Dear all,
I am trying to do nanoindentation using the diamond indenter on pure
nickel, for sake of convenience i am using cuboid as an indenter. i
used tersoff potential for the atom of indenter to create a diamond
lattice. eam potential for the Ni substrate and lj potential for the
interaction between Ni and C(diamond).
but there seem to problem with my definition of the potential as soon
as the indenter approaches the Ni substrate the atoms of the substrate
start behaving like a wave, with huge displacement and the indenter is
not able to indent the substrate even at high velocities.
the code for it is
#3d lattice
units metal
boundary p s p
atom_style atomic
region box block 0 40.0 0 80.0 0 40.0 units box
create_box 2 box
mass 1 12.0
mass 2 58.71
lattice diamond 3.57 #neighbor 0.3 bin #neigh_modify delay 5
region diamond block 13 27.28 57 78.42 13 27.28 units box
create_atoms 1 region diamond units box
group indent region diamond
lattice fcc 3.52
region Ni block 0 38.72 0 38.72 0 38.72 units box
create_atoms 2 region Ni units box
group substrate region Ni
region lower block 0 38.72 0 3.52 0 38.72 units box
group lower region lower
pair_style hybrid lj/cut 8.5 eam tersoff
pair_coeff * * tersoff SiC.tersoff C NULL
pair_coeff 2 2 eam Ni_u3.eam
pair_coeff 1 2 lj/cut 0.1034 2 8.5
#fix 1 all nve
fix 2 lower setforce 0.0 0.0 0.0
fix 1 all nve
thermo 10 #dump 1 all xyz 50 dump.metal.xyz
minimize 1.0e-6 1.0e-6 1000 1000
velocity indent set 0 -5 0 units box
dump 1 all xyz 50 dump.metal.xyz
run 5000
Indentation is meant to be done
slowly, especially at MD timescales
and speeds. Potentials have a cutoff
length. So the surface witll “feel” the
indenter before the atoms get there.
If you bring it in too fast, the surface
will respond in odd ways.
Thanks for your reply, Even if i try to do it at slower speed the diamond indenter did not succeed in doing the indent, whereas in the case of fix indent there is not any problem at any speed.
I am using the diamond indenter to bring the simulation close to the reality, as diamond indenter are generally used in the nanoindetation process, i faced similar problem when i was using airebo potential for the C (diamond), but i moved to tersoff when in literature i came to know that tersoff potential works fine in the case of diamond, but the problem with the indenter did not get solved.
One thing more i want to ask is my way of defining the tersoff potential is wright?
In most nanoindentation simulations, the indenter is made rigid. "Fix
indent" is in fact a rigid indenter. So you might want to exclude
your indenter from "fix nve". Also you want to use "fix move" to
drive your indenter into the substrate. If you use "velocity set"
then your indenter will stop moving after being repelled by the
substrate.
Dear Steve,
Thanks for your reply, Even if i try to do it at slower speed the diamond
indenter did not succeed in doing the indent, whereas in the case of fix
indent there is not any problem at any speed.
I am using the diamond indenter to bring the simulation close to the
reality, as diamond indenter are generally used in the nanoindetation
process, i faced similar problem when i was using airebo potential for the C
(diamond), but i moved to tersoff when in literature i came to know that
tersoff potential works fine in the case of diamond, but the problem with
the indenter did not get solved.
that is because of poor design of your simulation; you are not
simulating an indentation, but you are simply smashing a small thingy
onto a larger thingy with a speed of about mach 1.5.
even more so it looks like you chose the how you set up the Ni surface
equally poorly, because the "wave" you see is mostly due to the
surface slab trying to get into equilibrium. if you did the
calculation with fix indent in similar way, it will be similarly
wrong.
Dear Alex,
Thanks for your reply, i tried even at (0 -0.2 0) the indenter was not able to indent, i given that much velocity to check whether the indenter is able to touch the substrate or not, bu even at this velocity the indenter is not able to indent, it just stops before the metal surface, this is the thing i wanted to point out, i know indentation requires very slow even the speed of 1m/s is too high due to the limitation of the computing power we are doing at such high speeds.
I have used the minimize command before the start of the indentation so that it can be in equilibrium.
In the case of fix-indent there was no such wave, and the result were similar to the work done earlier.
Again I will like to point out that there might something wrong with the way I have used the tersoff potential.
Dear Alex,
Thanks for your reply, i tried even at (0 -0.2 0) the indenter was not able
to indent, i given that much velocity to check whether the indenter is able
to touch the substrate or not, bu even at this velocity the indenter is not
able to indent, it just stops before the metal surface, this is the thing i
wanted to point out, i know indentation requires very slow even the speed of
1m/s is too high due to the limitation of the computing power we are doing
at such high speeds.
I have used the minimize command before the start of the indentation so that
it can be in equilibrium.
In the case of fix-indent there was no such wave, and the result were
similar to the work done earlier.
Again I will like to point out that there might something wrong with the way
I have used the tersoff potential.
no it isn't.
ray and i explicitly pointed out your problems. fix them and it will
work. there are many and none of them have to do with the tersoff
potential.
Dear Alex,
Thanks for your reply, i tried even at (0 -0.2 0) the indenter was not able
to indent, i given that much velocity to check whether the indenter is able
to touch the substrate or not, bu even at this velocity the indenter is not
able to indent, it just stops before the metal surface, this is the thing i
wanted to point out, i know indentation requires very slow even the speed of
1m/s is too high due to the limitation of the computing power we are doing
at such high speeds.
limitation of compute power is a commonly used and very convenient
excuse for the most ridiculous simulations. the example you posted,
took less than 90 seconds to complete on a single core of my 5 year
old desktop. a mid-range smartphone has more compute power these days
that that single CPU core. if you have *this* little compute power,
don't do simulations.
axel.