Since the beginning of the industrial revolution there has been a constant search for a process that will provide metal a greater ability to resist wear. Industry has made incalculable investments in modifying alloys to enhance temperatures, increased pressures, or actual abrasive attack. High temperature nickel and cobalt alloys, as well as titanium and stainless steel alloys provided a partial answer. The difficulty lies in the problem of cost as well as the fact that even the best nickel alloys still fall victim to oxidation and erosion by the very nature of the metals that constitute the alloy. A certain degree of additional protection has been realized by electroplating the surface of these alloys. The problem here is that plating only establishes a mechanical bond. Another difficulty is the limited types of metal that can be applied to the surface. The ideal situation would be to have a method of applying a very hard and resistive metal to the surface and achieving more than an electrical based bond. If the surface coating (metal or alloy) could be chemically bonded to the base metal while not simultaneously diluting any of its resistive qualities, then lifetimes could be multiplied by factors of 10! Such a process presently exists. This process is known as diffusion alloying. Diffusion alloying has only been available for the last twenty years and has mostly been applied to airborne turbine engines. Platinum group diffusion alloys have only started being used for the last ten years.
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Diffusion is a metallurgical process, which is very easy to understand. One simple but important concept is that diffusion is the result of random motion of the individual atoms on the base metal surface. Because of thermal energy, the atoms in a metal crystal are in constant motion around their equilibrium lattice sites. Occasionally, as a result of this motion, an atom will jump to a neighboring site. At room temperature, the frequency with which any atom makes a move to a neighboring site is usually small. However, as the temperature increases, the atom jump frequencies increase, with the net rate of atomic migration eventually becoming large enough to provide readily observable effects, including transport of atoms over considerable distances and appreciable changes in chemical composition or atomic distribution.
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What this means is that an alloy of one metal can be chemically bonded to a base alloy. The resulting product could be one in which the surface is composed of 100% of the coating. As the diffusion alloy penetrates deeper and deeper into the substrate, alloying with the substrate occurs. For example, the surface of a nickel substrate that has been diffusion alloyed with, say, platinum, is 100% platinum. But as we proceed deeper and deeper into the nickel alloy, it becomes apparent that there is no clear barrier separating the two.
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The diffusion alloy is the layer of various intermetallic compounds that has been formed by the addition of different elements at elevated temperatures to the parts surface. The reaction between the base material and the applied coating is what forms the intermetallic compounds and provides the parts with various enhanced characteristics, one of them being life extension.
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By definition, an intermetallic compound is a true chemical bond. The advantage of a chemically bound atomic structure over conventional metallic alloying is significant. Typically, an alloy is composed of various elements melted together in a fashion to produce a uniform distribution of mixed metals. These bonds that have formed between the various elements are quite weak and are subject to easy chemical or mechanical attack. But since an intermetallic compound is chemically bound to each other, the bonds are exceptionally strong and are composed of a specific fixed ratio of the elements.
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Even though diffusion layers are thin, their resistance to mechanical attack by erosion could be ten times greater than that of the base material. This is for several reasons. The first being that a diffusion alloy is composed of a single phase system. This means that all of the elements are chemically bound and there are no soft binders that are holding the coating in place. Other coatings may also be hard, but materials that are soft and easily worn hold them together. As soon as these soft materials begin to wear, the harder particles will drop out of the coating and the surface will rapidly decay.

Secondly, diffusion alloys by having high atomic bond strength, minimizes the effect that one experiences between parts rubbing against each other. By generating a coating that has little desire to have individual metallic atoms move across the surface of the parts, galling and seizing can be minimized if not eliminated. Since the coefficient of friction has been reduced due to the prevention of metallic migration, because of the formation of various intermetallic compounds, wear will decrease and the life expectancy of the part will significantly increase.
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A materials hardness is usually measured either in Knoop Hardness Number (KHN or Rockwell on the "C" scale (see Hardness Chart). Diffusion alloying generally produces a KHN of anywhere from 1800 on Carbon Steel to over 5000 on Tungsten Carbide. As a comparison, Rockwell C90 equals roughly 2000 KHN while a Diamond is at 6500 KHN. But even though the diffusion alloy is hard, it is ductile enough to withstand as much as 5% deformation without cracking since there is no specific surface interface to contend with. Mechanical and thermal shocks have no effects upon the coating. Other coatings, sprayed or plated, have a specific interface shear point that will lead to premature failure when exposed to thermal differentials that cause different rates of expansion to exist and put stresses on the interface boundary.
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Just about all of the steels, nickel alloys, cobalt alloys, iron, nickel, titanium, molybdenum, and other exotic materials can be diffusion alloyed. Low temperature materials such as aluminum, copper, zinc, etc. cannot be diffusion alloyed. Parts are usually diffused in an annealed state and would have to be heat treated subsequent to the alloying if a higher base material strength is required.
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Any complete theory of diffusion must be concerned with basic diffusion mechanisms. Knowledge of specific mechanisms can allow measured diffusion quantities to be related to atomic parameters. The equations for the various diffusion coefficients, their relation to each other, and their relation to other measured quantities, such as the atomic drift mobility, often depend on the particular diffusion mechanisms that are operating in the materials being alloyed. Moreover, it frequently occurs that two large-scale diffusion processes can be related to the same atomic model and/or parameters. Then relations between the two processes can be established that could not be obtained without the use of atomic models.

1.  A very common type of diffusion mechanism is the vacancy mechanism. In thermodynamic equilibrium a certain number of vacant lattice sites can be expected to be present in a crystal. Any atoms neighboring on a vacancy can then diffuse by jumping into the vacancy, the result being an interchange of position of an atom and the vacancy.

2.  The simplest diffusion mechanism is the interstitial mechanism, also called the direct interstitial mechanism. Here, an interstitial atom diffuses by moving directly from one interstitial site to another interstitial site without causing net motion of any other atom.

3.  A separate mechanism, which is different in important respects from the interstitial mechanism, is the interstitialcy mechanism, also called the indirect interstitialcy mechanism. Here the interstitial atom moves by pushing a normal lattice atom into an interstitial site and moving into the lattice site itself. The region centered on an interstitial atom can be called an interstitialcy. The location of the interstitialcy during an elementary jump in this mechanism may move twice as far as does either of the individual atoms themselves.

4.  The exchange mechanism, which is simple to envision but occurs very infrequently in practice, involves the simultaneous motion of tow lattice atoms so that in a single jump they exchange places with one another.

DIFFUSION     MECHANISIMS