Synthesis of TiAlN based coating on mild steel substrate using combined laser/sol-gel technique M. Sundar ⁎, A.M. Kamara, P.T. Mativenga, L. Li Manufacturing and Laser Processing Group, School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, Manchester, M60 1QD, United Kingdom abstract article info Article history: Received 18 June 2009 Accepted in revised form 26 January 2010 Available online 10 February 2010 Keywords: Hard Coating Laser Sol-Gel Steel Titanium Aluminium Nitride In this work a novel method for synthesising a titanium aluminium nitride (TiAlN) based coating is reported. This was achieved by scanning a high-power diode laser over the surface of slurry of aluminium hydroxide and titanium hydroxide sol-gels, urea and graphite, which was uniformly spread over a mild steel substrate. The laser processing parameters giving rise to thermal conditions suitable for the synthesis of the coating were chosen through predictions from chemical thermodynamics analysis and computational fluid dynamics (CFD) modelling. The surface morphology and microstructure of the synthesised coatings were examined using optical microscopy, scanning electron microscopy and X-ray diffraction. The analyses revealed the formation of an inter-layer in the coating/substrate interface. This inter-layer is an indication that metallurgical diffusion took place across the coating/substrate interface and suggests an enhancement in coating/substrate interface bonding. The investigation demonstrated the potential of the laser/sol-gel process to deposit coatings with super-adhesive properties and good hardness and surface finish. © 2010 Elsevier B.V. All rights reserved. 1. Introduction TiAlN coatings exhibit unique properties such as low thermal conductivity and high wear resistance. At elevated temperatures, they retain relatively high hardness with good thermal and chemical stability [1,2]. Such properties are typically required in the protection of dies and moulds, which are often required to operate at high temperatures; and in cutting tools, where the coatings are used to extend tool life and enhance machining performance [3,4]. At high temperatures, TiAlN coatings form a protective Al 2 O 3 layer [5–7], which provides better thermal insulating properties compared to the base TiAlN coating and majority of other coatings. For these reasons, TiAlN base coatings are of choice for cutting tools in most dry and high speed machining operations. The synthesis of this very important TiAlN coating is normally undertaken by deposition techniques that include chemical vapour deposition (CVD) [8] and physical vapour deposition (PVD) [9]. Though these deposition techniques are extensively used in industries for the synthesis of these coatings, reservations have been held by some researchers regarding their coating/substrate adhesion [10], as influ- enced by factors such as difference in material properties of the coating and the substrate and the absence of a good coating and substrate inter- layer. Poor coating adhesion compromises tool life in machining. While coating adhesion has been improved for some processes [11,12] there is still a need to develop better adhesion compared to the majority of coating processes. The sol-gel method has been widely used for the synthesis of amorphous or crystalline oxides and was reported to be effective in enhancing coating corrosion and oxidation resistance [13,14]. The process allows the coating of complex geometries while avoiding the expensive cost associated with PVD/CVD equipment. However, disadvantages of conventional sol-gel method includes the formation of cracks in thicker coatings [15], which is a result of stress generated due to shrinkage and thermal expansion mismatch during gel drying and its subsequent heat treatment. Another limitation is poor bonding between the coating and the substrate [16], which similar to other conventional deposition techniques is a result of the absence of an inter-layer along the coating/substrate interface. Compared to the conventional sol-gel method, laser enhanced sol- gel surface coating or hybrid laser/sol-gel coating, utilises a laser power as a heat source to synthesise the sol-gel coatings. The combined process offers advantage of fast and well controlled heat input. Recent investigations [17,18] reported the effectiveness of the laser/sol-gel technique for the deposition of TiN coating. The work reported in the current paper is focused on exploring the capabilities of the laser/sol-gel method for synthesising TiAlN coating. Chemical thermodynamics analysis and computational fluid dynamics (CFD) simulation were used to aid the prediction of process conditions used in the experiments for the synthesis of this coating. 2. Thermodynamics analysis of TiAlN formation process The formation of TiAlN is commonly achieved through the use of titanium and aluminium as precursors in appropriate reactions. Surface & Coatings Technology 204 (2010) 2539–2545 ⁎ Corresponding author. Tel.: +44 1613063828; fax: +44 1613063755. E-mail address: Sundar.Marimuthu@manchester.ac.uk (M. Sundar). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.01.037 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat