Microstructure and properties

of PVD WC/C coatings

František Lofaj1, Milan Ferdinandy1, András Juhás2*

Institute of Materials Research of SAS, Košice

Eötvös Loránd Tudomány Egyetem, Budapešť

Outline� Introduction

Hard vs. Superhard coatings

� Experimental

Combined PE PVD technology

Nanoindentation

Results and Discussion

Microstructure & morphology

Chemistry

Nanohardness

� Conclusions

Introduction

Functional coatings ...

Potential of hard coating with low friction coefficients

- lower wear

- longer lifetime

- lower energy consumption

Hard ���� Superhard ���� Ultrahard coatings

<15 GPa ���� 15 – 40 GPa ���� >40 GPa

(Max. 105 GPa in nc-TiN/a-Si3N4)

Basic Principles of Hard Coatings

Approaches to obtain hard coatings:

1. Intrinsically hard material for coatings –(diamond, DLC, c-BN)

limits – hardness of the material

2. High compressive stresses at the surface

frequent decohesion, stress relaxation at higher temperatures

3. Nanocrystalline structure (TiN nanoparticles in Si3N4 matrix and others)

Superhard nanocrystalline coatings

Veprek’s model requirements:

- 2 immiscible components with

spinodal decomposition

dD

- D has to be nano (~ 5 nm) –

dislocation movement is

excluded

- d has to be around 1 nm,

amorphous, high E, continuous skeleton

Experimental ProcedurePVD apparatus

Vacuum pump

ARE electrode 0-300 V

0-5 kV

Sublimation chamber

substrate

Main parameters: total gas pressure, bias, current density

Substrates: HSS, Si single crystal; (Ti sublayers)

Microstructure and PropertiesSEM /EDX, X-ray

Nanohardness – UMIS 2000 (ELTE Budapest)

Berkovich

loading up to 1 mN

or 5 mN in 20 steps

5 s hold

Unloading in 10

steps

ResultsPreparation conditions

1.2

0.8

1

1

Beam current

[mA/cm2]

2004.50.8#21

70052#24

20051#20

3003.84#13, Ar

Thickness

[nm]

Bias

[kV]

Total pressure

[Pa]

Sample/

Microstructure of the coatings (OM)

Delamination of the coating

High contact stresses –

unsuitable conditions

#13, 300 nm: 4Pa (Ar)/ 3.8 kV / 1 mA.cm-2

Ti interlayer Without Ti

Limited delamination

Microstructure morphology and

chemistry of the coatings#13/Ti #13/no Ti

Big W-rich clusters on the

surface

Fe substrate

Ti

WC/C+O,

Relatively high content of oxygen...

Morphology of the coating#20: 200 nm

strong delamination, ?? layered structure??

1 Pa / 5 kV/ 1 mA.cm-2

Microstructure and morphology of the

coatings#24, 700 nm, 2 Pa/4.5 kV /1.2 mA.cm-2

Partial delamination

Si substrate

Microstructure of the WC/C coating

Close to optimum conditions

Steel substrate Si substrate

Phase composition of the coatings

Position [°2Theta]

30 40 50

Counts

0

200

400

W C

; Si

C

Si

C; W

C; Si

C

Si

C

W C

; Si

C

wc-c_si_2.rd

Sample #13 /Si substrate

WC (PDF 05-0728 ) identified – possibility for nanocrystalline phases

Repeatability of the measurements

at 1 mN load

0,000 0,005 0,010 0,015 0,020 0,025 0,030 0,0350,0

0,2

0,4

0,6

0,8

1,0 HSS 07924-1

2Pa/5 kV/0.8 mA/cm-2

Load, m

N

Penetration depth, µm

0,00 0,02 0,04 0,06 0,080,0

0,2

0,4

0,6

0,8

1,0 HSS 070913-1

4Pa/3.8kV/1 mA/cm-2

Load, m

N

Penetration depth, µm

Ti

Penetration depth ~ 30-80 nm ... << layer thickness

??Effect of residual stresses??, correlation with delamination

Repeatability at 5 mN load

0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,140

1

2

3

4

5HSS 070920

1 Pa/5kV/1mA/cm-2

Load m

N

Penetration depth, µm0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,140

1

2

3

4

5

HSS 070924

2Pa/5 kV/0.8 mA/cm-2

Load, m

N

Penetration depth, µm

0,00 0,02 0,04 0,06 0,08 0,10 0,120

1

2

3

4

5

HSS 070921

0.8 a/ 4.5 k / 1.2 mA/cm2

Load, m

N

Penetration depth, µm

Summary of the hardness and

elasticity

266 ± 128(106 – 350)

20.1 ± 11.9(7.1 – 38.9)

#21

265 ± 12(251-277)

19.9 ± 2.1(17.7-22.9)

#24

318 ± 24(297-355)

21.5 ± 3.1(18.0 – 25.7)

#20

323 ± 96(203-489)

24.1 ± 9.5

(16.9- 45.8)

#13

Elastic modulus [GPa]

(min. – max.)

Hardness [GPa]

(min. – max.)

Sample

Conclusions

� Delamination of the coating is a problem..

� WC/C coating preparation conditions were optimized ....

� Thin “nanocrystalline –like” coatings obtained but their hardness is only 20-25 GPa ...

� Serious problems with the measurement of nanohardness of thin coatings - influence of the residual stresses and morphology...

� TEM of the coatings and AFM/nanoindentation combination necessary ( direction of our future work)