  Revealing phase transitions of solids through mechanical anomalies in the
friction of nanotips sliding on their surfaces is an unconventional and
instructive tool for continuous transitions, unexplored for first-order ones.
Owing to slow nucleation, first-order structural transformations generally do
not occur at the precise crossing of free energies, but hysteretically, near
the spinodal temperatures where, below and above the thermodynamic transition
temperature, one or the other metastable free energy branches terminates. The
spinodal transformation, a collective one-shot event with no heat capacity
anomaly, is easy to trigger by a weak external perturbations. Here we propose
that even the gossamer mechanical action of an AFM tip may locally act as a
surface trigger, narrowly preempting the spontaneous spinodal transformation,
and making it observable as a nanofrictional anomaly. Confirming this
expectation, the CCDW-NCCDW first-order transition of the important layer
compound 1T-TaS$_2$ is shown to provide a demonstration of this effect.
