On the elastic constants of amorphous carbon nitride A. Champi a, ⁎, A.S. Ferlauto b , F. Alvarez b , S.R.P. Silva c , F.C. Marques b a Universidade de São Paulo, USP, Instituto de Fisica, 05508-090, SP, Brazil b Universidade Estadual de Campinas, Unicamp, Instituto de Física “Gleb Wataghin”, 13083-970, Campinas–SP, Brazil c School of Electronics and Physical Sciences, University of Surrey, Guildford, GU2 7XH United Kingdom article info abstract Article history: Received 5 June 2007 Received in revised form 5 February 2008 Accepted 18 February 2008 Available online 4 March 2008 Elastic and thermomechanical properties of amorphous carbon nitrite thin films as a function of nitrogen concentration are reported. The films were prepared by ion beam assisted deposition with nitrogen concentrations ranging from 0 to 33 at.%. By using a combination of the thermally induced bending technique and nano-indentation measurements it was possible to calculate independent values for the Young's modulus, the Poisson's ratio, as well as the thermal expansion coefficient of the films. The hardness and elastic recovery are discussed in terms of the Young's modulus and the Poisson's ratio. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Amorphous carbon (a-C) and carbon nitride films (a-CN x ) have been investigated for many reasons, mainly related to their hardness and chemical inertness [1]. In particular, carbon nitrogen films have been intensively investigated due to the theoretically predicted superhard compound β-C 3 N 4 [2]. A number of research groups have also tried the development of its amorphous counterpart, a-C 3 N 4 , aiming to achieve very hard amorphous films [3–5]. In spite of the high technological importance of these materials there are few reported data concerning the values of the Young's modulus (E), Poisson's ratio (υ) and the coefficient of thermal expansion (CTE) for a- CN x as function of nitrogen concentration [6]. The elastic constants reported thus far are mainly those obtained by nano-indentation measurements, which supply the E/(1-υ 2 ) ratio (using the Oliver and Parr model [7]), and not the pure Young's modulus. In this study, we adopt two different techniques, i. e., nanoindentation and thermally induced bending (TIB), to obtain distinctively separate values for both the Young's modulus and Poisson's ratio, as well as the coefficient of thermal expansion of carbon nitrogen alloys. 2. Experimental A series of a-C:N x were prepared by ion beam assisted deposition (IBAD). A high purity graphite target was sputtered by a 1500 eV argon ion beam. Simultaneously a 100 eV nitrogen ion beam was directed towards the substrate/film composite with ion current varying in the 0.06 mA/cm 2 to 3.0 mA/cm 2 range. The concentration of nitrogen was determined by means of the 14 N(d,p) 14 N nuclear reaction using a Van Der Graff (positron) reactor. Films of about 300 nm thick were deposited at constant 150 ° C substrate temperature. Additional details of the apparatus can be found elsewhere [8]. The sp 2 concentration was determined by electron energy-loss spectroscopy (EELS) from the area of the ⁎ peak in the carbon K edge (the 1s–2p transition) [9,10]. Nanoindentation measurements were conducted using a Berkovich indenter from a nanoindenter model Nanotst-100. 3. Results In order to determine the thermal expansion, we adopted the thermally induced bending (TIB) technique, which uses the tempera- ture dependence of the stress, σ, given by the relation [11]: Dr th ¼ E f = 1 À v f ð Þ a s À a f ð ÞDT ð1Þ where E f , and ν f are the Young's modulus and Poisson's ratio, respectively. ΔT is the temperature interval. The subscripts “s” and “f” refer to substrate and film, respectively. The total stress, σ, is given by the modified Stoney's equation [12] r ¼ E s = 1 À v f ð Þt 2 f =6t f 1=R À 1=R 0 ð Þ ð2Þ where 1/R 0 and 1/R are the curvature before and after the film deposition, respectively, and t is the film thickness. The three unknown parameters in Eq. (1) can be reduced to two by defining the biaxial modulus as E f /(1 - ν f ). The coefficient of thermal expansion and the biaxial modulus can be obtained from the stress vs. temperature measurements if one adopts at least two different substrates, with different thermal expansion. The inset in Fig. 1 displays the result of these measurements performed in one a-C:N x film, with 25% nitrogen, deposited on two substrates [c-Si (111) and Diamond & Related Materials 17 (2008) 1850–1852 ⁎ Corresponding author. E-mail addresses: farfan@if.usp.br, anamelvac@yahoo.com (A. Champi). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.02.024 Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond