Picture a pot of boiling water with a lid covering it. The steam rises to the lid, condenses, and little water droplets form. Now picture a pot of molten metal with a lid covering it. What happens when you heat the pot hot enough? The same process occurs, except the lid, instead of being coated with water gets coated with a layer of evaporated and resolidified metal. When this process is done in a vacuum, the lid will be covered with a smooth coat of resolidified metal. The pot, in this case, is called a crucible, and the lid is the deposition point. If you changed the shape of a crucible to look like a flattened Viking boat and made it smaller, it would be an evaporation boat.
Wait, how do crucibles get hot enough to boil metal?
Different types of metal have different melting points. That means that whatever you are trying to evaporate has to have a lower evaporation point than whatever the evaporation boat is made out of.
So, what has a high enough evaporation point to be used as a crucible or evaporation boat?
Right now, there are two industry standard metals alongside some ceramic composites.
- Molybdenum evaporation boats are incredibly durable, corrosion resistant, and have a high melting point.
- Tungsten evaporation boats share those qualities. Tungsten, in fact, has the highest melting point of any metal in pure form at 6,170 degrees Fahrenheit. That?s why tungsten is the typical go-to metal for crucibles and boats. It also has the highest tensile strength and lowest vapor pressure. Some evaporation material, however, will be alloyed by tungsten during this process, contaminating the evaporated metal.
What is this process used for?
Ever own an electronic device? The circuit boards inside your phone or computer are coated with gold and copper and all types of conductive alloys. The evaporation process using a crucible or boat is what made it possible. There are many other uses like copper plated wiring, saw blades, and airplane engine parts.
The uses for a tungsten or molybdenum evaporation boat are integral to various industries that make our society run smoothly. If you are looking for a boat or crucible, then you should know there’s a reason these metals and their alloys are standard across industries.