Ion-plasma erosion-resistant nanocoatings based on metal carbides and nitrides

The erosion, corrosion, and heat resistance of alloy/ion-plasma nanolayer coating compositions based on TiC and CrC carbides and TiN, CrN, ZrN, and AlN nitrides are studied. The effect of the nanolayer thickness, composition, and structure of the coatings based on the metal nitrides and carbides on the relative erosion resistance of alloy/coating compositions in a gas-abrasive quartz sand flux is studied at a sand grain size of 300–350 μm, abrasive supply rate of 200 g/min, and an angle of flux incidence of 20° (tangential flow) and 70° (near-head-on attack flow). It is shown that high erosion resistance is characteristic of 15–22 μm thick coatings that are produced by assisted deposition and consist of alternating TiN (70 nm) and CrN (70 nm) layers on a VT1-0 titanium alloy or TiC (70 nm) and CrC (70 nm) layers on an EP866 compressor steel.     

Properties of nanolayer erosion-resistant coatings based on metal carbides and nitrides

Vacuum-arc ion-assisted deposition is used to form nanolayer protective 2D coatings based on the nitrides or carbides of titanium and chromium, vanadium carbide, and aluminum nitride with a layer thickness of 5–80 nm and a total thickness up to 25 μm. The phase composition of the coatings is studied after deposition and tests. Titanium alloy VT1-0 (EP866 steel)-nanolayer coating compositions are subjected to hot-strength and rapid cyclic corrosion tests, and the erosion resistance of the 2D nanolayer coatings in a dust-air flux (the average fraction of quartz sand is 300–350 μm) is studied. Among the 2D nanolayer coatings on titanium and steel substrates, a composition of VT1-0 alloy with a TiN/CrN coating at a nanolayer thickness of 60–70 nm and a total thickness of 19 μm has the maximum erosion resistance. The erosion resistance of the TiN/CrN coating is shown to decrease with decreasing nanolayer thickness, and it has a high thermal stability after holding at 700°C for 100 h.     

Erosion-resistant coatings for gas turbine engine compressor blades

The erosion-resistant ZrN and Cr₃C₂ coatings intended for the protection of the titanium and steel blades in a GTE compressor are studied. The erosion resistance of the substrate–coating composition is shown to depend on the coating thickness, the deposition conditions, and the coating texture. Ion-assisted deposition changes the structure–phase state of a coating and substantially increases its erosion resistance. It is found that a nanolayer 2D TiN/CrN coating with an average nanolayer thickness of ~60 nm is the best erosion- corrosion-resistant coating for titanium alloys and that a (NiCrTiAlHf)C + CrC coating formed by ionassisted deposition is the best coating for steels. The testing of alloy VT8 compressor blades in an engine supported high protective properties of the nanolayer TiN/CrN coating.     

Deposition of Amorphous Hardening Coatings by Electrospark Treatment in a Mixture of Crystalline Granules

This article is devoted to the formation of amorphous coatings on the steel 35 surface by electrospark treatment in a mixture of crystalline granules. It is revealed by the energy dispersive X-ray spectroscopy (EDS) that formed coatings contain W, Mo, Co, and Ni in various ratios. The weight of granules of various compositions decreases by 11–16 wt % for 6-h treatment due to electric erosion. The mass transfer coefficient varies in a range from 33 to 54%. X-ray structural analysis showed the prevalence of an amorphous phase (81–99%) in the structure of deposited layers. Annealing of coatings at a temperature above 1150°C leads to the crystallization of the amorphous phase into boron carbide of the M23(C, B)6 type, as well as into α-Fe. The coatings have an increased hardness of 10–15 GPa, while their wear resistance in the dry sliding wear mode under loads of 10 and 50 N is higher than for steel 35 by a factor of 3.3 and 1.6. The coating friction coefficient is lower than for steel 35 by 13–30% and was 0.27–0.31. The wear resistance of coatings in a dry abrasive wear mode is higher by a factor of 3–5 when compared with uncoated steel 35. The best characteristics are inherent without nickel and worst are inherent without cobalt. Thus, it is established that tungsten and cobalt increase the wear resistance of iron-based amorphous alloys, while nickel and molybdenum tend to worsen their tribotechnical behavior.     

Effect of Coatings on the Heat Resistance of VT-41 and VIT1 Alloys during Isothermal Oxidation

The influence of aluminide coatings on high-temperature α + β VT-41 alloy and a VIT1 alloy based on the Ti₂NbAl orthophase on the isothermal heat resistance at 700°C is studied during oxidation in still air. NiCrAlY and Al(Si) coatings are found to substantially increase the heat resistance of the alloys at 700°C. The phase composition of the aluminosilicide layer (TiAl₃ + TiSi₂ + Ti₅Si₃) under the NiCrAlY coating significantly retards the titanium diffusion to the NiCrAlY layer and can be used as a diffusion barrier and a aluminum source for the NiCrAlY layer during high-temperature oxidation.     

Investigation of a wear-and corrosion-resistant coating of the TiCN−Ni-alloy system, obtained by high-speed gas-flame spraying

The structure and properties of high-speed gas-flame (HSGF) coating of the system TiC_0.14N_0.86−Ni(CrBSi)-alloy (70%) on high-strength cast iron have been studied. The HSGF coatings were deposited on the Struya installation at the Institute for Problems of Materials Science, operating on an oxygen-hydrogen fuel mixture in a nearly stoichiometric ratio. The pressure in the combustion chamber was 0.6 MPa, the gas consumption was 8.5 g/sec, the sprayed powder consumption was 2.2 g/sec, the transporting air consumption was 0.5 g/sec, and the spraying distance was 200 mm. The coating was about 1 mm thick with a porosity of less than 2% and had a typical layered structure. The alternating zones were enriched with a refractory phase or metal. In comparison with the alloy WC (8% Co) the coating had lower linear wear (<5 μm/km) and a lower friction coefficient (0.1), and its resistance to high-temperature corrosion was almost an order of magnitude higher. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(408), pp. 40–44, July–August, 1999.     

Effect of combination of gas phase deposition and electric-spark alloying on wear resistance of VK8 (WC-8% Co) hard alloy

The composition and structure of heterophase coatings on VK8 (WC-8% Co) hard alloy cutting blades (inserts) were studied. The coatings were obtained by the methods of CVD of TiC with double magnetic-abrasive treatment and electric-spark alloying (ESA) with a TiCN-AlN-based composite as well as by combining these two methods. The wear resistance of WC-Co cutting blades with combination coatings was the best in tests on cutting the materials from SCh20 cast iron. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(450), pp. 49–55, July–August 2006.     

Heat-resistant coatings for the high-pressure turbine blades of promising GTEs

Heat-resistant coatings are considered for the external surface of high-pressure turbine (HPT) single-crystal blades for promising gas turbine engines (GTEs) made from carbon-free nickel superalloys with rhenium or rhenium plus ruthenium. Nickel superalloys covered with heat-resistant coatings consisting of heat-resistant connecting layers and an external ZrO₂-(7–8 wt %)Y₂O₃ ceramic layer are subjected to heat resistance and high-temperature tests. The test results are used to choose the heat-resistant layer that ensures the highest properties of a composition heat-resistant coating. The use of sequential chemical and physical deposition methods for coating layers is shown to be required to protect HPT blades in promising GTEs. Medium-frequency magnetron plasmachemical deposition of ceramic layers in heat-resistant coatings with a low thermal conductivity is found to be promising.     

Plasma coatings of (TiCrC)-(FeCr) composite powder alloys: Structure and properties

The coatings of the (TiCrC)-(FeCr) composite are deposited on steel and titanium alloy by plasma method. The composition, structure, and tribotechnical properties of these coatings are studied in comparison with traditional materials based on the Ni-Cr alloy. The effect of preliminary surface treatment methods, i.e., sandblasting treatment and electrospark alloying, on coating properties is examined. The fretting corrosion of coatings is investigated. It is established that coatings based on double titanium-chromium carbide have considerably greater wear resistance than that of Ni-Cr alloys at almost equal friction coefficients. It is established that electrospark alloying is competitive with traditional sandblasting treatment in environmental effect and coating-to-based adhesion. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 3–4 (454), pp. 37–45, 2007.     

Correlation between Microstructures and Oxidation Resistance in Zr-Nb-Ti Alloys

Oxidation behavior of Zr-10Nb-10Ti and Zr-10Nb-20Ti (compositions are in atomic percent) alloys has been investigated in air between 300 °C and 700 °C. Higher Ti content in the alloy enhances the oxidation resistance. The calculated isotherms by PANDAT[1,2] show that 20Ti enters a three-phase (αZr-hcp, βNb-bcc, and βZr-bcc) region at 500 °C, while 10Ti alloy continues to be a two-phase (αZr and βNb) alloy until 550 °C and then enters the three-phase (αZr, βNb, and βZr) region. Both alloys have a single-phase βZr solid solution at 700 °C, which is detrimental for the oxidation resistance. The βNb phase greatly contributes to the oxidation resistance in these two alloys. The common oxidation products have been identified as TiO₂, ZrO₂, and Nb₂O₅. Both alloys suffer from pest oxidation at temperatures between 500 °C and 550 °C, respectively (20Ti and 10Ti), up to 700 °C. X-ray diffraction (XRD) indicates strong peaks for monoclinic structure of ZrO₂ at temperatures above 600 °C.     

Environmental Testing of Iron-Based Amorphous Alloys

Iron (Fe)-based amorphous alloys possess enhanced hardness and are highly resistant to corrosion, which make them desirable for wear applications in corrosive environments. It was of interest to examine the behavior of amorphous alloys during anodic polarization in concentrated salt solutions and in the salt-fog testing. Results from the testing of one amorphous material (SAM2X5) both in ribbon form and as an applied coating are reported here. Cyclic polarization tests were performed on SAM2X5 ribbon as well as on other nuclear engineering materials. SAM2X5 showed the highest resistance to localized corrosion in 5 M CaCl₂ solution at 105 °C. Salt fog tests of 316L SS and alloy 22 coupons coated with amorphous SAM2X5 powder showed resistance to rusting. Partial devitrification may be responsible for isolated pinpoint rust spots in some coatings. This article is based on a presentation given in the symposium entitled “Materials Issues for Advanced Nuclear Systems,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the Corrosion and Environmental Effects Committee of ASM-TMS.     

Mechanical properties of eutectic alloys β-NiAl + γ-Re and spray coatings

We have studied the structure and mechanical properties of eutectic alloys β-NiAl + γ-Re of the ternary system Ni-Al-Re. We have established that the best combination of mechanical characteristics, determined by local loading with a rigid indentor, is exhibited by the alloy containing 2.5 at.% Re, the structure of which consists of the eutectic β-NiAl + γ-Re. Rhenium inclusions can inhibit movement of cracks in the material, and also can play the role of traps for cracks. Brittle intercrystallite fracture is characteristic of the alloy consisting of one-phase intermetallic NiAl. Mixed fracture is typical of the eutectic alloy β-NiAl + γ-Re, with transcrystallite cleavage predominating. We have shown that plastic interlayers of a rhenium phase within the microstructure increase the crack resistance of a detonation coating made from eutectic alloys β-NiAl + γ-Re. __________ Translated from Poroshkovaya Metallurgiya, Nos. 3–4(448), pp. 78–87, March–April, 2006.     

Wear resistance of a plasma-electrospark zirconium diboride-based coating on aluminum alloy D16T

The paper examines the structure, phase composition, and mechanical properties of ZrB₂-based plasma coatings formed on D16T aluminum alloy under different conditions. It is established that coatings with an electrospark sublayer are characterized by stronger adhesion with the substrate as compared with that deposited on the base after conventional sandblasting. The wear resistance of this coating in dry friction is comparable with the monolithic VK15 hard alloy. Laser treatment of the coating in open air decreases the wear by 25% at low sliding rates and simultaneously decreases the hardness by 50%. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 5–6 (455), pp. 53–59, 2007.     

Examination of Galvanic Action between Fe-Based Bulk Metallic Glass and Crystalline Alloys

Fe-based bulk metallic glasses (amorphous metals) have been developed, and several compositions are shown to have excellent corrosion resistance in chloride solutions. Further, thermal-spray amorphous metals are being developed for use as a barrier coating layer, to protect substrate materials from corrosion. Galvanic action between dissimilar metals and the coating/substrate for the amorphous-alloy coatings is of practical interest for a number of applications. The mixed-potential theory provides a useful approach for examining the corrosion behavior of the component materials in the galvanic couple and is applied in this study. Galvanic action was studied for an Fe-based structurally amorphous metal (SAM) 1651 and several crystalline alloys that included 1018 C-steel, stainless steel (SS) 316L, and alloy 22. Anodic and cathodic polarization curves of each of the metals were measured by potentiodynamic polarization. Based on the mixed-potential theory, the behavior of the component materials in a galvanic cell was predicted. The predictions are compared to the measured behavior of galvanic couples with the crystalline alloys. This article is based on a presentation given in the symposium entitled “Iron-Based Amorphous Metals: An Important Family of High-Performance Corrosion-Resistant Materials,” which occurred during the MSandT meeting, September 16–20, 2007, in Detroit, Michigan, under the auspices of The American Ceramics Society (ACerS), The Association for Iron and Steel Technology (AIST), ASM International, and TMS.     

Nonsensitizing Zr–O–N Coatings for Jewelry Made of Nonprecious Alloys

In recent decades, jewelry manufacturers have started using nonprecious alloys to decrease production costs. A large number of customers, however, have an allergic (sensitizing) body reaction to jewelries made of such materials. The application of nonsensitizing coatings can decrease the negative influence of the jewelry material on the human organism. One material biologically inert towards human body tissues is zirconium. In this study, zirconium-based coatings applied by magnetron sputtering are examined. Eleven regimes of applying zirconium oxynitride coatings onto a substrate of AISI 430 grade steel are investigated. Microhardness and corrosion tests of the coatings in Hank’s solution are conducted and color performance is determined in the CIE 1976 L*a*b* and RGB color spaces. The coating width is 0.4–1.2 μm. It is established that the coatings have a microhardness of 2.5–3.0 GPa and are able to imitate colors of jewelries. Using energy dispersive X-ray spectroscopy, it is established that the coatings consist of Zr, N, and O. The authors select sputtering conditions that produced metallic coatings with high optical reflectance in an energy range near the infrared region of the spectrum (<1.7 eV) of golden color with high lightness. It has been experimentally proven that the coatings do not corrode in Hank’s solution. The allergy patch test of jewelry with the zirconium oxynitride coating conducted on a respondent with sensitizing reaction to nonprecious alloy jewelry has demonstrated a good result—no signs of skin allergy were observed. The results allow us to recommend magnetron sputtering as a technique for applying zirconium-based coatings onto jewelry of nonprecious alloys.     

Exploratory investigation of Y,La, and Hf coatings for nitridation protection of chromium alloys

Chromium alloys were coated with Y, La, and Hf, either singly or in combination, by pack cementation. Based on ductile-brittle transition temperature, a complex Cr + Y + La coating offered the most potential for protecting chromium alloys from nitrogen embrittlement. For example, the transition temperature for a chromium + 0.17 wt pct Y substrate coated with this complex Cr + Y + La coating was 450 K (350°F) after isothermal exposure in air at 1420 K (2100°F) for 200 hr, compared to a transition temperature of 840 K (1050°F) for the similarly exposed uncoated substrate. Simple yttrium coatings are more attractive as coatings for chromium alloys than simple lanthanum and hafnium coatings.     

Study of Molten Salt Corrosion of High Velocity Oxy-Fuel Sprayed Cermet and Nickel-Based Coatings at 900 °C

Absract Samples of Cr₃C₂-NiCr cermet and NiCrBSi coatings formed by the high velocity oxy-fuel (HVOF) process on Superni 718 superalloy have been corroded in the Na₂SO₄-V₂O₅ molten salt environment at 900 °C under cyclic conditions. The hot corrosion behavior of the coatings has been investigated by means of thermogravimetric analysis, X-ray diffraction, scanning electron microscopy/energy dispersive spectroscopy, and electron probe microanalyzer techniques. Efforts have been made to formulate the mode of corrosion attack. Both the coatings protected the substrate superalloy Superni 718 completely. While protecting the superalloy, the Cr₃C₂-NiCr cermet coating partially oxidized along the splat boundaries up to the coating substrate interface, whereas only the upper part of the coating, to about 100 μm from the surface, oxidized in the case of the NiCrBSi coating. The hot corrosion resistance of both the coatings has been attributed to the formation of protective oxides of chromium/silicon at the surface and at the splat boundaries of the coatings.     

Surface Hardening of Hard Tungsten-Carbide Alloys: A Review

Russian and non-Russian research on the surface hardening of hard tungsten-carbide alloys to improve the wear resistance is reviewed. There is great scope for improving the wear resistance and durability of hard-alloy components by surface strengthening on the basis of various coatings, including coatings with 100-nm structural components. On hard tungsten-carbide alloys, the most common coatings consist of titanium carbide TiC and nitride TiN, characterized by high lattice binding energy and high melting point. If such coatings are applied to hard-alloy tools, the frictional coefficient is reduced by a factor of 1.5–2.0 when cutting steel. The use of a TiN + ZrN ion-plasma coating reduces the frictional coefficient by a factor of 5.9. At present, multilayer coatings are widely employed. The most widespread are TiN + TiC and Al₂O₃ + TiC coatings. Their wear is proportional to the coating thickness. These multilayer coatings still leave room for improvement in the wear resistance of hard alloys. In Russia, the potential of hard alloys with a strength gradient from a ductile and high-strength core to a wear-resistant surface is being explored. At the Research Institute of Refractory Metals and Hard Alloys, a method has been developed for producing alloys with variable cobalt content over the thickness of the cutting insert. That permits change in alloy composition from VK20 to VK2 over the sample thickness. Correspondingly, the wear resistance of the insert’s working section is equivalent to that of VK2 alloy, while the base is able to withstand considerable flexural stress. Recently, cutting tools with a diamond coating on hard alloys have been adopted in practice. To increase the durability of hard-alloy VK inserts, strengthening based on concentrated energy fluxes may be employed. Examples include treatment of hard-alloy surfaces by γ quanta, ion beams, and laser beams, electroexplosive alloying, and electrospark strengthening.     

Oxidation resistance and high-temperature strength of WC-Co-Ni-Re(Mn) hard alloys

The basic regularities of hard alloy oxidation in the WC-Co-Ni-Re(Mn) system have been established. The temperature dependence of bending strength in the operation temperature range of turning tools has been investigated. Cutting blades from the investigated alloys with a modified metal binder have high operational properties, which is confirmed by tests. __________ Translated from Poroshkovaya Metallurgyia, Nos. 7–8(450), pp. 55–59, July–August 2006.