After the remelting process, a remelted layer of
less than about 100 ktm formed on the surface of the
PTAW overlay (Fig. 3a). This layer presented very
high etching resistance. Although many kinds of
etchants were selected and an etching period of as
long as 6h was tried, we failed to etch out the
microstructure of the remelted layer. In Fig. 3a,
which is a transversal cross-section of the PTAW
overlay, the white band on the top surface of the
overlay is the remelted layer. After etching in 40%
HNO3 + 00% CH3COOH at 50 °C for 2-3 h, etch pits began to form on the remelted top surface. The
morphology of the etch pits after 5 h etching is
presented in Fig. 3b.