Corrosion behavior of biocompatible coatings of porous NITI shape memory alloy

Dissertant

Sadiq, Murtada Abd al-Muin M.

Thesis advisor

al-Fatlawi, Abd al-Rahim Kazim Abd Ali
Ujayl, Sami Abu al-Nun

University

University of Technology

Faculty

-

Department

Department of Production Engineering and Metallurgy

University Country

Iraq

Degree

Ph.D.

Degree Date

2013

English Abstract

This study investigates the effect of biomimetic coatings on the corrosion properties and nickel ion release of equiatomic nickel- titanium shape memory alloy (SMA).

This alloy is prepared by powder metallurgy method, powder mixture containing nickel and titanium is pressed at different compacting pressures varying 200, 400,600 and 800 MPa.

Then three attempts are made to sinter the green compacted samples under protected atmosphere of argon gas under different conditions of time and temperature.

It is concluded that the attempt to switch the entire elemental nickel and titanium to phases of NiTi and Ni3Ti is developed in the case of sintering at a temperature of 975 º C for 6 hours.

Then heat treatment is conducted after this attempt of sintering by heating to a temperature of 900 º C and for a period of one hour.

The samples are also protected by argon gas atmosphere and then quenched by cold water.

The porosities of the samples are measured at each compacting pressure and the results of porosities after sintering are 32.5%,26.3%,20.3%, and 15.9% for 200,400,600,800 MPa respectively.

Then pore sizes are measured by using ImageJ program and the pore sizes are 111.4 , 87.35, 75.8 and 54 μm for 200,400,600,800 MPa respectively.

Shape memory effect also investigated by Vickers microhardness technique.

The results of shape memory effect are are 4.19%, 4.205%, 4.217% and 4.26% for porosities 32.5%, 26.3%,20.3% and 15.9% respectively.

The samples after sintering and heat treatment divided into three groups for coating processes.

The first group remains the same without subsequent coating.

The second and third groups are treated chemically with a boiling solution of hydrogen peroxide 35% H2O2 for two hours in order to obtain TiO2 layer.

The second group is kept on this treatment.

The third group then is treated with a solution of 4 M hydroxide potassium (KOH) for 30minutes at 120 º C and finally is immersed in a solution of high strength of simulated body fluid of 5 SBF at 37ºC, in order to deposit hydroxylapatite layer.

Phases analysis of samples before coatings during attempts of sintering is conducted by X-ray diffraction technique (XRD).

On the other hand phases analysis for samples after coatings is also conducted by using Raman technique in addition to X-ray diffraction technique.

The surfaces of samples are studied by using scanning electron microscope SEM and energy dispersive X- ray technique (EDX).

Corrosion test is conducted by two solutions, Hank's solution and artificial saliva for each of the three groups in three methods; open circuit potential with time, Tafel extrapolation and reversion anodic polarization.

Then corrosion behavior are studied depending on Ep, ip, Eb, Ecorr, icorr, Epp and ipp.

Corrosion tests, referred to increase of corrosion potential and pitting corrosion as the specimens coating with TiO2 or TiO2/HA.

The porosity affected the tendency to general and pitting corrosion.

The corrosion rate increases as porosity increases.

Nickel ion release test is achieved in immersing the samples from the three groups in the 0.9% NaCl solution at a 37 º C and then liquid of the samples are withdrawn from the solution every week for 7 weeks and are measured the amount of Ni ion release by using Atomic Absorption Spectrophotometer(AAS).

The results of X-ray diffraction show that the sintering at 975 °C for 6 hours for equiatomic compacted powders of Ni and Ti results in complete sintering reaction and all Ni and Ti transform for Austenite NiTi, Martensite NiTi and Ni3Ti phases.

The results of X-ray diffraction also show that the heat treatment reduces the Ni3Ti phase.

X-ray diffraction technique and Raman test show both crystalline forms of TiO2; anatase and rutile after chemical treatment with hydrogen peroxide.

X- ray diffraction technique and Raman test also show the formation of hydroxylapatite in addition to anatase and rutile after treatment with potassium hydroxide, then the samples are immersed in 5 SBF for 14 days.

The results of Ni ion release show a significant elimination in the releasing values after coatings.

But results from alloy coated with TiO2 only show that there are no intersections with a safety line (Slope 0.5 μg/cm2/w) while the results from nickel ion release of samples coated with composite coatings of TiO2/HA show intersect with a safety line and thus are more appropriate for use in medical applications.

SEM micrographs show the coatings of TiO2 and composite coatings of TiO2/HA.

Chemically surface analysis using the EDX indicate the surfaces consist of elements of phases that are formed after coatings.

The thickness of coatings is measured inside the SEM instrument by using ion beam cutting technique ,and the thickness of TiO2 is 635 nm and for composite coatings TiO2/HA is 1.3 μm.

Main Subjects

Materials Science , Minerals

Topics

• Alloys

• APA
• MLA
• AMA

Language

English

Data Type

Arab Theses

Record ID

BIM-418188