In general, strength and hardness are not synonymous
terms. Material strength can be evaluated on a number of criteria…..tensile
strength, hardness, brittleness, etc. Dependent on what your criteria
are for defining “strength” in the case of a metal….your answers
will be different. In terms of hardness, tungsten carbide is the probably
hardest material currently used in the manufacturing of watch cases. Rado, Movado, and a number of Asian manufacturers are working with
this material. Although extremely scratch resistant….it may be more
prone to stress fractures than steel in certain cases.
I decided to see what I could find on the subject of material hardness
as it applies to scratch resistance. In most cases, the term hardness/strength
is used somewhat synonymously to scratch resistance.
From what I have been able to find…..sapphire rates as a 9 on the
Mohs Hardness Scale, which is a commonly used hardness ranking.
Below is a revised Mohs’ Hardness Scale with some everyday items listed:
Gypsum (fingernail at 2.5)
Calcite (copper – old penny at 3.5)
Apatite (window glass or typical knife blade at under 5.5)
Orthoclase (good steel file at over 6.5)
Corundum (sapphire, ruby)
The Mohs’ Hardness Scale is relative, which means that fluorite at
4 is not twice as hard as gypsum at 2; nor is the difference between
calcite and fluorite similar to the difference between corundum and
diamond. An absolute hardness scale is somewhat different than the
relative scale. Using special equipment (which I do not really understand),
a comparison of the absolute hardness of minerals can be measured.
It turns out that most minerals are close in hardness. But as hardness
increases, the difference in hardness greatly increases as seen in
the scale below. Please note that the actual values of the hardnesses
have no units of measure….they are only useful when compared against
This is an absolute hardness scale:
As you can see, diamond is 4 times harder than sapphire, even though
the two are only separated by a single point on the Mohs scale. Another
important note is that many minerals display different hardnesses
based on the direction of the stress, and thus will actually behave
as softer or harder depending on the stress applied. Corundum (sapphire)
is not one of these minerals.
A variation of the aforementioned absolute scale is the Knoop hardness
scale. This absolute hardness scale provides a better method for direct
comparison of two materials. Sapphire has an absolute hardness of
between 2000 and 2050. Generally speaking, any material will scratch
another material that is lower on the scale.
Both ruby and sapphire are varieties of corundum (see above) Some
synthetic materials which have hardnesses of 9 (and thus would scratch
a sapphire crystal) or above are as follows:
|Synthetic (or natural)
|10 (Absolute 8000 to
||9.0 (Rado uses this in
some watch cases, as does Movado). The material is very heavy,
with density of about 15.7g/cubic centimeter. On the absolute
hardness scale, it ranks as very slightly harder than sapphire
(Absolute 2050 to 2150).
||About 9.4 (I don’t think
this is used in watches…generally used as an abrasive. Absolute
2150 to 2950)
||About 9.5 (I don’t think
this is used in watches. Absolute 2900 to 3900)
|Cubic Boron Nitride:
||In excess of 9.9 (Not
used in watches. Very difficult to manufacture, and can only
be made in very small pieces. Absolute 4500 to 4600) .
Most of these materials are not commonly occurring in everyday life,
with the exception of perhaps diamond. It is not likely that a sapphire
crystal will become very scratched.
It is important to note that in the literal sense, hardness is not
synonymous with strength. Brittleness basically indicates how
resistant the material is to plastic deformation. A very brittle material
will, when placed under stress, break/fracture rather than bend. In
the case of a sapphire crystal versus a glass crystal, the sapphire
is considerably more brittle. As a result, a sapphire crystal is more
likely to chip or crack than is glass counterpart if both are subjected
to an equally hostile stress (banging, etc.).
I hope this helps…and that is accurate. I am not an engineer…..so
my understanding of this subject is a bit limited.