To improve surface properties such as wear and/or corrosion resistance, thermo-chemical surface treatments in glow discharge plasmas are already well known. By a combination of diffusion and coating processes, the hard coatings attain an appropriate supporting effect, improved layer bonding and reduction of cracking underneath the surface. The following overview illustrates the possibilities of plasma-assisted processes.

Surface conditions determine a component’s wear and corrosion characteristics. Through a modification of the surface, the properties of the parts can essentially be changed with regard to the wear resistance, friction coefficient, chemical behavior, corrosion resistance and the optical and electrical behavior.

Fig. 1. RÜBIG PLASNIT®/PLASTIT® plant layout

PLASTIT Plasma Nitriding with Subsequent PACVD Coating

Plasma-assisted chemical vapor deposition (PACVD) combines the advantages of both the popular coating techniques – PVD and CVD – at a lower temperature.

Process temperature of the chemical vapor deposition can be reduced below 500°C (932°F) by activation of precursor gases of pulsed plasma. This enables alloyed steels previously hardened and with a set microstructure and core properties to be preserved.

Unlike conventional sputter CVD, a homogeneous PACVD deposition is possible even for complex geometries. In the PACVD process, the components to be coated do not need to be mechanically rotated as it is not dependent on the line of sight to produce uniform coatings. Therefore, the charging and manipulating labor can be significantly reduced.

A further advantage is that cleaning by sputtering and the nitriding process can be carried out in the same plant immediately before the coating process. The nitriding process generates a nitrogen-enriched diffusion layer that produces the necessary supporting effect for thin, hard coatings. As mentioned, the process parameters of the nitriding step can be optimized for every steel/coating combination.

Fig. 2. Example of the ignition with/without bipolar technology: left – 80µs, unipolar mode; right – 80 µs in bipolar mode

Furnace Technology

Figure 1 outlines a MICROPULS® plasma plant manufactured by Rübig. An important feature is that the charge is essentially heated by wall heating and not only by plasma. This permits an optimum adaptation of the plasma process parameters independent of the process temperature.

Especially in serial production, you have to guarantee excellent temperature uniformity to achieve minimum deviations in the results (surface hardness, nitriding depth, white layer) of your products. The new generation of plasma nitriding furnaces are equipped with three independent heating and cooling zones. The temperature difference between charging plates in different heights can therefore be reduced to less than 5°C. Additionally, it is possible to install a control-cooled inner anode. This leads to an excellent temperature uniformity and therefore to uniform nitriding results and a high reproducibility. By using four to six thermocouples distributed throughout the whole furnace, an excellent temperature uniformity can be guaranteed and documented.

The plasma voltage is applied in the form of rectangular pulses with a repetition frequency of up to 50 kHz. Pulses of positive and negative polarity are possible. In the event of arcing, the voltage is disconnected in less than one microsecond so that damage to the workpieces is prevented. The thermocouples can be attached directly to the workpieces by means of a special insulating amplifier.

Due to the capability of MICROPULS plasma generators, it is possible to treat very complicated geometries by using the bipolar technology. This technology supports the velocity of the plasma ignition and permits the treatment/coating of small gaps and holes (Fig. 2).

PACVD Hard Coatings for Industrial Applications

Increasing the lifetime of tools is one of the most important aims of tool manufacturers and users. The task for surface engineering is to understand the complex loading and wear conditions of the working parameters and develop countermeasures.

The PACVD technique is a well-suited method to deposit hard coatings on both large dies and molds as well as on small tools.

The aim of this study is to present and discuss results obtained with different PACVD PLASTIT® hard coatings such as TiN, Ti(C,N), Ti(B,N) and (Ti-Al)N in industrial applications.

Older Deposition Techniques
The problems associated with older industrial deposition techniques for molds and dies are as follows:
  • High cost of dies and inherent risks of handling damage
  • Rotation of big and heavy molds is difficult to impossible in PVD
  • Negative influences of spark erosion on adhesion of PVD coating
  • Low substrate hardness (29-48 HRC) and higher temperatures required with PCV and CVD result in insufficient load support to hard coating
  • Adhesion problems due to residual degassing during coating process
Solution with PACVD Hard Coatings
PACVD coatings solve the previously experienced problems:
  • No risk of losing core properties due to the coating temperature between 480 and 510°C (896-950°F)
  • Operating pressures in the range of positive mbar pressure allows coating of big and heavy tools without rotation
  • Substrate pre-treatment like sputtering and plasma etching to support adhesion of nitriding in the same process cycle is possible
  • Higher operating pressure allows lower pump-down times due to degassing
PACVD Conclusions
The benefits of the PACVD processes are:
  • Low process temperature in comparison to CVD
  • The possibility of combining pre-treatment methods like sputter cleaning and chemical etching followed surface treatments of plasma-ion nitriding and then PACVD coating in one chamber and batch run
  • Not dependent on line of sight as with PVC
  • The ability to coat large three-dimensional tools homogeneously without having to rotate parts to be treated
  • Deposition of new low-friction TiN-based hard coatings with low chlorine content


Fig. 3. Number of shots achieved in aluminum pressure die casting for core pins with different surface treatment

Practical Examples for Coating Development

Aluminum Pressure Die Casting
In aluminum die casting, the hard coating primarily is intended to reduce erosion, corrosion and soldering due to the chemical attack of liquid aluminum. Adhesion, hardness, soldering behavior, oxidation resistance and stress state conditions have to be carefully optimized before dies can be coated to achieve optimum tool performance. End-of-die lifetime is determined by heavy soldering of aluminum or insufficient surface quality of the casting.

Figure 3 illustrates the difference in mold performance with different surface treatments. The Tenifer® treated mold was heat checked after 8,500 shots, and after 50,000 shots the roughness was >10µm. In contrast, the PLASTIT® Ti(C,N) coated mold, while having slightly higher soldering at first, it then went 45,000 shots without interruption. After 65,000 shots the mold was recoated, and the total number of shots was 160,000.

Fig. 4. Injection mold for reflector; Fig. 5. Machining setup for metal forming

Plastic Injection Molding
In plastics injection molding, wear of the molds occurs due to corrosion caused by exhaust gases or decomposition products, abrasion from the flow of material in contact with tool surfaces, adhesion between tool surfaces and molten material.

An industrial application where surface quality is extremely important is the production of reflectors for automotive headlamps, e.g., made of polyetherimide (PEI, ULTEM® 1010). Figure 4 shows an injection mold made of ESR H-13 hot-work steel. Without coating, the mold had to be polished manually after a few hours of operation. After PLASTIT Ti(C,N) coating, the adhesion tendency was significantly reduced. The service life without polishing was increased to more than one week, resulting in significant cost reduction due to reduced polishing, scrap production and maintenance.

Fig. 6. Tribo-measurement PACVD TiN against unalloyed steel

Sheet-Metal Forming
In sheet-metal forming, the main failure mechanisms have been identified as:
  • Adhesive wear due to high cold-working loads applied by highly strain-hardened wear debris build-up on the tool’s working surface
  • Mechanical fatigue due to cyclic loading, resulting in tensile stress
For the tool in Figure 5, an uncoated AISI A-11 tool was lubricated every 20 strokes. After 2,000 parts, tools had to be disassembled and re-polished. After PACVD Ti(C,N) low-friction coating, tools were lubricated every 50 strokes, and 26,000 parts were produced without cleaning until the test was stopped, having reached production quantity.

The results of tribometer measurements (Fig. 6) show very low friction values of TiN and Ti(C,N) coatings. This is the friction coefficient of an unalloyed steel ball sliding against a PACVD, TiN coated disc (normal load, 2N; sliding speed, 10 cm/s; relative humidity, 35%).

Fig. 7. Multilayer

New Developments
With PACVD boron (TiB2) multilayer coatings, the layers are especially noticeable because they not only feature a high degree of hardness but also unusually fine layer microstructure. These layers are often applied as a multilayer coating. This means that two or more layers are alternately applied, which significantly improves layer toughness, hardness and gradient of intrinsic stresses.

Figure 7 shows a calowear section of a component that was coated using the multilayer technique. In this example, one TiN and one TiB2 layer were applied alternately, whereby the individual layers are less than 0.2 µm thick. The new coating systems are in nanoscale layer thickness. Depending on the layer thickness and the boron concentration, the abrasive wear characteristics change significantly (Fig. 9).

Fig. 9. Pin-on-disc 3-D test

Summary

The possibilities of plasma-assisted surface treatments are wide-ranging. They have been known for some time and should be considered a technical standard. The PLASTIT process, which combines a PACVD process with preceding plasma nitriding, is a relatively young process that Rübig has sold successfully worldwide for seven years. More than 15 PACVD plants were supplied to the U.S., Asia, Canada and Europe. Know-how exchange is accomplished through ongoing user meetings.

Large and intricate tools for the forming, die-casting and injection-molding industries are ideal for this process. In addition, corrosion resistance and sliding characteristics of components can be significantly improved by a simple post oxidation (PLASOX®) treatment after the plasma nitrocarburizing process. IH

For more information: Contact Terry Bachmeier, Canadian sales mgr., Rubig Engineering, Schafwiesenstr. 56, A-4600 Wels, Austria; tel: 877-967-7722; e-mail: terrybach@bellnet.ca; or Pat Sinnott, U.S. sales mgr., tel: 978-399-8326; e-mail: psinnott@comcast.net; web: www.rubig.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: plasma nitriding, PVD, CVD, pressure die casting, nanoscale