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 sialon:

Hardness of Alumina is 1800 Knoop.  Sialon is  2000 Knoop and Silicon Nitride is 2200 Knoop. Diamond is 6000 - 9000 Knoop.

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, 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.