Fatigue strength of coatings and materials under continuous load

Impact test during inspection of a coating on a piston ring.
© Fraunhofer IST
Impact test during inspection of a coating on a piston ring.

Impact test of coatings

The impact test is a test method used to test the behavior of thick or thin layers, but also of solid materials with regard to permanent load. A fixed test ball exerts a long-term cyclic load (up to 1 million cycles) on the component to be tested. Depending on the area of application, the impact test can generate a fluctuating or repeated load (contact mode or lifting mode). The impact test provides a statement concerning the fatigue strength of the surface. 

How the test is used

Die charakteristischen Prüfparameter beim Impact-Test sind:

  • The characteristic test parameters in the impact test are:
  • Load: 500 - 5000 N
  • Duration: 100 - 1,000,000 cycles
  • Frequency: 20 - 70 Hz
  • Counterbody: carbide or ceramic balls

The evaluation is usually carried out under an optical or scanning electron microscope, whereby deformations, cracking or spalling can be documented and quantified, including in cross-section if necessary. 

Load series (e.g. 1, 2, 3, 4, 5 kN) or cycle series  (z.B. 102, 103, 104, 105, 106 Zyklen) are possible in order to determine the damage threshold or service life of the materials. 

Load variation

Prüfung einer CrN-Schicht auf Stahl. Feste Zyklenzahl von 100.000, Variation der Last von 200 - 600N.
© Fraunhofer IST

Testing of a CrN coating on steel. Fixed number of cycles of 100,000, variation of load from 200 - 600N.

Variation of the load duration

Prüfung einer CrN-Schicht auf Stahl. Feste Last von 3000N, Variation der Dauer von 1000 - 100.000 Zyklen.
© Fraunhofer IST

Testing of a CrN coating on steel. Fixed load of 3000N, variation of duration from 1000 - 100,000 cycles. 

Impact test sample disruption

CrN-Schicht auf Stahl. Die Querschliffpräparation durch den Impactkrater zeigt die Rissbildung und Zerrüttung in der Schicht und der darunter liegenden Nitrierzone.
© Fraunhofer IST

CrN layer on steel. Cross-section preparation through the impact crater shows cracking and disruption in the layer and underlying nitrided zone. 

Publications

 

Fraunhofer IST in the journals

Publication

Reinhold Bethke, Antje Hipp, Hannes Meyer, Nadine Nöcker

Bewertung der Lebensdauer von dünnen Hartstoffschichten – Der Impact-Test als ergänzende Prüfmethode.

In: Vakuum in Forschung und Praxis, September 2017, Vol. 29 Nr. 4, S. 32-37