Analysis of cathode process changes at the plasma nitriding of machined parts locally protected with special paints.

Bibu, Marius ; Deac, Cristian ; Petrescu, Valentin 等

1. INTRODUCTORY NOTIONS

After the machining of a metallic part, sometimes it is necessary to apply a hardening by thermochemical processes on some of its surfaces, but others need to be left in the initial state. In this regard, the local protection of machined parts is a concept that is more and more used, some of its requirements and benefits being {Bibu, 1998; Vermesan et al., 1999}:

* avoiding the hardening of the protected surfaces by maintaining the chemical composition, the initial structure and internal tension state of metallic materials on protected areas;

* preserving the physical-chemical and mechanical characteristics of the protected areas;

* normal development of the thermochemical process in unprotected areas;

An important condition is, however, the quick and easy removal of the local protection after the process is finished (Vermesan & Deac, 1992; Rie, 1999).

In the specific context of the thermochemical ion nitriding treatment, the elaboration of special paints for protecting certain areas of the metallic parts against adsorption, absorption and nitrogen diffusion during the ion-nitriding process, is a very important accomplishment in the domain, thus being eliminated a big part of the insufficiencies that characterise other existing methods of local protection against this thermochemical process.

As a result of the theoretical and experimental researches made in the mentioned direction, there were realised in practice two original variants of special protective paints based on lamellar copper powder in mixture with a magnesium-based binding system (magnesium oxide), where polystyrene dissolved in an organic solvent (carbon tetrachloride) was added, paints labelled V-1 and V-2.

When plasma nitriding is applied to metallic parts with the protective paint applied, we can see that the elementary fundamental physical-chemical processes that take place at the cathode (where the parts are connected) can be divided in three different categories: processes attenuated by the protection, processes completely cancelled due to the protection and processes that develop normally without being influenced at all by the protective layer (Tracton, 2006).

In the following, we present an analysis of the way the basic cathode processes are influenced by the protection layers.

2. CATHODE PROCESSES

During the plasma nitriding process, plasma particles (electrons, positive and negative ions, atoms or neutral molecules, in fundamental or excited state, photons) are those that contribute and lead to the release and development of surface physical-chemical phenomena at the discharge cathode (Staines, 1990).

If the part's surface is covered with a protective layer of paint, a part of the cathode processes transfer from it to the surface of the protective lay (Bibu, 1998). The interaction of incident particles "i" with the cathode atoms is shown in figure 1.a and b.

The incident particles "i" (ions and fast neutral type particles: N+, H+N, [N.sup.+.sub.2], [H.sup.+.sub.2], Hi [N.sup.+.sub.j], NiHj) fall under the new circumstances on the surface of the protective layer, where either an elastic dispersion or an non-elastic dispersion occurs.

[FIGURE 1 OMITTED]

Depending on the impact conditions, the incident particle is adsorbed at the layer surface, but does not penetrate beyond a certain depth in its volume, because the nitrogen cannot form a chemical composition (copper nitride) with the copper particles in the layer at the regime temperature of 350...600[degrees]C, corresponding to the process. Thus the paint creates a chemical protection barrier of metallic surfaces that are under the copper layer (fig. 1.a and b).

At the same time the protection paint creates a physical barrier because during the development of energies typical to the ion-nitriding process, incident particles practically give away their kinetic energy in the protective layer. Under these circumstances the parts get warm from the protective layer and not directly by ionic bombardment.

Basic phenomena like ionic implantation, interstitial dislocations, cathode spraying, redepositing and secondary electronic emission do not take place anymore at the surface of the parts protected with special paints. There are no longer registered side effects like the increase of the number of faults, local temperature increase, desorption by diffusion or positive ions reflection. Most atoms and molecules of nitrogen and hydrogen remain by adsorption on the protective layer surface, and due to the thermal stirring can migrate on this, and if the temperature is high enough can evaporate leaving the paint surface as positive, negative ions or neutral atoms, as in the case of nitriding of non-protected parts.

On the other hand, the special protective paints are not perfectly airtight, so that the atoms and molecules of nitrogen and hydrogen can penetrate them occasionally, reaching the protected part's surface. Here, even if the activating, adsorption and absorption phenomena are very much attenuated, these still take place and consequently, a small quantity of nitrogen is transferred on 1 ... 5 [micro]m depth in the part interior.

At the same time, at 500 ... 550[degrees]C, the polystyrene used as a binding agent in the paints at the part surface an atmosphere rich in carbon, fact that leads to the appearance of a carburizing phenomenon at 1 ... 3 [micro]m depth. Simultaneously, a copper diffusion phenomenon from the protective layer to the part surface was pointed out at 1 ... 2 [micro]m depth.

The analysis of current densities obtained on unprotected and protected surfaces with paints based on copper and magnesium oxide lamellae indicates a superior value of the copper layer's secondary emission coefficient compared to that of the steel. This could lead to the overheating of the areas covered with protective paint, but luckily, the paint emission coefficient is higher than that of steel. But as a matter of fact, the energy excess determined by the superior value of secondary electronic emission coefficient is compensated by the high loses of radiant energy determined by the superior value of paint emission. The high electric conductivity of protections based on copper lamellae makes the luminescent discharge to be auto maintained as in the case of unprotected steel parts

Due to their high mobility and to the intense electrical field, determined by the potential cathode decrease, the secondary emission electrons reach in this situation, at a small distance from the cathode, a sufficient energy for determining the excitation and ionisation of atoms and gas molecules. This proves that the process develops normally, without interruptions or distortions that the considered protective layer may produce.

The analysed protective paints eliminate the chemical cathode spraying (this fact encourages superficial decarburizing in the case of paints neutralisation). In the new situation the carburizing is indicated as a side effect.

During the plasma nitriding process, due to the collision with gas molecules in the working space, a part of the sprayed particles redeposit at the discharge cathode. In the case of protected surfaces ionic bombardment, the sprayed atoms are copper and/or magnesium atoms that redeposit either on the protective surfaces or on the unprotected metallic surfaces.

Macrohardness tests and the spectrometrical analysis made on unprotected areas close to the protected areas proved that the lamellar copper powder or magnesium deposits on the unprotected surfaces are insignificant and do not influence in any way the non-isolated metallic layer.

With regard to the phenomena that currently develop in the cathode parts under-the protective layer, the forming of the phases [gamma]' - [Fe.sub.4]N and [epsilon] is totally suppressed while the absorption and diffusion are significantly attenuated by the use of the elaborated protective paints.

3. CONCLUSIONS

The theoretical studies together with experimental determinations carried out outlined a series of important conclusions:

* The existence of a protective paint layer determines a series of elementary processes at the cathode surface to be cancelled (ionic implantation, chemical adsorption, redepositing, forming of Fe-N compounds, physical and chemical cathode spraying, physical and chemical secondary electronic emission, interstitial dislocations, positive ions refection, absorption by diffusion, migration, evaporation, local increase of temperature) and other important phenomena to be reduced (activation, impurities desorption, thermal vibration, physical adsorption, absorption).

* As a consequence of protection with the promoted layers, in cathode under-layer no phase changes (Fe3C [right arrow] [alpha], [alpha] [right arrow] Fe3N, [alpha] [right arrow] e) take place, and adsorption and diffusion take place at much reduced intensities.

* Although the experiments indicated for certain the dielectric character of protective paints, their electric conductivity is still sufficiently high for a assuring the transmission from the cathode to the surface of the protective layer. This fact is very important because if the paints would act as real dielectrics, the discharge could not be stable on the covered metallic surfaces, passing in electric arc regime.

The protection with special paints of certain areas of parts that are to be ion nitrided leads to obtaining a series of advantages:

* reduction of the number of luminescent discharge transitions;

* reduction of degassing duration--preheating and heating;

* reduction of energy consumption in degassing stages preheating and heating;

* efficientisation and improvement of the thermochemical process;

* the decreasing of production costs.

It can thus be said that the experimentally realised special protection paints V-1 and V-2 offer a series of certain advantages that recommend them for a successfully usage on industrial scale.

4. REFERENCES

Bibu, M. (1998). Researches regarding the realising of local protection technologies at plasma thermochemical treatments, Doctor degree thesis, "Lucian Blaga" University of Sibiu, Sibiu

Rie, K.T. (1999). Recent advances in plasma diffusion processes, Surface and Coatings Technology, vol. 112, issues 1-3, pp 56-62

Staines, A.M. (1990). Trends in plasma-assisted surface engineering processes, Heat Treatment of Metals, Vol. 17, pp. 85-92

Tracton, A. (ed.) (2006). Coatings Technology. Fundamentals, Testing and Processing Technology. CRC Press, Taylor and Francis Group, Boca Raton. ISBN 9781420044065

Vermesan G. et al. (1999). Introducere in ingineria suprafejelor (Introduction to surfaces engineering), Dacia Publishing House, Cluj-Napoca, 1999

Vermesan G., Deac, V. (1992). Bazele tehnologice ale nitrurarii ionice (Technological Fundaments of Plasma Nitriding), Publishing House of the University of Sibiu, Sibiu. ISBN 973-95604-0-7