PVD and CVD coatings are usually associated with wear resistant films, which extend useful life of tools and machine parts. However, these coatings can also provide a variety of other unique surface properties. Ionbond’s logo ‘The Surface Engineers’ reflects the fact that we specialize in engineering of the surface to meet specific application needs.
French company Snecma (www.snecma.com) approached Ionbond with a request to develop a special coating for the new line of plasma propulsion engines, which are used in space to drive spacecraft and/or keep satellites on their orbit. Snecma manufactures these plasma thruster engines mainly for Telecom satellites, like Alphabus currently developed under contract with European Space Agency (ESA).
Plasma propulsion system is a reaction engine, which uses electric power and plasma as a means to generate the thrust. Conversely, chemical propulsion relies on chemical reaction for the same purpose. Interestingly, both engines use the same Newton’s third law as their fundamental operating principle. Chemical propulsion, while being the mainstream technique, has a physical limitation in the velocity of the exhaust gases and, respectively, the maximum speed a craft can achieve. In addition, its equivalent of ‘fuel economy’ (called ‘specific impulse’ by rocket scientists), is somewhat low. Plasma propulsion engines, while having only a modest value of thrust, can deliver it for a very long time and have much better fuel efficiency. Plasma thrusters can propel spacecraft up to the impressive speeds of 50 km/s (180 000 km/hour).
Snecma manufactures so-called Hall Effect plasma thrusters, which use ionized xenon gas as a propellant and electrostatic field as its accelerating means. Xenon plasma is formed inside the cylindrical anode through collisions of xenon atoms with electrons. Xenon ions are then accelerated and propelled out of the anode. At the exit stage the ions recombine with electrons, thus keeping the whole setup electrically neutral. Snecma engineers selected hollow cathode principle to generate electrons required for ionization. Hollow cathode uses the energy of gas ions to sustain high temperature required for electron emission. Unwanted chemical reactions start occurring at these temperatures between the used materials, which are usually inert at ambient temperature. Therefore specific coating barriers shall be added to prevent degradation of performance during the spacecraft lifetime.
Zirconium Nitride (ZrN) material was identified as an efficient means to block the diffusion. Ionbond engineers’ task was to develop a process capable of producing ZrN coating on the surfaces of the hollow cathode components. This task presented major challenges. Firstly, the coating had to be produced on the inner diameters of the parts with high aspect ratio. Secondly, a thick coating was necessary – unusually thick in comparison with regular requirements – which nevertheless had to adhere reliably to the used refractory metals. Thirdly, deposited ZrN film had to exhibit a particular morphology and crystallographic lattice to meet Snecma specifications.
Since PVD process was not suitable for the ID coating deposition, Ionbond process engineers zeroed on the CVD technology. In this technique, the coating is produced by chlorinating pure zirconium and transfer of the formed ZrCl4 vapor to the reactive zone, where it reacts with hydrogen and nitrogen or nitrogen-containing gases at high temperature to form a ZrN deposit. In conventional processes, the deposition rate of ZrN is less than a micron per hour, which would be unacceptable for deposition of thick films. For that reason, Ionbond engineers had to look for the possibilities to increase the deposition rate. Such parameters as concentration of reactants, chlorination and deposition temperatures, pressure and load setup were identified as factors affecting the deposition rate. After preliminary thermodynamic analysis, feasible ranges for every parameter were selected and a matrix of experiments was designed. Besides deposition rate, lattice and morphology types were monitored as the response variables.
Experimentation and analysis of the data allowed establishing the dependence between input parameters and response variables. It was determined that deposition temperature was the largest contributor to the deposition rate of ZrN. The ratio between concentrations of hydrogen and ZrCl4 played the second most important role. Special attention was paid to the setup of the load in order to produce a coating with acceptable thickness distribution and uniformity. Since the overall optimization task included three parameters (deposition rate, morphology and lattice type), substantial adjustments to the process parameters were necessary to produce a coating conforming to the specification, while maintaining the highest value of the deposition rate.
Ionbond engineers have successfully overcome these challenges and developed a specialized process of deposition ZrN coatings onto Snecma components. Besides satisfying all the requirements of the Snecma specification, a striking fourfold increase in the deposition rate was achieved as well. The first components are delivered to Snecma, where they will be tested and installed in the plasma thrusters. ”It was a complicated, daring task of multifactorial optimization. Ionbond engineers demonstrated professionalism, deep knowledge of the technology and very enthusiastic approach to the problem. Chapeau!” – says Gilles Turin, Snecma’s Program Manager.
For further questions please contact Val Lieberman