Fabrication of reaction bonded TiB 2 /Si/SiC composites for thermal applications

Titanium diboride (TiB 2 ) is an extremely hard ceramic which has excellent heat conductivity, oxidation stability, and resistance to mechanical erosion. Broader ap-plication of this material is inhibited by economic factors, particularly the cost of densifying a material with a high melting point. In this study, reaction bonded TiB 2 (RB-TiB 2 ) composites are fabricated by the reactive infiltration of molten Si into preforms of TiB 2 plus carbon. Microstructure analysis indicates uniform distribution of TiB 2 particles in the composites. RB-TiB 2 composites with fine particles show higher flex strength and fracture toughness, while composites with larger particles have higher thermal conductivity, measured to be 120 W/(m K) at room. The coefficient of thermal expansion (CTE) of the composites is insensitive to the particle size and can be controlled in the range of 4.0-5.2 ppm/K (room temperature). RB-TiB 2 shows the potential to be used for electronics thermal management applications with a combination of matching CTE, good thermal conductivity, high melting point, and attractive mechanical properties.


| INTRODUCTION
Titanium diboride (TiB 2 ) is an extremely hard ceramic which has excellent heat conductivity, oxidation stability, and resistance to mechanical erosion. Applications for TiB 2 include impact resistant armor, cutting tools, crucibles, and wear resistant coatings. TiB 2 is also resistant to oxidation in air at temperatures up to 1100°C. 1 Many TiB 2 applications are inhibited by economic factors, particularly the costs of densifying a high melting point material (2970°C). Furthermore, titanium dioxide layers that form on the surfaces of the particles increase the resistance to sintering. Reaction bonded ceramic composites offer the potential of improved, reliable high-temperature properties with near net-shape processing. Reaction bonded ceramic composites consolidate by filling the void space within the green part with reaction product, rather than shrinking as occurs during conventional sintering, so sintering aids are not required. The absence of sintering aids in the grain boundaries gives superior high temperature and corrosion properties. Processing temperatures for reaction bonding are usually lower than those used in conventional sintering processes with no applied pressure, which reduces capital and operating costs. Moreover, fine reactive powders capable of being densified are not required, which reduces raw material cost. 2 One of the most studied reaction bonded ceramics is reaction bonded SiC, first developed

Abstract
Titanium diboride (TiB 2 ) is an extremely hard ceramic which has excellent heat conductivity, oxidation stability, and resistance to mechanical erosion. Broader application of this material is inhibited by economic factors, particularly the cost of densifying a material with a high melting point. In this study, reaction bonded TiB 2 (RB-TiB 2 ) composites are fabricated by the reactive infiltration of molten Si into preforms of TiB 2 plus carbon. Microstructure analysis indicates uniform distribution of TiB 2 particles in the composites. RB-TiB 2 composites with fine particles show higher flex strength and fracture toughness, while composites with larger particles have higher thermal conductivity, measured to be 120 W/(m K) at room. The coefficient of thermal expansion (CTE) of the composites is insensitive to the particle size and can be controlled in the range of 4.0-5.2 ppm/K (room temperature). RB-TiB 2 shows the potential to be used for electronics thermal management applications with a combination of matching CTE, good thermal conductivity, high melting point, and attractive mechanical properties.

K E Y W O R D S
composites, reaction bonding, silicon, thermal properties, titanium diboride | 265 in the 1950s. [3][4][5] Conventionally, the process consists of Si infiltration (liquid or vapor) into preforms of SiC + carbon. During the infiltration step, the Si and carbon react to form SiC. Typically, all carbon is consumed, yielding a product of porous SiC (vapor infiltration) or dense Si/SiC (liquid infiltration).
In this study, a variation to the process has been evaluated. Preforms of TiB 2 + carbon were produced and subsequently infiltrated with molten Si. The resultant ceramic bodies contained TiB 2 , Si, and a small amount of reaction formed SiC (ie, TiB 2 /Si/SiC) as compared to typical reaction bonded ceramics with a formulation of SiC/Si. One disadvantage of SiC/Si F I G U R E 1 A, Typical rheological property of TiB 2 slip; B, Schematic of Reaction Bonded Ceramics F I G U R E 2 XRD pattern of the RB-TiB 2 composite is a coefficient of thermal expansion (CTE) that is too low for most electronics packaging applications. At room temperature, the CTE of SiC/Si is nominally 3 ppm/K, 6 whereas most applications desire CTEs in the 6-7 ppm/K range for matching to DBC AlN dielectrics, Al 2 O 3 dielectrics, and borosilicate glass for glass/metal sealing. The traditional material used for electronics packaging in this CTE range is Kovar, which has a room temperature CTE of 5.9 ppm/K and a thermal conductivity of 17 W/m K. Kovar's CTE is attractive; however, its thermal conductivity is too low to sufficiently dissipate heat in high power applications. 7 Aluminum-silicon carbide (Al/ SiC) metal matrix composites (MMCs) are also interesting thermal management candidates with common varieties having CTEs of nominally 8 ppm/K and thermal conductivities of about 180 W/m K. The Al/SiC MMCs, however, do not have sufficient melting point to be suited to hermetic glass to metal sealing, and they have CTEs that are higher than desired such that costly design features, such as dome shapes, are required for reliability. 8,9 The structural properties of TiB 2 /Si/ SiC have been previously disclosed. 10 The present work focuses on thermal properties, with the goal of achieving CTEs in the range of Kovar with a higher melting point than Al/ SiC MMCs (for glass to meal sealing) and a favorable thermal conductivity for dissipating heat from high power electronics.  and RBTB-1H) contain 60 wt% 42 μm + 40 wt% 12 μm TiB 2 powders. Type II slips (RBTB-5 and RBTB-13) contain only the 12 μm TiB 2 powder. The slips were prepared by mixing TiB 2 powders with DI water, a polymeric carbon source results in about 5 wt% carbon, and a sulfonic acid derivative dispersant. The rheological properties of the slips were optimized for slurry casting. Figure 1A shows a typical shear-thinning viscosity vs shear rate curve of the TiB 2 slips. Slips were cast into rectangular molds and high packing density (>60 vol%) powder compacts were achieved. Next, carbonization process, typically at 600°C in an inert N 2 atmosphere, pyrolyzed the binder into small carbon particles on the TiB 2 powder surface. After de-molding, the preforms were ready for Si infiltration.

| Preform infiltration
The industrial infiltration process was used, which allows the manufacture of large, complex shapes with minimal or no machining. Components measuring up to 60″ × 60″ with infiltration depth as thick as 6″ can be manufactured by this process. As shown schematically in Figure 1B, reaction bonded TiB 2 is produced in a vacuum furnace (The Furnace Source) by the reactive infiltration of molten high purity (>99%) silicon into preforms (90 mm × 50 mm × 15 mm) containing TiB 2 particles and carbon. 11,12 The infiltration temperature was set to about 1500°C with a heating rate of 2°C/min and a holding time of 3 hours. During the infiltration process, reaction occurs between the Si and carbon phases, yielding SiC that effectively bonds the TiB 2 particles into an interconnected ceramic structure, while remaining liquid Si infiltrates the open space in the preforms. The result is a composite of TiB 2 particles, residual Si, and reaction formed SiC. RBTB-13-HSC was infiltrated with a silicon alloy (30% Al) with higher CTE and thermal conductivity to modify the thermal properties of the composites.

| Characterization
The samples infiltrated from different slips were sectioned by electrical discharge machining (EDM), ground, and polished with 3, 1, and 0.1 μm diamond suspensions. Microstructure of polished samples was observed using an OLYMPUS GX41 optical microscope. An X-ray diffractometer using CuK α radiation (λ = 1.54 Å) was used to determine the phase composition of the composites. The volume fraction of components in TiB 2 -SiC-Si composites was determined using the quantitative image analysis software (ImageJ, NIH) on at least 5 optical micrographs for each composite. Density of the composites was determined by the water immersion technique in accordance with ASTM Standard B 311. Elastic properties were measured by an ultrasonic pulse-echo technique following ASTM Standard D 2845. Flexural strength in four-point bending was determined following ASTM C1161-18 (SIZE B). Fracture toughness was measured using a four-point-bend-chevronnotch technique following ASTM C1421-01B (CONFIG D). At least 5 specimens from each composite were tested for flexural strength and fracture toughness measurements. Fracture surfaces from flex strength and fracture toughness tests were observed using a FEI Quanta 250 FEG field emission scanning electron microscope. Thermal expansion was measured in accordance with ASTM E288-17, Standard Test Method for Linear Thermal Expansion of Solid Materials with a pushrod dilatometer, using a NETZSCH model DIL 402C pushrod dilatometer. Thermal diffusivity and specific heat capacity were measured by the laser flash method using a NETZSCH LFA 467 HyperFlash™ instrument and the test method conform to ASTM E1461-13, "Standard Test Method for Thermal Diffusivity by the Flash Method."

| Microstructure and elastic properties
All composite samples show the same phase composition in the XRD patterns. Figure 2 shows the XRD pattern of RBTB-1H.

F I G U R E 6 (A) Thermal conductivity and (B) CTE of RB-TiB 2 composites
The infiltrated composite consists three phases, TiB 2 (JCPDF 85-2083), infiltrated Si (JCPDF 27-1402), and reaction formed SiC (JCPDF 72-0018). Figure 3A,B is microstructure images of RBTB-1L and RBTB-1H. The microstructure contains: (a) interconnected TiB 2 particles (white); (b) Si matrix (gray); (c) reaction formed SiC (dark gray); and (d) infiltration defects (black). As shown in Table 1, the composites have 65-66 vol% of uniformly distributed TiB 2 , ~4 vol% reaction formed SiC and small amount of infiltration defects (1.8 vol% for RBTB-1L and 1.6 vol% for RBTB-1H). RBTB-1H has more particle agglomeration that led to larger particle size after higher temperature exposure. Both RBTB-5S and RBTB-13-HSC used 12 μm small TiB 2 powder and have slightly higher volume fraction of TiB 2 (67-69 vol%) and reaction formed SiC (~5.5 vol%), with porosity of 3.0 and 3.4 vol%. High porosity in RBTB-5S and RBTB-13-HSC is possibly due to the not optimized infiltration parameters. Binary powder mixtures with different particle sizes usually can reach higher packing density those that of single size powder systems. 13 In this case, low wettability of the large TiB 2 powder with DI H 2 O caused slightly lower packing density in the slips and the preforms. More surface area of 12 μm TiB 2 preforms absorbed more carbon source that results in higher volume fraction of reaction formed SiC. Both RBTB-5S and RBTB-13-HSC have more infiltration defects (~3 vol%) with lower elastic modulus, due to the smaller particle size and internal channel size that make it difficult for an effective Si infiltration.

| Flex strength and fracture toughness (effects of particle size)
Flex strength and fracture toughness data of RBTB-1 and RBTB-5S are shown in Table 2. All three composites used pure silicon as infiltration material. RBTB-5S has higher flex strength (205 MPa vs 164 and 158 MPa) and fracture toughness (4.95 vs 4.7 and 4.52 MPa m 1/2 ) than those of RBTB-1L and RBTB-1H, due to the finer microstructure using only small size 12 μm TiB 2 powder. RBTB-1H has slightly lower flex strength and fracture toughness than those of RBTB-1L, due to the larger particle size. All flex strength and fracture toughness samples show complex fracture surfaces with a mixture of intergranular and transgranular fracture (Figures 4 and 5) The flex strength and fracture toughness surfaces of RBTB-5S are rougher, due to the finer TiB 2 particles. Larger surface feature size of RBTB-1H than that of RBTB-1L is also noticed, which is coincident with the lower flex strength and fracture toughness.

| Thermal conductivity and CTE
Thermal conductivity of the composites was measured by the laser flash method ( Figure 6A). RBTB-1L and 1H samples have higher thermal conductivity (120 and 114 W/(m K) at 25°C) than those of RBTB-5S and RBTB-13-HSC (95 W/(m K) at 25°C). An interface between materials with different elastic properties (~500 GPa for TiB 2 and ~160 GPa for Si) 14 will introduce phonon scattering, the smaller the particle size, the more interface areas per unit volume and the more phonon scattering is expected. 15,16 Therefore, thermal conductivity decreases for the fine grain size RBTB-5S and RBTB-13-HSC that were made with only 12 μm TiB 2 powder. Also, more infiltration defects exist in RBTB-5S and RBTB-13-HSC and those pores add additional travel paths for phonons to bypass the porosities that further reduce the thermal conductivities of the composites. Nevertheless, the thermal conductivity of all RB-TiB 2 composites is significantly higher than those of Kovar (17 W/(m K) at 25°C) and Al 2 O 3 (37.2 W/m K at 25°C) and can be further improved by optimizing particle size, infiltration parameters that can reduce the porosity. TiB 2 has higher CTE than that of Si (7.4 vs 2.6 ppm/K at 25°C). RBTB-1 and RBTB-5S have similar CTE profiles over the temperature range of 20-500°C, which is between 4 and 6 ppm/K ( Figure 6B). By using a higher CTE (8-9 ppm/K) 70/30 Si-Al alloy, the CTE of RBTB-13-HSC ranges from 5 to 7 ppm/K over the temperature range of 20-500°C, which is similar to the CTE of Kovar and Al 2 O 3 in the same temperature range and the average CTE of the composites is identical to that of the Al 2 O 3 (7.1 ppm/K), 17 over the range of 20-500°C. The higher thermal conductivity and similar CTE to those of currently used electronics packaging materials, such as Kovar and Al 2 O 3 , indicate that RB-TiB 2 composites can be applied for electronics packaging and thermal management applications. Moreover, owing to its high melting point of 1410°C, RB-TiB 2 is suited to glass to metal sealing. The electrical resistivity of the RB-TiB 2 composites can be controlled by the purity of the Si matrix. Table 3 summarizes the thermal properties of RB-TiB 2 developed in this study, in comparison with some typically used electronics packaging and sealing materials. RB-TiB 2 composites combine the advantages of high melting point and good thermal conductivity with similar CTE values.

| SUMMARY
Reaction bonded TiB 2 /Si/SiC ceramics were developed for thermal applications. RB-TiB 2 composites with fine TiB 2 (12 μm) powder exhibit higher flex strength and fracture toughness. RB-TiB 2 composites that contain large TiB 2 (42 μm) powder show higher thermal conductivity, while CTE is not sensitive to the powder size. CTE of RB-TiB 2 can be further increased by using high CTE Si-Al alloy. The RB-TiB 2 composites have CTE similar to those materials used in electronics packaging, sealing, and thermal management applications, while combining good thermal conductivity, attractive mechanical properties, and high melting point for potential applications in those areas.