Interface tailoring and thermal conductivity enhancement in diamond particles reinforced metal matrix composites

Interface tailoring and thermal conductivity enhancement in diamond particles reinforced metal matrix composites

17

Hailong Zhang a , Xitao Wang a , Jinguo Wang b , Moon J. Kim b a University of Science and Technology Beijing, Beijing, China; bUniversity of Texas at Dallas, Richardson, TX, United States

The miniaturization and integration of electronic devices are producing very high power density [1]. Thermal management materials having high thermal conductivity are urgently demanded to dissipate the heat rapidly in order to maintain the performance of the devices. Traditional thermal management materials like Cu/W, Cu/Mo, Al-Si, SiC and Al/SiC have moderate thermal conductivities and may no longer meet the demand. Diamond has the highest thermal conductivity of w2000 W/mK in nature [2], but diamond alone is difficult to act as thermal management materials due to its supreme hardness and poor machinability. Compared with other materials, the diamond exhibits unique advantages, especially the high thermal conductivity; while how to effectively avoid its shortcomings and have a role, is still the research hotspot. It is an effective way to select materials with better thermal conductivity and ductility for composite preparation. Diamond particles reinforced metal matrix (metal/diamond) composites could be a competitive candidate for electronic packaging applications by combining high thermal conductivity of diamond and easy processing of metal matrix composites. The interface plays a critical role in determining the thermal conductivity of the metal/diamond composites by affecting the interfacial thermal conductance. The interface structure can be tailored by chemical composition design or by processing parameter optimization to enhance the thermal conductivity of the metal/diamond composites. Al and Cu are currently adopted as the metal matrix [3,4] in the metal/diamond composites due to their common use in industry and the excellent thermal conductivity. In the field, various routes like vacuum hot pressing [5,6], spark plasma sintering [7,8], pressure infiltration [9,10], gas pressure infiltration [11,12], and high pressure high temperature method [4,13] have been applied to produce Al/diamond and Cu/diamond composites. Generally, the thermal conductivities are reported to be 500e700 W/mK [5,7,9,11] for Al/diamond composites and 500e900 W/mK [4,6,8,10,12,13] for Cu/diamond composites. Nevertheless, the interface structure characterization is rarely reported [14e16] and the underlying mechanisms for interface formation and thermal property enhancement still remain unclear in the literature. Our group in University of Science and Technology Beijing (USTB) and University of Interfaces in Particle and Fibre Reinforced Composites https://doi.org/10.1016/B978-0-08-102665-6.00017-0 Copyright © 2020 Elsevier Ltd. All rights reserved.

474

Interfaces in Particle and Fibre Reinforced Composites

Texas at Dallas (UTD) has concentrated on metal/diamond composites for years. With the formation of appropriate quantity of aluminum carbide at the Al/diamond interface by gas pressure infiltration, we have attained a high thermal conductivity of 710 W/mK in the Al/diamond composite [11]. By producing discrete carbides at the Cu/diamond interface by alloying Ti element into Cu matrix, we have attained a high thermal conductivity of 735 W/mK in the Cu-Ti/diamond composite [17]. By optimizing thickness of carbide layer at the Cu/diamond interface by coating Ti element onto diamond surface, we have attained a high thermal conductivity of 811 W/mK in the Cu/Tidiamond composite [18]. With thorough characterization by focused ion beam (FIB) and scanning transmission electron microscopy (STEM), we have clarified the formation mechanism of the interface structure and demonstrated the thermal conductivity enhancement. In this chapter, the Al/diamond, Cu-Ti/diamond, and Cu/Ti-diamond composites are adopted as three examples to outline the development in the metal/diamond composites, with an emphasis on the tailoring of interface structure and the improvement of thermal properties.

1.

Diamond particles reinforced Al matrix composites

Al has a low density and the supply is abundant on the earth. The Al/diamond composites could find important applications in aerospace technology as an attractive thermal management material. Aluminum can react with carbon to form aluminum carbide (Al4C3). The reaction ensures sound interfacial bonding between the diamond reinforcement and the Al matrix in the composites. In the Al/diamond composite community, the manipulation of the interfacial reaction between Al and diamond is of great interest [19e21]. But fine characterization of the Al/diamond interface is sparsely reported [3,15,22]. Some reports are inconsistent regarding the formation of Al4C3 on the diamond surfaces [3,20]. Here we prepare Al/diamond composites and characterize the interfacial structure using STEM. The results could be of assistance to identify the interface formation mechanism in Al/diamond composites.

1.1

Fabrication of Al/diamond composites

Several processing techniques have been reported to prepare Al/diamond composites, including solid-state process like vacuum hot pressing [5] and spark plasma sintering [7] as well as liquid-state process like pressure infiltration [9]. Here we use a gas pressure infiltration technique to prepare Al/diamond composites [11]. The schematic illustration of the technique is shown in Fig. 17.1. Al bulks (99.97 wt%) and synthetic single-crystalline diamond particles (MBD8, 150e180 mm) were used as the starting materials. The cubo-octahedron shaped diamond particles are shown in Fig. 17.2. To prepare Al/diamond composites, the diamond particles were installed into a cylinder-shaped graphite mold, with the Al bulks covered

Interface tailoring and thermal conductivity enhancement in diamond

475

Gas pressure Metal bulks

Ar

Heating

Diamond particles

Fig. 17.1 Schematic illustration of gas pressure infiltration.

(a)

(b)

400 mm

Fig. 17.2 Starting materials of diamond particles as the filler in the composites. (a) at low magnification and (b) at high magnification.

on top of the mold. The assembly was moved to a furnace and a vacuum of <0.1 Pa was evacuated, before heating the mold to 800
C. Then, an argon gas was pumped into the furnace and a pressure of 1.0 MPa was maintained to facilitate the infiltration of liquid Al into diamond particle beds. The infiltration time was varied from 5 to 60 min to regulate the interface reaction. After turning off the heating power, the mold was furnace cooled down to room temperature. The diamond content in the composites was determined to be 67 vol% by measuring the sample density and assuming full densification of the composites.

1.2

Characterization of composite microstructure

Fig. 17.3 shows the XRD patterns of the prepared Al/diamond composites. Besides Al and diamond, Al4C3 was also detected in the composites. The interfacial carbides nucleate and grow by the reaction between Al atoms in the Al matrix and C atoms on the diamond surface. The interfacial Al4C3 phase is very important to connect the diamond reinforcement and the Al matrix. With increasing infiltration time from 5 to 60 min, the amount of Al4C3 increases accordingly. As seen from the polished and fractured surfaces of the Al/diamond composites in Fig. 17.4, the diamond particles are uniformly dispersed in the matrix. The Al matrix residue was found to cover on the diamond particles (Fig. 17.4(c)). This suggests that the interfacial bonding is sound so that the cracks propagate into the Al matrix.

476

Interfaces in Particle and Fibre Reinforced Composites

Al

Intensity ( a.u.)

Diamond

Al4C3

60 min 30 min 10 min 5 min

20

30

50

40

60

70

80

90

2 Theta (degrees)

Fig. 17.3 XRD patterns of the Al/diamond composites prepared by gas pressure infiltration.

(a)

(b)

800 µm

100 µm

(c)

10 µm

Fig. 17.4 Polished surface and fractured surface of the Al/diamond composites. (a) polished surface, (b) fractured surface at low magnification, and (c) fractured surface at high magnification.

1.3

Characterization of Al/diamond interface

STEM was used to characterize the interfacial structure of the Al/diamond composites. Since diamond is extremely hard and traditional mechanical milling is unable to prepare TEM foils, we used FIB to mill the composite samples to prepare eligible thin foils. Fig. 17.5(a) shows the process of the FIB milling and an intact interface can

Interface tailoring and thermal conductivity enhancement in diamond

477

Fig. 17.5 Interface between diamond (100) surface and Al in the Al/diamond composites. (a) FIB-prepared TEM sample, (b) TEM bright field image in the rectangular area of (a), and (c) HRTEM image in the rectangular area of (b).

be derived by this method. Figs. 17.5 and 17.6 show the distribution of Al4C3 phase on different diamond surfaces along the Al/diamond interface. The Al4C3 phase is not continuous and appears as a particle shape. The morphology of the Al4C3 phase differs greatly on diamond (100) and (111) surfaces. On diamond (100) surface (Fig. 17.5),

Fig. 17.6 Interface between diamond (111) surface and Al in the Al/diamond composites. (a) FIB-prepared TEM sample, (b) TEM bright field image in the rectangular area of (a), and (c) HRTEM image in the rectangular area of (b).

478

Interfaces in Particle and Fibre Reinforced Composites

many inverted pyramid pits are firstly formed and then Al4C3 nucleates on the pits. This can be finely outlined by the TEM image in Fig. 17.7. On diamond (111) surface (Fig. 17.6), Al4C3 nucleates directly on the (111) surface. As a result, triangular Al4C3 particles are formed on diamond (100) surface, but block Al4C3 particles are formed on diamond (111) surface. On both diamond surfaces, the crystallographic orientation relationship of diamond[110]//Al4C3[2110] and diamond(111)//Al4C3(0003) is characterized. This indicates the epitaxial growth of Al4C3 on the diamond surface. Earlier studies have shown that the interfacial reaction only occurs on diamond (100) surface, and this selective nucleation of Al4C3 is ascribed to different carbon atom arrangements on diamond (111) and (100) surfaces [3,15]. Actually, the Al/diamond composites are produced by pressure infiltration with a very short infiltration time of just 5 min [3], and thus molten Al reacts only with more active diamond (100) surface. Recently, other studies [19,20,23] have suggested that Al4C3 is formed on both diamond (111) and (100) surfaces but with different growth processes, and unequal density of carbide on different diamond surfaces is derived. The Al/diamond composites are also produced by gas pressure infiltration and the reference results are consistent with our study. The long infiltration time ensures the interfacial reaction on both diamond (100) and (111) surfaces. Our results in Figs. 17.5 and 17.6 further explain why unequal density of carbide is formed on diamond (111) and (100) surfaces.

1.4

Thermal conductivity of Al/diamond composites

The quality of the interface governs the interfacial thermal conductance and then the thermal conductivity of Al/diamond composites. Sound interfacial bonding will facilitate phonon transfer across the interface to increase the interfacial thermal conductance. The interface in the Al/diamond composites is mainly determined by the quantity and morphology of interfacial Al4C3 particles. In order to tailor the diamond surface condition for the nucleation of aluminum carbide, the diamond particles were kept at 800
C for a period of time from 10 to 30 min before infiltration. As shown in Fig. 17.8, the thermal conductivity of the Al/diamond composites increases from 540 to 710 W/mK with increasing pre-annealing time from 10 to 30 min.

Fig. 17.7 Nucleation of Al4C3 on diamond (100) surface during infiltration.

Interface tailoring and thermal conductivity enhancement in diamond

479

Thermal conductivity (Wm–1K–1)

800 750 700 650 600 550 500 450 400 10

20

30

Pre-annealing period of GPI method (min)

Fig. 17.8 Thermal conductivity of the Al/diamond composites prepared by gas pressure infiltration.

We have characterized the interface structure of the Al/diamond composites reinforced with diamond particles annealed for various lengths of time before infiltration [11]. We found that the pre-annealing of diamond before infiltration promotes the formation of (111) facets on diamond (100) surface and thus increases the quantity of nucleation sites of Al4C3. The increase in amount of the carbides strengthens the interfacial bonding and improves the interfacial thermal conductance. Accordingly, the thermal conductivity of the Al/diamond composites increases with prolonging pre-annealing time. This suggests that the thermal conductivity can be increased by tailoring the quantity of interfacial carbides in the Al/diamond composites. Some researchers [9,24] attempt to make interfacial modification in Al/diamond composites by coating carbide layer onto diamond surface. It is worthy to note that the modified Al/diamond composites exhibit otherwise lower thermal conductivities than the unmodified Al/diamond composites [11,25]. Because carbide usually has lower thermal conductivity than Al or diamond, the insertion of an interfacial carbide layer introduces additional interfacial thermal resistance between Al and diamond. Since Al and diamond have already reacted to form carbide at the interface, researchers have focused on the controlling of reaction between Al and diamond at the interface in recent years [26,27]. The treatment of diamond surface by Arþ bombardment [27] before composite preparation is beneficial to interfacial bonding and to thermal conductivity enhancement in the Al/diamond composites.

2. Diamond particles reinforced Cu matrix composites While Cu has a higher density than Al, Cu has a higher thermal conductivity than Al. Cu/diamond composites have aroused great interests for thermal management applications due to their high thermal conductivity. Different from the formation of Al4C3 at

480

Interfaces in Particle and Fibre Reinforced Composites

Al/diamond interface, Cu is inert to diamond because of full-state electronic structure in Cu 3d band and no carbide is formed between Cu and diamond. The Cu/diamond interface must be modified in order to attain sound interfacial bonding and achieve high thermal conductivity. The formation of carbides at Cu/diamond interface depends on the introduction of foreign carbide-forming elements. The elements will react with carbon atoms on the diamond surface to generate carbides at Cu/diamond interface. This strategy can be fulfilled by adding carbide-forming elements to the Cu matrix [28] or by coating carbide-forming elements onto the diamond surface [29] before composite preparation. Both the metal matrix alloying and the diamond surface coating methods are proved to enhance the thermal conductivity of Cu/diamond composites [28,29]. But the interface formation mechanism in the Cu/diamond composites is still not very clear, and further enhancement of the Cu/diamond composites is impeded. Here we employ both metal matrix alloying and diamond surface coating to modify the Cu/diamond interface, and the same carbide-forming element of Ti is used in the two methods so as to describe the difference between modification methods in tailoring interface structure of Cu/diamond composites.

2.1

Modification of Cu/diamond interface by metal matrix alloying

Alloying elements having a low solubility in Cu are generally selected as the carbideforming agent in the metal matrix alloying route. This makes sure that most of the alloying elements move out from the Cu matrix to take part in the reaction with carbon atoms on the diamond surface. In this study, Cu-Ti alloy was used as the metal matrix [17] since Ti has a low solubility in Cu and Ti is a carbide-forming element frequently used. During infiltration, the melting Cu-Ti alloy serves as a storage to provide Ti source for the reaction Ti þ C ¼ TiC.

2.1.1

Fabrication of Cu/diamond composites

Cu-xTi alloy (x ¼ 0.3, 0.5, 2.0 wt%) bulks were vacuum induction melted using pure Cu (99.999 wt%) and pure Ti (99.99 wt%) as the starting materials. Synthetic singlecrystalline diamond particles (HHD90, 212e250 mm) were used as the reinforcement. The Cu-Ti alloys and the diamond particles were used to prepare Cu-Ti/diamond composites with the above-mentioned gas pressure infiltration technique. The infiltration was conducted at 1150
C for 30 min under an argon gas pressure of 1.0 MPa. The diamond content in the Cu-Ti/diamond composites was determined to be 61 vol% by measuring the sample density and assuming full densification of the composites.

2.1.2

Characterization of composite microstructure

Fig. 17.9 shows the microstructure of the prepared Cu-Ti/diamond composites. The diamond particles are uniformly distributed in the Cu matrix. Some diamond fracture

Interface tailoring and thermal conductivity enhancement in diamond

481

(b)

(a)

300 µm

100 µm

Fig. 17.9 Polished surface and fracture surface of the Cu-Ti/diamond composites. (a) polished surface and (b) fracture surface.

surfaces are observed since the synthetic diamond might have some defects. However, the diamond fracture surfaces are not observed in the unmodified Cu/diamond composite. This kind of transgranular fracture mode suggests good interfacial bonding in the Cu-Ti/diamond composites. Fig. 17.10 shows the phase structure of the Cu-Ti/diamond composites. Since the amount of added Ti element is small, diamond particles are also extracted from the composites by an electrochemical etching method [23] to perform XRD analysis in order to clearly inspect carbides on the diamond surface. From the XRD patterns of the composites, the formation of TiC is observed. The carbide peaks are more apparent in the XRD patterns of the extracted diamond particles. The result confirms the formation of TiC phase at Cu/diamond interface by the reaction of Ti atoms in the Cu matrix and C atoms on the diamond surface.

2.1.3

Characterization of Cu/diamond interface

To detect the existing state of TiC phase at Cu/diamond interface, we prepare thin foils using FIB from the Cu-Ti/diamond composites and then conduct TEM analysis. The results are shown in Fig. 17.11. At low Ti content (0.3 and 0.5 wt%), discrete interfacial carbides are divided by the Cu matrix. The carbides protrude from the diamond into the Cu matrix and carbide islands are formed in between Cu matrix. Since the carbides grow in-situ from the diamond surface into the Cu matrix, they exhibit a strong pinning effect and the Cu/diamond interface is strengthened. Nevertheless, the carbides become continuous at high Ti content (2.0 wt%) owing to the sufficient Ti source. In this case, the Cu matrix and the diamond particles are completely separated by the interfacial TiC layer. The morphological evolution of interfacial carbides is also observed in the Cu/diamond composites alloyed with other metallic elements. The TEM observation by Li et al. [30] indicates that when Zr content in the Cu matrix is more than 0.5 wt%, the ZrC at the Cu/diamond interface evolves from discrete particles into a continuous layer. Bai et al. [31] suggest a threshold value of 0.3 wt% in the Cu/diamond composites alloyed with boron. It serves as a common law for the interfacial carbide evolution in the Cu/diamond composites modified by metal matrix alloying.

482

Interfaces in Particle and Fibre Reinforced Composites

(a) (200)

0.3 wt% 0.5 wt% 2.0 wt%

30

(220)

(220)

(200)

(111)

(220)

Intensity (a.u.)

(111) (111)

Diamond TiC Cu

40

50

60

80

70

2 Theta (Degrees) 0.3 wt.% 0.5 wt.%

(111)

(b)

(220) (222)

(311)

(200)

(111)

Intensity (a.u.)

Diamond TiC

(220)

30

40

50 60 2 Theta (Degrees)

70

80

Fig. 17.10 XRD patterns of (a) the Cu-Ti/diamond composites and (b) the diamond particles extracted from the composites.

2.1.4

Thermal conductivity of Cu/diamond composites

The thermal conductivity of the Cu-Ti/diamond composites are displayed in Fig. 17.12. The thermal conductivity firstly increases to 0.5 wt% Ti and then decreases, giving a maximum thermal conductivity of 752 W/mK. The variation of thermal conductivity is closely related to the evolution of interfacial titanium carbides. This can be explained by the schematic illustration in Fig. 17.13. On one hand, the formation of the carbides improves the interfacial bonding and thus increases the thermal conductivity. More amount of discrete carbides could improve the interfacial bonding more effectively, and the maximum thermal conductivity is obtained at 0.5 wt% Ti. On the other hand, TiC has a low thermal conductivity of 21 W/mK

Interface tailoring and thermal conductivity enhancement in diamond

483

Fig. 17.11 Interface between diamond and Cu in the Cu-Ti/diamond composites. (a) 0.3 wt% Ti, TEM, (b) 0.5 wt% Ti, TEM, and (c) 2.0 wt% Ti, SEM.

[32], lower than both 400 W/mK of Cu and w2000 W/mK of diamond. A thick and continuous TiC layer will impair phonon transfer across the interface. Therefore, the thermal conductivity is reduced at 2.0 wt% Ti. This suggests that in order to increase the thermal conductivity of the Cu/diamond composites modified by metal matrix alloying, the spacing between discrete carbides at the interface should be as small as possible, but it cannot be so small that a continuous carbide layer is formed to reduce the thermal conductivity.

2.2

Modification of Cu/diamond interface by diamond surface coating

By coating carbide-forming elements onto diamond particle surface, the elements will react with carbon atoms on the diamond surface to form carbides at the interface. This is a more direct route to modify the Cu/diamond interface compared with the metal matrix alloying method. The reduction of matrix thermal conductivity by the alloying element is also alleviated. However, the uniform carbide layer covered on the diamond surface could decrease the composite thermal conductivity in the absence of the discrete interfacial carbides. As suggested above, the discrete interfacial carbides generated by the metal matrix alloying method could provide efficient thermal

484

Interfaces in Particle and Fibre Reinforced Composites 1200

Thermal conductivity (W/mk)

TC predicted by DEM model 1000

800

752

710

600

TC of pure copper

400

200

141

123

0 0.0 wt%

0.3 wt%

0.5 wt%

2.0 wt%

Fig. 17.12 Thermal conductivity of the Cu-Ti/diamond composites prepared by gas pressure infiltration.

Heat transport

(a)

TiC Diamond

Cu-0.5%Ti

Cu-0.3%Ti

(b)

Cu alloy Cu-2.0%Ti

RTic RTic RCuTi

Fig. 17.13 A schematic explaining thermal conductivity of the Cu-Ti/diamond composites. (a) schematic illustration of Cu-TiC-diamond transition region and (b) analogue of interfacial thermal resistance in the transition region.

transport paths. Furthermore, the carbides are not formed in-situ by the diamond surface coating method and the interfacial bonding could not be as strong as that by the metal matrix alloying method. To compare the two methods for modification of Cu/ diamond interface, we coat Ti onto diamond particle surface and then fabricate Cu/ Ti-diamond composites [18].

Interface tailoring and thermal conductivity enhancement in diamond

2.2.1

485

Fabrication of Cu/diamond composites

Synthetic single-crystalline diamond particles (MBD8, 150e180 mm) were used as the starting material. Various thicknesses of Ti layers were coated onto the diamond particle surfaces by magnetron sputtering using pure Ti (99.99 wt%) as the target material and Ar gas (99.995 vol%) as the sputtering gas. The coated diamond particles were infiltrated by pure Cu (99.99 wt%) with the above-mentioned gas pressure infiltration technique. The infiltration was conducted at 1150
C for 15 min under an argon gas pressure of 1.0 MPa. The Cu/Ti-diamond composites are thus prepared. The diamond content in the Cu/Ti-diamond composites was determined to be 61 vol% by measuring the sample density and assuming full densification of the composites.

2.2.2

Characterization of composite microstructure

Fig. 17.14 shows the microstructure of the coated diamond particles. Four thicknesses of 65, 220, 340, and 850 nm were obtained for the Ti-coating on the diamond surface. Except for the very thin 65 nm, the Ti-coating for the other three thicknesses is uniform. The XRD patterns in Fig. 17.14(e) and the HRTEM image in Fig. 17.14(f) show that the coating is metallic Ti. The SiO2 and Pt layers were artificially deposited during FIB milling in order to protect the Ti-coating layer from Gaþ bombardment. During the infiltration of Cu/Ti-diamond composites, the SiO2 and Pt layers are not deposited onto the Ti-coated diamond particles. After infiltration at high temperature, the Ti-coating has transformed from metallic Ti into TiC, as shown in the XRD patterns of the Cu/Ti-diamond composites in Fig. 17.15(a). The TiC peaks are more obvious in the XRD patterns of the extracted diamond particles (Fig. 17.15(b)) and the intensity of TiC peaks steadily increases with increasing Ti-coating thickness. This suggests that the reaction of Ti þ C ¼ TiC is completed during the infiltration process. Fig. 17.16 shows that the diamond particles are uniformly distributed in the Cu matrix. No defects such as voids or cracks are found and some Cu matrix residue are observed on the diamond surface in the Cu/Ti-diamond composite (Fig. 17.16(d)), as compared with the unmodified Cu/diamond composite (Fig. 17.16(c)). The result proves the effectiveness of diamond surface coating with Ti to improve the interfacial bonding of Cu/diamond composites.

2.2.3

Characterization of Cu/diamond interface

Fig. 17.17 shows the interface structure of the Cu/Ti-diamond composites. The EELS spectrum and EELS elemental mapping further confirm the formation of TiC at the interface. It is noted that the thickness of the coating remains almost unchanged after the coating transforms from metallic Ti into TiC. This suggests that Ti barely diffuses into the diamond particles or the Cu matrix during infiltration. The STEM annular bright field (ABF) images show that the diamond/TiC (Fig. 17.17(c)) and TiC/Cu (Fig. 17.17(d)) interfaces are atomically sharp and chemically bonded. The interfacial TiC layer exhibits a crystallographic orientation relationship of TiC[110]//diamond

486

Interfaces in Particle and Fibre Reinforced Composites

(a)

(b)

50 nm

200 nm

200 nm

(c)

(d)

200 nm

200 nm

20

30

(220)

(f)

40 50 60 2 Theta (Degrees)

(103)

(111)

(110)

d c b a

(102)

Diamond Ti (100) (002) (101)

Intensity (a.u.)

(e)

70

80

1 nm

Fig. 17.14 Characterization of the Ti-coated diamond particles produced by magnetron sputtering. (a)e(d) TEM bright field images of the Ti coating layer with different thicknesses, (e) XRD patterns of the Ti-coated diamond particles, and (f) HRTEM image of the coating area marked in (b).

[110] and TiC(111)//diamond(111) with the diamond surface, but has no typical crystallographic orientation relationship with the Cu matrix. Zhang et al. [16] have applied molten salt method to coat Cr3C2 onto diamond surface and produced Cu/Cr-diamond composites by vacuum hot pressing. The TEM observation of the Cu/diamond interface shows that Cr3C2 grows directly on diamond (100) surface but there is an amorphous carbon layer between Cr3C2 and diamond (111) surface, which is ascribed to different atom arrangements on both surfaces. Such an amorphous carbon layer is not observed in the Cu/Ti-diamond composites. This suggests the difference in coating element effect on Cu/diamond interface modification.

2.2.4

Thermal conductivity of Cu/diamond composites

Fig. 17.18 shows the thermal conductivity of the Cu/Ti-diamond composites. The thermal conductivity firstly increases to 220 nm-thick Ti coating and then decreases to

(111)

(a) Diamond

(220) (220)

(311)

(220)

(200)

(111)

(200)

TiC Cu Intensity (a.u.)

487

(111)

Interface tailoring and thermal conductivity enhancement in diamond

850 nm Ti coating 340 nm Ti coating 220 nm Ti coating 65 nm Ti coating

30

40 50 60 2 Theta (Degrees) (111)

(b)

70

Diamond TiC

80

(220)

20

(311)

(220)

(200)

(200)

(111)

Intensity (a.u.)

Cu

850 nm Ti coating 340 nm Ti coating 220 nm Ti coating 65 nm Ti coating

20

30

50 40 60 2 Theta (Degrees)

70

80

Fig. 17.15 Phase structure of (a) the Cu/Ti-diamond composites and (b) the diamond particles extracted from the composites by an electrochemical etching method.

850 nm-thick Ti coating. The maximum thermal conductivity of 811 W/mK is obtained at 220 nm-thick Ti coating. The variation of the thermal conductivity can be explained by the thickness of Ti-coating on diamond surface. Too thin a layer cannot cover all the diamond surface and some defects occur inevitably, as shown in Fig. 17.14(a). This case differs from the in-situ formation of discrete interfacial carbides by the metal matrix alloying method, which shows strong pinning effect at the interface. The defects will

488

Interfaces in Particle and Fibre Reinforced Composites

(a)

(b)

200 µm

(c)

200 µm

(d)

100 µm

100 µm

Fig. 17.16 Polished surface and fracture surface of the unmodified Cu/diamond (a and c) and Cu/Ti-diamond (b and d) composites for comparison.

introduce interfacial thermal resistance and thus reduce the composite thermal conductivity. With increasing Ti-coating thickness, the coating layer becomes uniform and interfacial defects are removed. So the thermal conductivity increases gradually to 220 nm-thick Ti coating. However, the thermal conductivity starts to decrease with further increasing Ti-coating thickness. Since TiC has a thermal conductivity much lower than both Cu and diamond, too thick a TiC layer will increase interfacial thermal resistance and reduce the composite thermal conductivity. As a result, the maximum thermal conductivity is attained at an intermediate thickness of 220 nm-thick Ti coating. This suggests that in order to increase the thermal conductivity in the Cu/diamond composites produced by diamond surface coating, the coating layer should be as thin as possible with a prerequisite that no defects exist in the coating. We have compared the results with literature, where different techniques are used to prepare Cu/Ti-diamond composites. By optimizing the Ti-coating layer to a proper thickness, we obtain so far the highest thermal conductivity of 811 W/mK among Cu/diamond composites reinforced with Ti-coated diamond particles. When comparing the two modification methods of metal matrix alloying and diamond surface coating, the derived Cu-Ti/diamond composite and Cu/Ti-diamond composite exhibit thermal conductivities at a similar level of w800 W/mK (Figs 17.12 and 17.18). Since multiple factors affecting composite thermal conductivity like interfacial carbide morphology, diamond quality, diamond particle size are intertwined, it is difficult to estimate which method is more effective to modify Cu/diamond interface. But it is clear that the formation of interfacial carbide structure is essential to maintain high thermal properties of Cu/diamond composites.

(b)

(c)

Intensity (a.u.)

Atomic ratio C: 48.13% Ti: 51.87% Ti - L2,3

C K-edge

250

300

350

400

450

500

550

600

Energy loss (eV)

(d)

(e)

489

Fig. 17.17 Interface between diamond and Cu in the Cu/Ti-diamond composites. (a) TEM bright field image, (b) EELS spectrum collected from the interface, (c) STEM ABF image and SAED pattern of the interface marked in (a), (d) STEM ABF image of the interface marked in (a), and (e) EELS elemental mapping of C and Ti at the interface marked in (a).

Interface tailoring and thermal conductivity enhancement in diamond

(a)

490

Interfaces in Particle and Fibre Reinforced Composites

(a) Thermal conductivity (W m–1 K–1)

1000

Cu/Ti-diamond composites

811 800 619 600

590

405 400

200

133

0 0

340 220 65 Thickness of Ti-coating layer (nm)

850

(b) Thermal conductivity (W m–1 K–1)

800

Ti modified Cu/diamond composites This study Amelioration method

Reference Diamond particle size µm 75 70

600

100 100 Ti-coating

70 105 110

400

100 165 230 Ti-alloying

300

Ti-coating & Ti-alloying

180

100

200 40

70 50 60 80 Volume fraction of diamond (%)

90

Fig. 17.18 Thermal conductivity of the Cu/Ti-diamond composites prepared by gas pressure infiltration. (a) thermal conductivity as a function of thickness of Ti-coating layer and (b) comparison with literature.

3.

Summary

We have prepared Al/diamond composites and Cu/diamond composites by gas pressure infiltration and investigated their interface structure by TEM analysis. In the Al/diamond composites, the interface is characterized by discrete interfacial carbides that are formed by the reaction of Al and diamond. The interfacial Al4C3 phase is significant to connect the diamond reinforcement and the Al matrix. Triangular and block Al4C3 particles are formed on diamond (100) and (111) surfaces, respectively.

Interface tailoring and thermal conductivity enhancement in diamond

491

The increase in amount of the carbides strengthens the interfacial bonding and improves the interfacial thermal conductance. A high thermal conductivity of 710 W/mK is obtained by optimizing the quantity of interfacial carbides in the Al/diamond composites. Interface modification is additionally needed to overcome the inherently weak bonding between Cu and diamond in the Cu/diamond composites. Two different methods of metal matrix alloying and diamond surface coating are used to introduce interfacial carbides to the Cu/diamond interface. Alloying of Ti element into the Cu matrix is proved to be an effective way to improve the thermal conductivity. The carbides protrude from the diamond into the Cu matrix and carbide islands are formed in between Cu matrix. The maximum thermal conductivity of 752 W/mK is obtained at 0.5 wt% Ti by tailoring the spacing between discrete interfacial carbides in the Cu-Ti/ diamond composites. Various thicknesses of Ti layers can also be successfully coated onto the diamond particle surfaces, and the formation of TiC at the interface was confirmed. The Cu/Tidiamond composites exhibit the maximum thermal conductivity of 811 W/mK at 220 nm-thick Ti coating by optimizing the thickness of continuous carbide layer. The study demonstrates the importance of interface engineering in improving thermal properties of metal matrix composites.

Acknowledgements The authors thank the financial support from National Natural Science Foundation of China and Ministry of Science & Technology of China.

References [1] C. Zweben, Thermal materials solve power electronics challenges, Power Electron. Technol. 32 (2006) 40e47. [2] Y. Yamamoto, T. Imai, K. Tanabe, T. Tsuno, Y. Kumazawa, N. Fujimori, The measurement of thermal properties of diamond, Diam. Relat. Mater. 6 (1997) 1057e1061. [3] P.W. Ruch, O. Beffort, S. Kleiner, L. Weber, P.J. Uggowitzer, Selective interfacial bonding in Al(Si)-diamond composites and its effect on thermal conductivity, Compos. Sci. Technol. 66 (2006) 2677e2685. [4] K. Yoshida, H. Morigami, Thermal properties of diamond/copper composite material, Microelectron. Reliab. 44 (2004) 303e308. [5] Z.Q. Tan, Z.Q. Li, G.L. Fan, X.Z. Kai, G. Ji, L.T. Zhang, D. Zhang, Diamond/aluminum composites processed by vacuum hot pressing: microstructure characteristics and thermal properties, Diam. Relat. Mater. 31 (2013) 1e5. [6] H.Y. Wang, J. Tian, Thermal conductivity enhancement in Cu/diamond composites with surface-roughened diamonds, Appl. Phys. A 116 (2014) 265e271.

492

Interfaces in Particle and Fibre Reinforced Composites

[7] K. Mizuuchi, K. Inoue, Y. Agari, Y. Morisada, M. Sugioka, M. Tanaka, T. Takeuchi, J. Tani, M. Kawahara, Y. Makino, Processing of diamond particles dispersed aluminum matrix composites in continuous solid-liquid co-existent state by SPS and their thermal properties, Compos. Part B 42 (2011) 825e831. [8] J. Grzonka, M.J. Kruszewski, M. Rosinski, L. Ciupinski, A. Michalski, K.J. Kurzydlowski, Interfacial microstructure of copper/diamond composites fabricated via a powder metallurgical route, Mater. Char. 99 (2015) 188e194. [9] W.S. Yang, G.Q. Chen, P.P. Wang, J. Qiao, F.J. Hu, S.F. Liu, Q. Zhang, M. Hussain, R.H. Dong, G.H. Wu, Enhanced thermal conductivity in diamond/aluminum composites with tungsten coatings on diamond particles prepared by magnetron sputtering method, J. Alloy. Compd. 726 (2017) 623e631. [10] Q.P. Kang, X.B. He, S.B. Ren, L. Zhang, M. Wu, C.Y. Guo, W. Cui, X.H. Qu, Preparation of copper-diamond composites with chromium carbide coatings on diamond particles for heat sink applications, Appl. Therm. Eng. 60 (2013) 423e429. [11] Z.F. Che, J.W. Li, L.H. Wang, Y.X. Qi, Y. Zhang, H.L. Zhang, X.T. Wang, J.G. Wang, M.J. Kim, Effect of diamond surface chemistry and structure on the interfacial microstructure and properties of Al/diamond composites, RSC Adv. 6 (2016) 67252e67259. [12] L. Weber, R. Tavangar, On the influence of active element content on the thermal conductivity and thermal expansion of Cu-X (X ¼ Cr, B) diamond composites, Scr. Mater. 57 (2007) 988e991. [13] E.A. Ekimov, N.V. Suetin, A.F. Popovich, V.G. Ralchenko, Thermal conductivity of diamond composites sintered under high pressures, Diam. Relat. Mater. 17 (2008) 838e843. [14] O. Beffort, F.A. Khalid, L. Weber, P. Ruch, U.E. Klotz, S. Meier, S. Kleiner, Interface formation in infiltrated Al(Si)/diamond composites, Diam. Relat. Mater. 15 (2006) 1250e1260. [15] S. Kleiner, F.A. Khalid, P.W. Ruch, S. Meier, O. Beffort, Effect of diamond crystallographic orientation on dissolution and carbide formation in contact with liquid aluminium, Scr. Mater. 55 (2006) 291e294. [16] H.D. Zhang, J.J. Zhang, Y. Liu, F. Zhang, T.X. Fan, D. Zhang, Unveiling the interfacial configuration in diamond/Cu composites by using statistical analysis of metallized diamond surface, Scr. Mater. 152 (2018) 84e88. [17] J.W. Li, H.L. Zhang, L.H. Wang, Z.F. Che, Y. Zhang, J.G. Wang, M.J. Kim, X.T. Wang, Optimized thermal properties in diamond particles reinforced copper-titanium matrix composites produced by gas pressure infiltration, Compos. Part A 91 (2016) 189e194. [18] L.H. Wang, J.W. Li, M. Catalano, G.Z. Bai, N. Li, J.J. Dai, X.T. Wang, H.L. Zhang, J.G. Wang, M.J. Kim, Enhanced thermal conductivity in Cu/diamond composites by tailoring the thickness of interfacial TiC layer, Compos. Part A 113 (2018) 76e82. [19] I.E. Monje, E. Louis, J.M. Molina, Optimizing thermal conductivity in gas-pressure infiltrated aluminum/diamond composites by precise processing control, Compos. Part A 48 (2013) 9e14. [20] J.M. Molina, Design of composites for thermal management: aluminum reinforced with diamond-containing bimodal particle mixtures, Compos. Part A 70 (2015) 45e51. [21] Z.Q. Tan, G. Ji, A. Addad, Z.Q. Li, J.F. Silvain, D. Zhang, Tailoring interfacial bonding states of highly thermal performance diamond/Al composites: spark plasma sintering vs. vacuum hot pressing, Compos. Part A 91 (2016) 9e19. [22] G. Ji, Z.Q. Tan, Y.G. Lu, D. Schryvers, Z.Q. Li, D. Zhang, Heterogeneous interfacial chemical nature and bonds in a W-coated diamond/Al composite, Mater. Char. 112 (2016) 129e133.

Interface tailoring and thermal conductivity enhancement in diamond

493

[23] I.E. Monje, E. Louis, J.M. Molina, Aluminum/diamond composites: a preparative method to characterize reactivity and selectivity at the interface, Scr. Mater. 66 (2012) 789e792. [24] Z.F. Che, Q.X. Wang, L.H. Wang, J.W. Li, H.L. Zhang, Y. Zhang, X.T. Wang, J.G. Wang, M.J. Kim, Interfacial structure evolution of Ti-coated diamond particle reinforced Al matrix composite produced by gas pressure infiltration, Compos. Part B 113 (2017) 285e290. [25] Y. Zhang, J.W. Li, L.L. Zhao, X.T. Wang, Optimisation of high thermal conductivity Al/ diamond composites produced by gas pressure infiltration by controlling infiltration temperature and pressure, J. Mater. Sci. 50 (2015) 688e696. [26] I.E. Monje, E. Louis, J.M. Molina, On critical aspects of infiltrated Al/diamond composites for thermal management: diamond quality versus processing conditions, Compos. Part A 67 (2014) 70e76. [27] W.L. Yang, J.Q. Sang, L.P. Zhou, K. Peng, J.J. Zhu, D.Y. Li, Overcoming selective interfacial bonding and enhancing thermal conductivity of diamond/aluminum composite by an ion bombardment pretreatment, Diam. Relat. Mater. 81 (2018) 127e132. [28] K. Chu, C.C. Jia, H. Guo, W.S. Li, On the thermal conductivity of Cu-Zr/diamond composites, Mater. Des. 45 (2013) 36e42. [29] S.B. Ren, X.Y. Shen, C.Y. Guo, N. Liu, J.B. Zang, X.B. He, X.H. Qu, Effect of coating on the microstructure and thermal conductivities of diamond-Cu composites prepared by powder metallurgy, Compos. Sci. Technol. 71 (2011) 1550e1555. [30] J.W. Li, X.T. Wang, Y. Qiao, Y. Zhang, Z.B. He, H.L. Zhang, High thermal conductivity through interfacial layer optimization in diamond particles dispersed Zr-alloyed Cu matrix composites, Scr. Mater. 109 (2015) 72e75. [31] G.Z. Bai, N. Li, X.T. Wang, J.G. Wang, M.J. Kim, H.L. Zhang, High thermal conductivity of Cu-B/diamond composites prepared by gas pressure infiltration, J. Alloy. Compd. 735 (2018) 1648e1653. [32] W. Williams, The thermal conductivity of metallic ceramics, JOM 50 (1998) 62e66.