Why use iridium instead of wider chrome for the pen nib

The first correction is that most modern pure iridium particles do not contain pure iridium

In order to increase the wear resistance of the nib, the small particles of osmium and iridium raw ore in the free state of nature were welded on the nib in the early stage of foreign countries.  But the limitation and output and quality, with the development of metallurgical technology, osmium, iridium and other precious metals to join other metals into metal particles, because the alloy contains iridium, so called "iridium grain".  However, with the increase in the demand for pens, the limited price of iridium, osmium and other precious metals is high, which makes modern metallurgical technology requires the use of little or no rare metal to manufacture iridium particles, so all the alloy ball welded in the nib, no matter whether it contains "iridium", are referred to as iridium particles.  

Second, hardness is an important factor in determining the properties of iridium particles, but not the hardness of individual particles. Iridium particles are alloys, and the formula is complex.  The hardness of iridium and chromium has been clearly explained by friends.  

The main technical specifications of iridium particles are: weldability, machinability, polishing ability, abrasion resistance and corrosion resistance.  Hardness is only one aspect of wear resistance, too high hardness and machinability form a contradiction.  (Iridium particles need to be slit, chamfered and rounded after welding, and poor machinability can cause iridium particles to shatter, known as "flaking (small pieces peel off)" and "iridium flakes (large pieces fall off).")  

As for wear resistance, iridium particles wear away during writing in two main ways: the friction between the iridium particles and the paper, and the corrosion friction between the iridium particles and the ink.  Increasing hardness can reduce abrasive wear between iridium particles and paper, but too high hardness will cause micro-brittle shedding on the scratch surface of the abrasive particles, which is faster than normal wear rate. Therefore, iridium particles should not be too hard, but also need to have some toughness.  

Chromium is indeed the most commonly used material for making iridium particles.  Iridium fountain pen are commonly used in domestic perfection of iridium 188 grain, for example, among them 10% 90% chromium, nickel, chromium main ingredients high hardness, corrosion resistance, also high melting point, but poor machinability, toughness is poor, so join the bondability metal nickel to improve performance, but nickel corrosion resistance is poorer, after nitriding process, and can improve the corrosion resistance of iridium 188 grain and machinability.  

The 601, which is more advanced than the 188, is chromium-free. Its main material is ruthenium, with tungsten and a small amount of cobalt.