Surface analysis of failed vacuum reeds

The failure of vacuum reeds mainly appears as the switch is off but the current is still retained. People used to call it "adhesion" of the contacts. For new type of reeds, a thick layer of tungsten is plated on the blade surface for improving the operation life. Experimental results show that tungsten plating can greatly increase the operation life. However, failure of the reeds are still found. Micro-investigation of the failed reeds shows that "volcanos" or craters are observed on the contact surfaces which is the result of "explosion". It is reasonable to deduce that when arcing is formed at the contact surface, temperature may rise rapidly to very high rating. If the temperature on the tungsten plating is high beyond its softening point (1000/spl deg/C), the surface hardness is reduced. When the temperature of the blade underneath the tungsten plating increased above its melting point, the expansion of the iron alloy, and the vapor pressure may form an extremely high force upward to extrude through the plating. Metal vapor concentrated between cathode and anode may cause the retention of electric current. Cracks seem to have no responsibility for the failure.