Emily Skrutskie’s 2018 Hullmetal Girls is a standalone SF novel.
Three centuries ago, a fleet set out from the Solar System determined to find a new world to replace the one they had squandered. Worlds sufficiently Earth-like to support a human population proved rare; to date the only one known is the previous, pre-trashed Earth. Generations after launch, the fleet has settled into a regimented seven-tiered society. Life in first tier, where the administrators live, is tolerable. Life in the impoverished seventh tier is short.
Having learned the hard way that heavy weapons are a poor way to maintain peace in an environment one hull-breach away from mass death, the ruling General Body has turned enforcement over to an elite force of cyborgs, the Scela. Conversion is dangerous, even for teens, and only highly motivated people volunteer to become Scela. Poverty-stricken Aisha Un-Haad, for example, is determined to earn enough to pay for her brother’s medical treatment and to keep her little sister out of the dye factory.
First-tier teen Key Tanaka could not tell you why she volunteered, although her reasons were likely not financial. Where her pre-conversion memories should be is a great blank. Whatever her reasons for submitting to the operation, they must have been compelling.