Prakt. Met. Sonderband 38 (2006) 337 
4. SUMMARY 
Internal thread former in new condition, after 10.000, 20.000 produced parts and at the 
end of service life were investigated. The investigations showed that mainly the third and 
fourth tooth were damaged and the damaged area was concentrated on the tip and 
increased according to the run-time. The damage intensity on the other teeth was 
remarkable less. Therefore it was not possible to reconstruct the history by examination of 
a single thread former. To investigate how the failure mechanism starts metallographic 
pictures in various depths of grinding were taken. Cracks in the TiN-coating nearly parallel 
to the coating surface were found there. This leads to the conclusion that the TiN coating 
Zi pm collapses partly in small local areas in an early state of life time. Those flakes break away 
Co and the remaining TiN coating takes over the wear protective function. This cycle runs until 
the substrate material is reached. From this moment forth, abrasive wear starts destroying 
the geometry of the teeth. The SEM investigation showed that the substrate material in the 
damaged area is covered with zinc, which comes from the thread tube. This early starting 
mechanism mutates the tribological system which is enlarged of the substrate material. 
The more intense reaction to the etching process and the decrease of hardness in the 
zones of the massive material removal points to a high thermal stress. The failure 
mechanism could be identified as local coating collapse caused by local plastic 
deformation possibly because the substrate material does not sustain sufficiently the TiN- 
coating. Since the coating is removed, the substrate material is being worn and thus the 
geometry destroyed till the end of service life. 
20pm, 
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