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yttria-stabilized zirconia system
Materials Science and Engineering A327 (2002) 117– 127 www.elsevier.com/locate/msea
Surface tension, wettability and reactivity of molten titanium in Ti/yttria-stabilized zirconia system Jun Zhu a,*, Akira Kamiya a, Takahiko Yamada a, Wen Shi a, Katsuyoshi Naganuma a, Kusuhiro Mukai b a
Chubu Research Center, National Institute of Ad6anced Industrial Science and Technology, 1 -1 Hirate-cho, Kita ku, Nagoya 462 -8510, Japan b Department of Materials Science and Engineering, Kyushu Institute of Technology, Sensui-cho, Tobata-ku, Kitakyushu, Fukouka-ken 804 -8550, Japan Received 2 February 2001; received in revised form 11 July 2001
Abstract The wettability and the interaction between pure liquid titanium and yttria-stabilized zirconia were investigated by the sessile drop method in argon atmosphere at 1973 K. The micrographic observations made on cross sections perpendicular to the interface using EPMA show that interfacial reactions occurred at high temperature. However, the contact angles are relative stable and larger than 90° within experimental duration. The density and surface tension of molten titanium found equal to 4.12 90.01 g cm − 3 and 1.46 90.05 N m − 1, respectively. The contact angle increases with increasing the substrate porosity. The influence of porosity on the wettability can be explained by analogy with the influence of surface roughness. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Wettability; Surface tension; Interfacial reaction; Titanium; Zirconia; Yttria
1. Introduction Titanium and its alloys have by now found wide use in the aerospace, chemical, medical prostheses, marine and related industries as being high-temperature, highstrength/weight and high-corrosion resistance materials [1]. Thanks to the efforts of different investigators [2], various methods have been applied in titanium alloys processing such as forging, forming, casting, powder metallurgy, welding etc. On the other hand, titanium matrix composites with low density and excellent mechanical properties have been processed. Wettability and surface tension are key factors for making metal-matrix composites by liquid routes, casting, brazing ceramics or glazing metals, corrosion processes and recycling of metal scraps etc. Thus, a * Corresponding author. Tel.: + 82-52-911-2942; fax: +82-52-9162802. E-mail address: [email protected] (J. Zhu).
number of research teams [3,4] have conducted studies on the capillary and adhesive properties of metals in contact with different ceramics. Wetting of the ceramic by metal is determined by two types of interactions occurring at the interface, leading to non-reactive wetting and reactive wetting. Non-reactive wetting occurs in liquid/solid systems in which the mass transfer through the interfaces is very limited and has a negligible effect on the interfacial energies. The measurement of work of adhesion in dissimilar non-reactive materials provides useful information on interfacial interactions. Wetting involving a chemical change and/or diffusion of chemical species through the interface is referred to as reactive wetting, which appears frequently in metal/ ceramic systems at high temperature. Titanium is a metal with high reactivity and high melting point. It is always used as a reactive solute in a non-reactive matrix for studying the reactive wetting between the ceramics and liquid metals. Benko [5] studied the wettability of cubic boron nitride by tita-
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nium using BN- (Ag – Ti) as a model system. The results of the study revealed that titanium exhibits high chemical activity towards boron nitride. Chemical interactions occurring at the interface result in the formation of titanium nitride and borides in the regions close to the interface layer. The contact angle is reduced with time and is equal to 0° after 15 min of keeping the system at 1000 °C. Other investigators reported the effect of titanium on the wetting of oxides such as Ni –Ti/Al2O3 [6], Sn – Ti/Al2O3 [7], NiFeCr– Ti/Al2O3 [8], Cu–Ti/Al2O3 [9,10], Au– Ti/Al2O3 [11], Cu–Ti/ Y2O3 [9] systems. The effect was attributed to the strong interaction between O and Ti solutes inducing adsorption of O –Ti clusters at the liquid-side of the interface and formation of metallic oxides such as TiO at the solid-side of the interface [10]. The combination of these two effects leads to a dramatic decrease in contact angle caused by Ti additions in metals, which are non-reactive with oxide substrates. Naidich [12] also revealed that liquid Ti could react to dissolve several at.% O and form metal-like oxides such as TiO or even solid solutions of Ti with high oxygen contents in the Ti/MgO system. Nevertheless, only little research has been reported on the interfacial phenomena and wettability of ZrO2 by molten titanium. Furthermore, a very limited number of determinations of surface tension for molten titanium are available and there is a large variation range from 1200 to 1520 mN m − 1 among different researchers [1,13]. The purpose of the present work is to study by the sessile drop method the wettability and reactivity of the molten titanium/zirconia system and the surface tension of pure titanium. The effect of porosity of ZrO2 substrates on the wettability and reactivity is also studied.
2. Experimental procedure
2.1. Apparatus Fig. 1 shows the apparatus used to conduct sessile drop experiments. It consists of a graphite heating element furnace and an image analyzing system. Distortion in the circular symmetry of the drops can degrade the reliability of derived values of contact angles and surface tension and can also have some effect on the measurement of density. In order to avoid the effects of asymmetry, a rotating stage, which allows to verify symmetry in five directions, was adopted in this apparatus. The temperature was measured with a tungsten thermocouple, which was placed directly above the metal drop. The height of thermocouple can be adjusted to account for the expansion of specimen and substrate with increasing of temperature during the experimental procedure. The maximum temperature for the furnace is 2073 K, and the experimental temperature was 1973 K. The temperature was controlled by a PID digital program controller. In order to illuminate the drop placed on the substrate and to project its image, windows were fitted on each side of water-cooled stainless steel chamber. A spotlight was used as background illumination for obtaining sharp image profile. Argon gas was passed through an argon gas purifier with sponge Ti heated at 1273 K and further deoxidized by Mg chips heated at 823 K. The purified gas was then introduced into the furnace. A zirconia oxygen sensor used to verify the purity of gas leaving the sessile drop furnace. The oxygen partial pressure was kept in 10 − 21 –10 − 22 atm in the testing process. The droplet shape was captured using a CCD video camera, transferred into a computer and recorded by a videocassette.
Fig. 1. Schematic diagram of experimental apparatus.
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Table 1 Chemical composition of pure titanium (wt.%) Element
N
H
Fe
O
Ti
Composition
0.006
0.0013
0.043
0.057
Bal.
Fig. 4. Formation of TiC film on the surface of molten titanium droplet on ZrO2 substrate after 10 min being heated at 1973 K.
Fig. 2. Schematic diagram of specimen for surface tension and wettability measurement by the sessile drop method. Table 2 Porosity (in percentage) for ZrO2–8 mol% Y2O3 substrates Specimen
ZrO2-A
ZrO2-B
ZrO2-C
Porosity
0.5
19.5
24.7
Fig. 5. Density of molten titanium on ZrO2 – 8 mol% Y2O3 substrates with different porosity.
Fig. 3. Images of sessile droplets of molten titanium on ZrO2 – 8 mol% Y2O3 substrates with different porosity at 1973 K.
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2.2. Procedure High purity titanium forging bar (6 mm in diameter) was employed in this study. The composition of the pure titanium is shown in Table 1. The pure titanium bar was machined into the shape shown in Fig. 2, each specimen weighing about 0.68 g. The shape of the upper part of the specimen allows to obtain sharp profile image when raising temperature in the solid state, while the shape of lower part of specimen is to facilitate droplet formation at melting. The substrates adopted are plates of cubic structure zirconia containFig. 8. Contact angle and porosity versus substrate roughness.
Fig. 6. Surface tension of molten titanium measured on ZrO2 – 8 mol% Y2O3 substrates with different porosity. Table 3 Values of surface tension for pure molten titanium Author Retained in Boyer [1] Retained in Eustathopoulos et al. [13] Retained in Metals Reference Book [14] Calculated from Eustathopoulos et al.’s value [13] with d|/dT= −0.28 mN m−1 per K [15] Present study
Value (N m−1)
T (K)
1.2 1.52
1943
1.65
1958
1.51
1973
1.46 90.05
1973
ing 8 mol% Y2O3 with three kinds of porosity as shown in Table 2. These substrates were 20 mm in diameter and 5 mm in thickness. The surface roughness of substrates was characterized by the average displacement from the mean height of asperities Ra which was measured by a laser profile micrometer. Both metal specimen and substrates were ultrasound cleaned by acetone and ethanol and then dried in an air blast. As shown in Fig. 1 the metal sample was placed on the upper surface of a ZrO2 –8 mol% Y2O3 substrate and the horizon of the substrate was adjusted by a water level. The furnace chamber was evacuated and filled by argon gas three times. After that, the system was heated in an argon gas atmosphere. The flow rate of argon gas was held at 150 ml min − 1 in the experiment. When the temperature reached 1973 K, the images of droplet were taken every 30 s. The volume, contact angle and surface tension were obtained by fitting the image contour of metal specimen determined with an image analysis software to the theoretical contour calculated using the classical Laplace equation. The measurement accuracy depends mainly on the symmetry of the droplet. After the test, the surface of droplet was examined by X-ray diffraction (XRD) and Auger electron spectrometer (AES). The interface reactivity is characterized by electron probe microanalysis (EPMA).
3. Results and discussion
3.1. Characterization of the molten titanium surface
Fig. 7. Variation of contact angle of molten titanium on ZrO2 – 8 mol% Y2O3 substrates with different porosity.
Fig. 3 shows the video images of sessile drops of pure titanium on the ZrO2 –8 mol% Y2O3 substrate with three kinds of porosity at 1973 K. It can be seen that droplets with ideal smooth shapes are formed on all substrate. The contact angles are \ 90°. The profile of droplets is stable in time and the phenomena of drop mobility and spreading, which were reported in other M-Ti/oxide systems [6–12], were not observed in the
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procedure of experiment. However, as shown in Fig. 4 after heating at constant temperature 1973 K about 10 min the droplet was coated with a kind of film. By means of AES and XRD analyses on the surface of the solidified droplet, the film was identified as TiC, brought about by evaporation of carbon from graphite heating elements at high temperature. Analy-
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ses by EPMA of the sample cooled every 1 min reveal that the TiC is formed in about 7–8 min after metal melting. In order to eliminate the effect of TiC formation on the measured quantities, only the images taken within the first 5 min were used to calculate the density, contact angle and surface tension of molten metal.
Fig. 9. Back scattered electron micrographs of the cross-section perpendicular to the interface of pure titanium and ZrO2 – 8 mol% Y2O3 substrates cooled from 1973 K after being heated for 5 min. Note: black ‘particles’ are pores in substrates.
Fig. 10. Map analyses of Ti, Zr, Y and O elements on cross-section of Ti/ZrO2-A (porosity: 0.5%) system after cooling from 1973 K.
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Fig. 11. Backscattered electron images of cross-section at the triple line for the Ti/ZrO2 – 8 mol% Y2O3 system cooled from 1973 K.
3.2. Surface tension and contact angles Fig. 5 shows the time histories of molten pure titanium density measured at 1973 K. The values of density become stable after complete melting of the metal occurring in about 60 s. It can be seen that the values are independent of substrate porosity. The average density is 4.129 0.01 g cm − 3. Fig. 6 indicates the variation of surface tension of pure titanium with time in this study. The surface tension is nearly constant during the measurement and is also independent of substrate porosity. The average value is 1.4690.05 N m − 1 which is compared in Table 3 with data obtained by other authors [1,13,14]. It can be seen that a discrepancy exists among the values, which can be hardly explained by the difference of experimental temperatures of these determinations. Indeed Eustathopoulos et al. [15] calculated the temperature coefficient of surface tension of pure metals on the basis of Skapski’s nearest-neighbor interaction—broken-bond model. The calculated temperature coefficient of pure titanium (d|/dT) is − 0.28 mN m − 1 per K. Considering the difference of temperature, it can be seen that the surface tension measured in the present study closes to the value reported by Eustathopoulos et al. [13].
Fig. 7 illustrates the changes of contact angle with time for different porosity substrates. The experimental measurements reveal that contact angles depend on the porosity of substrate, and the contact angle becomes larger with increasing the porosity of substrate. It can be seen that the substrate surface changes from smooth to uneven with increasing of porosity. Fig. 8 describes the relations between the surface roughness Ra with contact angle and porosity of the substrate, which confirms that roughness Ra increases when the substrate porosity increases. Thus, the influence of porosity on contact angle could be explained through the influence on q of surface roughness.
3.3. Interfacial reaction of molten titanium with yttria-stabilized zirconia After the experiments, it is found that the solidified droplet adheres well to ZrO2-A substrate but does not hold tightly on ZrO2-B and ZrO2-C substrates. The micrographic observation made on the cross section perpendicular to the interface of samples using EPMA shows that for all substrates there are two distinct layers between titanium and ZrO2 as illustrated in Fig. 9. The observed area is situated near the drop center. The unaffected ZrO2 (zone A) is located at the right
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end, while the unaffected titanium (zone F) is at the left end of the micrograph. It can be seen that layer I in contact with the ZrO2 consists of zone B (black phase) and zone C (light gray phase around zone B). The other layer (layer II) mixed of darker gray (zone D) and needle like white (zone E) phases is observed between the layer I and titanium. The quantitative analyses of the specimens held 5 min at 1973 K by EPMA enable
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identification of their formation given in Table 4. In all cases the sum of concentrations (in wt.%) of elements in each phase differs from 100% by B 3%. Reaction between oxide ZrO2 and molten titanium can be described by the following equations:
ZrO2(s)= Zr +2O,
(1)
Fig. 12. Map analysis of Ti, Zr, Y and O elements on cross-section at triple line after cooling from 1973 K: a and b correspond to 2 and 4 in Fig. 11, respectively.
A: Unaffected ZrO2 B: Black phase in Layer I C: White phase in Layer I D: Dark gray phase in Layer II E: White phase in Layer II F: Unaffected titanium
ZrO2-C 27.87 13.24 26.44 9.50 17.87 0.02
ZrO2-A 4.80 5.68 5.62 0.00 0.00 0.00
ZrO2-B 27.99 14.25 26.35 9.75 16.26 0.02
ZrO2-A 29.04 15.63 26.27 10.53 16.18 0.02
ZrO2-C 66.99 30.37 43.82 22.99 23.50 0.72
ZrO2-A 66.11 25.39 45.08 21.14 20.69 1.04
ZrO2-B 66.94 30.42 44.79 21.07 25.13 0.90
Y
Zr
O
Table 4 Chemical compositions of reactive interface obtained by quantitative analyses (mol.%)
ZrO2-B 5.02 5.29 6.14 0.00 0.00 0.00
ZrO2-C 5.10 5.95 5.29 0.00 0.00 0.00
ZrO2-A 0.06 53.30 23.03 68.33 63.13 98.94
Ti ZrO2-B 0.05 50.04 22.71 69.18 58.61 99.08
ZrO2-C 0.04 50.45 24.44 67.51 58.63 99.27
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Fig. 13. Schematic configurations of a sessile drop resting on the substrate with smooth and rough surfaces in the case of q \90°.
where Zr and O denote Zr and O dissolved in molten titanium. At high temperature, the liquid titanium reacts with ZrO2 forming a chemical reaction zone (zone B). Oxygen is liberated from ZrO2 and dissolved in titanium melt. TiO2 or Ti2O3 could be formed if the reaction between zirconia and titanium is extensive. At the same time, oxygen deficient zones (zone C) were formed near the zone B. It can be deduced that zone B is a kind of titanium –zirconium oxide Tiy Oz ·ZrO2 − x [16]. The reaction can be described as following: ZrO2 +Ti(l)Tiy Oz · ZrO2-x
(2)
Layer II corresponds to the solid solution of Ti/Zr containing about 20– 25 mol.% dissolved oxygen. It can be seen that the part of unaffected titanium (zone F) contains after solidification 0.72– 1.04 mol.% of oxygen, which is likely due to substrate dissolution, not to the very negligible oxygen contained in the purified Ar gas. Table 4 shows that the zirconium content of the white phase (zone E) is about 1.5 times as high as that of darker gray phase (zone D). Thus, the formation of darker gray and needle-like white phases can be attributed to segregation of zirconium and titanium in the process of solidification. In order the reaction to proceed, it is necessary that the reaction products (Zr and O) should be removed from the interface as they are formed. Titanium has a high oxygen solubility and forms completely miscible solutions with zirconium, which would provide a driving force for the removal of reaction products from interface. The Ti diffuses into the ZrO2 – Y2O3 substrate while Zr and O diffuses into Ti (l) in the opposite direction. Although a reaction occurs at interface of Ti/ZrO2 –8 mol% Y2O3, the contact angles are relative stable and larger than 90°, which is different from the conventional opinions that dissolution of solid in the liquid leads to the increase of the contact radius and the decrease of the apparent, or visible contact angle [17]. The solid/liquid interface remains macroscopically flat and the modification of geometry at the triple line is
not observed within the experimental time as shown in Fig. 3. The results of quantitative analyses (Table 4) reveal that Y2O3 shows a good stability as no yttrium is detected in layer II. The result is in agreement with Suzuki et al.’s [18] reporting that Y2O3 exhibits the highest stability when a molten titanium alloy was solidified in a mulllite mold coated with a plasmasprayed refractory oxide layer. Saha et al. [19] pointed out that the stability of Y2O3 in contact with titanium correlates with thermodynamic data for the oxide formation and with the free energy of solution of oxygen and yttrium in titanium. McCoy [20] also indicated that Y2O3 stabilized ZrO2 had better stability than CaO-stabilized ZrO2 in contact with titanium. The X-ray images for the Ti/ZrO2-A system correspond to Fig. 9(a) confirm the higher stability of Y2O3 in the process of reaction with molten titanium shown in Fig. 10. The interface between Ti and substrate cannot be distinguished on the X-ray images of Ti, Zr and O, but it can be clearly observed on the X-ray image of yttrium. It means that Y2O3 remains at interface when the ZrO2 reacts with molten Ti and decreases its content at interface. Thus, it can be deduced that a layer that is a Ti–Zr oxide containing significantly more Y2O3 than the initial substrate may form at interface, which may slow down the progress of the Ti –ZrO2 reaction and the rate of element interdiffusion. However, the contact angle of 140° observed for Ti containing a relative high oxygen concentration as shown in Table 4 is very close to the contact angles observed on stable oxides for metals Cu and Ag having a much lower affinity for oxygen than Ti. Moreover, Naidich et al. [9] indicated that even Cu–Ti alloys containing only 10 mol.% Ti yield contact angles on Y2O3 at 1150 °C lower than 90°, in conditions in which there is no new compound formation at the metal– oxide interface. Also, molten U, a metal with a high affinity for oxygen as Ti, wets well (q 90°) Y2O3 substrates at 1400 °C [21]. It is very difficult to explain why pure Ti at 1700 °C would not wet an oxide layer rich in Y2O3. If on the contrary the intrinsic contact
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angle of Ti on the dense zirconia substrate should be much lower than 140°, then why the contact angle formed at melting remains stable at this value? A first possible explanation is that the drop surface is covered by a very thin TiC layer not in 7– 8 min after melting but much earlier. Such films can inhibit wetting without surpressing the interfacial reactions. However, if the profile of droplets is controlled by the TiC film, it should be independent of the composition of the solid oxide substrates. It means that the same results could be obtained using the various substrates such as Al2O3, MgO etc. But the experimental results indicated that the Ti droplet with ideal smooth shape cannot be formed on substrates of Al2O3, MgO and CaO-stabilized ZrO2 except Y2O3-stabilized ZrO2 substrate under the same condition. It infers that the TiC film is not a main reason for the stable droplet formation. Another possibility is that an oxide layer grows at the droplet free surface close to the triple line that governs the wetting in reactive systems [22,23], by reaction between Ti and oxygen provided by decomposition of the substrate. The backscattered electron and X-ray images of cross-section at the metal– oxide – vapor triple line for the Ti/ZrO2-A and Ti/ZrO2-C systems cooled from 1973 K are shown in Figs. 11 and 12, respectively. It can be seen that the reactive products formed near the triple line and Y displays a high concentration along the interface contrary to Zr. Although the composition of the reactive oxide compound is not clear yet, it may play an important part in inhibiting wetting of Ti on ZrO2 substrate. Furthermore, the stability of the Y2O3 controls the final interfacial chemistry at the triple line and limits the rate of chemical reaction, which blocks spreading of the droplet. The residual Y2O3 may form a rough surface that could block the movement of triple line. As the rate of reaction is slowed down by the Y2O3, it could be assumed that the interface configuration of the system is similar to that of a no-reactive wetting. Thus, a ‘composite’ interface would be formed when the liquid droplet set on the rough surface shown in Fig. 13. The influence of solid surface roughness on the wetting could be attributed to the pinning of triple line by sharp edges that act as obstacles to the spreading of the liquid. According to Wenzel [24] and Cassie and Baxter [25], in the region of wettability (qB 90°), roughness must cause the contact angle to decrease in comparison with the contact angle on a smooth surface. In the region of non-wettability (q \90°), the contact angle on the rough surface would be higher, that is q%\ q. In this study, in any case, the contact angle of the molten titanium/zirconia-yttria ceramic system is larger than 90° within measuring time. Thus, the contact angle would increase as the porosity increases. It also can be seen from Fig. 9 that the thickness of the reaction layer decreases with increasing substrate poros-
ity. The rough interface results in reduction of contact area between the molten metal and substrate as shown in Fig. 13. Thus, the whole reactive rate may decrease with uneven substrate surface.
4. Conclusions The study of high temperature wetting properties of molten titanium on the yttria-stabilized zirconia substrate at 1973 K, carried out by the sessile drop method led to the following results: (1) The density and surface tension measurements at 1973 K yield z1973 = 4.129 0.01 g cm − 3 and z1973 = 1.469 0.05 N m − 1. (2) The contact angle is increased from 140° to 165° as the porosity of substrate is changed from 0.5 to 24.7%. The influence of porosity on the wettability can be explained by analogy with the influence of surface roughness. (3) The occurrence of interfacial reactions between titanium and zirconia at high temperature is confirmed. Nevertheless, the contact angle of molten titanium on yttria-stabilized zirconia substrates is larger than 90° and relatively stable in time within 5 min. The yttria exhibits a higher stability in contact with molten titanium than ZrO2 and slows down the rate of Ti/ZrO2 reaction.
Acknowledgements The authors would like to thank Dr Wei Lin (Shinagawa Refectories Co. Ltd.) for providing the zirconia substrates to this experiment, as well as Mr Kunisuke Ando, technical assistant of the laboratory for his skillful experiment support of this work.
References [1] R. Boyer, Materials Properties Handbook: Titanium Alloys, vol. 3, 1994, pp. 1 – 55. [2] M.J. Donachie Jr., Titanium A Technical Guide, vol. 7, 1988, pp. 1– 19. [3] K. Mukai, J. Ceram. Soc. Jpn. 10 (1975) 44 – 58. [4] N. Eustathopoulos, Acta Mater. 46 (No. 7) (1998) 2319 –2327. [5] E. Benko, Ceram. Int. 21 (1995) 303 – 307. [6] Y.V. Naidich, V.S. Zhuravlev, V.G. Chuprina, Sov. Powder Metall. Met. Ceram. 13 (1974) 236. [7] A.P. Tomsia, E. Saiz, B.J. Dalgleish, R.M. Canon, in: Proceedings of the Fourth Japan International SAMPE Symposium, 1998, p. 347. [8] C. Wan, P. Kritsalis, B. Drevet, N. Eustathopoulos, Mater. Sci. Engng. A207 (1996) 181 – 187. [9] Y.V. Naidich, V.S. Zhuravljov, N.I. Frummina, J. Mater. Sci. 25 (1990) 1895 – 1901. [10] P. Kritsalis, L. Coudurier, N. Eustataopoulos, J. Mater. Sci. 26
J. Zhu et al. / Materials Science and Engineering A327 (2002) 117–127 (1991) 3400 – 3408. [11] V.S. Zhuravlev, M.A. Turchanin, Powder Metall. Met. Ceram. 36 (1997) 141. [12] Y.V. Naidich, Progress in Surface and Membrane Science, vol. 14, Academic Press, New York, 1981, p. 353. [13] N. Eustathopoulos, M.G. Nicholas, B. Drevet, Wettability at High Temperature, Pergamon, New York, 1999, p. 150. [14] Metals Reference Book, fifth ed., Butterworths, 1976. [15] N. Eustathopoulos, B. Drevet, E. Ricci, J. Crystal Growth 191 (1998) 268 – 274. [16] K.F. Lin, C.C. Lin, Scripta Materialia 39 (1998) 1333 – 1338. [17] J.A. Warren, W.J. Boettinger, A.R. Roosen, Acta Matererialia 46 (1998) 3247 – 3264. [18] K. Suzuki, K. Nishikawa, S. Watakabe, Mater. Trans. JIM 38
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(1997) 54 – 62. [19] R.L. Saha, T.K. Nandy, R.D.K. Misra, K.T. Jacob, Mater. Trans. B 21B (1990) 559 – 566. [20] L.G. McCoy, Air Force Materials Laboratoy, OH, AFML-TR72-238, Part II, 1974. [21] C. Tournier, B. Lorrain, F.Le. Guyadec, N. Eustathopoulos, J. Mater. Sci. Lett. 15 (1996) 485 – 489. [22] P. Kritsalis, B. Drevet, N. Valignat, N. Eustathopoulos, Scripta Metall. Mater. 30 (1994) 1127. [23] K. Landry, N. Eustathopoulos, Acta Matererialia B 44 (1996) 3923. [24] R.N. Wenzel, Industrial and Engineering Chemistry, vol. 28, 1936, pp. 988 – 994. [25] A.B. Cassie, S. Baxter, Trans. Farad. Soc. 3 (1966) 546.
Surface tension, wettability and reactivity of molten titanium in Ti/yttria-stabilized zirconia system Jun Zhu a,*, Akira Kamiya a, Takahiko Yamada a, Wen Shi a, Katsuyoshi Naganuma a, Kusuhiro Mukai b a
Chubu Research Center, National Institute of Ad6anced Industrial Science and Technology, 1 -1 Hirate-cho, Kita ku, Nagoya 462 -8510, Japan b Department of Materials Science and Engineering, Kyushu Institute of Technology, Sensui-cho, Tobata-ku, Kitakyushu, Fukouka-ken 804 -8550, Japan Received 2 February 2001; received in revised form 11 July 2001
Abstract The wettability and the interaction between pure liquid titanium and yttria-stabilized zirconia were investigated by the sessile drop method in argon atmosphere at 1973 K. The micrographic observations made on cross sections perpendicular to the interface using EPMA show that interfacial reactions occurred at high temperature. However, the contact angles are relative stable and larger than 90° within experimental duration. The density and surface tension of molten titanium found equal to 4.12 90.01 g cm − 3 and 1.46 90.05 N m − 1, respectively. The contact angle increases with increasing the substrate porosity. The influence of porosity on the wettability can be explained by analogy with the influence of surface roughness. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Wettability; Surface tension; Interfacial reaction; Titanium; Zirconia; Yttria
1. Introduction Titanium and its alloys have by now found wide use in the aerospace, chemical, medical prostheses, marine and related industries as being high-temperature, highstrength/weight and high-corrosion resistance materials [1]. Thanks to the efforts of different investigators [2], various methods have been applied in titanium alloys processing such as forging, forming, casting, powder metallurgy, welding etc. On the other hand, titanium matrix composites with low density and excellent mechanical properties have been processed. Wettability and surface tension are key factors for making metal-matrix composites by liquid routes, casting, brazing ceramics or glazing metals, corrosion processes and recycling of metal scraps etc. Thus, a * Corresponding author. Tel.: + 82-52-911-2942; fax: +82-52-9162802. E-mail address: [email protected] (J. Zhu).
number of research teams [3,4] have conducted studies on the capillary and adhesive properties of metals in contact with different ceramics. Wetting of the ceramic by metal is determined by two types of interactions occurring at the interface, leading to non-reactive wetting and reactive wetting. Non-reactive wetting occurs in liquid/solid systems in which the mass transfer through the interfaces is very limited and has a negligible effect on the interfacial energies. The measurement of work of adhesion in dissimilar non-reactive materials provides useful information on interfacial interactions. Wetting involving a chemical change and/or diffusion of chemical species through the interface is referred to as reactive wetting, which appears frequently in metal/ ceramic systems at high temperature. Titanium is a metal with high reactivity and high melting point. It is always used as a reactive solute in a non-reactive matrix for studying the reactive wetting between the ceramics and liquid metals. Benko [5] studied the wettability of cubic boron nitride by tita-
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 7 3 2 - 4
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nium using BN- (Ag – Ti) as a model system. The results of the study revealed that titanium exhibits high chemical activity towards boron nitride. Chemical interactions occurring at the interface result in the formation of titanium nitride and borides in the regions close to the interface layer. The contact angle is reduced with time and is equal to 0° after 15 min of keeping the system at 1000 °C. Other investigators reported the effect of titanium on the wetting of oxides such as Ni –Ti/Al2O3 [6], Sn – Ti/Al2O3 [7], NiFeCr– Ti/Al2O3 [8], Cu–Ti/Al2O3 [9,10], Au– Ti/Al2O3 [11], Cu–Ti/ Y2O3 [9] systems. The effect was attributed to the strong interaction between O and Ti solutes inducing adsorption of O –Ti clusters at the liquid-side of the interface and formation of metallic oxides such as TiO at the solid-side of the interface [10]. The combination of these two effects leads to a dramatic decrease in contact angle caused by Ti additions in metals, which are non-reactive with oxide substrates. Naidich [12] also revealed that liquid Ti could react to dissolve several at.% O and form metal-like oxides such as TiO or even solid solutions of Ti with high oxygen contents in the Ti/MgO system. Nevertheless, only little research has been reported on the interfacial phenomena and wettability of ZrO2 by molten titanium. Furthermore, a very limited number of determinations of surface tension for molten titanium are available and there is a large variation range from 1200 to 1520 mN m − 1 among different researchers [1,13]. The purpose of the present work is to study by the sessile drop method the wettability and reactivity of the molten titanium/zirconia system and the surface tension of pure titanium. The effect of porosity of ZrO2 substrates on the wettability and reactivity is also studied.
2. Experimental procedure
2.1. Apparatus Fig. 1 shows the apparatus used to conduct sessile drop experiments. It consists of a graphite heating element furnace and an image analyzing system. Distortion in the circular symmetry of the drops can degrade the reliability of derived values of contact angles and surface tension and can also have some effect on the measurement of density. In order to avoid the effects of asymmetry, a rotating stage, which allows to verify symmetry in five directions, was adopted in this apparatus. The temperature was measured with a tungsten thermocouple, which was placed directly above the metal drop. The height of thermocouple can be adjusted to account for the expansion of specimen and substrate with increasing of temperature during the experimental procedure. The maximum temperature for the furnace is 2073 K, and the experimental temperature was 1973 K. The temperature was controlled by a PID digital program controller. In order to illuminate the drop placed on the substrate and to project its image, windows were fitted on each side of water-cooled stainless steel chamber. A spotlight was used as background illumination for obtaining sharp image profile. Argon gas was passed through an argon gas purifier with sponge Ti heated at 1273 K and further deoxidized by Mg chips heated at 823 K. The purified gas was then introduced into the furnace. A zirconia oxygen sensor used to verify the purity of gas leaving the sessile drop furnace. The oxygen partial pressure was kept in 10 − 21 –10 − 22 atm in the testing process. The droplet shape was captured using a CCD video camera, transferred into a computer and recorded by a videocassette.
Fig. 1. Schematic diagram of experimental apparatus.
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Table 1 Chemical composition of pure titanium (wt.%) Element
N
H
Fe
O
Ti
Composition
0.006
0.0013
0.043
0.057
Bal.
Fig. 4. Formation of TiC film on the surface of molten titanium droplet on ZrO2 substrate after 10 min being heated at 1973 K.
Fig. 2. Schematic diagram of specimen for surface tension and wettability measurement by the sessile drop method. Table 2 Porosity (in percentage) for ZrO2–8 mol% Y2O3 substrates Specimen
ZrO2-A
ZrO2-B
ZrO2-C
Porosity
0.5
19.5
24.7
Fig. 5. Density of molten titanium on ZrO2 – 8 mol% Y2O3 substrates with different porosity.
Fig. 3. Images of sessile droplets of molten titanium on ZrO2 – 8 mol% Y2O3 substrates with different porosity at 1973 K.
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2.2. Procedure High purity titanium forging bar (6 mm in diameter) was employed in this study. The composition of the pure titanium is shown in Table 1. The pure titanium bar was machined into the shape shown in Fig. 2, each specimen weighing about 0.68 g. The shape of the upper part of the specimen allows to obtain sharp profile image when raising temperature in the solid state, while the shape of lower part of specimen is to facilitate droplet formation at melting. The substrates adopted are plates of cubic structure zirconia containFig. 8. Contact angle and porosity versus substrate roughness.
Fig. 6. Surface tension of molten titanium measured on ZrO2 – 8 mol% Y2O3 substrates with different porosity. Table 3 Values of surface tension for pure molten titanium Author Retained in Boyer [1] Retained in Eustathopoulos et al. [13] Retained in Metals Reference Book [14] Calculated from Eustathopoulos et al.’s value [13] with d|/dT= −0.28 mN m−1 per K [15] Present study
Value (N m−1)
T (K)
1.2 1.52
1943
1.65
1958
1.51
1973
1.46 90.05
1973
ing 8 mol% Y2O3 with three kinds of porosity as shown in Table 2. These substrates were 20 mm in diameter and 5 mm in thickness. The surface roughness of substrates was characterized by the average displacement from the mean height of asperities Ra which was measured by a laser profile micrometer. Both metal specimen and substrates were ultrasound cleaned by acetone and ethanol and then dried in an air blast. As shown in Fig. 1 the metal sample was placed on the upper surface of a ZrO2 –8 mol% Y2O3 substrate and the horizon of the substrate was adjusted by a water level. The furnace chamber was evacuated and filled by argon gas three times. After that, the system was heated in an argon gas atmosphere. The flow rate of argon gas was held at 150 ml min − 1 in the experiment. When the temperature reached 1973 K, the images of droplet were taken every 30 s. The volume, contact angle and surface tension were obtained by fitting the image contour of metal specimen determined with an image analysis software to the theoretical contour calculated using the classical Laplace equation. The measurement accuracy depends mainly on the symmetry of the droplet. After the test, the surface of droplet was examined by X-ray diffraction (XRD) and Auger electron spectrometer (AES). The interface reactivity is characterized by electron probe microanalysis (EPMA).
3. Results and discussion
3.1. Characterization of the molten titanium surface
Fig. 7. Variation of contact angle of molten titanium on ZrO2 – 8 mol% Y2O3 substrates with different porosity.
Fig. 3 shows the video images of sessile drops of pure titanium on the ZrO2 –8 mol% Y2O3 substrate with three kinds of porosity at 1973 K. It can be seen that droplets with ideal smooth shapes are formed on all substrate. The contact angles are \ 90°. The profile of droplets is stable in time and the phenomena of drop mobility and spreading, which were reported in other M-Ti/oxide systems [6–12], were not observed in the
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procedure of experiment. However, as shown in Fig. 4 after heating at constant temperature 1973 K about 10 min the droplet was coated with a kind of film. By means of AES and XRD analyses on the surface of the solidified droplet, the film was identified as TiC, brought about by evaporation of carbon from graphite heating elements at high temperature. Analy-
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ses by EPMA of the sample cooled every 1 min reveal that the TiC is formed in about 7–8 min after metal melting. In order to eliminate the effect of TiC formation on the measured quantities, only the images taken within the first 5 min were used to calculate the density, contact angle and surface tension of molten metal.
Fig. 9. Back scattered electron micrographs of the cross-section perpendicular to the interface of pure titanium and ZrO2 – 8 mol% Y2O3 substrates cooled from 1973 K after being heated for 5 min. Note: black ‘particles’ are pores in substrates.
Fig. 10. Map analyses of Ti, Zr, Y and O elements on cross-section of Ti/ZrO2-A (porosity: 0.5%) system after cooling from 1973 K.
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Fig. 11. Backscattered electron images of cross-section at the triple line for the Ti/ZrO2 – 8 mol% Y2O3 system cooled from 1973 K.
3.2. Surface tension and contact angles Fig. 5 shows the time histories of molten pure titanium density measured at 1973 K. The values of density become stable after complete melting of the metal occurring in about 60 s. It can be seen that the values are independent of substrate porosity. The average density is 4.129 0.01 g cm − 3. Fig. 6 indicates the variation of surface tension of pure titanium with time in this study. The surface tension is nearly constant during the measurement and is also independent of substrate porosity. The average value is 1.4690.05 N m − 1 which is compared in Table 3 with data obtained by other authors [1,13,14]. It can be seen that a discrepancy exists among the values, which can be hardly explained by the difference of experimental temperatures of these determinations. Indeed Eustathopoulos et al. [15] calculated the temperature coefficient of surface tension of pure metals on the basis of Skapski’s nearest-neighbor interaction—broken-bond model. The calculated temperature coefficient of pure titanium (d|/dT) is − 0.28 mN m − 1 per K. Considering the difference of temperature, it can be seen that the surface tension measured in the present study closes to the value reported by Eustathopoulos et al. [13].
Fig. 7 illustrates the changes of contact angle with time for different porosity substrates. The experimental measurements reveal that contact angles depend on the porosity of substrate, and the contact angle becomes larger with increasing the porosity of substrate. It can be seen that the substrate surface changes from smooth to uneven with increasing of porosity. Fig. 8 describes the relations between the surface roughness Ra with contact angle and porosity of the substrate, which confirms that roughness Ra increases when the substrate porosity increases. Thus, the influence of porosity on contact angle could be explained through the influence on q of surface roughness.
3.3. Interfacial reaction of molten titanium with yttria-stabilized zirconia After the experiments, it is found that the solidified droplet adheres well to ZrO2-A substrate but does not hold tightly on ZrO2-B and ZrO2-C substrates. The micrographic observation made on the cross section perpendicular to the interface of samples using EPMA shows that for all substrates there are two distinct layers between titanium and ZrO2 as illustrated in Fig. 9. The observed area is situated near the drop center. The unaffected ZrO2 (zone A) is located at the right
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end, while the unaffected titanium (zone F) is at the left end of the micrograph. It can be seen that layer I in contact with the ZrO2 consists of zone B (black phase) and zone C (light gray phase around zone B). The other layer (layer II) mixed of darker gray (zone D) and needle like white (zone E) phases is observed between the layer I and titanium. The quantitative analyses of the specimens held 5 min at 1973 K by EPMA enable
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identification of their formation given in Table 4. In all cases the sum of concentrations (in wt.%) of elements in each phase differs from 100% by B 3%. Reaction between oxide ZrO2 and molten titanium can be described by the following equations:
ZrO2(s)= Zr +2O,
(1)
Fig. 12. Map analysis of Ti, Zr, Y and O elements on cross-section at triple line after cooling from 1973 K: a and b correspond to 2 and 4 in Fig. 11, respectively.
A: Unaffected ZrO2 B: Black phase in Layer I C: White phase in Layer I D: Dark gray phase in Layer II E: White phase in Layer II F: Unaffected titanium
ZrO2-C 27.87 13.24 26.44 9.50 17.87 0.02
ZrO2-A 4.80 5.68 5.62 0.00 0.00 0.00
ZrO2-B 27.99 14.25 26.35 9.75 16.26 0.02
ZrO2-A 29.04 15.63 26.27 10.53 16.18 0.02
ZrO2-C 66.99 30.37 43.82 22.99 23.50 0.72
ZrO2-A 66.11 25.39 45.08 21.14 20.69 1.04
ZrO2-B 66.94 30.42 44.79 21.07 25.13 0.90
Y
Zr
O
Table 4 Chemical compositions of reactive interface obtained by quantitative analyses (mol.%)
ZrO2-B 5.02 5.29 6.14 0.00 0.00 0.00
ZrO2-C 5.10 5.95 5.29 0.00 0.00 0.00
ZrO2-A 0.06 53.30 23.03 68.33 63.13 98.94
Ti ZrO2-B 0.05 50.04 22.71 69.18 58.61 99.08
ZrO2-C 0.04 50.45 24.44 67.51 58.63 99.27
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Fig. 13. Schematic configurations of a sessile drop resting on the substrate with smooth and rough surfaces in the case of q \90°.
where Zr and O denote Zr and O dissolved in molten titanium. At high temperature, the liquid titanium reacts with ZrO2 forming a chemical reaction zone (zone B). Oxygen is liberated from ZrO2 and dissolved in titanium melt. TiO2 or Ti2O3 could be formed if the reaction between zirconia and titanium is extensive. At the same time, oxygen deficient zones (zone C) were formed near the zone B. It can be deduced that zone B is a kind of titanium –zirconium oxide Tiy Oz ·ZrO2 − x [16]. The reaction can be described as following: ZrO2 +Ti(l)Tiy Oz · ZrO2-x
(2)
Layer II corresponds to the solid solution of Ti/Zr containing about 20– 25 mol.% dissolved oxygen. It can be seen that the part of unaffected titanium (zone F) contains after solidification 0.72– 1.04 mol.% of oxygen, which is likely due to substrate dissolution, not to the very negligible oxygen contained in the purified Ar gas. Table 4 shows that the zirconium content of the white phase (zone E) is about 1.5 times as high as that of darker gray phase (zone D). Thus, the formation of darker gray and needle-like white phases can be attributed to segregation of zirconium and titanium in the process of solidification. In order the reaction to proceed, it is necessary that the reaction products (Zr and O) should be removed from the interface as they are formed. Titanium has a high oxygen solubility and forms completely miscible solutions with zirconium, which would provide a driving force for the removal of reaction products from interface. The Ti diffuses into the ZrO2 – Y2O3 substrate while Zr and O diffuses into Ti (l) in the opposite direction. Although a reaction occurs at interface of Ti/ZrO2 –8 mol% Y2O3, the contact angles are relative stable and larger than 90°, which is different from the conventional opinions that dissolution of solid in the liquid leads to the increase of the contact radius and the decrease of the apparent, or visible contact angle [17]. The solid/liquid interface remains macroscopically flat and the modification of geometry at the triple line is
not observed within the experimental time as shown in Fig. 3. The results of quantitative analyses (Table 4) reveal that Y2O3 shows a good stability as no yttrium is detected in layer II. The result is in agreement with Suzuki et al.’s [18] reporting that Y2O3 exhibits the highest stability when a molten titanium alloy was solidified in a mulllite mold coated with a plasmasprayed refractory oxide layer. Saha et al. [19] pointed out that the stability of Y2O3 in contact with titanium correlates with thermodynamic data for the oxide formation and with the free energy of solution of oxygen and yttrium in titanium. McCoy [20] also indicated that Y2O3 stabilized ZrO2 had better stability than CaO-stabilized ZrO2 in contact with titanium. The X-ray images for the Ti/ZrO2-A system correspond to Fig. 9(a) confirm the higher stability of Y2O3 in the process of reaction with molten titanium shown in Fig. 10. The interface between Ti and substrate cannot be distinguished on the X-ray images of Ti, Zr and O, but it can be clearly observed on the X-ray image of yttrium. It means that Y2O3 remains at interface when the ZrO2 reacts with molten Ti and decreases its content at interface. Thus, it can be deduced that a layer that is a Ti–Zr oxide containing significantly more Y2O3 than the initial substrate may form at interface, which may slow down the progress of the Ti –ZrO2 reaction and the rate of element interdiffusion. However, the contact angle of 140° observed for Ti containing a relative high oxygen concentration as shown in Table 4 is very close to the contact angles observed on stable oxides for metals Cu and Ag having a much lower affinity for oxygen than Ti. Moreover, Naidich et al. [9] indicated that even Cu–Ti alloys containing only 10 mol.% Ti yield contact angles on Y2O3 at 1150 °C lower than 90°, in conditions in which there is no new compound formation at the metal– oxide interface. Also, molten U, a metal with a high affinity for oxygen as Ti, wets well (q 90°) Y2O3 substrates at 1400 °C [21]. It is very difficult to explain why pure Ti at 1700 °C would not wet an oxide layer rich in Y2O3. If on the contrary the intrinsic contact
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angle of Ti on the dense zirconia substrate should be much lower than 140°, then why the contact angle formed at melting remains stable at this value? A first possible explanation is that the drop surface is covered by a very thin TiC layer not in 7– 8 min after melting but much earlier. Such films can inhibit wetting without surpressing the interfacial reactions. However, if the profile of droplets is controlled by the TiC film, it should be independent of the composition of the solid oxide substrates. It means that the same results could be obtained using the various substrates such as Al2O3, MgO etc. But the experimental results indicated that the Ti droplet with ideal smooth shape cannot be formed on substrates of Al2O3, MgO and CaO-stabilized ZrO2 except Y2O3-stabilized ZrO2 substrate under the same condition. It infers that the TiC film is not a main reason for the stable droplet formation. Another possibility is that an oxide layer grows at the droplet free surface close to the triple line that governs the wetting in reactive systems [22,23], by reaction between Ti and oxygen provided by decomposition of the substrate. The backscattered electron and X-ray images of cross-section at the metal– oxide – vapor triple line for the Ti/ZrO2-A and Ti/ZrO2-C systems cooled from 1973 K are shown in Figs. 11 and 12, respectively. It can be seen that the reactive products formed near the triple line and Y displays a high concentration along the interface contrary to Zr. Although the composition of the reactive oxide compound is not clear yet, it may play an important part in inhibiting wetting of Ti on ZrO2 substrate. Furthermore, the stability of the Y2O3 controls the final interfacial chemistry at the triple line and limits the rate of chemical reaction, which blocks spreading of the droplet. The residual Y2O3 may form a rough surface that could block the movement of triple line. As the rate of reaction is slowed down by the Y2O3, it could be assumed that the interface configuration of the system is similar to that of a no-reactive wetting. Thus, a ‘composite’ interface would be formed when the liquid droplet set on the rough surface shown in Fig. 13. The influence of solid surface roughness on the wetting could be attributed to the pinning of triple line by sharp edges that act as obstacles to the spreading of the liquid. According to Wenzel [24] and Cassie and Baxter [25], in the region of wettability (qB 90°), roughness must cause the contact angle to decrease in comparison with the contact angle on a smooth surface. In the region of non-wettability (q \90°), the contact angle on the rough surface would be higher, that is q%\ q. In this study, in any case, the contact angle of the molten titanium/zirconia-yttria ceramic system is larger than 90° within measuring time. Thus, the contact angle would increase as the porosity increases. It also can be seen from Fig. 9 that the thickness of the reaction layer decreases with increasing substrate poros-
ity. The rough interface results in reduction of contact area between the molten metal and substrate as shown in Fig. 13. Thus, the whole reactive rate may decrease with uneven substrate surface.
4. Conclusions The study of high temperature wetting properties of molten titanium on the yttria-stabilized zirconia substrate at 1973 K, carried out by the sessile drop method led to the following results: (1) The density and surface tension measurements at 1973 K yield z1973 = 4.129 0.01 g cm − 3 and z1973 = 1.469 0.05 N m − 1. (2) The contact angle is increased from 140° to 165° as the porosity of substrate is changed from 0.5 to 24.7%. The influence of porosity on the wettability can be explained by analogy with the influence of surface roughness. (3) The occurrence of interfacial reactions between titanium and zirconia at high temperature is confirmed. Nevertheless, the contact angle of molten titanium on yttria-stabilized zirconia substrates is larger than 90° and relatively stable in time within 5 min. The yttria exhibits a higher stability in contact with molten titanium than ZrO2 and slows down the rate of Ti/ZrO2 reaction.
Acknowledgements The authors would like to thank Dr Wei Lin (Shinagawa Refectories Co. Ltd.) for providing the zirconia substrates to this experiment, as well as Mr Kunisuke Ando, technical assistant of the laboratory for his skillful experiment support of this work.
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