微波烧结SiC_Cu_Al复合材料的工艺及机理_英文_

张海龙等：锆钛酸铅/银压电复合材料的烧结及其电学和力学性能· 1431 ·第34卷第12期微波烧结SiC–Cu/Al复合材料的工艺及机理王海龙1,2，张锐1，汪长安2，何小波1，黄勇2，胡行1(1. 郑州大学，材料物理教育部重点实验室，郑州 450002；2. 清华大学，新型陶瓷及精细工艺国家重点实验室，北京 100084)摘要：用微波烧结工艺成功制备了铜包裹碳化硅颗粒增强铝基(SiC–Cu/Al)复合材料，利用扫描电镜和X射线衍射分析仪对烧结样品进行表征，并讨论了烧结过程及机理。

研究表明：采用多晶莫来石纤维棉、硅碳棒和氧化铝坩锅组合设计的保温结构能很好地促进烧结。

烧结温度为720℃时，SiC–Cu/Al复合材料的密度取得最大值为2.53g/cm3。

SiC–Cu/Al复合材料的硬度随烧结温度的升高的变化成马鞍状。

烧结温度对样品显微结构的影响较大，随着烧结温度的升高，相分布的均匀性降低，在较高的烧结温度下会出现SiC颗粒的偏聚。

涡流损耗和界面极化损耗是促进微波烧结的主要动力。

关键词：金属基复合材料；微波烧结；致密化；界面极化；烧结机理中图分类号：TB333 文献标识码：A 文章编号：0454–5648(2006)12–1431–06PROCESS A ND MECHANISM OF MICROW A VE SINTERING OF SiC–Cu/Al COMPOSITESWANG Hailong1,2，ZHANG Rui1，WANG Chang′an2，HE Xiaobo1，HUANG Yong2，HU Xing1(1. Laboratory of Material Physics of the Ministry of Education of China, Zhengzhou University, Zhengzhou 450002；2.The State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China)Abstract: Aluminum matrix composites reinforced by silicon carbide/copper coated particles (SiC–Cu/Al) were prepared using mi-crowave sintering. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were used to characterize the sin-tered compacts. The process and mechanism of microwave sintering were discussed. It was found that the assistant-heating structure which was set up by mullite heat-preservation material, silicon carbide (SiC) rods and alumina crucible can effectively enhance the process of densification during sintering. The highest density of SiC–Cu/Al composites was 2.53g/cm3 at 720℃. The saddle-shaped profile of the hardness with sintering temperature was detected. Correspondingly, different microstructures were observed at different temperatures. The uniformity of disturbed phases decreased with increasing temperature. Segregation of SiC grains occurred at high temperatures. Induction losses caused by both eddy currents and interfacial polarization were contributed to successful sin-tering.Key word s: metal matrix composite; microwave sintering; densification; interfacial polarization; sintering mechanismMicrowave sintering is a promising novel tech-nique that was first used by Tinga in 1968 to fabricate materials. [1] The advantage of volumetric heating con-tributes to sintering of parts with complex shapes, espe-cially near-net-shape parts that can not be prepared by the conventional sintering method.[2–3] It shows many advantages such as fast heating rates, lower sintering temperatures, enhanced densification, smaller average grain sizes, and an apparent reduction in activation energy for sintering. A wide variety of materials have now been prepared using microwave sintering.[4–10] However, there are fewer reports about sintered metal or metal matrix composites (MMCs) using microwave sintering. The reason for this lack of interest is that microwaves have a low penetration depth in electrically conductive materials. Roy et al,[11] have used microwave sintering to prepare different metals and alloys in 1999. Some other scientists used microwave sintering to investigate TiAl/SiC, Al/TiO2, Al/Al2O3, and Al/SiC composites.[12–13] The microstructure of microwave sintered samples shows much higher density and finer grain size. Never-theless, as a new kind of rapid heating, the mechanisms of microwave sintering in different composite systems are still not clear.收稿日期：2006–07–11。

修改稿收到日期：2006–08–18。

第一作者：王海龙(1977～)，男，博士。

Received date:2006–07–11. Appr oved date: 2006–08–18. First author: WANG Hailong (1977—), male, doctor.E-m ail: 119whl@第34卷第12期2006年12月硅酸盐学报JOURNAL OF THE CHINESE CERAMIC SOCIETYVol. 34，No. 12December，2006硅 酸 盐 学 报· 1432 ·2006年Aluminum matrix composites reinforced by silicon carbide/copper coated particles (SiC–Cu/Al) were pre-pared by using microwave sintering. Some basic proper-ties were analyzed. The possible microwave sintering mechanisms were also investigated.1 Experimental procedures1.1 Raw materialsβ–SiC particles (8 µm in diameter, China White Dove Group) and aluminum (Al) particles (50 µm in dia- meter, China Great-Wall Aluminum Group) were com-mercially available products. The coated SiC/Cu particles were prepared by an electroless coating process, V (Cu)/ V (SiC)=1:1. Details of the coating process have beenpreviously described.[14] The Al powder and the SiC/Cu coated particles were mixed by ball milling. The mass fraction of Al was 90%. Samples were press-formed at 180 MPa.1.2 Structure of microwave chamberThe compacts were sintered in a microwave cham-ber with a resonance mode of TE444. The microwave frequency was 2.45 GHz, with power output in the range of 1—5 kW. One of the simplest versions of the micro-wave chamber is represented schematically in Fig.1a. The circulator and tuner were used to protect the magnetron through deflecting or barring the reflected wave. Tem-perature measurements were carried out with an infrared thermometer.Fig.1 Schematic drawing of microwave sintering furnace and insulation structure1.3 Sintering progressA hybrid heating system was used to avoid defects caused by non-uniform heating. An insulation structure was designed, as shown schematically in Fig.1b. It was composed of an alumina crucible surrounded by mullite fiber. Silicon carbide rods were placed around the sin-tered samples as supplementary heaters. The insulation material was made of mullite which does not absorb mi-crowaves in the lower temperature ranges. At the initial stage of heating, silicon carbide absorbs microwaves to heat the sintered samples via a conventional process. As the temperature increases, the samples absorb the micro-waves and are heated by a volumetric heating effect.The samples were placed at the center of the cruci-ble. The temperatures of the samples were monitored using an infrared pyrometer on top of the samples. The input power of the microwave furnace was 4.0 kW, and the sintering temperatures were 700, 720, 740, 760 ℃, and 780 ℃. The heat holding time were 1, 2, 3, 4, 5, 8 min, and 10 min.1.4 CharacterizationsThe sintered samples were cut along the pressingaxes with a diamond disc and polished. The microstruc-ture was characterized using an optical microscope (Olympus, BHT–M, Japan). The fraction surfaces were observed using a field-emission scanning electron mi-croscope (FESEM, JEOL JSM–6700F). Phases in the samples were determined by X-ray diffraction (XRD) analysis (D/max–2550V). The density measurement was carried out with the Archimedes method. The micro hardness of compacts was measured using a diamond Vickers hardness tester (A VK–A, Japan).2 Results and discussion2.1 Effect of sintering process on density of samplesThe effect of holding time during sintering on the density of samples was detected, and the results are shown in Fig.2. The density of compacts increased with the dura-tion of holding time of 5 min, and it did not increase any more after 5 min. Therefore, 5 min at the sintering tem-perature is enough to sinter under this condition.Figure 3 shows the changes in the density of com-pacts sintered at different temperatures. It was found that the highest density of composites is 2.53 g/cm 3, occurring王海龙等：微波烧结SiC–Cu/Al复合材料的工艺及机理· 1433 ·第34卷第12期Fig.2 Changes in density of composites vs hold timeFig.3 Changes in density of samples vs sintering temperature at 720℃. This indicated the effect of microwaves on the densification of compaction. First, the stage of 0—600℃required about 10min. Because of the selective heating of microwaves, silicon carbide rods were first heated by microwave, and then the samples were heated to 600℃by heat exchange from SiC rods, and this stage took about 10min. But the method for heating samples changed from heat exchange to volumetric heating after the samples were heated to 600℃. Then the aluminum particles around the SiC particles began to melt after being heated. The polarization by microwaves increases as a liquid becomes electrolyte.[15] So the first negative feedback occurred when the heating point of SiC disap-peared. The SiC particles were trapped by the surface tension of liquid, and the amount and size of pores in the compacts decreased. Then the densification of compacts was accomplished. At the stage of heating at which SiC particles change to liquid Al, the compacts achieved the highest density. However, as the temperature increased, on one hand, the reaction which formed Al2Cu at the in-terface was promoted, so the coated layer on the surface of the SiC particles was destroyed. Then a cluster of SiC formed due to the force of liquid Al. On the other hand, the local discharge of the Al in the microwave caused rapid melting, and “thermal runaway” occurred, which destroyed the compacts. In order to avoid thermal runaway, it is effective to control the power output of the microwave sintering furnace. Then the power output must be kept un-der 1 kW, especially at stage of holding the temperature. 2.2 X-ray spectrum analysisFigure 4 shows the XRD patterns of compacts sin-tered at different temperatures. Aluminum, SiC and Al2Cu were detected at every temperature, which indi-cates that there was no significant change of phase with the increase of temperature. The formation of Al2Cu was the outcome of the reaction between Al and Cu, and itwas enhanced by the microwaves.Fig.4 XRD patterns of microwave sintered samples atdifferent temperatures2.3 Effect of temperature on morphologyAs shown in Fig.5, the morphologies of the sintered SiC–Cu/Al compacts are different at different sintering temperatures. At 700℃ and 720℃, the Al around the SiC particles first melted as the heated SiC particles ab-sorbed the microwaves. With the amount of melted Al increasing, the densification of compacts progressed, as shown in Figs 5a and 5b. Copper particles coated in SiC were still present among the Al particles. The micro-structure of the compacts was homogeneous and the in-terface structure was dense and compact due to the capil-lary force of liquid and diffusion. Figure 5c shows the morphology of the compacts sintered at 740℃. It was found that the size of Al particles increased and the in-terphase was fragmented. The mode of heating in the compacts is changing at this stage. First, the amount of liquid Al increases which would allow them to absorb more microwaves than SiC particles could. Secondly, the硅 酸 盐 学 报· 1434 ·2006年Fig.5 Optical microscope photographs of microwave sintered samples at different temperaturesspace charge at the interphase polarizes to increase dissi-pation. Thirdly, an eddy-current occurs at the surface of Al particles due to the role of the microwaves. All these phenomena generated heat, causing the Al particles to grow. Figures 5d and 5e show the morphologies of com-pacts at 760 ℃ and 780 ℃. The biggest difference in microstructure compared to another compacts micro-structure is the appearance of agglomeration of SiC in the grain boundary. Because more heat was generated from the microwaves, a great amount of liquid of Al and Al 2Cu formed. Then most SiC separated from the melting Cu due to the poor wetting ability between SiC and Cu,and they tended to segregate at the tri-granular region. The recrystallization of melted Al lead to the appearance of fine Al particles.2.4 Effect of microwave on hardnessThe Vickers hardness of the compacts is illustrated in Fig.6. It was observed that the curve of hardness changed with temperature is saddle-shape, and the minimum hardness appeared at 740 ℃. The abnormal changes might be associated with the changes in the microstruc-ture with the sintering temperature. The structure was heterogeneous and the interphase was fragmented, so the hardness was lowest. The structure of compacts sintered王海龙 等：微波烧结SiC–Cu/Al 复合材料的工艺及机理· 1435 ·第34卷第12期Fig.6 Changes in hardness vs sintering temperature ofmicrowave sintered samplesat 700 ℃ and 720 ℃ were homogeneous, and the bond between matrix and reinforcement became tighter, so the material had better hardness and an improved micro-structure. The formation of both the cluster of SiC parti-cles and fine Al particles due to recrystallization leads to an increase the hardness of compacts sintered at 760 ℃ and 780 ℃.2.5 Mechanisms of microwave sintering MMCThe SiC–Cu/Al composites can be successfully synthesized by microwave; however, there are still many gaps of fundamental knowledge about the precise mechanism of metal reactions in microwaves. The inte- ractions between metal powders in microwaves are com-plex. Figure 7 shows the SEM photographs of the frac-ture structure for microwave sintering samples sintered at 740 ℃ and 760 ℃, which illuminate the microwave sintering mechanism. It was found that the large Al parti-cles were detected (shown in Fig.7a), which are the result of the induction losses caused by eddy currents. And Cherradi, et al . [16] recently claimed that in most ceramics the dielectric loss mechanism was a minor contribution to the power absorbed compared to the induction losses caused by eddy currents. Roy, et al . [11] also attributed the heating of metals to eddy-current losses from the electric field. Thus, an induction loss caused by eddy currents is one mechanism for microwave sintering of SiC–Cu/Al composites. The neck between particles is clearly shown in Fig.7b, which is powerful evidence for mass transfer due to the focusing of the electric field at the neck of the sintering. A similar phenomenon was also found by Booske, et al . [17] when they studied the microwave pon-deromotive forces in solid-state ionic plasmas. They found the net ponderomotive action of the enhanced electric field results in mass flow along the grain surfaces and away from the interior of the intergrain boundary.Furthermore, the temperature gradation due to the selectiveFig.7 SEM photographs of fracture structure for microwavesintering samplesheating of microwave increases the diffusion coefficient D , which contributes to sintering.[18] And at last, the liquid formed in sintering will reduce the viscidity to accelerate the sintering.3 ConclusionsAluminum matrix composites reinforced by silicon carbide/copper coated particles were sintered using a 2.45 GHz microwave field. A 5-minute period at a certain temperature is sufficient for sintering under these condi-tions. There were no obvious difference in phases of composites sintered at different temperatures within a range of 700 ℃ to 780 ℃. The uniformity of the distur-bances of phases decreased with the increasing tempera-ture, which affected the mechanical property of the com-pacts. The highest density of composites was 2.53 g/cm 3, which occurred at 720 ℃. The Vickers hardness of the compacts was 29—35 MPa, and changed with tempera-ture is in the curve of saddle-shape. Induction losses caused by both eddy currents and interfacial polarization contributed to successful sintering.References:[1] TINGA W R, VOSS W A G . Microwave Power Engineering [M]. NewYork: Academic Press, 1968. 73–78.硅酸盐学报· 1436 ·2006年[2] FANG Y, ROY R, AGRAWAL D K, et al. Transparent mullite ceramicsfrom diphasic aerogels by microwave and conventional processing [J].Mater Lett, 1996, 28: 11–15.[3] GUPTA M, WONG W L E. Enhancing overall mechanical performanceof metallic materials using two-directional microwave assisted rapid sintering [J]. Scripta Materialia, 2005, 52(6): 479–483.[4] DA VID E C, DIANE C F, JON K W. Processing materials with micro-wave energy [J]. Mater Sci Eng A, 2000, 287(2): 153–158.[5] XIE Zhipeng, HUANG Yong, WU Su. Microwave sintering of zirconiatoughened alumina ceramics [J]. J Chin Ceram Soc (in Chinese), 1995, 23(1): 7–13.[6] FANG Y, CHENG J P, AGRAWAL D K. Effect of powder reactivity onmicrowave sintering of alumina [J]. Mater Lett, 2004, 58(3–4): 498–501.[7] FANG Y, AGRAWAL D K, ROY D M, et al. Fabrication of transpa-rent hydroxyapatite ceramics by microwave processing [J]. Mater Lett, 1995, 23: 147–151.[8] LIN I N, LEE W C, LIU K S, et al. On the microwave sintering tech-nology for improving the properties of semiconducting electronic ce-ramics [J]. J Eur Ceram Soc, 2001, 21(10–11): 2085–2088.[9] CHENG J P, AGRAWAL D K, KOMARNENI S, et al. Microwaveprocessing of WC–Co composites and ferroic titanates[J].Mater Res Innov,1997(1): 44–52.[10] RAMESH D P, ROY R, AGRAWAL D. Anisothermal reaction syn-thesis of garnets, ferrites and spinels in microwave field [J]. Mater ResBull, 2001, 36: 2723–2739.[11] ROY R, AGRAWAL D, CHENG J P, et al. Full sintering of pow-dered-metal bodies in a microwave field [J]. Nature, 1999, 399: 668–670.[12] RAMAKRISHNAN K N. Powder particle size relationship in micro-wave synthesized ceramic powders [J]. Mater Sci Eng A, 1999, 259(1): 120–125.[13] SUSANNE L, SEBASTIEN V, OLIVIER B. Assessment of micro-wave heating for sintering of Al/SiC and for in-situ synthesis of TiC [J]. Adv Eng Mater, 2003, 5(6): 449–453.[14] ZHANG R, GAO L, GUO J K. Effect of Cu2O on the fabrication ofSiCp/Cu nanocomposites using coated particles and conventional sin-tering [J]. Composites Part A, 2004, 35(11): 1301–1305.[15] SPORTZ M S, SKAMSER D J. Thermal stability of ceramic materialsin microwave heating [J]. J Am Ceram Soc, 1995, 78(4): 1041–1048.[16] CHERRADI A, DESGARDIN G, PROVOST J, et al. Electric mag-netic field contributions to the microwave sintering of ceramics [A]. In: WASNER R, HOFFMANN S, BONNENBERG D, eds. Electrocera- mics IV vol II [C], Aachen, 1994. 1219–1224.[17] BOOSKE J H, COOPER R F, FREEMAN S A, et al. Microwaveponderomotive forces in solid-state ionic plasmas [J]. Phys Plasmas, 1998, 5(5): 1664–1670.[18] WU S, LU A, BA X Y, et al. Sintering mechanisms of materials bymicrowave [J]. J Aeronaut Mater, 1996, 16(4): 25–29.※※※※※※※※※《硅酸盐学报》获2006年度中国科协精品科技期刊工程项目资助为了促使中国科学技术协会(以下简称“中国科协”)及所属全国性学会主办的科技期刊更好地服务科技自主创新，加强学术交流功能，推进实施精品科技期刊战略，提高科技期刊核心竞争力，提高全国性学会为实施科教兴国战略、人才强国战略服务的能力，中国科协决定在“十一五”期间实施精品科技期刊工程，并于2006年6月发出《关于申报2006年度中国科协精品科技期刊工程项目资助的通知》。