Evaluation of surface coatings and layers by modern methods

Kříž

A., Beneš

P., Sosnová

M., Hrbáček P.University of

West

Bohemia -

Pilsen

Department of

Material

Science and

Technology

13th International Symposium on Metallography 2007, Stará

Lesná, Slovak Republic

The components in many industrial applications are exposed to intensive effects of contact stress.

Degradation occurs during the contact stress of two surfaces because of their interaction.

2/25

Degradation –

the

limited factor of lifetime

It is necessary to find the ways of suppressing this degradation or eliminate it to an acceptable limit.

Dynamic contact wear is not the basic kind of wear ⇒ it consists of basic kinds of wear (adhesive, abrasive, fatigue and fretting wear).

The solution to this problem with the bulk material is very expensive and very often useless.

It is not possible to prefer one property to the others and suppose that the problem is solved (↑

hardness ≠ ↑ resistance).

Surface properties is necessary to solve generally !!!

The suitable surface modification is adequate to assure the necessary function.

3/25

To analyse the entire surfaces’ behaviour during dynamic loadingit is necessary to describe generally the wear obtained by two

testing instruments.

IMPACT TESTER FRETTING TESTER• Fatigue strength

• Contact wear

• Fretting wear

• Wear from vibrations (oscillation)

• Fatigue wear

4/25

1) Steel with nitride layers:

2) Nodular cast iron with chrome coatings:

a)

galvanic porous chrome with content of Al2

O3

particles

–

sample no.4

b)

galvanic porous chrome without content of Al2

O3

particles

–

sample no.5

a)

steel ASL 817 with ion nitriding surface coating

–

sample no.1

b)

steel ASL 813 with ion nitriding surface coating

–

sample no.2

c)

steel ASL 813 with gas nitriding surface coating

–

sample no.3

Coatings and layers thicknesses [µm]Sample no.1 Sample no.2 Sample no.3 Sample no.4 Sample no.5

123 105 82 91 154

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Surfaces of nitride layersSample no.1 Sample no.2

Sample no.3

Different quality of sample surfaces

6/25

Impact test

< 10 N

4 – 6 mm

Pin holder

Pin

Weight

max

. 65

Coating

Substrate

• impact energy –

25 J

• „PIN“

ball –

WC

• number of impacts:

nitride layer : 1 000, 2 500, 5 000, 10 000

chrome coatings : 1 000, 2 500, 5 000 (due to smaller resistance)

•

impact craters were monitored by: light microscopy, confocal

microscopy,

electron microscopy

•

depth of craters was measured contactless by laser confocal microscope Olympus LEXT

7/25

enables testing of selected coatings and layers for fatigue strength (while e.g. tests based on the scratch test or tribological experiments can be inadequate for condition simulation, when the surface is exposed both to fatigue and contact wear)

truly simulates real situations within coatings’ lifetime

principle lies in periodic frequency impact under certain loaded “PIN” (e.g. Al2O3, tungsten carbide and ČSN 17 042 steel)

impact tester is equipped with blow apparatus ⇒ damaged and removed wear debris will not remain in the contact area and results are not influenced

Impact test

8/25

Fretting testis caused by the oscillating movement with small amplitude that may occur between contacting surfaces subjected to vibration is a special kind of fatigue surface wear is a dynamic process which is strongly effected by vibration, contact area, tribochemical influence and wear debris play an important role theredirect output is course of friction coefficient in dependence on number of cycles character and wear of “PIN”ball and wear track are observed 9/25

Fretting test •

“PIN”

-

ČSN 17042, 1000 cycles ⇒ for exact measuring

of friction coefficient

•

“PIN”

–

WC, 5000 and 10

000 cycles

⇒ to monitor resistance of each coatings and layers to

wear

• Track length of 1 cycle -

4250 µm

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Nitride layers

Number of impacts

Sample no.1 Sample no.2 Sample no.3 Sample no.4 Sample no.5

Depth of crater [µm] 9,71 13,77 15,9 22,5 27,75

Width of crater [µm] 644,9 683,65 717,35 789,8 911,2

Depth of crater [µm] 14,39 15,49 20,9 27,25 65

Width of crater [µm] 716,35 712,25 724,45 766,3 1420,45

Depth of crater [µm] 15,78 16,04 24,32 31 90

Width of crater [µm] 721,45 704,1 730,6 921,45 1521

Depth of crater [µm] 19,51 18,36 28,75 - -

Width of crater [µm] 751 739,8 781,65 - -

1000

2500

5000

10000

38,4

9 45,7 49

,79

66,4

1

40,2

9

43,8

9

45,9

8

49,6

4

27,1

9

30,0

4

34,6

7 45,1

3

0

10

20

30

40

50

60

70

10000 5000 2500 1000

number of impacts

wid

th -

dept

h ra

te

Sample no.1

Sample no.2

Sample no.3

90,0

65,0

27,831,027,3

22,5

0

10

20

30

40

50

60

70

80

90

100

5000 2500 1000

Number of impacts

Dep

th [μ

m]

with Al2O3 without Al2O3

Dependences of width -

depth rate of impact crater on number of impact of tested nitride layers

Depths of impact craters of chrome coatings

Strengthening processes which take place in contact area are evident from

width -

depth rate of impact crater

Depth increase in dependence on number of impacts is linear ⇒ low

resistance to

impact straining

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The gradual

growth of the crater illustrates, that the strengthening process

does not take place on its surface in the contact area.

Strengthening processes could stop or slow down the linear growth of impact depth, as in the case of samples no.1 and 2.

19,5

1

15,7

8

14,3

9

9,71

18,3

6

16,0

4

15,4

9

13,7

7

28,7

5

24,3

2

20,9

0

15,9

0

0

5

10

15

20

25

30

35

10000 5000 2500 1000

Number of impacts

Dep

th [μ

m]

Sample no. 1 Sample no. 2 Sample no. 3

Sample no.1 had the best resistance to

low-cycle dynamic straining.

Sample no.3 showed low resistance to

low and high-cycle dynamic straining

.

13/25

Width -

depth rate

38,4

9 45,7 49

,79

66,4

1

40,2

9

43,8

9

45,9

8

49,6

4

27,1

9

30,0

4

34,6

7 45,1

3

0

10

20

30

40

50

60

70

10000 5000 2500 1000

number of impacts

wid

th -

dept

h ra

te

Sample no.1

Sample no.2

Sample no.3

Sample no.2 –

small difference of width -

depth rate of impact crater between 10 000 and 1 000 impacts ⇒ gradual increasing of crater width without increasing of impact depth

The highest resistance to the repetitive dynamic impact straining of sample no.2

Intensive straining

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Sample

no.3 after

5000 impacts Sample

no.1 after

10000 impacts

– edge

of

impacts

Adhesive

failure

of

sample

no.2 in the

edge of

impacts.

Low

toughness

of

layers

Edge

of

impacts

= transition

of tensile

stress to compressive

⇒

contrarious

stress condition

15/25

Sample Number of cycles Load [N] “PIN” Width of track

[µm]Wear of track

[mm3]Average depth of track

[µm]

Sample no.1Sample no.1

10001000 22 ČČSN 17042SN 17042 148148 0,00020,0002 0,30,3

50005000 55 WCWC 171171 0,00080,0008 1,11,1

1000010000 55 WCWC 347347 0,00170,0017 1,21,2

Sample no.2Sample no.2

10001000 22 ČČSN 17042SN 17042 148148 0,00020,0002 0,30,3

50005000 55 WCWC 182182 0,00040,0004 0,50,5

1000010000 55 WCWC 217217 0,00050,0005 0,60,6

Sample no.3Sample no.3

10001000 22 ČČSN 17042SN 17042 142142 0,00020,0002 0,30,3

50005000 55 WCWC 221221 0,00060,0006 0,70,7

1000010000 55 WCWC 271271 0,00090,0009 0,80,8

•

sample no.1 exhibited the highest wear of all tested nitride layers when the “PIN” counterpart from tungsten carbide was used

(due to low hardness)

• sample no.2 exhibited the best wear resistance

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Fretting test

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0 2000 4000 6000 8000 10000

number of cycles

fric

tion

coef

ficie

nt

sample no.1

sample no. 2

sample no.3

•

the lowest friction coefficient (≈

0,4) was measured in sample no.2 with “PIN”

from tungsten carbide.• the highest friction coefficient which was evaluated in case of

sample no.3 (≈

0,63).

•

course of friction coefficient changed with the use of steel “PIN”. The lowest friction coefficient was measured in sample no.3, in comparison with sample no.2 exhibiting the highest one.

Course of friction coefficient of samples 1, 2, 3, F= 5 N, n = 10

000 cycles, “PIN”

–

WC.

Sample

no.1, 5000 cycles, 5N, WC PIN Sample

no.2, 5000 cycles, 5N, WC PIN Sample

no.3, 5000 cycles, 5N, WC PIN

Portion of elastic and total deformation

5060708090

100

10 m

N

20mN

30mN

50mN

70mN

100 m

N20

0mN

300m

N50

0mN

700m

N10

00mN

Load [mN]

Valu

e [%

]

Sample 1

Sample 2

Sample 3

Sample no.1 Sample no.2 Sample no.3 Sample no.4 Sample no.5

HV0,02 [GPa] 11,92±0,7 11,48±0,32 13,80±0,46 11,97±0,93 12,02±0,46

HV5 [GPa] 7,998±0,390 6,500±0,278 10,21±1,624 6,232±0,297 6,042±0,333

Hardnesses

and Microhardnesses

Sample no.3 had considerably higher elastic deformation ability (lower ability of plastic deformation)

• all analysed samples reach similar values of microhardness

• hardness values were quite different at HV5

load

•

with respect to contact stress, the indentations at F=294,3N were

carried out

-

initiation of surface cracks round about the imprints was

detected

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•

to evaluate both the cracks and imprints the laser confocal

microscope was used

•

sample no. 3 shows

the highest initiation of damage to

the wear neighbourhood where the imprints were carried out

⇒

it's

evidently

evoked by its high hardness, when minimum of strain is eliminated by plastic deformation and elastic deformation is insufficient to eliminate of

expand the cracks

Portion of elastic and total deformation

5060708090

100

10 m

N

20mN

30mN

50mN

70mN

100 m

N20

0mN

300m

N50

0mN

700m

N10

00mN

Load [mN]

Valu

e [%

]

Sample 1

Sample 2

Sample 3

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Chrome coatings

Chrome

coatings

with

content

of Al2

O3

Chrome

coatings

without

content

of Al2

O3

1 000 impacts

5 000 impacts

21/25

90,0

65,0

27,831,0 27,322,5

0102030405060708090

100

5000 2500 1000

Number of impacts

Dep

th [μ

m]

with Al2O3 without Al2O3

Depths

of

impact craters

The poruses

significantly influenced coating resistance.

Poruses have ability to eliminate accumulated tensile stress effects.

Al2

O3

particles

and their distribution can negatively affect cohesion of the coating.

X

Coating without Al2

O3

particles exhibited

the best wear resistance to impact straining.

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Decohesion of chrome coating with Al2

O3

particles

Gradual breaking off of the coating –

creation of the graded structure

Influence of surface roughness increase on impact wear resistance was

not proved.

Impacts

craters

after

5 000 impacts.

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Chrome coatings

00,10,20,30,40,50,6

0 5000 10000

number of cycles

firct

ion

coef

ficie

nt

Coating w ith Al2O3

Coating w ithoutAl2O3

•due to of

unequal surface topography in the case of chrome sample with Al2

O3

particles, original surface layer remained in the wear track

• wear debris consisted of material from layer and the “PIN”

counterpart

•

coating with Al2

O3

particles was more intensively worn; delamination of this coating was

not observed, the layer was only deformed or cohesive failure occurred.

•

surface topography of coating

without Al2

O3

particles was markedly higher than layer with Al2

O3

particles

⇒ a larger part of original surface preserved.

•

accumulation of wear debris in

the

wear track and delamination of layers were not observed

• plastic deformation was detected, which evoked cohesive failure of layers

Chrome coating with Al2

O3

particles

Chrome coating without Al2

O3

particles24/25

ConclusionsThe conformity between results from the fretting test and impact test were found. The best resistance to impact and fatigue failure was exhibited by the nitride layers sample no. 2 - steel ASL 813 with ion nitriding surface coatingIn chrome coatings the best wear resistance was monitored in porous chrome without the content of Al2O3 particles. Results of the microhardness measurement of nitride layers and coatings would not be sufficient for the explanation of fatigue behaviour and application possibilities of investigated samples ⇒ it is not possible from the microhardness measurement alone to predict the resistance of layers and coatings in industry applications.

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Thank you for your attention