The PTC is probably going to trip almost instantly once resistance is applied to the motor at those voltages, but the extra voltage should give a bump in current (same resistance + more voltage = higher current), so I think that yes the torque of the motor would increase at stall.
Just doing some quick math here:
The 393 motor is listed as having a stall current of 4.8A. This value was measured at 7.2V here:
We can then infer the effective resistance of the 393 motor when stalled using Ohm’s law (V= IR) rearranged in our case to be R = V/I, which works out to 1.5 Ohms.
Rearranging the equation again to I = V/R, we can calculate how much current would be pushed through a stalled 393 motor at 18v to be a whopping 12 Amps. Given that @jpearman measured the PTC to trip at 3A in about ~3 seconds, I wouldn’t expect it to last long at these kinds of currents. However, lets have some fun and say we’re really interested in how much torque we can get the 393 to produce in the time it takes for the PTC to trip.
Given the relationship between current and torque is basically linear, it’s easy to calculate how much torque the motor would be producing in our 18v/12A stall situation: 37.5 in/lbs
At least, for a small fraction of a second 