Originally Posted by
Mark Kelly
The information regarding possible surface stresses came from my interpretation of several articles on shot peening of Ti alloys, some of which showed measureable reductions in fatigue strength of anodised Ti alloy. See for instance
this one, esp fig 2.
I don't understand where your 30 nm figure comes from, I thought the colour effect needed a quarter wavelength to occur, that's a minimal thickness of 80 nm.
I know how hard it is to get anything including paint to stick to Ti. I understood that an agent which modified the surface activity (eg silane) was the best bet, on the grounds that etching Ti wasn't easy.
My apologies. That should have read 300nm. I didn't notice. Whoops! Above that, you're looking at heavily crystallised coating intended for gross corrorion resistance, or wear resistance (although on titanium, any very hard layer is really a bad idea - the substrate will never be hard enough to properly support it). Most oxide layers on Ti are about 3nm per volt and decorative anodizing doesn't go much above 20 v...
Anyways, a third of a micron isn't a sizeable enough percentage of sectional thickness to dominate the mechanical properties. More than two microns on the other hand is, especially in thin sections or rods, and especially when the crystals are coarse as they are in big, thick coats. The oxide films grown on those samples in the paper you've shown are heavy, heavy layers.
As for the surface modification, anodizing itself serves as a good etch. But it needs to be pretty thick. See where I'm going?