Brussels, 14 Jul 2004
Vacuum deposition of wear coatings is more environmentally friendly than galvanic methods – but also more costly. The TIPCOAT project has now developed lower cost physical vapour deposition processes to encourage take-up.
The demand for greater performance and lower cost from mechanical components is the driving force behind wider use of coatings. They can improve surface hardness, reduce friction and increase wear protection for a wide range of industrial components. Coatings also allow designers to use lighter, more workable alloys, with cost, performance and environmental benefits. The performance and economics of coating technology are therefore critical for industrial competitiveness.
Physical vapour deposition (PVD), chemical vapour deposition (CVD) and galvanic techniques are the most common coating technologies, the first two exploit vacuum systems while the latter employs 'wet chemistry'. Galvanic techniques require heavily polluting materials, presenting significant environmental problems for process waste disposal. In contrast, PVD and CVD are relatively environmentally benign, although CVD is more dangerous and not suitable for thermally and chemically sensitive substrates.
However, PVD has drawbacks. It produces superior coatings on flat surfaces, but on 3D components results in variable coating quality and poor adhesion. So, while PVD is applied in surface hardening of some 95% of machine tools, where small flat surfaces need to be coated over a small area, it is less used for 3D objects such as medical prostheses.
A further disadvantage is that, as a vacuum technology, PVD requires long pump-down times and has low deposition rates. This makes PVD techniques more expensive – limiting them to where the performance gain justifies the cost, such as titanium nitride coating of high precision cutting tools.
Lowering costs and improving performance
The EU-funded TIPCOAT project united seven partners to increase the industrialisation of PVD coating technologies through improved technical performance and lower costs. Led by Italian defence specialist Oto Melara, the consortium included end users, coating companies and research institutes.
"We set three overall objectives," explains project coordinator Dr GianCarlo Caligiani. "Firstly, to develop an isotropic coating process that would give excellent coating quality on 3D substrates; secondly, to lower coating costs by shortening processing times; and, thirdly, to demonstrate a mobile unit that could coat only the critical areas of large mechanical components in situ – this would broaden the field of application.
"Currently thin-film wear coatings are seen as a hi-tech solution for a hi-tech problem – we want to change this perception and help them compete with galvanic solutions in a wider market."
TIPCOAT technology was based around ion plating plasma assisted (IPPA) where, in a vacuum system, a negatively-charged component is bombarded by ions of the source material such as titanium (Ti) or chromium (Cr), often in a low pressure, reactive gas atmosphere (e.g. nitrogen) – to produce a metallic titanium or chromium nitride coating.
Better coating processes
Deposition at higher process pressures helped improve isotropy for 3D substrates as they cause more diffused deposition. Coating thickness ratios between vertical and horizontal surfaces were increased from 0.25 to 0.5 – a 100% improvement. A disadvantage was reduced deposition rates and lower hardness, but arc-evaporation sources gave acceptable results and, at the highest pressures, defect elimination was an unexpected benefit in CrN films, important for decorative coatings.
To reduce processing costs, the partners investigated shorter pump-down times. These times are heavily dependent on the cleanliness of the vacuum system, and its capacity to absorb water vapour onto the vessel walls when vacuum is broken. Heating the vacuum chamber to prevent absorption, using radiant heaters under vacuum to desorb water, and flushing the system with dry nitrogen during pumping were all studied.
The most effective method to reduce pump-down times was a cryogenic coil in the high vacuum pump port. Filled with liquid nitrogen, such a coil has a very high pumping speed for water vapour trapped on the cold surface – much higher than the conventional diffusion pumps used industrially. IonBond installed cryogenic coils on three production systems and is achieving a 10% productivity improvement.
Mobile treatment chambers
An innovative aim was to demonstrate a local coating chamber (LCC) for in-situ coating of specific areas of large components, such as car dies. CeTeV and DaimlerChrysler constructed prototype LCCs using various ion sources. Trials showed suitable ion energies could be obtained within the chambers for high quality coatings. However, adhesion remained a significant problem, as plasma cleaning of the surfaces prior to coating was not technically possible in the demonstrators.
Despite poor adhesion, the quality of the coatings produced is excellent and the work showed mobile coating units are feasible in an industrial environment. Only development of an effective initial cleaning step remains for LCC technology to be commercially exploited. Their economic potential through reducing downtime in mass production metal-forming operations is highly significant and is driving continuing development of LCCs.
Other direct benefits were also established. "IonBond adopted several TIPCOAT innovations into its coating lines – such as the cryogenic coils, lower process temperatures and reduced pumping times, combined with higher pressure deposition – and has doubled productivity on a high volume drill production line, allowing a 60% price reduction," says Dr Caligiani.
"High pressure plasma conditioning before coating gave marked improvements in the cosmetic appearance of coatings. These are of great interest for decorative coatings, where we are seeing industry switch to PVD from polluting galvanic methods."