Characterization and control of microstructure in combinatorially prepared aluminum-silicon thin film nanocomposites
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In this paper, we describe the application of thin film combinatorial deposition methods to systematically control the microstructure of AlxSi(1−x) alloys 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 a-(Al–Si) and amorphous a-Si plus crystalline c-Al. The microstructure was investigated using x-ray diffraction while atomic force microscopy techniques were used to obtain surface morphology and phase distribution.
I. INTRODUCTION
Due to 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 properties such as low wear, low coefficient of thermal expansion, high corrosion resistance, and high thermal conductivity make this material system one of practical importance. Several 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 nanocomposite matrix. Reports on bulk samples have shown that finer grain structure 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, leads to a harder material.4,5 Konno et al. presented a comprehensive investigation of the nucleation, crystallization, and phase formation in the Al–Si system.6,7 These researchers made five samples of SixAl1−x amorphous alloys (x ⳱ 0.65, 0.61, 0.37, 0.35, and 0.13) using co-sputtering deposition techniques. Significant among their results was that the as-deposited films with x 艌 0.61 show a single-phase amorphous structure [a(Al–Si)] while three alloys (x ⳱ 0.13, 0.35, and 0.37) show a two-phase structure in which an Al-based columnar crystal phase (c-Al) is embedded in an amorphous Si
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0164 J. Mater. Res., Vol. 21, No. 5, May 2006
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matrix (a-Si). 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. Since the SixAl1−x microstructure depends on the composition as well as the solidification conditions, a sample preparation method that can control both of these with precision must be used to thoroughly investigate their effects on microstructure. Such a method exists in the combinatorial technique for thin film deposition. Subsequent screening measurements and analysis should produce the knowledge needed to further refine growth and composition parameters and produce materials with improved properties. Combinatorial chemistry has a successful hi
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