Tungsten Filament becoming brittle
Tungsten Filament becoming brittle
(OP)
I have been winding 0.25mm tungsten wire to make heating filaments. The wire is of course malluable before use (which is why I can wind it) and highly elastic.
After passing current through the wire (in a vacuum) the wire becomes very brittle and breaks very easily.
Why is this so? what processes are involved?
The filament holds a crucible so the filament breaking means that I lose my crucible as well.
I am trying to figure out how to prevent or reduce the risk of the wire breaking.
thanks
After passing current through the wire (in a vacuum) the wire becomes very brittle and breaks very easily.
Why is this so? what processes are involved?
The filament holds a crucible so the filament breaking means that I lose my crucible as well.
I am trying to figure out how to prevent or reduce the risk of the wire breaking.
thanks





RE: Tungsten Filament becoming brittle
RE: Tungsten Filament becoming brittle
embrittlement depends on how the wire was drawn.
typically it is the result of iron contamination in the drawing process.
the fact that you are using a mandrel to form your filament may be an additional source of contamination.
what is the cruciable material and what temperature are you trying to achieve?
RE: Tungsten Filament becoming brittle
RE: Tungsten Filament becoming brittle
believe the application involves temps considerably greater than 900 oF and is being used in a relatively low stress application, but you've raised a good point
RE: Tungsten Filament becoming brittle
The driving force for grain growth is surface-tension forces between grains. JimMetalsCeramics was right in recommending using MgO inclusions to pin the grain boundaries against the sufrace tension forces. I hope this helps.
RE: Tungsten Filament becoming brittle
Pure tungsten will recrystallize to an equiaxed structure @1200C, and in this condition is extremely brittle at RT. Lamp grade tungsten is doped with potassium (~80ppm) which impedes GB growth, raising the re-x temp to @ 2000C, and imparting ductility when cool. However, Fe, Ni, and Al on the surface can cause equiaxed morphology to develop, and brittleness, so keep the wire clean. Also, make sure there are no residual carbon-containing gases in the furnace to prevent carbide growth. If you notice that the windings have sagged, check for poor vac (i.e. oxygen contamination)
good luck
RE: Tungsten Filament becoming brittle
How does the crucible fit into the filament? When does the filament break? Is it in use or during cooldown (differential expansion of the crucible and filament)?
RE: Tungsten Filament becoming brittle