Search Results for microstructure

This is part II of the two part paper based on international collaboration between the University of Limoges, France and the State University of New York, Stony Brook, USA, aimed at fundamental understanding the relationship between processing condition and microstructure development and properties of thermally sprayed materials. In this study, the effects of deposition temperature on the microstructure and properties development of molybdenum coating was investigated. It is found that with the increase of steel substrate temperature, the molybdenum splat morphology changes from fragmented to more contiguous disk-like shape. The splats on molybdenum substrate show predominantly disk shape. With the increase in deposition temperature, the coating exhibits better lamellar structure with less interlayer pores and debris. The fracture characteristics changes gradually from interlamellar to trans-lamellar and, thermal conductivity is enhanced. Higher deposition temperature improves dramatically the adhesion and bonding of the splats, therefore the physical and mechanical properties of coatings.

The microstructure of arc sprayed stainless steel 316L coatings appears mainly in bright white matrix, deteriorated layers (grey), and black pores under optical microscopy. The black pores and the chromium-depleted areas in the deteriorated layers are known as the factors for decreasing the ability of protecting substrate under corrosive environments. Results of experiments in this paper suggests, in the condition of this study besides the factors mentioned above, Fe-Cr oxides should be another factor of dominating the corrosion resistance in the coatings. It also describes that the quantity and the distributions of such oxides are great influence on the corrosion behaviors. In this study, two kinds of coatings were used, one with thick deteriorated layers and another with thin deteriorated layers, which were sprayed on mild steel substrate by air atomization and nitrogen atomization respectively. Salt spray test and salt-water dip test were carried out to investigate corrosion behavior in macro and micro view. An effect of sealing treatment on the performance of the coatings was also examined. Results of metallographic examination and image processing analysis are well supported by a detailed investigation of corrosion behaviors of individual phases.

The aim of this work is to determine how to control the microstructure and tribological properties of HVOF-sprayed TiC composite coatings. The powders used in the study were made by the SHS process and contained a mixture of TiC and either FeCr20Ni10 or FeCr18Ni15Mo3, which serve as a binder and give the sprayed coatings additional corrosion resistance. The composites produced were assessed based on metallographic examination and wear testing. The results show how the structure of the SHS powder changes due to the injection molding process and how the tribological properties of the HVOF layers are influenced by spraying conditions and the formation of mixed carbides. Paper text in German.

Laser post-treatments and plasma-laser hybrid spraying processes are increasingly being used to extend the service life of thermal barrier coatings by making them more resistant to thermal shock. Studies show that laser-induced cracking plays a major role in the improvements achieved. The investigation of such modified layers can be difficult, however, because the stresses associated with metallographic procedures can alter the structural features of segmented microcracks and damage the specimen. In this research, laser treated and laser hybrid sprayed thermal barrier coatings are vacuum impregnated with fluorescent epoxy resins in order to study their microstructure and its relationship with thermal shock resistance. All relevant processes are described along with crack formation behaviors. Paper includes a German-language abstract.

The microstructure of plasma-sprayed deposits (PSD) is dominated by two void systems - interlamellar pores and intralamellar cracks - each with a different anisotropy. Varying anisotropics and crack-to-pore ratios within PSDs are responsible for the anisotropic properties observed in the deposits. While it is difficult to apply standard porosity measurement techniques to the assessment of anisotropic microstructures, novel techniques utilizing different approaches have recently emerged. Image analysis (IA) of impregnated PSD samples is the most direct technique. The structure is stabilized by impregnation and then polished and imaged. The limitations of IA lie in the impregnation process and in the subsequent polishing. Also, the images produced from anisotropic materials can be difficult to interpret quantitatively. The technique of small-angle neutron scattering (SANS) has recently been successfully applied to the study of PSDs. The major advantages of SANS are that it does not require sample preparation and that quantitative information can be gotten about the separate crack and pore systems, including their distinctive anisotropics. However, the relationship between the SANS results and the underlying structure is more complex and less intuitive than for IA, and the availability of the SANS technique is limited by the need to have access to a powerful neutron source, such as a reactor. Also, the two techniques present different views of the microstructure because of the different sensitivities in different parts of the size range. This paper compares results from IA and SANS from a set of thick plasma-sprayed ceramic deposits possessing a range of crack/pore microstructures, and discusses how the two techniques might complement one another.

The evolution of the void microstructure of yttria-stabilized zirconia (YSZ) plasma-sprayed deposits (PSD) was studied as a function of heating in air from room temperature to 1400°C , and during a constant temperature hold at 1100°C for 19 hours. The samples were studied using the terminal slope (Porod scattering) of small-angle neutron scattering (SANS), modified for the analysis of anisotropic structures. The experiment was done in-situ using a special furnace built for use on the small-angle scattering instrument. SANS Porod scattering can distinguish between the two major void systems - interlamellar pores and intralamellar cracks - that are present in the PSD microstructure. Thus, changes in the void surfaces of the cracks and the pores could be followed separately as a function of temperature. The surface area attributable to the interlamellar cracks significantly decreased at temperatures below 1000°C, whereas the surface area of the interlamellar pores only began to decrease at temperatures above 1000°C. This suggests that there are significant differences in the sintering of these two void systems, probably associated with differences in their sizes and shapes. The first noticeable changes in the void surfaces were observed at temperatures just above 600°C, which is a very low temperature for YSZ. During annealing at 1100°C for 19 hours, there was a decrease in the interlamellar surfaces of about 10%.

In this paper, a diffusion kinetic model was applied to simulate the microstructure development in a MCrAlY-superalloy system at high temperatures. Both simulation and experimental results showed that γ+γ’ microstructure was obtained in the coatings due to Al depletion after oxidation. With the help of the modelling, the mechanism of the formation of the diffusion zones in the single crystal (SC) superalloy can be also analyzed. The results revealed that the inward diffusion of Al from coating affected the depth of secondary reaction zone (SRZ) with the precipitation of TCP phases while the depth of inter-diffusion zone (IDZ) was decided by the inward diffusion of Cr.

This study developed microstructure-based finite element (FE) models to investigate the behavior of cold-sprayed aluminum-alumina (Al-Al2O3) metal matrix composite (MMCs) coatings subject to indentation and quasi-static compression. Based on microstructural features (i.e., particle weight fraction, particle size, and porosity) of the MMC coatings, representative volume elements (RVEs) were generated by using Digimat software and then imported into ABAQUS/Explicit. State-of-the-art physics-based modelling approaches were incorporated into the model to account for particle cracking, interface debonding, and ductile failure of the matrix. This allowed for analysis and informing on the deformation and failure responses. The model was validated with experimental results for cold-sprayed Al-18 wt.% Al2O3, Al-34 wt.% Al2O3, and Al-46 wt.% Al2O3 metal matrix composite coatings under quasi-static compression by comparing the stress versus strain histories and observed failure mechanisms (e.g., matrix ductile failure). The results showed that the computational framework is able to capture the response of this cold-sprayed material system under compression and indentation, both qualitatively and quantitatively. The outcomes of this work have implications for extending the model to materials design and under different types of loading (e.g., erosion and fatigue).

High-entropy alloys (HEAs) represent an innovative development approach for new alloy systems. These materials have been found to yield promising properties, such as high strength in combination with sufficient ductility as well as high wear and corrosion resistance. Especially for alloys with a body-centered cubic (bcc) structure, advantageous surface properties have been revealed. However, typical HEA systems contain high contents of expensive or scarce elements. Consequently, applying them as coatings where their use is limited to the surface represents an exciting pathway enabling economical exploitation of their superior properties. Nevertheless, processing conditions strongly influence the resulting microstructure and phase formation, which in turn has a considerable effect on the functional properties of HEAs. In the presented study, microstructural differences between high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) sprayed coatings of the alloy AlCrFeCoNi are investigated. A metastable bcc structure is formed in both coating processes. Precipitation reactions are suppressed by the rapid solidification during atomization and by the relatively low thermal input during spraying. The coating resistance to corrosive media was investigated in detail, and an improved passivation behavior was observed in the HVAF coatings.

The effect of deposition pressure on the microstructure and ablation behavior of ZrB2 coatings deposited by very low pressure plasma spraying is investigated. The results show that under a chamber pressure less than 50 kPa, as the spray chamber pressure decreases, the porosity of the coating deposited at the same distance decreases, and the coating prepared under 100 Pa presents the lowest porosity of 1.79 %. Furthermore, among the ZrB2 coatings deposited at 100 Pa, 5 kPa, 10 kPa and 50 kPa, the dense coating deposited at 100 Pa showed the lowest ablation rate of 0.33 μm/s, 0.75±0.08 mg/s.

This study investigates the microstructure and hardness of coatings produced by atmospheric plasma spraying using a commercial (Al,Cr) 2 O 3 solid solution (ss) powder blended with various amounts of TiO 2 . The microstructures were analyzed using SEM, EDS, and XRD measurements. It was shown that blending with TiO 2 reduces porosity and defect density while increasing deposition efficiency and microhardness. Small amounts of Ti in ss (Al,Cr) 2 O 3 splats were detected in coatings prepared from blends with higher TiO 2 content. Variations in aluminum and chromium content were also observed.

A CoNiCrAlY-hBN/Polyester material has recently been developed for clearance control applications in gas turbine compressors that use titanium alloy blades. While engine tests serve as the final evaluation of the coating performance, quality assurance laboratories and production shops would rely upon the more readily available coating hardness values to predict performance. This paper will focus on the reproducibility of coating macrohardness with a plasma spray process. It is shown that plasma spray parameters affect the hardness of CoNiCrAlY-hBN/Polyester coatings by changing the level of polyesters retained in the coating and the volume percentage of metallic matrix. The correlation between hardness, retained polyester level and microstructure of these coatings is captured in a coating hardness map from which desired microstructure and polyester entrapment are determined. Based on the understanding of the correlation between coating hardness and microstructural features, the use of additional criteria other than hardness such as retained polyester level and non-metallic portion of the coating is recommended in order to assure the quality of the coating more effectively.

Plasma spraying was used to produce continuously graded and layered structures of molybdenum disilicide and alumina. These microstructures were achieved by manipulating the powder hoppers and plasma torch translation via in-house created computer software. The resultant microstructures sprayed uniformly and were crack free. The mechanical and thermal performance of these sprayed materials will be evaluated through C-ring tests and thermal cycling experiments respectively. The purpose of this study is two fold; firstly to demonstrate the ability of produce such composite ceramic microstructures using a conventional plasma spraying process, and secondly to quantify the improvements in thermo-mechanical performance provided by these composite microstructures over conventional monolithic microstructures.

Cold work and heat treatment influence the mechanical properties, residual stress-state, and corrosion resistance of austenitic stainless steels. In this study we have examined changes in the defect substructure and microstructure of Type 304 stainless steel resulting from surface preparation, and deposition of bond coats and thick ceramic coatings using plasma spray methods. The structure of the stainless steel was examined as a function of depth from the coating surface using optical and transmission electron microscopy, and x-ray diffraction. Grit blasting was found to severely cold work the material to a depth of tens of microns, and the amount of cold work varied with measured abrasive particle velocity. The heat input to the surface as a result of depositing a metallic bond coat or thick ceramic coating resulted in substantial annealing of the cold work imparted into the substrate by surface preparation. There was, however, no evidence of change in grain size near the substrate-coating interface that could be attributed to recrystallization or grain growth in the substrate.

Titanium carbide cermet spray powder was produced by the SHS process (Self-propagating High-temperature Synthesis) using elemental Ti, C, Mo and prealloyed CrNiMo powders as starting materials. The powder was characterised (particle size distribution, phase structure, morphology) and the internal structure of each cermet particle was found out to be dense consisting of fine distribution of carbides embedded in a metallic matrix. The particle size range suitable for thermal spraying was obtained by sieving and air classifying. The coatings were prepared by HVOF spraying (DJH2600 and DJH2700). The dry abrasion wear resistance was evaluated by the rubber wheel abrasion wear test and electrochemical corrosion behaviour by open circuit potential measurements. According to the XRD analysis the amount of retained carbides in the coatings is high and the carbide phase has a spherical shape also in the coatings. The microstructure of coatings obtained is dense and the coatings possess good properties in wear and corrosion tests. WC-Co-Cr and Cr3C2-NiCr powders were used for comparison.

Plasma spraying is a very complex process, controlled by a large number of process parameters. The spray gun parameters control the plasma plume and thereby the velocity and temperature of the particles in the plasma. Some of the spray gun parameters are difficult or impossible to control, but variations of them give rise to fluctuations in the microstructure of the sprayed thermal barrier coating and thereby low reproducibility. By movement of the control from the spray gun to direct control of the particle properties in the plasma this problem will be avoided, and it should result in better process control, higher quality of the final coating and thus improved reproducibility. In this study, the influence of the plasma spray process on the coating microstructure was investigated. An orthogonal factorial designed experiment was performed, where eight process parameters were varied, resulting in 16 different coatings. The particle properties were observed in-situ with the optical measurement system DPV 2000. The microstructure of the coatings was studied using optical microscopy and the amount of different features, i.e. cracks and pores, was quantified. Multiple linear regression was used to find models describing the relation between the spray gun parameters and the particle properties, between the spray gun parameters and the microstructure, and between the particle properties and the microstructure. The results showed that the spray gun parameters well describe the variation in particle velocity and particle temperature. Further, it was found that particle velocity, particle temperature, spray angle, and substrate temperature are the most important parameters concerning influence on the coating microstructure. However, their influence on the different microstructure features varied. The study implies that focus can be set on one or two particle properties measured in the plasma, instead of the numerous spray gun parameters.

Some applications of thermally sprayed coatings need a metallurgical bonding of substrate and coating. This can be reached by laser remelting of a thermally sprayed coating, which causes, on the other hand, a certain dilution of the substrate elements into the coating. This article discusses the influence of reaction enthalpies on the microstructure formation in the alloying systems Ni-Al and Ti-Al. Experimental work and simulation were done to examine the time constants of solidification influenced by laser dwell time and reaction enthalpy. It was observed that, for short dwell times, the reaction heat dominates the solidification process and the microstructure formation.

Improved understanding of microstructure-property relationship can help to shift from experiment-based to science-based development of thermally spray deposits. This should result in shorter and less expensive development as well as in higher functionality and reliability of the deposits. Significant amount of work has been done, however, nearly always studying deposits manufactured by only one of the thermal spray techniques. Results are therefore often spray technique specific. A broad study with samples manufactured by a number of different thermal spray techniques seems to be missing yet. Relationships valid across different techniques should provide better understanding of the generic relationships. This research employs number of different techniques - flame, HVOF, plasma (APS, VPS, WSP), to generate a wide range of microstructures. Various Ni-based alloys are studied starting from a simple chemistry (Ni) and ending with complex NiCrAlY alloys. Presented results were obtained with NiCr (80% Ni, 20% Cr) feedstock. Microstructures are characterized by various techniques-OM, SEM, XRD, small-angle neutron scattering (SANS) and others - to obtain the most comprehensive set of macro to micro structural parameters available today. The wear and corrosion properties of these deposits are measured together with internal coating stresses and the most generic microstructure-property relationships are sought.

316L stainless steel and Hastelloy C alloy powders were sprayed by an HVOF apparatus onto mild steel substrates. The microstructure, pore size distribution, composition and corrosion resistance of thus obtained coatings were evaluated experimentally. Corrosion resistance in sea-water was examined by monitoring the impedance and corrosion potential of samples immersed in artificial sea-water at 300 K over a period of more than 3 months and also by polarization measurement. It was found that the stainless coatings composed mainly of plastically deformed particles and some splats which were molten at the impact. By increasing the combustion pressure, the porosity as measured by mercury porosimeter could be reduced to below 1%. In comparison, Hastelloy C deposits sprayed under the standard condition were so dense that its porosity could not be measured by the porosimeter. The polarization curve and the results of impedance monitoring both exemplified that the Hastelloy C coatings possess much superior corrosion resistance to the stainless coatings in sea-water, which was attributed to the higher density and better adhesion of the Ni-base alloy coatings.

In cutting tool technologies WC-Co based materials are increasingly replaced by composites containing TiC as hard phase and Ni or Co based metallic binders to improve life time and the performance at higher temperatures. These new light-weight materials are also promising for wear resistant coatings. However, while the production of WC-Co coatings by thermal spray techniques, especially high velocity oxy-fuel flame (HVOF) spraying, is well-established, thermal spraying of TiC-based powders did not lead to satisfactory results so far. This could be attributed to the oxidation during the spray process and the insufficient bond between hard phases and the metallic binder. Strategies to improve the properties of TiC-based coatings aim for microstructural modifications, especially by alloying additives into the thermal spray powder. By HVOF and vacuum plasma spraying (VPS), modified TiC-based coatings are produced, which globally show similar microstructures but significantly differ in their oxide contents. Investigations of mechano-technological properties and wear mechanisms demonstrate that alloying Mo into the hard phases or the metallic binder of thermal spray powders can improve the adhesion between hard phases and metallic binder of the coatings. In addition, properties of the metallic matrix can be tuned up for specific applications by solution hardening. In case of HVOF-coatings these effects are partially compensated by high oxygen contents. The overall better performance of coatings produced by VPS demonstrates that the high potential to improve properties of TiC-based composites by alloying additives can only be attained by minimizing the oxidation during the spray process.