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The Right TiME For Etching Titanium An Editorial For Materials And Manufacturing Technologies November 2018

Published on 16/11/2018

Titanium is the 22nd element in the periodic table and the first element in a group of metals known as the Transition Metals. It is the 9th most abundant element in the Earth’s crust and the 4th most abundant structural metal after aluminium, iron, and magnesium. Like most metals, titanium does not occur in nature as a pure metal. Instead, it is found combined with other elements in the form of minerals. The main titanium minerals are ilmenite (iron and titanium oxide) and rutile (titanium dioxide).

The application of titanium and its alloys has seen rapid growth over the last few decades, mainly as a result of the metal’s high strength and low density. The density of titanium is almost half that of copper and less than 60% of the density of stainless steel. The tensile and yield strengths of titanium are comparable to those of most stainless steels. The resulting high strength-to-weight ratio accounts for the widespread use of titanium; most notably in the aerospace industry (nearly 80% of titanium is used in the aerospace industry).

Titanium alloys are also known to possess excellent corrosion resistance. While titanium itself is a very reactive metal, it exhibits excellent corrosion resistance in practice thanks to its ability to form a protective oxide layer upon exposure to oxygen.

Thanks to their biocompatibility, corrosion resistance and mechanical strength, titanium alloys have also found widespread use in medical applications, such as in medical implants. Chemical processing is another area where titanium is used due to its outstanding resistance to aggressive chemical environments.

Wet photochemical etching is a versatile technique that can be used to produce patterns and features from metal sheets, including titanium alloys. Etching works by selectively dissolving the metal using an oxidising chemical reagent. Areas where etching of the metal is required are left exposed to the etching solution, while the rest of the metal surface is covered with a protective polymeric film known as the photoresist. It is essential that the photoresist remains attached to the metal areas where etching is not required.

Etching of titanium is conventionally carried out using hydrofluoric acid (HF) or a mixture of hydrofluoric and nitric acids. The effective etching reagent is hydrofluoric acid while the optional nitric acid is used mainly to reduce hydrogen absorption and therefore hydrogen embrittlement in the final part. However, there are two major issues associated with the use of etching solutions containing HF and nitric acid. Firstly, HF is a major health and safety hazard. The fluoride ion (generated by the dissociation of HF in water) readily penetrates the skin, damaging deep tissue layers and bone. Secondly, both HF and nitric acid tend to attack most types of photoresists, ultimately causing detachment of the photoresist.

Recognising the potential of photochemical etching in the manufacture of titanium parts, Advanced Chemical Etching (ACE), a Telford-based SME specialising in photo-chemical etching, has invested in an intensive R&D programme to develop a non-HF process for the etching of titanium alloys. These R&D efforts have culminated in the development of the TiME™ process for etching titanium as well as nickel-titanium alloys. The new process uses a unique chemistry that is safer than conventional HF-based solutions. The process also includes a pre-treatment step to improve photoresist adhesion to the metal, as well as a post-etch treatment process to achieve the required surface finish.

The recent expansion of the company to double its previous size was driven in part by the success of the new TiME™ process. ACE has invested in a new etching department, including new staff and state-of-the-art etching, cleaning, measuring and inspection equipment.

Etching can produce complex features and geometries in titanium sheets. Sheet sizes of up to 330×1000 mm and thicknesses ranging from 70 µm to 1.0 mm can be processed using the TiME™ process. Moreover, the process does not affect the chemical and mechanical properties of the metal.

ACE believes in the value of scientific knowledge and research in advancing the state of the photochemical etching industry. Innovations in this industry can have a huge impact on other industries which use chemically etched components in the manufacture of their products, such as the medical, electronic, automotive and aerospace industries. Advances in etching technologies will allow more complex and intricate geometries to be produced and will make it possible to etch new materials that are difficult to etch using conventional methods.


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