Emerging Frontiers in Surface Modification: A Critical Investigation of Advanced Anodization Technique for Biomedical Application

Research Questions

• Whether the applied constant voltage on the titanium specimen will enhance the color quality or not?
• whether the voltage variation will change the color or not?
• Whether the elemental composition change after anodizing or not?
• Whether chemical composition, Type of chemical, surface preparation, processing parameters, etc. decrease the fingerprint issue?
• Whether the voltage variation alter the thickness of the oxide layer?

Selection of Material

In the current investigation, the material utilized is a cylindrical-shaped PureTitanium, Grade-4 having a 10x20

mm diameter. The pure titanium material was purchased from “India MART Inter MESH Ltd”. This material is selected due to it is a more corrosion-resistant alloy, highly ductile, softer & a greater percentage of titanium elements as compared to Ti6Al4V (Grade 5), etc. The chemical composition, mechanical properties & physical properties of PureTitanium, Grade-4 as per ISO 5832-2:2018 are given in Tables 1a-c respectively.

Table 1a: Chemical Properties of Pure Titanium.

Table 1b: Mechanical Properties of Pure Titanium.

Table 1c: Physical Properties of Pure Titanium.

Equipment and Parameter Used for Specimen Preparation

The titanium alloy of dimensions 10 × 20 mm (cylindrical shaped) specimens were cut using laser cut for color anodizing and Type II gray color anodizing. Thereafter samples were processed at different parameters as well as by using distinct chemical compositions.

General Principle of Titanium Anodizing

The general principles of the titanium anodizing process are as follows- (a) Electrochemical Reaction: Titanium anodizing is an electrochemical process that comprises the development of a thin oxide layer on the surface of titanium through an electrochemical reaction. (b). Oxidation: The surface of titanium is oxidized in the form of a thin layer of titanium oxide through the reaction with oxygen ions. (c) Anodic Reaction: The titanium surface acts as an anode and the oxygen ions as attracted to the surface, causing the oxidation reaction. (d) Cathodic reaction: The cathode (called +ve electrode) is where the reduction reaction occurs, mainly consisting of the hydrogen gas. (e) Electrolyte: The electrolytes (chemicals) comprise ions that provide the electrochemical reaction. (f) Current and Voltage: The process of titanium anodizing is controlled by mainly applying the specific voltage & current to the chemical solution/ electrolyte. (g) Temperature of Electrolyte: It also affects the process it must be controlled. (h) Anodizing time: It controls the oxide layer thickness, with a long duration of time resulting in a thicker layer. (i) Proper surface preparation: It is important for achieving a uniform & adherent oxide layer (e.g. etching, cleaning, polishing, powder blasting, etc.). (j). Post-treatment: is an important step for improving the properties of the anodized layer. Post-treatment includes sealing etc. The anodizing process principle and Tank sequence are shown in Figures 2 & 3 respectively.

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In addition the general steps, processing parameters used with detail description of the anodizing process given below Figure 4.

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Selection of Processing Parameters, Steps, and Chemicals Used During Anodizing

The extensive literature study experiments were used in prior current investigation, for the selection of the optimum combination of process parameters and chemicals. The detailed steps of the anodizing process used during anodizing are explained in the section below.

As the heating and forging process is completed, by the optimum parameters, the pickling process were utilized for removing surface contamination by immersion in a strong acidic chemicals of temperature 28-32° C for 38-40 seconds using a mixture of 5 ± 2% HF (Hydrofluoric acid) and 20 ± 2% HNO3 (Nitric acid), to ensure surface cleaning/removal of heating & bending defects. The pickling process removes the material but etching is greater to create a roughened surface texture. Remember that do not use the wire brush for cleaning because it can damage the surface, metal particles can introduce contamination such as metal particles, and debris onto the surface. After the Pickling process needs to use an ultrasonic cleaning process for remove the pickling solution effect by using cleaner/with a mild detergent etc., DM water rinsing & air drying respectively. Thereafter follow another step as explained in the section below.

Step 1. Surface Preparation Using Manual and Automatic Polishing

There are two methods i.e. manual & automatic, for part preparation which is essential to perform, and two methods can use any one of the methods to enhance the surface quality as well as for optimal anodized layer adhesion. Thus the detailed advantages and limitations of these processes step by step are given below.

• Manual Polishing: The manual polishing process is just performed before the degreasing (ultrasonic cleaning) & just after part production. Manual polishing is performed for removal of tool marks, minor scratches/ pits, imperfections, sharp edges, and burrs, which further help in creating a smooth/even surface, good appearance/luster & finally enhance the quality of the anodizing layer.
• Automatic Polishing: The role of the automatic polishing process is similar to that of the manual polishing process but the only difference is that it provides consistent surface finish & quality across all the parts, suitable for processing high quantities of parts while maintaining a similarity/maintaining the same surface finish, precision control on processing parameters (speed, time, coolant, water & media, processing method), greater luster. Thus it is a good method for part preparation as compared to manual polishing but it is a costlier process than manual polishing because of setup cost, media, water, and compound used. Note: If the heating and bending process is applied to parts then ensure the part surface is properly cleaned by using a wire brush, thereafter move the part for the polishing process, which further helps in effective anodization on the part and chances of related defect occurrence such as white dots after anodization or improper coloring/color fed issue, etc. removed totally. Hence effective surface preparation is necessary for effective color. In the current study manual polishing were carried out process the samples. (polishing wheel+AMAR AP Media) etc.

Step 2. Ultrasonic Cleaning Process (Primary)

The ultrasonic cleaning process is mandatory after the polishing process (manual/automatic) because it helps in the removal of polishing dust/contaminants, even complex shaped parts can be cleaned effectively, dirt, machining oils, greases, loose particles, small machining burrs, residues. Thus good cleaning is highly essential because it reduces the contaminant risk & ensures the further stronger bond between the part & anodized layer. In this study for fine cleaning the optimum parameters are (cleaning time of 5-8 minutes, a cleaning temperature (of 55°C-60°C), a high- quality cleaning agent good quality water (DM), of zero TDS, and a method of part dipping based upon geometry i.e. vertically/horizontally (e.g. hollow shaped part dipped in vertically), ultrasonic frequency (20-25 KHz) for removal of large particles, dust, in case of medical devices). Different tests can be used for the effectiveness of the cleaning process such as TOC, THC, Bioburden, etc. & monitoring of the process using foil test, etc.

Step 3. Normal Cold Water Rinsing Process

The water rinsing process is mandatory to consider after the ultrasonic process because this process helps to remove the chemical residue effect of the previous process etc. The dipping time of 10 minutes, with normal DM water temperature, was used. For cleaning purposes (for small burr removal) also used white nylon bristles 13x38x180mm reusable type in this study.

Step 4. Drying Process

The drying process is used after cleaning to prevent water spots or mineral deposits. To dry the part surface desiccant compressed air (dry compressed air) was moisture-free after the ultrasonic cleaning process. The gun material is made of an SS/polyurethane (PU) coated gun. The pressure used for drying is 2-4 bar, flow rate of 0.3-0.6m3/min. , drying time 35-50 sec. (based upon part geometry) Excessive pressure can damage the implant surface so ensure to use of optimum parameters. During cleaning of parts for wipe purposes, the white/blue color lint-free cloth (such as oil, coolant, ultrasonic detergents wipe) & lint free swap to make a free from deburrs, in case of complex shaped parts were used.

Step 5. Dry Blasting Process

The dry blasting process is performed after ultrasonic cleaning, cleaning & drying for surface cleaning /surface preparation (it removes contamination, and residual from the surface, improves mechanical interlocking for anodized layer, improves surface finish as well, and closes the manual polishing scratches, etc.).

This process is performed manually (by the operator) then it is not possible to develop a uniform roughness on the part surface, & for proper adhesion requirement (bone to part) osseointegration, the small roughness surface of the part needs to be maintained especially for medical devices. However, the limitation of this process is that the excessive blasting can damage the surface, creating scratches or pits, over roughness (causing surface fatigue). In this investigation the proper selection of blasting media (size) & optimum process parameters used are (S.O.D. 70-80mm), blasting time 120 sec., or depend upon part size, optimum blasting angle 95°, blasting pressure 4-5 bar), an experienced operator can help achieve the required surface roughness. Finally, use compressed air drying for escaping sand particles so that further color can be produced effectively. The glass bead powder designation AH size 45-90 microns (for less rough surfaces e.g. plate, nail, screw, etc.) and designation A particle size 600-850 (for high rough surface) which can be used based upon requirement (e.g. Hip implants required course surface while facial implants smooth/less rough surface).

Step 6. Ultrasonic Cleaning Process (Secondary)

Ultrasonic cleaning is an important step that is highly important to consider after dry blasting because it helps to remove remaining blasting media or debris from the part surface and remove surface contaminants, or residues introduced during blasting. The proper cleaning further helps in ensuring the strong bond between the titanium surface and the anodizing layer. It is important to consider avoiding harsh chemicals & use optimum process parameters (temp. cleaning time, ultrasonic frequency, power, etc.). Temperature: 40 ± 2 ° C; Chemicals: optimum combination of NaOH (sodium hydroxide) or caustic soda and surfactant + Time: 10 ± 2 minutes & Ultrasonic frequency 30-40 KHz were utilized for precision cleaning, removing small particles & surface finish. In addition, the anodizing chemicals are available & used in different concentrations of percentage & kg. So for calculation, the knowledge of the density of each chemical is important. In this study also used lint free white color cloth for visual strain, oil and coolant residue, and water mark for wiping purpose.

Step 7. Normal Cold Water Rinsing Process

Again repeat the normal cold water rinsing process after the ultrasonic process for chemical effect removal of the previous process etc.

Step 8. Drying Process

Apply the dry compressed air drying process after the normal water rinsing process for water spot removal etc.

Step 9. Etching Process

The etching process is very important to perform before color anodizing because it creates a uniform smooth surface with lesser roughness as compared to dry blasting which is essential to gain the effective anodizing color in the further process. For the etching process, different chemicals combinations in optimum ratio (Ammonium bi fluoride+Hydrofluoride+Nitric+DM etc.) can used at room temperature/25-35°C, but controlled etching (i.e. dipping time 15-30 sec.) is essential to consider. Because in this process material is removed, excess/incorrect etching time application surely decreases its part dimension or can create tolerance issues. The etching chemical can be changed by estimating the surface area of the part processed into the chemicals. If the chemical level in the tank due to continuous use decreases then do not add the chemical because its concentration imbalance, pH level change, contamination issues can arise, etc. Titration method/pH paper or color of the solution changes to dark green (if pH>2 to 2.5) then need to add new etching chemicals. In addition, if see no ending reaction on the standby condition &/or mix any other chemicals & water to the products then immediately need to change the etching chemical. So, do not leave any part in the etching tank & for regular monitoring, pH measurement is the best test for chemical change.

Step 10. Normal Cold Water Rinsing Process

Again repeat the normal cold water rinsing process after the etching process for the removal of chemical effect/ residue etc.

Step 11. Drying Process

Apply the dry compressed air drying process after the normal cold water rinsing process for water spot removal etc. Step 12. Color Anodizing Process In color anodizing a thin layer of Ti oxide is added when the electrical current is passed through an electrolyte solution, this may be due to the chemical reaction. The major roles of color anodizing on Ti alloy, especially for orthopedic applications are enhancement of appearance (aesthetics), providing resistance against corrosion naturally, helping to surgeon for identify specific implant types, batches, or sizes/ surgeon easy-to-handle implants during surgery, improving biocompatibility. FDA and CE recommend using different colors on implant surfaces such as brown, blue, silver, golden, pink, peacock, and green by setting the different voltages. The control over parameters such as dipping time & voltage is highly important during color anodizing. Processing parameters are the temperature of chemicals 20 ± 2 ° C, voltage 10 to 77V; electrolyte: ammonium sulfate (NH4)2SO4 at a optimum concentration of percentage. During the dipping of the parts in chemicals the fixed voltage is most important to maintain throughout the dipping time, in the anodizing setup because it ensures color uniformity and provides a consistent oxide layer. Thus the risk of over or under-anodizing can be reduced by constant voltage & can achieve better control over the process. If the chemical level in the tank due to continuous use if decreases then try to replace the total chemicals, because if we monitor the process chemical pH changes. In some cases first time only can add the chemical max. Upto 10%. If titration method/ pH paper or color is not obtained, (eg. working range pH>10.5 to 12.5) then need to add new etching chemicals. In addition if see no ending reaction on the stand-by condition &/or mix any other chemicals & water to the products, or if white deposits occur on the wall of the tanks & titanium products then immediately need to change the electrolyte. So, remember that do not leave any part in the chemical tank & for regular monitoring pH measurement is the best test for chemical change. In addition, remove the cathode plate when not used for more than 8 hours, because it can corrode, contaminating the solution & affecting the color anodizing process, The solution degrades the cathode plate if it is dipped into it for a long time and results in inconsistent color. So daily inspection for corrosion/wear is important to ensure consistent color anodizing results.

Step 13. Normal Cold Water Rinsing Process

Again repeat the normal cold water rinsing process after the removal of electrolyte for the residue etc.

Step 14. Drying Process

Apply the dry compressed air drying process after the normal cold water rinsing process for water spot removal etc. For Type II (Dark gray color) Repeat steps 1 to 10 thereafter move for Type II anodizing, which is explained below.

Step 15. Type II Anodizing

In this type of anodizing process, we obtained a dark gray color. When comparing the color anodizing with type II (Dark gray color) the highest oxide layer thickness can be achieved. However, the use of control parameters (dipping time 5 minutes & fixed voltage 133V) during processing of part is highly important for consistent uniform color. The different chemicals & their optimum ratio/composition were used such as orthophosphosphoric + Sulphuric + etc).

The type II chemical can be changed by estimating the surface area of the part processed into the chemicals. If the chemical level in the tank due to continuous use decreases then do not add the chemical because its concentration imbalance, pH level change, contamination issues can arise, etc. The apparatus such as the hydrometer working range (89-66 Baume eye check color is dark green black & if the color is not achieved at a constant voltage then need to change the chemicals. In addition, if see no ending reaction on the standby condition &/or mix any other chemicals & water to the products then you need to change the etching chemical. Also, do not leave any part in the chemical tank & for regular monitoring, pH measurement is the best test for chemical change by considering the surface area of the parts. To increase the high luster finish after the Type II color on the Ti alloy surface ceramic polishing media can be used because it does not damage the anodized layer/scratch the part surface, or not react with the Ti and anodized layer. However, the processing parameters need to be considered. Thus thoroughly cleaning the part after ceramic polishing is highly important to remove any residual media and from contamination risk.

Step 16. Normal Cold Water Rinsing Process

Again repeat the normal cold water rinsing process after the removal of electrolyte for the residue etc.

Step 17. Drying Process

Apply the dry compressed air drying process after the normal cold water rinsing process for water spot removal etc. Note: Powder-free sterile gloves, sterile cloth e.g. microfiber cloths (use a clean, lint-free, nonabrasive cloth that helps to prevent scratching, prevent contamination), cleaned basket, regular cleaning is highly important in the anodizing process because it prevents contaminants & residue, not introduce bacteria/other microorganisms, ensure clean handling, consistent anodizing that ensure consistent results. In this way, the anodizing process steps are completed.