**1. Introduction**

Nitriding and carbonitriding are used as basic processes for details and tools surface strengthening, during which a layer is formed on their surface, containing nitrogen or a combination of nitrogen and carbon.

When these two methods are used in conventional gas furnaces or in salt baths, the thickness of the layer or the composition of the resultant layers could not be reliably regulated, which necessitates varying with the potentials of nitrogen and carbon in the gas mixture or the liquid medium. The percentage of nitrogen and active carbon is defined by a few parameters, namely, temperature and composition of the gas mixture, and the possibilities for variation are limited.

During the process of carbonitriding and nitriding in glow discharge plasma these difficulties are resolved, which is an essential advantage of this method. The usage of glow electric discharge is a perspective method for nitriding and carbonitriding of materials in modern machine building.

The works [1,2,3,4,5] consider mainly the mechanism of building, the structure and the properties of the nitrided layers, obtained in low-temperature plasma, and present the peculiarities of carbonitriding in glow electric discharge. There is lack of data concerning carbonitriding in glow discharge plasma in an actuating medium, consisting of ammonia and gas corgon (82% Ar and 18% CO2), and there also lacks sufficient information of any comparative investigations between the two processes – nitriding and carbonitriding.

It is established in the works [3,5] that, when in the process of carbonitriding propanebutane is used as a carbon-carrier, the phase composition of the combined zone in the carbonitrided layer could not be precisely regulated. Better results could be obtained when

#### 114 Heat Treatment – Conventional and Novel Applications

using a mixture of methane and argon [15]. In metal welding the role of the protective gas is often taken by carbon, which contains both argon and carbon dioxide in a particular ratio.

Carbonitriding of Materials in Low Temperature Plasma 115

polymeric mass-spectroscopy, sensitive assessment of nano-materials, as well as analysis of very thin (<0.1 μm) layers [ 6, 7, 8, 9]. The glow discharge optical emission spectroscopy (GDOES) is an atomic emission process for carrying out deep profile analysis. It combines pulverization and atomic emissions in order to enable an extremely fast and sensitive analysis. The plasma is generated in the chamber by applying voltage between the anode and the cathode with the availability of argon under low pressure. The ionized argon atoms cause pulverization in the area of the sample. The deposited atoms are excited in the plasma

The GDOES is usually used for defining surface coatings, hidden connections, and deep profiles. The technique suggests quick, reliable and economically effective decisions. It

Ensuring high quality of manufactured products is directly related to increasing their reliability and durability, which, in turn, are determined to a large extent by the internal

One of the basic methods of increasing the wear resistance of details is the purposeful improvement of their surface layer properties by means of mechanical, thermal, chemical-

Since the values of the internal stresses are often below the limit of flow of the corresponding material, their measuring is highly demanding to the measuring equipment. There are plenty of methods for defining the internal stresses and they can be divided into the following two groups: destructive methods – the methods of disassembling, of hanging down (the slack method), of drilling, boring and trimming; non-destructive methods – the Roentgen method, the magnetic method, the ultrasound method and the neutron rays

The Roentgenographic method allows registering submicroscopic changes in the distances between the atoms corresponding to the measured planes in the crystal lattice of the grains for a mono-crystal material. It is a completely non-destructive method. Because of the limited depth of penetration of the X-rays, which, for steel is l ≤ 20μm, only the tense state of the closest to the surface layer is registered. The calculation principle used here allows

The distance between the atoms in the crystal lattice is normally about several nanometers. The wave length λ of the X-rays is also several dozens of nanometers, i.e., these quantities are of the same order. Therefore the Roentgen rays are considered to be among the most

The aim of the present work is to investigate the influence of the carbonitriding in a lowtemperature plasma in an actuating medium consisting of ammonia and corgon (82% Ar and 18% CO2) over the surface hardness, the total thickness of the carbonitrided layer, as well as its influence on the type and size of the formed compressive residual stresses on the surface and also to study the distribution of the nitrogen and carbon in depth of the formed

determining of only two-axial internal stresses, parallel to the surface.

suggests additional information for the rest of the surface analysis methods.

and radiate photons with characteristic wave lengths.

thermal and other types of hardening treatment.

reliable for investigating the crystal structure.

layer of Armco-Fe and 25CrMnSiNiMo steel.

stresses in the details.

method.

One of the aims of the present paper is to investigate the possibility to use gas corgon not only in welding but also as an indirect carbon-carrier in the process of simultaneous saturation of a metal surface with nitrogen and carbon (carbonitriding) at low temperatures. The small percentage (18%) of carbon dioxide in the gas corgon makes it possible to regulate the amount of carbon, introduced into the vacuum camera.

Despite the numerous investigations, conducted with the use of ammonia, nitrogen or a mixture of nitrogen and hydrogen as saturating media, there is no an integrated model yet, representing the mechanism of nitriding and carbonitriding in glow discharge. There are two principle methods concerning the question of forming diffusion capable nitrogen and carbon atoms on the surface of the treated articles.

According to the first method, iron nitrides are formed initially, which then dissociate into lower substances and release nitrogen, in its turn diffusing into the treated material. According to the model, developed by Kölbel, it is assumed that, as a result from the pulverization of iron in the glow discharge, iron nitrides rich of nitrogen are formed and they deposit on the surface of the treated articles. The deposited nitrides decompose and release nitrogen, which diffuses into the interior of the material. The availability of ions could increase the number of the centers of chemisorption. In nitrogen and hydrogen containing atmospheres the process goes with the participation of NH-radicals, which, after taking hydrogen, turn into the very active radical NH2. In result from the interaction between the iron and the neutral nitrogen atoms or radicals on the surface of the substrate iron nitrides are formed, which release diffusion capable nitrogen.

According to the second method, diffusion capable nitrogen is directly formed on the surface of the treated articles, i.e., without the preliminary forming of iron nitrides. Materials, having cathode, anode or floating potential, have been treated by the comparatively new method of nitriding in two-step vacuum-arc discharge under low pressure. The fact, that the samples with a positive potential can be nitrided renounces the idea of forming iron nitrides in the gas medium, as a surface, subject to electron impact does not pulverize [15]. The high activity of the saturating medium in this case is due to the neutral nitrogen atoms. The process depends only on the concentration of atomic nitrogen and the temperature of the article, while the electron/ion impact plays the role of a convenient tool for ensuring the temperature needed for the process. By investigating the area of the dark cathode space of a direct current glow discharge in the presence of hydrogen the work for electron detachment from the iron is reduced and thus the absorption of the nitrogen atoms is facilitated . Ultimately, during the process of glow discharge nitriding, atomic nitrogen is formed on the surface of the articles, which, depending on their temperature, diffuses into their interior.

Since the beginning of the 1970s glow discharge sources have been used predominantly in the field of investigating alloys. The scientific literature suggests a great number of applications based on glow discharge spectroscopes, not supposed so far, including polymeric mass-spectroscopy, sensitive assessment of nano-materials, as well as analysis of very thin (<0.1 μm) layers [ 6, 7, 8, 9]. The glow discharge optical emission spectroscopy (GDOES) is an atomic emission process for carrying out deep profile analysis. It combines pulverization and atomic emissions in order to enable an extremely fast and sensitive analysis. The plasma is generated in the chamber by applying voltage between the anode and the cathode with the availability of argon under low pressure. The ionized argon atoms cause pulverization in the area of the sample. The deposited atoms are excited in the plasma and radiate photons with characteristic wave lengths.

114 Heat Treatment – Conventional and Novel Applications

the amount of carbon, introduced into the vacuum camera.

iron nitrides are formed, which release diffusion capable nitrogen.

depending on their temperature, diffuses into their interior.

carbon atoms on the surface of the treated articles.

using a mixture of methane and argon [15]. In metal welding the role of the protective gas is often taken by carbon, which contains both argon and carbon dioxide in a particular ratio.

One of the aims of the present paper is to investigate the possibility to use gas corgon not only in welding but also as an indirect carbon-carrier in the process of simultaneous saturation of a metal surface with nitrogen and carbon (carbonitriding) at low temperatures. The small percentage (18%) of carbon dioxide in the gas corgon makes it possible to regulate

Despite the numerous investigations, conducted with the use of ammonia, nitrogen or a mixture of nitrogen and hydrogen as saturating media, there is no an integrated model yet, representing the mechanism of nitriding and carbonitriding in glow discharge. There are two principle methods concerning the question of forming diffusion capable nitrogen and

According to the first method, iron nitrides are formed initially, which then dissociate into lower substances and release nitrogen, in its turn diffusing into the treated material. According to the model, developed by Kölbel, it is assumed that, as a result from the pulverization of iron in the glow discharge, iron nitrides rich of nitrogen are formed and they deposit on the surface of the treated articles. The deposited nitrides decompose and release nitrogen, which diffuses into the interior of the material. The availability of ions could increase the number of the centers of chemisorption. In nitrogen and hydrogen containing atmospheres the process goes with the participation of NH-radicals, which, after taking hydrogen, turn into the very active radical NH2. In result from the interaction between the iron and the neutral nitrogen atoms or radicals on the surface of the substrate

According to the second method, diffusion capable nitrogen is directly formed on the surface of the treated articles, i.e., without the preliminary forming of iron nitrides. Materials, having cathode, anode or floating potential, have been treated by the comparatively new method of nitriding in two-step vacuum-arc discharge under low pressure. The fact, that the samples with a positive potential can be nitrided renounces the idea of forming iron nitrides in the gas medium, as a surface, subject to electron impact does not pulverize [15]. The high activity of the saturating medium in this case is due to the neutral nitrogen atoms. The process depends only on the concentration of atomic nitrogen and the temperature of the article, while the electron/ion impact plays the role of a convenient tool for ensuring the temperature needed for the process. By investigating the area of the dark cathode space of a direct current glow discharge in the presence of hydrogen the work for electron detachment from the iron is reduced and thus the absorption of the nitrogen atoms is facilitated . Ultimately, during the process of glow discharge nitriding, atomic nitrogen is formed on the surface of the articles, which,

Since the beginning of the 1970s glow discharge sources have been used predominantly in the field of investigating alloys. The scientific literature suggests a great number of applications based on glow discharge spectroscopes, not supposed so far, including The GDOES is usually used for defining surface coatings, hidden connections, and deep profiles. The technique suggests quick, reliable and economically effective decisions. It suggests additional information for the rest of the surface analysis methods.

Ensuring high quality of manufactured products is directly related to increasing their reliability and durability, which, in turn, are determined to a large extent by the internal stresses in the details.

One of the basic methods of increasing the wear resistance of details is the purposeful improvement of their surface layer properties by means of mechanical, thermal, chemicalthermal and other types of hardening treatment.

Since the values of the internal stresses are often below the limit of flow of the corresponding material, their measuring is highly demanding to the measuring equipment. There are plenty of methods for defining the internal stresses and they can be divided into the following two groups: destructive methods – the methods of disassembling, of hanging down (the slack method), of drilling, boring and trimming; non-destructive methods – the Roentgen method, the magnetic method, the ultrasound method and the neutron rays method.

The Roentgenographic method allows registering submicroscopic changes in the distances between the atoms corresponding to the measured planes in the crystal lattice of the grains for a mono-crystal material. It is a completely non-destructive method. Because of the limited depth of penetration of the X-rays, which, for steel is l ≤ 20μm, only the tense state of the closest to the surface layer is registered. The calculation principle used here allows determining of only two-axial internal stresses, parallel to the surface.

The distance between the atoms in the crystal lattice is normally about several nanometers. The wave length λ of the X-rays is also several dozens of nanometers, i.e., these quantities are of the same order. Therefore the Roentgen rays are considered to be among the most reliable for investigating the crystal structure.

The aim of the present work is to investigate the influence of the carbonitriding in a lowtemperature plasma in an actuating medium consisting of ammonia and corgon (82% Ar and 18% CO2) over the surface hardness, the total thickness of the carbonitrided layer, as well as its influence on the type and size of the formed compressive residual stresses on the surface and also to study the distribution of the nitrogen and carbon in depth of the formed layer of Armco-Fe and 25CrMnSiNiMo steel.
