Investigation Into Etching Effects on the Interface Strength Between Nickel Titanium and Bismuth Tin for the Creation of Metal Matrix Self Healing Composites

This paper presents recent research into the effects of etching treatments on the interface strength between nickel titanium (NiTi) fibers and a bismuth tin (BiSn) matrix composing a self-healing metal-metal composite.

Self-healing metallic composites have the potential to create a paradigm shift in the design of engineered structures by changing what is considered a “failure,” resulting in lighter weight and safer structures with reduced maintenance requirements.

Some of the most advanced self-healing metal-metal composites are composed of NiTi shape memory alloy fibers reinforcing an off-eutectic metal matrix. When activated these materials can recover from macroscopic deformations and matrix fracture, returning to their original geometry and then soldering cracks closed. This process restores nearly 100% of original strength without using any consumable adhesives. As a result, this healing process has the potential to be repeated indefinitely.

The strength of the interface between the fibers and matrix is critical to creating high-quality composites because it governs the ability to transfer loads between the matrix and fibers, through the structure. Unfortunately, NiTi forms an inert native titanium oxide (TiO₂) surface layer when exposed to air that is notoriously hard to bond to. To overcome this weak bonding most initial work in self-healing using NiTi fibers employed structural anchors within specimens for the NiTi to react against. This approach was functional for demonstrating basic components of self-healing abilities but negated distributed load transfer between the fibers and matrix of the composite structure, creating concentrated load-paths and resulting in a poor composite structure. Chemical etching in an inert environment has been performed to remove the inert TiO₂ surface layer from NiTi, but the resulting interface strength has not previously been quantified.

This paper presents an experimental investigation of the strength of the interface between NiTi and BiSn with both the native oxide layer present (control) and after undergoing a chemical etching process in an inert environment to remove the TiO₂ layer and coating the NiTi while in the inert environment to prevent re-formation of the TiO₂ layer.

Testing was performed using single fibers of NiTi cast into and extending out of a BiSn block. The fiber/wire was mechanically pulled out of the BiSn block while measuring the force applied and recording video of the process.

The experimental evaluation of these interface strengths in this research will enable the efficient design of composite materials to ensure proper load transfer between composite fibers and matrix to maximize strength and support self-healing capabilities.