Safran cooperates with Ionbond to develop coatings for outer space

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.

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.

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.