Low-Temperature Nitriding of Titanium Alloys VT1-0 and VT16 1

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1 Low-Temperature Nitriding of Titanium Alloys VT-0 and VT D.S. Vershinin, Yu.H. Akhmadeev*, I.V. Lopatin*, A.S. Mamaev**, and M.Yu. Smolyakova Centre Nanostructured materials and nanotechnologies of Belgorod State University, a, Koroleva str., Belgorod, 080, Russia Phone/Fax: +7(7) 8--0, * Institute of High Current Electronics SB RAS, Tomsk, Russia ** Institute of Electrophysics, Ural Division RAS, Ekaterinburg, Russia Abstract Results of nitriding of titanium alloys VT-0 (technically pure titanium) and VT (Ti Al.V Mo) are presented. Nitriding was done in three types of discharges: non-selfsustained lowpressure arcs discharge plasma, non-selfsustained glow discharge plasma and low-energy electron beam plasma. The principle possibility of nitriding of titanium alloys at 0 C has been shown. Modification of structure and properties of surface after nitriding in different types of discharges is investigated.. Introduction Titanium and its alloys are widely used materials for medical applications. At condition of additional hardening of surface the titanium alloys are considered as mostly perspective materials for medical instruments. Surface hardening of titanium and its alloys can be done by different methods e.g., by plasma vapor deposition of functional coating or micro-arc oxidation, by the nitriding [], by ion implantation and so on. Unfortunately, indicated methods have advantages and disadvantages. Using plasma vapor deposition technique it could be obtained coatings with high hardness, but the problem of adhesion between coating and substrate should be solved. Micro-arc oxidation allows forming thick (up to tens of microns) coatings, but surface morphology of the substrate would be changed essentially. Besides high surface hardness the ion implantation is characterize by low depth of ions penetration into substrate. For nitriding of titanium and its alloys the traditional gaseous nitriding should be performed at temperatures С and higher [], and the low-temperature ion-vacuum nitriding at 0 С []. At the same time the characteristics of materials can be improved by severe plastic deformation methods. As the result of such treatment the nano- or ultrafine grain state could be produced in the bulk of material. This leads to increasing of strength and plasticity of material []. Combining methods of severe plastic deformation and a nitriding technique the essential better results can be achieved comparing with traditional methods of improving of materials characteristics. But nitriding of technically pure titanium in nanoor ultra-fine grain state at temperature 0 С and higher will lead to the bulk recrystallization of material. Thus it is necessary to solve the problem of decreasing of nitriding temperature up to 0 С for titanium and its alloys. For investigations the following methods were chosen the nitriding in plasma of non-selfsustained low-pressure arc discharge [], the nitriding in plasma of non-selfsustained glow discharge [], and the nitriding in plasma of low-energy electron beam []. The materials for investigations were technically pure titanium VT-0 and α + β titanium alloy VT (Ti Al.V Mo). Nitriding was performed at temperature 0 С in gaseous mixtures of nitrogen-argon and nitrogen-helium during h.. Experimental schemes The nitriding in plasma of non-selfsustained low-pressure arc discharge (NSAD) was done on method described in []. Experimental scheme presented in Fig.. to pump Fig.. Experimental scheme on nitriding in plasma of NSAD: hollow cathode; heated cathode; magnetic coil; vacuum chamber; samples holder; samples; 7 thermocouple; B/V bias voltage power supply; PS- and PS- power supplies of plasma sources; E-PS power supply for e-regime 7 B/V E-PS PS- PS- The work was supported by State contracts Nos and P9. Analytical equipment of CJU Diagnostics of structure and properties of nanomaterials of BSU was used.

2 Poster Session The working gases (Ar, N ) are letting in through hollow cathodes of plasmagenerators in to vacuum chamber, preliminary pumped by diffusion pump up to 0 Pa. After setting pressure of p = 0.9 Pa the heated cathodes are igniting and voltage of 70 V is applying between hollow cathode and vacuum chamber. This leads to initiation of lowpressure diffusion arc with heated cathodes. Such discharge effectively generates low-temperature gas-discharge plasma without arc spot. An electrical scheme was developed for realizing the low-temperature nitriding. The scheme allows applying to the substrate holder a negative voltage (the regime of cleaning by ions and nitriding) or a positive voltage (the regime of heating by electrons). The requirement of usage of this scheme was caused by tendency to decrease bias voltage for reducing sputtering of surface by ions. In other hand during bombardment of surface by electrons the probability of generation of atomic nitrogen increases. It is well known fact that atomic nitrogen plays main role in nitriding of materials [7]. The position of keys in nitriding regime with negative bias is shown in Fig.. Changing position of keys leads to applying positive voltage to samples. The nitriding process was done by the alternating of keys positions. Scheme of nitriding in plasma of non-selfsustained glow discharge (NSGD) is presented in Fig.. Vacuum chamber with dimensions mm is a hollow cathode of glow discharge. It was pumped by turbo-molecular pump TMN-00 up to residual pressure 0 Pa. Water I gl U gl 7 discharge. They form equipotential gap for separation of plasma of main discharge and additional discharges. Thus in electrode system of such type the nonselfsustained glow discharge is realized. The discharge is ignited and sustained by selfsustained arc discharge with cold hollow cathode. The nitriding process in this scheme was done on following sequence. A mixture of working gases has filled in vacuum chamber up to pressure р = Pa. The arc discharge has ignited to forming initial plasma. Non-selfsustained glow discharge has ignited with current up to 0 A, when voltage (00 00 V) was applied between anode of glow discharge and vacuum chamber. In working regime ion current density was ma/cm and plasma density was cm. Samples were mounted under potential of hollow cathode. Samples temperature was measured by chromelalumel thermocouple. Heating of samples and cleaning of their surface were done by ion bombardment. Ions were extracted from plasma of non-selfsustained glow discharge and were accelerated in cathode layer. In experiments on nitriding in plasma of low-energy electron beam (LEEB) source of electrons with plasma cathode [], based on low-pressure glow discharge, was used. An electrode system of the source (Fig. ) consist of hollow cathode and hollow anode with diameter of 0 mm and length of 00 mm each one. N, Ar PS 0 kv PS 0 kv PS 0 00 kv Fig.. Experimental scheme on nitriding in plasma of NSGD: water-cooled tubular anode of glow discharge; vacuum chamber cathode of glow discharge; grid anode of arc discharge; ignition electrode of arc discharge; magnetic coil; diaphragm; 7 treated samples The pressure regulates in range of 0. Pa when working gas fills into the chamber. Glow discharge was initiated between water-cooled tubular anode (square 0 cm ) and hollow cathode (volume 0. m ). An electron source based on arc discharge with hollow cathode [] is used for relief of ignition of glow discharge and steady-state combustion at low pressures. An electron emission becomes from plasma, which is generated by arc discharge, through cells of finestructure ( mm) grids. Grids are anode for arc Pump N, Ar Fig.. Experimental scheme on nitriding in plasma of LEEB: hollow cathode; hollow anode; grid; electron beam; working chamber; samples holder The cathode and anode are connected through aperture with diameter of 0 mm. Electrons have been extracted through cells of titanium grid from plasma of anode part of glow discharge. A square of the grid is 80 cm and it defines the value of cross section of electron beam. Electron source is mounted on working chamber. Electrically isolated holder with 8 samples is situated inside the chamber. Distance between the grid and the holder is 0 mm. A measuring of temperature was done by a

3 thermocouple connected to one of samples by contact welding. The working chamber was pumped by turbomolecular pump TMN-00 up to residual pressure 0 Pa. Nitriding was done at pressure of Pa of gaseous mixture of argon-nitrogen or helium-nitrogen. Dual-channel gas feed system has provided separate feeding of gases in to electrode system of the source and in to vacuum chamber. Electron beam was formed in layer of space discharge between beam plasma and grid of plasma cathode. Beam current was A and accelerating voltage was 0 V. Negative bias voltage of 0 V was applied to samples. Temperature of samples was 0 С in given conditions. Ion sputtering of samples surface was performed before nitriding.. Experimental results After performing of experiments on nitriding in three types of discharges it was found following (the Table). Nitriding of technically pure titanium in plasma of NSGD allows increasing surface microhardness up to 7%, in plasma of NSAD up to %, and in plasma of LEEB up to 7.% (0%He 0%N ) and up to.% (0%Ar 0%N ). Increasing of hardness of samples surface is caused not only by formation of layer of diffusion saturation by nitrogen but by formation of thin nitrided layer. From Figure, a it can be seen that nitriding in plasma of NSAD leads to formation of dense layer of highly dispersed particles which could reduce diffusion rate of nitrogen in bulk of material. At the same time on surface of samples nitrided in plasma of LEEB (Figs., c and d) formation of such particles wasn t found. Accordingly the diffusion layer in this case will be deeper and stronger increasing of microhardness comparing with other discharges indicates on it. Nitriding of titanium alloy VT don t lead to significant increasing of surface microhardness. For instance Values of surface microhardness before and after nitriding in three different types of discharges Surface microhardness, GPa Type Mixture of gases VT-0 VT of discharge Initial After nitriding Initial After nitriding NSAD 0%Ar 0%N..7 0%Ar 0%N..7 LEEB.. 0%He 0%N. NSGD 0%He 0%N.8.8 a b c d Fig.. Surface morphology of technically pure titanium VT-0 after low-temperature nitriding: in plasma of NSAD (а), in plasma of NSGD (b), in plasma of LEEB (c and d). Scanning electron microscopy, 0 000

4 Poster Session a b c d Fig.. Surface morphology of titanium alloy VT after low-temperature nitriding: in plasma of NSAD (а), in plasma of NSGD (b), in plasma of LEEB (c and d). Scanning electron microscopy, instance, after nitriding in plasma of NSGD increasing of surface microhardness is %, in plasma of LEEB.% and 9%, and in plasma of NSAD % from initial value. Insignificant increasing of surface microhardness of titanium alloy VT can be explained by the fact that the presence of alloying elements Al, V, Mo in alloy VT causes low velocity of nitrogen diffusion comparing with α-ti [8]. Formation of layer of highly dispersed particles on surface in the aggregate with low temperature of nitriding is also constraining the velocity of nitrogen diffusion. As the result of these limitations under nitrided layer the diffusion layer of small depth was formed. The small depth of the layer doesn t allow possibility of measuring of changes of surface microhardness by method of microindentation. It is known that efficiency of nitriding is defined by diffusion processes in a bulk of modified material. The efficiency is increasing with increasing of material temperature and reducing of activation energy. The last is depended on density of such active particles coming to a surface of material as atomic nitrogen in ionized and neutral state [9]. In contrast to methods of nitriding in gas phase when conditions for passing of diffusion processes are proceeded at temperatures above 900 С [0], investigated methods of ionplasma nitriding (NSAD, NSGD, LEEB) provide fulfillment of conditions for diffusion of nitrogen in to titanium substrate at low temperatures (< 0 C). This achieved by cleaning of oxide film and other impurities which are diffusion barrier for nitrogen atoms, by activation of substrate surface by flows of charged particles and by effective generation of atomic nitrogen not only at a surface of samples but in a volume of plasma chamber.. Conclusion Thus, the principal possibility of low-temperature (< 0 С) nitriding of technically pure titanium VT- 0 and titanium alloy VT was shown with usage of three types of different discharge systems. Nitriding at 0 С has allowed increasing surface microhardness up to.% for technically pure titanium and up to % for titanium alloy VT. Performing of process in three different types of discharge systems has shown high efficiency of nitriding of technically pure titanium and titanium alloy VT. References [] B. Arzamasov, A. Bratukhin, Yu. Eliseev, and T. Panaioti, Ion Chemical-Thermal Treatment of Alloys, Moscow, Izd-vo MGTU im. N.E. Baumana, 999, 00 p. [] A. Il in and S.Skvortsova et al., Metally, 8 (00). [] Yu. Kolobov, Russian Nanotechnologies, (9 0), 9 (009). [] D. Vershinin, T. Vershinina, Yu. Kolobov, M. Smolyakova, and O. Druchinina, in Proc. of 8th Int.

5 Conf. Interaction of Radiations with Solids, 009, pp. 0. [] P. Schanin, N. Koval, Yu. Akhmadeev, and S. Grigor ev, Rus. J. Tech. Phys. 7, (00). [] N. Gavrilov and A. Mamaev, Letters in Rus. J. Tech. Physics,, 7 (009). [7] I. Pastukh, Teoriya i Praktika Bezvodorodnogo Azotirovaniya v Tleyuschem Razryade, Kharkov, NNTs KhFTI, 00, p. [8] A. Tumanov, S. Glazunov, and A. Khorev, Alloying and thermal treatment of titanium alloys, Moscow, 977, pp [9] N. Gavrilov, A. Mamaev, and A. Medvedev, Izv. Vysh. Ucheb. Zavedenii. Fizika,, (009). [0] S. Malinov, A. Zecheva, and V. Sha, Materialovedenie i Termicheskaya Obrabotka Metallov 7, (00).