Quote:
Originally Posted by
ash471 
You raise an interesting point. Can a material with a hardens of 6 scratch a material with a hardness of 7? I had to pause and think about this. However, I think it can.
And you'd be wrong.
By definition, an item will NOT scratch something above it on the Mohs scale.
http://chemistry.about.com/od/geoche.../mohsscale.htmQuote:
A mineral with a given hardness rating will scratch other minerals of the same hardness and all samples with lower hardness ratings.
This is complicated slightly by the fact that hardness can vary with the direction that you are scratching. For example, a mineral might be a 6.5 in one direction, but a 7.0 in another direction. In addition, most materials have impurities, so hardness can vary throughout a sample.
Quote:
Originally Posted by
ash471 
It depends on several different factors like the geometry of the contact surfaces and the force applied. An easy example that shows you don't have to have a harder material to etch a surface is the fact that steel can be cut with a high pressure water stream. Water has a hardness of 0 on the mohs scale.
Wrong there, too.
First water does not have a Mohs hardness. Since it's impossible to 'scratch' water, Mohs hardness is meaningless for a liquid.
Second, water jet cutting does not work by 'scratching' the surface, so it is not really relevant to Mohs hardness. And for cutting steel, an abrasive is usually added, anyway.