Choice of Heater for Vacuum Coating - Tungsten Heater

Vacuum coating is a sophisticated technique for the application of modern new technology, and it is also an important topic of applications and research for “modern physics experiment” of institutions higher learning. As for experimental teaching, the traditional approach is to use refractory metal—— tungsten， which has the highest melting point (3380℃) and molybdenum with the melting point of 2620 ℃ as a heating element. Plated metal is generally used lower melting point metals such as silver (melting point 960℃) and gold (melting point of 1063℃). In order to ensure the quality of coating, operators usually use high-purity gold and silver, but gold and silver need to go through a public purchase of the public security department for approval, so the price is very expensive. Therefore, in the choice of the coating material, it needs to consider all aspects of the problem.

Tungsten and molybdenum are refractory metal, and both of them have high temperature strength and excellent acid corrosion ability, which are an important material for military industry and the chemical industry. However, there are two issues that must be taken into account that “cold brittleness” of tungsten and molybdenum and they would act with the surrounding gas vehemently at high temperatures. Because of these two problems, these greatly limit the tungsten heater aluminized applications.

Thus, vacuum coating must note the followings:

1. The choice of heater material and the coated metals..
2. Maximize the degree of vacuum coating chamber (1 × 10-4mmHG ~ 1 × 10-5mmHG).
3. Coating chamber must be baked for a long time before coating the (30 to 45 minutes).
4. Using bombardment rod to bombard at low vacuum for a long time (20 to 30 minutes), these measures are to clean up miscellaneous ions and other gas molecules in coating chamber.
5. Control the preheat temperature and coating time to achieve the desired coating.  

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