In situ carbon nanotube reinforcements in a plasma-sprayed aluminum oxide nanocomposite coating

doi:10.1016/j.actamat.2007.10.038

Acta Materialia

Volume 56, Issue 3, February 2008, Pages 571-579

https://doi.org/10.1016/j.actamat.2007.10.038 Get rights and content

Abstract

Carbon nanotubes (CNT) are potential reinforcements for toughening the ceramic matrix. The critical issue of avoiding CNT agglomeration and introducing CNT-matrix anchoring has challenged many researchers to improve the mechanical properties of the CNT reinforced nanocomposite. In the current work, dispersed CNTs are grown on Al₂O₃ powder particles in situ by the catalytic chemical vapor deposition (CCVD) technique. Consequently, 0.5 wt.% CNT-reinforced Al₂O₃ particles were successfully plasma sprayed to obtain a 400 μm thick coating on the steel substrate. In situ CNTs grown on Al₂O₃ shows a promising enhancement in hardness and fracture toughness of the plasma-sprayed coating attributed to the existence of strong metallurgical bonding between Al₂O₃ particles and CNTs. In addition, CNT tentacles have imparted multi-directional reinforcement in securing the Al₂O₃ splats. High-resolution transmission electron microscopy shows interfacial fusion between Al₂O₃ and CNT and the formation of Y-junction nanotubes.

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 V_H) 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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In situ carbon nanotube reinforcements in a plasma-sprayed aluminum oxide nanocomposite coating

Abstract

Introduction

Section snippets

CCVD growth of in situ CNTs

Chemical bonding of in situ CNTs with Al2O3 powder

Conclusions

Acknowledgements

Carbon

Carbon

Carbon

Ceram Int

Mater Sci Eng A

Scripta Mater

Acta Mater

Mater Today

Ceram Int

Nature Mater

Diam Rel Mater

Chem Phys Lett

Acta Mater

Mater Sci Eng B

Mater Sci Eng A

Biomaterials

Mater Sci Eng A

Chemical bonding of in situ CNTs with Al₂O₃ powder