Fatigue Resistance and Crack Initiation Mechanisms in Nitrided Steel Treated with Selected Types of Surface Coating

Abstract

Surface treatment processes are usually applied to improve surface characteristics and resistance. Particularly duplex processes like nitriding combined with nanostructured based coatings are considered as the most advanced ones, with a large application potential in automotive industry. An example is an application in gear wheels, where fatigue loading in gear teeth roots exists besides contact fatigue on teeth sides. Contact fatigue, has to be therefore well balanced with pure fatigue resistance. Some surface treatments leading to excellent contact properties, wear resistance and extremely low friction coefficient may reduce resistance to pure fatigue crack initiation and subsequent growth to premature failure. In the paper, results of an experimental programme aimed at evaluation of fatigue resistance at repeated loading of nitrided steel after application of two types of coating, (i) nanostructured C-layer deposited by a modified cathodic arc technique according to Microcoat patent and (ii) Cr-WC:H-DLC layer produced according to Hauzer patent [1], are presented and discussed. Fatigue tests were carried out using small samples with 2.1 mm diameter, with nitrided layer of 0.35 mm thickness. Tests were completed with fractographical analyses of initiation areas. Results and analyses showed that coating effects depended on the specific technology and parameters. The Cr-WC:H-DLC layer affected fatigue limit favourably unlike the Microcoat C-layer, which resulted in reduction of fatigue strength. A detailed analysis of recorded data during static tensile tests indicated an occurrence of significant surface and subsurface residual stresses with a strong possible effect on fatigue crack initiation mechanisms. A significant, unfavourable effect of inclusions in the subsurface layer was demonstrated.