Materials and metallic structures
Deposit
Microwave Plasma Enhanced Chemical Vapour Deposition (MPECVD)
An increase of working temperature is always wished to improve the performances of aeronautic and gas turbines. To circumvent the limited progress foreseen in airfoil cooling design and high temperature properties of superalloys, thermal barrier coatings systems are used on both turbine blade and vanes to protect metallic part from the hot gas stream (>1400°C). Yttria partially stabilised zirconia (YPSZ) exhibits the best thermal and mechanical properties to accommodate the thermal expansion mismatch between 100 to 200µm thick ceramic coating and metallic substrate. Furthermore the morphology of the coating has a great importance to improve the TBC lifetime, a columnar morphology as obtained by electron beam physical vapour deposition (EBPVD) being far more resistant than a lamellar morphology created by plasma spraying. Presently, the most critical turbine components are coated by EBPVD. But this technique is quite expensive, partly because it requires accurately controlled high power electron guns and high vacuum equipment. As an alternative to EBPVD, a microwave PECVD process has been developed by ONERA with the aim of obtaining columnar deposits, better complex shaped part surface coverage, lower investment and running costs and a wide scale of composition or morphology (different porosity and single or multilayer deposit)without extensive process modifications.
Figure 1: Microwave PECVD reactor
(pre industrial scale)
The microwave PECVD reactor developed at ONERA is composed of three parts : the system for precursor vapour production and gas injection in the deposit chamber, the microwave plasma generation system and the heating substrate holder (figure1).
The precursor vapour production system integrates up to three ovens, the distribution of the different gas sources and injection of the gaseous mixture into the deposition chamber. To achieve a correct mixture, the different precursor vapours and the auxiliary gas flow (Ar/O2) have been distributed around a mixing chamber respecting the azimuth symmetry of the reactor. This distribution is very important to obtain deposits with a uniform chemical composition on large substrates at high deposition rates. Both gas flow rates and total pressure are controlled by mass flow controllers and throttle valve respectively.
The microwave plasma is generated with the help of a surfaguide system developed in the eighties by Moisan and al. The 2.45GHz microwave power is launched perpendicularly on a 100 mm diameter quartz tube constituting the deposition chamber, maintained at low vacuum by a Roots blower and a rotary vane pump. A surface wave is generated on the tube’s inner surface. The discharge creates its own propagation medium generating high electron densities. The microwave power (up to 2kW) is injected to ensure the generation of a quasi symmetric plasma in the quartz tube.
Finally, the heating substrate holder is located in front of the gas injector. Substrates are heated by a resistive wire (disc Ø20mm with laboratory reactor) or an oven (blade with pre industrial reactor) and their temperature is controlled in the range 300 to 900°C.
Different kind of precursors are used in this reactor. The sole limitation is their evaporation rates at temperatures lower than 250°C, which must be compatible with the wished deposition rate. This temperature limit has been given to allow the use of relatively simple oven heating techniques. Some chlorides (ZrCl4, HfCl4,etc) are compatible with our heating capability. An argon flow passes through a porous crucible filled with powder and is leas to the mixing chamber. The metal chlorides exhibiting low vapour pressures at low temperature can be replaced by metal organic compound as for yttrium. For instance, Y(thd)3 is vicious at about 170°C; it is placed in a crucible and an argon flow carries the vapour to the mixing chamber. This reactor allows the deposition of oxides at rates higher than 200 µm/h. In CVD process the chemical reactions take place in the boundary layer near the heated substrate. Plasma enhancement allows the initiation of chemical reactions far from the sample: the inelastic collisions between energetic electrons and heavy atoms promote the production of gaseous reactive species like atomic oxygen and metal sub oxide. The deposition rate is consequently increased. The experiments on tilted substrates and plasma diagnostics both show that the most efficient chemical reactions for deposition process take place in fact upstream in the plasma. An energetic plasma can actually compensate for the precursor concentration decrease due to the deposition and create a large zone where the deposition process is maintained at a high efficiency.
Finally, PECVD YPSZ coatings are very similar to that obtained by EBPVD, especially with respect to morphology, cristallographic properties, thermal conductivity and durability at high temperature. In addition, this reactor allows the deposition of many kind of oxides (new composition or multilayer) on laboratory scale substrate or real aeroengine parts.
