Surface-Modified-CNTs/Al Matrix Nanocomposites Produced via Spark Plasma Sintering: Microstructures, Properties, and Formation Mechanism

Abstract

The aim of this work was to perform surface modification of carbon nanotubes (CNTs) in order to utilize them as reinforcements into aluminum matrix nanocomposites. The surface modification of carbon nanotubes was carried out through a three-stage process. At stage 1, mechanical crushing of Ti and MWCNTs was performed via ultrasonication and wet milling. Then, in stage 2, additional pristine MWCNTs were added into the crushed mixture of Ti/MWCNT via ultrasonication and wet-milling methods in different mixing ratios. The mixing ratio of crushed mixture to pristine MWCNT varied from 1:2 to 5:2. Finally, pressureless spark plasma sintering (SPS) was applied in stage (3) to coat and/or decorate the pristine MWCNTs with nanocrystalline titanium carbide (TiC). The surface-modified CNTs, thereafter, were used to prepare Al-matrix composites (AMCs)Aluminum matrix composite (AMC) through SPS method. First of all, the optimum mixing ratio, in which optimal thermal and mechanical properties can be achieved, was determined using composites containing 1 wt.{%} surface-modified CNTs. Changing the CNT content (within the range of 0.5--5 wt.{%}) in the fixed optimum mixing ratio (4:2) revealed the effect of surface-modified CNTs content on mechanical and wear properties of composite samples. It was demonstrated that 1.5 wt.{%} carbon additive brought about the best properties in composite samples. The microstructure of the modified samples was investigated using scanning (SEM) and transmission electron microscopy (TEM) while X-ray diffraction (XRD) was used to identify phase structures. In order to evaluate thermal properties of the modified CNTs, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out. TGA results revealed significant improvement in thermal stability of modified CNTs comparing the unmodified ones, especially in 4:2 mixing ratio. Raman spectroscopy was also conducted to identify the quality and the defects over CNTs structures. Furthermore, a formation mechanism was presented for carbide nanostructures in this study. The nature of surface-modified regions was characterized by selected area electron diffraction (SAED) and convergent beam electron diffraction (CBED) patterns. It was found that nanocrystalline TiCs grow epitaxially on the surface of the pristine carbon nanotubes with an orientation relationship of CNT (002)//TiC (200) at the interface. Carbide morphologies, depending remarkably on the energy level of carbon sources, were mostly TiC nanoparticles and a smaller fraction of TiC nanoblocks. Moreover, a little carbide nanotubes and nanorods were observed in 4:2 mixing ratio. Preferential sites for TiC nucleation, e.g., bamboo-like defects and unzipping sites, were thoroughly investigated by high-resolution electron microscopy studies. Mechanical properties of composite samples were evaluated by microhardness, nanoindentation, and compression test. In this regard, the optimal sample exhibited 176{%} and 71{%} increase in Vickers hardness, compared to pure Al and 1.5 wt.{%} unmodified-CNT/Al composite sample, respectively. In addition, 82{%} and 45{%} increase in nanohardness as well as 48{%} and 25.5{%} increase in Young's modulus was achieved for the mentioned modified sample compared to pure Al and 1.5 wt.{%} unmodified sample, respectively. In the case of tribological characteristics, reduction of around 68{%} and 61{%} in mean friction coefficient and reduction of about 88{%} and 84.5{%} in wear weight loss were obtained for 1.5 wt.{%} modified-CNT/Al composite, compared to the pure Al and 1.5 wt.{%} unmodified sample, respectively. According to surface modification mechanism, it was found that carbide-modified regions enhanced load-bearing efficiency of carbon reinforcements owing to linking the outer walls to inner walls, allowing the naturally inactive inner walls to contribute in load transfer phenomenon through TiC nanoparticles.