Wednesday, March 5, 2008

Structure and Mechanical Properties - Nickel Plating

Structure and mechanical properties of nickel plating are affected by the operation condition of nickel bath plating. When nickel plated at 55 oC and pH 2.0, deposits show a columnar or conical structure which grows courser with increasing thickness. Grain refinement occurs as the pH or the chloride content is increased or the temperature is decreased. The effect of current density varies with conditions. The hardness, tensile strength and ductility of nickel deposits are almost always consistently related; that is an increase in hardness is accompanied by an increased in tensile strength and a decrease in ductility.

Soft ductile deposits are obtained at pH 4.5 or lower. For example, a tensile strength of 35 kg/mm2, an elongation of 37%, and a Vickers diamond pyramid hardness (10 kg load) of 100 in nickel deposited at pH 4.5. As pH was decreased below 3 the hardness rose slowly, but as pH was increased above 5 it rose rapidly, with a corresponding increase in tensile strength and reduction in ductility.

Current density over the range 1 to 5 A/dm2 had little effect on the mechanical properties of the deposit from the low pH bath, but at pH 5.0 there appeared to be some decrease in hardness and tensile strength with increasing current density in this range. Hardness reached in a minimum of 135 at 4 A/dm2 and then increased slowly with further rise in current density. An increase in temperature of the electrolyte had the expected effect of decreasing hardness and tensile strength and increasing ductility.

Nickel can be deposited with a wide variety of metallographic structure and with controlled physical and mechanical properties over a wide range of values. When other than a soft, ductile nickel is required, additive of bath composition quite different from the Watts solutions are employed.

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