The Role of the Organic Component in the Mechanical Behavior of Biomineralized Composites.
- PDF / 865,057 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 90 Downloads / 166 Views
1187-KK07-01
The Role of the Organic Component in the Mechanical Behavior of Biomineralized Composites.
George Mayer and Jason Zhou University of Washington, Department of Materials Science & Engineering, Box 352120, Seattle, WA 98195-2120, U.S.A. ABSTRACT The roles of minor organic layers in influencing the mechanical response of such biomineralized composites as mollusk shells and sponge spicules have been investigated. The mechanisms whereby such minor constituents govern energy dissipation in rigid biomineralized structures are discussed, and a rationale for new modes of toughening that may relate more generally to families of ceramic- or glass/organic composites is offered. New results of simple torsional tests conducted on spicule fibers of a hexactinellid sponge, Euplectella aspergillum (Euplectella a.), compared with those done on meltdrawn glass fibers, showed an enhanced ability to resist failure in torsion, whereas the glass fibers did not. This behavior was attributed to the presence of a very thin adhesive viscoelastic phase between the siliceous layers of the spicule fibers, combined with the architectural and surface features of the spicule fiber. INTRODUCTION The spicules of hexactinellid sponges have been studied extensively during the past several years in a number of investigations, e.g., (1-7). In addition to the excellent combinations of strength and stiffness possessed by such natural composites, they have also have exhibited features normally seen in ceramic composites, such as crack diversion and crack bridging (8), but some that are not, such as an unusual ability to damp out dynamic loads (9). The damping behavior of the sponge spicules observed in the latter work is indicated in Figure1. 4.5 4 3.5 Tan δ
3 2.5
Spicule
2
Glass
1.5 1 0.5 0 0
10
20
30
40
50
60
Freq (Hz)
Figure 1. Damping (represented by Tan δ) of spicule fiber compared to that of meltgrown glass fiber over a range of frequencies (9).
and was concluded to originate with the thin organic layers that are sandwiched between concentric rings of hydrated silica, as shown schematically in Figure 2.
Euplectella aspergillium -Cylindrical spicule fibers ~30-70 µm in diameter -Rings increase in thickness (0.1µm-1µm) proceeding inward -Solid silica cylinder is ~30-40% of diameter -Central protein core ~1µm2 -Number of rings vary, spicule to spicule
Figure 2. Schematic drawing of cross-section of the spicule fiber of Euplectella a. Note: drawing is not to scale.
The concentric ring structure in these hexactinellid spicules is composed of hydrated silica cylinders that are separated by very thin (on the order of 5-10 nm) layers of organic adhesives. The thicknesses of the silica cylinders decrease from inner to the outer surface layers, with the outer layer on the order of 0.2µm. After deformation, evidence of fibril or ligament formation in the organic phase had been seen in Monorhaphis chuni, (the largest living giant sponge) where they were termed “palisades” (Figure 3) by Müller et al. (10).
Figure 3. Separating lay
Data Loading...
