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26FEATURES OF DEPOSITION, STRUCTURE AND PROPERTIES OF ELECTROSPARK COATINGS OF Cr-Al-B-Si, Mo-Si-B AND Zr-Si-B ON NICKEL ALLOY
A. E. Kudryashov*", E. A. Levashov", Ph. V. Kiryukhantsev-Korneev", and A. N. Sheveiko"
^National University of Science and Technology MISiS, Moscow, 119049 Russia *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10073
Metals and their alloys, graphite, and hard alloys (mostly those based on tungsten carbide) are conventionally used as electrode materials in the electrospark deposition technology. SHS-electrode materials based on titanium carbide and titanium diboride have recently started to be used [1]. Broadening of the range of practical application of the electrospark deposition technology is associated with the development of novel compositions of electrode materials. In this connection, it was decided to use SHS alloys based on the Cr-Al-B-Si, Mo-Si-B, and Zr-B-Si systems [2-4]. This study was aimed at investigating the features of formation, structure, composition, and properties of electrospark coatings deposited onto substrates made of refractory EP718-ID nickel alloy using heat-resistant SHS-electrode materials. The Cr-Al-Si-B, Mo-Si-B, and Zr-B-Si alloys fabricated by self-propagating high-temperature synthesis (SHS) were used as electrodes (anodes). The commonly used EP718-ID heat-resistant nickel alloy was applied as a substrate (cathode). This alloy is widely used in aircraft engine technology. The alloy composition was as follows: C < 0.1 %; Cr ~ 14.0-16.0 %; Ni ~ 43.0-47.0 %; Fe ~ 22.0-33.0 %; Mo ~ 4.0-5.2 %; W ~ 2.5-3.2 %; Nb ~ 0.8-1.5 %; Ti ~ 1.9-2.4 %; Al ~ 0.9-1.4 %; Mn ~ 0.60 %; Si ~ 0.30 %; Zr ~ 0.02 %; Ce ~ 0.10 %; B ~ 0.008 %; S ~ 0.010 %; P ~ 0.015 %. The coatings were deposited using an Alier-Metal 303 setup. The applied three frequency-energy modes of electrospark treatment were characterized by current I = 120 A and total pulse energy IE = 9.2 kJxmin-1 but differed in terms of pulse frequency and length (3200 Hz (20 |s), 1600 Hz (40 |s), and 800 Hz (80 |s)). The heat-resistance tests were conducted at a temperature of 700°C and exposure duration of 40 h (48 h for Mo-Si-B). The oxidation weight gain (Ams) (changes in the sample weight due to oxidation per unit surface area) was determined using the following formula:
Ams = (m1 - m0)/S0 (1)
where m0 is the initial weight of the sample, g; m1 is the weight of the sample containing the oxidation products, g; and S0 is the area of the sample surface, m2.
The tribological properties of the coatings were determined in accordance with the international standards ASTMG 99-959 and DIN 50324 on a high-temperature tribometer (CSM Instruments) operating in the pin-on-disk mode. A ball made of AhO3 (6 mm in diameter) was used as a counter body. The coated samples were allowed to slide at a linear velocity of 10 cm/s under a load of 1 N. The temperature of the experiment was 500°C. The dependence between the coefficient of friction of the wearing pair and the path length of the counter pair (300 m) was plotted on a PC using the InstrumX software.The wear track profile and coating roughness (the arithmetic mean roughness value Ra) was evaluated using a Veeco WYKO NT1100 optical profilometer. The wear rate was calculated using the following formula:
W = (s x L)/(N x l) (2)
where Wis the wear rate, mm3-N-1-m-1; L is the perimeter of a circle, mm; s is the sectional area of the wear groove, mm2; N is the load, N; and l is the sliding distance, m.
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Cr-Al-B-Si system
The chemical and phase compositions of the applied electrode materials are listed in Table 1.
Table 1. The chemical and phase compositions of the Cr-Al-B-Si electrode materials.
Electrode material Cr Content, % Al B Si Phase composition, %
Composition 1 69.4 8.1 12.0 10.5 65CrB + 27Cr(Si,Al)2 + 8CrsSi3Bx
Composition 2 65.8 16.2 12.0 6.0 35CrB + 26Cr5Si3 + 39Cr4Aln
Composition 3 63.4 21.6 12.0 3.0 15CrB + 24Cr5Si3 + 57Cr4Aln + 4Cr5Si3Bx
It was found that application of the electrode with composition 1 (with the highest content of refractory CrB) caused weight loss on the kinetic curves showing the total weight gain of the cathode (£ACi) after ESD for 3 min in all treatment regimes. For the electrodes with compositions 2 and 3, which have a much lower content of the boride phase, weight gain on the cathode (the substrate) was observed throughout the entire treatment for 10 min in all ESD regimes. At higher contents of low-melting phases Cr(Si,Al)2, CrsSi3, and Cr4Alii (composition 3), coating formation became more vigorous as the amount of the melt in the discharge arc increased.
The high-frequency regime of coating treatment (f = 3200 Hz), characterized by steady-state mass transfer and minimal roughness, was chosen to be optimal.
Table 2 lists the properties and composition of the deposited coatings. All the coatings are characterized by 100% continuity.
The coating deposited from the electrode with composition 1 had the minimal oxidation weight gain of 0.20 g/m2 (the best heat resistance). The maximum oxidation weight gain values were observed for the coating deposited from the electrode with composition 3 and the uncoated sample (0.38 and 0.37 g/m2, respectively). It should be mentioned that when calculating this parameter, we did not take into account that the electrospark coating had a larger geometric surface area. The electrode with composition 1 was recommended to be used for treatment of items made of EP718-ID nickel alloy.
Table 2. Properties and composition of electrospark coatings.
Electrode material Thickness, ^m Hardness*, GPa Oxidation weight gain**, g/m2 Wear rate**, 10-6 mm3/(mxN) Phase composition of ESD coatings**** (after 10 min)
Composition 1 15 4.9 0.20 133 Solid solution based on Ni, y-Al20s, Cr2B
Composition 2 20 3.0 0.23 222 CrsSi, Cr, FesSi, &3B4, y-Al20s, Fe3B Cr3Si, Cr, Fe3Si,
Composition 3 25 3.0 0.38 225 Y-Al203, Cr5Si3, Cr2N, Cr5Al8
*Hardness of EP718-ID nickel alloy was 1.4 GPa; **Oxidation weight gain of EP718-ID nickel alloy was 0.37 g/m2; ***Wear rate of EP718-ID nickel alloy was 260x10-6 mm3/(m*N);****Phase composition of EP718-ID nickel alloy: Ni-based solid solution.
Mo-Si-B system
The chemical and phase compositions of the applied electrode materials are listed in Table 3.
Cathode weight loss was observed during the entire treatment with the Mo-Si-B electrode materials. The properties and composition of the deposited coatings are summarized in Table 4. All the coatings had a 100% continuity. The minimal roughness (Ra) was revealed for the coatings formed upon ESD treatment in the high-frequency regime.
It was found that the coatings deposited using the electrode materials with compositions 1-3 contained an amorphous phase. Weight loss was revealed upon oxidation of the coated
samples; the coatings oxidized using electrodes with compositions 1, 3, and 4 contained unoxidized spots. The electrodes with compositions 3 and 4 were recommended to be used to treat items made of EP718-ID nickel alloy.
Table 3. Chemical and phase compositions of Mo-Si-B electrode materials.
Electrode material Content, % Phase composition of the electrodes, %
Mo Si B
Composition 1 90.49 4.41 5.10 51 Mo2B + 2 MoB + 47 MosSiB2
Composition 2 74.12 21.7 4.18 57 MoSi2 + 41% MoB + 2 MosSis
Composition 3 90.61 5.31 4.08 96 MosSiB2 + 2 MosSi + 2 Mo
Composition 4 65.88 33.06 1.06 89 MoSi2 + 9 MoB + 2 MosSis
Table 4. Properties and composition of electrospark coatings.
Electrode material Thickness, ^m Hardness, GPa Roughness Ra, ^m Wear rate, 10-6 mm3/(m*N) Phase composition of ESD coatings (after 10 min)
Composition 1 35 4.4 6.26 153 Ni-based solid solution
Composition 2 35 3.8 6.42 169 Ni-based solid solution, Mo-based solid solution
Composition 3 45 3.8 8.76 58 solid solution based on Ni, Mo (Ni,Si)2, &3B4 Mo (Ni,Si)2, solid
Composition 6 45 3.2 8.05 67 solution based on Ni, &3B4
Zr-Si-B system
Electrode material with composition 64.7 % Zr-20 % Si-15.3 % B (66 % B2-6% Si-26% ZrSi2-2 % ZrO2) was used for coating deposition. The coatings were deposited in various media: in air, argon, and under vacuum. Automated treatment was used to deposit coatings under vacuum. When coatings were deposited in air, the nickel alloy was pre-treated with carbon-carbon composite material (C-CCM) in order to produce a sublayer. Heat resistance tests of the samples were conducted at 900°C and exposure duration of 5 h. Thickness of the oxidized layer in the samples was determined by glow discharge optical emission spectroscopy (GD-OES) on a Profiler instrument (Horiba Jobin Yvon).
The maximum cathode weight gain was observed after treatment in argon. Treatment under vacuum also resulted in cathode weight gain. Electrospark deposition in air led to cathode weight loss. Cathode weight loss after pre-treatment of nickel alloy with C-CCM was less significant. Properties and composition of the coatings deposited onto EP718-ID nickel alloy are listed in Table 5. Exposure to high temperature led to formation of an oxide layer on the surface of the uncoated sample, which could be easily removed from the surface. Thickness of the deposited coatings was 20-25 |im, being much greater than depth of the oxidized layer. Therefore, it is fair to say that these coatings enhance heat resistance of nickel alloys.
Table 5. Properties and composition of electrospark coatings.
Treatment medium Roughness Ra, ^m Thickness of the oxidized layer*, ^m Phase composition of electrospark coatings (after 10 min)
Vacuum 6.44 0.7 Si, ZrB2, Ni, Ni(1-x)Si(x)
Air 7.38 1.5 Ni(1-x)Si(x), ZrO2, Fe2Si, Ni4N
Argon Air 5.80 6.01 1.4 0.9 Si, ZrB2, Ni, Ni(1-x)Si(x) ZrB2, ZrC, Ni(1-x)Si(x), ZrO2,
(C-CCM + EM) Fe2Si, Ni4N
*Thickness of the oxidized layer of the EP718-ID alloy is 1 |im (without allowance for the oxidized layer that was removed).
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It was impossible to measure the wear track depth when the load exerted by the counter body on the sample was minimal (1 N), since only some wear marks could be seen on the sample surface. When the load was increased to 5 N, the wear rate of the coated samples was approximately twofold lower than that for nickel alloy samples.
According to these findings, it is recommended that the Zr-Si-B electrode materials are deposited in argon or under vacuum for obtaining high-quality coatings.
This work was carried out with financial support from the Ministry of Education and Science of the Russian Federation in the framework of state assignment no.11.7172.2017/8.9.
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