Applied Surface Science 256 (2010) 6213–6218 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Microscopic observation of laser glazed yttria-stabilized zirconia coatings M.F. Morks a,∗ , C.C. Berndt a , Y. Durandet a , M. Brandt b , J. Wang a a IRIS, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia b School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Bundoora, Victoria 3083, Australia article info Article history: Received 24 February 2010 Received in revised form 29 March 2010 Accepted 29 March 2010 Available online 4 April 2010 Keywords: Thermal barrier coating YSZ Plasma spraying Laser glazing Microstructure Vickers hardness abstract Thermal barrier coatings (TBCs) are frequently used as insulation system for hot components in gas- turbine, combustors and power plant industries. The corrosive gases which come from combustion of low grade fuels can penetrate into the TBCs and reach the metallic components and bond coat and cause hot corrosion and erosion damage. Glazing the top coat by laser beam is advanced approach to seal TBCs surface. The laser beam has the advantage of forming a dense thin layer composed of micrograins. Plasma- sprayed yttria-stabilized zirconia (YSZ) coating was glazed with Nd-YAG laser at different operating conditions. The surface morphologies, before and after laser treatment, were investigated by scanning electron microscopy. Laser beam assisted the densification of the surface by remelting a thin layer of the exposed surface. The laser glazing converted the rough surface of TBCs into smooth micron-size grains with size of 2–9 m and narrow grain boundaries. The glazed surfaces showed higher Vickers hardness compared to as-sprayed coatings. The results revealed that the hardness increases as the grain size decreases. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Yttria-stabilized zirconia (YSZ) is a ceramic thermal barrier coat- ing that has come of age in high temperature application industries. Thermal barrier coatings (TBCs) have been developed to improve the efficiency of high temperature components in various gas turbines for aircraft propulsion, power generation, and marine propulsion, among the principal ones [1–7]. TBCs offer protection for underline metallic components from creep, oxidation and/or localized melting by insulating the metal from hot gases in the engine core. The state-of-the-art aerospace TBCs typically comprise yttria-stabilized zirconia (YSZ) ceramic coat over a NiCrAlY superal- loy bond coat. Yttria-stabilized zirconia (YSZ) coat has low thermal conductivity (2 W/m K for bulk and 1.5–2.05 for plasma-sprayed YSZ) and remains stable at high operating temperatures [8]. The bond coat is usually MCrAlY (M: Ni, Co or NiCo) layers deposited by vacuum plasma spraying (VPS) and high velocity oxy-fuel (HVOF) [9]. During service at elevated temperatures, oxygen transported through the top coat by diffusion through the connected poros- ity or ionic conduction causes the bond coat to oxidize, primarily forming Al 2 O 3 . This is usually termed the thermally grown oxide (TGO). Formation and thickening of this layer may contribute to TBCs failure [10–16]. ∗ Corresponding author at: Faculty of Engineering and Industrial Sciences, Indus- trial Research Institute Swinburne, 543-545 Burwood Rd, Hawthorn, Melbourne, Victoria 3122, Australia. Fax: +61 3 9214 5050. E-mail address: mhanna@swin.edu.au (M.F. Morks). Some studies focused on sealing the porosity in plasma-sprayed ZrO 2 -based ceramic coatings by laser surface glazing [17–25] and laser cladding [26], which produces a combination of a porous coating with a fully dense top layer. Laser treatment is usually asso- ciated with the propagation of cracks. Cracks in the ceramic layer parallel to the surface are very detrimental to TBC lifetime because of spallation, but vertical cracks are generally considered to be ben- eficial to accommodate the strain occurring during thermal cycling [27]. It was shown that the vertical cracks in coatings induced by laser treatment improved the thermal shock resistance [18,19,22]. Coatings resulting from in-situ laser remelting present a colum- nar dendritic structure, which is closed to the vertically segmented microstructure deriving from the electron beam-physical vapor deposition (EBPVD). So, in comparison to the lamellar structure resulting from thermal spray, the columnar structure probably per- mits a better thermomechanical resistance during thermal cycling [28]. Miller and Berndt [29] showed that laser glazing prevented the surface spalling of TBCs subjected to a high heat flux environment. Another study reported that laser glazing increased the thermal shock resistance of TBCs in diesel simulating conditions up to 900 ◦ C [30]. In hot gases environment, the penetration of sulfur, sodium and vanadium contaminants contained in many industrial low- quality fuels through the porous and microcracked TBCs may attack the underlying metallic components by hot corrosion mechanisms [31–33]. Hot corrosion results from the presence of salt contami- nants such as Na 2 SO 4 , NaCl and V 2 O 5 which form molten deposits that react with the protective oxide coatings [34,35]. Laser glazing is an approach to improve the hot corrosion resistance of zirconia- 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.03.143