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Applied Surface Science 253 (2007) 3547–3556

Chemical reactivity of PVD-coated WC–Co tools with steel

S. Gimenez, S.G. Huang, O. Van der Biest, J. Vleugels *

Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium

Received 26 January 2006; received in revised form 21 July 2006; accepted 22 July 2006

Available online 1 September 2006

Abstract

The chemical reactivity of CrN, ZrN, TiCxN1�x and naCo1 PVD coatings on a WC–Co cemented carbide substrate with steel has been

evaluated by means of the static interaction couples technique. Diffusion experiments with coated and uncoated tools were carried out at 900, 1100

and 1300 8C in order to establish the maximum temperature at which the substrate–coating–workpiece combinations are chemically stable.

Computational equilibrium thermodynamics was used to identify the interaction products formed at elevated temperature and the chemical

solubility of the different coating materials into iron. A metallic (Fe, Co) fcc solid solution was identified at the steel side of the interface from

1100 8C on for all the coated tools and from 900 8C for the uncoated carbide. In addition to this interaction product, the E-carbide was identified at

1300 8C on the WC–Co side of the interface. Both of the experimental findings and thermodynamic equilibrium solubility calculations

demonstrated that the PVD-coated WC–Co tools exhibit a lower chemical reactivity with respect to the uncoated tools.

# 2006 Elsevier B.V. All rights reserved.

PACS: 81.15 Cd; 82.30 Hk; 82.60 Lf; 82.60 Lf; 82.60 Cx; 82.80

Keywords: Chemical reactivity; Interaction couples; PVD coatings; Chemical wear

1. Introduction

Physical vapour deposition (PVD) of hard layers of nitrides,

carbides or oxides on the surface of materials constitutes one of

the fast developing research lines in materials science due to

their high potential to increase the lifetime of functional

components [1]. The increased performance of PVD coatings

regarding wear and corrosion resistance, tribological behaviour

and thermal stability with respect to monolithic components is

directly related to the highly defected, amorphous structure and

smaller grain size [2]. One of the main driving forces for the

development of PVD coatings is connected to the cutting tools

industry. In the last 30 years, coating engineering has generated

different generations of coating materials ranging from TiN

monolithic monolayer coatings to nanocomposite multilayered

coatings [3], aiming at improving the performance of cutting

tools when operating under the most severe conditions, i.e.,

high speed machining, dry-machining, interrupted cutting, etc.

. . .. Under these machining conditions, high temperatures are

generated at the chip–tool and tool–workpiece contacts and

* Corresponding author. Tel.: +32 16 321244; fax: +32 16 321992.

E-mail address: jozef.vleugels@mtm.kuleuven.be (J. Vleugels).

0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2006.07.062

consequently, chemical wear, i.e., the dissolution and diffusion

of the tool material into the workpiece material, can be the

predominant tool wear mechanism. Uncoated cemented

carbides, cermets, polycrystalline diamond and Si3N4 inserts

for example are not suitable for high speed machining of steel

due to the chemical interaction [4].

Information on chemical wear can be obtained from static

interaction couples, where adhesive and abrasive wear mechan-

isms do not interfere as it is the case during machining operations.

In the present work, the chemical reactivity of different PVD

coatings with steel has been studied by assessing the extent of

interdiffusion of chemical species between the coating, substrate

and steel workpiece material. The experimental results obtained

are compared with equilibrium solubility calculations following

the approach of Kramer [5,6] which has been successfully used at

our lab to explain the chemical wear of Si3N4 and sialon ceramics

and composites when machining steel [4,7,8].

2. Materials and experimental techniques

2.1. Materials

CrN, TiCxN1�x, ZrN and the naCo1 nanocomposite

monolayer PVD coatings were applied by Platit AG (Grenchen,

S. Gimenez et al. / Applied Surface Science 253 (2007) 3547–35563548

Switzerland) on cylindrical WC + 10Co + 0.5Cr2C3 + 0.2VC

(wt%) cemented carbide blanks (grade DK460UF) provided by

Guhring oHG (Sigmaringen, Laiz, Germany). The CrN,

TiCxN1�x and ZrN coatings are monolithic whereas the naCo1

(Platit AG, Grenchen, Switzerland) is a nanocomposite coating

composed of AlxTi1�xN particles with a particle size � 3 nm

embedded in an amorphous Si3N4 matrix.

The coating materials were characterised by scanning

electron microscope (SEM, XL-30 FEG, FEI, Eindhoven, The

Netherlands) equipped with an energy dispersive analysis

system (EDS, EDAX, Tilburg, The Netherlands) for composi-

tional analysis. The coating crystallography was evaluated by

X-ray diffraction (XRD, 3003-TT, Seifert, Ahrensburg,

Germany). Fig. 1 shows the cross-sectioned cemented carbide

substrate–coating system revealing the coating thickness (t).

The nanostructure of the naCo1 coating cannot be visualised at

the magnification shown in Fig. 1c. More details on the

processing and properties of PVD nanocomposite coatings can

be found elsewhere [3].

From the crystallographic characterisation by XRD the CrN

and ZrN phases were clearly identified. Additionally, the

amorphous structure of the Si3N4 matrix in the naCo1 coating

Fig. 1. Microstructure of the different cross-sectioned cemented carbide-coating. (

coating. All micrographs were taken at the same magnification.

was confirmed by the absence of a diffraction peak for this

phase. Moreover, small diffraction peaks were identified

corresponding to the AlxTi1�xN phase. Since the unit cell

parameter of TiCxN1�x solid solutions is a linear function of x,

[9] the composition of this coating was identified from the 2u

values of the (1 1 1) and the (2 0 0) diffraction lines to be

TiC0.6N0.4.

2.2. Chemical reactivity

The chemical reactivity of the uncoated cemented carbide and

the different coatings with steel was studied by static interaction

couples assessing the extent of interdiffusion of chemical species

between the WC–Co cemented carbide substrate, the coating and

the steel, pressed together under a small load of 2.5 MPa during

1 h invacuum (<0.1 Pa) at different temperatures (900, 1100 and

1300 8C) for a predefined dwell time with a heating and cooling

rate of 50 8C/min. A W100/150-2200-50LAX hot press (FCT

Systeme, Rauenstein, Germany) was used. A structural carbon

steel (DIN 17100 St 37-2 with 0.2 wt% C, 0.05 wt% P and

0.05 wt% S) was chosen for the investigation to facilitate the

analysis of interdiffusion of species. Fig. 2 shows a schematic

a) CrN, (b) TiCxN1�x, (c) naCo1 and (d) ZrN. t indicates the thickness of the

S. Gimenez et al. / Applied Surface Science 253 (2007) 3547–3556 3549

Fig. 2. Configuration of the interaction couple set-up.

representation of the interaction couple set-up. Before assem-

bling, both steel and coated cemented carbide were ultrasonically

cleaned in ethanol. After cooling, the interaction couples were

cross-sectioned, polished, etched with 2 vol% Nital and

investigated by means of SEM and electron probe microanalysis

(EPMA, Superprobe 733, JEOL, Tokyo, Japan) equipped with a

Noran EDS system. More information on the experimental

procedure is provided in Refs. [4,7].

3. Results

Preliminarily, the interaction between the uncoated WC–Co

and steel was evaluated through interaction experiments at 900,

1100 and 1300 8C for 1 h (Fig. 3). At 900 8C, Co diffusion into

Fig. 3. Micrographs illustrating the interaction between the uncoated WC–Co and ste

(c) is taken at a lower magnification than (a and b).

the steel is evidenced in Fig. 3a by the pearlitic-free area in the

steel side adjacent to the interface indicated as ‘‘1’’. At

1100 8C, the same interaction product is observed, the pearlitic-

free area is more extended and W diffusion into the steel is

evidenced by the bright spots observed in the interaction area 1

very close to the interface (Fig. 3b). No clear signs of Fe

diffusion into the hardmetal are observed at 900 and 1100 8C.

Additionally, the structure of the hardmetal in contact with the

steel is not significantly affected by the high temperature

excursion. At 1300 8C (Fig. 3c), the interaction is much more

pronounced. The region labelled as 1 is associated with the

diffusion of Co and W across the interface as deduced from the

compositional profiles obtained by EPMA (Fig. 4). On the

cemented carbide side, a W–Co-rich carbide, labelled as 2, is

formed at the interface (Fig. 3c) dissolving the Fe diffused

across the interface, as evidenced by Fig. 4. Some globular Fe–

Co-rich particles (darker contrast) appear embedded in region

2, close to the interface between regions 1 and 2 (Fig. 3c). The

presence of these particles coincides with the Co peak observed

at the same location in Fig. 4.

3.1. Coatings

3.1.1. Chemical interaction at 900 and 1100 8CNo appreciable interaction was observed in the interaction

couples tested at 900 8C for 1 h since the constituent materials

spontaneously separated after thermal cycling. After 1 h at

1100 8C, however, all substrate–coating–steel combinations

tested showed a similar behaviour, i.e., Co diffusion across the

el at 900 8C (a), 1100 8C (b) and 1300 8C (c). For the sake of clarity, micrograph

S. Gimenez et al. / Applied Surface Science 253 (2007) 3547–35563550

Fig. 4. Compositional profile across the cemented carbide–steel interaction

couple after 1 h at 1300 8C.

coating from the cemented carbide to the steel was clearly

identified by EDS analysis. This fact was also reflected in a

pearlite-free region adjacent to the coating–steel material

interface with similar composition as the metallic phase 1

observed in the interaction between uncoated carbide and steel

(Fig. 5). It is believed that the major counterdiffusion element

from the steel side of the interface is Fe, although no hard

proof could be provided by EDS analysis of the cemented

carbide side of the interaction couple at 1100 8C. The coating

integrity, i.e., the absence of coating dissolution or damage

after the tests at 1100 8C was excellent for TiC0.6N0.4, naCo1

and ZrN. The CrN coating however showed a certain degree of

solution, evidenced by the irregular coating thickness and the

Fig. 5. Microstructure of the cross-sectioned interaction couples tested for 1 h at 110

indicated by the double arrows. All micrographs were taken at the same magnific

Cr-containing acicular structure formed in the pearlite-free

zone shown in Fig. 5a. Partial oxidation of the CrN coating due

to interaction with the furnace atmosphere was also evidenced

by EDS. Due to the lack of chemical interaction, a small

continuous gap is observed between the steel and the coating

in case of TiC0.6N0.4 and ZrN, whereas the interface between

the naCo1 coating and WC–Co substrate appears cracked.

3.2. Chemical interaction at 1300 8C

3.2.1. CrN

The microstructure of the cross-sectioned interaction couple

after 1 h at 1300 8C is presented in Fig. 6a. The CrN coating

completely dissolved and two different phases could be

identified at both sides of the original interface. On the steel

side, a pearlite-free zone adjacent to the interface is observed,

similar to that present after testing at 1100 8C (Fig. 5b) for

coated tools and from 900 8C for uncoated substrate. This

region, labelled as 1 in Fig. 6a, is associated with the diffusion

of Co and W across the interface as deduced from the

compositional profiles obtained by EPMA (Fig. 6b). Penetra-

tion of this phase into the cemented carbide side of the

interaction couple is also observed. Additionally, some porosity

can be identified in phase 1, originating from the decomposition

of the coating according to the following reaction: 2CrN

(sol)$ 2Cr (ss) + N2 (gas).

On the cemented carbide side, a W–Co-rich carbide, labelled

as 2, similar to that observed for the uncoated hardmetal at the

same temperature is formed at the interface dissolving the Fe

0 8C. (a) CrN, (b) TiC0.6N0.4, (c) naCo1, (d) ZrN. The pearlite-free region (1) is

ation.

S. Gimenez et al. / Applied Surface Science 253 (2007) 3547–3556 3551

Fig. 6. Overview (a) and EPMA composition profile (b) of the cross-sectioned

steel/CrN/cemented carbide interaction couple after 1 h at 1300 8C.Fig. 7. Overview (a) and compositional profile (b) of the cross-sectioned

interaction couple of the TiC0.6N0.4-coated cemented carbide with steel after

1 h at 1300 8C (C = coating).

diffused across the interface, as evidenced by Fig. 6b. The

penetration of phase 1 into phase 2, as observed in Fig. 6, is

responsible for the irregular Co profile and the abrupt decrease

of the W content near the interface of regions 1 and 2 (Fig. 6b).

The solution of Cr in both interaction zones is evidenced by the

compositional profile. Note the different scale for the Cr

content in Fig. 6b due to the much lower Cr content compared

to the Fe, W and Co content.

Since W diffuses across the original interface, the WC phase

in the cemented carbide must be decomposing, generating a C

gradient across the interface with the steel. The limitations of

EDS system used to quantify the C content makes a more

quantitative evaluation of this issue impossible. However, the

higher than expected for a 0.2 wt% C steel pearlite content on

the steel side of the interaction couple close to the interface

proofs the dissolution and diffusion of carbon into the steel.

3.2.2. TiC0.6N0.4

An overview of the interaction area observed after 1 h at

1300 8C is shown in Fig. 7a. Interaction phases 1 and 2 similar to

those observed for the uncoated carbide and for the experiment

with the CrN-coated cemented carbide can be identified and

correlated to Fe, Co and W (and C) diffusion across the coating

(Fig. 7b). The interaction layer on the cemented carbide side,

phase 2, contains cracks as a result of the strong bonding at the

interface and the thermal stresses during cooling of the

interaction couple. Pronounced interdiffusion across the coating

is evidenced by the local presence of the Co and W containing

pearlite-free material (phase 1) on both sides of the coating.

3.2.3. naCo1

A significant degree of interaction was observed for the

naCo1 coating at 1300 8C, as shown in Fig. 8. Similar to the

interaction couple with the CrN- and TiC0.6N0.4-coated carbide,

an interaction zone can be found on both sides of the coating

corresponding to phases 1 and 2. Additionally, a Fe, Co, W and

C containing phase, labelled as 3, is observed surrounding the

coating interface evidencing the high degree of interaction. The

interface between phases 1 and 3 appears cracked as indicated

in both micrographs. The EDS compositional profile is shown

in Fig. 8c. Note that a secondary axis is included for the

composition of the low content elements (Si, Al and Ti). A

magnified view of this profile with a superimposed micrograph

(Fig. 8d) revealed the segregation of Al to the external part of

the coating (dark areas at the coating/phase 3 interface). No Al

was detected in the interaction phase 3 adjacent to the coating.

The Al originates from the decomposition of the (Al, Ti)N solid

solution phase during the high temperature experiment,

revealing a higher affinity of Al to dissolve in the steel

compared to Ti.

The identification of the Si3N4 amorphous matrix phase in

the coating by EDS was not straightforward, since there is an

overlap of the W M and Si K EDS spectral lines. Qualitative

wavelength dispersive spectroscopy (WDS) analysis revealed

the presence of Si in phase 3, therefore it is believed that the

Si3N4 phase in the coating partially decomposed at 1300 8C.

S. Gimenez et al. / Applied Surface Science 253 (2007) 3547–35563552

Fig. 8. Overview (a) and detail (b) of the cross-sectioned interaction couple of the naCo1-coated carbide with steel after 1 h at 1300 8C. The interaction phases 1–3

and the coating (C) are indicated. Compositional profile (c) and magnified profile at the coating interface (d) of the interaction couple.

Fig. 9. Overview (a), detailed view (b) and compositional profile (c) of the cross-sectioned interaction couple of the ZrN-coated cemented carbide with steel after 1 h

at 1300 8C. Interaction phases 1 and 2 and the coating (C) are indicated.

S. Gimenez et al. / Applied Surface Science 253 (2007) 3547–3556 3553

3.2.4. ZrN

An overview of the cross-sectioned interaction couple

between the ZrN-coated cemented carbide and the steel after

1 h at 1300 8C is shown in Fig. 9a. Since the contrast of the

interaction phase 1, pearlite and the coating is very similar, a

magnified view of the coating is shown in Fig. 9b. Similar to

the other interaction couples, a diffusion layer is present on

both sides of the coating, previously labelled as phases 1 and

2. The corresponding cross-sectional compositional profile is

given in Fig. 9c, revealing the diffusion of Fe, Co and W

across the coating. Again, the irregular compositional

profiles for Fe and Co within phase 2 are due to the

presence of phase 1 pools within phase 2. At the high magn-

ification micrograph (Fig. 9b), the pronounced interdiffusion

of chemical species is also reflected on the degradation of the

coating structure.

The results of the interaction couple investigation are

summarized in Table 1. The coating integrity, i.e., resistance

to structural degradation, has been ranked from 0 (totally

dissolved coating) to ��� (best integrity). IA1 and IA2

correspond to the widths of the interaction zones 1 and 2,

respectively, given as the average of five measurements on

the steel and the cemented carbide side, respectively. IAtotal is

the total width of the interaction zone, taken as the sum of

IA1 and IA2. Considering both the coating integrity and

the extension of the interaction zone, the chemical

stability of the coatings can be ranked as CrN < naCo1 <TiC0.6N0.4 < ZrN.

4. Discussion

4.1. Identification of the interaction phases

Irrespectively of the coating material tested, a W–Co–Fe

carbide solid solution (phase 2) is formed on the cemented

carbide side of the coating whereas a W and Co containing

metallic diffusion zone (phase 1) is formed on the steel side of

the interaction couple after annealing at 1300 8C. Although

the TiC0.6N0.4 and ZrN coatings do not seem to be significantly

Table 1

Summary of the results of the static interaction couples

Coating T (8C) Time (h) Integrity

CrN 900 1 –

1100 1 �1300 1 0

TiCxN1�x 900 1 –

1100 1 ���1300 1 ��

naCo1 900 1 –

1100 1 ���1300 1 ��

ZrN 900 1 –

1100 1 ���1300 1 ��

IA1 and IA2 refer to the widths of the interaction layer on the steel (phase 1) and the c

qualitative estimation of the coating integrity.

dissolved, significant interdiffusion of material was observed

(Fig. 9b). This could be due to the columnar structure of the

PVD coatings that promotes diffusion of ions along the

columnar grain boundaries. The CrN coating dissolved

completely at 1300 8C, whereas the coating integrity of the

naCo1 was lower than that of TiC0.6N0.4 and ZrN due to the

partial dissolution of TiAlN and Si3N4. The irregular profiles

observed for Fe and Co within phase 2 at 1300 8C are due to

the presence of the metallic phase 1 within phase 2. This is

partly related to the roughness of the initial contact planes as

well as the shear forces generated during the interaction

couple test.

The higher than expected for a 0.2 wt% C steel pearlite

content on the steel side of for example the CrN-coated

cemented carbide–steel interaction couple after 1 h at

1300 8C close to the interface proofs the dissolution and

diffusion of carbon from the cemented carbide into the steel.

In order to identify the interaction products observed on both

sides of the interaction couple, i.e., phases 1 and 2,

thermodynamic calculations were carried out in the Fe–

Co–W–C–X system. X = 0 (uncoated carbide), Cr, Ti, Al, Si

and Zr were considered in the different calculations

depending on the corresponding coating studied. Since the

presence of N was not detected in any of the interaction

phases, this element has not been further considered in the

thermodynamic calculations. The Thermo-Calc software and

the SSOL database [10] were used. References related to

previous calculations and thermodynamic models are summ-

arized [11–15].

Fig. 10a shows a calculated phase relations in the Fe–W–

Co–C–Cr system, under the conditions of n = 1, P = 105 Pa,

T = 1300 8C, x(Co) = 0.1, x(Fe) = 0.7, x(C) � x(W) = 0,

x(Cr) = 0.02, where n is the number of moles of the whole

system, P the pressure, T the temperature and x is the molar

fraction of components. The simulation indicated that the

phase constitutes varied with the gross Fe content at 1300 8C.

The carbon-deficient M6C phase is observed due to the

increasing solution of C in fcc or liquid Fe. However, under the

high temperature annealing process, the diffusion rate of

IA1 (mm) IA2 (mm) IAtotal (mm)

– – –

15.6 � 0.7 0 15.6 � 0.7

43 � 5 73 � 3 106 � 8

– – –

8.9 � 0.7 0 8.9 � 0.7

37 � 2 70 � 10 107 � 11

– – –

8 � 3 0 8 � 3

57 � 4 39 � 2 97 � 6

– – –

5.6 � 0.7 0 5.6 � 0.7

41 � 1 47 � 9 87 � 10

emented carbide (phase 2) side. IAtotal = IA1 + IA2. The symbols ‘‘�’’ provide a

S. Gimenez et al. / Applied Surface Science 253 (2007) 3547–35563554

Fig. 10. Evolution of the weight fraction of the stable phases in the Fe–Co–W–C–Cr when x(C) � x(W) = 0 (a) and x(C) � 0.9x(W) = 0 (C depletion) (b) together

with the evolution of the composition of the M6C carbide (c) and the fcc (Fe, Co) phase (d) as function of the mole fraction of iron.

carbon atoms from the decomposed WC grains is much faster

compared to W atoms in Co or steel. The carbon-deficient h

phase can be more easily formed due to the insufficient carbon

content. In Fig. 10b, the condition x(C) � x(W) = 0 is changed

to x(C) � 0.9x(W) = 0, simulating a partial C depletion during

the high temperature experiment due to the possible oxidation

or the much faster diffusion rate of C atoms than that of the W

atoms. It is worth to mention that the selected parameters for

simulation can be considered as a very rough approximation of

the equilibrium conditions taking place at the WC–Co/Fe

interface, but a more complex calculation is considered to be

out of the scope of the present work. Since the presence of the

coating did not significantly affect the phases formed at both

sides of the coating, the main objective has been to identify the

phases formed at the WC–Co/Fe interface and include the

presence of coating elements to see how the stability of the

identified phases changed.

It is clearly observed that the stability of the M6C phase

increases with the presence of increasing of Fe content and the

decreased carbon content. Since some oxygen is present in the

atmosphere during the high temperature experiment

(P = 0.1 Pa) and some C diffuses to the iron side of the

interface, it is reasonable to associate the phase M6C with that

labelled as 2 in all the interaction couples. Multiple examples of

the formation of M6C phase in WC–Co cemented carbides

related to C depletion can be found in literature, e.g., Ref. [18].

This phase can dissolve some Cr (Fig. 10c), as observed in the

compositional profile given in Fig. 6b, as well as a significant

amount of Fe. Moreover, different starting compositions were

taken (0.3 < x(Fe) < 0.8) and the evolution for the M6C phase

was found to be similar (considerable increase of the

equilibrium volume fraction of this phase when decarburisation

takes place).

The pearlite-free zone on the steel side of the interaction

couple, i.e., phase 1, has been identified as a fcc (Fe–Co)

solid solution, which is the stable phase at the highest Fe

content as shown in Fig. 10a and b. The substitutional

solubility of Fe and Co in this phase is perfectly reflected in

both the compositional profile (Fig. 6b) and the calculation of

composition (Fig. 10d). Identically, the solubility of both W

and Cr is also in good agreement in both cases. It is believed

that the amount of Co and W in this phase is high enough to

stabilise the fcc phase at room temperature, explaining the

absence of carbides and the concomitant pearlite structure in

this region.

The phase identification of phases 1 and 2 carried out for the

CrN interaction couple can also be extended to the rest of the

studied systems, since identical phases are observed. The

thermodynamic calculations of the relevant Fe–Co–W–C–X

systems (including X = 0 for the uncoated carbide) revealed the

presence of the same fcc (Fe, Co) and M6C phases with

analogous compositions as those given in Fig. 10. With respect

to phase 3, observed at both sides of the naCo1 coating after

testing at 1300 8C, the correlation between thermochemical

calculations and the compositional profiles (Fig. 8c and d) did

not lead to the identification of any additional phase. It is

possible that this region is constituted by a mixture of fcc and

M6C phases, or to a fcc phase with a W content higher

compared with the zones identified as 1. Further work is needed

to clarify this issue.

S. Gimenez et al. / Applied Surface Science 253 (2007) 3547–3556 3555

4.2. Equilibrium solubility of tool materials and coatings

in Fe

The chemical stability or reactivity of the coating material

in contact with steel can be estimated from the calculated

equilibrium solubility of coating or substrate materials into

iron [4]. The effect of alloying elements in steel on the

solubility of coating materials was not considered here for the

simplification. The equilibrium calculation procedure was

briefly explained for the dissolution of a hypothetical AxBy

phase in pure iron according to the previous work [4–8]. The

phase transformation from bcc-Fe to fcc-Fe was taken into

account during the calculation. The formation energy (Gf) of

the AxBy phase was taken from the thermodynamic database

of Barin [16]. The relative partial molar excess Gibbs energy

of solution of the different elements A and B in iron was

obtained using the Thermo-Calc software and the SSOL

database [10]. The molar equilibrium solubility (mol/mol

solution) of the individual phases can be converted into a

volumetric solubility (cm3/mol solution) by means of the

molar volume of the phase, calculated from the density and

the molar weight.

Fig. 11 plots the equilibrium solubility of the substrate and

the different coating materials experimentally tested as a

function of temperature. TiC0.6N0.4 was the stoichiometry

selected for the TiCxN1�x coating as characterised by XRD.

Regarding the multiphase naCo1 coating, the mol% of the

different phases selected was 20% Si3N4 and 80% equally

distributed for TiN and AlN since this is the standard phase

distribution leading to the characteristic superhardness for this

coating (>40 GPa) [3,17]. The calculated solubility results

reported in Fig. 11 matched perfectly with the ranking of the

degree of interaction from the interaction couples study, i.e.,

CrN < naCo1 < TiCxN1�x < ZrN. It is also clear that the

weakest link in the naCo1 coating is the Si3N4 phase, which

exhibits a lower solubility compared to the TiAlN phase,

justifying the presence of Si in the phase 3 after the diffusion

test at 1300 8C during 1 h. The damage observed in the ZrN

coating (Fig. 9b) is not chemical in nature but a consequence of

the pronounced interdiffusion of species between the hardmetal

and the steel due to the columnar structure of the PVD coatings.

The benefit of the use of the coatings compared to the pure

Fig. 11. Calculated equilibrium solubility of the different coatings, substrates

and phases of the multiphase naCo1 coating in iron.

substrate material is also evident since solubility of WC and

WC–Co is much higher compared to the different coatings

discussed. This correlates well with the experimental results

obtained, since evidence of WC–Co/Fe interaction was

observed already at 900 8C.

5. Summary and conclusions

The chemical reactivity of CrN, TiCxN1�x, ZrN and naCo1

PVD-coated and uncoated WC–Co cemented carbide with

0.2 wt% carbon steel was investigated by means of static

interaction couples at 900, 1100 and 1300 8C. Independently on

the coating material, severe diffusion of Fe, Co and W across the

coating resulted in the formation of two distinct interaction

phases, i.e., an fcc (Fe, Co) metallic solid solution on the steel

side and a (Fe, W, Co)6C phase on the cemented carbide side of

the interaction couple. A similar interaction was observed for the

interaction with the uncoated cemented carbide. The coatings

were found to be stable in contact with the steel at 900 8C,

whereas clear interaction was observed after 1 h at 1100 and

1300 8C. Based on the extent of interaction, the coatings could be

ranked as CrN < TiCxN1�x < naCo1 < ZrN. The CrN coating

was found to completely dissolve at 1300 8C. Equilibrium

solubility calculations of the coating material in pure iron showed

a perfect agreement with the experimental interaction couple

results. It was also demonstrated that the PVD coatings reduce

the extent of interaction compared to the uncoated cemented

carbide.

Acknowledgements

The authors wish to thank Dr. Tibor Cselle from Platit AG

for supplying the coating materials, Michael Loeffler and

Manfred Schwenk from Guhring oHG for providing the

cemented carbide substrates and Prof. K.C. Hari Kumar from

the Indian Institute of Technology Madras for his support with

the thermodynamic calculations. The European Commission is

acknowledged for the financial support through the PM-MACH

Growth project (Contract No. G1RD-CT2002-00687).

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