Armor-plating

Though the spark is the true life of Transformers, 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 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 more foreign metals for increased durability.

“	No way, being a star rocks! If the titanium-alloy plating fits, wear it! And...I make it look good.	”
—Bumblebee wisecracking again, "Home Is Where the Spark Is"

Fiction

Live-action film series

Cybertronians' armor was made up of the elemental metal Transformium. The armor proved capable of taking hits from 30-mm Avenger gatling gun rounds and Maverick missiles but was vulnerable to the high heat of sabot rounds. Protoform endoskeletons are durable enough to survive atmospheric reentry in transition forms. When Scorponok's tail was severed in battle it was taken by the military for research. They discovered the severed tail's exterior was composed of a "self-regenerating molecular armor". Unfortunately, it could be breached by the gradual wear-down of human weapons fire and more easily by Cybertronian-based weaponry; strong blows to particular areas such as the plating protecting the Transformer's core or that of the joints were demonstrated to be especially vulnerable. Megatron's armor proved capable of surviving 2000-pound Jdam strikes while the other Decepticons were unable to. Transformers Revenge of the Fallen Dark of the Moon Corrupt CIA black-ops unit Cemetery Wind melted down the armor-plating of dead Transformers, more precisely Autobots to mass-produce lifeless remote-controlled drones on behalf of KSI. The armor-plating of the drones had apparently even greater resiliency than before; this may have been due to the drones' prominent ability to manipulate their bodies on a molecular level. Age of Extinction Optimus Prime was able to survive uncontrolled reentry to Cybertron's surface in his robot mode. The Last Knight

Animated

All Cybertronians had armor-plating formed from protomatter. The armor of Decepticons was normally more durable than that of Autobots. AllSpark Almanac II Armor-plating at one of its highest densities was strong 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 own armor after he was rebuilt proved capable of withstanding even the AllSpark-contained explosion of one Starscream-controlled Omega Supreme clone. When the Autobots stranded on Earth quickly became famous, Bumblebee said that if the armor-plating could fit they should just equip themselves with it. Home Is Where the Spark Is

Of course, it had its own limits with Optimus Prime's plating noticeably cracking after falling a great height, Transform and Roll Out Prometheus Black developing a hyper-corrosive acid from a sample of Bumblebee's mech fluid that proved able to melt through Autobot armor Total Meltdown Survival of the Fittest and Starscream's armor severely cracking to the point of bisection after receiving a point-blank blast from Megatron's fusion cannon. Only owing to his AllSpark fragment he survived. Endgame, Part I Despite the armor's importance, Megatron ultimately dismissed this as merely superficial knowledge regarding Cybertronians when compared to things such as 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 the whole galaxy should be armor-plated. The Dying of the Light Part 2: The Sun in Flight The armor of Cybertronians and some of their descendants were completely immune to human-produced 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 However, repeated and concentrated fire from multiple Neutralizers proved able to penetrate Cybertronian armor. Shining Armor issue 1