Bogdan Vasyliv

Effect of Corrosion Environment on the Fatigue Behavior of WC — Co Hard Alloy Teeth of Drill Bits

The fatigue crack growth in WC-Co hard alloys with cobalt content from 6 to 12 wt pct has been investigated in air and fluids of pH from 4.5 to 10.5. Significant difference in crack growth rates at low the stress intensity factor range was detected. It was established that the negative influence of corrosion environment on fatigue crack growth resistance of WC-Co hard alloy increases significantly for compositions with higher cobalt content. This effect is observed in fluids of various pH-values. Lower operating life of hard alloy inserts of drill bits in service environment corresponds to hydrogen embrittlement of cobalt phase.

Effect of the Additive of Y 2 O 3 on the Structure Formation and Properties of Composite Materials Based on AlN–SiC

The results of studies on the strength at bending and volumetric electrical resistance of composite materials based on AlN–SiC with additions from 2 to 6 wt % Y2O3. It is shown that at increasing the content of Y2O3 in the mixture from 2 to 6 wt % the compaction of the composites intensifies their electrical resistance from (1.4–5.4) × 106 to (1.8–5.94) × 107 Ohm·cm (at 20°C), which at the increasing temperature decreases exponentially and at 800°C for all composites is (5–6) × 104 Ohm·cm. It was determined that materials with the smaller content of Y2O3 have somewhat higher value of the ultimate strength during bending, namely, 110 MPa.

Effect of a Hydrogen Sulfide-Containing Atmosphere on the Physical and Mechanical Properties of Solid Oxide Fuel Cell Materials

The effect of hydrogen sulfide content in a hydrogenous atmosphere on the structure, physical, and mechanical properties of solid oxide fuel cell (SOFC) anode materials has been studied. A series of specimens of porous nickel and YSZ–Ni cermet have been investigated. In order to obtain the corresponding YSZ–Ni cermet structure, specimens of the YSZ–NiO ceramics were singly reduced in a hydrogenous atmosphere (either Ar–5 vol% H2 mixture or hydrogen of 99.99 vol% H2 purity) for 4 h at 600 °C under the pressure of 0.15 MPa. A part of the specimens of each series was then aged in “hydrogen sulfide in Ar–5 vol% H2 mixture” atmosphere for 4 h at 600 °C. According to a test mode, the atmosphere contained 7 or 18 vol% Н2S. Material microstructure and fracture surface morphology of the specimens as well as the physical and mechanical behaviors were investigated. It was revealed that the atmosphere containing up to 7 vol% Н2S does not affect the strength and electrical conductivity of the YSZ–Ni cermet. Increased content of Н2S (18 vol%) causes some changes in the YSZ–Ni cermet structure. A large number of completely reduced tiny nickel particles are formed. These nickel particles react with hydrogen sulfide. Sulfur is segregated on the boundaries between the zirconia and nickel phases and pores. Finally, multiple breaking of the zirconia–nickel bonds occurs that results in reduced strength of the cermet (by 39% as compared to as-received ceramics).

Effect of Porosity on Strength and Electrical Conductivity of NiO–3.5YSZ Composite and Its Ni–3.5YSZ Cermet

The change in porosity of the Ni–3.5YSZ (ZrO2 stabilized with 3.5 mol.% Y2O3) composite, when produced, and the effect of porosity on the strength and electrical conductivity is studied. The porosity was provided by adding granular starch to the mixture of NiO and 3.5YSZ powders. The content of 18 vol.% pore-forming agent in the Ni–3.5YSZ cermet provides open porosity 47.1%, while its strength and electrical conductivity are 74.3 MPa and 0.93 · 106 S/m, respectively.

Effect of Water Vapor Amount in Hydrogenous Atmospheres on Reducing Ability of the YSZ–NiO Fuel Cell Anode Material

Anode ceramics of YSZ–NiO system for solid oxide fuel cells (SOFCs) has been investigated. A series of specimens were singly reduced in a hydrogenous atmosphere (the Ar–5 vol%Н2 mixture) at 600°С under the pressure of 0.15 MPa or subjected to “reduction in the mixture–oxidation in air” (redox) cycling at 600°С. After both the treatment conditions, corresponding structures of YSZ–Ni cermets were formed and then subjected to dwell in “water vapor–Ar–5 vol% Н2 mixture” atmosphere at 600°С under the pressure of 0.15 MPa. Additionally, behavior of the as-received material in this atmosphere was studied. It was revealed that small amount of water vapor in Ar–5 vol% Н2 mixture (water vapor pressure below 0.03 MPa) does not affect the reduction of a nickel phase in YSZ–NiO ceramics but causes some changes in the YSZ–Ni cermet structure, in particular, growth of nanopores on tiny Ni particles. A higher concentration of water vapor in the mixture (water vapor pressure above 0.03–0.05 MPa) causes a converse change in the reduction kinetics. For as-received material, such amount of water vapor in the mixture is an obstacle for its reduction and causes reoxidation of a nickel phase at 600°С. For the material treated by redox cycling, better physical and mechanical properties were revealed after dwelling at 600°С in a water-depleted gas mixture. Contrary to this, after dwelling at 600°С in a water-enriched gas mixture, the material showed lower resistance against reoxidation. Based on the scanning electron microscopy and the data on the conductivity and strength, the dual effect of water vapor on durability of a nickel–zirconia anode is discussed.