Historical Articles

August, 1952 issue of Plating

 

Quantitative Measurement of Adhesion of Electrodeposited Metals

(Abstracted from a thesis presented by H.C. Schlaupitz in partial fulfillment of requirements for the degree of Master of Engineering, Yale University, 1951.)

H.C. Schlaupitz, R Wallace & Sons Manufacturing Company, Wallingford, Conn., and
W.D. Robertson, Hammond Metallurgical Laboratory, Yale University.

(Continued from the July, 1952 issue)

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MODIFIED NODULE METHOD FOR TESTING ADHESION
Inasmuch as the foregoing results indicated that the Ollard type of adhesion test does not meet the required specifications, an attempt was made to design a test that might more nearly approach the “ideal”. Brenner’s nodule methods was adopted in principle but modified to eliminate high stress concentration at the lase of the nodule and to provide symmetry across the interface. This was accomplished by electroforming identical concentric cylindrical nodules on both sides of the electroplated base metal, which gives a miniature tensile specimen with the electrodeposited base metal sandwiched between the two nodules. The specimen is shown schematically in Fig. 6. The material of the nodule should have a flow curve and a fracture curve at considerably higher values than those of the deposit and base metal in order that the nodule material does not fail prematurely but transmits the load to the deposit and the base metal in such a manner that they be subjected to a state of stress approaching “hydrostatic tension”. Furthermore, the deposit and base metal should be as thin as possible so that plastic deformation is minimized.

Fig. 6. Schematic view of an adhesion-test specimen with electroformed nodules for gripping the electrodeposits

One of the major problems associated with the modified nodule adhesion specimen was the development of an electroforming fixture in which the two nodules would be exactly in line on opposite sides of the test specimen to avoid eccentric loading. Also, the future must outline the nodule sharply on the base metal and not permit plating solution to penetrate onto the specimen surface underneath the fixture. The fixture which was finally developed, and which worked satisfactorily in both respects, was made of Lucite and is shown in Fig. 7. With this fixture five nodules are ,electroformed simultaneously on each plated specimen. They are 1/16 inch (1.6 mm) in diameter and have a stem 1/16 inch (1.6 mm) in height. The head is 5/32 inch (4.0 mm) in diameter and is plated to a thickness of approximately 1/16 inch (1.6 mm). For the initial experimental work, nickel nodules were electroformed from a chloride bath containing NiCI₂ . 6H₂O, 300 g/1 (40 oz/gal); H₃BO₃, 30 g/l (4 oz/gal), which had been purified according to standard procedures¹⁶.

Because it is necessary to have the base metal as thin as possible to restrict deformation, preliminary work was conducted on annealed copper foil 0.003 inch (76 µ) thick, with a grain size of 0.025 mm.

Fig. 7. Fixture for depositing nodules on foil specimens

The shape of the specimen cut from the foil is shown in Fig. 8. The foil test specimen extends beyond the Lucite fixture at both ends for current connections and for better definition of the current density. The technique followed in preparing a test specimen is as follows:

(1) The foil is treated in the manner prescribed by the purpose of the test, for example, by electropolishing and plating.

(2) The prepared foil is inserted in the Lucite plating fixture, and after proper alignment the screws are tightened and covered with a plating-stop off coating.

(3) The foil is chemically activated to receive nickel plate and carefully rinsed with water, care being taken not to trap bubbles in the counter-bored holes.

(4) Nickel nodules are formed at 24 asf (2.6 amp/dm²), 130-140°F (550°C) and pH 4.0-4.5. Under these conditions it takes about four days to build up the nodules. The appearance of the specimen after electroforming is shown in Fig. 9.

(5) The screws are removed and the Lucite dissolved in acetone.

Fig. 8. Dimensions of foil specimen, in top view, for nodule adhesion test

Fig. 9. Appearance of nodule-test specimen after removal from electroformed bath

Because of the small size of the specimen and the nature of the test, axiality of loading is very important. Consequently, grips (Fig. 10) similar in principle to those used by Burke for tensile loading of magnesium single crystals¹⁷ were employed. Axiality is insured by transmitting the load through flexible piano wire from the cross-head of the tensile-testing machine.

Fig. 10. Grips employed in tensile testing of nodule adhesion specimens

The results obtained on five nodules electroformed on one specimen of copper foil are presented in Table II. The copper foil was first electropolished in phosphoric acid, which reduced the thickness to 0.0025 (64 µ). The nickel nodules were then plated directly on the copper foil without tan intermediate deposit.

In all cases the fracture occurred entirely in the copper foil. This indicates an adhesive strength between the electrodeposited nickel and the copper foil greater than the fracture strength the copper foil. Furthermore, the load did not go through a maximum but rather increased continuously to fracture.

The reproducibility of the fracture strengths for three of the nodules is very good, and the average is 46,600 psi (32.8 kg/mm²). This value is in agreement with that presented by Gensamer¹⁸ for the fracture strength of copper wire, 48,000 psi (33.8 kg/mm²). It is interesting to compare the value with the technical tensile strength of annealed copper, which is approximately 30,000-35,000 psi (21-25 kg/mm²). The remaining two nodules exhibit fracture strengths approximately 5000 and 15,000 psi (3.5 and 10.5 kg/mm²) respectively below the above-mentioned average. The reason for this large deviation is not entirely clear. It may be that axiality of loading was not obtained in spite of precautions taken, or perhaps these deviations represent local variations in the fracture strength of the copper foil. Further investigation is indicated.

Examination of the fracture in the copper foil under the microscope indicated that little plastic flow had occurred during the tensile testing. Photomicrograph of the fracture of specimen 1c is shown in Fig. 11. It should be observed that grains, twin boundaries, and grain boundaries extend to the fracture surface without any sign of distortion. Another feature which indicated that little plastic flow had occurred in the nodule during the testing was that the final diameter of the stem after fracture was the same as the original diameter.

Fig. 11. Cross section through fractured copper foil, 700X. Above black line, copper foil; below black line, nickel nodule.

The foregoing results show that the general principle of the method is sound in that an approximation to the fracture strength may be obtained with little plastic deformation; furthermore, the procedure, although rather laborious, is perfectly feasible.

In a continued evaluation of the method, a limited number of tests were made to determine the adhesion of a silver deposit to copper foil, with and without a preliminary strike. After having been electropolished in phosphoric acid, one test specimen was given a silver strike at 22 asf (2.4 amp/dm²) and the other was placed directly in the silver plating solution without a previous strike; current connections to both specimens were made previous to immersion in the silver plating solution. They were silver plated at 6 asf (0.65 amp/dm²) to a thickness of 0.0001 inch (2.5 µ). Nickel nodules were then electroformed onto the silver plated foil. The results obtained on the two specimens of plated foil are given in Table III.

Although the data obtained on the silver plated specimens are less reproducible than those on the unplated copper foil (owing to pits in the nickel nodules) certain conclusions may still be deduced. It evident that the adhesion of silver to copper previously treated with a silver strike is at least equal to the fracture strength of the copper itself. Without a strike, failure occurred at the silver-copper interface and at much lower values. In view of the fact that failure occurred in the nickel nodules, it is difficult to say whether or not the wide scatter in adhesion values when fracture took place at the silver-copper interface is an inherent characteristic of plating without a strike. However, the data can be interpreted to mean that the adhesion of a silver deposit to copper without a strike is unreliable.

SUMMARY AND CONCLUSIONS
The general problem of evaluating the adhesion of electrodeposits has been considered, and appears analogous to that of measuring the fracture strength of metals in the absence of appreciable plastic deformation. To conduct this type of test and to obtain significant data, it is necessary to design a test specimen incorporating (1) symmetry across the interface in question, (2) absence of local stress concentration, (3) a minimum base-metal and deposit thickness to limit plastic deformation, and (4) a means for transmitting the load to the interface that subjects the base metal and deposit to a stress state of high triaxial tension.

Previously proposed methods do not meet adequately these requirements. It has been demonstrated in the present work that it is possible to cause non-ductile fracture in annealed copper foil when the design of the test specimen is such that a triaxial state of tensile stress is obtained. The method may be adapted to the evaluation of the adhesion of electrodeposits in a manner which conformed to the aforesaid four principal requirements of a quantitative adhesion test.

ACKNOWLEDGMENT
H. C. Schlaupitz wishes to express sincere gratitude to R. Wallace & Sons Manufacturing Company, which sponsored this investigation. Appreciation is also expressed to Mr. William Whitty and Mr. Austin Norton of the Wallingford Steel Company for the use of its tensile machine. Acknowledgment is also due Dr. H. Leper of Yale University for a helpful discussion of the problem of measuring adhesion.

BIBLIOGRAPHY
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17. E. Burke, Ph.D. Thesis, Yale University (1951).
18. M. Gensamer, “Strength of Metals Under Combined Stresses”, American Society for Metals (1940).

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