Carbon Dioxide Snow Cleaning
by Applied Surface Technologies
Metal surfaces successfully cleaned include high and ultra high vacuum parts and assemblies, substrate cleaning before coating, diamond turned metal optics, electrical contacts, relays, machined parts, mold cleaning of polymer residues. and many more applications. We present a few examples
Example 1
The first example involved a nonconducting and contaminated gold relay contact. Initial surface chemical analysis by XPS, shown below, indicated almost no gold near the surface; instead a carbon rich specie dominated the spectrum. The small Si peak suggests a silicone type contaminant. Before cleaning, the gold peak is rather small, its surface concentration is about 0.3%. The relative carbon to gold ratio is 281, implying a very dirty surface. Click to magnify image.
After cleaning, the gold peak dominated the spectrum and its composition increased from 0.3% to 24.3%, a factor of over 80. The surface is now conductive, the silicon and sodium peaks are gone, and the carbon content has been reduced. The carbon to gold ratio is now 2.62. Overall, the relative carbon to gold content decreased after snow cleaning from 281 to 2.6, over a factor of 100.
Example 2
We investigated aluminum sheets coated with a thin layer of rolling lubricants and other debris. XPS was done on a contaminated, CO2 snow cleaned, and solvent cleaned (reagent grade of acetone followed by methanol - ultrasonically cleaned) Al samples. The table below shows that CO2 snow cleaning reduced the hydrocarbon levels to levels similar to solvents (data not shown). The percentage hydrocarbon removed by snow cleaning was greater (45%) than by solvent cleaning (30%) and this may reflect the fact that the different samples though from the same sheet.
Element Initial CO2 Snow Cleaned
Al 12 28
O 22 37
C 60 33
Na 1 0
Si 4 1
N 1 0
Inspection of the spectra before and after CO2 snow cleaning highlights the changes in the above composition table. Snow cleaning reduces the amount of hydrocarbons on the surface without altering other substrate related chemistries. The C 1s hydrocarbon peak is reduced (the C-H bonds) but the C-O and C=O peaks did not decrease. This finding suggests that snow cleaning removes only the hydrocarbon peak and does not alter other bonding arrangements such as the hydroxyl and carbonyl compounds. This finding has been seen on almost all surfaces.
CO2 Snow cleaning can clean contaminated surfaces, remove machining oils, remove silicone mold agents, clean parts for vacuum chambers, clean contact pin surfaces, and many other items. It will not remove native or any adherent oxides.