Interphase tuning for stronger and tougher composites

The development of composite materials that are simultaneously strong and tough is one of the most active topics of current material science. Observations of biological structural materials show that adequate introduction of reinforcements and interfaces, or interphases, at different scales usually improves toughness, without reduction in strength. The prospect of interphase properties tuning may lead to further increases in material toughness. Here we use evaporation-driven self-assembly (EDSA) to deposit a thin network of multi-wall carbon nanotubes on ceramic surfaces, thereby generating an interphase reinforcing layer in a multiscale laminated ceramic composite. Both strength and toughness are improved by up to 90%, while keeping the overall volume fraction of nanotubes in a composite below 0.012%, making it a most effective toughening and reinforcement technique.


EDSA process
EDSA process is a result of a complex balance between the liquid's surface tension, the friction force of the contact line and evaporation. The simplest model often described in the literature is a single drop of liquid with dispersed particles ("coffee droplet") on a flat surface [S1].
When a drop of coffee is left on a solid surface to evaporate, it leaves a dense agglomeration of coffee particles at the periphery of the droplet, while much less particles are deposited inside the droplet. This "coffee ring" phenomenon is a well-known example of the general form of EDSA ( Figure S2a).

Figure S2a
: Dense deposition of coffee particles at the droplet periphery.
When a droplet spreads on a non-ideal surface (i.e. surface that has some degree of roughness) it (1) forms a cape due to the surface tension and (2) the contact line (the triple liquid-solid-gas interface) is getting pinned to the surface. Further, we assume that evaporation always takes place and is constant in an open system and in stable proper conditions (temperature/pressure). When a droplet is drying on a surface, evaporation reduces the height of the droplet at any point on the droplet surface. In a case of an ideal surface, where no contact line pinning takes place, the droplet would shrink to maintain its spherical shape, retained by the surface tension. In this ideal case, the system has a single contact angle. However, on non-ideal surfaces the contact line is pinned and the droplet cannot shrink to compensate on the liquid loss. In this case, the compensation is done by a flow of liquid from the center of the droplet to the periphery [S1], [S2].  The contact line is pinned. The black curve is the change in the droplet profile due to the contact line pinning and the evaporation. The red arrow points at a portion of solution that has been evaporated but needs to be compensates by the flow of liquid from inside [S1].

Ideal
When liquid flows from inside the droplet to the periphery, it will carry any particles that are suspended in it. Since evaporation of liquid continues as long as there is liquid available, a constant flow occurs simultaneously as well, thus taking a major portion of the particles to the periphery. Another important phenomenon associated with EDSA is the stick-slip movement of the contact line. The stick-slip movement of the contact line can be visually seen as discrete concentric rings with a gradient of deposit concentration. This phenomenon was thoroughly investigated in the literature [S3, S4, S5]. On an ideal surface, there is no pinning of contact line and therefore, the system maintains a single contact line, governed by the surface-tension of the three phases. But on a non-ideal surface, where the pinning force is stronger than the surface tension, the contact angle changes, and the shape of the droplet changes with it. At a certain point, the system is so far from equilibrium that the tension forces of the droplet overcome the pinning force. In that case, the contact line detaches from its initial position and recedes to a new point where the system is again in equilibrium (i.e. the contact angle is θi). The process then will continue, and a dense agglomeration of particle will form at the new location of the contact line. This simplified explanation assumes that while the droplet evaporates only one thermodynamic parameter is changing while the other is constant. Experimentally it is usually not the case, and on nonideal surfaces, both contact angle and contact line will vary together, which leads to less discrete, but rather continuous coating of the surface.
According to the literature [S3], [S4], [S5] this droplet model can be transferred to the case of a flat vertical substrate in the evaporating liquid. The function of the droplet contact line in this case is transferred to the meniscus contact line. The EDSA coating mechanism therefore results from competition between the meniscus surface tension, the coatings surface tension, and the friction force at the contact line.  Figure S5: Dimensions of an Al 2 O 3 -PVA layered composite specimen for 3-point bending. The specimen thickness varied according to the number of layers: from 0.5 mm for 2 layers to 1.5 mm for 6 layers; each layer added 0.25 mm.

Nanoindentation data
Nanoindentation was performed on a peeled-off polymer interphase (such as seen on Figure 3c,d in the main article body or on Figures S7 and S8a in this Supporting Information) with and without carbon nanotubes (plain and reinforced samples). The nanoindenter used was an Agilent XP nanoindenter, with a 10 micron spherical diamond tip. The continuous stiffness (CSM) method was applied to get continual reading of modulus with depth. Indentations were made to depth of 1 micron.
Hardness as measured by nanoindentation for the plain PVA interphase: 0.14 ± 0.04 GPa Hardness as measured by nanoindentation for the CNT-n reinforced PVA interphase: 0.26 ± 0.02 GPa It is important to note that the measurement was challenging due to high surface roughness and adhesion.