Porosity Kirkendall

Experimental diffusion data obtained by isothermal anneals of diffusion couples made of Rene-N4 and Rene-N5 in the single phase y region were compared to calculations using a multicomponent diffusion mobility database. The results compare favorably. The calculations also correctly predict the location of Kirkendall porosity. This initial success points to the suitability of the general approach and the desirability of further database refinements. 

The extent of diffusion in the interface depends on time and temperature. Differing diffusion rates of the film and substrate materials can create porosity in the interfacial material. Porosity formed by this mechanism is called Kirkendall porosity. This porosity can weaken the interfacial material and provide an easy fracture path for adhesion failure. The diffusion interface is generally conducive to good adhesion, but, if the diffusion region is too thick, the development of porosity may lead to poor adhesion. 

Diffusion-type interface (film formation) When the interfacial material (interphase material), formed during the deposition of A onto B along with subsequent diffusion, consists of an alloy of A and B with a gradation in composition. See also Interface Interphase material Kirkendall porosity. 

Kirkendall porosity (film formation, adhesion) Porosity that develops in the interfacial region between two materials when the first material diffuses faster into the second than the... 

Over-diffusion (adhesion) When the extent of the interdiffusion of materials causes a weakening of the material in the diffusion zone. Examples Weakening by formation of Kirkendall porosity microfracturing due to stresses caused by phase changes in the diffusion zone. 

If, within the diffusion zone, there is no active vacancy source or sink, then no drift of lattice planes could occur and the difference in the diffusion fluxes of substitutional chemical species would result in vacancy supersaturation and build-up of local stress states within the diffusion zone. Return to local equilibrium in a stress-free state could be achieved by the nucleation of pores leading to the well-known Kirkendall porosity (Fig. 2.2d). All intermediate situations are possible depending on local stress states and the density, distribution and efficiency of vacancy sources or sinks. However, it should be emphasized that complete Kirkendall shift would occur only in stress-free systems in local equihbrium. Therefore, all obstacles to the free relative displacement of lattice planes would lead to local non-equilibrium. Such a situation corresponds to the build-up of stress states that modify the conditions of local equilibrium and the action of vacancy sources or sinks these stress states must therefore be taken into account to define and analyse these local conditions and their spatial and temporal evolutions. 

Sometimes interdiffusion between two metals is uneven and may lead to the creation of vacancies or voids. This type of imbalance is the result of possible unequal mobilities between a metal couple. These voids occur individually near the common interface. The voids, like bubbles, coalesce, resulting in porosity and loss of strength. Many thin-fihn couples exhibit this phenomenon, which is referred to as Kirkendall void creation. Al-Au, Cu-Pt, and Cu-Au are just a few examples. To be specific, it has been found (7), for instance, that in the case of Au-Ni, about five times more Ni atoms diffuse into Au than Au atoms diffuse into Ni. 

These are depicted schematically in Figure 18.4 in the case of metal A deposited on metal B. Bulk diffusion, as noted above, is the transfer of B into A or A into B through the crystal lattice. This is characterized by the coefficient D in the figure. Defect path diffusion is the migration along lattice defects such as grain boundaries, characterized by the coefficient D in the figure. Ordered A B, possible phases are indicated between the metals. Finally, Kirkendall void porosity is indicated and will be expected to be present if the interdiffusion rates from one metal to the other are not equal in both directions. 

There were several types of genuine porosities observed by various researchers. The first type was a void observed in the magnetite layers formed on iron and steel at temperatures below 570°C [91,93], where the gas used could be moist air, dry air or CO2. The voids were very fine in size and were observed at grain boundaries as well as inside magnetite grains. This type of porosity is also known as Kirkendall voids [100]. The formation of voids appeared to be associated with the formation of a duplex scale structure [101]. Recently, some theoretical treatments using conventional diffusion theories were made by Maruyama etal. [102] and Ueda etal. [92] to provide a semi-quantitative and quantitative explanation of their formation mechanism and their location in the scale. 

---