The Research Of Al₂O₃Thermal Control Coatings Prepared By High Pulsed Power Magnetron Sputtering

Thermal control coating is an effcetive method to ensure the spacecraftoperating at the appropriate termperature. The two most important parameters of thethermal control coating are the solar absorption ratio and hemispherical emissivity.In this paper, with high power pulsed magnetron sputtering, the Al2O3thermalcontrol coating are prepared in two ways: The single magnetron with the modulatedplused power and the Twin-Mag with the symmetrical bipolar pulse power. Duringthe sputtering, the component and intensity of plasma in the system are diagnosedby fiber optic spectrometer. The microstructure and the phase composition of thecoating are analyzed by SEM, EDS and XRD. The solar absorption ratio andhemispherical emissivity are measured.The power used in the conventional sputtering is designed.on the basis ofmodulated pulsed power (MPP). The output frequency can be regulated in the rangeof20~500Hz. The most maximum pulse voltage and current can be900V and60Arespectively.In the single magnetron sputtering with the designed power, the main plasmacomponent is Ar and O ions. The intensity of Al ion is weak an the wave length of927.48nm. The increasing of the O2flow make the poison of the target get moreserious when the pressure is kept constant, and the deposition rate also decreases.Arcing occurred in the Al target causes the depostiton of large particles on thesurface of the coating. When the O2flow is large enough to make the targecompletely poisoned, the deposition rate decreases to a very low level. The pressurehas less impact on the deposition rate. The hemispherical emissity can reach0.65after deposition for15hour, however the solar absorption rate is lager that expected.The density of the plasma in the Twin-Mag sputtering with sysmetrical bipolarpulse power is much higher than the single target sputtering. Twin-Mag inhibits thesputtering process from arcing, and reducea the poison of the targets to a greatextent. Thus the deposition rate is much higher than the single magnetron sputtering,and the surface roughness of the coating is improved. We studied the relationshipbetween the plasma intensity and the O2flow, as well as the pressure by emissionspectrometry. The increasing of the O2flow causes a slight decreasing in the plasmaquantity. The electron density also decrease by the Stark broadening analysis of theAr+at the wavelenth of813.68nm. The deposition rate decreases and the surfacequality gets worse with the increasing of O2flow. The plasma density and thepressure is positively correlated, while the the change of the pressure still has a slight effect on the deposition rate. It seems that the reduction of the pressure canimprove the roughness of the coating.The Al2O3deposited on the substrate is amorphous. The coating also containsAl when the O2flow is low. After annealing at the temperature of750°C, thecrystallization occurs and γ-Al2O3is detected in the coating by the X-ray diffration...