Influence of TiN coating thickness on the rolling contact fatigue resistance of austempered ductile iron

• Three nominal coating thicknesses were analyzed: 0.7, 1.4 and 2.1 µm.
• Rolling contact fatigue (RCF) resistance increases as coating thickness decreases.
• A coating thickness of 0.7 µm enhances the RCF resistance of ADI.
• Samples with a coating thickness of 0.7 µm failed exclusively by substrate spalling.
• Samples with thicker coatings failed mainly by coating delamination.

The influence of TiN coating thickness on the rolling contact fatigue (RCF) behavior of austempered ductile iron (ADI) was investigated. Coatings were applied in an industrial PVD reactor using processing parameters specifically selected for ADI. Deposition times were adjusted to obtain nominal thicknesses of 0.7, 1.4 and 2.1 µm. RCF tests were performed in a flat washer type testing rig under a lubricated contact. The maximum Hertz contact pressure was 1400 MPa. RCF data were analyzed using the two-parameter Weibull distribution. The rolling track of the tested samples was examined by using SEM. As coating thickness increases, within the analyzed range, the arithmetic average roughness of the coated samples increases slightly while the bearing area decreases more significantly. The elastic modulus and residual stresses of the coatings also increase with thickness, but the coating hardness and the adhesion strength does not. Finally, the RCF resistance of the coated samples decreases as thickness increases. A coating thickness of about 0.7 µm enhances the RCF resistance of ADI substrates while thicker coatings worsen it. Failures in samples with a coating thickness of about 0.7 µm occurred exclusively by substrate spalling, while failures in samples with thicker coatings were mainly produced by coating delamination.