Analysis of the Microstructure and Thermal Shock Resistance of Laser Glazed Nanostructured Zirconia TBCs

Nanostructured zirconia thermal barrier coatings (TBCs) have been prepared by atmospheric plasma spraying using the reconstituted nanosized yttria partially stabilized zirconia powder. Field emission scanning electron microscope was applied to examine the microstructure of the resulting TBCs. The results showed that the TBCs exhibited a unique, complex structure including nonmelted or partially melted nanosized particles and columnar grains. A CO₂ continuous wave laser beam has been applied to laser glaze the nanostructured zirconia TBCs. The effect of laser energy density on the microstructure and thermal shock resistance of the as-glazed coatings has been systematically investigated. SEM observation indicated that the microstructure of the as-glazed coatings was very different from the microstructure of the as-sprayed nanostructured TBCs. It changed from single columnar grain to a combination of columnar grains in the fracture surface and equiaxed grains on the surface with increasing laser energy density. Thermal shock resistance tests have showed that laser glazing can double the lifetime of TBCs. The failure of the as-glazed coatings was mainly due to the thermal stress caused by the thermal expansion coefficient mismatch between the ceramic coat and metallic substrate.     

Transmission electron microscopy observation of microstructure of plasma-sprayed nanostructured zirconia coating

Nanostructured zirconia coatings deposited by plasma spraying technique were observed using transmission electron microscopy (TEM). It was found that the as-sprayed nanostructured zirconia coating had bimodal microstructures in terms of grain size distribution in the direction parallel to substrate surface. One was in the range 30–120 nm, which was the dominative structure of the coating, and the other was about 150–400 nm. The cross-section micrograph of the plasma sprayed nanostructured zirconia coating revealed that the coating still exhibited lamellar structure with columnar grains extending through its thickness. In conjunction with partially molten zirconia grains, amorphous regions were found. Domain structure and superlattice structure were observed in the plasma-sprayed nanostructured zirconia coating. The formations of the domain and superlattice structures are discussed in this paper.     

Nanostructured yttria stabilized zirconia coatings deposited by air plasma spraying

Nanostructured yttria partially stabilized zirconia coatings were deposited by air plasma spraying with reconstituted nanosized powder. The microstructures and phase compositions of the powder and the as-sprayed nanostructured coatings were characterized by transmission electron microscopy（TEM）, scanning electron microscopy（SEM） and X-ray diffxaction（XRD）. The results demonstrate that the microstructure of as-sprayed nanostructured zirconia coating exhibits a unique tri-modal distribution including the initial nanostructure of the powder, equiaxed grains and columnar grains. Air plasma sprayed nanostructured zirconia coatings consist of only the nontransformable tetragonal phase, though the reconstituted nanostructured powder shows the presence of the monoclinic, the tetragonal and the cubic phases. The mean grain size of the coating is about 42 nm.     

Chemical vapor deposition of rhenium on a gourd shaped graphite substrate

Chemical vapor deposition (CVD) of rhenium coatings on a gourd shaped graphite substrate is studied. Effects of deposition temperature, chlorine flow rate, total pressure and chlorination temperature on deposition rate, yield, morphology and texture of rhenium coating are investigated, respectively. Uniform rhenium coatings have been obtained by using proper combination of deposition conditions at an acceptable deposition rate and yield. The rhenium coatings consist of two sub-layers, i.e., an inner nucleation layer of fine equiaxed grains and an outer layer comprising oriented columnar grains. Although different surface morphologies have been observed, the grains of rhenium coatings are all <002> oriented. The tendency of the preferred orientation <002> is more significant with decreasing surface roughness of the coating.     

Improvement of the tribological behaviour of PVD nanostratified TiN/CrN coatings — An explanation

A hard TiN/CrN multilayered coating, consisting of alternating nanometer scale TiN and CrN layers (bilayer period of 40 nm), was deposited by arc evaporation process on M2 tool steel. Monolayered TiN and CrN are also deposited in the same conditions, and used as references. In order to get a better understanding of the tribological behaviour of coated parts, two types of experiments were performed. The dry-sliding wear resistance was evaluated with a ball-on-disk tribometer, while surface fatigue resistance was determined by a cyclic multi-impact test. The architecture of layers is measured by XRD and observed by TEM. The residual stress field was characterised using XRD and the sin²ψ method at a synchrotron radiation facility.All coatings present a columnar microstructure. TiN demonstrated better wear resistance than CrN and this characteristic is still increased two times by using the nanostratified coating. In the same way, the results of surface oligo-cyclic fatigue test confirm the high performance of the nanostructured coating with respect to the monolayered ones. The differences in mechanical properties of coatings evaluated through nanoindentation measurements do not lead to a direct correlation with the tribological results, and therefore cannot explain such differences. Moreover, a microscopic analysis of the samples after both tribological tests reveals two opposite cracking mechanisms. Monolayered TiN and CrN are subjected to a transversal crack propagation until the peeling of the coating, whereas the multilayered coating only undergoes cohesive cracks deviated in the TiN/CrN interface zones. Both opposite behaviours are the consequence of the distribution of stresses along the thickness of the film.     

Microstructure, spallation and corrosion of plasma sprayed Al2O3–13%TiO2 coatings

The electrochemical corrosion behaviours of the steel substrates coated with three different plasma sprayed Al₂O₃–13%TiO₂ coatings were studied in this paper. The three kinds of Al₂O₃–13%TiO₂ coatings were conventional ME coating, nanostructured NP coating and NS coating. There were micro cracks, laminar splats and straight columnar grains in ME coating. For the two nanostructured coatings, the laminar microstructure and columnar grains were not obvious. The NP coating had the highest hardness and spallation resistance. Electrochemical corrosion behaviour of the three coatings was mainly investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in aqueous Na₂SO₄ solution.     

Effect of Ti interlayer on corrosion behavior of nanostructured Ti/TiN multilayer coating deposited on TiAl6V4

To improve the corrosion properties of TiAl₆V₄ alloy, TiN monolayer and Ti/TiN multilayer coatings are deposited by reactive magnetron sputtering. The phase, structure, and morphology properties are investigated by grazing‐incidence X‐ray diffraction, field‐emission scanning electron microscopy, and atomic force microscopy, respectively, and the corrosion behavior is evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization. The TiN monolayer and Ti/TiN multilayer with thickness of 1,350 and 1,410 nm have the (111) and (002) preferred orientation and crystallite size of 42.5 and 24.3 nm, respectively. Columnar growth in TiN is hindered by the Ti interlayers and no cracking is observed between the layers indicating strong adhesion. The nanostructured Ti/TiN coating forms stable surface titanium oxide which improves the corrosion resistance by approximately 80 and four times compared with TiAl₆V₄ alloy and TiN coating, respectively. Hindrance of the columnar structure in TiN by the Ti interlayer decreases the local corrosion rate and enhances the galvanic corrosion resistance by forming a layer on the β‐phase enriched with vanadium as well as a TiO₂ stable layer. The nanostructured Ti/TiN coating demonstrates capacitive behavior with phase angles approximately ?50° and high impedance values at low frequency to be the corrosion resistance mechanism.     

Characterization of silicide phases formed during pack siliconizing coating on the Nb-1Zr-0.1C alloy

The present paper describes the morphology, chemistry and crystallography of the phases observed in the silicide coatings produced by pack cementation technique on Nb based alloys. Cross-sectional microstructures examined by transmission electron microscopy and scanning electron microscopy techniques have shown that the coating has two silicide layers: NbSi₂ and Nb₅Si₃. NbSi₂ formed at the surface of the sample and Nb₅Si₃ formed in between the substrate (Nb alloy) and NbSi₂ coating layer. Electron diffraction analyses revealed that NbSi₂ has hexagonal crystal structure with lattice parameters as a = 0.48 nm and c = 0.66 nm and Nb₅Si₃ has tetragonal crystal structure with lattice parameters as a = 0.65 nm and c = 1.19 nm. Nb₅Si₃ showed fine equiaxed grains, whereas, NbSi₂ exhibited duplex morphology having columnar grain morphology near to the Nb₅Si₃ layer and large equiaxed grains at the surface of the coating sample. The presence of duplex morphology was explained by estimating diffusion of various species and it was shown that columnar morphology of grains could be attributed to outward diffusion of Nb and equiaxed grains to inward diffusion of Si. In the case of Nb₅Si₃, growth takes place due to single element Si diffusion, leading to development of single equiaxed grain morphology of the Nb₅Si₃ phase.     

反应等离子喷涂TiN的反应过程及涂层形成机理研究

采用反应等离子喷涂技术制备了TiN涂层,并收集了少量TiN颗粒;采用SEM对涂层和TiN颗粒的横断面肜貌进行分析,研究了喷涂过程中的反应过程和涂层的形成过程.结果表明:Ti粉与N_2的反应为燃烧合成反应.反应过崔中释放出大量的热量,此反应在颗粒的表面进行;TiN涂层具有典型的层状组织结构,且层与层之间结合较好.     

Microstructural characterization of titanium matrix composite coatings reinforced by in situ synthesized TiB ＋ TiC fabricated on Ti6AI4V by laser cladding

Titanium-based composite coatings reinforced by in situ synthesized TiB and TiC particles between titanium and B₄C were successfully fabricated on Ti6Al4V by laser cladding. Phase constituents of the coatings were predicted by thermodynamic calculations in the Ti-B₄C-Al and Ti-B-C-Al systems, respectively, and were validated well by X-ray diffraction (XRD) analysis results. Microstructural and metallographic analyses were made by scanning electron microscopy (SEM) and electron probe micro-analysis (EPMA). The results show that the coatings are mainly composed of α-Ti cellular dendrites and the eutecticum in which a large number of needle-shaped TiB and a few equiaxial TiC particles are embedded. C is enriched in α-Ti cellular dendrites and far exceeds the theoretical maximum dissolubility, owing to the extension of saturation during laser cladding. The coatings have a good metallurgical bond with the substrate due to the existence of the dilution zone, in which a great amount of lamella β-Ti grains consisting of a thin needle-shaped martensitic microstructure are present and grow parallel to the heat flux direction; a few TiB and TiC reinforcements are observed at the boundaries of initial β-Ti grains.     

Characterization of TiN Coatings Deposited on H11 Tool Steel by PECVD Method

In this work, TiN nanostructured hard coating was deposited on the quartz and H11 hot work tool steel substrates using by plasma enhanced chemical vapor deposition (PECVD) technique. The process was performed in a hot wall chamber containing Ar, N₂, and H₂ gases along with TiCl₄ vapor for 2 h at the temperature deposition of 470°C. Field emission scanning electron microscopy (FE-SEM) and EDS were used to determine thickness and chemical composition of the coating, respectively. Crystal structure of the coating was investigated by X-ray diffraction (XRD) technique which indicated strong reflections of highly crystalline TiN. This was further confirmed by X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS). Potentiodynamic polarization test in 3.5% NaCl solution and pin-on-disc were used to evaluate the properties of the TiN coating. The results indicate that specimens coated with TiN coating has fine columnar grains and low friction coefficient (about 0.2). Also polarization curve exhibit a 10 times improvement in corrosion resistance compare to uncoated samples. This protection is attributed to formation a dense and adhesive layer of TiN nanostructured hard coating on the H11 substrate.     

Characteristics of tungsten coatings deposited by molten salt electro-deposition and thermal fatigue properties of electrodeposited tungsten coatings

Tungsten coatings were obtained on a tungsten substrate by molten salt electrodeposition (MSE) at 50 mA/cm² for 60 min in a NaCl-KCl-NaF-WO₃ (0.3385:0.3385:0.25:0.0 73mol) melt at 800 °C. The MSE tungsten coatings have rough rectangular pyramid surface and special columnar grains structure. Thermal fatigue tests have shown that the damage factor of MSE tungsten coatings was smaller than that of rolled tungsten under the same thermal loads and MSE tungsten coatings displayed better thermal fatigue properties compared to that of rolled tungsten, which was attributed to high purity, special columnar grains and rectangular pyramid surface morphology.     

Microstructure and properties of CVD coated Ti(C,N)-based cermets with varying WC additions

In this paper, multilayer coatings of TiN/TiCN/Al₂O₃/TiN are deposited on the Ti(C, N)-based cermets containing WC, and the effect of WC on the growth and adhesion strength as well as the mechanical properties of the coating are investigated. The multilayer coatings deposited by chemical vapor deposition (CVD) are uniform and dense. TiN coating exhibits a dense fine-grained structures and the Ti (C,N) on TiN coating shows dense columnar structure. The α-Al₂O₃ layer deposited on transition coating presents coarse grains with limited voids. The grain size of the columnar crystals deposited on the substrates gradually decreases with WC addition. The Al₂O₃ layer shows a preferred growth orientation of (104) plane. For TiN/TiCN phase, a change in orientation from (111) to (200) is observed. Generally, the (200) preferred orientation enhances and (111) preferred orientation diminishes with increasing WC addition. Strong adhesion of the CVD coating is obtained due to a sufficient amount of chemical elements, especially tungsten, diffusing from the substrate to the interfacial layer. Scratch tests show that the adhesion strength of TiN/TiCN/Al₂O₃/TiN films gradually increases firstly, and then decreases. With the addition of WC, the hardness, elastic modulus and plasticity index increase at the beginning, and then decrease. The change in nanohardness and elastic modulus is related to the grain size, elemental diffusion, and preferred orientation of the coating.     

Microstructure and thermal stability of a Ni-Cr-Co-Ti-V-Al high-entropy alloy coating by laser surface alloying

A Ni-Cr-Co-Ti-V-Al high-entropy alloy (HEA) coating with a BCC phase and (Ni, Co)Ti₂ compounds was synthesized successfully by laser surface alloying on a Ti-6Al-4V substrate. The microstructure of as-synthesized coatings is typical, namely, the microstructure from the coating to the substrate changes from equiaxed grains to columnar grains. After remaining at 900 °C for 8 h, the constituent phases remain unchanged. However, owing to the unceasing dissolution of the Ti element, the lattice parameter of the BCC HEA phase changes from 3.06 Å to 3.16 Å. The thermoanalysis results show that the oxidation film on the Ni-Cr-Co-Ti-V-Al HEA coating is mainly composed of TiO₂, V₂O₅, and NiO. The oxidation resistance of this HEA coating may be due to the existence of NiO and the alloying elements Al, Cr, and Co; the oxidation phenomenon should be responsible for the mass increase in the thermogravimetry process. The differential scanning calorimetry and the dynamic differential scanning calorimetry curves show that the synthesized HEA coating is stable below 1005 °C.     

Properties and performances of innovative coated tools for turning inconel

Three innovative nanostructured coatings have been developed to be applied on cutting tools for continuous cutting of nickel-based super-alloys, in Minimum Quantity Lubrication (MQL) or dry conditions.The coatings, TiN+AlTiN, TiN+AlTiN+MoS₂ and CrN+CrN:C+C, were applied by PVD techniques on WC-Co inserts, developing nanostructured layers, characterised by superior performances, as confirmed both by laboratory tests and machining experiments.Coatings surface qualification included SEM observations with EDS analysis, ball erosion test, nanoindentation and scratch tests, classic tribological evaluation by ball-on-disc set-up, surface texture analysis.Results were analysed in light of the outcome of machining experiments performed mainly in dry and MQL turning of Inconel 718. Ball-on-disc and scratch tests, as well as machining experiments, agreed in classifying the coatings in the following decreasing performance order: TiN+AlTiN+MoS₂, followed by TiN+AlTiN, and by CrN+CrN:C+C.     

Pretreatment of glass substrates by Ar/O2 ion beams for the as-sputtered rough Al doped zinc oxide thin films

Textured TCO surfaces are required in silicon thin film solar cells to gain efficient light trapping. Nowadays, magnetron sputtered ZnO:Al films are usually etched in HCl solution to obtain textured surface. This study introduces a method to achieve as-grown rough ZnO:Al films by ion beam pretreatment of the glass substrate. The reference ZnO:Al films deposited on untreated glass are composed of well aligned columnar grains. In contrast, in the as-grown rough films, additional large conical grains are observed. The large grains exhibit faster growth rate than the surrounding columnar grains, and therefore overgrow the columnar grains gradually and finally cover the whole surface. In order to investigate the ZnO:Al film structural properties, transmission electron microscopy and X-ray diffraction are employed. The crystal orientations of these two types of grains are further analyzed by selected area diffraction patterns. The columns in the as-grown rough ZnO:Al are similarly textured as the reference ZnO:Al film on untreated glass. The c-axis is well aligned nearly perpendicular to the substrate surface, but a tilt of 10° was observed with respect to the growth direction. The large conical grains show no strong out of plane texture and random in plane orientation.     

CVD钨沉积层组织控制

以WF6和H2为反应气体,采用间断供应反应气体方法改变CVD钨沉积层显微组织形貌。研究了间断沉积工艺参数对沉积层显微组织及性能的影响,讨论了间断沉积层的表面应力状态及断口裂纹扩展情况。结果表明:采用间断化学气相沉积法钨层的显微组织随周期沉积时间的缩短,柱状晶晶粒长度尺寸变小,形态逐渐接近等轴晶;沉积层表面形貌呈圆球状,沉积层生长界面不再趋向于单一方向;钨层保持了连续CVD钨的高纯度、高密度特性。且采用间断供应反应气体沉积方法显著降低了钨制品表面的残余应力,使裂纹扩展方向发生改变,有效阻碍了裂纹的深入扩展。     

等离子喷涂和激光熔覆热障涂层隔热性能比较

采用多种方法制备不同类型的Al2O3-13％TiO2热障涂层,即等离子喷涂常规涂层、纳米结构涂层及激光熔覆纳米结构涂层．在分析三类涂层微观组织的基础上,对其隔热性能进行了比较．结果表明,即等离子喷涂常规陶瓷涂层呈典型的层状堆积特征,纳米结构涂层都为特殊的两相结构,其中部分熔化区由类似的残留纳米粒子组成,等离子喷涂纳米结构涂层的完全熔化区为片层状结构,而相应的激光熔覆涂层的完全熔化区则为细小等轴晶．在相同条件下,等离子喷涂纳米结构热障涂层具有最好的隔热性能,而激光熔覆纳米结构涂层的隔热性能要好于等离子喷涂常规涂层．     

Effect of coating thickness on the deformation mechanisms in PVD TiN-coated steel

The deformation mechanisms of a range of TiN coatings with different thicknesses, deposited on a V820 steel substrate following nanoindentation were characterized using focused ion beam (FIB) cross-sectioning and imaging, as well as cross-sectional transmission electron microscopy (TEM) of the indented region. Four TiN coatings were examined, including a cathodic arc evaporation (CAE) coating with a thickness of ∼ 0.7 μm and low voltage electron beam (LVEB) evaporation coatings with thicknesses of ∼ 2.0, ∼ 3.7 and ∼ 4.0 μm. Based on a model developed by Xie et al., the intercolumnar shear stresses were calculated to be approximately 2.20, 3.05, 3.50 and 3.55 GPa in the ∼ 0.7, ∼ 2.0, ∼ 3.7 and ∼ 4.0 μm thick TiN coatings respectively, that is, increasing as the coating thickness increases. Columnar cracking and shear steps at the coating/substrate interface were observed more frequently in the thinner TiN coatings indicated that these coatings deformed predominantly by shear along the columnar grain boundaries. In contrast, inclined cracking was the more dominant fracture type in the thicker TiN coatings. It is suggested that increased grain boundary strength occurs together with a lack of direct crack path along the grain boundaries through the thicker coatings due to the more equiaxed grain structure. Clearly, the grain structure and/or thickness of the TiN coating play a highly significant role in the deformation mechanisms.     

AISI M2钢上磁控溅射TiN、TiAlN和TiAlVN薄膜的显微结构(英文)

为了研究Al和V掺杂对TiN薄膜微结构的影响,用磁控溅射法在AISI M2高速钢上沉积TiN、TiAlN和TiAlVN薄膜。采用XRD、SEM和TEM对薄膜的显微结构进行表征。结果表明,TiN薄膜中掺杂Al引起了晶格常数的降低,TiAlN中掺杂V则导致晶格常数的增加。另外,TiN、TiAlN和TiAlVN薄膜的生长形态显示,添加Al和V有改善柱状结构的倾向。在TiN、TiAlN和TiAlVN薄膜中鉴定出(111)和(200)晶向,ε(Fe3N-Fe2N)相的存在是因为薄膜中存在少量的Fe。TiAlN和TiAlVN薄膜夹层具有)0101(择优取向。在TiAlN和TiAlVN薄膜中观察到(111)和(200)晶向的织构(柱状)结构,在TiAlVN/M2夹层和回火马氏体之间存在)0101(α-Ti//(110)T.M的位向关系。