Thursday, November 29, 2018

"HOW DID IT BREAK AND WHY - 2018 Edition"

I am a Materials Science & Engineering Professor and this time of the year (November) it is time to introduce our freshmen to the discipline with a small project that I call "How did it break and why". First stop at the stereo microscope. Of course it is a....Waterman nib.
Top view of a Waterman nib with two cracks at the bottom of the heart-shaped breather hole viewed under a stereo microscope and photographed through the lens by my phone.

Bottom view of the same nib (only of the cracks are through the thickness)
After the optical microscope, I demonstrated to the freshmen the power of the scanning electron microscope. These are pictures of the same nib which had these two cracks at the breather hole.
Detailed examination of one the cracks reveals that it is actually a stress corrosion crack. Stress corrosion is when a material breaks under the combined action of stress (repeated stress here, thus fatigue) and corrosion. The students asked me how come gold corrodes. Actually it does [1] especially as the purity decreases (lower K). Then the asked me who did I infer that there was corrosion. The answer of this was a little more elaborate. The shape of the top crack gives it away. Non only there is a crack but there is material missing on the surface of the nib and the sides of the crack. Moreover the surface of that area is faceted and it indicates that the material was removed along the grain boundaries. Then they asked me what are grains (remember - they are freshmen). So I explained to them that metals are crystalline materials (i.e. their atoms are arranged in a lattice) but they are most of the times built from many, many crystals that are next to each other with different orientations (what we see sometimes on the surface of galvanized steel). The boundaries between the grains are more susceptible to corrosion - this is a sophomore year detail - I told them  In fact there were much more spectacular signs of stress corrosion at the broken tip, which I will show it to you in the next set of SEM photographs. Enjoy these for the time being 

To complete the story, the infamous Waterman nib that ended up under our electron microscope has a second interesting story. The tip is missing from both tines and one of the tines was broken around 0.5-0.7mm behind the tip. High magnifications show a clear intergranular fracture which is clear indication of corrosion stress cracking. Interesting are: (a) the area that show wear lines perhaps from the usage of the nib "sans iridium", and the second big trasnaverse crack below the main fracture on the right tine. Bad composition? Highly corrosive ink? Both? Possible. We will look futher into the details of the microstructure in an upper level class on "Case Studies in Materials"... I am not sure if the students enjoyed as much as I did. I was as giddy as a six year old with this project 

[1] Dugmore, J. M., & DesForges, C. D. (1979). Stress corrosion in gold alloys. Gold Bulletin, 12(4), 140-144.

1 comment:

Grail Pens said...

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