CONTACT FATIGUE DAMAGE OF MONOLAYER AND BILAYER CERAMIC COATINGS
DEPOSITED ON CEMENTED CARBIDES
L. Llanes1,2, E. Tarres1,3, G. Ramírez1,4, E. Jimenez-Piqué1,2, N. Salán1 and A. Mateo1
1
CIEFMA- Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica,
Universitat Politècnica de Catalunya, ETSEIB, Barcelona 08028, Spain
2
CRnE, Campus Diagonal Sud, Universitat Politècnica de Catalunya, Barcelona 08028, Spain
3
Sandvik Hard Materials, Coventry CV4 0XG, UK
4
CTM Centre Tecnològic, Manresa 08242, Spain
RESUMEN
En este trabajo se estudia el comportamiento a fatiga bajo solicitaciones de contacto de un carburo cementado WC-Co
recubierto con dos películas cerámicas distintas: TiN y WC/C, ya sea como monocapas o bicapas, mediante técnicas de
indentación. Los resultados obtenidos en ensayos de indentación esférica indican que la nucleación de fisuras circulares
en las capas es un criterio más apropiado que la delaminación interfacial para definir las condiciones de daño crítico en
los sistemas investigados. Desde esta perspectiva, se encuentra que los carburos cementados recubiertos son
susceptibles a ser degradados mecánicamente bajo solicitaciones de contacto cíclicas, aunque la sensibilidad a fatiga y
el escenario de daño efectivos difieren en cada caso, en función de la naturaleza cerámica (cristalina/amorfa) y la
arquitectura (mono- o bicapa) del recubrimiento.
ABSTRACT
The contact fatigue behavior of a WC-Co cemented carbide coated with two distinct ceramic films: TiN and WC/C,
either as monolayers or bilayers, is studied by means of indentation techniques. Spherical indentation tests indicate that
circular cracking at the coating is a more appropriate choice than interfacial delamination for defining critical damage in
the coated hardmetals studied. From this perspective, coated cemented carbides are found to be fatigue susceptible
under contact loading, although effective fatigue sensitivity and failure scenario are discerned to be dependent on
ceramic nature (crystalline/amorphous) and coating assemblage (mono- or bilayer).
KEYWORDS: contact fatigue; ceramic film; coated cemented carbide; mono- and bilayer assemblage
1. INTRODUCTION
Cemented carbides belong to a class of composite
materials, in which hard particles, tungsten carbide
(WC), are bound together by a soft and ductile metal
binder, cobalt (Co). Such a particular microstructure
assemblage, usually referred to as hardmetals, yields an
extraordinary combination of mechanical properties,
allowing the use of these materials in a wide range of
applications. From this perspective, the role of
cemented carbides in metal cutting and forming tools is
remarkable, the corresponding components being here
subjected to a complex state of loading (abrasive and
adhesive wear, impact, repetitive mechanical contact,
etc.) that determines their lifetime (e.g. Ref. [1]). On the
other hand, the need to extend the lifetime of parts
and/or, for example, minimize the use of lubricants in
order to reduce time, maintenance and manufacture
costs has led to coat base hard materials with different
thin films or coatings. In this regard, the development of
vacuum deposition technologies, physical vapor
deposition (PVD) and chemical vapor deposition
(CVD), has been of major impact, since they make it
possible to deposit a thin layer of only a few
micrometers on the surfaces of most engineering
materials The geometrical change is minimal and the
surface layers may have properties covering an
extremely wide range of values [2]. In the metal cutting
and forming areas, research efforts have mainly ranged
from productions of coatings designed for high hardness
for cutting applications to low-friction films for sheet
forming ones, although optimal choices usually involve
combined approaches [3].
There exists extensive literature on the tribomechanical
response of coated systems to be used as metal cutting
or forming tools, though the use of film deposition
technology in the former is far more established than in
the latter [4,5]. In general, it has centered primarily on
investigating hardness, scratch resistance, friction
coefficient and wear behavior as a function of either
film nature and architecture - single versus multilayer (e.g. Refs. [5-8]) or surface roughness (e.g. Refs.
[7,9,10]). However, these experimental approaches are
rather deficient in evaluating issues directly pertinent to
mechanical performance of coated tools or components
subjected to repetitive contact loading. On the other
hand, experimental and analytical approaches using
spherical indenters to deliver stresses over a small area
of specimen surface, i.e. testing protocols based on the
Hertzian theory, have proven to be successful on the
assessment of contact damage in bulk polycrystalline
ceramics as well as layered structures (e.g. Refs.
[11-13]). Within this framework, it is the aim of this
work to assess the contact mechanical response and the
corresponding damage mechanisms, under both
monotonic and cyclic loading conditions, on two
different PVD coatings deposited, as either individual
monolayers or a combined bilayer, on a fine-grained
hardmetal. In doing so, experimental protocols based on
spherical
indentation
testing
techniques
are
implemented.
2. EXPERIMENTAL PROCEDURE
A commercial fine-grained WC-10%wt Co cemented
carbide grade was used as substrate. Elastic modulus
and hardness for such base material, as determined in a
previous study [14], were 540 GPa and 14.5 GPa,
respectively. Two coatings were investigated, both
deposited following Oerlikon-Balzers’s PVD processes:
(1) a single-layer TiN film by arc ion plating, and (2) a
multilayer diamond-like carbon (DLC) one, in this study
referred to as WC/C, by magnetron sputtering. They
were deposited as 3.7 m (TiN) and 3.3 m (WC/C)
individual monolayers as well as a combined bilayer of
2.7 m WC/C on 3.5 m TiN (Figure 1). In all the
cases dense coatings were attained. TiN layers exhibited
a crystalline fine-grained structure whereas WC/C films
a)
b)
c)
Figure 1. SEM micrographs of: focused ion beam
milled cross-sections of a) TiN and b) WC/C
monolayers; and fractured cross-section of
WC/C//TiN bilayer.
consisted of a nanostratified arrangement of amorphous
WC and C lamellas. Young’s modulus and intrinsic
hardness for both coatings were determined by
nanoindentation (MTS Nanoindenter XP) equipped with
a continuous stiffness modulus. As expected, the
crystalline coating exhibited significantly higher
stiffness (430 GPa as compared to 135 GPa) and
hardness (29 GPa with respect to 12 GPa) than the
alloyed amorphous one [15]. For the bilayer system the
hardness – penetration depth showed an early initial
plateau of 12 GPa (for depths about 250 nm) associated
with the intrinsic hardness of the top WC/C layer. As
load was increased, hardness also rose up to values
close to 20 GPa, where it described a second plateau
behavior starting at 2 m and lasting up to final
penetration depth tested, i.e. 6 m. Rising hardness
values are intimately related with the influence ascribed
to harder base layer and substrate, although the intrinsic
hardness of TiN film was never attained. One possible
reason for such finding is the fact that local chipping
was discerned as penetration depth got levels of 3 m,
i.e. close to the internal bilayer interface.
Similar trend was also discerned regarding adhesion, as
given by values of 98 N and 38 N for TiN and WC/C
respectively, for the critical normal load related to initial
coating detachment under scratch testing. Such critical
load rose to 54 N for the bilayer system and it was very
close to the one needed (51 N) for inducing decohesion
at the internal interface between WC/C and TiN layers.
Although these findings are in agreement with previous
literature reports (e.g. Ref. [15]), it should be
highlighted that critical loads in all the cases were
higher than 30 N, a level generally described as
sufficient in scratch testing with a Rockwell C diamond
tip for tooling applications [4].
The mechanical contact response of the coated systems
was investigated by means of spherical indentation.
Hertzian tests were conducted in a servohydraulic
testing machine (Instron 8511) by using a hardmetal
spherical indenter with a curvature radius of 1.25 mm.
Definition of the critical failure event was done on the
basis of “crack prevention”, an approach different from
the coating delamination criteria usually invoked in
previous works involving cyclic impact testing (e.g.
Refs. [16,17]). As it will be seen later, this definition is
sustained on the fact that through-thickness circular
cracks at the coating lead to hardmetal substrate
cracking, independent of the loading conditions, before
any interfacial failure (at the coating/substrate level)
takes place. Accordingly, the first stage on the
experimental protocol followed was the determination
of the critical load for circular crack emergence at the
coating surface under monotonic loading, Pc. Once it
was assessed, fatigue testing was carried out by
applying fractions of such critical load as the maximum
cyclic load (Pmax). The cyclic loading was imposed by
means of a sinusoidal waveform at a frequency of 10 Hz
and corresponding load ratio of 0.1. Main outcome of
such cyclic tests was critical load for discerning similar
circular cracks at the films after a very high number of
cycles, namely 106 cycles, Pf.
Evolution of subsurface indentation damage, with
increasing load or number of cycles, was investigated
by means of specific monotonic and cyclic Hertzian
indentation tests conducted on “clamped-interface”
specimens. In doing so, a procedure similar to that
commonly employed in ceramics by Lawn’s group (e.g.
Ref. [18]) was followed; although here extended to
coated surfaces. Briefly, these may be described as
coated samples consisting of two previously ground and
polished cross-sections clamped together by artificial
means. Indentation across the surface trace of the
existing interface and subsequent mechanical separation
of the referred halves permits then examination of
damage features under the surface, through optical and
scanning electron microscopy (SEM).
3. RESULTS AND DISCUSSION
3.1 Monotonic spherical indentation
As applied load increases irreversible deformation of
the coated systems was evidenced through residual
surface traces. At relatively high level loads, first signs
of damage were discerned at the edge of the
corresponding residual imprints in terms of
circumferential cracks. The critical load for the
emergence of these cracks, Pc, for the three studied
systems is listed in Table 1. Nevertheless, plastic
yielding of the coated sets was required for the
subsequent cracking in the film, in agreement with
previous studies by other authors (e.g. Ref. [19]).
Regarding experimental data analysis, it should be
pointed out that data from Hertzian tests are usually
presented in terms of contact pressure or indentation
stress (p) as well as the resulting indentation strain (e.g.
Ref. [13]). Such approach was also implemented in this
study. Accordingly, the corresponding critical mean
contact pressures, given by pc = Pc /ac2 where ac is the
contact radius measured on the residual impression after
applying Pc, are also listed in Table 1. The results
attained clearly indicate that load and pressure levels for
discerning film rupture is higher for the DLC-coated
hardmetals, either as monolayer or as bilayer, than for
the one simply coated with TiN. Taking into
consideration the intrinsic properties of both coatings,
such a finding should be associated with the lower
stiffness and hardness exhibited by the WC/C
multilayer, which then render more elasticity and
tolerance to follow the deforming substrate before
experiencing brittle rupture. The above monotonic
critical loads and indentation stresses were then used as
baseline reference for comparison purposes with the
ones determined under cyclic loading conditions.
Table 1. Critical load and indentation stress under
monotonic contact loading for the appearance of
circular cracks in the mono- and bilayer ceramic
coatings deposited on the hardmetal substrate.
Coating
Critical monotonic
load for coating
cracking,
Pc (N)
Critical monotonic
indentation stress
for coating
cracking,
pc (GPa)
TiN
800
10.2
WC/C
1300
11.3
WC/C//TiN
1600
12.5
3.2 Cyclic spherical indentation
Cyclic indentation tests were conducted attempting to
evaluate the susceptibility to damage appearance for the
coated systems as either applied cyclic stress (for a
given number of cycles) or number of cycles (for a
given applied stress) increases. In agreement with
previous studies on other hard-coated systems
[16,20,21], they show that the critical damage
emergence is sensitive to cycle loading. However, two
interesting remarks should be done. First, different from
the trend observed under monotonic loading, maximum
cyclic loads for which time-differed damage is still
evidenced after a very large number of cycles, here
referred to as Pf, are lower for the DLC-coated systems,
either as monolayer or as bilayer, than for the TiN
coated one (Table 2). Second, damage mechanisms
distinct from the cohesive one observed under
monotonic loading (i.e. adhesive failure, for instance)
are not discerned, even after 106 cycles. Following the
analysis described in the previous section, critical cyclic
indentation stresses for coating cracking, pf, were
calculated from the determined Pf and corresponding
residual spherical imprint diameters. They are also
included in Table 2. It should be noticed that although
relative difference on contact fatigue strength between
DLC- and just TiN-coated systems decreases, it is still
higher for the latter.
From the critical parameters listed in Tables 1 and 2,
the fatigue susceptibility of the coated systems here
studied may be analyzed by means of the ratio
[1 – (pf / pc)] a parameter usually referred to in the
fatigue literature as fatigue sensitivity. In this sense, the
closer to 0 the ratio is, the less sensitive to contact
fatigue, as related to the failure mechanism defined as
critical event, the material should be, and vice versa. For
the TiN coated system the fatigue sensitivity is found to
be 0.13, i.e. a relatively low value, even when compared
to similar low values usually reported for other brittle-
Table 2. Critical load and indentation stress under
cyclic contact loading for the appearance of circular
cracks in the mono- and bilayer ceramic coatings
deposited on the hardmetal substrate.
Coating
Critical cyclic
load for coating
cracking,
Pf (N)
Critical cyclic
indentation stress
for coating
cracking,
pf (GPa)
TiN
400
8.9
WC/C
200
7.0
WC/C//TiN
200
7.1
like materials (e.g. see Refs. [22,23]). Considering that
the failure event chosen here as critical involves
cracking of the TiN film, it is speculated that such low
fatigue sensitivity should be intimately related to the
ceramic nature of the coating, especially if the high
compressive residual stresses exhibited by it are
considered [24]. Such a hypothesis may be somehow
supported by a recent work, by Cairney and coworkers
[25] on degradation of TiN coatings under cyclic
loading, where it is shown that differences between
intergranular shear stresses required for promoting
sliding along intercolumnar cracks, postulated by the
authors as the principal deformation mechanism in the
coating, under cyclic and monotonic (nanoindentation)
loading are minimal. However, the ceramic nature of the
coating does not seem to be a sufficient condition for
expecting low fatigue sensitivity. This is clearly
evidenced by the higher [1 – (pf / pc)] ratios: 0.38 and
0.43, determined for the monolayer and bilayer DLCcoated hardmetals, respectively. Such high fatigue
sensitivity level is close to those determined for PVDcoated steels under the consideration of interfacial
decohesion as critical damage event, i.e. one where a
metallic substrate is directly involved in the failure
mechanism [26]. In this regard, it is speculated that the
amorphous nature of the WC/C film may be playing a
key critical role on effectively determining the intrinsic
cyclic degradation susceptibility of this ceramic coating.
Indeed, the existence of alike microstructural features
(e.g. equiaxed/columnar assemblage or crystalline/
amorphous nature) has been pointed out by Jayaram et
al. [27] as plausible reason for the differences reported
by two distinct research groups [25,28] on the indenter
displacement response under cyclic loading of TiN
films.
3.3 Subsurface indentation damage
Spherical indentation tests are specially attractive
because, different from more conventional techniques
involving sharp-like indenters, they allow to monitor
damage evolution within an otherwise uncracked
microstructure as a function of increasing either applied
load (monotonic tests) or number of cycles (cyclic
tests). This is particularly interesting in the case of
coated systems due to the fact that here it is important to
evaluate not only the intrinsic competition between
deformation and fracture mechanisms but also where
they are first developing, i.e. within the coating or the
substrate, or even at the interface. Within this context,
the damage resulting from contact loading, under
monotonic and cyclic conditions, was inspected by
means of optical and scanning electron microscopy on
both the contact surface and below it, the latter by using
“clamped-interface” specimens. In doing so, the number
of testing conditions studied was limited and focused on
identifying damage micromechanims.
The evolution of Hertzian-induced damage with
increasing applied load or number of cycles was
qualitative similar. Optical inspections of imprint halfsurfaces and cross-sections in the bonded-interface
specimens revealed a significant plastic damage in the
substrate under the indentation contact area (Figure 2),
as expected from the range of contact stress imposed.
More accurate inspection by SEM showed that damage
within the coating evolves, beyond the assumed
stretching conjugated with the referred plastic
deformation of the substrate, into circular crack
nucleation at the periphery of the contact and at the
a)
Micron bar
b)
Figure 2. Induced damage under spherical indentation
for the WC/C-coated hardmetal: a) 1400 N (monotonic)
and b) 600 N – 105 cycles. Upper (darker) and lower
(brighter) optical micrographs in each image correspond
to half-surface and cross section views respectively of
clamped-interface specimens.
coating surface, and its subsequent advance through the
thin film up to the interface. The cohesive failure
through the whole coating thickness is quite straight for
monolayer films (Figure 3). However, similar finding
was not always evidenced for the bilayer-coated
hardmetal. Rather, in several imprints cracks were
observed to grow, from the surface into the film,
following inclined paths, and exhibiting deflection-like
features, particularly at the internal bilayer interface.
Considering that critical cyclic load for cracking is
higher for the inner TiN layer (Table 2), the role of the
referred interface as crack stopper for applied maximum
loads lower than 400 N was an interesting query to
answer in this study. However, validation of such
hypothesis was not possible in this investigation.
Nevertheless, it should be highlighted that crack - layer
assemblage interactions as the one detailed in Figure 3c
points out an improved contact response for the DLCcoated system if it is supported by a harder layer
underneath. Indeed, this could be one of the “unknown
reasons” for explaining the existing empirical
knowledge that additional mechanical support from hard
coatings results in enhanced tribomechanical response
of low- friction films (not only DLCs but also MoS2)
(e.g. Ref. [6]).
Finally, in all the cases such through-thickness fissures
lead to substrate cracking along the metallic binder
surrounding the ceramic particles. Here, it should be
noted (1) the absence of any intermediate interfacial
delamination stage (at substrate/coating level) before
substrate failure is observed, and (2) the role of coating
circular fissures as precursors of crack nucleation in the
underlying brittle-like hardmetal. Both experimental
facts are quite significant, within the context of the
investigation conducted here, because they sustain the
choice of the early circular cracking at the coating as the
critical damage event, mainly because it induces a loss
of mechanical integrity of the substrate before
anyadhesive failure takes place. Moreover, they also
point out the relevant influence of the substrate nature
on the contact response of the coated system, on the
basis that such final damage evolution stage (substrate
cracking) is usually not evidenced when considering
tougher substrates, e.g. steels [26].
4. CONCLUSIONS
b)
a)
Based on the experimental study of the mechanical
contact behavior, under monotonic and cyclic spherical
indentation, of a WC-Co cemented carbide coated with
two distinct ceramic films: TiN and WC/C, either as
monolayers or bilayers, the following conclusions can
be drawn:
(1) Circular cracking at the coating is a more
appropriate choice than interfacial delamination for
defining critical damage under spherical indentation for
the coated hardmetal here studied. Such a statement is
based on the fact that it not only emerges as the first
failure feature observed with increasing load or number
of cycles but also leads to substrate cracking without
any intermediate failure at the substrate-coating
interface.
(2) Coated cemented carbides are susceptible to “real”
contact fatigue. However, the effective fatigue
sensitivity is dependent on ceramic nature, i.e.
crystalline versus amorphous. Additionally, the
consequent failure scenario associated with the induced
damage may also be dependent on the coating
assemblage regarding mono- or bilayers, the latter in
terms of a hard coating offering mechanical support to a
softer low-friction top-layer film.
c)
Figure 3. Typical contact damage scenario for the
coated systems studied, as assessed from spherical
indentation of clamped-interface specimens:
a) TiN monolayer; b) WC/C monolayer;
and c) WC/C//TiN bilayer.
ACKNOWLEDGEMENTS
This investigation was partly funded by the Spanish
Ministerio de Ciencia e Innovación (MAT2009-14461).
The support received from Direcció General de Recerca
del Comissionat per a Universitats i Recerca de la
Generalitat de Catalunya through recognition of
CIEFMA as Grup de Recerca Consolidat 2009SGR
1285 is also acknowledged. The authors would also like
to acknowledge the scholarship received from the
Consejo Nacional de Ciencia y Tecnología de México
(G.R.).
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