Self-Sealing Nanostructured Ceramic Coatings for Corrosion Protection of Orthopedic Implants

University Of Wisconsin-Milwaukee, Milwaukee WI

Investigators

Abstract

9988892 Aita An SGER award will demonstrate feasibility of a novel approach to corrosion protection of metallic implants. A "smart" ceramic coating that adjusts to the human physiological environment will be developed to protect an underlying metal implant against corrosion. The coating structure is based on the repetition of three types of modules. Module I consists of nanocrystalline AlN or ZrN and provides corrosion protection via a self-sealing mechanism. A self-limiting hydrated/hydroxide layer forms at the surface of the nanocrystallites upon exposure to blood thereby protecting the underlying implant metal. Module II consists of a zirconia-alumina nanolaminate and provides mechanical toughness via a phase transformation mechanism. Module III consists of a zirconia-titania nanolaminate and provides hardness. Modules II and III together are expected to accommodate both external stress occurring upon implant use and internal stress that may accompany the chemical changes of the self-sealing process. The proof-of-concept question is whether the coating offers better corrosion protection than bare metal used for the same purpose. The experimental strategy is to grow coatings of various nanoarchitectures on room temperature 316L stainless steel substrates by sequential reactive sputter deposition from multiple targets. Electrochemical testing will be carried out in physiological saline solution electrolytes at 37o C (body temperature) and at three levels of pH, 6.5, 7.4, 8.0, representing hypo-, iso-, and hypertonic conditions to normal blood. Direct and alternating current (electrochemical impedance spectroscopy) polarization methods will be used to determine corrosion parameters known to affect successful implantation, including open circuit potential, reciprocal polarization resistance, breakdown potential, and protection potential. Pin and disk accelerated wear tests will be conducted to identify synergistic corrosion-wear effects. Physical and chemical changes in the coating at various stages of electrochemical and mechanical abuse will be studied using electron microscopy, electron spectroscopy including depth resolution, and diffraction techniques. If successful, the result will be a coating for metal or polymer implant that offers superior protection upon initial introduction into the body and retains its integrity over long-term use. This coating should find use in medical applications beyond the orthopedic field. In addition, the experimental data obtained in this study will form a base for understanding corrosion and corrosion-wear synergy in multicomponent layered nanostructured ceramics. ***