Novel transparent conducting sol-gel oxide coatings

This work focuses on the preparation of novel ternary transparent conducting oxide coatings on glass by the sol-gel method. The coatings were deposited by spin-coating from solutions of appropriate metal precursors and heat-treated at different heat-treatment procedures. An increase in electrical conductivity was achieved by a final forming gas treatment. Best electrical and optical properties have been obtained for coatings of crystalline Zn₂SnO₄, Zn₃In₂O₆ and Zn₅In₂O₈ and X-ray amorphous ZnSnO₃ with resistivities in the order of 10^− 2-10^− 1 Ω cm, an average transmission in the visible of 85% and an average surface roughness of ∼ 1 nm. ZnGa₂O₄ and GaSbO₄ coatings showed no electrical conductivity. For Zn₂SnO₄ coatings, a restricted crystallite growth was observed probably due to phase segregation effects. Electrical properties of coatings in the system ZnO-In₂O₃ were interpreted on the basis of the percolation theory.     

Surface modification of Cr3C2–NiCr cermet coatings by direct diode laser

Thermal spraying has emerged as an important tool of increasingly sophisticated surface engineering technology, and it is being used widely to repair and surface modification in metallic parts. The Cr₃C₂–NiCr sprayed coatings are frequently used as wear resistant coatings against abrasion and erosion at high temperature up to 1173 K, and in corrosive environments. Hardness and microstructure of Cr₃C₂–NiCr cermet coatings fused by direct diode laser process was compared with that formed by high-velocity oxygen fuel spraying (HVOF) process. The effect of beam characteristics (power density, power, scanning speed, etc.) was examined on the surface modification of sprayed coatings. In this study, we treated Cr₃C₂–25%NiCr cermet coatings by laser irradiation process and examined its hardness compared with that formed by HVOF process. Consequently, the average hardness of laser-treated Cr₃C₂–25%NiCr cermet coatings has been found out to be higher than that of HVOF coatings. Laser remelting improved markedly the wear resistance of HVOF sprayed Cr₃C₂–25%NiCr cermet coatings.     

Internal oxidation mechanism for Ta-Ru and Mo-Ru coatings

Ta-Ru and Mo-Ru alloy coatings with phases of intermetallic compounds are often used as protective coatings on die materials. After annealing under an oxygen containing atmosphere, the internal oxidation phenomenon resulted in the preferential oxidation of Ta or Mo in the coatings. This process created a phase separated structure consisting of continuous and alternative oxygen rich and deficient layers with a nano scale period beneath the free surface. The experiments in this study deposited Ta-Ru and Mo-Ru coatings with a Cr interlayer by direct current magnetron co-sputtering at 400 °C. Annealing treatments were conducted at 600 °C for short durations under controlled atmospheres consisting of 50 or 10,000 ppm oxygen with residual nitrogen or argon gas. The internal oxidation behavior in the initial stage was determined by evaluating the variations in crystalline structure, surface morphology and chemical composition. The laminated structure was examined by transmission electron microscopy. Results show distinct oxidation behaviors for Ta₄₇Ru₅₃ and Mo₄₆Ru₅₄ coatings. Internal oxidation phenomena appeared in Ta₄₇Ru₅₃ coatings annealed in both 50 and 10,000 ppm O₂ containing atmospheres. The Mo₄₆Ru₅₄ coatings exhibit external oxidation at 50 ppm O₂ containing atmosphere, and internal oxidation at 10,000 ppm O₂ containing atmosphere. Finally, this study proposes an internal oxidation mechanism for alloy coatings with an orientated structure.     

Deposition and characterization of TiAlN/Si3N4 superhard nanocomposite coatings prepared by reactive direct current unbalanced magnetron sputtering

Superhard nanocomposite coatings of TiAlN/Si₃N₄ with varying silicon contents were synthesized using reactive direct current (DC) unbalanced magnetron sputtering. The Si and TiAl targets were sputtered using an asymmetric bipolar-pulsed DC power supply and a DC power supply, respectively, in Ar+N₂ plasma. The structural and mechanical properties of the coatings were characterized using X-ray diffraction (XRD) and nanoindentation techniques, respectively. The elemental composition of the TiAlN/Si₃N₄ nanocomposite coatings was determined using energy-dispersive X-ray analysis and the bonding structure was characterized by X-ray photoelectron spectroscopy. The surface morphology of the coatings was studied using atomic force microscopy. The XRD data showed that the nanocomposite coatings exhibited (1 1 1) and (2 0 0) reflections of cubic TiAlN phase. The broadening of the diffraction peaks with an increase in the silicon content in the nanocomposite coatings, suggested a decrease in the average crystallite size. The TiAlN/Si₃N₄ nanocomposite coatings exhibited a maximum hardness of 43 GPa and an elastic modulus of 350 GPa at a silicon concentration of approximately 11 at%. The hardness and the elastic modulus of the nanocomposite coatings decreased significantly at higher silicon contents. Micro-Raman spectroscopy was used to characterize the structural changes as a result of heating of the nanocomposite coatings in air (400-850 °C) and in vacuum (900 °C). The Raman data of the nanocomposite coatings annealed in air and vacuum showed better thermal stability as compared to that of the TiAlN coatings. Similarly, the nanocomposite coatings deposited on mild steel substrates exhibited improved corrosion resistance.     

Sol-Gel-derived TiO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> implant coatings for direct tissue attachment. Part II: Evaluation of cell response

Silica-releasing sol-gel derived TiO₂–SiO₂ coatings with tailored nanostructure were evaluated in fibroblast and osteoblast cell cultures. The adhesion of both fibroblasts and osteoblasts proceeded within two hours. The highest fibroblast proliferation activities were observed on the TiO₂–SiO₂ (70:30) and (30:70) coatings. However, the cell layer on TiO₂–SiO₂ (30:70) coating was disordered. Prolonged osteoblast activity was observed on the coatings as a function of increased amount of released silica. At day 21 the surfaces were fully covered by the calcified nodules and extracellular matrix except for the coatings TiO₂–SiO₂ (10:90) i.e. having the highest SiO₂ amount. The results suggested that TiO₂–SiO₂ (70:30) was the best for fibroblasts and TiO₂–SiO₂ (30:70) for osteoblasts. The applicability of the sol-gel derived TiO₂ and TiO₂–SiO₂ coatings as an alternative for the calcium phosphate based implant coatings are discussed.     

Microstructure and oxidation-resistant of ZrO2/Ni coatings applied by high-speed jet electroplating

ZrO₂/Ni composite coatings with different contents of ZrO₂ particles were deposited on superalloy K17 substrate using high-speed jet electroplating process. The microhardness and microstructure of composite coatings were studied. The oxidation kinetic curves of uncoated and coated K17 alloys were obtained. The results indicated that ZrO₂/Ni composite coatings exhibit higher microhardness than that of pure nickel coatings under the same high-speed jet electrodeposition conditions. ZrO₂/Ni composite coatings exposure to air at 1000 °C for 5 h formed scale containing NiO and Cr₂O₃; after exposure to air at 1000 °C for 100 h the scale was comprised NiO, NiCr₂O₄, and Cr₂O₃. The formation of Cr₂O₃ scales on the ZrO₂/Ni composite coating directly improved the oxidation resistance of superalloy K17.     

Preparation of CVD diamond coatings on gamma titanium aluminide using MPECVD with various interlayers

CVD diamond coatings were deposited on to γ-TiAl surfaces using a microwave plasma enhanced CVD to improve wear properties and the performance of γ-TiAl. Diamond coatings were directly deposited on to γ-TiAl substrates and deposited on to TiC, Ti₅Si₃, Al₂O₃ + TiO₂, and Si interlayers prepared on γ-TiAl substrates. The diamond coatings deposited directly on γ-TiAl suffered severe delamination and cracked. Those deposited on TiC and Ti₅Si₃ interlayers partially delaminated, whereas those deposited on Al₂O₃ + TiO₂ and Si interlayers adhered well to the underlying surfaces. The diamond films obtained were characterized using scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. Raman spectra showed that polycrystalline and nanocrystalline diamond films grew on γ-TiAl. Residual internal stresses of the diamond coatings deposited on interlayered-γ-TiAl were estimated experimentally from Raman spectra. The coatings prepared on Al₂O₃ + TiO₂/γ-TiAl and Si/γ-TiAl showed lower residual stresses.     

Effects of different acetylene/nitrogen ratios on characteristics of carbon coatings on optical fibers prepared by thermal chemical vapor deposition

The effects of C₂H₂/(C₂H₂ + N₂) ratios on the characteristics of carbon coatings on optical fibers prepared by thermal chemical vapor deposition are investigated. The C₂H₂/(C₂H₂ + N₂) ratios are set to 60, 70, 80, 90, and 100%. Additionally, the deposition temperature, working pressure, and mass flow rate are 1003 K, 133 kPa, and 40 sccm, respectively. The deposition rate, microstructure, and electrical resistivity of carbon coatings are measured. The low-temperature surface morphology of carbon-coated optical fibers is elucidated. Experimental results indicate that the deposition rate increases with increasing the C₂H₂/(C₂H₂ + N₂) ratio, and the deposition process is located at a surface controlled regime. As the deposition rate increases, the electrical resistivity of carbon coatings increases, while the ordered degree, nano-crystallite size, and sp² carbon atoms of the carbon coatings decrease. Additionally, the low-temperature surface morphology of the carbon coatings shows that if the carbon coating thickness is not smaller than 289 nm, decreasing the deposition rate is good for producing hermetic optical fiber coatings.     

Sputter deposited low-friction and tough Cr-Si3N4 nanocomposite coatings on plasma nitrided M2 steel

Nanocomposite coatings of Cr-Si₃N₄ exhibiting low friction and high toughness were prepared on plasma nitrided AISI M2 steel substrates using an unbalanced magnetron sputtering system. The surface morphology and cross-sectional microstructure of the Cr-Si₃N₄ nanocomposite coatings were studied using field emission scanning electron microscopy (FESEM) techniques. Cr-Si₃N₄ nanocomposite coatings prepared at 48 at.% Cr exhibited a dense microstructure with nanoindentation hardness and toughness values of 18 GPa and 2.0 MPam^½, respectively. Nanoscratch measurements indicated that Cr-Si₃N₄ nanocomposite coatings exhibited good adhesion with a maximum critical load of 150 mN. Ball-on-disc reciprocating tests at a load of 2 N showed that Cr-Si₃N₄ nanocomposite coatings prepared at 48 at.% Cr exhibited an average friction coefficient of 0.30. FESEM studies of the wear tracks indicated that there was no significant wear loss and the Cr-Si₃N₄ nanocomposite coatings exhibited only mild wear due to oxidation.     

Structure and mechanical properties of low temperature magnetron sputtered nanocrystalline (nc-)Ti(N,C)/amorphous diamond like carbon (a-C:H) coatings

This paper reports on the structure and mechanical properties of ~ 2 μm thick nanocomposite (nc-) Ti(N,C)/amorphous diamond like carbon (a-C:H) coatings deposited on 100Cr6 steel substrates, using low temperature (~ 200 °C) DC reactive magnetron sputtering. The carbon content was varied with acetylene partial pressure in order to obtain single layer coatings with different a-C:H carbon phase fractions. The nanocrystalline Ti(N,C) phase is approximately stoichiometric for all coatings and the a-C:H phase fraction increases from 31 to 47 at.% as the coatings stoichiometry changed from TiC_1.34 N_0.51 to TiC_2.48 N_0.48, respectively. TiC_1.34 N_0.51 coatings showed the highest nanoindentation hardness (H) of ~ 14 GPa and a modulus (E_r) of ~ 144 GPa; H reduced to < 6 GPa and E_r to < 70 GPa for TiC_2.48 N_0.48 coatings. nc-Ti(N,C)/a-C:H coatings are promising candidates for applications where better matching of the modulus between a relatively low modulus substrate, hard loading support layer and low modulus-high H/E ratio top layer is required.     

Cyclic oxidation behavior of glass-ceramic composite coatings on superalloy K38G at 1100 °C

The SiO₂-Al₂O₃-ZrO₂-CaO-ZnO glass-ceramic composite coatings (GC), nanocrystalline NiCoCrAlY coating, and their combinations (bi-layer GC/NiCoCrAlY) were prepared on K38G specimens. The thicknesses of the glass-ceramic coatings and the NiCoCrAlY coatings were about 10 μm and 20 μm, respectively. Cyclic oxidation tests were carried out at 1100 °C for 120 cycles. Microstructures of the specimens before and after oxidation tests were characterized by SEM, EDS and XRD. The glass-ceramic coatings with or without a NiCoCrAlY intermediate layer improved the isothermal and cyclic oxidation resistance of the Ni-base superalloy K38G at 1100 °C, and performed better than the NiCoCrAlY coatings. An alumina layer formed at the glass/metal interfaces of the specimens coated by the glass-ceramic coatings with or without a NiCoCrAlY intermediate layer. The NiCoCrAlY intermediate layer was beneficial to the cyclic oxidation resistance of the glass-ceramic coatings.     

Effects of aging treatment on microstructure and tribological properties of nickel-based high-temperature self-lubrication wear resistant composite coatings by laser cladding

The NiCr/Cr₃C₂–WS₂ high-temperature self-lubrication wear resistant composite coatings were fabricated on substrate of a hot-rolled AISI304 austenitic stainless steel by laser cladding. The high-temperature phase stability of the composite coatings was evaluated by aging at 600 °C for 10 h, 30 h, 50 h, and the microstructures of the as-laser clad and aged coatings were examined by means of XRD, SEM, EDS, respectively. The sliding wear resistance of the as-laser clad and aged coatings was evaluated at 600 °C. The results show that NiCr/Cr₃C₂–WS₂ composite coating has excellent high-temperature phase stability, the γ-(Fe,Ni)/Cr₇C₃ eutectic phases, Cr₇C₃ and (Cr,W)C hard phases, CrS/WS₂ mixed solid lubricant phases all existed in the as-laser clad and aged coatings. The volume fraction of eutectic phases decreased gradually with the increasing of aged time due to their dissolution. The microhardness of the aged coating decreased slightly after aging the coating 50 h at 600 °C due to the dissolution of the eutectic phases and notable breaking or granulation of the Cr₇C₃ hard phase, but the tribological properties were not significantly affected by aging treatment.     

Microstructure and magnetic properties of plasma-sprayed Fe73.5Cu1Nb3Si13.5B9 coatings

Amorphous and nanocrystalline Fe_73.5Cu₁Nb₃Si_13.5B₉ coatings were formed by plasma-spraying micron-sized powders onto H62 brass substrates and aluminum pipes. The coatings are about 0.2-0.3 mm in thickness with fully dense and low porosity. The microstructure of the coatings is classified into two regions, namely, a full amorphous phase region and homogeneous dispersion of α-Fe nanoscale particles with a scale of 30-70 nm. The hardness of the amorphous and nanocrystalline coatings is about 960 HV_100g. Coercivity (Hc), saturation induction (B₈₀₀), and initial relative permeability (μ_i) of the coatings are 144 A/m, 0.27 T, 249, respectively, under 800 A/m direct current (DC) magnetic field. The magnetic shielding performance is good under DC magnetic field and its magnetic shielding effectiveness (SE) is 10-12 dB at coating thickness of 0.45 mm under static magnetic field of 2-40 Oe. The SE increases by increasing the coating thickness when the magnetic field frequencies are 50, 100 and 200 Hz with an intensity of 0.85 Oe. The results indicate that the amorphous and nanocrystalline alloy coatings can be good for some magnetic shielding applications.     

Modulation periods and ratios induced changes of microstructure and mechanical properties of AlN/ZrB2 multilayered coatings

AlN/ZrB₂ multilayered coatings were synthesized in a magnetron sputtering system. The extensive measurements were employed to investigate the influence of different nanoscale modulation periods and modulation ratios on microstructure and mechanical properties of the coatings. Analysis of X-ray diffraction, profiler and nanoindention indicated that multilayered coatings possessed much higher hardness and elastic modulus than monolithic AlN and ZrB₂ coatings. At the substrate negative bias of −80 V, maximum hardness (34.1 GPa) and elastic modulus (469.8 GPa) were obtained in the multilayer with Λ = 30 nm and t_AlN:t_ZrB2 = 1:3. This hardest multilayer showed a marked polycrystalline structure with the strong mixture of ZrB₂ (001), ZrB₂ (100), ZrB₂ (101), AlN (100) textures.     

Oxidation study of Cr-Ru hard coatings

Cr-Ru alloy coatings with Cr content ranging from 47 to 83 at.% were deposited at 400 °C by direct current magnetron co-sputtering with a Ti interlayer on silicon substrates. With a total input power of 300 W, the Cr content in the Cr-Ru coatings increased linearly with the increasing input power of Cr. The intermetallic compound phase Cr₂Ru with columnar structure was identified for the as-deposited Cr₅₆Ru₄₄ and Cr₆₅Ru₃₅ coatings, resulting in an increase of hardness up to 15-16 GPa. To evaluate the performance of Cr-Ru coatings as a protective coating on glass molding dies, the annealing treatment was conducted at 600 °C in a 50 ppm O₂-N₂ atmosphere. The outward diffusion and preferential oxidization of Cr in the Cr-Ru coatings resulted in the variations of the crystalline structure, chemical composition distribution, and surface hardness after annealing. X-ray diffraction and transmission electron microscopy (TEM) proved that an oxide scale consisting of Cr₂O₃ formed on the free surface. Scanning electron microscopy and TEM observed the surface morphology and structural variation. The chemical composition depth profiles were analyzed by Auger electron microscopy, verifying the presence of a Cr-depleted zone beneath the oxide scale. The hardness of Cr₅₆Ru₄₄ and Cr₆₅Ru₃₅ coatings decreased to 11-12 GPa after annealing, accompanied by the replacement of the Cr₂Ru phase by the Ru phase.     

A comparative study of CrNx coatings Synthesized by dc and pulsed dc magnetron sputtering

Chromium nitride (CrN_x) coatings were prepared by reactively sputtering chromium metal target with various nitrogen flow rate percentages (f_N2) using a closed field unbalanced magnetron sputtering system operated in dc and middle frequency pulsed condition (100 kHz and 50% duty cycle). In this study, plasma examination proved that a large amount of ions with a wide range of ion energies (up to 65 eV and mainly from 10-30 eV region) was identified in the pulsed plasma compared to the low ion flux and energy (0-10 eV) in a dc discharged plasma. The results showed that the phase structure of CrN_x coatings was changed from nitrogen doped Cr(N) to pure β-Cr₂N, and to a mixture of β-Cr₂N and c-CrN and then to pure c-CrN phases with an increase in the f_N2 in both dc and pulsed conditions. However, the pulsed CrN_x coatings exhibit lower N concentrations than dc CrN_x coatings prepared under the same f_N2, which leads to the existing of β-Cr₂N phase within a wide range of f_N2 (30-50%). In comparison with the typical large columnar structure in the dc sputtered coatings, the pulsed CrN_x coatings exhibit dramatic microstructure improvements which benefited from the improved plasma density and ion bombardment from the pulsed plasma, where the super dense and nearly equi-axial structures were observed in a wide range of f_N2. The microstructure improvements contributed to the enhancements in the hardness and wear resistance of pulsed CrN_x coatings. In the pulsed CrN_x coatings, the hardness values were above 30 GPa when the f_N2 is in the range of 30-40%, which is related to the formation of the β-Cr₂N phase. With the formation of a mixture of β-Cr₂N and c-CrN phases in the coatings deposited with 40-50% f_N2, a low COF of 0.36 and wear rate of 1.66 × 10^− 6 mm³ N^− 1 m^− 1 can be achieved.     

Effects of substrate bias voltage and target sputtering power on the structural and tribological properties of carbon nitride coatings

Effects of substrate bias voltage and target sputtering power on the structural and tribological properties of carbon nitride (CN_x) coatings are investigated. CN_x coatings are fabricated by a hybrid coating process with the combination of radio frequency plasma enhanced chemical vapor deposition (RF PECVD) and DC magnetron sputtering at various substrate bias voltage and target sputtering power in the order of −400 V 200 W, −400 V 100 W, −800 V 200 W, and −800 V 100 W. The deposition rate, N/C atomic ratio, and hardness of CN_x coatings as well as friction coefficient of CN_x coating sliding against AISI 52100 pin in N₂ gas stream decrease, while the residual stress of CN_x coatings increases with the increase of substrate bias voltage and the decrease of target sputtering power. The highest hardness measured under single stiffness mode of 15.0 GPa and lowest residual stress of 3.7 GPa of CN_x coatings are obtained at −400 V 200 W, whereas the lowest friction coefficient of 0.12 of CN_x coatings is achieved at −800 V 100 W. Raman and XPS analysis suggest that sp³ carbon bonding decreases and sp² carbon bonding increases with the variations in substrate bias voltage and target sputtering power. Optical images and Raman characterization of worn surfaces confirm that the friction behavior of CN_x coatings is controlled by the directly sliding between CN_x coating and steel pin. Therefore, the reduction of friction coefficient is attributed to the decrease of sp³ carbon bonding in the CN_x coating. It is concluded that substrate bias voltage and target sputtering power are effective parameters for tailoring the structural and tribological properties of CN_x coatings.     

Mechanical properties of gradient and multilayered TiAlSiN hard coatings

Multicomponent coatings based on different metallic and non-metallic elements possess the combined benefit of individual components leading to further improvement of coating properties. In this study, monolayered Ti-Al-N, multilayered Ti-Al-N/TiN, gradient Ti-Al-Si-N, and multilayered Ti-Al-Si-N/TiN coatings were synthesized by using a cathodic-arc evaporation (CAE) system. In addition to Ti, Ti₃₃Al₆₇ and Al₈₈Si₁₂ cathodes were used for the deposition of Ti-Al-N, and Ti-Al-Si-N coatings, respectively. The gradient Ti_0.50Al_0.43Si_0.07N, and multilayered Ti_0.50Al_0.43Si_0.07N/TiN with nanograins separated by disordered grain boundaries possessed lower residual stress (− 2.8 ~ − 4.8 GPa) than that of monolayered Ti-Al-N (− 6.8 GPa) and multilayered Ti-Al-N/TiN coatings (− 5.7 GPa). The highest hardness was obtained for the gradient Ti_0.50Al_0.43Si_0.07N (38 ± 2 GPa) with Ti/(Ti + Al + Si) content ratio being 0.5. On the contrary, the multilayered Ti_0.50Al_0.43Si_0.07N/TiN possessed the highest H³/E^?2 ratio of 0.182 ± 0.003 GPa, indicating the best resistance to plastic deformation, among the studied coatings.     

High emissivity coatings on titanium alloy prepared by micro-arc oxidation for high temperature application

Micro-arc oxidation coatings were prepared on Ti6Al4V alloy in Na₃PO₄-based electrolyte with different additives such as FeSO₄, Co(CH₃COO)₂, Ni(CH₃COO)₂, and K₂ZrF₆. The composition, structure, surface morphology, and chemical state of the coatings were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis system, and X-ray photoelectron spectroscopy, respectively. The spectral emissivity of the coating was measured by Fourier transform spectrometer apparatus. The bonding strength between the ceramic coating and substrate was studied by tensile strength test and shear strength test. The thermal shock resistance of the coatings was evaluated too. The results showed that the thermal emission of the coatings increased significantly with adding additives. The average spectral emissivity value of the coating with adding Co(CH₃COO)₂ is about 0.91 at wavelength of 3–20 μm. All the coatings showed a tensile strength higher than 30 MPa, and a shear strength higher than 10 MPa. In addition, after subjected to severe thermal shocking for 40 cycles, there was no peeling off of the coating occurred, the coatings possessed good thermal shock resistance.     

Effect of CeO2 on the microstructure and isothermal oxidation of Ni-Al alloy coatings transformed from electrodeposited Ni-Al films at 800 °C

The electrodeposited two-phase γ-Ni + γ′-Ni₃Al and single-phase γ′-Ni₃Al coatings with and without CeO₂ particles were developed by the conversion of electrodeposited Ni-Al or Ni-Al-CeO₂ films with dispersed Al microparticles in Ni matrix into γ′-Ni₃Al by vacuum annealing at 800 °C for 3 h. SEM/EDAX and TEM characterizations showed that the CeO₂-dispersed γ-Ni + γ′-Ni₃Al or γ′-Ni₃Al coatings exhibited finer grains compared to the CeO₂-free Ni-Al alloy coatings. The oxidation at 1000 °C for 20 h showed that for a given Al content the addition of CeO₂ significantly improved the oxidation resistance of the electrodeposited Ni-Al alloy coatings. The effect of CeO₂ particles on the microstructure and oxidation behavior of the electrodeposited Ni-Al alloy coating is discussed in detail.