Safran cooperates with Ionbond to develop coatings for outer space
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 the application needs.
Safran’s mission: development of highly efficient plasma thrusters for satellites
French company Safran Space Propulsion 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. Safran manufactures these engines mainly for European Space Agency (ESA), whose spacecraft rely on plasma thrusters to propel them in space.
Why plasma propulsion?
Plasma propulsion system is a reaction engine, which uses 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 very 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).
The hall effect thruster and the role of a hollow cathode
Safran 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 its 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. Safran 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. To lower the emission temperature and thus increase the lifetime of the cathode, special materials with low work function are often used. In case of a hollow cathode, the tube of the emissive material is inserted inside the refractory metal shell. However, at high temperatures constituents of such materials starts diffusing into the shell material, which leads to its quick deterioration. In order to prevent this, a diffusion barrier between emissive and shell materials is required.
Zirconium Nitride as a diffusion barrier
Zirconium Nitride (ZrN) material was identified as an efficient means to block such diffusion. Ionbond engineers’ task was to develop a process capable of producing this 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, the coating thickness had to be in excess of 20 microns – unusually thick in comparison with regular requirements and adhere well to refractory metals. Thirdly, deposited ZrN film had to exhibit a particular morphology and crystallographic lattice to meet Safran specification.
The Challenges: coating of the part ID with high thickness of the deposit
Since PVD process was not suitable for the ID coating deposition, Ionbond process engineers zeroed on the CVD technology. The coating is produced by chlorinating pure zirconium and transfer of the formed ZrCl4 vapor to the reactive zone, where it reacts with a mix of hydrogen and nitrogen-containing gases at high temperature to form ZrN. In the conventional process, the deposition rate of ZrN is less than a micron per hour, which would be unacceptable for deposition of 20+ microns 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 matrix of experiments was designed. Besides deposition rate, lattice and morphology types were monitored as the response variables.
Typical morphology of ZrN deposits: left – acceptable; right – not acceptable
Engineering a tailored CVD process
Experimentation and analysis of the data allowed establishing the dependence between input parameters and response variables. Special attention was paid to the setup of the load to produce a coating with acceptable thickness distribution and uniformity. Since the overall optimization task included three parameters (deposition rate, morphology and lattice), substantial adjustments to the process were necessary to produce the required types of the morphology and crystalline structure, while maintaining the highest value of the deposition rate.
From concept to solution: mission accomplished
Ionbond engineers have successfully overcome these challenges and developed a specialized process of deposition ZrN coatings onto Safran components with fourfold increase in deposition rate, satisfying all the requirements of Safran’s specification. The first components are delivered to Safran, 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 Safran’s Laurent Godard, Materials & Processes Qualification Manager.
Questions?
Discuss your challenges with Val Lieberman
Dr. Val Lieberman, Global Segment Manager Industrial Components, will be glad to support you.
We will get back to you as soon as possible.
Dr. Val Lieberman
Global Segment Manager Industrial Components
Dr. Val Lieberman
Global Segment Manager Industrial Components
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