Page 417 - Handbook of Materials Failure Analysis

2 Influence of Deposition Parameters on PVD Coating Properties 415

coatings are present some impurities and defects that limit the grains growth. The
most common impurity is oxygen, while micro-droplets are the most common
defects, particularly in coatings deposited by arc evaporation and by magnetron reac-
tive sputtering. The number and size of micro-droplets depend on deposition param-
eters like bias voltage [17], substrate temperature [39], argon and nitrogen flow rate
[11], and gas pressure [40].
The investigations of an influence of negative bias voltage U B on mechanical and
structural properties of produced coatings showed a great importance of a deposition
method. In case of an arc evaporation method, a decrease of bias voltage to 100 V
caused the decrease of grain size in a CrN coating, while in the case of reactive mag-
netron sputtering method, a slight increase of grain size was noted [17,24]. Further
decrease of bias voltage caused a slight increase of grain size in case of CrN arc-
evaporated coating, while in the CrN magnetron-sputtered coating—a significant
decrease of a grain size.
A decrease of bias voltage caused an increase of lattice parameter [8,25]. In the
case of arc evaporation method, the lattice parameter in TiAlN coatings increased
from 0.4173 to 0.4231 nm (1.4%) with a decrease of negative bias voltage from
40 to 140 V [8], while in the case of CrN x coatings deposited by reactive unbal-
anced magnetron sputtering method, the lattice parameter increased from 0.4117 to
0.4225 nm (2.6%) with a decrease of negative bias voltage from 0 to 200 V [25].
The structure of CrN x coatings changed from open dentritic film grown at low bias
voltages to dense structures at higher ones. Moreover, in arc-evaporated coatings,
with decreasing negative bias voltage decrease a coating roughness, a size of defects
and a defect density [17].
Bias voltage has an influence also on mass density [25,27], residual stresses
[8,24,27,41,42], adhesion [17,42], and hardness [8,25,29,41,42] of coating. In non-
hydrogenated a-C films deposited by DC magnetron sputtering, a decrease of bias
voltage caused an increase of mass density up to some saturation level that reached
at approximately 100 V [27]. This saturation level depends on argon pressure; with
increasing argon pressure the saturation level decreases and, as a consequence, the
mass density also decreases.
The decrease of bias voltage caused a decrease of intrinsic stresses in
magnetron-sputtered coatings (CrN and TiN) [24,41]. Investigations of an influence
of bias voltage decrease on intrinsic stresses in arc-evaporated coatings gave two
opposite results. According to Ahlgren and Blomqvist [8], in TiAlN coating the
intrinsic stresses decreased from 1.7 GPa for U B ¼ 40 V to 5 GPa for
U B ¼ 200 V (increased the compressive stress), while according to Grant et al.
[42] and Karlsson et al. [43], the decrease of substrate bias voltage caused an
increase of intrinsic stresses in CrN coatings (from 6 GPa for U B ¼ 50 V to
2 GPa for U B ¼ 200 V) and in TiN and TiC x N 1 x coatings (for TiN coating, stres-
ses increase from 6 GPa for U B ¼ 50 V to 1 GPa for U B ¼ 700 V); stresses
were changed from compressive to tensile stresses. However, in case of TiC x N 1 x
coatings, first intrinsic stresses decreased, but later they increased. The bias voltage
at which intrinsic stresses began to increase depending on the carbon content in a