-
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Hard coatings
Thickness
Microstructure
Friction
Wear
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gs
coating thickness. In the present work, the effects of
microstructure and coating thickness on the
easingis austrialed wit
[1,2] to forming dies [3,4]. In general, CrN coatings
exhibit
stateutionilureating,orse
that the PVD coating microstructure and morphology are
depen-
Contents lists available at SciVerse ScienceDirect
.e
Wea
Wear ] (]]]]) ]]]]]]problem has been developed. Plasma enhanced
magnetronE-mail address: [email protected] (C.
Lorenzo-Martin).superior ductility, fracture toughness, and
corrosion and dent on thickness [6]. In addition to deposition
parameters, theproperties of coatings, including their tribological
properties, willbe inuenced by the microstructure. Recently, a
technique cap-able of producing relatively thick CrN lms without
the stress
0043-1648/$ - see front matter & 2013 Published by Elsevier
B.V.
http://dx.doi.org/10.1016/j.wear.2013.02.005
n Corresponding author. Tel.: 1 630 252 8577; fax: 1 630 252
5568.PleasCrNlike carbon. Of these coatings, CrN is increasingly
being used fortribological applications, ranging from automotive
components
the adhesion to the substrate. There is often an optimal
coatingthickness in terms of failure and performance. It is also
knownis usually a suitable coating for a given engineering
application.Thin lm coatings of different composition are among the
mostcommon ones used for tribological purposes, because they
requireminimal to no post-deposition processing. Some examples
ofmaterials used for thin lm coatings are TiN, CrN, and
diamond-
thickness limitation. The thickness affects the residual
stressin PVD coatings and also determines the stress eld
distribunder contact conditions, both of which are relevant to
fapathways and mechanisms. Usually, the thicker the cothe larger
the residual tensile stress accumulation and the wthat produces new
coatings with desirable tribological perfor-mance as well as
mechanical properties. Currently, many com-mercial coatings are
available for tribological applications.Because of the variety of
available coatings in terms of thickness,hardness, and fracture
toughness, among other properties, there
tion (PVD) techniques, including magnetron sputtering and
arcevaporation. The average CrN coating thickness is 15 mm, andthis
material exhibits hardness values of HV 14.523.5 GPa and asurface
morphology consisting of dense, ne columnar structures.One
challenge for PVD coatings (including CrN) is related to the1.
Introduction
Thin lm coatings are being incrapplications. Surface
engineeringresearch because of the high indcontrol and wear
resistance, couple cite this article as: C. Lorenzo-Mcoatings, Wear
(2013), http://dx.doifriction and wear behavior of CrN coatings
were determined under unidirectional sliding conditions.
Tests were conducted with a dry ball-on-at contact conguration
using 1-, 5-, and 10-mm thickcoatings deposited on a hardened H-13
steel substrate by plasma enhanced magnetron sputtering. The
ball specimen was made of WC. The friction behavior was observed
to be strongly dependent on coating
thickness and microstructure, especially at relatively low loads
(5 N). At higher loads, however, the
thinnest coating (1 mm) was quickly worn through, while the
thicker ones (5 and 10 mm) remainedintact. Wear in both the
counterface WC ball material and the coatings also depended on
coating
thickness and microstructure. In all coatings, there was
localized damage but minimal wear. Additional
tests were done with Si3N4 and 52100 steel balls, and the
results indicated different wear and friction
behavior from that for WC balls. The observed effect of coating
thickness on tribological behavior is
attributed to differences in the microstructure and mechanical
behaviors of CrN coatings as a function
of thickness.
& 2013 Published by Elsevier B.V.
ly used for tribologicalfast growing area ofdemands for
frictionh enabling technology
oxidation resistance compared to the widely used TiN
counter-part. Also the lower coefcients of friction and higher
wearresistance under dry sliding conditions make CrN coatings
excel-lent candidates for a variety of applications, such as
metal-forming and die-casting tools [5].
CrN coatings are produced mainly by physical vapor
deposi-Keywords:Effect of microstructure and thicknessbehavior of
CrN coatings
C. Lorenzo-Martin a,n, O. Ajayi a, A. Erdemir a, G.R. Fa Argonne
National Laboratory, Tribology Section, Lemont, IL 60439, USAb
Southwest Research Institute, San Antonio, TX 78228, USA
a r t i c l e i n f o
Article history:
Received 31 August 2012
Received in revised form
4 February 2013
Accepted 7 February 2013
a b s t r a c t
One of the most common
deposited by physical vapo
and forming tools, dies an
engines. In selecting coatin
journal homepage: wwwartin, et al., Effect of
micr.org/10.1016/j.wear.2013.02the friction and wear
ske a, R. Wei b
sed tribological thin-lm coatings is chromium nitride (CrN),
typically
eposition. Examples of current applications of this coating
include cutting
tomotive components, such as injection valves and piston rings
for diesel
for different tribological applications, one of the critical
parameters is the
lsevier.com/locate/wear
rostructure and thickness on the friction and wear behavior
of.005i
-
sputter (PEMS) deposition is an improved version of
conventionalmagnetron sputtering. The PEMS technology can produce
CrNcoatings as thick as 20 mm [7]. With the advent of PEMS, it is
thuspossible to evaluate the impact of coating thickness on
tribologi-cal performance over a broad range of coating
thicknesses. Thistype of study was not possible prior to PEMS
because the largestthickness achievable, without stress problems,
was typically lessthan 3 mm.
In this study, we used a tungsten lament and a discharge
powersupply to generate a global plasma in the entire vacuum system
andfabricated CrN coatings of different thickness (110 mm). The
goalwas to assess the effect of CrN coating thickness and
microstructureon tribological performance.
2. Experimental details
2.1. Coating material
CrN coatings of different thicknesses were deposited onpolished
H-13 steel at specimens with dimensions of21.50.25 in.3 and
hardness of 58.5 Rc by the PEMS process.The H-13 material was
hardened by heat treatment throughquenching and tempering at 550
1C. Fig. 1 is a schematic of thePEMS system developed at Southwest
Research Institute [6]. Priorto coating deposition, substrates were
sputtered with Ar for6090 min to remove residual contaminant on the
surface.
Power
C. Lorenzo-Martin et al. / Wear ] (]]]]) ]]]]]]2Cr Target
Sam
ple
s
To Pump
Ar, N2
Worktable
Supply
-
Bias Power Supply
-
MagnetronA solid target of 170-mm diameter was employed as a
sourceof Chromium (Cr). The power was set at 1.5 kW. During
thedeposition process, a mixture of argon (Ar) and nitrogen
(N2)gases was introduced, and the chamber pressure was
maintainedbetween 0.33 and 0.47 Pa (2.53.5103 Torr). The
surfacetemperature of the substrate was maintained at about400 1C
[8]. Because this temperature is lower than the
temperingtemperature, no change in microstructure and properties
isexpected for the substrate during the coating process.
Coatingswere deposited to approximately 1-, 5-, and 10-mm
thicknesseswith a corresponding surface nish of 0.011, 0.024, and
0.025 mm
Filament
GlobalPlasma
Magnetron
AC
Discharge Power supply
+
+Magnetron-generated PlasmaFig. 1. Schematic of plasma enhanced
magnetron sputtering (PEMS).
Please cite this article as: C. Lorenzo-Martin, et al., Effect
of micrCrN coatings, Wear (2013),
http://dx.doi.org/10.1016/j.wear.2013.02Ra, respectively. The
roughness of the original polished steel atsurface was about 0.005
mm. Coatings surface morphologies, aswell as cross sections, were
examined by using optical andscanning electron microscopy (SEM)
(Quanta 400F ESEM) operat-ing at 10 kV.
2.2. Hardness measurement
Coating surface hardness (H) and reduced Youngs modulus(Er) were
measured by nanoindentation method using a triboin-denter TI-950
(Hysitron Inc.). A Berkovich diamond tip with anapproximate radius
of curvature of 50 nm was used for thehardness measurements. A xed
displacement of 100 nm wasselected, and indents were performed at a
loading/unloading rateof 20 nm/s. The reduced Youngs modulus was
obtained from theslope of the unloading part of the
load-displacement curve usingthe Oliver and Pharr method, as
described in Ref. [9]. Because theroughness of the coating seemed
to be too large to obtainconsistent measurements, indentations were
placed on a polishedannular ring of the coating produced by a
micro-abrasion ballcratering method to measure the hardness and
elastic modulusaccurately close to the coating surface (less than
0.25 mm fromthe top surface). A minimum of six indents were
considered for anaverage of hardness and reduced modulus
calculation.
The crystalline structure of the coatings was assessed by usinga
Philips X Pert X-ray diffractometer. A Cu Ka radiation
source(l0.1542 nm) was used, operating at 40 kV and 40 mA.
X-rayscans covered from 201 to 1001 at a step size of 0.011 and
scanspeed of 0.1251/s. The total number of steps was 8000. Peaks
wereidentied by using the X Pert data viewer software.
2.3. Friction and wear testing
Friction and wear tests were conducted in unidirectional
drysliding contact using a ball-on-at conguration (Fig. 2). The
ballshad 1/2 in. diameter, were made of WC, and had a hardness
valueof 16 GPa and a precision surface nishing grade of 25 (38
nmmeasured surface roughness). According to manufacturer
speci-cations, the ball material consisted of 9094% tungsten and
610%cobalt as a binder material. The steel substrate and CrN
coatingswith three thicknesses were tested against the WC balls.
Forcomparison, 52100 steel and Si3N4 balls were also tested.
Tests were conducted at normal loads of 5, 10, and 20 N.
Themaximum Hertzian contact pressures calculated for steel at
testedagainst WC ball were (0.91, 1.15, and 1.44 GPa,
respectively). ForSi3N4 ball tested against steel at, the maximum
Hertzian contactpressures were ( 0.77, 0.96, and 1.22 GPa,
respectively), while for the52100 steel ball on steel at the
maximum Hertzian contactpressures were (0.68, 0.86, and 1.09 GPa,
respectively). The effect ofcoating on contact pressure will be
dependent on coatings properties(see Table 1). Compared to steel
substrate, coatings with lowerelastic modulus than steel (such as
CrN-3) will produce lowercontact pressures, while coatings with
higher elastic modules thansteel (such as CrN-1 and CrN-2) will
produce higher contactpressures than steel substrate. The
contribution of the coating tocontact pressure will be also
dependent on load and coatingthickness. The higher the load and the
thinner the coating, the moreimportant contribution of substrate to
contact pressure. All testswere run for 60 min and at a linear
speed of 1 cm/s in ambient roomair (relative humidity of 65%). The
friction coefcient was continu-ously measured during the sliding.
Wear on ats and counterface ballmaterials was evaluated at the
conclusion of the test by using opticalprolometry. We obtained 3D
maps of the ball scars and measuredvolume wear for each track. To
calculate the wear volume of thecoatings, we measured the wear
track proles across the sliding
direction of the track at four locations, also using the
prolometry
ostructure and thickness on the friction and wear behavior
of.005i
-
spectroscopy (EDS) analysis.
r un
C. Lorenzo-Martin et al. / Wear ] (]]]]) ]]]]]] 33. Results and
discussion
3.1. Surface morphologytechnique. Wear rates were obtained by
normalizing wear volumeby the total sliding distance. A repeat test
was conducted for eachcondition. Worn surfaces were also
characterized by optical andscanning electron microscopy equipped
with energy dispersive X-ray
Fig. 2. Ball-on-at test rig fo
Table 1Properties of coatings used in this work.
Coatings Deposition
method
Thickness
(mm)Reduced elastic
modulus (GPa)
Hardness
(GPa)
Roughness
(nm)
CrN-1 PEMS 1 256 21 11
CrN-2 PEMS 5 269 21 25
CrN-3 PEMS 10 190 14 24
Steel none 0 212 8.4 5Fig. 3 shows the surface morphology for
the three CrN coat-ings. Average grains size (d), as determined by
surface imageanalysis, is dependent on coating thickness: d0.1 mm
for CrN-1,d0.40.5 mm for CrN-2, and d0.81.2 mm for CrN-3.
Althoughgrain size distribution is mostly homogeneous for all the
coatings,the surface morphology (specically the aspect ratio) is
depen-dent on coating thickness.
For the thinnest coating, the grains are uniaxial, while
thethickest coatings possess a clear columnar structure with
thegrains preferentially elongated perpendicular to the
surface.Cross-sectional micrographs of the coatings are shown in
Fig. 4.
For CrN-1, the coating is mostly uniaxial. Over a Cr bond
layerof 400 nm, a columnar CrN structure that is tens of
nanometersthick and few hundreds of nanometers high is starting to
grow.For the thicker coatings, CrN-2 and CrN-3, the structure is
clearlycolumnar with cauliower-like texture on the surface (Fig.
5b).The columns grow wider with thickness of the coatings, forming
afan shape. The columns are slightly tilted 351 with respect to
theperpendicular direction of the substrate. In the thickest
coating,CrN-3, the columns grow from a width of 50 nm (close to
thesubstrate) to 500750 nm at the thickest area, giving a
coarserstructural morphology. Also, surface localized defects
(known asmacro particles or macro droplets) were observed for all
coatings,especially for thicker ones.
Fig. 5 shows such a typical nodular-like defect. The
particledefect diameter is in the 210 mm range. These types of
defectsare produced during the deposition process, and because
thebonding of the macro particle with the coating is relatively
poor,
Please cite this article as: C. Lorenzo-Martin, et al., Effect
of micrCrN coatings, Wear (2013),
http://dx.doi.org/10.1016/j.wear.2013.02they can be easily removed
during the wear process, leavingcraters on the coating surface. The
presence of macro particlesmay be responsible for the accelerated
initial wear.
3.2. Crystal structure of the coatings
X-ray diffraction (XRD) spectra for the coatings are presentedin
Fig. 6. These spectra show how the relative peaks for Fe, Cr,
andCrN change with coating thickness. The uncoated steel
(baseline)shows a clear iron peak at 44.451. For the thinnest
coating, CrN-1,the most intense peak is located at 44.521. Two
possible peaksoverlap at this position. One peak is Fe-a from the
substrate;considering the relative thin coating of less than a
micron, theeffect of the substrate can be important.
The second possible overlapping peak is Cr (110), most
probablyfrom an initial Cr bond layer during deposition. Another
peak almostequally intense for the CrN-1 coating is CrN (111).
Several otherweaker peaks, corresponding to Cr and CrN, are also
observed. Asthe coating thickness is increased to 5 mm (CrN-2), the
CrN (111)peak dominates, followed by CrN (220) and several other
weakerpeaks for CrN and Cr. With coating thickness of 10 mm
(CrN-3), thestrongest peak is CrN (220), followed by CrN (200). The
CrN (111)peak is still present but is less intense than for the
thinner coatings,and a new peak corresponding to CrN (311) appears.
In sum, theXRD analysis showed that the three coatings consist of
CrN crystalsprimarily, regardless of coating thickness.
3.3. Nano-mechanical properties of the coatings
Table 1 summarizes the measured hardness and reducedYoungs
modulus obtained for the three CrN coatings, determined
idirectional sliding testing.using the nano-indentation method
described earlier in experi-mental details section. Other relevant
properties of the coatingand substrate material, such as thickness
and roughness, are alsoincluded.
Note that CrN-1 and CrN-2 have very similar hardness(21 GPa)
while CrN-3 exhibits a considerably lower hardness(14 GPa). The
reason for differences in the measured hardnessvalues is unclear,
perhaps a reection of differences in structuralmorphologies and/or
differences in residual stresses in the coatings.
3.4. Friction and wear results
The friction behavior of the three CrN coatings and
uncoatedsteel substrate tested under unidirectional dry sliding
with theWC ball is summarized in Fig. 7. For the uncoated H13 steel
at,the friction behavior is nearly identical at 5 and 10 N loads.
Inboth cases, the friction coefcient increased rapidly at the start
ofthe test to a maximum value of about 0.6, followed by a
gradual
ostructure and thickness on the friction and wear behavior
of.005i
-
ren
C. Lorenzo-Martin et al. / Wear ] (]]]]) ]]]]]]4Fig. 3. Surface
morphology of CrN coating of diffedecrease to a minimum value of
about 0.4, after which acontinuous gradual increase occurred to the
end of test. For therst 1000 s of sliding at 20 N (Fig. 7c), a
similar trend wasobserved: rapid increase, followed by both gradual
sequentialdecrease and increase. However, this trend was followed
by asudden rapid decrease to a near constant value of 0.35 for
theduration of the test.
This friction behavior was observed in repeat tests for the WC
andH13 steel sliding pair. It is attributed to the extensive
formation of atransfer lm on the uncoated steel at, as shown in
Fig. 8. Higherloads produced more transfer lm. In the 20-N load
test, the rapiddecrease in friction coefcient after 1000 s of
testing is attributed tolarge coverage of the contact area by the
WC transfer lm, such thatin the later stage of testing, the contact
interface consisted of thesliding of the WC transfer lm on the WC,
which is known to havefriction coefcient of about 0.350.45 [10].
There is also the possibilityof oxidation at the tribo contact
interface resulting in the formation ofiron and tungsten oxides.
Oxidation of wear debris trapped at thecontact interface can also
occur. Indeed, tribo layers are well known
Fig. 4. Cross-section SEM micrographs of CrN coatin
Fig. 5. Characteristic surface defect (m
Please cite this article as: C. Lorenzo-Martin, et al., Effect
of micrCrN coatings, Wear (2013),
http://dx.doi.org/10.1016/j.wear.2013.02t thicknesses: (a) 1 mm,
(b) 5 mm, and (c) 10 mm.to consist of a complex mixture of both
material pair in contact, aswell as species from the
environment.
The friction coefcients in the tests with CrN-coated ats
werealmost identical at 5 and 10 N loads. For the three coatings
atthese loads, the friction coefcient showed a general
gradualincrease with time. For both loads, the friction coefcient
(m) inCrN-1 was consistently higher than for the thicker coatings,
witha value of m0.7 at the test conclusion. The CrN-2 and
CrN-3coatings showed identical friction coefcients for the rst
fewhundred seconds of testing, and by the end of test, the
frictioncoefcient with CrN-2 was about the same or slightly lower
thanthat for CrN-3 (m0.5 vs. 0.55).
At 20 N, the friction behavior in tests with CrN-2 and
CrN-3remained unchanged, but a sudden rapid increase in noise
wasevident in the test with CrN-1 after 1500 s. This transition in
CrN-1 coincided with the coating being worn through, as shown
inFig. 9a. It is possible that the sudden increase in friction was
dueto the ball sliding against the Cr interlayer, although
unlikely.Rather, the friction increase is more likely due to the
abrasive
g: (a) 1 mm, (b) 5 mm, and (c) 10 mm thickness.
acro particles) for CrN coatings.
ostructure and thickness on the friction and wear behavior
of.005i
-
plowing action of the hard coating debris generated by
thecoating failure.
Subsequent WC transfer occurred from the ball to the
partiallyexposed steel substrate (Fig. 9b), as indicated by EDS
analysis(Fig. 9c). This WC transfer may account for the decrease in
frictioncoefcient from 0.7 to 0.5 at the very end of the test,
consideringthat the new contact interface consists of WC ball
material against aat surface with CrN-steel-WC areas. For the
thicker coatings (CrN-2and CrN-3), the friction behavior and
magnitude of coefcients aresimilar to those in the lower load
tests. Although no steady frictioncoefcient value was achieved by
the conclusion of testing for mostof the materials and loads
tested, an average friction coefcient was
calculated for the duration of the test, excluding the rst
fewminutes of testing. Average friction coefcient values are
summar-ized in Fig. 10.
The highest average friction coefcients are observed for
thethinnest coating (CrN-1), for which friction decreases with
load,exhibiting values ranging from 0.64 (for 5 N) to 0.58 (for 20
N). Forthe thickest coatings (CrN-2 and -3), the average friction
coefcientsvalues are similar to those of the uncoated steel
baseline. While forthe uncoated steel at average friction values
decrease with load too,from 0.51 (for 5 N load) to 0.37 (for 20 N
load), they do not for theCrN-2 and CrN-3 coatings. Of the coatings
CrN-2 exhibits the lowestaverage values: 0.38, 0.48, and 0.44 for 5
N, 10 N, and 20 N,respectively. For CrN-3 the values are slightly
higher: 0.44, 0.51,and 0.49 for 5 N, 10 N, and 20 N respectively.
While the frictioncoefcients for repeat tests were quiet close
(less than 4% variability),more variations were observed in the
wear values (up to 10%).
Fig. 11 shows the wear results, as measured by optical
prolo-metry, in the ball-and-at specimens after testing. For the
test withCrN-1 at highest load (20 N), in which the WC ball
counterface hadworn through the coating, the highest wear rates
were measured intheWC ball and at. With that exception, wear in
both WC balls andats appears to increase nearly linearly with load
for the other testpairs. Wear in the uncoated steel at was, on
average, at least anorder of magnitude higher than that for the
coating material. Coatingthickness seems to have a minor effect on
coating wear, with thetwo thickest coatings wearing slightly more
than the thinnest one atlow loads (Fig. 11a). The same tendency was
observed for ball wear(Fig. 11b). While coating thickness showed
minimal effect on WCball wear, the material tested against it had a
major effect on wear.
Fig. 7. Friction evolution with time for unidirectional dry
sl
20 30 40 50 60 70 80 90 100-50
0
50
100
150
200
250
300
350
2 Theta
Inte
nsity
(co
unts
)
H-13
CrN-1
CrN-2
CrN-3
Fig. 6. XRD peaks of CrN coatings deposited on H-13 steel
substrate.
C. Lorenzo-Martin et al. / Wear ] (]]]]) ]]]]]] 5Fig. 8. Steel
at tested against WC ball in dry unidirectional sliding: (a)
overa
Please cite this article as: C. Lorenzo-Martin, et al., Effect
of micrCrN coatings, Wear (2013),
http://dx.doi.org/10.1016/j.wear.2013.02The WC ball wear against
the uncoated steel at was, on average, atleast four times higher
than ball wear against CrN coatings. Althoughsteel hardness is
considerably lower than CrN coating hardness (seeTable 1), this
higher wear is attributed to the presence of relativelyhard
second-phase carbide particles, notably VC, MoC, and CrC,sliding
against the H-13 steel sample microstructure and surface.
iding at different loads: (a) 5 N, (b) 10 N, and (c) 20 N.ll
track, (b) transfer layer from ball, and (c) EDS analysis of
transfer layer.
ostructure and thickness on the friction and wear behavior
of.005i
-
rack
C. Lorenzo-Martin et al. / Wear ] (]]]]) ]]]]]]6Fig. 9. CrN-1
coating tested against WC ball in unidirectional sliding: (a)
overall tdamage and transfer layer.
0.1
0.2
0.3
0.4
0.5
0.6
0.7
5N10N20N
Ave
rage
Fric
tion
Coe
ffici
entWear on the uncoated steel at was predominantly abrasive
wear.The transfer layer of WC from the ball onto the surface
protected itpartially against further wear (as shown in Fig. 8). In
terms of wearmechanisms in the CrN coatings, polishing wear and
some transferfrom the ball material are the dominant mechanisms.
The exceptionis CrN-1 tested at high load (20 N). As the coating
was worn through,severe abrasive wear became the dominant mechanism
(as seen inFig. 9b). By contrast, CrN-2 showed mostly polishing
wear withminimal transfer lm formation regardless of the test load.
Somelocal damage, consisting of cracking and delamination of the
transfersurface layer, was observed (Fig. 12).
For CrN-3, load seemed to affect the degree of damage on thewear
track. For low load (5 N), there is minimal damage to theoriginal
surface morphology. Only mild polishing and some smooth-ing of the
wear track byminimal transfer are apparent. At higher load(20 N),
the amount of transfer is increased considerably, and exten-sive
patches of WC transfer are observed (Fig. 13b). This transferlayer,
which has a smoothing effect on the track, eventually grows toa
critical thickness and fails by cracking and chipping (Fig.
13c).
3.5. Effect of counterface material
Additional friction and wear tests were conducted with the
CrN-2coated at sliding against polished, commercially available
bearingballs composed of either hardened 52100 steel (Ra36 nm) or
Si3N4(Ra10 nm). The hardness of the 52100 balls is about 7.2 GPa(62
Rc), and that of the Si3N4 balls is about 14.5 GPa. Tests with
theseballs were conducted by the same procedure as used with WC
balls.
Fig. 14 shows the friction variation with time during the test
withthe three balls (WC, 52100 steel, and Si3N4) when sliding
against
0H-13 CrN-1 CrN-2 CrN-3
Fig. 10. Average friction coefcient for CrN coatings and
uncoated substratesliding against WC ball.
Please cite this article as: C. Lorenzo-Martin, et al., Effect
of micrCrN coatings, Wear (2013),
http://dx.doi.org/10.1016/j.wear.2013.02CrN-2 at 10 and 20 N loads.
For WC, the test started at relatively lowfriction coefcient of
0.2, but increased gradually for the 1-hourduration of the test,
ending with a nal value of about 0.5. The rate offriction increase
decreased with sliding distance or time. For thismaterial, the
friction coefcient appears to be independent of load, asthe tests
at 10 and 20 N showed nearly identical friction behavior.
Thepresence of the cobalt binder phase in the ball may have
contributedto the friction coefcient and noise reduction. For the
52100 steelballs, sliding started with a friction coefcient of
about 0.8 at both 10and 20 N load. This was followed by a slight
gradual decrease to asteady value of about 0.70 for 20 N and about
0.75 for 10 N. Thefriction coefcient data showed considerably more
noise in the testswith the steel ball compared to the WC ball. The
high friction andnoise in the test data for the steel ball are
attributed to extensivemetal transfer into the coated at surface,
as illustrated in Fig. 15.
The tests with Si3N4 balls started with a friction coefcient
ofabout 0.25 at both 10 and 20 N. In both cases, the friction
coefcientincreased rapidly during the rst 5 min (300 s) of sliding,
reaching anearly constant value of 0.7 for the remainder of the
one-hour test.Furthermore, the friction coefcient data increased in
noise through-out the test. This friction behavior is the result of
both transfer ofmaterial from the Si3N4 ball into the coating and
the wear anddamage of the coating, as shown in Fig. 16.
Fig. 17 compares the wear for the three different balls with
CrN-2coated ats. Wear was the greatest for the steel ball and the
least forthe WC ball (Fig. 17a). Although more wear occurred in the
Si3N4ball compared to the WC ball, it was substantially lower than
thewear in the steel ball. This behavior is perhaps a reection
ofdifferences in ball hardness. The steel ball produced no wear
interms of material removal from the coated at, while the Si3N4
ballproduced the most wear on the CrN coated at (Fig. 17b). The
wearproduced by the WC ball is signicantly less than that for the
Si3N4ball due to the more extensive formation of a transfer layer,
eventhough the WC ball is harder than the Si3N4 ball. Note that
thefriction coefcients in the test with the WC ball are also much
lower
, (b) transfer layer from ball and coating damage, and (c) EDS
analysis of coatingthatheof w
4.
promic
ostr.00n those in the test with the Si3N4 ball. This condition
will reduceshear stresses imposed on the coating and, hence, the
amountear.
Summary
Although CrN coatings with different thicknesses can beduced by
PEMS, they have signicant differences in terms ofrostructure and
surface morphology:
For the 1-mm thick coating the grain structure is uniaxial.
Thecrystalline structure is mostly CrN with possible smallamounts
of Cr. In terms of morphology, this coating surfaceis relatively
smooth.
ucture and thickness on the friction and wear behavior of5i
-
For the 5-mm thick coating, the grain shape distribution
isbimodal. There are uniaxial grains close to the substrate
andcolumnar-shaped grains in the upper region of the coating.The
crystal structure is mainly CrN. Surface morphology iscoarser with
small cauliower texture.
The 10-mm thick coating is mostly columnar with fan shape(wider
columns as the coating thickens). Also, the crystalstructure is
mainly CrN, and its surface morphology is thecoarsest with
cauliower texture.
These differences have a signicant impact on the
coatingproperties and tribological behavior. For example, high
hardness inthe CrN-1 and CrN-2 coatings is most probably due to the
grainmorphology. The CrN-3 has lower hardness due to grain
morphologyand/or residual stresses. In unidirectional sliding
against the WC ball,friction in the 1-mm coating is higher, while
it is similar for the 5- and10-mm coatings. Since CrN-1 is the only
coating with signicantlydifferent microstructure, its higher
friction coefcient may be due toits different microstructure,
including higher level of Cr content.Usually, differences in
surface morphologies result in differences in
: (a) overall track, (b) mild polishing, and (c) localized
damage.
ishing at low load, (b) extensive transfer at high load, and (c)
localized damage in
1.3
0.27 0.260.51
2
0.32 0.45 0.81
4.4
6.5
0.511.1
0
1
2
3
4
5
6
7
8
Steel CrN-1 CrN-2 CrN-3
5N
10N
20N
3.3 0.9 0.2 1.7
15.4
1.5 1.0 3.5
69.9
134.0
0.8 4.30
20
40
60
80
100
120
140
Steel CrN-1 CrN-2 CrN-3
5N
10N
20N
Flat Wear x103 m3/m Ball Wear x105 m3
Fig. 11. Flat-and-ball wear for CrN coatings and uncoated
substrate at different loads.
Fig. 14. Friction behavior of CrN-2 coating when sliding against
different ballmaterials at 10 and 20 N.
C. Lorenzo-Martin et al. / Wear ] (]]]]) ]]]]]] 7Fig.
tran
PC13. CrN-3 surface after testing in dry sliding against WC
ball: (a) mild polFig. 12. CrN-2 surface after testing in dry
sliding against WC ballsfer layer.
lease cite this article as: C. Lorenzo-Martin, et al., Effect of
microstructure and thickness on the friction and wear behavior ofrN
coatings, Wear (2013),
http://dx.doi.org/10.1016/j.wear.2013.02.005i
-
g du
C. Lorenzo-Martin et al. / Wear ] (]]]]) ]]]]]]8Fig. 15. (a)
Transfer from steel ball to CrN-2 coatininteractions between
coating and ball surface asperities. This effectmay result in
differences in the amount of material transfer and,hence, in
friction and wear behavior. The CrN-2 and CrN-3 samples(thicker
coatings) exhibited more local surface defects
(macro-parti-cles/macro-droplets). This condition resulted in local
surface damagein these coatings during sliding contact. There are
other possibledifferences in the structure and microstructure of
the three coatingsin this study that may account for some of our
observations. Morecomprehensive comparative structural analysis by
multiple techni-ques and tools are needed to further elucidate the
observationsreported in the present paper.
In general, thicker coatings showed better friction and
wearbehavior, while the thinnest coating was easily worn through at
highload, resulting in a substantial increase in friction and wear.
Com-pared with the uncoated steel surface, the coatings have
minimaleffect on friction but signicant effect on wear reduction.
Indeed,lower friction was observed in tests with uncoated steel
under someconditions after a good transfer layer had formed. This
transfer layeron steel at consists of a signicant amount of
tungsten. Thus, it ispossible that lower friction in uncoated steel
is due to the formationof the W-rich transfer layer [11].
Fig. 16. (a) Transfer from Si3N4 ball to CrN-2 coating d
0.0
1.0
2.0
3.0
4.0
5.0
WC Si3N4 Steel
Wea
r V
olu
me
(105
m
3 )
10N
20N
Fig. 17. Comparison of ball and at wear for tests at 10 and 20
N: (a) CrN-2 coatin
Please cite this article as: C. Lorenzo-Martin, et al., Effect
of micrCrN coatings, Wear (2013),
http://dx.doi.org/10.1016/j.wear.2013.02ring dry sliding; (b) EDS
analysis of transfer layer.When sliding against different
counterface materials, theCrN-2 coating showed signicant
differences in friction and wearbehavior. Although more studies are
needed to further elucidatethe role of counterface materials, this
preliminary study indicatesthat CrN coatings should be paired
carefully with counterfacematerials. The current paper suggests
that thicker coatings can bedeposited for tribological
applications, but more work is neededto optimize coating thickness
for different applications.
Acknowledgments
This work was supported by U.S. Department of Energy,Energy
Efciency and Renewable Energy, Ofce of Vehicle Tech-nologies, under
contract DE-AC0206CH11357. The electronmicroscopy was accomplished
at the EMC at Argonne NationalLaboratory, a U.S. Department of
Energy Ofce of ScienceLaboratory operated under Contract No.
DE-AC0206CH11357by UChicago Argonne, LLC.
uring dry sliding; (b) EDS analysis of transfer layer.
Wea
r V
olu
me
(105
m
3 ) 356.6
0
100
200
300
400
W C Si3N4 Steel
10N
20N
g wear against different balls and (b) wear of different balls
against CrN-2 at.
ostructure and thickness on the friction and wear behavior
of.005i
-
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this article as: C. Lorenzo-Martin, et al., Effect of micrCrN
coatings, Wear (2013),
http://dx.doi.org/10.1016/j.wear.2013.02ostructure and thickness on
the friction and wear behavior of.005i
Effect of microstructure and thickness on the friction and wear
behavior of CrN coatingsIntroductionExperimental detailsCoating
materialHardness measurementFriction and wear testing
Results and discussionSurface morphologyCrystal structure of the
coatingsNano-mechanical properties of the coatingsFriction and wear
resultsEffect of counterface material
SummaryAcknowledgmentsReferences