The relationship between crystal and hardness

[vinhphysics]

Hello all,
Who can tell me the ralationship between crystal and hardness of the coatings. I do XRD and measure hardness but the some coatings with high intensity of XRD peak show the low hardness. The questions are: what direction of planes is useful for hadrness? what elements affect on hardness?
Thanks so much!

 

[Astronuc]

In metals, the tensile/compressive yield strength may be correlated with hardness, so whatever element increases yeild strength would also increase hardness. Pure (unirradiated) elements have relatively low hardness, while alloying and radiation (which produces dislocation) generally increases strength and harness. Strength and hardness are greater in the direction of lowest interatomic spacing. For example, in hcp metals, the strength would be greatest paralle to the basal plan (a-direction) and lower in the direction of the c-axis (normal to basal plane). Cubic systems would be stronger along diagonals.

Ceramic materials have high strength and high hardness, and BN (http://en.wikipedia.org/wiki/Boron_nitride,http://www.azom.com/details.asp?ArticleID=78) is perhaps the hardest ceramic. Diamond is perhaps the hardest substance with a tetrahedral (A4) structure (http://cst-www.nrl.navy.mil/lattice/struk/a4.html) .

One might find this site useful - http://cst-www.nrl.navy.mil/lattice/index.html