In situ carbon nanotube reinforcements in a plasma-sprayed aluminum oxide nanocomposite coating
Introduction
In order to tap the advantages of mechanical properties of carbon nanotubes (CNTs), the incorporation of dispersed CNTs into a matrix is quite a challenging task [1], [2], [3], [4]. The inherent tendency of CNTs to exist as agglomerates is attributed to the presence of strong surface forces that annihilate natural CNT dispersion [3], [5], [6]. Functionalization of CNTs, the sol–gel technique, hetrocoagulation, spray-drying and molecular mixing have proven to be effective methods of introducing CNT dispersion in nanocomposites [1], [2], [3], [4], [7], [8], [9]. Recently, much work has been done on toughening Al2O3 ceramic by CNT reinforcement [4], [9], [10], [11], [12], [13], [14], [15], [16]. Processes techniques such as hot pressing, chemical vapor deposition, spark plasma sintering and plasma spraying have been developed to synthesize ceramic–CNT nanocomposites [9], [10], [11], [12], [13], [14], [15], [16], [17]. CNT reinforcements have been shown to improve the bending strength and fracture toughness up to 10 and 300%, respectively [2], [15].
Here, CNTs are grown onto Al2O3 surface using the catalytic chemical vapor deposition (CCVD) process. Growth of CNTs throughout the Al2O3 powder creates a metallurgical bond between Al2O3 powder particles and CNTs. Consequently, in situ CNT-reinforced Al2O3 powder feedstock is plasma sprayed to achieve enhanced toughening by CNT anchors. It is emphasized that the length of in situ grown CNTs should be long enough to provide good anchoring, but should be short enough to avoid CNT entanglement. CNT entanglement can clog the plasma gun nozzle during plasma spraying or induce CNT agglomeration in the sprayed coating, which will nullify the objective. Hence, the challenges are to: (i) achieve CNT dispersion; (ii) retain undamaged CNTs in the final structure; and (iii) effectively reinforce the matrix by CNTs. In the current work, apart from uniform CNT dispersion and retention in the ceramic matrix, the approach of growing in situ CNTs has offered excellent anchoring at the CNT/matrix interface.
Section snippets
CCVD growth of in situ CNTs
Dense, sintered and crushed Al2O3 powder (15–45 μm, 99.5% + purity) is used as the starting material for growing CNTs as shown in Fig. 1a. Dense nature of micron-sized Al2O3 particles can produce plasma-sprayed coatings with higher density, when compared with that of spray dried nano powder agglomerate which contains 30–40% porosity [9]. Cobalt(II) nitrate hexahydrate (Co (NO3)2 · 6H2O, 98% + pure) reagent was used as a catalyst for growing carbon nanotubes. Cobalt catalyst (1 wt.%) was mixed with Al2O3
Chemical bonding of in situ CNTs with Al2O3 powder
A TEM image of CNTs grown in situ on an Al2O3 surface (Fig. 2) clearly shows that the originating CNTs are anchored onto the Al2O3 particle. It also must be noted that the CNTs are well dispersed onto the Al2O3 surface without any agglomeration. Other consolidation processes, such as hot pressing [13], extrusion [20] and spark plasma sintering [14], [15], [16], have used starting powder where CNTs are distributed in the ceramic matrix by mechanical mixing. The novelty of using CCVD in
Conclusions
In situ CNTs were successfully grown onto Al2O3 particles via the catalytic chemical vapor deposition (CCVD) technique. Consequently, plasma spraying of the Al2O3–0.5 wt.% in situ grown CNT powder elicited splat-reinforcement by CNT bridging. It was inferred that CNTs were short enough to avoid nozzle clogging during plasma spraying, but long enough to provide CNT bridging. The ideal CNT length of 0.5–1.5 μm depicted multi-CNT tentacle anchoring in enhancing the hardness (from 806 to 906 VH) and
Acknowledgements
K.B. and A.A. acknowledge the support of the Office of Naval Research under Grant N00014-05-1-0398. T.Z. and W.Z.L. acknowledge the support of the National Science Foundation under the career Grant DMR-0548061. S.S. acknowledge the support from ONR YIP N000140210591.
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