D. V. Yurechko

Spark-deposited coatings on magnesium alloys

The paper examines the mass transfer kinetics, structure, and properties of spark-deposited coatings on magnesium alloys. They are obtained using composite ceramic electrodes in the AlN–Zr(Ti)B2 and B6Si–CaB6 systems. It is revealed that the microhardness and corrosion resistance of the coatings increase in a 3% NaCl solution and the abrasive wear decreases as compared with uncoated magnesium alloy.

Some trends in the development of wear-resistant functional coatings

Based on analysis of literature data on coatings playing a role of solid lubricants, prospective research areas have been identified. Two categories of coating materials are considered: with hardness lower than 10 GPa (polymers, soft metals, halides, dichalcogenides of transition metals, sulphates of alkaline earth metals, graphite) and with hardness higher than 10 GPa (oxides, carbides, nitrides, borides of transition metals, carbon-based composites). Methods of their deposition and surface structuring are discussed as well. The greatest effect is observed when coatings are deposited using combined techniques involving surface structuring. The development of new nanostructured high-temperature composite coatings that can adapt to extreme performance conditions is regarded as the most promising area of research for the next decade.

Laser ZrB2-Based Coating on Graphite

A coating ≤50 μm thick is produced on graphite by laser melting of a ZrB2-based powder layer with additions of zirconium and molybdenum silicides in air. The main coating phases are ZrB2, SiO2, and ZrSiO4. The surface is formed by spherical particles 10–80 μm in size, representing a two-phase eutectic in the ZrSiO4–SiO2 system as needlelike crystals. Microhardness of the coating is 2.5 times higher than that of the substrate and reaches 15 ± 1 GPa.

High-Energy Electrospark Surface Strengthening of Steels with Composite Ceramics

The surface and cross-sectional structure, composition, and microhardness of composite coatings on ShKh15 and R6M5 steels have been studied under high-energy electrospark deposition with electrode materials based on zirconium boride and titanium (zirconium) nitride. It is shown that mass transfer of doping components into a metallic substrate is determined by wetting of the refractory component by iron. The strengthening/softening of the steel substrate is ascertained under high-energy electrospark deposition.

Combined Functional Biocoatings on the VT-6 Alloy

Electrochemical oxidation of spark-deposited coatings from intermetallic TiAl3 and TiN–3AlN nitride ceramics in 3% NaCl solution leads to a nanosized surface layer composed of titanium oxides of various composition: from higher to lower oxides toward the substrate. The highest corrosion resistance is shown by the intermetallic coating, which is confirmed by anodic oxidation polarization curves and layerwise Auger spectroscopy of protective titanium and aluminum oxide films. A combined biocoating is produced as follows: electrospark deposition of TiAl3 layer onto VT-6 alloy—electrochemical oxidation in 3% NaCl solution—deposition of hydroxyapatite layer—laser fusion. Energy-dispersive X-ray spectroscopy has revealed a wide adhesion area at the interface between the spark-deposited coating and hydroxyapatite, which agrees with smooth variation in microhardness in this area. The coating shows strong adhesion to the substrate and biocompatibility with the bone tissue.

Effect of electrospark deposition of Al–Si alloy on the wear resistance of a hard-alloy cutting plate

The electrospark deposition of AL25 aluminum alloy onto a VK8 cutting plate is studied to show the effect produced by the composition of a tribofilm formed in the cutting process on the wear resistance of the tool. The mass transfer kinetics, microstructure, composition, and microhardness of the deposited layer, and distribution of elements over the cutting edge surface after cutting are determined. It is shown that complex oxides that form on the cutting edge and are more chemically stable than volatile W and Co oxides increase the wear resistance of the tool by a factor of 1.5 to 1.8.

Mechanism of High-Temperature Oxidation of ZrB 2 -Based Composite Ceramics in the ZrB 2 –SiC–AlN System

The paper examines the oxidation of the ZrB2–SiC–AlN composite at high temperatures (1550–1700°C) and focuses on the structure and phase composition of the starting ceramic material and scale layer. The oxide layer develops in two stages. At the low-temperature stage (1170–1250°C), an intermediate layer forms, consisting of ZrB2 and involving solid solutions in the AlON–SiC/SiO2 system. The high-temperature stage (1250–1350°C) gives rise to the main protective layer consisting of xAl2O3–ySiO2 mullite solid solutions and colonies of zirconium oxide crystals, located at the grain boundaries of the mullite phase. The resultant oxide layer is an effective potential barrier to the diffusion of oxygen into the sample.

High-Temperature ZrB2-Based Coatings on Metallic Alloys Produced by High-Velocity Air-Fuel Thermal Spraying

The composition and structure of ZrB2-based coatings with MoSi2, SiC, and AlN additions on metallic alloys produced by high-velocity air-fuel thermal spraying are studied. The use of composite powders with a low content of the NiCr binder (≤10 wt.%) is shown to be beneficial due to intensive adhesive interaction of the components. The phase transformations induced by high-temperature thermodynamically nonequilibrium processes, including oxygen-assisted ones, occur when the coatings are being formed. The coatings with a hardness of ~15 GPa and a porosity of ≤4% are characterized by a granular structure without cracks and have a 50–120 μm thick ceramic layer and an interfacial layer of uniform width (~10 μm) at the boundary between the NiCr alloy and stainless steel.

Gradient Laser ZrB2–MoSi2 Coating on Graphite

A high-temperature ZrB2–MoSi2 coating on graphite is produced by pulsed laser melting (in air) of a graphite sample with powder layers pre-deposited on the working surface. It is demonstrated that laser melting is accompanied by an intensive phase formation in the layers and at the interface. The main phases of the surface are: ZrB2, MoSi2, Mo5Si3, and ZrC. It is demonstrated that the coating forms with the liquid phase reinforced with ~1 μm thick fibers. It is characterized by a gradient change in the composition towards the substrate: the content of ZrC increases and the content of ZrB2 decreases towards the graphite substrate. Such a change in the composition leads to higher adhesive strength of the coating.

High-temperature laser coatings of the ZrB2–MoSi2 system on graphite

Coatings of the ZrB2–MoSi2 system on graphite (≤ 50 μm thick and 14–15 GPa in hardness) have been produced by pulse laser melting in air of two-layer powder coating with sublayers based on ZrB2 with additions of ZrSi2 and SiC. Different coating structures: eutectics of the ZrSiO4–SiO2 system based on zircon or a structure of micron-thick fibers of composition Zr(Mo)B2 are formed depending on the sublayer composition, which controls the coating heat conductivity and accordingly, the crystallization rate of the bath of melt. The prospects of this line of investigations are the improvement of a laser technology of the deposition of high-temperature coating using continuous wave lasers.