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In thermal spraying of refractory metal powders, two major issues need to be solved: particles of materials having high melting temperatures should be heated to reach a semi-molten/molten state or temperatures close to the melting point, while oxidation of the metals should be prevented. It has long been believed that it is rather difficult, if not impossible, to produce high-quality refractory metal coatings by detonation spraying. In this work, we demonstrated the capability of the detonation spraying method to produce tantalum-based and molybdenum coatings of low porosity. Using a computer-controlled detonation spray (CCDS2000) facility, the detonation spraying behaviour of a molybdenum powder and a partially oxidized tantalum powder was studied. Spraying was conducted onto steel substrates using an acetylene-oxygen mixture with O2/C2H2 = 1.1. The spraying process was studied by means of analyzing the splat morphology and calculating the particle temperatures and velocities. According to the X-ray diffraction phase analysis, the metals did not experience oxidation during the deposition. Rather, partial reduction of the oxide phase contained in the Ta-based powder occurred during spraying.

The analysis of material flow fields under the explosive collapse of two-layer metal/ceramics tubes is carried out. It is shown that the main reason for the absence of condensed jet is the explosion evaporation of the inner metal layer due to ultra-high temperatures in the compression area. For the case of the copper inner layer, very interesting and extraordinary features of the flows are observed. The non-stationarity of the process during the entire tube compression period attracts attention. The copper density at the point of contact decreases to 6.06 g/cm3 to the moment of time of 13 µs. This can only mean that copper is already partially vaporized before the collision point. To produce condensed hypervelocity jets, it is proposed to use a material of the inner layer with a high evaporation temperature. The analysis of properties of existing materials shows the perspective of using such refractory and plastic metals as niobium and tantalum as an inner layer of the tube.

Detonation spraying of WC/Co was conducted with two-component fuel using a computer-controlled detonation spraying system. The in-flight temperatures and velocities of the particles were calculated and optimized for C2H2/C3H8/O2 and C2H2/C4H10/O2 detonation gas mixtures. The stable spraying mode was realized in a short gun barrel thanks to the formation of a stratified explosive charge by means of precise computer-controlled supply of the gas components. The stand-off distance was varied from 50 to 400 mm and the substrate tilt angle relative to a plane normal to the spraying direction was varied from 0 to 60°. The formation of splats was analyzed to determine the optimal spraying mode. The microstructure and porosity of the coating were studied, and microhardness, bonding strength and wear resistance were measured. It was found that spraying with two-component fuels produces coatings with enhanced properties. For the WC-12 wt.%Co coatings, the following characteristics were achieved: porosity of G65 abrasion wear of

The possibility of obtaining alloy/composite coatings by detonation spraying of Fe + Co + Ni and Fe + Co + Ni + Cu powders mixtures was evaluated for the first time. The conditions of spraying were selected such that the particles of all three (four) metals were flying out of the gun barrel in the fully molten state, as confirmed by the calculated particle temperatures. The deposited layers had a dense composite structure consisting of metallic lamellae. A common feature of the deposited layers was a diminished concentration of iron relative to the feedstock mixtures. The results of the present work show that, upon a proper adjustment of the composition of the feedstock mixture, detonation spaying can be used to form dense precursors of high-entropy alloy coatings.

Tungsten is a promising material for plasma-facing components of thermonuclear reactors. In the present work, tungsten coatings of low residual porosity (1%) were deposited on copper substrates by detonation spraying. Calculations of the particle temperatures and velocities were conducted for tungsten particles of different sizes using a previously developed model. These calculations helped selecting the spraying parameters enabling the formation of high-quality tungsten coatings in the experiments. A proper selection of the stand-off distance allowed achieving a high deposition efficiency of the powder (60%). The coatings showed a high bonding strength to the copper substrates, which was found to be 100 MPa, and a high hardness (850 HV). The coatings were shown to accumulate compressive stresses, which are beneficial for the structural integrity of coatings up to 500 μm thick.

In this work, the spraying behavior of a copper powder in the Computer-Controlled Detonation Spraying (CCDS) process was studied both theoretically and experimentally. The dependences of the particle temperatures and velocities on the explosive charge were calculated for particles of copper 40 μm in diameter for different O 2 /C 2 H 2 ratios (1.1; 1.5; 2.0; 2.5) and nitrogen and air as carrier gases using models and software previously developed at Lavrentyev Institute of Hydrodynamics SB RAS. The explosive charge was varied between 30 and 60% of the barrel volume. Calculations showed that the particle temperatures increase and the particle velocities decrease monotonously as the O 2 /C 2 H 2 ratio increases from 1.1 to 2.5 at a constant explosive charge when nitrogen is used as a carrier gas. It was shown that the choice of the carrier gas (nitrogen or air) significantly influences the temperature of the particles when spraying is conducted at a low O 2 /C 2 H 2 ratio. In this case, the particle velocities are also affected. For experiments, an electrolytic copper powder with an average size of 40 μm was used. The morphological features of the deposits were such that could be well expected in the deposits formed by particles heated up to the calculated temperatures.

Detonation (D-gun) spraying uses detonation of hydrocarbon fuel to heat and accelerate the powder particles. Spraying using incomplete combustion can lead to changes in the composition of the sprayed material due to the presence of the in-situ formed carbon particles in the spraying atmosphere. In this work, we have studied the phase formation of the deposits obtained by D-gun spraying of Ni and “Ni-amorphous carbon mixtures”. It was found that solid solutions based on metastable hcp-Ni and fcc-Ni form in detonation deposits at O 2 /C 2 H 2 =0.7 and explosive charges of 50–70%. No metastable phases were found in the coatings sprayed at O 2 /C 2 H 2 greater than 1.1. Microstructural and elemental analyses confirmed that the in-situ formed carbon is preferentially incorporated in molten Ni particles. Carbon ex-situ introduced into the feedstock powder changes the sprinkling conditions of the melt over the substrate surface and lowers the cooling rate of the molten particles. As a result, deposits obtained at an explosive charge of 60% and 70% from powders containing added carbon tend to show lower concentrations of the hcp phase compared with deposits obtained from Ni.

Progress in design of thermal barrier coatings (TBCs) is based upon application of new materials and deposition techniques. In this work traditional NiCrAl bond coat YSZ top coat were deposited on Ni superalloy substrate by inexpensive dry detonation spraying, while finishing layers of oxide nanocomposites (LaAlO3–La2Zr2O7, LaAlO3–LaCuAl11O19) were deposited on YSZ by slip casting. Complex oxides were prepared via Pechini route. Thin (~ 20 μm) finishing layers were deposited by slip casting of suspensions of oxides mixture in isopropanol with addition of polyvinylbutyral. Genesis of the texture, composition and real/defect structure of bulk nanocomposite materials and deposited layers after annealing in air up to 1300 °C as well as after series of thermal shocks by heating up to 1200 °C by H2–O2 burner were studied by combination of diffraction (high resolution SEM and TEM with EDX, XRD on synchrotron radiation) and spectroscopic (UV–Vis, EXAFS, laser-excited Dy3 + luminescence spectra) methods. Thermal conductivity of nanocomposites and TBCs was determined by using NETZSCH LFA 457 MicroFlash. Nanocomposites were shown to retain porosity as well as nanosizes of disordered domains of oxide phases even after sintering at high temperatures. A good adhesion and compatibility of all layers in TBCs were demonstrated, while in general disordering of the oxides structure in deposited layers was higher than that in bulk materials due to the effect of depositing procedure and interaction between layers. This provides a low thermal conductivity of nanocomposites and functionally graded TBC. After 90 thermal shocks neither layers spallation nor cracks were revealed, 8YSZ and finishing layers retaining porosity, nanocrystallinity and disordering required for a low thermal conductivity and cracks trapping.

The phase composition and microstructure of coatings formed by detonation spraying of Ti-Cu powder mixtures using C2H2 + kO2 explosive charges with k varying from 0.7 to 2.0 were studied. The Ti-Cu system was selected to demonstrate the possibilities of forming metal matrix composites (MMCs) with in-situ synthesized ceramic particles during the coating deposition. The formation of composite microstructures needs to be considered when obtaining carbon-containing MMCs by detonation spraying using acetylene-oxygen mixtures at k ˂ 1. The focus of the present study is the phase formation, microstructure development and hardness variation of the coatings caused by carbon, oxygen and nitrogen uptake by the Ti-Cu mixtures during spraying. Carbon released in the form of graphene nanoplatelets as a result of incomplete combustion of acetylene at k = 0.7 reacted with titanium, and, in the presence of nitrogen (a carrier gas), the TiCxNy phase formed in the coatings. Copper was found to play a protective role, reducing the conversion degree of titanium into the ceramic phases. Deposits containing 21 – 44 wt.% of ceramic components formed when an explosive charge equal to 50% of the gun barrel volume was used, the nature and concentration of the ceramic phases depending on k. At k = 2.0 and an explosive charge of 50%, a composite coating reinforced with Ti2O3, TiO and TiN (comprising 41 wt.% in total) was obtained. This composite showed a microhardness of 480 HV, which was due to a high content of ceramic phases synthesized in situ during spraying.

In this work, the formation of coatings by chemical reaction-accompanied detonation spraying of TiC x –Ti composite powders differing in the microstructure and carbon content was studied. The powders were synthesized by a reaction between titanium and carbon in the presence of excess of titanium via self-propagating high-temperature synthesis followed by mechanical milling with an additional amount of titanium (which produced a composite product containing 4.2 wt% of carbon) or by thermal annealing of mechanically milled mixtures containing 2.1 wt% of carbon. It was found that in the detonation coatings, the relative amount of metallic titanium dramatically decreased in comparison with the powders due to its reactions with carbon and nitrogen contained in the spraying atmosphere and the formation of titanium carbonitrides TiN v C w . It was possible to distinguish regions in the cross-section of the coatings with a composite microstructure inherited from the powders and with a microstructure in situ formed by the products of chemical reactions that occurred during spraying. The volumes of the material with the in situ formed microstructure contained the major fraction of the porosity of the coatings. The average hardness of the TiC x –TiN v C w –Ti coatings ranged between 400 and 480 HV. Despite higher porosity of the coatings formed at higher degrees of chemical transformation, their average hardness increased. No significant differences in the hardness of the coatings produced from two feedstock powders were observed. The similarity between the coating formation processes from two TiC x –Ti powders of different microstructures and the similarity of those to the coating formation process from a metallic titanium powder show that the reactivity of titanium during detonation spraying is a crucial factor determining the coating microstructure and properties.

The phase development in the coatings formed by detonation spraying of titanium in a wide range of compositions of the spraying atmosphere is reported. The phase composition of the coatings was found to be very sensitive to the O 2 /C 2 H 2 ratio and the nature of the powder carrier gas. Using O 2 /C 2 H 2 = 1.1 and air as a carrier gas, titanium oxynitride-containing coatings were obtained, while at O 2 /C 2 H 2 = 2.5, titanium oxides were the reaction products. In highly reducing conditions at O 2 /C 2 H 2 = 0.7 and with the use of nitrogen as a carrier gas, titanium carbide and carbonitride formed. Higher contents of nitrides in the coatings were found when nitrogen was added into the O 2 + C 2 H 2 mixture. Metal-ceramic coatings formed at high transformation degrees of titanium were either composed of metallic titanium-rich particulate agglomerates distributed in a ceramic-rich matrix or contained alternating layers rich in metallic titanium and rich in ceramic compounds.

This article is aimed to discuss the chemical aspects of detonation spraying of powder materials. In this method of coating deposition, ceramic, metallic or composite powders are injected into the barrel of a detonation gun filled with an explosive gaseous mixture. When the latter is ignited, the powders are heated and accelerated toward the substrate. Subjected to high temperatures, the powders are prone to chemical reactions, the reaction products possibly becoming the major phase constituents of the coatings. What types of reactions are possible? Can these reactions be carried out in a controlled manner? We answer these questions considering the interactions of the sprayed powders with the gaseous environment of the barrel as well as those between the phases of a composite feedstock powder. In Computer-Controlled Detonation Spraying (CCDS), the explosive charge and stoichiometry of the fuel-oxygen mixtures are precisely measured and can be flexibly changed. Our studies demonstrate that with the introduction of a highly flexible process of CCDS, detonation spraying has entered a new development stage, at which it can be considered as a powerful method of composition and microstructure tailoring of thermally sprayed coatings. During CCDS of TiO2-containing powders, chemical reduction of titanium dioxide can be carried out to different levels to form either oxygen-deficient TiO2−x or Ti3O5 suboxide. CCDS of Ti3Al can produce titanium oxide coatings when oxidation by the detonation products dominates or titanium nitride-titanium aluminide coatings when oxidation is hindered but the interaction of the powders with nitrogen—a carrier gas component—is favored. During detonation spraying of Ti3SiC2–Cu composites, the Ti3SiC2 phase is preserved only in cold conditions; otherwise, Si de-intercalates from the Ti3SiC2 phase and dissolves in Cu resulting in the formation of the TiCx–Cu(Si) composite coatings.

We have examined the new phase formation and compositional variations in the coatings produced by detonation spraying of Ti3Al powders in atmospheres of different chemistry. The composition of the spraying atmosphere was varied by changing the O2/C2H2 ratio used for the detonation. In the coatings sprayed at an O2/C2H2 ratio of 0.87 and 1.05, certain deviations of the Al/Ti ratio have been observed relative to the sprayed powder. In these coatings, the Ti3Al phase was found along with TiN and Ti2N formed due to the reaction of the heated particles with nitrogen used for powder injection. The amount of TiN increased with increasing explosive charge and a concomitant rise in the particles temperature. Under O2/C2H2 ratios equal to 1.50 and 2.00, significant losses of Al were detected, the coatings being composed mainly of titanium oxides. This study shows that detonation spraying conditions excluding severe oxidation of Ti3Al can be found as well those facilitating oxidation with an associated loss of Al in the deposits.

Composite WC/Co + MoS 2 coatings were deposited onto steel substrates by Computer Controlled Detonation Spraying using three spraying modes: very cold, cold and normal. Maximal content of MoS 2 in a sprayed powder was 10 wt.%. Characterization of coatings was made with chemical and phase analyses, microhardness measurement, morphology and microstructure investigation. X-ray diffraction study shows that residual MoS 2 exists only in coatings obtained at very cold and cold spraying modes. At normal spraying mode complete decomposition of the solid lubricant occurs during spraying. From the engineering point of view, the coating applied at the cold mode using a powder containing 10 wt.% MoS 2 is the most promising. Such a coating has microhardness of 650 HV 0.2 and a porosity of 10%.

In the present work, rutile powders containing additions of metallic silver (2.5 vol.%) were detonation sprayed in a reducing atmosphere formed by gaseous detonation products of the C2H2 + 1.05O2 mixture. The initial volume of the C2H2 + 1.05O2 mixture – explosive charge – used for a detonation pulse was computer-controlled as the fraction of the barrel volume filled with the mixture. Using a previously developed model of the detonation process, the particle temperatures and velocities were calculated to explain the observed phase and microstructure development in the coatings. With increasing explosive charge, the temperature of the sprayed particles increased and rutile was partially reduced to oxygen-deficient TiO2−x and then to Ti3O5. When the melting temperature of rutile was not reached, the coatings were porous; semi-molten particles formed denser coatings obtained with higher spraying efficiency. Silver inclusions in the titanium oxide matrix experienced melting and substantial overheating, but remained well preserved in the coatings.