Structure and mechanical properties of

KRZYSZTOF LUKASZKOWICZ, JAROSŁAW MIKUŁA, KLAUDIUSZ GOŁOMBEK, LESZEK A. DOBRZAŃSKI, JANUSZ SZEWCZENKO, MIECZYSŁAW PANCIELEJKO

Structure and mechanical properties of nanocomposite coatings deposited by PVD process onto tool steel substrates

Struktura i własności mechaniczne nanokompozytowych powłok naniesionych w procesie PVD na podłoże ze stali narzędziowych ABSTRACT

STRESZCZENIE

This paper presents the research results on the structure and mechanical properties of nanocomposite coatings deposited by PVD process onto the tool steel substrates. The tests were carried out on TiAlSiN, CrAlSiN and AlTiCrN coatings. It was found that the structure of the PVD coatings consisted of fine crystallites, while their average size fitted within the range of 15÷30 nm, depending on the coating type. The coatings demonstrated columnar structure and dense cross-section morphology as well as good adherence to the substrate, the latter not only being the effect of adhesion but also by the transition zone between the coating and the substrate, developed as a result of diffusion and high-energy ion action that caused mixing of the elements in the interface zone. The critical load LC2 lies within the range of 46÷54 N, depending on the coating and substrate type. The coatings demonstrate a high hardness (~40 GPa) and corrosion resistance. The good properties of the PVD nanocomposite coatings make them suitable in various engineering and industrial applications.

W pracy przedstawiono wyniki badań struktury i własności mechanicznych nanokompozytowych powłok naniesionych w procesie PVD na podłoże ze stali narzędziowych. Badania wykonano na powłokach TiAlSiN, CrAlSiN i AlTiCrN. Stwierdzono, że struktura powłok PVD złożona jest z drobnych krystalitów, a ich średnia wielkość zawiera się w przedziale 15÷30 nm w zależności od rodzaju powłoki. Badane powłoki wykazują strukturę kolumnową oraz strukturę o zagęszczonych krystalitach, jak również dobrą przyczepność do podłoża, o której decyduje nie tylko adhezja, lecz również warstwa przejściowa pomiędzy powłoką a podłożem powstała w wyniku dyfuzji i na skutek działania jonów o dużej energii, powodujących przemieszanie się pierwiastków w strefie połączenia. Obciążenie krytyczne LC2 zawarte jest w przedziale 46÷54 N, w zależności od powłoki i materiału podłoża. Badane powłoki wykazują wysoką twardość (~40 GPa) i odporność korozyjną. Dobre własności nanokompozytowych powłok PVD powodują, że warstwy te są odpowiednie do różnych technicznych i przemysłowych zastosowań.

INTRODUCTION

a low friction coefficient (self-lubricating coatings) in numerous working environments, maintaining high hardness and increased resistance [8, 9]. The main concept in the achievement of high hardness of nanocrystal structure coatings and good mechanical properties and high strength related to it, particularly in case of nanocomposite coatings [10-12] is the restriction of the rise and the movement of dislocations. High hardness and strength of the nanocomposite coatings are due to the fact that the movement of dislocations is suppressed at small grains and in the spaces between them. When the grain size is reduced to that of nanometers, the activity of dislocations as the source of the material ductility is restricted. This type of coatings is also characterized with a large number grain boundaries with a crystalline/amorphous transition across grain-matrix interfaces, restricting the rise and development of cracks. Such mechanism explains the resistance to fragile cracking of nanocomposite coatings [13÷15]. Simultaneously, the equiaxial grain shapes, high angle grain boundaries), low surface energy and the presence of the amorphous boundary phase facilitating the slide along the grain boundaries causes a high plasticity of the nanocomposite coatings [3]. The purpose of this paper is to examine the structure, mechanical properties and corrosion resistance of nanocomposite coatings deposited by PVD process on substrates made from X40CrMoV5-1 hot-work tool steel and gradient tool materials obtained by using the powder metallurgy of the chemical composition corresponding to the HS6-5-2 high-speed steel reinforced with the WC and TiC type hard carbide phases with the growing portions of these phases in the outward direction from the core to the surface.

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 single-layer, multi-layer or gradient coatings using PVD methods [1, 2]. While selecting the coating material, we encounter a barrier caused by the fact that numerous properties expected from an ideal coating is impossible to be obtained simultaneously. For example, an increase of hardness and strength causes the reduction of the coating’s ductility and adherence to the substrate. The application of the nanostructural coatings is seen as the solution of this issue [3, 4]. According to the Hall-Petch equation, the strength properties of the material rise along with the reduction of the grain size. In case of the coatings deposited in the PVD processes, the structures obtained, with grain size ~10 nm cause the obtainment of the maximum mechanical properties. Coatings of such structure present very high hardness >40 GPa, ductility, stability in high temperatures, etc. [5÷7]. The known dependency between the hardness and abrasion resistance became the foundation for the development of harder and harder coating materials. 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 ___________________________ Ph. D. Krzysztof Lukszkowicz, Ph. D. Jarosław Mikuła, Ph. D. Klaudiusz Gołombek, Prof. Leszek A. Dobrzański ([email protected]), Ph. D. Janusz Szewczenko – Institute of Engineering Materials and Biomaterials, Silesian University of Technology, Gliwice, Ph. D. Mieczysław Pancielejko – Institute of Mechatronics, Nanotechnology and Vacuum Technique, Koszalin University of Technology, Koszalin

EXPERIMENTAL DETAILS The tests were made on samples of the X40CrMoV5-1 hot work tool steel with 56 HRC (2.1 GPa) hardness and the gradient tool materials

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obtained by using the powder metallurgy of the chemical composition corresponding to the HS6-5-2 high-speed steel reinforced with the WC and TiC type hard carbide phases with the growing portions of these phases in the outward direction from the core to the surface, deposited by PVD process with TiAlSiN, CrAlSiN, AlTiCrN hard coatings. The steel samples were mechanically ground and polished (Ra=0.03 µm), in order to provide the appropriate surface quality. The coating deposition process was made in a device based on the cathodic arc evaporation method. Cathodes containing pure metals (Cr, Ti), the AlTi (67:33 at. %) alloy and the AlSi (88:12 wt. %) alloy were used for deposition of the coatings. The base pressure was 5×10-6 mbar, the deposition temperature was approximately 300°C. The deposition conditions are summarized in Table 1.

RESULTS AND DISCUSSION The 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 (Figs. 1÷3). 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.

Table 1. Deposition parameters of the coatings Tabela 1. Parametry nanoszenia powłok Coating

Substrate bias voltage V

TiAlSiN

-90

CrAlSiN

-60

AlTiCrN

-60

Arc current source A Ti – 80 AlSi – 120 Cr – 70 AlSi – 120 Cr – 70 AlTi – 120

Pressure Pa 2,0 3,0 2,0

Observations of surface and structures 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 and 30,000× maximum magnification. The phase composition of the investigated coatings was determined by means of the DRON-2.0 X-ray diffractometer, using the filtered X-radiation of the cobalt anode lamp, at 40 kV supply voltage and 20 mA glow current intensity. The chemical concentration changes of the coating components, both perpendicular to their surface and in the transition zone between the coating and the substrate material, were evaluated in tests carried out in the GDOS-750 QDP glow-discharge optical spectrometer from Leco Instruments. Tests of the coatings’ adhesion to the substrate material were made using the scratch test on the CSEM REVETEST device, by moving the diamond indenter along the examined specimen’s surface with gradually increasing load. The radius of the diamond indenter used in the scratch test was 200 µm. The device registered the friction force, friction coefficient, indenter penetration depth and acoustic emission along the scratch track. The tests were made using the following parameters: • load range: 0÷100 N, • load increase rate (dL/dt): 100 N/min, • indenter’s sliding speed (dx/dt): 10 mm/min, • acoustic emission detector’s sensitivity AE: 1. The critical load LC, 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 recorded and observation of the damage developed in the scratch test on a LEICA MEF4A light microscope. The microhardness tests of coatings were made with the SHIMADZU DUH 202 ultra-microhardness tester. The test conditions were selected so as to obtain comparable test results for all coatings. Measurements were made at 0.05 N load, to eliminate the substrate impact on the coating hardness. X-ray line broadening technique was used to determine crystallite size of the coatings using Scherrer formula [16] with silicon as internal standard. (1) D=(0.899λ/BcosθB) where: D – crystallite size, B – full width at half-maximum XRD peak (111) in radians, λ – wavelength of the X-ray radiation, θB – the Bragg angle in radians.

Fig. 1. Fracture of the TiAlSiN coating deposited onto the X40CrMoV5-1 steel substrate Rys. 1. Powierzchnia przełomu powłoki TiAlSiN naniesionej na podłoże ze stali X40CrMoV5-1

Fig. 2. Fracture of the CrAlSiN coating deposited onto the X40CrMoV5-1 steel substrate Rys. 2. Powierzchnia przełomu powłoki CrAlSiN naniesionej na podłoże ze stali X40CrMoV5-1 Basing on the X-ray diffraction of the samples examined (Figs. 4÷6), the occurrence of fcc phases was only observed in the coatings. The hexagonal AlN of wurtzite type was not detected in the coatings examined, which could have been caused by a low amount of aluminium 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 2. The size of crystallites determined with Scherer method confirmed the results of tests made by means of transmission electron microscopy. The hardness of the coatings tested fits within the range from 40 to 42 GPa. The highest hardness was recorded in the case of the AlTiCrN coating (Tab. 2).

Nr 6/2008 ___________________ I N Ż Y N I E R I A M A T E R I A Ł O W A ________________________ 733

9000

Fe (011)

8000

6000

2000 1000

Fe (112)

3000

Fe (002)

4000

(Ti,Al,Si)N (200)

5000

(Ti,Al,Si)N (111)

Intensity, imp/s

7000

0 30

40

50

60

70

80

90

100

110

Reflection angle, 2Θ

50

60

70

80

90

100

110


Fig. 4. X-ray diffraction pattern of the CrAlSiN coating deposited onto the X40CrMoV5-1 steel substrate Rys. 4. Dyfraktogram rentgenowski powłoki CrAlSiN naniesionej na podłoże ze stali X40CrMoV5-1 Fe (011)

8000 7000

1000

45

55

65

75

85

0

95

1

2

3

4

5

6

7

8

9

Acoustic emission AE nn

20

0

Fig. 7. Diagram of the dependence of the acoustic emission (AE) and friction force Ft on the load for the X40CrMoV5-1 steel with the AlTiCrN coating Rys. 7. Wykres zależności emisji akustycznej (AE) i siły tarcia Ft od obciążenia dla powłoki AlTiCrN naniesionej na podłożu ze stali X40CrMoV5-1 Load force Fn, N 0 10 20 30 40 50 60 70 80 90 100 100

100

80

80

60

60

40

40

20

20

0

0 1

2

3

4

5

6

7

8

9

Path X, mm

0 35

40

20

0 Fe (112)

2000

(Al,Ti,Cr)N (311)

3000

Fe (002)

4000

(Al,Ti,Cr)N (200)

5000 (Al,Ti,Cr)N (111)

Intensity, imp/s

6000

60

40

105

115


Fig. 5. X-ray diffraction pattern of the AlTiCrN coating deposited onto the X40CrMoV5-1 steel substrate Rys. 5. Dyfraktogram rentgenowski powłoki AlTiCrN naniesionej na podłoże ze stali X40CrMoV5-1 The critical load values LC1 and LC2 were determined by the scratch test method (Figs. 7-9). The load at which the first coating defects appear is known as the first critical load LC1. The value of the first critical load is linked to the cohesive defects related to chipping of the material inside the coating, although without uncovering the substrate material (Figs. 10a, 11a, 12a). Such defect is represented by the first weak signal of noise emission (Figs. 7÷9). The second critical load LC2 is characterized with the total damage of the coating. The damage is considered the vertex of the ascending friction curve on the graph. The vertex corresponds to the first contact of the diamond indenter with the substrate when extensive chipping of the coating occurs (Figs. 10b, 11b, 12b). 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 2.

Fig. 8. Diagram of the dependence of the acoustic emission (AE) and friction force Ft on the load for the X40CrMoV5-1 steel with the CrAlSiN coating Rys. 8. Wykres zależności emisji akustycznej (AE) i siły tarcia Ft od obciążenia dla powłoki CrAlSiN naniesionej na podłożu ze stali X40CrMoV5-1 Load force Fn, N 0 10 20 30 40 50 60 70 80 90 100 100

100

80

80

60

60

40

40

20

20

0

0 0

1

2

3

4

5

6

7

8

9


40

80

60


0 30

80

Path X, mm

Friction force Ft, N ggg

1000

100

0

Friction force Ft, N ggg

2000

Fe (002)

3000

(Cr,Al,Si)N (200)

4000

(Cr,Al,Si)N (111)

Intensity, imp/s

5000

100

Fe (112)

6000

(Cr,Al,Si)N (311)

Fe (011)

7000

Load force Fn, N 0 10 19 29 38 48 58 67 77 87 96 Friction force Ft, N ggg

Fig. 3. Fracture of the AlTiCrN coating deposited onto the X40CrMoV5-1 steel substrate Rys. 3. Powierzchnia przełomu powłoki AlTiCrN naniesionej na podłoże ze stali X40CrMoV5-1

Fig. 6. X-ray diffraction pattern of the TiAlSiN coating deposited onto the X40CrMoV5-1 steel substrate Rys. 6. Dyfraktogram rentgenowski powłoki TiAlSiN naniesionej na podłoże ze stali X40CrMoV5-1

Path X, mm

Fig. 9. Diagram of the dependence of the acoustic emission (AE) and friction force Ft on the load force for the X40CrMoV5-1 steel with the TiAlSiN coating Rys. 9. Wykres zależności emisji akustycznej (AE) i siły tarcia Ft od obciążenia dla powłoki TiAlSiN naniesionej na podłożu ze stali X40CrMoV5-1


(a)

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. (a)

50 µm (b)

50 µm (b)

50 µm Fig. 10. Scratch failure pictures of the AlTiCrN coating on X40CrMoV5-1 steel substrate at: (a) LC1, (b) LC2 Rys. 10. Ślad zarysowania na powłoce AlTiCrN naniesionej na podłoże ze stali X40CrMoV5-1 przy: (a) LC1, (b) LC2 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 using the light microscope coupled with the measuring device, determining the critical load LC1 and LC2 in virtue of metalographic observations. In case of the coatings examined, it was found that coating AlTiCrN had the highest critical load value LC1=24 N and LC2=54 N deposited on substrate made of the X40CrMoV5-1 steel, while CrAlSiN and TiAlSiN coatings deposited on the substrate made of the gradient tool materials – had the lowest value. In general, the coatings deposited on the substrate made of the X40CrMoV5-1 hot-work tool steel show better adherence to the substrate than coatings deposited on the substrate made of the gradient tool materials. 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 (Figs. 10÷12). 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 substratecoating separation surface, which indicates a good adhesion of coatings to substrate. Changes of coating component concentration and substrate material made in GDOS were presented in Figures 13÷15. 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

50 µm Fig. 11. Scratch failure pictures of the CrAlSiN coating on X40CrMoV5-1 steel substrate at: (a) LC1, (b) LC2 Rys. 11. Ślad zarysowania na powłoce CrAlSiN naniesionej na podłoże ze stali X40CrMoV5-1 przy: (a) LC1, (b) LC2 The corrosion resistance test results of coatings deposited on substrate made of the X40CrMoV5-1 hot work tool steel with the method of potentiodynamic polarisation curves in 1 M HCl solution were presented in Figure 16. As a result of tests on the electrochemical corrosion, it was observed that the coatings deposited by PVD process on the substrate constitute effective protection of the substrate material against the corrosive affect of the aggressive factor. The potentiodynamic polarisation curve analysis (Fig. 17) and that of the corrosion rate confirm the better corrosion resistance of the samples with coatings layers in comparison to the uncovered sample (Tab. 3). 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 µA/cm2), 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 icor, i.e. the lowest anode process of the coatings and related corrosion protective properties are presented by the CrAlSiN 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.


(a)

100

Atomic concentration, %

90 80 Fe

70 N

60 50 40 30

Al

20

Cr

Cr

10

Ni

S

0 0

1

2

3

4

5

6

Analysis depth, µm

50 µm

Fig. 14. Changes of constituent concentration of the CrAlSiN and the substrate materials Rys. 14. Zmiany stężenia składników powłoki CrAlSiN oraz materiału podłoża

(b)

100


90

50 µm

Table 2. The characteristics of the tested coatings (G – gradient tool materials, H – hot work tool steel) Tabela 2. Charakterystyka badanych powłok (G-podłoże-gradientowe materiały narzędziowe, H – podłoże stal X40CrMoV5-1) Critical Critical load Crystallite load Thickness Hardness LC1, N LC2, N size Coating µm GPa nm G H G H 2.4 2.9 2.1

42 40 40

15 25 30

15 8 9

24 18 16

27 28 27

54 49 46

60 50 40 30

Ti

20

Al

3

4

5

6

The growth of the anodic current related to the transpassivation was observed within the 0÷0,4 mV potential range. The corrosion currents and corrosion rate were estimated according to the potentiodynamic curve courses (Tab. 3). The corrosion potential Ecor test results confirm the better corrosive resistance of the coatings (Fig. 17) in comparison to the uncoated steel samples. The relatively high potential initial value for some coatings is explained by the demand for the longer time the HCl solution needs to reach the substrate of the samples via diffusion through small surface defects. The impedance measurement results (Tab. 3) confirm the higher corrosive resistance of coated steel. The charge transfer resistance values are higher for coated samples. The CrAlSiN coating presents the best protective properties, which is also confirmed by the polarisation test results. 1,20 1,00

Fe

0,80

N

50 40 30

0,60 0,40 0,20 0,00

20

Cr

Cr

10

Al Ti

Ni

0

2

Fig. 15. Changes of constituent concentration of the TiAlSiN and the substrate materials Rys. 15. Zmiany stężenia składników powłoki TiAlSiN oraz materiału podłoża

1,40

0

1

Analysis depth, µm

80

60

Ni

Si 0

90

70

Cr

0

Potential, V


100

Fe

N

70

10

Fig. 12. Scratch failure pictures of the TiAlSiN coating on X40CrMoV5-1 steel substrate at: (a) LC1, (b) LC2 Rys. 12. Ślad zarysowania na powłoce TiAlSiN naniesionej na podłoże ze stali X40CrMoV5-1 przy: (a) LC1, (b) LC2

AlTiCrN CrAlSiN TiAlSiN

80

1

2

3

4

CrAlSiN

TiAlSiN

-0,20 -0,40

AlTiCrN

-0,60

5

6

Analysis depth, µm

Fig. 13. Changes of constituent concentration of the AlTiCrN and the substrate materials Rys. 13. Zmiany stężenia składników powłoki AlTiCrN oraz materiału podłoża

-10

-8

substrate -6

-4

-2

0

Current density log |i|

Fig. 16. Potentiodynamic polarization curves of the coatings in 1 M HCl solution Rys. 16. Krzywe potencjodynamiczne badanych powłok w 1 M roztworze HCl


-0,10

ACKNOWLEDGMENT

Potential, V

-0,15

CrAlSiN

Research was financed partially within the framework of the Polish State Committee for Scientific Research Project No N507 2068 33 headed by Dr Jaroslaw Mikula.

-0,20 AlTiCrN -0,25

REFERENCES

-0,30 TiAlSiN

substrate

-0,35 -0,40 0

600

1200

1800

2400

3000

3600

Time, s

Fig. 17. Open circuit potential curves of the coatings in 1 M HCl solution Rys. 17. Krzywe potencjału korozyjnego w funkcji czasu w 1 molowym roztworze HCl dla badanych powłok Table 3. Summary results of the electrochemical corrosion investigation Tabela 3. Zestawienie wyników badań odporności korozyjnej badanych powłok Coating type AlTiCrN CrAlSiN TiAlSiN Substrate

Current density icor µA/cm2 0.77 0.15 0.48 11.65

Corrosion potential Ecor mV -0.26 -0.22 -0.28 -0.30

Corrosion rate mm/year 9.0 1.7 5.6 136.2

Resistance polarizatio n Rp kΩcm2 17.1 178.2 67.5 1.25

SUMMARY The compact structure of the coatings without any visible delaminations was observed as a result of tests on a scanning electron microscope. The fracture morphology of the coatings tested is characterised with a dense structure. 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 X40CrMoV5-1 and 27÷28 N for coatings deposited on the substrate made of the gradient tool materials. 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 µA/cm2). The corrosion current density for the coatings tested fits within the range 0.15÷0.77 µA/cm2, which proves their good anticorrosion properties.

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