CHAPTER 4: The wetting behaviour and reaction of the diamond-si system

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1 CHAPTER 4: The wetting behaviour and reaction of the diamond-si system In this chapter, the wetting behaviour of diamond by silicon will be presented, followed by the study of the interaction between diamond and silicon to form SiC. These are the two main parameters that influence infiltration, hence their study will enable better understanding of the infiltration behaviour and factors playing a role in the limitation thereof. 4.1 The wetting behaviour Experiments to determine the wetting behaviour were conducted using a heating microscope optical dilatometer described in chapter three. The measured wetting angles of diamond and silicon and of graphite and silicon as a function of temperature are plotted and given in fig 4.1 below. Images of some cross sections of the reacting set up as captured at different temperatures are given in fig 4.2 and 4.3 following their respective wetting angle versus temperature plots. The wetting of diamond by silicon was also studied along with the wetting of graphite as well as that of carbon-coated diamond by silicon, for the sake of comparison Results At first the results of the wetting angles of diamond and graphite by Si are given. From fig 4.1a it can be noted that the wetting angle between diamond and silicon remains constant up to about 1300 o C, whereupon it starts to increase and then decreases sharply after the melting of silicon, to about 20 o. The increase in the wetting angle of diamond by Si around 1350 o C is due to the silicon minimising its surface energy after softening, before melting, by trying to take a spherical shape, rounding up at the corners (fig 4.2b) and hence increasing the contact angle with the diamond surface. 47

2 Wetting Angle, Temperature ( C) a) Wetting Angle, Temperature ( C) b) Fig 4.1: The wetting angle between silicon and a) diamond, b) graphite, as a function of temperature. 48

3 Piece of Si Diamond surface Si rounding up at the corners a) 1000 C, contact angle left: 93 b) 1350 C, contact angle left: 107 c) 1502 C, contact angle left: 18 Fig 4.2: The images showing the contact angle between diamond and silicon corresponding to a) 1000 o C, b) 1350 o C and c) 1502 o C. a) 1000 C, contact angle rightt: 93 b) 1500 C, contact angle left: 0 Fig 4.3: Images showing the contact angle between graphite and silicon, corresponding to temperatures at: a) 1000 o C, and b) 1500 o C. In contrast to the wetting behaviour observed for diamond, graphite does not exhibit the increase prior to the sharp decrease in the contact angle. Another difference is the sharp drop occurring before the silicon melts, unlike in the case of diamond where the drop occurred after the melting of silicon. A possible reason could be the fast reaction of the non-diamond carbon with solid silicon [1] resulting in an enhanced wetting as observed. 49

4 This explains the low wetting angle even before the melting of silicon. The final contact angle is below 20 o. Because the synthesis of the material of concern here involves infiltration of a preform comprising diamond particles coated with carbon from a resin binder, it was necessary that the wetting of thus-coated diamond be studied in order to get a true picture of what is taking place inside the system. The results of this investigation are shown below. Contact angle vs Temp of Carbon-coated Diamond by Si Wetting angle (oc) Temperature (oc) Fig 4.4: The wetting angle between carbon-coated diamond and silicon as a function of temperature. The measurement of the wetting behaviour of the coated diamond was complicated due to the flaking off of the coating as it is heated up. In fact, the coated plates could not be pyrolysed, as outlined in the procedure in chapter three because the coating detached from the diamond and the silicon piece could not sit properly and form a 90 o contact with the surface during the set-up. It was then decided that the piece be placed onto the coated diamond plate after curing but before pyrolysis. Pyrolysis would take place in-situ during the heating up of the system during wetting behaviour determination. Pyrolysis, at the heating rate used will take place and reach completion well in advance of 1000 o C and therefore will not interfere with the wetting because any change in contact angle is 50

5 expected to happen well above 1000 o C (as can be observed for both diamond and graphite). Looking at figure 4.4, one immediately observes a chaotic recording of the wetting angle. The carbon coating upon heating started to form flakes that were detached from the diamond surface (fig 4.5). This was also observed independently when the coated diamond was heated in a furnace to try to pyrolyse the resin coating. As the flakes were forming around and underneath the silicon, they kept tilting it back and forth causing the non-orderly change in the angle measured and untrue wetting angles recorded. Some images corresponding to certain temperatures of the contact angle are presented in fig 4.5. The rounding up of the Si piece, characteristic of the wetting behaviour of diamond (figure 4.2b) is observed here. Flakes are visibly forming and tilting the Si piece. a) 300 C, contact angle left: 71 b) 1150 C, contact angle left: 17 Si rounding itself up c) 1300 C, contact angle left: 27 d) 1477 C, contact angle left: 41 Fig 4.5: Images showing the contact angle between carbon-coated diamond and silicon, corresponding to temperatures of: a) 300 o C, b) 1150 o C, c) 1300 o C and d) 1477 o C. 51

6 4.1.2 Discussion Data of the observed wetting behaviour are summarized in table 4.1 and compared to literature data. The observed wetting angles fit well in the known data from the literature for pyrolytic or graphitic carbon and Si. For the interaction of Si with graphite it was observed that the good wetting is caused by the formation of a SiC interlayer and that the kinetics of wetting is determined by the rate of SiC formation [2]. The higher reactivity of graphite in comparison to diamond with Si could explain why the wetting angle of graphite starts to reduce below the melting temperature whereas the wetting of the diamond starts only at the melting temperature of silicon. The formation of a SiC interlayer also explains the similar wetting angles for diamond and graphite observed above the melting temperature of silicon. Table 4.1: The reported wetting angles by silicon on graphite, diamond and SiC MATERIAL TEMPERATURE CONTACT LITERATURE ( o C) ANGLE ( o ) Polycrystalline [3] pyrolytic graphite SiC [3] SiC [4] Graphite This work Diamond The results obtained from wetting experiments of the coated diamonds cannot be used to make any sensible conclusions except that the coating does not stick to the diamond as it heats up. The final contact angle observed is about 20 o and is characteristic of that of uncoated diamond. These similarities could be indicating that the coating has reacted with silicon and formed silicon carbide and that the similarity in the wetting angle observed in both cases could be a result of this silicon carbide formation. 52

7 4.2 Diamond/Si interaction For the study of the reaction between diamond and silicon, a hot press was used. The set up is described in detail in chapter three. Silicon wafer(s) were placed on top of a diamond plate and the assembly heat treated at 1450 C, 1475 C and 1500 C for different times from 0 minutes up to 150 minutes Results SiC forms between the diamond phase and the silicon phase resulting in a sandwich-like structure (fig 4.6 and 4.8). After reaction all samples showed some remaining free silicon above the SiC layer. Plots of the SiC thickness against reaction time were produced (fig 4.7a). It has been found that the growth of this interlayer is very fast and that even in the 0 minutes dwell time experiments continuous SiC layers were formed. The thickness of the SiC-layer is greater at low temperatures. SiC seems to grow both away from the diamond surface and into the diamond at SiC grain boundaries. Grooves formation is observed in the diamond grains well into the diamond phase (fig 4.8d). The groves seem to be accompanied by microcracks of the diamond phase. The morphology of the interface shows two different sizes of SiC grains, large well-faceted SiC grains at the interphase and nano size SiC grains in the groves. There was no evidence of a conversion of diamond into a non-diamond carbon found in all the samples produced and analysed with SEM and XRD. 53

8 Some SiC precipitates in the Si phase a) T = 1450 C; t = 30 minutes b) T = 1475 C; t = 30 minutes c) T = 1500 C; t = 30 minutes Cracks d) T = 1450 C; t = 150 minutes e) T = 1475 C; t = 150 minutes f) T = 1500 C; t = 150 minutes Some SiC forming at the diamond grain boundaries and microcracks. Fig 4.6: Backscattered SEM micrographs of the polished cross sections of the Diamond/SiC/ Si Interface as a function of reaction temperature and time. 54

9 a) SiC thickness (um) 1450 oc 1475 oc 1500 oc Reaction time (min) 1450 oc 1475 oc 1500 oc Relative roughness Reaction time (min) b) Fig 4.7: The influence of reaction time and temperature on a) the thickness of the SiC layer formed at the reaction interphase and b) the roughness of the SiC interface. Si SiC SiC D D a) b) Groves in diamond at SiC grain boundaries Fig 4.8: Backscatter SEM micrographs of cross sections of the polished interface showing the inroads of SiC into the diamond by the Si-diamond reaction at the SiC grain boundaries. (T=1475 o C; t = 0 minutes, D=Diamond) 55

10 Considering that before the reaction took place the diamond was well polished and flat with a surface roughness of ± 0.14µm, the reaction seems to be leaching away the diamond (fig 4.6 and 4.8) thereby increasing its roughness relative to the starting one (fig 4.7b). Some micrographs, especially for reactions at 1450 o C (fig 4.6a and d), show some crack formation in the silicon phase. Also, similar to Zhou and Singh [4], some random SiC was found to have formed in the Si phase away from the SiC interface (fig 4.6a and 4.8). SiC inside the Si melt seems to be more pronounced at a lower temperature (1450 o C) and lower reaction time (30 minutes) (fig 4.6 a versus d) and absent at the high temperature of 1500 o C (fig 4.6 c versus a) and increased dwelling/reaction time. The SiC interface after being leached with the Murikami reagent revealed a difference in the nature of the SiC grains formed during the reaction. While the SiC forming on top of the diamond (i.e. on the grains) are large facetted grains of about 15µm in length, the SiC forming in the grooves is of submicrometer to nano-size (fig 4.9). SiC Si SiC Si Diamond Diamond a) b) Si SiC Si Micron-SiC at interface Diamond c) d) Nano-SiC in grooves Fig 4.9: Backscatter mode SEM micrographs of etched cross sections of the sample reacted at 1475 o C for 100 minutes at different places in different magnifications. Diamond 56

11 a) b) c) d) Fig 4.10: SEM micrographs in SE mode of etched cross sections of the sample reacted at 1500 o C for 60 minutes (a,b,c), and 1450 o C for 15 minutes (d) showing the position of the groves and the cracks accompanying some groves (a) Discussion The dense layer was build up quite quickly even during the heating up of the reaction couple to the reaction temperature. The morphology of the SiC layer reveals that its formation starts with a few crystals growing along the interface. These grains are well faceted. Similarly faceted grains were also found for the reaction of Si with glassy carbon or graphite [2, 4]. These grains, for very short reaction times are isolated [5]. TEM investigations of the interface showed orientation relationships of the diamond and SiC lattice, implying the heterogeneous nucleation of the SiC on the diamond [6]. Based on this information and the observed morphology of the formed reaction layer, it can be concluded that during wetting of diamond by Si at a few sides (distance in the range of µm) the SiC nucleates and then quickly grows along the surface and in the 57

12 thickness. Between these nuclei (especially at the grain boundaries of the diamond) the carbon dissolves into silicon and then it is transported through the liquid to the growing crystals. This explains the leaching away of the diamond and the quite regular distance of grooves formed at the interface (Fig. 4.8). This process is schematically shown in fig 4.10 below. The rate of nucleation and the growth rate seem to increase with temperature, therefore the interface becomes less rough during reaction at higher temperatures. This would also explain the lower overall thickness at 1450 C at 0 min reaction time (Fig.4.7). After the crystals have coalesced, the overall growth rate strongly decreases. Further growth takes place by the diffusion of Si and C through the grain boundaries. This diffusion process is very slow hence the fast reaction at the beginning (before a dense layer forms) stops and the reaction mechanism changes (transport through liquid to transport along grain boundary). Si Si attacking and eating into diamond (D) SiC SiC SiC D Grooves forming in the diamond a) Si Si and C diffusing through SiC grain boundaries D b) Fig 4.10: A schematic showing the formation and growth of SiC a) in the starting period of layer formation and b) after the layer is formed. 58

13 Table 4.2: A comparison between results obtained in this study after a 150-minutes reaction time against those of Zhou and Singh [4] obtained after a 180-minutes reaction time. ZHOU & SINGH [5] THIS STUDY Temp Thickness data taken from a Temp Thickness (µm) ( o C) graph in [4] (µm) ( o C) 30 min 180 min 30 min 150 min ± 1 8 ± ± 1 10 ± ± 1 10 ± 2 Zhou and Singh [4] found similar thicknesses for the longest dwelling time of 180 minutes for the reaction of glassy carbon with silicon in the temperature range C as the thicknesses found in this study for the longest dwelling time of 150 minutes for the reaction of diamond with silicon. This indicates that the reaction rate of diamond and glassy carbon is very similar and that the reaction is likely controlled by some transport processes. Zhou and Singh [4] proposed a very complicated model including a forth power rate law. This study s results and also a critical analysis of the data of Zhou and Singh [4] would suggest that there is a change in the mechanism after the layer has formed. 59

14 SiC thickness (um) 1450 oc 1475 oc 1500 oc Reaction time (min) a) Another possible fit b) Fig. 4.11: The thickness of SiC layer formed from the reaction of Si with a) diamond (this study) and b) carbon (Zhou and Singh study) [4], as a function of reaction time. Alternatively, Zhou and Singh s data could be fitted into a straight line proposed in fig b. 60

15 Fig 4.12: Self-diffusion coefficients of 14C lattice and grain-boundary diffusion and 30Si lattice diffusion in -SiC as a function of 1/T.[8] Assuming such change in the reaction mechanism, there is no necessity for a forth power rate law to describe the data. The time dependence of the growth of the SiC film itself is difficult to determine due to the low growth rate and the relative low time of the experiments. The extrapolation of the diffusion constants of Si and C in the SiC lattice and grain boundary determined (fig 4.12) [7-8] for the temperature ranges o C (for C) and o C (for Si), to 1500 C resulted in values for C and Si in the lattice of 4.3 x and 6.1 x cm 2 sec -1 respectively and for C in the grain boundary as 1.11 x cm 2 sec -1. These values would suggest that the transport would take place only along the grain boundaries. This mechanism would also correspond with the process observed. It is likely from the above micrographs that the SiC growth proceeds in both directions, into the diamond as well as into the silicon as suggested by W. B. Hillig [9] for the reaction of Silicon with graphite, i.e. that a coupling of the carbon and silicon fluxes takes place across the SiC barrier (shown schematically in fig 4.10b). 61

16 The differences between this study s thicknesses and those of Zhou and Singh [4] for the shorter reaction time of 30 minutes could be explained by the fact that carbon, being less crystalline than diamond, would react much faster initially (i.e. when still directly in contact with silicon, before reaching the diffusion controlled growth) because the C-C bonds in it are weaker than those in diamond. It could also be caused by a faster solubility of glassy carbon in silicon or even by the scattering of the data observed in both experiments. There exist arguments for both reasons. However, once a uniform layer of SiC has formed, the SiC growth rate is expected to become diffusion controlled in both instances and similar thicknesses should be observed for long reaction times. Experiments with very short reactions times and extreme heating rates are necessary to determine this. The micrographs of the etched cross sections (Fig. 4.9) reveal that the presence of smaller SiC grains in the grooves. From the data, it is not possible to decide whether the grains are formed before the layer on the top was completely dense or after. Experiments with a faster heating rate (greater than the 50 o C/min used here) may give an answer to this question. The random SiC-crystals that are formed in the Si phase but not as part of the SiC interface were also observed by Zhou and Singh [4]. It could be the evidence of the solution-precipitation mechanism proposed by Pampuch et al [3] where the carbon after dissolving in the liquid silicon, is carried well into the Si phase and precipitates probably during cooling of the system. The reason SiC precipitates in the Si phase are observed at 1450 o C and not at 1500 o C (fig 4.6 a versus c) could be that at lower temperatures, the coalescence of the SiC nuclei that form in the initial stages of the reaction is much slower due to much slower reaction kinetics at lower temperatures promoting the carbon/silicon contact and therefore the carbon dissolution process. Longer dwelling times give enough time for the dissolved carbon to migrate through the liquid silicon to the interface and precipitate there. After forming the dense SiC-layer, no more intensive dissolution of carbon in the liquid is possible. Therefore the concentration of carbon in the liquid 62

17 silicon will decrease with reaction time and no precipitation of larger SiC grains is possible. 4.3 Conclusions Liquid silicon wets diamond; a wetting angle of approximately 20 could be detected. The wetting is caused by the fast formation of a SiC interlayer after melting of silicon. The wetting angle is very similar to those observed for glassy carbon or graphite. The investigation of the growth of the SiC interlayer has shown that it forms very fast (even during the heating up period) by nucleation of SiC grains on the surface of the diamond. After a dense layer is formed, the growth rate slows markedly down and further growth is likely to be diffusion controlled (diffusion of Si and C through the grain boundaries). The diffusion-controlled growth rate increases as temperature increases. 63

18 REFERENCES: 1. P. Sangsuwan, et al., Reactive Infiltration of Silicon Melt through Microporous Amorphous Carbon Preforms, metallurgical and Materials Transactions B, 1999, vol 30B, O. Dezellus et al, Wetting and infiltration of carbon by liquid silicon, J. Mater. Sci., 40, 2005, , R. Pampuch et al, Reaction Mechanism in Carbon-Liquid Silicon Systems at Elevated Temperatures, Ceramics International, 12, 1986, Hong Zhou and Raj N. Singh. Kinetics Model for the Growth of Silicon Carbide by the Reaction of Liquid Silicon with Carbon. J. Am. Ceram. Soc. 1995, vol 78 No. 9, p Hong Zhou and Raj N. Singh, Processing and microstructural characterization of melt-infiltrated Si/SiC composites, J. Mat. Synth. and Proc., Vol. 5, No. 2, 1997, pp Joon Seok Park et al, FIB and TEM studies of interface structure in diamond-sic composites, J. Mater. Sci., 41, 2006, M.H. Hon and R.F. Davis, Self-diffusion of 14 C in polycrystalline -SiC, J. Mater. Sci., 14, 1979, M.H. Hon and R.F. Davis, Self-diffusion of 30 Si in polycrystalline -SiC, J. Mater. Sci., 15, 1980, William B. Hillig, Making ceramic composites by Melt Infiltration, American Ceramic Society Bulletin, vol 73, no. 4, April 1994,