Ultrasonic Shot Peening: State of Technology for Nanocrystalline Surface Fabrication and Its Mechanical and Biological Enhancement in Bioimplant Materials

Ultrasonic shot peening (USSP) is an effective method for surface modification of metallic materials. It can modify the surface topography of the target surface, introduce compressive stress into the components, and produce gradient nanograined structures in the peened materials. This dissertation concentrates on the technical developments of ultrasonic shot peening, the ability of the ultrasonic shot peening for the gradient nanograined surface fabrication and the mechanical behavior and biological applications of the developed gradient nanograined surfaces. Dynamic behavior of the shot during ultrasonic shot peening was investigated experimentally and numerically. A physical model describing the motion behavior of the shot during ultrasonic shot peening was established and verified. To better control the topography of the peened surface, a finite element simulation model simulating the strain distribution of the peened surface and a numerical algorithm to predict the surface topography during ultrasonic shot peening process were developed. Gradient nanograined surfaces were successfully fabricated on low carbon steel, pure copper, and 316L stainless steel materials. Mechanical testing indicated significant improvement of the nanohardness of the gradient nanograined surface fabricated on the pure copper and 316L stainless steel samples. In addition, an in-situ method to produce nanograined metallic flakes/powders was discovered in the presence of fractures of the nanograined materials. The findings indicate that ultrasonic shot peening can be used as an effective tool for mass production of nanograined metallic flakes/powders. The formation mechanism of the nanograined metallic flakes includes surface nanostructuring and fatigue failure of the gradient nanograined surface. Requirements of the next generation biomaterials include long-term durability and fast osseointegration. Metallic materials have been widely used in orthopedic applications. Due to the strength-ductility synergy and increased surface hydrophilicity, the gradient nanograined structure fabricated by ultrasonic shot peening provides an effective and potential mass-production method to enhance the mechanical and biological performance of the bioimplant materials. In this dissertation, “net-like” ultrafine-grained (~200nm) surfaces and nanograined (below100nm) surfaces were successfully fabricated on 316L stainless steel via ultrasonic shot peening. Human osteoblast cell (SaoS-2) was cultured on these ultrafine grained/nanograined surfaces and on coarse grained surfaces. Research results showed that the “net-like” ultrafine-grained and nanograined surface can promote the attachment, spreading, and proliferation of human osteoblast. Human osteoblast cell’s biological functions can be modulated by changing the grain boundary density of the ultrafine grained/nanograined surface. In addition, aggregated protein particles were observed on the nanograined cell-substrate interface, affirming that nanograined surfaces can promote protein adsorption. To further strengthen the assertion that nanograined cell-substrate interface can be used to promote human osteoblast cell functions, long-term “in-vivo” investigations are proposed for future study