Mechanical Engineering ETDs

Author

Guanlin Tang

Publication Date

8-27-2009

Abstract

A numerical study was undertaken to investigate the mechanical properties of metal-ceramic nanolayered composites. The model system features alternating thin films of aluminum (Al) and silicon carbide (SiC). Finite element modeling was employed to analyze the nanoindentation and microcompression behavior. This modeling study treats the heterogeneous structure of the material explicitly, and seeks to correlate the overall material response with the intrinsic deformation characteristics. We first report on the nanoindentation behavior of the Al/SiC composites. Two material systems were considered: Al/SiC multilayers free of substrate and Al/SiC above the silicon (Si) substrate. The numerical model features a conical indenter within the axisymmetric simulation framework. For the Al/SiC multilayers free of substrate, a valid composite elastic response can be retrieved beyond a certain depth. The effective modulus was found to be representative of the out-of-plane modulus of the multilayer composite. For the Al/SiC multilayers above Si substrate, the effects of the substrate material and heterogeneity of the composite play an important role in the modulus and hardness determination. Significant tensile stresses can be generated locally along certain directions. The unloading process leads to an expansion of the tension-stressed area and continuation of plastic flow in parts of the Al layers. The unloading response is therefore much more complex than the conventional elastic recovery process as seen in homogeneous materials. Attention was then turned to the viscoplastic effects during indentation. Within the present modeling framework, we found that a hold time at the peak load can help to obtain a reliable hardness value, while elastic modulus does not seem to be affected by the hold. The multilayers display a less significant time-dependent behavior, compared to the case of a single-layer material. Finally we report on the composite pillar behavior under micro-compression tests. It was found that the base material connected to the pillar plays a significant role in the measured mechanical response. It is essential to take into account the base and indenter compliances to obtain a reliable stress-strain relationship. The multilayered pillar deforms in a non-uniform way under compression, especially when a tapered side wall included in the numerical model.

Keywords

Nanocomposites (Materials)--Mechanical properties, Nanocomposites (Materials)--Compression testing, Ceramic-matrix composites--Mechanical properties, Ceramic-matrix composites--Compression testing, Layer structure (Solids)

Degree Name

Mechanical Engineering

Level of Degree

Doctoral

Department Name

Mechanical Engineering

First Advisor

Shen, Yu-Lin

First Committee Member (Chair)

Shen, Yu-Lin

Second Committee Member

Al-Haik, Marwan

Third Committee Member

Khraishi, Tariq

Fourth Committee Member

Maji, Arup

Sponsors

National Science Foundation

Document Type

Dissertation

Language

English

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