Internal stresses electrodeposits

Electrodeposits of Pt can only be applied as relatively thin coatings that are porous. Although the porosity decreases with increase in deposit thickness, so does the internal stress and if the platinum adhesion is poor the coating may exfoliate. As a consequence, thicknesses of 2-5 to 1-5 fim Pt... 

Electrodeposits are usually in a state of internal stress. Two types of stress are recognised. First order, or macro-stress, is manifest when the deposit as a whole would, when released from the substrate, either contract (tensile stress) or expand (compressive stress) (Fig. 12.12). Second order or microstress, occurs when individual grains or localities in the metal are stressed, but the signs and directions of the micro-stresses cancel on the larger scale. The effects of first order stress are easily observed by a variety of techniques. 

Walker, R., Internal Stress in Electrodeposited Metal Coatings, Industrial Newspapers Ltd., London (1968)... 

Finally, as in electrodeposition, additives are often employed to obtain deposits with decreased internal stress and improved mechanical properties. This is an important area, but one that is not well documented in the technical literature. Since it is not in their best interest, information about such additives is usually not disclosed by vendors of plating solutions, yet additives are often the key ingredient in a plating solution that makes it more suitable than others for a particular application. A discussion of additives used in electrodeposition is available in [124], A brief discussion of additive selection for a new electroless Cu solution is also available [96],... 

Whiskers grow in the presence of large growth-inhibiting molecules, which usually show a preferential adsorption on certain crystal plains. Incorporation of adsorbed particles in the crystal [86] and internal stress generated during the electrodeposition are major causes of whisker... 

Thick chromium deposits have high residual internal stress and may be brittle due to the electrodeposition process, in which hydrogen can be incorporated in the deposited layer. Cracks result during plating when the stress exceeds the tensile stress of the chromium. As plating continues some cracks are filled. This led to the development of controlled cracking patterns, which produce wet-table porous surfaces that can spread oil, which is important for engine cylinders, liners, etc. 

A fused-Sn plating is one that has been reflowed (melted and resolidified) subsequent to electroplating. Typically, an electroplated part is dipped into a hot-oil bath and held at a temperature above the melting point of Sn. The objective of the fusing operation is to allow the Sn to slowly sohdify and, thereby, remove any internal stresses that may have resulted from the electrodeposition process. Fused Sn is reputed to not grow whiskers unless the surface of the tin is scratched, compressively stressed, or plastically deformed [2]. 

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