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4 Powder-Consolidated MEMS Development 4.1 Background The conventional LIGA process for the fabrication of high aspect-ratio metallic MEMS components is based on electroplating into a lithographically-defined mold. For this reason, conventional LIGA components can only be fabricated out of a very limited set of electroplatable metals (i.e. copper, nickel, gold, etc.). Recent work has focused on the development of an alternative processing route which enables the fabrication of a much wider variety of materials, including many metals and ceramics. This process uses the same lithographically-defined high aspect-ratio mold, but rather than electrodepositing the structural material, powder processing is used. A submicron powder of the desired material is compacted, along with binder, into the mold, followed by a sintering step to consolidate the green body. Examples are shown in Figure 4.1. While this technique has the important advantage of allowing flexibility in materials selection for MEMS applications, it also has potential issues associated with the shrinkage of the component during sintering, as well as the potentially detrimental effect of remnant pores on mechanical performance. Details on the micromolding process, resulting microstructure, and shrinkage can be found in Ref. [4.1]. This section summarizes the evaluation of mechanical behavior of powder-consolidated alumina, nickel and stainless steel. 600 µm 30 µm Fig. 4.1. Micro-molded 316L SS gears sintered at 1250°C for 1 hr in 3% H2. From [4.1] 4.2 Method for Evaluating Flexural Strength of Alumina Parts The mechanical integrity of ceramic materials is often evaluated through the use of a flexural strength test, as described in the ASTM C 1161-94. The flexure experiments were conducted under four-point bend conditions with the span between outer loading points being 2.6 mm and the span between inner loading points being 1.3 mm. The 4-point bending configuration 40 120 µm 30 µm
has the advantage of distributing the maximum stress evenly along the inner span, thereby sampling any defects along 1.3 mm of the sample length. Self-alignment was achieved by incorporating a doubly-articulated joint in the load train. Specimens were produced directly via the LIGA-mold process with a cross-section of nominally 100x500 µm and specimen lengths of ~3 mm. Flexure tests were carried out in a voice-coil actuated Nanoindenter XP which had been modified to accommodate a four-point bend fixture and control the monotonic load-displacement experiment, and shown in Figure 4.2.. A second set of tests were carried out in a custom-built MTS-based servohydraulic micromechanical test frame designed for low-load (< 25 N) testing. In both test setups, the bend bars were loaded at a maximum strain rate of ~0.5x10 -4 s -1 . Failure loads were typically on the order of 200-1000 mN. Fig 4.2. Micro-bend configuration actuated by a nanoindenter. The four pins of the 4-point bend can be seen end-on. 4.3 Results from Flexural Strength Test of Alumina Five powder-compacted bend bars were evaluated for flexural strength. For these five specimens, the average maximum stress at failure was 205 MPa with a standard deviation of 32 MPa. This value is somewhat lower than what is typically reported for conventionally-prepared alumina, which is generally higher than 250 MPa. The somewhat low value of flexure strength exhibited in the micromold alumina is likely to be related to the relative flaw size. While the micromold specimen cross-section is considerably smaller than that of conventional alumina components, the flaw size may be of a similar magnitude. Therefore, the flaw in a micromold specimen takes up a considerably larger fraction of the total cross-section thereby resulting in a diminished strength. 41
Page 1 and 2: SANDIA REPORT SAND2004-1319 Unlimit
Page 3 and 4: SAND2004-1319 Unlimited Release Pri
Page 5 and 6: Contents ACKNOWLEDGEMENTS..........
Page 7 and 8: 6.5 Summary........................
Page 9 and 10: List of Figures Figure 1.1 SEM micr
Page 11 and 12: Figure 6.2 Contact between an elast
Page 13 and 14: Preface Our team was involved in a
Page 15 and 16: strength. The subsequent chapter di
Page 17 and 18: Fig. 1.3. SEM image of cylindrical
Page 19 and 20: Fig. 1.5. Schematic of primary feat
Page 21 and 22: Fig 1.7 Double-hinged pull-tab tens
Page 23 and 24: Fig 1.10. SEM image of optical forc
Page 25 and 26: 2 Strength Distributions in SUMMiT
Page 27 and 28: 2.2 Critical Flaw Size Evaluation T
Page 29 and 30: Fig. 2.5. SEM micrograph of pull-ta
Page 31 and 32: (1) Differences in surface roughnes
Page 33 and 34: 3 The Role Of Microstructure In SUM
Page 35 and 36: crystallographic orientation of a g
Page 37 and 38: direction. Also, maximum stress loc
Page 39: 3.4 References 3.1 Randle, V., Micr
Page 43 and 44: and servovalve at a strain rate of
Page 45 and 46: 4.6 References 4.1. TJ Garino, AM M
Page 47 and 48: microscopy studies. As a first step
Page 49 and 50: Fig. 5.2. Orientation imaging of a
Page 51 and 52: Fig. 5.5. Orientation imaging of a
Page 53 and 54: 6 Novel Techniques for Measurement
Page 55 and 56: of adhesion, and, (iv) a smaller co
Page 57 and 58: Force (mN) Fig. 6.5. Typical load-d
Page 59 and 60: 7 Impact of Silane Degradation Due
Page 61 and 62: All samples were coated with alkyls
Page 63 and 64: the friction structure and the cali
Page 65 and 66: 7.4 RESULTS 7.4.1 Simulation of dos
Page 67 and 68: Dose (MeV/g) Fig. 7.4. Results of X
Page 69 and 70: heating on contact angle for PFTS.
Page 71 and 72: the C-C backbone of the PFTS molecu
Page 73 and 74: The increase in static friction for
Page 75 and 76: 8 Friction and Wear of Selective Tu
Page 77 and 78: During processing of microelectrome
Page 79 and 80: actuator. After unloading, the devi
Page 81 and 82: 9 Conclusions and Recommendations T
Page 83: 10 Distribution 3 MS 0889 M.T. Dugg

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