Structural Characterization of Fine γ (cid:48) -Fe 4 N Nitrides Formed by Active Screen Plasma Nitriding

: The paper presents the structural characterization of γ (cid:48) -Fe 4 N nitrides produced by active screen plasma nitriding (ASPN) processes. Experiments were performed on the Fe-Armco model material at 693, 773, and 853 K for 6 h. Investigation of the properties of the substrate was realized using scanning electron microscopy (SEM, SEM–EBSD / Kikuchi lines), energy-ﬁltered transmission electron microscopy (TEM-EFTEM), X-ray di ﬀ raction (GID, grazing incidence di ﬀ raction, micro-XRD), and secondary ion mass spectroscopy (SIMS). Results have conﬁrmed that the γ (cid:48) -Fe 4 N nitrides’ structure and morphology depend considerably on the nitriding process’s plasma conditions and cooling rate. In addition to that, γ (cid:48) -Fe 4 N nitrides’ formation can be correlated with the surface layer saturation mechanism and recombination e ﬀ ect. It has been shown that the γ (cid:48) -Fe 4 N structure depends considerably on several phenomena that occur in the di ﬀ usive layer (e.g., top layer decomposition, nitrogen, and carbon atoms’ migration). Our research proves that the nitrogen concentration gradient is a driving force of nitrogen migration atoms during the recombination of γ (cid:48) -Fe 4 N nitrides. Finally, realized processes have allowed us to optimize active screen plasma nitriding to produce a surface layer of ﬁne nitrides. orientations of (200) and (210). However, the phase composition of the di ﬀ usive layer of Fe substrate was investigated with the µ -XRD method. Results obtained show super-net reﬂections originating


Introduction
Nitriding in plasma conditions is a group of processes that involve the formation of a diffusion layer on the metallic substrates. During this process, the material is submitted to low-temperature, non-isothermal, and low-pressure plasma. The primary role of plasma in the production of nitride layers is to generate charged electrons, ions, excited atoms, and particles with increased energy levels. During the ion-nitriding process, nitrogen in atomic and ionic form is obtained due to excitation, ionization, and dissociation of the nitrogen/hydrogen or ammonia atmosphere [1]. Nitrogen active species formed in the gaseous plasma changes the reaction at the surface and affects the diffusion zone formation's kinetics by changing the substrate energy and inducing sorption processes. This increases the number of new reactions at the plasma/substrate interface and highly improves the layer's adhesion [2]. Generally, conventional cathode plasma nitriding (CPN) is realized in vacuum reactors at a 400-1000 V; the low vacuum of 300 Pa-1 hPa; pressure ≥1 Pa and a temperature range 693-853 K. The composition of the

Materials and Methods
Pure iron (Fe-Armco) substrates (20 × 20 × 5 mm) were mechanically activated with metallographic paper (up to 2000 grit). In the next stage, samples were cleaned with ethanol and placed into the plasma nitriding reactor. Two variants of the nitriding process were realized: I. CPN; and II. ASPN. Processes were carried out using the cooled anode reactor shown in Figure 1. The cathode of the reactor and active screen material was AISI 1010 low-carbon steel so that there is no influence of alloying elements on the substrate surface. During the ASPN nitriding, the distance between the substrate and the active screen was 90 mm. Surface ion activation (sputtering) was performed during the workpiece's heating up to the nitriding temperature. Detailed parameters of the plasma nitriding processes are presented in Table 1. In order to investigate the structure of the nitrided layer produced with CPN and ASPN processes, a series of studies were conducted. Microscopic observations to verify morphology, structure, and dispersion rate of the layers were conducted by scanning electron microscopy (SEM, JEOL JSM-6610LV microscope, Tokyo Japan). Phase composition analysis was realized using the SEM-EBSD/Kikuchi lines method (HITACHI SU70 microscope with Thermo Scientific (Ibaraki, Japan) analytical equipment) and the X-ray diffraction methods grazing incidence diffraction (GIXRD) and micro-X-ray diffraction (μ-XRD). Structural characterization of the γ′-Fe4N compounds and nitrogen concentration maps were realized by the energy-filtered transmission electron microscopy (EFTEM) method (FEI TITAN 80-300 microscope, Waltham, MA, USA). The nitrided layer depth was analyzed by the secondary ion mass spectroscopy (SIMS) method (CAMECA IMS The cathode of the reactor and active screen material was AISI 1010 low-carbon steel so that there is no influence of alloying elements on the substrate surface. During the ASPN nitriding, the distance between the substrate and the active screen was 90 mm. Surface ion activation (sputtering) was performed during the workpiece's heating up to the nitriding temperature. Detailed parameters of the plasma nitriding processes are presented in Table 1. In order to investigate the structure of the nitrided layer produced with CPN and ASPN processes, a series of studies were conducted. Microscopic observations to verify morphology, structure, and dispersion rate of the layers were conducted by scanning electron microscopy (SEM, JEOL JSM-6610LV microscope, Tokyo Japan). Phase composition analysis was realized using the SEM-EBSD/Kikuchi lines method (HITACHI SU70 microscope with Thermo Scientific (Ibaraki, Japan) analytical equipment) and the X-ray diffraction methods grazing incidence diffraction (GIXRD) and micro-X-ray diffraction (µ-XRD). Structural characterization of the γ -Fe 4 N compounds and nitrogen concentration maps were realized by the energy-filtered transmission electron microscopy (EFTEM) method (FEI TITAN 80-300 microscope, Waltham, MA, USA). The nitrided layer depth was analyzed by the secondary ion mass spectroscopy (SIMS) method (CAMECA IMS 6F spectrometer, Gennevilliers, France).

Results and Discussion
The results of the plasma nitriding processes (CPN and ASPN) allowed us to identify the type of γ -Fe 4 N nitrides morphology and verify the influence of different process conditions on nitrogen transport and its concentration in the nitride layer. Formation of γ -Fe 4 N nitrides in the active screen plasma nitriding is a consequence of the recombination mechanism as a result of the rapid transition from a metastable state to a stable state, where non-stoichiometric nitrides γ -Fe 4 N 1-x , in consequence of high-energy nitrogen species saturation, tend to obtain stoichiometry and create fine γ -Fe 4 N phases, whereas in the cathode plasma nitriding variant, the γ -Fe 4 N nitrides are in the form of thick, brittle needles. The active screen effect is here particularly useful due to the reduction in the temperature of the process and changing the driving force of the γ -Fe 4 N nitride phase nucleation during nitriding, especially at a temperature below 793 K. The studies conducted have also shown that the γ -Fe 4 N nitrides formed by the ASPN process have higher activity and can more easily adsorb nitrogen particles from the Fe 2 N, ε-Fe 2-3 N + γ -Fe 4 N compounds finally to form desired fine structures [26][27][28]. Figure 2 shows the Fe-Armco surface's images after cathode plasma nitriding and active screen plasma nitriding processes at 693, 773, and 853 K, respectively. In the nitride layer formed, the following zones and interfaces can be observed: zone I: ε-Fe 2-3 N 1-x nitrides (white layer) with nitrogen concentration ranging from 4.55% to 11% (mass volume); zone II: ε-Fe 2-3 N 1-x + γ -Fe 4 N 1-x layer which is a mixture of ε-Fe 2-3 N 1-x and γ -Fe 4 N 1-x nitrides and is formed from the decomposition of the white layer; zone III: Fe α (N,C) which is a solid solution of nitrogen in ferrite; zone IV: γ -Fe 4 N 1-x nitrides forming a nitrogen solid solution based on the Fe 4 N compound. In zone IV, nitrogen is located in the middle point of the elementary cell with a 0 = 3.791 ÷ 3.801 and consists of precipitates from Fe α (N,C). These precipitates have often a needle or fine shape and are formed in the plasma process and during the cooling stage from process temperature to room temperature.
Substrates nitrided at 693 K for 6 hours with the ASPN method are characterized by a smaller layer thickness than those obtained during the CPN process. Besides, a defected zone which is being created in the process of surface ion sputtering. The results suggest that surface slowly undergoes recrystallization processes related to the slower stress relaxation of the defected surface at the stage preceding nitriding in the presence of the active screen. This phenomenon occurs by the process of linear defects elimination from the structure, i.e., annihilation. In the ASPN process, nitrogen is located in the ε+γ zone and forms an approximately 2 µm compound zone. It is a consequence of ion bombardment and more effective saturation of the surface layer. This zone is being formed in the deformed surface region, suggesting that already at a low temperature of plasma nitriding (693 K), saturation effects and nitrogen concentration are more significant in the ASPN process. The role of the active screen is also enhancing samples heating to the temperature of the nitriding process, as a result of both ion bombardment and thermal radiation from the surface of the heated active screen, as well as to continuously deliver highly energetic excited nitrogen species to the surface of the substrate in order to produce a high concentration of nitrogen at the surface. These effects are associated with increasing the gas phase activity (change of electric field intensity and pressure) due to ionization, which influences nitrogen ions transported to the substrate and changes the equilibrium at the plasma/substrate interface. As the temperature under the active screen rises, at constant pressure, there is an increase in the free path between successive collisions of the plasma components, thus giving them more kinetic energy, which increases the ionization and allows the more ionized gas particles to carry more electric charges from the power source. However in cathode nitriding, there are difficulties resulting from the lack of such highly energetic particles and an identical atmosphere flow on the nitrided substrate's surface, which finally inhibits the incubation stage of nitride phase formation. In the case of nitriding performed at 773 K for 6 h, the ASPN nitrided layer composed of fine γ -Fe 4 N phase is ca. 38 µm thick, and the compound zone (ε+γ ) is 3.5 µm. In the nitrided layer obtained by ASPN, few coarse γ -Fe 4 N nitrides formed at the grain boundaries can be spotted (Figure 2), which is a consequence of the recombination of fine γ -Fe 4 N precipitates, which effect will be further discussed and confirmed by the EFTEM results (see Figure 7). Important information on the phenomena and effects during plasma nitriding is given by the results obtained at 853 K. In this case, needle-shaped γ -Fe 4 N nitride layer was visible both after CPN and ASPN process and is approx. 250 µm thick. The porous and flaky white layer (ε-Fe 2-3 N) zone of thickness up to 3 µm was also observed (Figure 2, images for T = 853 K). The pores in the white layer were from 0.5 to 1 µm. At 853 K nitriding process substrates, the surface compound layer's depth after the ASPN and CPN process were 4 and 5.5 µm, respectively. It was also confirmed by SIMS results (Figure 3b,c). process and is approx. 250 μm thick. The porous and flaky white layer (ε-Fe2-3N) zone of thickness up to 3 µ m was also observed (Figure 2, images for T = 853 K). The pores in the white layer were from 0.5 to 1 µ m. At 853 K nitriding process substrates, the surface compound layer's depth after the ASPN and CPN process were 4 and 5.5 µ m, respectively. It was also confirmed by SIMS results (Figure 3b,c).  The next step of the nitrides phase characterization was X-ray grazing incidence diffraction experiments. Results showed that in the substrates after the ASPN process at 693 K for 6 h, no reflections from the ε-Fe2-3N phase (white layer), located around 2θ angle = 44°, can be seen. These reflections are visible in the CPN variant (blue and black peaks in Figure 3a). This phenomenon may result from the solution zone's intensive saturation, limiting ε-Fe2-3N phase thickness, or total disappearance at low temperatures due to the complete transition to the γ′-Fe4N phase. GIXRD patterns recorded at 853 K temperature (Figure 4b) depict this phase regardless of the treatment method. Low-intensity reflections characteristic for the ε-Fe2-3N phase in the 2θ = 44° angle can be spotted, see Figure 4b.   The next step of the nitrides phase characterization was X-ray grazing incidence diffraction experiments. Results showed that in the substrates after the ASPN process at 693 K for 6 h, no reflections from the ε-Fe 2-3 N phase (white layer), located around 2θ angle = 44 • , can be seen. These reflections are visible in the CPN variant (blue and black peaks in Figure 3a). This phenomenon may result from the solution zone's intensive saturation, limiting ε-Fe 2-3 N phase thickness, or total disappearance at low temperatures due to the complete transition to the γ -Fe 4 N phase. GIXRD patterns recorded at 853 K temperature (Figure 4b) depict this phase regardless of the treatment method. Low-intensity reflections characteristic for the ε-Fe 2-3 N phase in the 2θ = 44 • angle can be spotted, see Figure 4b.  The next step of the nitrides phase characterization was X-ray grazing incidence diffraction experiments. Results showed that in the substrates after the ASPN process at 693 K for 6 h, no reflections from the ε-Fe2-3N phase (white layer), located around 2θ angle = 44°, can be seen. These reflections are visible in the CPN variant (blue and black peaks in Figure 3a). This phenomenon may result from the solution zone's intensive saturation, limiting ε-Fe2-3N phase thickness, or total disappearance at low temperatures due to the complete transition to the γ′-Fe4N phase. GIXRD patterns recorded at 853 K temperature (Figure 4b) depict this phase regardless of the treatment method. Low-intensity reflections characteristic for the ε-Fe2-3N phase in the 2θ = 44° angle can be spotted, see Figure 4b.   GIXRD analysis after both plasma nitriding variants indicated the presence of the ε-Fe 2-3 N + γ -Fe 4 N phase (compound layer) with the crystallographic orientation (111)//(002), which corresponds to the reflex at 2θ = 41.5 • , and was registered for both phases. The intensive reflex was also noticed from the γ -Fe 4 N nitride phase at an angle of 2θ = 47.5 • . This reflex corresponds to the crystallographic orientations of (200) and (210). However, the phase composition of the diffusive layer of Fe substrate was investigated with the µ-XRD method. Results obtained show super-net reflections originating from the γ -Fe 4 N phase, with crystallographic orientations (210), (211), and (220). The intensity of the reflections differs significantly depending on the nitriding method. In ASPN, the most intensive reflections are from the γ -Fe 4 N phase with the crystallographic orientation of (111) and (200) ( Figure 5). This zone in the ASPN method consists of fine γ -Fe 4 N nitrides and nitrogen saturated ferrite. γ -Fe 4 N nitride crystallizes in a regular system with a wall-centered cell (fcc), and nitrogen atoms are placed in the spaces between Fe atoms. Simultaneously, the non-stoichiometric phase of ε-Fe 2-3 N has a dense-packed hexagonal crystallographic structure (hcp), and the stoichiometric ε phase changes its crystallographic structure to orthorhombic and forms the so-called ζ (Fe 2 N) phase. This change is related to the re-arrangement of nitrogen atoms and filling up octahedral gaps and interstitial spaces during the increase in the nitrogen concentration and expansion of the Fe α elementary cell. Analysis of the EBSD map shows a different orientation of the crystallographic planes at the diffusion zone's surface (γ -Fe 4 N + Fe α N,C). Results suggest that easier nucleation of nitrides in the diffusion zone occurs in the grains with orientation (001). This effect may be related to the change in the diffusion flux velocity for this orientation compared to the (111) orientation. It is expected that it occurs due to the incomplete filling of free areas in octahedral gaps. Consequently, in ASPN, opposite to CPN nitriding, slower delivery of nitrogen atoms (diffusion flux) to grains with orientation (111) leads to faster surface saturation of the substrate. Therefore, a reduction in the formation of brittle coarse needle-shaped γ -Fe 4 N nitrides occurs. Such complex nucleation mechanisms of fine γ -Fe 4 N nitrides and their nucleation in nitrogen ferrite grains are triggered by re-combination phenomena. Additionally, the use of Kikuchi line maps was made to determine the intersected planes' location (hkl) on which the deflection occurred for a given crystalline orientation ( Figure 6). Maps have indicated the following phases: γ -Fe 4 N nitride cell in a regular face-centered-cubic (fcc) system, Fe α (N,C) nitrogen ferrite in a body-centered-cubic (bcc) regular system, and ε-Fe 2-3 N nitride in close-packed hexagonal (hcp) system see Figure 6. The driving force of diffusion in plasma nitriding is a gradient of the chemical potential of atoms diffusing to the matrix of the substrate. Therefore atoms, depending on their physicochemical The driving force of diffusion in plasma nitriding is a gradient of the chemical potential of atoms diffusing to the matrix of the substrate. Therefore atoms, depending on their physicochemical properties, can diffuse from a higher concentration to a single-phase area. This phenomenon changes the diffusion flux during the diffusion zone formation and γ -Fe 4 N nitrides. It is known that the enthalpy of nitride formation is ca. 11.08 kJ·mol −1 [29]. Therefore, the nucleation of γ -Fe 4 N nitride phases can occur with their complete coherence and mismatch of the network from the matrix through dislocations to the grain boundaries. The fine γ -Fe 4 N zones can be associated with the formation of metastable, non-stoichiometric nitrides γ -Fe 4 N 1-x , which in consequence of further nitrogen saturation tend to achieve stoichiometry and as a result of the recombination process transit from a metastable to a more stable state and form fine γ -Fe 4 N structures (Figure 7). properties, can diffuse from a higher concentration to a single-phase area. This phenomenon changes the diffusion flux during the diffusion zone formation and γ′-Fe4N nitrides. It is known that the enthalpy of nitride formation is ca. 11.08 kJ‧mol −1 [29]. Therefore, the nucleation of γ′-Fe4N nitride phases can occur with their complete coherence and mismatch of the network from the matrix through dislocations to the grain boundaries. The fine γ′-Fe4N zones can be associated with the formation of metastable, non-stoichiometric nitrides γ′-Fe4N1-x, which in consequence of further nitrogen saturation tend to achieve stoichiometry and as a result of the recombination process transit from a metastable to a more stable state and form fine γ′-Fe4N structures (Figure 7). Therefore, when considering issues related to the kinetics of the nitrided layer growth, not only the phenomena occurring in plasma and process conditions should be taken into account but also the nitride phase formation mechanism, including: I. Nucleation, II. Precipitation, and III. Recombination. One of the hypotheses regarding the absence of coarse brittle needle-shaped γ′-Fe4N nitrides in the ASPN is the reduced intensity of cooling in active screen presence. It is expected that slow cooling after the process hence the formation of brittle needle-shaped γ′-Fe4N phase. In the case of active screen plasma nitriding, such effect is of great importance and will be discussed in our further research.

Conclusions
This study investigated the effects of plasma nitriding processes on the γ′-Fe4N nitrides structure and morphology. It has been shown that the γ′-Fe4N nitrides produced by cathode plasma nitriding Therefore, when considering issues related to the kinetics of the nitrided layer growth, not only the phenomena occurring in plasma and process conditions should be taken into account but also the nitride phase formation mechanism, including: I. Nucleation, II. Precipitation, and III. Recombination. One of the hypotheses regarding the absence of coarse brittle needle-shaped γ -Fe 4 N nitrides in the ASPN is the reduced intensity of cooling in active screen presence. It is expected that slow cooling after the process hence the formation of brittle needle-shaped γ -Fe 4 N phase. In the case of active screen plasma nitriding, such effect is of great importance and will be discussed in our further research.

Conclusions
This study investigated the effects of plasma nitriding processes on the γ -Fe 4 N nitrides structure and morphology. It has been shown that the γ -Fe 4 N nitrides produced by cathode plasma nitriding have a needle shape, where fine structures characterize nitrides produced in the active screen plasma nitriding. We also investigated the γ -Fe 4 N phase nucleation mechanism by the recombination effect induced by nitrogen diffusion from the compound layer into the substrate matrix. The influence of increased nitrogen concentration induced by active screen application on the plasma/substrate surface interface has also been demonstrated. Such an effect results from the increasing frequency of collisions between hydrogen and nitrogen particles, and the density increase of highly energetic N 2 + , N + ions, increased amount of N 2 * excited species and NH X radicals. The active screen plasma nitriding method's attractiveness is also associated with the change in morphology of γ -Fe 4 N nitride precipitates in the diffusion zone. It has been proved that the active screen method, opposite to cathode plasma nitriding, results in the creation of diffusive zones without adverse coarse, brittle needle-shaped γ -Fe 4 N nitrides and reduced ε+γ layer. Finally, the conducted research has shown the advantages of the ASPN technique, especially at 773 K process temperature, and has shown the possibility of optimizing the diffusion layer of metallic substrates, important from a technological point of view.