For the most part, a variety of steel (such as mild and stainless) can be etched with no coating.
0.12 interval, 100mm/min workspeed, and 100% power. No type of coating, bare SS.
Most other metals will require a coating that actually gets vaporized (or to absorb enough energy to impart in the metal) such as anodized aluminum.
Black anodized aluminum, 400mm/min, 0-100 min-max, 140dpi, grayscale.
Basically you’ll just have to do some tests. Coating the metal with either black paint or molylube will give you the best results (however I’ve not done this, haven’t had the need yet). Stainless shouldn’t need it, aluminum definitely does.