STRUCTURE AND TRIBOLOGICAL PROPERTIES OF Cu-Sn ALLOYS BASED ON MECHANICAL ACTIVATED POWDERS FORMED BY ELECTROCONTACT SINTERING

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1 STRUCTURE AND TRIBOLOGICAL PROPERTIES OF Cu-Sn ALLOYS BASED ON MECHANICAL ACTIVATED POWDERS FORMED BY ELECTROCONTACT SINTERING Vityaz 1 P. A., Kovaliova 1 S. A., Zornik 1 V. I., Kukareko 1 V.A., Grigorieva 2 T. F., Gamzeleva 3 T. V., Lyakhov 1 N. Z. 1 Joint Institute of Mechanical Engineering of the NAS of Belarus, , Minsk, 12 Akademicheskaya str., Belarus, 2 Institute of Solid State Chemistry and Mechanochemistry, Novosibirsk, 18 Kutateladze str., Russia, 3 State Scientific Institution "Powder Metallurgy Institute", Minsk, 41 Platonov Str., Belarus sveta_kovaleva@tut.by Electrocontact sintering is a highly effective and intensively developed metod to consolidate the powders at present [1, 2]. High efficiency, the ability to implement without creating a protective atmosphere, environmental cleanliness are advantages of this technology. The principal difference of this method of sintering of powder materials from conventional technologies is the high concentration of the released energy on contacts of powder particles. The most advantage of electrocontact sintering is the combination forming and sintering in one operation, which allows to receive a significant reduction temporal and energy costs. The bronze coating produced by conventional methods of sintering through the formation of the liquid phase has a number of the main disadvantage associated with a high degree of liquation in the copper-tin alloys. In recent years mechanochemical approach to create new materials is actively developed and make possible not only to grind up materials, but also synthesize the composites from different materials [3]. Phase composition, shape and size of powders obtained in this method are determined by the mechanical modes of activation. The aim of the present work is study the effect of preliminary mechanical activation (MA) of powders of Cu-20%Sn on the structure and tribological properties of coatings produced by electrocontact sintering. Copper powder PMS-1 and tin powder PTE-1 with size particle about μk were used in the work. Mechanical activation of the 181

2 composition powders Cu+20 mas. % Sn was carried out during 20 and 40s, 4, 8 and 20 min in the high-energy ball planetary mill AGO-2 with water cooling, in argon atmosphere (cylinder volume: 200 cm 3, ball diameter: 5mm, loaded mass 200: g, the weighed portion of the sample under treatment: 10g, the frequency of rotation of the cylinders around the common axis about 1000 r.p.m.). Electrocontact sintering of powder samples of size Ø10x10 mm has been performed using the SSM-02 module and with a current density of 10,8 ka/sm 2 and a pressure of 10 MPa during the 45 s. The pulse duration and pause were 0,02 s. X-ray diffraction measurements were carried out on a diffractometer D8 Advance Bruker (Germany) with a step of 0,05 in CuKα radiation. The phase analyses were accomplished with using a database PDF-2. X-ray diffraction analysis (XRD) were executed by means of Whole-Powder-Pattern-Decomposion with Rietveld refinement analysis in the TOPAS software (Bruker AXS). The peak shape was assumed to be a pseudo-voigt (pv) function. The structure of the resulting samples was studied using optical microscope Micro 200 and high-resolution scanning electron microscope (SEM) MIRA\TESCAN with an attachment for micro-x-ray spectral analysis (MXSA). The diameter of the electronic probe was 5,2 nm. Images were obtained in the mode of recording secondary and backward scattered electrons, which allowed us to investigate the distribution of chemical elements over the surface and to establish its composition inhomogeneity. Quantitative stereological analysis of the sample was carried out using the program complex image processing and analysis «Autoscan». Tribological testing was carried out in the mode of friction without lubrication by automated tribometer ATVP [4] on the scheme reciprocating motion at the counterbody. As a counterbody the plate made of steel with a hardness of Hµ = 7840 MPa was used. The sliding velocity of the sample during the tests were ~ 0.1m/s. Nominal load was P = 1 MPa. Measurement of wear was carried out on analytical scales ADV-200M. Wear intensity was determined as the ratio of the loss mass to sliding distance. The studies of phase analyses of precursor powders showed that a some formation of an intermediate intermetallic on the early stages of mechanical activation (about 20s) eta-cu6, 26Sn5 is formed. Phase composition of powder after 40 s MA consist of copper (PDF

3 0836), tin (PDF ) and a small amount of intermetallic eta- Cu6, 26Sn5 (PDF ) (Fig. 1). Fig. 1. X-ray diffractograms for powder Сu+20%Sn after mechanical activation during: а 40 s, b 4 min, c 20 min The maximum content of the intermetallic compound are reached to 4 min MA (~12%). Increasing of the time of mechanical activation more then 4 min leads to decreases the tin content and then it extincts to 8 min MA. The data of lattice parameter copper depend of the time activation time. The lattice parameters the copper is almost unchanged in during activation 8 min. The formation of single-phase powder of the solid solution tin in copper begins after 8 min (fig 2). Fig 2. Lattice parameter a of copper as functions of MA time 183

4 SEM image of the microstructure of powders obtained with different durations of mechanical activation are shown on Fig. 3. а b c Fig. 3. SEM images in backscattered electrons of the Сu-20%Sn powders: а - 40 s; b - 4 min; c - 20 min It may be noted that the activation within 40 s decreases particle size of the copper powder in average to 1-2 microns and agglomerates with a broad distribution of sizes (from 5 to 60 microns) are formed (Fig.4). The large aggregates (~ 1% by number and ~ 19% by weight) with the size of microns contain tin. MXSA analysis showed that tin was not is distributed homogeneously (Fig. 3a, b). Fig 4. Size distribution of aggregate for powder of Cu-20%Sn with different time of mechanical activation 184

5 Increasing time of the activation up to 4 min leads to the formation of the flake agglomerates which have the laminated structure and heterogeneity of the tin distribution. The average size of particles have increased to 60 microns. The range of sizes of the solid solution Cu (Sn) aggregates after 20 min of activation are microns. Although the size of the powder particles after activation for 20 min significantly more big than on the initial stages, it has a higher bulk density about 3.76 g/cm 3 (after 40s of MA it is 3,04g/cm 3 ). This effect may be explained by the formation of dendrite shape of particles powders from an early stage of activation. However, with increasing duration of mechanical activation compressibility of powder is reduced, which is probably associated with an increase in the content of a more solidity phase of the solid solution Cu (Sn). Thus, on initial stages of mechanical activation of powder Cu+20%Sn take place particles grinding and forming small aggregates with large crystallite size. An increase of the activation time is accompanied by a plastic deformation and changing the shapes of aggregates. At same time the size of the crystallite decreases. In the case of activation for 20 min the solubility of tin in copper are increased and oversaturated single-phase of the solid solution Cu (Sn) with a crystallite size <L> about 8 nm are formed. Electrocontact sintering is characterized by some physical processes in the contact zones, leading to high spatial inhomogeneity and non-stationarity of the processes in time. There are the heating to very high temperatures and melting of the most fusible phases and the reactions of them interaction at a moment of a short electric discharge. To study the effect of preliminary MA of powders on the formation of the alloys by electric pulse sintering were chosen precursors with the various durations of the activation and with different structures: 1 - the grinded initial components Cu+Sn (40s MA), 2 - with intermediate intermetallic compounds Cu+Sn+η-Cu 6,26 Sn 5 (4 min MA), 3 - the single phase oversaturated solid solution Cu(Sn) (20 min MA). The diffraction patterns obtained alloys are shown in Figure 5. It may be noted that an intermediate phase eta-cu6, 26Sn5 in the sintered composites is missing. In the alloys based on powders with phases compositions 1 and 2 are formed an intermetallic compound Cu 41 Sn 11 (PDF ), as well as a α-phase solid solutions based on copper with a variable concentration of tin. The content of solid solution 185

6 Fig. 5. X-ray diffractograms for alloys obtained by electric pulse sintering of the precursors Cu +20% Sn after time MA: а 40 s, b 4 min, c 20 min with a maximum tin concentration increases for the precursor with more time activation. Electric pulse sintering of a single-phase composition solid solution of Cu (Sn) forms single-phase composition of the solid solution based on copper with a tin concentration of ~ 8at.% Sn [5]. The values of the structural parameters of the sintered alloys are shown in the table1. Table 1 Parameters of the alloys obtained by electric pulse sintering of the precursors Cu +20% Sn with different duration of MA Time of MA precursors Сu Lattice parameter а, Å Cu-Sn, 8 at.% Sn Cu 41 Sn 11 Сu Cu-Sn, 8 at.% Sn Crystallite size <L>, nm Cu 41 Sn 11 Standart 3,615 3,694 17, param. 40 s 3,616(9) 3,702(0) 17,991(4) min 3,624(3) 3,698(7) 17,977(3) min - 3,697(8)

7 Depending of mechanical activation time of precursors on produced the structure parameters alloys changes significantly. Figure 6 shows the optical images of the surface alloys. It may be noted that the coatings obtained by sintering of powder precursors with less time of activation, have a more dense structure and small number of discontinuities. Using of single phases solid solution precursor leads to the discontinuities increases to about 8% and their size can be up to~60 µm. Fig. 6. Optical images of the polish coating surface on the basis of precursor Cu +20% Sn with the MA time: a - 40 s, b - 4 min, c - 20 min Microprobe studies (Fig.7) established that the using of activation powders with fusible phase tin leads to form the structure of the α-phase solid solution of tin in copper and the eutectoid (intermetallic compound δ-phase Cu 41 Sn 11 + α-cu). Intermetallic compound Cu 41 Sn 11 is characterized by high hardness and brittleness. The using of single-phase solid solution allows to achieve a homogeneous distribution of tin in the coating to its content ~ 16,7 wt.% (9.7 at.%). Analysis of the structural parameters of initial powders and obtained coatings shows that the initial phase composition has a significant effect on the mechanism of sintering and structures alloys formation. The presence tin in the initial phase of the powders leads to the melting of fusible phase during electric-pulse sintering and the eutectoid transformation has been observed. Sintering of the solid solution Cu (Sn) powders occurs at interparticle boundaries by way solid-phase reaction. In this case, a coarse-grained structure is formed. It may be noted that recrystallization take occurs during the sintering. The crystallites size of the solid solution grows from 8 nm in the precursor powder to 157 nm in the coating. 187

8 a b Fig. 7. SEM in the backscattered electrons and microprobe analysis data from the coatings based on precursors with phase content: a -Cu+Sn; b-cu(sn) The results of measurements of microhardness, friction and wear resistance of copper alloys are shown in Table 2 and Figure 8. Table 2 Received data of Microhardness and Friction Coefficient of copper alloys based on mechanical activation precursors Phases composition of powder precursor Time of MA precursor Microhardness Hμ, МПа ( ± 40 МПа) Friction Сoefficient Wear intensity, mg/m Cu+Sn 40 s ,02-1,11 0,0049 Cu+Sn+ 4 min ,00-1,04 0,053 η-cu 6,26 Sn 5 Cu(Sn) 20 min ,8-0,85 0,087 The microhardness of the alloy on the basis of single-phase powder of the solid solution Cu (Sn) is bigger than of alloys with the eutectoid structure while recent alloys are more brittle. When measuring the microhardness at low loads have appeared ductile cracks about an impress. Tribological tests conducted in the mode of dry friction against the steel counterbody showed that the duration of mechanical activation of precursor powder of Cu-20%Sn the influences on the initial stages of 188

9 intensive wear (running-in period) (Fig. 8b).The alloys based on precursor with 4 min of MA has longer running-in period 600 m as against 100 and 300 m for precursors with 40 s and 20 min accordingly. When the process moves to the stage of the friction of steady state wear has linear subjection of the wear from the friction path. It should be noted, that in the steady state wear stage the friction coefficient for the sample based on Cu (Sn) is lower ( f = ), than the alloys with eutectoid structure, and but its mass wear is large. This effect is caused mainly the fact that the sintering of the particles of the solid solution Cu (Sn) at this mode electrocontact impact has the character of the initial stage of solid-phase sintering, in which the particles retain their individuality, and the alloys are characterized by high inter-particle porosity. At tribology testing of alloys based on powder of solid solution Cu (Sn) the particles are chipping. a b Fig. 8. Mass wear (a) and wear intensity (b) of copper alloys as functions of the friction path at the precursors with different time of MA Conclusion As a result of the carried out studies of the structures of mechanical activation powders compounds Cu +20% Sn and alloys based on them received by the electrocontact pulse sintering have shown that preliminary mechanical activation leads not only to change the size 189

10 distribution of powders, but also to change in the phase-structural state. Formation of powder passes through the grinding stage, the plastic deformation with the formation of intermetallic compounds and the growth of aggregates of the solid solution. It is established that the electrocontact sintering of precursors which has got a fusible phases of tin leads to the melting of it and the formation of the alloy passes through the liquid-phase sintering mechanism. The result is structure of α-solid solution of copper and eutectoid. Electrocontact sintering the powder of the solid solution passes through the mechanism of solid-phase sintering and leads to the formation of a homogeneous coating on the basis of solid solution of Cu (Sn) with a significant number of discontinuities and coarse-grained structure. Acknowledgement The work is carried out under the Integration Project of SB RAS No. 19 and BRFFI No.X12CO-009. References 1. Khasanov A. O., Dvilis E.S, Kachaev A.A. The method of collector compacting nano-and polydisperse powders: tutorial. Tomsk: Publishing house of Tomsk Polytechnic University, p. 2. Grigorev E.G. Modeling of physical processes of electric-pulse sintering of powder materials// Scientific session of the MiFi T. 3. Grigorieva T. F., Barinova A. P., Lyakhov N. Z. Mechanochemical synthesis in metal systems. Novosibirsk, M.G. Karavaev, V.A. Kukareko. In Book of Materials of Intern. Scientific and Technical Conf. "The reliability of machines and technical systems", Minsk (2001), T. 1, P De M. Thermal expansion of some Cu-and Ag-base alloys at high temperatures Indian Journal of Physics (or Proceedings of the Indian Association for the Cultivation of Science) (1969) 43,