Mechanical testing of microsamples from weldments and MEMS materials

This dissertation describes the development of three microsample testing machines and measurements of the mechanical properties of specimens taken from a weldment and of polysilicon thin films. The first test machine is used for tension test of metal microspecimens which are 3 mm long and about 0.2 mm square in cross-section. The second test machine is constructed for fatigue test of microsamples from weldments where the specimen has a cross-sectional area about 0.3 mm by 0.3 mm and an overall length of 7.6 mm. The third test machine and the preparation processes for thin film specimens are developed for mechanical testing of MEMS materials that are a few microns thick; both tensile and tension-tension fatigue tests can be conducted with this thin film test machine. A unique feature of these three test machines is the application of the Interferometric Strain/Displacement Gage (ISDG) technique which enables direct strain measurement for mechanical testing of microsamples.; Fatigue microspecimens were taken from three different regions of an HY-100 steel weldment and tested under fully reversed cyclic loads with strain control. The specimens are believed to be the smallest fatigue specimens that have ever been tested under fully reversed cyclic loads, and the results are similar to those from larger specimens.; Extensive measurements of mechanical properties were performed for polysilicon thin film specimens that were 3.5 {dollar}mu{dollar}m thick, 0.6 mm wide and 4 mm long. Forty eight tests were conducted for measuring Young's modulus and tensile strength, while nineteen of them also measured Poisson's ratio. The measured properties of these thin film specimens are Young's modulus = 169 {dollar}pm{dollar} 6 GPa, tensile strength = 1.20 {dollar}pm{dollar} 0.15 GPa, and Poisson's ratio = 0.22 {dollar}pm{dollar} 0.01. The direct measurements of the stress-strain curves and Poisson's ratio are the first for polysilicon thin films. Two specimens were tested under tension-tension fatigue loads and showed no significant damages caused by cyclic loads. Seven tensile tests were conducted for specimens with etch holes which resulted in considerable decrease of tensile strength of the specimens. Finally, nine tests were performed for investigating the effect of HF exposure on mechanical properties of polysilicon, but no significant differences have been observed among these tests; this observation contradicts previous reports on the HF effect.; In comparison with other testing approaches, the techniques developed in this dissertation possess the features of direct, accurate, and versatile measurements. A quantitative analysis of the effect of rigid body motion on the ISDG measurement is performed for a thin film test and this effect is proved to be negligible. The relative uncertainty of the ISDG is estimated as {dollar}pm{dollar}4%, while the uncertainty of load measurement can be controlled within 0.5%, which meets the ASTM test standard.