wear resistance of multilayered pvd tin/tan on hss

Surface and Coatings Technology 120–121 (1999) 528–534www.elsevier.nl/locate/surfcoat

Wear resistance of multilayered PVD TiN/TaN on HSS

Maria Nordin *, Mats Larsson, Sture HogmarkDepartment of Materials Science, The Angstrom laboratory, Uppsala University, Box 534, 751 21 Uppsala, Sweden

Abstract

For a given substrate material the tribological performance of thin hard coatings is mainly governed by one or several of thefollowing parameters: coating/substrate hardness/stiffness, coating fracture resistance, coating adhesion, the contact temperatureand chemistry (in the prevailing tribosystem). Thus, for a given application, an improved tribological performance, i.e. anincreased wear resistance, can often be accomplished by increasing the coating fracture resistance while retaining its hardness orvice versa. It has been shown that a possible way to achieve this is to give the coating a multilayered structure, which may act ascrack inhibitor. In fact, such a structure can even cause an increase in hardness, simultaneously to the improvement of toughness.

PVD TiN/TaN multilayer coatings with three different multilayer periods (thickness of one TiN lamella together with one TaNlamella) of 10, 50 and 200 nm thickness, were deposited on high speed steel substrates. The morphology, the coating hardness,Young’s modulus, residual stress, cohesion and adhesion to the substrate as well as abrasive and erosive wear resistance of thecoatings were determined. The wear mechanisms in abrasion and erosion are discussed. Single layered TiN and TaN were includedfor comparison.

It can be concluded that multilayering of TiN and TaN is a possible means to obtain a very wear-resistant PVD coating. Aprerequisite is, however, that the lamella thickness is kept thin (11 nm or possibly less). A smaller multilayer period resulted in aslightly higher coating hardness and acted beneficially on the abrasive and the erosive wear resistance. For all coatings, there wasa strong correlation between the hardness and the abrasive wear resistance. In erosion, where a high coating toughness is necessaryin order to achieve a good wear resistance, the multilayered coatings displayed a much better wear resistance than bothhomogenous TiN and TaN. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: Multilayered coating; PVD; TaN; TiN; Wear

1. Introduction result of several different mechanisms: crack deflectiondue to weak interfaces [1], crack tip shielding by plastic

Today’s thin hard tribological coatings usually fail as deformation in combination with strong interfaces [2,3],a result of excessively high chemical or mechanical a favourable residual stress distribution [4], crackloading, or a combination of both. In many cases, the deflection due to large differences in cohesive strengthlifetime is governed by fragmentation due to poor between adjacent lamellae [5] and differences in elastictoughness. Improvement of the tribological performance properties and/or coating morphology between adjacentcan thus be accomplished by, e.g., increasing the coating lamellae [6 ].fracture toughness while retaining its hardness, or vice In several investigations it has been shown thatversa. ceramic multilayered coatings can exhibit an improved

A possible approach is to replace single layered fracture resistance as compared with today’s single lay-coatings with multilayered ones. Such coatings are ered ceramic coatings [6–8]. In addition, multilayeredobtained by alternately depositing two (or more) coatings have also been shown to possess very highmechanically different materials. The multilayered struc- hardnesses [3,9,10] and excellent corrosion resistancesture will act as a crack inhibitor and thereby increase [11].the coating fracture resistance. This effect can be the In this work a new PVD coating consisting of the

two metal nitrides, TiN and TaN, combined into amultilayered coating structure has been evaluated. As* Corresponding author. Tel.: +46 18 471 72 66;compared with PVD TiN, information about PVD TaNfax: +46 18 471 35 72.

E-mail address: [email protected] (M. Nordin) as a tribological material is relatively scarce. The few

0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved.PII: S0257-8972 ( 99 ) 00493-4

529M. Nordin et al. / Surface and Coatings Technology 120–121 (1999) 528–534

reports that can be found indicate that TaN is a very was regulated in order to maintain a constant totalpressure of 2.5×10−3 mbar, while the magnetron sput-hard and brittle material. PVD TaN (d.c. magnetron

sputtered) also seems to suffer from poor adhesion to tering power was kept constant at 5 kW.Three TiN/TaN multilayered coatings with differentvarious substrates. The reason suggested is that very

high compressive stresses develop in the coating [12]. multilayer periods were deposited simply by varying thesubstrate rotational speed. The nominal periods were 10,The aim of this work was to evaluate the influence

of the multilayer period (L, i.e. the thickness of one 50 and 200 nm. For the coating with the largest period,TiN/TaN(L200), the substrates were alternately heldlamella of TiN together with one TaN) in multilayered

TiN/TaN coatings on their mechanical and tribological stationary over the e-gun and d.c. magnetron, for 35 and12 s, respectively. All multilayered processes ended upproperties. Homogenous TiN and TaN were included

as reference coatings. with deposition of a TiN top layer. The growth rate ofthe multilayered coatings deposited with continuous sub-strate rotation was approx. 100 nm min−1 and for themultilayered coating grown with the alternately stationary2. Experimentalsubstrates approx. 200 nm min−1.

Single layered TiN was obtained using reactive2.1. Substrate materialelectron beam evaporation of Ti and a nitrogen flow of140 sccm. The Ar pressure was 1.2×10−3 mbar and thePowder metallurgical high speed steel (PM HSStotal pressure (Ar+N2) was kept constant atASP2030; Erasteel Kloster AB designation) was used as1.7×10−3 mbar. Single layered TaN was deposited usingthe substrate material. The nominal chemical composi-reactive magnetron sputtering at a power of 5 kW. Thetion of the HSS was (wt.%): 1.28 C; 4.1 Cr; 5.0 Mo; 6.4nitrogen flow was 40 sccm and the Ar pressure wasW; 3.1 V; 8.4 Co. The HSS was heat treated by3×10−3 mbar. The growth rate of TiN and TaN wasaustenitisation at 1180°C followed by tempering 3×1 h70 nm min−1 (rotating substrates) and 470 nm min−1at 560°C, resulting in a primary carbide volume fraction(stationary substrates), respectively. After deposition,of 13% and a hardness of 920 HV30 kgf. The carbidesall coated specimens were cooled in He for 20 min.were primarily of MC and M6C type with average size

of 2 mm. All substrates were polished to mirror finish,corresponding to an Ra value of approximately 5 nm. 2.3. Evaluation techniques

2.2. Deposition procedures Field emission gun scanning electron microscopy(FEG-SEM, LEO 1530) was utilised to measure thecoating thickness. To determine the multilayer period,All substrates were thoroughly degreased and dried

with N2 gas before inserted into the coating chamber. cross-sectional transmission electron microscopy( XTEM, JEOL 2000 FXII, fitted with an LaB6 filamentFirst the substrates were resistively heated to 450°C for

60 min using an electron beam. Thereafter, Ar+ ion and a 200 kV voltage) was employed. The morphologyof the coatings was studied in the scanning electronetching was performed for 15 min using a negative

substrate bias (−200 V ). microscope, while the surface roughness was determinedusing surface profilometry (Alfa step 200 with a stylusAll coatings were deposited in a commercial Balzers

BAI 640R apparatus, fitted with an electron beam radius of 12 mm).The coating hardness and Young’s modulus wereevaporation source (e-gun) and a planar d.c. magnetron

sputtering source. Deposition of the multilayered obtained using the nanoindentation technique [13]. Anindentation depth of 150 nm and a Berkowich tip wereTiN/TaN coating was performed using a hybrid process

consisting of reactive electron beam evaporation (TiN) used. A beam deflection method was used to measurethe residual stress in the coatings [14]. Cohesion andand reactive d.c. magnetron sputtering (TaN). The

deposition temperature and the substrate bias were adhesion of the coatings were evaluated using scratchtesting (CSEM Revetest). A loading rate of450°C and −110 V, respectively.

Before the hybrid deposition process commenced, the 10 N mm−1 (total range 0–100 N) and a Rockwell Cdiamond stylus with a radius of 200 mm were utilisedTa target was Ar+ ion etched for 1 min at an Ar pressure

of 2.0×10−3 mbar and using a magnetron power of [15]. The critical load (FN,C), i.e. the load at the firstremoval of coating material, was determined post-exper-5 kW. Simultaneously with this, the substrates were held

stationary over the e-gun and a Ti interlayer (approxi- imentally using light optical microscopy [16 ].For determination of the abrasive wear resistance ofmately 30 nm), followed by a TiN layer (250–500 nm)

were deposited. The nitrogen flow at the e-gun was the coatings the so-called ‘dimple grinder test’ (Gatanmodel 656 precision dimple grinder, normally used for140 sccm. Thereafter, 15 sccm of nitrogen was intro-

duced at the magnetron and the substrate rotation was TEM sample preparation) was used [17]. As abrasives2.5 mm diamond grits were used and the applied normalengaged, i.e. the multilayer deposition commenced.

During multilayer deposition, the e-gun emission current load was 20 gf. The worn volume was measured at

530 M. Nordin et al. / Surface and Coatings Technology 120–121 (1999) 528–534

Table 1Total coating thickness (tf), lamella thickness (tTiN and tTaN), multilayer period (L), and surface roughness (Ra)

Coating tf (nm) tTiN (nm) tTaN (nm) L (nm) Ra (nm)

TiN 4900±200 – – – 10±2TaN 5600±300 – – – 30±5TiN/TaN(L10) 3900±200 7±1 4±1 11±2 12±3TiN/TaN(L50) 6100±200 24±6 24±1 48±7 7±3TiN/TaN(L200) 6100±100 110±30 110±5 220±40 6±4

regular intervals (approximate sliding distance of (L50)<TiN/TaN(L10)<TaN, see Table 2. Within themeasurement scatter the multilayered coatings displayed6000 mm) until wear through of the coating was detected

using white light optical profilometry ( WYKO the same Young’s modulus, intermediate between thatof TiN and TaN (Table 2). All coatings showed aNT2000).

Last, but not least, an erosion test was performed compressive residual stress in the range 3.5–7.2 GPa,see Table 2. The stress increased in the following order:[18]. Erosion testing has proven to be a suitable tool

for studies of crack initiation and propagation in thin TiN<TiN/TaN(L50)#TiN/TaN(L200)<TaN<TiN/TaN(L10).hard coatings [19]. To focus on coating erosive wear,

i.e. to minimise the influence from the underlying sub-strate (based on an earlier investigation [19]), the 3.3. Coating cohesion and adhesionfollowing experimental parameters were chosen: highpurity (99.7%) angular silicon carbide particles In all cases the critical normal load corresponded to

a cohesive coating failure, i.e. chipping within the coat-(2700 HV25 gf, size distribution: 20–30 mm) as erodents,an impingement angle of 40° and a particle velocity of ing at the rim of the scratch, and ranged from 27 to

69 N, see Table 2. Homogeneous TiN showed the highest20 m s−1. The worn depths were determined by measure-ment of the depth of the eroded area using surface critical load, while TaN the lowest.profilometry (a-step) at regular intervals. In order tostudy the crack formation and propagation in the coat- 3.4. Abrasive and erosive wear resistancesings, polished cross-sections of the eroded coatings wereprepared and studied using SEM. The highest abrasive wear resistance was found for

single layered TaN and the lowest for TiN. Intermediatewear resistances were found for all multilayered coatings(Fig. 1). Furthermore, for the multilayered coatings, the3. Resultsabrasive resistance was found to increase with decreasingL. Very fine grooves were found in all wear scars3.1. Coating thickness, multilayer period, coating

morphology and surface roughness indicating that the predominant wear mechanism waspure abrasion, see Fig. 2. For the multilayered coatingwith thickest lamellae some areas of spalling at interfacesThe coating thickness ranged from 3900 to 6100 nm,

see Table 1. For the multilayered coatings the multilayer between two adjacent lamellae was also found (see AAin Fig. 3). These spalled areas reveal unscratched TaNperiods obtained were 11, 48 and 220 nm. All multilay-

ered coatings displayed a more or less columnar struc- lamellae. For this coating also areas in the wear scarshaving a wavy surface appearance were found, see BBture. It was also observed that the TaN single layered

coating had the thinnest columns, whereas TiN displayed in Fig. 3. All other coatings displayed much smootherwear scar surfaces.a very pronounced columnar structure and the thickest

columns of all coatings. All three multilayered coatings showed significantlyAll coatings, except TaN, had a surface roughness

(Ra) between 6 and 12 nm, see Table 1, and they dis- Table 2played more or less the same visual appearance. The Coating hardness (H ), Young’s modulus (E ), residual stress (sr) and

critical load (FN,C)relatively rough topography of the TaN (30 nm) wasdue to small droplets found on the surface.

Coating H (GPa) E (GPa) Lr (GPa) FN,C (N)

3.2. Coating hardness, Young’s modulus and coating TiN 31±6 500±50 − 3.5±0.4 69±3TaN 42±4 440±30 − 6.0±0.6 27±1residual stressTiN/TaN(L10) 41±3 480±20 − 7.2±0.7 39±3TiN/TaN(L50) 38±4 500±40 − 4.9±0.5 56±6The coating hardness was found to increase in theTiN/TaN(L200) 37±2 470±20 − 5.0±0.5 36±5

following order: TiN%TiN/TaN(L200)<TiN/TaN


erosive fatigue wear resulting in removal of small coatingfragments. The wear mechanism is illustrated by micro-graphs in Fig. 4. It can be seen that repetitive particleimpacts generated lateral cracks at a certain depth inthe coating (Fig. 4(b)), which propagated, deflected,interacted and caused removal of coating fragments, seeFig. 4(c) and (d). It can also be concluded from thepolished cross-sections that a large number of lateralcracks are formed during erosion that do not necessarilyresult in instant coating removal.

4. Discussion

Fig. 1. Abrasive and erosive wear resistance for all coatings. An interesting finding was that the hardness of themultilayered TiN/TaN coatings was higher than thehardness value suggested by a simple rule of mixtureobtained from the hardness (H ) and the lamella thick-ness (t) of TiN and TaN, respectively, see Eq. (1):

Hmultilayer=HTiNtTiN+HTaNtTaN

tTiN+tTaNFig. 2. Surface profile of an abrasive wear scar in TiN (optical

=HTiNtTiNL

+HTaNtTaN

L. (1)profilometry).

higher erosive wear resistance than the two single layered Another interesting finding was that the hardnessincreased slightly with decreasing L. This can, to somecoatings, cf. Fig. 1. TaN was found to have the lowest

erosive wear resistance. As for the abrasive wear resis- extent, be attributed to the multilayered structurethrough a mechanism similar to the Hall–Petch phenom-tance, the erosive wear resistance increased slightly with

decreasing L. The predominant wear mechanism was enon [20,21]. i.e. an increased hardness with a decreased

Fig. 3. Surface profile of a part of the abrasive wear scar in TiN/TaN (L200). Two line scans have been included, one over an area where flakinghas occurred (AA) and one over an unflaked area (BB).


Fig. 4. Micrographs (SEM) of polished cross-sections of (a) an uneroded TiN/TaN (L200) and of (b)–(d) eroded TiN/TaN(L200). It can be seenthat the particle bombardment of the coating results in (b) lateral cracks at a certain depth from the surface and, in a later stage, (c) and (d)spalling of coating material. (c) and d) show typical flaking damages in cross-section and normal surface view (60° tilt), respectively.

grain size. In the case of multilayered coatings, alllamellae of TiN and TaN can be considered as ‘sub-grains’. Consequently coatings with a small L value maybenefit from this with respect to their hardness. It shouldbe noted, however, that the smallest L investigated inthis work (11 nm) is larger than the L corresponding tothe hardness maximum found for other systems such asTiN/VN and TiN/NbN (3–10 nm) [9,22–24]. It shouldalso be noted that the TiN/TaN(L10) had the highestcompressive residual stress, which may contribute to itshigh hardness. Another likely contribution to theincreased hardness as the lamellae are made thinnercould be the phase change from mainly hexagonal TaN,which is present in TiN/TaN(L200) and TiN/TaN(L50),to cubic NaCl TaN, which is present in TiN/TaN(L10).This theory provides, however, that cubic TaN is harderthan hexagonal TaN, a requirement not confirmed by Fig. 5. Abrasive and erosive wear resistance as a function of coatingthe authors. hardness.

The abrasive wear resistance, V, was strongly corre-lated to the hardness of the coatings, see Fig. 5. It was

coatings, can be written as:also interesting to note that the multilayered coatingwith the thinnest lamellae displayed a wear resistance

Vmultilayer=VTiNtTiNL

+VTaNtTaN

L, (2)(0.091 mm N mm−3) higher than suggested by the rule

of mixture (0.053 mm N mm−3) obtained using Eq. (2).This equation was derived by Axen et al. to predict the where tTiN/L is the volume fraction of TiN and tTaN/Labrasive wear resistance of multiphase materials [25]. is the volume fraction of TaN and VTiN and VTaN are

the wear resistances of the two individual phases asThe equation, modified to be valid for multilayered


Fig. 6. Micrographs (SEM ) of polished cross-sections of (a) the eroded TiN/TaN (L10) coating, and (b) the eroded TiN/TaN (L200) coating.Note the large number of plateaus, formed as a result of the cracks formed, in the coating with thin lamellae as compared with the coating withthick lamellae.

determined for the homogenous coatings. The coatings the multilayered coatings can be even higher than thatof the constituent materials. It can also be concludedwith thicker lamellae displayed a wear resistance slightly

lower than the value suggested by the rule of mixture that the multilayer period (L) in a multilayered PVDTiN/TaN coating should be kept thin, 11 nm or possi-(0.062 mm N mm−3 for both TiN/TaN(L50) and

TiN/TaN(L200)). This is probably a result of the intro- bly less.duction of a new wear mechanism, i.e. spalling in thelamella interfaces, in addition to pure abrasion.

In the erosion wear test the multilayered coatingsAcknowledgementsdisplayed a much higher wear resistance than both single

layered coatings. The fact that TaN possessed a lowerThe financial support from AB Sandvik Coromantwear resistance than the multilayered coatings indicated

and the Swedish Research Council for Engineeringthat the resistance is not determined solely by theSciences (TFR) is gratefully acknowledged. Dr. Leifhardness, see Fig. 5. The higher wear resistance of theWestin of Erasteel Kloster AB is recognised for provid-multilayered coatings is believed to be a result of theing the substrate materials. Assoc. Prof. Niklas Axen issuperior toughness of these coatings. It is suggested thatacknowledged for valuable discussions.the toughening mechanism is crack deflection at the

lamella interfaces. This mechanism also explains theincrease in toughness with decreasing L. Namely that,the more interfaces in the coating, the more crack

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wear resistance of multilayered pvd tin/tan on hss

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