Vacuum Web Coating

Should we be satisfied with a boat life of ~15 hours?

            Systems can be driven hard and the boat life suffers.  There are low cost sources of boats but this too tends to lead to shorter boat life as the refining of the powers for the ceramic composite tend to be coarser & less densely packed.

            Compared to this is a small trickle of information that occasionally leaks out of Japan.  A company wished to use their existing induction heated crucibles for the deposition of aluminium but were advised against trying. The reasons given were that molten aluminium is extremely corrosive & the crucibles would not survive. Despite the warnings they tried & sure enough the crucible lifetime was short, less than 10 hours.  Rather than this being a deterrent they decided to improve the crucibles to increase the lifetime until they became acceptable.  Thus over the years they have improved to around 40 hours.

            I was trawling through some references looking for something else & I cam across an article about using air-to-air metallizers. In this article it referred to using an induction heated linear crucible of 75" length for depositing aluminium where they had increased the lifetime up to & beyond 120 hours.

            Now having read that I am asking myself why should the crucibles for an induction-heated source last for 120 hours & for resistance heated sources only last around 15 hours. Are the materials so radically different?

            The answer appears to be, no, the basic crucible material can be the same.  What is different is they spend time in making sure they have produced the densest material possible & then where the aluminium contacts the surface they have deposited alumina to fill the pores.  Thus the aluminium cannot penetrate the surface & corrode the crucible.

            So I now ask myself who is going to be the first to offer this same technology for resistance-heated boats?  Perhaps they are already available from Japan & I just do not know where to buy them.  If anyone out there knows of a source of this material or a supplier of these boats I would be interested to know more. 

HELP - overflowing boat problem

            I was recently sent a question.  A problem of the molten pool overflowing one end of the evaporation boat and reaching the water-cooled copper contact.  The result of this is that the level of spitting increases.

            My first response was to suggest that the boat was checked to make sure it was fitted level.  The response back was that the boats were level & that was not the problem.

            Whilst I know that hot spots on boats can sometimes change the shape of the molten pool I have never come across a pool variation such that the liquid all collects at one end of the boat to the point of overflowing the recess & reaching the copper blocks.

            My suspicion is that despite the protests that the boats are level they are, in fact, not as level as believed.

            What do you think?   

Are there any other reasons for this behaviour?

Evaporation Boats

What is the difference in two component and three component evaporators?

The boats can made up of combinations of 2 or three ceramic powders that are blended & compressed together. The three materials are titanium diboride, an electrical conductor that is easily wet by the molten aluminium, boron nitride, an electrical insulator that has good thermal shock resistance and is easily machined and finally aluminium nitride that is an electrical insulator but has good thermal conductivity.

Using different combinations gives the boats different properties of wetability, electrical conductivity, thermal stability, wear resistance and lifetime.