Self-destruct

Self-destruct or auto-destruct was an ability installed in many robotic mechanisms. It could be used as a weapon or a failsafe. For many cyborgs and robots, initiation of the self-destruct mechanism was akin to suicide.

Cybermen
The Cybermen under 's control could self-destruct in such a manner that they unleashed clouds of Cyber-pollen, a liquid metal that would turn organic life into more of their kind. When this substance was released in the skies of Earth, the Twelfth Doctor turned control of the Cyberman army over to Danny Pink, himself partially upgraded, who ordered all the Cybermen to fly into the clouds and conventionally self-destruct, dispersing the clouds and ending the invasion. The only Cyberman who was known to not follow this directive was Alistair Gordon Lethbridge-Stewart, (TV: Death in Heaven) though William Bishop claimed that the former Brigadier had later self-destructed of his own accord. (PROSE: An Ordinary Man)

Daleks
Daleks could also self-destruct, via self-destruct mechanisms, (PROSE: Prisoner of the Daleks) for various reasons. (TV: Remembrance of the Daleks, Dalek, Asylum of the Daleks, COMIC: Extermination of the Daleks)

During the Exxilon Gambit, a Dalek self-destructed for its failure following the escape of Jill Tarrant, killing itself while leaving its casing intact. (TV: Death to the Daleks) At the end of the Shoreditch Incident, the Supreme Dalek self-destructed when the Seventh Doctor revealed that Skaro was destroyed, reducing itself to "ashes" and dust". (TV: Remembrance of the Daleks) Another Dalek, who believed it was the last of its kind, asked Rose Tyler to order it to self-destruct after it gained emotions from her. Preferring death to a life with emotions, this Dalek ejected all the sense globes from its base unit to form a force field in which it was completely destroyed. (TV: Dalek) Unbeknownst to the Daleks themselves, their self-destruct would be triggered if they attempted to exterminate Davros. (PROSE: Father of the Daleks)

In the 22nd century, a group of Daleks on Earth initiated self-destruction after the Tenth Doctor used a proton cannon to make them intangible. (COMIC: Extermination of the Daleks)

When the Eleventh Doctor found himself in the Dalek Asylum, a damaged Dalek, deprived of its gunstick, attempted to use its self-destruct to kill him. Before it casing detonated, the Doctor managed to hack into its locomotion systems, sending the Dalek hurtling towards others of its kind, the resulting explosion destroying it and them. Despite bearing the casing utilized in the Last Great Time War, this Dalek's self-destruct lacked the forcefield and manifested itself merely as a large explosion, possibly due to its damaged state. (TV: Asylum of the Daleks)

Others
Kahler-Tek had a self-destruct mechanism, which he intended to use when he came to believe his only potential was as a war weapon, but the Eleventh Doctor convinced him otherwise. (TV: A Town Called Mercy)

Ships
In the 24th century of a parallel universe, Captain Jean-Luc Picard, formerly Locutus of Borg, and Lieutenant Commander Data were able to induce a Borg cube to self-destruct before it could begin the process of assimilation on Earth. (COMIC: Assimilation²)

The Prison Station had a self-destruct procedure which was operated from a self-destruct chamber. Ultimately, this self-destruct device was activated by Stien, destroying the station along with the Dalek Battlecruiser which had docked with it. (TV: Resurrection of the Daleks)

Following his defection to the Combined Galactic Resistance, Rusty transmitted a retreat signal to the Daleks, deceiving them into believing that the Aristotle had self-destructed. (TV: Into the Dalek)

During the 2021 Sontaran invasion of Earth, Dan Lewis attempted to operate controls on an Imperial Sontaran Time Carrier, hoping that it would not self-destruct. (TV: War of the Sontarans)

When P.R.O.B.E. discovered a Cyberon pod buried beneath Clapham Junction, Giles assumed offhand that the ship would have a self-destruct, something soon proven correct. Maxie Masters identified its trigger mechanism as a red wire on the ceiling of the pilot's cabin. He pulled it, and he and the rest of P.R.O.B.E. made their escape from the ship while its Cyberon pilot turned the whole of its attention to trying to halt the countdown. This was in vain and the ship exploded, partially collapsing the cavern in which it stood in the process. (HOMEVID: The Only Cure)

TARDISes
At least one battle TARDIS did have a working self-destruct, which the Sensorites activated telepathically, on Gallifrey's orders, as the Daleks had taken control of that TARDIS, among others, to seize the Time Lords' major power source, the Eye of Harmony. (AUDIO: Sphere of Influence)

The Eleventh Doctor twice pretended to have initiated, or be about to initiate, a "TARDIS self-destruct" to cow his enemies into a stalemate: first during the Ironside Incident opposite the Daleks, (TV: Victory of the Daleks) and again with the Van Baalen brothers. (TV: Journey to the Centre of the TARDIS) The Doctor's TARDIS did seem to possess an authentic self-destruct mechanism, however, as the Doctor knew to use it to allow them to "die" within the Dream Lord's highly realistic dream-world so that they could wake up back in the real world. (TV: Amy's Choice)

Self-destruct systems may also have been involved in smaller components of the timeships: Susan once suggested that the key of the Doctor's TARDIS forced the user to insert it precisely or the lock would self-destruct. (TV: "The Survivors")