耐高温金属与硬质合金.pdf.doc

Int. Journal of Refractory Metals and Hard Materials 41 (2013) 8589Contents lists available at ScienceDirectInt. Journal of Refractory Metals and Hard Materialsj o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / I J R M H MFabrication of diamond particles reinforced Al-matrix composites by hot-press sinteringJianping Long, Xin Li , Dedi Fang, Peng Peng, Qiang HeCollege of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, Peoples Republic of Chinaa r t i c l e i n f oArticle history:Received 22 October 2012 Accepted 8 February 2013Keywords:Vacuum hot-pressing sintering Diamond/AlElectronic packaging Thermal conductivityCoefficient of thermal expansiona b s t r a c tAl-matrix composites reinforced by 50 vol.% diamond particles under a 60 MPa sintering pressure were fab-ricated by a vacuum hot-pressing method. The composite obtained a relative density of 96.5%. The coefficient of thermal expansion (CTE) and the thermal conductivity (TC) of the diamond/Al composites were measured by the laser flash method and differential dilatometry, respectively. Results showed that diamond/Al composites have high TC and low CTE with high sintering pressure (60 MPa) and high volume fractions of diamond particles (50 vol.%). The TC of the 50 vol.% diamond/Al composite was 321 W/mK, which is 112 W/mK higher than that of pure Al (209 W/mK). At temperatures ranging from 298 K to 573 K, the com-posite obtained low CTEs in the range of 13.2 106/K to 8.3 106/K, which satisfied the CTE of electronic packaging materials. The CTE values obtained in the experiment were approximately equal to the CTE calcu-lated by the Kerner model 17. The effects of volume fractions of diamond particles and sintering pressure on the density, TC, and CTE of diamond/Al composites were investigated. 2013 Elsevier Ltd. All rights reserved.1. IntroductionBoth miniaturization of electronic components and increase in packaging density in microelectronic and semiconductor devices lead to problems that prevent currently used electronic packaging materials from meeting the requirements of a heat sink and a heat spreader 1,2. A new electronic packaging material with high thermal conductivity (TC) and low coefficient of thermal expansion (CTE) must be developed to increase the reliability of electronic packaging materials. The CTE of these materials must match that of semiconduc-tor materials, including GaAs and Si 3. Low CTE combined with slightly high TC (less than 200 W/mK) of metal matrix composites (MMCs), such as aluminum (Al) reinforced with SiC particle or carbon fiber composites 47, are used in electronic packaging. Despite the above, the TC of the MMCs used in electronic packaging cannot meet the future requirement because the highest power density of high-power integrated circuit will reach to 100 Wcm2.Considering the continuing decrease in the cost of diamond in re-cent years and the superior properties of high TC (maximum ranging from 1800 W/mK to 2000 W/mK at 298 K) combined with low CTE 8,9, diamond should be an ideal reinforcement for MMCs used in electronic packaging. In a previous study, diamond/Al composites were produced by spark plasma sintering 19, gas pressure infiltration, and squeeze casting infiltration 10. Squeeze casting infiltration and gas pressure infiltration are regarded as the most promising amongCorresponding author. Tel.: +86 28 84079027. E-mail address: (X. Li).these methods. However, these techniques require high energy and cause the formation of Al2OC in the interface layer of diamond and Al during fabrication. These carbides may lead to adverse effects on the interface bonding 10.The present study examined the fabrication of diamond/Al com-posites by a simple hot press (HP) method. The HP method can pro-duce diamond/Al composites below the melting point of Al. Therefore, harmful interface reactions between diamond and Al can be prevented. However, inappropriate amounts of HP sintering pressure can easily lead to a low relative density of composites prepared by the HP method. Consequently, varying levels of sintering pressure were applied to in-vestigate the influence on RD, TC, and CTE.2. Experimental2.1. Materials and methodsAl powders (purity 99.5%, Kelong, China) were used as matrix materials. Single-crystal diamond particles (Yangzhou Grinding Tools Co. Ltd., China) with sizes ranging from 55 m to 70 m were used as reinforcement for composites.The diamond/Al mixture powders were prepared by planetary ball mill mixing (Experiment Instrument Research Institute, China) at 300 rpm for 4 h with a ball-to-powder ratio of 2:1. The obtained powders were cold pressed in a 50 mm diameter steel mold.The acquired cold-pressed compacts were sintered in a 50 mm diameter graphite mold by using an HP device with induction heating (Huachen Electric Furnace Co., Ltd., China). As illustrated in Fig. 1,0263-4368/$ see front matter 2013 Elsevier Ltd. All rights reserved. /10.1016/j.ijrmhm.2013.02.00786J. Long et al. / Int. Journal of Refractory Metals and Hard Materials 41 (2013) 8589Table 1Some properties of diamond/Al composites.Fig. 1. HP sintering.sintering was performed as follows: The acquired cold-pressed com-pacts were first heated to 673 K and held for 20 min. The compacts were then heated to 878 K and held for 25 min under sintering pres-sures of 25, 35, 40, 45, 50, 55, 60, and 65 MPa. The sintering pressure was loaded at 698 K. Considering the difference in the CTE values of the Al-matrix and diamond particles, the sintering pressure was unloaded after the sample was cooled to 373 K. The HP parameters were assumed to have been optimized in our previous study.SampleSinteringRelativeCTETCRef.pressuredensity(298 K573 K)(MPa)%(106/K)(W/mK)(a) 50 vol.%2580.112.520.494diamond/Al(b) 50 vol.%3585.511.418.7148diamond/Al(c) 50 vol.%4091.010.115.9189diamond/Al(d) 50 vol.%45201diamond/Al(e) 50 vol.%5095.88.611.3255diamond/Al(f) 50 vol.%5596.48.613.5308diamond/Al(g) 50 vol.%6096.68.313.2321diamond/Al(h) 50 vol.%65319diamond/Al(i) 10 vol.%6098.9207diamond/Al(j) 30 vol.%6098.59.514.9278diamond/Al(g) 50 vol.%6096.68.313.2321diamond/Al(k) 60 vol.%6092.1227diamond/AlPure Al6020.626.2209Diamond1180020008,9,222.2. CharacterizationComposite density was measured by the Archimedes principle. Thermal diffusivity and the CTE of the samples were cut from the as-hot pressed cylindrical samples by electrical discharge wire cutting. The resulting samples exhibited cylindrical shapes of 12.7 mm 2.5 mm and 5 mm 25 mm for the measurements of TC and CTE, respectively. The thermal diffusivity of the samples was measured at room temperature by the laser flash method (LFA447 Micro-Flash, NETZSCH, Germany). The TC () of the samples can be calculated by the following equation 9: CP;where is the thermal diffusivity, is the bulk density of the com-posites, and CP is the specific heat capacity. The CTE was measured on a differential dilatometer (DIL402 PC, NETZSCH, Germany) at a heating rate of 5 K/min at heating temperatures ranging from 298 K to 573 K under continuous flow and 50 ml/min of argon to prevent oxidation-induced degradation. The dilatometer was cali-brated by an alumina standard sample to diminish systematic errors. The micro-shape of the diamond particles and the microstructure of the diamond/Al composites under varying sintering pressure were ob-served by a scanning electron microscope (SEM, S-530, HITACHI, Japan).2.3. Properties of diamond/Al compositesTable 1 and Fig. 2 present experimentally obtained properties of diamond/Al composite materials.First, the effect of relative density in the 50 vol.% diamond/Al com-posites under varying sintering pressure (25, 35, 40, 45, 50, 55, 60, and 65 MPa) was investigated, as shown in Table 1(a), (b), (c), (d), (e), (f), (g), and (h), respectively. This table reveals the following observations:(1) The relative density of diamond/Al composites increased with the increase in sintering pressure.(2) The relative density of composites reached 96.5% when sintering pressure was increased to 60 MPa. However, as sintering pres-sure increased to 65 MPa, the relative density of diamond/Alincreased to 96.6% only, reflecting a very slight improvement. Considering the energy consumption and useful lifespan of the graphite mold, a sintering pressure of 60 MPa seemed optimal.Second, Table 1 and Fig. 2 also show the influence of the volume fractions of diamond particles and sintering pressure on TC.(1) The TC increased when the sintering pressure increased from 25 MPa to 65 MPa. The TC values of the 50 vol.% diamond/Al composites under sintering pressures of 25, 35, 40, and 45 MPa were 94, 148, 189, and 201 W/mK, respectively. These values were lower than that of pure Al (209 W/mK). The TC of diamond/Al composites sintered with sintering pressures of 50, 55, and 60 reached 255, 308, and 321 W/mK, respectively. These values were higher than the TC of pure Al.(2) The TC of diamond/Al composites under a 60 MPa sintering pres-sure and containing 10 vol.% diamond particles was 207 W/mK. The TC of the composite containing 30 vol.% diamond particles was 278 W/mK, which was 69 W/mK higher than that of pureAl. With the increase in the volume fraction of diamond up to 50 vol.%, the TC of the diamond/Al composites reached 321 W/mK. Considering the higher volume fractions of diamond particles and the higher TC values, the composite containing 60 vol.% diamond particles was chosen as reinforcement to mea-sure the effect of the higher volume fractions on TC. However, the TC of the 60 vol.% diamond/Al composite could not exceed 321 W/mK and was limited to 227 W/mK.Third, the CTE of diamond/Al composites sintered under different sintering pressures were measured, as shown in Table 1 and Fig. 2(b) at temperatures ranging from 298 K to 573 K. Some features are noted as follows:(1) Al-matrix achieved the highest CTE within the range of20.6 106/K to 26.2 106/K. However, the CTE values of the 50 vol.% diamond/Al composite under 60 MPa sintering pressure were significantly decreased to a value ranging from8.3 106/K to 13.2 106/K.J. Long et al. / Int. Journal of Refractory Metals and Hard Materials 41 (2013) 858987Fig. 2. (a) The relationship of relative packing density and TC of diamond/Al composites containing 50 vol.% diamond particles with varying sintering pressure by HP sintering and(b) thermal expansion of pure Al and diamond/Al composites: temperature dependence of CTE.(2) The CTEs of composites decreased with increasing sintering pressure, except for that sintered under 50 MPa (8.6 106/K to 11.3 106/K), which was lower than the CTE under 60 MPa (8.3 106/K to 13.2 106/K).(3) The CTE of composites increased with the increase in temperature.2.4. Microstructure and properties of diamond/Al compositesThe SEM images present the distribution of diamond and the bonding of diamond and Al-matrix. The diamond particles were uniformly distributed in the Al-matrix under optimized sintering pa-rameters, as shown in Fig. 3(d).Khalid 11 and Kleiner 12 found aluminum carbide phases in the interface between diamond and Al during squeeze casting and gas pressure infiltration. However, aluminum carbide phases be-tween Al-matrix and diamond fabricated by HP were not detected by SEM observation in the current study, as shown in Fig. 3(d) and(e). The absence of formation of aluminum carbides in the present study was primarily attributed to the sintering temperature below the aluminum melting point during HP sintering. However, further microstructure investigation is needed to clarify whether the alumi-num carbides were formed in the interfaces of composites fabricated in the present study. Microstructural investigations by transmission electron microscopy are being conducted. Table 1 shows that theFig. 3. SEM images of the 50 vol.% diamond/Al composites under various sintering pressures of (a) 25 (b) 50, and (c) 60 MPa, as well as (d) the distribution of diamond particles and the enlarged image of diamond/Al composites under 60 MPa sintering pressure.88J. Long et al. / Int. Journal of Refractory Metals and Hard Materials 41 (2013) 8589relative density of 50 vol.% diamond/Al composites under sintering pressures of 25, 50, and 60 MPa were 80.1%, 95.5%, and 96.6%, respec-tively. These results are attributed to the changing microstructure. Fig. 3(a), (b), and (c) show the microstructure of the diamond/Al composites under varying sintering pressures (25, 50, and 60 MPa, respectively). Fig. 3(a) shows that compressed Al powders and dia-mond particles were segregated and many pores were present. This phenomenon typically occurs under insufficient pressure. Thus, the relative density of the diamond/Al composite was low. Few pores were observed between the diamond particle and the Al-matrix when the sintering pressure was increased to 50 MPa, as shown in Fig. 3(b). Gaps between the diamond particles and the Al-matrix nearly disappeared. Therefore, the relative density of the diamond/ Al composite was increased to 95.5%. Pores and gaps between the diamond particle and the Al-matrix were not observed for composites under a higher sintering pressure (e.g., 60 MPa), as shown in Fig. 3(c). The number of pores and gaps in the composite decreased with the increase in sintering pressure for two reasons: (1) Increasing the sintering pressure can improve the plastic yield of the Al particle contact area, thereby accelerating the contact area to the formation of the exponential creep area. In the exponential creep area, the volume diffusion and grain boundary diffusion of Al atoms or vacan-cies promoted the integration between the Al particles; thus, the relative packing density of composites was improved. (2) Higher sintering pressure can enhance the liquidity of the Al-matrix. There-fore, increasing the sintering pressure significantly improved the relative density of the composite during HP.The TC values of diamond/Al composites are presented in Fig. 1(a). The results showed that the TC of the diamond/Al composites in-creased as sintering pressure increased. The TC of 50 vol.% diamond/ Al composites reached 321 W/mK as sintering pressure increased to 60 MPa. A similar trend was observed between the TC and the rela-tive density of diamond/Al composites. TC and relative density in-creased because of the interface adhesion of the Al matrix to the diamond particles, which was improved by the increasing sintering pressure. The improvement in the adhesion reduced the interfacial thermal resistance between diamond and Al during heat conduction. Thus, the TC of diamond/Al composites increased. The TC of diamond/ Al composites increased as the volume fractions of diamond particles increased. However, the sample with 60 vol.% diamond content failed to achieve the highest TC, as expected, although it had the highest volume fraction. The relative density of 60 vol.% diamond/Al compos-ite was only 92.1%, which was lower than that of 50 vol.%. The results indicated that the promotion of relative density was the key to improving the TC of diamond/Al composites. In general, the TC of 45.5 vol.% diamond/AlMg composites was 403 W/mK by SPS 19. In the present study, the TC of 50 vol.% diamond/Al composites under 60 MPa sintering pressure was 321 W/mK, which was not sig-nificantly higher. Lower TC was mainly attributed to the following reasons. (1) Comparison of HP with SPS indicated that the diamond surfaces were not modified. However, the surfaces could be modified by the formation of micro-liquid aluminum between the diamond particles and the AlMg matrix by SPS. (2) The size and shape of dia-mond particles were also considered as factors.Figs. 2(b), and 3(a), (b), and (c) show that increased sintering pressure reduced the pores and gaps within the Al-matrix or between the Al-matrix and the diamond particles, resulting in a decrease in CTE. The CTEs of diamond/Al composites were related to the exis-tence of the interface and plastic deformation in the Al-matrix 13. Interfacial bonding and increased sintering pressure were the key approaches to limiting the CTEs of diamond/Al composites and im-proving interfacial bonding, respectively. Thus, higher sintering pres-sure results in lower CTE, except for the CTE of composites sintered at 50 MPa. Hansang and Hiroshi 14,15 considered that CTE can be achieved not only by continuous interface bonding but also by maintaining the point of contact and micro-pores in the system ofdiamond-reinforced Al-matrix composites. In the thermal stress field in a particulate composite, the CTE and elastic modulus of the diamond particles are assumed lower and higher than those of Al-matrix, respec-tively. The increase in temperature ( T 0) induced the CTE mismatch strain, causing compressive and tensile stress fields in the Al-matri