Wear-Mapping to Optimise Overlay Coating Design in Rolling Sliding Contacts

R. AhmedHeriot-Watt University, United

Kingdom

STRUCTURE of PRESENTATION

• Introduction• Design Considerations• Failure Modes• Wear Mapping• Conclusions and Future Work

TRIBOLOGICAL APPLICATIONS OF THERMAL SPRAY COATINGS

WHY THERMAL SPRAYING?

•Cost effectiveness

•Thicker coatings

•Technically competitive

•Environmental friendly

dies, shafts, rollers, gears, bearings in oil, chemical and food processing industry

COATING PROCESS ?

DESIGN CONSIDERATIONS

SUBSTRATE MATERIAL?

COATING MATERIAL?

COATING THICKNESS ?

M-50, 440C & Mild Steel

20 ~ 300 Micron

WC-Co & Al2O3

HVOF, APS & D-GunTRIBOLOGICAL CONDITIONS?

STRESS ?

LUBRICATION ?

SURFACE FINISH ?

(Moderate to High)

1.5 ~ 5.5 GPa

(Boundary, Mixed & Full Regime)

Dry, BF+H2O, Hitec-174, Exxon-2389

(Moderate to High)

0.5 ~ 0.05 Micron (Rq)

• 1- Coated cone1- Coated cone• 2- Planetary balls2- Planetary balls• 3- Spindle3- Spindle• 4- Loading lever4- Loading lever• 5- Driving motor5- Driving motor• 6- Heater plate6- Heater plate• 7- Loading piston7- Loading piston• 8- Belt drive8- Belt drive

SURFACE OBSERVATION –Suspended Test ( 70 million

stress cycles)

WC-Co coating (contact stress of 2.7 GPa)

Wea

r tr

ack

FAILURE MODES

• Delamination• Bulk Failure• Abrasion• Spalling

COATING

DELAMINATION

MECHANISM OF COATING DELAMINATION•Stress concentrations leading to cracks

•Cracks ALWAYS propagate at the depth of orth. and max. shear stress

•Crack propagation is accelerated if shear stress is located at interface

•Sheet like debris

•Ratio of coating thickness to the depth of max. shear (Design Criteria)

Depth of Maximum Shear stress

Depth of Orthogonal Shear stress

SUBSTRATE

DELAMINATION

INFLUENCE OF COATING THICKNESS ON COATING DELAMINATION

0

10

20

30

40

50

60

70

80

0 0.5 1 1.5 2 2.5 3

Normalised coating thickness ( = / )

Number of stress cycles (106)

Normalised pressure () = 1.5Normalised pressure () = 1.5

Normalised pressure () = 1.7Normalised pressure () = 1.7

SuspendedTests - No

Failure

BULK FAILURE

Substrate Migration to the SurfaceSubstrate Migration to the SurfaceMECHANISM OF BULK FAILURE•Substrate yielding

•Migration of substrate to the edge of wear track

•No debris generated

•Ratio of contact pressure to the substrate yield stress is the design criterion to combat bulk failure

Yielding of substrate

Migration of Substrate

Tensile stress within coating

Cracks in middle of wear track

PO

Wear track

Substrate

INFLUENCE OF SUBSTRATE YIELD STRENGTH ON COATING PERFORMANCE

4.55

5.56

6.57

7.58

8.5

1 1.5 2 2.5 3

Mild steel substrateMild steel substrate

Suspended tests – No failureSuspended tests – No failure

M-50 or 440-C steel substrate

M-50 or 440-C steel substrate

Normalised Pressure ( = Po/)

Log (Stress cycles)

•Ability to support the coating

OTHER CONSIDERATIONS

•Coefficient of thermal expansion

•Ability to withstand pre-heat temperature

•Ability to deform during shot-penning

DRIVE ROLLING DRIVE ROLLING ELEMENTELEMENT

DRIVEN DRIVEN ROLLING ROLLING ELEMENTELEMENT

ADHERED/ ADHERED/ EMBEDDEEMBEDDED DEBRISD DEBRIS

SMALL SMALL DEBRISDEBRIS

ASPERITY ASPERITY CONTACTCONTACT

ROLLING ROLLING DIRECTIONDIRECTION

ABRASION

MECHANISM OF COATING ABRASION•Asperity contact

•Micro-fracture leading to particle pull out

•Small wear debris lead to three body abrasion to accelerate the process

WEAR WEAR DEBRISDEBRIS

MICRO MICRO SLIP/SLIDINGSLIP/SLIDING

TRACTIONTRACTION

RATIO OF CONTACTIN

G PAIR

HARDNESS

SPALLING

•Surface or Subsurface crack initiation and propagation

•Possibilities of lubricant entrapment

INFLUENCE OF COATING MATERIAL

CERMETS PERFORMED BETTER THAN CERAMICS

HIGHER MELTING POINT OF CERAMICS

POROSITY and MIROCRACKING

QUENCHING STRESS

Coefficient o

f therm

al expansion

INFLUNCE OF COATING PROCESS

KIC = 0.025 E1/2bP1/2C-3/2 = 1.8 MNm-3/2

where

E =Young’s Modulus; 2b = Indentation Diagonal; C = Crack length; P= Applied Load

Crack

0

2

4

6

8

10

12

14

0 0.5 1 1.5 2 2.5 3

<3, Mixed lubrication regime

3, Full lubrication regime

Normalised Coating Thickness (=/)

Normalised Contact Pressure (=Po/)

NO FAILURE 70106 cycles

BULK FAILUREDELAMINATION

SPALLING

DELAMINATION (Cohesive)

andABRASIVE

WEAR

CohesiveAdhesive

Lubrication Regime (=Hmin/Rqa)

AB

RA

SIV

EW

EA

R

WEAR MAP OF WC-Co COATINGS (KIC 1.8 MNm-3/2)

CONCLUSIONS

• Wear map has been introduced to benchmark coating design process

• By appropriate design of coated components, it is possible to achieve a fatigue life in excess of 70 million stress cycles.

• Four failure modes I.e. Delamination, Bulk failure, Abrasion and Spalling can lead to the failure of thermal spray coatings.