How, Aluminium is the strongest nitride former among the alloying elements, and AlN has very high hardness, low COF and self lubricating ability?
A little more background on your statement/question would be helpful to get a good answer to your question. Your statment to be confirmed is largely anecdotic, not scientific. "strong" is not a very scientific expression. Likely you refer to steel with alloying elements which may have different thermodynamic affinity towards nitrogen, and Al seems to have the highest affinity (smallest equilbrium constant c(M)*c(N) (possibly decorated by stoichiometric coefficients). However, I doubt that under such circumstances Al is really "strong" against Ti.
To know which element will form Nitrides you have to consider the Ellingham diagram of free entalpy for nitrides. You will see that Ti and Zr have even a more stronger affinity to nitrogen than Al.
But it is not enough because you need also to consider the cinetic of formation. AlN has a very low cinetic to precipitate in steels so that even if it is predicted by thermodynamics at equilibrium it will not appear. It will appear with very long solidification time or very large thickness of ingots (min 100mm). Some diagrams exist to predict the precipitation or not.
Perhaps aluminium has extra 3 electrons in outer shell and nitrogen has 3 missing from complete shell. Both ions assumes neon configuration and fits really well with valency fulfilled.............................
Well, sort of. This is what high-school chemistry says. Here the Thermodyamics and kinetics up to some extant is dependent on Gibbs free energy (reaction rate... arrhenius...), and thus enthalpy and entropy. Entropy loss of sucking up whole gas is minute compared to what is gained by disordering metal structure. For enthalpy, relative bond strength between AlN, N2 gas and Al metal has to consider. Nitride vs metal enthalpy on one size, and disorderliness of nitride on the other, that compares Gibbs free energy.
Higher Transition metals have larger shells so the ionic stability gain is not much with heavier transition metal nitrides, and late transition metals also can have some interaction amoung (n-1)d and ns shells that complicate issues. Note, higher transition metals cannot have valency and size high enough to accomodate many nitrogen ions around the metal than Al, Ti and Zr can due to balance in size and density. But entropy makes gibbs free energy dependent on temperature as well, that is related to defect generation rate in nitrides. The nitride thet generates less defect at higher temperature and metal that undergoes structural disorientation more at high temperature undergoes more negative entropy change by nitriding and thus nitride becomes incerasingly more and more stable at higher themperature.
For kinetics, solid and liquid diffusion coefficients, and average interactomic distances also play significant role. and there are binary and ternary and yet higher order interaction coefficients and parameters that complicate up the situation.
Sorry for being unable to give a definite answer. hope you can find your own!
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