metals 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 Roman Stankevich 3 , Igor Batraev 4 and Filippo Berto 6 1 2 3 4 5 6
*
ID
, Vladimir Ulianitsky 4 ,
College of Physics, Jilin University, Changchun 130022, China Institute of Strength Physics and Materials Science SB RAS, Tomsk 634055, Russia;
[email protected] Department of Materials Science in Mechanical Engineering, National Research Tomsk Polytechnic University, Tomsk 634050, Russia;
[email protected] 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.) Department of Mechanical Engineering and Technologies, Novosibirsk State Technical University, Novosibirsk 630073, Russia Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway;
[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 /C2 H2 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 /C2 H2 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
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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.
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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 /C2 H2 molar ratios of 0.7, 1.1 and 2.5 (Table 1). 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 /C2 H2 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 Regime
O2 /C2 H2
Spraying Distance, mm
Explosive Charge, %
Carrier Gas
Coating Thickness (hcoat ), µm
1 2 3 4 5 6 7 8
1.1 2.5 1.1 0.7 0.7 0.7 1.1 + added 33 vol % N2 1.1 + added 33 vol % N2
100 100 10 10 10 100 10 100
30 30 40 40 50 50 60 60
air air nitrogen nitrogen nitrogen nitrogen nitrogen nitrogen
270 275 160 291 190 750 235 285
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]: σ=
Mmax , W
(1)
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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 Mmax = , (2) 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). W=
b × h2 . 6
(3)
The shear stress was calculated by Equation (4) [26]: τ=
hcoat × σdelam , Lc
(4)
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]: Kc = τ × (π × Lc )1/2 .
(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 (Figure 1a, Table 2) 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.
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(a)
(b)
(c)
Figure 1. 1. (a) (a) three-point three-pointbending bendingdiagram diagramfor forcoated coatedtitanium titaniumspecimens; specimens; (b,c) scratching diagrams Figure (b,c) scratching diagrams of of the coatings. the coatings. Table 2. 2. Mechanical Mechanical properties properties of of the the coatings coatings and and coated coated specimens specimens measured measured by by scratch scratch tests tests and and Table 3-point bending. 3-point bending. σ2.5,
1 2 3 4 5 6 7 8
σcr,
σdelam
Kc,
τ, MPa σ , σ , σ , Kc , τ, No. MPa 2.5 MPa cr MPadelam MPa·m1/2 1/2 MPa MPa MPa MPa·m MPa
No.
1 2 3 4 5 6 7 8
955 98.4 98.4 850 955 140.5 850 140.5 825 825 87.987.9 10551055 44.944.9 945 945 94.294.2 870 870 10 10 945 372.1 945 875 372.1 33.5 875 33.5
540 630 540 630 620 620 70 70 730 730 650 650 800 800 560 560
10.4 12.710.4 12.7 4.8 4.8 0.9 0.9 7.6 7.6 - 11.6 11.66.8 6.8
237.3 237.3 295.3 295.3 73.1 73.1 10.9 10.9 132.9 132.9 -228.7 228.7 93.1 93.1
Hμ, GPa
H µ , GPa
3.70 ± 0.03
3.70 ± 0.03 3.84 ± 0.07 3.84 ± 0.07 2.68 0.07 2.68 ±±0.07 2.45 ± 0.11 2.45 ± 0.11 2.72 2.72±±0.09 0.09 2.02 2.02±±0.01 0.01 2.74 ± 0.09 2.74 ± 0.09 2.29 ± 0.02
2.29 ± 0.02
Friction Friction μ0 Coefficient, Coefficient, µ0 0.406 0.406 0.412 0.412 0.637 0.637 0.661 0.661 0.580 0.580 0.594 0.594 0.675 0.675 0.585 0.585
Maximum Depth Maximum Depthμm of the Scratch, of the Scratch, µm
66.6
66.6 63.4 63.4 83.3 83.3 140.5 140.5 50.7 50.7 102.2 102.2 103.5 103.5 80.9
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 (Figure 1, Table 2). 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 (Figure 1b, 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 bending delamination stress (σdelam = 540 MPa) and a high interface fracture toughness (Kc = 10.4 MPa·m1/2 ). (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 With increasing load,2).the depth the pyramid duringa the scratch test changed linearly (Figure 1c, curve Atpenetration the end of the test,ofthe indenter reached depth of 63.4 µm, which almost linearly (Figure 1c, curve 2). At the end of the test, the indenter reached a depth of 63.4 μm, is close to the value observed during testing of specimen produced in regime No. 1. This agrees whicha is close microhardness to the value observed duringproduced testing ofin specimen produced in regimetoNo. 1. This agrees with higher of specimen regime No. 2 as compared specimen No. 1. with a higher microhardness of specimen produced in regime No. 2 as compared to specimen In addition, compared with specimen produced in regime No. 1, specimen produced in regimeNo. No.1. 2 In addition, compared with toughness, specimen produced regime strength, No. 1, specimen regime No. possesses a higher fracture a higher in interface a higherproduced bending in delamination 2 possesses fracture a higher interface strength, bending stress σdelama=higher 630 MPa and atoughness, higher interface fracture toughness (Kca=higher 12.7 MPa ·m1/2 ,delamination which is the stress σ delam = 630 MPa and a higher interface fracture toughness (Kc = 12.7 MPa·m1/2, which is the maximum value among the specimens studied). maximum value among the specimens studied).
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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 (Figure 1b, curve 3). This correlates well with a decrease in the microhardness down to 2.68 GPa. However, this coating has a higher bending delamination stress (σdelam = 620 MPa). The interface fracture toughness is moderate (Kc = 4.8 MPa·m1/2 ). 3.2.4. Spraying Regime 4 The maximum indentation depth was 140.5 µm (Figure 1c, 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 (Figure 1b, 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 (Figure 1c, 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 (Figure 1b, 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 slightly higher than that of specimen No. 2 (Kc = 11.6 MPa·m1/2 ). 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 (Figure 1c, 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 (Figure 2) 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
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XRD, the phase composition of the coating and the size of the crystallites were determined (Table 3). It was found that the nanohardness of the regions corresponding to titanium oxides is 6 times higher than Metals that of metallic titanium-rich regions (Figure 3, Table 3). 2017, 7, 355 7 of 16
(a)
(b)
(c)
(d)
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). Table 3. Microstructure nanohardness of theagainst coatings detonation sprayed under Metallic titanium in thecharacteristics micrographs isand seen as bright regions the background of a darker various regimes. matrix of 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 was found that the nanohardness of the regions correspondingPhases to titanium oxides is 6 times higher Chemical Composition Crystallite NanohardRegime No. Regions on the Detected by than that of metallic titanium-rich regions (Figure 3). Determined by EDS,3, at.Table % Size, nm Ness, GPa Cross-Section
the XRD
Bright areas characteristics Ti ~99%, ~1% 2.7 ± 1 Table 3. Microstructure and O nanohardness of the coatings detonation-sprayed under TiNx Oy 20 1various regimes. TiO 25 Ti ~65%, O ~35% 16.3 ± 3 Dark areas Phases Characteristic Ti O 30 2 3 Crystallite NanohardChemical Composition Regime Detected by Regions on the Bright areas Ti ~92%, ~8%at. % ±2 Size, nm Ness,7.8 GPa Determined by O EDS, No. the XRD Cross-Section Bright areas Ti ~99%, O ~1% -TiO 2.7 ± 1 2 Ti2 O3 xO y 20 TiN Ti ~55%, O ~45% 9.7 ± 2 Dark areas 1 Ti O 16.3 ± 3 Dark areas Ti ~65%, O ~35% TiO3 5 25 TiO2 30 Ti2O3 Bright areas ~94%,OO~8% ~6% 3.9± ± Bright areas TiTi~92%, - - 7.8 2 0.6 3 TiN 20 TiO Ti ~50%, O ~15%, C ~35% 7.7 ± 1.4 Dark areas 20 2 TiTiN 2O3 0.3 Dark areas Ti ~55%, O ~45% 9.7 ± 2 Ti3O-5 Bright areas Ti ~94%, O ~6% 1.8 ± 0.8 4 TiO2 Dark areas Ti ~70%, O ~5%, C ~25% TiNv Cw 20 3.6 ± 1.5 Bright areas Ti ~94%, O ~6% 3.9 ± 0.6 Bright areas Ti ~93%, O ~7% 4.6 ± 0.8 TiN 20 53 Dark areas Ti ~50%, O ~15%, C ~35% 7.7 ± 1.4 Dark areas Ti ~70%, O ~10%, C ~20% TiN0.3v Cw 7.6 ± 1.7 TiN 20 20 Bright areas Ti ~94%, O ~6% 1.8 ± 0.8 Bright areas Ti ~97%, O ~3% 1.8 ± 0.6 4 Dark areas Ti ~70%, O ~5%, C ~25% TiNvCw 20 3.6 ± 1.5 TiNv Cw 30 Bright areas Ti ~93%, O ~7% - TiC - 80 4.6 ± 0.8 65 ~60%,OO~10%, ~15%,CC~20% ~25% ± 1.6 Dark areas Dark areas TiTi~70%, TiNvTiN Cw 20 25 7.66.8 ± 1.7 Bright areas Ti ~97%, O ~3% 1.8 ± 0.6 TiN0.3 10 30 TiNvCw 6 6.8 ± 1.6 Dark areas Ti ~60%, O ~15%, C ~25% TiC 80 TiN 25
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Table 3. Cont.
Regime No.
Characteristic Regions on the Cross-Section
Metals 2017, 7, 355 Bright areas
7
7 8 8
Chemical Composition Determined by EDS, at. %
Phases Detected by the XRD
Crystallite Size, nm
NanohardNess, GPa
Ti ~95%, O ~5%
-
-
16 2.5 8±of0.4
Dark areas
Ti ~64%, O ~30%, C ~6%
Bright areas Bright areas Dark areas Dark areas Bright areas
Ti ~95%, O ~5% Ti ~95%, O ~5% Ti ~64%, O ~30%, C ~6% Ti ~77%, O ~15%, C ~8% Ti ~95%, O ~5%
Dark areas
Ti ~77%, O ~15%, C ~8%
TiN TiN 0.3 TiN 0.3 TiN TiNTiN 0.3 TiN - 0.3 TiN TiN0.3
35 10 25 35 25 -
-
4.5 ± 1
2.5 ± 0.4 4.6 ± 0.5 4.5 ± 1 6.8 ± 1.8 4.6 ± 0.5 6.8 ± 1.8
So, detonation spraying under the selected regime allows depositing 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, which can attributed uniform distribution over the entire volume structure consisting of be titanium and to its aoxides. Thephase coating shows a low porosity andcoating a uniform (Figure 2). Coating which produced in regime No. a similar however, largercoating number of microstructure, can be attributed to 2a has uniform phasestructure; distribution over thea entire splat-shaped inclusions of smaller sizes are observed. noticeable ofhowever, micropores of round volume (Figure 2). Coating produced in regime No. 2Ahas a similarnumber structure; a larger shapenumber are a characteristic feature of the coating.sizes Theare oxygen content in the oxide-rich is about of splat-shaped inclusions of smaller observed. A noticeable number ofregions micropores of round shape are a characteristic feature of the coating. The oxygen content in the oxide-rich regions 45%, which is higher than in specimen produced in regime No. 1 (Figure 3, Table 3). In the regions is about 45%, which is higher than in the specimen produced in regime No. 1 (Figure 3, Table In the consisting mostly of metallic titanium, presence of oxygen is also detected (less than3). 8%). regions consisting mostly of metallic titanium, the presence of oxygen is also detected (less than The key mechanical characteristics of coating produced in regime No. 2 are improved8%). 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%, (Table 2). These changes delamination stress—by 17%, and the interface fracture toughness—by 22%, (Table 2). These changes are associated withwith the the formation ofofa ahigh oxide-based matrix. are associated formation highstrength strength titanium titanium oxide-based matrix.
(a)
(b)
(c)
(d)
(e)
(f)
Figure 3. Cont.
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(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 (g) (h) red rectangle shows the specimen produced in regime No. 1 (a–d) and No. 2 (e–h). A circle within location where EDS analysis was carried out. location where EDS analysis was carried out. Figure 3. Micrographs of the coating cross-section and Energy Dispersive Spectroscopy (EDS) results: regime No. 1 (a–d) and No. 2 (e–h). A circle within red rectangle shows the 3.3.2.specimen Sprayingproduced RegimesinNo. 3 and No. 4 locationRegimes where EDSNo. analysis was carried 3.3.2. Spraying 3 and No. 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 A reduction the oxygen content 3.3.2. SprayinginRegimes No. 3 and No. 4in the explosive mixture should result in a decrease in the nitrides. content of the oxides in the coating. The use of nitrogen as a carrier gas can lead the formation A reduction inmicrographs the oxygen content in the explosive mixture should decrease in the From the SEM of the specimen’s cross section (Figure 4),result it can in beaseen that during of nitrides. content of the oxides in the coating. The use of nitrogen as a carrier gas can lead the formation of the grinding/polishing process some particles detached from the surface. This indicates a reduced From the SEM micrographs of the specimen’s cross section (Figure 4), it can be seen that during nitrides. cohesive strength in comparison with specimens produced in regime No. 1 and No. 2. It is also clear the grinding/polishing process some detached fromshape the surface. This a(their reduced From theparticles SEM micrographs of the specimen’s cross section (Figure 4),detonation it can be indicates seen that during that titanium have retained aparticles predominantly globular after spraying cohesive strength in comparison with specimens produced in regime No. 1 and No. 2. It is also the grinding/polishing process some particles detached from the surface. This indicates a reduced impact energy during spraying was not high). This result is most likely related to a substantial clear cohesive strength inhave comparison with specimens produced inin regime No. 1 and No. 2. It isspraying alsoinclear that titanium particles retained a predominantly globular shape after detonation decrease in the spraying distance. A higher oxygen amount the explosive mixture results the(their that titanium particles have retained a predominantly globular shape after detonation spraying (their impact energy during spraying not strength high). This result is most related a substantial decrease formation of a coating with awas higher (Figure 4a,b). This likely can also be thetoreason for a higher impact energy during was high). result is most likely related to strength of the coating. As isnot seen fromThis the 4c,d, a greater number of adark in theadhesion spraying distance. Aspraying higher oxygen amount in Figure the explosive mixture results insubstantial theregions formation decrease in the sprayingproduced distance.inAregime higherNo. oxygen amount in the explosive mixture the are observed ina coating 4, This which correspond to titanium carbonitrides. of a coating with higher strength (Figure 4a,b). can also be the reason for a results higherinadhesion 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 Figure 4c,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 coating produced in regime No. 4, which correspond to titanium carbonitrides. are observed in coating produced in regime No. 4, which correspond to titanium carbonitrides.
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(b)
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(c)
(d)
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) Figure SEM-micrographs of the coating cross-section of specimens in No. regime No. 3 (a,b) A high4.porosity is a characteristic feature of specimen producedproduced in regime 3 (Figure 5). Thin Figure 4. SEM-micrographs of the coating cross-section of specimens produced in regime No. 3 (a,b) 4 (c,d).along the boundaries of the titanium particles. The oxygen content in these layers layersand areNo. located 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 layers located is along the boundaries of theof titanium particles. The oxygen content layers A highare porosity a characteristic feature specimen produced in regime No.in3 these (Figure 5). Thin exceed 15% the (Table 3), which of is the significantly than the content in in specimens layersdoes are not located along boundaries titaniumlower particles. Theoxygen oxygen content these layers
does not exceed 15% (Table 3), which is significantly lower than the oxygen content in specimens
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produced and No. 2. In In large large pores pores of of the the coating, coating, aa higher higher content content of of carbon carbon is is produced in in regime regime No. No. 11 and No. 2. detected than in small pores (up to 35%). With the help of XRD, it was found that for both specimens, detected than in small pores (up to 35%). With the help of XRD, it was found that for both specimens, titanium nitride titanium nitride prevails prevails as as the the reaction reaction product. product. Coating produced in regime denser than A higher higher content content Coating produced in regime No. No. 44 is is denser than one one produced produced in in regime regime No. No. 3. 3. A of C H in the explosive mixture has led to an increase in the carbon content in the coating, as is is of C22H22 in the explosive mixture has led to an increase in the carbon content in the coating, as evidenced from from the the EDS (Table 3). boundaries is is evidenced EDS data data (Table 3). The The carbon carbon content content along along the the titanium titanium particle particle boundaries ~25%, reaching 45% in large pores. ~25%, reaching 45% in large pores.
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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 carried out. out.
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3.3.3. Spraying Regimes Regimes No. No. 55 and and No. No. 66 3.3.3. Spraying 3.3.3. Spraying Regimes No. 5 and No. 6 For specimens produced produced in inregime regimeNo. No.5 5and and No. 6, the spraying distance 10 mm For specimens No. 6, the spraying distance waswas 10 mm and and 100 For specimens produced in regime No. 5 and No.these 6, thecoatings sprayinghave distance was 10 mmstructure and 100 100 mm, respectively. According to the SEM data, a fine-grained mm, respectively. According to the SEM data, these coatings have a fine-grained structure (Figure 6), mm, respectively. According to the SEM these coatings havegrains. a fine-grained structure (Figure 6), (Figure in which titaniumare particles aredata, surrounded ceramic in which6),titanium particles surrounded by ceramicby grains. in which titanium particles are surrounded by ceramic grains. According to the 5, the According to the EDS EDS of of specimen specimen produced produced in in regime regime No. No. 5, the oxygen oxygen content content outside outside of of According to the EDS ofphases specimen produced in10%, regime No.the 5, content the oxygen content outside of unreacted titanium particles does not exceed while of carbon is about 20%. unreacted titanium particles phases does not exceed 10%, while the content of carbon is about 20%. unreacted titanium particles phases does not exceed 10%, while the content of carbon6,is the about 20%. In thesimilar similar regions microstructure of specimen produced in regime In the regions ofof thethe microstructure of specimen produced in regime No. 6,No. the oxygen oxygen content In the similar regions of the microstructure of specimen produced in regime No. 6, the oxygen content content is about 15%, while the carbon content is 25% (Figure 7, Table 3). It can be assumed that is about 15%, while the carbon content is 25% (Figure 7, Table 3). It can be assumed that when a longer is about 15%, while the carbon content is 25% (Figure 7, Table 3). It candegree be assumed that when a longer when a longer spraying distance is used, a greater transformation of titanium is achieved. spraying distance is used, a greater transformation degree of titanium is achieved. Therefore, the spraying distance is used,boundaries a greater transformation degreewhile of titanium is achieved. Therefore, the Therefore, interphase distinct, carbon in interphase the boundaries appear more appear distinct,more while the oxygen the andoxygen carbonand contents incontents specimen interphase boundaries appear more distinct, while the oxygen and carbon contents in specimen specimen produced in regime No. 6 are higher (Figure 6d). Using a shorter spraying distance, a coating produced in regime No. 6 are higher (Figure 6d). Using a shorter spraying distance, a coating with produced in regime No. are higher (Figure 6d). Using a shorter (specimen spraying distance, a coating with with increased values of 6nanoand microhardness obtained 5, Tables 2 and 3). increased values of nanoand microhardness was was obtained (specimen No. No. 5, Tables 2 and 3). As increased values of nanoand microhardness was obtained (specimen No. 5, Tables 2 and 3). As As compared to specimens produced in regime No. 3 and No. 4, the nanohardness value is almost compared to specimens produced in regime No. 3 and No. 4, the nanohardness value is almost twice compared to specimens produced in regime No. 3 and No. 4, the nanohardness value is almost twice twice as(Table high (Table as high 3). 3). as high (Table 3).
(a) (a)
(b) (b)
(c) (d) (c) (d) Figure 6. SEM-micrographs taken at the cross section of the specimens produced in regime No. 55 (a,b) Figure 6. SEM-micrographs SEM-micrographs taken taken at at the the cross cross section section of of the the specimens specimens produced produced in in regime regime No. No. 5 (a,b) Figure 6. (a,b) and No. 6 (c,d). and No. 6 (c,d). and No. 6 (c,d).
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(b) (b) Figure 7. Cont.
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(c)
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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 From comparison of the mechanical properties the specimens, theNo. following be drawn. The thickness of the coating in specimen of produced in regime 6 was 4conclusions times greatercan bethan drawn. thickness of the coating in specimen produced in regime No. resistance 6 was 4 times greater thatThe in the rest of the specimens), which significantly affected the crack measured than that in the rest of the specimens), which the crack resistance measured during bending and scratching (σcr and Kc). significantly Nevertheless,affected the bending cracking resistance of specimen produced in regime(σ No. 5 is higher than that of specimen produced in regime No. 6. This during bending and scratching and K ). Nevertheless, the bending cracking resistance of specimen cr c can be attributed more a uniform structure of specimen specimen produced At This the same produced in regimetoNo. 5 is higher than that of producedininregime regimeNo. No.5. 6. can be time, theto bending stress of produced in regime No. No. 5 (Table 2) the is noticeably attributed more adelamination uniform structure of coating specimen produced in regime 5. At same time, thandelamination the values observed in coating coatings produced produced in No. 4. thehigher bending stress of in regime regimeNo. No.3 5and (Table 2) 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. Spraying Regimes No. was 7 and The spraying distance 10No. mm8for specimen produced in regime No. 7 while it was 100 mm forThe specimen produced in regime Additionally, nitrogen was added No. to the O2/C2Hit2 was mixture spraying distance was 10 No. mm8.for specimen produced in regime 7 while 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 /C2 H2 mixture would favorbythe formation of nitrides in the in mixture an amount of 33% volume. It was assumed thatcoatings. 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 Figure 8, 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 interphase boundaries due content to a higher chemical transformation than 6%, while the oxygen is about 30% (Figure 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% (Figure 9, Table 3).
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(c) (d) (c) (d) Figure 8. SEM micrographs of the cross sections of specimens produced in regime No. 7 (a,b) and No. Figure 8. EM micrographs of the cross sections of specimens produced in regime No. 7 (a,b) and Figure 8 (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).
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Figure 9. Cont.
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(g)
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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 In specimen in regime No. 8,could the oxygen contentfor was lower (15%), the content of carbon did notproduced change substantially. This be the reason a decrease in the while microhardness. of The carbon did not change substantially. Thisincould beNo. the7reason for8 ahas, decrease in the microhardness. nanohardness of specimens produced regime and No. on the contrary, increased. Innanohardness both specimens, nitrides were found to dominate in the regions. The oftitanium specimens produced in regime No. 7 and No.dark 8 has, on the contrary, increased. bending titanium delamination stress for found specimen produced in in the regime 7 is higher than that of In bothThe specimens, nitrides were to dominate darkNo. regions. specimen No. 8 by 30%, while the cracking stress σcrproduced increases in by regime a factorNo. of ~10. the same The bending delamination stress for specimen 7 isAt higher thantime, that of the interface fracture toughness K c is increased by 40% (Table 2). The introduction of nitrogen specimen No. 8 by 30%, while the cracking stress σcr increases by a factor of ~10. At the sameinto time, explosive mixture has increased the amount titanium This appearsoftonitrogen be the reason thethe interface fracture toughness Kc is increased by of 40% (Table nitrides. 2). The introduction into the for the improvement of the mechanical properties relative to specimens produced regime explosive mixture has increased the amount of titanium nitrides. This appears to bein the reasonNo. for5the and No. 6. Specimen No. 7 possesses therelative maximum cracking resistance the No. compositions improvement of the mechanical properties to specimens producedamong in regime 5 and No. 6. studied. Specimen No. 7 possesses the maximum cracking resistance among the compositions studied. Conclusions 4. 4. 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 properties of the coatings were investigated. For the three characteristic modes of the coating formation coating formation (the composition of the explosive mixture and the carrier gas), the following (the composition ofwere the formulated: explosive mixture and the carrier gas), the following recommendations recommendations were formulated: 1. In case of the spraying with air as a carrier gas, no nitrogen added into the explosive mixture, an in the O2/C2Hwith 2 from 2.5 leads increase in theinto cracking resistance and 1. Inincrease case of the spraying air1.1 as and a carrier gas, to noan nitrogen added the explosive mixture, delamination under three-point as well as the interface fracture ancoating increase in the O2 /C2stress H2 from 1.1 and 2.5 leadsbending, to an increase in the cracking resistance and toughness under the scratch test due to formation of a hierarchically organized coating structure coating delamination stress under three-point bending, as well as the interface fracture toughness with the clearly exhibited phase boundaries. of titanium at Ocoating 2/C2H2 = 2.5 allows under scratch test due to formation of Deposition a hierarchically organized structure with obtaining coatings with a high fracture toughness, a high adhesion strength, a high clearly exhibited phase boundaries. Deposition of titanium at O2 /C2 H2 = 2.5 allows obtaining microhardness and a low friction coefficient. coatings with a high fracture toughness, 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, a short spraying distance (10 mm) and a low oxygen content in the explosive mixture mechanical properties comparable to those of the coatings, in which the oxide phases were (Opredominantly possible to form a heterogeneous structure of the coating. This ensures 2 /C2 H2 = 0.7) makes formed.it The coatings show a moderate crack resistance, a moderate adhesion mechanical properties comparable to those of the coatings, in which the oxide phases were strength, a high microhardness (H100 = 2.72 GPa) and a high friction coefficient (μ0 ~0.58). predominantly formed. The coatings show a moderate crack resistance, a moderate adhesion 3. In case of the spraying with nitrogen as a carrier gas and nitrogen added to the explosive strength, high microhardness (H100heterogeneous = 2.72 GPa) and a highmakes friction coefficient (µ0 ~0.58). mixture,athe formation of a complex structure it possible to achieve a high 3. Incrack case resistance of the spraying nitrogen as a carrier gas and nitrogen to theunder explosive mixture, underwith three-point bending, a high interface fractureadded toughness the scratch test and a high (H100 = 2.74 GPa). The makes friction itcoefficient of achieve the coating was crack μ0 the formation of microhardness a complex heterogeneous structure possible to a high ~0.68. resistance 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.
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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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