The ability of laser nitriding to improve water droplet erosion resist
ance of Ti-6Al-4V alloy was studied. Using a CO2 continuous laser, a l
ayer of about 400 mu m thickness was nitrided and another layer of 400
-500 mu m was only heat affected. Electron microscopy observations sho
wed that the microstructure of nitrided lavers consists essentially of
TiN compounds which are embedded in Ti(alpha) matrix. Depending on th
e nitrogen concentrations within the feeding gas, the titanium nitride
s exhibited plate-like shape or dendritic morphologies. Below the nitr
ided layer a thickness of 50-100 mu m of samples underwent martensitic
structure which in its turn gives rise progressively to bimodal (alph
a + beta) base material. Laser nitriding increased microhardness from
370-400 to 650-800 kgf mm(-2), and enhanced erosion resistance signifi
cantly compared with untreated Ti-6Al-4V and hardened 12% Cr stainless
steel. The mechanism of material removal by erosion was changed from
work hardening and platelets detachment in untreated samples to brittl
e fracture by formation of large Bakes and spalling in nitrided layers
. Advanced stages of erosion are accompanied by the appearance of macr
ocracks often in the nitrided zone, but some of them propagate even in
to heat-affected area. The annealing at 650 and 700 degrees C of the l
aser-nitrided samples results in precipitation of beta phase rich in v
anadium.