Chemical structure and micro-mechanical properties of ultra-thin ﬁlms of boron carbide prepared by pulsed-laser deposition J. Sun a , H. Ling a , W.J. Pan a , N. Xu a , Z.F. Ying a , W.D. Shen b , and J.D. Wu a a State Key Laboratory for Materials Modiﬁcation by Laser, Ion and Electron Beams & Department of Optical Science and Engineering, Fudan University, Shanghai 200433, P.R. China b Department of Physics and Astronomy, Eastern Michigan University, Ypsilanti, Michigan 48197, USA Received 24 June 2003; accepted 22 October 2003 Ultra-thin boron carbide ﬁlms with a thickness of about 40 nm were deposited on silicon substrates by means of pulsed-laser ablation of a sintered B 4 C target in vacuum. Together with the determination of the ﬁlm composition by X-ray photoelectron spectroscopy (XPS) and the observation of the surface topography by atomic force microscopy (AFM), the chemical structure of the ﬁlms was studied by Fourier transform infrared (FTIR) spectroscopy. Mechanical characterization of the ﬁlms was performed on the micron and sub-micron scales by means of nano-indentation and micro-scratch tests, from which the hardness, Young’s modulus and micro mar resistance of the ﬁlms were determined. The optimal values were obtained for the ﬁlms prepared at elevated temperature of 600 8C, with hardness of 39 GPa, Young’s modulus of 348 GPa and micro mar resistance (MMR) of 5:0  10 3 GPa; in comparison with those of 23, 252, and 7:1  10 2 GPa, respectively, for the ﬁlms prepared at room temperature. KEY WORDS: boron carbide, ultra-thin ﬁlm, pulsed laser deposition, chemical structure, hardness, mar resistance 1. Introduction Boron carbide is well known as a material with interesting properties such as being hard, light, refrac- tory, semiconducting, etc., which make it promising for a wide range of applications. Boron carbide also offers excellent chemical and thermal stability, mainly due to its covalent bonding. Most of its applications nowadays are based on the combination of its low speciﬁc density and extreme hardness, since rhombohedral boron carbide, often denoted B 4 C; is the third hardest material, surpassed only by diamond and cubic boron nitride [1], and even higher than diamond and cubic boron nitride at temperatures over 1100 8C [2]. More- over, boron carbide is an interesting compound, in which the boron-to-carbon atomic ratio can vary over a broad range and has been found to exist as a stable single phase in a large homogeneous region from B 4 C to B 10:4 C [1,3,4], although the stoichiometric compound of rhombohedral boron carbide is normally B 4 C: The wide composition and structure variation, excellent proper- ties and potential applications of boron carbide have received great attention. The synthesis, characterization and property studies of boron carbide have become a prominent topic of recent research. Despite these excellent properties, boron carbide has not been investigated extensively in comparison with other refractory materials, such as diamond, boron nitride, silicon carbide, and relatively few efforts has been devoted to develop methods for preparing boron carbide ﬁlms. However, signiﬁcant progress has been made since the early 1990s. Boron carbide thin ﬁlms can now be grown from several gaseous precursors by chemical vapor deposition (CVD) processes [5] includ- ing plasma assisted CVD [1,6] and laser assisted CVD [7,8]. Various physical vapor deposition (PVD) methods, such as diode sputtering [9], RF and magne- tron sputtering [10–13], as well as pulsed laser deposi- tion (PLD) [14–16] are frequently used in laboratories for the preparation of boron carbide thin ﬁlms. Among those preparation methods, pulsed laser deposition (PLD) has several advantages. Due to its ability to transfer the stoichiometry from the target to the ﬁlms as well as the high energies carried by the ablation-created species expanding from the target to the substrate, PLD is an attractive technique for preparation of various compound thin ﬁlms [17]. Some work has been done for mechanical properties of boron carbide ﬁlms, but they are mainly based on indentation tests for hardness determination. In some applications, however, resis- tance to marring and scratching is more important. In this paper we present our recent work on the prepara- tion of ultra-thin boron carbide (BC) ﬁlms by PLD. The elemental composition and chemical structure of the prepared ﬁlms were analyzed. The mechanical proper- ties of the ﬁlms were studied on the micron and sub- micron scales, based on nano-indentation and micro- scratch tests, from which the hardness, Young’s modulus and micro mar resistance were determined. We also investigated the inﬂuence of preparation temperature on the ﬁlm properties.  To whom correspondence should be addressed. E-mail: jdwu@fudan.edu.cn Tribology Letters, Vol. 17, No. 1, July 2004 (# 2004) 99 1023-8883/04/0700–0099/0 # 2004 Plenum Publishing Corporation