Materials Science and Engineering A 425 (2006) 94–106 Effects of variations in coating materials and process conditions on the thermal cycle properties of NiCrAlY/YSZ thermal barrier coatings Feng Tang a,∗ , Leonardo Ajdelsztajn a , George E. Kim b , Virgil Provenzano c , Julie M. Schoenung a a Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA b Perpetual Technologies, Montreal, Que., Canada H3E 1T8 c National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Received 23 October 2005; accepted 11 March 2006 Abstract Thermal cycle tests were conducted on a variety of thermal barrier coating (TBC) specimens with bond coats that had been prepared in different ways. Variables include: (1) different thermal spray processes (high velocity oxy-fuel (HVOF) spray and low pressure plasma spray (LPPS)), (2) different feedstock powder (gas-atomized and cryomilled), (3) the introduction of nano-sized alumina additives (particles and whiskers) and (4) with and without a post-spray vacuum heat treatment. The results show that the cryomilling of the NiCrAlY powder and the post-spray heat treatment in vacuum can both lead to significant improvement in the thermal cycle lifetime of the TBCs. The TBC specimens with LPPS bond coats also generally showed longer lifetimes than those with HVOF bond coats. In contrast, the intentional dispersion of alumina particles or whiskers in the NiCrAlY powders during cryomilling did not result in the further improvement of the lifetime of the TBCs. Microstructural evolution, including the thermally grown oxide (TGO) formation, the distribution of the dispersoids in the bond coat, the internal oxidation of the bond coat, the bond coat shrinkage during the thermal cycle tests and the reduction of the ZrO 2 in the top coat during the heat treatment in vacuum, was investigated. © 2006 Elsevier B.V. All rights reserved. Keywords: Thermal barrier coating (TBC); Thermal cycle test; Cryomilling; Heat treatment; Thermal spray; Dispersoid 1. Introduction Thermal barrier coatings (TBCs), consisting of an yttria- partially-stabilized zirconia (YSZ) top coat and a metallic bond coat, are applied on the surfaces of the hot-section components, such as transition ducts, combustor liners and first stage vanes, in gas turbine engines. In combination with cooling systems inside the hot-section components, TBCs enable the engines to operate at higher temperatures without raising the base metal tempera- tures, and thus enhance the operating efficiency of the engines [1]. At present, there are two methods most often used for deposit- ing the YSZ top coats: electron beam physical vapor deposition (EB-PVD) and air plasma spray (APS) [1]. EB-PVD TBCs (i.e., TBCs with EB-PVD top coats) generally provide longer ther- mal cycle lifetimes than those observed with APS TBCs as a ∗ Corresponding author. Tel.: +1 530 7529819; fax: +1 530 7529819. E-mail address: ftang@ucdavis.edu (F. Tang). result of a more strain tolerant structure in the top coat. How- ever, the APS TBCs offer lower thermal conductivity and lower processing costs, and thus they are still widely applied as thermal protection systems. Various efforts have been made to enhance the thermal per- formance of APS TBCs. It has been reported that top coat modi- fication can significantly improve the thermal cycle life of TBCs. For instance, it has been found that the thermal cycle life of APS TBCs can be enhanced about four-fold by modifying the surface of the YSZ top coats with a laser glazing technique [2]. A thermal barrier coating graded in porosity along the cross-section also showed improved thermal shock resistance [3]. Recently, Gell et al. suggested a new solution precursor plasma spray (SPPS) method for the deposition of YSZ coatings; TBCs with top coats deposited by such a method exhibited an average life of more than 2.5 times that of conventional APS TBCs [4]. Efforts to modify the bond coat, including the use of various thermal spray methods, post-spray treatments, grain size con- trol, oxide dispersoids and new alloy development, have also resulted in improvements in the thermal behavior. Various ther- 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.03.043