Yes, it is. You can grow, for example, nitrides by ALD. There are several literature works proposing specific recipes for such kind of films, and precursors are also available.
Yes- Laura is right. Even oxides like Al2O3 or HfO2 are pretty tough when grown by ALD, largely because the reactions concerned are quite exothermic
From the industrial point of view ALD films are "medium hard". PVD guys can make harder films (by ion bombarding) and DLC films are course much harder.
In addition to ALD oxides and nitrides, ALD carbides and carbide containing films are quite "hard".
Hardness is a difficult topic. There are many ways to measure it. At the end the tests tend to be practical comparisons like "womans hand bag", "drilling", "sliding".
My opininion is based of several industrial cases over the past decades. If the application simply requires "hard" coating like drill head, use DLC, CVD, PVD, if the are other requirements, then it may fall closer to the ALD.
We explored mechanical properties of alumina thin films deposited onto polymers. Since the nucleation behavior is different compared with deposition on an 'ideal' planar substrate like Si, the mechanical properties might be slightly different but the results are sumarized in the following publication:
http://onlinelibrary.wiley.com/doi/10.1002/cvde.201207042/abstract
We used nanoindentation to explore the mechanical properties of the ALD coatings.
We used to fabricate TiN/AlN multilayers which is proved to be an excellent couple for super-hardness film in PVD. But the result is frustrating, it only show a little harder than Al2O3 film, we think it might be the poor crystallinity and higher oxide concentration ruined the whole system. But ALD is unique for conformality, it might be very useful for special applications like filter anti-abrasion.
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