Rolling Contact Fatigue of Hot Isostatic Pressed WC-NiCrBSi Thermal Spray Coatings S. Stewart Supervisor : Dr R. Ahmed.

Rolling Contact Fatigue of Hot Isostatic Pressed WC-NiCrBSi Thermal Spray

Coatings

S. Stewart

Supervisor : Dr R. Ahmed

Introduction

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Thermal Spray Coatings are used in a number of industrial applications ranging from the automotive and aerospace industries to biomedical applications. However, in many types of industrial machinery such as gears, camshafts and rolling element bearings, surface damage generated by rolling / sliding contact limits the life of the component and hence reduces durability and product reliability. This drives the development and implementation of state of the art surface coatings which enable improved life reliability and load bearing capacity in more hostile environments.

AIMS AND OBJECTIVES

Subjecting Thermal Sprayed coatings to the post treatment, Hot Isostatic Pressing (HIPing), leads to significant densification within the microstructure. The combination of high temperatures and equi-axial pressure reduces porosity and leads to the formation of a more lamellar microstructure. This preliminary study marks the first investigation in published literature in which the rolling contact fatigue performance of HIPed functional graded WC-NiCrBSi coatings are studied.

FURNACE

PRESSURE VESSEL

WORK-PIECE

HIP Unit

Coating fabrication process

440-C Bearing Steel

WC-40%NiCrBSi (100m)

WC-10%NiCrBSi (300m)

Gun type : JP5000Spray distance : 380 mmBarrel length : 4”Fuel gas : KerosenePowder Carrier gas : Oxygen

WC

Ni-7.56%Cr-3.69%Si-2.57%Fe-1.55%B-0.25%

(sintered and agglomerated)

+

HVOF Process Parameters

Analysis of coating microstructure

100 (µm)

As -sprayed HIP 850ºC HIP 1200ºC

Reduction in porosity and change in carbide shape indicates significant densification has occurred within the microstructure of the coating HIPed at 1200ºC.

substrate

WC-10%NiCrBSi

WC-40%NiCrBSi

10 (µm) 10 (µm)

100 (µm) 100 (µm)

0

200

400

600

800

1000

1200

1400

0 50 100 150 200 250 300 350 400

0

50

100

150

200

250

300

350

400

450

0 50 100 150 200 250 300 350 400

As-Sprayed

HIP850

HIP1200

Distance from Surface (µm)

Vick

ers Hard

ness (H

V)

ºC

ºC

WC-10%NiCrBSi WC-40%NiCrBSi

Elastic M

odu

lus (G

Pa)

Micro-hardness measurements show increased hardness values at elevated temperatures of HIPing verifying the observation of densification with the coating

microstructure.

Elastic Modulus results show increased values with HIPing . This indicates improved bonding between the splats after HIPing which can improve resistance of the coating to rolling contact fatigue.

A modified four ball machine was used to study the rolling contact fatigue performance of the HIPed thermal spray coatings. A ceramic ball was placed below the three planetary balls to obtain the correct rolling / sliding contact kinematics.

Drive shaft connected via belt to motor rotates coated disc at 4000 rpm

Air pressure from bellows generates required contact

load between balls and disc.

M

Rc

ωAωs

ωp

R2RA

Rp

O

AB

CD

θ

β δ

Rolling Contact Fatigue Rig

0

10

20

30

40

50

60

70

80

2 2.3 2.7 2.7 (ceramicballs)

RCF test results

Contact stress (GPa)

Stress cycles to

failure (m

illions)

As-SprayedHIPed at 850ºC

HIPed at 1200ºC

• A high viscosity lubricant was used to prevent asperity contact between the coating surface and the planetary balls during testing and hence maintain full film elasto-hydrodynamic lubrication.

• Tests were performed using either 440-C Bearing Steel or HIPed Silicon Nitride Ceramic planetary balls.

Significant improvement

in RCF performance

over as-sprayed coating

Post test Analysis

Under identical experimental conditions, no failure was observed on the wear track of the coating HIPed at 1200ºC after 70 million stress cycles. However, a micro-pit 56µm in depth occurred on the wear track of the as -sprayed coating after only 10 million stress cycles.

1000 (µm)

200 (µm)

as-sprayed

HIP 1200ºC

56µm

Mechanism of failuresubstrate

coating

Planetary ball

aDepth of orthogonal shear stress

Depth of maximum

shear stress

Micro-crack initiates from pore and

propagates. Crack leads to formation of micro-pit at surface.

contact width

Wear track forms from cyclic stresses

Micro-pit forms on wear track from sub-surface

defects

Debris from micro-pit forms larger failure areas

on wear track

Conclusions

• HIPing at elevated temperatures of 1200 ºC lead to significant improvement in rcf performance at low levels of contact stress. No failure occurred at 2GPa, and improvement was attributed to increased densification within the upper layer of the coating.

• The post treatment HIPing was shown to increase elastic modulus and micro-hardness. At elevated temperatures of HIPing, densification occurred which was verified by an increase in micro-hardness within the upper layer of the coating.• Mechanism of failure in as-sprayed coatings was identified as delamination which initiated from sub surface defects.

Acknowledgments• Dr Susan Davies at Bodycote HIP ltd / Infutec ltd

• Dr T. Itsukaichi at Fujimi Inc.• Prof. S.Tobe at Agishaka Institute of Technology