A survey on spark plasma sinterability of CNT-added TiC ceramics

Highlights

• Role of CNT on the sinterability and characteristics of TiC ceramics was studied.

• TiC-CNT ceramics were produced by spark plasma sintering process at 1900 °C.

• CNT-agglomerated areas provided suitable place for precipitation of C from TiC.

Abstract

The objective of this study was to assess the impacts of carbon nanotubes (CNTs) incorporation on the mechanical characteristics, thermal conductivity, microstructure, and sintering behavior of titanium carbide (TiC) matrix ceramics. To this aim, two samples of TiC-CNTs and monolithic TiC were produced using the SPS process at 1900 °C. The results indicated the obvious deposition of carbon from the crystalline lattice of TiC in the undoped sample; however, the addition of CNTs hindered this phenomenon remarkably. Moreover, in the carbon-doped sample, the areas where CNTs were agglomerated provided suitable places for limited precipitation of carbon from TiC. As a result, no graphite flake was observed in the microstructure of the CNT-doped specimen. Additionally, the amount of residual amorphous TiO₂ in the final microstructure of carbon-doped ceramic was significantly declined by introducing CNTs to TiC. The thermal conductivity of 13.9 W/mK, the flexural strength of 413 MPa, and the hardness of 3016 HV_{0.1 kg} were obtained for the TiC-CNTs ceramic, which all of them were lower than those obtained for the monolithic TiC.

Introduction

Titanium carbide (TiC) has lately attracted many advocates thanks to providing physical-mechanical properties of ceramics, and the electronic characteristics of metals as well. High melting point, almost low density, high hardness, good corrosion and high abrasion resistance, low neutron absorption cross-section, and low friction coefficient to metallic parts are amongst the most significant features of this unique material [[1], [2], [3], [4], [5], [6]]. Such qualifications have made TiC a suitable candidate for many high-performance utilizations in different industries, including nuclear, defense, aerospace, and automotive. However, TiC possesses poor sinterability because of the low self-diffusion coefficient, along with the vigorous covalent bonding [[7], [8], [9], [10], [11], [12]]. Although the implementation of elevated sintering temperatures may fix the problem of poor sinterability, this solution can also decrease the mechanical characteristics of TiC thanks to the excessive grain growth at high temperatures [[13], [14], [15], [16], [17]]. Additionally, poor fracture toughness and low thermal shock resistance of this material, especially in the monolithic form, are two other issues that should be addressed too [[18], [19], [20], [21]]. In line with the previous investigations on TiC materials, both compositing and utilizing advanced sintering techniques could lead to noticeable improvements in the sintering behavior and various features of this outstanding ceramic [[22], [23], [24], [25]].

The implementation of some advanced sintering techniques, including reactive hot-pressing (RHP) and spark plasma sintering (SPS), can contribute to improving the sinterability of TiC [[26], [27], [28]]. Nowadays, the SPS process is being broadly used for manufacturing high-quality ceramic-based composites due to its combining aspects, namely short soaking time, external pressure, and approximately low working temperature [[29], [30], [31], [32], [33], [34], [35], [36]]. On the other hand, quite a few investigators have studied the effects of several ceramic dopants and metallic binders on the microstructural and mechanical-physical qualifications of TiC-based materials. Although metallic additives can significantly enhance the sinterability via the formation of a molten phase, they affect the high-temperature performance of such materials negatively [[37], [38], [39]]. Hence, as far as the applications at elevated temperatures are concerned, ceramic additives are regarded as better choices. Carbonaceous phases have invariably been amongst the frequent additives studied on different sintering systems by many researchers [[40], [41], [42]].

Fattahi et al. [43] scrutinized the TiC-graphite system under the SPS circumstances of 1900 °C, 10 min, 40 MPa. According to their results, incorporating nano-graphite could slightly improve the relative density of TiC, standing next to 97%. It was found that the introduction of graphite had a noticeable impact on the oxide removal of the TiC particles. As a result, the flexural strength of the ceramic increased by around 25%, standing at 633 MPa. In addition, Nguyen et al. [44] studied the influence of graphene on the mechanical aspects and microstructural development of the TiC-based materials. Both monolithic and graphene-doped ceramics reached roughly the same relative density, namely ~95.5%; however, graphene could marginally strengthen TiC by almost 5%. Moreover, graphene nano-platelets contributed to grain refining of the TiC matrix, obtaining a finer microstructure. Besides, Nguyen et al. [45] inspected the TiC-nano diamonds system, too, adding 5 wt% nano-sized diamond to the matrix. Based on this investigation, the graphitization of nano-diamond over the SPS process hindered the densification process to some extent, and as a result, the relative density of TiC-diamond ceramic dropped by almost 6% compared to the monolithic TiC. Such a phenomenon resulted in a decrease in both flexural strength and hardness of composite specimens. Nevertheless, the thermal conductivity of TiC was considerably enhanced (by almost 30%) thanks to the creation of a continual net of the graphitized nano-diamond at the grain boundary of the TiC matrix. Finally, it was reported that carbon black had remarkable impacts on the physicomechanical properties and sinterability of TiC [23]. In accordance with this research paper, the incorporation of 5 wt% carbon black to titanium carbide led to an approximately fully dense part, which manifests the advantageous role of carbon black in the sinterability of titanium carbide compared to other carbonaceous compounds. This improvement in the relative density, as well as the impact of carbon black in grain refining of TiC, led to increments in all flexural strength, thermal conductivity, and hardness of the TiC‑carbon black sample compared to the undoped ceramic.

The present study has tried to assess the role of CNTs in modifying the microstructure of the TiC matrix, as well as its impact on the mechanical and physical properties of TiC. To this aim, two samples of TiC-5 wt% CNTs and monolithic TiC were produced via the SPS method at 1900 °C and characterized using FESEM, XRD, and EDS. Apart from the microstructure and sintering behavior, some mechanical and physical properties of these two ceramics were also measured and compared to each other.