Stripping processes for hard-wearing coatings
New techniques for stripping hard-wearing non-friction coatings from the surfaces of steel, titanium alloys and hard metal tungsten carbide manufacturing tools and components have been developed by partners of an Anglo-Spanish Eureka project. Eureka is a pan-European network that sponsors market-oriented, industrial R&D.
Titanium- (Ti), chromium- (Cr) and carbon- (C) based ceramic phase vapour deposition (PVD) and plasma assisted chemical vapour deposition (PACVD) coatings are employed in sectors such as aerospace and biomedicine, to reduce wear and prolong the service life of components (see image above). But as well as ease of application, the ability to remove these layers is essential. Dr Jonathan Housden, Head of R&D at coatings specialist Tecvac, Cambridge, UK, (one of the members of the new research project) identifies three main reasons for this
Quality assurance - to remove a sub-standard coating, to prevent build up of stresses and delamination on items that have been recoated several times, to enable reapplication where a coating has been damaged through wear and tear.
Stripping these layers, however, has proved problematic, resulting in the establishment of the European project CREST (Coatings Removal for Environmental Surface Technologies). The programme ran during 2003-6 and comprises Tecvac and their Spanish counterparts Comercial de Tratamientos Termicos Carreras (TTC) and Asocation de la Industria Navarra (AIN).
Housden explains, ‘Initially, coatings were removed by grit or vapour blasting - but these caused considerable erosion of the substrate.' Immersion in hydrogen peroxide solutions, on the other hand, only strips Ti-based coatings from steel items, damaging hard metal substrates. New techniques are also required to remove Cr- and C-based coatings. These ‘have excellent corrosion resistant properties [making them] difficult to strip without using highly aggressive chemicals which would damage the [underlying material]', adds Housden.
He says, ‘A great deal of knowledge [in stripping coatings] exists in the microelectronics industry and CREST aimed to investigate the possibilities. Once suitable methods are developed for the engineering coatings industry, the size of the market could be doubled.'
The team has subsequently developed a range of wet-chemical and plasma stripping processes. These can be employed with the three main PVD coating techniques—electron beam, arc and magnetron—and can remove layers at a rate of a few microns per hour.
Selection of process depends on the substrate [and coating] material,' says Housden. For example, by immersing tools or components in a heated water bath, containing one of a number of proprietary chemical mixtures (wet stripping), researchers found they could remove Cr-based coatings from steel substrates.
Having achieved this, a gold titanium nitride (TiN) marker layer was deposited on the tool. It becomes visible as the chromium nitride (CrN) coating wears away, indicating the need for stripping and reapplication of the CrN layer before damage occurs to the underlying instrument.
Housden adds, ‘TiN acts as perfect marker layer because the process to remove Cr-based coatings does not touch Ti-based coatings.' The team will now scale up these procedures. Tecvac has already installed an industrial scale vacuum plant for plasma stripping.
Jonathan Housden, email: firstname.lastname@example.org.