Original Research Paper Studies on nano-crystalline CoNiCrAlY consolidated by conventional and microwave sintering Prashant Sharma, Jyotsna Dutta Majumdar ⇑ Department of Metallurgical & Materials Engineering, India Institute of Technology Kharagpur, W. B. 721302, India article info Article history: Received 4 April 2015 Received in revised form 14 October 2015 Accepted 18 October 2015 Available online xxxx Keywords: Nano-crystalline Mechanical milling Microwave sintering Thermal expansion High temperature oxidation abstract The present study concerns mechanical milling of commercially available CoNiCrAlY powder to produce nanocrystalline CoNiCrAlY and its consolidation by conventional and microwave sintering techniques. Detailed studies were undertaken to understand the effect of processing history (conventional and microwave sintering), materials parameters (as-received and mechanically milled for different time), and process parameters (temperature and time of sintering) on the density, microstructures, composition and phase distribution, coefficient of thermal expansion and high temperature oxidation resistance property. The density of the consolidated product increases with increase in milling time; increase in temperature of sintering in both conventional sintered and microwave sintered samples, with a maximum density achieved due to microwave sintering. There is a significant reduction in coefficient of thermal expansion due to sintering with a maximum reduction occurred when sintering is conducted by microwave sintering. Isothermal oxidation study showed a significant improvement in oxidation resistance due to nano-crystallization with a maximum improvement observed for 48 h milled sample microwave sintered at 1100 °C for 48 h. Ó 2015 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. 1. Introduction CoNiCrAlY is a widely used bond coat applied on metallic substrate prior to application of ceramic top coat and aimed at increasing the high temperature oxidation resistance [1,2]. The microstructure of the bond coat plays a crucial role in determining the oxidation resistance or hot corrosion resistance properties of thermal barrier coating [2]. Though a large number of research studies have been undertaken to understand the effect of the composition ((Ni, Co)–CrAlY) on the oxidation resistance of the bond coat, however, the studies related to the effect of grain size on the oxidation behavior of the bond coat have not been studied extensively [3–5]. Ajdelsztajn et al. [6] studied the effect of nanocrystallization (by cryo-milling of NiCrAlY powder) on oxidation behavior of high velocity oxy-fuel (HVOF) spray depos- ited NiCrAlY coating. The oxidation behavior of the nanocrystalline coating showed the formation of thermodynamically stable slow growing a-Al 2 O 3 layer in contrast to spinels developed in the same coating using as-received (micro-crystalline) powder [6]. In an another study, it was observed that HVOF spraying of nano crystalline (developed by cryo-milling) CoNiCrAlY coating with improved oxidation resistance could be achieved by dissolution of b-NiAl and subsequent enrichment of grain boundary with Al leading to stable a-Al 2 O 3 phase formation along grain boundaries and on the surface [4]. It is also observed that oxidation of nanocrystalline NiCoCrAlY coating showed the formation of duplex oxide layers with the presence of Cr 2 O 3 , NiAl 2 O 4 , CoAl 2 O 4 on the surface and the a-Al 2 O 3 at the subsurface region. Surface polishing prior to oxidation suppressed the formation of mixed oxide layer on the top surface [5]. High energy ball milling induced nano- crystallization of NiCrAlY powder and its coating on Inconel 738 superalloy by cold spraying was reported to form a uniform oxide layer when post treated by shot peening during high temperature oxidation [7]. In all the above mentioned investigations, the effect of nano crystallization on morphology of coating and its oxidation behavior has been studied. However, the effect of milling parameter on the crystallite size and subsequently, its effect on characteristics and oxidation behavior has not been studied in details. Apart from spraying, evaluation of high temperature oxidation resistance of milled nano-crystalline powder is difficult as it is believed that milled powders are more reactive and they have a higher kinetics of grain growth during conventional sintering. In the past, compaction of nano-crystalline powders has been http://dx.doi.org/10.1016/j.apt.2015.10.007 0921-8831/Ó 2015 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. ⇑ Corresponding author. Tel.: +91 3222 283288. E-mail address: jyotsna@metal.iitkgp.ernet.in (J. Dutta Majumdar). Advanced Powder Technology xxx (2015) xxx–xxx Contents lists available at ScienceDirect Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt Please cite this article in press as: P. Sharma, J. Dutta Majumdar, Studies on nano-crystalline CoNiCrAlY consolidated by conventional and microwave sin- tering, Advanced Powder Technology (2015), http://dx.doi.org/10.1016/j.apt.2015.10.007