Coating technologies to improve the wear performance of titanium

For components subject to sliding forces or friction titanium and its alloys do not offer adequate wear resistance owing to a susceptibility to welding. Lasting improvements to wear properties can be achieved by applying hard coatings such as titanium nitride or titanium carbide that exhibit extremely high hardness. The following technologies are used to apply these coatings.

1. Nitriding

The nitriding processes that have been used to coat titanium parts include case hardening in a cyanide-containing salt bath (TIDURAN process), high-pressure nitriding (TIDUNIT process) and plasma nitriding.

A nitrided coating consists as a rule of a roughly 10 μm thick connecting layer and 50 to 200 μm thick diffusion layer. The nitrogen content in the connecting layer is virtually constant over the thickness of the coating, whereas in the diffusion layer it decreases continuously with distance from the surface. Surface hardness levels of approx. 1000 HV 0.025 can be achieved.

2. Thin coat technologies

The thin coating technologies used with titanium are PVD (Physical Vapour Deposition) and CVD (Chemical Vapour Deposition). These are chemical/physical vapor deposition methods used to manufacture protective coatings. Coating thickness for purely decorative purposes is about 0.5 mm, whereas for technical applications coating thicknesses of 3 - 5 μm are usually necessary.

The PVD processes include evaporation, sputtering and ion plating. For coating titanium and titanium alloys the reactive variants of the ion plating method are mainly used. These differ in the way the metallic components of the hard-material coatings are evaporated. Evaporation is either by electron beam (anodic material source), with the aid of a thermal non-stationary arc (arc evaporation) or by a sputtering process (magnetron sputtering).

With regard to process temperatures, coating tests with the titanium alloy TiAl6V4, material No. 3.7165 have shown that to avoid microstructural changes coating temperatures of 600°C should not be substantially exceeded. In tests using temperatures of up to 500°C no changes to the microstructure were observed.

Compared with the PVD processes, the CVD processes exhibit better throwing power, which allows even intricately shaped parts to be provided with a uniform coating. A major disadvantage of the classic CVD processes is the high process temperature, in the range from 800 to 1400°C, necessary for coating formation.

Lower process temperatures are possible with the PACVD process (Plasma Assisted CVD). In this process the gas/substrate system is exposed to a low-temperature plasma that supplies the necessary energy to activate the reaction. The process temperatures used in PACVD lie between 450 and 650°C.

Alongside high temperatures, another disadvantage of the CVD processes is the toxic and corrosive nature of the gases.

3. Thermal spraying

Of the various thermal spraying methods the one suitable for coating titanium is vacuum plasma spraying (VPS).

As with all plasma spraying methods, the source of heat and energy in VPS is a high-temperature plasma. In a plasma burner an arc is produced which heats an inert gas stream by ionization and recombination reactions to temperatures of up to 20,000 K. The material to be deposited is fed in powder form into this high-energy plasma stream with the aid of a carrier gas. The powder particles are accelerated, heated to a molten state and projected at high speed onto the substrate, which causes them to flatten and form a lamellar coating. Depending on the duration of the spraying operation coating thicknesses from several μm to a few cm can be produced.

VPS has several decisive advantages over atmospheric spraying processes. Because they are applied in a low-pressure chamber the coatings display high mechanical quality and chemical purity, which is reflected in denser coatings with low residual porosity and smoother surfaces. Another advantage is the greater adhesion and stability of the coatings as the plasma flame removes thin oxide layers and even moisture from the part surface directly prior to coating. In addition, the VPS process allows precise temperature treatment, i.e. heating of the substrate up to its thermal stability limit without the risk of oxidation, and the production of protective coatings in which stresses in the layers and between coating and substrate can be kept at a low level.

The thermal spraying processes and particularly the vacuum plasma method can be used to deposit oxide as well as metal coatings.

4. Laser technology

Laser gas alloying is a process for coating titanium surfaces that originated in laser technology. The high energy of the laser beam causes the metal surface to melt. This process takes place in a nitrogen-containing atmosphere to produce titanium nitride coatings. The amount of nitride formed depends on the melting time, i.e. on the scanning velocity of the laser beam. Using this process titanium nitride coatings of several 100 μm thickness can be produced.

The hardness levels obtainable with this process lie in the range of 1000 HV in nitrogen-containing atmospheres and up to 1500 HV in pure nitrogen. Pure nitrogen atmospheres however result in the formation of surface cracks, the number of which decreases with decreasing nitrogen content. Crack-free surfaces were obtained in tests using pure helium and with atmospheres containing 50% and 30% nitrogen.

5. References

For the sake of completeness mention should be made of the possibility of electrolytically depositing metallic coatings on titanium such as copper, nickel, chromium, gold and silver. These are not dealt with in detail here.

The most appropriate method for coating a particular titanium part should be chosen on a case to case basis. We refer you to the relevant publications, a selection of which are listed below.

1. 1. G. Sepold, H.-D. Steffens "Beitrag zur Beschichtung von Metallen" Zeitschrift für Werkstofftechnik 8 (1979), p. 22/32
2. F. Preißer, P. Minarski, P. Mayr, F. Hoffmann "Hochdrucknitrieren von Titanwerkstoffen" Härterei-Technische-Mitteilungen issue 6 (1991), p. 361/66
3. J. Langan "Properties of Plasma Nitrided Titanium Alloys" Proceedings of the Sixth World Conference on Titanium, Cannes 1988 Edited by P. Lecombé, R. Tricot, G. Béranger Published by les éditions de physique
4. J. Müller "Stand und Perspektiven der Dünnschichttechnik" Metall 41 (1987), issue 3, p. 248/55
5. G. Kienel "PVD-Verfahren zur Beschichtung technischer Oberflächen" wt Werkstattstechnik 79 (1989), p. 224/27
6. Société Francaise de Métallurgie, Vol. 4 Proceedings of Sixth World Conference on Titanium, Cannes 1988 Edited by P. Lacombé, R. Tricot, G. Béranger Published by les èditions de physique
   • p. 1811/16: S.Z. Lee, H.W. Bergmann "Laser Surface Alloying of Titanium and Titanium Alloys"
   • p. 1795/800: H. Matsumoto et al "Alumina Coating on Titanium by Plasma Spraying"
7. H. Eschnauer, E. Lugscheider "Fortschritte beim thermischen Spritzen" Metall, issue 5 (1991), p. 458/66
8. Titanium'95: Science and Technologie, Vol.III Proceedings of the Eighth World Conference on Titanium, Birmingham 1995 Edited by P.A. Blenkinsop, W.J. Evans, H.M. Flower Published by The Institute of Materials
   • p. 1959/66: C. Hu, S. Mridha, H.S. Ubhi, P. Holdway, A.W. Bowen, T.N. Baker "Hardness, Dendrite Population and Microstructure under Different Nitrogen Environments in the Laser Nitrided Ti-6Al-V Alloy"
   • p. 1975/82: A. Bloyce, H. Dong, B. Hanson "Comparative Evaluation of Treatments for Wear Protection of Ti6Al4V"
   • p. 1983/90: B. Tesi, A. Molinari, G. Straffelini, T. Bacci, G. Pradelli "Ion-Nitriding Temperature Influence on Tribological Characteristics of Ti6Al4V Alloy"
   • p. 1991/98: P. Guiraldenq, B. Haenggi, F. Gaillard "Comparison of Different Coatings to Improve the Ti-6Al-4V Alloy against Wear in Sea Water"
   • p. 2023/30: A.I.P. Nwobu, D.R.F. West, R.D. Rawlings "Reactive Laser Cladding of Titanium Alloys with Wear-Resistant Aluminide-Ceramic Composites"
   • p. 2031/38: H. Xin, T.N. Baker "The Development of Wear Resistant Surfaces on CPTi and Ti-6Al-4V Alloys by Laser Nitriding"
   • p. 2096/103: T. Yashiki, T. Nakayama, J. Kato, Y. Terada, Y. Ito "The Wear Resistance of Electronic Ni-P Plated Titanium and ist Alloys"

Deutsche Titan, Nov. 2000

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