www.sciencemag.org/cgi/content/full/322/5907/1516/DC1
Supporting Online Material for
Tough, Bio-Inspired Hybrid Materials
E. Munch, M. E. Launey, D. H. Alsem, E. Saiz, A. P. Tomsia, R. O. Ritchie*
*To whom correspondence should be addressed. E-mail: roritchie@lbl.gov
Published 5 December 2008, Science 322, 1516 (2008)
DOI: 10.1126/science.1164865
This PDF file includes:
Materials and Methods
Figs. S1 and S2
References
SUPPORTING ONLINE MATERIAL
Tough, bio-inspired hybrid materials
E. Munch, 1 M. E. Launey,1 D. H. Alsem,1,2 E. Saiz,1 A.P. Tomsia,1 R. O. Ritchie1,3∗
MATERIALS AND METHODS
PROCESSING
The lamellar structures are prepared by freeze-casting de-ionized water based
suspensions. Alumina powder (SM8 high purity sub-micrometer alumina powder,
Baikowski) and deionized water are mixed in equal weight proportions. An ammonium
polymethacrylate anionic dispersant (Darvan CN, R.T. Vanderbilt Co., Norwalk, CT, 1.4
wt.% of the powder) and an organic binder (polyvinylalcohol, 1.4 wt.% of the powder)
are added to the suspension. This slurry leads to a structure that contains 66 vol. %
porosity after sintering. Sucrose (β-D-fructofuranosyl α-D-glucopyranoside) is used as
an additive in the suspension (4 wt.% of the water content) to modify the structure of ice
crystals, leading to homogeneous lamellae connected with thin bridges and a
characteristic roughness close to a micrometer in size. The macroscopic structure of the
scaffold can be tailored by controlling the roughness of the cold finger. Unidirectional
scratching of the cold surface with silicon-carbide paper (grade P400) leads to the
formation of homogeneous parallel lamellae. The suspension is cooled down to -80°C at
a rate of 20°C/min close to the maximal rate allowed by the thermal inertia of the system
in order to refine the wavelength of the structure. The frozen samples are freeze-dried
(Labconco, Freeze Dry system FreeZone 4.5) to remove the water. Subsequently, the
organic components are burnt in an air furnace (2 hr at 400°C) (Rapid Temp Furnace,
CM Inc. Bloomfield, NJ.) and the ceramic scaffolds are sintered (2 hr at 1500°C) to
prepare cylinders (height 30 mm, diameter 15 mm) that consist of macroscopically
oriented dense lamellae (thickness 5-10 μm) with a characteristic microscopic roughness
connected to each other by micrometer-sized inorganic bridges (Fig. S1).
“Brick-and-mortar” scaffolds (~20 vol.% porosity) are prepared from the lamellar
structures obtained after freeze casting. “Lamellar” structures are infiltrated by a short
chain polymer with a low melting point (Paraffin Wax) that acts as a sacrificial phase.
Infiltrated scaffolds are uniaxially pressed at a temperature close to the melting point
(80°C) of the organic phase. While liquid, the polymer flows and allows the
densification of the ceramic structure. After cooling the second phase holds the bricks
together for transfer into an air furnace. A thermal treatment in air is applied to remove
the organic phase from the structure (2 hr at 400°C) and a sintering step (2 hr at 1500°C)
promotes the formation of inorganic bridges between bricks. A higher densification is
1
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
3
Department of Materials Science and Engineering, University of California, Berkeley, California, 94720, USA
∗
To whom correspondence should be addressed. E-mail: roritchie@lbl.gov
2
achieved by cold isostatic pressing (at a global pressure of 1.4 GPa) of the brick-andmortar structure followed by a new sintering step (2 hr at 1500°C).
These porous scaffolds are subsequently infiltrated by a free radical polymerization of
polymethylmethacrylate in bulk. A chemical grafting procedure based on silanation
techniques is used to improve the adhesion between the ceramic and polymer phases.
The surface of both the brick-and-mortar and lamellar alumina scaffolds are made
reactive (presence of hydroxyl groups) by chemically etching in a Piranha solution (50
vol. %, H2O2 30%, 50% H2SO4) for 10 mins. The scaffolds are rinsed in DI water and
dried at room temperature for 12 hr. The grafting agent (γ-MPS: 3(Trimethoxysilyl)propyl methacrylate), diluted in acetone, reacts with the hydroxyl
groups and bonds to the ceramic surface. A first free radical polymerization step of
methyl methacrylate (MMA) initiated by 2,2′-Azobisisobutyronitrile (AIBN) is run in a
solvent (toluene) with a relatively high initiator concentration (1 wt.% AIBN in MMA, 1
h at 70°C). This first reaction maximizes the chance for radicals to develop on γ-MPS
molecules bonded to the ceramic surface, leading to a chemically-grafted PMMA chain.
A second free radical polymerization is done in bulk with a lower initiator concentration
(0.5 wt.% of AIBN in MMA) to complete the infiltration of the scaffold (8 hr at 50°C).
Non-grafted samples are subjected to this last polymerization step only. The thickness of
the organic layer ranges between 10-20 μm in the lamellar structures and has an average
value of ~1-2 μm in the brick-and-mortar materials. However, in brick-and-mortar
structures exhibit large regions with sub-micrometer polymer layers. Fig. S2 summarizes
these processing steps. The polymer fraction in the final composite was determined by
thermogravimetry and image analysis. The results obtained using both methods are
consistent.
The mechanical response of the hierarchical materials was compared to that of
homogeneous nanocomposites and dense alumina samples. The nanocomposites were
prepared by pressing uniaxially (20 MPa) the alumina powders. The resulting pellets
(diameter 30 mm, height 15 mm) were sintered in air at 1100°C for 2 hr. The resulting
samples were infiltrated with PMMA following the procedure described above. The
dense alumina samples were obtained by slip casting of a water-based ceramic
suspension (60 wt.% of alumina powder in water). These samples and the ceramic
scaffolds were sintered under the same conditions. The pure polymethylmethacrylate
samples were obtained by the polymerization in bulk of methylmethacrylate initiated by
AIBN (0.5 wt.% of AIBN in MMA) for 8 hr at 50°C. The use of identical
polymerization conditions lead to identical molecular weight characteristics.
MECHANICAL CHARACTERIZATION
Flexure measurements performed on beams were used to probe the overall
mechanical response of the hybrid composite. Beams were sectioned from the infiltrated
scaffolds by using a water-cooled, low-speed diamond saw and oriented such that the
tensile surface was parallel to the lamellae/brick boundaries. The specimens, of 18-28
mm, were machined with a thickness B ~ 1.3-1.5 mm and width W ~ 3.0-3.2 mm. Threepoint bend tests were performed to generate quantitative stress-strain information with a
support span of 12.5 mm (following ASTM Standard D790-03 (S1)) on an EnduraTec Elf
3200 testing machine (BOSE, Eden Prairie, MN) at a displacement rate of 1 μm/s. The
plane-strain fracture toughness, KIc, and crack resistance-curve (R-curve) measurements
were performed on single-edge notched bend, SE(B), specimens loaded in three-point
bending also with a support span of 12.5 mm. The notches were first introduced using a
low-speed diamond saw, and then sharpened using a razor micro-notching technique.
Micro-notches, with root radius ~3-5 μm, were obtained by repeatedly sliding a razor
blade over the saw-cut notch using a custom-made rig, while continually irrigating with a
1 μm diamond slurry. Sharp cracks with initial crack length, a ~ 1.6-1.7 μm, were
generated in general accordance with ASTM standards. Alumina SE(B) specimens were
fatigue pre-cracked using a half-chevron starter notch in addition to a Vickers hardness
indent placed at the notch tip in order to facilitate crack initiation. SE(B) specimens of
PMMA were also micro-notched. Prior to testing, specimens were polished to a 1 μm
diamond suspension surface finish on both faces. KIc values were determined by
monotonically loading the specimens to failure under displacement control with a
displacement rate of 1 μm/s on the EnduraTec testing machine. All toughness tests
satisfied the plane-strain and small-scale yielding requirements for valid KIc
measurements, as per ASTM Standard E399-06 (S2). All the values presented represent
at least an average of three measurements per configuration (N ≥ 3).
Nonlinear-elastic fracture mechanics methods were used to evaluate the fracture
toughness, involving R-curve characterization in terms of the J-integral. R-curves were
measured on micro-notched specimens in situ in a Hitachi S-4300SE/N ESEM (Hitachi
America, Pleasanton, CA) using a Gatan Microtest three-point bending stage (Gatan,
Abington, UK), again with N ≥ 3. The crosshead displacement was measured with a
linear variable displacement transducer, while the load was recorded using a 150 N load
cell. Crack extension was monitored in secondary electron mode in vacuo (10-4 Pa) and a
15 kV excitation voltage. Prior to in situ testing, specimens were coated with a 20 nm
thin layer of gold. R-curve determination was limited to small-scale bridging conditions,
where the size of the zone of crack bridges behind the crack tip remained small compared
to the in-plane test specimen dimensions. The use of nonlinear elastic fracture mechanics
with the J-integral as the driving force for crack initiation and growth was employed to
capture the contribution from inelastic deformation in the evaluation of the toughness of
the hybrid composites. J values were calculated from the applied load and instantaneous
crack length according to ASTM Standard E1820-06 (S3), and was decomposed into
elastic and plastic contributions:
J = Jel + Jpl.
The elastic contribution Jel is based on linear elastic fracture mechanics:
Jel =
K2
E'
,
where K is the stress-intensity factor and E′ is Young’s modulus appropriate to plane
strain, E′ = E/(1-ν2), where ν is Poisson’s ratio.
The plastic component Jpl is given by:
J pl =
2 A pl
Bb
,
where Apl is the plastic area under force vs. displacement curve, B is the specimen
thickness and b is the uncracked ligament length (W – a). The conditions for Jdominance and plane strain were met such that all specimen dimensions:
B, b ≥ 25
JQ
σy
.
J values were then converted to equivalent K values through the J-K equivalence
relationship for nominally mode I fracture:
K J = JE ′.
The structure of the hybrid composites (lamellar and brick-and-mortar) results in an
orthotropic material therefore making the assessment of the elastic modulus, E, nontrivial. Given this anisotropy the values of the elastic modulus will fall somewhere in
between the upper and lower bound of the “rule of mixtures”, as defined by the Reuss
and Voigt models. Measured values from the three-point bend tests suggest that the
values of the modulus of elasticity fall approximately at the mid-point between the upper
and lower bound. The values used for the calculations presented above were calculated
using this trend and the elastic moduli of the constituents of the hybrid composite,
PMMA and Al2O3 (3 and 300 GPa respectively). Note that any error in the modulus of
elasticity, E, only impacts the equivalent toughness, KJ, in a fairly limited way, such that
a ±20% error on E only results in a roughly ±1% error on KJ.
References
S1.
ASTM D790-03. Standard Test Methods for Flexural Properties of Unreinforced
and Reinforced Plastics and Electrical Insulating Materials Annual Book of
ASTM Standards, Vol. 08.01: Plastics (I) (ASTM International, West
Conshohocken, Pennsylvania, USA, 2003).
S2.
ASTM E399-06. Standard Test Method for Linear-Elastic Plane-Strain Fracture
Toughness KIc of Metallic Materials Annual Book of ASTM Standards, Vol. 03.01:
Metals - Mechanical Testing; Elevated and Low-temperature Tests; Metallography
(ASTM International, West Conshohocken, Pennsylvania, USA, 2006).
S3.
ASTM E1820-06. Standard Test Method for Measurement of Fracture Toughness
Annual Book of ASTM Standards, Vol. 03.01: Metals - Mechanical Testing;
Elevated and Low-temperature Tests; Metallography (ASTM International, West
Conshohocken, Pennsylvania, USA, 2006).
Figure S1. Porous scaffolds of practical dimensions obtained by freeze casting of ceramic
suspensions. The control of the processing conditions leads to macroscopic samples that exhibit
lamellar structures oriented over several centimeters.
Figure S2. Flow chart summarizing the processing of the hybrid composites