It's my understanding that HSS for tools is actually less hard than what you see knives at. For example, in my machine tooling course, we're using M2 to turn aluminum, and this is at 58Rc. Benchmade however hardens their M2 steeled knives to 60-62.
Carbide tools/bits are significantly harder, but notice how you almost never see an actual carbide tool. It's always the
tip that's carbide, and it's affixed to a softer, more flexible base. One that won't shatter under tool pressure.
This is because carbides are much more brittle and inflexible, something that doesn't jive with knives. A fully carbide knife would snap or shatter doing things we wouldn't think twice about doing with a normal knife.
Cobalt is a different beast entirely than steels. There are a couple cobalt alloys that are used in knives (mostly high-end customs) such as Stellite 6K or Talonite. These are also more brittle than steel, and have significantly less impact resistance. What they do have instead is slickness. The friction between cobalt and whatever you're cutting is much less than steel and whatever you're cutting. I read somewhere that these cobalts were originally developed for jet engines, because they have less friction with the air than steels or titaniums. It's the slickness that makes them better (in some applications) than steel (plus the fact they don't rust is appealing).
It's important to note that while cobalt alloys like Talonite are very comparable to high-end knife steels like CMPS30V in terms of edge retention, that they are actually
softer, not harder. The lower friction and abrasiveness of the metal compared to steel is what makes it a viable alternative and gives it comparable wear characteristics.
Steels themselves get very brittle above 62Rc, which is why you'll almost never see a knife that hard. Rarely, you'll see a forged knife advertized at up to 65Rc, but this is differentially hardened, meaning only the edge is that hard. The rest of the knife is much softer. Otherwise, the knife would break or shatter without much stress. You'll only see a couple steels (M2, D2, A2 - all tool steels) taken above 60Rc, with 62Rc as the upper limit. The upper limit for modern uber-steels (154CM, ATS-34, VG-10, S30V) is 59-60Rc.
All this technical mumbo-jumbo aside, knives are entirely different than machining tools. Most machining tools in HSS or carbide will easily bore through or turn aluminum without ever needing sharpening, or for a long, long time. However, this same tool couldn't cut skin or slice paper worth a rat's ass. On the flip side, trying to machine aluminum (like chamfer it or something) with a knife would destroy the knife's edge almost instantly. Even the "sharpened prybar" kind of knives like Striders can't handle that kind of thing. They're two completely different applications, and require two completely different kinds of tools. Think about this: you can easilly drill aluminum with a 118 degree carbide drill. Would you try drilling stainless steel with a 118 degree drill? Heeeeellllll no! You'd be resharpening every hundered thousandths. Different applications require different tools.
The point of all this is that "ease of sharpening" really isn't the key factor. In almost every case you'll see that a high-end knife will be hardened to the point where it's as hard (and thus wear-resistant) as possible, but not so brittle that it chips away. One notable exception is Chris Reeve's knives, as he hardens the S30V on his knives to "only" 58-59Rc, but this is almost more a property of that particular steel; S30V's structure is exceptionally wear resistant compared to many other high-end steels and is softer perhaps for "ease of sharpening" and maybe to reduce the possiblility of chipping, yet still hard enough to resist rolling of the edge, a problem if a steel is too soft.
Manufacturers give their customers what they want, and right now, what we want in our high-end knives is blades that last a long time without sharpening. If this was possible with steel blades at 68Rc, trust me, that's what we'd see. But knives with steel blades
that hard simply
don't work. The edge would chip, the tip would break off when you set it on the table, and the blade would shatter when you dropped it on the ground.
That's with current technology anyway. In the future, I'm sure we'll see even better stuff.
BTW, I might be talking out of my @$$ on a couple points, since I'm mostly repeating what I've heard/read/learned, so if I'm wrong, please correct me.
