Mechanism of conductive powder microstructure evolution in the process of SPS

258Science in China Ser. E Engineering & Materials Science 2005 Vol.48 No.3 258- 269Mechanism of conductive powdermicrostructure evolution in the process of SPSSONG Xiaoyan, LIU Xuemei & ZHANG JiuxingCollege of Materials Science and Engineering, Key Laboratory of Advanced Functional Materials of Min-istry of Education of China, Beijing Univrsity of Technology, Being 100022, ChinaCorrespondence should be addressed to Song Xiaoyan (email: xysong@bjut.edu.cn)Received November 22, 2004Abstract Spark plasma sintering (SPS) is an advanced sintering technology that hasbeen recently developed in the world. Contrast to many reports about experimentalinvestigations on the methods of new materials preparation, there are very few systematicstudies on the special sintering mechanism of SPS technology. In the present paper, byusing the pure electrolytic copper powders as the raw material, a series of sinteringexperiments have been designed and carried out. The evolution of the powdermicrostructures during SPS has been systematically studied, and for the first time a“self- adjusting mechanism”of the microstructure evolution is proposed, from which theessential for the advantages of materials preparation by SPS in the respects of highdensity, homogeneity and fine grain structure can be well understood. In addition, thechanges of the relative density during SPS are quantitatively predicted by a theoreticalmodel and confirmed by the experimental measurements.Keywords: spark plasma sintering, microstructure evolution, sintering mechanism.DOI: 10.1360/ 04ye0265Spark plasma sintering (SPS) is an advanced sintering technology that has been 1e-cently developed mainly in Japan. It has the advantages of the rapid heating rate (1000°C/min), short holding time (3- -5 min), rapid cooling, controllable pressure and sinteringatmosphere, as well as the special feature of the environment compatibilityh. Up to nowit has been used to fabricate various materials such as pure metal, alloys, ceramics,composites and polymer materials, especially, some new-type materials, e.g. bulkamorphous and nano materials, multi-scaled structure- and function- graded materials,and etc.24.The major parts of the SPS equipment are: a DC pulse generator, pressure unit, a-mosphere control unit, processing parameters control unit, measuring unit, cooling unit,punch and mould. The powder is held in the die, a nressure normallv of at least 10 MPais exerted on the upper and lower punches, and an中国煤化工arrent withan order of magnitude as high as one thousand ampMYHCNMHGowing,theCopyright by Science in China Press 2005Mechanism of conductive powder microstructure evolution in the process of SPS259sintering process is controlled by computer program with the designed parameters ofsintering temperature, heating rate and holding time as the input. The whole sinteringprocess from the very beginning of powder installation till the acquirement of the con-solidated bulk material can be completed easily in 30 minutes.As an advanced and highly efficient technique of materials preparation, SPS has at-tracted the attention of more and more researchers in the field of materials science andthose related. It is noted that the annual international publications concerning SPS in-creased from only one in 1992 to 90 in 200351. However, contrast to many reports aboutexperimental investigations on the methods of new materials preparation, there are veryfew systematic studies on the sintering mechanism of SPS technology. In literature, sofar there are only a few publications concerning the complex effects from multi- fieldsthat may coexist during the process of SPS- ), but the quantitative descriptions in themodels have not been identified by experiments yet, and very few common conclusionshave been drawn in this field. Moreover, the consolidation mechanisms of the conduc-tive and insulative powders may be very different, which causes even more difficultiesto obtain a compre hensive understanding of the SPS mechanism. At the present, a quali-tative analysis on the consolidation mechanism for the conductive powders, which isagreed by a majority of researchersl6 8 is that the pulsed electric current flowing in fromthe upper punch is divided into a few branches: the part through the graphite die leads toa big amount of Joule heat that will be transferred to the powder sample; the partthrough the powder itself produces discharging between neighboring powder particles,consequently, some gas molecules are ionized, the generated ions and electrons movetowards the cathode and the anode 1espectively, thus the plasma is formed between thesurfaces of particles. With the increase of the plasma density, there is a bigger impactpressure on the surfaces of the powders exerted by the rapidly moving plasma flux,which will accelerate the sintering process by eliminating the gas and impurities ad-sorbed by the surface on one hand and the activating effect on the other hand. Owing tothe discharging heat produced between the disconnected particles and the Joule heatbetween the contact particles, locally very high-temperature zones are formed instanta-neously hence resulting in the local melting at the particle surfaces. Under the pressure,the neck between particles is formed when the melting zones are jointed and solidifiedafter the heat is transferred away rapidly along the surfaces. As described above, verydifferent from the conventional sintering technology, the special heating mechanism ofSPS makes this technology capable of sintering at a lower temperature and a fairlyshorter time than those of the conventional methods, whereas a rather high relative den-sity can be achieved. Therefore, the key problem in the conventional technologies thatthe relative density and the grain size tend to evolve in the opposite directions during thesintering can be well solved in the SPS technique.Due to the lack of the quantitative description and prediction of the SPS mechanism,at present, materials fabrication by SPS is usually中国煤化工atemptingof processing parameters combination. In thisYHCNMH c2on of newwww.scichina.com260Science in China Ser. E Engineering & Materials Science 2005 Vol.48 No.3 258- 269materials is apparently extended. To develop the model of SPS mechanism that describesthe kinetics of microstructure evolution during the consolidation process is significant todetermine the optimistic SPS processing for new materials preparation, as well as to ex-plore the potential applications of SPS technology. Therefore, in this paper, by using thepure copper powders as an example of the conductive materials, a series of sinteringexperiments have been designed. By means of studying the characteristics of micro-structures at different sintering stages, the mechanism for advantages of short time, highrelative density and fine grain structure of SPS technology is analyzed. In addition, aquantitative model that describes the consolidation process during SPS is proposed.1 Experimental procedureThe experiments are carried out in the SPS-3.2-MV sintering system with the pureelectrolytic copper powders as the raw materials. Samples for the optical microscope andthe scanning electron microscope (Quanta 2000) are prepared from the sintered materialto observe the microstructure morphology and the local configuration of neighboringparticles at different sintering stages.First, the characteristic sintering stages, which will be focused on in the followingstudy, should be distinguished based on the recorded curves of SPS process. Fig. 1shows variations of the sintering parameters with time till the relative density of 98.5%of the sintered sample is achieved. For the process, parameters of the heating rate 104C/min, the pressure 20 MPa and no holding time are given as constants. For the purposeof comparison and correspondence, all the varying parameters, i.e. pressure, voltage,electric current, temperature, displacement of the punch and the displacement rate, aredisplayed in the same figure (Fig. 1(a)) with the normalized values. We consider thatchanges of the displacement rate can express more precisely the consolidation processduring sintering. The temperature and time corresponding to the biggest peak of the dis-placement rate curve represents the period with the highest statistic probability that neckformation and growth of powders occur. Therefore, different peaks in the displacementrate curve may be used to determine the different sintering stages during SPS. The varia-tions of the absolute values of the electric current (I), temperature (T) and the punch dis-placement rate (dr) are shown in Fig. 1(b). According to the characteristics of the dis-placement rate curve, the different sintering states corresponding to the biggest peak, thetwo second-biggest peaks before the biggest one and the moment when the electric cur-rent was taken away (the point at which the displacement rate just became zero) are de-termined respectively. In other words, the characteristic sintering stages can be obtainedby using the temperatures of 201C, 315"C, 415°C and 827°C (see Fig. 1(b)) separatelyas the final sintering temperatures at the given heating rate, pressure and holding time.2 Experimental results and analysis2.1 Microstructure evolution in the process of SPS中国煤化工Microstructures of the original powders and thrent stages,observed with the high resolution scanning micros:MYHCNMH Gi in Fig. 2.Copyright by Science in China Press 2005Mechanism of conductive powder microstructure evolution in the process of SPS2617F(a)DisplacementDisplacement rateWlTemperature4VoltageCurentPressureo100200300400/s2100 [(b)2.11800Current1500 t1.51200.2≤台900.9.600.6415 Cl Y -315c03I T=201 C0Fig. 1. The recorded sintering curves of SPS process (a) parameters normalized to be shown in the same figure; (b)variations of the absolute values of the electric current, temperature and punch displacement rate.At 201C (Fig. 2(b)), the powder particles are just at the beginning stage of sintering.Compared to the raw powders, the particles are more densely arranged and local segre-gation is observed. Neck formation is only found between few particles (it should benoted that the electrolytic powders usually have the arborization morphology, neckforms between the initially separate neighboring particles). Increase of the sample den-sity at this stage (corresponding to the first second.中国煤化工ement ratecurve in Fig. 1(b)) is a combined effect of the discYHCN MH Gessure. Aswww.scichina.com262Science in China Ser. E Engineering & Materials Science 2005 Vol.48 No.3 258- 26910 umFig. 2. Microstructures of the original powders and at different sintering stages (HRSEM observations). (a) Rawpowders, 25'C; (b) 201 C; (c) 315C; (d)415C.the electric current density is low at the initial stage when the particles are looselypacked, the increase of the displacement rate may be mostly contributed by the pressurethrough causing movement and densely packing of the particles.At 315°C (Fig. 2()), there is an obvious change in the morphology of the powdersample. Compared with the lower temperature, the trend of glomeration of particles isapparent (here one can see there is an advantage of the electrolytic powders with thearborization morphology to judge the special sintering stage). It is deduced that localmelting occurs preferentially at the edges and corners of the raw powders, smooth globalsurface forms as a result of the effect of the minimum surface energy. We consider thatthis stage corresponds to the period of strong spark discharging which occurs amongmost particles. The surfaces of particles are clean and active due to the discharging pulseand impact of the plasma. More importantly, there is a high (hundreds of ampere) elec-tric current flowing through the very small contact area between neighboring particles,which at the moment results in a big amount of Joule heat, hence the contact zone ismelted and jointed. Therefore, this stage can be noted as the neck formation period dur-ing which there is the highest probability of partic中国煤化amalysis ofthis stage, it can be understood that owing to theYHCNMHGI formedCopyright by Science in China Press 2005Mechanism of conductive powder microstructure evolution in the process of SPS263by special mechanisms of SPS, neck formation can be completed both rapidly and at alower temperature contrast to the conventional sintering.At 415°C (Fig. 2(d)), the microstructure is at the stage of neck growth. The numberof the scattered particles reduces, and the contact area between particles increases (neckcoarsening). Moreover, the trend of directional neck growth is observed, and is consid-ered as the result of the anisotropic distribution of the electric current. At this stage,since the pore density is quite low, the probability of discharging becomes much smaller,the Joule heating produced while the electric current flows through the connected parti-cles dominates the process of densification. The high-temperature field and the big tem-perature gradients between the neck and the center of particles promote efficiently themass diffusion. With the pressure and through the bulk-, surface- and grain bound-ary-diffusion mechanismslo, neck coarsening is the essential of the remarkable densifi-cation at this stage (corresponding to the maximum peak of the displacement rate curvein Fig. 1(b)).The local inhomogeneous microstructures are observed obviously during the processof SPS, as shown in Fig. 3. As there is a wide size distribution of the raw powders, a50 umrig. 3. The local inhomogencous microstructures at different sint中国煤化工(c)415C;(d)827CMYHCNMHGwww.scichina.com264Science in China Ser. E Engineering & Materials Science 2005 Vol.48 No.3 258- 269few segregations of small particles form at early stages (e.g. see Fig. 3a), in whichglomeration and necks formation occur the fastest among the powders. At the middlestage (Fig. 3(b)), those necks grow rapidly, at the same time the neighboring small parti-cles get melted continuously and adhere to the surfaces of the connected particles, thusthe coarse compact bulks form locally at the following stages (Fig. 3(c)). It is found thatat the higher temperature, there is obvious directional growth of the neck microstructure(see Fig. 3(c)). The reason is that the electric current always chooses the path of thesmallest resistance, i.e. the current flowing through the locally dense material must belarger than that through the particles piling with a large number of interfaces, thus le-sulting in a stronger Joule heating effect, consequently, melting and adhering of theneighboring particles along the flowing direction of the current leads to the directionalgrowth of the neck microstructure. When the sintered sample attains nearly the theoreti-cal density, the same as the copper bulk material, the dimples are observed in the rupturemorphology of the sintered sample (Fig. 3(d)). Judging from the dimple sizes, it can beconcluded that the grain size distribution is homogeneous and no abnormal big grainsare found to exist in the microstructure.According to the analysis above, a complete SPS process can be divided into severalcharacteristic stages as dense pileup of particles, neck formation and neck coarsening tillfull densification.2.2 Evolution of grain size distribution in the sintered sampleUsing the linear intercept method the mean grain size and the grain size distributionhave been measured for different sintering stages. There is only a small increase of themean grain size through the whole SPS process. In the conventional sintering, graingrowth occurs as soon as the pore density starts to decrease, and becomes faster with theincreasing temperaturel2. There are two main factors confining grain growth during SPSprocess: one is the rapid heating, which leads to a very short stay of the sample in thelow-temperature period, thus grain growth controlled by surface diffusion is hard tocarry out; the other is that no holding or very short holding time at the high temperaturebefore rapid cooling, from which grain growth can hardly occur and the fine grainstructure is maintained.The evolution of the grain size distribution during sintering is shown in Fig. 4. It canbe seen that the grain size distribution is wider at the middle stage T=315°C in Fig.4(b)). This appears because when most of particles are at the stage of neck formation,grains in the local compact bulk will grow preferentially, leading to a wide grain sizedistribution, whereas at the final stage the grain size distribution is narrow (T=827°C inFig. 4(d)), indicating that the relative grain sizes are finally homogeneous, which impliesa faster growth rate of small grains than that of the big grains at later stages of SPS. Itmeans that grains in the local compact bulk cannot fast grow continuously, with the neckformation and growth in other zones, the Joule he中国煤化工resistancestends to be distributed homogeneously in the sinterdYHCNMHGisity.Copyright by Science in China Press 2005Mechanism of conductive powder microstructure evolution in the process of SPS26512 |102t昌87=201 CT=315C642(a)b)| h.u .m.0 0.5 1.01.52.02.53.0 3.5 4.0 4.50.0 0.51.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5L/Fig. 4. The evolution of the grain size distribution during SPS.3 Discussion of SPS mechanism3.1 The self adjusting mechanism of the powder microstructure evolutionDue to the inhomogeneous particle sizes of raw powders, as well as the non-uniformdistribution of pressure among the particles, at the early sintering stages, some big parti-cles are loosely packed while particles in the small size group tend to segregate andclosely pile. Fig. 5 shows the diagram of particles contacting and the distribution of localelectric current. Assume the contact area between two neighboring particles is S and thelitte thickness of the deformed contact zone is I. Then the resistance of the contact zoneis calculated aslR=ρ=(1)ρ is the resistance coefficient of the material, which can be taken as a constant at lowtemperature (<300°C), but at higher temperatures it is expressed as a function9:ρ=Po +CT+C中国煤化工MYHCNMHGwww.scichina.com266Science in China Ser. E Engineering & Materials Science 2005 Vol.48 No.3 258- 269(a(bc)Fig. 5. The diagram of particles contacting and the distribution of local electric current.Po is the resistance coefficient at the room temperature, and C1 and C2 are the tempera-ture factors. At early stages, with the small current and low temperature, there is littlechange of the resistance coefficient, and the influence of the contact area is more impor-tant. The resistance in the zone with bigger contact area is smaller than that with smallercontact area, hence there is a bigger current flowing through this zone, i.e. 1>2 (Fig.5(a)). The Joule heat produced by current I causes a fast temperature increase in thecontact zone, local melting or plastic deformation under the pressure may occur, leadingto the formation of the neck. With the coarsening of the neck, the contact area increases(Fig. 5(b)) and consequently a bigger current is caused. Then with the high temperaturegenerated in the neck zone, the resistance coefficient increases (eq. (2)) and hence thelocal resistance increases. When the resistance of the neck zone exceeds that of otherzones with smaller contact area between particles, the current will flow through theconnecting particles with smaller contact area, i.e. l>I (Fig. 5(C)). Therefore, at hightemperatures the resistance coefficient is dominant to the flowing direction of the localcurrent. Thus neck forms and grows between particles with originally smaller contactarea (Fig. 5(c)). Through the above alternative procedure, finally the sintered sampleachieves the full densification. Accordingly, the grain structure in the local compact bulkformed at early stages cannot keep coarsening during the following sintering, but growsrelatively slowly compared with that of other less compact zones, consequently the ho-mogeneous grain size distribution is obtained at the final stage of sintering. The aboveconsiderations open out the essential for the advantages of materials preparation by SPSin the respects of high density, homogeneity and fine grain structure. We propose for thefirst time the above analysis as the“self-adjusting mechanism of powder microstructureevolution in the process of SPS".3.2Prediction on the variations of the relative density of the sintered sampleFig. 6 is the diagram of the punch displacement during SPS. P is the given pressureexerted on the punches, M is the mass of the powder sample, Vo and V, are the volumesof powders at the very beginning and that at the time t respectively,A is the cross-sectionarea of the punch (equal to the cross-section area af the. die inner, cavitv) S. is the dis-placement of the punch at the time t. The density o中国煤化工be calu-YHCNMHGCopyright by Science in China Press 2005Mechanism of conductive powder microstructure evolution in the process of SPS267PimchPulse DCSamplePunchFig. 6. The diagram of the punch displacement during SPS.lated with the expression:D, =M(3)V, V,-As,The initial volume Vo is evaluated byVo= A(hg + Smax),(4)where hf is the final height of the sintered sample, Smax is the largest displacement ob-tained from the recorded curve of SPS process. Thus,(5)A(hg + Smax -s)and the changes of the relative density of the sintered sample Ct with the time can bepredictedC, =D，(6)D。~ DoA(hy+中国煤化工TYHCNMHGwww.scichina.com268Science in China Ser. E Engineering & Materials Science 2005 Vol.48 No.3 258- 269Do is the theoretical density of the sample, the data of SI are obtained from SPS recordingcurves.The prediction on the changes of the relative density of the sintered sample is com-pared with the experimental measurements in Fig. 7. Good agreement is found whent> 100s, while the predicted value is higher than the experimental at the early stage. Thereason is that there are very few necks formed at the early stage, substantial connectionhas not formed yet, elastic reversion may occur after the pressure is released and thesample is taken out, resulting in a lower experimental measurement of the relative den-sity. It can be seen from Fig. 7 that the change of the relative density is different fromthat of the temperature with a linear increase via time. At early stages, there is lttlechange of the relative density in spite of the rapid increase of current. At the mid-dle-temperature period, the densification proceeds the fastest; at high temperatures thedensification process becomes slowly and gradually attains the maximum value.1400Current1200- - - TemperatureModel prediction10001 100Experiments800480员心600H 60400| 40200| 2010300/sFig. 7. Comparison of changes of the relative density of the sintered sample between theoretical predictions andexperimental measurements.4 ConclusionsBy using the pure electrolytic copper powders as the raw material, a series of sinter-ing experiments have been designed and carried out based on the analysis of the SPSrecording curves. The microstructures, mean grain size and size distribution, the relativedensity, as well as their evolution during sintering are systematically studied in the pre-sent paper. The mechanism of microstructure evolution is discussed, and the changes ofthe relative density during SPS are predicted with a theoretical model. The conclusionsare drawn as follows: .1) A complete SPS process can be divided into several characteristic stages as densepileup of particles, neck formation and neck coarsening till full densification, for whichthe pressure, discharging and Joule heating are dominant fartnre roenentivaly中国煤化工2) From the observations on the evolution of theTYHC N M H GrostructureCopyright by Science in China Press 2005Mechanism of conductive powder microstructure evolution in the process of SPS269and quantitative analysis on the grain size distributions, it is shown that the grain struc-ture in the local compact bulk formed at early stages cannot keep coarsening during thefollowing sintering process, the grain sizes tend to a homogeneous distribution at thefinal stage of sintering.3) Based on the analysis of distribution of the local current, for the first time a“self- adjusting mechanism” of powder microstructure evolution in the process of SPShas been proposed, from which the essential for the advantages of materials preparationby SPS in the respects of high density, homogeneity and fine grain structure can be wellunderstood.4) The changes of the relative density during SPS are quantitatively predicted andconfirmed by the experimental measurements.Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.50401001) and the Program of Bejing New Star of Science and Technology (Grant No.2004B04).References1. 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S. et al., Spark-plasma sintering of oxidized spherical Fe -powder, Proceedingof NEDO Intermational Symposium on Functionally Graded Materials, Kyoto, Japan, 1999, 123- 126.8. Omori, M., Sintering, consolidation, reaction and crystal growth by the spark plasma system (SPS), Mater.Sci. Eng. A, 2000, 287: 183-188.9. Matsugi, K., Kuramoto, H, Hatayama, T. et al, Temperature distribution at steady state under constant cur-rent discharge in spark sintering process of Ti and AI2O3 powders, J. Mater. Proc. Tech, 2003, 134: 225-232.中国煤化工MHCNMHGwww.scichina.com