Hard plasma chemical coatings based on silicon carbon nitride

Silicon carbon nitride (SiCN) coatings deposited on a silicon substrate are produced by plasma-enhanced chemical vapor deposition (PECVD) using methyltrichlorosilane (MTCS), nitrogen, and hydrogen as starting materials. The coatings are characterized with AFM, XRD, and FTIR. Their mechanical properties are determined with nanoindentation. The abrasion wear resistance is examined using a ball-on-plane (calowear) test and adhesion to the base using a scratch test. The x-ray diffraction indicates that the coatings produced at moderate F_N are nanostructured and represent β-C₃N₄ crystallites embedded into the amorphous a-SiCN matrix. The coatings deposited at a higher nitrogen flow rate are amorphous. The nanostructure is supposed to result from an increase in hardness (25 GPa) and Young’s modulus (above 200 GPa). The tribological tests have revealed that the friction coefficients of the coatings containing nitrogen are two to three times smaller than those based on SiC and deposited on a silicon substrate. The ball-on-plane tests show that the nanostructured coatings also exhibit the highest abrasive wear resistance. These findings demonstrate that the SiCN films deposited using MTCS show good mechanical and tribological properties and can be used as wear-resistant coatings.     

Electroerosion resistance and structural phase transformations in electrospark and laser deposition of titanium alloys using composite ceramics based on ZrB2-ZrSi2 and TiN-Cr3C2 systems

The paper examines the mass transfer kinetics, structure, phase and chemical compositions, and micromechanical properties of electrospark and laser coatings on titanium alloys (including their combination) deposited using composite materials based on the ZrB₂-ZrSi₂ and TiN-Cr₃C₂ systems. The electrospark deposition of both materials is characterized by a relatively high mass-transfer coefficient (∼40–60%) over a wide range of treatment time t ≥ 1 min/cm². It is determined that after prolonged electrospark deposition (t = 7 min/cm²), ZrB₂-ZrSi₂ coatings have structurally heterogeneous surface with smoothed Ti-alloy localities caused by the melt crystallization and modified with alloying components. It is shown that ZrB₂-based coatings are promising along with conventional wear-resistant coatings based on refractory titanium compounds. __________ Translated from Poroshkovaya Metallurgiya, Vol. 47, No. 1–2 (459), pp. 151–161, 2008.     

Ion-plasma Ti-Al-N coatings on a cutting hard-alloy tool operating under conditions of constant and alternating-sign loads

Structure and phase formation during the deposition of coatings of the Ti-Al-Ni system (Al ≈ 3.5 at %) by the ion-plasma method are investigated. The controlled process parameter was the bias potential (U _b) applied to the substrate made of the VK6 hard alloy. At U _b = 120 V (samples of group 1), titanium nitride close to the stoichiometric composition and solid solution of Al in α-Ti are formed, while at U _b = 120 V (group 2), nonstoichiometric titanium nitride and complex nitride (Ti, Al)N are formed. The hardness and elasticity modulus for coatings of group 1 were equal to 23.8 GPa and 462 GPa, and for group 2 they were 30.8 GPa and 565 GPa, respectively. The latter are characterized by a level of adhesion strength of 53–55 N as opposed to 39–40 N for coatings of group 1. Qualification tests for the durability of the cutting tool with developed coatings are performed. For example, upon turning steel 45, it increases by a factor of 6.3 and for gray cast iron it increases by a factor of 5; during end milling cut of the EI 698-VD alloy it increases by a factor of 2.5.     

Effect of Surface Treatment with Concentrated Energy Fluxes on the Properties of Composite Materials Based on Silicon Carbide and Coatings Made from Such Composites

Based on a study of the structure and composition of the composite ceramic SiC - Al₂O₃ - ZrO₂, its tribomechanical properties and behavior in high-temperature corrosion, we recommend the material for use as sealing elements and for deposition of wear-resistant and corrosion-resistant coatings. We have studied the formation of gradient layers when the ceramic surface is modified with refractory titanium compounds TiN - TiB₂ (1:1) with an Fe(Ni) - Cr - Al undercoat using concentrated solar radiation and when the steel surface is modified with laser irradiation of the SiC - Al₂O₃ - ZrO₂ coats. We have shown that laser modification of steel by the silicon carbide-based composite increases its corrosion resistance by a factor of 4–5 at 800–900 °C. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(444), pp. 91–99, July–August, 2005.     

Characteristics and Plasmochemical Deposition of Coatings Based on Amorphous Hydrogenated Silicon Carbide

The effects of major technological parameters in plasmochemical deposition have been examined as regards the properties of thin-film coatings based on amorphous hydrogenated silicon carbide (a-SiC : H) formed by the use of methyltrichlorosilane. Effects have been established from the substrate temperature, discharge power, and pressure of the reagent gases in the reaction chamber on the plasma performance. The films have been examined by secondary mass spectroscopy, and by infrared and optical spectroscopy, as well as by hardness testing by nanoindentation. Films based on amorphous hydrogenated silicon carbide can be used as wear-resistant coatings for metal-cutting tools and as active layers in semiconductor devices. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(444), pp. 69–79, July–August, 2005.     

Preparation and control mechanism of anisotropic Sm-Pr-Co/Co nanocomposites with unusual continuous soft-phase coatings

To produce nanocomposite materials with high magnetic properties, studies concerning nanostructural processing technologies and control mechanisms are urgently required in aspects of achieving perfect alignment of the hard phase while keeping desired sizes and distributions of the soft phase. In the present study, a designed low-rate electroless deposition method is found to be an effective way in producing strong textured anisotropic Sm-Pr-Co/Co nanocomposites with unusual continuous soft-phase coatings when assembling Co particles on the ball-milled anisotropic Sm-Pr-Co hard phase. The average particle size of the soft-phase coatings is 18–50 nm and the obtained Sm-Pr-Co/Co composites exhibit a high intrinsic coercivity of H_ci = 748 kA/m with an enhanced remanence of M_r = 79 A·m²/g, as compared to H_ci = 836 kA/m and M_r = 68 A·m²/kg for uncoated Sm-Pr-Co hard phase. Moreover, the coating process study reveals a nucleation control mechanism for the formation of the continuous coating structures. Down-sized Sm-Pr-Co/Co nanocomposites with tailored size below 300 nm or even below 100 nm were also produced by this designed method. This study is of theoretical and practical importance for developing advanced nanostructures including the next generation permanent magnets.     

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.     

Nanostructured Ti-Cr-B-N and Ti-Cr-Si-C-N coatings for hard-alloy cutting tools

Nanostructured Ti-Cr-B-N and Ti-Cr-Si-C-N coatings with various contents of chromium and nitrogen are obtained by the magnetron sputtering of multiphase composite targets. Their structure and phase composition are investigated by X-ray phase analysis, transmission and scanning electron microscopy, X-ray photoelectron spectroscopy, and optical emission glow-discharge spectroscopy. The Ti-Cr-B-N and Ti-Cr-Si-C-N coatings are based on the fcc phase with texture (100) and crystallite size <25 nm. The Si₃N₄-based hexagonal phase was also revealed in the Ti-Cr-Si-C-N coatings. An investigation into the properties of coatings with the use of methods of nanoindentation, scratch testing, and by performing tribological tests showed that they have a hardness of up to 30 GPa, an adhesion strength no lower than 35 N, and their friction coefficient falls in the range of 0.35–0.57. Coatings also possess high thermal stability, resistance to oxidation, and corrosion stability in a 1N H₂SO₄ solution. The data obtained in tests of hard-alloy cutting tools indicate that the deposition of nanostructured Ti-Cr-B-N and Ti-Cr-Si-C-N coatings increases its resistance by a factor of 11–17.     

Erosion-resistant coatings for gas turbine compressor blades

The effect of ion-plasma coatings made from high-hardness metal compounds on the erosion and corrosion resistance and the mechanical properties of alloy (substrate) + coating compositions is comprehensively studied. The effects of the thickness, composition, deposition conditions, and design of coatings based on metal nitrides and carbides on the relative gas-abrasive wear of alloy + coating compositions in a gas-abrasive flux are analyzed. The flux contains quartz sand with an average fraction of 300–350 μm; the abrasive feed rate is 200 g/min; and the angles of flux incidence are 20° (tangential flow) and 70° (near-head-on attack flow). Alloy + coating compositions based on VN, VC, Cr₃C₂, ZrN, and TiN coatings 15–30 μ m thick or more are shown to have high erosion resistance. A detailed examination of the coatings with high erosion resistance demonstrates that a zirconium nitride coating is most appropriate for protecting gas turbine compressor blades made of titanium alloys; this coating does not decrease the fatigue strength of these alloys. A chromium carbide coating is the best coating for protecting compressor steel blades.     

Changes in Structure and Physicochemical Properties of Spark Coatings Based on TiN and TiB2 after Treatment with Concentrated Solar Radiation

Studies have been made on the structure and properties of spark coatings made of TiN and TiB₂ on steel U8 after treatment with concentrated solar radiation. It is found that the absorption capacity of the steel raised by a factor 2–3. The concentrated radiation reduces the coefficient of friction of the spark coatings made of TiN and TiB₂ by a factor 1.4, while the wear rate is reduced by a factor 1.6–2 by comparison with the untreated material. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(444), pp. 64–69, July–August, 2005.     

Formation of Nanocrystalline Structure of TaSi2 Films on Silicon

The nanocrystalline structure and mechanical properties of TaSi₂ films deposited by sputtering of TaSi₂ target have been investigated by x-ray diffraction, cross-sectional transmission electron microscopy (TEM), four-point electrical resistance measurement, and cyclic depth-sensitive nanoindentation. The purpose of this work is to study the formation of nanocrystalline structure in TaSi₂ films on a silicon substrate. As revealed, a decrease in the deposition rate leads to an increase in the O and C impurity content in the films. Contamination of the film by O and C atoms during a low-rate deposition causes the formation of an amorphous phase in the deposited films. Upon annealing, the amorphous structures crystallize into mixtures of disilicide and a small amount of polysilicide, i.e. TaSi₂ and Ta₅Si₃, respectively. After annealing at 970 K, the formation of a nanocrystalline structure with a grain size about 10 nm takes place in the film produced at a deposition rate of 0.2 nm/sec. The formation of a nanocrystalline structure changes drastically the mechanical properties of the film. The nanohardness and elastic modulus increase significantly, and the film becomes brittle and overstressed. After deposition in the film produced at the 1 nm/sec deposition rate mainly Ta disilicide and the amorphous phase are observed. After annealing, the amorphous phase near the Si substrate coexists with column-shape grains of Ta disilicide of size 150 × 500 nm. The annealed thin film becomes nonuniform in thickness. The nanohardness and elastic modulus increase.     

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.     

Ni-Ni3B composite coating

The paper compares the microstructure and tribotechnical characteristics of composite coatings of 70 wt.% Ni-20 wt.% Cr-5 wt.% Si-5 wt.% B. These coatings are produced by slip casting and unidirectional solidification. The coating composition is substantiated and coating restructuring mechanisms in the forming process are discussed. Natural composite Ni-Ni₃B forms the basis of the coating. The fine lamellar eutectic, which occupies ∼50.5% of the coating, consists of Ni₃B, a nickel solid solution doped with chrome and silicon, and CrB crystals. The microhardness of the coating and phase constituents are determined. The method of contact eutectic vacuum melting with controlled cooling permits obtaining the nonporous coating without slag inclusions and with homogeneous oriented structure. The mechanism of 3 to 5 times increase in the wear resistance of homogeneous nonporous coatings and their potential application in friction units of machinery are discussed. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 1–2(453), pp. 40–47, 2007.     

Structural aspects of strength and methods to increase the serviceability of carbide coatings

The effect of composition and structure on the microhardness, plasticity parameter (δ_H) and fracture toughness (K_Ic) of the widely used carbide layers (TiC, ZrC, VC, NbC, Cr₂₃C₆, Cr₇C₃) was investigated. It was found that the microhardness and plasticity parameter were basically determined by the composition of the carbide coatings and did not substantially vary with the structure, which had a preferential effect on fracture toughness. The laws governing the influence of grain shape, size, and preferred orientation on the fracture toughness of the coatings were determined. Recommendations for the control of composition and structure were formulated, and methods developed for the deposition of carbide coatings which increased the durability of a hard-alloy tool used under severe conditions two-fold compared to the best known analogues. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(411), pp. 97–106, January–February, 2000.     

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.     

Residual stresses in high-velocity oxy-fuel metallic coatings

X-ray based residual stress measurements were made on type 316 stainless steel and Fe₃Al coatings that were high-velocity oxy-fuel (HVOF) sprayed onto low-carbon and stainless steel substrates. Nominal coating thicknesses varied from 250 to 1500 μm. The effect of HVOF spray particle velocity on residual stress and deposition efficiency was assessed by preparing coatings at three different torch chamber pressures. The effect of substrate thickness on residual stress was determined by spraying coatings onto thick (6.4 mm) and thin (1.4 mm) substrates. Residual stresses were compressive for both coating materials and increased in magnitude with spray velocity. For coatings applied to thick substrates, near-surface residual stresses were essentially constant with increasing coating thickness. Differences in thermal expansion coefficient between low-carbon and stainless steels led to a 180 MPa difference in residual stress for Fe₃Al coatings. Deposition efficiency for both materials is maximized at an intermediate (∼600 m/s) velocity. Considerations for X-ray measurement of residual stresses in HVOF coatings are also presented.     

Wear-resistant amorphous and nanocomposite steel coatings

In this article, amorphous and nanocomposite thermally deposited steel coatings have been formed by using both plasma and high-velocity oxy-fuel (HVOF) spraying techniques. This was accomplished by developing a specialized iron-based composition with a low critical cooling rate (≈10⁴ K/s) for metallic glass formation, processing the alloy by inert gas atomization to form micron-sized amorphous spherical powders, and then spraying the classified powder to form coatings. A primarily amorphous structure was formed in the as-sprayed coatings, independent of coating thickness. After a heat treatment above the crystallization temperature (568 °C), the structure of the coatings self-assembled (i.e., devitrified) into a multiphase nanocomposite microstructure with 75 to 125 nm grains containing a distribution of 20 nm second-phase grain-boundary precipitates. Vickers microhardness testing revealed that the amorphous coatings were very hard (10.2 to 10.7 GPa), with further increases in hardness after devitrification (11.4 to 12.8 GPa). The wear characteristics of the amorphous and nanocomposite coatings were determined using both two-body pin-on-disk and three-body rubber wheel wet-slurry sand tests. The results indicate that the amorphous and nanocomposite steel coatings are candidates for a wide variety of wear-resistant applications.     

Preparation and properties of oxide films of the rare earth metals

The properties of films of rare earth metal oxides obtained by evaporation of the metal in an oxygen atmosphere are given. The deposits were highly dispersed and crystallized with a cubic type-Mn₂O₃ structure. The data obtained include: width of the forbidden band (4–6 eV), refractive index (1.89–2.05), specific electrical resistance (10¹⁴–10¹⁵ ω·cm), breakdown voltage (10⁶–10⁷ V/cm), etc. It is shown that films of REM oxides are promising as passive elements of microcircuits, transparent coatings, temperature and heat-flow sensors. These examples by far do not exhaust the possibilities for use of such films. Materials Science Institute. Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 107–111, January–February, 1998.     

Contribution of electrospark alloying to the oxidation resistance of hard tungsten alloys

The paper examines the contribution of electrospark alloying to the oxidation resistance of hard tungsten alloys. It is established that the oxidation of carbides results from their electronic structure. When WC and hard tungsten alloys are heated to 1000°C, a brittle scale consisting of WO₃ and CoWO₄ rapidly forms. The oxidation resistance reduces as follows: TiC → Co → W → HTA (if TiC is more than 10%) → WC-Co → WC. The oxidation rate of hard tungsten alloys may be a criterion of their serviceability. It is shown that the oxidation resistance of hard tungsten alloys becomes much higher after their electrospark alloying with aluminum, titanium, and chromium and with wear-resistant composite TsLAB-2 ceramics based on the ZrB₂-ZrSi₂-LaB₆ system with Ni-Cr-Al (30 mole%) binder. __________ Translated from Poroshkovaya Metallurgiya, Vol. 47, No. 1–2 (459), pp. 145–150, 2008.     

The deposition of aluminide and silicide coatings on γ-TiAl using the halide-activated pack cementation method

The halide-activated pack cementation method (HAPC) was utilized to deposit aluminide and silicide coatings on nominally stoichiometric γ-TiAl. The deposition temperature was 1000°C and deposition times ranged from 2 to 12 hours. The growth rates of the coatings were diffusion controlled, with the rate of aluminide growth being about a factor of 2 greater than that of silicide growth. The aluminide coating was inward growing and consisted of a thick, uniform outer layer of TiAl₃ and a thin inner layer of TiAl₂, with the rate-controlling step being the diffusion of aluminum from the pack into the substrate. Annealing experiments at 1100 °C showed that the interdiffusion between the aluminide coating and the γ-TiAl substrate was rapid. In contrast to the aluminide coating, the silicide coating was nonuniform and porous, consisting primarily of TiSi₂, TiSi, and Ti₅Si₄, with the rate-controlling step for the coating growth believed to be the diffusion of aluminum into the γ-TiAl ahead of the silicide/γ-TiAl interface. The microstructural evolution of the aluminide and silicide coating structures is discussed qualitatively.