Examination of PolyimideCarbon Coating on Uncabled Fiber

1
Examination of Polyimide/Carbon Coating on
Un-cabled Fiber

• C. T. Mueller
• J. G. Coffer
• June 9, 2000

2
Microscopic Examination of Un-cabled Fiber Samples

• Physical inspection of CDO588XD CDO384XB fiber
  samples
• 100 - 400 x
• illumination reflection transmission
• polarized light

3
Cat Scratches

• Periodically-spaced gouges across the polyimide
  coating

200 µm
Illumination reflection, polarized
4
CDO384XB (baseline)

• Bubbles found in polyimide
• 12 µm diameter
• cracks in outer surface
• gouges found in polyimide
• depth appears to be less than polyimide thickness

cracks
bubble
gouge
10 µm
5
Carbon Breach

• CDO384XB
• illumination transmission (light behind the
  fiber), polarized

pinhole
10 µm
6
Depth Estimate

• Polyimide coating measured to be 13 µm using
  optical microscope
• Depth (surface to pinhole) approximately 14.8 -
  7.5 7.3 x 1.78 13 µm
• Believe all features are within the polyimide
  layer, not in the glass

7
Observations / Comments

• Found periodically-spaced gouges across the
  polyimide (small arcs)
• Discovered bubbles in polyimide coating in
  baseline (un-cabled) fiber samples
• CDO384XB (now examining CDO588XD)
• Bubble features
• 12 µm in diameter
• cracks
• does not appear to extend below the polyimide
  layer (will verify by examining the
  glass/carbon/polyimide interface)
• appears to pass light suggesting that the carbon
  coating has been breached
• Origin of bubbles in polyimide now under
  investigation
• incomplete curing or bubbles present in coating
  from manufacture
• ESD may be a factor if non-uniformities (i.e.
  thickness variations) in the polyimide coating
  are proven to be the norm
• breakdown voltage scales with thickness
• field strength will generally increase at sharp
  edges, lowering breakdown voltage

8
Conclusions

• Bubbles in polyimide can be created before
  cabling process
• mechanism being investigated
• Bubble features consistent with those observed
  above each low-strength break found in the
  strength testing performed by Boeing
• Carbon coating appears to have been breached
• The presence of a bubble in the polyimide does
  not mean that the fiber has low strength, but the
  strength of the fiber will most likely degrade
  during the cabling process as shown in the
  strength testing data
• both moisture and elevated temperature present
  during the cabling process (In this case, HF may
  not be needed to weaken the fiber)
• The polyimide/carbon coating on the fiber appears
  to be damaged before it starts the cabling
  process