M.B. Lewis

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.

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.

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.

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.

Migration behavior of helium under displacive irradiation in stainless steel, nickel, iron and zirconium

Abstract The techniques of ion implantation and nuclear reaction depth profiling are used to measure helium migration during irradiation at elevated temperatures in a number of base metals of importance to nuclear reactor technology. In stainless steel and Ni, gross trapping persists to the highest test temperature of 973 K. In α-Fe. significant migration begins between 773 and 898 K, and in α-Zr the helium is mobile above 700 K. Comparison of these data with helium transport theories suggests that the controlling mechanism is closer to substitutional migration than to dissociative migration; for α-Zr the analysis is inconclusive because of uncertain diffusion parameters. Helium moving from the implanted regions is captured in surface films in α-Fe and α-Zr. These results are discussed in terms of their relevance to radiation-induced swelling and helium embrittlement, and to helium degassing experiments.

Triple ion beam irradiation facility

Abstract A unique ion irradiation facility consisting of three accelerators is described. The accelerators can be operated concurrently to deliver three ion beams on one target sample as large as 100 mm2 in area. The energy ranges of the ions are 50 to 400 keV, 200 keV to 2.5 MeV and 1.0 to 5.0 MeV, which allows three different ions in the appropriate mass range to be simultaneously implanted to the same depth in a target specimen. Typical depth ranges are 0.1 to 1.0 μm. The X-Y profiles of all three ion beams are measured by a system of miniature Faraday cups. The ion beam energy of the low voltage accelerator can be ramped periodically during the implantation. Three different types of target chambers are in use at this facility. A triple-beam high vacuum chamber can hold nine TEM specimens at controlled temperatures in the range 400–800 °C during an irradiation by the three simultaneous beams. A second high vacuum chamber on the medium voltage accelerator beam line houses a low- and high-temperature translator and a two-axis goniometer for ion channeling measurements. The third chamber on the high energy beam line can be gas-filled for special stressed specimen irradiations. Applications of this facility to surface modification of materials are described.

Chemical G-values of ion-irradiated polymers

Abstract In order to understand the ion-polymer interactions which lead to improved surface properties of ion irradiated polymers, we have made quantitative measurements of the gases evolved from the polymers polyethylene and kapton during the irradiation. The singly charged ions used for irradiation in this work were 200 keV He+2, 200 keV He+, 200 keV B+, and 1000 keV Ar+. The penetration depth of each of these ions is approximately one micrometer. Chemical G-values (number of molecules released per 100 eV of absorbed energy) for the gases H2, CH4, C2H2, CO, and CO2 were measured. It was found that the G-values were strongly dependent upon the ion atomic number. It was shown that this atomic number dependency could be related to the probability of atomic recoil or displacement in the bulk of the polymer. In addition, the G-values appear to be correlated with the large increases in surface hardness after ion irradiation previously measured for these polymers.

Implantation of gases into sapphire

Ions of neon, argon, and bromine were implanted into single crystals of α-Al 2 O 3 . Implantation energies of 125 keV to 3 MeV and fluences of 1 10 16 to 3 × 10 17 ions/cm 2 were used. Some samples were subjected to post-implantation annealing. Scanning electron micrographs showed profuse (30–50% of surface area) blistering for implants of 1 × 10 17 Ar/cm 2 (230 keV) and 3 × 10 17 Ne/cm 2 (3 MeV). Specimens implanted with 4 × 10 16 Br/cm 2 (175 keV) contained a few ruptured blisters on the as-implanted surface. The implanted layer was severely embrittled and readily spalled from the substrate. Heating the specimens caused further blistering, cracking, and exfoliation in each case.

The reactivity of ion-implanted SiC

Abstract Implantation of chromium into single-crystal or polycrystalline α-SiC produces a surface amorphous layer for displacement damage greater than about 0.2 displacements per atom at room temperature. The enhanced chemical reactivity of such specimens was studied by two methods: chemical etching rate and oxidation rate. The chemical etching rates in a saturated solution of 50%K 3 Fe(CN) 6 plus 50% KOH were measured. The etching rate for the amorphous layer was 2.4−3.7 times that of the polycrystalline samples and 3.0–4.1 times that of the single-crystal samples. Polycrystalline specimens were exposed to flowing oxygen for 1 h at 1300 °C. Rutherford backscattering and the nuclear reaction 16 O(d,p) 17 O ∗ were used to determine the amount of oxygen on the surface. The amount of oxygen (and the thickness of oxide) over the amorphous region was 1.67 times that over the crystalline region. The relative thicknesses of the oxide on the amorphous and crystalline regions were confirmed by measuring the sputtering time required to remove the oxygen signal in an Auger spectrometer.

Effects of rapidly pulsed ion bombardment on microstructure and phase stability in a Ti-modified stainless steel

Abstract A low-swelling Ti-modified austenitic stainless steel has been bombarded with pulsed 4 MeV Ni 2+ and in some cases simultaneously with 0.2–0.4 MeV He + ion beams at 948 K over a dose range from 1 to 70 dpa. The interruption periods studied were 60, 1, 10 −3 ,or 10 −5 s on time with equal off time. Continuous irradiation was also employed as a comparison. At 1 dpa, pulsed irradiation caused interstitial dislocation loops (and subsequent MC precipitates) to be more refined with decreasing pulse periods down to the realm of the vacancy lifetime (10 −1 to 10 −3 s); still faster pulsing at 10 −5 s yielded little difference from steady irradiation. At higher doses, 40–70 dpa, pulsing reduced the amount of radiation-induced G phase. Long pulse periods promoted the thermally-stable MC phase, while shorter periods like 10 −3 s aided M 6 C and M 23 C 6 development. In helium-implanted material bubbles were present for all pulsing conditions but bias-driven voids only developed in continuously-irradiated specimens. The results are interpreted with regard to the inherent fluctuations in point defect concentrations that result from cascade formation. The sensitivity of damage microstructure to pulsed bombardment is seen as a possible new tool for the control of microstructure in ion implantation experiments.