Atomistic Insights on the Wear/Friction Behavior of Nanocrystalline Ferrite During Nanoscratching as Revealed by Molecular Dynamics
Tribol Lett
Atomistic Insights on the Wear/Friction Behavior of Nanocrystalline Ferrite During Nanoscratching as Revealed by Molecular Dynamics
A. T. AlMotasem 0 1 2
L. J. Holleboom 0 1 2
J. Bergstro¨m 0 1 2
A. Ga˚a˚rd 0 1 2
P. Krakhmalev 0 1 2
0 Department of Physics, Faculty of Science, Assiut University , Assiut 71516 , Egypt
1 Department of Mechanical and Materials Engineering, Karlstad University , Universitetsgatan 2, 65637 Karlstad , Sweden
2 & A. T. AlMotasem
Using embedded atom method potential, extensive large-scale molecular dynamics (MD) simulations of nanoindentation/nanoscratching of nanocrystalline (nc) iron have been carried out to explore grain size dependence of wear response. MD results show no clear dependence of the frictional and normal forces on the grain size, and the single-crystal (sc) iron has higher frictional and normal force compared to nc-samples. For all samples, the dislocation-mediated mechanism is the primary cause of plastic deformation in both nanoindentation/nanoscratch. However, secondary cooperative mechanisms are varied significantly according to grain size. Pileup formation was observed in the front of and sideways of the tool, and they exhibit strong dependence on grain orientation rather than grain size. Tip size has significant impact on nanoscratch characteristics; both frictional and normal forces monotonically increase as tip radii increase, while the friction coefficient value drops by about 38%. Additionally, the increase in scratch depth leads to an increase in frictional and normal forces as well as friction coefficient. To elucidate the relevance of indentation/scratch results with mechanical properties, uniaxial tensile test was performed for nc-samples, and the result indicates the existence of both the regular and inverse Hall-Petch relations at critical grain size of 110.9 A˚ . The present results suggest that indentation/scratch hardness has no apparent correlation with the mechanical properties of the substrate, whereas the plastic deformation has.
Atomistic; Polycrystalline iron; Scratch hardness; Wear; Dislocations; Twinning
1 Introduction
Understanding wear, friction and mechanical properties of
a material at nanoscale is crucial for further development in
technological applications. Experimentally,
nanoindentation and nanoscratching techniques are commonly used for
nanoscale mechanical testing as they can provide accurate
information of hardness, friction and wear. As a
compliment to experimental technique, atomistic modeling
becomes a powerful tool to deepen the understanding of
wear and failure modes of materials at the atomic scale. In
the literature, numerous studies of nanoscratching of metals
are available both theoretically and experimentally and a
comprehensive review can be found in [
1, 2
].
Molecular dynamics simulation has been used to
investigate nanoscale machining and the factors governing the
nanomachining process: tip geometry, machining speed,
rake angle and surface roughness. However, most of these
simulations usually adopt defect-free monocrystalline
structures as the work material [
3–9
]. On the other hand,
most engineering materials exist in polycrystalline forms
and mechanical properties such as flow stress, yield stress
and hardness of metals and alloys [
10, 11
] dramatically scale
with grain size. Thus, grain size in polycrystalline structures
is a controlling factor for material properties and material
responses to deformation. For example, it has been shown
that grain refinement to the nanometer scale leads to an
increased yield stress Hall–Petch (H–P) relationship, while
further refinement led to inverse H–P [
12, 13
]. The influence
of grain boundaries (GBs) during nanomachining has been
extensively studied [
14–18
]; nevertheless, most of the
considered samples are either nc-fcc metals, nc-diamond or
nczinc blend ceramics but rarely for nc-bcc materials.
For instance, Shi et al. [
15
] performed MD simulations
to investigate the effect of grain size and nanomachining
parameters of polycrystalline copper. It was discovered that
for all cutting conditions simulated, the polycrystalline
structure requires smaller cutting forces compared with the
monocrystalline structure. The authors attributed this
behavior to the reduction in material strength with grain
refinement. They also verified that the behavior of
frictional and normal forces for different grains is coupled with
the associated failure mode, i.e., (H–P) or inverse (H–P) as
verified by tensile testing.
Mishra et al. [
18
] simulated the wear of a
nanocrystalline silicon carbide substrate by tools with a rounded
end. They demonstrated that the primary mechanism for
nanoscale wear of silicon carbide is GBs sliding and the
compatible stress is accommodated by nucleation of partial
dislocations, void formation and grains pullout.
Furthermore, they compared the results with sc-silicon carbide
wear response and demonstrated that nc-silicon ca (...truncated)