The β to α phase transition of tantalum coatings deposited by modulated pulsed power magnetron sputtering Sterling Myers a, ⁎, Jianliang Lin a , Roberto Martins Souza b , William D. Sproul a, c , John J. Moore a a Advanced Coatings and Surface Engineering Laboratory (ACSEL), Department of Metallurgical & Materials Engineering, Colorado School of Mines, Golden, CO 80401, USA b Surface Phenomena Laboratory, Department of Mechanical Engineering, Polytechnic School of the University of Sao Paulo, Sao Paulo, Brazil c Reactive Sputtering, Inc., 2152 Goya Place, San Marcos, CA 92078, USA abstract article info Article history: Received 6 July 2012 Accepted in revised form 25 October 2012 Available online 2 November 2012 Keywords: Modulated pulsed power (MPP) magnetron sputtering High power pulsed magnetron sputtering (HPPMS) Tantalum coatings Phase transition Thick coating Tantalum coatings are of particular interest today as promising candidates to replace potentially hazardous electrodeposited chromium coatings for tribological and corrosion resistant applications, such as the internal lining on large-caliber gun barrels. Tantalum coatings have two crystalline phases, α-Ta (body-centered-cubic) and β-Ta (metastable tetragonal) that exhibit relatively different properties. Alpha-Ta is typically preferred for wear and corrosion resistant applications and unfortunately, is very difficult to deposit without the assistance of substrate heating or post-annealing treatments. Furthermore, there is no general consensus on the mechanism which causes α or β to form or if there is a phase transition or transformation from β →α during coating depo- sition. In this study, modulated pulsed power (MPP) magnetron sputtering was used to deposit tantalum coat- ings with thicknesses between 2 and 20 μm without external substrate heating. The MPP Ta coatings showed good adhesion and low residual stress. This study shows there is an abrupt β →α phase transition when the coat- ing is 5–7 μm thick and not a total phase transformation. Thermocouple measurements reveal substrate temper- ature increases as a function of deposition time until reaching a saturation temperature of ~388 °C. The importance of substrate temperature evolution on the β →α phase transition is also explained. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Tantalum (Ta) thin films have been of importance since the early 1960s for their use in fabricating resistors and capacitors [1]. Tanta- lum is of particular interest today because it is a strong candidate to replace electrodeposited (ED) chromium coatings often used for var- ious tribological and corrosion resistant applications. The substitution of these coatings is warranted because the associated ED chromium wastes contain hexavalent chromium, a known carcinogen and envi- ronmental hazard [2]. Recently, thick Ta coatings have been consid- ered to be used as the replacement of ED chromium coatings for the internal lining on large-scale gun barrels [3]. Tantalum films exhibit two crystalline phases, Alpha-Tantalum (α-Ta), which is body-centered-cubic and Beta-Tantalum (β-Ta), which is metastable-tetragonal [2]. The properties of each phase are quite different. Alpha-Tantalum is the phase commonly found in bulk tantalum, which exhibits good ductility, high melting temperature (T melting = 3269 K), good mechanical properties (Hardness = 8–12 GPa), and low resistivity (15–80 μΩ·cm). Beta-tantalum has high resistivity (150–200 μΩ·cm), is brittle (Hardness = 18–20 GPa) and is thermally unstable at temperatures above ~1023 K because of the β →α phase change, which translates to mechanical and thermal properties that are not as advantageous as the alpha phase. In general, α-tantalum coatings are difficult to deposit without the assistance of substrate heating or post-annealing treatments [4]. Lee et al. [2,3] emphasized the requirement of tantalum coatings used in gun barrel interiors to exhibit mainly the α-phase and to have thicknesses of at least 75 μm to properly protect against thermal shock and high shear forces in order to assure longer service life. The coating must also maintain adequate adhesion on three dimensional substrates. Traditional sputtering methods are often limited by these prerequisites due to the tendency of coating delamination on thicker (>10 μm) coatings because of large residual stresses intrinsically produced during deposition, which weakens the bonding strength be- tween the coating and the substrate. Lee et al. [3] found that depositing Ta with magnetron sputtering techniques that greatly enhance metal ionized plasma density such as, high power pulsed magnetron sputtering (HPPMS) [5], modulated pulsed power (MPP) magnetron sputtering [6], and plasma enhanced magnetron sputtering (PEMS) [7] can create films that meet the specified requirements. The development of HPPMS by Kouznetsov [5] and MPP by Chistyakov and co-workers [8] have shown great advantages as com- pared to the conventional magnetron sputtering techniques in terms of the quality and adhesion of coatings [9–11]. Both techniques create a highly ionized metal plasma using a pulsed, high peak target power density for a short period of time. The main difference between HPPMS and MPP is the magnitude, duration and shape of the high Surface & Coatings Technology 214 (2013) 38–45 ⁎ Corresponding author. Tel.: +1 3032508491. E-mail address: myers.sterling@gmail.com (S. Myers). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.10.061 Contents lists available at SciVerse ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat