Inﬂuence of pulsed plasma treatment on phase composition and hardness of Cr 3 C 2 -NiCr coatings

In this study, the research results of the inﬂuence of pulsed plasma treatment on phase composition, hardness, and roughness of Cr 3 C 2 -NiCr coatings are presented. The Cr 3 C 2 -NiCr coating was applied to substrate 12Kh18N10T stainless steel by detonation spraying method. To change the physical and mechanical properties of sprayed coating’s surface layers, subsequent pulse-plasma treatment was used. The pulse-plasma treatment leads to changing the roughness of Cr 3 C 2 -NiCr coating. The results of mechanical tests showed that after pulsed plasma treatment, the hardness of Cr 3 C 2 -NiCr coating is increased. Based on X-ray diffraction analysis, it was found that the hardness increasing of coating is associated with phase transformations on the surface layer, in particular, the formation of the oxide phase and an increase in the number of carbide particles.


Introduction
Currently, the application of protective coatings to improve the mechanical, physico-chemical and tribotechnical properties of metals and alloys, also products made from them are an actual area of materials science, solid state physics, plasma physics, chemistry, etc. [1][2][3]. Cr 3 C 2 -NiCr coating are widely used to protect against wear of working parts at high temperatures. The NiCr plastic metal matrix acted as a binder, supporting the brittle reinforcing phase of Cr 3 C 2 [4]. The application of such Cr 3 C 2 -NiCr coating can be produce using various gas thermal spraying methods, such as High Velocity Oxygen Fuel (HVOF) spraying, cold spraying, detonation spraying, atmospheric plasma spraying, etc. [5]. However, the presence of porosity and insufficient hardness of the coating can lead to premature failure of the structural elements working under conditions of intense wear. The sprayed coatings' physical and mechanical properties can be improved by subsequent treatment, such as laser beam-reflow, electron beam treatment, pulsed plasma treatment, etc. [6][7][8][9][10]. In this paper, we propose applying the pulsed-plasma method to treat Cr 3 C 2 -NiCr-based coatings to increase their hardness. Pulsedplasma modification of the products' surface is carried out by the combined thermal and dynamic action of the plasma jet and the electromagnetic field. In this case, a shock-compressed layer of alloying elements from the ionic and atomic components of the plasma is formed on the product's surface. By changing the composition of the plasma, can be controlled the composition of the surface layer. Pulsed plasma surface treatment is a high-performance process of surface doping. The supply of the fuel mixture components to the pulsed plasma torch is carried out continuously, which simplifies (reduces the cost) the hardening technology and technological devices [11,12].
It this paper cermet coatings (Cr 3 C 2 -NiCr) were studied. The coatings were manufactured by means of multi-chamber detonation spraying (MCDS). The influence of subsequent pulsed plasma treatment (PPT) of coatings on their phase composition and physical and mechanical properties is studied.

Materials and research methods
The commercially available Cr 3 C 2 -25 wt% NiCr spray powder manufactured by H.C. Starck (584.054) was used as a feedstock material. The particle size distribution was in range -45+10 µm. The coatings were applied to 12Kh18N10T ssubstrate by detonation spraying on the MCDS installation [13]. In MCDS is realised mode of compressed detonation combustion of a gas mixture in specially profiled chambers. The accumulation of energy combustion from the two chambers in the cylindrical barrel provides the formation of a high-speed jet of combustion products, which accelerates and heats the sprayed powder. The detonation frequency rate was 20 Hz. The gases supply and powder was carried out continuously. Consumption of powder -0.9 kg/h. The composition and consumption of components of the combustible gas mixture are shown in Table 1. Some of the coated samples were treated with pulsed plasma at the Impulse-6 set up. A feature of the used pulsed coaxial plasma generator is the possibility of commuting the electric current by the ionized gas region behind the detonation wave front [14]. This makes it possible to generate pulsed plasma with a frequency of 1-4 Hz and energy of up to 7 kJ. As a result, the surface experiences multiple pulsed thermal actions. The power density of the heat flow is 10 4 ...10 6 W/cm 2 . It is also possible to enter a complex of various doping elements into the plasma. PPT provides quick heating (heating time 10 −3 ...10 −4 s) of the surface layer, followed by intensive cooling by removing heat into the product volume. The high rate (up to 10 7 K/s) of melting and crystallization of the surface layers contribute to the formation of a finely dispersed crystal structure and a high density of dislocations. Pulsed thermal action, elastic-plastic deformation of the structure of processed material, in combination with electromagnetic action due to the flow of pulsed current (up to 10 kA/cm 2 ) through the treated surface layers, intensifies various physical and chemical processes. The pulse-plasma treatment parameters are given in Table 2. The phase composition was studied using traditional X-ray diffraction (XRD) methods, namely, the analysis of angular positions, intensities, and diffraction reflection profiles. The samples microhardness was measured in accordance with GOST 9450-76 (ASTM E384-11) on a Metolab 502 microhardness tester, with loads on the indenter P=1 N and a holding time of 10 s. Nanoindentation was performed on the nano-hardness tester NanoScan-4D Compact. In accordance with ASTM E384-11 Berkovich indenter 15 injections were conducted at a load of 100 mN. The Young's modulus and hardness were determined according to the method of Oliver and Pharr. The coatings surface roughness Ra was estimated using a profilometer model 130.

Results and discussions
Obtained coatings thickness is about 200-230 µm. Figure 1 shows surface micrographs and the measuring results of coatings roughness based on Cr 3 C 2 +NiCr. Coatings surfaces have an inhomogeneous structure with pores. We chose parameter R a , the arithmetic mean deviation of the profile, to be the main parameter characterizing coating roughness. The roughness parameter of the coatings obtained before plasma treatment has the values R a =11.2. And the coating, after 1 pass plasma treatment has R a =5.73 roughness value and after 2 passes have R a =5.31, which is almost 2 times less than the roughness index before treatment. This is related the treatment of coating surface with a plasma jet, after which the particles or fragments of powder rising above the surface melt, and the surface roughness is reduced by ≈ 48%.  Figure 2 shows results of X ray analysis of coating surface before and after pulsed plasma treatment. The following components of the phase components were detected in the coatings before treatment with a pulsed plasma jet: Ni-Cr-Fe, Cr 3 C 2 , Ni-Cr-Fe/Cr 7 C 3 and Cr 7 C 3 phases (Figure 2a). After treatment of the coating surface by the pulsed plasma method was revealed formation of the chromium oxide phase Cr 2 O 3 (Figure 2 (b, c)). With this, after 2 passes of the pulsed plasma jet on the diffractogram is observed increasing peaks intensity of Cr 3 C 2 chromium carbide, which indicates an increase in the content of Cr 3 C 2 carbide (Figure 2c). Figure 3 shows a graph of the microhardness distribution over the coating thickness before and after pulsed plasma treatment. The graph of microhardness dependence on the depth of the Cr 3 C 2 -NiCr coating shows an uneven distribution of microhardness: the coating near the transition layer has a lower microhardness value, with differences to the near-surface layers. Coatings microhardness before pulsed plasma treatment was ≈ 12, GPa (Figure 3a). Visible that after pulsed plasma treatment (Figure 3 (b, c)), the microhardness value increases and has a value after 1 pass of ≈ 14.3 GPa and after 2 passes of ≈ 15.6 GPa. Figure 4 shows the dependences of the nanohardness (H, GPa) and the elastic modulus (Young's modulus E, GPa) on the coating depth. The nanohardness before the pulsed plasma treatment has values of 10.8 GPa (Figure 4a), and after the pulsed plasma treatment the coating nanohardness has values of 12.8  (Figure 4c). The high hardness of the Cr 3 C 2 -NiCr coating after pulsed plasma treatment is explained by structural and phase transformations [15]. Based on X-ray diffraction analysis, it was found that the increase in the hardness of Cr 3 C 2 -NiCr coatings after pulsed plasma hardening is associated with increasing carbide particles in the near-surface layer, and it is associated with the formation of Cr 2 O 3 chromium oxide.

Conclusion
Based on the obtained experimental data and their analysis, we reached the following conclusions: The pulsed plasma treatment leads to changing the surface morphology, in particular, after pulsed plasma treatment, the surface roughness reduced by ≈ 48%. After pulsed plasma treatment the microhardness of Cr 3 C 2 -NiCr coatings increases from ≈ 12, GPa to ≈ 15.6 GPa. Based on X-ray diffraction analysis, it was found that the increase in the hardness of Cr 3 C 2 -NiCr coatings after pulsed plasma hardening is associated with increasing carbide particles in the near-surface layer, and it is associated with the formation of Cr 2 O 3 chromium oxide. As a result, the PPT can be recommended as a method for improving the complex physical and mechanical properties of pre-sprayed gas-thermal coatings.