Tungsten Heater for Vacuum Coating

A vacuum is a kind of gas state whose pressure is less than atmospheric pressure. Natural vacuum is a kind of vacuum that the air pressure decreases with the increasing of altitude and present in the astrospace. Man-made vacuum is to use of a vacuum pump to draw out the gas from vacuum vessel so that the pressure of container can be lower than atmospheric pressure. Generally, vacuum coating is operating by physical vapor deposition and chemical vapor deposition methods. Normally, it can be divided into vapor deposition, sputtering and ion plating. Vacuum coating often adopts tungsten heater to evaporate aluminum. The life time of the heater is also restricted by many factors, such as the degree of vacuum, current, voltage, temperature, and the amount of aluminum and evaporation time.

It is proved that the amount of insertion of the aluminum mandrel has a decisive role on its life time. On the premise of ensuring the plating thickness and unchanging other conditions, the amount of insertion of the aluminum mandrel should be minimized, the greater the amount of aluminum is inserted, the heater's life will be shorter. Because that there are some residuary aluminum on the tungsten heater after the completion of vapor deposition, and aluminum will react with tungsten, leading to corrosion of tungsten. As a result, with a long-term cumulative, it will greatly shorten the life of the tungsten heater.

In aluminum case of residual aluminum, the life time of tungsten heater is one-third less than those heaters which have no residual aluminum. Therefore, according to statistical data, the amount of the insertion of aluminum should be controlled within 5%~8% of the weight of the heater. Under the premise of ensuring the quality of coating, the fewer amounts of aluminum, the higher the rate of evaporation, and the degree of corrosion of the heater is also lower, therefore, life time of tungsten heater will be longer.

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