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MIT 3 37 - Lecture Notes

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3.37 (Class 3) Review: The inherent strength of all bonds (even van der Waals) is extremely high Primary (1-3eV)  1,000,000 – 3,000,000 psi van der Waals (0.1-0.2eV)  100,000 – 200,000 psi graph of energy vs. distance graph of force vs. distance interatomic distance can get bulk compressibility, modulus, etc from these graphs Today: Question: What is the difference between “strength” and “toughness”? • Strength is how much force to break something • Toughness is how much energy something will absorb o Rubber is tough, glass is brittle • If a brittle material has no defects it can be very strong, small defect makes it very weak • Strength is a property of material in absence of defects Metallurgists know that defects control properties Two fundamental reasons why two materials don’t stick together when brought in close contact: • Contamination • Surface Roughness Science of welding is opposite the science of lubrication, tribology (from Gr. “surface”) Graphite as a lubricant: • Early 60’s space program used graphite as a lubricant, but up in space, in absence of oxygen graphite is no longer a lubricant • Graphite: atoms line up in planes, hexagonal close-packed structure • Oxygen in air lines up on basal planes of graphite, carbon-oxygen bond is a satisfied bond • NASA became interested in surfaces and cold welding • In lab carbon brushes in motor wear out very fast, change carbon brushes to aluminum disulfide • 1967 conference book, research on cold welding, what does it take in a rarified atmosphere• lubrication breaks down when you clean off the “contaminant”, in this case oxygen Can think of surface energy as surface tension • Surface energy o Units of J/m • Surface tension o Units of N/m^2 • Can think of as either as energy or force Cleaving a material in a vacuum, then if stick it back together (no contamination) will bond with much of its original strength • Time to form a monolayer • Langmuir on order of 10^-8 atm*sec • In an ultrahigh vacuum, this time is much longer, then the only issue is surface roughness • Experiments with flat foils in vacuum, 20-30% of base material’s strength Lubrication • Lubrication book: draw ligands (sp?), stearates (sp?), oleyates (sp?), linate (sp?), number of carbons in the chain • Polar on one side, non-polar on the other • Stick to the surface, other side has no interest in forming a bond • Lubrication is intentionally trying to contaminate the surface Why is Teflon such a good lubricant? • Covalently bonded structure, all bonds satisfied • Adhesives don’t stick to Teflon (even eggs ☺: proteins as adhesives) Question: How do they get the Teflon on pan? • Mechanical interlocking • abrade the surface to get a mechanical interlock • Farberware: o Spray on porous surface, then melt Teflon into it Eagle Scout Project to clean off peeling paint • Acetic acid to roughen the surface of galvanized steel • Then primer coat • Then paint • 15 years later paint is worn down, but not peeling Pepsi better than Coke as a cleaning solution Peter Holcroft chart in packet: one axis how to control contamination, other is how to control surface roughnessHandout: How much contact do I get when surfaces touch • Asperity contact • Squeeze it together: Can I get 100% contact? No • Compressive yield strength (sigma-yield) of the material when two small pieces pushed together • 3 times sigma-yield to indent a flat object with a punch since need to push aside material on sides (drawn as slip line flow field, volume is about 3 times) • See handouts for formulas o Macroscopic (apparent area) o Microscopic (true area of contact) o Only get about 30% contact area • Can’t solve surface roughness by just squeezing things together (even in absence of contamination) Why interested in cold welding? More of these types of welds, semiconductor industry • Gold wires in semiconductors, gold wire squeezed into surface, mushrooms on bottom • How many? o Say 100 bonds/chip (Pentium may have 300-400 bonds around perimeter, others not as complex) o Semiconductor industry approx $100 billion / year o Say average chip costs $10 o This results in approximately 10^12 bonds/year • Why gold? Use gold since it doesn’t contaminate with oxygen (doesn’t have a surface oxide problem o Still have water, oils in air o Can drive off this contamination with temperature • “Thermal-compression bonding” • Need shear to break down surface • 2nd best material for this is aluminum, which has a tenacious

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