At temperatures above 2012° F (1100° C), graphite dust from graphite liners\/shields or heating elements can attack and carburize molybdenum sheaths. This causes the sheath to crystallize and become porous, losing its protective properties. The thermal elements in turn crystallize and open. Coating the heat-affected length of the molybdenum sheath with a thick layer of tungsten reduces this carburization action and extends probe life. The tungsten coating also provides increased abrasion resistance.