We have used contact-resonance-frequency atomic force microscopy techniques to nondestructively image variations in adhesion at a buried interface. Images were acquired on a sample containing a 20nm gold (Au) blanket film on silicon (Si) with a 1nm patterned interlayer of titanium (Ti). This design produced regions of very weak adhesion (Si∕Au) and regions of strong adhesion (Si∕Ti∕Au). Values of the contact stiffness were 5% lower in the regions of weak adhesion. The observed behavior is consistent with theoretical predictions for layered systems with disbonds. Our results represent progress towards quantitative measurement of adhesion parameters on the nanoscale.
We compare two types of tests for measurement of mechanical properties of thin films and small scale structures: a microtensile test and a thermomechanical fatigue test induced by alternating current at low frequency and high current density. The microtensile test was used as a reference for evaluating the feasibility of using the electrical test for measurement of mechanical properties. Tests were performed on structures cofabricated from thin film Al deposited on Si to ensure comparable mechanical properties. The films had a grain diameter of 220 nm and a thickness of 1.9 lm. The electrical test resulted in an estimated ultimate tensile strength of 250 ± 40 MPa. This value was based on extrapolation of high-cycle fatigue data to one reversal through a modified Basquin equation while accounting for varying mean stress. An ultimate tensile strength of 239 ± 4 MPa was determined from the microtensile test. Differences between these values are explained in terms of the effects of substrate constraint on the strength of the thin film. We conclude that electrical testing methods offer a feasible means for measuring mechanical properties of individual patterned structures.
A B S T R A C T Hydrogen is known to have a deleterious effect on most engineering alloys. It has been shown repeatedly that the strength of steels is inversely related to the ductility of the material in hydrogen gas. However, the fatigue properties with respect to strength are not as well documented or understood. Here, we present the results of tests of the fatigue crack growth rate (FCGR) on API X70 from two sources. The two materials were tested in air, 5.5 and 34 MPa pressurized hydrogen gas, and at both 1 and 0.1 Hz. At these hydrogen pressures, the FCGR increases above that of air for all values of the stress intensity factor range (ΔK) greater than~7 MPa · m 1/2 . The effect of hydrogen is particularly sensitive at values of ΔK below~15 MPa · m 1/2 . That is, for values of ΔK between 7 and 15 MPa · m 1/2 , the FCGR rapidly increases from approximately that found in air to as much as two orders of magnitude above that in air. Above 15 MPa · m 1/2 , the FCGR remains approximately one to two orders of magnitude higher than that of air.Keywords fatigue; fatigue crack growth rate; hydrogen environment assisted cracking; hydrogen-induced cracking; pipeline steel; X70 steel. N O M E N C L A T U R Ea = crack length A, b, a1, a2, B1, B2, m1 and d1 = fitting parameters API = American Petroleum Institute ASME = American Society of Mechanical Engineers ASTM = American Society for Testing and Materials C(T) = compact tension da/dN = fatigue crack growth rate FCGR = fatigue crack growth rate FPZ = fatigue process zone H (Heaviside step function) = 0 or 1 HA-FCG = hydrogen-assisted fatigue crack growth N = number of cycles P H = hydrogen pressure P H th = Threshold Hydrogen Pressure Q = effective activation energy R = universal gas constant (8.314 J/mol K) R a = average surface roughness T = absolute temperature V = partial molar volume of hydrogen w = width of the specimen ΔK = stress intensity factor range σ h = hydrostatic stress at the crack tip σ y = yield stress σ UTS = ultimate tensile stress Correspondence: E. S. Drexler.
A specially designed microtensile apparatus capable of carrying out a series of tests on microscale thin films for microelectromechanical system (MEMS) applications at room temperature and at temperature up to 400 degrees C has been developed and tested, and is described here. Several MEMS-applicable thin films were measured with it, including thermally grown silicon dioxide, gold, and gold-vanadium. The silicon dioxide was tested at room temperature. Gold and gold-vanadium films were tested at room temperature and at 200 and 400 degrees C. Examples of these results are presented
The LIGA process (Lithographie, Galvanoformung, Abformung, or lithography, electrodeposition, shaping) offers the possibility of mass producing strong mesoscale (order of a few mm or less) metal components of nearly any planar shape. Multilayer deposition has been developed to create three-dimensional shapes. The overall objective of this multi-year project is to enable the use of the Johnson-Cook flow and fracture models for evaluation of structural elements made from two LIGA alloys of interest, by providing the material property data at the microscale needed to carry out structural analyses utilizing these models. This report documents the first milestone in the project, which is the measurement of the room-temperature tensile properties of two LIGA alloys of one commercial vendor's proprietary materials -one optimized for strength and the other for ductility, with nominal thicknesses 190 µm and 170 µm respectively -at strain rates 0.001/s and 1/s, utilizing four specimen geometries with gauge widths ranging from 75 µm to 700 µm. Test methods adapted to the scale of these materials were applied. Results, measurement uncertainties, and statistical variations for the ultimate tensile strength and apparent Young's modulus are quantified for all combinations of these material/geometry/rate variations. In addition, preliminary studies were conducted into the effects of low-temperature annealing on the materials' strength, and use of electron back scatter diffraction to observe the microstructure. Keywords Executive SummaryThis report presents the results of room-temperature tensile tests on microfabricated specimens of two LIGA Ni alloys of interest to the project sponsor and supplied by a commercial vendor. Microtensile specimens of the vendor's proprietary materials -"Alpha" and "C" -were supplied with nominal thicknesses of 190 µm and 170 µm, respectively, and of four different sizes with gauge widths ranging from 0.075 mm to 0.7 mm. The Alpha material is very strong, with consistent strength around 1900 MPa over several fabrication runs of the material. The C material has lower strength -around 600 MPa -and is more ductile. Both materials were tested at strain rates 0.001/s and 1 /s; Figure 1 shows typical engineering stress-strain curves. Small size-and rate effects were found in both materials in the ranges tested. Pin-loaded tensile specimens were used with general purpose tensile test apparatus consistent with the scale of the specimens. This approach succeeded in measurements of the strength and ductility. However, attempts to measure the Young's modulus were not fully successful because of large experimental uncertainties and a microplasticity phenomenon similar to that reported as material instability in the previous literature on LIGA Ni materials. Figure 1. shows stressstrain curves for 2 nominal specimens, one of each material, tested at the two strain rates. The Alpha material clearly exhibits much higher strength and lower ductility than the C material.(a) (b) Figure 1. Engineering stress-...
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