From Teletraan I: The Transformers Wiki

Cybertonuron is a metal from the Headmasters portion of the Generation 1 continuity family.

Cybertonuron was an alloy developed by Vector Sigma, many times stronger than cybertonium.

Japanese name: Seibertonuron

[edit] Fiction

[edit] Headmasters animated series

While Vector Sigma was in the process of developing cybertonuron, Rumble was able to penetrate the computer’s chamber and learn of the alloy. Soundblaster reported back to Galvatron, who immediately arranged for a massive attack on Cybertron, intending to secure the metal upon its completion. Lost in the fantasy of the power that the metal would bring him, he imagined forging it into a massive sword, with which he could defeat even Fortress Maximus.

During the battle, however, Scorponok became fearful of the level of power Galvatron could attain with cybertonuron, and arranged to dispose of both him and the alloy in one swoop. Shortly thereafter, Vector Sigma perfected the cybertonuron formula, but bombs planted by Scorponok’s operatives destroyed Vector Sigma and devastated Cybertron. Galvatron vanished in the blast, and cybertonuron was lost.