Hard chrome coating is widely used for its high hardness, wear resistance, low friction, and corrosion protection. However, it may reduce fatigue strength due to surface cracks and tensile residual stresses introduced during the coating process. This study investigates the effect of coating thickness on the fatigue performance of full-scale rods used in hydraulic systems.

The specimens were made of 42CrMo4 alloy steel, with a diameter of 50 mm, a length of 400 mm, and an induction-hardened surface layer of 3 mm. Three configurations were tested: NR (uncoated), R20 (20 µm chrome coating), and R50 (50 µm chrome coating). Fatigue tests were performed using three-point bending at a stress ratio of , with a frequency of 3 Hz, up to 2 million cycles, in air at room temperature.

Fractographic analysis was conducted to identify crack initiation and propagation mechanisms. The Kitagawa–Takahashi diagram was used to evaluate the influence of coating thickness on the fatigue limit. A predictive S–N curve model was developed to estimate fatigue life as a function of chrome thickness. The results provide practical guidance for optimizing coating parameters in fatigue-critical applications.

Keywords: Hard chrome coating, Fatigue, Kitagawa–Takahashi diagram, S–N curve prediction