Hardness and surface studies on electrodeposited alloys using optical and interferometric methods

Abstract

This thesis presents the results of hardness and surface studies carried out on electrodeposited tin-nickel and tin-copper alloys using optical and interferometric methods. The importance of hardness testing of electrodeposits is surveyed with a brief account giving the main reasons for choosing tin-nickel and tin-copper alloys for hardness study. The various methods used to study the hardness of electrodeposits are reviewed. The structure and hardness values of the tin-nickel alloy deposits are found to be affected by each of the operating conditions of electrolysis, such as current density, pH of the solution, temperature of deposition, etc. There appears to be a close relationship between the structure and the hardness properties of these deposits. Thus the brighter and smoother the deposit, the harder it is. Conversely, as the grain size in the deposit increases with a consequent fall in reflectivity, it is found that the deposit becomes softer. Though the change in any of the operating conditions affects the structure as well as the hardness properties of the deposit, the deposits of tin-nickel are still found to be compact, homogeneous and isotropic. The isotropic nature of the deposits is revealed by the phenomenon of Ring cracking and further supplemented by the observations with the double-cone indenter. As a result of this study it has now "been possible to determine the optimum conditions for the electrodeposition of tin-nickel alloy for maximum hardness. These are: current density 2k amp. per sq.ft.; temperature of bath 70 - 80°C., pH 2.5. Under these conditions the deposit is hardest (710 D.P.H.) and has a high reflectivity with very fine grain. Furthermore, the present study has shown that the hardness of tin-nickel deposits is largely independent of the thickness of the coating, except possibly for very thin coatings. The hardness properties of tin-copper alloy deposits of various compositions (in the range 10 - 86 Sn) have been studied. It is found that the deposit composition has a marked effect on the hardness value of the deposit. Amongst the different tin-copper alloy deposits studied, speculum is found to possess the highest hardness (D.P.H. 520 +/- 9). Moreover, these deposits, like tin-nickel, are found to be compact, homogeneous and isotropic. The influence of the base metal on the measurement of hardness of both tin-nickel said tin-copper alloy coatings has been investigated in detail. For thinner coatings, the depth of the indentation and hence the apparent hardness may be affected by the hardness of the underlying metal. A minimum thickness of coating is necessary to give the true hardness figure of the coating independent of the nature of the base metal. The important results obtained may be summarised as follows: (1) If the deposit has the same hardness as the base, then its hardness value is completely unaffected "by the "base whatever is the depth of penetration of the indenter, (2) if it is softer than the base, its indicated hardness remains relatively unaffected by the base until the indenter has penetrated almost the whole thickness of the plating and then gradually rises towards the hardness value of the base and (3) if. the hardness of the deposit is greater than that of the base metal, its true hardness is not measured if the indenter penetrates beyond a characteristic depth, as for example, in the case of tin-nickel alloy deposits, this is l/l6th of the thickness of the deposit and in the case of tin-copper alloy deposits it is l/8th of the thickness. If, however, the diamond pyramid penetration exceeds the characteristic depth in the respective deposit, then the indicated hardness in general decreases with load initially rapidly and then gradually, finally approaching the value of the base metal.