Reviewers' Comments:
Reviewer #1:
Remarks to the Author:
The manuscript on “Additive Manufacturing of 3D Nano
-Architected Metals” by Vyatskikh et al.
describes the two
-photon lithography and subsequent pyrolysis of a metal-
rich photo-
curable
polymer
resin in the shape of octet lattices. The structure and composition of the lattices are
characterized using EDS and TEM, and the compressive properties of the lattices are evaluated
inside of a SEM. Researchers should be able to reproduce the work given th
e level of detail
provided in the manuscript. This manuscript presents an interesting fabrication process with novel
elements (new photoresist chemistry and pyrolysis) that addresses an important challenge in the
3D printing community -
the manufacture of 3D metallic structures with sub
-micron features, but
suffers from major flaws in analysis. I recommend publication in Nature Communications if the
following major and minor issues can be addressed.
Major issues:
1. I find it very odd that previous repo
rts from the Greer group and others on the fabrication and
mechanical properties of nano and micro
-lattices are not cited in this manuscript (Schaedler et al.,
Science; Jang et al., Nature Materials; Meza et al., Science; etc.), especially those that discu
ss the
fabrication and mechanical properties of metallic lattices (Schaedler et al., Science; Gu and Greer,
Extreme Mechanics Letters (2015); Lee et al., Nano Letters (2015)). The current work must be
placed in the context of these previous studies with re
gard to mechanical properties (fig. 4f) and
lattice beam size (fig. 5) to evaluate its novelty and achievements. I believe that the specific yield
strength reported here is lower than that reported in previous studies on metallic nano-
lattices,
and that th
e statement that metallic lattices with slender beams have lower strength is untrue
(line 152-
153).
2. It does not make sense to extrapolate the specific strength of the cited macroscopic samples to
smaller length scales to obtain the “expected” nano-
lat tice specific strength in figure 4f. Due to the
differences in microstructure, structural geometry and composition between the large and small
lattices, there is no reason to expect the mechanical properties of the small lattices to be similar to
those of the large lattices. Again, a much better comparison would be to previous studies on nano
and micro
-lattices with the same octet geometry and similar composition.
3. The authors state that the porosity of their nanolattices is detrimental to mechanical strength
(line 156-
158). This is counter to the large body of literature on nanoporous metallic foams
(Biener et al., Nano Letters (2006) among many others), and mechanical size effects at small
scales (Greer and De Hosson, (2011)). The authors must discuss the microstructural differences
between their work and previous studies that lead to the observed differences in mechanical
properties to support their views.
4. Alternative, is the specific strength evaluated based on the density of the porous lattice
structure, or an equivalent non-
porous structure? If the density of the non
-porous structure is
used, this would obviously lead to the wrong conclusions about the effect of porosity.
5. Is the vertical spring support removed before mechanical testing? If not, how does it factor into
the measured mechanical response of the lattice?
6. Mechanical properties are only measured on four samples. Given the large variation in
mechanical response, and the alleged ease of fabricating these lattices, a larger number of tests in
line with norms in the field of nano-
mechanical testing should be performed to determine the
source of this variation (differences in sample structure, or differences in testing conditions).
Minor issues:
7. Figure 3f. Explain the meaning
of mu and sigma in the figure caption.
8. What is the total processing time (lithography and pyrolysis) to make these structures? How
does this compare to other additive manufacturing techniques?
9. Was the TEM analysis of elemental composition in fig
ure 3e performed on a single particle or
multiple particles? This should be included in the text.
Writing and organization:
10. The quality of writing should be improved. For example, lines 39-
43 is very confusing, written
in a different verb tense as the rest of the paragraph, and possibly grammatically incorrect.
11. The conclusion section is labeled as the discussion section. The manuscript should be re
-
structured to have an actual discussion section.
12. Line 406: characterization is written twice
13. Citations should be included in the text as “ref. #” when referred to directly. Example: lines
258 to 266.
Reviewer #2:
Remarks to the Author:
General comments:
Overall this is a very good manuscript which is worthy of publication in Nature
Communications.
The authors report on a new way to achieve metallic nanoscale architectures using additive
manufacturing and advanced resin chemistries. These chemistries allow for the production of
nanolattices in a two-
photon lithography system and repr
esent the key contribution of the paper.
This was achieved through incorporating a Ni
-based MOF into a photo-
resin, fabrication with a
NanoScribe two
-photon system, and subsequent thermal post
-processing in the form of
pyrolization of the polymer leaving only the Ni. The process was successful in yielding nanolattices
primarily made of Ni. This is an interesting and useful result as it is a more direct way to generate
metallic nanostructures which have been shown to be extremely interesting by the correspon
ding
author’s group in prior papers as well as by other reputable groups around the world. While the
work is generally excellent, I have two overall requests/suggestions which the authors should
address prior to publication, as well as some detailed commen
ts:
• The claims regarding scalability which appear throughout the manuscript seem to be a bit of an
over
-reach. Two
-photon lithography, in its current form, has not been scaled effectively. While the
write speeds discussed in this paper are fast, it is still very difficult to build substantial structure
with a NanoScribe and this needs to be acknowledged. Write speed is not the only requirement for
scalability
– overall build volume is still very small in this process and the smaller the feature size,
the longer it takes to build substantial structure. The authors use “voxels per second” as a metric
but this is deceptive as all processes have different voxel sizes and this would over
-state the
speed/scalability of this process. While there are some groups
working on scale
-up of the process,
it has not yet been realized.
• The resultant mechanical properties are very good due to the very fine ~20nm nanocrystalline
structure. However, the paper lacks some in
-depth discussion of the deformation mechanism.
Particularly, how does the very long plateau shown in the data come about? Is this plateau a result
of the residual high porosity of 10
-30%? Some discussion clarifying this would be useful.
Detailed comments:
1. Line 25-
26: Authors claim, “scalable pathw
ay” to nanolattices –
this is a bit overstated. Two-
photon has not been shown to be particularly scalable and this work does not address this.
2. Line 32-
33: While I generally agree with the authors claim that “no established method is
available for print
ing 3D features below these dimensions” it is notable that they do not compare
this method to those where nanoscale coatings and/or nanoparticle suspensions have been used.
This includes work by the corresponding author's group. I believe that the work in this paper
shows a superior method to achieve metallic lattices, so this comparison should be favorable.
3. Line 35-
37: I agree with the statement “Accessing these phenomena requires developing a
process to fabricate 3D metallic architectures with macroscopic overall dimensions and individual
constituents in the sub
-micron regime.” However, as previously stated regarding scalability, I’m
not sure this paper addresses the “macroscopic” part of this.
4. Line 56: “We developed a scalable...” –
same comment reg
arding scalability. Suggest softening
these statements.
5. Line 73: “~80% shrinkage” –
is this isotropic (assume it is but maybe state this) and is this
volumetric?
6. Line 92: Composition is reported as 91.8% Ni, 5.0% O, 3.2% C –
is this purity level go
od or
bad? Can it be better? Does it impact properties?
7. Line 139: “None of the nanolattices recovered after deformation.” Would you expect them to
recover if strut diameters begin to approach the grain size?
8. Line 156-
158: Reports “10-
30% residual porosity..” What is impact of this on mechanical
properties? Have you considered something like a hot isostatic press to remove porosity?
9. Line 166-
175: This section again discusses write speeds and scalability. The authors use
“voxels/sec” as a metric o
f comparison to other printing methods. However, they do not define
“voxel” very well. Is a voxel the same volume for all processes? I don’t think it is. Why not use
actual volume of fabricated material per second? This would be more appropriate. The NanoS
cribe
generally has a very small voxel element so using this as a metric will overstate its scalability.
Again, I believe these claims are bit over
-stated. A more transparent metric should be used.
Comment 1
Reviewer 1:
1. I find it very odd that previous reports from the Greer group and others on the fabrication and
mechanical properties of nano and micro
-lattices are not cited in this manuscript (Schaedler
et al.,
Science; Jang et al., Nature Materials; Meza et al., Science; etc.), especially those that discuss the
fabrication and mechanical properties of metallic lattices (Schaedler et al., Science; Gu and Greer,
Extreme Mechanics Letters (2015); Lee et al
., Nano Letters (2015)). The current work must be placed
in the context of these previous studies with regard to mechanical properties (fig. 4f) and lattice beam
size (fig. 5) to evaluate its novelty and achievements. I believe that the specific yield strength reported
here is lower than that reported in previous studies on metallic nano
-lattices, and that the statement
that metallic lattices with slender beams have lower strength is untrue (line 152
-153).
Response:
We
thank the reviewer for the suggestion
to include the specified references
into the
manuscript. We have now included references to
Schaedler et al., Science
(2011)
; Jang et al., Nature
Materials
(2013)
; Meza et al., Science
(2014)
; Gu and Greer, Extreme Mechanics Letters (2015)
; Lee
et al., Nano Letters (2015)
; Montemayor and Greer, Journal of Applied Mechanics (
2015); and
Liontas and Greer, Acta Materialia (2017) in the revised
manuscript.
We
chose to ex
clude references
to the works previously published by Greer Group in F
ig. 4f
because its aim is to convey
the
specific
strength
s of
metallic
3D architectures fabricated using
only
metal additive manufacturing processes
.
The references
above
describe either metal structures with monolithic beams
fabricated
by
electroplating into a template (
Gu and Greer, Extreme Mechanics Letters (2015)
) or
shell
-beam
architectures made
by coating a template with a layer of metal (
Schaedler et al., Science
(2011);
Lee
et al., Nano Letters (2015)
; Montemayor
and Greer, Journal of Applied Mechanics (2015); and
Liontas and Greer, Acta Materialia (2017)
), which are not
metal AM processes. Similarly,
Fig.
5
focuses on the
resolution of the existing
metal AM processes.
To address the reviewer’s
critique
, we have included the specific strength of
the solid
-beam Cu
lattices reported by
Gu and Greer
in the
discussion section of the revised manuscript and in the
revised
Supplementary Table 2
. A direct
comparison
between
the compressive strength
s of
nanocryst
alline Ni nanolattices in this work
with
those
of the
metallic lattices
reported by
Montemayor and Greer
, Schaedler et al.
, Lee et al.
, and Liontas and Greer
may be misleading because
this work is focused on solid
-beam metallic
nanolattices
, which deform via compression and plastic
flow upon uniaxial compression; all others contain hollow shell beam
s and undergo
a different
deformation mechanism
upon compression that includes shell buckling and layer
-by
-layer collapse.
Comment 2
Reviewer
1:
2. It does not make sense to extrapolate the specific strength of the cited macroscopic samples to
smaller length scales to obtain the “expected” nano
-lattice specific strength in figure 4f. Due to the
differences in microstructure, structural geometry
and composition between the large and small
lattices, there is no reason to expect the mechanical properties of the small lattices to be similar to
those of the large lattices. Again, a much better comparison would be to previous studies on nano and
micro
-lattices with the same octet geometry and similar composition.
Response:
We thank the reviewer for this comment
. It may indeed be misleading to extrapolate the
specific strength plot to the micron
-sized beam dimensions precisely for the reasons that the
reviewer
brings up: among others, the differences in microstructure, composition, and flaw distribution, may
lead to drastically different mechanical performance at smaller sizes. We have now revised
Figure 4
accordingly
and included a comparison to the c
ompressive strength of the structures with monolithic
Cu beams fabricated by electroplating into a template (
Gu and Greer, Extreme Mechanics Letters
(2015)
) in the discussion section of the manuscript
.
Comment 3
Reviewer 1:
3. The authors state that the porosity of their nanolattices is detrimental to mechanical strength (line
156-158). This is counter to the large body of literature on nanoporous metallic foams (Biener et al.,
Nano Letters (2006) among many others), and mechanical size effects at small scales (Greer and De
Hosson, (2011)). The authors must discuss the microstructural differences between their work and
previous studies that lead to the observed differences in mechanical properties to support their views.
Response
:
It is true that some of the existing literature, including the publications on the deformation
of nanoporous metallic f
oams
and of the individual metallic nano-pillars brought up by the reviewer,
report higher strengths upon uniaxia
l compression
(Biener’s and Volkert’s groups)
. The key
difference
between the strength reported in this work and previous reports is that it is representative of
the
structural
strength of the nanolattice, where each beam
has heterogeneous porous microstructure
,
as well as each nodal junction, and both are
subjected to a complex stress state
upon global
compression
. The microstructure that comprises nanolattices in this work is nanocrystalline and
nanoporous,
and has different levels of hierarchy in the sense that each in
dividual beam is
nanocrystalline and nanoporous, as well as the entire structure. This microstructure within the
individual beams
stems from sintering of the Ni nanoparticles after the organic components violalize;
its in distinct contrast to the monolithi
c metallic beams in all other literature on the deformation of
nanoporous materials
. This microstructure is detrimental to the overall structural strength in two
ways: (1) the additional porosity within each beam lowers the overall relative density of the
architecture and (2) upon mechanical deformation, each
sintered junction experiences a local stress
state, which creates an effective stress concentration in the material at an adjacent pore. The
pores
that
border these regions of local stress concentratio
ns can be viewed as “notch
es” or “flaws
” that serve as
locations of failure initiation upon mechanical loads. The distribution of nano-pores in each beam that
comprises the nanolattices
in this work leads to a distribution in the local failure strengths, which –
in
combination with the detrimental effects of lower relative density and the presence of junctions –
serves to lower the overall structural strength.
In all the nano
- and micro
-pillar studies, the individual nano structure was subjected to the state of
(nearly) uniaxial compression, with the competing effects of intrinsic and extrinsic length scales
contributing to the overall deformation and strength (see
Greer and De Hosson, (
2011)
). For example,
the majority of single crystalline metallic nano
- and micro
-pillars exhibit the so
-called “smaller is
stronger” size effect in a power law fashion, which stems from the competing rates of dislocation
annihilation and dislocation nuclea
tion. Nanocrystalline metallic nano
-pillars have been shown to
become weaker with size reduction (
Gu et al., Nano Letters (2012)
, Jang and Greer, Scripta Materialia
(2011)
, Yang, B.; et al Philos. Mag. (2012)
) because of the activation of grain boundary de
formation
and sliding, particularly in the grains that are adjacent to the free surfaces, in response to uniaxial
deformation. To the best of the authors’ knowledge, no in
-depth literature exists on
the deformation
mechanisms
in individual
nanoporous nanoc
rystalline nano
- or micro
-pillars.
Literature on the deformation of nanoporous metals –
for example, the work of Biener, Hodge,
Volkert, et al Nano Letters 2006 -
reveals
a close-to
-
μ
/10 compressive strength of nanoporous
gold
cylinders with micron
-sized diameters and 10 nm
-sized individual ligands
. Their overall strength is
reported to be close to that of monolithic gold because each ligament is a virtually defect
-free
, single
crystalline
beam, whose strength approaches i
deal strength of gold. These foams have a
fundamentally different microstructure compared to the nanol
attices in this work in that
they are
stochastic foams with
relatively slender, curved
single
-crystalline
pristine beams.
The overall
structural strength is governed by the strength of each ligand (reported to be 4.6 GPa), which is close
to the theoretical shear stress for gold (4.3 GPa).
These foams have a single level of hierarchy in that the beams that comprise the porous architecture
are homogeneous. The nanolattices in this work have multiple levels of hierarchy: the beams that
comprise the micron
-level architecture have complex microstructure that is nanocrystalline and
nanoporous
; with short sintered necks between nanoparticles and no long or slender sub
-beams within
it. Upon mechanical deformation, each individual beam and each junction experiences a distribution
of local stresses wi
thin it, which create a landscape of stress concentrations around “flaws” and lead
to failure at lower applied stresses.
The importance and the significant contribution of the nodes to the
deformation and the lower
-than
-expected scaling of stiffness (and strength) with relative density was
recently published in Meza et al.,
Acta Materialia
(2017)
. The above discussion has now been
included in the revised manuscript
.
Comment 4
Reviewer 1:
4. Alternative, is the specific strength evaluated based on the density of the porous lattice structure, or
an equivalent non-porous structure? If the density of the non-porous structure is used, this would
obviously lead to the wrong conclusions about the effect of porosity.
Response:
This is a very important distin
ction. It would be incorrect to only account for the density of
the prescribed architecture because the density of the “parent solid” is not that of the monolithic, non
-
porous nickel.
We evaluated the specific strength as the measured strength of the lattice divided by
the lattice density
, which is
a product of the relative density of the structure and the material density.
For nickel nano-lattices fabricated in this work the relative density was estimated using a CAD model
with average unit cell sizes and beam diameters measured from the SEM images assuming fully
-dense
beams.
This assumption means that the calculated values are lower
-bound estimates of the specific
strength
. We estimated the porosity of the nanoporous nickel that comprises each beam to be ~
10-
30%
using SEM and TEM images
. In our ongoing
current work
on this project
, we are exploring
ways to accurately quantify the beam porosity.
Comment 5
Reviewer 1:
5. Is the vertical spring support removed before mechanical testing? If not, how does it
factor into the
measured mechanical response of the lattice?
Response:
The four corner springs
in the support structure were no longer connecting the sample to
the substrate
after pyrolysis, as can be seen in Fig. 4a, and did not contribute to the mechanical
response of the lattice. The central pillar in the support structure failed
after
the initial contact, which
allowed for establishing full contact between the sample and
the substrate and for sample alignment.
This contributed to the mechanical response of the sample
in the toe region, which was
not included
in the compression data
for
the compression strength
calculation
.
Comment 6
Reviewer 1:
6. Mechanical properties are only measured on four samples. Given the large variation in mechanical
response, and the alleged ease of fabricating these lattices, a larger number of tests in line with norms
in the field of nano-mechanical testing should be performed to determine the source of this variation
(differences in sample structure, or differences in testing conditions).
Response:
To comply with the reviewer’s request, we have now fabricated six
additional samples
following the AM process described
in the manuscript and compressed them using the same
methodology. The a
dditional stress
-strain data is
provided in Supplementary Fig. S
4 and conveys its
close resemblance to the original data shown in Figure 4e
. Data for additional samples was added to
Su
pplementary Table S
2 and to Fig. 4f.
We chose not to modify Figure 4e
in the main manuscript
because it shows representative stress
-strain data
for four samples out of ten, and it would crowd the
plot
. We revised the language
in the
manuscript to reflect this change.
Comment 7
Reviewer 1:
7. Figure 3f. Explain the meaning of mu and sigma in the figure caption.
Response:
We have modified
the figure caption to read as follows:
Fig. 3
TEM characterization of the nano
-architected Ni
.
a
SEM image of the
nickel beams fabricated
directly on a 200 nm
-thick SiN membrane within the
TEM grid
b
.
Low
-magnification TEM image
of
a 100 nm
-diameter
nickel beam overhanging the edge of a
1.25 μm
-diameter hole in the
SiN
membrane.
c
TEM image of the metal sample in the region where the diffraction pattern was
obtained.
d
Electron diffraction pattern shows that the printed beam consists mostly out of
polycrystalline nickel with a small amount of nickel oxide.
e
HRTEM image of a printed metal beam.
Analysis of atomic plane
distances using FFT shows predominantly polycrystalline nickel (region 1)
with some amount of nickel carbide within the beams
(region 2) and nickel oxide at the surface
(region 3).
f
Grain size histogram for n=40 particles measured from a TEM image showin
g 95%
confidence intervals for the mean grain size (
휇
) and the standard deviation (
휎
) (see Figure S
2 and
Supplementary Table S
1)
Comment 8
Reviewer 1:
8. What is the total processing time (lithography and pyrolysis) to make these structures? How does
this compare to other additive manufacturing techniques?
Response:
The lithography time to fabricate a single
structure shown in Fig. 1f is ~1 hr
, and the total
pyrolysis time is ~20 hr. We compare the throughput of the developed process to other state
-of
-the
-art
micro
-scale AM processes in the Discussion section of the manuscript:
For a typical 300-600 nm feature size printed by TPL
35
, writing speeds in this work correspond to
defining 6700
– 20000 voxels s
-1
, a printing speed that is out of reach for state
-of
-the
-art micro
-scale
metal AM techniques, i.e. electrohydrodynamic printing (0.05-300 voxels s
-1
), local el
ectroplating
(0.04-1.0 voxels s
-1
), focused beam methods (0.01-0.8 voxels s
-1
), and direct ink writing (0.7
-3000
voxels s
-1
)
7
.
We have also included a com
parison of
linear writing speeds and
volumetric throughputs of these
processes in the manuscript (
see Supplementary Table S
4).
Comment 9
Reviewer 1:
9. Was the TEM analysis of elemental composition in figure 3e performed on a single particle or
multiple particles? This should be included in the text.
Response:
FFT patterns shown in Fig. 3e were taken from regions that include a single particle or a
region within a particle. The diffraction pattern in Fig. 3d was collected from a conglomerate of
particles shown in Fig. 3c. We have included a clarification in the
Methods section
regarding
representative
regions 1,2, and 3
in Fig. 3e
tha
t were chosen to show the phases of the materials
present:
Phases and Miller indices for the phases in HRTEM image (Fig. 3e) were assigned based on the two
lattice distances
푑
ℎ푘푙
and the angle measured from FFT patterns within the outlined regions.
Representative regions 1, 2, and 3 for the FFT analysis were chosen to include a single particle or a
region within a particle of interest.
Comment 1
0
Reviewer 1:
10. The quality of
writing should be improved. For example, lines 39-43 is very confusing, written in
a different verb tense as the rest of the paragraph, and possibly grammatically incorrect.
Response:
We have revised this paragraph accordingly
:
Minimum feature size in
metal AM is generally limited by the material feedstock, i.e. the method of
supplying metal in powder, wire, sheet or ink form during fabrication. Inkjet
-based methods
12,13
manipulate 40-
60 μm droplets of metal inks, limiting th
e smallest features to at least the size of a
solidified droplet. Wire
- and filament
-based processes, such as Plasma Deposition
4
and Electron
Beam Freeform
Fabrication (EBF3)
14
, rely on locally melting a >100 μm
-diameter metal wire, which
produces millimeter
-sized features. Powder
-based processes, such as Selective Laser Melting (SLM)
and Laser Engineered Net Shaping (LENS)
15
, consolidate ~0.3
-
10 μm metal powder particles, which
limits the smallest feature size to about 20 μm
6,16
. Overcoming these resolution limitations requires a
capability to manipulate nanoscale quantities of metals in a stable and scalable 3D printing process.
Comment 1
1
Reviewer 1:
11. The conclusion section is labeled as the discussion section. The manuscript should be re-
structured to have an actual discussion section.
Response:
We thank the reviewer for the evaluation and feedback. We have re-structured the
manuscript
to include separate
discussion and conclusion sections.
Comment 1
2
Reviewer 1:
12. Line 406: characterization is written twice
Response:
We have corrected the figure caption accordingly.
Comment 1
3
Reviewer 1:
13. Citations should be included in the text as “ref. #” when referred to directly. Example: lines 258 to
266.
Response:
We thank the reviewer for pointing this out. We have modified
direct references
accordingly
.
Comment 1
4
Reviewer 2
:
• The claims regarding scalability which appear throughout the manuscript seem to be a bit of an
over
-reach. Two
-photon lithography, in its current form, has not been scaled effectively. While the
write speeds discussed in this paper are fast, it is still very difficult to build substantial structure with a
NanoScribe and this needs to be acknowledged. Write speed is not the only requirement for scalability
– overall build volume is still very small in this process and the smaller the feature size, the long
er it
takes to build substantial structure. The authors use “voxels per second” as a metric but this is
deceptive as all processes have different voxel sizes and this would over
-state the speed/scalability of
this process. While there are some groups worki
ng
on scale-up of the process, it has not yet been
realized.
Response:
We thank the reviewer for this comment. We have adjusted statements regarding
scalability throughout the manuscript.
We agree with the reviewer that “the smaller the feature size,
the longer it takes to build substantial structure
”, which is
the reason
why the “voxel s
-1
” metric was
suggested previously to compare writing speeds between AM processes with different resolutions
(Hirt
h et al., Adv. Mater., 2017)
. Please see a
more
detailed response about this in
Comment 24.
Comment 1
5
Reviewer 2
:
• The resultant mechanical properties are very good due to the very fine ~20nm nanocrystalline
structure. However, the paper lacks some in
-depth discussion of the deformation mechanism.
Particularly, how does the very long plateau shown in the data come about? Is this plateau a result of
the residual high porosity of 10-30%? Some discussion cl
arify
ing this would be useful.
Response:
We thank the reviewer for the question.
A detailed
discussion
on
the deformation
mechanisms
as a function of microstructure in small
-scale features and architectures has now been
included in the revised manuscript
. For
a more detailed
discussion, we refer the reviewer to the
response
to Comment 3.
Comment 1
6
Reviewer 2
:
1. Line 25
-26: Authors claim, “scalable pathway” to nanolattices
– this is a bit overstated. Two
-
photon has not been shown to be particularly scalable and this work does not address this.
Response:
We thank the reviewer for this comment. We have adjusted this statement accordingly
.
Comment 1
7
Reviewer 2
:
2. Line 32
-33: While I generally agree with the authors claim that “no established method is available
for printing 3D features below these dimensions” it is notable that they do not compare this method to
those where nanoscale coatings and/or nanoparticle suspensions have been used. This includes work
by the corresponding author's group. I believe that the work in this paper shows a superior method to
achieve metallic lattices, so this comparison should be favorable.
Response:
We thank the reviewer for this sugg
estion. In this manuscript, we
focus
ed on comparing
the
Additive Manufacturing
method
we developed to
the
previously reported metal additive
manufacturing processes
and metal AM
-fabricated structures. Based on the reviewer’s suggestion and
scope of the manuscript, we have included a comparison of specific yield strength of the structures
reported in this work to
some other relevant literature, which includes
solid beam metal mesolattices
fabricated by
electroplating process (Gu and Greer, Extreme Mechanics Letters (2015)
). A direct
comparison between the compressive strengths of nanocrystalline Ni nanolattices in this work with
those of the shell
-beam architectures made by coating a template with a lay
er of metal (
Schaedler et
al., Science
(2011);
Lee et al., Nano Letters (2015)
; Montemayor and Greer, Journal of Applied
Mechanics (2015);
Montemayor et al., Advanced Engineering Materials (2013);
and Liontas and
Greer, Acta Materialia (2017))
may be misleading because this work is focused on solid-beam
metallic nanolattices, which deform via compression and plastic flow;
shell
-beam structures
undergo
a different deformation mechanism upon compression that includes shell buckling and layer
-by
-layer
collapse.
Comment 1
8
Reviewer 2
:
3. Line 35
-37: I agree with the statement “Accessing these phenomena requires developing a process
to fabricate 3D metallic architectures with macroscopic overall dimensions and individual
constituents in the sub-micron regime.”
However, as previously stated regarding scalability, I’m not
sure this paper addresses the “macroscopic” part of this.
Response:
We thank the reviewer for this comment. We have adjusted the statements regarding
scalability throughout the manuscript.
Comment 1
9
Reviewer 2
:
4. Line 56: “We developed a scalable...” –
same comment regarding scalability. Suggest softening
these statements.
Response:
We thank the reviewer for this comment. We have adjusted this statement accordingly
in
the revised manuscript.
Comment 20
Reviewer 2
:
5. Line 73: “~80% shrinkage” –
is this isotropic (assume it is but maybe state this) and is this
volumetric?
Response:
The shrinkage is isotropic and results in ~80% smaller linear dimensions.
We have added
a clarifying sentence
to the Results section
.
Comment 21
Reviewer 2
:
6. Line 92: Composition is reported as 91.8% Ni, 5.0% O, 3.2% C –
is this purity level good or bad?
Can it be better? Does it impact properties?
Response:
This is a very important point.
The
SEM
EDS
analysis
is generally not the most accurate
method to evaluate carbon content and is highly dependent on carbon deposition in the SEM chamber
(Donovan et al., Scanning Electron Microscopy,
Chapter
in
Characterization of Materials
(John
Wiley and Sons, 2002)).
We
observe
<1%
by volume of
Ni
3
C
in the TEM, which confirms
the
presence of carbon in the as
-fabricated structure.
It is reasonable to expect some traces
of
carbon in
the final pyrolized
structure
due to high solubi
lity of carbon in nickel at 1000°C
(~0.1 wt%, Lander et
al.,
Journal of Applied Physics
23, 1305 (1952)
), which leads to carbon precipitation at nickel surface
upon cooling down to room temperature.
The
presence
of the oxygen peak in the EDS signal is consistent with nickel oxidation in air, which
leads to the formation of native oxide and full oxidation of surface particles smaller than 6 nm (Wang
et al.,
J Nanosci Nanotechnol
., 2011). We expect this effect to b
e especially pronounced in nano-
porous nickel beams fabricated in this work
since these were exposed to air before SEM and TEM
analysis.
TEM analysis reveals that these elements –
O and C –
are bound to Ni and form small
amounts of NiO and Ni
3
C. Lower oxygen content can be achieved by decreasing the
porosity of the
structural elements. Carbon content can be reduced by using oxygen plasma after the pyrolysis is
complete, but another reduction step would then be needed to decrease the amount of oxygen in the
sample.
It is likely that the ~5
wt% of O and ~3wt
% of C contribute negligibly to t
he mechanical
properties of the overall structure.
Comment 22
Reviewer 2
:
7. Line 139: “None of the nanolattices recovered after deformation.” Would you expect them to
recover if strut diameters begin to approach the grain size?
Response:
The question
of whether the nanolattices would recover if the struts had a bamboo
-like
structure
(with or without the pores)
, i.e. each grain spanned the entire diameter of the strut
, is an
interesting one to explore
. The recovery in the elastic, pre-yield regime is g
overned by the slenderness
ratio of the beams and the boundary conditions at the nodes. This type of elastic recovery would be
relatively insensitive to the microstructure, depending on it
only in terms of the modulus. Intuitively,
we do not expect
metalli
c nanolattices
with bamboo microstructure within the beams to recover in the
global post
-elastic regime
because
the local yielding events will occur at various points within the
structure, with dislocation
- and grain boundary
-driven deformation processes govern
ing local
plasticity in the grains within the struts
. The deformation of
mechanical behavior of copper
mesolattices with strut diameters
on the order of the grain size
(albeit in the micron range)
were
previously explored in
Gu and Greer, Extreme Mec
hanics Letters (2015)
and
showed
no recovery
after
yielding.
Our current efforts in this project are dedicated to a more in
-depth analysis of the
material microstructure and its effect on the mechanical deformation of the overall architecture and
will be p
repared in a separate manuscript.
Comment 23
Reviewer 2
:
8. Line 156
-158: Reports “10-30% residual porosity..” What is impact of this on mechanical
properties? Have you considered something like a hot isostatic press to remove porosity?
Authors:
This is a very good question.
Please see our response to Comment 15 for a detailed
discussion of the effect of porosity on the mechanical properties.
We thank the reviewer for suggesting
using
a hot isostatic press. Our current efforts are dedicated in
exploring pathways to accurately quantify the porosity within individual beams and to assess its effect
on the overall deformation
and strength. Methods to reduce
porosity are also being explored.
Comment 24
Reviewer 2
:
9. Line 166
-175: This section again discusses write speeds and scalability. The authors use
“voxels/sec” as a metric of comparison to other printing methods. However, they do not define
“voxel” very well. Is a voxel the same volume for all processes? I don’t think it is. Why not use actual
volume of fabricated material per second? This woul
d be more appropriate. The Nanoscribe generally
has a very small voxel element so using this as a metric will overstate its scalability. Again, I believe
these claims are bit over
-stated. A more transparent metric should be used.
Response:
We thank the reviewer for this comment.
The metric “
voxels s
-1
” to compare the write speeds of metal
AM processes with different resolutions has been proposed in a recent review of micro
-scale metal
AM
processes
(Hirt et al., Adv. Mater., 2017). The reasoning behind using this metric is the
correlation between the process resolution and the volumetric throughput. Writing with higher
resolution requires larger number of voxels to define the same geometry, which tran
slates into longer
processing times and lower volumetric throughputs. Comparing the speed between processes with
different resolutions can be accomplished by normalizing the write speed (
μm s
-1
) by the feature size
(
μ
m) or by normalizing the volumetric thr
oughput (
μm
3
s
-1
) by the voxel volume (
μ
m
3
) (Hirt et al.,
Adv. Mater., 2017)
. We have added a clarification regarding
the
metric
definition into the Discussion
part of the manuscript
and adjusted the statements regarding scalability.
Another key aspect of
any metal AM process is the throughput. Using hybrid organic
-inorganic
photoresist developed in this work allows for writing speeds of 4-6 mm s
-1
, which is ~100 times faster
than that for TPL of metal salts
20
. Comparing the speed between processes
with different resolutions
can be accomplished by normalizing the write speed (μm s
-1
) by the feature size (μm) or by
normalizing the volumetric throughput (μm
3
s
-1
) by the voxel volume (μm
3
)
7
. For a ty
pical 300-600
nm feature size printed by TPL
35
, writing speeds in this work correspond to defining 6700
– 20000
voxels s
-1
, a printing speed that is out of reach for state
-of
-the
-art micro
-scale metal AM techniques,
i.e. e
lectrohydrodynamic printing (0.05-300 voxels s
-1
), local electroplating (0.04
-1.0 voxels s
-1
),
focused beam methods (0.01-0.8 voxels s
-1
), and direct ink writing (0.7-3000 voxels s
-1
)
7
. High
scanning speeds and intrinsic advantage of parallelizing light delivery using lithographic methods
suggest that the presented AM process lends itself to streamlined and efficient manufacturing of metal
nano-architectures.
We have also added Supplementary Table 4,
which shows a comparison between
writing speeds and
volumetric throughputs of
state-of
-the
-art micro
-scale metal AM methods:
Supplementary Table 4
Comparison of linear and volumetric throughputs of representative
micro
-scale metal
additive manufacturing technologies (data adopted from ref.
26
)
#
Technology
Material
Feature size, μm
Writing speed
*
Ref.
1
Direct
Ink Writing
(DIW)
Ag
0.6
-20
500-
2000 μm s
-1
25
2
Electrohydrodynamic
(EHD) Printing
Ag, Co, Cu
0.7
-3.0
0.16
-
3.3 μm s
-1
27
3
Laser
-
Induced
Forward Transfer
(LIFT)
Au, Cu
4.0
-6.0
3000 μm
3
s
-1
24
4
Focused Electron
Beam Induced
Deposition (FEBID)
Pt
0.15
-0.23
0.0002
-
0.0009 μm
3
s
-1
28
5
Cryo
-FEBID
Pt
0.022
-0.31
10 μm
3
s
-1
29
6
Meniscus
-
confined
electroplating
Cu
12.0-
15.0
0.18
-
0.4 μm s
-1
30
7
Local electrophoretic
deposition
Au
0.5
-2.0
0.30
-
0.67 μm s
-1
22
8
This work
Ni
0.025
-0.4
4000
-
6000 μm s
-1
*Volumetric (
μm
3
s
-1
) or linear (
μm s
-1
) writing speed is given when available
Reviewers' Comments:
Reviewer #1:
Remarks to the Author:
This version of the manuscript is much improved, especially the discussion session. The additional
mechanical tests also add value to the manuscript. I recommend publication after the following
(new) issues are addressed:
Major issue:
Lines 24-
26 in the abstract state that the specific strength of the structures is higher than other
small truss structures making using metal AM. This directly contradicts Figure 4F, which shows
that many AM fab
ricated trusses have higher specific strength. It also directly contradicts lines 68
-
70, which state that the Ni nanolattices have comparable strength to other metal lattices. I am ok
with publication if the phrase on line 25
-26 (“which is up to an order o
f magnitude higher than that
of the smallest truss architectures...”) is removed.
Minor issue:
Line 194: violalize –
misspelling
Line 209 –
missing an “of”
Reviewer #2:
Remarks to the Author:
The authors have done an excellent job of addressing my
few concerns and this paper is surely
ready for publication. I thank the authors for clarifying the comparison of write speeds among the
various technologies. It is now a lot more clear and the normalization by feature size makes sense.
Additionally, the new text expanding on the mechanical performance of the structures adds detail
and value to the manuscript. Finally, the inclusion of additional references, primarily from the
authors own work, is appropriate. I congratulate the authors on excellent work. Thank you.
Comment 1
Reviewer 1:
Lines 24-
26 in the abstract state that the specific strength of the structures is higher than
other small truss structures making using metal AM. This directly contradicts Figure 4F, which shows
that many AM fabricated trusses have higher specific strength
. It also directly contradicts lines 68
-70,
which state that the Ni nanolattices have comparable strength to other metal lattices. I am ok with
publication if the phrase on line 25-
26 (“which is up to an order of magnitude higher than that of the
smallest truss architectures...”) is removed.
Response
:
We have corrected the statement in the abstract accordingly.