Appl Phys A (2009) 94: 861–870 DOI 10.1007/s00339-008-4990-4 Effect of laser melting on plasma-sprayed aluminum oxide coatings reinforced with carbon nanotubes Yao Chen · Anoop Samant · Kantesh Balani · Narendra B. Dahotre · Arvind Agarwal Received: 21 April 2008 / Accepted: 16 November 2008 / Published online: 12 December 2008 © Springer-Verlag 2008 Abstract The effect of laser melting on the microstructure and mechanical properties of plasma-sprayed aluminum ox- ide composite coating reinforced with 4 wt% multi-walled carbon nanotubes (CNTs) is reported. Laser-melted layer consists of dense, coarse columnar microstructure which is significantly different from plasma-sprayed coating that consists of splats and porosity. CNTs retained their original cylindrical graphitic structure after undergoing laser irradi- ation. Three dimensional heat flow model has been devel- oped to estimate temperature variation in the laser-melted composite layer. Laser-melted layers show an increase in the microhardness at the expanse of degradation of fracture toughness. Nanoindentation study indicates an increase in the elastic modulus and yield strength of the laser-melted layer which is attributed to dense microstructure with ab- sence of weak-bonding splats and porosity. PACS 79.20.Ds · 77.84.Lf · 73.63.Fg · 74.25.Bt 1 Introduction Aluminum oxide coating on the metallic substrates has at- tracted attention for a broad range of applications includ- ing high wear resistance, oxidation and hot corrosion resis- tance, heat and thermal shock resistance, thermal stability Y. Chen · K. Balani · A. Agarwal ( ) Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, USA e-mail: agarwala@fiu.edu Fax: +1-305-3481932 A. Samant · N.B. Dahotre Materials Science and Engineering, University of Tennessee at Knoxville, Knoxville, TN 37922, USA and electrical insulation [1, 2]. However, the natural brit- tleness is a major factor that limits its practical applica- tions [3–5]. Carbon nanotubes (CNTs) are known to pos- sess exceptional mechanical properties with tensile strength of 200 GPa, elastic modulus of the order of 1 TPa and fracture strain of 10–30% [6, 7]. Hence, CNTs are ex- pected to be ideal reinforcements in a metallic, ceramic and polymeric matrix to improve their mechanical proper- ties. It is reported that the fracture toughness of 10 vol.% CNT introduced into aluminum oxide was increased up to 24% compared to monolithic aluminum oxide [8]. Zhang et al. [9] fabricated a CNT/aluminum oxide nanocompos- ite by blending dispersed single-walled carbon nanotubes with nanocrystalline aluminum oxide powders, followed by spark plasma sintering (SPS) process. Fracture toughness improved three times higher than that of an unreinforced nanocrystalline aluminum oxide [9]. In our previous re- search [10], aluminum oxide composite coating reinforced with 4 wt% multi-walled carbon nanotubes has been suc- cessfully synthesized using plasma spray technique. The coatings displayed improved CNT dispersion and fracture toughness increases up to ∼43% when compared to coating without CNT. It is noted that plasma-sprayed coatings often suffers from the presence of porosity and weakly bonded splat structure which subsequently lead to degradation of mechanical properties [11]. Crystal structure of aluminum oxide is affected by the cooling rate experienced during ther- mal spray [12]. Higher cooling rates result in the formation of γ -aluminum oxide, whereas lower cooling rates cause formation of δ -aluminum oxide [13] and α-aluminum ox- ide [14]. Shrinkage associated with aluminum oxide phase transformation also contributes to the degradation of its me- chanical properties. Therefore, post-treatment such as laser melting could lead to improvement in mechanical properties via reconsolidation of plasma-sprayed microstructure.