This paper compares two methods for determining the composition of Ti/TiN coatings deposited by reactive plasma spraying. The coatings were obtained by spraying titanium powder in a low-pressure N2/Ar atmosphere. The resulting film has a variable nitrogen content in the form of titanium nitrides, depending on gas partial pressure, total pressure, sample-source distance, and other parameters. The composition of the film was determined using X-ray diffraction and X-ray photoelectron spectroscopy. The two techniques provide similar results and either can be used for the compositional characterization of these coatings.

Ultra-fine hydroxyapatite powders were successfully synthesized using radio frequency (RF) suspension plasma spraying (SPS). This novel technique utilises the inherent characteristics of the RF plasma to axially feed and spheroidise a liquid suspension to produce spherical ultra-fine HA powders. This offers an alternative approach over conventional D.C. and flame spheroidising techniques which are better suited for solid feed stocks. Rietveld analysis was subsequently applied using Rietquan Quantitative Analysis software package to determine the amount of decomposed phases and amorphous content of the as-sprayed powder. This was also compared against quantitative XRD analysis employing internal and external standards. However, pure phases needed for calibration is scarce and amorphous calcium phosphate (ACP) is virtually impossible to isolate. In addition, the long and laborious task of obtaining calibration curves makes this technique unpopular. Nevertheless, conventional quantitative phase analysis (QPA) was carried out, using relative peak height ratios of HA and the phase involved, but the calculated decomposition only shows relative trends for a particular parameter variation. Determining the actual phase content is critical because of possible variations in biological responses when used as coatings and inserts in restorative orthopaedic implants. Varying tissue responses can arise from decomposed phases such as α and β-tricalcium phosphate (TCP) and tetra-calcium phosphate (TTCP) as well as ACP which generally have higher solubility as compared to crystalline. QPA via the Rietveld method provides a powerful tool that offers the user simultaneous quantitative phase determination of multiphase systems containing amorphous content. Unlike XRD QPA, the amorphous content could be indirectly calculated using crystalline alumina standard. XRD QPA results showed that decomposition generally rose with plate power without considering the amorphous content. With Rietveld QPA, the results showed an initial rise in decomposition before decreasing at higher plate powers. The amorphous phase content was calculated at different plate powers and concentration of suspension with the aid of alumina as an external standard. Results showed that the amorphous content increased substantially at higher powers. This study demonstrates the ability of Rietveld analysis to completely quantify all associated amorphous and crystalline phases within a multiphase system for any thermally treated material.

The monoclinic and tetragonal phase compositions and distribution in air plasma sprayed (APS) yttria-partially stabilized zirconia (YPSZ) thermal barrier coatings were studied. The coatings were produced from powders with varying phase concentrations, chemical purity and powder production processes. Both the powder and coatings were characterized using X-ray diffraction (XRD) and Raman spectroscopy. The use of environmental scanning electron microscopy (ESEM) and X-ray energy dispersive analysis (EDS) added morphological and elemental information to the study. XRD and Raman spectroscopy were shown to be powerful combined tools and shows an overall decrease in the monoclinic phase within the coatings produced from the different powders. The distribution of both the monoclinic and the tetragonal phases could be highlighted both in the coatings and the individual powder particles. This indicated changes in monoclinic concentration in the less dense areas of some of the coatings and a varying distribution across particles in some of the powders. Raman mapping over small areas also showed how phase surface distribution, on the coatings surfaces, could be assessed.

Producing nanostructured materials through metastable phases is an interesting novel route in the field of ceramic materials. Due to their small grain size and uniform structure, these nanostructured bulk materials exhibit very interesting properties. Metastable coatings can be produced starting from microstructured powders through atmospheric plasma spray (APS) technique, followed by a quenching route. The initial powders are melted during the spraying and deposited over a substrate that is quenched with liquid nitrogen (LIN) feeders, producing metastable coatings. The thermal sprayed coatings have been characterized using XRD, SEM, FESEM and EDS in the Thermal Spray Centre (CPT) of the University of Barcelona. The properties of such coatings have been also studied obtaining promising results.

The low temperature characteristic of cold spraying makes it possible to deposit the coating of temperature sensitive materials, such as nanostructured material, without any significant change in the microstructure of feedstock. In the present study, the Fe and Si powders of particle size less than 75 µm were mixed at a composition of 10wt%Si and ball-milled to produce the nanostructured feedstock. Cold spraying process was used to deposit coating with nitrogen as a driving gas at different temperatures. The microstructure of the as-sprayed nanostructured Fe-Si coating was characterized using optical microscopy, scanning electron microscopy and transmission electron microscopy (TEM). The grain sizes of the feedstock and as-sprayed coating were estimated based on X-ray diffraction analysis. The results showed that the nanostructured Fe-Si coating can be deposited by cold spraying using the ball-milled powders as feedstock. The as-sprayed coating presented a dense microstructure. The average grain size of the as-sprayed coating was comparable to that of the corresponding milled feedstock. No significant effect of the temperature of driving gas on the microstructure of cold-sprayed nanostructured Fe-Si coating was recognized. Moreover, TEM analysis showed that the amorphous phase was present in the as-milled powders and the as-sprayed coating along with the nanocrystalline.

In the present work, pure Al and Al-Al 2 O 3 composite coatings are deposited by cold spraying while measuring in-flight particle velocities. Residual stresses, evaluated using the Almen curvature method, X-day diffraction, and modified layer removal, are correlated with particle velocity, coating thickness, and alumina content. Peening stresses due to plastic deformation were estimated to be less than 100 MPa and are shown to be nearly constant through the thickness of the coatings.

Most of the work published to date on thermally sprayed titanium has been carried out in controlled atmospheres, yielding little information about the reaction of titanium with nitrogen and oxygen. The aim of this study is to investigate the influence of atomization gas on the formation of titanium nitrides and oxides during wire arc spraying. In the experiments, three types of gases (air, nitrogen, and argon) are used to deposit Ti on steel substrates and the microstructure and composition of the coatings, as well as the wire feedstock, are assessed by means of SEM and XRD analysis. The effect of spraying distance on crystal structure and nitrogen content is also investigated in the case of the argon-atomized coating. Paper includes a German-language abstract.

A reaction-formed NiAl intermetallic compound (IMC) powder has been deposited as a coating onto low carbon steel test coupons by the High Velocity Oxy-Fuel (HVOF) process using both gaseous and liquid fuels. The microstructure of this coating has been examined using scanning electron microscopy and x-ray diffraction and was found to depend on spraying conditions. Oxidation tests on the coating in air, between the temperatures of 800°C-1200°C, revealed that an α-alumina (Al 2 O 3 ) scale formed on the coating's surface. At 1200°C, a nickel spinel (NiO/NiAl 2 O 4 ) and haematite (Fe 2 O 3 ) phases were observed. Diffusion studies were performed to calculate an activation energy for iron ion diffusion in NiAl.

Zircon is widely used as a refractory material, because of its excellent mechanical and chemical properties. Several studies on plasma sprayed zircon were reported since 70's, and it is known that zircon dissociate into silica and zirconia during the plasma spray process. Authors have been studied on plasma sprayed zircon for a protective coating application, and successfully obtained very dense coating with excellent adhesive strength by optimizing the spray parameter. However, it was also revealed that the coating had poor stability above 1500K. In this study, the effects of two different oxides additive (yttria and ceria) on the structure and stability of the plasma sprayed zircon coating above 1500K are evaluated. The addition of these oxides enhanced the amount of residual zirconia and decreased zircon after the heat treatments. Addition of yttria resulted in the coating composed of cubic zirconia and zircon, while monoclinic zirconia was formed by ceria addition.

Molybdenum disilicide (MoSi 2 ) is a suitable material for high temperature applications especially because of its excellent high temperature oxidation resistance. For several high temperature applications MoSi 2 shows high potential to be used as a protective coating. The oxidation behaviour of HVOF sprayed MoSi 2 coatings is studied at 1500 °C. The oxidation tests are carried out in a simultaneous thermogravimetric device and the mass change is measured in dependence on the oxidation time. The microstructure of the coatings before and after oxidation is examined by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXS). The mass of the coating increases according to a parabolic function. During the oxidation test the microstructure changes significantly from a typical thermal spray coating microstructure with lamellae, pores and a phase mixture of MoSi 2 and Mo 5 Si 3 to a two phase system with sharply separated grain boundaries. On the surface of the coating a silicon dioxide layer with a thickness of less than 10 µm is formed.

Obtaining dense ceramic coatings by thermal spraying still remains a challenge. Compared to metals, ceramics have a lower thermal conductivity and a larger melting enthalpy. These factors limit the heat transfer from the plasma to the particles and consequently do not necessarily allow their total melting. Problems linked to this heat transfer can be avoided, or at least limited, by using agglomerated particles made of a mixture of reactive powders yielding the ceramic material, via SHS (Self-propagating High-temperature Synthesis) reaction. In this case, the reaction can be ignited by the heat transfer at the particle surface of an agglomerate and propagate towards the centre during its flight through the plasma. The application of this process to Ti, C mixtures leads to the formation of a dense TiC based coating. The composition of the coating, influenced by the contamination of the surrounding gas entrainment during the spray process, belongs to the TiC-TiO solid solution. The influence of experimental parameters on the coating composition is discussed.

In our earlier studies we reported the excellent steam oxidation resistance of 50Ni-50Cr coatings produced by high velocity oxyfuel (HVOF) spray process. The coating produced by atmospheric plasma spray (APS) for the thickness of 40 µm yielded the scale incorporation into the coating structure on the steam oxidation due to its porous structure compared to HVOF process. The steam could penetrate into the pores and resulted in the scale initiation at the coating-substrate interface. To improve the steam oxidation resistance of the coating, thick coatings (~450 µm) of 50Ni-50Cr with sealant layer were tested in the steam. The sealant layer was applied to fill the pores produced by the APS process. The results showed that the thick coating of 50Ni-50Cr with sealant was able to protect against the steam oxidation till 3000 hours of tested duration.

This work investigates the biofunctionality and corrosion resistance of titanium (Ti) and Ti/bioglass composite coatings. Commercially pure Ti (CP-Ti) and 45S5 bioglass powders were deposited on CP-Ti plates by air plasma spraying and the coating samples were placed in Hanks’ balanced salt solution for simulated body fluid (SBF) testing. After four weeks of immersion, the coatings were examined by SEM imaging and EDS and XRD analysis. EDS analysis showed that the Ca content on the Ti/bioglass coatings increased from 4 to 16 wt%, while no increase in Ca was observed on the Ti coatings. Hydroxyapatite formation was found on both coatings, although the relative intensity of HA on the XRD spectrum was higher for the Ti/bioglass composite layers. Weight measurements before and after immersion showed that the CP-Ti samples experienced a mass gain and that the Ti/bioglass samples underwent a mass loss likely due to the dissolution of calcium and phosphate.

The cold spray process was used to prepare nanostructured WC-Co coatings. The coating microstructural characteristics and phase composition were analyzed via optical microscopy, SEM and XRD. The morphology and microstructure of the nanostructured WC-Co powder were also analyzed by SEM and XRD. A 10µm thick coating was achieved. The results show that there is no degradation of the WC-Co powder during the cold spray process and well-bonded and phase-pure WC coating can be produced by the cold spray process.

In this study, HVOF sprayed NiCr alloy coatings on A213 TP347H boiler steel are evaluated by severe corrosion tests in a high-temperature molten salt environment. XRD and FE-SEM/EDS analysis results are presented and discussed and correlated with corrosion kinetics.

FeAl iron-aluminide based materials with the ordered B2 structure are excellent candidates for use in high temperature applications because of the combination of good mechanical properties, low density, low cost and availability of raw materials, and improved oxidation resistance. The aim of this article is to produce an ultra-fine grained FeAl coating by HVOF thermal spraying of milled powders and characterize the fine scale features of its microstructure. Comparison is made with a more conventional coating obtained by projection of powders obtained by atomization. Starting powders and coatings were investigated using X-ray diffraction and transmission electron microscopy. It was observed that the coating obtained from milled powders had a microstructure essentially characterised by a nanometer grain size and the presence of a disordered FeAl phase.

In low-pressure plasma spraying, a plasma jet generator with a supersonic expansion nozzle is useful for spray coating hard and large-area films adhering strongly to substrates. In the expansion nozzle, the pressure and the electron density drastically decrease downstream, and therefore the plasma is in thermodynamic nonequilibrium state. Additionally, the supersonic expanding plasma jet is expected to be in chemical nonequilibrium state in which excited plasma particles are carried downstream in chemically-active state. In this study, titanium nitride (TiN) reactive spraying was carried out under a low-pressure environment using a DC arc plasma jet generator with a supersonic expansion nozzle. Titanium powders were injected using a hollow cathode with argon gas, and the plasma gas was nitrogen or nitrogen and hydrogen mixture. Microstructure and properties of the coatings were examined using scanning electron microscope (SEM) and X-ray diffraction (XRD). A dense and high-quality TiN coating with a Vickers hardness of 2000 was formed at a low substrate temperature of 700 °C with a low input power of 5.3 kW. All results showed that the supersonic plasma jet in thermodynamic and chemical nonequilibrium state had high potentials for reactive spraying.

Today, powder particles diameter used for thermal spraying is generally comprised between 5 and 100µm with a preferred range around 40µm for APS applications. Actually, the future trends in plasma spraying are directed to the use of fine or ultrafine powders and the reduction of the steps between raw materials and coatings. So, the present paper investigates the way to use directly spray dried ceramic powders in suppressing the sintering stage. AI2O3 based powders were obtained by the spray drying process. By optimizing the parameters (slurry composition and injection as well as drying characteristics), a narrow grain size distribution was achieved. Chemical composition and shape of synthesized powders were analyzed by scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS). The crystallographic structure was identified by X-ray diffraction (XRD). Demonstration was made that it is possible to obtain coatings using directly spray dried ceramic powders. The plasma spray process parameters (such as current intensity, gas flow rate, powder feed rate and injection mode, cooling stage,...) have to be managed to achieve cohesive coatings. The structure and chemical composition of these coatings were studied. In this way, the direct use of spray dried powders appears as a promising way to realize ceramic coatings.

TiO 2 photocatalyst in the form of coatings are of many advantages over those in powder form in practical applications. Various processes have been used to form TiO 2 coatings including sputtering, Sol-Gel, vapor deposition, thermal spraying, etc. In all those processes, the coatings are deposited under temperatures from 300 to more than 2000 °C. High temperature during those processing may change the microstructure of the as-received TiO 2 to a less effective one for photocataltyic performance. In the present study, TiO 2 coating was deposited through cold spraying process using two types of powders, which were agglomerated with anatase ultra-fine particles in micro-size and nano-size. Microstructures of both powders and deposited coatings were characterized using X-ray diffraction and scanning electron microscopy. The photocatalytic performance was examined through acetaldehyde degradation under ultraviolet illumination. The results showed that the nanostructured TiO 2 coatings were evenly deposited on stainless steel substrate through cold spraying. The thickness of the deposits reached up to 15 µm. The coating presented a rough surface and porous structure. Owing to the low temperature of spray powders, no change occurred to the phase and grain size of TiO 2 during deposition process. It was also found that the cold sprayed TiO 2 deposits were photocatalytically active for photodegradation of acetaldehyde.

Wollastonite and its composite coatings with zirconia and titania have been deposited by plasma spraying. The bioactivity of coatings was evaluated using a simulated body fluid soaking test. The fetal rat’s osteoblasts were seeded on the surface of the coatings to examine their biocompatibility. The SEM and XRD technologies were used to examine the morphologies, structure and composition of the surface of the coatings soaked in the simulated body fluid. The results obtained showed that carbonate-containing hydroxyapatite was formed on the surfaces of wollastonite, wollastonite/ZrO 2 and wollastonite/TiO 2 coatings, while was not formed on the surfaces of ZrO 2 and TiO 2 , indicating the wollastonite improved the bioactivity of ZrO 2 and TiO 2 coatings. Osteoblasts are able to survive and proliferate on the surfaces of wollastonite, wollastonite/ZrO 2 and wollastonite/TiO 2 coatings. It is enough to prove that these coatings possess excellent biocompatibility.