Fatigue strength of nickel electrodeposits prepared in ultrasonically agitated bath

Abstract

Nickel coatings are widely used 1:o prevent wear of structural steels resulting from abrasion and corrosion. When used for this purpose it may be expected that the coatings should also influence other characteristics such as mechanical woperties. As the plating alters the surface characteristics of the base metal, the fatigue properties of steels may be markedly affected by the presence of such coatings. Fatigue crack initiation generally occurs at the surface of a specimen. The internal stress of the deposit, its inherent fatigue strength relative to that of the substrate, its hardness and its thickness have all been shown to affect the fatigue limit [1-3]. In recent years, the effect of ultrasonic agitation during electrodeposition has gained popularity [4-6]. Because of the catastrophic failure due to hydrogen embrittlement in many plated highstrength steels, any effort to improve plating should attempt reduction, if not total eJimination, of this undesirable feature. Research is being continuously carried out with this as the main objective. Ultrasonics provide a solution to this problem. The present work is concerned with studying the effect of ultrasonics of the fatigue properties of nickel electrodeposits. A comparison has been made between the deposits obtained from a still bath and an ultrasonically agitated bath. The composition of a nickel Watts bath used for the present study is given in Table I. Fatigue tests were carried out in a rotating, bending fatigue testing machine of type PUNZ. Mild steel specimens prepared according to DIN 50113 standards (Fig. 1) and coated with nickel (thickness 20/~m) were employed. The mechanical properties of the base metal were: yield strength 475 MPa, tensile strength 650 MPa and hardness 194 Vicker's Hardness Number (VHN). All the fatigue tests were carried out in air at room temperature (25 °C). The thickness of deposit was measured using the gauge, Mikrotest III. The surface roughness of the plated specimens was measured by a perthometer, Perthon S5P. Fig. 2 shows the fatigue life of the specimens at different applied stresses. Three samples were tested