2022 年 63 巻 4 号 p. 644-648
This study reports on an interfacial design method of Cu–SiC composites by means of nano-diamond/SiC composite particles in melt-infiltration process. In the case of Cu–SiC composites fabricated by melt-infiltration process, reaction layers consisting of spherical carbon particles and Cu–Si solid-solution alloys were observed as SiC particles reacted with melt Cu. Formation of reaction layers is not appropriate to obtain the predesigned characteristics such as physical and mechanical properties of the Cu–SiC composites. To overcome this problem, nano-diamonds, which were chemically inert with Cu, were bonded to the surfaces of SiC particles by amorphous silica as a bond material in order to shield the SiC particles from reacting with melt Cu. It was found that this shielding method enabled us to disperse SiC particles homogeneously in the Cu matrix without reaction layers. This interfacial design based on the composite particles should be expected to be applied to a promising mass production technology of metal-matrix composites (MMCs) due to its short processing time and large amount of production for processing composite particles.
This Paper was Originally Published in Japanese in J. Japan Inst. Copper 60 (2021) 271–275.
Fig. 6 SEM BSE-C images showing interfaces around SiC particles in the (a) Cu–SiC and (b) Cu–ND/SiC composites fabricated by CSC process.
As silicon carbide (SiC) and diamond possess high thermal conductivities, numerous studies about copper (Cu)-based composites with SiC or diamond particles have been conducted for the application to heat sink materials with high thermal conductivities and low coefficients of thermal expansion.1,2) The Cu-based composites with SiC or diamond particles as abrasive grains have also been studied for the application to metal-bonded grinding wheels.3,4) Various properties of composites are greatly affected by interfacial condition between matrices and dispersed particles. Several problems should be considered in fabricating metal-matrix composites (MMCs) by melt processes such as casting.5) In the case of Cu–SiC composites with dispersed SiC particles, one of the problems is forming reaction layers between the SiC particles and molten Cu.1) Forming reaction layers is not desirable in most cases to obtain designed physical and mechanical properties of the composites.
In this study, nano-diamond (ND)/SiC composite particles were fabricated by combining ND particles, which did not easily react with molten Cu due to their chemical inertness,6) on the surfaces of SiC particles with micron scale using amorphous silica powder as a bond material. These core-shell structured composite particles are usually fabricated by coating surfaces of core particles with a shell structure composed of a different material that has a special function or excellent machining performance, etc. For example, composite particles have been studied for efficient uses of scarce resources or adding useful functions in the field of chemical mechanical polishing.7–9) Composite particles have also been studied for applications in the field of biomaterials.10) In the present study, the core-shell structured composite particles were used to suppress forming reaction layers during a melt process. SiC particles-dispersed and ND/SiC composite particles-dispersed Cu-based composites were respectively fabricated by centrifugal-sintered casting (CSC),11) which is a kind of pressure infiltration casting. Hereafter, these Cu-based composites are referred to as Cu–SiC and Cu–ND/SiC composites. Microstructural observations of the Cu–SiC and Cu–ND/SiC composites were performed by focusing on differences in their interfacial microstructures between matrices and dispersed particles. Effects of combining ND particles on the surfaces of SiC particles in the fabrication of Cu–SiC composites by the melt process were investigated.
As raw materials, SiC particles with an average particle size of 150 µm (Showa Denko K.K., Tokyo, Japan) and ND particles with a particle size of 250 nm or less (Tomei Diamond Co., Ltd., Tokyo, Japan) were used. Amorphous silica powder (Tokuyama Corporation) was also used as a bond material in order to improve the bonding strength between SiC particles and ND particles in the ND/SiC composite particles. Secondary electron (SE) images of these particles taken by a scanning electron microscope (SEM) are shown in Figs. 1(a)–(c), respectively. For the fabrication process of the ND/SiC composite particles, a mechanical particles-combining method was used.4,12,13) First, a mixed powder composed of three kinds of powders (SiC: 300 g, ND: 6.92 g, and amorphous silica: 6.92 g) was made in a container. After that, the ND/SiC composite particles were fabricated by applying compression, shear, and impact forces to the predesigned mixed powder using a dry particle composing machine (Nobilta: NOB-130, Hosokawa Micron Corporation, Osaka, Japan). The processing conditions of the dry particle composing machine were chosen as follows: a blade rotation speed of 2220–2240 rpm and a blade rotation time of 3 min. The fabricated ND/SiC composite particles were put in an alumina crucible embedded in a carbon powder bed, and subjected to heat treatment at 1273 K in a strong reducing atmosphere to improve the bonding strength of the amorphous silica as bond material.
SEM secondary electron (SE) images showing (a) SiC particles, (b) nano-diamond (ND) particles, and (c) amorphous silica powder.
The Cu–SiC and Cu–ND/SiC composites were respectively fabricated by CSC process11) as described above. First, a Cu–25 vol%SiC mixed powder was prepared by mixing SiC particles and Cu powder. A cylindrical preform was obtained by centrifugal sintering the mixed powder under a centrifugal force of 1100 G at 1173 K for 1 h in a vacuumed cylindrical-mold. A Cu–SiC composite was fabricated by pouring molten Cu into the cylindrical preform by means of centrifugal casting using a vacuum centrifugal casting machine (Yasui & Co., Tokyo, Japan) under a centrifugal force of 78 G at 1393 K. In the case of fabricating MMCs by conventional centrifugal-casting, compositional gradient or compositional separation usually occurs due to a difference in a volumetric mass density between a molten metal and dispersed particles.14–16) However, in the case of CSC process, solidification completion is achieved in a relatively short time because heat of a molten metal is used to dissolve metal matrix in a sintered preform. Besides, as a centrifugal force is applied in a highly viscous state during pouring a molten metal by centrifugal casting, migration of dispersed particles is suppressed in CSC process. In this way, different materials with significantly different densities can be uniformly dispersed in a metal matrix without separation by pouring a molten metal into a sintered preform.11) However, if a molten Cu with a temperature higher than 1393 K is used, an MMC with uniformly dispersed ceramic particles cannot be obtained as the Cu matrix in the sintered preform is completely melted.11) In the same way, a Cu–ND/SiC composite was fabricated using a Cu–25 vol%ND/SiC mixed powder. The microstructural observations of these composites were performed using a field emission-SEM (FE-SEM: JSM-7100F, JEOL, Tokyo, Japan), focusing on interfacial microstructures between Cu matrices and SiC particles. The chemical compositions of the samples were analyzed by a detector for energy-dispersive X-ray spectroscopy (EDS: X-MaxN, Oxford Instruments plc, Abingdon, UK) included in the FE-SEM.
Figures 2(a)–(c) show SEM SE images of the fabricated ND/SiC composite particles observed at different magnifications. As shown in Fig. 1(a), the surfaces of the SiC particles before combining ND particles consisted of cleavage surfaces. No attached material on the surfaces of the SiC particles was observed. On the other hand, ND particles and amorphous silica combined on the surfaces of the SiC particles can be observed in the images of the ND/SiC composite particles shown in Figs. 2(a) and (b). Figure 2(c) shows a magnified image of a surface of the ND/SiC composite particle. It can be confirmed that the entire surface of the ND/SiC composite particle was completely covered with ND particles and amorphous silica.
SEM SE images showing (a) ND/SiC composite particles, (b) an ND/SiC composite particle, and (c) a surface of an ND/SiC composite particle.
Figure 3(a) shows an SEM backscattered electron composition (BSE-C) image of the microstructure in the Cu–SiC composite. In Fig. 3(a), the brighter colored-area and the black colored-areas are the Cu matrix and the SiC particles, respectively. The reaction layers with gray color were observed between the Cu matrix and the SiC particles. These reaction layers, formed by reacting molten Cu with SiC particles, can be observed around the all SiC particles. The thicknesses of the reaction layers were in the range of about 10–40 µm. No interfacial debonding was observed at the interfaces between the SiC particles and the reaction layers, while the one was observed in some places at the interfaces between the Cu matrix and the reaction layers.
SEM backscattered electron composition (BSE-C) images showing microstructures of the (a) Cu–SiC, and (b) Cu–ND/SiC composites fabricated by centrifugal-sintered casting (CSC) process.
Figure 3(b) shows an SEM BSE-C image of the microstructure in the Cu–ND/SiC composite. Unlike the case of the Cu–SiC composite, no reaction layer was observed around the ND/SiC composite particles.
3.3 Interfacial microstructures between matrices and dispersed particles in the Cu–SiC and Cu–ND/SiC compositesFigure 4(a) shows a reaction layer formed at an interface between the Cu matrix and an SiC particle in the Cu–SiC composite. It is observed that the reaction layer has a spotted pattern and interfacial debonding at the interface between the Cu matrix and the reaction layer. It can be considered that the interfacial debonding was caused by solidification shrinkage and the difference in the thermal expansion coefficient between the Cu matrix and the reaction layer during cooling after centrifugal casting. An enlarged image of the reaction layer is shown in Fig. 4(b). Fine spherical particles with a diameter of about 1 µm or less were observed in the reaction layers. It was observed that the size of the fine spherical particles tended to decrease from about 1 µm on the SiC particle side to about 40 nm on the Cu matrix side.
SEM BSE-C images showing (a) an interface between Cu matrix and SiC particle, and (b) an enlarged image of the reaction layer in the Cu–SiC composite fabricated by CSC process.
Figures 5(a)–(d) show an SEM BSE-C image and corresponding SEM-EDS elemental mappings for Cu, Si, and C maps, respectively, taken at the interface between the SiC particle and reaction layer in the Cu–SiC composite. By considering these results together with the ones of SEM-EDS point analyses, it was found that these fine spherical particles were carbon crystallized in the reaction layer during centrifugal casting. The matrix of the reaction layer was composed of a Cu–6 mol%Si solid-solution alloy. In addition, Si was detected in the Cu matrix which was far enough away from the reaction layer shown in Fig. 3(a). It was confirmed that the Cu matrix of the Cu–SiC composite was a Cu–2 mol%Si solid-solution alloy with a lower Si concentration than that of the matrix in the reaction layer. Kang et al. described that the following decomposition may occur as a possible reaction in Cu–SiC composites at high temperature.17)
| \begin{equation} \text{SiC (s)} \rightarrow \text{Si (s)} + \text{C (s)} \end{equation} | (1) |
(a) SEM BSE-C image, and SEM-EDS elemental mappings: (b) Cu, (c) Si, and (d) C maps taken at the interface between the SiC particle and reaction layer in the Cu–SiC composite fabricated by CSC process.
Figures 6(a) and (b) show SEM BSE-C images of the interfacial microstructures of the (a) Cu–SiC and (b) Cu–ND/SiC composites. Compared with the microstructure of the Cu–SiC composite shown in Fig. 6(a), it is clear that no reaction layer was observed at all in the Cu–ND/SiC composite shown in Fig. 6(b). In addition, it should be noted that no Si was detected in the Cu matrix adjacent to the SiC particles by measuring Si concentration through SEM-EDS point analyses; that is, the matrix of the Cu–ND/SiC was pure Cu. Compared with SiC, it can be considered that diamond and carbon possess poor wettability to Cu,6,20–23) and are chemically inert with Cu as described above.6,18) These results can be attributed to the poor-wettability and chemical inertness of the ND particles, which were bonded on the surfaces of the SiC particles, with molten Cu in the melt process. It can be concluded that this controlling method for interfacial reactions can suppress the reaction between molten Cu and SiC particles during centrifugal casting, and enables us to disperse SiC particles homogeneously without forming reaction layers in the Cu matrix.
SEM BSE-C images showing interfaces around SiC particles in the (a) Cu–SiC and (b) Cu–ND/SiC composites fabricated by CSC process.
This controlling method for interfacial reactions can be expected to be applied to other melt processes such as thermal spraying and additive manufacturing. Besides, the interfacial design method based on the composite particles should be expected to be applied to a promising mass production technology of MMCs due to its short processing time and large amount of production for processing composite particles.
In the present study, the ND/SiC composite particles, which did not easily react with molten Cu, were fabricated. Effects of combining ND particles on the surfaces of SiC particles in the fabrication of Cu–SiC composites by the melt process was investigated. The obtained results can be summarized as follows.
This research was supported by the Regional Innovation Cluster Program (Global Type) “Tokai Region Nanotechnology Manufacturing Cluster” through the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.
