2. Wear-Mapping to Optimise Overlay Coating Design in Rolling Sliding Contacts R. Ahmed Heriot-Watt University, United Kingdom

3. STRUCTURE of PRESENTATION Introduction Design Considerations Failure Modes Wear Mapping Conclusions and Future Work

4. 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

5. COATING PROCESS ? DESIGN CONSIDERATIONS SUBSTRATE MATERIAL? COATING MATERIAL? COATING THICKNESS ? M-50, 440C & Mild Steel20 ~ 300 Micron WC-Co & Al 2 O 3 HVOF, APS & D-Gun TRIBOLOGICAL CONDITIONS? STRESS ? LUBRICATION ? SURFACE FINISH ? (Moderate to High) 1.5 ~ 5.5 GPa (Boundary, Mixed & Full Regime) Dry, BF+H 2 O, 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

6. SURFACE OBSERVATION –Suspended Test ( 70 million stress cycles) WC-Co coating (contact stress of 2.7 GPa) Wear track

7. FAILURE MODES Delamination Bulk Failure Abrasion Spalling

8. 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

9. INFLUENCE OF COATING THICKNESS ON COATING DELAMINATION Normalised coating thickness (  =  /  ) Number of stress cycles (  10 6 ) Normalised pressure (  ) = 1.5 Normalised pressure (  ) = 1.7 Suspended Tests - No Failure

10. BULK FAILURE Substrate Migration to the Surface MECHANISM 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 POPO Wear track Substrate

11. INFLUENCE OF SUBSTRATE YIELD STRENGTH ON COATING PERFORMANCE Mild steel substrate Suspended tests – No failure M-50 or 440-C steel substrate Normalised Pressure (  = P o /  ) 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

12. DRIVE ROLLING ELEMENT DRIVEN ROLLING ELEMENT ADHERED/ EMBEDDE D DEBRIS SMALL DEBRIS ASPERITY CONTACT ROLLING DIRECTION 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 DEBRIS MICRO SLIP/SLIDING TRACTION RATIO OF CONTACTING PAIR HARDNESS

13. SPALLING Surface or Subsurface crack initiation and propagation Possibilities of lubricant entrapment

14. INFLUENCE OF COATING MATERIAL CERMETS PERFORMED BETTER THAN CERAMICS HIGHER MELTING POINT OF CERAMICS POROSITY and MIROCRACKING QUENCHING STRESS Coefficient of thermal expansion

15. INFLUNCE OF COATING PROCESS K IC = 0.025 E 1/2 bP 1/2 C -3/2 = 1.8 MNm -3/2 where E =Young’s Modulus; 2b = Indentation Diagonal; C = Crack length; P= Applied Load Crack

16. 0 2 4 6 8 10 12 14 00.511.522.53 <3, Mixed lubrication regime  3, Full lubrication regime Normalised Coating Thickness (  =  /  ) Normalised Contact Pressure (  =P o /  ) NO FAILURE 70  10 6 cycles BULK FAILURE DELAMINATION SPALLING DELAMINATION (Cohesive) and ABRASIVE WEAR Cohesive Adhesive Lubrication Regime ( =H min /R qa ) ABRASIVE WEAR WEAR MAP OF WC-Co COATINGS (K IC  1.8 MNm -3/2 )

17. 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.