Microscopic cracking and spalling behavior of the duplex layer of a gas-nitrided gun bore surface

In the present study, the initial wear behavior of the duplex layer consisting of an oxide film and a compound layer on gun bores was investigated. The duplex layer was produced by the gas-nitriding and the post-oxidization to the surface of the 5.56 mm gun bores. The thickness of the duplex layer was measured from the specimen taken from a new gun barrel with an optical microscope. After real firing, the specimens were cut from the barrels. The microscopic wear behavior and phase identification of the duplex layer were observed by using a SEM and a XRD, respectively. A lot of surface cracks linked together to form the network of the cracks on the surface and the parallel cracks occurred in the interface between the oxide film and the compound layer. When the surface cracks and the parallel cracks join orthogonally, the spalling takes place.