Microstructural, Tribology, and Densification Studies of Spark Plasma Sintered Titanium Matrix Composites Reinforced with Silicon Carbide

Spark plasma sintered titanium matrix composites (TMCs) with varying SiC contents were fabricated at 900 o C, 150 o C/min, 30 MPa, and with 5 min of holding time, and were studied for structural, mechanical and tribology performances. The phase identification and microstructure analysis of the sintered specimens were examined using X-ray diffraction and scanning electron microscope equipped with EDS. The results indicate that influence of the varied SiC content dictate the physical properties of the sintered TMCs. The densification study showed that relative density was inversely related, while Vickers hardness value was directly proportional (238.84 Hv at 1 wt. % SiC and 361.81 Hv 6 wt. % SiC). The tribology study showed that both wear rate and coefficient of friction have inverse relationship while wear resistance has a direct trend with the composite reinforcement. Sample TiNiAl – 6 wt. % SiC, with the optimum composition had the best wear performance under the constant load of 20 N. This wear performance can be attributed to the good interfacial bond formed between the TiNiAl matrix and the SiC reinforcement in the developed composites contributed from the synergetic processing conditions of the SPS process.


Introduction
Novel materials with tailored properties evolve daily through composite technology. This makes the engineering use of materials more plausible for critical applications. Good temperature strength and improved tribological characteristics are the primary performance criteria for the selection of composites for most engineering applications, particularly in aerospace and automotive sectors. Primarily, titanium is considered an ideal aerospace material due to its high strength-to-weight ratio, lightweight, corrosion-resistant, and substantial thermal stability [1][2][3][4]. In the aerospace industry, titanium plays a leading role as a base material with the flexibility to combine with a significant number of metals (especially nickel and aluminium) and ceramics (oxides, carbides, or organic base) to form their alloys and composites [5][6][7][8]. This flexibility enables powder metallurgist to fabricate new titanium matrix-based materials with enhanced properties suitable for sustaining the system prone to mechanical vibration conditions, chemical species, and stringent temperature profiles.
Titanium matrix composites (TMCs) can be fabricated from monolithic titanium or its alloy, which serves as the base matrix for the infusion of second phase particles (reinforcements). Odetola et al. [9] established that the property profiles of TMCs can be easily upgraded through the right reinforcement match. Generally, the strength of composites is determined by the fabrication process, grain size (microns or nanoscales), composition (measuring the distribution of the weight percentage of the matrix phase compared to the reinforcement phase), and microstructure [10,11].
These dictate physical and mechanical properties of composite materials [11,13].
In TMCs fabrication, the use of discontinuous reinforcement as second-phase particles has gained the most recognition as low-cost sintering aids with the ability to enhance material efficiency. A good discontinuous reinforcement is such that is thermodynamically stable at sintering temperature, and insoluble in titanium or its alloy matrix. In terms of reinforcing materials, silicon carbide (SiC) has been identified as a promising sintering aid for enhancing strength-to-weight ratio, wear capacity, high temperature strength, and corrosion performance of TMCs [14][15][16][17].
In the production of particulate-reinforced composite, homogeneous distribution of the secondphase particles in the matrix is very vital for superior performance. Besides this, the reinforcing phase is also susceptible to agglomeration, which may deteriorate mechanical properties.
However, the difficulty of obtaining a clean matrix-reinforcement interface free from undesired secondary phases can be easily rectified with processing conditions that can minimize or eliminate common challenges such as porosity, segregation, agglomeration, de-bonding, grain pullout and atomic misfit. Spark plasma sintering is an exceptional powder metallurgy technique capable of bridging deficiency gap in the solid sintering of powders at elevated temperatures due to its remarkable processing conditions such as faster heating rates and lower temperature requirements at short holding times [18]. Therefore, difficult-to-sinter powder materials with very high or low melting points or excessive reactivity can easily be sintered with ease [4,13, 19-21].
In this work, spark plasma sintering was used to fabricate silicon reinforced titanium matrix composites (TiNiAl-SiC). Densification studies, as well as microstructural and mechanical characterization, were used to evaluate and determine property profiles in a variety of reinforcement compositions (1,3, and 6 wt. %). Densification studies are used to evaluate the quality of SPSed composites in terms of porosity level, as well as accompanied properties.

Materials
The powder combination chosen for fabrication are titanium, nickel, and aluminium (matrix supplied by TLS Technik, particle size of range 45-90 µm, and percentage purity of 99.9%) and silicon carbide (reinforcement supplied by F.J. Brodman & Co., L.L.C., approximate particle size  Table 1.

Powder preparation
The design of the titanium-based alloy matrix composites was done according to the compositions presented in Table 1. The successive powder batch was thoroughly mixed in a plastic container placed inside a Tubular Shaker Mixer (T2F) operated at a constant rotational speed of 72 rpm for duration of 12 h to attain homogenization.

Composite fabrication
The homogenized powder mixture was charged into ∅30 mm graphite die of the SPS HPD5, FCT Systeme GmbH at sintering conditions of 900 °C, 150 °C/min, 40 MPa and holding time of 5 min.
In addition to the control specimen, three different batches of TMCs were fabricated as shown in Table 1. For easy removal of sintered specimen and lowering of temperature gradient across the specimen, graphite sheets were used to demarcate the enclosed powders from direct contact with the die and also from the upper and lower punches as recommended by Shongwe et al. [22]. At completion of the sintering, the sintered specimens were sand blasted to get rid of graphite contaminations. Prior to any further characterization process, the relative densities of the sintered specimens were measured using Archimedes principle. The theoretical or bulk density of the specimens was calculated using the rule of mixtures based on the densities of raw powders according to equation 1 for control sample and specimens sintered at 1%, 3%, and 6% SiC contents respectively (see Table 1). .
represents relative density in ⁄ and .
represents theoretical density of bulk in ⁄ .

Microstructural characterization
Firstly, the specimens were sectioned, properly ground, and polished to reveal smooth mirror-like surface. This is followed by dipping into Nital reagent (100 ml ethanol in 1-10 ml HNO3) for 10-20 s prior to microstructural examination. The microstructures and the elemental compositions of the composites were examined with TESCAN scanning electron microscope equipped with an EDS. Also, the microstructures were also viewed under optical microscopy for further in-depth studies.

Mechanical characterization
The hardness values were evaluated on a Future-tech 700 microhardness testing machine using the Vickers hardness scale. The specimens were subjected to a 100 gf load with a dwell time of 10 s to evaluate the hardness. An average of seven different hardness indentations was computed for the hardness measure of each sintered specimen.

Densification study
Relative density is an index that illustrates densification progress of the as-sintered powder compacts as well as influences the mechanical performance of sintered specimen such as hardness.
The densification study was carried out by comparing the theoretical density of each specimen with their respective relative density using rule of mixtures. Spark plasma sintering has emerged as an efficient sintering technique to consolidate the powders to achieve density almost equal to theoretical density. Densification behavior is expected to be a function of linear shrinkage which in turn depends on the sintering temperature, pressure, time and current intensity.

Tribological study
The tribological characteristics of the control and composites specimens were studied using Universal Tribometer s/n RTEC 2441, USA. The wear test was conducted using samples prepared in form of discs Ø10mm and length of 10 mm under load of 20 N for 1000 s and at a speed of 5 Hz. The specimens were weighed initially against hard steel alloy of 350 mm.   Table 2. The highest content of SiC in the TiNiAl matrix is represented by Fig. 4 (a), which showed well developed grain boundaries with interlock SiC particulates. The EDS spectra in Fig. 4 (b) also confirmed the maximum abundance of the second-phase particles. This may be responsible for the highest hardness value recorded through strain hardening at the crowded interface between the matrix and the reinforcement phase.
Intermetallic like AlNi6Si3, Ti3SiC2, TiNiSi present in this microstructure further support the reason for the enhanced hardness at this optimum addition.    With subsequent increase in composition of SiC, the growth spread and interlocked mostly at the grain boundaries as shown in Fig. 5 (b). Also, in Fig. 5 (c), there was less pronouncement of the black flecks and numerous needle-like growth, which was as a result of active intermetallic compounds that have been formed. These intermetallic compounds are hard materials formed from the matrix and second phase particle. They gradually build up at the grain boundaries, thereby preventing dislocation movement and causing stain hardness.

Phase (XRD) Analysis
The XRD results for the developed TiNiAl -SiC composites are shown in Fig. 6. The spectra The confirmed presence of these intermetallics are responsible for the enhanced hardness profiles of the sintered specimens with increase in the wt. % composition SiC. At sintering temperature of 900 o C, these intermetallics were synergetically formed from the matrix elements (Ti, Ni, Al) and SiC. It is noteworthy that these intermetallic compounds have excellent match of properties viable for tailored applications that require high temperature strength, good corrosion resistance, and electrical integrity. Aside from the control sample, Ti3SiC2 intermetallic was present in XRD spetra of all the sintered composite speimens. Primarily, Ti3SiC2 is a hard carbide with dual properties of metal and ceramic, which makes it an exceptional material with better combination of thermal and mechanical property than monolithic titanium [23 -28]. Furthermore, it has a superior thermal shock resistance, higher temperature oxidation resistance, superior fracture toughness and better damage resilience compared to titanium.

Relative density and hardness
At a constant sintering temperature of 900 o C , relative density was inversely related while hardness value was directly proportional. Fig. 7 shows the variation of relative density and hardness with addition of SiC as a function of wt. % composition. It is obvious that the relative density of the sintered specimen decreased from 98.92% to 97.57% when the SiC content increased from 1% to 6%. This can be explained on the basis of residual pores generated within the matrix since higher activation energy is required for volume diffusion compared to grain boundary diffusion. The sintering was more of volume control than thermodynamically controlled and increase in SiC content at the constant temperature only permits active volume diffusion of grains. On the other hand, hardness enhancement may be attributed to homogeneous dispersion of particles, as well as increasing reinforcement particle content, which eventually lockup with the matrix at the grain boundary [29,30]. The lowest, medium, and highest hardness values are 238.84 Hv, 330.57 Hv, 361.81 Hv respectively. Therefore, there was tangible improvement in hardness of about 66% between minimum and optimum reinforcement.

Conclusion
In the developed titanium matrix composites (TMCs), the influence of the varied SiC content dictates the densification, microhardness, and tribological performance with the following deduced conclusions 1. Relative density was inversely related to the SiC content (98.92% at 1 wt. % SiC and 97.57% at 6 wt. % SiC). This can be explained on the basis of residual pores generated within the matrix since higher activation energy is required for volume diffusion than for grain boundary diffusion. The sintering being carried out at the same temperature of 900 o C was more of volume control than thermodynamically controlled. Increase in SiC content at this constant temperature only permits active volume diffusion of grains.
2. Vickers hardness value was directly proportional to SiC content (238.84 Hv at 1 wt. % SiC and 361.81 Hv 6 wt. % SiC). The hardness enhancement may be attributed to homogeneous dispersion of particles, as well as increasing reinforcement particle content, which eventually lockup with the matrix at the grain boundaries.
3. Wear rate and coefficient of friction have inverse relationship while wear resistance has a direct trend with the composite reinforcement, which introduced hard resisting particles into the TiNiAl matrix. Therefore, TiNiAl -6 wt. % SiC, which has the optimum composition had the best wear performance under the constant load of 20 N. This shows that increasing the SiC content is beneficial to reducing wear of the developed titanium matrix composites.
4. Titanium matrix composites have been successfully fabricated with outstanding property profiles, which are attributed to the good interfacial bond formed between the TiNiAl matrix and the SiC reinforcement in the developed composites contributed from the synergetic processing conditions of the SPS process.