200801-science-Tough, bio-inspired hybrid materials-1
https://www.360docs.net/doc/43836180.html,/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@https://www.360docs.net/doc/43836180.html,
Published 5 December 2008, Science322, 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
2 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@https://www.360docs.net/doc/43836180.html,
achieved by cold isostatic pressing (at a global pressure of 1.4 GPa) of the brick-and-mortar 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. Three-point 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, K Ic , 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. K Ic 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 K Ic 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 = J el + J pl .
The elastic contribution J el is based on linear elastic fracture mechanics:
,'2
E K J el =
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 J pl is given by:
,2Bb A J pl
pl =
where A pl 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 J -dominance and plane strain were met such that all specimen dimensions:
.25,y Q
J b B σ≥
J values were then converted to equivalent K values through the J-K equivalence relationship for nominally mode I fracture:
.E J K J ′=
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 , non-trivial. 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 Al 2O 3 (3 and 300 GPa respectively). Note that any error in the modulus of elasticity, E , only impacts the equivalent toughness, K J , in a fairly limited way, such that a ±20% error on E only results in a roughly ±1% error on K J .
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 K Ic 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