Thermal barrier coatings produced by chemical vapor deposition

Yttria stabilized zirconia (YSZ) can be employed as thermal barrier coatings (TBCs) on Ni-based super alloys in gas turbines and aircraft engines. The YSZ coatings have been fabricated by atmospheric plasma spraying or electron-beam physical vapor deposition. The increase in operation temperature of gas turbines demands another fabrication process to obtain high quality TBCs. Chemical vapor deposition (CVD) can be an alternative route to prepare TBCs due to excellent conformal coverage and columnar microstructure. This paper reviews the fabrication of YSZ films by conventional thermal CVD and plasma CVD intended for TBCs. A new laser CVD developed by our group with a high deposition rate of 660 µμh^-1 was also briefly introduced.     

Thick boride coatings by chemical vapor deposition

In this paper we describe an experimental study of the chemical vapor deposition (CVD) of TiB₂ by the hydrogen reduction of TiCl₄ and BCl₃ with the purpose of obtaining very thick (more than 100 μm) and uniform coatings. The optimum deposition conditions were as follows: temperature, 900–950 °C; source gas ratios, $\text{[}\text{Ti}\text{]/[}\text{B}\text{] =}\text{1}\text{2}$ and $\text{[}\text{H}\text{]/[}\text{Cl}\text{] =}\text{6}\text{1}$. With these conditions, deposition rates greater than 25 μm h^-1 were obtained. The composition was very uniform with an excess of boron over stoichiometry (probably in the form of free boron). A small amount of chlorine remained incorporated in the deposit probably as TiCl₂ (0.05 wt.% at 950 °C). The coatings were very hard (about 3700 kgf mm^-2) and hardness was uniform through the thickness. With careful control of the deposition parameters, the CVD of uniform coatings with thickness of 1 mm or more appears feasible.     

Architecture of thermal barrier coatings produced by electron beam-physical vapor deposition (EB-PVD)

Extremely high temperatures and severe atmospheric conditions in the hot section of aircraft engines during operation result in degradation and structural failures of turbine components. Replacing these components is very expensive. Thermal barrier coatings (TBC) composed of ZrO₂-8wt%Y₂O₃(8YSZ) applied by Electron Beam-Physical Vapor Deposition (EB-PVD) to turbine components offer excellent properties for thermal protection and resistance against oxidation - induced erosion and corrosion. However, the life of turbine components is still limited due to premature failure of the TBC. It is hypothesized that the life of the coated components can be extended by lowering the thermal conductivity of the TBC by creating multiple non-distinct or distinct interfaces and alloy additions such as Nb-oxide which will result in a reduction in the thermal conductivity and oxygen transport through the coating. This paper presents the microstructural results of standard 8YSZ, layered 8YSZ, Nb-oxide alloyed 8YSZ and functionally graded 8YSZ with Nb-oxide deposited by EB-PVD. TBC samples were examined by various methods including scanning electron microscopy (SEM), high-resolution optical microscopy (OM), X-ray diffraction (XRD), and thermal cycling tests. The preliminary results strongly suggest that multiple interfaced TBC exhibits better oxidation resistant properties as compared to standard and alloyed TBC.     

Thermal barrier coatings produced by laser cladding

A 2kW C0₂ laser has been used to clad a mild steel substrate with two different ceramic coatings, namely yttria partially stabilized zirconia (8 wt% YPSZ) or a mixture of YPSZ and pure alumina powder. A range of laser processing parameters has been investigated. Results have been obtained showing the possibility of using the laser beam for producing a clad layer of thermal barrier coating with different topography depending on the processing conditions.On leave from Scientific Research Council, Baghdad, Iraq.     

Formation of technological protective coatings by chemical vapor deposition

We perform the theoretical substantiation of the choice of modifiers of the processes of thermal treatment used to form nanometer and thin-film technological protective coatings (nanolayers), which give new properties to the metal surfaces of the products. This technological procedure is quite new, very promising, and does not require significant changes in the existing technological schemes. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 5, pp. 51–55, September–October, 2006.     

Initiated chemical vapor deposition of biopassivation coatings

Organosilicon polymers show great utility as both biocompatible and electrically insulating materials. In this work, thin films of a novel organosilicon polymer are synthesized by initiated CVD utilizing trivinyl-trimethyl cyclotrisiloxane as a monomer and terbutyl peroxide as a free radical generating initiator. Use of an initiator allows for formation of polymer films at filament temperatures as low as 250 °C which allows for retention of all siloxane ring moieties within the resulting polymer. The all-dry deposition process generates a highly crosslinked matrix material in which over 95% of the vinyl moieties present on the monomer units have been reacted out to form linear polymerized hydrocarbon chains. The material possesses an electrical resistivity of 4 × 10¹⁵ Ω cm. In addition, biased soak testing of material samples in a simulated biological environment demonstrates retention of electrical material properties for greater than 2 years.     

Carbon helixes produced by hot filament assisted chemical vapor deposition

Carbon helix nanofibers were synthesized by a hot filament assisted chemical vapor deposition at a substrate temperature of 600 °C. It was observed that the formation of a carbon helix structure was attributed to the mixing of cobalt catalyst particles with copper. The diameter of carbon helixes varied from 20 to 500 nm. The growth mechanism and the structure of these carbon helix nanofibers are discussed based on scanning electron microscopy and Raman spectroscopy measurements.     

Scratch resistant optical coatings on polymers by magnetron-plasma-enhanced chemical vapor deposition

The magnetron-plasma-enhanced chemical vapor deposition (magPECVD) provides silicon-organic thin films for optical, electrically insulating or diffusion barrier coating applications. With process pressures of ≤ 1 Pa this technology is well adapted to the sputtering process of optical interference coatings and also facilitates an inline-process implementation. This paper describes the deposition process for scratch resistant coatings on polycarbonate (PC) and allyl diglycol carbonate substrates. Based on the optical, chemical and mechanical characterization of single magPECVD thin films of varied chemical composition, several types of layer stacks (e.g. of gradient or alternating hardness distribution) were deposited with varied total thickness on PC substrates. Abrasion test results indicate two main effects: the resistance against scratches of high load abrasion (50 N) mainly depends on the total coating thickness. The durability against scratches of low load abrasion (5 N) shows a clear advantage for the multilayer design in contrast to homogeneous single layers even of higher thickness. Finally a 5-layer antireflective system was reactively sputtered onto the magPECVD coating and successfully passed adhesion and environmental tests.     

以生物质为催化剂化学气相沉积制备碳纳米管

以富含过渡金属元素铁的天然生物质黑木耳、紫菜、香菇、黑芝麻的炭化粉末作为催化剂前驱体,天然气为碳源,采用化学气相沉积工艺制备了碳纳米管(CNTs)。所制CNTs阵列的管径较窄,排列整齐,归因于蘑菇、紫菜和黑芝蔴的碳含量高及铁基纳米颗粒分散均匀。采用黑木耳为催化剂所制CNTs的直径也较窄,但杂乱生长,可能是由于黑木耳的碳含量低及铁基纳米颗粒分布不均匀所致。     

以液态碳硅烷为先驱体制备CVD SiC涂层

合成了液态碳硅烷并对其结构进行了分析,以液态碳硅烷为先驱体在900 ℃、低压的条件下采用化学气相沉积工艺制备了SiC涂层.实验结果表明,液态碳硅烷是以Si-C键为主链的数种低分子聚合物的混合物,分子组成中不含氧和腐蚀性元素,可通过分馏得到具有合适沸点的先驱体.涂层表面光滑且质硬,沉积产物为较纯的部分结晶的β-SiC.     

Tribological behavior at elevated temperature of multilayer TiCN/TiC/TiN hard coatings produced by chemical vapor deposition

Multilayer hard coatings of TiCN/TiC/TiN on high speed steel substrates were deposited using a chemical vapor deposition system. Evaluations of microstructure, wear morphology of coatings were characterized by scanning electron microscopy, and optical microscopy. Friction coefficient and wear rates of coatings were investigated using ball-on-disk tester sliding against a WC ball at a constant load of 20 N. Tribological behavior of the coatings at room and elevated temperature were discussed. Different changing tendency of friction coefficient were observed from ball-on-disc experiments. Results showed that the friction coefficient of coatings increased gradually to a highest value, then to a relatively constant value at room temperature dry sliding wear. The friction coefficient exhibited a reverse variation tendency at temperature of 550 °C. It got a higher value at the first sliding friction cycles. Then the value of friction coefficient decreased, suffered irregular oscillations and kept a relatively lower value with increasing sliding time. Reasons of the variation of friction coefficient with sliding time and wear mechanism were analyzed and discussed at room and elevated temperatures, respectively.     

Cold deposition of large-area amorphous hydrogenated silicon films by dielectric barrier discharge chemical vapor deposition

Amorphous hydrogenated silicon films were deposited on glass substrates at room temperature. This cold deposition process was operated in a dielectric barrier discharge CVD reactor with a fixed strip-shaped plasma matched with a moving substrate holder. The maximum film area was 300 × 600 mm². The film deposition rate as a function of applied peak voltage of DBD power was investigated under different hydrogen-diluted silane concentrations, and the film surface smoothness, continuity, and film/glass adherence were also studied. The maximum deposition rate was 12.2 Å/s, which was performed under the applied peak voltage of 16 kV and a hydrogen-diluted silane concentration of 50%. IR measurements reveal that the silane concentration plays a key role in determining the hydrogen-silicon bonding configurations. With increasing hydrogen-diluted silane concentration, the H-Si bonding configurations shift gradually from Si-H₃ to Si-H. The variation of photo/dark conductivity ratio and optical bandgap versus hydrogen-diluted silane concentration were investigated. The use of DBD-CVD for deposition of a-Si:H films offers certain advantages, such as colder substrate, faster film growth rate, and larger deposition area. However, the consumption of silane for the DBD-PECVD procedure is much greater than for the RF-PECVD process.     

催化化学气相沉积过程中硫元素引发的炭形貌衍变

以甲苯为碳源,二茂铁为催化剂,噻吩为生长促进剂,通过化学气相沉积方法得到了多分叉结构的炭.借助SEM、XRD和EDX考察了硫元素对产物的形貌和微观结构的影响.结果表明,当甲苯中的噻吩体积分数在0.01%～1%变化时,产物形貌由树状逐渐衍变为分支状.树状炭南较长而笔直的多分叉炭纤维构成,而分支状炭则由较短且弯曲的炭纤维构成.XRD结果表明硫元素对产物的微观结构没有明显的影响.     

Low pressure chemical vapor deposition of niobium coatings on graphite

Niobium coatings were prepared on graphite by low pressure chemical vapor deposition using niobium chloride and hydrogen as the reactant gases. The effects of deposition temperature on the morphology, phase, and deposition rate of niobium coatings were studied. The as-deposited niobium coatings were characterized by scanning electron microscopy and X-ray diffraction. The results indicate that the niobium coatings exhibit a granular hillock structure at 850-900 °C while a laminar structure at 950-1100 °C. The deposition is dominated by surface chemical kinetics with an apparent activation energy of 93.2 kJ/mol at 850-950 °C, while it is dominated by mass transport with an apparent activation energy of 7.9 kJ/mol at 950-1050 °C. At temperatures below 1100 °C, the deposited coatings mainly contain niobium. At temperatures above 1100 °C, the deposited coatings mainly contain niobium carbides. Considering the deposition kinetics and interfacial reactions, the deposition temperature should be controlled below 950 °C.     

Functional metal oxide coatings by molecule-based thermal and plasma chemical vapor deposition techniques

Deposition of thin films through vaccum processes plays an important role in industrial processing of decorative and functional coatings. Many metal oxides have been prepared as thin films using different techniques, however obtaining compositionally uniform phases with a control over grain size and distribution remains an enduring challenge. The difficulties are largely related to complex compositions of functional oxide materials, which makes a control over kinetics of nucleation and growth processes rather difficult to control thus resulting in non-uniform material and inhomogeneous grain size distribution. Application of tailor-made molecular precursors in low pressure or plasma-enhanced chemical vapor deposition (CVD) techniques offers a viable solution for overcoming thermodynamic impediments involved in thin film growth. In this paper molecule-based CVD of functional coatings is demonstrated for iron oxide (Fe2O3, Fe3O4), vanadium oxide (V2O5, VO2) and hafnium oxide (HfO2) phases followed by the characterization of their microstructural, compositional and functional properties which support the advantages of chemical design in simplifying deposition processes and optimizing functional behavior.     

Single-step fabrication of nanolamellar structured oxide ceramic coatings by metal-organic chemical vapor deposition

Oxide ceramic coatings in the system Y2O3-Al2O3-ZrO2 were fabricated in laboratory scale by using a MOCVD unit. A hot wall reactor was used along with different precursor feeding systems. Most experiments were carried out by using powder flash evaporation including a screw feeder for precursor powder delivery. For comparison, further samples were fabricated by using band flash evaporation and continuous evaporation from a crucible. Oxygen was used in all cases as reactant gas. Aluminium-tris-2,4-pentanedione (Al(acac)3), yttrium-tris-2,2,6,6-tetramethyl-3,5-heptanedione (Y(thd)3) and zirconium-tetrakis-2,2,6,6-tetramethyl-3,5-heptanedione (Zr(thd)4) were applied as metal-organic precursors because of their similar vaporization behaviour under the given conditions. The coating stoichiometry was varied from pure alumina to complex ternary compositions in the system Y2O3-Al2O3-ZrO2. Both kinds of ternary coatings fabricated by using flash evaporation methods show a nanolamellar microstructure in the as deposited state. Heat treating experiments at 1200 degrees C for up to 5 days enhance the lamellar character of the coating deposited by using powder flash evaporation. The lamellar microstructure is due to alternating YSZ enriched layers and YAG enriched layers in this state. However, the coating fabricated by using band flash evaporation shows a dense interpenetrating network of YSZ and YAG after heat treating instead of a lamellar microstructure observed in the as deposited state.     

Polymer layers by initiated chemical vapor deposition for thin film gas barrier encapsulation

A combination of SiN_x and polymer layers, in our case poly(glycidyl methacrylate) (PGMA) is very suitable as a permeation barrier layer on sensitive electronic devices. Our experiments thus far concentrate on increasing the stability and deposition rate of the polymer layers. To reach the thermal stability needed for the deposition of SiN_x on PGMA by HWCVD, the PGMA chain length must be large. PGMA with a very high molecular weight (M_W) (78,000 Da, ~ 548 monomers) was deposited at a high deposition rate (> 60 nm/min). To mimic the reactive atomic H ambient during SiN_x deposition conditions during HWCVD, the polymer layers were exposed to an atomic hydrogen environment for 0 to 550 s. Surprisingly, the most important factor for stability under these conditions was the filament temperature which was used during PGMA deposition, rather than the expected parameters such as M_W or surface roughness. Using lower filament temperatures for PGMA deposition, the layers were much more stable in atomic H ambient.