Effect of the rarefaction of an oxygen-containing medium on the formation of titanium oxynitrides
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EFFECT OF THE RAREFACTION OF AN OXYGEN-CONTAINING MEDIUM ON THE FORMATION OF TITANIUM OXYNITRIDES I. M. Pohrelyuk, V. M. Fedirko, and O. V. Tkachuk
UDC 669.295:621.795
We study the effect of the rarefaction of an oxygen-containing medium (0.001 to 10 Pa) on the formation of oxynitrides on the surface of VT1-0 titanium and its OT4, PT-7M, VT20, and VT6s alloys in the course of modification of nitrides formed in molecular nitrogen at a temperature of 850°C. The initiation of titanium oxynitrides becomes more intense as the rarefaction increases from 10 to 0.001 Pa. We also show that the surface microhardness increases after oxynitriding as compared with nitriding, and the depth of the hardened layer is determined by isothermal holding in nitrogen.
At present, coatings based on ternary interstitial compounds, in particular, titanium oxynitrides become more and more important. As compared with binary compounds (oxides, nitrides), these coatings possess better operating properties (hardness, wear and corrosion resistance, high-temperature strength, etc.) [1 – 3]. This can be explained by a nonlinear change in the parameters of the electron structure of oxynitrides and in the type of chemical bond between atoms in the crystal lattice depending on the composition of this compound [4, 5]. For the oxynitriding of titanium alloys by means of modification of the nitride layers with oxygen, the optimal temperature range (750 to 950°C) has been determined earlier, but the optimal degree of rarefaction of the oxygencontaining medium has not been substantiated [6]. In the present work, we study the effect of oxygen partial pressure on the oxynitriding of titanium alloys by means of modification of the nitride layers with oxygen. Procedure of Investigations We used specimens made of commercially pure VT1-0 titanium, PT-7M α-alloy, OT4 and VT20 pseudoα-alloys, and VT6s ( α + β )-alloy, 10 × 15 × 1 mm in size. The specimens were polished ( R a = 0.4 μm ), washed in alcohol, and dried. Afterwards, they were heated to the nitriding temperature in vacuum 10 mPa and saturated in molecular nitrogen under atmospheric pressure at 850°C. We used commercially pure nitrogen, which was cleaned by passing it through a capsule with silica gel and heated titanium chip so that the oxygen and moisture content should is not more than 0.05%. Then the specimens were subjected to isothermal holding in nitrogen for 5 h and cooled to 500°C in a rarefied oxygen-containing medium (0.001 to 10 Pa), and afterwards the chamber was vacuumized. We determined the phase composition of the surface layers by X-ray phase analysis on a DRON-3.0 diffractometer in monochromatic CuKα-radiation with focusing by the Bragg–Brentano scheme. The voltage on the anode of the X-ray tube was 30 kV, and the current through it was 20 mA. For data processing, we used DHN-PDS and CSD program packages. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, Lviv, Ukraine. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 1, pp. 56 – 60, January – Feb
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