Chapter 3 Materials and Methods
3. Materials and Methods 1. Materials and Solution
3.1.1 Samples
CP-Titanium Grade 2 (Goodfellow Cambridge Limited, UK) squared samples (20mm x 20mm x 1mm) were used to perform the tests. The samples were acid-etched (H2O: HF: HNO3
(1:1:1)). After the attack the samples were ultrasonic cleaned for 10 min in propanol followed by 10 min in distilled water. The samples were ground down to 1200 mesh size SiC paper. After grinding, the samples were ultrasonic cleaned in the same conditions as before. The samples were kept in desiccator for 24 h before starting the tests in order to obtained similar surface conditions.
3.1.2. Artificial Saliva
The chosen electrolyte was Fusayama´s artificial saliva. It was chosen due to the previous reported works [3], [4], and also by the electrochemical similarity with the oral environment [11], [79]. On Table 4 is given the chemical composition of the solution.
Table 4 – Fusayama’s artificial saliva composition [3], [4], [11], [79]
Compounds Artificial Saliva
Chapter 3 – Materials and Methods
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3.1.3. Hydrogen peroxide
Two different amounts of hydrogen peroxide were used to understand the corrosive and tribocorrosive behavior of CP-Ti. Concentrations of 0,1% and 6% (%vol.) of hydrogen peroxide were added to the artificial saliva in order to mimic the two boundaries that are mentioned in a directive of European Commission, in a plenary meeting on 28th of March, 2006 [78]. This guidance states that the use of tooth whitening products up to 0,1% hydrogen peroxide are safe and that an amount of 6% is also considered safe but it should only be used with an approval of a dentist.
All solutions were kept agitated using a magnetic stirrer up to the moment of the beginning of the test.
After this point the solutions will be called as the Table 5 suggests.
Table 5 – Abbreviations of solutions’ denomination
Solution Denomination
Artificial Saliva AS
Artificial Saliva + 0,1% of H2O2 AS-0.1
Artificial Saliva + 6% of H2O2 AS-6
3.2. Methods
3.2.1. Tribocorrosion Measurements
The tribocorrosion measurements were conducted on polished CP-Ti samples immersed in AS, AS + 0,1% and AS + 6% hydrogen peroxide. These solutions were prepared recurring to Fusayama’s Artificial Saliva recipe. It was prepared and maintained in rotation to avoid deposition. The addition of the hydrogen peroxide was done right before the beginning of the tests. The used electrochemical cell was designed for this purpose. The exposed area in this case is 4,52 cm2 and the cell can hold a solution volume of approximately 40 mL. The tests were performed at body temperature of (37º±1º C).
Figure 4 – Schematic representation of the test apparatus
Legend:
1- Saturated Calomel Electrode (SCE) 2- Platinum (Pt) Electrode
3- Acrylic Cell
4- CP-Ti Sample / Working Electrode
5- Acrylic Sample Holder 6- Alumina ball 7- Alumina ball support
A CETR tribometer (UMT-2, Campbell, USA) was used in ball-on-plane configuration (Figure 4) and for the reciprocating sliding parameters were:
- 1000 cycles;
- 1 mm of stroke length;
- 1,5 N of normal load;
- 1 Hz of frequency
The sliding tests were performed against an alumina ball (Al2O3) (Ceratec Technical Ceramics BV) with a diameter of 10 mm. A fresh part of the alumina ball was used for each trial to avoid contamination by transfer of wear debris from one test to another. The applied load, tangential force and COF were recorded and analyzed by the UMT-2 software (Campbell, USA).
The initial Hertzian contact pressure was calculated based on the materials properties and dimensions. The initial value for the contact pressure was 0.47 GPa.
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Electrochemical tests were performed recurring to a Gamry Potentiostat/Galvanostat (Model Reference 600).
A conventional three electrode set up was mounted to measure all the electrochemical aspects on the trials, although, two different configurations were used. A saturated calomel electrode (SCE) was used as reference electrode, a platinum (Pt) electrode was used as a counter electrode and the CP-Ti sample was placed as working electrode. The sample was connected to the potentiostat through an electrical copper connection placed under it.
The Open Circuit Potential (OCP) was measured right after the temperature of 37º C was reached. As soon as a stable potential plateau was reached on the OCP, the value is used as a corrosion potential in the Electrochemical Impedance Spectroscopy (EIS), over a frequency range from 63 kHz to 10 mHz, with 10 points per decade, and a sinusoidal wave of 10 mV of amplitude. OCP was measured before, during the reciprocating sliding contact and after it. At the end, another EIS was measured to understand the differences between the pre-sliding electrochemical characteristics of the system and its differences after of it. At least three samples were tested per condition in order to guarantee the reproducibility of the results.
After tests, a wear track analysis was performed using Leica DM2500 optical
H2O2. To avoid any kind of decomposition of hydrogen peroxide, the addition of H2O2 happened in the moment right before the electrochemical test.
First, the immersion of the electrochemical cell in a bath, in order to achieve the desired temperature, 37º C. as soon as reached, an OCP was performed for an hour (3600 s) in order to get a stabilized state of the system’s potential. The stabilized potential is observed and a potentiodynamic test is set up. With the value of stabilized potential, it starts at 0,5 V below Eocp
and moving into anodic direction up to 1 V. The potential scan rate used for these tests was 1 mV/s.
At least three samples were tested under each condition. The surface exposed to the solution was 0,64 cm2.
Chapter 4
Corrosion
Chapter 4 - Corrosion
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4. Corrosion
The study of corrosion behavior of the electrochemical system has an enormous importance, not only in dentistry but also in industrial filed. In this chapter, results obtained from potentiodynamic curves are going to be showed. Some curves as OCP and EIS, are going to be presented only on Chapter 5 due to its relation with the tribocorrosion tests. Potentiodynamic curves were measured to obtain information related to the corrosion rate and passivity of the electrochemical system.
4.1. Potentiodynamic Polarization
Potentiodynamic measurements are a very important tool to understand how both cathodic and anodic reactions influence the active/passive behavior of the material [80], [81].
Figure 5 shows the potentiodynamic curves of the three CP-Ti samples in artificial saliva containing H2O2 or not.
These curves were obtained by increasing the potential, dV/dt while the current was recorded. The magnitude of the measured current is related to the driving force of the reactions taking place on the working electrode. It shows which electrochemical processes take place at the anode and cathode, as well as their rate.
The curves show a similar behavior, typical of passive materials. All samples present a large passivation region which goes in concordance with the fact that titanium can get protected by a protective oxide film [82], [83], Corrosion parameters obtained from potentiodynamic polarization scan can be found in Table 6.
Figure 5 – Potentiodynamic polarization curve on CP-Ti immersed in AS with different amounts of H2O2. Surface area: 0,38 cm2, potential scan rate: 1mV/s, potential scan rate from OCP – 0,5V to
1V vs SCE
Table 6 – Corrosion potential and passive current density for potentiodynamic measurements on CP-Ti immersed on different AS solutions
Electrolyte Corrosion potential (Ecorr vs SCE)
Passive current density ipass(µA/cm2)
AS -0,48±0,05 8,67±2,44
AS-0,1 -0,03±0,07 8,34±4,22
AS-6% 0,05±0,03 14,2±7,44
The tests started in the cathodic part of the curve (range differ for each condition, potentials below E ). Under these conditions of potential, the reduction of the oxide film of the
Chapter 4 - Corrosion
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surface takes place [83]; the reduction of oxygen takes place [20]. When the potential reaches Ecorr, the current density that is measured has a value near 0. Under these conditions a steady state is achieved where the cathodic and anodic reactions happen at same but opposite rate [59]. Regarding the corrosion potential for all the samples, an increase of this value with the addition of H2O2, takes place leading to a nobler titanium surface potential. The potential shift in the positive direction is attributed to an enhanced oxide film growth rate according to the high-field theory which relates the oxide thickness to the potential difference across the film [65], [76].
On the contrary, the samples immersed only in AS showed a negative corrosion potential, near -0,5 V vs SCE, the ones in contact with H2O2 suffered an increase of about 0,5 V, getting a corrosion potential near 0 V vs SCE. This increase gives information related to the corrosion tendency, although, it doesn’t contribute with any information related to the reaction rate. This kind of information can be obtained by the analysis of passive current density values.
The anodic part of the curve, unrolls from Ecorr until the passivation plateau is achieved (this concept applies only if an oxide layer is formed on the surface). In Figure 5, it is possible to observe that the different curves don’t correspond it. The increase of the H2O2 concentration leads to a quicker oxidation of the surface. It is also possible to observe that the passivation plateau had moved to the right, to higher current densities. It can be said that the presence of hydrogen peroxide increases the corrosion rate of titanium. Comparing the values of Ecorr and Ipass, they seem to point out in different directions. On one hand the tendency to corrosion seems to decrease but on the other hand the corrosion rate increases. This can be explained by a non-stabilized element on the surface. This result suggests that the film growing on is not TiO2 but a hydroxy-titanium- and titanium-peroxy rich layer that is porous [29]. This aspect can be due to the variation on electrical properties, which suggests the presence of residual currents, created by the reduction followed by oxidation of H2O2 [29], as suggested on the next equation.
Tengvall proposed in 1989 a model to explain the interaction between titanium and hydrogen peroxide. When in contact, a different chemical state is achieved; the oxidized titanium surface is covered with a hydrated TiOOH matrix.
As a conclusion, regarding only potentiodynamic polarization results so far, it can be said that, the immersion of titanium in hydrogen peroxide decreases the tendency to corrosion of
the material but on the other hand, it increases the reaction rate. It is also important to keep in mind that the outer porous layer that is formed might not be completely composed by TiO2.
Chapter 5
Tribocorrosion
5. Tribocorrosion
The understanding of tribological and corrosive mechanisms on dental implants has a huge importance. Since the interactions between mechanical, chemical and electrochemical phenomena are not well known, a full understanding is needed to avoid implant failure. The extension of the study of these systems includes tribocorrosive and corrosive behavior in several physiological fluids, although, the influence of hydrogen peroxide as an agent in tribocorrosion of titanium implants had never been studied.
In Chapter 4, the influence of the concentration of H2O2 was studied based on potentiodynamic curves. The results showed that the presence of the H2O2 decreases the tendency to corrosion of CP-Ti but it increases its susceptibility. In this chapter, the influence of a reciprocating sliding applied on the sample surfaces is studied based on the analysis of the coefficient of friction, the wear volume loss, as well as OCP and EIS measurements.
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5.1. Reciprocating Sliding Contact
Figure 6 shows the recorded OCP values before, during and after reciprocating sliding on polished CP-Ti samples immersed in artificial saliva containing different amounts of hydrogen peroxide.
Figure 6 – OCP evolution of CP-Ti polished samples immersed in AS with different amounts of H2O2 (Reciprocating sliding at a frequency of 1 HZ, applied normal load of 1,5 N, amplitude of displacement –
1 mm, 1000 cycles)
The open circuit potential of the samples stabilized at different values. Samples immersed in AS had the lowest electrochemical potential (stabilized ≈ -0,4 V vs SCE), followed by AS-0,1(≈0 V vs SCE) and, the highest electrochemical potential is recorded on the samples immersed in AS-6. The presence of H2O2 shifts gradually the potential value to a nobler one. The stabilization time involves the transformation of the naturally formed passive film in a solution formed one [4], [53].
At the beginning of the contact it is possible to observe a decrease of potential in all the curves. This behavior can be explained by the contact pressure of the alumina ball on the sample’ surface, which is smaller than the yield strength of titanium causing the destruction of the outer layer, but smaller enough to avoid plastic deformation [6], [84]. The process of destruction and removal of the oxide layer on the surface is known as depassivation [84], allowing the exposition of an active titanium surface part, increasing the tendency to corrosion of the sample. Regarding the decreasing of the electrochemical potential, this phenomenon didn’t occur with the same impact in all the samples. The one immersed in AS suffered a potential decrease of almost 0,4 V, in contrast with the 0,3 V and 0,1 V of the ones immersed in AS-0,1 and AS-6, respectively. Actually, the fact that neither of the samples in contact with H2O2 ever reaches the potential displayed in AS, indicates that the passive layer hasn’t been completely removed. The oscillations in this part of the plot can be explained by the constant depassivation and repassivation of the outer layer.
At the end of the sliding tests, when the counter body is lifted away from the surfaces, the increase of the potential happens, recovering the stabilized electrochemical values observed at the beginning of contact, although, this happens at different rates. The sample immersed in AS took approximately 1000 s to reach the same potential while the one in AS-0.1took half of that time, namely 500 s. The sample in contact with the higher concentration of hydrogen peroxide, AS-6, had a different behavior, reaching the initial potential even before the sliding test ended.
Two explanations can be introduced in here. First it can be said that the repassivation rate is higher than the depassivation one [85]–[87]. The observation of the detachment of some wear debris can be pointed out, which might expose some fresh titanium area, accelerating the decomposition of H2O2. The presence of bubbles on the surface of the samples shows the model explained on Chapter 4, regarding the reduction and oxidation of hydrogen peroxide leading to the release of molecular oxygen.
An interpretation of OCP values should not be enough to fully characterize an electrochemical system. Since the OCP is a mean value determined by the ratio between active to passive, material in the wear track depending on, repassivation rate, contact frequency and normal load [81], some more detailed information regarding the kinetics of reactions should be taken into account. EIS measurements are such a good complement to OCP.
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5.2. COF
Concerning the COF evolution, Figure 7, no significant differences were found between the samples immersed in AS and in contact with 0,1 % of H2O2. The mean values under these conditions were 0,46 (±0,02) and 0,50 (±0,02) respectively. On the other hand, the curve related to AS-6 has a different shape, acquiring a mean COF of 0,57 (±0,06).
Figure 7 – Evolution of COF of CP-Ti polished samples immersed in AS containing different amounts of H2O2 (Reciprocating sliding at a frequency of 1 HZ, applied normal load of 1,5 N,
amplitude of displacement – 1 mm, 1000 cycles)
If the COF curves are compared to the OCP curves, some interesting fact points out.
Both curves follow the part, these means that AS and AS-0,1 remain constant during the contact, tough, the AS-6 samples´ COF and OCP increase. In Figures 8, 9 and 10 show the curves and their evolution for each condition. The oscillations observed in all the curves might be explained by fluctuations in the normal force and/or the reciprocating movement [3]
Figure 8 - Evolution of OCP and COF over time during contact sliding, for samples immersed in AS
Figure 9 - Evolution of OCP and COF over time during contact sliding, for samples immersed in AS-0.1
Figure 10 - Evolution of OCP and COF over time during contact sliding, for samples immersed in AS-6
The phenomena observed for each condition can be explained by the nature of the passive film and the chemical state of the surface. The adhesion of the passive film to the substrate and the resistance to delamination is shown by a stable COF which do not change considerably with potential [6]. Samples immersed in AS and AS-0.1 show a decrease on the measured potential, probably, bulk titanium was exposed. The fact that an active bulk titanium is in contact with the electrolyte lead to a synergism on the mechanism, the presence of a corrosive and mechanical part on the wear of the sample. On AS-6 samples, the potential never suffers such a drastic decreasing, which can indicate that the oxide film was never completely removed.
The OCP and COF increase in AS-6 immersed samples can be explained by the reactions that took place on the surface. Several studies showed that the presence of H2O2 leads to the
Chapter 5 - Tribocorrosion
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formation of a porous surface [29], [76], [86], [88], [89]. As already said on Chapter 4, the possibility of a TiOOH formed layer can lead to different electrochemical and mechanical properties [90]–[92]. With the data showed in Figure 6 and 7, it is possible to admit that the newly formed layer has different properties than the oxide layer previously formed. Although, further tests are still needed to fully understand the differences between them.
5.3. Wear Track Analysis
In Figure 11, 12 and 13 it is possible to observe the wear track, observed in Optical Microscope (OM), of CP-Ti immersed on different solutions.
Figure 11 - OM image of CP-Ti wear tack immersed in AS Figure 12 - OM image of CP-Ti wear tack immersed in AS-0.1
Figure 13 - OM image of CP-Ti wear tack immersed in AS-6
This observation technique can give information related to the scar dimensions, although, it is not possible to evaluate rigorously its depth. Also the wear mechanisms involved cannot be
derived from it. The wear track analysis was based on optical microscope images and the calculations for volume loss are theoretical, considering a perfect contact sliding. Figure 14 shows the wear scar obtained in a perfect contact.
Figure 14 - Schematic representation of a perfect sliding contact
It is considered that when in contact, the counter body will scratch the sample surface in a certain way that, the widest part of the scar is indicative of the depth of the track. Since the radius of the counter body is known and the scratch can be measured, the depth of the contact can be calculated with the following equation:
In which, r is the radius of the counter body, a the distance between the center of the contact and the edge and h the depth. The equation can be arranged in order of h, being possible to solve it.
As soon as the value depth is known, the calculation of the wear volume can be done by the following equation:
This equation is derived from the volume calculation of a scalene ellipsoid, an ellipsoid with three different dimensions, which is the case. In this, a is the distance between the center and the edge, transversal, h is the deepness and b is the distance between the center and the
This equation is derived from the volume calculation of a scalene ellipsoid, an ellipsoid with three different dimensions, which is the case. In this, a is the distance between the center and the edge, transversal, h is the deepness and b is the distance between the center and the