Exploitable results Deposition of hard metallic oxides at low temperatures (DOHMOLT) This Basic Research (Type BR) project present project was aimed at achieving a fundamental understanding of the physical processes governing the growth of alphaalumina coatings by Physical Vapour Deposition. It involved different research groups, which provided input on theoretical modelling, controlled growth by Molecular Beam Epitaxy (MBE), practical deposition using magnetron sputtering, and comprehensive analysis and testing activities. In addition, two industrial companies gave guidance on the practical applicability of the research. - The key results of importance in the project have arisen from this multidisciplinary approach. Thus, for example, results from the theoretical studies have shown that one of the major problems in growing alphaAl2O3 stems from the fact that small island growth is the preferred growth mode. This is to be contrasted with Cr2O3, which has a similar crystal structure, but growth occurs as extremely large islands. Consequently, the latter material is easier to grow in general than Al2O3. Both the MBE and magnetron sputtering investigations have confirmed this. These same theoretical considerations have revealed that attempts to grow multilayers of Al2O3/Cr2O3 in the corundum structure presents severe difficulties (due to destabilising effects emanating in the Al2O3 layer). This fact has also been confirmed experimentally via MBE studies, which have shown that when an abrupt change from the growth of Cr2O3 on steel to Al2O3 is made, the corundum structure reverts to an amorphous phase over a thickness of the order of a few nm to a few tens of nm. - Conversely, the theoretical studies indicated that the growth of an alloy of Al and Cr oxides would be more likely to produce the crystalline phase. Again, this has been confirmed by both MBE and magnetron sputtering. However, the experimental limit of the Al2O3 content appears to be low (about 35 at % from MBE experiments to date, but somewhat higher by sputtering). The theory shows also that the dynamics of crystal growth dictate that even under the most favourably controlled conditions (e.g. MBE where the incident species and their energies are well defined) it will be difficult to ensure a planar (i.e. essentially 2D-like) growth mode, unless the ratio of the fluxes of the incident species is kept within certain critical limits. Furthermore, this ratio varies with the plane of growth. Along certain directions one requires an O2 rich environment, whereas along other directions one needs an Al rich environment. If planar growth is lost in favour of 3D growth, the theory shows that native lattice defects can enter the structure in rapidly increasing self-perpetuating numbers. This has two implications; either a multi-phase layer of Al2O3 and/or an amorphous phase will occur. These results have been confirmed via MBE. For example, growth of alphaalumina on r-plane sapphire (which the theory shows requires an O2 rich environment) has been attained. However, growth on c-plane sapphire (which theory predicts requires an Al rich environment) has not been attained. This is because growth in an Al rich environment would face the problem that any excess Al would form a second phase Al metal (as is consistent with the MBE data). - Other results of importance are that the present theoretical investigations suggest that the growth of alphaalumina should be possible at low temperatures (~500C) via low energy incident species. This fact has been confirmed clearly by MBE. Similarly, the theory shows that diffusion of Fe from a steel substrate will destabilise the alphaphase. This is also consistent with the MBE result that aluminium deposition on steel substrates is essential prior to growth. Further, we note that, from a theoretical viewpoint, incident energetic species should assist in meeting the aim of defect-free planar growth by virtue of rearrangement collisions with the growing film, or by providing additional energy to migrating adatoms. In this connection, the results from magnetron sputtering have been confirmatory in showing that the reason why such energetic ions have been only partly successful in this regard is due to the formation of aluminium hydroxides. The latter have been shown to inhibit corundum formation and occur during growth as a result of the water background usually present even in a high vacuum system. Overall, the project has provided a unique and hitherto unavailable insight into the mechanisms critical to the formation of these films of the hard alpha phase of alumina. The original impetus in the project was to research the fundamental mechanisms for alumina growth and the potential application of this material as a cutting tool coating. Subsequent applications studies have revealed that there is considerable potential for a wide range of applications, not only in the wear, friction and corrosion control domain, but also for many applications in electronic, optical, opto-electronic and even magnetic thin film devices. The project has thus laid the foundations for successful coating process development for many sectors of industry. In meeting the objectives of the project, the partners have acquired considerable know-how in processing, testing and analysing alumina-based coatings. They have, for example, developed software codes to model growth and deposition processes, and have optimised characterisation methods, which are critical to the future commercial development of many coatings and processes, especially those based on oxide deposition. È in corso la ricerca di dati su OpenAIRE... Si è verificato un errore durante la ricerca dei dati su OpenAIRE Nessun risultato disponibile
