Here is the non engineering version

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There is another type of anodizing that is more than 20 times harder than the one used on the iPhone and is equally thin. It is used on on high quality aluminum flashlights. This anodizing is so hard that a surefire or Phenix flashlight can cut trough a cheaper maglite flashlight when used as a saw.
In summary the hardness of anodizing is not dependent on thickness but on type. Apple can use the Type III and make the iPhone almost scratchproof. Im sure the increase in manufacturing cost will be less than what they are loosing from the returned scratched units.