Hardness is ‘hard’ to characterize in any material. The different quantities that go into hardness vary not only with temperature and pressure. They also vary with the history and macroscopic structure of the material. Materials have some hysteresis in their resistance to stress. A hard material can be also ‘brittle’, so it is difficult to determine how the materials will respond to impact. However, general trends can be demonstrated.
However, the general trends in hardness are available. It is seen that
Water ice comes in various forms, such as crystalline and amorphous. These forms get harder as they cool down. The ice on the comet right now should be something like 30 K. So the large crystals should be very hard. I am not sure how this relates to the electric comet ‘theory’.
http://link.springer.com/article/10.1023/A:1021134128038#page-2
The mechanical properties of ice and snow are reviewed. The tensile strength of ice varies from 0.7 to 3.1 MPa and the compressive strength of ice varies from 5-25 MPA over the temperature range -10C to -20 C.’
The following shows that the hardness of an ice crystal increases dramatically as the temperature decreases. A comparison isn’t made with other materials. However, it is clear that the colder water ice gets, the harder it is.
http://www.minsocam.org/ammin/AM43/AM43_48.pdf
'Frc. 1. Brinell hardness of sirlgle ice crystals vs. temperatule. Twelve second application of load. (Above) Normal to c-axis. (Below) Parallel to c-axis.'
The graphs show that any piece of ice will get much harder as it cools down. I don't know what Brinell hardness really is. However, I note that compressive and tensile strength increases in ice as the temperature goes down.
Like most materials, water ice has hysteresis. One of the things that scientists hope to find out through Rossetta is what form different materials, including water ice, take. So hardness is an unknown.
Apparently scientists underestimated the hardness of the surface when they designed those harpoons. However, I don't really know what difference it makes to the comet. The ice will start to sublimate as it gets warmer.