Influence of Acid Composition on Morphology and Physical Properties of Titanium Alloy 

Introduction

Surface modification of titanium significantly enhances its topography and composition for dental implants, optimizing roughness, wettability, and antibacterial properties to promote osteoblast attachment and facilitate osseointegration. The method of implant surface modification can be categorized into two groups: subtraction and addition. Subtraction techniques, such as sandblasting, acid etching, sandblasting acid-etched (SLA), anodizing, and laser radiation, effectively modify the titanium surface’s roughness, hardness, and oxidation of Ti. In contrast, addition techniques involve coating the titanium surface with various substances, significantly altering its composition and enhancing biological properties through plasma spraying and sol-gel.^1,2 Acid-etching is often performed using hydrofluoric (HF), nitric (HNO₃), sulfuric (H₂SO₄), hydrochloric (HCL) acids, and combinations thereof. The surface features after acid etching may vary depending on the time, type, and concentration of the acid.¹ Regarding implant surface roughness, previous research concludes that osteoblast activity increases with a moderately micro-roughness profile with a Ra value range from 1 to 2 μm. The implant surface topography can be classified as smooth (Ra < 0.5 μm), minimally rough (Ra 0.5–1.0 μm), moderately rough (Ra 1.0–2.0 μm), and highly rough (Ra > 2.0 μm).³ The microtopographic features of the implant surface, such as peaks and valleys, are an essential factor in the biological response and the configuration of the bone-implant interface. However, optimal roughness and superficial morphology are still controversial and must be more clearly defined. Therefore, the present study aimed to investigate the effect of the combination of acid etching on titanium alloys’ surface morphology, roughness, and hydrophilicity. SEM analysis has shown that the combination of sandblasting followed by acid etching produces a honeycomb-like pattern.

Methods

Sample preparation

Titanium alloy (Ti6Al4V) discs with a diameter of 12 mm and thickness of 2 mm (n=12) were machined using a 5-axis milling machine, as shown in Figure 1. Ti-disc was blasted with large grit aluminum oxide Al₂O₃ by particle size 100 μm, 250 μm, and 500 μm for 30 seconds from a distance of 10 mm, with a pressure of 5 bar. These machine Ti-discs were cleaned in an ultrasonic cleaner using acetone, ethanol, and distilled water for 5 minutes each to remove machining chips, oil, and any organic contamination. After cleaning, samples were dried in a vacuum oven 60-80°C. Acid etching was carried out on these samples in hydrofluoric acid (HF) 48%, hydrochloric acid (HCL) 37%, and sulfuric acid (H₂SO₄) 97%. The etching process in this research was carried out in two methods, which included immersion of titanium in HF at room temperature for 30 seconds followed by immersion in a mixture of HCL and H₂SO₄ in a ratio of 2:1 at 120 degrees for 5 minutes and the other method was immersion of titanium in a mixture of HCL, H₂SO₄, H2O in a ratio of 2:1:1 at 120 degrees for 5 minutes. After the completion of the etching process, the specimens were subjected to ultrasonic cleaning using distilled water and then dried.

Figure 1. (a) Ti-disc specimen blasted with large grit aluminum oxide Al₂O₃ 100 μm, 250 μm, and 500 μm (b) SEM images of Ti after sandblasted with large grit aluminum oxide Al₂O₃ 100 μm, 250 μm, and 500 μm.

Results

The acid type significantly affects the titanium alloy’s morphology (Ti6Al4V) surface. Figure 1 presents the Ti-disc specimen and SEM images after sandblasting with Al₂O₃ particles. After sandblasting, SEM results show a heterogeneous and irregular titanium surface with varying degrees of roughness, depending on the size of the Al₂O₃ particles used. SEM results show that the sandblasting process on the surface of titanium alloy using Al₂O₃ particles produces a unique and irregular surface topography with peaks and valleys of varying sizes ranging from shallow and small depressions to more extensive and more profound depressions. The difference in Al₂O₃ particle size also affects the surface morphology. SEM results show that titanium sandblasted using Al₂O₃ 500 μm produces a more extensive and deeper valley shape when compared to Al₂O₃ 100 μm.

The results of etching specimens using HF at room temperature for 30 seconds and continued with a mixture of HCL and H₂SO₄ (2:1) at 120 degrees for 5 minutes are shown in Figure 2. Based on the SEM results, the titanium surface topography still shows an irregular surface with varying width and depth, but the etching process creates micro-roughness on the surface after sandblasting. The etching results with this method produce a topographic structure of the titanium surface that shows an irregular surface, more of a rounded porous structure, the peak area is still clearly visible, and the surface texture is less homogeneous. The white structure resulting from the reaction of titanium with H₂SO₄ and HCL appears to produce deposits and may be attached to the titanium surface.

Figure 2. SEM images of Ti after etching using HF at room temperature for 30 seconds, followed by etching using a mixture of HCL and H₂SO₄ in a ratio of 2:1 at 120 degrees for 5 minutes.

The results of etching with HCL, H₂SO₄, and H₂O (2:1:1) at 120 degrees for 5 minutes are shown in Figure 3. Based on the SEM results, the peak area is reduced and becomes flatter, the sandblast character is less visible, and the resulting surface texture is more homogeneous than the previous method. The combination of sandblasting followed by acid etching with this method results in a honeycomb-like pattern on the titanium surface. In addition, SEM analysis also shows the surface characteristics of titanium with a more even texture and a smaller cavity size than the previous method.

Figure 3. SEM images of Ti after etching using a mixture of HCL, H₂SO₄, H₂O in a ratio of 2:1:1 at 120 degrees for 5 minutes.

Based on Table 1, the average roughness of titanium alloy after etching using a combination of HF, HCL, and H₂SO₄ is higher when compared to using only HCL and H₂SO₄. Titanium alloy specimen has roughness between 1.369 μm – 4.304 μm depending on the size of Al₂O_3. There is no significant difference in surface roughness between using a combination of HF, HCL, H₂SO₄ and only the combination of HCL and H₂SO₄ without HF.

Titanium surface modification not only shows an increase in surface roughness but also undergoes chemical modification that will affect the wettability and interaction with biological fluids. The hydrophilicity of the titanium alloy surface after sandblasting and acid etching was determined by measuring the contact angle using the sessile drop method. The results of contact angle measurement are shown in Figure 4. In general, all titanium specimens show contact angle values < 90°, so it can be concluded that the surface is hydrophilic.

Figure 4. Contact angle evaluation after sandblast and acid etching of Ti-disc specimen.

Discussion

The present study’s finding has proved that acid etching is a relatively dominant factor in deciding the surface texture and roughness. The SEM figure shows that the different acids used to modify the implant surfaces lead to different surface morphology. In this study, the etching of titanium alloy (Ti6Al4V) was optimized to obtain micro- and nano-topography and to improve hydrophilicity and surface roughness. The etching process in this study uses three different acids: hydrofluoric acid (HF) 48%, hydrochloric acid (HCL) 37%, and sulfuric acid (H₂SO₄) 97%. Etching on titanium surfaces significantly affects titanium’s surface morphology and mechanical properties. Each acid contributes uniquely during the etching process to improve surface roughness. HF is known to have aggressive etching ability even at low concentrations. In addition, HF and HCL also contribute effectively to removing the passive oxide layer on the titanium surface, while H₂SO₄ increases the overall surface roughness due to its strong corrosive properties. Combining these acids will produce a more significant etching effect than using each acid individually.⁴

Liang et al.⁵ showed that HF can create micro- and nanostructured surfaces on titanium surfaces, significantly improving osteogenic activity and cell attachment. HF effectively removes the passive oxide layer because it naturally reacts with titanium in the TiO₂ layer to promote better cell adhesion. In addition, the concentration and duration of HF etching also play an essential role in determining the surface characteristics of titanium. Zahran et al.⁶ found that etching time significantly affects the topography and chemical composition of the titanium surface. In that study, it was observed that fluoride ions began to appear on the surface after the titanium was etched for two minutes using HF.

HCL and H₂SO₄ are the most widely used acid types for etching titanium. The etching process is done by mixing HCL and H₂SO₄ to accelerate the etching process. In addition, no reaction occurs due to the mixing of the two acids. Previous research showed that the etching process of titanium using HCL and H₂SO₄ at room temperature takes a very long time to obtain micro-textures on the titanium surface because both acids have a low etching rate at room temperature. Nadar et al.⁷ found that although the sandblasting process increased surface roughness, etching performed afterward at high temperatures could produce smoother titanium alloy surfaces. Another study by Hung et al.⁴ also showed that higher etching temperatures resulted in lower surface roughness for titanium alloy (Ti6Al4V) when etched using HCL. This phenomenon is related to increased activation energy at higher temperatures, which results in more uniform etching and reduces the formation of sharp microstructures that affect roughness. Although higher temperatures can reduce roughness, it is crucial to consider the balance between achieving the desired surface characteristics and maintaining the roughness required for biological interactions.

In this study, titanium alloy etching with HF, HCL, and H₂SO₄ showed an irregular surface, and the surface texture is less homogeneous than the acid etching method without HF. These results align with a previous study by Madaan et al., which found that using HF has disadvantages, including the potential to create a non-uniform surface profile. Prolonged etching time can also lead to the dissolution of the titanium oxide layer, which can potentially cause excessive roughness and impair mechanical properties, including reduced ductility and increased brittleness of the titanium alloy. This is particularly relevant in the development of dental implants, where the mechanical stability of the implant is crucial.⁸ Etching using HF can also contribute to the release of fluoride ions into the surrounding environment, which can potentially cause cytotoxicity and have negative impacts on cell function and viability. Due to these factors, the use of HF for etching titanium alloys requires careful consideration and strict safety protocols, so additional etching methods are needed to achieve optimal results for titanium surface modification.^8,9 In this study, an HF solution with a concentration of 48% was used. The etching rate of titanium in 48% HF is very high, which can be advantageous for achieving precise desired surface characteristics within a short time.

In this study, titanium alloy etching with HF, HCL, and H₂SO₄ showed a higher roughness value with a 1.482 μm – 4.312 μm range. However, there is no significant difference in surface roughness between using a combination of HF, HCL, H₂SO₄ and the combination of HCL and H₂SO₄. Based on previous research, the average roughness of commercial titanium implants was about 1-2 μm.¹⁰ Therefore, in this study, the Ti-disc specimen, after being sandblasted using 250 μm Al₂O₃ followed by etched with a combination of HCL and H₂SO₄ had a surface roughness close to the roughness of commercial implants and had the desired surface morphology. The effect of sandblasting and acid etching on surface roughness has been highlighted in various studies.¹¹ Acid etching, in conjunction with sandblasting, refines the surface topography further. This technique introduces micro-roughness on the already roughened surface, creating a more complex topography. The roughness of titanium surfaces and irregular surface features created by these methods can significantly improve the mechanical interlocking between the implant and surrounding bone tissue and improve biological responses from osteoblasts.^12,13

All titanium specimens show contact angle values < 90°, so it can be concluded that the surface is hydrophilic. The contact angle of dental implants is an important factor affecting their wettability, which can affect osseointegration and overall implant success. Various surface treatments can significantly change the contact angle of titanium implants, thereby improving their biological performance. This study’s results align with research conducted by Yang and Huang, which showed that the titanium surface after sandblasting and acid etching indicated a moderate level of wettability. Although titanium surfaces are not entirely hydrophilic, they have better wettability than untreated ones, which usually have a higher contact angle.¹⁴ This relationship is crucial for the design of titanium implants to improve osseointegration and overall biocompatibility.

Conclusion

From the result of this study, it was suggested that sandblast followed by acid etching method was an effective way to roughen titanium alloy surface for dental implant application. Although acid etching creates a rough surface, the etching process must proceed with a control condition. A combination of HCL and H₂SO₄ seems to be an effective solution for etching in getting the desirable roughness and surface morphology in terms of the micro pit presence and homogenous texture, which may lead to good attachment of cells and might increase bone-to-implant contact. However, further study must evaluate the in vitro and in vivo bone response to these surfaces.