Tribological Properties of Metal-on-Metal Hip Implants

Tribological

Properties of

Metal-on-Metal

Hip Implants

Dr. Yılmaz Özmen

Outline

Identify different types of Biomaterials

Wear of Metal-on-Metal Hip Implants

Diamond-like Carbon Coating for Bio-

medical Implant Materials

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Biomaterials Classes

Metals

Ceramics

Polymers

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Metals

Metal is any element that

positively ionized in solution.

Metallic Bond: positively

charged ions in a cloud of

electrons.

Metals are crystalline

solids. (Regular Atomic

Arrangement)

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Metals

Metals composed of one or more than one element are alloys

The surface of metals are often oxides, if inert (Passive) leads to protection, if active leads to corrosion

Typical metal properties include:

• High melting points

• High density

• High thermal and electrical conductivities

• High stiffness, strength and hardness

• High ductility, fatigue resistance

Their properties depend on the composition and the processing method

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Metallic Biomaterials

Stainless steel alloys

Titanium and Titanium alloys

Cobalt Chromium alloys

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Stainless steel alloys

Alloys of iron (Fe), chromium (Cr), Nickel (Ni) and carbon (C)

•Chromium: increase corrosion resistance.

•Nickel : increase strength

•Carbon : increase corrosion in human body & strength

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Titanium and Titanium Alloys

Forms a very stable oxide layer of Titanium oxide (passive metal)

• Titanium Alloys: addition of Aluminium and Vanadium, enables it to have a wide range of properties.

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Co Cr Mo alloys

It is an alloy of cobalt (Co), Chromium (Cr),Nickel (Ni), Molybdenum (Mo) and Carbon (C)

•Chromium : Passivation element (Chromium oxide)

•Nickel : increase strength

•Molybdenum : increase strength and hardness

•Carbon : increase corrosion in the body and increase strength

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Mechanical Properties of Metals

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High strength, stiffness (E), fatigue resistance,

High wear resistance

But, it may wear when metal-on-metal hip replacement is used, wear particles may reduce its biocompatibility.

Wear of Metal-on-Metal

Hip Implants

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What is Tribology?4/29/201512

Tribology tries to describe everything that happens when things rub together

TRIBOLOGY TRIANGLE

Would Life Be Possible Without

Tribology ?????

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Contact of Two Surfaces

Surfaces may look smooth, but on a microscopic scale

they are rough.

If we want to slide one surface

over the other then we have to apply a force to

break those junctions.

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Keeping the Surfaces Apart

(Lubrication)

Definition: introduction of a substance between the contact surfaces of moving parts to reduce friction and to dissipate heat.

• Boundary lub.

• Elasto-hydrodynamic lub.

• Hydrodynamic lub.

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Origins of friction:

adhesion between surfaces

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The degree of adhesion depends on:

- type of bond established between contacting surfaces

- deformation capability of the material (real area of contact,

orientation)

- surface oxidation, adsorption of molecules, contaminations

Origins of friction: deformation

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The deformation work (frictional work) depends on:

- geometry of indenter

- hardness of metal

Friction and Wear:

system-dependent phenomenon

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Wear

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Wear is the progressive material loss from solid surfaces in contact which occurs as a result of friction.

Wear is not a material property. It is a complex response of a material to the solicitations induced by the system in which it operates.

One can distinguish 4 fundamental mechanisms of wear :

•adhesive wear mechanism

•abrasive wear mechanism

•fatigue (delamination) wear mechanism

•tribochemical mechanism

Adhesive wear by strong interfacial bonding

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Critical factors

• size of contacting area

• bonding (ionic, covalent, metallic, Van der Waals)

• surface contamination

• surface oxydationSteel against steel contact

Abrasive wear by plastic deformation and microcutting

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Critical factors

• hardness ratio between indenting and abraded material > 1

• hardness of abraded material

• roughness of indenting material Aluminium abraded by SiC paper

Fatigue wear by repeated load and unload cycles

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Critical factors

•stress and number of cycles

• resistance to fatigue of the material

•surface defects and residual stresses

Fatigue failure of a steel ball bearing

Tribochemical wear by removal of reaction layers

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Critical factors

•mechanical properites of the reaction layer

• reaction kinetics

•kinematics and stresses

Agglomerated oxide

particles after wear of a

passive steel.

Tribological contacts in hip

joint implants

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Tribological tests for implants

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Tribological requirements for

contacting materials

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No seizure (blocking of joint by too strong interfacial adhesion).

High resistance against wear

High resistance against scratching

High resistance against fatigue and impacts

high hardness

high toughness

Materials for “low wear„ hip

joints

E = 200 E = 390 E = 200

K = 20 K = 2 – 4 K = 6 – 9

H = 350 H =2000 H = 1200E: Young's modulus [GPa]

K: Fracture toughness [MPa√m]

H: Vickers hardness [HV]

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In-vivo wear rates

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Annual linear wear rate of different material combinations as used

for cup and head in total hip replacement

Diamond-like Carbon

Coating for Bio-medical

Implant Materials

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Surface Engineering

Definition: Changing the properties of the surface of a material to give performance which cannot be achieved by the surface layer or bulk alone

Surface Engineering Processes

•Mechanical treatment (e.g. peening, shot blasting)

•Surface transformation (e.g. induction hardening, laser treatment)

•Surface composition changes (e.g. thermo chemical treatment, ion implantation)

•Chemical treatment (e.g. conversion coatings)

•Coating (e.g. painting, spraying, plating, vapor deposition)

•Surface activation (etching, plasma treatment of polymers)

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Diamond-like Carbon :

as a Strong Candidate Coating

Biological Compatibility

• Nontoxic, Noncarcinogenic, Noninflammatory

Chemical Compatibility

• Corrosion Resistance

Mechanical Compatibility

• Surface Hardness, Wear Resistance

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t-aC

sp2 H

sp3

DLC

No film

diamond

graphite

Polymer-like

Graphitic

t-aC:H

DLC: A Group of Carbon Mat’l4/29/201532

Use of DLC for

Bio-medical Implant Materials

Blood Contacting Applications

•Stents, Heart valves, Flow Accelerators

Load Bearing Applications

•Hip Joints, Knee Joints, Artificial Disk

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Load Bearing Implants

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Artificial Joints Application

•Needs an improved test method to obtain clinically relevant results.

•DLC/DLC or DLC/metal combinations show more promising results than other combinations.

ThanksDr. Yılmaz ÖZMEN

[email protected]

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