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.

Fiction

Live-action film series

Transformers' 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. 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

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

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 as superficial knowledge regarding Cybertronians when compared to the Spark. Megatron Rising - Part 2

IDW Generation 1 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 Dying of the Light Part 2: 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 issue 1