I also think it's neat. My read of it was that they were saying it like "hardening at the surface", e.g., https://www.sciencedirect.com/topics/materials-science

I also think it's neat. My read of it was that they were saying it like "hardening at the surface", e.g., https://www.sciencedirect.com/topics/materials-science/surface-hardening https://www.sciencedirect.com/topics/materials-science/surfa... Though technically oxidization can either strengthen or weaken a surface, so it's not a perfect phrasing, if so