Cog or gear with a rusty PBR material with texture from freepbr. All cred to the amazing work of freepbr. HDRI are an old sunset HDRI i had laying around. Do not know where it come from. As i am an Design engineer at a big Swedish manufacturing company i see this kind of gear every day and thought ... Hey! how hard are they to draw in 3D. It was hard :-) this cog have a convex cog surface that turns in tree directions like the real cog in a gearbox or engine do. Not straight cogs like the once you see in cartoons. A real cog. I plan to do the the other cog that fits in this cog when i have the time. This to maybe do a set of cogs. Let me know if you find that ide with a set of cogs that you can match together to make a nice gearbox or something finds you appealing and i try to find the time to do it :-)
I also added the texture i use and the model as 3DS, FBX, OBJ for you that do not use Blender.