Combinatorial investigation of the mechanical properties of aluminum-silicon thin film nanocomposites
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D.B. Haddad School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907 (Received 10 October 2006; accepted 21 December 2006)
We have undertaken the exploration of AlxSi1−x material systems to discover new alloys with enhanced mechanical properties. Combinatorial methods were used to systematically control thin film microstructure through variations in composition and growth temperature. Discrete libraries of compositionally graded films have been sputter deposited onto silicon substrates to produce two structural phase regions: amorphous Al–Si and amorphous Si plus crystalline Al. The mechanical properties of the thin films were determined through indentation experiments by analyzing the load–displacement traces based on the Oliver–Pharr method. X-ray diffraction was used to investigate the microstructures and determine the crystallite sizes. In samples with an Al concentration near 55 at.%, the data show that decreasing the Al crystallite size by nearly one half increases the nanocomposite hardness by nearly a factor of two. I. INTRODUCTION
Because of their high strength-to-weight ratio and good mechanical properties, Al–Si alloys are favorable candidates for replacement materials in many critical automotive components. Exceptional physical properties such as low wear, low coefficient of thermal expansion, high corrosion resistance, and high thermal conductivity make this materials system a model for investigation. Most of the enhancements in these properties have been attributed to particular features in the microstructure including the size, growth orientation, and the distribution of the Al crystallites within the Si matrix. Reports have shown that finer grain structures as well as engineered surfaces lead to significant mechanical property improvements.1–3 It is generally accepted that higher hardness in a material leads to better wear resistance; thus, increasing the fraction of the harder phase, Si, in this compound should, and has been reported to, do so.4,5 Konno et al. presented a comprehensive investigation of the nucleation, crystallization, and phase formation in the Al–Si material system.6,7 These researchers made five thin film samples of SixAl1−x amorphous alloys (x ⳱ 0.65, 0.61, 0.37, 0.35, and 0.13). Significant among their results was that the as-deposited films with x 艌 0.61 show a single-phase amorphous structure while three alloys (x ⳱ 0.13, 0.35, and 0.37) show a two-phase a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0139 J. Mater. Res., Vol. 22, No. 4, Apr 2007
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structure in which an Al-based columnar metal phase is embedded in an amorphous Si matrix. It was also reported that the decomposition and crystallization temperatures for these alloys suggest that the reaction proceeds by the creation of crystalline Si and the simultaneous growth of Al grains. Because the SixAl(1−x) microstructure depends on the composition as well as the solidification conditions, a
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