V. V. Pasichnyi

Corrosion Resistance of Ultrahigh-Temperature Zirconium Boride Ceramics Under Concentrated Solar Radiation

The behavior of ZrB2–MoSi2(SiC) and ZrB2–SiC–ZrSi2 ceramics under concentrated solar radiation is examined. The effect of zirconium silicide and molybdenum silicide admixtures on the formation of protective oxide layers on the ultrahigh-temperature ceramics in air at elevated temperatures is studied with the purpose of controlled development of dense protective films ensuring high service characteristics. The structure and composition of the oxidized materials are analyzed. The highest corrosion resistance is shown by the ZrB2–MoSi2 ceramics.

A method of determining phase-transition temperatures using solar heating

The phase-transition temperatures in oxide systems were determined on the basis of the process of solidification of specimens which were partially melted on the surface in the focus of a solar furnace. In the ZrO/sub 2/-Y/sub 2/O/sub 3/ binary systems, the specimens with intermediate composition were prepared by combined precipitation of zirconium and yttrium hydroxides from nitric acid solutions followed by drying, roasting at 800 C, briquetting, and melting in the solar furnace. The described method of determination of the phase-transition temperatures in the oxide systems on the basis of the surface of the specimens partially melted in the focus of the solar furnace has a number of advantages in comparison with the method of evaluating the temperatures from the radiation of the rotating cavity: it is not necessary to form the cavity with the emissivity properties similar to those of the absolute blackbody; and the oxide melt formed in the course of melting is distributed mainly on the surface of the specimen so that after interrupting the radiation flux the melt solidifies in its entire volume.

Interaction of ZrB2–MoSi2 Ceramics with Basalt Melt

The interaction mechanism in the (ZrB2–MoSi2) composite–molten basalt system is studied at different temperatures to examine the use of boride ceramics to replace platinum alloy for the manufacture of bushings. The wetting in the (ZrB2–MoSi2)–basalt system and the formation of a stable interaction area of limited thickness testify that the material is suitable for the manufacture of bushings to produce basalt fibers.

Behavior of MoxCryTazSi2 Solid Solutions During Cyclic Heating in Air

New oxidation- and heat-resistant materials based on a molybdenum solid solution (MoxCryTazSi2) have been tested by high-temperature cyclic heating and cooling in air in the range 300–1600°C. Compact samples of the solid solutions show high heat resistance. The oxidation resistance of the solid solution samples is much higher than that of MoSi2 in experimental conditions. The oxidation resistance of the solid solution is found to depend on its composition: the oxidation resistance of Mo0.60Cr0.36Ta0.04Si2 is two times higher and of Mo0.90Cr0.18Ta0.02Si2 is one order of magnitude higher than that of MoSi2. The oxidation resistance increases with Cr content up to 18 mol.%, but higher chromium amounts cause the characteristics to degrade because the strength of the protective surface film decreases. The MoSi2 solid solutions may be recommended as heat- and oxidationresistant materials for cyclic operation in air at temperatures up to 1600°C.

STRUCTURE AND PROPERTIES OF CERAMICS BASED ON MONETITE AND NANODISPERSED SILICA

Calcium–phosphate ceramics based on monetite and nanosized silica are produced by sintering at 500°C. It is established that phase changes and solid-phase reactions occur during sintering to form ceramics that contain β-calcium pyrophosphate (Ca2P2O7), calcium silicate (wollastonite, CaSiO3), and insignificant amount of β-tricalcium phosphate (Ca3(PO4)2). It is shown that the addition of silica inhibits re-crystallization of calcium pyrophosphate into tricalcium phosphate, if compared with ceramics prepared from monetite with no silica added. It is established that increasing the silica content in the starting composition leads to a decrease in the minimum pore size from 0.83 to 0.21 μm. Highly pure nanosized silica (content of SiO2 > 99%) prepared by heat-treating of silicon carbide waste in a solar furnace is used in our research. The porosity of ceramics produced reaches 43.5–46.8% and the compressive strength is 16–24 MPa.

Effect of substrate temperature on the electrospark deposition, structure, and mechanical properties of coatings. III. effect of substrate preliminary treatment on the electrospark deposition and phase composition of coatings

The effect of preliminary treatment of a steel substrate on the dynamic properties of cathode jets during electrospark deposition is studied. The energy of cathode jets is determined indirectly using the strain hardening of the electrode surface layer. It is established that the dynamic properties of cathode jets are determined by the electrode gap and structural and heat-transfer properties of the cathode material.

Formation of graded structures under high-temperature oxidation conditions with surface light-heat treatment

Complex alloying of low-carbon steel with treatment by artificial concentrated solar radiation (CSR) in air using a LaB6-based composite is studied. The distribution of elements through the thickness of the alloyed layer along the axis of CSR entry and at the periphery of the working zone is established. On the basis of electron-probe microanalysis, x-radiographic, and metallographic data it is established that as a result of high-temperature oxidation of the composite, formation of new phases, and convective mass transfer under CSR conditions, a graded structure forms at the steel surface in the form of a dense three-layer oxide film. Its outer layer contains lanthanum and nickel chromites that provide high resistance to marine corrosion.

Investigation of a wear-and corrosion-resistant coating of the TiCN−Ni-alloy system, obtained by high-speed gas-flame spraying

The structure and properties of high-speed gas-flame (HSGF) coating of the system TiC0.14N0.86−Ni(CrBSi)-alloy (70%) on high-strength cast iron have been studied. The HSGF coatings were deposited on the Struya installation at the Institute for Problems of Materials Science, operating on an oxygen-hydrogen fuel mixture in a nearly stoichiometric ratio. The pressure in the combustion chamber was 0.6 MPa, the gas consumption was 8.5 g/sec, the sprayed powder consumption was 2.2 g/sec, the transporting air consumption was 0.5 g/sec, and the spraying distance was 200 mm. The coating was about 1 mm thick with a porosity of less than 2% and had a typical layered structure. The alternating zones were enriched with a refractory phase or metal. In comparison with the alloy WC (8% Co) the coating had lower linear wear (<5 μm/km) and a lower friction coefficient (0.1), and its resistance to high-temperature corrosion was almost an order of magnitude higher.