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Matching the Grade of Sapphire to the Application


The physical, chemical and optical properties of sapphire make it one of the most versatile materials from which various types of windows can be fabricated.  Some applications, such as precision optical components, however, demand the highest level of quality while others such as mechanical applications permit use of less perfect material.

 

In order to optimize the cost/performance ratio of the window being fabricated, it is necessary to match the grade of sapphire to the application.  Failure to do so can lead to over-specification, increasing the cost of the window unnecessarily, or under-specification, resulting in unsatisfactory performance.

 

Why are there different grades?

Natural sapphire is a gemstone variety of the mineral corundum, or aluminium oxide, commonly referred to as alumina or oxide, one of nature's most abundant compounds.  In its natural state, aluminum oxide is a white powdery material used extensively as an industrial abrasive.  When heated to about 2050 centigrade, the powder melts and can then be formed into a single crystal using any of several crystal growth methods.

 

Many methods of manufacturing single crystal sapphire today are variations of the Czochralski process, which was invented in 1916.  In this process a tiny sapphire seed crystal is dipped into a crucible containing molten alumina and then slowly withdrawn upward at a rate of one to 100 mm per hour.  The alumina crystallizes on the end, creating long carrot shaped boules of large size, up to 400 mm in diameter and weighing up to 500 kg.

 

Intentional or unintentional variations in the manufacturing process, however, can result in different grades of sapphire, some with nearly perfect optical properties and others with acceptable but imperfect properties.  The following is a discussion of how the various different qualities are graded.

 

How sapphire is graded?

Sapphire quality is graded based on its optical and physical properties. There is no single globally accepted grading system that is used by all manufacturers of synthetic sapphire. Instead, synthetic sapphire is graded by what is important for a particular application, either optical or mechanical

In general, there arethree main grades of sapphire:

  • A high grade of sapphire thathas little or no light scatter or lattice distortion and is used mainly for the most demanding optical applications.
  • An intermediate, ultraviolet grade sapphire that will not darken on exposure to UV light.
  • A lower grade of sapphire that may have extensive light scatter or lattice distortion and is used primarily for mechanical and structural applications such as bearings or fixtures. It can also be used for less demanding optical applications.

 

In another commonly used grading system for sapphire, shown below, grades 1 through 4 are considered to be of optical quality while grades 5 and 6 are considered technical quality.

  • Grade 1:Free of insertions, block boundaries, twins, micro bubbles and scattering centers.
  • Grade 2: Free of insertions, block boundaries and twins. Individual scattering centers (micro-bubbles less than 10 micro meters located no closer to each other than 10 mm) are allowed.
  • Grade 3: Free of insertions, block boundaries and twins.  Individual bubbles less than 20 micro meters located no closer to each other than 10 mm are allowed.
  • Grade 4:Free of insertions, block boundaries and twins. Bubbles less than 20 micro meters located no closer to each other than 2 mm, well as bubble clusters (which may include individual bubbles to 50 micro meters) less than  200 micro meters scattered no closer to each other than 10 mm within an effective volume are allowed.
  • Grade 5:Free of insertions, block boundaries and twins. Bubbles less than 20 micro meters located no closer to each other than 2 mm. as well as bubble clusters (which may include individual bubbles to 50 micro meters) less than 500 micro meters scattered no closer to each other than 5 mm within an effective volume are allowed.
  • Grade 6:Free of insertions, block boundaries and twins. Defective areas with bubble clusters greater than 500 micro meters are allowed.

 

Note that neither blue nor green coloration is allowed for any of the optical grades 1 to 4 above, while such coloration is allowed for either of the technical optical grades 5 to 6.