Why titanium "writes" on glass , meanwhile don't scratching it? It is possible to "write" on a piece of glass with a bar of titanium without scratching the glass.
I think the explanation is that molecular connections in glass are stronger than in titanium and little pieces of it detach when some force is applied. It seems that the same happens with the chalk and chalkboard.
But I really want a thorough explanation of the process from molecular physics perspective to understand how it works. So if possible show me every little nuance.
 A: The process is known as galling, or more popularly adhesive transfer or adhesive wear. It isn't unique to titanium-glass but is seen in many systems.
Exactly why it happens in the titanium glass system I don't know, though there are some obvious general points. Clean glass is an exceedingly high energy surface with a very high friction. When we meet glass in everyday life it has a surface layer of crud absorbed from its environment, but rubbing the glass with a hard material like a metal will displace this layer and expose clean glass with reactive hydroxyl groups on the surface. Titanium also has a highly reactive surface that is normally passivated by an oxide layer, and again rubbing the titanium on a hard surface (like glass) will displace the oxide layer and expose reactive titanium.
Exactly why titanium should react strongly with a glass surface I don't know. I would guess that there's nothing special about the titanium glass reaction, and it's just that titanium has just the right hardness - it's hard enough to scrape away the surface layers between the titanium and the glass but soft enough to gall onto the glass.
Titanium is not the only metal that will mark glass. In my youth I used to keep lead shot for fishing in a glass jar, and that would leave silver streaks on the jar that would not wash off. However titanium seems to be unusually effective at marking glass.
