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Christopher Williams

Publications and source records attributed to Christopher Williams.

5 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2024

Plasma Rapid Oxidation Technique for Extending Component Tenability (PROTECT)

Additive manufacturing (AM) is increasingly utilized in aerospace applications. However, post-processing is required for many applications, so intricate designs are limited. Plasma electrolytic oxidation (PEO) was investigated as a post-processing treatment on Ti-6-4 (or Ti-6Al-4V: Alpha-beta titanium alloy with a high specific strength and excellent corrosion resistance)additively manufactured specimens. The “Keronite Black” PEO treatment reduced the surface roughness (Ra) of test specimens by approximately 0.5 μm. This reduction in Ra compared to the as-printed sample was hypothesized to benefit corrosion and fatigue properties because increased Ra is known to negatively impact corrosion and fatigue characteristics. Up to a 101% improvement in the corrosion potential (Ɛcorr) was observed. Although fatigue characteristics varied by PEO parameters, there was no significant fatigue debit due to the PEO treatment. Additionally, no effect to adhesion characteristics was observed due to PEO treatment. It is recommended that additional studies investigate the ability of PEO treatments to improve the corrosion, fatigue, chemical compatibility, radiation hardening, particle shedding characteristics, and flammability/oxidizer compatibility of aerospace additively manufactured components.

Jennifer Williams↗
NASA NTRS2022

A Process-Model Perspective on Recent Changes in the Carbon Cycle of North America

Continental North America has been found to be a carbon (C) sink over recent decades by multiple studies employing a variety of estimation approaches. However, several key questions and uncertainties remain with these assessments. Here we used results from an ensemble of 19 state-of-the-art dynamic global vegetation models from the TRENDYv9 project to improve these estimates and study the drivers of its interannual variability. Our results show that North America has been a C sink with a magnitude of 0.37 ± 0.38 (mean and one standard deviation) PgC year−1 for the period 2000–2019 (0.31 and 0.44 PgC year−1 in each decade); split into 0.18 ± 0.12 PgC year−1 in Canada (0.15 and 0.20), 0.16 ± 0.17 in the United States (0.14 and 0.17), 0.02 ± 0.05 PgC year−1 in Mexico (0.02 and 0.02) and 0.01 ± 0.02 in Central America and the Caribbean (0.01 and 0.01). About 57% of the new C assimilated by terrestrial ecosystems is allocated into vegetation, 30% into soils, and 13% into litter. Losses of C due to fire account for 41% of the interannual variability of the mean net biome productivity for all North America in the model ensemble. Finally, we show that drought years (e.g., 2002) have the potential to shift the region to a small net C source in the simulations (−0.02 ± 0.46 PgC year−1). Our results highlight the importance of identifying the major drivers of the interannual variability of the continental-scale land C cycle along with the spatial distribution of local sink-source dynamics.

Guillermo Murray-Tortarolo↗