EXPERIMENTAL STUDIES OF THE COMPOSITION AND PROPERTIES OF COATING BASED ON Zr-Nb

ABSTRACT

The article presents the use of vacuum equipment for obtaining coatings based on zirconium-niobium, as well as the results of experimental studies of their phase and chemical composition and properties.

АННОТАЦИЯ

В статье приведены результаты экспериментальных исследований состава и свойств покрытий на основе циркония и ниобия, влияние параметров технологического процесса на толщину и микротвердость, а также их коррозионную стойкость покрытий.

Keywords: coating, result, experimental studies, phase, chemical, composition, zirconium, niobium, oxide, nitride, Auger spectroscopy, properties, thickness, microhardness, corrosion resistance, technological process, diffractogram, arc current.

Ключевые слова: покрытие, результат, экспериментальные исследования, фаза, химический состав, цирконий, ниобий, оксид, нитрид, оже-спектроскопия, свойства, толщина, микротвердость, коррозионная стойкость, технологический процесс, дифрактограмма, ток дуги.

Introduction. Zirconium alloys and the processes of their formation are widely studied using various methods. Of great interest are zirconium alloys alloyed with niobium, especially in the aviation and engineering industries, where such characteristics of coatings as wear resistance, corrosion resistance, chemical inertness, strength, and others are key. Niobium increases the strength characteristics of zirconium alloys and stabilizes their corrosion resistance, neutralizing the effect of harmful impurities [1, 2, 3]. Alloys based on the Zr-Nb system are characterized by increased strength based on a heterogeneous structure consisting of an α-solid solution with inclusions of β-Nb dispersed particles, which makes it possible to use this compound as a protective coating.

Discussion. Coatings based on zirconium doped with niobium in this work were obtained by magnetron sputtering. To obtain coatings, a UVN-75R-1 type vacuum deposition unit was used. Coatings were applied to samples of high-alloy steel 12Kh18N10T. Zircaloy-2 alloy was used as the cathode material. In the course of experimental studies, coatings based on zirconium and niobium - Zr-Nb, (Zr-Nb)N, (Zr-Nb)O were obtained, phase and chemical analysis of the obtained coatings were determined, their properties were investigated.

The phase composition of the coatings was studied by X-ray diffraction analysis using a general-purpose DRON-2.0 diffractometer. Phase X-ray diffraction analysis shows that when applying coatings based on Zr-Nb, it consists of a phase - Zr with an hcp lattice. The experimental value of the crystal lattice period is greater than the tabulated value (a_table = 0.4235 nm, с_table = 0.5147). Thus, it can be concluded that the coating on the samples mainly consists of Zr. A characteristic feature of X-ray diffraction analysis data processing is the wide use of constants previously determined experimentally or calculated theoretically. This greatly speeds up and facilitates the process of processing the results of the study. In cases where it is possible to assume which substances are present in the sample under study, a qualitative X-ray diffraction analysis consists in comparing the experimental values ​​of interplanar distances and relative line intensities with reference X-ray diffraction patterns [2]. When comparing, it should be borne in mind that the data in the tables mainly refer only to compounds of stoichiometric composition, and when solid solutions are formed, the values of interplanar distances naturally change.

Statistical background fluctuations also affect the accuracy of the qualitative analysis. A special analysis of the errors shows that peaks with a value three times greater than the value of the average deviations for the background can be taken as diffraction maxima with reliable accuracy.

The phase composition of the coatings was studied in CoK_a radiation using an iron selective absorbing filter. Operating current - 10 mA; high voltage - 30 kV; detector movement speed – 1 deg/min. Three slits with dimensions of 1x2x1 mm were used in the measurements. The scale of the diffractogram is 1 – 20 mm. Interplanar distances of coatings based on zirconium-niobium were calculated from the centers of gravity. No Zr-Nb nitrides were detected on the diffraction pattern, since the coating thickness is approximately 1 μm, X-rays pass through and only the substrate (12X18H10T) reflects (on the diffraction pattern).

Fragments of diffractograms for the corresponding modes of coating deposition are shown in Fig. 1, the calculation of the phase composition of coatings based on Zr-Nb was performed.

Figure 1. X-ray diffraction pattern of coatings based on Zr-Nb

For the true value of the period of the crystal lattice, the value corresponding to the angle 2Q = 41.3 degrees is taken.

The crystal lattice period was calculated for various modes of coating formation, the results are summarized in Table 1.

The study of the chemical composition of the coatings was carried out by Auger electron spectroscopy. The energy of primary electrons was 3 keV, the current density was 5×10^-6 А/сm², the anode voltage was 200 V, and the pressure in the chamber was ~ 0.000013 Pa. A qualitative analysis of the elements in the coating was carried out by comparing the energy of secondary electrons taken from the Auger spectrum with tabular data for the energies of Auger electrons for various elements [4–7].

Table 1.

Interplanar distances of the crystal lattice parameters of the coating based on Zr-Nb

The intensity of the maximum peak is taken as the intensity of the Auger peak. The elemental sensitivity values ​​for each element present in the spectrum were plotted depending on the atomic number and the possibility of the corresponding energy transition (KLL, LMM, MNN transitions) [8, 9]. Auger spectroscopy of the coatings showed that the coating contains 94.2% zirconium and 1.3% niobium. As a result of the calculation of the chemical composition of coatings based on Zr-Nb, the following were determined  the intensity of Auger peaks, elemental sensitivity and ratio for each element present in the coating. The study of the phase and chemical composition of coatings based on Zr-Nb nitrides was carried out using X-ray diffraction analysis and Auger spectroscopy. Auger - spectroscopy of the coatings showed that the coating contains nitrogen in the amount of 2.9% and zirconium 91.6%, as well as niobium 1.1%.

Based on chemical analysis, it can be assumed that the coating contains Zr-Nb nitrides. Studies of the phase and chemical composition of coatings based on Zr-Nb oxides were carried out on samples of steel 12Kh18N10T. The calculation of the diffraction patterns showed that the coating consists of a ZrO₂ phase with a monoclinic crystal lattice, the lattice period a=0.463 nm. Also found traces of Zr. No niobium oxides were found on the diffraction pattern, only traces of niobium are present.

Chemical analysis showed that the Zr-Nb oxide contains approximately 90% zirconium, 1% niobium and about 3% oxygen.

The microhardness, corrosion resistance and thickness of coatings based on Zr-Nb, (Zr-Nb)N, (Zr-Nb)O have been studied.

The effect of technological process parameters, such as discharge current I and deposition time T, on the thickness and microhardness of coatings based on zirconium and niobium, as well as their nitrides and oxides, has been studied.

Coatings based on Zr-Nb, their oxides and nitrides formed on a magnetron sputtering unit. The discharge current varied from 1 to 3A, the duration of deposition was from 20 to 100 min (see Fig. 2-5).

Thickness of the coating, mcm.

Figure 2 The influence of the duration of the magnetron spraying on the thickness of the Zr-Nb coating

Figure 3. Influence of thickness on the microhardness of coatings based on Zr-Nb

Figure 4. Influence of the arc current on the thickness of the coating based on oxides Zr-Nb

The study of the microhardness of the coatings was carried out by the method of restored indentation (the dimensions of the indentation are determined after the removal of the load), when an imprint is applied to the surface of the coating under the action of a static load applied to the diamond tip for a certain time.

Figure 5. Influence of the arc current on the microhardness of the coating based on Zr-Nb oxides.

The value of microhardness is determined as the ratio of the applied load to the conditional area of the side surface of the resulting imprint.

The results of measurements of microhardness, with the corresponding thickness of the coating are presented in table. 2.

Table 2.

Influence of technological parameters on the properties of coatings

Note: T - duration; I- arc current; U- voltage on the target is 400V; t=100 °С; pressure in the chamber 2·10^-3 mm Hg; substrate stainless steel 12Х18Н10Т, H_m=20 =336 kg/mm², H_m=50=340 kg/mm².

Measurements of the microhardness of the coatings obtained on samples of high-speed steel grade R6M5 were carried out on a PMT-3 microhardness tester according to the standard procedure for conducting experimental studies. Microhardness tests were carried out at loads of 0.196N (20gs) and 0.490N (50gs). A four-sided diamond pyramid with a square base was used as an indenter.

Based on the conducted studies, it was found that with an increase in thickness from 3 to 8 μm, the microhardness of coatings based on Zr-Nb increases from 400-500 kgs/mm2, respectively. The microhardness of coatings based on niobium and zirconium oxides is about two times higher than that of the substrate made of steel 12X18N10T, R6M5. High microhardness is achieved with a coating thickness of 3-4 µm. The microhardness of coatings based on Zr-Nb nitride at a coating thickness of ~ 1 μm does not differ significantly from the microhardness of the substrate. This is due to the small thickness of the coating, since a diamond indenter at such a thickness does not provide an objective assessment of microhardness (it is recommended to measure microhardness at a thickness of more than 3-4 microns). The arc current affects the coating thickness, so with an increase in the arc current from 1 to 3A, the coating thickness changes by ~ 4 times.

In this work, the microscopic method was chosen to determine the coatings, since it allows one to determine the absolute thickness of the coating at any point in the section where it is necessary, and not the average thickness as in the gravimetric method. According to the measurements, the thickness of the coatings based on zirconium-niobium, depending on the deposition time, was 3, 6, 8 μm. Studies have shown that the thickness of the zirconium-niobium coatings is proportional to the deposition time in the range of 15-25 minutes.

Experimental studies of the corrosion resistance of coatings based on Zr-Nb were carried out by the method of destroying the area occupied by corrosion. To assess the protective ability of corrosion inhibitors, a standard ten-point scale is used, given in GOST 9.041-74. This method was chosen to study the corrosion resistance of coatings, as it is the simplest and gives a qualitative assessment of corrosion damage.

The area affected by corrosion of the working surface and the depth of etching were carried out on a microscope with an MII-4 interferometer. Based on these data, the percentage of corrosion damage to the surface under study was calculated. The research results are summarized in table 3.

Table 3.

Corrosion resistance of coatings based on Zr-Nb, (Zr-Nb)N, (Zr-Nb)O

Conclusion. Based on the conducted research, the following conclusions can be drawn:

• The operational properties of coatings based on Zr–Nb, their nitrides and oxides, obtained by the ion-plasma method of magnetron sputtering, are affected by the thickness of the coating, as well as the arc current.

• Coatings based on Zr-Nb oxide with a thickness of 3 µm have maximum corrosion resistance.

• Zr-Nb oxide is the most corrosion-resistant of the investigated coatings.

• coatings based on zirconium-niobium, their oxides and nitrides can be used as protective functional and decorative layers on steels and glass products, resistant to acids and alkalis.

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