Actual Crack Size and Shape of Electrodeposited Chromium Layers

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Actual Crack Size and Shape of Electro-deposited
Chromium Layers

• 2007. 10. 26
• Y. Choi and Y. S. Hahn
• Sunmoon University, Korea
• Korea Atomic Energy Research Institute

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Backgrounds

• Microstructure of coating layers such as chromium
  deposit for automobile industry and copper thin
  film for electronic industry is very important
  because it significantly influences its
  performance. Sometimes, we need dense or porous
  structure for various applications.
• Cracks are inevitably formed during
  electroplating because hydrogen evolution on
  cathode surface and abnormal growth of columnar
  grains.
• Hence, quantitative analysis of crack size and
  distribution is so important, however, it is
  difficult to find a consistency in previous
  research data.

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Previous Cracks Observation

• Fig. 1 Trivalent chromium layers observed by (L)
  optical microscopy and (M) scanning electron
  microscopy (R) transmission electron microscopy
  and corresponding selected area diffraction
  pattern.
• It is difficult to observe any real size of
  inner cracks by using destructive sample
  preparation methods. So we should use
  non-destructive observation methods.

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Objective

• Find a reliable observation method to collect
  quantitative data of the crack size and
  distribution of thin films.

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Why small angle neutron scattering ?

• Neutrons reveal structure and dynamics because
  neutron show where atoms are and what atoms do.
• Neutrons see more than X-rays because X-rays are
  scattered by electron whereas, neutron by atomic
  nuclei.
• Neutrons reveal inner stress because neutron
  diffraction can show how much the distance
  between the atoms has changed.
• Neutron shows what atoms remember of their
  earlier position when they are move randomly in
  relation to each other in liquid and melt.

• ref Shull and Brockhouse got Nobel Prize in
  physics 1994 for their pioneering contributions
  to the development of neutron scattering
  techniques for studies of condensed matter.

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1. Crack Size and Distribution by SANS

• SANS shows that most of inner crack are about 40
  nm in size.
• recall Braggs law, d2p/Q

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Actual Crack Morphology
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Micro-size Cracks
Micro-size cracks seem to be calabash shape
defects
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3. How to evaluate Calabash Shape crack ?

• Porod Law applicable for the relatively large
  crack size

Scattering intensity expressed by following
equation. where Kp is expressed by scattering
length density (Dr) and specific surface area
(Sv). Defect size can be estimated by using
that specific surface area is equal to 3/R with
the assumption of spherical shape.
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7. Defect Analysis by SANS
Fig. 4. Calabash crack size of eco-friendly
trivalent chromium layers with poor NSF life
average size of crack are 4.9 mm for D.C plating,
3.3-7.9 mm for the pulse plating
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Summary

• Most of nano-size crack of trivalent chromium
  layer has about 40 nm in size, whereas,
  micro-size cracks are 4.9 mm for direct current
  plating, 3.3-7.9 mm for various pulse plating
  conditions. The crack size was decreased with
  decreasing plating current density and voltage.
• The micro-size crack seems to be Calabash shape
  which is formed by the interconnection of several
  nano-size cracks.
• Small angle neutron scattering (SANS) is one of
  useful methods for evaluating of cracks in
  eco-friendly trivalent chromium layer and other
  thin films.

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