Comparison of thermal shock behaviors between plasma-sprayed nanostructured and conventional zirconia thermal barrier coatings

NiCoCrAlTaY bond coat was deposited on pure nickel substrate by low pressure plasma spraying(LPPS), and ZrO2-8%Y2O3 (mass fraction) nanostructured and ZrO2-7%Y2O3 (mass fraction) conventional thermal barrier coatings(TBCs) were deposited by air plasma spraying(APS). The thermal shock behaviors of the nanostructured and conventional TBCs were investigated by quenching the coating samples in cold water from 1 150, 1 200 and 1 250 ℃, respectively. Scanning electron microscopy(SEM) was used to examine the microstructures of the samples after thermal shock testing. Energy dispersive analysis of X-ray(EDAX) was used to analyze the interface diffusion behavior of the bond coat elements. X-ray diffractometry(XRD) was used to analyze the constituent phases of the samples. Experimental results indicate that the nanostructured TBC is superior to the conventional TBC in thermal shock performance. Both the nanostructured and conventional TBCs fail along the bond coat/substrate interface. The constituent phase of the as-sprayed conventional TBC is diffusionless-transformed tetragonal(t′). However, the constituent phase of the as-sprayed nanostructured TBC is cubic(c). There is a difference in the crystal size at the spalled surfaces of the nanostructured and conventional TBCs. The constituent phases of the spalled surfaces are mainly composed of Ni2.88Cr1.12 and oxides of bond coat elements.     

An analytical model for simulation of heat flow in plasma-sprayed thermal barrier coatings

Numerical (finite difference) and analytical models have been developed for the simulation of heat flow through plasma-sprayed coatings, allowing the effective thermal conductivity to be predicted as a function of microstructural parameters. The structure is assumed to be composed of lamellar material (splats), separated by (thin) pores, within which there are areas of contact (bridges). The analytical model is based on dividing the material into two regimes, within which the heat flow occurs either by unidirectional serial flow through lamellae and pores or by being funneled through the regions of the lamellae above and below the bridges. The validity of this model is demonstrated by a comparison of the predictions obtained from it and those obtained from the numerical model. The effects of pore geometry on conductive and radiative heat transfer within the coating have been investigated over a range of temperatures and gas pressures. It is shown that the main factor controlling the conductivity is the intersplat bridge area. Comparisons are also presented with experimental conductivity data, for cases in which some attempt has been made to characterize the key microstructural features. The study is oriented toward thermal barrier coatings, based on zirconiayttria top coats. It is noted that the effect of microstructural sintering, which tends to occur in these coatings under service conditions, can be predicted using this model.     

Thermal cycling behavior of plasma-sprayed thermal barrier coatings with various MCrAlX bond coats

The influence of bond coat composition on the spallation resistance of plasma-sprayed thermal barrier coatings (TBCs) on single-crystal René N5 substrates was assessed by furnace thermal cycle testing of TBCs with various vacuum plasma spray (VPS) or air plasma-spray (APS) MCrAlX (M=Ni and/or Co; and X=Y, Hf, and/or Si) bond coats. The TBC specimens with VPS bond coats were fabricated using identical parameters, with the exception of bond coat composition. The TBC lifetimes were compared with respect to MCrAlX composition (before and after oxidation testing) and MCrAlX properties (surface roughness, thermal expansion, hardness, and Young’s modulus). The average TBC spallation lifetimes varied significantly (from 174 to 344 1 h cycles at 1150 °C) as a function of bond coat composition. Results suggested a relationship between TBC durability and bond coat thermal expansion behavior below 900 °C. Although there were only slight differences in their relative rates of cyclic oxidation weight gain, VPS MCrAlX bond coats with better oxide scale adhesion provided superior TBC lifetimes.     

Ultrasonic characterization of elastic anisotropy in plasma-sprayed alumina coatings

An ultrasonic, nondestructive contact measurement technique was employed to detect and characterize the elastic anisotropy of a free-standing, plasma-sprayed alumina coating. Following this initial evalu-ation, a computer-assisted, ultrasonic anisotropic test bed was used to determine the anisotropic elastic stiffness constants of coatings produced by plasma gun currents of 600 and 400 A. The results showed that the plasma-sprayed alumina coatings are transversely isotropic; i.e., isotropic in the spraying direction. These coatings were characterized by five independent elastic stiffness constants. Coatings produced at 600 A plasma gun current showed higher elastic stiffness constants than those produced at 400 A plasma gun current. This increase appeared to be related to a decrease in the porosity content of the coatings pro-duced at the higher plasma gun current.     

Evaluation of wear damage in zirconia plasma-sprayed coatings using scanning white light interferometry

The mechanical and tribological properties of thermal barrier coatings (TBCs) can be improved by means of a thermal treatment. The evolution of the mechanical and tribological properties in a NiCr-ZrO₂ TBC with different times of thermal treatment has been measured. In this work, scanning white light interferometry (SWLI) is used to observe and quantify the ZrO₂ wear damage. ZrO₂ shows very poor light reflection, and a sputtering process over the coating has been made to achieve a proper light reflection and make the use of SWLI possible. It has been observed that thermal treatments at 1000 °C produce a decrease of the wear damage and an increase of hardness. The ball-on-disk test and the wear mechanisms are described and include the intersplat delamination of the main wear process in the as-sprayed coatings and thermally treated samples. The volume loss after 18 h at 1000 °C is 38% less than the as-sprayed coating. The erosion test and hardness measures show the same evolution as the ball-on-disk test.     

Anisotropic thermal conductivities of plasma-sprayed thermal barrier coatings in relation to the microstructure

Anisotropic thermal conductivities of the plasma-sprayed ceramic coating are explicitly expressed in terms of the microstructural parameters. The dominant features of the porous space are identified as strongly oblate (cracklike) pores that tend to be either parallel or normal to the substrate. The scatter in pore orientations is shown to have a pronounced effect on the effective conductivities. The established quantitative microstructure-property relations, if combined with the knowledge of the processing parameters-resulting microstructure connections, can be utilized for controlling the conductivities in the desired way.     

提高等离子喷涂热障涂层隔热性能的方法

为进一步提高等离子喷涂热障涂层的隔热性能,对陶瓷材料的导热理论及热障涂层的热导率进行了研究.提出了包括寻求新型热障涂层陶瓷材料、添加掺杂剂、制备纳米涂层及双陶瓷层热障涂层等能够改善等离子喷涂涂层隔热性能的方法;并指出,采用等离子喷涂技术制备带颜色的稀土锆酸盐纳米双陶瓷层热障涂层,将会进一步改善热障涂层的隔热性能.     

Superior thermal barrier coatings using solution precursor plasma spray

A novel process, solution precursor plasma spray (SPPS), is presented for depositing thermal barrier coatings (TBCs), in which aqueous chemical precursors are injected into a standard direct current plasma spray system. The resulting coatings microstructure has three unique features: (1) ultra fine splats (1 μm), (2) nanometer and micron-sized interconnected porosity, and (3) closely spaced, through-thickness cracks. Coatings over 3 mm thick can be readily deposited using the SPPS process. Coating durability is excellent, with SPPS coatings showing, in furnace cycling tests, 2.5 times the spallation life of air plasma coatings (APS) and 1.5 times the life of electron beam physical vapor deposited (EB-PVD) coatings. The conductivity of SPPS coatings is lower than EB-PVD coatings and higher than the best APS coatings. Manufacturing cost is expected to be similar to APS coatings and much lower than EB-PVD coatings. The SPPS deposition process includes droplet break-up and material arriving at the deposition surface in various physical states ranging from aqueous solution, gel phase, to fully-molten ceramic. The relation between the arrival state of the material and the microstructure is described.     

Influence of hydrogen on the microstructure of plasma-sprayed yttria-stabilized zirconia coatings

The influence of secondary hydrogen and current on the deposition efficiency (DE) and microstructure of yttria-stabilized zirconia (YSZ) coatings was evaluated. To better understand the influence of the spray process on coating consistency, a YSZ powder, −125 +44 μm, was sprayed with nitrogen/hydrogen parameters and a 9 MB plasma gun from Sulzer Metco. DE and coating porosity, which were produced using two different spray gun conditions yielding the same input power, were compared. Amperage was allowed to vary between 500 and 560 A, and hydrogen was adjusted to maintain constant power, while nitrogen flow was kept at a fixed level. Several power conditions, ranging from 32 to 39 kW, were tested. Different injection geometries (i.e., radial with and without a backward component) were also compared. The latter was found to produce higher in-flight temperatures due to a longer residence time of the powder particles in the hotter portion of the plasma. Porosity was based on cross-sectional micrographs. In-flight particle temperature and velocity measurements were also carried out with a special sensor for each condition. Test results showed that DE and coating density could vary significantly when a different hydrogen flow rate was used to maintain constant input power. On the other hand, DE was found to correlate very well with the temperature of the in-flight particles. Therefore, to obtain more consistent and reproducible DE and microstructures, it is preferable to maintain the in-flight particle temperature around a constant value instead of keeping a constant input power by adjusting the secondary hydrogen flow rate.     

Laser glazing of plasma-sprayed zirconia coatings

A CO₂ laser with cylindrical focal lens has been used to glaze the surface layer of plasma-sprayed ZrO₂-20wt% Y₂O₃/MCrAlY coatings. Both a continuous-wave laser and a pulsed laser were used in this study. Different parameter settings for power, travel speed, and pulse frequency were used, and their effects on the melting width, melting depth, coupling efficiency, microstructure, surface roughness, and process defects have been evaluated. Results show that the melting width of the glazed track was slightly smaller than the diameter of the raw beam. The melting depth increased with increasing energy density for both a continuous-wave laser and a pulsed laser. The coupling efficiency as about 40 to 65% for a continuous-wave laser, which increased with increasing laser travel speed, but decreased with an increase in energy density. The power density has no significant effect on coupling efficiency. Defects, such as bubbles or depressions, occur easily with a continuous wave laser. A high-quality glazed layer is successfully produced using a pulsed laser. The surface roughness of the plasma-sprayed ceramic coatings was significantly improved by laser glazing. Surface roughness decreased slightly as the pulse frequency increased for the glazed surface. Based on this study, proper processing parameters have been suggested.     

等离子喷涂热障涂层的隔热性分析

采用大气等离子喷涂方法制备不同类型的氧化钇部分稳定氧化锆热障涂层：传统涂层、纳米团聚粉末制备的纳米涂层和空心球粉末制备的空心球涂层。通过扫描电镜、透射电镜、压汞仪和激光脉冲法观察和测试各种涂层的组织形貌、空隙分布和导热系数,并在相同条件下测试各种涂层的隔热性能。结果表明：纳米涂层空隙率最低,内部孔洞细小。空心球涂层组织相对疏松,内部层片更薄,有最高的空隙率和最大的平均空隙大小。传统涂层介于二者之间。纳米涂层和传统涂层均表现出双态空隙大小分布。涂层的导热系数均随着温度的上升而升高。传统涂层的热导率最高,纳米涂层与空心球涂层的热导率相接近。纳米涂层具有最好的隔热性能,空心球涂层接近纳米涂层的隔热效果。隔热效果与涂层厚度呈线性关系。随着厚度增加,导热系数低的纳米涂层和空心球涂层的隔热效果增长幅度高于传统涂层。     

New material concepts for the next generation of plasma-sprayed thermal barrier coatings

In application as a thermal barrier coating (TBC), partially stabilized zirconia (Zr) approaches some limits of performance. To further enhance the efficiency of gas turbines, higher temperature capability and a longer lifetime of the coating are needed for the next generation of TBCs. This paper presents the development of new materials and concepts for application as TBC. Materials whose compositions have the pyrochlore structure or doped Zr are presented in contrast with new concepts like nanolayers between the top and bond coat, metal-glass composites, and double-layer structures. In the last concept, the new compositions are used in a combination with Zr, as a double, multi, or graded layer coating. In this case, the benefits of Zr will be combined with the promising properties of the new top coating. In the case of metal-glass composites, the paper will be focused on the influences of different plasma spraying processes on the microstructure. The performance of all these different coating systems has been evaluated by burner rig tests. The results will be presented and discussed.     

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.     

Structure of plasma-sprayed zirconia coatings tailored by controlling the temperature and velocity of the sprayed particles

The correlation between particle temperature and velocity with the structure of plasma-sprayed zirconia coatings is studied to determine which parameter most strongly influences the coating structure. The particle temperature and velocity are measured using an integrated optical monitoring system positioned normal to the spraying axis. The total porosity, angular crack distribution, and thermal diffusivity are correlated with the particle temperature and velocity. Results show that the temperature of the sprayed particles has a larger effect on the coating properties than the velocity in the conditions investigated.     

Wear characteristics of plasma-sprayed nanostructured yttria partially stabilized zirconia coatings

Reconstituted nanostructured and conventional yttria partially stabilized zirconia coatings were deposited by atmospheric plasma spray. The tribologic properties of the coatings against 100C6 steel were evaluated with a ball-on-disc configuration under dry friction conditions at room temperature. Microstructure and the phase composition of the powders and the coatings were examined using a scanning electron microscope, optical microscope, and x-ray diffraction. Microhardness and the Young’s modulus of coatings were measured by indentation testing. Results showed that the wear resistance of the coatings produced using the nanostructured powder is improved compared with the coating produced using the conventional powder. The wear rates of nanostructured zirconia coatings are about four-fifths of those of conventional counterparts under a load of 5 N. The wear mechanism is also discussed. The original version of this paper was published as part of the DVS Proceedings: “Thermal Spray Solutions: Advances in Technology and Application,” International Thermal Spray Conference, Osaka, Japan, 10–12 May 2004, CD-Rom, DVS-Verlag GmbH, Düsseldorf, Germany.     

Elastic modulus measurements in plasma sprayed deposits

A commercial hardness indenter has been modified to record load displacement as a spherical ball is elastically loaded onto the surface of the material to be measured. The resulting data are used to calculate the elastic modulus. This technique has been used to characterize the elastic modulus of zirconium oxide-8% yttrium oxide plasma sprayed deposits. Moduli were measured both on the cross section and on the plan section, and the differences were correlated with the microstructure. Since relatively small areas of the material were sampled by the indenter, local mapping of elastic modulus variations on the size scale of the microstructure was possible. A periodic variation in modulus with position in the cross section was found on a length scale that corresponded to the average plasma spray pass thickness. Elastic modulus variations also have been found on a macro scale through the thickness of freestanding plasma sprayed deposits. These large scale variations were probably a result of self annealing during the production of these thick samples. Finally, significant increases in elastic modulus have been found in samples annealed for a total of 2.5 h at 1100 °C. These changes have been correlated with small angle neutron scattering measurements of void surface area.     

Sintering and creep processes in plasma-sprayed thermal barrier coatings

During operation at elevated temperatures, sintering processes can significantly influence the mechanical properties of thermal barrier coatings (TBCs) by increasing Young’s modulus and reducing strain tolerance. These changes of the mechanical response of TBCs were investigated using free-standing plasma-sprayed TBCs in a thermomechanical analysis (TMA) facility. The time-dependent change of Young’s modulus was determined in situ in a flexure mode at different annealing temperatures. In addition, relaxation processes during loading and unloading were monitored. The time-dependent deformation behavior of the TBC sample can be described by a simple viscoelastic approach (Burgers model). Viscosity data are determined as a function of annealing temperature and time.     

激光重熔对等离子喷涂热障涂层冲蚀行为影响

研究了等离子喷涂和激光重熔ZrO<,2>-7%Y<,2>O<,3>热障涂层的微观结构,同时考察了两种涂层的抗冲蚀性能,并探讨了其冲蚀破坏机理.试验发现,等离子喷涂热障陶瓷涂层呈典型的层状堆积特征;经过激光重熔处理后,涂层表面形成了沿热流方向生长的柱状品重熔区;相对于等离子喷涂试样,激光重熔涂层有较好的抗冲蚀性能;不管等...     

X-ray diffraction characterization of crystallinity and phase composition in plasma-sprayed hydroxyapatite coatings

Orthopedic and dental implants consisting of a metallic substrate plasma spray coated with hydroxyapatite (HA) are currently used in reconstructive surgery. The crystalline phases present in the calcium phosphate ceramic and the degree of crystallinity must be controlled for medical applications. X-ray diffraction (XRD) is routinely employed to characterize the phase composition and percent crystallinity in both biological and sintered HA. However, application of the same XRD methods to plasma-sprayed coatings is complicated by the potential presence of several crystalline contaminant phases and an amorphous component. To overcome the complexities of characterizing plasma-sprayed HA coatings, an external standard method of XRD quantitative analysis has been developed that can be applied nondestructively. Data collection and reduction strategies allowing separation of intensity diffracted from commonly occurring phases and the amorphous fraction are presented. The method is applied to coating samples, and detection limits and sources of error are discussed. Repeability and accuracy are demonstrated with powder mixtures of known composition.     

Failure of physical vapor deposition/plasma-sprayed thermal barrier coatings during thermal cycling

ZrO₂-7 wt.% Y₂O₃ plasma-sprayed (PS) coatings were applied on high-temperature Ni-based alloys precoated by physical vapor deposition with a thin, dense, stabilized zirconia coating (PVD bond coat). The PS coatings were applied by atmospheric plasma spraying (APS) and inert gas plasma spraying (IPS) at 2 bar for different substrate temperatures. The thermal barrier coatings (TBCs) were tested by furnace isothermal cycling and flame thermal cycling at maximum temperatures between 1000 and 1150 °C. The temperature gradients within the duplex PVD/PS thermal barrier coatings during the thermal cycling process were modeled using an unsteady heat transfer program. This modeling enables calculation of the transient thermal strains and stresses, which contributes to a better understanding of the failure mechanisms of the TBC during thermal cycling. The adherence and failure modes of these coating systems were experimentally studied during the high-temperature testing. The TBC failure mechanism during thermal cycling is discussed in light of coating transient stresses and substrate oxidation.