Ž .Wear 232 1999 221–225www.elsevier.comrlocaterwear

Mechanical and tribological properties of multilayered PVD TiNrCrN

Maria Nordin ), Mats Larsson, Sture HogmarkUppsala UniÕersity, Materials Science DiÕision, Box 534, 751 21 Uppsala, Sweden

Abstract

Ž . Ž .By combining thin layers of PVD titanium nitride TiN with thin layers preferably less than 10 nm of another nitride, e.g., niobiumŽ . Ž .nitride NbN or aluminum nitride AlN , into a lamellae structure, a coating with superior properties to the homogenous coatings made

from the constituents can be obtained. For example a very high hardness in combination with a high toughness, and thereby a highresistance to abrasive wear, have been reported for such multilayered coatings. In this work a new multilayered PVD coating, TiNrCrN,deposited on cemented carbide have been evaluated with respect to the hardness, residual stress, coating cohesionradhesion and the

Žabrasive wear resistance. Especially the influence of the chemical modulation period thickness of one lamella of TiN together with one. Ž .lamella of CrN and thickness ratio TiNrCrN ratio of the individual layers on the above properties have been investigated. In addition,

the effect of substrate bias, magnetron power and nitrogen flow during deposition have been studied. As reference material, homogenousTiN and CrN have been used. The TiNrCrN coatings were deposited using a hybrid process which combines reactive electron beamevaporation of TiN and reactive dc magnetron sputtering of CrN. In general, multilayered TiNrCrN was found to have superiorproperties to both homogenous TiN and CrN. It could also be seen that the lamella thickness in the multilayered coating and especiallythe lamella thickness of the CrN should be kept thin in order to obtain as good mechanical and tribological properties of the coating aspossible. q 1999 Elsevier Science S.A. All rights reserved.

Keywords: PVD; Multilayer; TiNrCrN; Cohesion; Adhesion and abrasion

1. Introduction

During the last years multilayered PVD coatings havebeen subjected to great interest. The reasons for this arethat the multilayered coatings possess a high hardness aswell as a high toughness as compared to today’s homoge-nous coatings. The latter is very important, since in manyapplications the life of thin hard coatings is governed byfragmentation.

Toughening of lamellae coatings can be a result ofseveral mechanisms, e.g., energy dissipation by ductilefracture of the metal phase, delamination of interfaces andsliding or shearing at interfaces. For metalrceramic multi-layered coatings this means that modifications which in-

Žcrease the work of fracture of the ductile phase increasedcoating thickness, increased tensile strength and greater

. w xfailure strain will improve the toughness 1 . In agreementwith this, experimental work have shown that multilayered

) Corresponding author. Tel.: q46-18-471-72-66; fax: q46-18-471-35-72; E-mail: maria.nordin@material.uu.se

coatings consisting of ceramic and metallic material havew xhigher fracture toughness 2 as compared to single layered

ceramic coatings. However, this have been at the expenseof a reduced hardness and wear resistance.

The problem with reduced hardness can be solved bydepositing a multilayered coating consisting of two differ-ent ceramic materials, e.g., thin lamellae of titanium nitrideŽ . Ž . Ž .TiN and niobium nitride NbN less than 10 nm each .Such multilayered coatings have been found to yield anincreased toughness as compared to single layered TiN andNbN and in addition retained or even increased hardness.Additionally, improved mechanical and tribological prop-erties such as reduced residual stress and higher micro

w xabrasive wear resistance are also reported 3 .Today, deposition of both PVD TiN and CrN are well

established techniques. TiN and especially CrN have shownhigh fracture toughness as compared to other commercialPVD coatings. The aim of this work was to evaluate themechanical and tribological properties of multilayered PVDTiNrCrN. More precisely the influence of chemical modu-lation period and the thickness ratio of the individual TiNand CrN layers as well as the substrate bias, magnetron

0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0043-1648 99 00149-0

( )M. Nordin et al.rWear 232 1999 221–225222

Table 1Ž .The coatings and the process parameters utilised together with the chemical modulation period. TiNrCrN x indicates a multilayered TiNrCrN coating

Ž .where x is the parameter varied in comparison to the coating designated TiNrCrN ref.

Coating Substrate Magnetron N flow Substrate rotational Chemical modulation2Ž . Ž . Ž . Ž . Ž .bias V power kW sccm velocity rpm period, L nm

TiN y110 – 140 9.5 –CrN y110 2 40 9.5 –

Ž .TiNrCrN ref. y110 2 155 9.5 10.2Ž .TiNrCrN 0 V 0 2 155 9.5 9.8Ž .TiNrCrN y200 V y200 2 155 9.5 9.5Ž .TiNrCrN 4 kW y110 4 155 9.5 12.0Ž .TiNrCrN 6 kW y110 6 155 9.5 14.5

aŽ .TiNrCrN 115 sccm y110 2 115 9.5 6.1Ž .TiNrCrN 195 sccm y110 2 195 9.5 12.4Ž .TiNrCrN L 20 y110 2 155 4.8 19.1Ž .TiNrCrN L 110 y110 2 155 0.9 110.0

a Ž .The lamellae thickness for TiNrCrN 115 sccm is calculated from the rotational speed during deposition and the total coating thickness.

power and nitrogen flow during deposition on the abovecoating properties have been studied.

2. Experimental

2.1. Substrate materials

Ž .Cemented carbide inserts 10 wt.% Co were used assubstrate material. The hardness was 1540 HV . All500 g

substrates were polished to a surface roughness of approxi-mately 5 nm. Before coating deposition all substrates wereultrasonically cleaned in an alkaline solution heated to333 K and thereafter in ethanol for 5 min each and thendried in nitrogen air.

2.2. Coating deposition

The coatings were deposited in a commercial BalzersBAI 640R equipment in which TiN was deposited usingreactive electron beam evaporation and CrN was depositedusing reactive dc magnetron sputtering.

The samples were resistively heated to 4508C for60 min and thereafter Ar ion etched for 15 min using a

negative substrate bias of 200 V. The single layered coat-ings were deposited at a total pressure of 1.7=10y3 and3=10y3 mbar for TiN and CrN, respectively. For allmultilayered coatings the deposition sequence started with

Ž .deposition of a thin approximately 30 nm Ti layer fol-lowed by a 200 nm TiN layer. Simultaneously, Ar ionetching of the sputter target was performed. The totalpressure was kept constant at 2=10y3 mbar during depo-sition. A multilayered structure was grown by keepingboth sources active and rotating the substrates to allowthem to alternately pass the Ti source and the Cr source.The deposition parameters were varied in accordance toTable 1. The chemical modulation period, i.e., the thick-ness of one lamella of TiN together with one lamella of

ŽCrN, have been determined using FEG-SEM field emis-.sion gun-scanning electron microscope . The total deposi-

tion time was 60 and 160 min for TiN and CrN, respec-tively, and 40 min for the multilayered coatings.

2.3. Coating eÕaluation

The coating thickness was determined using a SEM.The coatingrsubstrate composite hardness was measuredusing a conventional Vickers hardness micro indenter and

Table 2Coating thickness, Vickers hardness, critical load and abrasive wear rate

Coating Coating Vickers hardness Critical Abrasive wear rate2 3Ž . Ž . Ž . Ž .thickness mm kgrmm load N mm rmm N

TiN 3.8"0.1 2310"380 78"2 236"20CrN 4.4"0.1 1950"160 36"6 1340"90

Ž .TiNrCrN ref. 3.7"0.1 2480"160 73"3 56"2Ž .TiNrCrN 0 V 3.9"0.1 2330"130 11"7 94"4Ž .TiNrCrN 200 V 3.4"0.1 2350"300 65"3 62"6Ž .TiNrCrN 4 kW 4.2"0.1 2470"320 71"6 74"8Ž .TiNrCrN 6 kW 5.0"0.1 2330"180 11"4 70"4Ž .TiNrCrN 115 sccm 2.5"0.1 2440"250 8"1 66"6Ž .TiNrCrN 195 sccm 4.8"0.1 2590"200 11"2 100"4Ž .TiNrCrN L 20 3.6"0.1 2350"260 69"14 82"4Ž .TiNrCrN L 110 3.9"0.1 2380"270 11"2 112"2

( )M. Nordin et al.rWear 232 1999 221–225 223

Fig. 1. Compressive residual stress.

a load of 50 g. The coatings residual stress was obtainedw xusing a beam deflection method 4 .

The cohesion and adhesion of the coatings were studiedŽ .using a commercial scratch tester CSEM Revetest . A

Ž .loading rate of 10 Nrmm total range 0–100 N and aRockwell C diamond stylus with a radius of 200 mm was

w xutilised 5 . The critical load, i.e., the normal load at thefirst coating failure as detected using an acoustic emission

w xdetector 6 , was used as a measure of the coating quality.Analysis of the coating failure mechanisms was performed

Ž .using light optical microscopy LOM .

The abrasive wear rate of the coatings was determinedŽusing a commercial dimple grinder Gatan model 656

. w xprecision dimple grinder 7 . As abrasives 2.5 mm dia-mond grits were used. The applied normal load was 20 g.

Ž .The worn volumes of the coating V and the substratecŽ . ŽV were determined at regular intervals of an approxi-s

.mate sliding distance of 6000 mm using a white lightoptical profilometer. These values were then inserted in thefollowing expression;

V Vc sSLs q 1Ž .

k kc s

where S is the sliding distance, L is the applied load, andk and k are the wear constants for the coating and thec s

substrate, respectively, for determination of the coatingwear constant. The wear constant of the substrate materialwas determined in advance for an uncoated specimen.

3. Results

3.1. Coating thickness and growth rate

ŽThe coating thickness ranged from 2.5 to 5.0 mm see.Table 2 . The growth rate increased with nitrogen flow and

magnetron sputtering power, but was unaffected by the

Ž . Ž Ž . Ž .Fig. 2. Representative examples of the different coating failures encountered. a Small cohesive failures TiN, TiNrCrN ref. , TiNrCrN 4 kW ,Ž . Ž .. Ž . Ž Ž . Ž .. Ž .TiNrCrN 200 V and TiNrCrN L 20 ; b small semicircular cracks at critical load TiNrCrN L 110 and TiNrCrN 115 sccm ; c large semi-circular

Ž Ž . Ž .. Ž . Ž Ž ..cracks at critical load CrN, TiNrCrN 6 kW and TiNrCrN 195 sccm ; and d substrate exposure TiNrCrN 0 V .

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rate of substrate rotation during deposition, i.e., the lamellathickness, and only slightly affected by the substrate bias.

3.2. Hardness and residual stress

The hardness of the multilayered coatings were allfound to be of the same order, ranging from 2330 to 2590

Ž .HV see Table 2 . The lowest hardness was found forsingle layered CrN, while the highest was found for thecoating deposited with highest nitrogen flow. All the othercoatings displayed a similar hardness of approximately2400 HV.

In all cases the residual stress was found to be compres-sive and varied between y0.3 GPa for the single layeredCrN and y3.2 GPa for the multilayered coatings deposited

Ž .with high and low nitrogen flow Fig. 1 . TiN had aresidual stress of y2.5 GPa. It was found that the residualstress increased with substrate bias.

3.3. Coating cohesion and adhesion

Ž .The critical load ranged from 8 to 78 N see Table 2 .The coating deposited using the intermediate nitrogen flow,i.e., a nitrogen flow of 155 sccm, displayed the highestcritical load of the multilayered coatings. This coating,

Ž .TiNrCrN ref. , displayed small cohesive failures at theŽ .rim of the crack see Fig. 2a . A much more brittle

behaviour was found for the coatings deposited with theŽhigher and lower nitrogen flow see Fig. 2b and c; the

.thicker coating display larger cohesive failures . It couldalso be seen that substrate bias yielded a coating with goodadhesion and cohesion and hence a high critical load.Moreover, it was found that the critical load was very lowfor the coating deposited using the highest magnetron

Ž .sputtering power, TiNrCrN 6 kW , and the coating withŽ .the thickest modulation period, TiNrCrN L 110 , i.e., the

two coatings with the thickest lamellae of CrN. BothŽ . Ž .TiNrCrN 6 kW and TiNrCrN L 110 were found to be

very brittle, displaying semicircular cracks at the criticalŽ .load see Fig. 2b and c . The only coating that showed a

Ž .poor adhesion to the substrate was TiNrCrN 0 V whichŽ .displayed large areas of substrate exposure Fig. 2d .

Ž .Fig. 3. Surface profile of a dimple in TiNrCrN 0 V showing adhesivefailure at A.

3.4. AbrasiÕe wear rate

The abrasive wear rates, ranging from 56 to 1340mm3rmm N, are presented in Table 2. The single layeredcoating displayed the highest abrasive wear rate, with CrNhaving more than a factor of five higher than TiN. Thewear rate was higher for the coating deposited with nosubstrate bias than the coatings deposited at y110 andy200 V, respectively. That is, substrate bias was neces-sary in order to get good adhesion of the coating to the

Ž .substrate see Fig. 3 and cohesion within the coating. Thisis in agreement with the results from the scratch test. Theabrasive wear rate was also found to increase with lamellathickness.

4. Discussion

The present results show that the reactive hybrid pro-cess consisting of reactive electron beam evaporation ofTiN and reactive magnetron sputtering of CrN is well-suitedfor deposition of dense multilayered TiNrCrN coatings. Itwas found that the coatings deposited at an intermediatebias, intermediate nitrogen flow and having very thinlamellae had the overall best mechanical and tribologicalproperties. In the following, the influence of the processparameters on the mechanical and tribological propertiesstudied will be discussed.

To obtain a beneficial influence from the multilayeredstructure on the properties of TiNrCrN, it was found to becrucial to keep the chemical modulation period, and morespecifically the thickness of the CrN lamellae, very thin.The reasons for this are not clear but it is believed thatdifferences in the inherent properties of the two coatingdeposition processes used, partly can explain the results.From earlier investigations it is clear that the dc magnetronsputtering process utilised in this work yields coatings of amuch poorer mechanical and tribological quality as com-pared to the coatings produced using the ion-plating tech-nique, e.g., reactive e-gun evaporation in combination withapplying a negative substrate bias. The explanation for thisis that ion-plated coatings continuously are Ar ion etchedduring deposition. Hence, a very dense and strong coatingwill be deposited, since material loosely attached to thesurface will be etched away and only strongly attachedmaterial will remain. In the sputtering process, however,the degree of ionisation is much lower, i.e., only a smallnumber of ions will be accelerated towards the substrate.This, in turn, means little Ar ion etching of the growingcoating or lamella and consequently a coating with highamount of defects and poor mechanical and tribologicalproperties will be obtained.

In this case it is important to note that Ar ion etchingmainly is performed outside the area of the magnetron, i.e.,

( )M. Nordin et al.rWear 232 1999 221–225 225

above the e-gun. Thus, a coating deposited using thehighest possible rotational speed will be subjected to ahigher degree of Ar ion etching per unit volume of sput-tered material than a coating deposited using a lowerrotational speed. Therefore, the latter type of coating willbecome more brittle and possess less attractive mechanicaland tribological properties, in agreement with the resultsobtained.

5. Conclusions

In this work, multilayered PVD TiNrCrN coatings oncemented carbide substrates have been evaluated withrespect to some fundamental properties such as hardness,residual stress, cohesionradhesion and abrasive wear resis-tance. The major conclusions are the following.

Ø The deposition process works well for deposition ofwell-adhering, multilayered TiNrCrN coatings that pos-sess superior properties to both homogenous TiN and CrN.

Ø The overall best mechanical and tribological proper-ties was found for the multilayered coatings deposited atan intermediate substrate bias, intermediate nitrogen flowand having very thin lamellae. The latter was found to bevery important since a reduced CrN lamella thickness is

Žbelieved to decrease the amount of defects due to larger

.Ar ion sputtering during deposition in the coating andthereby result in a less brittle coating.

Acknowledgements

The financial support from Sandvik Coromant and theŽ .Swedish Research Council for Engineering Sciences TFR

is gratefully acknowledged by the authors. In addition, Dr.Torbjorn Selinder, Sandvik Coromant, Sweden, is ac-¨knowledged for providing the substrate materials.

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