Wear- and Corrosion-Resistant PVD Coatings for Tools and Machine Parts

A. S. Korhonen, Helsinki University of Technology/Finland - Submitted by M. S. Sulonen ( l ) , Helsinki/Finland Received on January 22,1990

SUMMARY The applications of wear-resistant physically vapour deposited nitride coatings are briefly reviewed. It is shown that the coatings contain defects called pinholes which in an agressive environment lead to a rapid corrosion of the underlying less-noble substrate and flaking of the coating itself. Methods of improving the corrosion resistance methods, like chemical passivation, deposition of multilayer coatings and noble metal alloying of the coating, are discussed.

Key words: Wear, Corrosion, Coatings, Physical vapour deposition, Titanium nitride, Zirconium nitride.

Physically vapour-deposited (PVD) hard titanium nitride coatings first became generally available from various sources about ten years ago. The growth of applications of thin-film nitride coatings is reflected in the number of papers published on the subject. This is illustrated in Fig. 1, which shows how the number of papers dealing with thin films of TiN, ZrN and HM increased from 1970 to 1987. The interest in PVDcoatings focused primarily on transition metal nitrides rather than carbides, which had already been successfully deposited by chemical vapour deposition (CVD) for about ten years.

The reasons for this were that it is considerably simpler to deposit nitrides from nitrogen than it is to deposit carbides from hydrocarbons. Moreover, the nitrides offer a number of interesting properties which can be exploited in various applications /l/. Examples include - - good electrical conductivity - relative chemical inertness - -

golden colour or selective optical absorption

high melting point and hardness low coefficient of friction against steel

sometimes even unexpectedly. Thus, for example, prolonged use in aggressive chemical environments may lead to corrosion and flaking of the coating. Examples include:

- - - -

tools in the plastics industry which are in contact with corrosive liquids dental instruments and corrosive washing liquids machine parts which a subjected to aggressive environments (pumps in the chemical industry etc.) cutting tools, where wrong types of cutting fluids may have adverse effects on coating performance

Although it has not yet been possible to explain all the phenomena related to the chemical effects of the working environment, much has been learned, and in what follows. some new ways of improving both the wear and corrosion resistance of the PVD nitride coatings will be given. 2 2 A typical PVD nitride coating is fairly thin, usually just a few micrometres thick. Although it basically reproduces underlying surface roughness and looks shiny and uniform, it always contains numerous small defects, so-called pinholes, which may go throughout the whole coating.

The formation of pinholes is nearly impossible to avoid. This is due to the fact that the coated surfaces are always non-uniform and also the coating tends to start to grow in a non-uniform manner. Various growth models have been developed to describe the growth process. A general feature of these is that after the original nucleation stage the growth takes place in isolated islands which then grow together often leaving voids between them. The general growth morphology of the coatings is typically columnar, although it may sometimes be difficult to detect and may even require the use of cross-sectional transmission electron microscopy /3/ before it can be confirmed. Although various techniques can be used to minimize the amount of pinholes, they cannot usually be totally eliminated. They occur commonly in all kinds of coatings on all kinds of substrates.

&J. Published papers dealing with TiN-, ZrN- and HfN-films from 1970 to 1987. Data collected from Chemical Abstracts.

Although TiN, ZrN and H M all show similar properties. so far most interest has been concentrated on TiN and more recently on ZrN. Since hafnium is usually found only as a minor impurity among the other two, i.e. Ti and Zr, it is also much more expensive and has not been studied to the same extent a s titanium and zirconium.

In mechanical applications like in cutting tools and machine parts, the good wear resistance and low coefficient of friction make the nitride coatings an interesting alternative. However, under some conditions the coated components may fail fairly quickly and Coated and corroded dental instruments.

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Although the pinholes are generally invisible to the naked eye, they can soon be distinguished as black spots when corrosion proceeds and the coating finally starts to peel away. Fig. 2 shows a practical example of the corrosion of a) coated dental instruments and b) a detail of a corroded knife blade. Fig. 3 shows how the number of spots increases in the coated steel test rods as the exposure to the corrosive ambient increases.

Corrosion becomes possible if the substrate is less noble than the coating. Since both titanium and zirconium and the corresponding nitrides are clectrochemically rather stable compared to the steel substrate, even small defects in the coating can lead to the formation of a galvanic cell and result in corrosion of the substrate.

The structure of the coating naturally affects its corrosion resistance. The more defects there are, the poorer is the corrosion resistance. The first PVD-TiN coatings tested were found to be rather columnar, were stained easily with fmgerprints and had poor corrosion resistance /2J. When the processing parameters were better understood, more and more uniform coatings could be produced and it even became difficult to distinguish any structural details from the fracture cross-section by scanning electron microscopy D/. Fig. 4 shows examples of columnar and uniform coatings.

Generally, also other properties like cutting performance tend to improve with increasing uniformity of the coating, as can be seen from Fig. 4 for (Ti,Al)N-coatings.

3. METHODS OF PREVENTING CORROSION

It would naturally be of considerable interest if one could improve the corrosion resistance of the PVD nitride coatings without affecting their good wear resistance.

A number of ways of achieving this have been suggested. Nishida /2/ suggested chromic acid treatment to passivate the bottoms of pinholes in TiN coatings deposited on stainless steel. Erdemir et al. /4/ suggested that one should try to make the coatings less

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Effect of improved coating uniformity on the wear life in turning with (Ti,Al)N-coated high-speed steel inserts.

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columnar. Molarius /5/ and Tavi et al. 161 suggested the deposition of an intermediate metal layer between the nitride film and the substrate. This method was developed further by Wiiala et al. /7/ and Harju et al. (81, who studied the corrosion and wear resistance of multilayer Ti-N and Zr-N coatings.

Fig. 5 shows some typical corrosion results for TiN-coated high- speed steel. Non-coated high-speed steel (C45) and a Td-coated glass rod were used as references. All the measured values were found between these two extremes. Table I shows the compositions of the individual coatings. The best multilayer coatings on high- speed stccl came rather close to TiN 011 the glass rod and a clear passive region could be observed.

Corrosion current vs. potential curves for various multilayer TiN-coatings on high-speed steel. Non-coated C45 high-speed steel and a TiN-coated glass rod served as references. The composition of the coatings 1-4 is shown in Table I below. The tests were carried out in 0.1 N H$O,-solution at 25 O C /7/.

Table I Composition of the multilayer Ti-N-coatings.

Coating Nitrogen flow Thickness Phases number (cm3min-I) (pm) (x-ray diffraction)

1 0-50-0-50 1.2 TiN,Ti 2 50-30-50 3.5 Td,e-Ti,N 3 50-15-50 2.6 TiN,e-Ti,N,Ti 4 50-0-50-0-50 3.0 TiN,e-T&N.Ti

Turning tests were carried out by Harju et al. /8/ to find the multilayer coatings which showed both good wear and corrosion resistance. It was found that a pure metal layer between individual nitride layers was detrimental to wear resistance since it was too soft and led to shearing in a sandwich-like structure. However, some of the multilayer coatings showed fairly good values for both wear and corrosion resistance, and optimal coatings could therefore be found both in Ti-N and Zr-N systems.

Despite the success in producing corrosion-resistant multilayer coatings as illustrated in Fig. 6 and in optimizing their wear resistance, it has not been possible to eliminate the pinholes totally. Subsequent passivation treatments with the multilayer coatings /I/ showed further slight improvement in the corrosion resistance, indicating that pinholes had decreased in number though some still existed. It turned out that even the best multilayer coatings tended to corrode easily in solutions containing chlorides.

A new solution was therefore developed to overcome the corrosion problem in chloride-containing solutions PI. It consists of alloying the nitride coating with small amounts of noble metal such as palladium or platinum. Minute amounts of an alloying element increase the current going through the cathodic coating in such a way that the critical current for passivity of the anodic substrate is exceeded and the bottoms of the pinholes passivate. The preliminary fiidings have shown encouraging test results for various cases. It is especially noteworthy that owing to the small amount of alloying elements required (a couple of atomic percent at most), the wear resistance is not affected at all but remains excellent.

An example of a multilayer TiN-coating. An SEM- micrograph of the fracture cross section. The coating was deposied on high-speed steel and consisted of alternating titanium- and TiN- rich layers (coating number 4 in Table I).

4. SUMMARY AND CONCLUSIOm

The applications of new physically vapour deposited nitride coatings were briefly reviewed. It was shown that while the wear resistance generally poses no problems, the thin coatings contain defects called pinholes which may lead to rapid corrosion of the underlying less noble substrate material and subsequent flaking of the coating itself.

Various ways of overcoming the corrosion problem have been suggested. These include e.g. various passivation treatments and deposition multilayer coatings. Although some success has been obtained by the deposition of alternating metal- and nitrogen-rich layers, it has not been possible to eliminate the corrosion totally. In addition, some multilayer coatings show decreased wear resistance and still do not mis t corrosion in chloride-containing solutions. As a new method for solving the corrosion problem, the addition of minute amounts of noble alloying elements in the coating is sug- gested. This leads to a passivation of the boaoms of the pinholes, while the good wear resistance is not affected at all. The preliminary results seem very promising.

REFERENCES Wittmer, M.A., 1985, J. Vac. Sci. Technol. 3: 1797-1803.

Nishida, N. 1980. Ornamental part for watches and method of producing the same. U.S. Patent 4 226 082. Oct. 7, 1980.

Korhonen, A.S., Molarius, J.M., Penttinen, I. and Harju, E.. 1988, Mater. Sci. Eng. A1051106: 497-501.

Erdemir, A., Carter, W.B. and Hochman. R.F., 1985, Mater. Sci. Eng. 69: 89-93.

Molarius, J., 1987, Structure and properties of titanium nitride and zirconium nitride thin films prepmd by reactive ion plating, Dissertation. Helsinki University of Technology, Espoo, Finland.

Tavi, M., Aromaa. J., For&, 0. and Yllsaari, S., 1988. The development of corrosion resistance of zirconium and titanium nitride films. 12th World Congr. on Surface Finishing, Pans, Oct. 4-7.1988.

Wiiala, U.K., Penttinen, LM., Korhonen, AS., Aromaa, J. and Ristolaincn, E., 1989, Surface and Coatings Technology, in press.

Harju, E. J., Penttinen LM. and Korhonen. AS.. 1989. Ibid.

Yliisaari, S., ForsCn O., Harju. E., Korhonen, A.. Penttinen, I. and Tavi, M., 1990. Finnish patent application.

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