AES Research Project 3, Adhesion of Electrodeposits, Part 2, General Considerations
This is Part 2 of a four-part article consisting of the full report of AES Research Project #3, Adhesion of Electrodeposits, done at the University of Michigan in the mid-1940s, following the end of World War II. It outlines the general concepts involved on plate adhesion.
Project Director: A.L. Ferguson
Associate: Elmer F. Stephan
Chemistry Department, University of Michigan, Ann Arbor, Michigan
Part 2. General considerations
This is Part 2 of a four-part article consisting of the full report of AES Research Project #3, Adhesion of Electrodeposits, done at the University of Michigan in the mid-1940s, following the end of World War II. It outlines the general concepts involved on plate adhesion. A printable version of this section can be downloaded by clicking HERE.
Adhesion of electrodeposited metals to the base metal has always been of major concern to the electroplating profession. Its importance has increased rather than decreased with time and even more so during recent years since thick electrodeposits have come into increasing general use through such processes as building up worn parts by iron, chromium and nickel and through the development of electrodeposited metals for bearings.
There are two major factors involved in adhesion; one is concerned with methods for securing adhesion and the other with methods for measurement. These are so intimately related that it is difficult to consider them separately, however, the present paper will be confined as closely as possible to the question of methods of measurement.
The whole matter of adhesion is further complicated by some factors of uncertainty that should receive consideration before entering upon the main topic of the paper. In the first place, one might ask what is adhesion? What is adhesion due to? What causes it? What forces are involved? In the second place, one might ask whether the conclusions arrived at concerning the methods for securing and for measuring adhesion of one electroplated metal to one base metal, or electrodeposited metals to different base metals. In other words, do different electrodeposited metals and different base metals possess personalities of their own and require individual attention. In the third place, one might ask whether the definition of adhesion should have universal application. In other words, may a deposit be said to be adherent for some purposes and not for others? Can one set of standards of specifications for relative degrees of satisfaction in adhesion be set up to apply to all cases, or should several sets be used each for a given purpose? And in the fourth place, one might even ask the question what is to be considered perfect adhesion. Strangely enough there is a distinct lack of agreement on the answer to this last most basic question. Each one of these factors will be briefly considered.*
The forces involved in adhesion
As pointed out above, one question which should be considered in any problem related to adhesion is, to what is adhesion due, or what are the forces involved in adhesion? From reading text books and published literature, one would get the impression that there are many different kinds of forces producing adhesion. Some of the more commonly named forces are: gravitational, magnetic, electrical, adhesive, cohesive, van der Waals', chemical, valence, covalence, atomic, etc. It is a very serious question as to whether these different names mean that there are many distinct kinds of forces or whether many of the different names are used simply for convenience or due to historical development, but actually refer only to different manifestations of the same force or different degrees of the same force. A whole book might be written on this subject, but attention will be drawn here to only a few points. It is just about universally accepted now that all matter is made up of molecules. Molecules of different kinds of matter are held together by so-called adhesive forces, those of the same kind by cohesive forces. Molecules are composed of atoms held together by so-called chemical, van der Waals’, electrovalent and/or covalent forces. Atoms in turn are made up of a central part which carried the mass of the atom and is positively charged, and an outer part made up of enough electrons or negative charges to neutralize the positive charge of the inner part. Since all matter is made up of a combination of atoms into molecules and, since pure material is only made up of the same kind of molecules, and since all other material since all other material is made only of different kinds of molecules, it is difficult to account for the existence of any basically different types of forces other than those found in the ultimate building blocks of all molecules, namely, the electrical and magnetic forces in the atom. If this point of view is accepted, then the different names for forces involved refer to different manifestations or degrees of intensity of the same basic forces within the atoms themselves.
The point of view expressed above that the forces involved in adhesion are identical with those involved in cohesion, adsorption, chemical reactions and in the, formation of crystals, in other words, that they are all atomic in nature is supported by much experimental evidence and many outstanding authorities in chemistry. One of the best known among these is Irving Langmuir, of the General Electric Company, Schenectady, NY. In one of his articles2 he makes the following remarks:
"In the past, it has been customary to consider that solids and liquids are held together by the "forces of cohesion" and are called "physical forces" as distinguished from chemical forces. In the following pages, the writer hopes to show that there is no present justification for this distinction between chemical and physical forces. Cohesion, adsorption and surface tension are all manifestations of forces similar in their nature to those acting between the atoms of solid bodies. It is therefore advantageous to look upon these forces as direct results of chemical affinity. The surface of a crystal or solid must be looked upon as a sort of checker-board containing a definite number of atoms of definite kinds, arranged in a plane lattice formation. The space between and immediately above these atoms is surrounded by a field of electromagnetic forces more intense than that between the atoms inside the crystal."
The following is one of the conclusions stated in this article by Langmuir:
"There is no present justification for dividing interatomic or intermolecular forces into physical and chemical forces. It is much more profitable to consider all such forces as strictly chemical in nature. Evaporation, condensation, solution, crystallization, adsorption, surface tension, etc. should all be regarded as typical chemical phenomena."
As part of another conclusion he states:
"A large number of experimental results are given which prove conclusively that adsorption is very frequently the result of the strongest kind of chemical union (primary valences) between the atoms of the adsorbed substance and the atoms of the solid."
Another independent type of evidence which also demonstrates that the forces of adsorption are chemical in nature is that gained from heats of adsorption. This evidence is summarized in a statement by S. Glasstone:100
"The chemical nature of the forces involved (in adsorption) is indicated by the heats of adsorption which are of the order of 20 to 100 kg-cal/mol, so that the bonds formed between the metal and the adsorbed gas are almost as strong as those existing in stable stoichiometric compounds."
It is a question why Glasstone uses the qualifying word "almost" since these heats of adsorption are within the range of heats of chemical reactions.
There are several instances known in which the strength of the force between the adsorbed material and the metal is greater than the cohesive force or tensile strength of the metal itself. A commonly presented illustration is the case of oxygen adsorbed on tungsten. In the course of time the normal oxide of tungsten WO3 distills from the tungsten surface and is deposited on the walls of the vessel. This proves that the so-called adsorptive force between the oxygen and the surface atoms of the tungsten wire is even greater than the so-called chemical force between these surface atoms of tungsten and the other tungsten atoms surrounding them.
It is universally accepted that the strength of a force increases with decrease in the distance over which it operates, the rate at which the strength increases with decrease in distance becomes much greater the shorter the distance, ultimately reaching a value of a high order of magnitude at interatomic distances. This means that the greatest manifestation of force strength is within the atom; the next in magnitude would be between the atoms within molecules, commonly called atomic or valence or chemical forces; the next would be between molecules or atoms of the same composition commonly called molecular or cohesive forces; and next those between molecules of different kinds, and commonly known as adhesive.
According to the modern concept of the structure of metals, only atoms are present. This means that all the forces, including those at the surface which result in adhesion are atomic or chemical in nature.
If, the above analysis is correct, then the degree of, adhesion for a given deposit and base metal becomes largely a matter of how intimate a contact can be made between atoms of deposited metal and the atoms of the base metal. It should he possible, theoretically, to make this contact so intimate that the force or bond between the deposited and base metals might be even greater, than the forces between the atoms of deposited metal or atoms of base metal themselves, which are commonly called cohesive forces. Or, to put it differently, the tensile strength of the bond might be greater than the tensile strength of either the deposited or base metal. This means that the greatest, secret to perfect bonding (the expression perfect adhesion is purposely avoided) is an absolutely clean base metal surface.