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Scripta Mate&h_ Vol. 35, No. 11, pp.1265-1270.1996 Ekevier Science Ltd Couvrkht 0 1996 Acta Metallweica Inc. b&ted in the USA. All rightsreserved 1359-6462/96 S12.00 + .OO
1NFLUE:NCE OF SUBSTRATE ORIENTATION ON WETTING KINETICS IN REACTIVE METAL/CERAMIC SYSTEMS B. Drevet(*), K. Landry, P. Vikner and N. Eustathopoulos Laboratoire de Thermodynamique et Physico-Chimie M&allurgiques, INP Grenoble, URA 29 CNRS, Domaine Universitaire, BP 75,38402 Saint Martin d’H&res Cedex, France (Received June 3, 1996) (Accepted June 13,1996) Introduction During the last few years, several studies have been devoted to the mechanisms of reactive wetting in metal/metal and metal/ceramic systems (l-7). From sessile drop experiments carried out with model metal/ceramic couples (8-IO), the following conclusions have been drawn. (i) The main effect of interfacial reaction:; on wetting is a change in the relevant interfacial energies of the system (2,8) and not, as previously proposed (1 l), linked to the free energy produced by the reaction. (ii) The steady-state contact angle is roughly equal to the Young contact angle of the liquid on the reaction product (8,12). (iii) The spreading kinetics is controlled by the lateral growth of the reaction product at the triple line (10). In the frame:work of this description, it is expected that crystallographic factors, like the orientation of the substrate surface, which can affect reaction kinetics, can also influence wetting kinetics. The effect of subwrate orientation will be studied in two systems. The first one is a Cu-Ti alloy on c1monocrystalline alumina. For this system, wettability and reactivity of Cu-Ti alloys of various compositions on A&(& monocrystals of random orientation were studied in detail by Kritsalis et al. (13). A two-step wetting process was observed: after a very rapid decrease of contact angle in less than one second, explained by the formation of an adsorption layer of Ti at the interface, a much slower decrease arises in about IO’s, attributed to the formation of the wettable Ti monoxide at the interface. In the present study, sessile drop experiments are carried out with a specific alloy composition (Cu-10.8 at.% Ti) on three different crystallographic faces of alumina, as well as on surfaces of random orientation. The second system investigated here is pure Al on carbon. Wetting and interfacial reactions have already been studied by Landry et al. (10) on vitreous carbon. After a first decrease of contact angle owing to deoxidation of the Al drop and A&C3 formation in transient conditions, a quasi-stationary regime of carbide growth at the triple line is established, leading to a linear variation of the drop base radius as a function of time. In the present work, sessile drop experiments are performed on other varieties of carbon, i.e. pyrocarbon and pseudo-monocrystalline carbon.
“‘Now
at CEA G-renoble, DTA/CEREM/DEIWSPCM,
17 rue des Martyrs, 38054 Grenoble Cedex 9, France. 1265
1266
INFLUENCE OF SUBSTRATE ORIENTATION
Experimental For the
Vol. 35. No. 11
Method
Al/C system, three types of carbon are investigated: vitreous carbon (C,), pyrolytic carbon (C,) and graphite pseudo-monocrystals (C,). All these substrates have no open porosity, an ash content less than 50 ppm, and are polycrystalline graphite materials with different degrees of crystalline perfection and microstructures. The degree of graphitization G and the inclinaison of the graphite basal planes with respect to the substrate surface are determined from X-ray diffraction and pole figures. For C,, G is close to zero and planes are randomly inclined. For C,, the value of G is 15% and the maximum inclinaison 25”. For Cpe,the value of G reaches 97% and the maximum inclinaison 3”. The C, and C, substrates are polished down to a 0.1 pm diamond finish. In the case of C,, the average roughness obtained is smaller than 2 nm while C, presents a greater average roughness in the 1O-l 00 nm range. The pseudo-monocrystal basal plane surface obtained by cleavage presents an average roughness of 1000 nm because of high-wavelength roughness of the surface. If measured locally (on areas of 20x20 pm*), the average roughness falls to 2-4 nm. This last value is the relevant one as wetting is controlled only by the local area of the triple line. Before the experiments, all substrates are ultrasonically cleaned in acetone and then annealed in vacuum at 1300 K for 2h. For the Cu-Ti/A1203 system, the substrates are platelets of a-monocrystalline alumina (99.998% purity) with different surface crystallographic planes: random, (ll?O), (1702) and (0001). After mechanical polishing, the substrates are ultrasonically cleaned in acetone and annealed under vacuum at 1473 K for 2h in order to restore surface order which has been disturbed by polishing and eliminate any hydroxyl groups and carbon that may have been adsorbed on the surface. A second annealing is performed in air at 1763 K for 4h in order that oxygen deficient surfaces become stoichiometric. After annealing, the average roughness is l-2 nm. Contact angles are measured by the sessile drop method in a high vacuum metallic furnace. The apparatus consists essentially of a molybdenum resistance furnace fitted with two windows, enabling the illumination of the sessile drop on the substrate and the projection of its image on a screen. Contact angles 8, as well as linear dimensions of the drop (contact radius R and height H), are measured directly from the image of the drop section with an accuracy of + 2” for 0 and + 2% for R and H. The pressure in the experimental chamber can be reduced to 10W5-1 0” Pa at room temperature by a system of two pumps (a rotary vane pump and an oil-diffusion pump both connected to a nitrogen-cooled trap). In order to limit evaporation of Al and Cu, once this pressure level is reached, the experiment is performed in a dynamic vacuum of 10e3Pa obtained by controlled helium micro-leaks. The helium is purified before introduction in the furnace by passing through a Zr-Al getter. In the case of Al/C experiments, the Al specimens (99.998% purity and about 10 mg weight) are freshly cut on all their faces a few minutes before introduction into the furnace, since very short exposure to air limits the thickness of the native superficial oxide layer. Contact angle measurements are performed during isothermal hold at 1100 K. For Cu-Ti/ AlzOJ experiments, the Cu-10.8 at.% Ti alloy, weighting about 100 mg, is processed in situ during the sessile drop experiment by direct melting on the substrate of a piece of Cu (99.999% purity) with a piece of Ti (99.96% purity) over the copper. This procedure, used previously in (13), is carried out to avoid any contact between Ti and the ceramic before complete fusion of the alloy; however, the volume V of the liquid drops is not exactly the same in all experiments. The radius of the equivalent spheres of the drops R,=(3V/47t)“3 is equal to 1.45 f 0.20 mm (Table 1). The heating rate of the furnace is 33 K/min and the holding period at the working temperature Tw, ranging between 13 19 K and 1413 K, lasts one hour. This duration, which is much greater than the time needed to reach a stationary contact angle, is necessary to obtain thickness of reaction product easy to characterize by scanning electron microscope.
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TABLE 1 Wetting Parameters for Cu-10.8 at.% Ti on
Various CrystallographicFaces of Alumina (See Figure 1 for Meaningof Parameters).
Results
Figure 1 shows variations of drop base radius R and contact angle 8 of a Cu-10.8 at.?.. Ti alloy on (1120) face of Al*03 as a function of time. The first measurement is taken when complete fusion is achieved. The initial contact angle, obtained in less than one second, is equal to about 65” and remains nearly constant for about 50 seconds. Thereafter, 8 decreases and reaches a stationary value in about 135 s. No significant variation of 8 (lower than 3”) is observed at the working temperature TWduring one hour. Note that in the experiment of Figure 1, as well as in experiments of Table 1, the steady-state contact angle 8, is attained during the temperature rise, at T=1323 + 15 K, i.e. at a temperature lower than the working temperature. Clearly, the significant variations of 9 and R with time are obtained in non-isothermal conditions. The main features of results of Table 1 are the following. (i) The final contact angle f& is nearly the same for all substrates (OF=29 f 4’). The 4” scattering is typical of 0 measurements, even in nonreactive metaUceramic systems. Thus, any effect of anisotropy of A1203 is lower than this experimental accuracy. Note that the eF value is in good agreement with the previous value measured by Kritsalis et 1450 T
time (4
Figure 1. Variations of contact angle and drop base radius during the temperature rise at Tel403 ( 1150) face of alumina.
K for Cu-10.8 at.% Ti on the
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R (mm)
Figure 2. Variations of contact angle and drop base radius at T==l100 K for Al on pyrocarbon.
al. (13) for the same alloy on a random surface. (ii) Both total spreading time tF and the difference (tFto) do not exhibit any significant variation, the differences between the experiments being of the order of -I: 10% (tF=130 4 15 s and (t&)=83& 8 s). These differences are close to the differences in drop size R,, (Tablel). Although the R(t) curve is not linear (Figure I), an average triple line velocity can be defined as v=(R(tF)-R(to))/(t&). It is found equal for all experiments to 11 f 2 pm.<‘. Micrographic observations made on a perpendicular section to the interface using a scanning electron microscope show a continuous reaction layer, identified as TiO by means of X-ray diffraction. The average thickness of this layer, obtained from several measurements performed along the interface, is about I l_trn.No significant variation of this average thickness was observed between the different crystallographic orientations of the alumina substrates (14). On Figure 2, contact angle and drop base radius are given for the Al/C,, system as a function of time at T=l 100 K. As in the Al/C, system, discussed in detail elsewhere (lo), after a first decrease of 8, due to deoxidation of the Al drop and to carbide formation in transient conditions, a quasi-linear spreading is observed with a rate of 20 nm.d’. Note that even after 2 IO4seconds, no steady-state contact angle is reached. Results of contact angle measured on C,, C, and C,, are compared on Figure 3. With regard to C,, wetting on C, is significantly faster with a triple line velocity of 40 nm.s-‘, and a stationary contact angle is obtained after 16000 s. Wetting kinetics on C,, is much slower, and the spreading rate is only 10 nm.s-‘. For all varieties of carbon, a continuous layer of A14C3a few microns in thickness (15,16) was observed at the Al/C interface.
Figure 3. Contact angle kinetics at T=l (C,).
100
K of Al on vitreous carbon (C,), pyrocarbon (C,) and graphite pseudo-monocrystal
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Figure 4. If the growth of the reaction layer is controlled by the diffision of a reactive solute B from the bulk liquid A-B to the triple line, the reduction of the diffusion field during the spreading process will decrease the growth rate and consequently the velocity of the triple line.
Discussion
By analogy with reactions occuring at the interface between bulk phases, kinetics of reactions between a component of the drop and the solid substrate at the triple line may be composed of two steps: (i) a step including processes localized at the triple line, namely dissociation of the solid, short-range diffision and nucle;ation and growth of the reaction product, and (ii) a step of transport of reactive solutes from bulk drop to the triple line, or, conversely, of soluble reaction products from the triple line to the bulk liquid. In the Al/C (couple, the growth of aluminium carbide at the triple line does not need any long-range diffusion. Thus, reaction kinetics is probably controlled by phenomena occuring at the triple line. When a steady-state configuration is established at the triple line, a regime with a constant triple line velocity is attained (the so-called “linear spreading” (lo)), as indeed observed in the Al/C, system (Figure 2). The presence of a linear region on the R(t) curve testifies strongly in favour of wetting kinetics contralled by a triple line step. For this type of kinetics, crystallographic parameters, such as the orientation of the substrate surface, may affect the spreading rate, as indeed observed in the Al/C system (Figure 3). With regard to the Al/C system, the Cu-Ti/AlzOj couple differs in the following points. (i) As shown on Figure 1, th.e R(t) curves do not exhibit the quasi-linear regime observed in the Al/C system during spreading of the drops (Figure 2). Thus, different phenomena may control spreading kinetics in the two investigated systems. (ii) The lateral growth rate of the TiO reaction product, which is equal to the triple line velocity, is higher by more than two orders of magnitude than that measured in the Al/C system (v=lO pm.s“ instead of v=40 nm.d’ for Al/C, and v=lO nm.s*’ for Al/C,,). For systems with such high reaction rates, mass transport may become the limiting step of the reaction. (iii) Precisely, in view of the oxido-reduction reaction 3(Ti) + A&O, + 3TiO + 2(Al) occuring at the triple line, the progress of the reaction requires transport of Ti from the bulk liquid to the triple line and, conversely, of Al from the triple line to the bulk liquid. Thus, mass transport may be the rate limiting process, explaining why, in this case, the overall process is not sensitive to crystallographic parameters of the substrate. Transport OFa reactive solute B in a A-B alloy may occur either by pure diffusion or by a convection plus diffusion process. The main feature of the diffusion problem in the sessile drop configuration is that the diffusion field is continuously reduced during the spreading process (Figure 4). Consequently, no constant velocity of the triple line is expected in this case (10). It can be easily shown that this conclusion remains valid when convection participates to solute transport. Calculations of wetting
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kinetics controlled by diffusion, taking into account the particular configuration of the triple line in sessile drop experiments, will be given elsewhere (17). Conclusion
Two types of wetting kinetics in reactive metal/ceramic systems can be found. First, in systems with low reaction rates such as AK, dynamics of wetting is controlled by the processes localized at the triple line, featuring a constant velocity of the triple line. For this type of system, spreading kinetics can be influenced by crystallographic factors of the substrate, which is effectively observed in the Al/C system. Secondly, in systems with high reaction rates like Cu-Ti/A1203, wetting is controlled by the transport of species involved in the reaction from the bulk liquid to the triple line. Contrary to the first type of system, spreading kinetics does not exhibit a constant velocity regime, and is not influenced by crystallographic factors of the substrate. Acknowledgements
The authors thank Professor A. Mortensen and Doctor M. Nicholas for critical reading of the manuscript. References 1, 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
J. C. Ambrose, M. G. Nicholas and A. M. Stoneham, Acta Metall. Mater. 41,239s (1993). N. Eustathopoulos and B. Drevet, MRS Symp. Proc. 3 14, 15 (1993). H. Fujii, H. Nakae and K. Okada, Acta Metall. Mater. 41,2963 (1993). S. Hara, K. Nogi and K. Ogino, Proc. Int. Conf. High Temperature Capillarity, Smolenice Castle May 1994, N. Eustathopoulos Ed. (Reproprint, Bratislava, 1995) p.43. X. H. Wang and H. Conrad, Metall. Mater, Trans. 26A, 459 (1995). R. Cannon, E. Saiz, A. Tomsia and W. Carter, MRS Symp. Proc. 357,279 (1995). X. Zhou and J. de Hosson, Acta Mater. 44,421 (1996). L. Espie, B. Drevet and N. Eustathopoulos, Metall. Trans. 25A, 599 (1994). P. Kritsalis, B. Drevet, N. Vahgnat and N. Eustathopoulos, Scripta Metall. Mater. 30, 1127 (1994). K. Landry and N. Eustathopoulos, Acta Materiala, in press. L. A. Aksay, C. E. Hoge and J. A. Pask, J. Phys. Chem. 78, 1178 (1974). K. Landry, C. Rado, R. Voitovich and N. Eustathopoulos, Acta Materiala, submitted. P. Kritsalis, L. Coudurier and N. Eustathopoulos, J. Mater. Sci. 26, 3400 (1991). P. Vikner, DEA Report, INP Grenoble, France (1993). K. Landry, Ph.D. Thesis, INP Grenoble (1995). K. Landry, S. Kalogeropoulou, N. Eustathopoulos, Y. Naidich and V. Krasovsky, Scripta Materiala 34,841 (1996). A. Mortensen, B. Drevet, N. Eustathopoulos, to be published.
Scripta Mate&h_ Vol. 35, No. 11, pp.1265-1270.1996 Ekevier Science Ltd Couvrkht 0 1996 Acta Metallweica Inc. b&ted in the USA. All rightsreserved 1359-6462/96 S12.00 + .OO
1NFLUE:NCE OF SUBSTRATE ORIENTATION ON WETTING KINETICS IN REACTIVE METAL/CERAMIC SYSTEMS B. Drevet(*), K. Landry, P. Vikner and N. Eustathopoulos Laboratoire de Thermodynamique et Physico-Chimie M&allurgiques, INP Grenoble, URA 29 CNRS, Domaine Universitaire, BP 75,38402 Saint Martin d’H&res Cedex, France (Received June 3, 1996) (Accepted June 13,1996) Introduction During the last few years, several studies have been devoted to the mechanisms of reactive wetting in metal/metal and metal/ceramic systems (l-7). From sessile drop experiments carried out with model metal/ceramic couples (8-IO), the following conclusions have been drawn. (i) The main effect of interfacial reaction:; on wetting is a change in the relevant interfacial energies of the system (2,8) and not, as previously proposed (1 l), linked to the free energy produced by the reaction. (ii) The steady-state contact angle is roughly equal to the Young contact angle of the liquid on the reaction product (8,12). (iii) The spreading kinetics is controlled by the lateral growth of the reaction product at the triple line (10). In the frame:work of this description, it is expected that crystallographic factors, like the orientation of the substrate surface, which can affect reaction kinetics, can also influence wetting kinetics. The effect of subwrate orientation will be studied in two systems. The first one is a Cu-Ti alloy on c1monocrystalline alumina. For this system, wettability and reactivity of Cu-Ti alloys of various compositions on A&(& monocrystals of random orientation were studied in detail by Kritsalis et al. (13). A two-step wetting process was observed: after a very rapid decrease of contact angle in less than one second, explained by the formation of an adsorption layer of Ti at the interface, a much slower decrease arises in about IO’s, attributed to the formation of the wettable Ti monoxide at the interface. In the present study, sessile drop experiments are carried out with a specific alloy composition (Cu-10.8 at.% Ti) on three different crystallographic faces of alumina, as well as on surfaces of random orientation. The second system investigated here is pure Al on carbon. Wetting and interfacial reactions have already been studied by Landry et al. (10) on vitreous carbon. After a first decrease of contact angle owing to deoxidation of the Al drop and A&C3 formation in transient conditions, a quasi-stationary regime of carbide growth at the triple line is established, leading to a linear variation of the drop base radius as a function of time. In the present work, sessile drop experiments are performed on other varieties of carbon, i.e. pyrocarbon and pseudo-monocrystalline carbon.
“‘Now
at CEA G-renoble, DTA/CEREM/DEIWSPCM,
17 rue des Martyrs, 38054 Grenoble Cedex 9, France. 1265
1266
INFLUENCE OF SUBSTRATE ORIENTATION
Experimental For the
Vol. 35. No. 11
Method
Al/C system, three types of carbon are investigated: vitreous carbon (C,), pyrolytic carbon (C,) and graphite pseudo-monocrystals (C,). All these substrates have no open porosity, an ash content less than 50 ppm, and are polycrystalline graphite materials with different degrees of crystalline perfection and microstructures. The degree of graphitization G and the inclinaison of the graphite basal planes with respect to the substrate surface are determined from X-ray diffraction and pole figures. For C,, G is close to zero and planes are randomly inclined. For C,, the value of G is 15% and the maximum inclinaison 25”. For Cpe,the value of G reaches 97% and the maximum inclinaison 3”. The C, and C, substrates are polished down to a 0.1 pm diamond finish. In the case of C,, the average roughness obtained is smaller than 2 nm while C, presents a greater average roughness in the 1O-l 00 nm range. The pseudo-monocrystal basal plane surface obtained by cleavage presents an average roughness of 1000 nm because of high-wavelength roughness of the surface. If measured locally (on areas of 20x20 pm*), the average roughness falls to 2-4 nm. This last value is the relevant one as wetting is controlled only by the local area of the triple line. Before the experiments, all substrates are ultrasonically cleaned in acetone and then annealed in vacuum at 1300 K for 2h. For the Cu-Ti/A1203 system, the substrates are platelets of a-monocrystalline alumina (99.998% purity) with different surface crystallographic planes: random, (ll?O), (1702) and (0001). After mechanical polishing, the substrates are ultrasonically cleaned in acetone and annealed under vacuum at 1473 K for 2h in order to restore surface order which has been disturbed by polishing and eliminate any hydroxyl groups and carbon that may have been adsorbed on the surface. A second annealing is performed in air at 1763 K for 4h in order that oxygen deficient surfaces become stoichiometric. After annealing, the average roughness is l-2 nm. Contact angles are measured by the sessile drop method in a high vacuum metallic furnace. The apparatus consists essentially of a molybdenum resistance furnace fitted with two windows, enabling the illumination of the sessile drop on the substrate and the projection of its image on a screen. Contact angles 8, as well as linear dimensions of the drop (contact radius R and height H), are measured directly from the image of the drop section with an accuracy of + 2” for 0 and + 2% for R and H. The pressure in the experimental chamber can be reduced to 10W5-1 0” Pa at room temperature by a system of two pumps (a rotary vane pump and an oil-diffusion pump both connected to a nitrogen-cooled trap). In order to limit evaporation of Al and Cu, once this pressure level is reached, the experiment is performed in a dynamic vacuum of 10e3Pa obtained by controlled helium micro-leaks. The helium is purified before introduction in the furnace by passing through a Zr-Al getter. In the case of Al/C experiments, the Al specimens (99.998% purity and about 10 mg weight) are freshly cut on all their faces a few minutes before introduction into the furnace, since very short exposure to air limits the thickness of the native superficial oxide layer. Contact angle measurements are performed during isothermal hold at 1100 K. For Cu-Ti/ AlzOJ experiments, the Cu-10.8 at.% Ti alloy, weighting about 100 mg, is processed in situ during the sessile drop experiment by direct melting on the substrate of a piece of Cu (99.999% purity) with a piece of Ti (99.96% purity) over the copper. This procedure, used previously in (13), is carried out to avoid any contact between Ti and the ceramic before complete fusion of the alloy; however, the volume V of the liquid drops is not exactly the same in all experiments. The radius of the equivalent spheres of the drops R,=(3V/47t)“3 is equal to 1.45 f 0.20 mm (Table 1). The heating rate of the furnace is 33 K/min and the holding period at the working temperature Tw, ranging between 13 19 K and 1413 K, lasts one hour. This duration, which is much greater than the time needed to reach a stationary contact angle, is necessary to obtain thickness of reaction product easy to characterize by scanning electron microscope.
Vol. 35, No. 11
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TABLE 1 Wetting Parameters for Cu-10.8 at.% Ti on
Various CrystallographicFaces of Alumina (See Figure 1 for Meaningof Parameters).
Results
Figure 1 shows variations of drop base radius R and contact angle 8 of a Cu-10.8 at.?.. Ti alloy on (1120) face of Al*03 as a function of time. The first measurement is taken when complete fusion is achieved. The initial contact angle, obtained in less than one second, is equal to about 65” and remains nearly constant for about 50 seconds. Thereafter, 8 decreases and reaches a stationary value in about 135 s. No significant variation of 8 (lower than 3”) is observed at the working temperature TWduring one hour. Note that in the experiment of Figure 1, as well as in experiments of Table 1, the steady-state contact angle 8, is attained during the temperature rise, at T=1323 + 15 K, i.e. at a temperature lower than the working temperature. Clearly, the significant variations of 9 and R with time are obtained in non-isothermal conditions. The main features of results of Table 1 are the following. (i) The final contact angle f& is nearly the same for all substrates (OF=29 f 4’). The 4” scattering is typical of 0 measurements, even in nonreactive metaUceramic systems. Thus, any effect of anisotropy of A1203 is lower than this experimental accuracy. Note that the eF value is in good agreement with the previous value measured by Kritsalis et 1450 T
time (4
Figure 1. Variations of contact angle and drop base radius during the temperature rise at Tel403 ( 1150) face of alumina.
K for Cu-10.8 at.% Ti on the
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R (mm)
Figure 2. Variations of contact angle and drop base radius at T==l100 K for Al on pyrocarbon.
al. (13) for the same alloy on a random surface. (ii) Both total spreading time tF and the difference (tFto) do not exhibit any significant variation, the differences between the experiments being of the order of -I: 10% (tF=130 4 15 s and (t&)=83& 8 s). These differences are close to the differences in drop size R,, (Tablel). Although the R(t) curve is not linear (Figure I), an average triple line velocity can be defined as v=(R(tF)-R(to))/(t&). It is found equal for all experiments to 11 f 2 pm.<‘. Micrographic observations made on a perpendicular section to the interface using a scanning electron microscope show a continuous reaction layer, identified as TiO by means of X-ray diffraction. The average thickness of this layer, obtained from several measurements performed along the interface, is about I l_trn.No significant variation of this average thickness was observed between the different crystallographic orientations of the alumina substrates (14). On Figure 2, contact angle and drop base radius are given for the Al/C,, system as a function of time at T=l 100 K. As in the Al/C, system, discussed in detail elsewhere (lo), after a first decrease of 8, due to deoxidation of the Al drop and to carbide formation in transient conditions, a quasi-linear spreading is observed with a rate of 20 nm.d’. Note that even after 2 IO4seconds, no steady-state contact angle is reached. Results of contact angle measured on C,, C, and C,, are compared on Figure 3. With regard to C,, wetting on C, is significantly faster with a triple line velocity of 40 nm.s-‘, and a stationary contact angle is obtained after 16000 s. Wetting kinetics on C,, is much slower, and the spreading rate is only 10 nm.s-‘. For all varieties of carbon, a continuous layer of A14C3a few microns in thickness (15,16) was observed at the Al/C interface.
Figure 3. Contact angle kinetics at T=l (C,).
100
K of Al on vitreous carbon (C,), pyrocarbon (C,) and graphite pseudo-monocrystal
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Figure 4. If the growth of the reaction layer is controlled by the diffision of a reactive solute B from the bulk liquid A-B to the triple line, the reduction of the diffusion field during the spreading process will decrease the growth rate and consequently the velocity of the triple line.
Discussion
By analogy with reactions occuring at the interface between bulk phases, kinetics of reactions between a component of the drop and the solid substrate at the triple line may be composed of two steps: (i) a step including processes localized at the triple line, namely dissociation of the solid, short-range diffision and nucle;ation and growth of the reaction product, and (ii) a step of transport of reactive solutes from bulk drop to the triple line, or, conversely, of soluble reaction products from the triple line to the bulk liquid. In the Al/C (couple, the growth of aluminium carbide at the triple line does not need any long-range diffusion. Thus, reaction kinetics is probably controlled by phenomena occuring at the triple line. When a steady-state configuration is established at the triple line, a regime with a constant triple line velocity is attained (the so-called “linear spreading” (lo)), as indeed observed in the Al/C, system (Figure 2). The presence of a linear region on the R(t) curve testifies strongly in favour of wetting kinetics contralled by a triple line step. For this type of kinetics, crystallographic parameters, such as the orientation of the substrate surface, may affect the spreading rate, as indeed observed in the Al/C system (Figure 3). With regard to the Al/C system, the Cu-Ti/AlzOj couple differs in the following points. (i) As shown on Figure 1, th.e R(t) curves do not exhibit the quasi-linear regime observed in the Al/C system during spreading of the drops (Figure 2). Thus, different phenomena may control spreading kinetics in the two investigated systems. (ii) The lateral growth rate of the TiO reaction product, which is equal to the triple line velocity, is higher by more than two orders of magnitude than that measured in the Al/C system (v=lO pm.s“ instead of v=40 nm.d’ for Al/C, and v=lO nm.s*’ for Al/C,,). For systems with such high reaction rates, mass transport may become the limiting step of the reaction. (iii) Precisely, in view of the oxido-reduction reaction 3(Ti) + A&O, + 3TiO + 2(Al) occuring at the triple line, the progress of the reaction requires transport of Ti from the bulk liquid to the triple line and, conversely, of Al from the triple line to the bulk liquid. Thus, mass transport may be the rate limiting process, explaining why, in this case, the overall process is not sensitive to crystallographic parameters of the substrate. Transport OFa reactive solute B in a A-B alloy may occur either by pure diffusion or by a convection plus diffusion process. The main feature of the diffusion problem in the sessile drop configuration is that the diffusion field is continuously reduced during the spreading process (Figure 4). Consequently, no constant velocity of the triple line is expected in this case (10). It can be easily shown that this conclusion remains valid when convection participates to solute transport. Calculations of wetting
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kinetics controlled by diffusion, taking into account the particular configuration of the triple line in sessile drop experiments, will be given elsewhere (17). Conclusion
Two types of wetting kinetics in reactive metal/ceramic systems can be found. First, in systems with low reaction rates such as AK, dynamics of wetting is controlled by the processes localized at the triple line, featuring a constant velocity of the triple line. For this type of system, spreading kinetics can be influenced by crystallographic factors of the substrate, which is effectively observed in the Al/C system. Secondly, in systems with high reaction rates like Cu-Ti/A1203, wetting is controlled by the transport of species involved in the reaction from the bulk liquid to the triple line. Contrary to the first type of system, spreading kinetics does not exhibit a constant velocity regime, and is not influenced by crystallographic factors of the substrate. Acknowledgements
The authors thank Professor A. Mortensen and Doctor M. Nicholas for critical reading of the manuscript. References 1, 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
J. C. Ambrose, M. G. Nicholas and A. M. Stoneham, Acta Metall. Mater. 41,239s (1993). N. Eustathopoulos and B. Drevet, MRS Symp. Proc. 3 14, 15 (1993). H. Fujii, H. Nakae and K. Okada, Acta Metall. Mater. 41,2963 (1993). S. Hara, K. Nogi and K. Ogino, Proc. Int. Conf. High Temperature Capillarity, Smolenice Castle May 1994, N. Eustathopoulos Ed. (Reproprint, Bratislava, 1995) p.43. X. H. Wang and H. Conrad, Metall. Mater, Trans. 26A, 459 (1995). R. Cannon, E. Saiz, A. Tomsia and W. Carter, MRS Symp. Proc. 357,279 (1995). X. Zhou and J. de Hosson, Acta Mater. 44,421 (1996). L. Espie, B. Drevet and N. Eustathopoulos, Metall. Trans. 25A, 599 (1994). P. Kritsalis, B. Drevet, N. Vahgnat and N. Eustathopoulos, Scripta Metall. Mater. 30, 1127 (1994). K. Landry and N. Eustathopoulos, Acta Materiala, in press. L. A. Aksay, C. E. Hoge and J. A. Pask, J. Phys. Chem. 78, 1178 (1974). K. Landry, C. Rado, R. Voitovich and N. Eustathopoulos, Acta Materiala, submitted. P. Kritsalis, L. Coudurier and N. Eustathopoulos, J. Mater. Sci. 26, 3400 (1991). P. Vikner, DEA Report, INP Grenoble, France (1993). K. Landry, Ph.D. Thesis, INP Grenoble (1995). K. Landry, S. Kalogeropoulou, N. Eustathopoulos, Y. Naidich and V. Krasovsky, Scripta Materiala 34,841 (1996). A. Mortensen, B. Drevet, N. Eustathopoulos, to be published.