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copper composites with near-net-shape by pressureless infiltration
Materials Science and Engineering B 177 (2012) 1524–1530
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Fabrication and infiltration kinetics analysis of Ti-coated diamond/copper composites with near-net-shape by pressureless infiltration YingHu Dong a,b,∗ , RuiQing Zhang d , XinBo He c , ZhiGuo Ye b , XuanHui Qu c a
Key Laboratory for Microstructural Control of Metallic Materials of Jiangxi Province, Nanchang Hangkong University, Nanchang 330063, China School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China d School of Economics and Management, Nanchang Hangkong University, Nanchang 330063, China b c
a r t i c l e
i n f o
Article history: Received 23 April 2012 Received in revised form 16 June 2012 Accepted 13 August 2012 Available online 28 August 2012 Keywords: Heat management materials Diamond Pressureless infiltration
a b s t r a c t The Ti-coated diamond/copper composites with near-net-shape are manufactured by pressurelessly infiltrating liquid copper into porous Ti-coated diamond preforms. The contact angle between diamond and copper, relative density, thermal conductivity (TC), coefficient of thermal expansion (CTE), leak rate and microstructure are evaluated and characterized. In addition, the numerical analysis of the pressureless infiltration kinetics is also discussed. The results indicate that the relative density, TC and CTE of composites are 99.3%, 385 Wm−1 k−1 and 3–8 × 10−6 K−1 , respectively. It can meet heat-sink package requirement of high-power electronic devices as LED, insulated gate bipolar transistor (IGBT), etc. The liquid copper exhibits a turbulent flow with the Reynolds number in the range of 27.83–49.7. The porosity and the pressure drop p are the main influence factors controlling the velocity of liquid copper. Moreover, under vacuum condition of 8.7 × 10−3 Pa, the maximum theoretical infiltration length Lmax of Ti-coated diamond/copper composites is found to be 552 mm. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Diamond/metal composites are widely employed as electrical contact, wear resistant and machining tools on account of their high hardness and wear resistance [1,2]. In recent years, diamond/metal composites also arouse many researchers’ enthusiasm in heat management research fields because of the excellent thermal conductivity (TC) and very low coefficient of thermal expansion (CTE) of the diamond [3–7]. In addition, the amelioration of properties and depreciation of the price of synthetic diamond render the diamond/metal composites (especially for heat management materials) a new attractive prospect on both research and application. At present, the fabricating techniques of diamond/metal composites mainly involve powder metallurgy (PM), hot pressing (HP), high-temperature high-pressure (HTHP), squeeze casting, gas pressure infiltration and chemical vapor co-deposition (CVD). As for heat management materials, the thermal properties such as TC and CTE, the related theories researches such as the heat conduction mechanism across the metal–diamond interface are also discussed [4–7]. However, systematic investigations pertaining to the novel
∗ Corresponding author at: Key Laboratory for Microstructural Control of Metallic Materials of Jiangxi Province, Nanchang Hangkong University, Nanchang 330063, China. Tel.: +86 791 83953320; fax: +86 791 83953320. E-mail address: [email protected] (Y. Dong). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.08.009
fabrication techniques and their related mechanisms of diamond/copper composites acting as heat managing materials are scarce. For example, how to enhance the thermal conductivity to meet large heat releasing requirement, how to fabricate near-netshape package parts to avoid machine cut, how to realize pressureless infiltration to diminish the cost of fabrication, and how to reveal the liquid metal flowing through the porous diamond preforms to understand the kinetics mechanism of pressureless infiltration. The liquid copper exhibits a poor wettability with carbon. According to the experimental result of Dorfman et al., the contact angle between carbon and copper is over 170◦ [8]. In order to know the actual wettability between diamond and copper, the contact angles between them are measured with temperature arranging from 1100 ◦ C to1400 ◦ C in this work. The results indicate that diamond can hardly be wetted by liquid copper. To improve the wettability between diamond and copper, some researchers attempt to add the carbide former elements such as Cr, B, Ti or Co into copper, and their research results indicate that it is an efficient way [9,10]. In our experiments, we improve the wettability through coating Ti-layer on diamond surface by salt-bath. At present, the conventional fabricating techniques of the diamond/metal composites (mentioned above) can only yield materials with simple-shapes such as cylinder, cube or slice, and it is very difficult to cut them into other shapes of actual parts by machine ways. Eventually, these simple-shaped materials can hardly meet the actual shapes with various requirements for the
Y. Dong et al. / Materials Science and Engineering B 177 (2012) 1524–1530
1525
Fig. 1. SEM and EDS results of Ti-coated diamond.
electronic packaging industry. Though the pressureless infiltration technique, as a traditional fabricating technique, is widely used to fabricate SiC/metal, Mo/metal, W/metal and AlN/metal composite parts exhibiting complicated shapes, the researches about the fabricating techique of diamond/metal composites by pressureless infiltration are very rare (It is mainly because that there are poor wettability and high interfacial thermal resistance between diamonds and copper). If introduce pressureless infiltration and near-net-shape forming techniques into the diamond/metal composites fabricating, the most difficult problem, cutting and machining process, will be solved efficiently. In addition, this technique will decrease the cost of production efficiently. In this work, the fabrication of Ti-coated diamond/copper composites exhibiting near-net-shape is accomplished successfully by pressureless infiltration technique. The properties of the composites are evaluated. Furthermore, the mechanism of pressureless infiltration is also researched and discussed. 2. Experimental The synthetic diamond with mean size of 110 m and type of MBD6 is supplied by Henan Famous Diamond Industrial Co., Ltd of
China. The high purity copper (99.999%) and Ti powder with mean size of 300 mesh are offered by GRIPM Advanced Materials Co., Ltd, Beijing, China. Ti and diamond powder are mixed thoroughly in mill machine for 6 h. Then, the mixed powders are put into a crucible and coverd with NaCl–KCl mixture (with the weight rate of 49:51). The function of the salt are mainly to pretect diamond and Ti at high temperature, and disperse the Ti powder equably around the diamond particles. The Ti plating course is carried on at 850 ◦ C for 2 h. Lastly, the composite powder are separated by precipitate dissolution method, and the Ti-coated diamond powder is selected by sample screen. The micrograph and EDS analysis results of Ti-coated diamond powder are shown in Fig. 1(a and b). The thickness of the Ti coatings is about 2.5 m (shown in Fig. 1(c)) and the composition of coatings are mainly Ti and TiC (shown in Fig. 2). The Ti-coated diamond particles are firstly mixed with moderate binders (polyvinyl alcohol solution) in mill machine for 8 h. Then, the mixed powders are packed into the mould and compressed at pressure of 140 MPa for 3 min to obtain the near-net-shape Ticoated diamond preforms. The photographs of Ti-coated diamond preforms are shown in Fig. 3(a). The porosity of the porous preforms after debonding is about 36–44% which is estimated roughly
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Y. Dong et al. / Materials Science and Engineering B 177 (2012) 1524–1530
Fig. 2. Composition of Ti-coatings on diamond surface.
by fracture micrographs of porous Ti-coated diamond preforms (shown in Fig. 4, debonding not thoroughly at 210 ◦ C for 20 min due to the requirement of strength of the preforms). Subsequently, the Ti-coated diamond preforms are put into a crucible and covered with copper blocks. The pressureless infiltration was performed at 1250–1450 ◦ C for 30–150 min at vacuum. Lastly, the specimens are grinded and polished to wipe off the redundant copper on the outside surface of composites. The photographs of Ti-coated diamond/copper composites parts are shown in Fig. 3(b). The bulk density of composites is measured according to Archimedes’ principle. Thermal conductivity and coefficient of thermal expansion are determined by LFA427/3/G type laser flash
Fig. 4. Fracture micrographs of the porous diamond preform debinded at 210 ◦ C for 20 min.
technique and DIL402PC-type thermal dilatometer. The leak rate of Ti-coated diamond/copper composites and is determined by leak detector. The contact angle is measured by Kruss DSAHT (drop shape analysis system for extremely high temperature). The microstructure is analyzed by S-6 type scanning electron microscopy (SEM). The composing of coating on diamond surface is analyzed by energy dispersive spectrometer (EDS) and XRD. 3. Results and discussion 3.1. Wettability between diamond and copper Paving status and contact angle between pure liquid copper and diamond or Ti-coated diamond substrates at various temperatures are shown in Figs. 5 and 6, respectively. In experiment, the CVD diamond substrate and pure Ti substrate is selected as a substitute for pure diamond and Ti-coated diamond substrate. Fig. 5 reveals that the contact angle between diamond substrate and liquid copper changes imperceptibly (only from 128.7◦ to 132.2◦ ) with a temperature range of 1100–1400 ◦ C. It means that diamond cannot be wetted and pressurelessly infiltrated by pure copper without any treatment. Therefore, to obtain diamond/copper composites by infiltration, extra pressure (including mechanical or gas pressure) must be put on liquid metals. However, the contact angle between Ti-coated diamonds and copper decreases dramatically after 1100 ◦ C (near the melting point of copper, show in Fig. 6). At temperature of 1300 ◦ C, the contact angle of them becomes 25◦ (also show in Fig. 6(e)), and this means that Ti-coated diamonds can be wetted well by liquid copper. In addition, above the melting point of copper, Ti can dissolve easily in copper and form mutually soluble liquids. Therefore, the soluble edge of Ti-coated diamond substrate and copper can move ahead spontaneously, fleetly and becomes unevenly (show in Fig. 6(e and f)). It makes liquid copper can infiltrate Ti-coated diamond preforms easily and form diamond/copper composite with high relative density and well thermal properties due to the well wettability. 3.2. Microstructure and properties of Ti-coated diamond/copper composites
Fig. 3. Near-net-shape Ti-caoted diamond/copper composites by pressureless infiltration: (a) porous Ti-coated diamond preform; (b) Ti-coated diamond/copper composite parts.
The SEM of Ti-coated diamond/copper composites is shown in Fig. 7. It reveals that the Ti-coated diamond particles distribute well in the copper matrix and form compact Ti-coated diamond/copper composites. The two phases of Ti-coated diamond and copper construct the continuous and well-proportioned nets in composites.
Y. Dong et al. / Materials Science and Engineering B 177 (2012) 1524–1530
Fig. 5. Paving status of liquid copper on pure diamond substrates at various temperatures.
Fig. 6. Paving status of liquid copper on Ti substrate (substitute for Ti-coated diamond) at various temperatures.
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where is the density of liquid, is the velocity vector of the flow, d is the equivalent channel diameter, and is the viscosity of molten copper. Bear [17] has derived a relation between the thermodynamic radius R and a shape coefficient f related to the pores and the particle shape. d=f ·R
(2)
and R=
Vp 1 = Sp Sv
(3)
where Vp is the volume of pores, Sp is the surface area of pores, and Sv is the specific surface area. For the irregular particles, the value of f is about 20. With the introduction of a factor relating to pore f(), the permeability of porous medium k, the particle diameter Dd , and the porosity , Sv can be written as
Many researches [11–15] have proved that this morphology and microstructure presents better performances both in the mechanical and thermal properties than the homogeneous microstructure. In addition, by our experimental verification, the untreated diamond cannot be infiltrated pressurelessly by liquid copper due to the poor wettabilities between the diamond and copper. On the contrary, not only the Ti-coated diamond can be infiltrated well by copper, but also the superabundant copper is dispersed smoothly and uniformly on the whole surfaces of Ti-coated diamond/copper composites. This also shows that the Ti-coated diamond and copper reveals excellent wettabilities. The relative density of the Ti-coated diamond/copper is about 99.3%. The thermal conductivity of Ti-coated diamond/copper composites is about 385 Wm−1 k−1 , which is much higher than that of AlN, W/Cu, and SiC/Al composites. The coefficient of thermal expansion and leak rate of Ti-coated diamond/copper are 3–8 × 10−6 K−1 and lower than 5 × 10−10 Pa m3 s−1 , respectively. Such high value of TC, low CTE and leak rate suggest that these materials can act as electronic packaging parts efficiently. Comparing with monolithic Cu or diamond, though Ti-coated diamond/copper composites exhibit TC between that of Cu and diamond (In our experiments, the TC of composites is near monolithic Cu), it possesses much more compatible CTE for packaging ICs or other electronical apparatus. Ti-coated diamond/copper composites have outstanding performance mainly because that they combine the well plasticity of Cu with excellent thermal conductivity performance of diamond. In addition, the near-net-shape composites by pressureless infiltration technique also present promising outlook for application in many fields. 3.3. Infiltration kinetics The infiltration of diamond preforms by molten metals can be described in terms of the filtration theory dealing with the flow through a porous medium. Darcy’s law is the widely used theory to describe the infiltration [16]. However, Darcy’s law can only be applied for the laminar flow rather than the turbulent flow. Critical Reynolds number is generally used to determine which type of the flow belongs. For example, the laminar flow transfers into turbulent flow when the value of practical Reynolds number (Re) surpasses the critical Reynolds. As for metal matrix composites, the critical Reynolds number lies in the range of 1–10, and the Reynolds number can be calculated from Eq. (1) as follows: Re =
d
C0 f () , k
Sv =
Fig. 7. SEM of Ti-coated diamond/copper composites.
(1)
3
f () =
(1 − )
2
(4)
.
(5)
where k = f Dd 2 .
(6)
Substituting the above equations into Eq. (2), the equivalent channel diameter d transfers into fD (1 − ) d= d
f . C0
(7)
where f’ is the shape factor and C0 is the constant. For the diamond preforms, f’ and C0 are 6.54 × 10−4 and 0.2, respectively. Therefore, Eq. (7) can be written as d = 1.144Dd (1 − )−3/2 .
(8)
In this work, the diamond particle diameter Cd is 105 m and the porosity of diamond preforms is 0.36–0.44. The equivalent channel diameter d calculated from Eq. (8) is 230.48–411.52 m. Robert et al. [18] has found that the density of copper is 8.458, 7.92, 7.75 and 7.6 g cm−3 at temperatures of 1200, 1400, 1600 and 1800 K, respectively. The viscosity of molten copper is 3.41 × 10−3 N s m−2 and practical velocity vector of the flow v is about 5.2 × 10−2 m s−1 . At the temperatures of 1400 K and 1600 K, the practical Reynolds number (Re), calculated from Eq. (8), is Re1 = 27.83–49.7 and Re2 = 27.24–48.63, respectively. The minimum value of practical Reynolds number is Re(min) = 27.24 > 10, therefore, the flow of molten copper in porous diamond preforms obeys turbulent nature and the Darcy’s law cannot applied here any more. Hence, another model must be adopted to investigate the infiltration course of molten copper flowing in porous diamond preforms. In order to research the mechanism of molten copper flowing through porous diamond preforms, the schematic illustration of uni-dimensional infiltration process is depicted in Fig. 8. Assuming that the molten copper flows in unilateral movement, e.g. axial direction x, the porosity of diamond preforms can be written as =
Vp . Vp + Vs
(9)
where Vs and Vp are the volume of solid particles and pores respectively. Introducing the contact area As between the flow and solid, while assuming all the pores are open, it can be calculated from following equation.
V p
As
p
=
Vs . As (1 − )
(10)
Y. Dong et al. / Materials Science and Engineering B 177 (2012) 1524–1530
Fig. 8. The schematic illustration of uni-dimensional infiltration process.
In addition, if the length of a capillary is L, capillary diameter is dc , then (Vp /Vs )c of n capillaries can be written as
V p
As
c
n(/4)dc = nLdc
2
d = . 4
4Vs dc = As (1 − )
(12)
8LQ 128LQ = R4 d4
(13)
where p is the pressure gradient, and Q = A = R2 = d2 /4. Substituting the practical velocity of flow (/)(Le /L), Eqs. (12) and (13) can be written as p =
128Le d2 32Le v Le v= 4 d4 d2 L
or
v=
pd2 L 32Le
2
=
3 1 p L (1 − )2 ˇ(Vs /As )2
(14)
Here  = 2(Le /L)2 is Kozeny’s constant with a value in the range of about 3.5–6.0 at the middling porosity [19]. Eq. (14) indicates that the effect of porosity on the velocity v is significant. 3 /(1 − )2 in Eq. (14) is influential efficiency. Therefore, a small variation in the porosity will render a remarkable difference in the velocity of the flow. In addition, Eq. (14) clearly depicts that an enhancement in p is another way to ameliorate the velocity of molten copper. Considering the diamond particles as spherical with radius R = 52.5 m, Kozeny’s constant  = 4.75, and = 3.41 × 10−3 N s m−2 , results in (Vs /As ) = (4R3 /3)/(4r2 ) = R/3 = 1.75 × 10–5 m−1 . Therefore, Eq. (14) can be simplified as
v=
3 2 × 1011 p . 2 L (1 − )
derived from Laplace equation and Posieuille’s Law, respectively [20]. pc =
As the molten metal flows in porous media, only a small part of porosity at cross sectional area may let the liquid flow through. Therefore, the mean velocity of the flow is /. Additionally, the practical route Le of the flow becomes flexuous and longer than axial length of capillary L. The practical velocity of flow can be modified as (/)(Le /L). Then the Posieuille’s Law can be written in terms of the volumetric flow rate Q p =
Fig. 9. Relative densities of diamond/copper composites at different length and infiltration time.
(11)
Taking (Vp /Vs )p and (Vp /Vs )c as equal, dc can be derived as:
(15)
The various types of pressures exerted on the flow include atmosphere pressure po , capillary pressure pc , viscous resistance pressure p and the gravity pressure pm . The pc and p can be
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2 lg cos R
(16)
8 p = − 2 vx R
where ␥lg is surface tension and the contact angle. Taking H + x as H0 , the total pressure on the liquid flow with length of x is p = po + pc + pm + p = po + = po +
2 lg cos r
2 lg cos r
+ g(H + x) −
8 vx r2
(17)
8 + gH0 − 2 vx r
Here R is the radius of capillary, H or H0 is the length (e.g., the height) of flow, and is the density of flow. It can be deduced from the Eq. (17) that if the adscititious pressure po and the meaning velocity v are constant, the contact angle and the infiltration length x become the most influential factors on the total pressure exerted on the flow. As the contact angle becomes an obtuse angle i.e. beyond 90◦ , the capillary pressure will transfer from propulsion to resistance. Moreover, Eq. (17) also indicates that the length of the molten metal flows through the porous media is limited. Taking diamond/copper composites for example, at the condition of T: 1200 ◦ C, po : 8.7 × 10−3 Pa, v: 1.81.8 × 10−2 ms−1 , : 7.92 × 103 kg m−3 , : 18◦ , H0 : 5 mm, R: 115.24 m and lg :1.25 N m−2 which is obtained from an empirical formula (␥lg = 1275–0.20(T-1356) for temperature range of T = 1287–1998 K) [21], and assuming that the infiltration has terminated with p = 0, the maximal theoretical infiltration length L calculated from Eq. (17) is Lmax = 552 mm. Therefore, it can be summarize that in order to fabricate the high density diamond/copper composites by pressureless infiltration technique, the length of the porous preform should be kept below 552 mm. In this work, in order to further make clear the status of liquid copper flowing through the diamond preforms, the practical verification experiments are carried on. The diamond powder is firstly filled into the mold with the interior diameter of 10 mm and the length of 120 mm. Then, the pressureless infiltration is finished at various times ranges from 30 min to 150 min. Following pressureless infiltration, the diamond/copper composites are all cut into six sections and the relative densities of all specimens are measured. Fig. 9 indicates the relative densities of diamond/copper composites at different length and infiltration
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Y. Dong et al. / Materials Science and Engineering B 177 (2012) 1524–1530
time. It is found that at the same infiltration time the relative density decreases with an increase of infiltration length. This implies that the pressure acting on the liquid copper at different depth in diamond preforms is different, i.e., the pressure declines with the infiltration depth increasing. Accordingly, the velocity of the liquid copper flowing through diamond preforms also slows down. How ever, as for diamond preforms with length of 120 mm, if prolonging the infiltration time from 30 min to 150 min, the relative densities of composites at end section can reach 98.3%. Therefore, it can be concluded that the diamond/copper composites with high performance can be obtained as long as the length of diamond preforms is below the theoretical maximal infiltration length of 522 mm and the infiltration time is sufficient. 4. Conclusions (1) The near-net-shape diamond/copper composites are manufactured by pressureless infiltration successfully, the composites exhibit the relative density of 99.3%, the TC of 385 Wm−1 k−1 , the CTEs of 3–8 × 10−6 k−1 and the leak rates lower than 5 × 10−6 Pam3 s−1 , respectively. (2) The Ti coatings on diamond surface can improve the wettabilities between diamond and liquid copper efficiently. (3) The molten copper flows through porous diamond preform as turbulent flow with the Reynolds number (Re) of 27.83–49.7. The porosity and the pressure drop p exerted on the flow are the main factors which determine the flow of molten copper through porous preforms. As for the pressureless infiltration of diamond/metal composites, it can be ameliorated efficiently by increasing the porosity and decreasing the contact angle . (4) The results of theoretical calculation and some experimental verified results demonstrate that in order to yield diamond/copper composites with high relative density and properties, the maximal theoretical length of the composites should be less than 552 mm and the infiltrating time should be prolonged moderately.
Acknowledgements This work is financially supported by the Open Foundation of Key Laboratory for Microstructural Control of Metallic Materials of Jiangxi Province, China (JW201123003), the Start-up Funds from University of Nanchang Hangkong University of China (EA201101190), the National Nature Science Foundation of China (50774005) and the National Natural Science Foundation of China (21103085). Grateful acknowledgement goes to them! References [1] X.P. Xu, X.R. Tie, Y.Q. Yu, Journal of Materials Processing Technology 187–188 (2007) 421–424. [2] Y.F. Ge, J.H. Xu, H. Yang, Wear 269 (2010) 699–708. [3] K. Yoshida, H. Morigami, Microelectronics Reliability 44 (2004) 303–308. [4] O. Beffort, S. Vaucher, F.A. Khalid, Diamond and Related Materials 13 (2004) 1834–1843. [5] P.W. Ruch., O. Beffort, S. Kleiner, L. Weber, P.J. Uggowitzer, Composites Science and Technology 66 (2006) 2677–2685. [6] E.A. Ekimov, N.V. Suetin, A.F. Popovich, V.G. Ralchenko, Diamond and Related Materials 17 (2008) 838–843. [7] M. Battabyal, O. Beffort, S. Kleiner, S. Vaucher, L. Rohr, Diamond and Related Materials 17 (2008) 1438–1442. [8] S. Dorfman, D. Fuks, M. Suery, Journal of Materials Science 34 (1999) 77–81. [9] L. Weber, R. Tavangar, Scripta Materialia 57 (2007) 988–991. [10] M.G. de Azevedo, A. Potemkin, A.L.D. Skury, R.N. de Azevedo Faria Jr., Diamond and Related Materials 10 (2001) 1607–1611. [11] H.X. Peng, Z. Fan, J.R.G. Evans, Journal of Microscopy 201 (2001) 333–338. [12] H.X. Peng, Materials Science and Engineering A 396 (2005) 1–2. [13] L.J. Huang, L. Geng, A.B. Li, F.Y. Yang, H.X. Peng, Scripta Materialia 60 (2009) 996–999. [14] L.J. Huang, L. Geng, H.X. Peng, J. Zhang, Scripta Materialia 64 (2011) 844–847. [15] L.J. Huang, L. Geng, H.Y. Xu, H.X. Peng, Materials Science and Engineering A 528 (2011) 2859–2862. [16] E. Candan, H.V. Atkinson, H. Jones, Journal of Materials Science Letters 32 (1997) 289–294. [17] J. Bear, Dynamics of Fluids in Porous Media, Dover publication Inc, New York, 1972, pp.91–128. [18] H.P. Robert, W.G. Don, O.M. James, Chemical Engineers’ Handbook, 7th ed., McGraw-Hill Professional, New York, 1999. [19] C. Orr, Filtration Principles and Practices Part 1, Marcel Dekker, New York, 1977, p.176. [20] R. Mises, K.O. Fredricks, Fluid Dynamics, Rhode Island: Brown University, Providence, 1941, p.140. [21] T. Matsumoto, H. Fujii, T. Ueda, M. Janau, K. Nogi, Measurement Science and Technology 16 (2005) 432–437.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Fabrication and infiltration kinetics analysis of Ti-coated diamond/copper composites with near-net-shape by pressureless infiltration YingHu Dong a,b,∗ , RuiQing Zhang d , XinBo He c , ZhiGuo Ye b , XuanHui Qu c a
Key Laboratory for Microstructural Control of Metallic Materials of Jiangxi Province, Nanchang Hangkong University, Nanchang 330063, China School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China d School of Economics and Management, Nanchang Hangkong University, Nanchang 330063, China b c
a r t i c l e
i n f o
Article history: Received 23 April 2012 Received in revised form 16 June 2012 Accepted 13 August 2012 Available online 28 August 2012 Keywords: Heat management materials Diamond Pressureless infiltration
a b s t r a c t The Ti-coated diamond/copper composites with near-net-shape are manufactured by pressurelessly infiltrating liquid copper into porous Ti-coated diamond preforms. The contact angle between diamond and copper, relative density, thermal conductivity (TC), coefficient of thermal expansion (CTE), leak rate and microstructure are evaluated and characterized. In addition, the numerical analysis of the pressureless infiltration kinetics is also discussed. The results indicate that the relative density, TC and CTE of composites are 99.3%, 385 Wm−1 k−1 and 3–8 × 10−6 K−1 , respectively. It can meet heat-sink package requirement of high-power electronic devices as LED, insulated gate bipolar transistor (IGBT), etc. The liquid copper exhibits a turbulent flow with the Reynolds number in the range of 27.83–49.7. The porosity and the pressure drop p are the main influence factors controlling the velocity of liquid copper. Moreover, under vacuum condition of 8.7 × 10−3 Pa, the maximum theoretical infiltration length Lmax of Ti-coated diamond/copper composites is found to be 552 mm. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Diamond/metal composites are widely employed as electrical contact, wear resistant and machining tools on account of their high hardness and wear resistance [1,2]. In recent years, diamond/metal composites also arouse many researchers’ enthusiasm in heat management research fields because of the excellent thermal conductivity (TC) and very low coefficient of thermal expansion (CTE) of the diamond [3–7]. In addition, the amelioration of properties and depreciation of the price of synthetic diamond render the diamond/metal composites (especially for heat management materials) a new attractive prospect on both research and application. At present, the fabricating techniques of diamond/metal composites mainly involve powder metallurgy (PM), hot pressing (HP), high-temperature high-pressure (HTHP), squeeze casting, gas pressure infiltration and chemical vapor co-deposition (CVD). As for heat management materials, the thermal properties such as TC and CTE, the related theories researches such as the heat conduction mechanism across the metal–diamond interface are also discussed [4–7]. However, systematic investigations pertaining to the novel
∗ Corresponding author at: Key Laboratory for Microstructural Control of Metallic Materials of Jiangxi Province, Nanchang Hangkong University, Nanchang 330063, China. Tel.: +86 791 83953320; fax: +86 791 83953320. E-mail address: [email protected] (Y. Dong). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.08.009
fabrication techniques and their related mechanisms of diamond/copper composites acting as heat managing materials are scarce. For example, how to enhance the thermal conductivity to meet large heat releasing requirement, how to fabricate near-netshape package parts to avoid machine cut, how to realize pressureless infiltration to diminish the cost of fabrication, and how to reveal the liquid metal flowing through the porous diamond preforms to understand the kinetics mechanism of pressureless infiltration. The liquid copper exhibits a poor wettability with carbon. According to the experimental result of Dorfman et al., the contact angle between carbon and copper is over 170◦ [8]. In order to know the actual wettability between diamond and copper, the contact angles between them are measured with temperature arranging from 1100 ◦ C to1400 ◦ C in this work. The results indicate that diamond can hardly be wetted by liquid copper. To improve the wettability between diamond and copper, some researchers attempt to add the carbide former elements such as Cr, B, Ti or Co into copper, and their research results indicate that it is an efficient way [9,10]. In our experiments, we improve the wettability through coating Ti-layer on diamond surface by salt-bath. At present, the conventional fabricating techniques of the diamond/metal composites (mentioned above) can only yield materials with simple-shapes such as cylinder, cube or slice, and it is very difficult to cut them into other shapes of actual parts by machine ways. Eventually, these simple-shaped materials can hardly meet the actual shapes with various requirements for the
Y. Dong et al. / Materials Science and Engineering B 177 (2012) 1524–1530
1525
Fig. 1. SEM and EDS results of Ti-coated diamond.
electronic packaging industry. Though the pressureless infiltration technique, as a traditional fabricating technique, is widely used to fabricate SiC/metal, Mo/metal, W/metal and AlN/metal composite parts exhibiting complicated shapes, the researches about the fabricating techique of diamond/metal composites by pressureless infiltration are very rare (It is mainly because that there are poor wettability and high interfacial thermal resistance between diamonds and copper). If introduce pressureless infiltration and near-net-shape forming techniques into the diamond/metal composites fabricating, the most difficult problem, cutting and machining process, will be solved efficiently. In addition, this technique will decrease the cost of production efficiently. In this work, the fabrication of Ti-coated diamond/copper composites exhibiting near-net-shape is accomplished successfully by pressureless infiltration technique. The properties of the composites are evaluated. Furthermore, the mechanism of pressureless infiltration is also researched and discussed. 2. Experimental The synthetic diamond with mean size of 110 m and type of MBD6 is supplied by Henan Famous Diamond Industrial Co., Ltd of
China. The high purity copper (99.999%) and Ti powder with mean size of 300 mesh are offered by GRIPM Advanced Materials Co., Ltd, Beijing, China. Ti and diamond powder are mixed thoroughly in mill machine for 6 h. Then, the mixed powders are put into a crucible and coverd with NaCl–KCl mixture (with the weight rate of 49:51). The function of the salt are mainly to pretect diamond and Ti at high temperature, and disperse the Ti powder equably around the diamond particles. The Ti plating course is carried on at 850 ◦ C for 2 h. Lastly, the composite powder are separated by precipitate dissolution method, and the Ti-coated diamond powder is selected by sample screen. The micrograph and EDS analysis results of Ti-coated diamond powder are shown in Fig. 1(a and b). The thickness of the Ti coatings is about 2.5 m (shown in Fig. 1(c)) and the composition of coatings are mainly Ti and TiC (shown in Fig. 2). The Ti-coated diamond particles are firstly mixed with moderate binders (polyvinyl alcohol solution) in mill machine for 8 h. Then, the mixed powders are packed into the mould and compressed at pressure of 140 MPa for 3 min to obtain the near-net-shape Ticoated diamond preforms. The photographs of Ti-coated diamond preforms are shown in Fig. 3(a). The porosity of the porous preforms after debonding is about 36–44% which is estimated roughly
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Fig. 2. Composition of Ti-coatings on diamond surface.
by fracture micrographs of porous Ti-coated diamond preforms (shown in Fig. 4, debonding not thoroughly at 210 ◦ C for 20 min due to the requirement of strength of the preforms). Subsequently, the Ti-coated diamond preforms are put into a crucible and covered with copper blocks. The pressureless infiltration was performed at 1250–1450 ◦ C for 30–150 min at vacuum. Lastly, the specimens are grinded and polished to wipe off the redundant copper on the outside surface of composites. The photographs of Ti-coated diamond/copper composites parts are shown in Fig. 3(b). The bulk density of composites is measured according to Archimedes’ principle. Thermal conductivity and coefficient of thermal expansion are determined by LFA427/3/G type laser flash
Fig. 4. Fracture micrographs of the porous diamond preform debinded at 210 ◦ C for 20 min.
technique and DIL402PC-type thermal dilatometer. The leak rate of Ti-coated diamond/copper composites and is determined by leak detector. The contact angle is measured by Kruss DSAHT (drop shape analysis system for extremely high temperature). The microstructure is analyzed by S-6 type scanning electron microscopy (SEM). The composing of coating on diamond surface is analyzed by energy dispersive spectrometer (EDS) and XRD. 3. Results and discussion 3.1. Wettability between diamond and copper Paving status and contact angle between pure liquid copper and diamond or Ti-coated diamond substrates at various temperatures are shown in Figs. 5 and 6, respectively. In experiment, the CVD diamond substrate and pure Ti substrate is selected as a substitute for pure diamond and Ti-coated diamond substrate. Fig. 5 reveals that the contact angle between diamond substrate and liquid copper changes imperceptibly (only from 128.7◦ to 132.2◦ ) with a temperature range of 1100–1400 ◦ C. It means that diamond cannot be wetted and pressurelessly infiltrated by pure copper without any treatment. Therefore, to obtain diamond/copper composites by infiltration, extra pressure (including mechanical or gas pressure) must be put on liquid metals. However, the contact angle between Ti-coated diamonds and copper decreases dramatically after 1100 ◦ C (near the melting point of copper, show in Fig. 6). At temperature of 1300 ◦ C, the contact angle of them becomes 25◦ (also show in Fig. 6(e)), and this means that Ti-coated diamonds can be wetted well by liquid copper. In addition, above the melting point of copper, Ti can dissolve easily in copper and form mutually soluble liquids. Therefore, the soluble edge of Ti-coated diamond substrate and copper can move ahead spontaneously, fleetly and becomes unevenly (show in Fig. 6(e and f)). It makes liquid copper can infiltrate Ti-coated diamond preforms easily and form diamond/copper composite with high relative density and well thermal properties due to the well wettability. 3.2. Microstructure and properties of Ti-coated diamond/copper composites
Fig. 3. Near-net-shape Ti-caoted diamond/copper composites by pressureless infiltration: (a) porous Ti-coated diamond preform; (b) Ti-coated diamond/copper composite parts.
The SEM of Ti-coated diamond/copper composites is shown in Fig. 7. It reveals that the Ti-coated diamond particles distribute well in the copper matrix and form compact Ti-coated diamond/copper composites. The two phases of Ti-coated diamond and copper construct the continuous and well-proportioned nets in composites.
Y. Dong et al. / Materials Science and Engineering B 177 (2012) 1524–1530
Fig. 5. Paving status of liquid copper on pure diamond substrates at various temperatures.
Fig. 6. Paving status of liquid copper on Ti substrate (substitute for Ti-coated diamond) at various temperatures.
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where is the density of liquid, is the velocity vector of the flow, d is the equivalent channel diameter, and is the viscosity of molten copper. Bear [17] has derived a relation between the thermodynamic radius R and a shape coefficient f related to the pores and the particle shape. d=f ·R
(2)
and R=
Vp 1 = Sp Sv
(3)
where Vp is the volume of pores, Sp is the surface area of pores, and Sv is the specific surface area. For the irregular particles, the value of f is about 20. With the introduction of a factor relating to pore f(), the permeability of porous medium k, the particle diameter Dd , and the porosity , Sv can be written as
Many researches [11–15] have proved that this morphology and microstructure presents better performances both in the mechanical and thermal properties than the homogeneous microstructure. In addition, by our experimental verification, the untreated diamond cannot be infiltrated pressurelessly by liquid copper due to the poor wettabilities between the diamond and copper. On the contrary, not only the Ti-coated diamond can be infiltrated well by copper, but also the superabundant copper is dispersed smoothly and uniformly on the whole surfaces of Ti-coated diamond/copper composites. This also shows that the Ti-coated diamond and copper reveals excellent wettabilities. The relative density of the Ti-coated diamond/copper is about 99.3%. The thermal conductivity of Ti-coated diamond/copper composites is about 385 Wm−1 k−1 , which is much higher than that of AlN, W/Cu, and SiC/Al composites. The coefficient of thermal expansion and leak rate of Ti-coated diamond/copper are 3–8 × 10−6 K−1 and lower than 5 × 10−10 Pa m3 s−1 , respectively. Such high value of TC, low CTE and leak rate suggest that these materials can act as electronic packaging parts efficiently. Comparing with monolithic Cu or diamond, though Ti-coated diamond/copper composites exhibit TC between that of Cu and diamond (In our experiments, the TC of composites is near monolithic Cu), it possesses much more compatible CTE for packaging ICs or other electronical apparatus. Ti-coated diamond/copper composites have outstanding performance mainly because that they combine the well plasticity of Cu with excellent thermal conductivity performance of diamond. In addition, the near-net-shape composites by pressureless infiltration technique also present promising outlook for application in many fields. 3.3. Infiltration kinetics The infiltration of diamond preforms by molten metals can be described in terms of the filtration theory dealing with the flow through a porous medium. Darcy’s law is the widely used theory to describe the infiltration [16]. However, Darcy’s law can only be applied for the laminar flow rather than the turbulent flow. Critical Reynolds number is generally used to determine which type of the flow belongs. For example, the laminar flow transfers into turbulent flow when the value of practical Reynolds number (Re) surpasses the critical Reynolds. As for metal matrix composites, the critical Reynolds number lies in the range of 1–10, and the Reynolds number can be calculated from Eq. (1) as follows: Re =
d
C0 f () , k
Sv =
Fig. 7. SEM of Ti-coated diamond/copper composites.
(1)
3
f () =
(1 − )
2
(4)
.
(5)
where k = f Dd 2 .
(6)
Substituting the above equations into Eq. (2), the equivalent channel diameter d transfers into fD (1 − ) d= d
f . C0
(7)
where f’ is the shape factor and C0 is the constant. For the diamond preforms, f’ and C0 are 6.54 × 10−4 and 0.2, respectively. Therefore, Eq. (7) can be written as d = 1.144Dd (1 − )−3/2 .
(8)
In this work, the diamond particle diameter Cd is 105 m and the porosity of diamond preforms is 0.36–0.44. The equivalent channel diameter d calculated from Eq. (8) is 230.48–411.52 m. Robert et al. [18] has found that the density of copper is 8.458, 7.92, 7.75 and 7.6 g cm−3 at temperatures of 1200, 1400, 1600 and 1800 K, respectively. The viscosity of molten copper is 3.41 × 10−3 N s m−2 and practical velocity vector of the flow v is about 5.2 × 10−2 m s−1 . At the temperatures of 1400 K and 1600 K, the practical Reynolds number (Re), calculated from Eq. (8), is Re1 = 27.83–49.7 and Re2 = 27.24–48.63, respectively. The minimum value of practical Reynolds number is Re(min) = 27.24 > 10, therefore, the flow of molten copper in porous diamond preforms obeys turbulent nature and the Darcy’s law cannot applied here any more. Hence, another model must be adopted to investigate the infiltration course of molten copper flowing in porous diamond preforms. In order to research the mechanism of molten copper flowing through porous diamond preforms, the schematic illustration of uni-dimensional infiltration process is depicted in Fig. 8. Assuming that the molten copper flows in unilateral movement, e.g. axial direction x, the porosity of diamond preforms can be written as =
Vp . Vp + Vs
(9)
where Vs and Vp are the volume of solid particles and pores respectively. Introducing the contact area As between the flow and solid, while assuming all the pores are open, it can be calculated from following equation.
V p
As
p
=
Vs . As (1 − )
(10)
Y. Dong et al. / Materials Science and Engineering B 177 (2012) 1524–1530
Fig. 8. The schematic illustration of uni-dimensional infiltration process.
In addition, if the length of a capillary is L, capillary diameter is dc , then (Vp /Vs )c of n capillaries can be written as
V p
As
c
n(/4)dc = nLdc
2
d = . 4
4Vs dc = As (1 − )
(12)
8LQ 128LQ = R4 d4
(13)
where p is the pressure gradient, and Q = A = R2 = d2 /4. Substituting the practical velocity of flow (/)(Le /L), Eqs. (12) and (13) can be written as p =
128Le d2 32Le v Le v= 4 d4 d2 L
or
v=
pd2 L 32Le
2
=
3 1 p L (1 − )2 ˇ(Vs /As )2
(14)
Here  = 2(Le /L)2 is Kozeny’s constant with a value in the range of about 3.5–6.0 at the middling porosity [19]. Eq. (14) indicates that the effect of porosity on the velocity v is significant. 3 /(1 − )2 in Eq. (14) is influential efficiency. Therefore, a small variation in the porosity will render a remarkable difference in the velocity of the flow. In addition, Eq. (14) clearly depicts that an enhancement in p is another way to ameliorate the velocity of molten copper. Considering the diamond particles as spherical with radius R = 52.5 m, Kozeny’s constant  = 4.75, and = 3.41 × 10−3 N s m−2 , results in (Vs /As ) = (4R3 /3)/(4r2 ) = R/3 = 1.75 × 10–5 m−1 . Therefore, Eq. (14) can be simplified as
v=
3 2 × 1011 p . 2 L (1 − )
derived from Laplace equation and Posieuille’s Law, respectively [20]. pc =
As the molten metal flows in porous media, only a small part of porosity at cross sectional area may let the liquid flow through. Therefore, the mean velocity of the flow is /. Additionally, the practical route Le of the flow becomes flexuous and longer than axial length of capillary L. The practical velocity of flow can be modified as (/)(Le /L). Then the Posieuille’s Law can be written in terms of the volumetric flow rate Q p =
Fig. 9. Relative densities of diamond/copper composites at different length and infiltration time.
(11)
Taking (Vp /Vs )p and (Vp /Vs )c as equal, dc can be derived as:
(15)
The various types of pressures exerted on the flow include atmosphere pressure po , capillary pressure pc , viscous resistance pressure p and the gravity pressure pm . The pc and p can be
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2 lg cos R
(16)
8 p = − 2 vx R
where ␥lg is surface tension and the contact angle. Taking H + x as H0 , the total pressure on the liquid flow with length of x is p = po + pc + pm + p = po + = po +
2 lg cos r
2 lg cos r
+ g(H + x) −
8 vx r2
(17)
8 + gH0 − 2 vx r
Here R is the radius of capillary, H or H0 is the length (e.g., the height) of flow, and is the density of flow. It can be deduced from the Eq. (17) that if the adscititious pressure po and the meaning velocity v are constant, the contact angle and the infiltration length x become the most influential factors on the total pressure exerted on the flow. As the contact angle becomes an obtuse angle i.e. beyond 90◦ , the capillary pressure will transfer from propulsion to resistance. Moreover, Eq. (17) also indicates that the length of the molten metal flows through the porous media is limited. Taking diamond/copper composites for example, at the condition of T: 1200 ◦ C, po : 8.7 × 10−3 Pa, v: 1.81.8 × 10−2 ms−1 , : 7.92 × 103 kg m−3 , : 18◦ , H0 : 5 mm, R: 115.24 m and lg :1.25 N m−2 which is obtained from an empirical formula (␥lg = 1275–0.20(T-1356) for temperature range of T = 1287–1998 K) [21], and assuming that the infiltration has terminated with p = 0, the maximal theoretical infiltration length L calculated from Eq. (17) is Lmax = 552 mm. Therefore, it can be summarize that in order to fabricate the high density diamond/copper composites by pressureless infiltration technique, the length of the porous preform should be kept below 552 mm. In this work, in order to further make clear the status of liquid copper flowing through the diamond preforms, the practical verification experiments are carried on. The diamond powder is firstly filled into the mold with the interior diameter of 10 mm and the length of 120 mm. Then, the pressureless infiltration is finished at various times ranges from 30 min to 150 min. Following pressureless infiltration, the diamond/copper composites are all cut into six sections and the relative densities of all specimens are measured. Fig. 9 indicates the relative densities of diamond/copper composites at different length and infiltration
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time. It is found that at the same infiltration time the relative density decreases with an increase of infiltration length. This implies that the pressure acting on the liquid copper at different depth in diamond preforms is different, i.e., the pressure declines with the infiltration depth increasing. Accordingly, the velocity of the liquid copper flowing through diamond preforms also slows down. How ever, as for diamond preforms with length of 120 mm, if prolonging the infiltration time from 30 min to 150 min, the relative densities of composites at end section can reach 98.3%. Therefore, it can be concluded that the diamond/copper composites with high performance can be obtained as long as the length of diamond preforms is below the theoretical maximal infiltration length of 522 mm and the infiltration time is sufficient. 4. Conclusions (1) The near-net-shape diamond/copper composites are manufactured by pressureless infiltration successfully, the composites exhibit the relative density of 99.3%, the TC of 385 Wm−1 k−1 , the CTEs of 3–8 × 10−6 k−1 and the leak rates lower than 5 × 10−6 Pam3 s−1 , respectively. (2) The Ti coatings on diamond surface can improve the wettabilities between diamond and liquid copper efficiently. (3) The molten copper flows through porous diamond preform as turbulent flow with the Reynolds number (Re) of 27.83–49.7. The porosity and the pressure drop p exerted on the flow are the main factors which determine the flow of molten copper through porous preforms. As for the pressureless infiltration of diamond/metal composites, it can be ameliorated efficiently by increasing the porosity and decreasing the contact angle . (4) The results of theoretical calculation and some experimental verified results demonstrate that in order to yield diamond/copper composites with high relative density and properties, the maximal theoretical length of the composites should be less than 552 mm and the infiltrating time should be prolonged moderately.
Acknowledgements This work is financially supported by the Open Foundation of Key Laboratory for Microstructural Control of Metallic Materials of Jiangxi Province, China (JW201123003), the Start-up Funds from University of Nanchang Hangkong University of China (EA201101190), the National Nature Science Foundation of China (50774005) and the National Natural Science Foundation of China (21103085). Grateful acknowledgement goes to them! References [1] X.P. Xu, X.R. Tie, Y.Q. Yu, Journal of Materials Processing Technology 187–188 (2007) 421–424. [2] Y.F. Ge, J.H. Xu, H. Yang, Wear 269 (2010) 699–708. [3] K. Yoshida, H. Morigami, Microelectronics Reliability 44 (2004) 303–308. [4] O. Beffort, S. Vaucher, F.A. Khalid, Diamond and Related Materials 13 (2004) 1834–1843. [5] P.W. Ruch., O. Beffort, S. Kleiner, L. Weber, P.J. Uggowitzer, Composites Science and Technology 66 (2006) 2677–2685. [6] E.A. Ekimov, N.V. Suetin, A.F. Popovich, V.G. Ralchenko, Diamond and Related Materials 17 (2008) 838–843. [7] M. Battabyal, O. Beffort, S. Kleiner, S. Vaucher, L. Rohr, Diamond and Related Materials 17 (2008) 1438–1442. [8] S. Dorfman, D. Fuks, M. Suery, Journal of Materials Science 34 (1999) 77–81. [9] L. Weber, R. Tavangar, Scripta Materialia 57 (2007) 988–991. [10] M.G. de Azevedo, A. Potemkin, A.L.D. Skury, R.N. de Azevedo Faria Jr., Diamond and Related Materials 10 (2001) 1607–1611. [11] H.X. Peng, Z. Fan, J.R.G. Evans, Journal of Microscopy 201 (2001) 333–338. [12] H.X. Peng, Materials Science and Engineering A 396 (2005) 1–2. [13] L.J. Huang, L. Geng, A.B. Li, F.Y. Yang, H.X. Peng, Scripta Materialia 60 (2009) 996–999. [14] L.J. Huang, L. Geng, H.X. Peng, J. Zhang, Scripta Materialia 64 (2011) 844–847. [15] L.J. Huang, L. Geng, H.Y. Xu, H.X. Peng, Materials Science and Engineering A 528 (2011) 2859–2862. [16] E. Candan, H.V. Atkinson, H. Jones, Journal of Materials Science Letters 32 (1997) 289–294. [17] J. Bear, Dynamics of Fluids in Porous Media, Dover publication Inc, New York, 1972, pp.91–128. [18] H.P. Robert, W.G. Don, O.M. James, Chemical Engineers’ Handbook, 7th ed., McGraw-Hill Professional, New York, 1999. [19] C. Orr, Filtration Principles and Practices Part 1, Marcel Dekker, New York, 1977, p.176. [20] R. Mises, K.O. Fredricks, Fluid Dynamics, Rhode Island: Brown University, Providence, 1941, p.140. [21] T. Matsumoto, H. Fujii, T. Ueda, M. Janau, K. Nogi, Measurement Science and Technology 16 (2005) 432–437.