diffusion coating

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DIFFUSION

Many reactions and processes that are important in the treatment of materials relyon the transfer of mass either within a specific solid (ordinarily on a microscopic

level) or from a liquid, a gas, or another solid phase. This is necessarilyaccomplished by diffusion , the phenomenon of material transport by atomicmotion.

DIFFUSION MECHANISMS

From an atomic perspective, diffusion is just the stepwise migration of atoms fromlattice site to lattice site. In fact, the atoms in solid materials are in constantmotion, rapidly changing positions.

For an atom to make such a move, two conditions must be met:

1. There must be an empty adjacent site, and

2. The atom must have sufficient energy to break bonds with its neighbor atoms and then cause some lattice distortion during the displacement.

This energy is vibrational in nature. At a specific temperature some small fractionof the total number of atoms is capable of diffusive motion, by virtue of the

magnitudes of their vibrational energies.

VACANCY DIFFUSION

One mechanism involves the interchange of an atom from a normal lattice positionto an adjacent vacant lattice site or vacancy, as represented schematically in Fig. 1.

This mechanism is aptly termed vacancy diffusion. Of course, this processnecessitates the presence of vacancies, and the extent to which vacancy diffusion

can occur is a function of the number of these defects that are present; significantconcentrations of vacancies may exist in metals at elevated temperatures.

Both self-diffusion and inter diffusion occur by this mechanism; for the latter, theimpurity atoms must substitute for host atoms.

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INTERSTITIAL DIFFUSION

The second type of diffusion involves atoms that migrate from an interstitialposition to a neighboring one that is empty. This mechanism is found for inter

diffusion of impurities such as hydrogen, carbon, nitrogen, and oxygen, whichhave atoms that are small enough to fit into the interstitial positions. Host orsubstitutional impurity atoms rarely form interstitials and do not normally diffusevia this mechanism.

This phenomenon is appropriately termed interstitial diffusion Fig.1b. In mostmetal alloys, interstitial diffusion occurs much more rapidly than diffusion by thevacancy mode, since the interstitial atoms are smaller and thus more mobile.

Furthermore, there are more empty interstitial positions than vacancies; hence, theprobability of interstitial atomic movement is greater than for vacancy diffusion.

Fig.1 Schematic representations of ( a ) vacancy diffusion and ( b) interstitial diffusion

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STEADY-STATE DIFFUSION

Diffusion is a time-dependent process that is, in a macroscopic sense, the quantityof an element that is transported within another is a function of time.

= J = diffusion flux, defined as the mass (or, equivalently, the number of atoms) M diffusing through and perpendicular to a unit cross-sectional area of solid per unitof time. The units for J (kg/m 2-s or atoms/m 2-s).

If the diffusion flux does not change with time, a steady-state condition exists.

= =

Concentration in terms of mass of diffusing species per unit volume of solid (kg/m 3 or g/cm 3).

= The constant of proportionality D is called the diffusion coefficient, which is

expressed in square meters per second. The negative sign in this expressionindicates that the direction of diffusion is down the concentration gradient, from ahigh to a low concentration. Above equation is sometimes called Ficks first law.

One practical example of steady-state diffusion is found in the purification of hydrogen gas. One side of a thin sheet of palladium metal is exposed to the impuregas composed of hydrogen and other gaseous species such as nitrogen, oxygen,and water vapor. The hydrogen selectively diffuses through the sheet to theopposite side, which is maintained at a constant and lower hydrogen pressure.

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Fig.2 ( a ) Steady-state diffusion across a thin plate. ( b) A linear concentration profilefor the diffusion situation in ( a ).

NONSTEADY-STATE DIFFUSION

Most practical diffusion situations are nonsteady-state ones. That is, the diffusionflux and the concentration gradient at some particular point in a solid vary withtime, with a net accumulation or depletion of the diffusing species resulting.

=

known as Ficks second law, is used. If the diffusion coefficient is independent of composition.

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Fig. 3 Concentration profiles for nonsteady-state diffusion taken at three differenttimes, t1, t2, and t3.

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DIFFUSION COATINGS

Diffusion coating(s) are element(s) intentionally deposited on the substratematerial for the purpose of producing diffusion saturated layers on the substrate

material, using chemo-physical processes, particularly thermo-and electro-chemical reactions.

TERMINOLOGY

Diffusion coatings are also known as pack cementation, but this term should bereserved only for one particular method diffusion coating in solid phase in closedreactors otherwise, the word cementation should be eliminated.

Depending on the diffused element, an ending -izing is added to the elementsname, e.g., boron-izing, carbonizing, chromizing etc. Aluminum diffusion coatingseven have two names: aluminizing and calorizing.

PRINCIPLES

1. The mechanism of diffusion coatings generally can be described in thefollowing ways:

There initially must be higher concentration of the material to be diffused into the

substrate, and then a translocation of the atoms occurs from the region with higherconcentration into the region with lower concentration. Thus, diffusion results inthe equalizing and stabilizing of material concentration.

2. There are three basic stages caused by the thermochemical mobility of theatoms participating in the diffusion process:

Formation of active atoms of the material to be diffused into the substrate,depending on the composition of the diffusion phase;

Adsorption of active atoms by the substrate material, which depends on thecharacter of mutual inter-reaction between the components of the diffusion

phase and with the substrate;

Diffusion of the element(s) atoms into the metal or alloy, which is controlled by: the substrate of the active atoms to be diffused (e.g., their atom radius)

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and substrate material, and by the activation energy, mutual solubility of alloying or inter-reacting elements, etc. the active atoms can more easilydiffuse in vacancies, at phase and grain boundaries, and at dislocations and other defects of the crystalline structure.

The element(s) to be diffused share the following characteristics with the substratematerial: a mutually unlimited or limited solubility forming intermetalliccompounds, and/ or a chemical bond.

In case they do not have the above bonds, the elements to be diffused can form socalled independent structures (e.g., active non-metallic elements with small atomradius, like B, C, or N with non metallic elements having big atom radiusesforming substitutional solid solutions).

3. The process causes the diffusion coating material to disperse in the direction of the lower concentration of the substrate. The amount is equal to the differencebetween the amount of diffusion coating elements passing into the substrate andin the reverse direction (substrate elements into the coating). Therefore, theamount is proportional to the gradient of concentration (or the decrease of concentration at distance X from a reference interface). This relation isexpressed by Ficks First Law

=

In turn, D depends on the frequency (V o) of atoms jumps from one position toanother, and their atomic diameters (a); therefore

D = a 2.Vo

The concentration charge of the diffused element with time (t) is expressed byFicks second law

=

The relationship between the diffusion coefficient (D) and the temperature followsArrhenius Law:

= ( )

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Do= a2.Vo= Frequency factor (cm

2 /sec);Q= Activation energy (cal/gm atom);R= Universal gas constant (cal/gm mole);T= Temperature (K)

Activation energy is the energy required to produce the diffusive motion of onemole of atoms. A Large activation energy results in a relatively small diffusioncoefficient.

4. In summary, the diffusion coefficient increase (along with factors) when thefollowing occur:

Increased diffusion temperature Increased vacancies, and other defects in the crystalline structure of the

substrate material, including a less perfect crystalline lattice; Decreased atomic radius of the elements to be diffused, and an increase of

its concentration Lower diffusion activation

PROCESS DESCRIPTION

The pack cementation, or pack diffusion, process diffuses the coating material into

the substrate, generally to impart oxidation and high temperature corrosionresistance to the coated part. Most often, the coating material is a powder of aluminum, chromium cobalt or alloys of these materials.

The parts to be coated are placed in a retort or sled in a mixture of the coatingmaterial and an inert powder, such as aluminum oxide, along with a halide salt.The retort is then placed in a protective atmosphere (hydrogen or argon) furnaceand brought up to the coating temperature. The salt vaporizes and combines withthe coating to generate the transporting vapor species. The retort is placed in afurnace and brought to a temperature at which the coating material will react withthe salt to form a metallic halide vapor, which comes in contact with the surface of the parts to form the coating.

In the aluminizing process, a source of Al reacts with a chemical activator onheating to form a gaseous compound (e.g., pure Al with NaF to form AlF). Thisgas is the transfer medium that carries aluminum to the component surface. The

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gas decomposes at the substrate surface depositing Al and releasing the halogenactivator. The halogen activator returns to the pack and reacts with the Al again.Thus, the transfer process continues until all of the aluminum in the pack is used oruntil the process is stopped by cooling. The coating forms at temperatures rangingfrom 700 to 1100 oC over a period of several hours.

Features of the Pack Cementation Process

Metallurgically bonded to the substrate Batch processing for high production rates Can be used to coat large or small components Coats both external and internal surfaces, even deep, small bores Can be tailored to meet specific requirements Economical process