Young Sik Pyun

Low friction and high strength of 316L stainless steel tubing for biomedical applications

We propose herein a nondestructive surface modification technique called ultrasonic nanocrystalline surface modification (UNSM) to increase the strength and to improve the tribological performance of 316L stainless steel (SS) tubing. Nanocrystallization along nearly the complete tube thickness of 200μm was achieved by UNSM technique that was confirmed by electron backscatter diffraction (EBSD). Nano-hardness of the untreated and UNSM-treated specimens was measured using a nanoindentation. Results revealed that a substantial increase in hardness was obtained for the UNSM-treated specimen that may be attributed to the nanocrystallization and refined grains. Stress-strain behavior of the untreated and UNSM-treated specimens was assessed by a 3-point bending test. It was found that the UNSM-treated specimen exhibited a much higher strength than that of the untreated specimen. In addition, the tribological behavior of the untreated and UNSM-treated specimens with an outer diameter (OD) of 1.6mm and an inner diameter (ID) of 1.2mm was investigated using a cylinder-on-cylinder (crossed tubes of equal radius) tribo-tester against itself under dry conditions at ambient temperature. The friction coefficient and wear resistance of the UNSM-treated specimen were remarkably improved compared to that of the untreated specimen. The significant increase in hardness after UNSM treatment is responsible for the improved friction coefficient and wear resistance of the tubing. Thus, the UNSM technique was found to be beneficial to improving the mechanical and tribological properties of 316L SS tubing for various potential biomedical applications, in particular for coronary artery stents.

THE CONCEPTS AND PROPERTIES OF NANO-SKIN MATERIALS AND COMPONENTS CREATED BY ULTRASONIC NANOCRYSTAL SURFACE MODIFICATION

Material surface and immediate subsurface layers can be called skin. A novel Ultrasonic Nanocrystal Surface Modification (UNSM) technology produces uniformed micro dimples on the top surface and nanometer grain in the subsurface; increases surface hardness and induces compressive residual stress, therefore mechanical characteristics related to fatigue, wear, friction, etc. can be improved. The concepts and properties of nanoskin materials and components are proposed with their potential application.

Microstructural Characterization and Mechanical Properties of Stainless Steel Inlay Welded Dissimilar Materials

This present work was carried out to characterize the microstructure and mechanical properties of austenitic stainless steel (SS) 316L to dissimilar welding of Inconel 52 and Inconel 82 fillers that were treated by ultrasonic nanocrystalline surface modification (UNSM) technique. The microstructure and hardness map of the specimens were characterized by optical microscope (OM) and micro-hardness tester. The increase in hardness and refinement in grain size after UNSM treatment were found for the SS 316L/52 and SS 316L/82. In addition, the induced compressive residual stress in the surface layer of base SS 316L was measured. It is expected that the increased hardness into several microns in depth and modified microstructure with a severe plastically deformed surface layer by UNSM technique can increase the resistance to corrosion and extend the service life of pressurized water reactors (PWRs).

Microstructure and Friction Behavior of AISI 52100 and D2 Steels Subjected to Ultrasonic Nanocrystalline Surface Modification (UNSM) Technique at a High Temperature

In this study, two different AISI 52100 bearing and D2 tool steels were subjected to ultrasonic nanocrystalline surface modification (UNSM) technique at ambient and high temperature of 500 °C. The objective of this study is to characterize the microstructure and to investigate the effectiveness of UNSM technique on the friction and wear behavior of those steels. The friction and wear behavior of the specimens against AISI52100 bearing steel ball with a diameter of 10 mm was carried out using a micro-tribo tester under dry conditions. The hardness with respect to depth from the top surface was measured using a microhardness. The change in the microstructure of the specimens before and after UNSM treatment was characterized by scanning electron microscopy (SEM). The findings from this preliminary study are expected to be implemented to the bearings and tools to increase the efficiency and performance of the components.

The Effects of Ultrasonic Nanocrystal Surface Modification Technique Temperature on Microstructure and Wear of Alloy 600

Alloy 600 (UNS N06600) is an austenitic nickel-based alloy with superior corrosion resistance and high-temperature endurance, which determines its widespread applications in aeronautical, aerospace, marine and nuclear industries. Particularly, a number of nuclear components used Alloy 600 as their structure materials due to their high corrosion resistance, high-temperature endurance and excellent fabricant characteristics. Many failures have occurred in Alloy 600 with various forms of environmental degradations during long-term operation. In this study, an ultrasonic nanocrystal surface modification (UNSM) technique was applied to Alloy 600 at a room and a high temperature of 500 OC. The effects of UNSM treatment temperature on the microstructure and wear behavior including a compressive residual stress were investigated. The hardness, compressive residual stress with respect to depth from the top surface were measured. Also, the wear behavior of UNSM-treated at a room and a high temperature Alloy 600 specimen was compared to that of the untreated specimen. The increase in wear resistance by UNSM technique was discussed in terms of increased hardness, refined grain size and induced compressive residual stress.

Mechanical and Fatigue Characteristics of Ti-6Al-4V Extra Low Interstitial and Solution-Treated and Annealed Alloys After Ultrasonic Nanocrystal Surface Modification Treatment

This study investigates the influence of ultrasonic nanocrystal surface modification (UNSM), a novel surface treatment technology, on the mechanical characteristics and rotary bending fatigue performance of solution-treated and annealed (STA) Ti-6Al-4V and Ti-6AI-4V extra low interstitial (ELI) alloys. Various techniques have been developed to use surface modification in order to improve fatigue performance of machine parts. The UNSM treatment emerges due to its ability to induce a distinctive combination of modifications in the top surface and the subsurface of the material. The mechanical properties are acquired and compared for UNSM-treated and untreated specimens. Fatigue data is acquired and compared for UNSM-treated and untreated specimens of Ti-6Al-4V ELI alloy by subjecting specimens to rotary-bending fatigue tests. Fracture analysis of failed specimens is performed. The improvement in the fatigue performance is observed for UNSM treated specimens. This improvement is attributed mainly to the UNSM-induced combination of following modifications: compressive residual stress, micro-hardness, and nanograin in the subsurface of the material.