Laurence Romana

Microstructural characterization of iron ion implantation of silicon carbide

In order to better use implantation to improve the properties of ceramics, a detailed understanding of the nature of implanted species is needed. In this investigation, analytical electron microscopy (AEM) and Rutherford backscattering spectroscopy-ion channeling (RBS-C) have been used to characterize single crystal α-silicon carbide implanted at room temperature with 160 keV 57Fe ions to fluences of 1, 3, and 6 × 1016 ions/cm2. RBS-ion channeling demonstrated that the implanted region was amorphous even at the lowest fluence. AEM was performed for both cross-sectional and back-thinned specimens with the highest fluence. The depth of the peak iron concentration measured with energy dispersive X-ray spectroscopy and with RBS were in approximate agreement with the value calculated by the TRIM code assuming a specimen density equal to crystalline silicon carbide. The AEM examinations of the implanted regions in this study did not reveal the presence of any precipitates, suggesting that the iron has not coalesced into precipitates (observation limit ≈ 2 nm). Inspection of the electron energy loss fine structure for iron in the implanted specimens suggests that the iron is not metallically-bonded; this result supports the conclusion from conversion electron Mossbauer spectroscopy (CEMS) that the iron is covalently bonded.

Plastic flow induced by ionization processes in ion-damaged MgO

Abstract The change in mechanical properties induced by ionizing radiation in pre-implanted MgO single crystals has been investigated by surface-strain measurements and defect characterization. Optically polished, oriented MgO substrates were implanted with 500 keV Xe+ ions followed by 2 MeV He+ ion bombardment at fluences up to 1.5 × 1016 He+/cm2. In order to use RBS to study the atomic migration accompanying the strain, some MgO samples were implanted with 150 keV Ca+ “marker” ions and were then irradiated with 2 MeV He+ ions. The defects induced in the material were analyzed before and after each type of irradiation by using RBS channeling and optical absorption. It appears that the ionizing radiation induces an out-of-plane strain composed of a volume expansion and a plastic flow which relax the stress in the pre-implanted layer. No long-range atomic migration appears to be involved in this effect.

Ion bombardment, ultrasonic, and pulsed laser beam effects on small metallic clusters of potassium in MgO

Abstract Small metallic precipitates of potassium were produced in ion-implanted layers (5 × 10 16 K + ions cm -2 , 150 keV, 300 K) on {100} faces of single crystals of MgO by thermal annealing in air at 1000 K. The resulting potassium aggregates had a mean size of 7 nm as characterized by optical absorption measurements and analytical electron microscopy. The effects of three distinct treatments on these precipitates and the implanted layer as a whole were investigated. Ion bombardment with Ar 2+ (1 × 10 16 ions cm -2 , 300 keV, 300 K) results in a dissolution of the metallic clusters and the production of F + and V - centers, as evidenced by the relative changes in the corresponding absorption bands. Rutherford backscattering spectroscopy indicates that this dissolution of precipitates does not result in any long-range redistribution of the potassium. Laser irradiation at a frequency near the absorption resonance of the potassium metal (1 μm wavelength) produces an exfoliation of the implanted layer. A similar effect occurs in ultrasonically treated samples where the entire implanted layer is removed after a 12 min exposure to an energy flux of 120 W cm -2 . The similarities and differences among these treatments are discussed along with suggestions for further study.

Ion-beam-enhanced adhesion of iron films to sapphire substrates

Abstract The effect of implantation of different ion species on the adhesion of iron films to sapphire substrates was investigated. The implantation energies were adjusted to ensure that the ion concentration profiles, damage profiles, and recoil distributions were the same for each species. For all implantations, the peak ion concentration was at the film-substrate interface. The adhesion of the films was measured by a pull test and a scratch test. For a fluence of 1 × 10 15 ions cm -2 , implantation of chromium (300 keV) and iron (320 keV) increased the bond strength whereas implantation of nickel (340 keV) did not. The effect is proposed to be due to changes in the interfacial energy resulting from the presence of the ion species at the interface. Only a narrow zone is affected; the mixing at the interface is less than 10 nm.

The effect of post-implantation annealing on the microstructure of Al2O3 implanted with iron at − 185°C☆

Abstract Analytical electron microscopy and Rutherford backscattering spectrometry have been used to investigate the effect of post-implantation annealing on the microstructure of Al 2 O 3 implanted with iron at −185°C. By varying the implantation dose, annealing temperature, and annealing environment it has been possible to investigate the Fe 2 O 3 -Al 2 O 3 and FeAl 2 O 3 phase relationships. It was found that the as-implanted amorphous surface layer crystallizes according to a two-stage process. The amorphous layer first transforms to γ-Al 2 O 3 and then epitactical regrowth of α-Al 2 O 3 follows. The redistribution of Fe during annealing is related to the Al 2 O 3 phase present and is dependent on the annealing environment, O 2 or 96% Ar-4% H 2 . The microstructure which develops during annealing at 1100°C in oxygen depends on the implantation dose (Fe concentration). Results show that as the dose increases the solubility limit of Fe 2 O 3 in Al 2 O 3 is exceeded and the excess Fe precipitates in the form of Fe 2 O 3 . During annealing at 1100°C in a reducing environment the majority of the Fe is driven towards the surface and precipitated as α-Fe. The observed phase transformations are in good agreement with the Fe 2 O 3 -Al 2 O 3 phase diagram.

Electrical conductivity in niobium implanted TiO2 rutile

Abstract Single crystals of TiO 2 rutile were implanted at 300 K with niobium ions at fluences in the range 5×10 13 to 2.5×10 17 ions cm −2 , corresponding to a mean local concentration of 6×10 18 to 3×10 22 Nbcm −3 , respectively. Rutherford backscattering spectroscopy (RBS) in channeling geometry and X-ray photoemission spectroscopy (XPS) have been performed to determine the Nb substitutional fraction and the Nb oxidation state as a function of fluence. The microstructural evolution has been followed by X-ray diffraction at glancing incidence. The dc conductivity measurements were performed using the four probes technique in the temperature range of 150 to 850 K. The conductivity as a function of the Nb fluence exhibits two different behaviours: (i) at low fluence, between 5×10 13 and 5×10 14 Nbcm −2 , the conductivity increases by four orders of magnitude, (ii) while at high fluence, between 5×10 15 and 2.5×10 17 Nbcm −2 , it varies slowly from 10 2 to 2×10 3 Ω −1 cm −1 . The conductivity is thermally activated and the activation energy deduced from Arrhenius plots ( σ α (1/ T )) depends on the local concentration of implanted niobium. It decreases from 0.12 eV (5×10 14 Nbcm −2 ) to 0.01 eV (2.5×10 17 Nbcm −2 ). A high oxidation state of niobium was observed at low fluences and the conductivity could be described by a polaron process. At high fluences the niobium oxidation state vanishes and then a hopping process between metallic clusters is superimposed on the polaron mechanism.

Distribution and Characterization of Iron in Implanted Silicon Carbide

Analytical electron microscopy (AEM) and Rutherford backscattering spectroscopy-ion channeling (RBS-C) have been used to characterize single crystal {alpha}-silicon carbide implanted at room temperature with 160 keV {sup 57}Fe ions to fluences of 1, 3, and 6 {times} 10{sup 16} ions/cm{sup 2}. Best correlations among AEM, RBS, and TRIM calculations were obtained assuming a density of the amorphized implanted regions equal to that of crystalline SiC. No iron-rich precipitates or clusters were detected by AEM. Inspection of the electron energy loss fine structure for iron in the implanted specimens suggests that the iron is not metallically-bonded, supporting conclusions from earlier conversion electron Moessbauer spectroscopy (CEMS) studies. In-situ annealing surprisingly resulted in crystallization at 600{degrees}C with some redistribution of the implanted iron.

Macro- and Nanotribological Properties of Graphite Tribofilms: Influence of the Sliding Interface

Macrotribological studies of microcrystalline graphite powder reveal a drastic decrease in the friction coefficient when the experiments are carried out in the presence of low-viscosity liquids. The friction reduction is attributed to the simultaneous presence of particles and liquid in the sliding contact, but the mechanisms involved remain unclear. In order to contribute to the understanding of liquid action in friction reduction mechanisms, nanoscale investigations of the tribofilms have been performed using lateral force microscopy. Attention is devoted to the nanostructure of the film surfaces and their nanofriction behavior using an atomic force microscope. The influence of the tip/sample interfaces on friction properties is investigated by using AFM tips constituted of different compounds (silicon, gold/chromium alloy, silicon nitride or carbon-covered AFM tip) and by performing the nanofriction tests in air or liquid environments. The results indicate that the friction reduction observed at macroscale is attributed neither to the lowering of the shear strength of the carbon/carbon interface in the presence of liquid nor to the nanostructure of the film surface. Collective liquid/particles effects inside the contact during sliding are probably involved.

Influence of the Volume Structure on the Tribological Properties of Lamellar Tribofilms

It has been shown that friction coefficient of powdered lamellar compounds measured at macroscale undergoes a drastic decrease when the experiments are performed in the presence of a low-viscosity liquid. The effect was attributed to the simultaneous presence of the solid particles and the liquid in the area of contact. However, the friction reduction mechanisms are still not yet fully understood. Recently, it was found by Atomic Force Microscopy that the presence of liquid during graphite tribofilm built-up induced a nanostructuration of the film surface and it has been demonstrated that such structuration has no influence on the tribological properties measured at macroscale. In this work, we report structural and friction analyses of MoS2 tribofilms performed at nano- and macroscales, in comparison with graphite tribofilms. In addition, MoS2 and graphite tribofilm structure are investigated by means of transmission electron microscopy. The results clearly indicate that, at macroscale, friction does not always take place at the surface but could occur inside of the tribofilm depending on its nanostructuration.

Stability of ion‐implanted layers on MgO under ultrasonic cavitation

The effects of ion implantation and ultrasonic cavitation on the near‐surface region of MgO single crystals were investigated. For 150 keV, room‐temperature implantations of Ar+ or K+ at 5×1016–1017 ions cm−2 into a (100)‐oriented surface of MgO, a dilatometric swelling of the implanted layer is observed perpendicular to the implanted surface. When these strained surface layers are then subjected to an ultrasonic (∼20 kHz) cavitation treatment, uniform layers (∼100–200 nm) can be removed from the MgO surface in a controlled manner. The thickness of the removed layer is directly related to the duration of the ultrasonic irradiation, but does not exceed the depth of the implanted species. When MgO implanted with Ar+ or Kr+ at fluences of 5×1016–1017 ions cm−2 is subsequently irradiated with 2 MeV He+ ions at 1016–1017 ions cm−2, the region exposed to the He+ beam is effectively ‘‘stabilized’’ and is not removed by the ultrasonic treatment. K+ or Ar+ implantations at <5×1016 ions cm−2 produced no swelling of...