Armor-plating

Though the spark is the life of the Transformer, they are still made up of circuits and processors that are vulnerable. And years of warfare have left those circuits and processors even more vulnerable. To protect them, armor-plating is placed over their structures to preserve their vital functions. Though the armor-plating is not invulnerable, and can succumb with enough weapons fire, the armor is still more resilient than most things on Earth. It is typically formed from the same living metal as the rest of a Transformer's body, but has been known to be reinforced with other metals for increased durability.

Fiction

Live-action film series

Cybertronians' armor was made up of Transformium. The armor proved capable of taking hits from 30 mm rounds from Avenger gatling guns and maverick missiles but was vulnerable to the high heat of sabot rounds. Protoform endoskeleton is durable enough to survive reentry in transition form. When Scorponok's tail was severed in battle, it was taken by the military for research. It was discovered that the exterior of the severed tail was composed of a kind of "self-regenerating molecular armor". Transformers Megatron's armor proved capable of surviving 2000 pound Jdam strike while the other Decepticons were unable to. Transformers: Revenge of the Fallen Optimus Prime was able to survive uncontrolled reentry to Cybertron's surface in robot mode. Transformers: The Last Knight

IDW movie comics

Jetfire survived a reentry impact to Earth surface in robot mode, but was knocked into stasis lock by the shock. Tales of the Fallen issue 3 Shockwave survived the crash that caused the Tunguska event, but was knocked into stasis lock as well. Rising Storm issue 2 Blitzwing survived the detonation of an ICBM with a nuclear warhead near him that destroyed a Decepticon base, though he used Dead End's corpse as makeshift cover. From Cybertron with Love Starscream survived with some injuries the complete destruction of Nemesis while he was inside it. The explosion also seemingly turned the Trypticon city it crashed on into a smoldering wreck. The Reign of Starscream issue 5

Animated

All Cybertronians had armor plating formed from protomatter. The armor of the Decepticons was more durable than that of the Autobots. AllSpark Almanac II The armor plating was durable enough to survive the heat of atmospheric reentry largely intact, Transform and Roll Out and according to Megatron was capable of handling far more than the harshest of Earth environments. Megatron Rising - Part 1 Blurr's armor was capable of protecting his Spark from compression forces that crushed his body into a small cube. TransWarped The Return of Blurr Megatron's armor later proved durable enough to withstand being at ground zero of an exploding Starscream Supreme. Endgame, Part II

It had its limits however with Optimus Prime's plating noticeably cracking after falling a great height, Transform and Roll Out and Prometheus Black developing a hyper-corrosive acid that proved able to melt through Autobot armor. Total Meltdown Starscream's armor was severely cracked to the point of bisection after receiving a point blank blast from Megatron's fusion cannon. Owing to his AllSpark fragment however he survived. Endgame, Part I Despite the importance of the armor however, Megatron dismissed it as superficial knowledge regarding Cybertronians when compared to the spark. Megatron Rising - Part 2

2005 IDW continuity

Armor-plating was formed from sentio metallico. Silent Light When Megatron began his crusade against organic lifeforms, he did so under the belief that the whole galaxy should be armor-plated. The Sun in Flight The armor of both Cybertronians and Camiens was completely immune to human bullets. White Light Concorde Hymn Optimus Prime's armor proved capable of taking the full blast of a Space Knight Neutralizer though he was blown back the impact. O Ship of State Concentrated fire from multiple Neutralizers however proved able to penetrate Cybertronian armor. Shining Armor #1