M. STASZUK et al.: INVESTIGATION STUDIES INVOLVING WEAR-RESISTANT ALD/PVD ...755–759

INVESTIGATION STUDIES INVOLVING WEAR-RESISTANTALD/PVD HYBRID COATINGS ON SINTERED TOOL

SUBSTRATES

PREISKAVE OBRABNE ODPORNOSTI HIBRIDNEGA NANOSAALD/PVD NA SINTRANEM ORODJU

Marcin Staszuk, Daniel Paku³a, Tomasz TañskiSilesian University of Technology, Institute of Engineering Materials and Biomaterials, Konarskiego Street 18A, 44-100 Gliwice, Poland

marcin.staszuk@polsl.pl

Prejem rokopisa – received: 2015-07-22; sprejem za objavo – accepted for publication: 2015-10-13

doi:10.17222/mit.2015.236

This paper is an important research contribution to the development of PVD coatings, in particular, on ceramic substrates,which, due to their dielectric properties, are difficult materials to coat using this technique. In order to satisfy the desiredexpectations relating to the PVD coatings, one of the basic properties must be provided for, the adherence to the substrate. Themain aim of this research is to investigate the structure and mechanical properties of the coatings deposited in a hybrid process,comprising the atomic-layer deposition (ALD) and cathodic-arc evaporation (CAE-PVD) on sintered carbides and multipointceramic cutting tools. The concept of this research study involves an execution and investigation of ALD + PVD hybrid coatingson sintered carbides and a sialon-ceramic substrate, and defining the influence of the ALD interlayer on the adherence of theinvestigated coatings. The critical load Lc, which is the adhesion measure of coats, was determined with the scratch-test methodand a tribological test made with a pin-on-disk tester. Observations of the surface topography and wear mechanism wereperformed using a scanning electron microscope and atomic-force microscopy. The investigation studies showed that an ALDlayer considerably improves the adherence of the PVD layer to the tool-ceramic substrate. The research of the coatings onsintered-carbide substrates showed that the adherence of the PVD coating to the substrate deteriorates in the case of applying anALD interlayer.

Keywords: PVD, ALD, hybrid coatings, tool ceramics, sintered carbides

^lanek je pomemben raziskovalni prispevek k razvoju PVD nanosov, {e posebno na kerami~ni podlagi, ki je zaradi dielektri~nihlastnosti zahteven material za nana{anje s to tehniko. Da bi zadovoljili pri~akovanja na delu s PVD nanosom, je oprijemljivostna podlago osnovna lastnost, na katero je potrebno biti pozoren. Glavni namen raziskave je preiskati strukturo in mehanskelastnosti nanosa, nane{enega s hibridnim postopkom, ki obsega nanos atomskih plasti (ALD) in katodno izparevanje v obloku(CAE-PVD) na sintrane karbide in ve~to~kovno kerami~no rezilno orodje. Koncept te raziskave vklju~uje izvedbo in preiskavohibridnega nanosa ALD + PVD na podlago iz sintranih karbidov in sialon keramike ter dolo~itev vpliva vmesne plasti ALD naoprijemljivost preiskovanih nanosov. Kriti~na obremenitev Lc, ki je merilo oprijemljivosti nanosov, je bilo dolo~eno spreizkusom razenja in s tribolo{kim preizkusom trn na plo{~i. S pomo~jo vrsti~nega elektronskega mikroskopa in mikroskopana atomsko silo je bilo izvedeno opazovanje topografije povr{ine in mehanizma obrabe. Izvedene preiskave ka`ejo, da ALDnanos mo~no izbolj{a oprijemljivost PVD nanosa na orodno keramiko. Pri nanosu na podlago iz sintranega karbida je raziskavapokazala, da je pri uporabi ALD vmesne plasti, oprijemljivost PVD nanosa slab{a.

Klju~ne besede: PVD, ALD, hibridni nanosi, keramika za orodja, sintrani karbidi

1 INTRODUCTION

Sialon-tool ceramics are a group of tool materialscombining the mechanical properties of silicon nitrideSi3N4 and the chemical properties of Al2O3. Ceramicsinters are fabricated with powder-metallurgy methods,but in contrast to sintered carbides, they do not contain ametallic binder. The use of tool ceramics as compared tosintered carbides is still considerably small but it isgrowing, also due to the application of thin wear-resis-tant layers PVD and CVD.1,2

Surface engineering technologies play a significantrole in material engineering and are viewed as the fun-damental scope of knowledge in this field of engi-neering. The research entities working in this field try toinvestigate and identify the phenomena taking place onthe surfaces of the machined materials. In order to

improve the operating properties of products, varioussurface-machining technologies are applied.2–6 Theadvantages arising from the application of PVD coatingson cutting tools such as a high microhardness and resis-tance to abrasion, a low friction coefficient of the toolcovered with the coat, the resistance to oxidation or tothe formation of build-up edges on the tool, combinedwith the possibility to machine materials without liquidcooling lubricants are important contributions advocatingthe development of this technological field. However, inorder to improve the desired properties of PVD coatings,one of the basic properties must be taken care of, whichis the adherence to the substrate. The coating of ceramicmaterials in PVD processes, including also tool cera-mics, is difficult due to dielectric properties, since theinability of the substrate polarization during the coating

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UDK 620.19:620.178.16:621.793.8 ISSN 1580-2949Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(5)755(2016)

process makes it difficult to obtain coatings with a goodadherence to ceramic substrates.2,7–11

The fabrication technology of stratified coatings ofthe type "adhesive layer/PVD layer" using the multi-stage surface machining is one of the most modernmethods to modify the properties of the surface layer andit consists, in a successive application, of two (or more)technologies of surface engineering. In terms of materialeffects, we obtain multilayer systems whereof one part ismade up of an appropriately selected adhesive layerfabricated on the surface of the substrate, which protectsthe "proper" PVD layer against a loss of internal cohe-sion and an insufficient adhesion to the substrate. And onthe other end of the system, we obtain a PVD coatingwhich efficiently insulates the substrate, limiting theimpact of the external harmful factors encountered in theoperating process.2,12

The coatings of the (Ti,Al)N type are isomorphic,with titanium nitride which is still widely applied. Thepresence of aluminum in the coatings of this type bringsabout the situation where the service-life temperature ofthese coatings exceeds 970 K, and in such operating con-ditions of raised temperature, a layer of Al2O3 is formedon the surface, which forms a diffusive barrier for theatmospheric oxygen.13,14

Zinc oxide is a semiconductor characterized by ahigh energy gap (3.4 eV), high exciton binding energy(60 meV) and easy n-type doping.15,16 Zinc oxide, ZnO,obtained with the use of the ALD technique on a sialon-ceramic substrate enables the polarization of this sub-strate during the technological process of PVD and,hence, the objective of this paper is to investigate the im-pact of the ZnO layer obtained with the use of the ALDtechnique on the adhesion of the hybrid coatingALD/PVD of the ZnO/(Ti,Al)N type to the sialon sub-strate. For the sake of comparison, we also investigatedthe coatings on sintered carbide substrates.

2 MATERIALS AND METHOD

The research studies were carried out on multipointcutting tools made of sintered carbides WC-Co andsialon-tool ceramics covered in the PVD and ALD/PVDprocesses. The tools were covered during the cathodic-arc evaporation process CAE-PVD with the (Ti,Al)Ncoating type and in the ALD/PVD hybrid process usingthe ZnO/(Ti,Al)N coating type.

The topography of the surfaces and wear mechanismsof the coatings was viewed with a Supra 35 scanningelectron microscope from Zeiss. The secondary-electron(SE) detection was used to obtain the images of thetested samples, with an acceleration voltage of 5–20 kV.

The topography of the coatings was tested using aPark Systems XE-100 atomic-force microscope. Thetests were carried out in the non-contact mode.

The adherence of the coatings to the substrate wasassessed with a scratch test on a Revetest device from

CSEM. The method consists of moving a Rockwell Cdiamond indenter through the surface of a tested sampleat a constant speed, with the applied force growinglinearly. The ranges of the applied load were 0–100 Nand 0–200 N. The Lc critical load, at which the coatingadhesion is lost, was determined according to the acou-stic-emission value registered during the measurementand by observing the scratches formed during the scratchtest.

The abrasive-wear-resistance tests and the wear-fac-tor tests for the tested coatings were performed with thepin-on-plate method using a CSEM Tribometer (THT).A 6 mm Al2O3 ball was used as the counter specimen.The tests were made at room temperature. The followingtest conditions were applied: a normal force of FN = 10 Nand a movement speed of v = 0.1 m/s. The total distanceof 1 km was set for all the tested specimens.

3 DISCUSSION AND TEST RESULTS

The value of critical load Lc, being the adherencemeasure for the investigated coatings on sintered-carbidesubstrates or sialon-tool ceramic substrates, was deter-mined using a scratch test (Figure 1). The researchstudies show that the ALD/PVD hybrid coat has a much

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756 Materiali in tehnologije / Materials and technology 50 (2016) 5, 755–759

Figure 1: a) Acoustic emission (AE) and friction force Ft dependingon load Fn for the ZnO/(Ti,Al)N coating on sialon ceramics, b) scratchfailure at Lc (opt) = 65 N, magnified 200×Slika 1: a) Akusti~na emisija (AE) in sila trenja Ft v odvisnosti odobremenitve Fn pri ZnO/(Ti,Al)N nanosu na sialon keramiko, b) po-{kodba z razo pri Lc (opt) = 65 N, pove~ano 200×

higher adherence to the sialon-ceramic substrate than thePVD coating without the adhesive layer (Table 1). A re-verse situation takes place in the case of coatings onsintered-carbide substrates. Due to high adhesion of thePVD coating to the sintered-carbide substrate, the ALDlayer causes a drop in the critical load from 109 N to55 N.

Table 1: Critical loads Lc of the investigated coatingsTabela 1: Kriti~ne obremenitve Lc preiskovanih nanosov

Type of coatingCritical load Lc, N

Sialon substrate Cemented-carbidesubstrate

(Ti,Al)N 21 109ZrO/(Ti,Al)N 64 55

The damage done to the coats during the performedadhesion/scratch tests was identified on the basis ofobservations with the scanning electron microscope. Thestudies carried out show that it is the delamination,which is the principle mechanism of the coat damage onthe sialon-ceramic substrate after the critical load Lc hasbeen exceeded. In the case of the PVD coat, which ischaracterised by a low adherence to the ceramic substra-te, a total delamination with vast chipping was identified.Furthermore, a low adherence of the PVD coat to theceramic substrate indicates a considerable morphologicalnon-homogeneity and, in particular, the presence ofchippings found on the whole surface of the coat (exceptfor the scratch area) (Figure 2). A low adherence of thecoat to the substrate results in its unprompted chippingdue to the internal strain of the coat. In the case of theZnO/(Ti,Al)N hybrid coat on the sialon substrate, dela-mination was identified on both sides of the sample(Figure 3). The chipping on both sides of the scratchedsample is definitely less vast than that observed in thecase of the (Ti,Al)N coat on the same substrate. The onlymorphological defects of the hybrid coat are the drop-

shaped microparticles, inseparably connected with thecathodic-arc evaporation process (Figure 4). In the case

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Figure 4: Surface topography of the ZnO/(Ti,Al)N coating depositedonto the sialon-ceramic substrate: a) SEM, b) AFMSlika 4: Topografija povr{ine ZnO/(Ti,Al)N nanosa, nane{enega napodlago iz sialonske keramike: a) SEM, b) AFM

Figure 2: Characteristic failure obtained with the scratch test of the(TiAl)N coating deposited on the sialon-ceramic substrateSlika 2: Zna~ilna po{kodba pri preizkusu z razenjem nanosa (Ti,Al)N,nane{enega na podlagi iz sialonske keramike

Figure 3: Characteristic failure obtained with the scratch test of theZnO/(TiAl)N coating deposited on the sialon-ceramic substrateSlika 3: Zna~ilna po{kodba pri preizkusu razenja ZnO/(Ti,Al)Nnanosa, nane{enega na podlago iz sialonske keramike

of the coats on sintered-carbide substrates, friction is thedominant mechanism. In the case of the PVD coat on thesintered-carbide substrate, which is characterised by thehighest adherence among the investigated samples, noabrasion deep down in the substrate was identified untilthe maximum load was reached during the scratch test.However, numerous cohesive cracks were identified aftersurpassing the critical load Lc. In the ZnO/(Ti,Al)N coaton the same substrate, we identified a uniform abrasionspreading down into the substrate after the critical loadwas exceeded.

With the tests on the abrasive-wear resistance of thecoats deposited on sialon ceramics and on sintered car-bides using the pin-on-plate method, we found that inalmost all the cases where the coats were applied thecoats were damaged down to the substrate zone (Figures5 and 6). The dominant wear mechanism of the investi-gated coats was attrition. We also found that in somecases the damaged coat sticks onto the material of thecounter specimen, which has a direct impact on thechanging values of the friction coefficient. The researchstudies also confirm a positive impact of the ZrO layeron the attrition resistance as compared to the specimenwithout such a layer. At one end of the scratch, we ob-served a lot of damage to the coat deposited on the sialonsubstrate without the ZrO layer. The damage is ofadhesive character – we can observe stratifications andcracks of the layer (Figure 5). And in the case of thecoat deposited with the ZrO layer, the end of the scratchis uniformly grated without any adhesive damage (Fig-ure 6).

4 CONCLUSION

Nowadays, the improvement of the wear resistance ofcutting tools mainly involves surface machining, whilethe selection of the technology or coating material aims

at ensuring the proper resistance of a tool to dominantwear mechanisms. The operating properties of the coatsresistant to the wear are the results of many components,primarily the microhardness, the grain size and theadherence to the substrate. Especially the last property isof key importance, and in view of the performed researchstudies2, the grain size, the thickness and the micro-hardness of the obtained coats have smaller impacts thanthe adhesion on the durability of cutting tools since thechanges in these properties have smaller impacts onservice life.

The paper presents the results of research studies in-volving the application of an ALD layer to obtain abetter adhesion of the PVD coat to the tool sialon-cera-mic substrate or sintered-carbide substrate. We inve-stigated the (Ti,Al)N coat deposited on both substrates,with the ZnO layer and without it. On the basis of theinvestigation studies, we can state that zinc oxide con-siderably improves the adherence of the PVD coat to theceramic substrate, which was confirmed with scratchtests and observations of the damage made during thetests, using a scanning electron microscope. The criticalload, being the adherence measure of the PVD coat onthe ceramic substrate, increased due to the application ofthe adhesive layer by over 200 %. With respect to theinvestigated coats on sintered carbides, the adherence ofthe PVD coat to the ALD layer decreased as compared tothe coat without ALD.

The improvement in the adherence of the PVD coatto the ceramic substrate is undaubtly connected with thepossibility to polarize the substrate during the coatingprocess involving the application of the ZnO layer. Theresults of the research studies are important since thedeposition of ceramics in PVD processes is hindered dueto the dielectric properties of ceramics.

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Figure 6: Trace of tribological damage on the surface of theZnO/(Ti,Al)N coat deposited on the sialon-ceramic substrateSlika 6: Sled tribolo{ke po{kodbe na povr{ini ZnO/(Ti,Al)N nanosa,nane{enega na podlago iz sialonske keramike

Figure 5: Trace of tribological damage on the surface of the (Ti,Al)Ncoat deposited on the sialon-ceramic substrateSlika 5: Sled tribolo{ke po{kodbe na povr{ini (Ti,Al)N nanosa,nane{enega na podlago iz sialonske keramike

Acknowledgements

The publication was co-financed by the statutorygrant of the Faculty of Mechanical Engineering of theSilesian University of Technology in 2015.

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