Surface & Coatings Technology 200 (2006) 3856 – 3860
www.elsevier.com/locate/surfcoat
Scratch resistance of CrN coatings on nitrided steel
R. Hoya, J.-D. Kammingaa, G.C.A.M. Janssenb,*
a
Netherlands Institute for Metals Research, Rotterdamseweg 137, 2628 AL Delft, Netherlands
Delft University of Technology, Department of Materials Science and Engineering, Rotterdamseweg 137, 2628 AL Delft, Netherlands
b
Received 9 September 2004; accepted in revised form 4 November 2004
Available online 16 December 2004
Abstract
Duplex coatings consisting of a nitrided steel covered by a hard CrN coating have been produced in an industrial Hauzer HC 750 PVD
machine. Hot work tool steel was used as substrate and was plasma nitrided up to a depth of 65 Am. The nitriding parameters were chosen
such that nitriding was achieved without formation of iron nitrides at the surface. Hard CrN0.94 coatings with thickness up to 10 Am were
deposited on all nitrided specimens in the same PVD machine. A set of 42 specimens was obtained with independent variation of nitriding
depth and coating thicknesses.
The samples were mechanically characterized by scratch testing and nanoindentation. The hardness of the steel substrates measured by
nanoindentation increased from 10 to 16 GPa due to nitriding. Increasing the nitriding depth of the duplex coatings results in a significant
increase of the scratch hardness (up to two to three times). However, increasing the coating thickness up to 10 Am hardly influences the
scratch hardness.
The relative importance of coating thickness and nitriding depth for scratch resistance and load carrying capacity of the substrate is
discussed.
D 2004 Elsevier B.V. All rights reserved.
PACS: 46.55.+d; 62.20.Qp; 68.60.Bs; 81.15.Cd
Keywords: [B] Scratch test; [C] Reactive sputtering; [C] Nitriding; [D] Chromium nitride
1. Introduction
Ceramic coatings of high hardness and excellent wear
resistance can be produced by physical vapour deposition
(PVD). Currently, such coatings are successfully used for
wear protection in various engineering applications.
An important fail mechanism for hard coatings is
eggshell-like failure. The hard but brittle coating fails as
the substrate is deformed. Duplex coatings consisting of a
nitrided layer plus a hard coating are a solution for this fail
mechanism. The nitrided zone at the steel surface supports
the hard coating [1].
Nitriding of steel and production of duplex coatings are
reported in Refs. [2–7]. In order to ensure good adhesion
* Corresponding author. Tel.: +31 15 2781684; fax: +31 15 2786730.
E-mail address: [email protected] (G.C.A.M. Janssen).
0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2004.11.013
between the nitrided steel and the hard coating, the
formation of iron nitrides on top of the nitrided steel has
to be prevented. In our experiments, bright nitriding was
achieved by a previously published procedure [8].
The CrN coatings, deposited in our laboratory, have high
values of hardness, density and elastic modulus and show
good coating to substrate adhesion [9,10].
In the present paper, the relative importance of coating
thickness and nitriding depth on scratch resistance of duplex
coatings is treated.
2. Experimental
A set of 42 samples was produced in an industrial Hauzer
HC 750 PVD machine using seven nitriding treatment times
and six coating thicknesses. Plasma nitriding was performed
for range of times (15 min to 8 h) in an Ar–N2 plasma at a
R. Hoy et al. / Surface & Coatings Technology 200 (2006) 3856–3860
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3. Results
Fig. 1. Nitrogen peak intensity versus depth measured by EPMA on crosssectional samples. Different markers represent the specimens with different
nitriding times. The nitrogen peak intensity is proportional to the nitrogen
content.
temperature of 540 8C. A detailed description of the
nitriding treatment is given in Refs. [2,8]. Nitriding depth
varied from 5 to 65 Am depending on nitriding time. Bright
nitriding was achieved, eliminating the need for polishing
prior to coating deposition.
Hard CrN0.94 coatings were deposited by means of
reactive sputter deposition in the same Hauzer PVD
machine equipped with a secondary plasma source [9]. A
detailed description of the PVD machine used is given in
Ref. [11]. The deposition time varied from 7 min to 12 h
leading to coating thickness of 0.1–10 Am (deposition rate
0.8 Am/h). The nitrogen flow of 53 sccm and argon flow of
115 sccm were kept constant during all depositions. The
deposition temperature was 450 8C.
The thickness of the coatings was measured by
observing the cross-section using a scanning electron
microscope (SEM). The chemical composition of the
coatings and the nitrogen depth profiles of the nitrided
substrates were determined by electron-probe X-ray microanalysis (EPMA) using a JEOL JKA 8900 R microanalyser. The substrate hardness, coating hardness and
reduced modulus (E r=E/(1 m 2)) for coatings were determined using a Hysitron nanoindenter (a load of 10 mN
was applied).
Scratch tests were performed with constant load on all
samples using a diamond Rockwell indenter with a
spherical tip (radius 200 Am). Applied loads were in the
range 10–100 N—with 10 N steps. Scratches (length 7
mm) were made with a linear speed of 5 mm/min. During
the tests, the coefficient of friction was measured. The
width of the scratch tracks was measured by optical
microscopy.
Optical micrographs of the scratch tracks were made
using a confocal laser scanning microscope (CLSM).
Micrographs were constructed from a large amount of
images (10–50) taken at different focal depths. From these
images the depth profiles of the scratch tracks were
constructed.
Fig. 1 shows the nitrogen depth distribution for all the
specimens produced with different plasma nitriding times.
Nitriding of steel substrates leads to an increase in hardness
from 10 to 16 GPa, measured on the steel surface by
nanoindentation.
The nitrogen content at the surface increases with
increasing nitriding time until the nitriding time is 1 h.
For higher nitriding times of more than 1 h, the nitrogen
depth profiles can be separated in two zones. The first
zone is close to the surface, where the nitrogen content is
more or less constant—dnitriding saturation levelT (for the
specimen that was nitrided for 8 h this zone extends from
0 to 50 Am depth). The second zone is a dtransitionT zone
with a depth of approximately 30 Am for all samples,
where the nitrogen content steadily reduces to zero (from
50 to 80 Am for the 8-h specimen). The distance between
interface (depth zero in Fig. 1) and the position where the
nitrogen content is half of the value at the interface was
taken as the nitriding depth. Fig. 2 shows the nitriding
depth versus nitriding time—only for specimens, where
the saturation level was reached (after 1–8 h of nitriding).
As can be seen from Fig. 2, the nitriding depth increases
approximately linearly with nitriding time (nitriding rate
approx. 6.2 Am/h).
Hard CrN0.94 coatings of various thickness were deposited
on all nitrided samples. The hardness and reduced modulus of
deposited coatings are 30 and 270 GPa, respectively. The
stress is approximately 1.5 GPa compressive.
Fig. 3 shows CLSM micrographs of scratch tracks on
specimens with a nitriding depth of 16 Am and various
coating thickness. Fig. 4 shows CLSM micrographs of
scratch tracks performed on specimens of various nitriding depth and coating thickness of 1 Am. All scratches
were made with constant load of 40 N. For all micrographs, the scratch direction is from bottom to top.
Fig. 2. Nitriding depth versus nitriding time. For the nitriding depth, we
take the depth at which the nitrogen content is half of the value of the
nitrogen content at the interface.
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R. Hoy et al. / Surface & Coatings Technology 200 (2006) 3856–3860
track is much smaller (Fig. 3). Cracks occur at the edge
of scratch tracks only in samples with a small nitriding
depth. Upon increasing the nitriding depth in duplex
coatings cracks shift from outside the scratch track to
inside (Fig. 4).
Figs. 3a,b and 4b show examples of damage after scratch
testing in duplex coatings, which is known as tensile
cracking. Tensile cracks form in the wake of the moving
Fig. 3. Micrographs of the scratches, made on duplex coatings with a
nitriding depth of 16 Am (nitriding time 1 h) and various coating thickness:
(a) 0.1 Am, (b) 1 Am, (c) 10 Am.
Depending on nitriding depth and coating thickness of
duplex coatings we observed different failure mechanisms
after scratch testing. As can be seen from Fig. 4,
increasing of the nitriding depth for the duplex coatings
reduces the width of the scratch tracks, considerably. The
influence of coating thickness on the width of the scratch
Fig. 4. Micrographs of the scratches, made on duplex coatings consisting of
a nitrided layer of various depth and coating thickness of 1 Am. Nitriding
depth: (a) 5 Am, (b) 16 Am, (c) 62 Am.
R. Hoy et al. / Surface & Coatings Technology 200 (2006) 3856–3860
Fig. 5. Track width versus nitriding depth. Different markers represent
different coating thicknesses.
indenter. This failure mechanism is typical for duplex
coatings (see Ref. [2]). The resistance against tensile cracking
improves with increasing nitriding depth (62 Am) and coating
thickness (10 Am). For the thickest layer (Fig. 3) and the
largest nitriding depth (Fig. 4), tensile cracks after scratch
testing are hardly observed. Increasing the coating thickness
up to 10 Am leads to deterioration of the coatings. The surface
becomes very rough.
For all specimens (various nitriding depths and various
coating thicknesses), the width of the scratch tracks, made
by applying a load of 40 N, is presented in Fig. 5. The
width range is from 65 Am for duplex coating (10 Am
coating and 62 Am nitriding depth) to 120 Am for
nontreated steel without coating. From the track width,
we calculated the hardness—the load divided by projected
area pd 2/4 (where d is the width of scratch track). We will
use here the term bapparent scratch hardnessQ—because the
exact size of load carrying area is unknown [3,12,13]. In
fact, only the front half of the indenter is fully supported
by the surface, while the rear half of the indenter may not
be fully supported due to the plastic deformation of the
Fig. 6. Apparent scratch hardness versus nitriding depth. Different markers
represent different coating thicknesses.
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Fig. 7. Apparent scratch hardness versus the sum of nitriding depth and
coating thickness. Different markers represent different coating thicknesses.
Data of uncoated steels and steels coated with the 10-Am coating are
connected by dotted lines.
specimen. Apparent scratch hardness versus nitriding depth
is presented in Fig. 6.
4. Discussion
For all our specimens, the adhesion between the nitrided
steel and the coating is good. Good coating-to-substrate
adhesion of our hard PVD coatings, confirmed by micrographs obtained using a focused ion beam, was already
reported previously [3,11].
From Figs. 5 and 6, we can see that increasing the
nitriding time for the duplex coatings results in a reduction
of the width of the scratch tracks almost up to two times
and, consequently, a significant increase of the apparent
scratch hardness up to three times. The load carrying
capacity of the steel substrate increases considerably with
nitriding causing an increase of apparent scratch hardness.
Increasing the coating thickness up to 10 Am has a relatively
small effect on the scratch track width and the apparent
scratch hardness.
Fig. 8. Depth profiles of the scratch tracks for specimens with coating
thickness of 1 Am and various nitriding depth. Different markers represent
different nitriding times.
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specimens in Fig. 9, the nitriding time was 1 h corresponding to a nitriding depth of 16 Am) is relatively small. All
tracks have about the same depth (5 Am) and there is limited
pile-up at the track edge for all measured samples.
5. Conclusions
Fig. 9. Depth profiles of the scratch tracks for specimens with nitriding
depth of 16 Am (1 h nitriding) and various coating thickness. Different
markers represent different coating thicknesses.
In view of the relatively high hardness of the CrN coating
(30 GPa) as compared to the nitrided steel (16 GPa), one
might expect that a coating of 10 Am on top of the (not
nitrided) steel would yield more pronounced hardening than
nitriding the steel to a depth of 10 Am. Fig. 7 shows the
scratch hardness versus the sum of nitriding depth and
coating thickness. If a given coating thickness has a
significantly larger effect on the hardness than the same
nitriding depth, a line though the data points of the thicker
coatings should be positioned above a line through the data
points of the thinner coatings. This seems not to be the case
(unfortunately the data in Fig. 7 show considerable scatter).
We conclude that for the present experiments a coating of a
certain thickness does not induce hardening to a significantly greater extent than a nitrided zone of the same
thickness. Because of the relatively high nitriding rate (6.2
Am/h) as compared to the coating deposition rate (0.8 Am/h),
it is therefore more efficient to increase the nitriding time
than the coating thickness if the apparent scratch hardness
has to be maximised.
Upon indenting hard coatings on soft substrates, deformation starts in the substrate. Some analysis of deformation
after scratch testing performed on hard CrN coatings on
nitrided and non-nitrided steel has already been published in
Refs. [2,9]. Here, we present the scratch track profiles,
obtained after scratch testing (performed at 40 N) on
samples with various nitriding depth and coating thickness,
using CLSM. As can be seen from Fig. 8, for substrates
covered with a 1-Am CrN coating, increasing nitriding depth
from 5 to 62 Am leads to a significant reduction of scratch
track depth (from 5 to 2 Am). The scratch track on the
duplex coating with the smallest nitriding depth (5 Am)
shows significant pile-up at the track edge.
Because the nitriding depth and the coating thickness
have a similar effect on scratch hardness, the coating
thickness is not expected to have a large effect on scratch
track depth profiles for specimens with considerable
nitriding depth. Indeed, the effect of the coating thickness
(0.1–10 Am) on the depth profiles in Fig. 9 (for the
Bright nitriding up to 60 Am is possible in a Hauzer HC
750 PVD machine. After reaching the saturation level at the
surface of the steel, the nitriding depth increases approximately linearly with nitriding time.
Increasing the nitriding depth in duplex coatings has a
large influence on scratch resistance. The load carrying
capacity of duplex coatings increases leading to an increase
of the apparent scratch hardness. Increasing the coating
thickness also increases the scratch resistance. However, in
view of the (1) similar effect of coating thickness and
nitriding depth on apparent scratch hardness and the (2)
relatively low deposition rate as compared to the nitriding
rate, increasing the coating thickness is a less efficient way
to maximise the scratch hardness.
Acknowledgements
C. Kwakernaak and H. Kiersch of our department are
acknowledged for the electron probe X-ray micro-analysis.
This research was carried out under project number
MC7.01087 in the framework of the Strategic Research
program of the Netherlands Institute for Metals Research in
The Netherlands (www.nimr.nl).
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