Looks like I have to repeat myself, and explain more in depth.
HARDENING AND INVISIBLE HAKI ARE TWO SEPERATE APPLICATIONS OF COA.
THERE IS DIFFERENT LEVELS TO HARDENING, AND DIFFERENT LEVELS TO INVISIBLE HAKI.
FOR INVISIBLE HAKI YOU HAVE THREE LEVELS
Level 1 (Beginner) - Barrier - You perform a protective invisible barrier with your haki.
Level 2 (Intermediate) - Penetration - Your haki penetrates the persons body to allow for your attacks to hit internally.
Level 3 (Advanced) - Internal Destruction - Once you can penetrate a persons body, you can explode your haki for massive amounts of damage.
For Invisible Haki, Zoro is at level 2, penetration Haki.
For Hardening, we don't know much about it, except that the basic application is to harden ones body, or weapon. We know Katakuri had achieved at least level 2, which gave his armament haki a much more metallic sheen to it, and caused his armament to completely overpower Luffy's armament Haki. However, there wasn't really given any explanation behind the principle of it at the time.
We don't know what the advanced verion of Armament Haki is yet, but I have a suspicion that it is going to be related to Black Blades.