Mathematics – Logic
Scientific paper
Jan 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002iaf..confe.460f&link_type=abstract
IAF abstracts, 34th COSPAR Scientific Assembly, The Second World Space Congress, held 10-19 October, 2002 in Houston, TX, USA.,
Mathematics
Logic
Scientific paper
Nowadays there is an important problem to increase the resistance to high-temperature corrosion of molybdenum as material of jet and space industry. The electric-spark alloying (ESA) being characterized by expedient application and ecological safety is one of the ways to decide such task. The goal of this study is to investigate the composition, structure and corrosion properties of Mo after its ESA with AlN-TiB2/ZrB2-based composite ceramics. The choice of coating materials was conditioned by high corrosion resistance of these ceramics till 1400oC. ESA was implemented under high-frequent alloying in air at the following regime: short curcuit - 0.9 A, impuls energy - 0.08 J, impulses frequency 1200 Hrz. The electrodes (3 x 4 x 35 mm) with the porosity ~ 2.0 - 2.5% were fabricated by powder metallurgy methods. ESA was carried out using layer-by-layer procedure. The outer layer was formed with the use of MoSi2 electrode, the latter being applied to silicide the surface. The composition and structure of surface were studied using XRD, EPMA, SEM, and metallographic methods. The kinetics of interaction with oxygen under isothermal conditions (up to 1400oC) was studied by TG method. It was found that the coating obtained had an island-like nature and consisted of three parts. The surface of sample with coating (a thickness ~ 5- 20 μm) proved to be reinforced by globules of fine-dispersion sintered composite material on the base of oxidized electric erosion solid-phase products. They occupied approximately 20-30% of working area whereas the greater part of coating was Mo, mainly, alloyed with aluminum, and also contained the uncoated molybdenum, the latter being explained by peculiarities of ESA method. According to XRD data as well as EPMA spectra of a cross section of globule for both the ALN- TiB2 and ALN-ZrB2-ZrSi2 electrode materials, one can conclude that the gradient structure of globule sintering layer is formed on the Mo surface. In the case of ALN-TiB2 alloying electrode, the outer globule later proved to be the titanium diboride while the layer near Mo substrate is double titanium - molybdenum diboride, and the middle one is alumina. In the case of ALN-ZrB2-ZrSi2 alloying electrode, the surface sintered layer consists of the alumina whereas the lower layer is zirconium borone silicide with the additive of Al2SiO5 mullite. Hereby on the "globule-Mo" boundary there is a transition zone of about 1 μm, enriched, mainly, with aluminum. It was shown that the deposition of ESA coating promoted to a rise of corrosion resistance of technical Mo samples (up to 1200oC) by ~ 8 times. Hereby the vaporization of MoO3 oxide was the process limiting high-temperature corrosion, on the whole. On the base of both data concerning the temperature dependence of linear vaporization rate and thermodynamic calculation of diminution of oxidation/ vaporization active centers amount one can evaluate the magnitude of working area of uncoated sample surface that proved to be equal to ~ 16%. The total affect of an increase of corrosion resistance of molybdenum is conditioned by formation of complex compounds of the Ti-Zr-Al-Si system in the material of sintered layer as well as a presence of molybdenum regions alloying with aluminum.
Frolov G. A.
Lavrenko V. A.
Panasyuk A. D.
Podchernyaeva I. A.
Teplenko M. A.
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