Nanocoatings
Micrograin vacuum coatings on the basis of high-melting metals feature high hardness (20...35 GPa), wear resistance, and low friction coefficients; though their disadvantage lies in significant brittleness that impairs their performance.
At present there is an opportunity to obtain new vacuum nanocoatings with grain size below 100 nm. Such coatings have improved mechanical properties. Reviewing existing nanocoatings enables to divide them into three main groups:
– Nanostructural coatings unlike traditional coatings combine higher microhardness and adequate plastic properties. Behavior of nanocrystalline materials with grain size below 10 nm is mostly governed by the processes within boundary areas since the number of atoms in grains is comparable or less that in their boundaries. This condition causes significant change to nature of interaction between neighboring grains, for instance, decelerates generation of dislocations and prevent cracks from spreading due to strengthening of grain boundaries. Dislocation in nanograins are absent in these conditions.
– Nanocomposite coatings are composed of basis nanocrystalline solid phase of which grain boundaries host a thin layer of the second nanocrystalline or nanoamprphous phase. Thus, solid grains of the strengthening phase are divided by thin atomic layers of the other phase. Such coatings have super high hardness, high coefficients of recovery, and high thermal resistance.
Nanocomposite structure
– Nanolayer coatings feature higher cracking resistance. Preconditions for obtaining such coatings existed as far back as 20 years in process of design of multi-layer coatings on the basis of Ti-N/Nb-N, Ti-N/V-N, (Ti-Al)N/Cr-N etc. alternating with layers of metals or metal compounds. Nanolayer coatings possess different inherent stresses (elastic modulus) and similar thermal expansion factors. Thickness of individual layers is to be so small as to prevent source of dislocations, while dislocations moving to the interface from a softer layer due to stresses would be repelled by forces generated by elastic stresses in the harder layer. Total number of layers in a coating depends on the process purpose and sometimes may reach two hundred.
Nanolayer structure