by WarAngel and Motoyasu

Titanium was catapulted to public awareness with the launching of NASA's first Space Shuttle which sported heat shield insulation tiles containing the rare metal as part of its reusable thermal protection system. Recently, Titanium has been rave by those desiring blades of superior strength, unmatched lightness and improved corrosion resistence. It is highly resistant to to acids, bases and solvents, and has a strength-to-weight ratio superior to steel.

Is Titanium the miracle metal that sword collectors and sword smiths have been waiting for?

Terminology

It is important to establish correct terminology before we proceed. The common misconception is that when people talk about "titanium", they're talking about pure elemental titanium. This is not the case, however. Unalloyed, pure titanium is very soft, with a Brinell hardness of ~100 (although the Rockwell "C" scale doesn't go down this far, if it did, the equivalent would be about 10-12 RC) and have a yield strength of ~150 MPa (compared with something like W1 steel which has a yield strength of ~1000-1500 MPa in hardened condition.) Based on this data, pure titanium is unsuitable for structural use.

In actuality, all structural "titanium" stock is actually a titanium alloy! The same goes for "aluminum", which is also an alloy, but we always just call it "aluminum", with the understanding that we are really talking about an alloy rather than a pure material!

Lightening the Load

Initially, knife makers manufactured folding knives sporting titanium liners, springs, and frames resulting in a lighter knife. The logical next step would be to make the blade in titanium as well right? Well, unfortunately, it is impossible for most titanium alloys to be hardened beyond the mid-40's on the Rockwell hardness "C" scale. For knife blades this becomes a major detriment as most knives need to be heat treated to high-50s, low 60's RC in order to hold a keen edge for cutting. Swords, having higher shock tolerance requirements than knives are ideally treated to low-to-mid 50's Rc.

Some knifemakers sought to get around this limitation and created a laminate consisting of outer plates of titanium alloy sandwiching a core of high quality cutlery steel, where the steel provided the cutting edge of the knife and the titanium providing support and flexibility as well as reducing weight (two-thirds of the blade is made of titanium alloy).

Other makers looked at different solutions: Benchmade - one of the leading manufacturers of commercial knives such as combat folders - produced a number of knives with titanium surface coatings and also the model 970ST Emerson combat knife sporting a blade made completely of titanium alloy and a special surface coating (note: due to termination of relations between Emerson and Benchmade, this model is no longer available).

Improving Edge Retention?

Titanium-coating is one of the latest attempts at incorporating the exotic metal into the knifemaking industry. Titanium-coating offers a less expensive alternative to blades completely made with titanium, as titanium costs approximately ten times more than steel. The process of titanium-coating involves micro-welding and alloying an extremely thin layer (anywhere from 0.25 to 12 microns) of titanium carbide (TiC), titanium nitride (TiN) or titanium-carbo-nitride (TiCN - the black-colored version of the coating) to softer base metals such as steel - a process which has already been an industrial and commercial process performed on anything from drill bits (the "gold" bits you can buy in the hardware store), dental instruments and medical devices to surgical implants and aerospace components. Other materials, usually tungsten and cobalt based, can also be coated onto softer base materials to increase their hardness and durability (the coating material that Benchmade ended up chosing for the 970ST was not a titanium coating but tungsten tantalum carbide).

TiC, TiN and other such coatings by themselves are extremely hard and brittle materials - the hardness has been rated at 2,000 kg per square millimeter, or an equivalent of over 85 points on the Rockwell "C" scale. However, when used as a micron-thin coating on a softer material, it forms a very hard and durable "case" around it. Such coatings have already been in wide use on steel knife blades (including some of Benchmade's other steel knives) and it was surmised that such a coating on soft titanium structural alloys would likewise increase its edge holding potential, allowing an all-titanium blade to become a reality.

Sounding strong on paper, coated blades were put to cutting tests. Initially it was surmised that the carbide coating was performing it's theoretical function and improving edge retention, but upon microscopic examination, it was discovered that due to its extreme hardness, the carbide/nitride layer was wearing away at the edge, and in so doing many micro-serrations were forming, which in turn were aiding the cutting ability of the blades.

On the cutting performance of their model 970ST, Benchmade states: "The model 970ST was built at the request of a military organization who needed a strong, lightweight, non magnetic, non corrosive, folding prybar with some limited cutting ability. In our efforts to increase the cutting ability we discovered the tungsten tantalum carbide that is deposited on the cutting edge of the blade. While the Sharptek carbide treatment dramatically increases the cutting potential of the titanium blades, it is not a magic coating and is not superior to steel in terms of edge retention. We do not recommend the 970ST as an every day working knife. You would be much better served with one of the steel blades for every day use. Where the titanium blade would really shine is as an emergency tool in a corrosive environment where it would require very little maintenance but would be there, and work, when you really need it. "

A Goliath of Strength? Or just a Goliath?

Titanium has been known for its high strength and low density. "The strength-to-weight ratios for titanium-based alloys are superior to almost all other metals," states Reactive Alloy Manufacturing of Sweet Home, Oregon.

Is Titanium stronger than steel, then? If the word "weight" is disregarded, many may conclude that titanium is indeed stronger due to its "superior strength-to-weight" ratio. However, except where weight is an issue such as in aircraft parts, steel is in most cases still a structurally superior material. In most applications, weight is somewhat less important than other factors. In knives, you want the material to be hardenable, so that it can hold an edge and cut. If the material you choose can't be hardened, then it doesn't matter what its other properties are like. For example, like titanium, aluminum also has a better strength-to-weight ratio than steel, but you certainly don't want a knife made out of aluminum! Aluminum and titanium are simply not the right materials for the purpose.

As mentioned above, where these materials are of use are in applications where weight is paramount and other properties are less important. But forgetting this for a moment, here's an example using some average values for titanium alloys and high carbon steels: A steel when hardened to RC 60 has tensile yield strength of 1,500 units/cross-sectional area (there are other more appropriate measures of strength for swords/knives, but we'll stick to tensile strength as an example), it also weighs 7,800 units per volume; an alpha-titanium alloy can be hardened to RC 40 and has yield strength of 850 units/cross-sectional-area and it weighs 4,500 units per volume. So, for a given volume of material at the hardnesses mentioned, the titanium is about half as strong and twice as light as the steel.

So you see that in order to have the same strength, the titanium bar must be made much larger in cross section than the steel, but because of its good strength-to-weight ratio, the much larger titanium bar would still lighter than the steel bar. Or, in other words, if the bars were exactly the same size, the titanium bar would be much weaker than the steel!! Ti alloys have superior strength/weight, but nobody said anything about strength/size!!

To illustrate graphically:

You also see from the example numbers that at knife hardnesses, the strength-to-weight ratio of the steel when hardened to RC 60 is actually a bit better than that of the Ti alloy! This is a fact often omitted by the comparisons: the strength-to-weight ratios are calculated based on materials at structural hardness (e.g. steel at RC 30-40 used as I-beams) rather than at knife hardness (RC 50-60) - strength of materials is greater at higher hardnesses, so in fact, when used for knives, steels can in fact have a similar strength/weight ratio to Ti alloys! If we redid this example with a lower carbon steel at RC 35-40, then the Ti alloy would look much better (approx 3 or 4 times better; aluminum is about 2 times better) in comparison.

What are the implications for cutlery? Well, it means that although Ti compares favorably to steel in an I-beam, this isn't true when you start hardening the steel for use in cutlery: let's take a sword for example. If you made a typical broadsword in steel, its blade would be between 1"-1.5" wide, 1/4"-3/8" thick and about 2-3 feet long. Now, if we ignore the fact that Ti alloys don't hold an edge for now, to make a Ti blade that is as strong as the steel, you'd need to make the cross section about twice the area, so you'd end up with something about 1/2-3/4" thick and 1-3/4"-2" wide! Such a blade would look more like a 2x4 than a sword!! On the other hand, the blade would be lighter than the equivalent steel blade.. Here's what I'm talking about.

(Okay, so I exaggerated the dimensions somewhat, but you get the idea!)

Look at this from another point of view: if you have two blades made out of the exact shape and size, one of tempered steel, and one of titanium, the steel blade would in this case be stronger and because of it higher hardness, would likely cut the titanium sword in half, but the titanium sword would weigh but a small fraction of the weight of the steel blade. Titanium's only advantage here is low weight. Essentially, it's a glorified version of aluminum.

Now there are Beta-titanium alloys available which have yield strengths that are as good or better than that of steel, but the majority still suffer from a lack of hardenability. There are a few of this class of alloys that can be hardened to RC 50, but these materials have less flexibility than steel, so use in sword-lengths is probably not feasible, and at the current time, the cost of such materials is so high as to be prohibitive.

Conclusion

In an industry where there is a constant thirst for swords that are "indestructable", it is easy to fall prey to marketing hype and other fads. What may seemingly work for other industries may not necessarily work for the knife industry (a knife is not an aircraft frame), and what works for knives does not necessarily work for swords, as swords have different physical requirements than knives.

Titanium alloys cannot be heat treated to a sufficient hardness to hold a cutting edge. Steel will not only out-cut titanium alloy blades but may indeed cut titanium alloy blades in two!

In addition, Titanium is far too cost-prohibitive in sword lengths and, save for TiN and TiC coatings (which are at higher hardness by virtue of molecular composition), titanium is generally too soft as swords. Even at high Rockwell hardnesses, titanium-coated edges do show signs of wear; TiN- or TiC-coated swords appear as black, gold, or a small variety of other colors that bear no resemblance to steel. And despite popular belief, when used as cutlery material, titanium is significantly weaker than steel in the same blade cross-sectional shape.

While there is no such thing as an indestructable steel sword, careful selection of metallurgies and attention to heat-treating, combined with proper sword geometries and designs can create some light weight and incredibly strong swords that may surpass the expectations of the custom-sword collector. For example, in European swords, a distal taper - where the thickness of the blade decreases towards the point - results in improved weight and balance. In other swords, the presence of a fuller or groove (but not "blood groove" as in marketing literature) lightens the sword considerably and maintains its stiffness (think of an I-beam).

Proper heat-treating, etc. and various processes can achieve favorable degrees of flexibility in European swords, as well as a good balance between hardness and toughness for Japanese style swords. The presence of various trace elements in certain metallurgies can yield unprecedented results, such as a high degree of shock tolerance or edge-holding capabilities, etc.

In short, steel - with all its features and possibilities - is still the best material for swords. A new "miracle metal" is not necessarily required, only improvements upon a metal that mankind has worked upon for centuries. Without proper heat treating, etc. and proper sword design, a sword will become inferior regardless of what superior metal is used for its blade.

WarAngel (Editor in Chief) and Motoyasu (Japanese Sword Editor) met for the first time in July of 1998 after having shared their sword interests online for a year. Discussions on the birth of Sword Forum Magazine and eating foul-tasting sandwiches comprised a four hour evening, during which they found the strange coincidence of sharing the same Chinese character for their first names.

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