Is Sapphire Malleable?

 

The word “malleable” comes from the Latin malleus meaning hammer or mallet.  It refers to the property—most often associated with metals—of being susceptible to plastic deformation (particularly at room temperature).  In other words, something that is ductile and can be physically hammered into a new shape.  The most common malleable materials are metals with relatively low melting points and high crystalline symmetry such as gold, which can be pounded into sheets thinner than paper.  We tend to think of ceramic materials as being distinctly non-malleable.  They deform elastically until they reach their elastic limit at which point they fail by brittle fracture.  Sapphire is no exception – or is it?

A few years ago I was setting up an experimental growth run of a sapphire crystal by the EFG (Edge-defined, Film-fed Growth) method.  I was using a long sapphire seed about 10 mm in diameter (itself growFig 1n by EFG).  The first figure shows the seed hanging in the hot zone which is mostly graphite.  The die structure has some sapphire “crackle” placed on top of it as an indicator that the right growth temperature has been reached (on heating up it melts).  I had intentionally suspended the seed from a flexible mount to ensure that it hung vertically.  The problem was that the surface tension of sapphire is so strong that, unless the die assembly and seed are perfectly symmetrical and perfectly aligned, surface tension will pull the crystal to one side until gravity pulls it back to center, at which point it overshoots and surface tension pulls it to the other side.  This continues until you grow a zigzag crystal—which by 3 o’clock in the morning more closely resembles the cloven hoof of Satan.  You can see this in tZig-zag xtalhe second photo where I have graphically indicated the original seed.

On the next growth attempt the same behavior began so I decided to force the seed down hard on the die.  The next thing I knew was that the seed had disappeared from view.  (As you can see from the first photo, the sight window does not provide a very wide angle of view.)  At first I assumed that I had simply broken the seed, except that I couldn’t see any pieces of seed crystal lying on or near the die.  I was totally flummoxed by this until I cooled the system down to room temperature and opened the chamber.  Then I saw that I had actually bent the sapphire seed!  Since the seed is hanging in the afterheater section of the furnace at a temperature above 1900 C, we can conclude that, at a sufficiently high temperature, sapphire can indeed be plastically deformed.  (The dotted lines show the extent of the bending in the seed.)

Fig 3
Once I redesigned the seed holder assembly I was able to grow the sapphire plates with no problem.  The next picture shows the early stages of growth of a typical plate produced by this system.  The fully grown plates can be seen on other pages of my website and the last photo shows a typical end use application—windows for the infrared sensor pod on the F-22 Raptor.  Sapphire’s extreme hardness combined with its wide rangeof transparency make it an ideal material for this application.

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F-22