Morphological characterization of diamond coatings grown by MWPECVD on hexagonal Boron Nitride
Abstract
A relevant and still unsolved issue in the characterization of diamond coatings deposited on ceramic materials such as hexagonal boron nitride (h-BN) is to enhance the resistance of the substrate to erosion by Kr+ or Xe+ ions generated in the plasma propulsion systems. In this work, diamond films were grown by microwave plasma enhanced chemical vapour deposition on h-BN substrates untreated, for the first time, and pre-treated for short (31-65 min) and long (285-296 min) process times. The morphology of diamond films was analysed by scanning electron microscopy, and atomic force microscopy, and their chemistry and structure by Raman spectroscopy. Microscopy analysis revealed that noncontinuous (at short process time) and continuous (at long process time) films were formed, respectively, on both untreated and pre-treated h-BN substrates. In particular, diamond films grown on untreated h-BN substrates exhibited roughness values higher than those of h-BN substrates pre-treated by a conventional ultrasonic method.
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Introduction
The space exploration program faces enormous challenges as it seeks to achieve significant improvements in safety, cost and speed of missions to the frontiers of space. Plasma propulsion systems have been recognized to be more efficient than chemical thrusters, but substrates in contact with the discharge need to be highly resistant to ion erosion. This fact led to the development of highly efficient electric propulsion thrusters that are currently the only feasible technology in many deep space missions.1However, the sputter erosion of the discharge channel walls due to ion impingement is one of the main lifelimiting factors of the system. During a normal operation, a small fraction of accelerated ions impacts the interior of the main discharge channel, thus causing its gradual erosion during long operational times (from thousands to tens of thousands of hours).1
Materials such as Mo, which shows sputter-resistant surfaces, have been fabricated and used in electric propulsion thrusters to lengthen their life.2 Further, carbon-based materials have shown an improvement of nearly an order-of-magnitude over Mo in sputter erosion resistance.3 In particular, among the tested carbon-based materials, diamond films prepared by chemical vapour deposition (CVD) have provided an improvement by a factor of 1.5 over other materials in sputter erosion rate.4
Boron Nitride (BN) has also raised a high interest in the area of material sciences and its use as an insulator in space electric propulsion thrusters. This is due to the special bonding features of boron and nitrogen, its chemical inertness at hightemperature and its high thermal conductivity and low dielectric constant.5-7BN exists in many different structures, of which two forms, i.e. its only two stable phases, are of particular interest: the cubic allotrope of BN (c-BN), which exhibits an extreme hardness that makes it a promising material for protective and abrasive coatings; and the hexagonal allotrope of BN (h-BN), which shows optimal insulating features. The h-BN has characteristics similar and is iso structural to graphite, i.e. it shows layered sp2 -bonded structure where each carbon atom is replaced by boron and a nitrogen atom, respectively. For its colour it is also known as “white graphite”6 . The major differences between h-BN and graphite are the electrical and optical properties, i.e., graphite is an electrical conductor, whereas h-BN is an insulator. Further, h-BN is the only BN phase found in nature and can be easily synthesized and machinable.
Bulk h-BN is widely used in several applications because of its interesting properties, including low density, chemical inertness, stability in air at high temperatures (up to 1000°C), stability to thermal shocks, easy workability as hot-pressed shapes, and excellent electrical insulating features as a ceramic material. Due to these specific properties, h-BN has been used in space electric propulsion thrusters,1,8to construct crucibles and molds for melting glass and metals for high-temperature applications, as a very high thermal conductor,6,7as a protective layer on surfaces, 9-16 as a substrate for electronic devices,17-23 as an additive in engine oils and plastics,24-27 and as a solid lubricant.5
Diamond is well known to feature a high sputter resistance to bombardment by Xe ions, the highest thermal conductivity of all insulators, and the lowest dielectric constant. The limited experimental data available on Xe-ion induced sputter yield suggest that, when comparing various wall material options including diamond, alumina (Al2O3) and BN, the sputter resistance of polycrystalline diamond is 25% better than that of BN.28In a preliminary study by Yap et al.,2 ion erosion of multiwall nanotubes by Kr ions generated by a Hall effect thruster was compared to that of CVD diamond films, c-BN and amorphous-carbon (a-C) films. The CVD diamond films were found to have a resistance to ion erosion higher than that of vertically-aligned multiwall nanotubes.
Most studies on diamond growth on BN used bulk c-BN crystal substrates obtained by high-pressure methods,29,30 whereas only one study has dealt with diamond growth on h-BN films.31The latter authors demonstrated the feasibility of growing high quality polycrystalline continuous diamond films by hot filament CVD onto a pre-treated PECVD h-BN film about 2- µm thick deposited on a Si (100) substrate.
The main objective of the present work was to find a means to avoid the sputtering of h-BN insulator walls of Hall thrusters by growing diamond coatings on untreated, for the first time, and pre-treated h-BN substrates by microwave plasma enhanced chemical vapour deposition (MWPECVD) using CH4/H2 gas mixtures. The first part of this research was focused on the evaluation of growth of diamond coatings by pyrometric interferometry (PI). In the second part, the morphological characterization of diamond films grown at short (31-65 min) and long (285-296 min) process times was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM), and the nature of chemical bonding by Raman spectroscopy.
Conclusion
This work for the first time investigated comparatively untreated and pre-treated h-BN substrates, and studied the influence of substrate nature on the morphology of diamond coatings synthesized by MWPECVD.
The diamond coating growth on untreated h-BN samples has shown an unexpected higher RRMS value compared to the pre-treated h-BN samples as measured by AFM and confirmed by the lower amplitude of interference fringes of PI curve. This behaviour was due to the initial rough topography of the h-BN substrate. A number of diamond crystals with peculiar shapes and showing a five-fold symmetry due to multiple twinning were observed by SEM and AFM. These materials appear promising for use as protecting coatings of surfaces against erosion when in contact with particles of various size range and/or by high velocity fluid impingement.