By Darrin Radatz, Ani Zhecheva and Sid Clouser
High strength and low weight, coupled with the ability to readily form a tenacious surface oxide film, make titanium and its alloys useful in many applications in the aerospace, industrial and medical fields.
A limitation of titanium alloys is the relatively poor resistance to adhesive wear, which results in galling and cold welding, poor fretting behavior and a high coefficient of friction. One can overcome this limitation by providing a surface coating. Coatings are also applied for heat reflection, emissivity, corrosion resistance in hot acidic environments, conductivity, lubricity, brazing and resizing.
Titanium is very reactive and rapidly forms an oxide film whenever the metal surface is exposed to air or any environment containing available oxygen. This oxide layer should be removed before electroplating or other surface treatment, but its tenacity makes removal problematic.
Surface roughening can improve coating adhesion, and can be accomplished by abrasion, grit blasting and etching. Surface preparation is key to achieving robust adhesion of any coating to titanium, as nickel brush plated over the oxide film results in poor adhesion in localized areas.
SIFCO Applied Surface Concepts has performed multiple experiments on titanium for surface preparation and selective plating. Our R&D department obtained titanium sheets 1.1 mm thick and tubes 0.83 mm thick in three substrate materials: Ti-6Al-4V, Ti-6Al-6V-2Sn and commercially pure Grade 2 titanium, and mechanically finished the surfaces using several techniques including dry or wet abrasion, wire brushing and abrasive blasting.
R&D used mechanical methods to improve adhesion by increasing the substrate surface area and exposing a fresh, clean titanium surface. Mechanical working of the surface by abrasion with grinding media, wire brushes or by blasting with silicon carbide, or wet or dry alumina increased surface area and improved deposit adhesion. But, adhesion was still not high enough to routinely survive a 180° bend test.
They then undertook to identify an electrochemical treatment method with the capability to increase the substrate surface area in a controlled manner and provide an oxide-free surface that enabled good deposit adhesion. The resulting electrochemical treatment includes both an electrolyte and an anodic/cathodic etch/activate methodology to promote microetching of the titanium surface to increase surface area and reduce the surface oxide. This electrochemical treatment resulted in excellent adhesion. The plating procedure given in Table 1 was used to make a quality deposit.
Several factors contribute to the excellent adhesion: mechanical interlocking, increased surface area and lack of an oxide film. These three attributes were generated during the brush plating process. Brush plating is particularly suited for generating these attributes because of the small volume of electrolyte, close contact between the anode and the cathode, and the rapidity with which electrolytes can be switched from activation to strike plating.
Important considerations for the procedure are:
- Keep the titanium under potential control at all times
- Keep the plated area 100% covered by the wrapped anode
- Use rapid switching from anodic to cathodic
- Allow no rinsing between steps
- Do not reuse the solution.
The surface of titanium alloy stubs were pretreated by machining or SiC grit-blasting, then abrading, etching and activating using the process in Table 1. A 50-μm thick nickel deposit was plated from two acid electrolytes. The failure mode in all specimens was adhesive, at the nickel coating – titanium interface.
Hydrogen embrittlement was tested according to General Motors Engineering Standard GM3661P, and all samples were satisfactory for hydrogen embrittlement — that is, no failure or cracking was observed on the any of the coupons.
This technology also performed well with Ti-6Al-6V-2Sn alloy, and deposit adhesion was satisfactory. However, the procedure does not provide a deposit with adequate adhesion on Grade 2 titanium. Deposits on Grade 2 generally passed tape tests but failed bend tests.
Future research work will continue to develop principles for good adhesion of plated deposits to titanium alloys, and identify a process to deposit coatings with improved adhesion on Grade 2 titanium. Deposition of other materials with better wear resistance than titanium will also be investigated.