- Email: [email protected]
Ti joints produced by liquid state bonding
Applied Radiation and Isotopes 98 (2015) 1–6
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Interface behavior of Al2O3/Ti joints produced by liquid state bonding J. Lemus-Ruiz a,n, A.O. Guevara-Laureano a, J. Zarate-Medina a, A. Arellano-Lara a, L. Ceja-Cárdenas b a Instituto de Investigaciones en Metalurgia y Materiales, Universidad Michoacana de San Nicolás de Hidalgo, Fco. J. Mújica S/N, Edif. U, C.U., Apdo. Postal 888, C.P. 58000, Morelia, Mich., México b Departamento de Metal-Mecánica, Instituto Tecnológico de Morelia, Avenida Tecnológico 1500, Col. Lomas de Santiaguito, Morelia, Mich., México
H I G H L I G H T S
The interface behavior of Al2O3–Mo/Au/Ti joints was studied. Al2O3–Mo/Au/Ti combinations were produced by brazing at 1100 °C in vacuum. Defect-free interfaces and continuous thin reaction layers of Ti3Au and TiAu phases were obtained. SEM images revealed an homogenous diffusion zone on the metal side of the joint.
art ic l e i nf o
a b s t r a c t
Article history: Received 29 July 2014 Received in revised form 7 January 2015 Accepted 8 January 2015 Available online 9 January 2015
In this work we study brazing of Al2O3 to Ti with biocompatibility properties, using a Au-foil as joining element. Al2O3 was produced by sintering of powder at 1550 °C. Al2O3 samples were coated with a 2 and 4 μm thick of Mo layer and then stacked with Ti. Al2O3–Mo/Au/Ti combinations were joined at 1100 °C in vacuum. Successful joining of Mo–Al2O3 to Ti was observed. Interface shows the formation of a homogeneous diffusion zone. Mo diffused inside Au forming a concentration line. Ti3Au and TiAu phases were observed. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Metal/ceramic joining Alumina Titanium Brazing
1. Introduction In order to increase the practical use of advanced materials, bonding constitutes an alternative to produce metal/ceramic joints for medical applications; so it is necessary to develop simple techniques that allow us to bond these materials. The appropriate selections of the bonding materials represent the technological key of this method; properties such as physical and thermodynamic compatibility, mechanical and physical properties, and thermal expansion coefficient of thermal expansion must be considered (Yong et al., 2006). Alumina (Al2O3) is an important material used in a wide range of applications, including refractories, structural materials, electronic packaging, catalysts, and sensors. Alumina precursors, depending on the starting materials, processing route, presence of impurities, and heating conditions, undergo a variety of transitions until the most stable structure n
Corresponding author. E-mail addresses: [email protected] (J. Lemus-Ruiz), [email protected] (J. Zarate-Medina), [email protected] (L. Ceja-Cárdenas). http://dx.doi.org/10.1016/j.apradiso.2015.01.010 0969-8043/& 2015 Elsevier Ltd. All rights reserved.
(α-Al2O3) is formed at high temperature (Bahlawane and Watanabe, 2000; Ebadzadehw and Sharifi, 2008). Alumina has many useful properties such as high electrical resistivity, high temperature strength and chemical inertness. These properties make alumina the material of choice for various applications especially in situations with hostile environmental conditions such as high temperatures, strong electric fields and intense radiation fluxes. Alumina substrates can be found in electronic components used in portable electronic devices. The material is radiation sensitive and can be applied in dosimetry using thermally or optically stimulated luminescence. Electronic portable devices which are worn close to the body, can represent personal dosemeters for members of the general public in situations of large-scale radiation accident or malevolent acts with radioactive materials. Ekendahl and Judas (2015) investigated dosimetric properties of alumina substrates and aspects of using mobile phones as personal dosemeters and they found that alumina substrates exhibited favorable dosimetry characteristics. Though only a small amount of volume fraction of these systems is composed of the components which contain alumina or other similar candidate ceramic materials, the
2
J. Lemus-Ruiz et al. / Applied Radiation and Isotopes 98 (2015) 1–6
successful operation of these systems depends strongly on these critical components. Any degradation of mechanical and/or electrical properties of these ceramic materials might seriously cause malfunction of the systems and shorten their life expectancy. On the other hand, Kirm et al. (2010) studied luminescence properties and electronic excitation of δ- and Θ-phases of Al2O3 nano-powders using time-resolved luminescence spectroscopy under vacuum ultraviolet excitation, and Zvonarev et al. (2015) studied the effect of structural changes on luminescent and dosimetric properties through variations of the particle size and porosity of alumina ceramics synthesized by annealing compacted nanopowders, showing that formation of fine grains and the growth of their fraction with the increasing annealing temperature increases the thermoluminescence intensity and expands the usable dose range upwards. The manufacturing of component shapes from monolithic engineering ceramics is difficult, and this has generated a continued interest in the use of joining technologies to produce complex configurations from assemblies of simple shapes. In most cases it is required to join these ceramics to metals with sufficient bond strength as metal can compensate for the poor workability and brittleness of ceramics. There are three general categories or types of joining process. The first is mechanical and is achieved through the use of mechanical interlocking of components. The second approach is direct joining, in which components are bonded either by a solid-state process or by fusion. The third approach could be referred as indirect joining in the sense that an intermediate layer of material, such as an adhesive, cement, or braze, is used to bond the two components (Asthana and Singh, 2009; He et al., 2006). In metal–ceramic bonding, it is a common practice to introduce a metal interlayer between the components, producing a joint structure similar to that created by brazing but with higher temperature capabilities. The interlayer should be ductile so that it can deform readily to achieve intimate contact with both mating surfaces at various pressures and temperatures. Moreover, it will act as a stress relieving buffer layer if the thermal expansion of the metal and ceramic components differ significantly, and of course it must adhere and bond strongly to both the metal and ceramic components (Saeed et al., 2010). Various techniques for joining ceramic to metal are available, some need an intermediate liquid phase, brazing, and others are produced by diffusion bonding (Liu et al., 2009; Firmanto et al., 2011). One widely-used method for joining ceramics consists of brazing with a reactive metal alloy; however, the highest obstacle of successful brazing of ceramics to metals is the fact that in most conventional brazing materials, in general, the first problem is the poor wettability of ceramics by most metals and alloys. The second problem is the difference in the physical properties between ceramics and metals which is manifested by extreme differences in the thermal expansion coefficients (α) and Young's modulus. Brazing process increases contact between the materials promoting interfacial interaction through the liquid material. To improve wetting and adherence of filler metal with the surfaces in contact during the joining process it is essential that the surfaces preparation (cleanliness) and/or state condition of the materials to be joint, i.e. an effective metallization (through wetting or flow) is required (Sugar et al., 2006; Lee et al., 2006). In diffusion bonding chemical reaction takes place between the metal and the ceramics, and the properties of the reaction product layer dictate the usefulness of the bind and thus of the whole assembly at high temperature. The reaction product layer should provide a strong bond between the two dissimilar materials, which means: (i) it should accommodate for the thermo-mechanical mismatch resulting from differences in thermal expansion coefficients, and (ii) the reaction product layer should not consist of compounds that have mechanical properties significantly inferior to those of the metal and the ceramic (He et al.,
2010; Roy et al., 2009). Al2O3 has been widely used as a very promising structural ceramics in a variety of engineering applications due its excellent performance in various severe working environments (Walker and Hodges, 2008; Nono et al., 2006). The use of copper as a reactive system for joining metals to Al2O3 has significant potential as an active element, in particular for the active brazing. However, studies of the diffusion bonding kinetics, interfacial reaction and their effects on the properties of the joints are far from complete. The practical use of advanced ceramics depends on the reliability of ceramic/metal joining techniques and the properties of the resulting interfaces. Therefore, it is important to study the microstructure and mechanism of interface formation between the metal and ceramic. Therefore the main objective of this work was to determine various aspects during brazing of Al2O3 samples to commercial titanium alloy grade 4 with biocompatibility properties, using a Au-foil as joining element.
2. Materials and methods The starting materials used were cylindrical monolithic Al2O3 samples, produced by isothermal sintering of commercial powders, α-Al2O3, in a CARBOLITE furnace at 1550 °C for 120 min, and commercially pure Ti-rod and Au-foil (99.98%), Aldyrich Chemical Company., Inc. USA. The alumina samples were produced with the same diameter of the Ti rod (10 mm), and 3 mm thick. The success of diffusion bonding processes depends on a combination of factors, one of the most important is the surface preparation of the materials because it controls the initial contact area between the diffusion couples. From the above, the diffusion bonding experiments started with the preparation of the materials to be joined. Prior to joining experiments, the surface of Al2O3 samples were coated by chemical vapor depositions (CVD) process, with a 2 and 4 μm thick of Mo layer. Sandwich joints samples consisted of a block of Mo–Al2O3 mounted axially with a block of Ti and Au-foil in between, the Mo coated alumina surface was facing the metallic phase. The specimens to be joined were placed in a graphite die embedded in a boron nitride (99.5% pure) powder bed shown in Fig. 1. The purpose of the powder-bed was to avoid contact between the sample and the internal walls of the graphite die. The experimental apparatus used to join the sample combinations consists of a resistance furnace with a square chamber. Once the sample was assembled in the graphite die, it was positioned in the furnace and it was closed and filled with argon prior to be evacuated to the vacuum joining atmosphere. After the joining environment was established, the furnace was heated up to the preset joining temperature. Joining experiments were carried out on Al2O3–Mo/Au/Ti sandwich-like combinations at a temperature of 1100 °C using different holding times for both, 2 and 4 μm thick of Mo-coating, under vacuum. The microstructural examination was performed in cross-sections of polished experimental joints. Micrographs and analysis of the interfaces were obtained from a JEOL JSM-6400 scanning electron microscope.
3. Results and discussion 3.1. Sintering of Al2O3 Cylindrical samples of 10 mm of diameter and 3 mm thick were produced by sintering at 1550 °C for 120 min of highly homogeneous commercial Al2O3 with size distributions lower than 100 nm. Fig. 2a shows a free surface of Al2O3 consisting of grains formed out of nanometric particles.
J. Lemus-Ruiz et al. / Applied Radiation and Isotopes 98 (2015) 1–6
3
Fig. 1. Schematic representation of the samples assembly inside the graphite die.
It is possible to observe some residual porosity because the relative density of this specimen is higher than 90%. X ray diffractions analysis carried out in alumina sample produced at high sintering temperature confirms the α-Al2O3 phase structure as shown in Fig. 3b. On the other hand, Fig. 3 shows a cross-section and surface of the alumina samples coated with Mo. The estimated thickness of the surface Mo-coating of (a) 2 μm and (b) 4 μm, respectively can be observed. 3.2. Metal/ceramic joint behavior Diffusion bonding of sandwich-like samples of Al2O3/Ti was investigated by Al2O3–Mo/Au/Ti combinations joined at temperature of 1100 °C in vacuum atmosphere and varying holding times for both, 2 and 4 μm thick of Mo-coating. In general, successful joining of alumina to titanium can be observed, achieved at joining times of 2.5 and 5 min when the alumina surface was coated with Mo. However, in the case of samples treated without Mo-coating, separations during metallographic preparations process occurred for the samples bonded at 1100 °C. It could be associated to the low bonding strength of the sample produced by the diffusion process. On the other hand, in order to determine the interfacial interaction during the joint, a microstructural examination was performed in cross-section of polished experimental joined samples produced at 1100 °C. Joints of Al2O3–Mo/Au/Ti combinations are formed through the formation of a reactive interface on the metal side of the sample as a result of diffusion and interaction of Au with Ti and Mo-coating. A cross-section of Al2O3–Mo/Au/Ti interface for samples joined at 1100 °C for (a) 2 μm of Mo-coating for 5 min and (b) 4 μm of Mo-coating for 2.5 min is shown in Fig. 4. It can be observed in Fig. 4a that starting Mo-coating of 2 μm
a
thick stay disperse inside the Au joining element even for a joining time of 5 min; however when the starting Mo-coating increases to 4 μm thick, it is possible to observe different Mo concentrations inside the joining element. It can be observed that, when the joining time increases from 2.5 min (Fig. 4b) to 5 min the Mo concentration is observed along region forming a line close to alumina side, as is shown in Fig. 5. On the other side, interaction of Ti and Au produced by atomic diffusion generates the formation of intermetallic phases, following the phase diagram (Okamoto and Massalski, 1990), Ti3Au and TiAu, on the interface along the bonding line with the metal size Ti. Composition profile was measured using electron probe microanalysis carried out in the cross-section of the Al2O3–Mo/Au/Ti interface which shows the main diffusion atomic elements during bonding. The analysis were performed on different points along the line illustrated at the interface of the joint made at 1100 °C for 5 min using Al2O3 with 4 mm of Mo-coating (Fig. 5) and the results are shown in the graphic in the same Fig. 5. The scan line was chosen to start on the Ti side of the sample through the bonding line interface passing through different interacting phases up to the ceramic Al2O3. The composition profile measured in the interdiffusion zone of Ti and Au signals shows that it is evident that there is a reasonable penetration of both elements, Ti and Au, showing a narrow homogeneity range corresponding to atomic compositions of these phases, Ti3Au and TiAu, respectively. This can be visualized from the composition profile shown in the zone (a) and zone (b) of Fig. 5. This kind of behavior is very common, especially, if the phase grows mainly from one interface because of relatively high diffusion rate of one species compared to the other; this is in agreement with results obtained by Kumar and Paul (2010). The Ti signal reached its maximum at the metal side; however, Au signal reached its maximum at the interface and the minimum inside the
b
Fig. 2. (a) Fracture image and (b) x-ray diffractions pattern of Al2O3 samples sintered at 1550 °C for 120 min.
4
J. Lemus-Ruiz et al. / Applied Radiation and Isotopes 98 (2015) 1–6
a
b
Fig. 3. Cross-section and surface examination of (a) 2 μm and (b) 4 μm Mo-coating of Al2O3 samples sintered at 1550 °C for 120 min.
Ti metal and in the size in the interface where the Mo signal reached its maximum. It can be observed that the Mo signals are distributed inside all the interfaces. Reactions formed inside the interface, Ti3Au and TiAu, produce a different type of joint between the metal and ceramic and the relative efficiency of these reaction products on the strength of Ti/Al2O3 interfaces is not fully understood. However, most reaction layers are brittle, and therefore potentially detrimental to the interface properties. The thickness of the reaction zone in joints is a function of the joining temperature and may dominate in the final strength. In general, when a ceramic is in contact with a metal a reaction is expected to occur. Joining temperature, time, and Mo-coating thickness were the main process parameters studied. Temperature is the most important one due to the fact that in thermally activated processes, a small change in temperature will result in a significant change in process kinetics compared with other parameters, and virtually all mechanisms including plastic deformation and diffusion are sensitive to temperature. On the other hand, the key to a successful joining technology lies in the ability to modify
a
the interface to accommodate the vastly different types of chemical bonding, from the metallic bonding of the metal to the ionic or covalent bond of the ceramic in order to reduce the electronic discontinuity at the joined surfaces. This is usually achieved by producing a graded interface. This graded interface is also essential in reducing the unfavorable effects due to two kinds of factors: the thermal mismatch and the differences between the two classes of materials, metals and ceramics (Paiva and Barbosa, 2008). In our case, the role of Mo coated film has two main proposes, first to increase the initial area of contact with the Al2O3 (TEC: 8.1 10 6 K 1) and to promote a graded interlayer with hard metals that have a relatively low thermal expansion coefficient (TEC: 5.3 10 6 K 1) that can avoid premature failures. Joints of Al2O3–Mo/Au/Ti combinations are formed through the formation of a reactive interface on the metal side of the sample as a result of diffusion and interaction of Au with Ti and Mo-coating and it can be explained since diffusion is more difficult in ceramics than metals (Lesage, 1994). Liquid formation plays an important role in the joining process, because it increases the rate of the interface
b
Fig. 4. Cross-section of Al2O3–Mo/Au/Ti interface for samples joined at 1100 °C for (a) 2 μm of Mo-coating for 5 min and (b) 4 μm of Mo-coating for 2.5 min.
J. Lemus-Ruiz et al. / Applied Radiation and Isotopes 98 (2015) 1–6
5
clear that the amount of interfacial reaction played a major role in determining the final mechanical properties of the joints. Furthermore, the nature of the reaction products may also have influenced the mechanical properties of the joints. In summary, the choice of suitable conditions to prepare ceramic/metal joints requires knowledge about the mechanism of reaction between the materials and the evolution of the interface.
4. Conclusions On the basis of the results presented in this work, we have shown that it is possible to join alumina with a Mo-coating layer to titanium using an Au foil interlayer as joining element. Successful bonding was observed at 1100 °C under vacuum and Mo-coating. Joining of Al2O3 to Ti occurred through the formation of a homogenous diffusion zone on the metal side of the joint, with no severe interfacial cracking or porosity. Diffusion of the Mo-coating inside the interface takes place forming a continuous region line close to the bonding line with the Al2O3 when 4 μm of coating and 5 min were used.
Acknowledgments The authors wish to thank CONACYT-México (Grant 167286) and Universidad Michoacana de San Nicolás de Hidalgo for the financial support and facilities of this research.
References
Fig. 5. Line analysis across the Al2O3–Mo/Au/Ti interface bonded at 1100 °C for samples with 4 μm of Mo-coating for 5 min.
formation, improving the contact area between the bonding materials, and consequently the interaction is higher, promoting rapid diffusion of the material, since liquid diffusion is much faster than the diffusion in solid-state (Lemus and Drew, 2003). The interface consisted of a homogeneous reaction layer produced by chemical interaction, as well as the high affinity of Au and Ti forming Ti3Au and TiAu phases. These results are in good agreement with the observation realized by Kumar and Paul (2010). It can be observed that the joints formed at the same temperature and different times exhibit similar microstructures with a continuum and homogeneous interface free of crack and porosity. It can be observed that increasing the holding time from 2.5 to 5 min in samples joined with alumina, coated with 4 μm of Mo, did not significantly increase the interface thickness; however, the formation of a region of Mo forming a line inside the reaction layer and close to the bonding line with the Al2O3 can be observed. The strength of diffusion-bonded ceramic/metal joints depends on the nature and microstructure of the interface between the joining materials. The effect of a reaction layer on the interface strength depends on a number of factors such as the mechanical properties of the reaction layer, its thickness and morphology. Lemus–Ruiz and Aguilar–Reyes (2004) observed that as soon as the thickness of these phases increases, the joint strength, at first, rises due to the creation of a strong, integral bond and then reaches a maximum at a certain thickness and then decreases as the interface continues to grow. Therefore, the reaction layer thickness must be controlled to ensure good joint strength. It is
Asthana, R., Singh, M., 2009. Evaluation of Pd-based brazes to join silicon nitride to copper-clad-molybdenum. Ceram. Int. 35, 3511–3515. Bahlawane, N., Watanabe, T., 2000. New sol–gel route for the preparation of pure αalumina at 950 °C. J. Am. Ceram. Soc. 83 (9), 2324–2326. Ebadzadehw, T., Sharifi, L., 2008. Synthesis of γ-Al2O3 from a mixture of γ-aluminum nitrate and carboxymethyl cellulose. J. Am. Ceram. Soc. 91 (10), 3408–3409. Ekendahl, D., Judas, L., 2015. Retrospective dosimetry with alumina substrate from electronic components. Radiat. Prot. Dosim. 150 (2), 134–141. Firmanto, H., Hussain, P., Mamat, O., 2011. Reaction layers of the diffusion-bonded sialon and high-chromium steel. J. Appl. Sci. 11, 1809–1814. He, P., Feng, J.C., Xu, W., 2006. Microstructure and kinetics of induction brazing TiAl-based intermetallics to steel 35CrMo using AgCuTi filler metal. Mater. Sci. Eng. A 418 (1–2), 53–60. He, Y.M., Zhang, J., Liu, C.F., Sun, Y., 2010. Microstructure and mechanical properties of Si3N4/Si3N4 joint brazed with Ag–Cu–Ti þ SiCp composite filler. Mater. Sci. Eng. A 527 (12), 2819–2825. Kirm, M., Feldbach, E., Kotlov, A., Liblik, P., Lushchik, A., Oja, M., Palcevskis, E., 2010. VUV spectroscopy and electronic excitations in nano-size alumina. Radiat. Meas. 45 (3–6), 618–620. Kumar, A.K., Paul, A., 2010. Interdiffusion studies in bulk Au–Ti system. J. Mater. Sci.: Mater. Electron. 21, 1202–1206. Lee, J., Ishimura, H., Tanaka, T., 2006. Novel method determining contact angle of liquid Au on solid Al2O3 single crystal (0001) surface at 1373 K. Mater. Sci. Forum 512, 306–312. Lemus, J., Drew, R.A.L., 2003. Joining of silicon nitride ceramics using a titanium foil interlayer. Mater. Sci. Eng. A 352, 169–178. Lemus-Ruiz, J., Aguilar-Reyes, E.A., 2004. Mechanical properties of silicon nitride joints using a Ti-foil interlayer. Mater. Lett. 58, 2340–2344. Lesage, B., 1994. Some aspects of diffusion in ceramics. J. Phys. III 4, 1833–1850. Liu, C.F., Zhang, J., Zhou, Y., Yi, H.L., Naka, M., 2009. Effect of holding time on the self-joining of silicon nitride. J. Alloy. Compd. 471 (1–2), 217–221. Nono, M.C.A., Barroso, J.J., Castro, P.J., 2006. Mechanical behavior and microstructural analysis of alumina–titanium brazed interfaces. Mater. Sci. Eng. A 435–436, 602–605. Paiva, O.C., Barbosa, M.A., 2008. Brazing parameters determine the degradation and mechanical behaviour of alumina/titanium brazed joints. J. Mater. Sci. 35, 1165–1175. Roy, R.K., Panda, A.K., Das, S.K., Govind, Mitra A., 2009. Development of a copper based filler alloy for brazing stainless steels. Mater. Sci. Eng. A 523 (1–2), 312–315. Saeed, A.M., Hussain, Z., Badri, A., Ariga, T., 2010. The effects of welding parameters on the weldability of different materials using brazing alloy fillers. Mater. Des. 31 (7), 3339–3345.
6
J. Lemus-Ruiz et al. / Applied Radiation and Isotopes 98 (2015) 1–6
Sugar, J.D., McKeown, J.T., Akashi, T., Hong, S.M., Nakashima, K., Glaeser, A.M., 2006. Transient-liquid-phase and liquid-film-assisted joining of ceramics. J. Eur. Ceram. Soc. 26, 363–372. Walker, C.A., Hodges, V.C., 2008. Comparing metal–ceramic brazing methods. Weld. J., 43–50. Yong, C.Z., Di, F., Yong, H.Z., Chun, C.X., 2006. Progress in joining ceramic to metals.
J. Iron Steel Res. Int. 13 (2), 1–5. Zvonarev, S.V., Kortov, V.S., Shtang, T.V., Ananchenko, D.V., Petrovykh, K.A., 2015. Effect of structural changes on luminescent and dosimetric properties of nanoscale aluminum oxide. Appl. Radiat. Isot. 95, 44–47.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Interface behavior of Al2O3/Ti joints produced by liquid state bonding J. Lemus-Ruiz a,n, A.O. Guevara-Laureano a, J. Zarate-Medina a, A. Arellano-Lara a, L. Ceja-Cárdenas b a Instituto de Investigaciones en Metalurgia y Materiales, Universidad Michoacana de San Nicolás de Hidalgo, Fco. J. Mújica S/N, Edif. U, C.U., Apdo. Postal 888, C.P. 58000, Morelia, Mich., México b Departamento de Metal-Mecánica, Instituto Tecnológico de Morelia, Avenida Tecnológico 1500, Col. Lomas de Santiaguito, Morelia, Mich., México
H I G H L I G H T S
The interface behavior of Al2O3–Mo/Au/Ti joints was studied. Al2O3–Mo/Au/Ti combinations were produced by brazing at 1100 °C in vacuum. Defect-free interfaces and continuous thin reaction layers of Ti3Au and TiAu phases were obtained. SEM images revealed an homogenous diffusion zone on the metal side of the joint.
art ic l e i nf o
a b s t r a c t
Article history: Received 29 July 2014 Received in revised form 7 January 2015 Accepted 8 January 2015 Available online 9 January 2015
In this work we study brazing of Al2O3 to Ti with biocompatibility properties, using a Au-foil as joining element. Al2O3 was produced by sintering of powder at 1550 °C. Al2O3 samples were coated with a 2 and 4 μm thick of Mo layer and then stacked with Ti. Al2O3–Mo/Au/Ti combinations were joined at 1100 °C in vacuum. Successful joining of Mo–Al2O3 to Ti was observed. Interface shows the formation of a homogeneous diffusion zone. Mo diffused inside Au forming a concentration line. Ti3Au and TiAu phases were observed. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Metal/ceramic joining Alumina Titanium Brazing
1. Introduction In order to increase the practical use of advanced materials, bonding constitutes an alternative to produce metal/ceramic joints for medical applications; so it is necessary to develop simple techniques that allow us to bond these materials. The appropriate selections of the bonding materials represent the technological key of this method; properties such as physical and thermodynamic compatibility, mechanical and physical properties, and thermal expansion coefficient of thermal expansion must be considered (Yong et al., 2006). Alumina (Al2O3) is an important material used in a wide range of applications, including refractories, structural materials, electronic packaging, catalysts, and sensors. Alumina precursors, depending on the starting materials, processing route, presence of impurities, and heating conditions, undergo a variety of transitions until the most stable structure n
Corresponding author. E-mail addresses: [email protected] (J. Lemus-Ruiz), [email protected] (J. Zarate-Medina), [email protected] (L. Ceja-Cárdenas). http://dx.doi.org/10.1016/j.apradiso.2015.01.010 0969-8043/& 2015 Elsevier Ltd. All rights reserved.
(α-Al2O3) is formed at high temperature (Bahlawane and Watanabe, 2000; Ebadzadehw and Sharifi, 2008). Alumina has many useful properties such as high electrical resistivity, high temperature strength and chemical inertness. These properties make alumina the material of choice for various applications especially in situations with hostile environmental conditions such as high temperatures, strong electric fields and intense radiation fluxes. Alumina substrates can be found in electronic components used in portable electronic devices. The material is radiation sensitive and can be applied in dosimetry using thermally or optically stimulated luminescence. Electronic portable devices which are worn close to the body, can represent personal dosemeters for members of the general public in situations of large-scale radiation accident or malevolent acts with radioactive materials. Ekendahl and Judas (2015) investigated dosimetric properties of alumina substrates and aspects of using mobile phones as personal dosemeters and they found that alumina substrates exhibited favorable dosimetry characteristics. Though only a small amount of volume fraction of these systems is composed of the components which contain alumina or other similar candidate ceramic materials, the
2
J. Lemus-Ruiz et al. / Applied Radiation and Isotopes 98 (2015) 1–6
successful operation of these systems depends strongly on these critical components. Any degradation of mechanical and/or electrical properties of these ceramic materials might seriously cause malfunction of the systems and shorten their life expectancy. On the other hand, Kirm et al. (2010) studied luminescence properties and electronic excitation of δ- and Θ-phases of Al2O3 nano-powders using time-resolved luminescence spectroscopy under vacuum ultraviolet excitation, and Zvonarev et al. (2015) studied the effect of structural changes on luminescent and dosimetric properties through variations of the particle size and porosity of alumina ceramics synthesized by annealing compacted nanopowders, showing that formation of fine grains and the growth of their fraction with the increasing annealing temperature increases the thermoluminescence intensity and expands the usable dose range upwards. The manufacturing of component shapes from monolithic engineering ceramics is difficult, and this has generated a continued interest in the use of joining technologies to produce complex configurations from assemblies of simple shapes. In most cases it is required to join these ceramics to metals with sufficient bond strength as metal can compensate for the poor workability and brittleness of ceramics. There are three general categories or types of joining process. The first is mechanical and is achieved through the use of mechanical interlocking of components. The second approach is direct joining, in which components are bonded either by a solid-state process or by fusion. The third approach could be referred as indirect joining in the sense that an intermediate layer of material, such as an adhesive, cement, or braze, is used to bond the two components (Asthana and Singh, 2009; He et al., 2006). In metal–ceramic bonding, it is a common practice to introduce a metal interlayer between the components, producing a joint structure similar to that created by brazing but with higher temperature capabilities. The interlayer should be ductile so that it can deform readily to achieve intimate contact with both mating surfaces at various pressures and temperatures. Moreover, it will act as a stress relieving buffer layer if the thermal expansion of the metal and ceramic components differ significantly, and of course it must adhere and bond strongly to both the metal and ceramic components (Saeed et al., 2010). Various techniques for joining ceramic to metal are available, some need an intermediate liquid phase, brazing, and others are produced by diffusion bonding (Liu et al., 2009; Firmanto et al., 2011). One widely-used method for joining ceramics consists of brazing with a reactive metal alloy; however, the highest obstacle of successful brazing of ceramics to metals is the fact that in most conventional brazing materials, in general, the first problem is the poor wettability of ceramics by most metals and alloys. The second problem is the difference in the physical properties between ceramics and metals which is manifested by extreme differences in the thermal expansion coefficients (α) and Young's modulus. Brazing process increases contact between the materials promoting interfacial interaction through the liquid material. To improve wetting and adherence of filler metal with the surfaces in contact during the joining process it is essential that the surfaces preparation (cleanliness) and/or state condition of the materials to be joint, i.e. an effective metallization (through wetting or flow) is required (Sugar et al., 2006; Lee et al., 2006). In diffusion bonding chemical reaction takes place between the metal and the ceramics, and the properties of the reaction product layer dictate the usefulness of the bind and thus of the whole assembly at high temperature. The reaction product layer should provide a strong bond between the two dissimilar materials, which means: (i) it should accommodate for the thermo-mechanical mismatch resulting from differences in thermal expansion coefficients, and (ii) the reaction product layer should not consist of compounds that have mechanical properties significantly inferior to those of the metal and the ceramic (He et al.,
2010; Roy et al., 2009). Al2O3 has been widely used as a very promising structural ceramics in a variety of engineering applications due its excellent performance in various severe working environments (Walker and Hodges, 2008; Nono et al., 2006). The use of copper as a reactive system for joining metals to Al2O3 has significant potential as an active element, in particular for the active brazing. However, studies of the diffusion bonding kinetics, interfacial reaction and their effects on the properties of the joints are far from complete. The practical use of advanced ceramics depends on the reliability of ceramic/metal joining techniques and the properties of the resulting interfaces. Therefore, it is important to study the microstructure and mechanism of interface formation between the metal and ceramic. Therefore the main objective of this work was to determine various aspects during brazing of Al2O3 samples to commercial titanium alloy grade 4 with biocompatibility properties, using a Au-foil as joining element.
2. Materials and methods The starting materials used were cylindrical monolithic Al2O3 samples, produced by isothermal sintering of commercial powders, α-Al2O3, in a CARBOLITE furnace at 1550 °C for 120 min, and commercially pure Ti-rod and Au-foil (99.98%), Aldyrich Chemical Company., Inc. USA. The alumina samples were produced with the same diameter of the Ti rod (10 mm), and 3 mm thick. The success of diffusion bonding processes depends on a combination of factors, one of the most important is the surface preparation of the materials because it controls the initial contact area between the diffusion couples. From the above, the diffusion bonding experiments started with the preparation of the materials to be joined. Prior to joining experiments, the surface of Al2O3 samples were coated by chemical vapor depositions (CVD) process, with a 2 and 4 μm thick of Mo layer. Sandwich joints samples consisted of a block of Mo–Al2O3 mounted axially with a block of Ti and Au-foil in between, the Mo coated alumina surface was facing the metallic phase. The specimens to be joined were placed in a graphite die embedded in a boron nitride (99.5% pure) powder bed shown in Fig. 1. The purpose of the powder-bed was to avoid contact between the sample and the internal walls of the graphite die. The experimental apparatus used to join the sample combinations consists of a resistance furnace with a square chamber. Once the sample was assembled in the graphite die, it was positioned in the furnace and it was closed and filled with argon prior to be evacuated to the vacuum joining atmosphere. After the joining environment was established, the furnace was heated up to the preset joining temperature. Joining experiments were carried out on Al2O3–Mo/Au/Ti sandwich-like combinations at a temperature of 1100 °C using different holding times for both, 2 and 4 μm thick of Mo-coating, under vacuum. The microstructural examination was performed in cross-sections of polished experimental joints. Micrographs and analysis of the interfaces were obtained from a JEOL JSM-6400 scanning electron microscope.
3. Results and discussion 3.1. Sintering of Al2O3 Cylindrical samples of 10 mm of diameter and 3 mm thick were produced by sintering at 1550 °C for 120 min of highly homogeneous commercial Al2O3 with size distributions lower than 100 nm. Fig. 2a shows a free surface of Al2O3 consisting of grains formed out of nanometric particles.
J. Lemus-Ruiz et al. / Applied Radiation and Isotopes 98 (2015) 1–6
3
Fig. 1. Schematic representation of the samples assembly inside the graphite die.
It is possible to observe some residual porosity because the relative density of this specimen is higher than 90%. X ray diffractions analysis carried out in alumina sample produced at high sintering temperature confirms the α-Al2O3 phase structure as shown in Fig. 3b. On the other hand, Fig. 3 shows a cross-section and surface of the alumina samples coated with Mo. The estimated thickness of the surface Mo-coating of (a) 2 μm and (b) 4 μm, respectively can be observed. 3.2. Metal/ceramic joint behavior Diffusion bonding of sandwich-like samples of Al2O3/Ti was investigated by Al2O3–Mo/Au/Ti combinations joined at temperature of 1100 °C in vacuum atmosphere and varying holding times for both, 2 and 4 μm thick of Mo-coating. In general, successful joining of alumina to titanium can be observed, achieved at joining times of 2.5 and 5 min when the alumina surface was coated with Mo. However, in the case of samples treated without Mo-coating, separations during metallographic preparations process occurred for the samples bonded at 1100 °C. It could be associated to the low bonding strength of the sample produced by the diffusion process. On the other hand, in order to determine the interfacial interaction during the joint, a microstructural examination was performed in cross-section of polished experimental joined samples produced at 1100 °C. Joints of Al2O3–Mo/Au/Ti combinations are formed through the formation of a reactive interface on the metal side of the sample as a result of diffusion and interaction of Au with Ti and Mo-coating. A cross-section of Al2O3–Mo/Au/Ti interface for samples joined at 1100 °C for (a) 2 μm of Mo-coating for 5 min and (b) 4 μm of Mo-coating for 2.5 min is shown in Fig. 4. It can be observed in Fig. 4a that starting Mo-coating of 2 μm
a
thick stay disperse inside the Au joining element even for a joining time of 5 min; however when the starting Mo-coating increases to 4 μm thick, it is possible to observe different Mo concentrations inside the joining element. It can be observed that, when the joining time increases from 2.5 min (Fig. 4b) to 5 min the Mo concentration is observed along region forming a line close to alumina side, as is shown in Fig. 5. On the other side, interaction of Ti and Au produced by atomic diffusion generates the formation of intermetallic phases, following the phase diagram (Okamoto and Massalski, 1990), Ti3Au and TiAu, on the interface along the bonding line with the metal size Ti. Composition profile was measured using electron probe microanalysis carried out in the cross-section of the Al2O3–Mo/Au/Ti interface which shows the main diffusion atomic elements during bonding. The analysis were performed on different points along the line illustrated at the interface of the joint made at 1100 °C for 5 min using Al2O3 with 4 mm of Mo-coating (Fig. 5) and the results are shown in the graphic in the same Fig. 5. The scan line was chosen to start on the Ti side of the sample through the bonding line interface passing through different interacting phases up to the ceramic Al2O3. The composition profile measured in the interdiffusion zone of Ti and Au signals shows that it is evident that there is a reasonable penetration of both elements, Ti and Au, showing a narrow homogeneity range corresponding to atomic compositions of these phases, Ti3Au and TiAu, respectively. This can be visualized from the composition profile shown in the zone (a) and zone (b) of Fig. 5. This kind of behavior is very common, especially, if the phase grows mainly from one interface because of relatively high diffusion rate of one species compared to the other; this is in agreement with results obtained by Kumar and Paul (2010). The Ti signal reached its maximum at the metal side; however, Au signal reached its maximum at the interface and the minimum inside the
b
Fig. 2. (a) Fracture image and (b) x-ray diffractions pattern of Al2O3 samples sintered at 1550 °C for 120 min.
4
J. Lemus-Ruiz et al. / Applied Radiation and Isotopes 98 (2015) 1–6
a
b
Fig. 3. Cross-section and surface examination of (a) 2 μm and (b) 4 μm Mo-coating of Al2O3 samples sintered at 1550 °C for 120 min.
Ti metal and in the size in the interface where the Mo signal reached its maximum. It can be observed that the Mo signals are distributed inside all the interfaces. Reactions formed inside the interface, Ti3Au and TiAu, produce a different type of joint between the metal and ceramic and the relative efficiency of these reaction products on the strength of Ti/Al2O3 interfaces is not fully understood. However, most reaction layers are brittle, and therefore potentially detrimental to the interface properties. The thickness of the reaction zone in joints is a function of the joining temperature and may dominate in the final strength. In general, when a ceramic is in contact with a metal a reaction is expected to occur. Joining temperature, time, and Mo-coating thickness were the main process parameters studied. Temperature is the most important one due to the fact that in thermally activated processes, a small change in temperature will result in a significant change in process kinetics compared with other parameters, and virtually all mechanisms including plastic deformation and diffusion are sensitive to temperature. On the other hand, the key to a successful joining technology lies in the ability to modify
a
the interface to accommodate the vastly different types of chemical bonding, from the metallic bonding of the metal to the ionic or covalent bond of the ceramic in order to reduce the electronic discontinuity at the joined surfaces. This is usually achieved by producing a graded interface. This graded interface is also essential in reducing the unfavorable effects due to two kinds of factors: the thermal mismatch and the differences between the two classes of materials, metals and ceramics (Paiva and Barbosa, 2008). In our case, the role of Mo coated film has two main proposes, first to increase the initial area of contact with the Al2O3 (TEC: 8.1 10 6 K 1) and to promote a graded interlayer with hard metals that have a relatively low thermal expansion coefficient (TEC: 5.3 10 6 K 1) that can avoid premature failures. Joints of Al2O3–Mo/Au/Ti combinations are formed through the formation of a reactive interface on the metal side of the sample as a result of diffusion and interaction of Au with Ti and Mo-coating and it can be explained since diffusion is more difficult in ceramics than metals (Lesage, 1994). Liquid formation plays an important role in the joining process, because it increases the rate of the interface
b
Fig. 4. Cross-section of Al2O3–Mo/Au/Ti interface for samples joined at 1100 °C for (a) 2 μm of Mo-coating for 5 min and (b) 4 μm of Mo-coating for 2.5 min.
J. Lemus-Ruiz et al. / Applied Radiation and Isotopes 98 (2015) 1–6
5
clear that the amount of interfacial reaction played a major role in determining the final mechanical properties of the joints. Furthermore, the nature of the reaction products may also have influenced the mechanical properties of the joints. In summary, the choice of suitable conditions to prepare ceramic/metal joints requires knowledge about the mechanism of reaction between the materials and the evolution of the interface.
4. Conclusions On the basis of the results presented in this work, we have shown that it is possible to join alumina with a Mo-coating layer to titanium using an Au foil interlayer as joining element. Successful bonding was observed at 1100 °C under vacuum and Mo-coating. Joining of Al2O3 to Ti occurred through the formation of a homogenous diffusion zone on the metal side of the joint, with no severe interfacial cracking or porosity. Diffusion of the Mo-coating inside the interface takes place forming a continuous region line close to the bonding line with the Al2O3 when 4 μm of coating and 5 min were used.
Acknowledgments The authors wish to thank CONACYT-México (Grant 167286) and Universidad Michoacana de San Nicolás de Hidalgo for the financial support and facilities of this research.
References
Fig. 5. Line analysis across the Al2O3–Mo/Au/Ti interface bonded at 1100 °C for samples with 4 μm of Mo-coating for 5 min.
formation, improving the contact area between the bonding materials, and consequently the interaction is higher, promoting rapid diffusion of the material, since liquid diffusion is much faster than the diffusion in solid-state (Lemus and Drew, 2003). The interface consisted of a homogeneous reaction layer produced by chemical interaction, as well as the high affinity of Au and Ti forming Ti3Au and TiAu phases. These results are in good agreement with the observation realized by Kumar and Paul (2010). It can be observed that the joints formed at the same temperature and different times exhibit similar microstructures with a continuum and homogeneous interface free of crack and porosity. It can be observed that increasing the holding time from 2.5 to 5 min in samples joined with alumina, coated with 4 μm of Mo, did not significantly increase the interface thickness; however, the formation of a region of Mo forming a line inside the reaction layer and close to the bonding line with the Al2O3 can be observed. The strength of diffusion-bonded ceramic/metal joints depends on the nature and microstructure of the interface between the joining materials. The effect of a reaction layer on the interface strength depends on a number of factors such as the mechanical properties of the reaction layer, its thickness and morphology. Lemus–Ruiz and Aguilar–Reyes (2004) observed that as soon as the thickness of these phases increases, the joint strength, at first, rises due to the creation of a strong, integral bond and then reaches a maximum at a certain thickness and then decreases as the interface continues to grow. Therefore, the reaction layer thickness must be controlled to ensure good joint strength. It is
Asthana, R., Singh, M., 2009. Evaluation of Pd-based brazes to join silicon nitride to copper-clad-molybdenum. Ceram. Int. 35, 3511–3515. Bahlawane, N., Watanabe, T., 2000. New sol–gel route for the preparation of pure αalumina at 950 °C. J. Am. Ceram. Soc. 83 (9), 2324–2326. Ebadzadehw, T., Sharifi, L., 2008. Synthesis of γ-Al2O3 from a mixture of γ-aluminum nitrate and carboxymethyl cellulose. J. Am. Ceram. Soc. 91 (10), 3408–3409. Ekendahl, D., Judas, L., 2015. Retrospective dosimetry with alumina substrate from electronic components. Radiat. Prot. Dosim. 150 (2), 134–141. Firmanto, H., Hussain, P., Mamat, O., 2011. Reaction layers of the diffusion-bonded sialon and high-chromium steel. J. Appl. Sci. 11, 1809–1814. He, P., Feng, J.C., Xu, W., 2006. Microstructure and kinetics of induction brazing TiAl-based intermetallics to steel 35CrMo using AgCuTi filler metal. Mater. Sci. Eng. A 418 (1–2), 53–60. He, Y.M., Zhang, J., Liu, C.F., Sun, Y., 2010. Microstructure and mechanical properties of Si3N4/Si3N4 joint brazed with Ag–Cu–Ti þ SiCp composite filler. Mater. Sci. Eng. A 527 (12), 2819–2825. Kirm, M., Feldbach, E., Kotlov, A., Liblik, P., Lushchik, A., Oja, M., Palcevskis, E., 2010. VUV spectroscopy and electronic excitations in nano-size alumina. Radiat. Meas. 45 (3–6), 618–620. Kumar, A.K., Paul, A., 2010. Interdiffusion studies in bulk Au–Ti system. J. Mater. Sci.: Mater. Electron. 21, 1202–1206. Lee, J., Ishimura, H., Tanaka, T., 2006. Novel method determining contact angle of liquid Au on solid Al2O3 single crystal (0001) surface at 1373 K. Mater. Sci. Forum 512, 306–312. Lemus, J., Drew, R.A.L., 2003. Joining of silicon nitride ceramics using a titanium foil interlayer. Mater. Sci. Eng. A 352, 169–178. Lemus-Ruiz, J., Aguilar-Reyes, E.A., 2004. Mechanical properties of silicon nitride joints using a Ti-foil interlayer. Mater. Lett. 58, 2340–2344. Lesage, B., 1994. Some aspects of diffusion in ceramics. J. Phys. III 4, 1833–1850. Liu, C.F., Zhang, J., Zhou, Y., Yi, H.L., Naka, M., 2009. Effect of holding time on the self-joining of silicon nitride. J. Alloy. Compd. 471 (1–2), 217–221. Nono, M.C.A., Barroso, J.J., Castro, P.J., 2006. Mechanical behavior and microstructural analysis of alumina–titanium brazed interfaces. Mater. Sci. Eng. A 435–436, 602–605. Paiva, O.C., Barbosa, M.A., 2008. Brazing parameters determine the degradation and mechanical behaviour of alumina/titanium brazed joints. J. Mater. Sci. 35, 1165–1175. Roy, R.K., Panda, A.K., Das, S.K., Govind, Mitra A., 2009. Development of a copper based filler alloy for brazing stainless steels. Mater. Sci. Eng. A 523 (1–2), 312–315. Saeed, A.M., Hussain, Z., Badri, A., Ariga, T., 2010. The effects of welding parameters on the weldability of different materials using brazing alloy fillers. Mater. Des. 31 (7), 3339–3345.
6
J. Lemus-Ruiz et al. / Applied Radiation and Isotopes 98 (2015) 1–6
Sugar, J.D., McKeown, J.T., Akashi, T., Hong, S.M., Nakashima, K., Glaeser, A.M., 2006. Transient-liquid-phase and liquid-film-assisted joining of ceramics. J. Eur. Ceram. Soc. 26, 363–372. Walker, C.A., Hodges, V.C., 2008. Comparing metal–ceramic brazing methods. Weld. J., 43–50. Yong, C.Z., Di, F., Yong, H.Z., Chun, C.X., 2006. Progress in joining ceramic to metals.
J. Iron Steel Res. Int. 13 (2), 1–5. Zvonarev, S.V., Kortov, V.S., Shtang, T.V., Ananchenko, D.V., Petrovykh, K.A., 2015. Effect of structural changes on luminescent and dosimetric properties of nanoscale aluminum oxide. Appl. Radiat. Isot. 95, 44–47.