48 November 2009 - IndustrialHeating.com

FEATURE | Heat Treating

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.

urface conditions determine a components wear and corrosion characteristics. Through a modifi cation of

the surface, the properties of the parts can essentially be changed with regard to the wear resistance, friction coeffi cient, chemical behavior, corrosion resistance and the optical and electrical behavior.

PLASTIT Plasma Nitriding with Subsequent PACVD CoatingPlasma-assisted chemical vapor deposi-tion (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 500C (932F) by activation of precursor gases of pulsed plasma. This enables al-loyed steels previously hardened and with a set microstructure and core properties to be preserved. Unlike conventional sputter CVD, a homogeneous PACVD deposition is pos-sible even for complex geometries. In the PACVD process, the components to be coated do not need to be mechanically ro-tated as it is not dependent on the line of sight to produce uniform coatings. There-fore, the charging and manipulating labor

can be signifi cantly reduced. A further advantage is that cleaning by sputtering and the nitriding process can be carried out in the same plant im-mediately before the coating process. The nitriding process generates a nitro-gen-enriched diffusion layer that pro-duces 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.

Furnace TechnologyFigure 1 outlines a MICROPULS plasma plant manufactured by Rbig. An impor-tant feature is that the charge is essen-tially heated by wall heating and not only by plasma. This permits an optimum ad-aptation of the plasma process parameters independent of the process temperature. Especially in serial production, you have to guarantee excellent temperature uni-formity to achieve minimum deviations in the results (surface hardness, nitrid-

Th fSS

FEATURE | Heat Treating

Reduced Wear and Higher Corrosion Resistance by Combined Heat-Treatment and Coating Procedures Using Plasma-Assisted ProcessesT. Mueller, A. Gebeshuber, R. Kullmer, C. Lugmair Rbig GmbH & Co. KG, Wels, AUSTRIA

S. Perlot, M. Stoiber Materials Center, Leoben, AUSTRIA

Visu

PC

PLC

Processparameter

MICROPULSplasma

generator

H2, N2, Ar, CH4TiCl4, AlCl3, BCl3

Vacuum chamber (anodic)

Wall heating

Cooler

Workpiece (cathodic)

Fig. 1. RBIG PLASNIT/PLASTIT plant layout

IndustrialHeating.com - November 2009 49

ing depth, white layer) of your products. The new generation of plasma nitriding furnaces are equipped with three inde-pendent heating and cooling zones. The temperature difference between charging plates in different heights can therefore be reduced to less than 5C. Additionally, it is possible to install a control-cooled inner anode. This leads to an excellent tempera-ture uniformity and therefore to uniform nitriding results and a high reproducibil-ity. 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 repeti-tion frequency of up to 50 kHz. Pulses of positive and negative polarity are possible. In the event of arcing, the voltage is dis-connected in less than one microsecond so that damage to the workpieces is prevent-ed. The thermocouples can be attached directly to the workpieces by means of a special insulating amplifi er. 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 igni-

tion and permits the treatment/coating of small gaps and holes (Fig. 2).

PACVD Hard Coatings for Industrial ApplicationsIncreasing 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 indus-trial 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 dif-

fi cult to impossible in PVD Negative infl uences of spark erosion on

adhesion of PVD coating Low substrate hardness (29-48 HRC)

and higher temperatures required with PCV and CVD result in insuffi cient load support to hard coating

Adhesion problems due to residual de-gassing during coating process

Solution with PACVD Hard CoatingsPACVD coatings solve the previously ex-perienced problems: No risk of losing core properties due to

the coating temperature between 480 and 510C (896-950F)

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

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

50,000

40,000

30,000

20,000

10,000

0 Tenifer TiN TiCN

Num

ber o

f sho

ts

Surface treatment

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

50 November 2009 - IndustrialHeating.com

FEATURE | Heat Treating

pump-down times due to degassing

PACVD ConclusionsThe benefi ts of the PACVD processes are: Low process temperature in comparison to CVD The possibility of combining pre-treatment methods like sput-

ter cleaning and chemical etching followed surface treatments of plasma-ion nitriding and then PACVD coating in one cham-ber and batch run

Not dependent on line of sight as with PVC The ability to coat large three-dimensional tools homoge-

neously without having to rotate parts to be treated Deposition of new low-friction TiN-based hard coatings with

low chlorine content

Practical Examples for Coating DevelopmentAluminum Pressure Die Casting In aluminum die casting, the hard coating primarily is intended to reduce erosion, corrosion and soldering due to the chemical at-tack of liquid aluminum. Adhesion, hardness, soldering behavior, oxidation resistance and stress state conditions have to be care-fully optimized before dies can be coated to achieve optimum tool performance. End-of-die lifetime is determined by heavy soldering of aluminum or insuffi cient 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 >10m. In contrast, the PLASTIT Ti(C,N) coated mold, while having slightly higher soldering at fi rst, it then went 45,000 shots without interruption. After 65,000 shots the mold was re-coated, and the total number of shots was 160,000.

Plastic Injection MoldingIn plastics injection molding, wear of the molds occurs due to cor-rosion caused by exhaust gases or decomposition products, abra-sion from the fl ow of material in contact with tool surfaces, adhe-sion between tool surfaces and molten material. An industrial application where surface quality is extremely im-portant is the production of refl ectors 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 ten-dency was signifi cantly reduced. The service life without polishing was increased to more than one week, resulting in signifi cant cost reduction due to reduced polishing, scrap production and mainte-nance.

Sheet-Metal FormingIn sheet-metal forming, the main failure mechanisms have been identifi ed as: Adhesive wear due to high cold-working loads applied by high-

ly strain-hardened wear debris build-up on the tools 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 lu-bricated every 20 strokes. After 2,000 parts, tools had to be dis-assembled 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 - online only) show very low friction values of TiN and Ti(C,N) coatings. This is the friction coeffi cient of an unalloyed steel ball sliding against a PACVD, TiN coated disc (normal load, 2N; sliding speed, 10 cm/s; relative humidity, 35%). IH

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IH07094Cen.indd 1 6/11/09 9:26:48 AM

Fig. 4. Injection mold for refl ector

Fig. 5. Machining setup for metal forming