MIXTRON FACEPLATES - and their wear
Most manufacturers of microwave moisture
sensors for concrete mixers use a ceramic known as alumina, which is
aluminum oxide, Al2O3. This
material is very hard and tough, lasting months to years in concrete mixer
applications, depending on the volume of concrete produced and the abrasive
qualities of the aggregates. One other manufacturer has switched to
sialon, which is an aluminum oxide / silicon nitride composite giving
superior wear characteristics. In 2002 we investigated sialon and silicon
nitride, settling on silicon nitride, Si3N4
for all future production. This material is harder than both alumina and
Hardness of Alumina is 1800 Knoop.
Sialon is 2000 Knoop and Silicon Nitride is 2200 Knoop. Diamond is 6000 -
Although the difference in hardness
does not seem a lot, the difference in wear is amazing. The reason
is in the mechanism of wear; aggregates are just hard enough to
scratch and abrade the surface of alumina, but they cannot make an
impression on the slightly harder silicon nitride. Thus, it
suffers practically no abrasion from even very aggressive aggregates.
The cap at right is alumina and has been in a high production concrete
mixer using very abrasive aggregates for only 4 months. Note the
wear at the left hand edge. The cap at left has been in a similar
mixer that had a similar wear problem until the cap was replaced with
silicon nitride. Measured wear is only 0.010" over one year of
operation, 50 hours of continuous operation per week with granite aggregates, making its projected lifetime 10 years - an increase of 20 to
30 times! Note the high polish on its surface, showing that the
aggregates cannot score this material.
Since 2002 we have eliminated
replaceable caps. The wear characteristic is so good that we don't need to
replace them - they last for the life of the sensor. And the proof
is that we've only had one back, as of January 2014.
faceplate at right has been returned in 2013, at the end
of its useful life at the same plant, A. Jandris and Sons,
Gardner, MA. After 9 years of use with granite aggregate, although worn down to the metal on the right
side, the left side is almost untouched. If it had been
rotated occasionally to even out the wear, it would have lasted longer.
Call us for this reference if you would like to verify it for
Note that the faceplate is twice as
thick as it seems from the outside. It's stepped so that
an equal depth of material lies inside the metal tube.
This explains its enormous strength and freeedom from breakage
when used as recommended.