Recrystallization of amorphous nano-tracks and uniform layers generated by swift-ion-beam irradiation in lithium niobate.

The thermal annealing of amorphous tracks of nanometer-size diameter generated in lithium niobate (LiNbO3) by Bromine ions at 45 MeV, i.e., in the electronic stopping regime, has been investigated by RBS/C spectrometry in the temperature range from 250°C to 350°C. Relatively low fluences have been used (<1012 cm−2) to produce isolated tracks. However, the possible effect of track overlapping has been investigated by varying the fluence between 3×1011 cm−2 and 1012 cm−2. The annealing process follows a two-step kinetics. In a first stage (I) the track radius decreases linearly with the annealing time. It obeys an Arrhenius-type dependence on annealing temperature with activation energy around 1.5 eV. The second stage (II) operates after the track radius has decreased down to around 2.5 nm and shows a much lower radial velocity. The data for stage I appear consistent with a solid-phase epitaxial process that yields a constant recrystallization rate at the amorphous-crystalline boundary. HRTEM has been used to monitor the existence and the size of the annealed isolated tracks in the second stage. On the other hand, the thermal annealing of homogeneous (buried) amorphous layers has been investigated within the same temperature range, on samples irradiated with Fluorine at 20 MeV and fluences of ∼1014 cm−2. Optical techniques are very suitable for this case and have been used to monitor the recrystallization of the layers. The annealing process induces a displacement of the crystalline-amorphous boundary that is also linear with annealing time, and the recrystallization rates are consistent with those measured for tracks. The comparison of these data with those previously obtained for the heavily damaged (amorphous) layers produced by elastic nuclear collisions is summarily discussed

Evidence of an ion-beam induced crystalline-to-crystalline phase transformation in hafnia

Samples of monoclinic hafnia were irradiated with increasing fluences of 800 and 300 MeV Kr ions giving rise to a slowing down essentially caused by high electronic excitations. Their structural evolution was monitored in situ by the X-ray diffraction technique. The results indicate, for the first time to our knowledge, the occurrence in monoclinic hafnia of an ion-beam induced crystalline-to-crystalline phase transition. The new formed phase is very likely tetragonal and appears with an effective threshold in the deposited electronic energy loss which is around 20 keV nm -1 . In addition, the evolution of the amount of the produced phase with the ion fluence exhibits a sigmoidal shape suggesting a mechanism for phase transformation which needs two ion impacts. Some features of this phase transition are compared with those obtained in the case of zirconia, a well-known isomorphic material with hafnia. Copyright Springer-Verlag Berlin/Heidelberg 2003

Evidence for a strong correlation between the amount of surface carbon and the tribological behaviour of ion-treated steels

Samples of 100Cr6-bearing steel were treated by different ion beams in order to study the evolution of their tribological properties. Complementary physico-chemical characterisation techniques give evidence for a strong correlation between the amount of surface carbon, whatever its origin (contamination, direct C implantation or ion beam mixing of a deposited carbon layer), and the reduction of the friction coefficient as well as the increase of the wear resistance. It is found that the friction coefficient is improved by a factor 5 and the wear volume by two orders of magnitude when the amount of surface carbon exceeds $10^{17}\;{\rm C\,cm}^{-2}$

Dramatic change of the kinetics of the phase transition induced in pure zirconia by swift heavy-ion irradiation

It is generally admitted that the irradiation of crystalline materials with high-energy heavy ions often leads to the formation of highly defective or amorphous tracks. In the latter case, the amorphization process follows a single-ion-impact mechanism. However, recent results demonstrate that in certain circumstances crystalline-to-crystalline phase transitions can also occur. This is well illustrated with the case of pure zirconia which can undergo a transition from the monoclinic to the tetragonal phase with a kinetics characterized by a sigmoidal shape. The present study provides new evidence that this transformation is driven by a double-ion-impact mechanism. Another important finding is that the steepness of the sigmoid depends drastically on the amount of the electronic-energy loss: a small change in this parameter leads to a catastrophic modification of the kinetics of the phase transition