Pure metals and metallic alloys

Why these are to first approximation a bad choice for stiff cog&gear style nanomachinery nanomachinery and gemstone-like compounds are the way to go instead:

• tendency to surface diffusion – some atoms do not stay where put at room temperature
• tendency to oxidation – for all but a few noble metals which are generally too rare for large scale structural purposes
• incompatibility with nanoscale passivation – making seamless covalent welding the default. (Related: Catenation)
• ...

Stoichometric intermetallic compounds sometimes fair somewhat better:
Despite still being electrically conductive and optically reflective in the human visible spectral range,
their bonds have a bore covalent character.
This makes them less ductile and more brittle, which is ...

• an undesirable property form macroscale parts with many high strain internal defects
• a desirable property for nanoscale parts with on average no defects

Some gemstones that contain non-mentals too are also still metallic in optical and conductivity and optical properties (similar to inter-metallic compounds). Examples include:

• pyrite FeS (iron sulfides)
• hematite and magnetide (iron oxides)
• titanium nitride and some other XY-type compounds where X=metal Y=nonmetal

Related

• Oxidation & Diffusion
• Metallic contact welding (related to Seamless covalent welding)

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• Macroscale style machinery at the nanoscale
• Atomic precision

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• Passivation (disambiguation)
• Nanoscale surface passivation (old: Surface passivation)
• Macroscale surface passivation
• Passivation layer mineral

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Complementary:

• Gem-gum: The defining traits of gem-gum-tec
• Gemstone-like compound