Strange DLC Properties

P

Portholes

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Dec 19, 2009
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So a short while back I snapped up a Ritter RSK mk.1 version of BM's Griptilian with a CPM-M4 blade with a DLC finish. For the most part I feel pretty positive about the DLC. It is very hard, shows no wear from use. It almost looks like a very dark blued finish, you can see the thin, uniform machine marks on the sides of the blade that give it a low gloss look, and the grain of the steel on the blade's spine. Very nice appearance, coupled with the slickness of the coating that probably could aid in cutting some materials. What really puzzles me is the fact that the steel still seems to corrode. A couple days ago I pulled the knife out of my drawer, and to my dismay, when I opened it I saw what appeared to be thin rust spots. It looked like typical rust on steel coupled with a tarnished look on the surrounding DLC. Since I didn't have my oil with me in the room, I scrubbed the affected area with zippo lighter fluid and a rifle cleaning brush. That cut through it pretty well and just left tiny tarnished looking spots, which the following day, I completely cleared off with some soy lube I use on my bicycle chain. Looks good as new now. What I want to know is how this happened. The DLC is so micro thin, I feel that if the corrosion came from it, there would be bare patches on the finish now. I'm thinking that perhaps whatever caused the corrosion, somehow penetrated the DLC coating and the corrosion built up above it. Does anyone know if I'm correct there? Is DLC not intended to inhibit rust?
 
yes, DLC is not a rust preventative. There are several types of PVD coatings, and their ability to inhibit corrosion varies. DLC helps, but not as much as one might expect a coating to do. It is only a few microns thick.
 
that's suprising. i've noticed that the coating on my user retained some "stains" that resisted a simple wiping but not a good washing with hot soapy water. this happened only when eating with the knife but i doubt it's oxydation as it doesn't stay after washing.

what could be the effect of oxydation in this case ? is it going to stay under the coating and develop or is it going to strip the coating from the inside ??

i don't really understand this but i'm curious.
 
it would seem that if he is able to wash out the stain through the coating, that the coating is porous, correct?

so i don't think that it would rust BENEATH it persay, but more, through it?
 
Interesting topic.
Before I purchased my new Spyderco Military S30V, I was considering the DLC-coated version. I emailed Spyderco, asking whether or not DLC aided in corrosion resistance, and they said that it does not. So I ended up buying the standard satin version.
Jim
 
it is hard and low friction. and the porous coatings will hold oil to aid in rust prevention & lubricity. probably mostly that it looks cool
 
James Y said:
I emailed Spyderco, asking whether or not DLC aided in corrosion resistance, and they said that it does not.
Click to expand...

Wow! I learn something new every day. I had no idea this coating did not help prevent rust. I prefer TiNitride coatings, anyway. Do THEY help resist rust? I thought they did...
 
"Looks cool" is the right answer, I think. A blade that doesn't cut well is not going to be magically restored with a coating. And the cutting edge is never coated. With the first hard use the coating will scratch and make the blade even more susceptible to rust. Get an uncoated blade and oil it is my $.02 advice.
 
If it is a true DLC the stuff you're cutting usually ends up on the blade. What appears to be scratches are actually the other material. The coating also prevents light reflection for all the tactical sandwich cutters out there. LOL
 
IMHO coatings have two purposes, corrosion control and looking cool.

The ones that look the coolest sometimes provide lower protection, with the possible exception of a properly applied industrial hard chrome.

The ones that look awful, such as the various epoxy-based coatings, will generally provide better protection, except for the edge and wherever they wear off.

Even the 'slickest' of the coatings offer MORE drag than 320 grit finished bare steel.
 
With most stainless steel knives I don't see rust proofing as too serious of an issue. The DLC is an improvement in my opinion because it is very durable, very slick, looks nice, and I suppose it has some tactical value. I just expected a rust resistant finish on my non-stainless blade.
 
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