Nanomechanical properties of hafnium nitride coating Yao Chen, a Tapas Laha, a,b Kantesh Balani a and Arvind Agarwal a, * a Department of Mechanical and Materials Engineering, Florida International University, 10555 West Flagler Street, EC 3464, Miami, FL 33174, USA b Department of Chemical Engineering and Materials Science, University of California, One Shields Avenue, Davis, CA 95616-5294, USA Received 10 January 2008; revised 7 February 2008; accepted 7 February 2008 Available online 20 February 2008 Nanomechanical properties of plasma-sprayed HfN coating with and without hot isostatic pressing (HIP) treatment were eval- uated using nanoindentation. For HIPed HfN coating, the elastic modulus (E) and yield strength increase whereas the hardness (H), H/E ratio and fraction of the elastic work decrease. HIPed HfN coating shows a larger pile-up around the indent as compared to as- sprayed HfN. HIPing causes densification and improvement in inter-splat bonding which subsequently lead to increase in nanome- chanical properties. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nitrides; Nanoindentation; Mechanical properties; Plasma spray Hafnium nitride is characterized by a high melting point, chemical inertness and relatively good oxidation resistance in extreme environments, and is referred to as ultra-high temperature ceramic (UHTC) [1–3]. Hf- based ceramics are expected to be promising materials for hypersonic flight, reusable orbital vehicles and sharp leading edges on future generations of reentry vehicles due to their high melting points and relatively good oxi- dation resistance in a propulsion environment [4]. Haf- nium nitride has been also investigated for potential applications as hard and wear-resistant coating and dif- fusion barriers [5]. Fabrication of UHTCs into useful shapes and components is a major challenge due to their ultra-high melting point and brittle behavior. Our previ- ous research has shown that Hf-based ceramics can be successfully synthesized as coatings and near net struc- tures by a vacuum plasma spraying (VPS) technique [6]. In the present study, nanomechanical properties of vacuum plasma sprayed HfN coatings have been evalu- ated using nanoindentation technique. Basic elastic–plastic responses of coatings can be characterized through tensile or bending tests [7]. How- ever, it is noted that such methods always require com- plicated sample fabrication, which is extremely difficult for UHTCs such as HfN [8]. Nanoindentation, which does not require complicated sample preparation, can record load and depth continuously throughout the indentation cycle. The load–depth curves can be then analyzed to determine hardness, elastic modulus and fracture toughness. Therefore, nanoindentation test has been widely used to analyze micromechanical re- sponse of brittle ceramic materials. Commercially available HfN powder (average size: 5 lm) with Hf addition (30 wt.%) was employed as the powder feedstock for synthesizing hafnium nitride cermet coating using vacuum plasma spray. Hf was added to pro- vide improved fracture toughness to HfN ceramic with some compromise in the high temperature properties. The details of the processing can be found elsewhere [6]. To obtain fully dense coating, hot isostatic pressing (HIP) was carried out on as-sprayed HfN coating. HIP treatment was performed at 1500 °C and 170 MPa in nitrogen atmosphere for 2 h. Nanoindentation tests were carried out using Hysitron Triboindenter (Hysitron, Min- neapolis, MN) with a diamond Berkovich indenter tip. A matrix of 7  7 indentations (49 indents) was made, spaced 12 lm apart, giving an overall area of 72 lm  72 lm. During nanoindentation of as-sprayed HfN, the load was applied at the rate of 25 lNs 1 up to a peak load of 1500 lN, where it was held for 2 s and then unloaded completely at negative rate of 25 lNs 1 . On the other hand, HIPed HfN coatings experienced a peak load 1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.02.012 * Corresponding author. Tel.: +305 348 1701; fax: +305 348 1932; e-mail: agarwala@fiu.edu Available online at www.sciencedirect.com Scripta Materialia 58 (2008) 1121–1124 www.elsevier.com/locate/scriptamat