Class 8 Notes: Surface Preparation

3.37 (Class8) Review C4 (Area Array) 1000-2000 I/O Cold welding • Aluminum is the second easiest metal to cold weld • Make near perfect welds in aluminum wire Adhesive Bonding • Unique in that it does not remove surface contamination • Type I Adhesive Bonding results from attractive force of wetted liquid at the interface (lowers the interfacial energy, van der Waals bonds, inherently weaker than primary bonding) o Example: adhesive to attach rear-view mirror to the windshield • Type II AB mechanical interlocking • Contact angle <30deg required for wetting (usually want something less than 10deg), young’s equation: metals are nice, strongest solder/braze joints Today Surface preparation • Rough surfaces wet more easily (surface area is greater, reduced surface energy) • Anodizing aluminum o Aluminum oxide growth makes a great mechanical interlocking surface, anodized coating o If want corrosion resistance, need to seal the surface, otherwise have channels down to the metal o Seal it by boiling it in hot water to grow oxide between cells, sometimes also use sulfuric acid • Phosphate steel (coke/pepsi are phosphoric acid on iron) o First used for lubrication, if want lubricant to adhere to the surface, want to have a porous surface, use calcium stearate (soap), have a thick layer of lubricant o Cold heading, start with rod, shear it, then phosphate it so that it can be worked multiple times, couldn’t do this unless surface was prepared • Titanium anodized, surgical instruments, can anodize with different voltages, change thickness, changes color o Anodized titanium jewelry • Chromates can also be used (but create environmental hazards in applying) Stefan equation • Time to squeeze a viscous liquid between plates • • • • • o J.J. Bikerman, The Science of Adhesive Joints, Acad Press, 1961 o None of the modern books on adhesives go through this Force*time product = see equation on board o Viscosity o Initial and final separations o Radius for a circular disc Looking at different forces, viscosities, radii, and separations o Water at given parameters 7.5ms o As the joint gets thinner, time gets longer o Also works in reverse, how long will it take the joint to separate as the viscous liquid flows with time o Start with something that forms quickly, then change it so that it lasts a long time (by changing the viscosity) o Viscosity is measured as a shear stress, how fast it moves at a given shear stress Gas approx 10^-3 Water approx 1 Molasses about 100, 1000 Solid approx 10^10 or 10^14 Highest viscosity ever measured, of Finnish coastline 10^22 o Can increase viscosity by an order of 10^10 10^10 sec is many years Filling bottles at rate of about 24/sec, also need to put the label on within a short period of time, so need an adhesive that doesn’t take long to form How to change a liquid into a solid o Freeze it o Dissolve something in a solvent and then let it evaporate (like licking a postage stamp) o Chemical reaction (epoxy with two elements that solidify based on a chemical reaction) Homework: think of all sorts of adhesive, ask self by what method this was done.