Strengthening Mechanisms

The hardening mechanism of nanocomposites should be more complicated [165, 385, 386] than that in coarse-grained polycrystalline materials. The hardness of nanocomposites is most precisely approximated by the Hall-Petch equation, but currently there is no agreement on the strengthening mechanism of nanocomposites. A few strengthening models of nanostructured materials at nanoscale thicknesses of the bilayer repeat period below the Hall-Petch breakdown were proposed:

• Orowan mechanism [344, 387-393], which shows that dislocation slip can occur by bowing of dislocations within a nanolayer between interfaces [394].

• Koehler mechanism [395], which takes into account a shear modulus mismatch between the nanolayers of nanolayered nanocomposites [396].

• Coherency stress mechanism, which is based on the idea that alternation of in-plane coherency stresses from tensile to compressive between nanolayers resists dislocation movement across nanolayers [397-401].

• Electronic structure model [402-405], which arises from the interaction between the Fermi surface and the Brillouin zones created by the periodicity of nanolay-ered composites.

It has been estimated [393, 395, 406] that nanolayered composites consisting of two periodically repeated high-strength materials can possess high hardness because the alternating stress fields in the nanolayers prohibit any dislocation movement. The difference in shear moduli means that dislocations have lower energy in the phase with the lower elastic modulus. The role of the second-phase nanolayer, having a higher modulus, is to act as an elastic barrier inhibiting dislocation movement through the nanolayered composite. Therefore, it acts as an elastic barrier to dislocation propagation into the harder layer.

A model of two-component nanolayered coatings has been analyzed for the prediction of the critical shear stress that influences a dislocation slip transmission across the interface [407] as well as crack formation [408]. The investigation was based on a dislocation pileup model in which the Frank-Read dislocation source was considered for dislocation generation. This effect generated by changes in the modulus across the interface in a thin layer indicates strong dislocation interaction with the interface. This analysis is based on [161, 354, 393, 395, 408, 409]:

• Effects of modulus changes through the interfaces

• Coherency stress

• Barriers for dislocation slip across the interfaces The effect of increasing hardness is controlled by [354]:

• Option of a bilayer repeat period

• Constituent modulus of material nanolayers

• Epitaxy of nanolayers

The movement of dislocations through the interface between two neighboring nanolayers with different shear moduli has also been considered [407], where a dislocation source length was the critical parameter. The mechanism of strengthening, that is, increasing hardness with the bilayer repeat period approaching an optimum value, in nanolay-ered composite coatings is due to the fact [394-396, 410,

411] that the strain induced prevents a dislocation movement across the interface from a softer material into a harder one and assists the dislocation movement through the interface from the harder material. Therefore, the harder nanolayer is usually free from dislocations or, at least, has a minimum number of them. The critical value (Ac) of the dislocation spacing in the pileup can be calculated from the equation [341]:

where Ac is the dislocation spacing in the pileup, G is the shear modulus, b is the numerical value of Burger's vector of dislocation, v is the Poisson ratio, and H is the hardness.

Another suggested explanation of superhardness [277,

412] is that the stress appears because of mismatch in the lattice constants of the two phases which inhibits dislocation movement as well. For example, the residual stresses in the nanolayers of Cu/Cr were found [413] to reach a peak value of approximately 1 GPa for a bilayer repeat period of about 50 nm. The stress level in nanolayered coatings is higher when there is a lattice mismatch. Because the lattice plains fit across the nanolayers of monocrystal type, the mismatch in the spacings should be compensated for by elastic strains in the nanolayers; that is, one of them must be compressed and the other should be tensed [398]. These strains prevent movement of dislocations across interfaces and, therefore, increase the hardness of the nanolayered coatings. The elastic modulus has been found to be up to 2-3 times greater as compared with homogeneous coatings of the same average composition [404, 405]. When a nanolayered composite coating is on a substrate, the substrate imposes an additional deformation on the nanolayers as well.

A detailed analysis of the strengthening mechanism has been considered elsewhere [414] where a phenomenolog-ical model for the one-dimensional elastic reaction of the nanolayered composite coatings was developed. The model demonstrated the capability of reproducing the elastic anomalies, which were then confirmed experimentally. The critical shear stress of two-phase nanolayered coatings for dislocation slip transmission was found [397, 398, 407, 415] to be dependent on layer thickness, shear moduli of the phases, slip conditions of interfaces, properties of dislocation source, and orientation of pileup. An anomalous change of elastic constants of compositionally modulated structures is explained by the different shapes of the Fermi surface [403] and the electronic structure of nanolayered composites [402].

There are a few approaches to explain the superhardness effect in nanolayered composite coatings, namely:

• Strain distribution within individual nanolayers [416, 417]

• Difference in elastic moduli of nanolayers

• Lattice mismatch of nanolayers

The above preconditions help explain aspects of the hardening mechanisms, but a comprehensive explanation of the strengthening phenomenon does not currently exist.

0 0

Post a comment