Page 416 - Handbook of Materials Failure Analysis

414 CHAPTER 16 Degradation of protective PVD coatings

Columnar grains
Transition structure
consisting of densely
packed fibrous grains
Porous structure
consisting of tapered Recrystallized
grain structure
crystallites separated
by voids
ZONE 3
ZONE 2

1.0
ZONE 1 0.9
ZONE T
0.8
0.7
30
0.6
0.5 Substrate
20
0.4
temperature (T/T M )
Argon 0.3
pressure 10 0.2
(m torr) 0.1
1
Influence of substrate temperature
and argon pressure on microstructure
of sputtered films
FIGURE 16.2
Thornton’s model of thin coating structure.

In the Thornton’s model exists four zones, in which different structures are
formed. At low substrate temperature below 0.1 T m (zone 1), the nucleation density
is high and the coating growth proceeds in vertical direction. In this zone, fiber-like
porous structure develops. The diameter of fibers is in the range between 1 and
10 nm; with increasing T s /T m , the fiber diameter increases. At higher temperature
(0.2<T s /T m <0.4) exists a zone T, in which coating has an inhomogeneous structure
along its thickness. At the substrate, fine crystallites grow, and next cone-shaped
columnar grains, while in the upper part of thick coating are columnar grains. Further
increase of substrate temperature (T s /T m >0.4), causes the coating to have a homo-
geneous structure that is composed of grains in the form of columns, whose height is
as same as the coating thickness. The grain boundaries are nearly perpendicular to the
coating plane. In zone 3, the structure is characterized by equiaxed three-dimensional
grains.
Most PVD coatings are deposited under conditions that allow obtaining coatings
with columnar grains oriented approximately parallel to the direction of growth.
However, not all columnar grains extend the coating thickness, indicating that some
grains are nucleated during the coating growth. This indicates that structures
described in the Thornton’s model are only present in an ideal coating. In real