E.H. Lee

Ion-beam modification of polymeric materials – fundamental principles and applications

Abstract When an energetic ion traverses a polymer medium, it loses its energy by electronic and nuclear processes. In the past, considerable research efforts have been devoted to understand the effects of the electronic and nuclear processes in materials. There have been, however, some conflicting reports regarding the roles of electronic and nuclear stopping in producing property changes in polymeric materials, namely the magnitude of cross-linking and scission. A consensus derived from the work conducted at ORNL indicates that electronic stopping is largely responsible for cross-linking and nuclear stopping for scission, although both processes can cause cross-linking as well as scission. The most important parameter for cross-linking is found to be the energy deposited per unit ion path length or linear energy transfer (LET). The mechanisms involved with property changes are discussed by clarifying the concepts of nuclear and electronic stopping, LET, tracks, and spurs. Experimental evidence to support the views are presented. Also addressed are specific property changes induced by ion-beams, which may be of use for industrial applications.

Improved surface properties of polymer materials by multiple ion beam treatment

Ion beam treatment studies have been carried out to investigate the potential for improvements in surface-sensitive properties of polymers. Kapton, Teflon, Tefzel, and Mylar have been implanted with boron, nitrogen, carbon, silicon, and iron ions, singly or simultaneously with dual or triple beams. The implanted materials were characterized by optical microscopy, transmission electron microscopy, nano-hardness indentation, wear testing, scanning tunneling microscopy, x-ray analysis, nuclear reaction analysis, Fourier transform infrared spectroscopy, and Raman spectroscopy. Although the polymers showed a color change and varying degrees of measurable surface depression in the bombarded area, the implanted surface revealed substantial improvements in surface smoothness, hardness, and wear resistance. In particular, B, N, C triple-beam implanted Kapton showed over 30 times larger hardness than unimpanted material, making it more than three times harder than stainless steel. Sliding wear properties were characterized using an oscillating nylon or high carbon steel wear ball. Severe wear tracks were observed in virgin Kapton, but no appreciable wear was observed in ion implanted Kapton. Mechanisms underlying the improved surface properties are addressed.

Defect and void evolution in oxide dispersion strengthened ferritic steels under 3.2 MeV Fe+ ion irradiation with simultaneous helium injection

Abstract In an attempt to explore the potential of oxide dispersion strengthened (ODS) ferritic steels for fission and fusion structural materials applications, a set of ODS steels with varying oxide particle dispersion were irradiated at 650°C, using 3.2 MeV Fe + and 330 keV He + ions simultaneously. The void formation mechanisms in these ODS steels were studied by juxtaposing the response of a 9Cr–2WVTa ferritic/martensitic steel and solution annealed AISI 316LN austenitic stainless steel under the same irradiation conditions. The results showed that void formation was suppressed progressively by introducing and retaining a higher dislocation density and finer precipitate particles. Theoretical analyses suggest that the delayed onset of void formation in ODS steels stems from the enhanced point defect recombination in the high density dislocation microstructure, lower dislocation bias due to oxide particle pinning, and a very fine dispersion of helium bubbles caused by trapping helium atoms at the particle–matrix interfaces.

Control of helium effects in irradiated materials based on theory and experiment

Helium produced in materials by (n,..cap alpha..) transmutation reactions during neutron irradiations or subjected in ion bombardment experiments causes substantial changes in the response to displacement damage. In particular, swelling, phase transformations and embrittlement are strongly affected. Present understanding of the mechanisms underlying these effects is reviewed. Key theoretical relationships describing helium effects on swelling and helium diffusion are described. Experimental data in the areas of helium effects on swelling and precipitation is reviewed with emphasis on critical experiments that have been designed and evaluated in conjunction with theory. Confirmed principles for alloy design to control irradiation performance are described.

Effects of electronic and recoil processes in polymers during ion implantation

It has been shown that ion implantation produces remarkable improvements in surface-sensitive mechanical properties, as well as other physical and chemical properties in polymers. To understand mechanisms underlying such property changes, various polymeric materials were subjected to bombardment by energetic ions in the range of 200 keV to 2 meV. The magnitude of property changes is strongly dependent upon ion species, energy, and dose. Analysis indicated that hardness and electrical conductivity increased by employing ion species with larger electronic cross sections and with increasing ion energy and dose. The results showed that electronic stopping or linear energy transfer (LET, energy deposited per unit track length per ion) for ionization was the most important factor for the enhancement of hardness, while nuclear stopping or linear energy transfer for displacement generally appeared to reduce hardness.

LET effect on cross-linking and scission mechanisms of PMMA during irradiation

Abstract Mechanisms of scission and cross-linking are investigated for poly(methyl methacrylate) subjected to various irradiation sources, such as low LET radiation sources (MeV e -beams, Co 60 γ-rays) and high LET ions (MeV He + , Ar + ). PMMA properties were degraded upon irradiation by e -beam and γ-rays, but were improved upon bombardment with high energy ion beams (HEIB) as a result of cross-linking. The results indicate that high LET produces a high concentration of free radicals over many neighboring molecular chains, facilitating track overlap and enhancing cross-linking over scission, while low LET affects only a single molecular chain, leading to chain scission.

Ion beam application for improved polymer surface properties

Abstract Various polymeric materials were subjected to bombardment by different energetic ions with energies ranging from 200 to 1000 keV. Tests showed substantial improvements in hardness, wear resistance, oxidation resistance, resistance to chemicals, and electrical conductivity. The magnitude of property changes was strongly dependent upon ion species, energy, dose, and polymer structure. Both hardness and electrical conductivity increased with ion energy and dose. These properties were apparently related to the effectiveness of cross-linking. Ion species with a large electronic stopping cross-section are expected to produce more cross-linking. It is believed that the polymer property improvements are commensurate with the extent of cross-linking, which is responsible for the formation of three-dimensionally-connected, carbon-rich, rigid networks.

On the origin of deformation microstructures in austenitic stainless steel: part I—microstructures

Abstract A comprehensive characterization of room temperature deformation microstructures was carried out by transmission electron microscopy for ion irradiated and deformed AISI 316LN austenitic stainless steel. Deformation microstructures were produced by a recently developed disk-bend test method and also by a uniaxial tensile test. Cross-slip was dramatically suppressed by the radiation-induced defects and slip occurred predominantly by planar glide of Shockley partial dislocations. Deformed microstructures consisted of piled-up dislocations, nanotwin layers, stacking faults, and defect-reduced dislocation channel bands. Analyses revealed that all these features were different manifestations of the same type of deformation band, namely a composite of overlapping faulted layers produced by Shockley partial dislocations.

Residual gas and ion-beam analysis of ion-irradiated polymers☆

Abstract In order to understand the ion-polymer interactions which lead to improved polymer-surface properties of ion-irradiated polymers, we have measured both the gases evolved from the polymers Teflon and Kapton during irradiation and the change in the composition of Kapton after irradiation. Ion beams of helium, nitrogen and silicon in the energy range 0.2 to 2.0 MeV were used to irradiate the targets. The primary residual gases observed were CF and CF3 from Teflon and H2, CO, and CO2 from Kapton. Teflon appeared to decompose by fragmentation of its molecular chain. Ion beam analysis of the ion-irradiated Kapton was carried out by Rutherford backscattering (RBS), by elastic recoil detection analysis (ERDA), and by nuclear reaction analysis (NRA). The results from these analyses show that the heavy-ion irradiation creates a complex depth dependence in the composition of the target. Radiolysis and radiation induced chemical reactions during the ion irradiation are discussed.

Plastic deformation in 316LN stainless steel – characterization of deformation microstructures

The effects of irradiation, test temperature, and strain on the deformation microstructures of a 316LN stainless steel have been investigated using a disk-bend method and transmission electron microscopy. Deformation microstructure changed progressively from a dislocation network dominant to a large stacking fault/twin band dominant microstructure with increasing radiation dose and with decreasing test temperature. Also, an increased strain level enhanced the propensity of deformation twinning. Since the stress was considered to be a key external parameter controlling deformation mechanism in 316LN austenitic stainless steel, the equivalent stress level was estimated for the examined surface of the disk sample. It was possible to categorize the deformation microstructures in terms of the equivalent stress range. A key conclusion is that the austenitic material will deform by forming bands of large stacking faults and twins when the stress exceeds a critical equivalent stress level of about 600 MPa by any of several possible strengthening measures: irradiation, increasing strain level, and decreasing test temperature.