Chemical Sculpturing of Al Micro-Particles for Polymer Composites and Universal Polymer-Polymer Joints
Authors: Baytekin-Gerngross Melike , Gerngross Mark-Daniel , Carstensen Jürgen , Adelung Rainer 
Published: February 2020

Composite materials attract more and more attention due to their unique properties in many fields. For certain applications metal particle-polymer composites are very attractive, where e.g. lightweight heat-conductive composites are needed. For particle-polymer composites the typical problem is the deterioration of the mechanical properties of the polymer matrix due to poor adhesion at the polymer-metal particle interface. Our approach solves this problem by generating a mechanical interlocking surface structure on the metal particle surface via wet chemical etching which we call sculpturing.
In this work commercially available granular Al micro-particles with a typical size distribution between 200 - 800 µm and a purity of 99.7 % is used. These Al particles are structured in an optimized aqueous HCl based electrolyte. The key idea is to balance the passivation and dissolution of the metal surface in such a way that the desired interlocking structures form. Similar to HNO3-HF based Si etchants, the employed electrolyte makes use of in-situ formed reaction products controlling the structuring homogeneity and reaction speed. Figure 1a) provides a top view on Al particles sculptured in such way with a magnification view in Fig. 1b).
The sculptured surface structure consists of a multitude of rectangular structures with flat faces in a step-like arrangement or as free-standing hooks. These rectangular structures vary in size and are tilted towards each other depending on the crystal orientation. The EDX analysis confirms that the surface of these Al micro-particles after sculpturing is covered only by a native oxide layer.
By adjusting the chemical fabrication process the Al micro-particles could be fully or partially sculptured. Correspondingly, the Al micro-particles showed a change in their optical properties depending on the etching duration. Fully-sculptured Al micro-particles are dark gray in color, partially structured ones are light-gray, and the untreated ones are silver colored.
With these sculptured Al micro-particles polymer composites can be formed as shown in Fig. 1c. Here, a densely packed polythiourethane (PTU)-sculptured Al particle composite is formed in the shape of a dog bone. Tensile test results show that the addition of sculptured particles results in a strongly increased Young’s modulus and reaching at least the maximum tensile strength of the pure polymer, unlike in case of the addition of untreated Al micro-particles. In case of polydimethylsiloxane (PDMS) which is well-known for its poor adhesion to metals, polymers and ceramics, the effect on the increase of Young’s modulus is even far more pronounced as well as a tremendous increase in the maximum tensile strength. The fracture surfaces of all sculptured Al micro-particle-polymer composites showed always cohesive failure where even the particles themselves are torn apart, which is the best reachable adhesion strength.
Another very important application for sculptured Al micro-particles could be adhesive-free joining of immiscible polymers like ethylene-vinyl acetate (EVA) and polyoximethylen (POM) or other low surface energy polymers. Here, the sculptured micro-particles are partially imprinted into the thermally-softened POM surface and then joined with the softened EVA. Owing to the hook and step structures of the sculptured particles a robust connection between the Al micro-particles and any polymer matrix is obtained. No matter how the sculptured particles are partially imprinted in the polymer surface, they stick to it and hinder delamination. This is impressively shown via tensile testing, where only cohesive failure within the EVA component is found, while the direct joint without particles and via untreated particles fail adhesively directly at the POM-EVA interface.
Fig. 1: SEM top view on chemically sculptured a) Al micro-particles, b) in high magnification of the structured Al micro-particle surface and c) Al micro-particle filled PTU composite in the shape of dog bone.

Figure 1