Carbon is a very hard substance that is sometimes used as brushes for electric motors or generators. When it forms inside an engine cylinder, it happens because of the intense heat in an air cooled engine. And even more so in a two stroke, because of the added oil. It can build up so much, and when it does chip off, it usually just gets swept out the exhaust. But sometimes it can get caught in between the piston and cylinder wall, and put quite a nasty scratch the cylinder wall. And that equates to it being harder than steel.
The reason I ask is because of my previous experience. Gasoline has gone through many changes over the years and if I said "gasoline isn't what it used to be" I believe many people would agree with me. If we take a look at the history of gasoline we find that lead used to be added to gasoline to help boost octane and as a side benefit it would lubricate the valves. U.S. EPA laws mandated that lead be removed from gasoline due to the toxicity level increase in our atmosphere. This change happened some time in the mid 1970's and since then many more changes have taken place such as MTBE (Methyl Tertiary Butyl Ether), Ethanol, etc.
In years past, about 25% of a barrel of crude oil would be made into gasoline but now the percentage is over 60% (and I've heard up to 80% but can't verify) and that translates to more of the heavier & lighter fractions of crude oil become gasoline in which they must be formulated to stay mixed. As a result of these changes the carbon deposits we are familiar with have changed over the years. In the 1960's & 1970's the carbon deposit on intake valves was actually a softer deposit that could be easily removed during a valve job and some of the older mechanics on this forum probably remember what I'm referring to. I worked with a mechanic that performed "hillbilly tune ups" in which he would get the car out on the freeway and accelerate as hard as he could. Lots of black smoke poured out the exhaust in which some was due to a rich condition but some was also soft carbon literally being blown off the valves, piston crown and combustion chamber. An old excuse given to police officers as to why a driver was speeding or accelerating so hard was: "honest officer, I was just blowing the carbon out". The excuse rarely, if ever, worked and the driver got a much-deserved ticket however the practice was successful in removing soft carbon deposits. In recent years if you wanted to remove deposits from an intake valve manually you most likely use a bench grinder with a wire wheel and would have to push much harder to remove the deposit compared to decades ago.
Carbon itself has not changed but what has changed are the other by-products that bond carbon molecules together and this is the reason we no longer have soft carbon deposits. Another contributing factor is that engine operating temperatures have increased over the years and this bakes the carbon on even more. For instance, the temperature where the cooling fan turns on in some cases is 235 degrees.
I do agree that brushes for electric motors have carbon in them but they also need a bonding agent to hold the carbon in place. However if carbon got between the cylinder and piston, as you say, we would see just about every engine out there with scored pistons/cylinders. What I believe happens is the two stroke engines are run too lean and get too hot or lack of proper lubrication which causes metal transfer and since the cylinder wall (steel) is harder than the piston (aluminum) we see scored pistons. Carbon is definitely a hard substance but the bonding agent would break under the pressure of being squeezed between the piston and cylinder and if it didn't we would see many more engines with carbon scoring or scratches in the cylinder and/or piston - thankfully we don't have that problem.
Fortunately there are some good products on the market that will remove carbon deposits safely and quickly. Polyeither amine is one of the known best products to do the job safely and effectively by dissolving the bonding agent that holds carbon molecules together and allowing the carbon to exit the exhaust system in such small particles that they cannot be seen by the naked eye. Polyeither amine is found in products such as BG 44K and Chevron Techron. Of the two products mentioned, BG 44K has a greater concentration of Polyeither amine and will achieve the desired result sooner. No, I don't work for either company or even a chemical producing company, I've learned some things along the way and wanted to share them. Thanks for listening.