March 15, 2012
by Ariel Adams
Please welcome Mr. İlker Özcan who will collaborate with aBlogtoRead.com on a few articles discussing materials and processes used in watch cases and other components. Ozcan is a professional Materials Engineer and owns a small R&D company (www.ndsgn.com) – and he loves mechanical watches and loves the exotic materials used in those watches. The purpose of this article series is to enlighten watch lovers about many aspects of watch materials so that everyone can make more educated decisions about the watches they are buying. Now on to Mr. Ozcan:
In this first article I have chosen to start with Titanium. Titanium has recently become very popular in high-end watches, and it is not without reason. For me, titanium is the best overall engineering metal ever! Is it however the best material for a watch case? Let’s take a look…
We engineers measure many properties to evaluate materials. Probably more types of properties than you have heard of if you are not a materials engineer. The most widely known of these properties are: strength, hardness, toughness, lightness and durability.
There are many alloys of titanium – like all engineering metals. The most widely known one is Ti-6Al-4V also known as Grade 5. It is the alloy that we will mostly denote when we say titanium in this article. It is used in very demanding applications such as aircraft components, missiles etc. Grade 2 titanium for instance is considered Commercial Purity and has inferior properties, however still a very good material.
Titanium has been around for the last two centuries, mostly in labs. In the last century methods to extract titanium in industrial amounts were developed. Until recently it has been used exclusively as an aerospace material, and it is slowly penetrating many markets, including watch industry. It is prohibitively expensive for many applications, with a price around 50 times of plain carbon steels. Still its price is much lower than precious metals.
Now strength and toughness is where Titanium excels. Durability is not a scientific term used, we can think of it as the life of a material under cyclic loads, like a turning shaft i.e. fatigue life. Titanium is also great in this aspect, but it is quite irrelevant in a watch case, as a case never fails under fatigue. However it can be important if it is used inside the movement or for a crown or pushers. Strength denotes how much load a material can bear before failing. Titanium has very high strength around 1000 MPa for Grade 5. Five times that of ordinary steels. Even pure titanium is quite strong with Grade 2 being around 350 MPa. However there are alloys of steel that have even more strength than titanium, such as the tool-grade steel that we mentioned above, or some special Aerospace steels (e.g. 300M) have around 2000 MPa strength. Toughness denotes how big an impact a material can absorb before breaking apart. Titanium is really great here, absorbs lots of energy around 20 joules on impact test. However once again there are steel alloys that are better up to 150-200J for Austenitic stainless steels (there are also steels with very poor toughness such as Martensitic steels). Toughness beyond a point is however not important for watch related applications. It is not likely that you will smash you watch to such a degree that its case will break. Well before that would happen the delicate movement would fail.
As you can see steel is very good, but where titanium actually pulls ahead is in terms of lightness (weight). It is circa half the weight of steel for the same volume with a density of 4.5 g/cc vs 7.8 for steels. So for a case that has the same strength, titanium case would be half the weight of a high strength steel case. And actually very high strength steels are almost never used in watches (they are used for aircraft components and tools for industry). Stainless steel is widely used, and it has lower strength (unless it is forged, which is again almost never used in watch industry). So a titanium case would be less than half the weight for the same strength.
Another area where titanium is excellent at is corrosion resistance (e.g. its resistance to rusting). It is so good that it is almost impossible to rust titanium. It is impervious to all acids but nitric acid. And nitric acid is something you do not come nearby in your daily life. It’s corrosion resistance is similar to that of platinum, and in terms of engineering metals only zirconium can beat titanium for corrosion resistance. This corrosion resistance is the key to the hypoallergenic properties of titanium. It is so inert due to the oxide layer that forms on its surface, that it does not react with human body – thus being the material of choice for many medical applications.
There are some areas that titanium is beaten by steels. For instance the stiffness, which denotes how much the material deflects under loads. Steel has higher stiffness, much higher. But I think in a watch case it is not an important property. Maybe, except for diving watches.
The hardness of titanium is lower than some steels, so it scratches easier than most steel. However, titanium is much much harder than gold, platinum and aluminum. Coatings can improve the hardness of titanium as well, and there are many coatings out there that we might cover in another article. Grade 5 titanium has around 35 Rockwell C (Rc) hardness. Steels have a range of hardness from low values all the way up to 55 Rc for hardened carbon steels to 65 Rc for tool steels and even higher for special steels used for knifes (e.g. D-2 tool steel, S30V knife steel). Gold, platinum and aluminum are so soft that they are usually not even shown on the same Hardness scale (there are many hardness scales).
Machining titanium is tricky. Welding, forging, casting and heat treating it are tough. So this trickiness adds to the high price of the titanium. Many properties that makes titanium such a good metal also make it hard to manufacture. With certain techniques that were developed in aerospace industry these hardships can be overcome. This is how we have nice titanium cases at affordable prices these days. However casting and welding processes are still very tricky, and we are unlikely to see a cast titanium case, or welded one at very affordable prices.
Titanium is such a good material that its usage is still increasing in fighter aircraft. For instance the F-22 utilizes more titanium than any western aircraft ever did. Its biggest competitor as a material is Carbon Fiber Reinforced Polymers (CFRP) and we will cover that in another article.
This article was written to generally inform people about titanium as a watch case material and has been obviously simplified. A larger discussion of titanium would expand on each of these topics and include many more. Though as watch lovers you hopefully have a new perspective on the popular metal. In conclusion, if you buy a titanium watch, especially one with a good coating, you will be happy with your decision.