Fabrication of TiCx-TiB2/Al Composites for Application as a Heat Sink

Metal matrix composites reinforced with ceramic particles have become the most attractive material in the research and development of new materials for thermal management applications. In this work, 40–60 vol. % TiCx-TiB2/Al composites were successfully fabricated by the method of combustion synthesis and hot press consolidation in an Al-Ti-B4C system. The effect of the TiCx-TiB2 content on the microstructure and compression properties of the composites was investigated. Moreover, the abrasive wear behavior and thermo-physics properties of the TiCx-TiB2/Al composite were studied and compared with the TiCx/Al composite. The compression properties, abrasive wear behavior and thermo-physics properties of the TiCx-TiB2/Al composite are all better than those of the TiCx/Al composite, which confirms that the TiCx-TiB2/Al composite is more appropriate for application as a heat sink.


Introduction
Heat sinks are used to dissipate the heat generated from electronic components during their working process and protect them from external damage and mechanical forces [1,2]. Accordingly, ideal heat sink materials should exhibit superior thermal conductivity (TC) to effectively dissipate the heat generated by the electronic components and high strength and good abrasive wear behavior to protect the electronic components [3,4]. In addition, the heat sink materials also need possess a suitable coefficient of thermal expansion (CTE) to avoid high thermal stresses generated at the boundary between the heat sink and electronic components [5,6].
Aluminum possesses a relatively high TC and low density, which makes it become a material commonly used as heat sinks [7,8]. However, the CTE of aluminum (~23ˆ10´6/˝C) is so high compared to those of semiconductor materials (~7ˆ10´6/˝C) that thermal stress and strain may occur at the boundary between them [9,10]. Aluminum matrix composites offer the possibility of tailoring the properties of aluminum by adding an appropriate reinforcement phase to meet the demands for low CTE in thermal management [11]. Wei et al. [12] fabricated the Al-20Si matrix composites reinforced with SiC particles by high pressure solidification and investigated the effects of solidification pressure and SiC volume fraction on the thermal expansion behavior. They concluded that the CTE of the composite decreases with increasing the SiC content and the high pressure solidification technique would be helpful to develop the SiC/Al-Si matrix composite as a packaging material. Zhang et al. [13] investigated the effect of a metalloid silicon addition on the thermal-physical properties of diamond/Al composites. They reported that the silicon addition would effectively increase the TC and decrease the CTE of the diamond/Al composites. It can be seen from the above-mentioned work that the reinforcing particles used in aluminum matrix composites for heat sink applications are mainly limited to SiC and diamond, which are usually introduced into the Al matrix by an ex situ method. The shortcoming is that the interface bonding between the reinforcement and the Al matrix in the composites fabricated by the ex situ method is usually imperfect, which limits the further improvement of the mechanical and thermo-physical properties of the composites. TiC x -TiB 2 particles offer the advantages of enhanced strength, fracture toughness and wear resistance over the monolithic constituent (TiC x or TiB 2 ). In particular, the TiC x -TiB 2 particle-reinforced metal matrix composites could be fabricated by an in situ method, which takes advantage of the cleaner particle-matrix interface and the better comprehensive properties. The TiC x -TiB 2 particle-reinforced Ni, Fe and Mg matrix composites were extensively investigated as engineering materials [14][15][16]. However, to the authors' knowledge, the research on the TiC x -TiB 2 particle-reinforced aluminum matrix composites has mainly focused on the study of the microstructure and reaction mechanism [17,18]. Investigation on the properties and heat sink application of the TiC x -TiB 2 /Al composite has never been reported.
For heat sink materials, the aim of adding reinforcements is to decrease their coefficient of thermal expansion (CTE). A low content addition of reinforcement cannot decrease the CTE sufficiently. So, aluminum matrix composites with low content reinforcement (<40 vol. %) are usually used as engineering materials. Those reinforced with high content particles (>40 vol. %) are used as electric packaging materials. However, if the content of the reinforcements is too high (>60 vol. %), the thermal conductivity (TC) and processing ability of the composites become worse. So, in this paper, the content of the reinforcement was selected as 40-60 vol. %. The TiC x -TiB 2 /Al composites were fabricated by the in situ method of combustion synthesis combined with hot press consolidation in the Al-Ti-B 4 C system, and the effect of the TiC x -TiB 2 content on the microstructure and compression properties of the composites was investigated. Moreover, the abrasive wear behavior and thermo-physics properties of the TiC x -TiB 2 /Al composite were studied and compared with the TiC x /Al composite. This will offer some guidance to fabrication and investigation for the aluminum matrix composite in a heat sink application, and even more to the other metal matrix composite in a heat sink application.

Fabrication of the TiC x -TiB 2 /Al Composites
The raw materials used were commercial powders of aluminum (99% purity,~47 µm), titanium (99.5% purity,~25 µm) and B 4 C (98% purity,~3.5 µm). The theoretical molar ratios of TiC x : TiB 2 in the aluminum matrix composites were predetermined to be 1:2 with 40, 50 and 60 vol. % volume fraction based on the following reaction [19]: The blended powders of the Al-Ti-B 4 C systems were sufficiently mixed in a stainless steel die by ball milling for 8 h at a low speed (~35 rpm). After that, the mixtures were cold pressed into cylindrical compacts of 28 mm in diameter and 30 mm in height, and then the compacts were put in a self-made vacuum thermal explosion furnace. When the temperature which was measured by Ni-Cr/Ni-Si thermocouples suddenly rose rapidly, indicating that the formation reaction of the ceramics was ignited, the sample was quickly pressed just when it was still hot and soft, and then was cooled down to the ambient temperature. In addition, for comparison, the sample of 50 vol. % TiC x /Al composite was also fabricated by the above method in a Al-Ti-C system.

Characterization
The phase constituent of the products was investigated by X-ray diffraction (XRD, Moldel D/Max 2500PC, Rigaku, Tokyo, Japan) with Cu K α radiation. The microstructure and worn abrasive surfaces were investigated by scanning electron microscopy (SEM, Moldel Evo18 Carl Zeiss, Oberkochen, Germany) and field emission scanning electron microscope (FESEM, JSM 6700F, JEOL, Tokyo, Japan).

Property Tests
Uniaxial compression tests were carried out under a servo-hydraulic materials testing system (MTS 810, MTS Systems Corporation, Minneapolis, MN, USA) with a strain rate of 1ˆ10´4 s´1. Micro-hardness of the composites was measured by a Vickers hardness tester (Model 1600-5122VD, Newage, Feasterville, PA, USA) using a static load of 2 N and a dwell time of 15 s. The sizes of the sample for the compression and micro-hardness tests are 3 mm in diameter and 6 mm in a height.
Abrasive wear tests were conducted on a pin-on-disk machine under loads ranging from 5 to 25 N with the Al 2 O 3 abrasive papers as counter face. The sizes of the sample for abrasive wear tests are 4 mmˆ4 mmˆ12 mm dimensions. The wear rate was defined as the volume loss suffered per unit sliding distance, and the volume loss is obtained from the ratio of weight loss to the density of the composites. The weight loss was measured using an electronic balance with a resolution of 0.1 mg, and the density (ρ) was measured by Archimedes' water-immersion method.
The thermal conductivity, defined as K, can be calculated from K = αρc [20,21], where α is thermal diffusivity, ρ is density and c is specific heat. The thermal diffusivity was measured using a laser flash method (NETZSCH LFA427, NETZSCH, Selb, Germany), and the specific heat was determined using Differential Thermal Analysis (DTA, SDT-Q600, TA, New Castle, DE, USA) under argon atmosphere.
The coefficient of thermal expansions of the composites were measured between 100˝C and 300˝C using a Dilatometer (NETZSCH DIL402C, NETZSCH, Selb, Germany) at heating and cooling rates of 5 K/min under argon atmosphere. Each result presented here is an average of three distinct experiments. The sizes of the measured samples were 4 mmˆ4 mmˆ25 mm. Figure 1 shows the X-ray diffraction results for the composites reinforced with various TiC x -TiB 2 contents. The products in these samples are mainly Al, TiC x and TiB 2 phases, without any intermediate phases that can be detected. It indicates that the pure in situ dual-reinforcement TiC x -TiB 2 /Al composites could be successfully fabricated by the method of combustion synthesis and hot press consolidation. In addition, as the Al content decreases, the content of the synthesized TiC x and TiB 2 would be increased consequently, which is consistent with the XRD results as shown in Figure 1 where the intensities of TiC x and TiB 2 peaks increase with the decrease in the Al content. Figure 2a-c shows the SEM images of the etched surfaces of the TiC x -TiB 2 /Al composites with 40, 50 and 60 vol. % TiC x -TiB 2 particles, respectively. As shown in Figure 2a-c, the composites have no appreciable porosity and the synthesized ceramic particles present a uniform distribution in the aluminum matrix. Figure 2d-f shows the FESEM images of the morphology of the extracted TiC x and TiB 2 particles from the composites with various ceramic contents, respectively. As indicated, the TiB 2 particles formed in these composites are in the typical hexagonal prismatic or rectangular shape, while the TiC x particles are in the spherical shape. As is known, the combustion temperature increases with the synthesized ceramic content [18,22], and the growth of the ceramic particles is an exponential function of the combustion temperature [23]; that is, the higher the combustion temperature is, the bigger the size of the ceramic particles will be. So, with the ceramic content increase from 40 to 60 vol. %, the size of TiB 2 particles increases from 500 nm to 1 m, and the size of TiC x particles increases from 1 µm to 3 µm.   Figure 2d-f shows the FESEM images of the morphology of the extracted TiCx and TiB2 particles from the composites with various ceramic contents, respectively. As indicated, the TiB2 particles formed in these composites are in the typical hexagonal prismatic or rectangular shape, while the TiCx particles are in the spherical shape. As is known, the combustion temperature increases with the synthesized ceramic content [18,22], and the growth of the ceramic particles is an exponential function of the combustion temperature [23]; that is, the higher the combustion temperature is, the bigger the size of the ceramic particles will be. So, with the ceramic content increase from 40 to 60 vol. %, the size of TiB2 particles increases from 500 nm to 1 m, and the size of TiCx particles increases from 1 μm to 3 μm.     Figure 2d-f shows the FESEM images of the morphology of the extracted TiCx and TiB2 particles from the composites with various ceramic contents, respectively. As indicated, the TiB2 particles formed in these composites are in the typical hexagonal prismatic or rectangular shape, while the TiCx particles are in the spherical shape. As is known, the combustion temperature increases with the synthesized ceramic content [18,22], and the growth of the ceramic particles is an exponential function of the combustion temperature [23]; that is, the higher the combustion temperature is, the bigger the size of the ceramic particles will be. So, with the ceramic content increase from 40 to 60 vol. %, the size of TiB2 particles increases from 500 nm to 1 m, and the size of TiCx particles increases from 1 μm to 3 μm.

Compression Properties
Besides heat dissipation, the other functions of the heat sink are used to protect electronic components from external damage and mechanical forces. Thus, heat sink materials also need to possess high strength and good abrasive wear behavior. Based on this consideration, the compression properties and abrasive wear behavior of the fabricated composites were studied in this section and the next section, respectively. Figure 3 shows the compression engineering stress-strain curves of the TiC x -TiB 2 /Al composites with various TiC x -TiB 2 contents, and the compression properties and hardness of the composites are summarized in Table 1. The yielding strength (σ 0.2 ), ultimate compression strength (σ UCS ) and hardness (Hv) of the composites increase with the increase in the content of TiC x -TiB 2 particles, while the fracture strain (ε f ) decreases. Usually, the compression strength of the composites is mainly controlled by the content of the reinforcing particles. Thus, the σ 0.2 and σ UCS of the composites increase with the increase in the content of the TiC x -TiB 2 particles. with various TiCx-TiB2 contents, and the compression properties and hardness of the composites are summarized in Table 1. The yielding strength (σ0.2), ultimate compression strength (σUCS) and hardness (Hv) of the composites increase with the increase in the content of TiCx-TiB2 particles, while the fracture strain (εf) decreases. Usually, the compression strength of the composites is mainly controlled by the content of the reinforcing particles. Thus, the σ0.2 and σUCS of the composites increase with the increase in the content of the TiCx-TiB2 particles.        Figure 5 shows the variation in ware rate with the applied loads (5-25 N) for the 50 vol. % TiCx-TiB2/Al composite and the compared sample composite (50 vol. % TiCx/Al) tested against the Al2O3 abrasive particles of 10 μm. It is apparent that the wear rate of the two composites increases  Figure 5 shows the variation in ware rate with the applied loads (5-25 N) for the 50 vol. % TiC x -TiB 2 /Al composite and the compared sample composite (50 vol. % TiC x /Al) tested against the Al 2 O 3 abrasive particles of 10 µm. It is apparent that the wear rate of the two composites increases with the increase in the applied loads. The reason is that the penetration depth of the abrasive particles into the surface of the composites increases with the increase in the applied load, leading to the reinforcing particles being more easily spalled [25].

Abrasive Wear Properties
properties. So, we thought the most suitable content of TiCx-TiB2 is 50 vol. % for the fabrication and application of TiCx-TiB2/Al composite. The yielding strength (σ0.2), ultimate compression strength (σUCS) and hardness (Hv) of the 50 vol. % TiCx/Al composite fabricated in our previous work are 468 MPa, 714 MPa and 216.5 Hv, respectively [24]. The σ0.2, σUCS and hardness of the 50 vol. % TiCx-TiB2/Al composite are 315 MPa, 350 MPa and 81.3 Hv higher than those of the 50 vol. % TiCx/Al composite, respectively. Figure 5 shows the variation in ware rate with the applied loads (5-25 N) for the 50 vol. % TiCx-TiB2/Al composite and the compared sample composite (50 vol. % TiCx/Al) tested against the Al2O3 abrasive particles of 10 μm. It is apparent that the wear rate of the two composites increases with the increase in the applied loads. The reason is that the penetration depth of the abrasive particles into the surface of the composites increases with the increase in the applied load, leading to the reinforcing particles being more easily spalled [25]. Another observation in Figure 5 is that the 50 vol. % TiCx/Al composite experiences an extremely high wear rate compared to the 50 vol. % TiCx-TiB2/Al composite under all applied loads. It indicates that the Al2O3 abrasives can penetrate easily into the 50 vol. % TiCx/Al composite during sliding, resulting in excessive material removal from the surface. This can also be confirmed by the worn surfaces of the composites which are shown in Figure 6a,b. As can be seen in Figure 6a, the Al2O3 particle has deeply penetrated into the 50 vol. % TiCx/Al composite, which could lead to the extensive deformation and fracture in the surfaces of the 50 vol. % TiCx/Al composite. The worn surfaces of the 50 vol. % TiCx-TiB2/Al composite shown in Figure 6b are relatively smoother than that of the 50 vol. % TiCx/Al composite. As discussed above, the hardness of the TiCx-TiB2/Al composite is higher than that of the TiCx/Al composite. Thus, the depth of the abrasive particles penetrating into the TiCx/Al composite would be deeper than that of the TiCx-TiB2/Al composite. Another observation in Figure 5 is that the 50 vol. % TiC x /Al composite experiences an extremely high wear rate compared to the 50 vol. % TiC x -TiB 2 /Al composite under all applied loads. It indicates that the Al 2 O 3 abrasives can penetrate easily into the 50 vol. % TiC x /Al composite during sliding, resulting in excessive material removal from the surface. This can also be confirmed by the worn surfaces of the composites which are shown in Figure 6a,b. As can be seen in Figure 6a, the Al 2 O 3 particle has deeply penetrated into the 50 vol. % TiC x /Al composite, which could lead to the extensive deformation and fracture in the surfaces of the 50 vol. % TiC x /Al composite. The worn surfaces of the 50 vol. % TiC x -TiB 2 /Al composite shown in Figure 6b are relatively smoother than that of the 50 vol. % TiC x /Al composite. As discussed above, the hardness of the TiC x -TiB 2 /Al composite is higher than that of the TiC x /Al composite. Thus, the depth of the abrasive particles penetrating into the TiC x /Al composite would be deeper than that of the TiC x -TiB 2 /Al composite. According to these results, we presume that the reinforcing particles with different sizes and shapes possesses better properties effective in acting as a barrier to reduce the cutting efficiency of the Al 2 O 3 abrasive particles and the plastic deformation of the aluminum matrix during wear. Thus, the abrasive wear resistance of the TiC x -TiB 2 /Al composite is better than that of the TiC/Al composite. According to these results, we presume that the reinforcing particles with different sizes and shapes possesses better properties effective in acting as a barrier to reduce the cutting efficiency of the Al2O3 abrasive particles and the plastic deformation of the aluminum matrix during wear. Thus, the abrasive wear resistance of the TiCx-TiB2/Al composite is better than that of the TiC/Al composite.

Thermo-Physical Properties
For heat sink materials, a high thermal conductivity (TC) and a low coefficient of thermal expansion (CTE) are often necessary for electronic packaging applications. The TC of TiC/Al and TiCx-TiB2/Al composite in the range of 30 °C to 100 °C is shown in Figure 7a. This temperature range was selected to fit the working temperature range for electronic packaging materials. The TC of the

Thermo-Physical Properties
For heat sink materials, a high thermal conductivity (TC) and a low coefficient of thermal expansion (CTE) are often necessary for electronic packaging applications. The TC of TiC/Al and TiC x -TiB 2 /Al composite in the range of 30˝C to 100˝C is shown in Figure 7a. This temperature range was selected to fit the working temperature range for electronic packaging materials. The TC of the two composites decreases marginally with the increase in temperature up to 100˝C, and during the whole temperature range, the TC of the TiC x -TiB 2 /Al composite is higher than that of the TiC x /Al composite. The TiC x -TiB 2 /Al composite had a thermal conductivity of 160 W¨m´1¨˝C´1 at the temperature of 30˝C, which, though lower than that of Al alloy (200 W¨m´1¨˝C´1), is significantly higher than of the TiC x /Al composite (100 W¨m´1¨˝C´1).
The CTE is expressed as the change in the dimensions of the material as a function of the temperature, which is desirable for electronic packing applications. In the present study, the coefficients of thermal expansion of the TiCx-TiB2/Al and TiCx/Al composites are 9.3 × 10 −6 /°C and 9.6 × 10 −6 /°C at the temperature of 100 °C, which is much smaller than that of Al alloy (~23 × 10 −6 /°C) [9]. This sufficient reduction of CTE indicates a perfect interface bonding between the particles and the matrix. Moreover, the CTE of the TiCx-TiB2/Al composite was about two-fifths of that of the Al alloy, which indicates that the TiCx-TiB2 particle filler effectively constrains the thermal expansion of the matrix. Moreover, during the whole range of 100 °C-300 °C, the CTE of the TiCx-TiB2/Al composite is totally lower than that of the TiCx/Al composite. This indicates that the dual-reinforcement TiCx-TiB2 particles are a promising filler to lower the CTE of electronic packaging materials.  Table 2 summarizes the thermo-physical properties of the aluminum matrix composites usually used in heat sink applications. It can be seen that the thermal conductivities of the fabricated aluminum matrix composites reinforced with different reinforcements are in the range of The CTE is expressed as the change in the dimensions of the material as a function of the temperature, which is desirable for electronic packing applications. In the present study, the coefficients of thermal expansion of the TiC x -TiB 2 /Al and TiC x /Al composites are 9.3ˆ10´6/˝C and 9.6ˆ10´6/˝C at the temperature of 100˝C, which is much smaller than that of Al alloy (~23ˆ10´6/˝C) [9]. This sufficient reduction of CTE indicates a perfect interface bonding between the particles and the matrix. Moreover, the CTE of the TiC x -TiB 2 /Al composite was about two-fifths of that of the Al alloy, which indicates that the TiC x -TiB 2 particle filler effectively constrains the thermal expansion of the matrix. Moreover, during the whole range of 100˝C-300˝C, the CTE of the TiC x -TiB 2 /Al composite is totally lower than that of the TiC x /Al composite. This indicates that the dual-reinforcement TiC x -TiB 2 particles are a promising filler to lower the CTE of electronic packaging materials. Table 2 summarizes the thermo-physical properties of the aluminum matrix composites usually used in heat sink applications. It can be seen that the thermal conductivities of the fabricated aluminum matrix composites reinforced with different reinforcements are in the range of 120-321 W¨m´1¨˝C´1 and the coefficients of thermal expansion are all larger than 10ˆ10´6/˝C. Thus, the TiC x -TiB 2 /Al composite fabricated in this work with the TC of about 160 W¨m´1¨˝C´1 and CTE of 9.3ˆ10´6/˝C would satisfy the heat sink application requirements in electronic packaging.

Conclusions
The TiC x -TiB 2 /Al composites have been successfully fabricated by combustion synthesis and hot press consolidation in an Al-Ti-B 4 C system. The ceramic particles distribute uniformly in the composites and their sizes decrease with the increasing Al content. The compression strength and micro-hardness of the composites increase with the increasing TiC x -TiB 2 content. The incorporation of TiC x and TiB 2 particles can act as a barrier to effectively protect the Al matrix from plastic deformation and destructive action, leading to the lower wear rate compared with the TiC x /Al composite. Moreover, the thermo-physical properties of the TiC x -TiB 2 /Al composites are more promising compared to the TiC x /Al composite. The combination of a TC of about 160 W¨m -1¨˝C-1 and a CTE of 9.3ˆ10 -6 /˝C was achieved. The TE of the Al matrix would be restricted by the TiC x -TiB 2 particles via the interfacial restriction effect. They are close to that of an Si chip and can meet the CTE requirements in electronic packaging well. The investigation demonstrated that the excellent mechanical and thermo-physical properties of TiC x -TiB 2 /Al composites make them ideal candidate materials for electronic packaging applications.