Effect of c-BN Size and Content on the Self-Propagating High-Temperature Synthesis of c-BN Composites Bonded with Ti-Al-C System Multiphase Products

Introduction

Ceramic-bonded c-BN tools possess high strength, high grinding efficiency and low thermal expansion coefficient [1,2]. Ceramic-bonded c-BN wheels, as one of the rapidly developed superhard grinding materials, have been applied in various engineering applications including in the machining of cemented carbide, metal ceramics, ferrite and sapphire. The self-propagating high-temperature synthesis (SHS) method is interesting because of its advantages such as very short processing time, low energy requirement and high-purity products [3,4]. Diamond tools have been fabricated by SHS method [5–9]. The SHS systems include Ti-Al-C-diamond, Ti-B-diamond, Ni-Al-Cu-Ti-diamond and Ni-Al-diamond. However, few studies have reported on the SHS fabrication of c-BN abrasive tools.

MAX phase ceramics, such as Ti₂AlC, have both metallic and ceramic properties [10,11]. These materials are relatively ductile and machinable. They also possess low density, good electrical and thermal conductivities as well as excellent anti-oxidation and thermal shock resistance. Thus, MAX phase ceramics have good potential as carrier-bonded materials for c-BN. We fabricated Ti₂AlC- and Ti₃AlC₂-bonded polycrystalline c-BN tools by SHS technology in our previous study [12]. This result shows that the diamond bonds well with Ti₂AlC or Ti₃AlC₂ matrix.

In this paper, we report the fabrication of a c-BN material bonded with Ti-Al-C system multiphase products using the SHS method. The effect of c-BN size and content on the fabrication process was investigated.

Experimental procedure

Powders of Ti (99.5% pure, ~53 μm), Al (99.6% pure, ~53 μm) and graphite (99% pure, ~53 μm) were mixed in a mole ratio calculated from 2Ti/Al/C. Then, c-BN powders (W20; 170/200, 120/140 and 80/100 mesh) were added in the above mixed powders. These powders were ball-milled in a planetary ball mill (XM-4×05) for 5 h using agate balls to achieve homogeneous mixed mixtures. The rotation speed was set to 50 rpm to avoid alterations in the c-BN size. Finally, the mixtures were cold-pressed into 4 mm × Φ13 mm bars. The samples were mounted in a container, in which argon was continuously supplied to provide an inert environment. The combustion reactions were conducted in a CO₂ laser. The laser was operated at an output power of 2 kW and beam diameter of 8 mm. The temperature was measured using a high-temperature infrared radiation thermometer (Model EC8300). The X-ray diffraction (XRD) experiments were performed in a rotating anode X-ray diffractometer (Rigaku Ultima IV) with Cu Kα radiation. Scanning electron microscopy was conducted using a VEGA3 TESCAN microscope equipped with an energy-dispersive spectroscope (EDS).

Results and discussion

Figure 1 shows the typical temperature profile recorded from the combustion of the Ti/Al/C/c-BN sample. The maximum combustion temperature reached 1,977°C. The high combustion temperature evidently enhanced the synthesis of Ti-Al-C system multiphase products.

Figure 1:

A typical temperature profile recorded from the combustion of the Ti/Al/C/c-BN reactants.

The XRD patterns of the samples with different c-BN (120/140 mesh) contents obtained by SHS are shown in Figure 2. The main product phases of the sample were Ti₂AlC, Ti₃AlC₂, Al₃Ti, TiN, TiC, AlN, graphite and TiB₂. c-BN evidently participated in the reaction of the Ti-Al-C system and formed some related compounds. The possible reactions were as follows:

Figure 2:

The XRD patterns of the samples that contained different c-BN contents obtained by SHS.

The c-BN peaks were relatively weaker at low c-BN content (10%, 20% and 30%). However, the c-BN peaks became very strong when the c-BN content reached 40% and 50%. Additionally, Al peaks appeared and the titanium aluminium carbide peak became weak in the samples containing 40% and 50% c-BN. These results show that c-BN sufficiently reacted with the matrix element when its content was lower than 40% in the raw materials. Moreover, the addition of excess c-BN may inhibit the synthesis of titanium aluminium carbide and lead to insufficient reaction of the raw materials.

These results indicate that c-BN reacted with Ti and Al to some extent. The addition of different amounts of c-BN obviously affected the reaction process and result of the Ti-Al-C system.

Figure 3 shows a fracture micrograph of the synthesized sample containing 20% c-BN (120/140 mesh). A dense transition layer with a thickness of approximately 10 µm presented the interface between c-BN and the matrix (Figure 3(a)). The transition layer was composed of TiN, AlN and TiB₂, which is in accordance with the XRD result. The interface between c-BN and the matrix was well-knit, indicating that c-BN bonds well with the matrix in this sample (Figure 3(b)). The matrix (Figure 3(c)) was basically composed of lamellar Ti₂AlC or Ti₃AlC₂ grains and particle TiC grains, which were identified by EDS. The Ti₂AlC or Ti₃AlC₂ grains had approximate lengths of 10 µm to 20 µm. TiC grains had an average size of approximately 2 µm. Figure 4 shows a fracture micrograph of the synthesized sample that contained 0% c-BN. The c-BN was unequally enwrapped by one coating (Figure 4(a)), and many particles with varied sizes were dispersed in the coating (Figure 4(b)). The coating had a thickness of approximately 2 µm. The XRD and EDS results show that the coating was composed of Al, TiN, TiB₂ and AlN. Additionally, the matrix was composed of some titanium aluminium carbide and amounts of particles (Figure 4(b)). Notably, the Ti₂AlC or Ti₃AlC₂ grains became obviously fine.

Figure 3:

Fracture micrograph of the synthesized sample containing 20% c-BN (120/140 mesh). (a) low latitude morphology, (b) the interface morphology between c-BN and the matrix, (c) the morphology of matrix.

Figure 4:

Fracture micrograph of the synthesized sample containing 50% c-BN (120/140 mesh).(a) low latitude morphology, (b) the surface morphology of c-BN,(c) the morphology of matrix.

Thus, the c-BN additive amount obviously affected the reaction of Ti-Al-graphite-c-BN systems. The primary reasons are as follows.

The SHS adiabatic temperature of 2Ti/Al/C system was approximately 1,600–1,800°C [11]. The transform temperature of c-BN to h-BN was usually 1,400°C. Some amounts of c-BN may transform to h-BN because of the extremely short reaction time (2–3 s) of SHS. Therefore, the B and N elements on the surface of c-BN reacted with Ti and Al to form the TiB₂, AlN and TiN phases, which ensured a good bond between c-BN and Ti₂AlC matrix. However, this phenomenon led to the shortage of Al and Ti, which reacted with graphite. Some graphite cannot react, and residue was found in the products. Hence, the composition of the raw materials deviated from the stoichiometric ratio of Ti₂AlC phase to some extent. Finally, the Ti₂AlC peaks in the bonded samples were slightly weaker than that of the samples without c-BN [11].

However, at 40% and 50% c-BN content, the good heat conduction of c-BN played a key role, that is, it apparently decreased the adiabatic temperature of the system. When c-BN content was lower than 40%, c-BN reacted adequately with the Ti and Al. The reaction degree was reduced with increasing c-BN content. Therefore, complete c-BN particles were observed in the sample that contained 50% c-BN (Figure 4(a)). Excessive c-BN addition (40% or higher) seriously decreased the adiabatic temperature, which inhibited the reaction synthesis of Ti-Al-C system. This phenomenon caused the incomplete reaction of the raw materials. Consequently, Ti₂AlC peaks obviously became weak, and Al peaks appeared in the samples that contained 40% and 50% c-BN.

Hence, addition of different c-BN contents affected not only the reaction characteristic of the Ti-Al-C system, but also the bonding condition between c-BN and the matrix. Considering the good bond of c-BN with the matrix in the sample that contained 10% c-BN, 10% c-BN addition was used in the following experiment.

Figure 5 illustrates the XRD patterns of the samples containing 10% c-BN with different particle sizes obtained by SHS. The product phase of the sample was Ti₂AlC, Ti₃AlC₂, Al₃Ti, TiN, TiC, AlN, graphite and TiB₂ in the products. Ti₂AlC and Ti₃AlC₂ peaks in the samples containing coarser c-BN were much stronger, which indicates that Ti₂AlC and Ti₃AlC₂ contents were higher in the samples containing coarser c-BN. However, when 20 µm c-BN was added, Al peak appeared, and the titanium aluminium carbide peaks vanished. The main phases of this sample were TiC, TiN, AlN and TiB₂. These results show that the c-BN size obviously affects the phase composition of the matrix. Finer c-BN size may inhibit the synthesis of Ti₂AlC.

Figure 5:

The XRD patterns of the samples that contained different c-BN (10%) particle sizes obtained by SHS.

Figure 6 shows a fracture micrograph of the synthesized sample containing 20 µm c-BN. The sample was basically composed of granules. The c-BN particles were difficult to find in its microstructure. Figure 7 shows a fracture micrograph of the sample containing 20 µm c-BN. The c-BN particles were closely inserted in the matrix (Figure 7). A transition layer with a thickness of 2–5 µm was formed on the surface of c-BN (Figure 6). The layer had a good bond with c-BN and the matrix. Ti₂AlC and Ti₃AlC₂ lathed grains were found near the c-BN. Figure 8 shows a fracture micrograph of the sample containing 125 µm c-BN, which shows a morphology that was very similar to that of the 120/140 mesh c-BN (Figure 3). c-BN also bonded well with the matrix.

Figure 6:

Fracture micrograph of the synthesized sample that contained 20 µm c-BN.

Figure 7:

Fracture micrograph of the synthesized sample containing 170/200 mesh c-BN.

Figure 8:

Fracture micrograph of the synthesized sample containing 80/100 mesh c-BN the morphology of (a) c-BN and (b) the matrix.

Thus, the addition of different c-BN particle sizes apparently affects the SHS process. Under high-temperature SHS reaction, c-BN with fine particle size can fully transform to h-BN. Therefore, finer c-BN particles more adequately participate in the reaction with the matrix element. Consequently, the addition of coarse c-BN particle can yield the Ti₂AlC main phase matrix, and c-BN can adequately react with Ti and Al to form TiN, AlN and TiB₂ accordingly. Obviously, fine c-BN was not suitable as the additive for these composites.

Conclusions

c-BN materials bonded with Ti-Al-C system multiphase products can be obtained by SHS method. The main product phases were Ti₂AlC, Ti₃AlC₂, Al₃Ti, TiN, TiC, AlN, graphite and TiB₂. The addition of various c-BN particle sizes and contents evidently affects the SHS process between c-BN and matrix. The following conclusions could be drawn:

1. At 10–30% c-BN (120/140 mesh) content, the matrix was mainly composed of Ti₂AlC, Ti₃AlC₂, Al₃Ti and TiC. A dense transition layer with a thickness of approximately 10 µm appeared as the interface of c-BN in these samples. However, Al appeared and titanium aluminium carbide content decreased in the samples containing 40% and 50% c-BN. c-BN was unequally enwrapped by the coating with a thickness of approximately 2 µm.

2. When 20 µm c-BN was added, c-BN adequately reacted with Ti and Al to form TiN, AlN and TiB₂. The synthesis of titanium aluminium carbide was inhibited. Addition of coarser c-BN particles (80/100, 120/140 and 170/200 mesh) yielded a dense transition layer in the samples. Additionally, the main phase matrix of titanium aluminium carbide was also obtained.