What is the main difference between a steel knife and a ceramic knife?
Aside from the obvious difference in their material composition, the greatest contrast between a steel knife and a ceramic knife is how long each holds a sharp edge. This is primarily a function of the level of hardness.
The hardness of steel is calibrated on the Rockwell Scale, invented by Stanley Rockwell, a New England metallurgist, in 1919. The particular Rockwell Scale used for knives is the "C" scale. The test takes less than 10 seconds. A cone shaped diamond is pressed into the steel with about 330 pounds of force. Then they measure the depth of penetration and one Rockwell number is equal to penetration of 0.0002 millimeters. So a penetration of 0.12 mm would rate a 60 on the Rockwell Scale. Most steel knives rate between RC58 and RC63.
There is nothing like a Rockwell Scale for ceramics so we can compare materials using another scale, the MOHS (Material Order of Hardness Scale of mineral hardness). This scale was devised in 1812 by German mineralogist Friedrich Moh. On this scale talc powder rates a 1, a fingernail is 2.5, glass is a 5, quartz is a 7 and a diamond rates a 10. A white ceramic knife comes in at 8.5 and a black ceramic knife comes in at 9.
By comparison a steel knife comes in at 5.5. It's the hardness of the ceramic knife that's responsible for it's ability to hold an edge so much longer than a steel knife with a RC63. The tradeoff for the hardness of a ceramic knife is brittleness. The ceramic knife is more likely to chip or break when it encounters a bone or gets dropped on the floor.
With the sharpening equipment we use either material can be sharpened easily and quickly.