G.R. Rao

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.

Metal ion implantation effects on surface properties of polymers

Abstract Polyethylene (PE), polycarbonate (PC) and polyetherimide (PEI) have been implanted with a variety of metallic ions from a metal vapor arc source at low energies yielding shallow depth implants. PC and PEI have been implanted with Cr, Ti, Si and Pt, while PE was implanted with Cr and Ti. Nanoindentation hardness and surface resistivity changes were measured for the implanted polymers. Results showed that the Ti and Si implantations caused similar hardness increases for PC and PEI while Cr implantation resulted in the largest hardness improvement for the two polymers. These effects were attributed to energy and dose effects. Pt implantation, on the contrary, had very little effect on hardness. This was attributed to a greater extent of chain scission caused by nuclear collisions by the heavy Pt ions than cross-linking caused by electronic excitation. In the case of PE, Ti and Cr yielded similar hardness increases although the measured hardness values were smaller than those for PC and PEI. The implantations also decreased electrical resistivities of the polymers. Ti, Si and Cr implantations exhibited trends for a decrease in resistivity similar to those for the increase in hardness suggesting similar underlying causes, namely cross-linking. It appears that irradiation-induced cross-links provide paths for increased mobility of carriers contributing towards decreased resistivity. Pt implantation yielded the largest increase in conductivity as a result of the high dose used as well as the deposition of microdroplets of Pt which form at the metal ion source causing Pt to accumulate at the polymer surface leading to metallic conduction. This study represents the first use of a mixed energy spectrum of ion implantation to modify surface properties of polymers.

Triple ion beam studies of radiation damage in 9Cr–2WVTa ferritic/martensitic steel for a high power spallation neutron source

Abstract To simulate radiation damage under a future Spallation Neutron Source (SNS) environment, irradiation experiments were conducted on a candidate 9Cr–2WVTa ferritic/martensitic steel using the Triple Ion Facility (TIF) at ORNL. Irradiation was conducted in single, dual and triple ion beam modes using 3.5 MeV Fe ++ , 360 keV He + , and 180 keV H + at 80°C, 200°C and 350°C. These irradiations produced various defects comprising black dots, dislocation loops, line dislocations, and gas bubbles, which led to hardening. The largest increase in hardness, over 63%, was observed after 50 dpa for triple beam irradiation conditions, revealing that both He and H are augmenting the hardening. Hardness increased less than 30% after 30 dpa at 200°C by triple beams, compatible with neutron irradiation data from previous work which showed about a 30% increase in yield strength after 27.2 dpa at 365°C. However, the very large concentrations of gas bubbles in the matrix and on lath and grain boundaries after these simulated SNS irradiations make predictions of fracture behavior from fission reactor irradiations to spallation target conditions inadvisable.

Structure and dose effects on improved wear properties of ion-implanted polymers

Abstract Polyethylene, polypropylene, polystyrene and polyethersulfone, representing an increasing complexity in molecular structure, were implanted with 200 keV boron to three doses of 1.7, 5 and 17 × 1018 ions m−2. Polystyrene was also implanted with 100 keV boron to the same three doses. The polymers were investigated for near-surface micromechanical property changes using a nanoindentation technique. Wear properties of the polymers were studied using a reciprocating tribometer with a nylon ball as the counterface. Tests were conducted for 10000 sliding cycles using a 1 N normal load, a stroke length of 3 mm and an oscillation frequency of 100 cycles min−1. The ion implantation increased the near-surface hardness of the four polymers and the increase was proportional to the dose and beam energy. A clear structure dependence was observed for the hardness changes that were related to cross-linking of molecular chains caused by the ion irradiation. In general, the implantation also significantly improved the wear properties of the four polymers. For each polymer, an optimum dose was identified that yielded the best wear improvement. With increasing dose, the dominant wear mechanism shifted from adhesive and abrasive wear to no observable wear to surface fatigue. Remarkable wear improvements were observed for polystyrene and polyethersulfone for which, at the optimum dose, no wear damage was visible even after 10000 sliding cycles.

Friction microprobe studies of ion implanted polymer surfaces

Abstract Recent studies at Oak Ridge National Laboratory (ORNL) have shown that ion implantation of polymers can significantly improve wear properties, sometimes dramatically. In several cases, the “best-wear resistance” condition for a specific counterface material was associated with the lowest friction coefficient values. To explore the friction behavior of ion implanted polymers further, microfriction studies were conducted on 1 MeV Ar + implanted poly (ether ether ketone) (PEEK) and polystyrene (PS), implanted to fluences of 5, 10 and 50 × 10 18 ions m −2 . The studies were conducted using the ORNL friction microprobe, a specialized micro-contact tribometer. The results were compared with macro-friction values obtained using standard pin-on-disk type tests. The polymers were also characterized for surface mechanical properties using a nanoindentation technique. Results indicated that the most striking aspect of the microfriction tests on the ion implanted polymers was a marked “stick-slip” behavior, which was not observed for the unirradiated polymers. This friction behavior was related to the three-dimensionally cross-linked structure of the ion irradiated polymer material, since, with increasing fluence, the polymer surface becomes harder and less elastic due to a greater extent of cross-linking. The stick-slip behavior was thus caused by adhesion of the two surfaces, and periodic elastic extension and sudden release of the cross-linked structure of the implanted layer. The microfriction results were correlated with nanoindentation hardness measurements, which are an indirect measure of the extent of cross-linking.

Wear properties of argon implanted poly(ether ether ketone)

Abstract The sliding wear properties of Ar + -implanted poly(ether ether ketone) (PEEK) using nylon 66 and SAE 52100 high carbon chrome steel ball counterfaces have been investigated in this study. PEEK was implanted with 1 MeV Ar + to fluences of 5 × 10 18 , 1 × 10 19 and 5 × 10 19 ions m −2 . Reciprocating sliding wear tests were conducted using a 1 N normal load, 3 mm sliding distance and 100 cpm frequency for 10 000 sliding cycles. The implantation significantly improved the wear properties of PEEK for the nylon ball tests to the extent that no wear tracks were evident on the PEEK surface. This was attributed to significant strengthening of the PEEK surface by ion beam induced cross-linking which, in turn, caused corresponding wear of the softer nylon counterface. For the steel counterface tests, the intermediate-fluence implanted specimen showed no wear damage on the surface. There was wear damage for the unimplanted, lower and higher fluence implanted specimens. The results indicate that the best wear properties appeared at some optimum level of ion-beam induced cross-linking of PEEK for the steel counterface. Friction coefficient data were correlated with the observed wear behavior. Mechanisms of energy transfer from the energetic ions to the polymer molecules were analyzed to explain the wear improvements. This study shows that ion implantation is a promising technique for significant improvements in the tribological properties of PEEK.

Recoil effects on chemical G-values during ion irradiation of polystyrene

Abstract Measurements of G -values (number of molecules emitted per 100 eV of absorbed radiation energy) have been made for helium and boron ion irradiations of polystyrene (PS) films of different thicknesses. These ions were chosen because the electronic linear energy transfer (LET) values for B + with energy near 200 keV is comparable to the corresponding LET for He + near 400 keV. In contrast, the nuclear of ‘recoil’ LET of B + is several times larger than that of He + throughout the B + depth range. When the G -values for H 2 and C 2 H 2 gas production were measured for these ions, three important findings were noted: (1) The G -values for H 2 and C 2 H 2 showed only a slight increase and decrease respectively with increasing electronic LET. (2) The G ( H 2 ) for B + irradiation was approximately 50% larger than that for He + irradiation. (3) The G ( C 2 H 2 ) was about a factor of five larger for the B + irradiation compared to the He + . In contrast to earlier speculation, no evidence was seen for an electronic LET threshold, above which the G -values rapidly increase. It appears that this anomaly was primarily due to changing the bombarding ion (increasing the atomic number) to reach a larger LET in the measurement. While G -value effects due to different ion-track energy densities are not yet resolved, our findings imply that most of the molecular gases formed from radiochemical reactions in polymers during typical ion irradiations are dependent not simply upon electronic energy, but upon a mechanism involving momentum transfer from the ion to the atomic nuclei of the target.

Effects of Simultaneous Boron and Nitrogen Implantation on Microhardness and Fatigue Properties of Fe-13Cr-15Ni Alloys

Eight complex austenitic stainless steel alloys based on the composition Fe-13Cr-15Ni-2Mo-2Mn-0.2Ti-0.8Si-0.06C were implanted simultaneously with 400-keV B+ and 550-keV N+ ions and were investigated for changes in fatigue properties and surface microhardness. The nearsurface hardness of all eight alloys improved, but the fatigue life of each decreased. These findings were contrary to those obtained in an earlier study using four simple Fe-13Cr-15Ni alloys, where the dual implantation improved fatigue life by up to 250 pct. While unimplanted specimens failed by slip-band crack initiation, it was hypothesized that the dual implantation suppressed slip to the extent that fewer slip-band cracks were initiated and these were subjected to accelerated crack propagation. In addition, grain-boundary cracking was promoted, yielding a lower fatigue life. Support for this hypothesis was obtained by a study of single crystals of Fe-15Cr-15Ni, which were also implanted with B+ and N+. The dual implantation caused a lower fatigue life due to concentration of slip along a few slip bands to relieve applied stress. Evidence of grain-boundary cracking was obtained using the four simple alloys, which were subjected to triple ion implantation with B+, N+, and C+. The triple implantation decreased the fatigue life of the alloys and caused accelerated growth of fewer slip bands and grain-boundary cracking due to suppression of surface slip bands. This study thus shows the existence of an optimum level of strengthening when multiple ion implantation is used to improve the fatigue properties of alloys.