Texture structure and ablation behavior of TaC coating on carbon/carbon composites

TaC coatings with hybrid, (2 0 0) and (2 2 0) texture structure were prepared on carbon/carbon (C/C) composites by isothermal chemical vapor deposition with TaCl₅-Ar-C₃H₆ system. The residual stress, hardness and ablation behaviors of the different coatings were characterized by Raman spectra, nano-indentation and oxyacetylene flame ablation machine respectively. Results shown tensile stress exists in the TaC coatings and increases when texture orientation turns from hybrid to (2 2 0) and (2 0 0), while nano-indentation hardness of the coatings also obeys the same trend. The deposited coatings could improve the ablation-resistance properties of C/C composites effectively. The texture structure also had great effects on the ablation properties and ablation morphologies of the coatings. The mass ablation rate obviously decreases when the texture structure changes from hybrid orientation to (2 0 0) and (2 2 0) orientations. The hybrid orientation and (2 0 0) texture coatings exhibit coarse oxide morphologies with crater or some breakage existed; while the (2 2 0) texture coating shows dense, molten oxide morphology. The main ablation behaviors of the hybrid, (2 0 0) and (2 2 0) texture TaC coatings are oxidation and particle denudation and block denudation, oxidation and block denudation, oxidation and mechanical erosion and block denudation, respectively.     

Influence of high temperature annealing on the structure, hardness and tribological properties of diamond-like carbon and TiAlSiCN nanocomposite coatings

Diamond-like carbon (DLC) and TiAlSiCN nanocomposite coatings were synthesized and annealed at different temperatures in a vacuum environment. The microstructure, hardness and tribological properties of as-deposited and annealed DLC-TiAlSiCN nanocomposite coatings were characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Raman spectroscopy, nano-indentation and friction tests. The TEM results reveal that the as-deposited DLC-TiAlSiCN coating has a unique nanocomposite structure consisting of TiCN nanocrystals embedded in an amorphous matrix consisting of a-Si₃N₄, a-SiC, a-CN and DLC, and the structure changed little after annealing at 800 °C. However, XPS and Raman results show that an obvious graphitization of the DLC phase occurred during the annealing process and it worsened with annealing temperature. Because of the graphitization, the hardness of the DLC-TiAlSiCN coating after annealing at 800 °C decreased from 45 to 36 GPa. In addition, the DLC-TiAlSiCN coating after annealing at 800 °C has a similar friction coefficient to the as-deposited coating.     

Characteristics and heat treatment of cold-sprayed Al-Sn binary alloy coatings

In this study, Al-Sn binary alloy coatings were prepared with Al-5 wt.% Sn (Al-5Sn) and Al-10 wt.% Sn (Al-10Sn) gas atomized powders by low pressure and high pressure cold spray process. The microstructure and microhardness of the coatings were characterized. To understand the coarsening of tin in the coating, the as-sprayed coatings were annealed at 150, 200, 250 and 300 °C for 1 h, respectively. The effect of annealing on microstructure and the bond strength of the coatings were investigated. The results show that Al-5Sn coating can be deposited by high pressure cold spray with nitrogen while Al-10Sn can only be deposited by low pressure cold spray with helium gas. Both Al-5Sn and Al-10Sn coatings present dense structures. The fraction of Sn in as-sprayed coatings is consistent with that in feed stock powders. The coarsening and/or migration of Sn phase in the coatings were observed when the annealing temperature exceeds 200 °C. Furthermore, the microhardness of the coatings decreased significantly at the annealing temperature of 250 °C. EDXA analysis shows that the heat treatment has no significant effect on fraction of Sn phase in Al-5Sn coatings. Bonding strength of as-sprayed Al-10Sn coating is slightly higher than that of Al-5Sn coating. Annealing at 200 °C can increase the bonding strength of Al-5Sn coatings.     

Role of growth temperature and oxygen partial pressure on the structural and electrical properties of pulsed laser deposited La1−xSrxnO3−δ thin films

The paper presents the fabrication and characterization of La_0.65Sr_0.35MnO_3−δ (LSMO) polycrystalline thin films deposited directly on Si (1 0 0) substrates using pulsed laser deposition technique. Various deposition parameters like substrate temperature and oxygen partial pressure have been varied systematically to obtain stoichiometric, crack-free films with smooth surface morphology having nearly monodisperse grain size distribution. The substrate temperature variation from 600 to 800 °C had profound effects on the microstructure and topography of the deposited film, with optimum result being obtained at 700 °C. The variation of partial pressure of oxygen controls the deposition kinetics as well as the stoichiometry of the film in terms of oxygen vacancy, which influences the magnetic and electrical transport properties of the manganate films. The microstructure and crystallinity of the deposited films have been studied using X-ray diffraction, scanning electron microscopy and atomic force microscopy. A correlation between the oxygen stoichiometry and micro-structural and transport properties of the deposited films has been obtained.     

Preparation and in vitro characterization of aerosol-deposited hydroxyapatite coatings with different surface roughnesses

Hydroxyapatite (HA) coatings with different surface roughnesses were deposited on a Ti substrate via aerosol deposition (AD). The effect of the surface roughness on the cellular response to the coating was investigated. The surface roughness was controlled by manipulating the particle size distribution of the raw powder used for deposition and by varying the coating thickness. The coatings obtained from the 1100 °C-heated powder exhibited relatively smooth surfaces, whereas those fabricated using the 1050 °C-heated powder had network-structured rough surfaces with large surface areas and were superior in terms of their adhesion strengths and in vitro cell responses. The surface roughness (R_a) values of the coatings fabricated using the 1050 °C-heated powder increased from approximately 0.65 to 1.03 μm as the coating thickness increased to 10 μm. The coatings with a rough surface had good adhesion to the Ti substrate, exhibiting high adhesion strengths ranging from 37.6 to 29.5 MPa, depending on the coating thickness. The optimum biological performance was observed for the 5 μm-thick HA coating with an intermediate surface roughness value of 0.82 μm.     

Formation of Cr-modified silicide coatings on a Ti-Nb-Si based ultrahigh-temperature alloy by pack cementation process

Cr-modified silicide coatings were prepared on a Ti-Nb-Si based ultrahigh temperature alloy by Si-Cr co-deposition at 1250 °C, 1350 °C and 1400 °C for 5-20 h respectively. It was found that both coating structure and phase constituents changed significantly with increase in the co-deposition temperature and holding time. The outer layers in all coatings prepared at 1250 °C for 5-20 h consisted of (Ti,X)₅Si₃ (X represents Nb, Cr and Hf elements). (Ti,X)₅Si₄ was found as the only phase constituent in the intermediate layers in both coatings prepared at 1250 °C for 5 and 10 h, but the intermediate layers in the coatings prepared at 1250 °C for 15 and 20 h were mainly composed of (Ti,X)₅Si₃ phase that was derived from the decomposition of (Ti,X)₅Si₄ phase. In the coating prepared at 1350 °C for 5 h, single (Ti,X)₅Si₃ phase was found in its outmost layer, the same as that in the outer layers in the coatings prepared at 1250 °C; but in the coatings prepared at 1350 °C for 10-20 h, (Nb_1.95Cr_1.05)Cr₂Si₃ ternary phase was found in the outmost layers besides (Ti,X)₅Si₃ phase. In the coatings prepared at 1400 °C for 5-20 h, (Nb_1.95Cr_1.05)Cr₂Si₃ ternary phase was the single phase constituent in their outmost layers. The phase transformation (Ti,X)₅Si₄ → (Ti,X)₅Si₃ + Si occurred in the intermediate layers of the coatings prepared at 1350 and 1400 °C with prolonging co-deposition time, similar to the situation in the coatings prepared at 1250 °C for 15 and 20 h, but this transformation has been speeded up by increase in the co-deposition temperature. The transitional layers were mainly composed of (Ti,X)₅Si₃ phase in all coatings. The influence of co-deposition temperature on the diffusion ability of Cr atoms was greater than that of Si atoms in the Si-Cr co-deposition processes investigated. The growth of coatings obeyed inverse logarithmic laws at all three co-deposition temperatures. The Si-Cr co-deposition coating prepared at 1350 °C for 10 h showed a good oxidation resistance due to the formation of SiO₂ and Nb, Cr-doped TiO₂ scale after oxidation at 1250 °C for 10 h.     

Cycle oxidation behavior of nanostructured Ni60-TiB2 composite coating sprayed by HVOF technique

Cycle oxidation resistance at 800 °C in static air was investigated for a nanostructured Ni60-TiB₂ composite coating sprayed by high velocity oxy-fuel (HVOF). For comparison, a Ni60-TiB₂ conventional composite coating was also studied. The results indicate that, the oxidation processes of both composite coatings are controlled by diffusion mechanism, and the nanostructured composite coating has better cycle oxidation resistance than that of the conventional composite coating. The reasons for this improvement can be attributed to the formation of the intact SiO₂ and Cr₂O₃ protective layer, and the enhanced adhesion between oxide film and nanostructure coating.     

Diamond deposition on siliconized stainless steel

Silicon diffusion layers in AISI 304 and AISI 316 type stainless steels were investigated as an alternative to surface barrier coatings for diamond film growth. Uniform 2 μm thick silicon rich interlayers were obtained by coating the surface of the steels with silicon and performing diffusion treatments at 800 °C. Adherent diamond films with low sp² carbon content were deposited on the diffused silicon layers by a modified hot filament assisted chemical vapor deposition (HFCVD) method. Characterization of as-siliconized layers and diamond coatings was performed by energy dispersive X-ray analysis, scanning electron microscopy, X-ray diffraction and Raman spectroscopy.     

Preparation and annealing study of TaNx coatings on WC-Co substrates

To prevent Co diffusion from cemented carbides at high temperatures, we fabricated TaN_x coatings by reactive direct current (d.c.) magnetron sputtering onto 6 wt.% cobalt cemented carbide substrates, to form diffusion barrier layers. Varying the nitrogen flow ratio, N₂/(Ar + N₂), from 0.05 to 0.4 during the sputtering process had a significant effect on coating structure and content. Deposition rate reduced as the nitrogen flow ratio increased. The effects of nitrogen flow ratio on the crystalline characteristics of the TaN_x coatings were examined by X-ray diffraction. The TaN_x coatings annealing conditions were 500, 600, 700, and 800 °C for 4 h in air. We evaluated the performance of the diffusion barrier using both Auger electron spectroscopy depth-profiles and X-ray diffraction techniques. We also investigated oxidation resistance of the TaN_x coatings annealed in air, and under a 50 ppm O₂-N₂ atmosphere, to evaluate the fabricated layers effectiveness as a protective coating for glass molding dies.     

Diffusion research between Ni3Al coating and titanium alloy produced by plasma spraying process

A Ni₃Al coating was prepared by plasma spraying technique on the surface of titanium alloy. Ni-Al mixed powders, coatings and reaction products were investigated by scanning electron microscope, EDS, DSC and XRD. A tight bonding between the coating and the substrate was formed. The X-ray diffraction analysis of the patterns showed that the coating not only had Ni₃Al phase, but also had NiO and Al₂O₃ phase microcontent. Comparing Ni coated Al to Ni₃Al at 900 °C, the diffusion was stronger and the diffusion layer was thicker. A minute pore structure was formed at 1200 °C in the front edge of solid-state reaction layer. So Ni₃Al restrained the solid-state reaction of the coating with the substrate, and as a whole weakened the entry of oxygen atoms into the substrate and quenched the out-diffusion of titanium.     

Effect of relative humidity on morphology of YBCO films prepared by CSD method

YBa₂Cu₃O_7−x (YBCO) films have been prepared on LaAlO₃ (1 0 0) substrates by chemical solution deposition using metal trifluoroacetates. The influence of relative humidity in coating circumstance on the morphology of YBCO films was examined by optical microscopy and atomic force microscopy. The crack-free YBCO film was obtained if the relative humidity in coating circumstance was lower than 30%. During the decomposition step, the influences of the water vapor partial pressure and the heating rate within the temperature range from 200 °C to 320 °C were discussed. A smooth YBCO pyrolzed film could be obtained if the suitable water vapor partial pressure and the suitable heating rate within the temperature range from 200 °C to 320 °C were chosen. Furthermore, the smooth crystallized films show a high critical current density of 3 MA/cm² at 77 K.     

Fabrication of protective tantalum carbide coatings on carbon fibers using a molten salt method

Protective tantalum carbide (TaC) coatings were fabricated successfully on carbon fibers in the temperature range of 950-1100 °C using a molten salt method. A salt mixture composed of LiCl-KCl-KF was used as a reaction medium in which the tantalum and the carbon fiber substrates reacted to form the TaC coatings. The structure and morphology of the TaC coatings were characterized by XRD, SEM and EDX analyses. The results show that the reaction temperature and time have significant influence on the thickness, integrity and surface morphology of the TaC coatings. A uniform, adherent and crack-free TaC coating can be obtained by controlling the reaction temperature and time. Thermo-gravimetric analysis indicated that the oxidation resistance of the carbon fibers was improved remarkably by coating them with a high-quality TaC layer.     

A study of TaxC1−x coatings deposited on biomedical 316L stainless steel by radio-frequency magnetron sputtering

In this paper, Ta_xC_1−x coatings were deposited on 316L stainless steel (316L SS) by radio-frequency (RF) magnetron sputtering at various substrate temperatures (T_s) in order to improve its corrosion resistance and hemocompatibility. XRD results indicated that T_s could significantly change the microstructure of Ta_xC_1−x coatings. When T_s was <150 °C, the Ta_xC_1−x coatings were in amorphous condition, whereas when T_s was ≥150 °C, TaC phase was formed, exhibiting in the form of particulates with the crystallite sizes of about 15-25 nm (T_s = 300 °C). Atomic force microscope (AFM) results showed that with the increase of T_s, the root-mean-square (RMS) values of the Ta_xC_1−x coatings decreased. The nano-indentation experiments indicated that the Ta_xC_1−x coating deposited at 300 °C had a higher hardness and modulus. The scratch test results demonstrated that Ta_xC_1−x coatings deposited above 150 °C exhibited good adhesion performance. Tribology tests results demonstrated that Ta_xC_1−x coatings exhibited excellent wear resistance. The results of potentiodynamic polarization showed that the corrosion resistance of the 316L SS was improved significantly because of the deposited Ta_xC_1−x coatings. The platelet adhesion test results indicated that the Ta_xC_1−x coatings deposited at T_s of 150 °C and 300 °C possessed better hemocompatibility than the coating deposited at T_s of 25 °C. Additionally, the hemocompatibility of the Ta_xC_1−x coating on the 316L SS was found to be influenced by its surface roughness, hydrophilicity and the surface energy.     

Low pressure chemical vapor deposition of niobium coating on silicon carbide

Nb coatings were prepared on a SiC substrate by low pressure chemical vapor deposition using NbCl₅. Thermodynamic calculations were performed to study the effect of temperature and partial pressure of NbCl₅ on the final products. The as-deposited coatings were characterized by scanning electron microscopy, X-ray diffraction, and energy dispersive spectroscopy. The Nb coatings are oriented and grow in the preferred (2 0 0) plane and (2 1 1) plane, at 1173 K and 1223-1423 K, respectively. At 1123-1273 K, the deposition is controlled by the surface kinetic processes. The activation energy is found to be 133 kJ/mol. At 1273-1373 K, the deposition is controlled by the mass transport processes. The activation energy is found to be 46 kJ/mol. The growth mechanism of the chemical vapor deposited Nb is also discussed based on the morphologies and the deposition rates.     

Preparation and characterization of boron nitride coatings on carbon fibers from borazine by chemical vapor deposition

Boron nitride (BN) coatings were deposited on carbon fibers by chemical vapor deposition (CVD) using borazine as single source precursor. The deposited coatings were characterized by scanning electron microscopy (SEM), Auger electron spectroscopy (AES), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The effect of temperatures on growth kinetics, morphology, composition and structure of the coatings was investigated. In the low temperature range of 900 °C-1000 °C, the growth rate increased with increasing temperature complying with Arrhenius law, and an apparent active energy of 72 kJ/mol was calculated. The coating surface was smooth and compact, and the coatings uniformly deposited on individual fibers of carbon fiber bundles. The growth was controlled by surface reaction. At 1000 °C, the deposition rate reached a maximum (2.5 μm/h). At the same time, the limiting step of the growth translated to be mass-transportation. Above 1100 °C, the growth rate decreased drastically due to the occurrence of gas-phase nucleation. Moreover, the coating surface became loose and rough. Composition and structure examinations revealed that stoichiometric BN coatings with turbostratic structure were obtained below 1000 °C, while hexagonal BN coatings were deposited above 1100 °C. A penetration of carbon element from the fibers to the coatings was observed.     

Investigation of Ni/Ta contacts on 4H silicon carbide upon thermal annealing

Nickel and Tantalum thin films with 3:5 thickness ratios were deposited in succession onto 4H-SiC substrate at room temperature. The samples were then heated in situ in vacuum at 650, 800 or 950 °C for 30 min. Glancing angle X-ray diffraction (XRD), Auger electron spectroscopy (AES) and current-voltage (I-V) technique were used for characterising the interfacial reactions and electrical properties. Amorphous Ni-Ta can be formed by solid-state reaction at 650 °C. The minor dissolved Ni in the Ta metal promotes the reaction between Ta and SiC. With increasing annealing temperature up to 950 °C, the dominant carbide changes from Ta₂C to TaC and a layer structure is developed. Electrical measurements show that ohmic contact is formed after annealing at or above 800 °C.     

Influence of heat treatment on tribological properties of electroless Ni-P and Ni-P-Al2O3 coatings on Al-Si casting alloy

Evolution of tribological properties of electroless Ni-P and Ni-P-Al₂O₃ coating on an Al-10Si-0.3Mg casting alloy during heat treatment is investigated in this work. The pre-treated substrate was plated using a bath containing nickel hypophosphite, nickel lactate and lactic acid. For preparation of fiber-reinforced coating Al₂O₃ Saffil fibers pre-treated in demineralised water were used. The coated samples were heat treated at 400-550 °C/1-8 h. Tribological properties were studied using the pin-on-disc method. It is found that the best coating performance is obtained using optimal heat treatment regime (400 °C/1 h). Annealing at higher temperatures (450 °C and above) leads to the formation of intermetallic compounds that reduce the coating wear resistance. The reason is that the intermetallic phases adversely affect the coating adherence to the substrate. The analysis of wear tracks proves that abrasion is major wear mechanism, however due to the formed intermetallic sub-layers, partial coating delamination may occur during the pin-on-disc test on the samples annealed at 450 °C and above. It was found that fiber reinforcement reduces this scaling and increases wear resistance of coatings as compared to the non-reinforced Ni-P coatings.     

High temperature oxidation behavior of interconnect coated with LSCF and LSM for solid oxide fuel cell by screen printing

The current study examined the effect of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) and La_0.7Sr_0.3MnO₃ (LSM) coatings on the electrical properties and oxidation resistance of Crofer22 APU at 800 °C hot air. LSCF and LSM were coated on Crofer22 APU by screen printing and sintered over temperatures ranging from 1000 to 1100 °C in N₂. The coated alloy was first checked for compositions, morphology and interface conditions and then treated in a simulated oxidizing environment at 800 °C for 200 h. After measuring the long-term electrical resistance, the area specific resistance (ASR) at 800 °C for the alloy coated with LSCF was less than its counterpart coated with LSM. This work used LSCF coating as a metallic interconnect to reduce working temperature for the solid oxide fuel cell.     

Characteristic and biocompatibility of the TiO2-based coatings containing amorphous calcium phosphate before and after heat treatment

TiO₂-based coating containing amorphous calcium phosphate (CaP) was prepared on titanium alloy by microarc oxidation (MAO). The increase in the EDTA-2Na concentration was unfavorable for the crystallization of TiO₂. After heat treatment, the amorphous CaP was crystallized. The thickness of the MAO coatings did not change when heat-treated at 400, 600 and 700 °C; while it increased slightly after heat treatment at 800 °C due to the crystallization of amorphous CaP and growth of TiO₂. No apparent discontinuity between the coatings and substrates was observed at various heat-treatment temperatures, indicating the MAO coatings with good interfacial bonding to the substrate. The heat treatment did not alter the chemical composition of the MAO coating and the chemical states of Ti, Ca and P elements. However, it increased the roughness (Ra) of the MAO coating and improved the wetting ability of the MAO coating. In this work, preliminary investigation of the MG63 cell proliferation on the surface of the MAO and heat-treated MAO coatings was conducted. The MAO coating surface with about Ra = 220 nm may be suitable for the MG63 cell adhesion and proliferation. The increased roughness of the heat-treated MAO coatings may result in a decrease in the ability for cell adhesion and proliferation.     

Preparation and properties of electroless Ni-P-SiO2 composite coatings

Dispersible SiO₂ nanoparticles were co-deposited with electroless Ni-P coating onto AISI-1045 steel substrates in the absence of any surfactants in plating bath. The resulting Ni-P/nano-SiO₂ composite coatings were heat-treated for 1 h at 200 °C, 400 °C, and 600 °C, respectively. The hardness and wear resistance of the heat-treated composite coatings were measured. Moreover, the structural changes of the composite coatings before and after heat treatment were investigated by means of X-ray diffraction (XRD), while their elemental composition and morphology were analyzed using an energy dispersive spectrometer (EDS) and a scanning electron microscope (SEM). Results show that co-deposited SiO₂ particles contributed to increase the hardness and wear resistance of electroless Ni-P coating, and the composite coating heat-treated at about 400 °C had the maximum hardness and wear resistance.