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.     

High weibull modulus HVOF titania coatings

The mechanical behavior of high-velocity oxyfuel (HVOF) sprayed titania (TiO₂) coatings was evaluated using Vickers hardness measurements on the cross section and top surface. The distribution of hardness values for the cross-section and top surface under 25, 50, 100, 300, 500, and 1000 g loads was analyzed via Weibull statistics. The coating microstructure was evaluated using scanning electron microscopy (SEM). It was observed that the microstructural features were similar in the top surface and cross-section, different from the lamellar structure commonly found in thermal spray coatings. X-ray diffraction (XRD) analysis identified rutile as the major coating phase. The in-flight sprayed particle parameters such as temperature and velocity were determined using a commercial diagnostic system based on pyrometry and time-of-flight measurements. The uniformity of the microstructure resulted in a near isotropic behavior of the mechanical properties, such as hardness, in the coating cross-section and top surface. High Weibull modulus values were observed when compared with results of other thermal spray coatings available in the literature. These initial results should contribute to a more general understanding of the conditions necessary to achieve coatings with high uniformity and assist in the engineering of coating microstructures for specific applications.     

Towards engineering isotropic behaviour of mechanical properties in thermally sprayed ceramic coatings

It is widely recognized by the scientific community that thermal spray coatings exhibit anisotropic behaviour of mechanical properties, e.g., the elastic modulus values of the coating in-plane (i.e., parallel to the substrate surface) or through-thickness (i.e., perpendicular to the substrate surface) will tend to be significantly different due to their anisotropic microstructures. This work shows that thermally sprayed ceramic coatings may exhibit isotropic mechanical behaviour similar to that of bulk materials even when exhibiting the typical anisotropic coating microstructure. Elastic modulus values on the in-plane and through-thickness directions were measured via Knoop indention and laser-ultrasonic techniques on a coating produced via flame spray (FS) using a nanostructured titania (TiO₂) powder. No significant differences were found between the coating directions. In addition, four major cracks with similar lengths were observed originating near or at the corners of Vickers indentation impressions on the coating cross-section (i.e., a typical characteristic of bulk ceramics), instead of two major cracks propagating parallel to the substrate surface, which is normally the case for these types of coatings. It was observed by scanning electron microscopy (SEM) that coatings tended to exhibit an isotropic behaviour when the average length of microcracks within the coating structure oriented perpendicular to the substrate surface was about twice that of the microcracks aligned parallel to the substrate surface. Modelling, based on scalar crack densities of horizontal and vertical cracks, was also used to estimate when thermal spray coatings tend to exhibit isotropic behaviour.     

Significance of quenching stress in the cohesion and adhesion of thermally sprayed coatings

In thermal spraying, molten particles strike a solid surface, where they are flattened and quenched within a very short time. Considerable in-plane tensile stress on the order of 100 MPa can develop within each splat during quenching after solidification because thermal contraction of the particle is constrained by the underlying solid. Ni-20Cr alloy and alumina powders have been plasma sprayed in air onto steel substrates that were maintained at about 473 K. The influence of spraying conditions such as spray distance on the magnitude of the quenching stress have been studied by measuring the curvature of the substrate during spraying. Mechanical properties such as Young’s modulus and bend strength of the deposited coatings have also been measured. A strong correlation was found between the quenching stress and the strength of Ni-20Cr coatings, which suggests that the strength of interlamellar bonding limits the quenching stress at such temperature. Presented at ITSC ’92, June, 1992, Orlando, Florida.     

Elastic and inelastic effects in compression in plasma-sprayed ceramic coatings

Four-point bend tests of a plasma-sprayed zircon (ZrSiO₄) coating are presented, the coating being prepared by atmospheric plasma spraying (APS) spraying onto a titanium alloy substrate, using a water-stabilized plasma (WSP) torch. The mechanical behavior in compression of the coating material is nonlinear, including hysteresis (9% of the maximum strain) and permanent deformation. The effective in-plane Young’s modulus is about 13 GPa for very small strains and doubles for a compressive strain of −0.18%. Possible micromechanisms to explain this behavior are proposed, and some general protocols concerning the testing of plasma-sprayed ceramic coatings are deduced. Mr. Harok has since left the Institute of Plasma Physics, and can now be contacted at Bontaz Centre CZ, V. Noveho 973, 33701 Rokycany, Czech Republic.     

Structure/property relationships in sintered and thermally sprayed WC-Co

Thermally sprayed WC-Co is widely used as a wear-resistant coating for a variety of applications. Although it is well established that thermal spray processes significantly affect chemistry, microstructure, and the phase distribution of WC-Co coatings, little is known about how these changes influence wear resistance. In this study, the microstructure and wear behavior of sintered and thermally sprayed WC-Co materials are examined. Powders of WC-12 wt% Co and WC-17 wt% Co were pressed and sintered, as well as thermally sprayed by high-velocity oxy-fuel (HVOF), air plasma spray (APS), and vacuum plasma spray (VPS) techniques. Results indicated considerable differences in the resulting microstructures, mechanical properties, and wear resistance. The thermally sprayed coatings showed anisotropic fracture toughness, whereas the sintered materials did not. It was also shown that a combined mechanical property/microstructure parameter, based on considerations of indentation fracture mechanisms, can be used in most cases to describe abrasive and erosive wear resistance of thermally sprayed WC-Co materials as follows: Wear resistance a whereK _ic is the indentation fracture toughness,H is hardness, andV ^Co _f is the volume fraction of cobalt. This relationship provides a means for assessing wear resistance of WC-Co coatings intended for industrial applications requiring abrasion and/or erosion resistance.     

Elastic modulus of SiC_w/6061Al alloy composites as-squeeze-cast

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Laser-assisted spraying and laser treatment of thermally sprayed coatings

In the present study, surface engineering research related to thermally sprayed wear and corrosion resistant coatings modified by in-situ and post-treatment by novel high power lasers (Nd:YAG and diode lasers) has been carried out. The main aim of the study was to create experimental based information and knowledge on simultaneous remelting of thermally sprayed (plasma, HVOF, flame) single layers for production of metallurgically bonded, dense corrosion and wear resistant coatings on surfaces of steels. The main focus of research was to estimate the needed processing parameters, which enable simultaneous laser-assisted thermal spraying. The microstructures and hardness-profiles for the coatings prepared by laser-assisted thermal spraying are also introduced.     

Young's modulus and stress intensity factor determination of high velocity electric arc sprayed metal-based ceramic coatings

In this paper, a commercial metal-based ceramic coating has been used to prepare the specimen adopting high velocity electric arc spraying (HVAS) technology. The three-point bend tests were proposed to measure the Young's modulus (E) of the coating. A test specimen capable of measuring the complex stress intensity factor of bi-material interfacial crack has been devised. The finite element method (FEM) was conducted to investigate the stress fields in the vicinity of the crack for the bend specimen, and the complex stress intensity factor and its phase angle have been computed through the stress field. The test results of determining E of the coatings showed the Young's modulus increased with the increase of coating thickness. The reason was that the rise of coating thickness resulted in the reduction of porosity in the coatings, so the Young's modulus followed it. The toughness test results showed that crack initiation of the interfacial crack occurred in the range of linear elastic for the whole specimen. In addition, the analysis on validity of K showed that the stress intensity factor was able to be used as the fracture-controlled parameter of interface crack for the specimen geometry and loading pattern in this paper.     

Determination of residual stress distribution from in situ curvature measurements for thermally sprayed WC/Co coatings

An in situ monitoring of curvature of the specimen during spraying using a high speed video system was implemented to determine stresses in thermally sprayed WC/Co coatings. Influences of different spray-ing techniques (atmospheric plasma spraying and high-velocity oxygen fuel) and cooling levels were con-sidered using a mathematical model. Results show that temperature history of a part is of paramount importance in stress generation and distribution.     

Thermal conductivity and elastic modulus evolution of thermal barrier coatings under high heat flux conditions

Laser high heat flux test approaches have been established to obtain critical properties of ceramic thermal barrier coatings (TBCs) under near-realistic temperature and thermal gradients that may be encountered in advanced engine systems. Thermal conductivity change kinetics of a thin ceramic coating were continuously monitored in real time at various test temperatures. A significant thermal conductivity increase was observed during the laser-simulated engine heat flux tests. For a 0.25 mm thick ZrO₂-8% Y₂O₃ coating system, the overall thermal conductivity increased from the initial value of 1.0 W/m K to 1.15, 1.19, and 1.5 W/m K after 30 h of testing at surface temperatures of 990, 1100, and 1320 °C, respectively, Hardness and elastic modulus gradients across a 1.5 mm thick TBC system were also determined as a function of laser testing time using the laser sintering/creep and microindentation techniques. The coating Knoop hardness values increased from the initial hardness value of 4 GPa to 5 GPa near the ceramic/bond coat interface and to 7.5 GPa at the ceramic coating surface after 120 h of testing. The ceramic surface modulus increased from an initial value of about 70 GPa to a final value of 125 GPa. The increase in thermal conductivity and the evolution of significant hardness and modulus gradients in the TBC systems are attributed to sintering-induced microporosity gradients under the laser-imposed high thermal gradient conditions. The test techniques provide a viable means for obtaining coating data for use in design, development, stress modeling, and life prediction for various TBC applications.     

A cantilever beam method for evaluating Young’s modulus and Poisson’s ratio of thermal spray coatings

Young’s modulus and Poisson’s ratio for thermal spray coatings are needed to evaluate properties and characteristics of thermal spray coatings such as residual stresses, fracture toughness, and fatigue crack growth rates. It is difficult to evaluate Young’s modulus and Poisson’s ratio of thermal spray coatings be-cause coatings are usually thin and attached to a thicker and much stiffer substrate. Under loading, the substrate restricts the coating from deforming. Since coatings are used while bonded to a substrate, it is desirable to have a procedure to evaluate Young’s modulus and Poisson’s ratio in situ. The cantilever beam method to evaluate the Young’s modulus and Poisson’s ratio of thermal spray coat-ings is presented. The method uses strain gages located on the coating and substrate surfaces. A series of increasing loads is applied to the end of the cantilever beam. The moment at the gaged section is calcu-lated. Using a laminated plate bending theory, the Young’s modulus and Poisson’s ratio are inferred based on a least squares fit of the equilibrium equations. The method is verified by comparing predicted values of Young’s modulus and Poisson’s ratio with reference values from a three-dimensional finite ele-ment analysis of the thermal spray coated cantilever beam. The sensitivity of the method is examined with respect to the accuracy of measured quantities such as strain gage readings, specimen dimensions, ap-plied bending moment, and substrate mechanical properties. The method is applied to evaluate the Young’s modulus and Poisson’s ratio of four thermal spray coatings of industrial importance.     

纳米压痕法测定微铸件硬度及弹性模量

利用新的金属型微精密铸造工艺制备微米尺度的ZnAl4微齿轮铸件,并用纳米压痕仪测试微铸件的硬度和弹性模量。结果表明,微齿轮铸件不同部位的硬度表现出梯度性,随晶粒尺寸的增加而减小,弹性模量的值比较分散。与常规尺寸铸件的显微硬度相比,微铸件硬度明显提高,最大达到1．70倍,而弹性模量则下降约50％。分析表明快冷造成的非平衡凝固组织是性能改变的主要原因。     

Corrosion performance of HVOF and APS thermally sprayed NiTi intermetallic coatings in 3.5% NaCl solution

Amorphous/nanocrystalline Ni-Ti powders produced by low energy mechanical alloying were used as feedstock to deposit NiTi intermetallic coatings on 316L stainless steel substrate using high velocity oxy-fuel (HVOF) and air plasma spraying (APS) processes. Electrochemical impedance spectroscopy (EIS) and polarization tests indicated that the corrosion performance and passive behaviour of HVOF coating were far better than those of APS coating. The study also showed that the solution had penetrated through the coating microcracks and caused interior corrosion of APS coating, while the HVOF coating was immune from interior corrosion attack and consequently exhibited a good passive behaviour during long-term immersion.     

Use of neutron diffraction for stress measurements in thin and thick thermal sprayed coatings

Abstract

Thermal spraying is a widely used and cost effective technique for the surface protection of engineering components. The spectrum of applications is vast: corrosion protection, wear resistance and abrasion resistance, thermal barriers, electrical (dielectric) coatings, etc. Process induced residual stress has long been recognised as an important factor influencing the integrity and overall performance of coatings. Residual stress generation during thermal spraying is a complex phenomenon. Significant efforts have been made to improve understanding of the evolution of residual stresses during deposition and to develop practical models for numerical prediction of stress distributions in coatings. Owing to the high penetrating power of neutrons and spatial resolution in the millimetre and submillimetre range, neutron diffraction is, perhaps, the most versatile method for stress determination, and has been used extensively for experimental validation of theoretical predictions. Examples of neutron diffraction residual stress results are presented to illustrate the capabilities of the technique: a thin (～0·3 mm) Mo/Mo₂C composite HVOF coating, several examples of millimetre thick ceramic and metallic coatings, and thick coatings (～10 mm) of iron made by spray forming.     

Characteristics of the nanostructures in thermal sprayed hydroxyapatite coatings and their influence on coating properties

There are generally two methods for depositing nanostructured coatings, retaining the nanostructures from starting feedstock and forming novel nanostructures through quenching. The present study utilized spray-dried nanostructured hydroxyapatite (nSD-HA) feedstock for coating/splat deposition. The nanostructures were characterized by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). Results revealed that the rod-shaped nano-grains in the starting HA particles (< 500 nm in length and 40-70 nm in diameter) encountered two major experiences: enlargement due to unmelt state and reorganization due to melting-solidification. The molten part of the particles results in formation of spherical nanosized particles with grain sizes of 30-110 nm. TEM observation of the HA splats demonstrates consistent nanostructures. The unmelted part of individual nSD-HA particles showed significantly enlarged grains along radial direction (< 550 nm in length and < 400 nm in diameter). In addition, individual hexagonal grains were observed in the HVOF coating made from 30 ± 10 μm powder. The grains have the size of < 250 nm in height and < 50 nm in side length and are perpendicular to the coating/substrate interface. The nanostructures within the coatings contribute to an increased Young's modulus with up to 60.11 GPa, however, they showed detrimental effect on adhesion of the coatings. In vitro cell culturing revealed marked attachment and proliferation of the osteoblast cells on the nanostructured coatings. However, the results suggest that the nanostructures possess less importance than the phases (preferably high content of crystalline HA) on enhancing the cell proliferation.     

Corrosion behaviour of thermally sprayed Al and Al/SiCp composite coatings on ZE41 magnesium alloy in chloride medium

Thermally sprayed Al and Al/SiCp composite coatings have been deposited on ZE41 magnesium alloy and mechanical compaction at room temperature was applied to the Al and Al/SiCp coatings to reduce their porosity. Corrosion behaviour of coated samples was evaluated and compared to that of uncoated substrate in 3.5 wt.% NaCl solution using electrochemical measurements. Al and Al/SiCp composite coatings reduced the corrosion current density of Mg-Zn alloys by three and two orders of magnitude, respectively, and reductions up to four orders of magnitude were obtained after mechanical compaction.     

低温处理对SiCw／Al复合材料声学特性及弹性模量的影响

对压铸态20%（体积分数）SiCw/6061Al复合材料进行了低温处理,研究了低温处理前后复合材料声学特性及显微组织的变化,揭示出低温处理对复合材料弹性模量的影响。结果表明,低温处理可造成复合材料超声波声速及超声波衰减系数增大,同时复合材料的弹性模量得到改善。     

Microstructure and stresses in HVOF sprayed iron aluminide coatings

The microstructure and state of stress present in Fe₃Al coatings produced by high velocity oxygen fuel (HVOF) thermal spraying in air at varying particle velocities were characterized using metallography, curvature measurements, x-ray analysis, and microhardness measurements. Sound coatings were produced for all conditions. The microstructures of coatings prepared at higher velocities showed fewer unmelted particles and a greater extent of deformation. Residual stresses in the coatings were compressive and varied from nearly zero at the lowest velocity to approximately −450 MPa at the highest velocity. X-ray line broadening analyses revealed a corresponding increase in the extent of cold work present in the coating, which was also reflected in increased microhardness. Values of mean coefficient of thermal expansion obtained for assprayed coatings using x-ray analysis were significantly lower than those for powder and bulk alloy.     

Elastic properties of Ni and Ni + Mo coatings electrodeposited on stainless steel substrate

Adhesion coefficient and Young's modulus of Ni and Ni + Mo coatings electrochemically deposited on stainless steel were examined by applying vibrating reed technique. It was shown that adhesion coefficient of the Ni coating slightly decreases (about 8%) with increasing layer thickness (5-40 μm). Young's modulus E_f of these coatings at room temperature was found to be about 130 GPa. The relative adhesion coefficient of the Ni layer decreases with increasing temperature (300-600 K) in relation to the thinnest examined layer (5 μm). Young's modulus of the Ni + Mo coatings decreases with increasing Mo content; for 9 wt.% of Mo E_f = 40 GPa and for 32 wt.% of Mo E_f = 23 GPa.