Various industries employ myriad consumables for vacuum operations. These consumables may cost anywhere from a couple of hundreds to lakhs of rupees. This gives rise to the question: Are low cost consumables lesser in importance to high cost ones? Is that the reason why they cost less?
What is the cost of an O-ring seal?
And what is the total cost of the entire Space Programme?
Is there any comparison?
Did anyone imagine even in their wildest dreams that ignoring the quality of such a small product can have such far reaching consequences? Not at that time surely. But after the accident they do!
In NASA, now no one takes O-rings lightly. Or for that matter, seemingly low value items. Low in cost, yes, but not low in value!
Having made my point, I want to highlight the importance of two low-cost consumables in the vacuum industry: Graphite Foil/Tape and Graphite Suspension/Paste.
This is a thin paper-like material with a smooth satin finish. In actuality, it’s not paper but highly compacted graphite that has been cut into rolls for ease of use.
Inside the vacuum chamber it is used in between the copper clamp and the evaporation boat. Its uses are-
1. It fills the uneven gaps between the clamp end and the evaporation boat in order to prevent sparking.
2. While filling up the minute gaps it allows electrical current to flow easily from the clamp into the boat in order to heat it up.
This seemingly simple and unimportant consumable can cause absolute havoc to the entire vacuum system if chosen badly.
- Graphite foil meant for vacuum applications should never have even trace amounts of binders or bonding agents like glue, resin or rubber compounds in it. Reason: Under vacuum, these binders (a) release unwanted vapors inside the chamber and affect the vacuum and (b) stick to the substrate, thereby ruining the product quality.
- Plumber-grade graphite foil may seem smooth in texture but is not compressible enough. Note: Compressibility is one factor which determines how well the foil will be able to fill the minute gaps existing in between the copper clamp and the evaporation boat. If not filled with foil properly, sparking occurs in these gaps.
The more the compressibility, better will be the void fill. Good graphite foil will be highly compressible and will not allow any sparking to take place.
- 100% purity is an absolute must in graphite foil.
Reason: This will ensure that the graphite foil will be high on electrical conductivity and that conductivity will remain constant. If there are impurities the amount of current conduction fluctuates. This causes costly power supply transformers to burn out.
- The foil should never, under any circumstances, heat up. Resistance to high temperatures is another absolute must for graphite foils.
Reason: In the vacuum chamber it may come in contact with molten metals like aluminum etc. Low-grade foil will heat up and burn or melt when subjected to temperatures as high as 1500°c. This can have disastrous consequences in the sense that now Hot Evaporation Boat will come directly in contact with Copper clamp and melt it. This further escalates the damage to Metallizer.
- High grade graphite foil will always be inert to most molten metals as well as the process gasses. Caution: Presence of binders in low grade graphite foil fails to make them inert, this causes chemical reaction and production of harmful substances.