A STUDY OF AN EFFECT OF THE PARAMETERS OF NIOBIUM-BASED ION CLEANING OF A SURFACE ON ITS STRUCTURE AND PROPERTIES

Nowadays, a vacuum arc method is widely used for applying wear-resistant coatings based on nitrides of refractory metals, in particular titanium [1], zirconium [2], chromium [3, 4], molybdenum [5, 6], as well as multi-element alloys with them [7, 8]. These coatings have a high heat of forming the implementation phases and, consequently, a strong force of the covalent bonding. However, there is virtually no information on the materials of coatings with a strong metallic bond, such as niobium. Quite often in practice, when large size products are sputtered for coating in a vacuum chamber, there happen gassing from the sides and a possible opening of pores in the metal, which results in a pressure spike in the chamber. These gases are mostly nitrogen and oxygen, which react with ionized niobium to form compounds; consequently, instead of cleaning and activating the workpiece surface, there occurs sedimentation of the coating. Such a coating does not have any good diffusing connection with the hardened piece, so the subsequent deposition of the protective coating will only lead to delamination. Therefore, it is very important to know the parameters of sputtering on the surface of products under different conditions of using reactive gases. This information allows suggesting a method with improved conditions of applying such coatings. Thus, the relevance of the study lies in the scientific novelty of developing a technology of ionic sputtering in the plasma with heavy ions of Nb. This technology can be practically used in the preparation of steel surfaces with low roughness and high mechanical properties.


Introduction
Nowadays, a vacuum arc method is widely used for applying wear-resistant coatings based on nitrides of refractory metals, in particular titanium [1], zirconium [2], chromium [3,4], molybdenum [5,6], as well as multi-element alloys with them [7,8].These coatings have a high heat of forming the implementation phases and, consequently, a strong force of the covalent bonding.However, there is virtually no information on the materials of coatings with a strong metallic bond, such as niobium.
Quite often in practice, when large size products are sputtered for coating in a vacuum chamber, there happen gassing from the sides and a possible opening of pores in the metal, which results in a pressure spike in the chamber.These gases are mostly nitrogen and oxygen, which react with ionized niobium to form compounds; consequently, instead of cleaning and activating the workpiece surface, there occurs sedimentation of the coating.Such a coating does not have any good diffusing connection with the hardened piece, so the subsequent deposition of the protective coating will only lead to delamination.Therefore, it is very important to know the parameters of sputtering on the surface of products under different conditions of using reactive gases.This information allows suggesting a method with improved conditions of applying such coatings.
Thus, the relevance of the study lies in the scientific novelty of developing a technology of ionic sputtering in the plasma with heavy ions of Nb.This technology can be practically used in the preparation of steel surfaces with low roughness and high mechanical properties.

Literature review and problem statement
The vacuum arc coating technology has been successfully used to increase the resistance of machine parts and tools by increasing anti-corrosion, anti-friction and strength properties [9,10].To achieve high mechanical properties of a protective coating prior to applying such a coating to a
It is known that bombardment with ions of a vaporizable material at a negative displacement potential (U b ) of about -1000 V and a subsequent application of a wear-resistant coating provide mutual penetration of the metal atoms of the coating and the substrate.This produces the desired diffusion layer with high adhesion bonding [12].
To create a diffusion layer, it is necessary to produce ion bombardment (purification) in high vacuum (about 0.001 Pa) to ensure a sputtering of the surface layers that contain various oxides and other contaminants which may impede the diffusion of the atoms of the vaporizable material into the substrate [13,14].There simultaneously happens an intensive heating of the surface layers, since the kinetic energy of the bombarding ions is mostly converted into heat [15].These layers have abnormally low thermal conductivity, so their temperature can be several times higher than the temperature of the substrate itself [16], which in turn intensifies the diffusion processes [17,18].However, in case of the presence of gases in the vacuum chamber, the gasses (nitrogen, oxygen, etc.) can create a compound with the vaporizable metal, and these compounds can be deposited on the surface of the substrate, limiting the diffusion processes [19].The actual processing conditions involve large loads of products in a vacuum chamber.Besides, almost always there is an intention to reduce the time of the production processes.Therefore, the process of cleaning the surface of a product in practice begins at higher pressures in the chamber, which often leads to delamination of the coatings [20].

The purpose and objectives of the study
The aim of the study is to determine the effect of the value of U b (substrate potential) and the pressures of various gases in a vacuum chamber on the substrate sputtering processes under bombardment with ions of niobium.It is an important goal of research, both for the scientific bases of plasma physics and for industrial use.
The goal is achieved by doing the following tasks: -to choose the most efficient modes of cleaning the samples' surfaces prior to coating them by means of bombarding with niobium ions; -to determine the effect of the presence of argon and reactive gases (nitrogen) in the process of ion bombardment on the sputtering rate.

The procedure for preparing and studying the samples
The experiments were performed on samples obtained by the vacuum-arc method in the modernized vacuum chamber Bulat-6 (Ukraine) [21].The cathode material was niobium Nb1.The coatings were applied to the surface of the samples such as 18×18×3 mm of stainless steel 12X18H9T that had been prepared by the standard methods of grinding and polishing.After polishing and washing the samples in an ultrasonic bath with an alkaline solution and wiping with calico soaked in Nefras C2 80/120, they were weighed on the analytical scales VLR-200.These scales were also used to weigh the samples after the experiments.
The distance from the evaporator (niobium) to the substrate was 300 mm or 485 mm.The arc current in the evaporators was 130 A, and the chamber pressure was varied from 0.002 Pa to 0.66 Pa by admitting argon, nitrogen or their mixtures in various proportions.In the experiments, the substrate was supplied with U b in a range of -50...-1300 V.The bombardment time in each experiment was 10 minutes.The temperature of the substrate, as measured by the thermocouple chromel-alumel, was 150….550ºС.The surface morphology was investigated on the metallographic microscope XDS-3.
Microhardness was measured on the microhardness tester PMT-3, with a load of 10 g and 50 g, taking the average by the results of ten measurements.

The influence of the modes of bombardment with Nb ions on the processes of sputtering and depositing
Fig. 1 shows the results of changes in the volume weights in the cases of the coating deposition and the substrate sputtering after the niobium ion bombardment.When the values were positive, the calculation was based on the weight of niobium, and when the values were negative, the calculation was based on the weight of the stainless steel.It should be noted that at a U b of -900 and -1300 V, the substrate temperature increased to 550 ºC and 600 ºC, respectively.
Of particular interest is the effect of impurity gases on the amount of sputtering the material.Fig. 1 shows the effect of the inert argon gas pressure on the rate of sputtering on the surface of the samples.
As can be seen from Fig. 1, a, when in a high vacuum the U b is from -50 V to -350 V, the niobium coating is deposed, and its thickness decreases as the U b increases as the result of sputtering.When U b =-350 V, there is an equilibrium between the processes of sputtering and depositing, but with a further increase of the U b , there prevail the sputtering processes.A significant effect on the rate of sputtering across the surface is produced by the distance from the samples to the cathode: when the distance decreases, the rate of sputtering increases.When argon pressure in the vacuum chamber increases, it significantly increases the sputtering rate on the samples within the range of all pressure values.And the equilibrium point "sputtering-depositing" (Fig. 1, с, d) is shifted to U b =-250 V at a distance of 300 mm.The increase in the sputtering rate at the increasing argon pressure in the chamber is associated with a significant increase in the concentration of niobium ions due to the intensive interaction of the plasma with the gas, which increases the intensity of niobium sputtering and diffusion into the substrate material.
For a fuller understanding of the nature of sputtering the substrate material, Fig. 2 shows a graph of the dependence of the sputtering magnitude on the U b on the substrate under various pressures of argon in the chamber.

The influence of a potential displacement under the plasma processing on the mechanical properties and surface preparation of steel 12Х18Н9Т
The use of the argon medium for bombarding steel with niobium ions increases the microhardness (Fig. 3).
It is necessary to take into account that bombardment with metal ions leads not only to sputtering on a sample surface but also to deposition followed by diffusion.Such sputtering and depositing on the surface of a sample increase the microhardness of the sample surface due to the formation of a solid solution (Fig. 3).
From the graphs of the microhardness dependence (Fig. 3) on the U b , there appears to be an increase in the microhardness at all pressures of argon.The highest degree of microhardness arises at the maximum intensity of the surface sputtering when U b =-1,300 V.This is due to the diffusion of niobium in the substrate and the creation of a saturated layer.
For a sample at a distance of 485 mm, the microhardness increases from the baseline to 1.8...2.0 GPa.However, it should be taken into account that the thickness of the coating is relatively small, and when the microhardness is measured at a load of 50 grams, it becomes pressed through.The most significant increase in the microhardness occurs in the process of sputtering on the surface of a sample at a high value of U b =-1,300 V at a distance of 300 mm (Fig. 3), which is associated with the diffusion of niobium deep into steel.However, in the presence of nitrogen in the vacuum chamber, an increased pressure produces a displacement so that the equilibrium between sputtering and depositing is shifted towards a higher U b (from a value of about -400 V at a P N =0.02Pa to -500 V at a P N =0.08 Pa) (Fig. 4).It can be noticed that the balance point between the sputtering and sputtering shifts (respectively, to -400 V and -500 V), depending on the distance of 300 mm and 485 mm (Fig. 4, a).A subsequent increase in the U b also cleans the surface.
By increasing the nitrogen pressure in the vacuum chamber to P=0.08 Pa (Fig. 4, b), there is even a greater shift of the equilibrium point between the sputtering and sputtering.Thus, for the sample at a distance of 300 mm, the equilibrium point is -600 V, and the sample at a distance of 485 mm has an equilibrium point even at the level of -800 V.
The increased microhardness of the samples after adding nitrogen to the chamber is connected with the formation of niobium nitride on the substrate of the solid coating (Fig. 5).
The nitride phase creation leads to an increased microhardness of up to 2.2 GPa at small U b =-200...-350 V, which is associated with the NbN coating deposition.
Fig. 6 shows photographs of the samples' surfaces where it is obvious that an increase in the U b increases the sputtering intensity.At a small U b , there is a deposition of the niobium-based coating (Fig. 6, a By increasing the U b to -250 V and -350 V (depending on the distance from the sample to the cathode: 300 mm and 485 mm, respectively), there occurs its sputtering on the surface, as shown in Fig. 6, b.
At a further increase of the U b in the substrate up to -900 V, there is a significant change in its surface topography (Fig. 6, c).
The most efficient surface preparation takes place for the samples that have been treated at U b =-1,300 V (Fig. 6, d).
Fig. 7 shows a coating that was created at a nitrogen pressure of 0.08 Pa and under U b =-200 V.
The coating that was received at the lowest pressure is brittle, which is manifested by its spontaneous cracking.the chamber results in two competing processes: on the one hand, the supply of nitrogen (as well as argon) results in an increase of niobium ion concentration in the plasma volume and intensifies the ion bombardment of the surface.On the other hand, there appear niobium nitrides on the substrate surface, for sputtering which it is required to have a high energy of ions.As a result, there is a predominant influence of the mechanism of forming the nitrides, and the U b , corresponding to the equilibrium process of "sputtering-depositing" is shifted towards higher values.A significant increase in the substrate surface hardness after cleaning with the Nb ion bombardment apparently happens due to a large diffusion saturation of the surface with Nb ions.The findings are of interest because the cleaning technology of surface preparation is the key to ensuring adhesive strength of any vacuum plasma coatings.Undoubtedly, this study is far from complete; it requires additional research.It is planned to undertake further microscopic and structur-al-phase tests to obtain a complete and unambiguous picture of the process of ion cleaning.

Conclusions
1.It has been determined that the cleaning process produces a diffusion saturation of steel with niobium, which leads to an increase in its hardness and resistance to crack formation.In this article, an optimal cleaning mode is suggested at the following parameters: the distance from the cathode to the substrate should be 300 mm, with U b = =-1300 V and P Ar =0.002 Pa.
2. It has been found that the presence of argon in the ion bombardment process increases the sputtering rate at an increased pressure, and the presence of reactive gases (nitrogen) leads to a decrease in the sputtering rate due to the formation of niobium nitride on the surface.

Fig. 1 .
Graphs of changes in the specific weight depending on the U b when being bombarded with ions of niobium at various pressures of argon and distances to the samples: ■ -H=300 mm, • -H=485 mm; a -P=0.002Pa, b -P Ar =0.04 Pa, c -P Ar =0.09 Pa, and d -P Ar =0.66 Pa

Fig. 2 .
Fig. 2. A graph of changes in the specific weight depending on the U b when being bombarded with ions of niobium at a distance of 300 mm from the cathode under the following argon pressures P Ar : а -0.66 Pа, b -0.267 Pа, c -0.09 Pа, d -0.04 Pа and e -0.002 Pа

Fig. 4 .
Fig. 4. Graphs of changes in the specific weight depending on the U b when being bombarded with ions of niobium at various distances to the samples: ■ -Н=300 mm and • -Н=485 mm; а -Р N =0.02Pа and b -Р N =0.08 Pа

Fig. 5 .
Fig. 5. Graphs of changes in the substrate microhardness depending on the U b after the bombardment with niobium ions at various distances to the samples: ■ -Н=300 mm, • -Н=485 mm and -the original; а -Р N =0.02Pа and b -Р N =0.08 Pа

Fig. 7 . 7 .Fig. 6 .
Fig. 7.A photo of the surface substrate after the bombardment with niobium ions with a thin coating layer of NbN that was created at a nitrogen pressure of 0.08 Pa and under U b =-200 V