Preparation of cross sections of thermal spray coatings for TEM investigation

A technique for the preparation of cross sections for transmission electron microscopy (TEM) of thermal spray coatings has been developed. The procedure is designed to minimize specimen damage during mechanical thinning and to reduce the effect of differential thinning during ion milling. Specimens were made by two different coating systems— WC- Co coating produced by the FARE Gun process on a mild steel substrate and Tribaloy T- 800 sprayed by the HVOF process on a nickel- base superalloy. These specimens have large areas that are electron transparent on either side of the interface, and the results have shown the atomic scale microstructure of the interface between the thermal spray coating and the substrate.     

Optimized multiaxis robot kinematic for HVOF spray coatings on complex shaped substrates

High accuracy robot systems are required for an efficient and reproducible application of thermal spray processes. Due to the influence of the torch trajectory on the heat and mass transfer processes during the deposition of APS and HVOF coatings, specific software tools for the simulation, generation and implementation of these trajectories must be developed. The internal HVOF series coating process of cylinder bores in lightweight engine manufacturing, the series production of optimized oxide ceramic coatings on textile surfaces and the trajectory generation and simulation for the coating of a complex 3D surface will be presented as case studies of these technologies.     

Study of microstructure, phases and microhardness of metallic coatings deposited by flame thermal spray

The recharging technique by thermal spraying offers the opportunity of renovating the worn surface parts of a machine element to give it again a new technical life despite its previous degradation in service. This process has consequently interesting economic impacts. In order to improve the adherence between 100Cr6 steel deposits and the substrate material (left worn crankshafts), company SNC ATRA of Béjaïa uses at present a composite formed by (100Cr6 steel/molybdenum bond coat of 0.2 mm thick/crankshaft substrate). As a matter of fact, it is shown in the present work that the molybdenum bond coat is not appropriate since, for the 0.2 mm thickness, lateral cracks are observed in the middle of the bond coat. On the other hand, our experiment is that a deposit of 100Cr6 steel projected directly on the substrate seems more promising since no gaps or cracks were detected at the “deposit/substrate” interface of this two-material composite. Lastly, phase analysis using X-ray diffraction confirmed that during spraying process, a stable -phase (bcc) of 100Cr6 wire was transformed to a new phase of γ-phase (fcc). The coatings exhibited the higher microhardness which would contribute to increase wear resistance.     

Microstructure evolution and thermal stability of an Fe-based amorphous alloy powder and thermally sprayed coatings

High velocity oxy-fuel (HVOF) thermal spraying has been used to produce coatings of an Fe–18.9%Cr–16.1%B–4.0%C–2.8%Si–2.4%Mo–1.9%Mn–1.7%W (in at.%) alloy from a commercially available powder (Nanosteel SHS7170). X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) were employed to investigate the powder, as-sprayed coatings and annealed coatings which had been heated to temperatures in the range of 550–925 °C for times ranging from 60 to 3900 min. Microhardness changes of the coatings were also measured as a function of annealing time and temperature. The powder was found to comprise amorphous and crystalline particles; the former had a maximum diameter of around 22 μm. The coating was composed of splat like regions, arising from rapid solidification of fully molten powder, and near-spherical regions from partially melted powder which had a largely retained its microstructure. The amorphous fraction of the coating was around 50% compared with 18% for the powder. The enthalpies and activation energies for crystallization of the amorphous phase were determined. Crystallization occurred in a two stage process leading to the formation of α-Fe (bcc), Fe_1.1Cr_0.9B_0.9 and M₂₃C₆ phases. DSC measurements showed that the first stage occurred at 650 °C. Annealing the coating gave a hardening response which depended on temperature and time. The as-sprayed coating had a hardness of 9.2 GPa and peak hardnesses of 12.5 and 11.8 GPa were obtained at 650 and 750 °C, respectively. With longer annealing times hardness decreased rapidly from the peak.     

Hydroxyapatite coatings for biomedical applications deposited by different thermal spray techniques

Thermally sprayed hydroxyapatite (HAp) coatings are widely used for various biomedical applications due to the fact that HAp is a bioactive, osteoconductive material capable of forming a direct and firm biological fixation with surrounding bone tissue.Bioceramic coatings based on nanoscale HAp suspension and microscale HAp powder were thermally sprayed on Ti plates by high-velocity suspension flame spraying (HVSFS) technique and atmospheric plasma spraying (APS) as well as high velocity oxy fuel spraying (HVOF) technique. HVSFS is a novel thermal spray process developed at IMTCCC, for direct processing of submicron and nano-sized particles dispersed in a liquid feedstock.The deposited coatings were mechanically characterized including surface roughness, micro hardness and coating porosity. The bond strength of the layer composites were analyzed by the pull-off method and compared for the different spray techniques. Phase content and crystallinity of the coatings were evaluated using X-ray diffraction (XRD). The coating composite specimen and initial feedstock were further analysed by scanning electron microscope (SEM) and rheology analysis.     

Microstructural and mechanical characterization of Ni-base thermal spray coatings deposited by HVOF

The present work has been conducted in order to determine the microstructural features, hardness and elastic modulus of two different Ni-base coatings deposited by means of HVOF thermal spray, onto a SAE 1045 plain carbon steel substrate. The morphology and chemical composition of the phases that are present in the coatings were characterized by means of SEM, EDS and XRD techniques. Image analysis was used for the evaluation of the coatings porosity. Both conventional and instrumented indentation tests were also carried out on the surface and cross section of the coatings, in order to evaluate the effect of coating microstructure on hardness and elastic modulus. Conventional indentation tests were conducted using a Knoop indenter and a maximum load of 9.8 N. Instrumented indentation tests, in which the indenter depth and applied load were recorded continuously, were carried out employing a Vickers indenter and maximum loads of 0.49, 0.98, 1.96, 4.9 and 9.8 N. Instrumented nanoindentation tests (in a continuous stiffness measurement mode) were also conducted employing a Berkovich indenter with a maximum load of 9.8 N. The elastic modulus was computed by means of the Oliver and Pharr method and compared with the values determined by means of the method earlier advanced by Marshall et al. The results obtained indicate that the elastic modulus values determined on the cross section of the coatings are higher than those obtained on the surface, clearly indicating the anisotropy of the structure. Also, the values found employing a Berkovich indenter are very similar to those derived by means of the Vickers indenter. In addition, the these values are in agreement with those determined by taking into consideration the elastic recovery of the short Knoop diagonal after removal of the load.     

Advanced microstructural study of suspension plasma sprayed titanium oxide coatings

Suspension plasma sprayed titanium oxide coatings were analyzed using transmission electron microscope (TEM) and using Raman spectroscopy. The suspensions used to spray were formulated using fine rutile pigment, water, alcohol or their mixtures, and a small quantity of dispersant. TEM study realized using a “face-to-face” preparation technique enabled to visualize a lamellar shape of grains and their columnar growth. The Raman spectroscopy was made for the samples prepared using different operational spray parameters. The investigations showed the presence of rutile and anatase phase in coatings sprayed using rutile fine powder.     

TEM study of TiN coatings fabricated by mechanical milling using vibration technique

A mechanical milling method was used for the deposition of TiN coatings. The principle of this method is that a substrate and powder were placed along with balls into the vibration chamber that was vibrated by a mechano-reactor. During mechanical milling process, the substrate surface was impacted by a large number of flying balls. The TiN particles trapped in between the balls and the substrate became cold welded to the surface. The repeated substrate-to-ball collisions forged TiN particles into a coating on the substrate surface. The process allowed the thick TiN coatings to be produced at room temperature in an ambient atmosphere. TEM study of the as-fabricated coatings was carried out. The coating formation depended on the size of the initial TiN particles. The 50-nm TiN nanoparticles were more easily cold welded than 1.5-μm microparticles. The nanoparticles had a tendency to consolidate and densify into the bulk material under the applied compressive loading. The TiN particles better consolidated and densified on the hard Ti surface than on the soft Al one.     

Comparison of aluminum coatings deposited by flame spray and by electric arc spray

Aluminum coatings are widely used as a protection against corrosion on steels substrates. Among the various deposition processes, thermal spray has proved to be a process that is easy to use and offers a long-lasting alternative in maintenance operations. Recent studies [Rodriguez, R.M.H.P. Formação de óxidos nos revestimentos de alumínio depositados por aspersão térmica. Curitiba-PR: ST, UFPR, 2003. Tese (Doutorado) -- Programa Interdisciplinar de Pós -- Graduação em Engenharia -- PIPE, Universidade Federal do Paraná, 2003. 118p; Paredes, R.S.C. Estudo de revestimentos de alumínio depositados por três processos de aspersão térmica para a proteção do aço contra a corrosão marinha. Florianópolis-SC:ST, UFSC, 1998. Tese (Doutorado) - Departameto de Engenharia Mecânica, Universidade Federal de Santa Catarina, 1998. 245p] demonstrated that the source and the gas used for aluminum spraying generate dissimilar corrosion protection behaviors. This paper purports to contribute toward a better understanding of the differences between the flame spray (FS) and electric arc (EA) processes. Initially, the coatings were analyzed by XPS (X-ray Photoelectron Spectroscopy), which revealed differences resulting from the deposition processes. To deepen the investigation into the differences relating to the heat source utilized, the coatings were subjected to a 4000-hour saline mist assay. The corrosion products formed on the surface of the coatings were evaluated based on photographic records, SEM, and X-ray diffraction. The XPS analysis of the aluminum coatings revealed that the aluminum oxide/hydroxide layer persisted at a greater depth in the coating deposited by EA than in that deposited by FS. This difference may also indicate the formation of different aluminum compounds. An analysis of the coatings subjected to the saline mist assay revealed clear differences in the corrosion products formed, which were strongly adherent in the FS coating and only slight adherent in the EA coating, leading to loss in thickness. X-ray diffraction revealed that the principal phase was bayerite in the FS process and boehmite in the coating deposited by EA. It should be noted that the heat source and the deposition material are determining factors in the protection process. In the period during which the coatings of this study were subjected to saline mist, both the FS and EA deposition methods provided satisfactory barrier protection.     

Interfacial indentation and shear tests to determine the adhesion of thermal spray coatings

Adhesion is one of the most important parameters which influences the development of thermal spray coatings. Therefore, the level of adhesion should be known for a given application. Apart from the standardized Tensile Adhesive Test (TAT), more than 80 methods are reported to measure the coating adhesion. Most of them are energy consuming in terms of time, cost and equipment. Moreover, they do not fulfil the necessary requirements of accuracy, confidence and representation of the real delamination process observed in service. To address this problem, the interfacial indentation test is used here to initiate and propagate a crack at the interface between the substrate and the coating. Studying the extension of the crack, an interfacial toughness is defined and deduced analytically from the experimental results. The new shear test, developed in the frame of the EU-CRAFT-project “Shear Test for Thermally Sprayed Coatings”, is also employed to assess the coating adhesion. Both tests are compared to the standardized TAT for various spraying systems, materials, substrate roughness and coating thickness. Advantages and disadvantages of the three tests are discussed. Correlations between the tests results obtained for different coating-substrate combinations are presented and general trends are described.     

The effect of heat treatment on thermal stability of Ti matrix composite

The microstructures of in situ synthesized (TiB + La₂O₃)/Ti composite after β and TRIPLEX heat treatment are investigated. The room temperature tensile properties of the titanium matrix composites (TMCs) are tested, and thermal stability is carried out at 600, 650 and 700 °C for 100 h, respectively. The results show that the microstructure of specimen after β heat treatment is widmanstätten, while it is similar to basketweave after TRIPLEX heat treatment. Room tensile properties of specimen after TRIPLEX heat treatment are better than those of β heat treatment. After thermal exposure, the strength of specimens treated by β and TRIPLEX heat treatment increases, while the ductility decreases sharply, this is attributed to the precipitation of Ti₃Al and silicides. The thermal stability of specimen after TRIPLEX heat treatment is better than that after β heat treatment.     

Tialite formation from its quasi-amorphous stoichiometric co-precipitated powder by thermal spray process

In this work, the formation of tialite from powders obtained by the co-precipitation method with inorganic salts was investigated. For the co-precipitation process, a solution of aluminum nitrate (Al(NO₃)₃  9H₂O) and titanium tetrachloride (TiCl₄) in ethyl alcohol was co-precipitated with an increase in ammonium hydroxide base. Phase transformations of the quasi-amorphous dried gel with stoichiometric composition of tialite treated in a muffle furnace and in an atmospheric plasma thermal spray deposition were explored. The effect of calcination thermal treating on the tialite formation was evaluated for different temperatures (400, 600, 800, 900, 1000, 1200, 1300, and 1400 °C). The powder was thermally treated by calcination at 600 °C for 6 h prior to the thermal spray process. Dried gel pieces were also calcined at 600 °C for 6 h and ground, the produced powders were thermally sprayed onto a stainless steel substrate and the formed coating was mechanically detached from the substrate and then characterized. It was confirmed that the thermal spray process transformed the quasi-amorphous powders to single tialite phase. The absence of the corundum and rutile phases was attributed to the high heating and cooling rates (~ 10⁶ K/s) supplied by thermal spray processing.     

High temperature erosion of Cr3C2-NiCr thermal spray coatings — The role of phase microstructure

Cr₃C₂-NiCr thermal spray coatings are extensively applied to mitigate erosion at temperatures above 450-550 °C. The aim of this work was to extend the current comparison based knowledge towards a mechanistic interpretation of the high temperature erosion of Cr₃C₂ based thermal spray coatings. Coatings that span the range of industrial quality were assessed. They were eroded under high temperature (700 °C and 800 °C), aggressive (impact velocity 225-235 m/s) conditions designed to simulate the high velocity erodent impacts within a turbine environment. The influence on the erosion response of high temperature induced changes in the coating microstructure and composition with extended in-service exposure was assessed by heat treating selected samples to generate a steady state microstructure prior to testing. In spite of the marked variation in coating microstructure the erosion rates were comparable across the range of coatings tested. The significance of this conclusion is discussed in terms of the erosion mechanism.     

Monoclinic zirconia distributions in plasma-sprayed thermal barrier coatings

Phase composition in an air plasma-sprayed Y₂O₃-stabilized ZrO₂ (YSZ) top coating of a thermal barrier coating (TBC) system was characterized. Both the bulk phase content and localized pockets of monoclinic zirconia were measured with Raman spectroscopy. The starting powder consisted of ∼15 vol.% monoclinic zirconia, which decreased to ∼2 vol.% in the as-sprayed coating. Monoclinic zirconia was concentrated in porous pockets that were evenly distributed throughout the TBC. The pockets resulted from the presence of unmelted granules in the starting powder. The potential effect of the distributed monoclinic pockets on TBC performance is discussed.     

Evaluating the oxidation resistance of bond coats on thermal barrier coatings

Coatings are being increasingly used as an engineering alternative for advanced projects. Various techniques and processes are available for applying coatings, depending on the specific situation they are intended for. Thermal barrier coatings, known as TBCs, form part of a particular series of metal-ceramic coatings that are traditionally used in the aeronautical industry and are being increasingly applied in the automotive industries and for industrial turbines. One of the main issues with TBCs is degradation owing to the oxidation of the bond coat in high temperatures, resulting in the failure of the coating due to peel off. This study investigates and compares the behaviour of the oxidation of the TBC bond coat when the material used is NiAl alloy; this alloy is commonly used because of its characteristics at high temperatures and to ensure strong adhesion on various substrates. The bond coat was applied on an AISI 1020 Steel substrate using the flame spraying process. In order to carry out isothermal oxidation tests, the furnace used was regulated at a temperature of 1000 °C in static air. The samples were exposed for 24, 48 and 96 h and cooling was carried out in atmospheric air at ambient temperature. The analysis of the thermally grown oxide for each sample was carried out based on the exposure times and the oxide rates were evaluated by measuring the mass gained by the samples with oxidized coatings and using Scanning Electron Microscopy and Optical Microscopy.     

Efficient stochastic simulation of thermal spray processes

Thermal spray processes allow the production of coatings with a wide variety of useful properties, and therefore are used in a lot of different industrial applications. However, in specific applications, it can be a challenging and time consuming task to tune the parameters of a thermal spraying system so that required surface properties, such as wear- , corrosion- or heat resistance, can be achieved. Computer-aided stochastic simulations of thermal spray processes try to overcome these problems by modelling the coating build-up on a per-droplet basis, and by deriving the properties of interest from the generated structure afterwards. Because of the large number of droplets that need to be treated during the simulation to generate a sufficiently large virtual coating, the use of efficient algorithms is essential. On the other hand more complex algorithms are needed to increase the accuracy of the simulation.In this report we present novel extensions to a simulation model by Ghafouri et al. concerning plasma spraying. The extensions include a physically inspired splat spreading approach, a method to generate splat fingers, and a filtering technique that allows for the calculation of pores below overlapping splats. Furthermore, features characterizing coatings beyond the commonly used porosity or surface roughness are presented and analyzed with respect to their appropriateness. Features are considered appropriate if they are depending in a unique way on the parameters of the simulation model. This kind of features might be used in an automatic adaptation of the simulation model to real processes. Finally, several contributions are made in order to increasing the computational efficiency of the simulation. Most important, dexel models are introduced to represent the coating.     

Ti- 6Al-4V/hydroxyapatite composite coatings prepared by thermal spray techniques

The poor mechanical properties of hydroxyapatite (HA) can be enhanced by forming a composite with a bioinert and mechanically strong metal alloy such as Ti-6A1-4V. Biomedical composites composed of titanium alloys and HA can offer concomitant bioactive properties as well as good mechanical strength and toughness. This paper describes an attempt to improve coating mechanical properties by forming a composite composed of HA and Ti-6A1-4V. Several compositions (20, 33, and 80 wt % HA) were prepared. Subsequent examination of the plasma-sprayed coatings revealed alternating HA-rich and titanium-rich lamella microstructures. The HA-rich regions appeared porous as a result of poor interparticle adhesion, with the 80 wt% HA coatings having the highest porosity. Mechanical property analysis showed the 20 wt% HA coating to have the highest storage modulus (∼60 GPa). This coating also had the highest bond strength (≥20 MPa max). The coatings tended to exhibit increased bond strength at thicknesses less than or equal to 60 μm. The excellent bond strength of the Ti-6A1-4V/HA composite is caused by the superior interfacial bond between the Ti-6Al-4V-rich splats and the substrate. The encouraging development of this composite raises the possibility of its use as a bond coat for plasma-sprayed HA on titanium-alloy implants.     

Hydrogen-sorption and thermodynamic characteristics of mechanically grinded TiH1.9 as studied using thermal desorption spectroscopy

Isobaric thermal desorption spectroscopy and X-ray diffraction analysis were used to study the influence of mechanical dispersion during high-energy milling γ-TiH_1.9 hydride upon its hydrogen-sorption characteristics, temperature and enthalpy of the γ → β (TiH_1.9 → Ti[H]_β) phase transition at isobaric heating the sample under hydrogen atmosphere with speed 3°/min. Isobars of hydrogen thermal desorption in the regions of the γ and β phases of the Ti-H₂ system at pressures of 0.1, 0.25, 0.315 and 0.45 MPa of hydrogen in the reactor have been derived. Experimental data obtained for initial titanium hydride and mechanically grinded for 20 min in a planetary ball mill have been used for construction of Van’t Hoff plots and for determination of enthalpy of formation of γ-hydride from solid solution of hydrogen in bcc titanium. Our experimental data reveal that 20 min high-energy influence on titanium hydride powder leads to increasing the specific surface of the samples from 0.13 to 8.58 m²/g and to significant (more than 250°) decreasing the temperature of the beginning of hydrogen release when heating the sample (i.e., to a decrease of thermal stability of mechanically activated TiH_1.9). However, mechanical dispersion does not change the temperature of the γ → β phase transition. It has been established that high-energy milling TiH_1.9 powder causes the effect of a decrease of enthalpy of the formation of γ-hydride from 248 kJ/mole H₂ to 175 kJ/mole H₂.     

Fundamentals of adhesion of thermal spray coatings: Adhesion of single splats

Indentation experiments were performed inside a scanning electron microscope to measure adhesive strength of individual alumina splats on a steel substrate. The in situ nature of experimental evaluations made characterization of interfacial crack propagation possible by direct observation. The increase in the strain energy of brittle alumina splats originating from indentation deformation was correlated to the strain energy release rate through the characterization of interfacial crack propagation. An analytical model previously reported and evaluated in studies of the adhesive strength of thin films was employed. An average calculated strain energy release rate of 80 J m⁻² was found for single splats. This high value suggests that splat adhesion can make a significant contribution to the adhesion of thermal sprayed coatings.     

An investigation of thermal aging effects on the mechanical properties of a Ni3Al-based alloy by nanoindentation

Ni₃Al-based intermetallic alloys are composed of a γ network and γ′ domains. In general, through precipitation, the γ′ phase is used as a hardening source for this alloy. This study examined how γ′ acts as a hardening material in a cast Ni₃Al-based intermetallic alloy. Specimens cut from a centrifugally cast tube were aged in Ar at elevated temperatures for up to 1600 h. Hardness tests were then performed in air at room temperature. Vickers microhardness and nanoindentation tests were carried out on specimens aged at 900 °C and 1100 °C. The microstructures were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microhardness of bulk Ni₃Al decreased dramatically with increasing thermal aging time at 900 °C, but the nanohardness measured by the nanoindenter did not significantly decrease. The nanohardness data suggested that the hardening effects were caused by the precipitation of the γ′ phase on the γ and γ′ cells.