Dr. Andy Bloyce, Product Manager- Components, Balzers Ltd.

Originally Published in "Coatings" October 2000 

Engineering PVD coatings are now widely used across a wide range of industries. The gold colour of titanium nitride (TiN) is very familiar to engineer, for example, on tools used in metal cutting and forming machinery. It has been available for some 20 years now and is used on a significant proportion of quality tools. The benefits available from using this type of coating  vary according to the specific application, but may be simplified in terms of cost savings on tooling and/or increased productivity.

Tools Coatings

Different coatings have been developed throughout the last twenty years, with new materials for specific applications being  developed. These changes are not easily observed since the newer coatings do not have an appearance so strikingly different from the substrate materials.

PVD stands for physical vapour deposition. Substrates are placed in a vacuum chamber, heated, coated, cooled and removed.

• Commercially successful TiN coating processes, such as BALINIT^®A, are generally heated to temperature in the range 450-500°C, with a coating thickness usually in the the range 2-5 mm, and a hardness of about 2200HV.
• BALINIT^®D, chromium nitride (CrN) is similar to electrolytic Cr plate in colour, has a hardness of  approximately 1700HV. It finds application in plastics moulding, particularly of PVC.
• BALINIT^®B, titanium carbonitride (TiCN) is a blue-grey colour. It has a hardness of approximately 3000HV and surprisingly is tougher than TiN. Its oxidation temperature is lower however (400 vs. 600°C) therefore it finds use in interrupted cutting operations such as milling, where the temperature is relatively low but toughness is useful when the tool contacts the work-piece.
• The need for tools to work at higher temperatures arise from machining hard/tough materials, reduced lubricant efficiency or the need for dry machining, and higher production rates necessitating higher speeds and feeds. BALINIT^® FUTURA and BALINIT^® X.TREME are based on titanium aluminium nitride (TiALN. These coatings are purple-grey in colour.
• Specifically for dry machining, BALINIT^®HARDLUBE is a two stage coating, carried out sequentially in one process chamber, comprising of a lubricious surface layer on a TiAlN sub layer. It has high hardness and high temperature stability to cut or form the work material, whereas the lubricious C-based layer is to minimise heat generation through reduced friction during chip removal or sliding against the work material.

Component Coatings

Drivers for the use of PVD coatings on engineering components are greater functionality, increase lifetimes, increased energy efficiency resulting in reduced fuel consumption or reduced lubricant requirements.

For coatings of ~3mm thickness to function, they must usually be supported by a high strength substrate. The ~500°C process temperature of tool coatings limits the substrate materials to higher tempering temperature tool steels and cermets. This, in turn, means that they are far more widely used on tooling rather than engineering components. A major breakthrough has been made with the advent of carbon-based PVD coatings. These are available with various chemistries, the amount of diamond-like bonding significantly affecting the hardness of the layer(1). They are available with hardness value of between 1000-5000HV, but one common feature is a low coefficient of friction, dry against steel, of between 0.1-0.2, (steel vs. steel, ~0.7).

BALINIT^®C has been designed specifically for engineering components. Its properties include:

• a hardness of 1000HV, that will not abrade a hardened and tempered steel counter face.
• a coefficient of friction against steel of ~0.15.
• a process temperature of ~200°C, which can be as low as 160°C.

This low process temperature allows a very large range of conventional engineering steels to be coated without changing their mechanical properties. Significant groups of materials include carburising steels, with tempering temperatures between 160-220°C and bearing steels, with tempering temperatures between 160-200°C.

It is the combination of low friction and wear resistance which has lead to  the success of BALINIT^®C. Conventional wear resistant materials, when sliding against hard steel, exhibit high friction (>0.5) and cold welding. Low friction materials tend to be softer or contain relatively low hardness constituents, and they wear badly, Figure 1

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Figure 1 Friction vs. time for low wear and low friction surfaces.

The coating comprises a lamellar structure of amorphous carbon and tungsten carbide precipitates (WC/C), which helps maintain a desirable residual stress distribution which in turn contributes to the excellent adhesion of coating, which is of course fundamental to a successful engineering coating.

Applications where this type of coating is used comprise sliding motion, rolling or a combination of the two, often in a regime of reduced lubrication. The largest current sector is the diesel fuel injection systems, where emissions legislation has meant that injection pressures have risen significantly, reducing clearances between , for example, plungers and barrels. The reduced clearances has meant scuffing has become an increased problem and C-based coatings are now considered an underpinning technology by the leaders in the diesel fuel injection industry, Figure 2. 

Figure 2 Coated plungers from large diesel fuel injection systems.

Transmission performance can be improved by coating ears with BALINIT^®C. In poorly lubricated environments, the coating can reduce the wear of carburised gears. The pitting fatigue limit of case carburised gears can be raised by 10-15% with the use f the WC/C coating, Figure 3. At high loads and high speeds, as in the case of fuel injection plungers, scuffing can be offset.

Figure 3 Low friction PVD coating on gear to reduce wear and contact fatigue.

PVD engineering coatings move on. The QS 9000 quality standard is in some Balzers Components coating centres. Batch processes are currently the predominant method of processing, but multi-chamber equipment which can significantly increase production rates is now being installed. The potential applications of low friction, wear resistant coatings are widespread. Using carbon coating in the cam/tappet contacts of a 'family car' can produce a measurable reduction in fuel consumption. As environmental issues become still more stringent, new areas of application will become viable and the technology will continue to develop.

(1) S.Neuville & A.Matthews, "Hard Carbon Coatings: The Way Forward"
MRS Bulletin, Sept. 1997 22-26

For further information about this article please contact:

Dr. Andy Bloyce , Product Manager- Components, Balzers Ltd.

For other articles presented at the TSSEA conferences or printed in "Coatings" see the publications page or contact TSSEA for further information.

© Copyright 2001 TSSEA