September_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 0 2 7 MRSA bacteria, one with 2.5 wt% Cu (sample#2)andtheother,sample#2with 12 wt% Cr added (sample #2+Cr), both after heat treatment to form nanome- ter-size copper precipitates. Incubation at 37°C for 30 minutes is sufficient to in- activate 96% and 90%, respectively, on average for all MRSA bacteria exposed to these steel samples (Fig. 2). The authors were curious about what might happen when Cu atoms on surfaces of these precipitates come into direct contact with microbes or active molecules in the vicinity. To ex- plore this idea, molecular dynam- ics simulation using a large-scale atomic/molecular massively parallel simulator was conducted on the inter- action between a bcc Cu (111) stepped surface and 150 hydrogen peroxide (H 2 O 2 ) molecules at room temperature. H 2 O 2 is produced by human cells and many bacteria species. Figure 3 shows the evolution of the number of H 2 O 2 , hydroxyl radicals, mo- lecular oxygen, and water molecules as a function of time. The fragmentation of H 2 O 2 upon contact with Cu is almost im- mediate, producing hydroxyl radicals, oxygen, and water. These changes are accompanied by the oxidation of Cu to form Cu + , strongly suggestive of Fenton- like chemical reactions. The process ap- pears to reach steady state after about 60 ps, which indicates the occurrence of a back-reaction, i.e., formation of H 2 O 2 from initial reaction products. Both H 2 O 2 and hydroxyl radicals are reactive oxygen species that play an important role in antimicrobial action. IS MORE COPPER BETTER? Cu is known to cause “hot short- ness” in steels—embrittlement of steel due to formation of Cu-rich liquid and its penetration into grain boundaries during hot rolling or forging. Excessive Cu amplifies this problem. Also, how adding Cu affects the corrosion perfor- mance of stainless steels must also be considered. When Cu is in solid solu- tion, it has no detrimental effect on the stability of passivation oxide films of ferritic or austenitic stainless steels [15] . However, when present as nano- meter-size precipitates after aging, Cu Fig. 1 — 3D copper atommap showing Cu precipitates in one of the authors’ Cu-precipitation- strengthened alloys containing 2.5 wt%Cu. Sample was solution-treated at 950°C, followed by water quenching and aging at 500°C for 2 hours. Average precipitate radius is 2.0 ± 0.6 nm; precipitate number density is 68.7 ± 3.2 × 10 22 /m 3 . Each red dot represents one Cu atom; Cu precipitates appear as regions with high density of red dots. Only Cu atoms are shown for clarity. Fig. 2 — Log 10 (reduction factor) of MRSA bacteria remaining after 30 minutes exposure at 37°C to pure Cu and two polished steel samples, one with 2.5 wt%Cu (sample #2) and the other, sample #2 with 12 wt%Cr added (sample #2+Cr), both after heat treatment to form nanometer- size copper precipitates. Median reduction percentage is noted next to each box. Fig. 3 — Evolution of the number of H 2 O 2 , hydroxyl radicals, oxygen, and water molecules as a function of time at room temperature.