New and Novel Superhard Coatings Prepared by Electron-Cyclotron-Resonance Plasma-Enhanced Chemical Vapour Deposition
Final Report Abstract
The central goal of this project was to seek new synthetic routes to novel hard and superhard phases based on the light elements Si-B-C-N-O system. A central part of this project was to utilize the high density of electron cyclotron resonance (ECR) enhanced microwave generated plasma to deposit films at relatively low substrate temperatures and to provide an understanding of the growth mechanisms of the films. In addition, the project focuses on the synthesis of new hard phases and nanocrystalline composites based on the aforementioned system and to understand the relationship between the structure and properties. The work aims to explore the extent to which ECR-MWCVD can be employed in order to synthesise new hard and superhard phases in the light elements system. The potential of such new materials for the thin film industry is very high. However, progress in this area is only possible by following an approach which can be characterised as being both systematic and innovative and by employing a vast range of materials and surface sensitive characterisation tools in order to highlight materials effect at low film thicknesses. Although in certain situations, we had to deviate from the proposed investigation plan in order to overcome some technical issues and practical hindrances, it was not only possible to synthesise new hard phases in the Si-B-C-N system but also to go beyond the initial plan and realize several promising hard nanocomposites. By adjusting and adding to certain parts of the plan, we have managed to benefit from several effects that we observed during the course of our investigations in order to achieve the synthesis of well-defined and novel nanocomposite films which to date have not been reported elsewhere. Furthermore, we drew on additional resources to highlight specific mechanical and tribological properties of our materials. This was only possible due to the multidisciplinary approach which we followed in conducting this work. Our new findings have quickly captured the attention of the scientific community as marked by a series of invited talks at an international level. The new results will be used as a starting point for further investigations and sequential proposals. In this project, very promising results have been achieved in the context of synthesising amorphous hard phases in the Si-C-N and Si-B-C-N systems featuring hardness up to 25GPa along with very high elastic recovery of more than 90%. While BCN films, which were synthesise at low DC substrate biases, were soft in nature and featured a hardness that exceeded only slightly that of hexagonal boron nitride, attempts to synthesise film at increasing substrate DC- bias lead to catastrophic film de-bonding and delamination. However, the addition of silicon to the B-C-N system produced several positive effects including outstanding adhesion, resistance to film hydrolysis and de-bonding as well as reduced film stresses. As part of the accompanying sub-studies, we noticed that the Si-B-C-N system demonstrate an exceptional structural stability and oxidation resistance at temperatures as high as 1350°C in air. Furthermore, the mechanical and tribological testing confirmed that while the film hardness could not exceed 25-30GPa at low DC substrate bias, the amorphous and very smooth films demonstrated remarkably low friction coefficient values which are able to compete with stateof-the-art amorphous carbon- based coatings. These results will open new industrial application fields which were not thought of previously. As an additional study, it was possible to synthesise a new class of nanocomposites, as mentioned above, by incorporating a metal into the light element network. This was achieved by combining the ECR-method as a hybrid technique with an ion-beam sputtering method. These findings demonstrated that due to specific reaction conditions this method allows for the establishment of a process resulting in a two phase nanocomposite film whose composition could be controlled via the corresponding ratio of the reactant gases. By employing an additional in-situ ion beam assisted deposition it was possible to simultaneously deposit the nitride of a metal (Molybdenum in this case) along with a non-metal (boron as an example). The mass fraction of either phase; Mo2N and BN can be varied by controlling the ratio of BF3/H2 and the bias voltage of the Mo sputtering source. With this method, films with controllable microstructure and properties can be prepared under well defined conditions with a range of hardness and elasticity. In this project, it is demonstrated that by using such high density plasma a range of new hard materials can be synthesised with controllable microstructure. The results pave the way to prepare a new class of hard and tribological coatings which should arouse great interest in wear resistant applications. The results obtained in this work demonstrate that several thin film systems can be prepared with very attractive combination of mechanical, tribological and oxidation resistance properties which will enable the technical exploitation in a number of applications. As a matter of fact, we have already started defining further strategic implementation pathways of the hard amorphous Si-C-N coatings, which were obtained in this work, with leading industrial manufacturer of automobile parts to explore new pathways of applying the material in applications under sever and extreme conditions. Due to the fact that with the ECR method, not only thin but also thick film can be synthesised on a wide range of technically important substrates at mild deposition temperatures. The additional benefits arising from including Si in the B-C-N network have enabled the deposition of films with significantly reduced growth stresses and outstanding coating adhesion also to metallic substrates. This progress, which was only possible as a result of this project, will have far-reaching consequences. These will not only have an impact on the fundamental understanding but also on the level of technical applications. On a side note, the several additional studies which we conducted in the context of this project on the development of new hard nanocrystalline films in the Mo-B-N system will also open new fundamental research possibilities in the near future. Although this part of the work was not initially a part of the original plan, it was possible to exploit unexpected observations and phenomena during the bias-assisted film growth to design a series of sub-investigations of a totally new hard phase. The multidisciplinary nature of the research which is conducted in the group of Prof. Jiang enabled to initiate further investigations using the group’s expertise and infrastructure. With the development of completely new kind of hard nanocrystalline and amorphous films by the ECR method, new understanding of growth mechanism and propertystructure is gained. This will form a basis of further fundamental research investigations in the near future.
Publications
- Structural stability, mechanical and electronic properties of cubic BCxN crystals within a random solid solution model. J. Phys.: Condens. Matter 21, 405401 (2009)
Chunqiang Zhuang, Jijun Zhao, and Xin Jiang
- Tuning bond contents in B-C-N films via temperature and bias voltage within RF magnetron sputtering. Surf. Coat. Technol. 204, 713 (2009)
Chunqiang Zhuang, Jijun Zhao, Fuchao Jia, Changyu Guan, Yizhen Bai, and Xin Jiang
- “Deposition and characterisation of nanocrystalline Mo2N/BN composite coatings by ECR plasma assisted CVD”. Surface and Coatings Technology 204, 1919 (2010)
H. Abu Samra, T. Staedler, J. Xia, I. Aronov, C. Jia, B. Wenclawiak, and X. Jiang
- “Structure and mechanical properties of cubic BC2N crystals within a random solid solution model”. Diamond and Related Materials, 19, 1419 (2010)
J. Zhao, C. Zhuang, X. Jiang
