TaC-based wear-resistant coatings obtained by magnetron sputtering and electro-spark deposition for wedge gate valve protection

Introduction

One of the problems that modern production faces is the wear of parts and metal structures during operation. To enhance wear resistance and extend the service life of these components, a viable approach involves modifying only the surface of the products through the application of protective coatings. Two prominent coating deposition technologies, electro-spark deposition (ESD) [1] and magnetron sputtering (MS) [2] are promising coating deposition technologies in demand in the industry.

ESD is based on the phenomenon of electric erosion occurring during spark discharge, which leads to the transfer of erosion products from the electrode surface to the substrate. This method offers several advantages, including high adhesion, the ability to perform localized surface treatment, minimal thermal impact on the substrate, and relaxed requirements for surface preparation before application [3–5]. On the other hand, the technology of MS involves coating formation through atomic fluxes during vacuum atomization of the cathode material, facilitated by the flow of anomalous glow discharge. This method is characterized by its versatility in working with various substrates, and the resulting coatings demonstrate a low concentration of defects, a dense homogeneous structure, and a uniform distribution of elements throughout the coating’s thickness [6–8].

Until recently, the utilization of ESD and MS techniques had primarily been confined to applications within the domain of metalworking tools, encompassing cutting, stamping, or rolling tools. The effectiveness of the ESD approach in enhancing the durability of tools such as rolling mills [9], drills [10] and stamps [11] is noteworthy. Similarly, the favorable outcomes of employing MS coatings have been documented in the reinforcement of components like punches [12], cold rolling rollers [13], cutting lathe plates, end mills [14–16], and stamps [17].

In recent years, the range of applications for strengthening protective coatings through ESD and MS processes has significantly broadened. Notably, efforts have been directed towards the application of MS coatings to heat-generating element pipes [18; 19] and boiler components for biomass combustion [20]. ESD coatings have exhibited impressive performance when utilized on blades within the high-temperature zone of gas turbine engines [21], gas turbines within thermal and nuclear power plants [22], as well as on bearings [23], components of internal combustion engines [24], elements of power hydraulic cylinders [25], pumps within tractor hydraulic systems and agricultural machinery [26], and marine infrastructure installations [27], among others. Additionally, the application of ESD coatings to rods of hydraulic cylinders for drilling pumps [28] has shown promising outcomes.

The potential effectiveness of the MS and ESD methods finds applicability in a range of objects, including components within isolation valves employed within the oil and gas sector. These components are susceptible to significant wear resulting from abrasion and the impact of abrasive particles. The wear endured by these valve components stands as a prevalent cause of equipment malfunctions and incidents [29]. The matter of intensified wear and ensuing failures of these elements is explored in scholarly works [30; 31]. A remedy to address this challenge lies in the coating of isolation valve components. Metallic coatings (Zn, Cu, Al–Cr) applied through galvanic and ion-plasma techniques, as well as via plasma sputtering and laser cladding, have been examined in this context. These investigations have indicated the potential of utilizing coatings characterized by heightened density and enhanced resistance to corrosion [32]. Numerous strategies aimed at extending the operational life of isolation valve elements through ion-plasma technologies, particularly MS, have been detailed in [33].

Tantalum carbide is emerging as a promising material for electrode applications in both magnetron sputtering and electro-spark deposition methods. It finds application as a protective coating due to its notable properties: high hardness (from 25 to 45 GPa), elastic modulus (300–450 GPa), resistance to abrasion, corrosion and oxidation, as well as excellent thermal stability (up to 2000 °C) [34; 35]. To mitigate the propensity of TaC to exhibit brittle behavior under loading and the potential for fracture due to cracking, binary coatings are imbued with specific elements. These elements include:

– elements that demonstrate solubility within the basic phase (Cr, Mo, V, Ni, Zr, etc.). The integration of these elements significantly enhances the properties of base coatings by inducing lattice deformation through the creation of new solid solutions [36];

– amorphous elements like Si and B, which play a pivotal role in reshaping the structure of the coating. This leads to the formation of a nanocomposite characterized by superior mechanical properties, wear and corrosion resistance [37–39].

The objective of this study was to generate wear-resistant coatings using tantalum carbide as a foundation, employing both magnetron sputtering and electro-spark deposition techniques. These coatings were intended to provide protection for components utilized in isolation valves.

Experimental

Ta–Zr–Si–B–C coatings were deposited by magnetron sputtering using a TaSi₂–Ta₃B₄–(Ta, Zr)B₂ ceramic target (composition, wt. %: 70.8 Ta, 18.6 Si, 7.4 Zr and 2.9 B). The target with a diameter of 120 mm and a thickness of 6 mm, was produced by hot pressing of milled products obtained from self-propagating high-temperature synthesis [40]. The coatings were deposited using a UVN-2M vacuum installation (AO Quartz, Russia) [41]. The magnetron was powered by a Pinnacle+ 5×5 source (Advanced Energy, USA). Power, voltage and current parameters were 1 kW, 500 V and 2 A, respectively. The coatings were applied in an Ar + C₂H₄ gas mixture, where Ar (99.9995 %) and C₂H₄ (99.95 %) gases were used. The gas flow rate was controlled by gas supply system (Eltochpribor, Russia), maintained a flow rate of 15 sccm Ar and 10 sccm C₂H₄ . The operating conditions were a residual pressure of ~10\(^–\)³ Pa and a working gas pressure of 0.1–0.2 Pa. The deposition procedure was carried out for 40 min.

The vacuum electro-spark deposition method [36; 42] was employed to administer a TaC–Cr–Mo–Ni coating, using a TaC–Fe–Cr–Mo–Ni electrode. The preparation of electrodes involved the cold pressing of powders: Cr grade RN-1S (particle size fraction <60 μm), Ni (PNK-0T2, <20 μm), Mo (PM99.95, <5 μm) and TaC (MRTU 9-09-03443-77, <5 μm). The powders were blended in specified proportions (wt. %: 67.5%TaC–12.5%Mo–7.5%Ni–12.5%Cr) using a planetary mill, Activator-4M (Russia). The application of coatings was carried out under the subsequent technological conditions:

– electrode rotation speed 1000 rpm;

– electrode scanning rat 500 mm/min;

– scanning step 0.5 mm;

– electrical pulse frequency 100 Hz;

– pulse voltage 100 V;

– pulse duration 50 μs;

– working pressure within the vacuum chamber 0.5 Pa;

– application medium – Ar.

Model substrates for coating deposition consisted of PH1 steel discs (wt. %: 77.2%Fe–14.6%Cr–3.8%Ni–3.6%Cu–0.8%Si) measuring 45 mm in diameter. Coatings were also applied onto the wedge and seats of wedge gate valves composed of PH1 steel. Prior to coating, the substrates underwent cleaning in isopropyl alcohol using a UZDN-2T unit (Russia), operating at a frequency of 22 kHz for 5 min. Preceding the application of coatings via the MS method, the substrates were further cleaned in a vacuum environment using an ion source (Ar⁺ ions, 2 keV) for a duration of 20 min.

The coatings elemental composition and structure were studied using the following methods:

– scanning electron microscopy (SEM) employing an S-3400 microscope (Hitachi, Japan) equipped with a Noran-7 Thermo attachment for energy dispersive spectroscopy (EDS);

– glow discharge optical emission spectroscopy (GDOES) using a Profiler 2 installation (Horiba JY, France);

– X-ray diffraction (XRD) employing a D2 Phaser diffractometer (Bruker, Germany).

The mechanical properties of the coatings were determined using nanoindentation carried out with a precision Nano-hardness tester (CSM Instruments, Switzerland) equipped with a Berkovich indenter at a load of 8 mN.

For the purpose of tribological assessment, coatings and steel substrates underwent testing at Tribometer (CSM Instruments, Switzerland), equipped with a reciprocating module. When using a steel counterbody, intensive sticking of wear products to the surface of a harder coating occurs, which makes it difficult to assess wear resistance [43]. Test parameters included a load of 2 N, linear velocity of 0.3 cm/s and 300 cycles. Wear tracks of the coatings were examined through optical profilometry, utilizing a WYKO NT1100 instrument (Veeco, USA). The wear areas of the counterbody were analyzed using an Axiovert 25 optical microscope (Carl Zeiss, Germany). The calculation of reduced wear values for both the coatings and counterbody followed the methodology detailed in [44].

Pilot tests were conducted to assess the tightness of the steel gate valve with a rising spindle, featuring a coating applied on the wedge and seats of the locking mechanism. The tests were carried out on a certified stand in compliance with the State Standard GOST 33257 and Specifications TU 3741-001-22986183-2009. The testing took place on a certified test bench utilizing precision control instruments. The testing was performed using water as the testing medium at a temperature of 20 ± 5 °C, with a pressure of 18.0 MPa. The tests were carried out continuously until the maximum number of “open-close” cycles was reached, as determined by the criterion indicating the onset of valve leakage (loss of tightness).

Results and discussion

Table 1 provides the elemental composition and coa­ting thickness details. The MS coating contains a higher content of TaC (45 at. %), which is 30 % more than the carbide phase content in the ESD coating (32 at. %).

Figures 1, a, b show cross sections SEM images and XRD patterns of the MS and ESD coatings. These images show a homogeneous and defect-free microstructure. The coating thickness was measured as 7 µm for MS and 54 µm for ESD coatings. In the ESD coating, tantalum carbide particles uniformly distributed throughout the entire volume, were observed. These particles had sizes up to 0.2 µm and were embedded in an iron-based metal matrix containing alloyed by Cr, Ni, and Mo. Additionally, localized areas featured larger TaC grains, measuring up to 5 μm. The initial roughness of the ESD coating was determined as R_a = 2.2 µm, while that of the MS coating was 15 nm.

The XRD patterns of the MS and ESD coatings show peaks at 2θ = 34.9, 40.5 and 58.6°, corresponding to the (111), (200), and (220) planes of the TaC FCC phase (JCPDS 89–3831) (as shown in Fig. 1, c).

The broadened peaks observed in the MS coating suggest the existence of an amorphous matrix around TaSi₂, with the inclusion of zirconium and boron in a dissolved state [38]. In the XRD pattern of the ESD coating, supplementary peaks at positions 44.5 and 64.8° correspond to a solid solution founded on alpha iron α-Fe(Cr, Ni, Mo). The crystallite size of the TaC phase, deduced from the most prominent line (111), was ~3 nm for the MS coating and ~30 nm for the ESD coating. For the MS coating the lattice parameter (a) was measured at 0.447 nm, whereas it was 0.441 nm for the ESD coating. This slightly deviates from the value of a = 0.445 nm, established for the TaC powder standard (JCPDS 89-3831 card). Such variation could be attributed to the presence of compressive stresses (for the MS coating) [45] and tensile stresses (for ESD) [46], or a divergence in the composition of the TaC phase from its stoichiometric state [47].

The mechanical properties of the coatings and substrate, including hardness (H), Young’s modulus (E) and elastic recovery (W) are shown in Table 2.

The enhanced mechanical properties exhibited by the MS coating could be attributed to its significant content of the hard carbide phase TaC [49; 50] as well as its finely crystalline structure [38; 51; 52].

Both the ESD and MS coatings demonstrated consistently low coefficients of friction, measuring µ = 0.15 (as illustrated in Figure 2, a, and detailed in Table 2). Conversely, for the steel substrate, the friction coefficient (µ) displayed a gradual increase from 0.17 to 0.65 during the initial 120 cycles. Following the initial wear-in phase, the friction coefficient stabilized at 0.73. Consequently, the deposition of MS and ESD coatings reduced this parameter by 5 times compared to a steel substrate. This reduced friction coefficient exhibited might be attributed to the beneficial effect of free carbon, which in certain circumstances can be released during the oversaturation of the crystalline carbide phase. This unbound carbon could function as a solid lubricant during friction [53].

In Fig. 2, b 2D profiles of wear tracks are presented. The track depth for both MS and ESD coatings remained within the range of surface roughness and did not exceed 150 nm. Conversely, for the steel substrate, the track depth measured 8 µm. The reduced wear (V_w), calculated from these profiles, was below 10\(^–\)⁷ mm³/(N·m) for both MS and ESD coatings, while for the steel substrate, it was 1.2·10\(^–\)³ mm³/(N·m) (as indicated in Table 2). Notably, the application of MS and ESD coatings substantially enhanced the wear resistance of the steel substrate.

Micrographs of the tribological contact zones on the surface of the Al₂O₃ ball are depicted in Figure 3. In the instance of the MS coating and the steel substrate, a slight sticking of wear products was identified, making it challenging to precisely determine the Vw values for the counterbody. On the other hand, for the ESD coating, the contact area is free from sticking. In this particular scenario, the reduced wear of the Al₂O₃ ball was 4.7·10\(^–\)⁷ mm³/(N·m).

Tests conducted on a steel wedge gate valve revealed that the number of operational cycles before valve leakage for components with MS and ESD coatings reached 3750 and 5100 cycles, respectively. In contrast, steel components managed to withstand only up to 3000 cycles before experiencing leakage. This indicates that the application of MS and ESD coatings extends the service life of a steel wedge valve by 25 and 70 %, respectively.

Fig. 4, a presents the appearance of parts coated with MS, along with SEM micrographs of tribological contact regions following pilot tests. Scratches were evident on the part’s surface within the tribological contact area, signifying the abrasive nature of wear. Based on SEM and EDS data, three distinct zones can be identified along the inner edges of the tribological contact segments:

1 – corresponds to the original MS coating with a high carbon content on the surface;

2 – corresponds to the coating and oxidized wear products of the substrate;

3 – refers to the substrate material.

In figure 4, b the surface of a steel wedge gate with ESD coating is depicted, accompanied by SEM images of segments within the region of tribological contact following pilot tests. Prior to the tests, ESD coatings were subjected to lapping on a lapping plate utilizing 6 µm dispersed diamond powder, achieving a roughness of 500 nm. The SEM image of the ESD-coated part surface exhibits two distinctive areas with varying contrast: the first, appearing as light gray, corresponds to the worn surface of the steel sample; the second, appearing as dark gray, corresponds to the coating with the following composition in at. %: 10 Ta, 11 C, 67 Fe, 9 Cr, 2 Mo and 1 Ni. Within the region of tribological contact, wear products emerged, consisting of a blend of iron oxide and chromium. These wear products were incorporated into scratches through the frictional process (as depicted in Fig. 4, b).

Consequently, no differences were observed in the wear mechanism between the MS and ESD coatings. The thickness of the coating stands out as a pivotal factor influencing wear resistance. The ESD coating, boasting a greater thickness, exhibited the most favorable wear resistance. An advantage of MS coatings lies in the elimination of the need for additional smoothing machining.

Conclusions

1. Ta–Zr–Si–B–C and TaC–Fe–Cr–Mo–Ni coatings based on tantalum carbide were successfully manufactured using magnetron sputtering and electro-spark deposition. The MS coating 7 μm thick consisted of TaSi₂-based amorphous matrix with dissolved zirconium and boron and TaC crystallites up to 3 nm in size. Meanwhile, the ESD coating 54 µm thick had an alpha-iron matrix and TaC crystallites of up to 30 nm.

2. The TaC concentration in the MS coating exhibited a 30 % increase compared to the ESD coating, consequently endowing it with higher hardness (H = 28 GPa versus 10 GPa).

3. Both coatings exhibited a low friction coefficient of 0.15. The minimized wear was below 10\(^–\)⁷ mm³/(N·m), while for the steel substrate, it measured 1.2·10\(^–\)³ mm³/(N·m). The application of the developed MS and ESD coatings led to a substantial fivefold reduction in the friction coefficient and an enhancement of the service life of the steel wedge gate valve by 25 and 70 %, respectively.