Effect of addition agents on the structure, internal stresses, hardness and annealing behavior of chromium electrodeposits.

摘要

Chromium was deposited on electroless-nickel, copper and steel substrates from four solutions which differed in the addition-agent composition. Scanning (SEM) transmission (TEM) electron microscopy, scanning tunneling microscopy (STM) and X-ray diffraction (XRD) were the materials characterization techniques. The internal stresses were determined. The effect of annealing on the hardness, grain size and microstrain were also determined.; The orientation of very thin chromium coatings was determined by that of the substrate. A plane containing the close-packed {dollar}langle 111rangle{dollar} direction of chromium tended to be preferentially parallel to the surface and to one in the substrate which led to lateral growth. Scanning tunneling microscopy and transmission electron microscopy showed that the initial and subsequent chromium layers developed by three-dimensional crystallite formation and coalescence. After reaching a certain thickness, deposits from three of the plating solutions had the {dollar}langle 111rangle{dollar} close-packed direction perpendicular to the surface and correspondingly exhibited a fiber-like structure in the cross section. The deposit from the fourth solution had the {dollar}langle 100rangle{dollar} direction preferentially perpendicular to the surface which led to a more equiaxed grain structure. Additions of sulfoacetic acid to the plating solutions produced crevices in the chromium deposits due to a non-random distribution of occluded sulfur. The deposits also resulted in small crystallites having smoother side walls.; The development of an initial tensile-stress maximum coincided with crystallite coalescence. Deposits plated from solutions containing sulfoacetic acid exhibited the highest tensile stress due to smaller crystallites and greater misorientation between them. The smoother side walls of the crystallites facilitated coalescence and thus contributed to the tensile stress. The decrease in tensile stress after the maximum was attributed to deposition between existing crystallites and microcrack formation. Microcracks were found to propagate along areas high in occluded sulfur. Deposits on electroless nickel had the lowest internal tensile stress probably due to the substrate being in compression.; The microstrains determined from X-ray line broadening were directly related to the hardness and proportional to the oxygen content. The hardness decrease in deposits from three solutions was due to grain growth. An increase in hardness upon annealing in deposits plated from the solution containing sulfoacetic acid was attributed to a precipitation-hardening mechanism.; Chromium deposits plated over a compressively stressed electroless-nickel layer protected steel from corrosion due to a lower microcrack density.