out. and Energy Dispersive Spectroscopy (EDS) results: 3.3.2.specimen Spraying produced Regimes in No. regime 3 and No. No. 1 (4a –d) and No. 2 (e–h). A circle within red rectangle shows the 3.3.2. Sprayinglocation Regimes where EDS No. analysis 3 and was No. carried 4 out. A reduction in the oxygen content in the explosive mixture should result in a decrease in the content of the oxides in the coating. The use of nitrogen as a carrier gas can lead the formation of A3.3.2. reduction Spraying in Regimes the oxygen No. 3 and content No. 4 in the explosive mixture should result in a decrease in the contentnitrides. of the oxides in the coating. The use of nitrogen as a carrier gas can lead the formation FromA reduction the SEM in micrographs the oxygen contentof the specimen’s in the explosiv crosse section mixture (Figure should 4), result it can inbe a seen decrease that during in the of nitrides. thecontent grinding/polishing of the oxides in process the coating. some Theparticles use of detac nitrogenhed from as a thecarrier surface. gas can This lead indicates the formation a reduced of From the SEM micrographs of the specimen’s cross section (Figure4), it can be seen that during cohesivenitrides. strength in comparison with specimens produced in regime No. 1 and No. 2. It is also clear the grinding/polishingthat titaniumFrom the particles SEM micrographs process have retained some of the a particles predominantly specimen’s detached cross globular section from shape (Figure the surface.after 4), detonation it can This be indicatesseen spraying that during a(their reduced cohesiveimpactthe grinding/polishing strength energy in during comparison sprayingprocesswith some was specimens particlesnot high). detac producedThished result from inis the regimemost surface. likely No. Thisrelated 1 and indicates No.to a2. substantial a It reduced is also clear that titaniumdecreasecohesive strengthin particles the spraying in have comparison retaineddistance. with A a predominantlyhigher specimens oxygen produced amount globular in in regime shapethe explosive No. after 1 and detonation mixture No. 2. Itresults is spraying also in clear the (their impactformationthat energy titanium duringof aparticles coating spraying havewith retaineda was higher not a strength high).predominantly This (Figure result globular4a,b). is most This shape likelycan afteralso related bedetonation the to reason a substantial spraying for a higher (their decrease in theadhesionimpact spraying energy strength distance. during of the Aspraying coating. higher oxygenwasAs is not seen amounthigh). from This the in Figure theresult explosive 4c,d,is most a greater likely mixture numberrelated results toof adark in substantial the regions formation of a coatingaredecrease observed with in the in a coatingspraying higher produced strength distance. in (FigureA regime higher4 a,b).No. oxygen 4, This which amount can correspond also in the be explosive the to titanium reason mixture forcarbonitrides. a higherresults in adhesion the formation of a coating with a higher strength (Figure 4a,b). This can also be the reason for a higher strength of the coating. As is seen from the Figure4c,d, a greater number of dark regions are observed adhesion strength of the coating. As is seen from the Figure 4c,d, a greater number of dark regions in coatingare observed produced in coating in regime produced No. 4, in which regime correspond No. 4, which to correspond titanium carbonitrides.to titanium carbonitrides.

(a) (b)

Figure 4. SEM-micrographs of the coating cross-section of specimens produced in regime No. 3 (a,b) and No. 4 (c,d). (c) (d)

AFigure high 4. porosity SEM-micrographs is a characteristic of the coating feature cross-sectio of specimenn of specimens produced produced in regime in No.regime 3 (Figure No. 3 ( a5).,b) Thin Figure 4. SEM-micrographs of the coating cross-section of specimens produced in regime No. 3 (a,b) layersand are No. located 4 (c,d ).along the boundaries of the titanium particles. The oxygen content in these layers and No. 4 (c,d). does not exceed 15% (Table 3), which is significantly lower than the oxygen content in specimens A high porosity is a characteristic feature of specimen produced in regime No. 3 (Figure 5). Thin A layers high are porosity located isalong a characteristic the boundaries feature of the of titani specimenum particles. produced The oxygen in regime content No. in 3 these (Figure layers5). Thin layersdoes are not located exceed along 15% the(Table boundaries 3), which ofis thesignifican titaniumtly lower particles. than the The oxygen oxygen content content in inspecimens these layers does not exceed 15% (Table3), which is signiﬁcantly lower than the oxygen content in specimens

Metals 2017, 7, 355 10 of 16 Metals 2017, 7, 355 10 of 16 produced in regime No. 1 and No. 2. 2. In In large large pores pores of of the the coating, a a higher content of carbon is detected than in small pores (up to 35%). With With the he helplp of XRD, XRD, it it was was found found that that for for both both specimens, specimens, titanium nitride prevails as the reaction product.product. Coating produced inin regime No.No. 4 is denser than one produced in regime No. 3. A higher higher content of C2HH22 inin the the explosive explosive mixture mixture has has led led to to an an increase increase in in the carbon content in the coating, as is evidenced from the EDS data (Table 3 3).). TheThe carboncarbon contentcontent alongalong thethe titaniumtitanium particleparticle boundariesboundaries isis ~25%,~25%, reaching 45% in large pores.

(a) (b)

(e) (f)

(g) (h)

Figure 5. Micrographs of the coating cross-section and EDS results: specimen produced in regime No. Figure 5. Micrographs of the coating cross-section and EDS results: specimen produced in regime 3 (a–d) and No. 4 (e–h). A circle within red rectangle shows the location where EDS analysis was No. 3 (a–d) and No. 4 (e–h). A circle within red rectangle shows the location where EDS analysis was carried out.

Metals 2017, 7, 355 11 of 16 Metals 2017, 7, 355 11 of 16 Metals 2017, 7, 355 11 of 16 3.3.3. Spraying Regimes No. 5 and No. 6 6 3.3.3. Spraying Regimes No. 5 and No. 6 For specimens produced produced in in regime regime No. No. 5 5and and No. No. 6, the 6, the spraying spraying distance distance was was 10 mm 10 mmand and100 For specimens produced in regime No. 5 and No. 6, the spraying distance was 10 mm and 100 100mm, mm, respectively. respectively. According According to the SEM to the data, SEM these data, coatings these coatingshave a fine-grained have a ﬁne-grained structure (Figure structure 6), mm, respectively. According to the SEM data, these coatings have a fine-grained structure (Figure 6), (Figurein which6), titanium in which particles titanium are particles surrounded are surrounded by ceramic by grains. ceramic grains. in which titanium particles are surrounded by ceramic grains. According to the EDS of specimenspecimen producedproduced inin regimeregime No.No. 5, the oxygen content outside of According to the EDS of specimen produced in regime No. 5, the oxygen content outside of unreacted titanium particles phases does not exceed 10%, 10%, while while the the content content of of carbon carbon is is about about 20%. 20%. unreacted titanium particles phases does not exceed 10%, while the content of carbon is about 20%. In the the similar similar regions regions of of the the microstructure microstructure of spec of specimenimen produced produced in regime in regime No. 6, No. the oxygen 6, the oxygencontent In the similar regions of the microstructure of specimen produced in regime No. 6, the oxygen content contentis about 15%, is about while 15%, the whilecarbon the content carbon is 25% content (Figure is 25% 7, Table (Figure 3). It7, can Table be3 assumed). It can that be assumed when a longer that is about 15%, while the carbon content is 25% (Figure 7, Table 3). It can be assumed that when a longer whenspraying a longer distance spraying is used, distance a greater is used,transformation a greater transformationdegree of titanium degree is achieved. of titanium Therefore, is achieved. the spraying distance is used, a greater transformation degree of titanium is achieved. Therefore, the Therefore,interphase theboundaries interphase appear boundaries more appeardistinct, more while distinct, the oxygen while theandoxygen carbon andcontents carbon in contents specimen in interphase boundaries appear more distinct, while the oxygen and carbon contents in specimen specimenproduced producedin regime in No. regime 6 are No. higher 6 are (Figure higher (Figure6d). Using6d). a Using shorter a shorter spraying spraying distance, distance, a coating a coating with produced in regime No. 6 are higher (Figure 6d). Using a shorter spraying distance, a coating with withincreased increased values values of nano- of nano- and microhardness and microhardness was wasobtained obtained (specimen (specimen No. No.5, Tables 5, Tables 2 and2 and 3). 3As). increased values of nano- and microhardness was obtained (specimen No. 5, Tables 2 and 3). As Ascompared compared to specimens to specimens produced produced in regime in regime No. No.3 and 3 No. and 4, No. the 4, nanohardness the nanohardness value valueis almost is almost twice compared to specimens produced in regime No. 3 and No. 4, the nanohardness value is almost twice twiceas high as (Table high (Table 3). 3). as high (Table 3).

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

(c) (d) (c) (d) Figure 6. SEM-micrographs taken at the cross section of the specimens produced in regime No. 5 (a,b) Figure 6. SEM-micrographsSEM-micrographs taken at the cross section of the specimens produced in regime No. 5 ( (aa,,b)) and No. 6 (c,d). and No. 6 ( c,d).

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

Figure 7. Cont.

Metals 2017, 7, 355 12 of 16 Metals 2017, 7, 355 12 of 16

(e) (f)

(g) (h)

Figure 7. SEM-micrographs of the coating cross-section and EDS results: specimen produced in Figure 7. SEM-micrographs of the coating cross-section and EDS results: specimen produced in regime regime No. 5 (a–d) and No. 6 (e–h). A circle within red rectangle shows the location where EDS No. 5 (a–d) and No. 6 (e–h). A circle within red rectangle shows the location where EDS analysis was analysis was carried out carried out From comparison of the mechanical properties of the specimens, the following conclusions can be Fromdrawn. comparison The thickness of the of the mechanical coating in properties specimen ofproduced the specimens, in regime the No. following 6 was 4 conclusionstimes greater can bethan drawn. that Thein the thickness rest of the of thespecimens), coating inwhich specimen significantly produced affected in regime the crack No. resistance 6 was 4 times measured greater thanduring that inbending the rest and of thescratching specimens), (σcr and which Kc). signiﬁcantlyNevertheless, affected the bending the crack cracking resistance resistance measured of duringspecimen bending produced and scratching in regime ( σNo.cr and 5 is Khigherc). Nevertheless, than that of thespecimen bending produced cracking in resistance regime No. of specimen6. This producedcan be attributed in regime to No. more 5 isa uniform higher than structure that ofof specimen produced produced in in regime regime No. No. 5. 6.At Thisthe same can be attributedtime, the tobending more adelamination uniform structure stress of of coating specimen produced produced in regime in regime No. No.5 (Table 5. At2) theis noticeably same time, thehigher bending than delamination the values observed stress ofin coatingcoatings producedproduced in regime regime No. No. 3 5and (Table No. 24.) is noticeably higher than the values observed in coatings produced in regime No. 3 and No. 4. 3.3.4. Spraying Regimes No. 7 and No. 8 3.3.4. SprayingThe spraying Regimes distance No. was 7 and 10 No.mm 8for specimen produced in regime No. 7 while it was 100 mm forThe specimen spraying produced distance in regime was 10 No. mm 8. for Additionally, specimen produced nitrogen was in regime added No.to the 7 whileO2/C2H it2 wasmixture 100 in mm an amount of 33% by volume. It was assumed that the introduction of nitrogen into the explosive for specimen produced in regime No. 8. Additionally, nitrogen was added to the O2/C2H2 mixture inmixture an amount would of 33%favor by the volume. formation It was of nitrides assumed in the that coatings. the introduction of nitrogen into the explosive As seen in Figure 8, the coating structure is rather uniform and is represented by metallic mixture would favor the formation of nitrides in the coatings. titanium and ceramic phases. A number of small microcracks and round pores can be observed. An As seen in Figure8, the coating structure is rather uniform and is represented by metallic titanium increase in the spraying distance for specimen produced in regime No. 8 has led to the formation of and ceramic phases. A number of small microcracks and round pores can be observed. An increase in pronounced interphase boundaries due to a higher chemical transformation degree of titanium. The the spraying distance for specimen produced in regime No. 8 has led to the formation of pronounced EDS data for specimen produced in regime No. 7 show that the carbon content in the coating is less interphasethan 6%, while boundaries the oxygen due content to a higher is about chemical 30% (Figure transformation 9, Table 3). degree of titanium. The EDS data for specimen produced in regime No. 7 show that the carbon content in the coating is less than 6%, while the oxygen content is about 30% (Figure9, Table3).

Metals 2017, 7, 355 13 of 16 Metals 2017, 7, 355 13 of 16 Metals 2017, 7, 355 13 of 16

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

(c) (d) (c) (d) FigureFigure 8. 8.EM SEM micrographs micrographs of of thethe cross sections sections of of specimens specimens produced produced in regime in regime No. 7 No. (a,b) 7 and (a,b No.) and Figure8 (c,d). 8. SEM micrographs of the cross sections of specimens produced in regime No. 7 (a,b) and No. No. 8 (c,d). 8 (c,d).

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

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

Figure 9. Cont.

Metals 2017, 7, 355 14 of 16 Metals 2017, 7, 355 14 of 16

(g) (h)

Figure 9. Micrographs of the coating cross-section and EDS results: specimen produced in regime No. Figure 9. Micrographs of the coating cross-section and EDS results: specimen produced in regime 7 (a–d) and No. 8 (e–h). A circle within red rectangle shows the location where EDS analysis was No. 7 (a–d) and No. 8 (e–h). A circle within red rectangle shows the location where EDS analysis was carried out. carried out. In specimen produced in regime No. 8, the oxygen content was lower (15%), while the content of carbonIn specimen did not produced change substantially. in regime No. This 8, could the oxygen be the reason content for was a decrease lower (15%), in the whilemicrohardness. the content ofThe carbon nanohardness did not change of specimens substantially. produced This in could regime be No the. 7 reason and No. for 8 ahas, decrease on the contrary, in the microhardness. increased. TheIn nanohardnessboth specimens, of titanium specimens nitrides produced were found in regime to dominate No. 7 and in the No. dark 8 has, regions. on the contrary, increased. In bothThe specimens, bending titaniumdelamination nitrides stress were for foundspecimen to dominateproduced inin theregime dark No. regions. 7 is higher than that of specimenThe bending No. 8 by delamination 30%, while the stress cracking for specimen stress σcr produced increases inby regimea factor No.of ~10. 7 is At higher the same than time, that of specimenthe interface No. 8fracture by 30%, toughness while the K crackingc is increased stress byσ 40%cr increases (Table 2). by The a factor introduction of ~10. of At nitrogen the same into time, the explosive mixture has increased the amount of titanium nitrides. This appears to be the reason the interface fracture toughness Kc is increased by 40% (Table2). The introduction of nitrogen into the explosivefor the improvement mixture has increased of the mechanical the amount properties of titanium relative nitrides. to specimens This appears produced to be in the regime reason No. for 5 the improvementand No. 6. Specimen of the mechanical No. 7 possesses properties the relativemaximu tom specimenscracking resistance produced among in regime the No.compositions 5 and No. 6. studied. Specimen No. 7 possesses the maximum cracking resistance among the compositions studied.

4.4. Conclusions Conclusions The structure and mechanical properties of the coatings deposited by reactive detonation The structure and mechanical properties of the coatings deposited by reactive detonation spraying spraying of a titanium powder were studied for three characteristic compositions of the explosive of a titanium powder were studied for three characteristic compositions of the explosive mixture mixture and simultaneous variation of other deposition parameters. The composition and and simultaneous variation of other deposition parameters. The composition and nanohardness of nanohardness of the individual phases of the coatings as well as the effect of the spraying regimes on the individual phases of the coatings as well as the effect of the spraying regimes on the mechanical the mechanical properties of the coatings were investigated. For the three characteristic modes of the propertiescoating formation of the coatings (the werecomposition investigated. of the For explosive the three mixture characteristic and the modes carrier of gas), the coating the following formation (therecommendations composition of were the formulated: explosive mixture and the carrier gas), the following recommendations were formulated: 1. In case of the spraying with air as a carrier gas, no nitrogen added into the explosive mixture, an 1. Inincrease case of thein the spraying O2/C2H with2 from air 1.1 as and a carrier 2.5 leads gas, to no an nitrogen increase added in the into cracking the explosive resistance mixture, and ancoating increase delamination in the O2/C 2stressH2 from under 1.1 andthree-point 2.5 leads bending, to an increase as well in theas the cracking interface resistance fracture and coatingtoughness delamination under the stressscratch under test due three-point to formation bending, of a hierarchically as well as the organized interface coating fracture structure toughness underwith theclearly scratch exhibited test due phase to formationboundaries. of Deposition a hierarchically of titanium organized at O coating2/C2H2 = structure 2.5 allows with obtaining coatings with a high fracture toughness, a high adhesion strength, a high clearly exhibited phase boundaries. Deposition of titanium at O2/C2H2 = 2.5 allows obtaining coatingsmicrohardness with a highand a fracture low friction toughness, coefficient. a high adhesion strength, a high microhardness and a 2. In the event of spraying with nitrogen as a carrier gas, no nitrogen added into the explosive low friction coefficient. mixture, a short spraying distance (10 mm) and a low oxygen content in the explosive mixture 2. In the event of spraying with nitrogen as a carrier gas, no nitrogen added into the explosive (O2/C2H2 = 0.7) makes it possible to form a heterogeneous structure of the coating. This ensures mixture,mechanical a short properties spraying comparable distance (10to mm)those andof the a low coatings, oxygen in contentwhich the in theoxide explosive phases mixturewere (Opredominantly2/C2H2 = 0.7) makesformed. it The possible coatings to form show a heterogeneousa moderate crack structure resistance, of the a moderate coating. This adhesion ensures mechanicalstrength, a propertieshigh microhardness comparable (H100 to= 2.72 those GPa) of and the a coatings, high friction in which coefficient the oxide(μ0 ~0.58). phases were 3. predominantlyIn case of the formed.spraying Thewith coatings nitrogen show as a acarrier moderate gas and crack nitrogen resistance, added a moderate to the explosive adhesion strength,mixture, a the high formation microhardness of a complex (H100 heterogeneous= 2.72 GPa) and structure a high makes friction it possible coefficient to achieve (µ0 ~0.58). a high 3. Incrack case resistance of the spraying under with three-point nitrogen bending, as a carrier a high gas interface and nitrogen fracture added toughness to the under explosive the scratch mixture, thetest formation and a high of microhardness a complex heterogeneous (H100 = 2.74 GPa). structure The makesfriction itcoefficient possible toof achievethe coating a high was crackμ0 resistance~0.68. under three-point bending, a high interface fracture toughness under the scratch test and a high microhardness (H100 = 2.74 GPa). The friction coefficient of the coating was µ0 ~0.68.

Metals 2017, 7, 355 15 of 16

Based on the results obtained, two detonation spraying regimes of titanium powders can be recommended for deeper R&D and practical applications. The ﬁrst ensures strengthening of the coating by titanium oxides (use of the air as a carrier gas and high oxygen content in the explosive mixture). A more advanced deposition alternative produces strengthening by titanium carbonitrides with blurred interphase boundaries (short spraying distances, large explosive charges with nitrogen as a carrier gas and addition of N2 into the explosive mixture). The experimental data on the possibilities of obtaining detonation coatings containing titanium nitrides and oxides and having different structural characteristics can be used in the development of coatings for biomedical and catalytic applications, respectively.

Acknowledgments: The work was partially supported by Cheung Kong Scholar of Jilin University, and performed with a partial support by Grant No. 11.2 of the Russian Academy of Sciences (Department of Power Engineering, Mechanical Engineering, Mechanics, and Control Processes). The authors are grateful to “Nanotech” Shared Use Center of ISPMS SB RAS for the assistance in running fractographic investigations. The nanoindentation tests were carried out at Tomsk Polytechnic University within the framework of Tomsk Polytechnic University Competitiveness Enhancement Program grant. Author Contributions: Igor Batraev performed detonation spraying, Ilya Vlasov, Roman Stankevich and Dina Dudina carried out experimental studies of the coatings, Sergei Panin, Ilya Vlasov, Dina Dudina and Vladimir Ulianitsky analyzed the data; Sergei Panin, Ilya Vlasov, Dina Dudina, Vladimir Ulianitsky, Filippo Berto wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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