ng-Tse Chu

Root cause determination of on-orbit degradation of the VIIRS rotating telescope assembly

The Visible Infrared Imaging Radiometer Suite (VIIRS) is a sensor onboard the recently launched Suomi NPP spacecraft. Shortly after launch, VIIRS was found to exhibit a pronounced decrease in the optical throughput of several bands, with the near-infrared bands being more affected than those in the visible. The anomaly investigation team performed several experiments that concluded the primary source of degradation was throughput loss in the VIIRS rotating telescope assembly, likely caused by ultraviolet light illumination. This paper will discuss the laboratory investigation that determined the root cause of the telescope degradation to be UV photo-darkening of a tungsten oxide contaminant film that had been inadvertently deposited during the mirror manufacturing process. We will present data from experiments conducted on witness mirrors manufactured along with the telescope, as well as other mirrors of the same type that were not contaminated.

Corrosion characterization of durable silver coatings by electrochemical impedance spectroscopy and accelerated environmental testing

Highly reflective front-surface silver mirrors are needed for many optical applications. While various protective dielectric coating schemes have been developed, the long-term durability of Ag mirrors is still of great concern in the optics community for a variety of applications under harsh environments. The corrosion protection behavior of a SiNx-coated silver-mirror coating scheme was tested with electrochemical impedance spectroscopy (EIS) and accelerated environmental testing, including humidity and salt fog tests. The EIS data obtained were fitted with different equivalent circuit models. The results suggested that the 100A thick SiNx coating produced by rf magnetron sputtering was porous and acted as a leaky capacitor on the Ag film, whereas the addition of a NiCrNx interlayer as thin as 3A between SiNx and Ag films resulted in a much denser SiNx coating with a low-frequency impedance value of 2 orders of magnitude higher than that without the interlayer. Humidity and salt fog testing of different silver coatings showed similar results. The 100A SiNx/3A-NiCrNx/Ag coating exhibited excellent corrosion resistance against the corrosive environments used in this study.

Environmental durability of protected silver mirrors prepared by plasma beam sputtering

Various overcoat layers have been developed to protect silver mirrors from tarnish and corrosion. However, the mechanisms by which these protective layers improve mirror durability are not fully understood. Mixed flowing gas exposure was used to investigate the corrosion behavior of plasma beam sputtered silver mirrors with different adhesion layer materials. A small amount of nickel in the adhesion layer had a significant impact on the silver-dielectric interface. Additionally, lateral transport of silver was found to be an important factor in the corrosion process. Better stability at all layer interfaces is suggested to improve mirror durability.

Control of stress in protected silver mirrors prepared by plasma beam sputtering

Plasma beam sputtering was used to deposit dielectric-protected silver mirrors that exhibited excellent durability and controlled stress. The durability of the mirrors was strongly dependent upon the presence of a very thin chromium adhesion layer between the silver layer and the dielectric overcoat. The stress of the five-layer mirror was balanced by controlling the compressive stress in the top dielectric layer, offsetting the net tensile stress of the combination of layers below.

Lessons Learned from Optics Flown on the Materials International Space Station Experiment

The Materials International Space Station Experiment (MISSE) provided opportunities to study the effect of the low earth orbit space environment on materials. This paper presents observations made on optical samples flown as part of MISSE.

Effects of adhesion layer composition on the environmental durability of protected silver mirrors

While various layer schemes have been developed to protect thin film silver mirrors from tarnish and corrosion, the mechanisms by which these protective layers improve mirror durability are not fully understood. Mixed flowing gas exposure of plasma beam sputtered silver mirrors was used to investigate how the composition of the very thin adhesion layer changes the mechanism of corrosion feature growth. Two model mirror coatings were analyzed in which the composition of the base layer below the silver and the adhesion layer above were varied. Optical measurements and microscopy, SEM, TEM, and EDS were used to characterize the compositional and chemical effects at the layer interfaces. Large circular corrosion features formed along the silver-chromium interfaces; the addition of nickel to the layers on either side of the silver limited the growth of these features, but resulted in the corrosive attack of the silver itself.

Spacecraft materials HCl susceptibility assessments

The susceptibility of spacecraft materials to HCl exposure was investigated in light of concerns to potential contamination during evolved expendable launch vehicle (EELV) overflight scenarios. Overflight refers to the circumstance where one spacecraft, resident on a launch pad, may be exposed to HCl generated from an earlier solid rocket launch at an adjacent pad. One aspect of the overflight risk assessments involves spacecraft materials susceptibility to HCl exposure. This study examined a wide range of spacecraft materials after being exposed to HCl vapor in a well-characterized facility. Sample thermal/optical and electrostatic dissipation properties, as well as surface chemical and morphological features, were characterized before and after the HCl exposure. All materials tested, except for indium tin oxide (ITO) coated Kapton film, showed no significant degradation after HCl exposure of up to 4800 ppb-hr. The ITO coated Kapton sample showed slight signs of degradation after being exposed to 500 ppb-hr HCl, as the surface resistance was increased by a factor of 5. However, the potential HCl dose inside the payload fairing (PLF) was estimated to be far below 500 ppb-hr in an EELV overflight event. These results, along with other relevant laboratory test data on the HCl removal efficiency of the filtration media used on the launch sites, provide the technical rationale that properly filtered air as the PLF purge should pose little risk in terms of HCl contamination under EELV overflight scenarios.

On-Orbit Degradation of Silver Mirrors Exposed to Ultraviolet Radiation

Silver mirrors on the Suomi-NPP spacecraft exhibited significant degradation during early orbit operations. This paper describes the investigation that identified the source of the loss as a UV-sensitive contaminant deposited during manufacturing.

Micromachined reference samples for particle counting

Three micromachined reference sample prototypes for particle counting have been fabricated by using dark ceramic spots in a transparent glass wafer to simulate particles on a surface. The direct write approach permits the spots to be positioned at random locations within an indicated area of the sample with sizes and numbers that are consistent with a distribution of particles. The goal of this work is to provide a path to creating a set of particle counting and sizing samples that can be used to establish the accuracy and precision of different measurements.

Control of Stress in Protected Silver Mirrors Prepared by Plasma Beam Sputtering

Dielectric protected silver mirrors were prepared by plasma beam sputtering. Stress in the coatings was modified by adjusting deposition parameters. Reductions in stress were achieved without impacting coating durability.