The questions of consolidation of nanopowders concerning hot compaction by pressing activated by electric current action are considered. Mechanisms of grain boundary creep-sliding which are sequentially prevalent in a forming of compacted structures under influence of temperature factor and in the presence of a direct electric heating are discussed. Structural-transformational sources and conditions of forming of high physical-mechanical properties of nanopowder refractory solid-state products are described

Edwin GEVORKYAN

Yury GUTSALENKO

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                                                  Fiabilitate si Durabilitate - Fiability & Durability     nr.1/2010                                 Editura “Academica Brâncuşi” , Târgu Jiu, ISSN 1844 – 640X  15 ACTIVATED HOT PRESSING BEHAVIOR OF WC NANOPOWDERS  Edwin GEVORKYAN1, Yury GUTSALENKO2 1Ukrainian State Academy of Railway Transport, Ukraine 2National Technical University “Kharkov Polytechnic Institute”, Ukraine  Abstract: The questions of consolidation of nanopowders concerning hot compaction by pressing activated by electric current action are considered. Mechanisms of grain boundary creep-sliding which are sequentially prevalent in a forming of compacted structures under influence of temperature factor and in the presence of a direct electric heating are discussed. Structural-transformational sources and conditions of forming of high physical-mechanical properties of nanopowder refractory solid-state products are described.  Keywords: growth, graphite, powder.   1. Introduction The urgency of a consolidation problem of nanopowders refractory materials of a tool assignment  is  connected  to  formation  of  the  structure  determining  physicomechanical properties of solid-state result of sintering. In research [1] we considered questions of sintering of submicronic powders Al2O3 and WC activated by an electric field (FAST technology developed at the Californian university USA by J. Groza, et al. [2-4]). At the same time, in our opinion, from all methods of compacting nanopowder materials for refractory powders the method of hot pressing with current straight transmission is the most effective. This point of view is explained with a number of factors: an opportunity of fast heating of the graphite form up to high temperature; relative simplicity of installation and an opportunity of use of a usual alternating current of industrial frequency that excludes use of expensive pulse generators; an opportunity of reception of preparations concerning sizable; fast heating allows to limit intensive growth nanograins an initial powder, leaving their sizes in nanolimits. If process of an electric current influence on sintering of metal powders is investigated enough  [5],  process  of  hot  pressing  with  application  of  an  electric  current  at  sintering nanopowders,  despite  of  numerous  foreign  publications  in  the  field  of  sintering  these powders,  is  investigated  a  little,  is  especial  regarding  mechanisms  of  refractory  powders sintering connections without which it is practically impossible to present the modern tool industry.  Processes  of  sintering  of  usual  powders  of  refractory  connections  are  in  detail considered in work [6]. Authors [4, 7] investigate also the certain regularities of sintering various nonmetallic nanopowders, including refractory connections. In given article some results of research of sintering mechanisms regularities mainly monocarbide nanopowders tungsten are submitted during hot pressing in graphite forms at transmission  through  a  powder  of  an  electric  current  (direct  electroheating)  with  use  in researches approved [1] foreign WC powders (Aldrich Co, Austria). Carried  out  researches  have  shown,  that  from  monocarbide  nanopowders  tungsten  (40-70 nanometers)  it  is  possible  to  receive  practically  a  pore-free  material  with  rather  high physicomechanical  properties  [8].  It  is  obvious,  that  on  contact  sites  between  the  next particles under influence of the big electric current should occur intensive mass transfer. In result in powder pressings there is a process of fast sintering. Depending on parameters of process (pressure, force of a current, a pressure, speed of heating, time of heating) the course of sintering can occur differently. In this connection over a wide range can change structure and properties of a material.                                                   Fiabilitate si Durabilitate - Fiability & Durability     nr.1/2010                                 Editura “Academica Brâncuşi” , Târgu Jiu, ISSN 1844 – 640X  16 2. Experimental conditions Preliminary preforming carbide powder tungsten was exposed to sintering in installation with the special vacuum chamber. The maximal pressure for used graphite МПГ-7 about 20 кN at temperature is higher 1200ºС, therefore for a compression mould d=20 mm created the maximal effort no more than 15 KN. Density of samples after grinding a surface determined by hydrostatic weighing, and chips on their surfaces, as well as a kind of a powder, by raster electronic microscope JSM-840.  3. Investigation The  temperature  of  the  termination  shrinkage  WC  product  has  made  1650ºС.  The received  material  has  shown  high  hardness  and  density.  Some  properties  of  the  received samples at various effective modes of hot pressing are resulted in tab. 1. As have shown experiments, optimum modes are in rather narrow limits, in particular the temperature factor most determining from them 1700-1800 ºС. Pressure of hot pressing is limited by durability of  a  graphite  compression  mould  at  the  given  temperatures,  and  optimum  speed  of  rise temperature is within the limits of 400-500 ºС/min though, with the purpose of the warning of breakage of the form, up to 900ºС the temperature rose more smoothly, with a speed of 50-250ºС/min.  Table 1 - Some physicomechanical properties received WC samples  Р, МPа  Т, °С  ,%facttheoryrr  HRA  σflex, МPа  dmean, μm 40,0  1700  96,2  93  530  0,350 50,0  1730  99,0  95  720  0,420 50,0  1800  99,2  95  670  0,750  Process of sintering is accompanied by a number phenomena which were fixed by us with the help of various devices and tools. Supervision over these phenomena and over their change under change of technological parameters allows to operate process of sintering in a required  direction  with  the  purpose  of  reception  of  materials  with  the  set  level  of  the properties providing reception of a product set form and the sizes.  The refractory body of tungsten monocarbide combines high hardness and the raised fragility. Shrinkage of preparations, as a rule, it is accompanied with reduction of quantity of defects of crystal structure, a roughness of a particles surface, porosity, and also growth of grains and homogenization. Directly it is possible to relate to the process of sintering only two phenomena:  growth  of  the  area  of  contact  between  particles  and  rapproachement  of  the centers of particles, i.e. shrinkage actually, other listed phenomena are accompanying.  4. Analysis and generalization of results As  is  known,  sintering  approaches  the  system  consisting  of  separate  particles,  to thermodynamic  balance,  i.e.  it  is  process  during  which  superfluous  energy  of  system decreases. It is possible to assume, what exactly this energy of a free surface, surfaces of borders and defects of a crystal lattice is the main driving force of sintering.                                                    Fiabilitate si Durabilitate - Fiability & Durability     nr.1/2010                                 Editura “Academica Brâncuşi” , Târgu Jiu, ISSN 1844 – 640X  17 The size of this driving force can be estimated, taking into account the sizes of particles from which preparation is formed. For example, the specific surface of carbide powders after milling usually makes 1-5 m2/g, the powders received by plasmo-chemical synthesis, - 10-40 m2/g [6]. If to accept, that as a result of any formation method preparations by porosity of 40 % it is obvious turn out, that superfluous energy of such system is  great enough.  It  also performs the basic work on condensation, causing current of substance in emptiness (pores). With some approximation, following authors [6], we can admit, that current of substance to a surface  with  negative  curvature  is  carried  out  under  some  effective  pressure  Р  caused superficial tension: Р=2γ /r,  (1)  where γ - free superficial energy, kJ/μm2; r - the size of a grain, μm. This pressure is great enough for pressing from nanodispersed particles where the size of pores is comparable with the size of particles. The equivalent pressure working at least in an initial stage of sintering, for nanodispersed powders is very great and, naturally, causes fast condensation έ. In general temperature έ-Т dependences έ=A Р Vexp(-Eап/RT),   (2)  where A, R, v - constants (R – a gas constant); Еan-energy of activation, kJ, in an initial stage of sintering on the mechanism of creep P=Pcreep. The following driving force of sintering is energy of a crystal lattice imperfections. It serves as the reason of the accelerated exchange of places: nа = nоехр(-Еаан/RТ),  (3) where nа - number of atoms which have left the place in a lattice and have occupied vacancy unit;  n0  -  the  structural  factor,  not  dependent  on  temperature;  Еaан  -  energy  of  activation annihilator defects, kJ. This size in the broken lattice strongly goes down. Therefore for active compaction of a material it is necessary to break structure as it is possible strongly, crushing substance,  as  that  occurs  in  nanopowders  received  in  the  plasmo-chemical  way.  During sintering the quantity  of imperfections of a  crystal  lattice considerably decreases.  So, for example, in monocarbide tungsten with a specific surface up to 1m2/g the size of coherent areas  makes  20000-50000  nanometers,  and  after  sintering  at  temperature  0,8  [Т]  ([Т]  – melting temperature) makes already 200000-500000 nanometers. Besides it has been noticed, that at sintering at the moment of an electric current passage of enough the big size (up to 5000-8000 А) in zones of interpartial contacts arise electric discharges which according to physics of this process should cause formation of plasma, that naturally results in clearing and activization of a surface of sintered powders. Clearing of a surface  of  particles of a powder in turn results in  formation  of  pure  borders  of  grains  and activation of the sintering process.    5. Conclusion Thus,  the  lead  researches  allow  to  assume,  that  rather  high  physicomechanical properties of the received samples from monocarbide nanopowders tungsten are caused, first of all, by superfine grains and strong borders between them, that is provided with high speed of rise of temperature up to 1700ºС during hot pressing in graphite forms.                                                   Fiabilitate si Durabilitate - Fiability & Durability     nr.1/2010                                 Editura “Academica Brâncuşi” , Târgu Jiu, ISSN 1844 – 640X  18   The given researches allow to assume, that the method of hot pressing nanopowders at heating to straight transmission an alternating current accelerates a stream of vacancies on a interstice surface, and fast reduction of porosity on borders of grains conducts to thermally activated sliding nanograins from each other, that finally, together with imposing of a variable electric field, provides the best stacking grains, and, hence, provides their faster compaction. At the same time, supervision for  shrinkage of  nanopowder samples at  hot pressing with direct  transmission  a current  at  temperatures  up to 900-1000ºС allow to assume, that the initial mechanism of consolidation is the mechanism of creep.   References [1] Gevorkyan, E.S.; Gutsalenko Yu.G.: Sintering of AL2O3 and WC powders activated by electric field. In: Proceedings of 5th International Conference “Research and Development in Mechanical Industry – RaDMI 2005”, Vrnjacka Banja, Serbia and Montenegro, 04-07. September 2005. Edited by Predreg Dasic. Trstenik: High Technical Mechanical School, 2005, pp. 694-695. ISBN 86-83803-20-1. [2] Bourell D.L.; Groza J.R.: Consolidation of ultrafine and nanocrystalline powders. ASM Handbook, Powder Metallurgy, 1998, vol. 7, pp. 504-515. [3] Groza J.R.: Field assisted sintering. ASM Handbook, Powder Metallurgy, 1998, vol. 7, pp. 583-589. [4] Groza J.R. Powder consolidation. Chapter in: Non-Equilibrium Processing of Materials. Edited  by  C.  Suryanarayana.  In:  Pergamon  Materials  Series.  Edited  by  R.W.  Cahn. Pergamon Press, Elsevier, 1999, pp. 345-372. [5] Raychenko A.I.: Foundation of powder sintering process by electric current. Moscow, Russia, Publishing House “Metallurgy”, 1987, 128 p. (in Russian). [6] KISLYI, P.S.; BODNARUK, N.I.; GORICHOK, I.O.; et al.: Physicochemical foundation of receipt of superhard materials. Kiev, Ukraine, Publishing House “Naukova dumka”, 1986, 208 p. (in Russian). [7]  Skorohod  V.V.;  Uvarova  A.V.;  Ragulya  A.V.:  Physicochemical  kinetics  in nanostructural systems. Kiev, Ukraine, Publishing House “Institute of Material Science Problem”, 2001, 180 p. ISBN 966-8002-09-1 (in Russian). [8]  Kodash  V.Yu.;  Gevorkyan  E.S.:  Tungsten  curbide  cutting  tool  materials.  U.S.  Pat. №6617271, Sept. 9, 2003.                

ACTIVATED HOT PRESSING BEHAVIOR OF WC NANOPOWDERS

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