Γ (1

11

)

HA

P

HA

P

HA

P

HA

P

Γ (2

20

)

Γ (2

00

)

ε (1

01

)

CoCrMo-0%CaP

CoCrMo-3%CaP

R TCP

Laser Processed CoCrMo-Calcium Phosphate Composites for Articulating Surfaces Himanshu Sahasrabudhe, Thomas Gualtieri, Susmita Bose and Amit Bandyopadhyay

W. M. Keck Biomedical Materials Research Laboratory, School of Mechanical and Materials Engineering

Washington State University, Pullman, WA 99164 USA. E-mail: amitband@wsu.edu

Laser Engineered Net Shaping

Laser Engineered Net Shaping (LENSTM) is a additive manufacturing technique in

which a powdered material is simultaneously melted and deposited by a high power

Nd:YAG laser. The powder is carried through Argon carried gas. Successive layer wise

deposition of the material finally yields a complete 3D part. The design is specified through

a CAD software.

CoCrMo is a widely used material for implants. CoCrMo is hard and shows good wear and corrosion resistance. However, release of Co and Cr ions in vivo is a serious concern. Various attempts have been

made to reduce metal ion release from CoCrMo. In this research, we have used Laser Engineered Net Shaping (LENSTM) to form a meal-ceramic composite layer by the addition of calcium phosphate to

CoCrMo. The objective of our study is to measure the effect of CaP on the tribological properties of CoCrMo. Our hypothesis is that CaP will act as a solid lubricant and reduce metal ion release from CoCrMo

during articulation. If successful, these composites hold the potential to considerably improve the life of metal-on-metal implants without altering the material significantly.

Acknowledgement

References

Authors would like to acknowledge the financial support from the Life Sciences Discovery

Fund (LSDF) Authority, Washington.

Processing of CoCrMo-CaP by LENSTM

Three different compositions: CoCrMo-0%, 1% & 3% by weight of Hydroxyapatite was

premixed.

Processed at a power of 425W on SS410 base plate

Each sample was ~15mm thick with layers each 0.15mm thick

Cross section SEM and top surface XRD analysis was done to verify phases.

50 µm 50 µm

CoCrMo-3%CaP structure appears more discontinuous as compared to the CoCrMo-0%CaP

structure. Structure of CoCrM-0% shows more equiaxed grained with random orientation.

Both 1% and 3% composites do not show CaP particles.

50 µm

Addition of CaP to

CoCrMo alloy stabilizes

the ε phase.

Γphase of CoCrMo is lost

after CaP addition and

LENSTM processing.

HAP though not seen from

SEM images, is prominent

in the diffraction pattern.

XRD Analysis of CoCrMo-CaP Composites

Hardness Testing of CoCrMo-CaP Composites

Sample Vickers Hardness

(HV0.1)

CoCrMo-0%CaP 564±10

CoCrMo-1%CaP 568±7

CoCrMo-3%CaP 570±8

Addition of CaP to

CoCrMo does not

change the hardness of

the CoCrMo alloy.

Wear Testing of CoCrMo-CaP Composites

Linear Reciprocating Wear Tests performed in DI Water Medium

Short distance wear test for 1km and long distance wear test for 3km

Constant Load of 5N, constant speed of 1200mm/min and 10mm long wear track

CoCrMo-0%CaP CoCrMo-3%CaP

500 µm 500 µm

Wear Track

Wear track in DI medium for CoCrMo-3%CaP sample is not as wide and as deep

as the wear track for CoCrMo-0%CaP tested under the same conditions.

Wear track is difficult to locate on SEM because of its small depth and width.

Sample Wear Rate x 10-5

(mm3/Nm)

CoCrMo-0%CaP 3.02±0.32

CoCrMo-1%CaP 1.71±0.29

CoCrMo-3%CaP 1.02±0.37

Wear rate decreases by

almost 3 times by the

addition of 3%CaP to

CoCrMo alloy and LENSTM

processing.

Addition of CaP to CoCrMo alloy

does not reduce the coefficient of

friction; it slightly increases it.

All samples achieve a steady state

at a distance of ~100m, after which

the COF is more or less the same.

CaP does not reduce friction but

reduces surface wear.

CoCrMo-3%CaP CoCrMo-0%CaP

Formation of Tribofilm During Wear

High magnification SEM reveals the formation of a tribofilm in the CoCrMo alloy samples’

wear tracks with CaP addition. It was not seen on the wear track of CoCrMo-0%CaP.

50 µm

Tribofilm

CoCrMo alloy underneath

Tribofilm was discontinuous for

1km wear track of CoCrMo-3%CaP

samples.

It was relatively more continuous

towards the center of the wear track

than towards the edges.

CoCrMo-3%CaP- 1km Wear Distance

20 µm

CoCrMo-3%CaP- 3km Wear Distance

There was full coverage of the tribofilm after

longer wear distance of 3000m.

Development of tribofilm during wear is time

dependent. It may be absent or highly scattered

during initial phases of wear. It becomes more

continuous with time. Thus wear rate reduces with

distance.

Tribofilm After Etching The Wear Tracks

50 µm 40 µm

Partially Etched Wear Track Deep Etched Wear Track

Partial etching of the

wear track removed

some of the tribofilm

from the surface.

Complete etching

shows the

microstructure of the

wear track, completely

etching out the

tribofilm.

Summary

1. Addition of CaP to CoCrMo alloy has found to reduce the wear in DI medium.

2. Addition of CaP does not however decrease the coefficient of friction of CoCrMo alloy. CaP

seems to be the preferentially wearing phase that prevents the wear of the alloy.

3. A distinct tribofilm forms during wear. It is discontinuous in shorter distances of wear and

more continuous for longer distances of wear.

1. R. Pourzal et al., Wear (2011), 271-9-10, pp 1658-1666.

2. S.A. Dittrick et al., MSE ‘C’(2011), 31-4, pp 809-814.

2014 Annual Meeting

in Denver, CO.