Mechanical Characterization of Composite Coatings Formed by Reactive Detonation Spraying of Titanium
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 O 2 /C 2 H 2 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
... anohardness 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 O 2 /C 2 H 2 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. 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  . 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  . 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  . 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  . 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  . 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  . The method can also be applied for metallization of ceramics and periodic restoration of the worn machine parts    . In Ref. , 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.  . 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  . Also, machining of the thermally sprayed coatings is needed to achieve dimensional tolerances  . 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. TiO 2 coatings formed by the HVOF technique were studied in Ref.  . 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.