DOI: 10.1007/s00339-008-4536-9 Appl. Phys. A 92, 387–395 (2008) Materials Science & Processing Applied Physics A k. mergia 1, ✉ d. lafatzis 1 n. moutis 1 t. speliotis 2 g. apostolopoulos 1 f. cousin 3 Oxidation behaviour of SiC coatings 1 Institute of Nuclear Technology and Radiation Protection, National Centre for Scientific Research ‘Demokritos’, 15310 Aghia Paraskevi, Attikis, Greece 2 Institute of Materials Science, National Centre for Scientific Research ‘Demokritos’, 15310 Aghia Paraskevi, Attikis, Greece 3 Laboratoire L´ eon Brillouin, CEA Saclay, 91191 Gif sur Yvette, France Received: 8 February 2008/Accepted: 3 April 2008 Published online: 29 May 2008 • © Springer-Verlag 2008 ABSTRACT Amorphous silicon carbide (SiC) films were de- posited on silicon substrates by radio-frequency magnetron sputtering. The films were oxidized in air in the temperature range 400–900 ◦ C and for times from 1 to 16 h. Neutron re- flectivity measurements provided information on the thickness, density and roughness of the SiC and on the formed SiO 2 layers. Fourier transform infrared spectroscopy was used to determine the bond structure of the formed SiO 2 and changes in the bond- ing of SiC after exposure at the oxidation temperature. The surface morphology of the oxidized films was characterized by atomic force microscopy measurements. The oxidation kinetics is initially fast and as the SiO 2 layer is formed it slows down. The SiC consumption varies linearly with time at all oxidation temperatures. Exposure of the SiC at the oxidation temperature affects its density and to some degree its bond structure, while the formed SiO 2 has density and bond structure as that formed by oxidation of Si under the same conditions. PACS 66.30.Ny; 68.47.Gh; 68.55.J- 1 Introduction C f /SiC and SiC f /SiC materials (ceramic matrix composite materials, CMCs) and ceramic multilayers with the functionalities of oxidation and abrasion resistance, thermal protection and low friction are candidate materials as thermal protection systems for space vehicles, air-breathing propul- sion and moveable aerospace bearing systems [1, 2]. For the application of these bulk ceramics in hostile en- vironments appropriate SiC-based coatings have to be de- veloped. Large-scale SiC/SiO 2 structures can serve as high temperature protection coatings in aerospace applications, as they can protect materials from oxygen diffusion within their structure but also should be able to remain intact in complex thermo-mechanical loads and corrosive environments [3–5]. First-principles calculations [6] suggest that oxygen inter- stitials in the Si–C bond form O 3 metastable clusters which transform to a stable defect by the emission of CO. The global ✉ Fax: +30-210-6533431, E-mail: kmergia@ipta.demokritos.gr process resulting in the emission of CO is exothermic with energy of about 0.7 eV. Amorphous SiO 2 grows with the con- tinuation of the same process. Once the SiC–SiO 2 interface is formed, it advances by the diffusion of oxygen in SiO 2 , insertion in the next available Si–C bond and so on. The ejected CO molecules very likely diffuse through the SiO 2 and are released from the surface. However, CO can break either in the bulk SiC or the advancing interface where it can either contribute further to the oxidation or nucleate and form C complexes. There is indirect evidence that during ther- mal oxidation of SiC excess carbon in the form of clusters or graphite precipitates remain at or near the interface [7]. Also, from first-principles calculations it has been found that both CO and CO 2 are emitted during SiC oxidation, as CO molecules passing through SiO 2 form oxygen-deficient clus- ters emitting CO 2 [8]. The purpose of this work is to understand the oxidation of SiC in air at elevated temperatures, the degree to which the formed SiO 2 film acts as a protection to further oxida- tion of the underlying structure and approaches to improve the desired functionalities of the oxide film. As the degree of crys- tallinity of the SiC protection layers employed for different applications varies broadly depending on growth techniques and conditions, it is more productive to study such a phe- nomenon using amorphous SiC films. In this manner the SiO 2 function as a barrier will be more apparent and also the oxida- tion of SiC as it transforms from amorphous to crystalline will be revealed. In this paper the oxidation properties of amorphous sput- tered SiC films are evaluated by neutron reflectivity (NR), Fourier transform infrared (FTIR) spectroscopy and atomic force microscopy (AFM) measurements. NR can provide im- portant information in nano-scale layers and multilayered sys- tems, such as the density and the thickness of each layer, interfacial roughness and inter-diffusion profile. On the other hand, by FTIR measurements the bond structure of the ma- terials, either SiC or SiO 2 , and their changes through heat treatments can be determined. Comparison between NR and FTIR measurements offers an additional advantage since NR results can be used as an absolute calibrant of the FTIR meas- urements. Further, this FTIR calibration may also be used for oxidation studies of other SiC systems. This is very im-