The flyback diode also known as snubber diode or freewheeling diode is connected in paralle to the inductive load either motor or relay.
So care must be taken that the reverse breakdown voltage of the diode should be greater than the voltage driving the motor.
The flyback diode protects the driving circuit from voltage spikes.
Any diode whether signal or power can be used as the flyback diode
jackmicro:
Any diode whether signal or power can be used as the flyback diode
Are you sure? If the inductance of the coil is large enough to store enough energy to drive a forward current through said diode, that is greater than it's ability to survive said current, well then, that might not be the proper selection, right?
I'm asking for a friend. 
jackmicro:
Any diode whether signal or power can be used as the flyback diode
That is almost true. 
The common 1N914/ 1N4148 has a nominal current rating of 300 mA and a voltage rating of 75 to 100 V. So if the stall current of the motor is less that 300 mA as I described in reply #1, then it is suitable for the job.