Computer simulation studies of titanium surface etching with hf, hci and hbr acids

Abstract

Titanium metal and its alloy components find extensive applications in various industries such as aerospace, medicine and automotive due to its light weight-strength ratio. More importantly, titanium-based components with better surface finishing are required in the titanium manufacturing industries. However, these components suffer from surface roughness and brittleness due to the formation of alpha-case layer. Recently, the etching process has been widely used for metal surface modification, but the etching mechanism and the choice of etchant are not well prescribed. On the other hand, the adsorption of halogen molecules on the metal surface has received much attention due to their technological applications and relevance for material surface processing, corrosion and etching. This is considered as a promising approach towards selecting an effective etchant for the metal surface etching process. In this study, the first-principle approach has been used to study the adsorption behaviour of halogen molecules and ions on Ti (100) and (110) surfaces. Their adsorption mechanism was deduced from the calculated adsorption energy, heats of formation, desorption energy, work function, charge density difference and density of states. In particular, to understand how different etchant can influence the properties of titanium metal surface during etching process. Firstly, the free halogen molecules (HF, HCl, HBr and HI), as well as the clean Ti (100) and (110) surfaces were investigated to deduce the reactivity and surface stability, respectively. It was established that the HF dissociate easily due to its lowest dissociation energy and higher electronegativity, which suggest stronger interaction with the Ti surfaces. The halogen molecules stability trend was found to follow the order of HF>HCl>HBr>HI consistent with the electronegativity strength. Furthermore, it was also found that Ti (110) is the most stable surface displaying the lowest surface energy as compared to Ti (100) surface. v Secondly, the adsorption of halogen molecules on Ti (100) and Ti (110) surfaces was studied to investigate the chemical interaction and their reactivity. The halogens were adsorbed on three possible adsorption sites (Top, Bridge and Hollow) and their reaction is spontaneous. Moreover, the bridge and top sites were found to be the most favourable sites on Ti (100) and Ti (110) surfaces, respectively. Our results showed that all halogen molecules dissociate spontaneously on both Ti surfaces. The findings revealed that the adsorption of halogen molecules on Ti surfaces is energetically favourable suggesting adsorption energy strength order of 𝐸𝑎𝑑𝑠𝐻𝐹>𝐸𝑎𝑑𝑠𝐻𝐶𝑙>𝐸𝑎𝑑𝑠𝐻𝐵𝑟>𝐸𝑎𝑑𝑠𝐻𝐼. This indicates that the adsorption of HF molecule on these surfaces is thermodynamically more stable than HCl, HBr and HI molecules. Also, our results revealed that the adsorption of halogen ions (F-, Cl-, Br- and I-) is more favourable than the adsorption of halogen molecules (HF, HCl, HBr and I) on the bridge site in both Ti surfaces considered. The F ion was found to be the most preferable than Cl, Br, and I ions. In addition, the interaction of halogen ions with Ti surfaces was deduced with regards to electron charge. We found that the amount of electron charge transferred depends on the adsorption energy strength. In particular, it was found that the F atom accepts more electrons than other halogen ions. Moreover, the spherical shape was observed, this suggests that the charge density distribution between Ti atom and halogen exhibit ionic bonding behaviour. We also found that the adsorption of halogen has a stronger effect on the work function of Ti surfaces depending on the halogen ion. The magnitude of the induced work function varies from the halogen ionic order of F>Cl>Br>I. Lastly, in order to describe the dependence of the surface coverage of an adsorbed molecule, F2 and Cl2 molecules were adsorbed on Ti (100) surface at different coverages. We observed the formation of etching products TixFy and TixCly species on the surface. The heats of formation (EHF) and desorption energy of volatile etch products were calculated. Our findings show that the formed volatile molecules (TixFy and TixCly) are energetically favourable (EHF<0), suggesting an vi exothermic process. We also found that the TixFy clusters is more stable with lower heats of formation than the TixCly species. Moreover, the desorption energy of the formed volatile (TiF4) species was found to be lower than TiCl4 indicating that that TiF4 species desorb easily. This demonstrates that F2 is more suitable for surface etching as compared to Cl2.