The Copper Thin Layer Deposition by Pulse Vacuum-Arc Sprayer

In this work, the deposition process features of a copper coating on stainless steel substrate, using the pulse vacuum arc spraying method were researched. A continuous layer of copper was deposited on a stainless steel substrate, and a high degree of coating adhesion to the substrate was demonstrated. The thickness of coating using application time was calculated, and the surface unevenness was estimated. Inside the coating layer substrate material elements were observed, that could appear as a result of simultaneously plasma spraying on the surface of the substrate. The feature of this method was discovered, that surface plasma plays a significant role in the deposition process. Finally, it was shown that the device with a pulsed arc could effectively be used in industry, since it is possible to change the layer thickness in the range of hundreds of microns by varying the deposition time.


Introduction
The actual problems of materials science include the development of technologies for improving surface properties of various constructional materials and tools. One of the solutions to the tasks may be using plasma-spraying devices [1]. Plasma spraying is widely used in those branches of industry and technology, where it is necessary to protect the machine parts and tools from intensive wear, corrosion, erosion and to create the electrical layers [2]. Plasma vacuum deposition of functional coatings is obtained in vacuum equipment with a controlled environment and composition. Vacuum-arc accelerators (VAA) are widely used to change the structure of materials and for surface treatment of metals and alloys [3]. By applying a protective layer on the surface of materials, using VAA can improve their chemical composition and physical properties [4]. Therefore, the goal of this work is to develop the pulse vacuum arc sprayer and research technological advantages of this method to deposit copper layers on constructional steel.

Experimental Devices and Samples
In this paper, deposition experiments were carried out by experimental installation VAA-1, shown in Figure 1. The main elements of this accelerator are vacuum chamber, electrode system, DC power supply with energy store unit (capacitor battery) and pulsed ignition units. Electrode system of (VDU-1 installation) (Figure 1): the removable cathode with diameter D = 50 mm is mounted on cathode holder with cylinder shape, connected to the cooling system (not shown in the figure). On cathode axis there is an orifice anode with D = 90 mm, which is connected to the power supply through a copper conductor placed inside ceramic insulation 2. The other conductor connected to the vacuum arc ignition electrode 4. The ignition electrode is fixed at distance of 50 mm from the cathode surface. Plasma 5 formed at the anode outlet is deposited on substrate put on the holder6. Calorimeter 7 is included to research distribution of plasma flow 5. The anode pulse current is measured by Rogowski belt 8, which covers the anode wire in place close to the contact of vacuum chamber. The power supply source of VDU-1 consists of the capacitor battery with summarising the capacitance 600 μF, 5000 V, DC source for 500/1000V, 10 A and a pulse generator with the passing frequency of 5 -50 Hz.
In the experiment, the samples of the test material are loaded into the working chamber at a residual air pressure of 1.33 × 10 −3 Pa (1 × 10 −5 mbar). After the gas inflow, the sprayer mode sets and the pulse power generator switching on. In order to initiate the arc discharge the device design provides an ignition electrode made of stainless steel, situated in the shell of insulating material (ceramics), The surface morphology of the copper coatings was studied using scanning electron microscope Quanta 2000. The chemical composition of the surface layer of material was determined by the X-ray secondary electron spectroscopy (RSA) on device Pegasus 2000. The microhardness measurements were carried out on metallographic microscope "METAVAL" at P = 300 N. Table 1 shows composition of the substrate surface before (No. 7) and after de- 3 was placed in the center, which increased the efficiency of the deposition. In addition, it was found the increase in carbon content from 2.09% to 16.76% and a decrease in iron and chromium ~ 7 and 12 times, respectively, which is evidence of internal rearrangements of the structure. The carbon apparently comes from the residual air or because of erosion of electrodes insulating of the accelerator.   (1)

Results and Discussion
Knowing the average time for one working cycle t = 40 min, we can to calculate the copper layer mass for spraying: where ρ-density of copper; S-square of the deposited area, L-thickness of the sprayed layer. The density of copper is a tabular value of 8.96 g/cm 3 . If deposition area is 100 cm 2 (the diameter of plasma flow about 10 cm), the thickness of the layer evaluated from formula (2) will be The results of calculation presented on Table 2. So, the thin copper layer with thickness 400 µm was deposited by VAA-1 device in time about 40 min. This result demonstrated the good efficiency of researched method for coating obtaining.
The results of microhardness measurements were given in Table 3. As seen the copper layer was quite thin and did not affect substrate hardness. But changes in samples No. 1 and No. 3 could be traced, one of which was at the stream periphery. The thickness of the copper coating on sample No. 3 was maximum; therefore, when measuring microhardness, the imprint on this sample was the deepest, and this could be affected the value measured by the device.
But the in homogeneity of hardness was about 255/261 or 2% and we can say that the homogeneous coating was deposited.

Conclusions
This work shows the good efficiency of deposition of copper coating on stainless steel substrate using the pulsed vacuum arc spraying method. A continuous layer  of a copper was deposited on stainless steel substrate, which was a result of high degree adhesion of the coating to the substrate. The coating thickness altered using different depositing time was evaluated, and the non-uniformity of coating along the surface was calculated. Inside the coating, substrate material elements where observed, which could be attributed to simultaneously plasma spraying on the surface of the substrate. The feature of this method was found to be that surface plasma played a significant role in the deposition process.
The results can be used to develop industrial applications of the vacuum arc method. Currently, great prospects for development receive technology increases in the durability of structural materials and products from them. Proposed in this paper, time limit method for deposition of metallic materials using equipment of the type VAA-1 allows obtaining metallic coatings on substrates of structural steel with high efficiency and, requires low cost for production.