COMPARISON OF NANOCOMPOSITE AND GRADIENT COATINGS DEPOSITED BY PVD TECHNOLOGY ONTO THE X40CrMoV5-1 STEEL SUBSTRATE

Krzysztof Lukaszkowicz1, Jozef Sondor2, Antonin Kriz3
1Institute of Materials and Biomaterials Engineering, Silesian University of Technology, Konarskiego St. 18A,
44-100 Gliwice, Poland, Tel.: +48322371245, e-mail: krzysztof.lukaszkowicz@polsl.pl
2LISS, a.s., Zuberska 2603, 756-61 Roznov p.R., Czech Republic, Tel.: +420571842681, e-mail: j.sondor@liss.cz
3Departament of Materials Science and Technology, University of West Bohemia, Univerzitni 22, 306-14 Plzen,
Czech Republic, Tel.: +420377638315, e-mail: kriz@kmm.zcu.cz

Abstract
The aim of the paper was the investigation of the microstructure and the mechanical properties of the nanocomposite TiAlSiN, CrAlSiN, AlTiCrN and the gradient TiAlN, TiCN, AlSiCrN coatings deposited by PVD technology onto hot work tool steel substrate. It was found that the microstructure of the nanocomposite coatings consisted of fine crystallites, while their average size fitted within the range of 11€25 nm, depending on the coating type. The coatings demonstrated columnar structure and dense cross-section morphology. The critical load LC2 lies within the range of 46€54 N. In case of the gradient coatings it was found that the microstructure consisted of crystallites while their average size fitted within the range of 25€50 nm, depending on the coating type. The coatings demonstrated columnar structure as well as good adhesion to the substrate. The critical load LC2 lies within the range 46€59 N.

INTRODUCTION
The research issues concerning the production of coatings is one of the more important directions of surface engineering development, ensuring the obtainment of coatings of high usable properties in the scope of mechanical characteristics and wear resistance. Giving new operating characteristics to commonly known materials is frequently obtained by laying simple monolayer, multilayer or gradient coatings using PVD methods [1,2]. The progress in the field of producing coatings in the physical vapour deposition process enables the obtainment of coatings of nanocrystal structure presenting high mechanical and usable properties. The coatings of such structure are able to maintain a low friction coefficient in numerous working environments, maintaining high hardness and increased resistance [3,4]. The main concept in the achievement of high hardness of nanostructure coatings and good mechanical properties and high strength related to it, particularly in case of nanocomposite coatings [5-7] is the restriction of the rise and the movement of dislocations. Nanocomposite coatings comprise at least two phases, a nanocrystalline phase and a matrix phase, where the matrix can be either nanocrystalline or amorphous phase [8,9]. Functional gradient coatings create a new class of coatings with properties and structure changing gradually. Gradient coatings deposited on the tool material substrate and providing appropriately high resistance to abrasive wear in tool operating conditions, core ductility and stress relaxation between the particular coating layers and between the gradient coating and tool material coating are seen as a solution to the issue due to the inappropriate adhesion of the layer produced or the excessive stresses between the surface layer and the substrate [10-12]. Frequently a rapid difference between the coating and substrate properties occurs causing a stress concentration in this area, both during the manufacturing and operation of the tools. This causes fast degradation demonstrated by cracks and delamination of the coatings. The application of functional gradient coatings offers a possible solution to the issue. Gradient coatings can be applied in manufacturing modern machining tools, due to their resistance to high-temperature oxidation and erosion as well as abrasive wear. The purpose of this paper is to examine the microstructure, mechanical properties and corrosion resistance of nanocomposite and gradient coatings deposited by PVD technique on the X40CrMoV5-1 hot work tool steel substrate.

EXPERIMENTAL PROCEDURE
The tests were made on samples of the X40CrMoV5-1 hot work tool steel deposited by PVD process with TiAlSiN, CrAlSiN, AlTiCrN nanocomposite coatings and TiAlN, TiCN, AlSiCrN gradient coatings. The coating deposition process was made in a device based on the cathodic arc evaporation method in an Ar and N2 atmosphere. Cathodes containing pure metals (Cr, Ti) and the AlSi (88:12 wt. %) alloy were used for deposition of the coatings. The base pressure was 5~10-6 mbar. The deposition conditions are summarized in Table 1 and 2.




Observations of surface and microstructures of the deposited coatings were carried out on cross sections in the SUPRA 25 scanning electron microscope. Detection of secondary electron was used for generation of fracture images with 15 kV bias voltage. Phase identification of the investigated coatings was performed by glancing angle X-ray diffraction (GAXRD). The cross-sectional atomic composition of the samples (coating and substrate) was obtained by using a glow discharge optical spectrometer, GDOS-750 QDP from Leco Instruments. Tests of the coatingsf adhesion to the substrate material were made using the scratch test on the CSEM REVETEST device. The critical load LC2, causing the loss of the coating adhesion to the material, was determined on the basis of the values of the acoustic emission, AE, and friction force, Ft, and observation of the damage developed in the track using a LEICA MEF4A optical microscope. The microhardness tests of coatings were made with the SHIMADZU DUH 202 ultra-microhardness tester. The test conditions were selected in order as to be comparable for all coatings. Measurements were made with 50 mN load, to eliminate the substrate influence on the coating hardness. X-ray line broadening technique was used to determine crystallite size of the coatings using Scherrer formula with silicon as internal standard:

D = (0.9ă/BcosĮB)

where: D . crystallite size, B . full width at half-maximum XRD peak in radians, ƒÉ . wavelength of the X-ray radiation, ƒÆB . the Bragg angle in radians.

Investigation of the electrochemical corrosion behaviour of the samples was done in a PGP 201 Potentiostat/Galvanostat, using a conventional three-electrode cell consisting of a saturated calomel reference electrode (SCE), a platinum counter electrode, and the studied specimens as the working electrode. To simulate the aggressive media, a HCl 1M solution was used under aerated conditions and room temperature. The aqueous corrosion behavior of the coatings was studied first by measuring the open circuit potential (OCP) for 1h.Subsequently, a potentiodynamic polarization curve has been recorded. The curve started at a potential of approx. 100 mV below the corrosion potential and ended at +1200 mV or a threshold intensity level set at 100 mA/cm2. Once this level was reached, the reverse cycle was started. The scan rate was 15 mV/min. The corrosion current densities and the polarization resistance were obtained on the basis of the Tafel analysis after potentiodynamic
polarization measurements.

RESULTS AND DISCUSSION
The nanocomposite coatings present a compact structure, without any visible delaminations or defects. The morphology of the fracture of coatings is characterized with a dense structure, in some cases there is a columnar structure (Fig. 1 a, b). The fractographic studies of the fractures of the steel samples examined, with the coatings deposited on their surface show a sharp transition zone between the substrate and the coating. The gradient coatingsf structure appears to be compact without any visible delamination or defects. The investigated coatings tested show a columnar structure that may be considered compatible with the Thornton model (zone I), except the fact that in TiCN the cross-section morphology is dense (Figs. 1 c, d).





Fig. 1. Fracture of the: a) TiAlSiN, b) CrAlSiN, c)TiAlN, d) TiCN coating deposited onto the X40CrMoV5-1 steel
substrate.


Basing on the glancing angle X-ray diffraction (GAXRD) of the samples examined, the occurrence of fcc phases was only observed in the coatings. The hexagonal AlN of wurtzite type was not discovered in the coatings examined, which could have been caused by a low amount of aluminium in the coatings. In case of the TiAlSiN coating a lattice parameter of 0.426 nm was derived, which is greater than that of bulk TiN (0.424 nm). The highest lattice parameter corresponds to a system where a partial Si segregation occurred, which might be enough to nucleate and develop a Si3N4 amorphous phase. Also, decrease intensities of the reflections, showing an increase in the amorphous content in the coatings. Basing on the results obtained, using Scherer method, the size of crystallites in the coatings examined was determined. The results were presented in Table 3.

The hardness of the X40CrMoV5-1 steel substrate without coating is 2.1 GPa, as settled upon hardness tests. The deposition of the PVD coatings onto the specimens causes the growth of hardness of the surface layer rainging from 30 to 32 GPa in case of gradient coatings and from 40 to 42 GPa in case of nanocomposite coatings (Table 3). The critical load values LC1 and LC2 were determined by the scratch test method (Fig. 2, 3). The load at which the first coating defects appear is known in the references as the first critical load LC1. The first critical load LC1 corresponds to the point at which first damage is observed; the first appearance of microcraking, surface flaking outside or inside the track without any exposure of the substrate material . the first cohesion-related failure event. LC1 corresponds to the first small jump on the acoustic emission signal, as well as on the friction force curve. The second critical load LC2 is the point at which complete delamination of the coatings starts; the first appearance of
cracking, chipping, spallation and delamination outside or inside the track with the exposure of the substrate material . the first adhesion-related failure event. After this point the acoustic emission graph and friction forces have a disturbed run (become noisier). The cumulative specification of the test results are presented in Table 3.

In order to establish the nature of damage responsible for the occurring increase of the acoustic emission intensity, the cracks produced during the test were examined by the light microscope coupled with the measuring device, determining the critical load LC1 and LC2 in virtue of metallographic observations. In case of the nanocomposite coatings examined, it was found that coating AlTiCrN had the highest critical load value LC1=24 N and LC2=54 N. In case of the gradient coatings examined, it was found that the TiCN coating had the highest critical load of LC1 = 33 and LC2 = 59 N, whereas AlSiCrN and the TiAlN coatings had the lowest LC1 = 19 and LC2 = 46 N.

The TiCN coatings show the best adhesion to the substrate of all the gradient coatings tested, which is not only due to the adhesion itself, but also due to mixing of the elements in the transition zone between the coating and the substrate as a result of diffusion, because the TiCN coatingsf deposition process temperature was 500‹C. The first symptoms of damage in most of the coatings examined are in the form of arch cracks caused by tension or scaling occurring on the bottom of the scratch that appears during the scratch test. Occasionally, there are some small chippings on the scratch edges. Along with the load increase, semicircles are formed caused by conformal cracking, leading to delaminations and chippings, resulting in a local delamination of the coating. As a result of the steel fracture test against the coatings deposited, made after prior cooling in liquid nitrogen, no case delaminations were revealed along the substrate-coating separation surface, which indicates a good adhesion of coatings to substrate.



Fig. 2. Diagram of the dependence of the acoustic emission (AE) and friction force Ft on the load for the
X40CrMoV5-1 steel with the: a) AlTiCrN, b) TiAlSiN coating.





Fig. 3. Scratch failure pictures of the AlTiCrN coating on X40CrMoV5-1 steel substrate at: (a) LC1, (b) LC2.

Changes of coating component concentration and substrate material made in GDOS were presented in Fig. 4. The tests carried out with the use of GDOS indicate the occurrence of a transition zone between the substrate material and the coating, which results in the improved adhesion between the coatings and the substrate. In the transition zone between the coatings and the substrate the concentration of the elements of the substrate increases with simultaneous rapid decrease of concentration of elements contained in the coatings. The existence of the transition zone should be connected with the increase of desorption of the substrate surface and the occurrence of defects in the substrate and the relocation of the elements within the connection zone as a result of a high energy ion reaction. Such results, however, cannot be interpreted explicitly, due to the non-homogeneous evaporation of the material from the sample surface.



Fig. 4. Changes of constituent concentration of the: a) AlTiCrN, b) TiCN coating and the substrate materials.

As a result of tests on the electrochemical corrosion, it was observed that the coatings deposited by PVD process on the substrate made of theX40CrMoV5-1 steel constitute effective protection of the substrate material against the corrosive affect of the aggressive factor. The potentiodynamic polarisation curve analysis (Fig. 5) and that of the corrosion rate confirm the better corrosion resistance of the samples with coatings layers in comparison to the uncovered sample (Table 4). During the anode scanning the current density is always lower for the sample with a coating deposited on its surface in comparison to the uncovered sample (11,56 ƒÊAcm-2), which indicates a good protective effect. The potentiodynamic polarisation curve course is the evidence of the active process of the uncoated steel surface. The lowest corrosion current density of the investigated coatings are obtained (from Tafel plot) for the nanocomposite CrAlSiN coating. This can be explain by the relatively low porosity of this coating. The current density for the other coatings is significantly higher than the one obtained for the CrAlSiN coating. The curve course in the cathodic range indicates a strong inhibition of the reactions taking place on coated steel. The behaviour of the systems tested within the anodic range may evidence the porosity or defect of the coatings. Some of the coatings tested within the anodic range were subjected to self-passivation, however, the passive state occurs within a narrow range of the potentials. The growth of the anodic current related to the transpassivation was
observed within the 0€0,4 mV potential range. The corrosion current density and corrosion rate were estimated according to the potentiodynamic curve courses (Table 4). The corrosion potential Ecor test results confirm the better corrosive resistance of the coatings (Table 4) in comparison to the uncoated steel samples. The fact that the corrosive potential of the uncoated substrate significantly grows after a 60-minute experiment, is also worth noting.



Fig. 5. Potentiodynamic polarization curves of the: a) nanocomposite, b) gradient coatings in 1 M HCl solution.



SUMMARY
In case of nanocomposite coatings the compact structure of the coatings without any visible delaminations was observed as a result of tests in the scanning electron microscope. The fracture morphology of the coatings tested is characterised with a dense structure. Basing on the thin film test in the transmission electron microscope, it was observed that the coatings are built of fine crystallites. Their size is 11€25 nm. The coating adhesion scratch tests disclose the cohesion and adhesion properties of the coatings tested. In virtue of the tests carried out, it was found
that the critical load LC2 fitted within the range 46€54 N for the coatings deposited on a substrate made of hot work tool steel. The tests made with the use of GDOS indicate the occurrence of a transition zone between the substrate material and the coating, which affects the improved adhesion between the coatings and the substrate. As a result of the potentiodynamic polarisation curve analysis, the corrosion current density . corrosion rate was determined. It confirms the better corrosion resistance of samples coated with the use of the PVD technique to the uncoated samples made of the austenitic steel (11,65 ƒÊAcm-2). The corrosion current density for the coatings tested fits within the range 0,15€0,77 ƒÊAcm-2, which proves their good anti-corrosion properties.

In case of gradient coatings the compact structure of the coatings without any visible delamination was observed in the scanning electron microscope. The investigated TiAlN and CrAlSiN coatings show columnar structure, which may be considered compatible with the Thornton model (zone I). Upon examination of the thin films obtained from TiAlN and TiCN coatings, it was found that the coatings were composed of fine crystallites. The scratch tests on coating adhesion the reveal the cohesive and adhesive properties of the coatings deposited on the substrate of the X40CrMoV5-1 hot work tool steel. On the basis of the above examinations, it was found that the critical load of LC2 is between 46€59 N. The biggest value of the critical load was obtained for the TiCN coating. The GDOS investigations indicate the existence of the transition zone between the substrate material and the coating resulting in the improved adhesion of the coatings deposited to the substrate.

ACKNOWLEDGEMENT
Research was financed partially within the framework of the Polish State Committee for Scientific Research
Project No N N507 550738 headed by Dr Krzysztof Lukaszkowicz.

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Fig. 1. Fracture of the: a) TiAlSiN, b) CrAlSiN, c)TiAlN, d) TiCN coating deposited onto the X40CrMoV5-1 steel substrate.


Fig. 2. Diagram of the dependence of the acoustic emission (AE) and friction force Ft on the load for the X40CrMoV5-1 steel with the: a) AlTiCrN, b) TiAlSiN coating.


Fig. 3. Scratch failure pictures of the AlTiCrN coating on X40CrMoV5-1 steel substrate at: (a) LC1, (b) LC2.


Fig. 4. Changes of constituent concentration of the: a) AlTiCrN, b) TiCN coating and the substrate materials.


Fig. 5. Potentiodynamic polarization curves of the: a) nanocomposite, b) gradient coatings in 1 M HCl solution.


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