Takashi Nagoshi

Large increase in fracture resistance of stishovite with crack extension less than one micrometer

The development of strong, tough, and damage-tolerant ceramics requires nano/microstructure design to utilize toughening mechanisms operating at different length scales. The toughening mechanisms so far known are effective in micro-scale, then, they require the crack extension of more than a few micrometers to increase the fracture resistance. Here, we developed a micro-mechanical test method using micro-cantilever beam specimens to determine the very early part of resistance-curve of nanocrystalline SiO2 stishovite, which exhibited fracture-induced amorphization. We revealed that this novel toughening mechanism was effective even at length scale of nanometer due to narrow transformation zone width of a few tens of nanometers and large dilatational strain (from 60 to 95%) associated with the transition of crystal to amorphous state. This testing method will be a powerful tool to search for toughening mechanisms that may operate at nanoscale for attaining both reliability and strength of structural materials.

Micro-Compression Test of Nanocrystalline Nickel Deposited by Supercritical Carbon Dioxide Emulsion

This paper reports experimental results of compression test on non-tapered rectangular shaped micro-pillar fabricated by focused ion beam techniques. The pillar is composed of electrodeposited nickel in additive-free Watts bath emulsified with supercritical carbon dioxide. We found that the electroplated film does not contain any defects or pores and has grain size of 8 nm. Maximum compression flow stress exceeds 3.5 GPa without any failure up to 9 % of permanent strain. This is 10 times higher than the strength of the single crystal nickel counterparts fabricated using the same focused ion beam techniques loaded along . This is because of the enhanced mechanical properties by grain boundary strengthening in nanocrystalline nickel and defect-free nickel film. Carbon impurity observed in the nickel film fabricated by electroplating with supercritical carbon dioxide emulsion enhances cohesion of the grain boundary and inhibits grain boundary sliding, which is the predominant deformation mechanisms in this grain size regime.

Size Effect on the Electrodeposited Nickel Investigated by Micro-compression Test

Effect of the grain size and sample size were examined with different grain size below 20 nm and sample size from 30 to 5 μm fabricated from electroplated nickel. TEM observation confirmed smallest grain size of 8 nm obtained at applied pressure of 15 MPa. On the contrary to the Hall–Petch relationship reported before, inverse Hall–Petch was not observed in our nanocrystalline nickel even when the grain size at 8 nm. Sample size effect on the 8 nm grained nickel was smaller than that of single crystal nickel as Hall–Petch exponent of −0.25 and −0.125 respectively. Suppressed Hall–Petch breakdown and sample size effect were explained by the physics of grain boundaries in association with impurity carbon.

Mechanical Property Evaluation of Electrodeposited Nanocrystalline Metals by Micro-testing

Electrodeposition is a very important technology in the fabrication of micro-compo‐ nents for micro-electro-mechanical systems (MEMS) or integrated circuits. Evalua‐ tions of the materials used in these devices as 3D components should be conducted using micro-sized specimens due to the sample size effect on the practical use of the components. Nanocrystalline metals could be deposited using an electrodeposition method with supercritical CO2 emulsion. Our experiment on the micro-specimens provides information on micro-mechanical testing of electrodeposited metals in‐ cluding the effect of sample size, grain size, and anisotropic structures on mechani‐ cal properties. In this chapter, recent studies on crystal growth in electrodeposition of metals and its evaluation using micron-sized testing will be presented.

Intact Ultrathin Ni Films Fabricated by Electroplating with Supercritical CO2 Emulsion

Ultrathin (2 emulsion (SCE). Incomplete coverage of the Cu plate, the working electrode, by electroplated Ni and non-uniform Ni films with defects were obtained when conventional electroplating at 1 A/dm2 with 30 sec of deposition time was used. When electroplating with SCE (ESCE) was applied, complete coverage, defect-free and uniform UTNFs were obtained. SEM and AFM showed surface morphology of the UTNFs was covered by spherical-shaped particles with ~10 nm in diameter, which was expected to be individual Ni grains because the size was consistent with grain size of Ni films reported when ESCE was applied. High H2 solubility in CO2, periodic-plating-characteristic after applying ESCE, and improved transport efficiency of the reactive species are believed to be the main reasons to cause effects of grain refinement and suppression in formation of the defects. Thickness of the UTNFs was 11.97±1.82 nm when the deposition time was 15 sec, and the thickness increased to 38.45±1.71 nm when the deposition time was increased to 45 sec.