Case Histories » Aerospace

Spalling of Nitrided Surface Layer

Cracked nitrided surface layer from an aircraft landing gear
Overall view of the localized damage observed on the landing gear component (Magnification ˜ 40X)
SEM of cracked nitrided surface layer
Scanning Electron Microscopy (SEM) image of the surface spalling of the nitrided surface layer (Magnification ˜ 500X)

Specimen

Nitrided landing gear component (Aerospace).

Material

Age-hardenable nitriding steel.

Background

The nitrided component was part of an aircraft landing gear. A visual inspection revealed it had suffered surface damage (Figure A).

Service Life

The landing gear had been in service for approximately 500 hours, with a total of 596 landings. At this time it was removed from the aircraft and subjected to several load tests, estimated at approximately 1000 – 3000 landing load cycles. The landing gear assembly was disassembled, at which point it was noticed that one of the four nitrided components was damaged.

Findings

The localized damage observed on the surface of the nitrided component was caused by an excessively hard nitride surface layer (1117 – 1148HK). The impact energy the landing gear absorbed as a result of the aircraft touching down during landing, in conjunction with the sliding motion between the nitrided surface and landing gear parts, had resulted in the initiation of subsurface cracking. The subsurface cracking propagated parallel to the surface and led to the spalling of the nitride layer on the surface (Figure B).