HVOF coatings of 80Ni-20Cr and 50Ni-50Cr powders were carried out on 9Cr-1Mo steel substrate. The coating thickness was around 60 µm. The coated specimens were steam oxidized in four different temperatures, ranging from 600 to 750°C. The steam oxidized specimens were taken out from the chamber at 10, 100 and 1000 hours to examine their protectiveness against scale growth. X-ray diffraction, scanning electron microscope and electron probe microanalysis were carried out on the steam-oxidized specimens in reference with as-coated conditions. Both 80Ni- 20Cr and 50Ni-50Cr coatings show neither scale growth at the interface nor de-lamination in the coating structure. Fe, and Ni diffusion was found in the case of 80Ni-20Cr coatings. The diffusion increased with increase in the temperature and test duration. On the other hand, 50Ni-50Cr coatings, showed an excellent performance in all tested temperatures. The effect of Cr content in the coating, change in phase and compound formation during oxidation, and their influence on the diffusion process will be discussed in detail.

Thermal spray of Al was carried out on the modified 9Cr-1Mo steel to evaluate the steam oxidation resistance of the sprayed Al coating. Atmospheric plasma spray process (APS) was used to coat aluminum on sandblasted 9Cr-1Mo steel substrate. The coating thickness was around 40 µm. The coated specimens were steam oxidized in four different temperatures, ranging from 600 to 750°C. The results show that the scale growth occurred in the interface between coating and substrate subsequently it penetrated into the coating structure. Al diffused into the alloy substrate with high solubility. The diffusion increased with increase in the steam temperature and test duration. Diffused aluminum formed the high hardness intermetallic compound in the substrate near the coating/substrate interface. With increase in the test duration, the intermetallic compound moved towards the bulk and at prolonged aging, it became dissolved. This was identified from the decrease in the micro hardness values at coating/substrate interface at prolonged duration. The scale growth at the substrate surface of Al sprayed steel was much controlled compared to the uncoated specimens.

This study investigates the influence of heat treatment on the oxidation resistance of thermal barrier coatings. The coating systems examined incorporate an APS YSZ topcoat and a HVOF NiCo bond coat. Isothermal heating tests were conducted to determine how heat treat sequences affect interdiffusion between the bond layer and substrate and the microstructure of the bond coat and ceramic topcoat. Before and after heating, specimen cross-sections were studied by SEM and EDX analysis. Preliminary results indicate that the oxidation resistance of YSZ-NiCo coatings can be improved with a suitable heat treatment, in some cases, extending elevated-temperature service life by as much as 500 h. Abstract only; no full-text paper available.

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.

In this work, SiC coatings varying in content were prepared on carbon-fiber-reinforced silicon-carbide composite (C/SiC) substrates in order to study the effect of free silicon on oxidation resistance. The coatings were formed in a vacuum atmosphere by means of pack cementation using a powder mixture ranging in content as follows: 20-50 wt% SiC, 20-60 wt% Si, 7-12 wt% graphite, and 6-10 wt% Al 2 O 3 . Coating surface and cross-sectional morphologies were examined using SEM, EDS, and semiquantitative XRD analysis and oxidation resistance was determined by cyclic oxidation testing in air at 1300 °C. The results show that cracks and voids decrease with increasing free silicon content and that coatings with an appropriate amount of free silicon have better oxidation resistance than those with no free silicon at all. However, further increases in silicon content were found to be detrimental to oxidation behavior for a number of reasons that are discussed.

The basic aim of presented article is related with comparison of internal morphology of atmospheric plasma sprayed NiCrAlY coating and sintered alloy obtained from the same feedstock powder. This part of investigations consisted characterization of phases composition and internal morphology characterization by SEM/EDS method. The second level of this comparing process is characterization of oxidation process in both cases. In order to properly perform these tests the top surfaces of sintered sample was sand blasted to the roughness level adequate to surface condition after plasma spraying process. The oxidation tests were performed at temperature 1000 and 1100°C during the time to 1000h of exposure in air. The specimens after determined intervals were moving out from the furnace and characterized by visual inspection of top surface as well as by SEM/EDS method and XRD analysis to characterization of phases generated during oxidation. The special emphasis was on the characterization of TGO zone on cross sectioned samples where detailed assessment of oxides zone morphology was made by SEM/EDS method. The oxide thickness was measured as well with dividing of overall oxides zone thickness on the sublayers related with alumina and spinels formation.

The present study describes the development of agglomerated and sintered WC and Cr 3 C 2 based cermet powders with complex and highly corrosion resistant Fe and Ni matrix alloys. In addition a Fe-Cr-C with a Ni matrix was tested as a potential low cost alternative to standard Cr 3 C 2 -NiCr powders. The powders were sprayed by HVOF using liquid and gaseous fuel. Microstructure and phase composition of the coatings were analyzed by microscopy and X-ray diffraction. The technical properties of the coatings were examined by hardness measurements, abrasive wear and cavitation tests as well as corrosion tests. The results were compared with state-of-the-art coatings of WC-Co-Cr 86/10/4 and Cr 3 C 2 -NiCr 75/25.

MCrAlY(M=Ni, Co, or Ni-Co)coatings with good high temperature oxidation resistance have attracted great interest. They are widely used in gas turbines as protecting layers, such as thermal barrier coatings and seal coatings. Among many methods developed for preparing MCrAlY coatings, electroplating has drawn great attention due to its perfect bond strength, precise controllability, good coating ability for complex shape and so on. In this paper, the MCrAlY coatings have been prepared by a composite plating way. During this process, the CrAlY particles are wrapped with Ni clad layer. The thickness of the composite coatings is controlled at 150- 200 μm. The plating tests results indicate that the density of the clad layers mainly depend on the electroplating time. After that, these coatings are heat treated under the vacuum condition to make elements diffuse, forming homogeneous M(Ni)CrAlY component. The high-temperature oxidation resistance tests of the prepared coatings show good antioxidant ability at 1000 °C under air condition.

Thermal cycle resistance of Ni-20Cr, Ni-50Cr and CoNiCrAlY coatings manufactured by air plasma spraying was investigated according to a Japanese Industrial Standard "Testing method for thermal cycle resistance of oxidation resistant metallic coatings (JIS H 8452)” established in 2008. The specimens were exposed at 1000 °C and 1093 °C in air under cyclic heating and cooling condition up to 100 times. The thermal cycle resistance of oxidation-resistant metallic coatings was found to depend strongly on the testing temperature and the chemical composition of the coating materials. In the thermal cycle test at 1000 °C, the remarkable failure was not observed in any specimen. However, in the thermal cycle test at 1093 °C, although the Ni-20Cr coating caused the spalling on the whole surface of coating, the Ni-50Cr and the CoNiCrAlY coatings exhibited the excellent thermal cycle resistance even after applying the thermal cycles of 100 times, The CoNiCrAlY coating showed the mass gain with increasing the numbers of thermal cycle due to the preferential oxidation between the splats of the thermal spray particles. Furthermore, the failure behavior of specimens was investigated in detail by SEM, XRD and EPMA etc.

Light water reactors (LWR) use zirconium-alloy fuel claddings, the tubes that hold the uranium-dioxide fuel pellets. Zr-alloys have very good neutron transparency, but during a loss of coolant accident or beyond design basis accident (BDBA) they can undergo excessive oxidation in reaction with the surrounding steam environment. Relatively thin oxidation-resistant coatings on Zr-alloy fuel cladding tubes can potentially buy coping time in these off-normal scenarios. In this study, cold spraying, solid-state powder-based materials deposition technology has been developed for deposition of oxidation-resistant Cr coatings on Zr-alloy cladding tubes, and the ensuing microstructure and properties of the coatings have been investigated. The coatings when deposited under optimum conditions have very good hydrothermal corrosion resistance as well as oxidation resistance in air and steam environments at temperatures in excess of 1100 °C, while maintaining excellent adhesion to the substrate. These and other results of this study, including mechanical property evaluations, will be presented.

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.

The present work evaluates the oxidation and hot corrosion resistance of high velocity oxy-fuel (HVOF) sprayed WC-NiCrFeSiB coating deposited on Ni-based superalloy (Superni 75) and Fe-based superalloy (Superfer 800H). The coated as well as uncoated specimens were exposed to air and molten salt (Na 2 SO 4 -25%NaCl) environment at 800 °C under cyclic conditions. The thermogravimetric technique was used to establish the kinetics of corrosion. The corrosion products were characterized using the combined techniques of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe micro analyser (EPMA). The WC-NiCrFeSiB coating provides necessary resistance against oxidation and hot corrosion to both the nickel and iron based superalloys in the given environmental conditions at 800 °C. The oxides of active elements of the coatings, formed in the surface scale as well as at the boundaries of nickel and tungsten rich splats, have contributed for the oxidation and hot corrosion resistance of WC-NiCrFeSiB coatings, as these oxides act as barriers for the diffusion/penetration of the corrosive species through the coatings.

Several recently published studies have shown remarkable improvements in dry abrasion resistance and corrosion resistance of aluminum phosphate sealed oxide coatings when compared to unsealed ones. There are numerous applications in chemical industry where a corrosive environment is accompanied with abrasive or erosive particles. In this study the wet abrasion resistance and slurry erosion resistance of aluminum phosphate-sealed and unsealed oxide coatings were studied and compared to their dry abrasion resistance. In wet abrasion tests kaolin and water mixture was used as the abrasive. In slurry erosion tests several abrasives in water with various pH values was used as the erosive medium. The coatings were characterized for microstructure and their wear mechanisms were analyzed using SEM. The results from wear tests are reported and correlated with coating properties. The influence of coating quality to the relative improvement achieved by sealing is presented and discussed.

This work is concentrated on plasma sprayed cermet coatings consisting of stainless steel (SS) (17 wt % Cr and 12 wt % Ni) and chromium oxide (Cr 2 O 3 ). These powders were sprayed simultaneously, however being injected separately to account for the drastic difference in their thermo-physical properties. Chromium oxide was injected internally and stainless steel externally. The plasma parameters (arc current, hydrogen vol %, mass flow rate) were optimized as well as the injector positions. Coatings were achieved with different mass ratios of SS and Cr 2 O 3 . All exhibited a lamellar structure with well distributed Cr 2 O 3 and SS lamellae. They were then systematically characterized by their phase content (XRD), composition (EDS), Vickers micro hardness, morphology (SEM), slurry and dry wear resistances. Finally the best dry linear abrasion resistance was obtained for the pure chromium oxide coatings, while the best slurry wear resistance corresponded to coatings with a mass ratio SS/ Cr 2 O 3 of 56/44.

The paper deals with wear properties of various advanced oxide ceramic coatings deposited by plasma and HVOF spray processes. Several types of ceramic oxide coatings on the base of Al 2 O 3 , Cr 2 O 3 and TiO 2 are studied in the work. The coatings are characterised by their wear properties in slurry abrasion wear test with fine abrasives (kaolin) and in dry abrasion conditions with coarse abrasives (quartz sand). The surfaces of the wear tested coatings were studied by SEM. The results show that coatings with high homogeneity and minor porosity, especially HVOF sprayed and some plasma sprayed coatings perform extremely well in both of the wear tests used in the study. In the test with hard abrasive and high load, the coatings with lower interlamellar strengths seem to be more prone to wear than coatings with higher homogeneity, e.g. HVOF sprayed oxide ceramic coatings. The results can be explained on basis of type of the coating material and the coating microstructure.

In this paper Al-Al 2 O 3 -CrC nanostructured composite coating was presented, fatigue and fracture of the composite coating were investigated by nanoindentation, and in situ experiments performed in a scanning electron microscope to permit examination of freshly exposed surfaces. Crystallographic and morphological texture was characterized and the fracture resistance measured using fracture-mechanics. A CrC layer may improve the fracture resistance of an oxide aluminum layer. A CrC layer produced by pyrolitic deposition effectively heals the pores and defects of an oxide aluminum layer. It results in high load rating of the coating. Experiments revealed that in all cases, detection of an acoustic signal corresponded to an appearance of circular cracks seen on the surface; in a very few cases, examination of the surface after detection of a signal revealed the presence of two ring cracks. The degree of toughening associated with crack healing is determined by the number of healed defects and the effectiveness of the individual healing. Macroscopically, a crack path in the oxide aluminum appears to be straight, propagating along pores and internal voids. However, microscopically, a crack path exhibits a high degree of intergranular fracture. Because cracks generally deflect at small angles in the oxide aluminum layer, a crack path moves through pores and internal voids that usually concentrate internal stresses. The crack path is primarily intergranular at all velocities.

Previous work on thermal barrier coatings (TBCs) has shown that Ce-containing bond coats promote the formation of a wedge-like interface oxide that improves delamination resistance. The oxide was found to form at temperatures greater than 1100 °C, which in many applications, may not be reached. In this study, TBC samples consisting of a YSZ topcoat and various cold-sprayed bond coats were prepared. In order to obtain a wedge-like thermally grown oxide (TGO), pre-oxidation was carried out for 20 h at 1100 °C prior to high-temperature testing for 1000 h at 1000 °C. It was confirmed that the pre-oxidation treatment produced a wedge-like TGO that continued to grow at 1000 °C, which improved delamination resistance as four-point bend tests showed. A wedge-like oxide was also observed in some TBCs exposed to temperatures of 1000 °C, without pre-oxidation.

This study investigates the influence of plasma spray conditions on the structural, mechanical, and plasma-erosion properties of yttrium oxide coatings. Powder feed rate, plasma power, and primary particle size appear to have little effect on plasma erosion properties, hardness, surface morphology, and coating structure. However, a Y 2 O 3 coating produced with fine powder retained a smooth erosion surface despite its porosity and relatively low hardness.

In the current investigation plasma spray technique was used for depositing hydroxyapatite (HA) and hydroxyapatite – silicon oxide (SiO2) coatings on 316L SS substrate. In HA-SiO2 coating, 20 wt% SiO2 was mixed with HA. The feedstock and coatings were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) / energy-dispersive X-ray spectroscopy (EDX) analyses. The corrosion resistance of the uncoated, HA coated and HA + 20 wt% SiO2 coated 316L SS was investigated by electrochemical corrosion testing in simulated human body fluid (Ringer’s solution). After the corrosion testing, the samples were analyzed by XRD and SEM / EDX analyses. The addition of SiO2 reduces the crystallinity of the coating. The corrosion resistance of the 316L SS was found to increase after the deposition of the HA + 20 wt% SiO2 and HA coatings.

The CoCrAlSiY alloy powder with Si mass concentrations of 0, 2% and 5% was prepared in this work. The oxidation kinetics curves of all three kinds of powders after 300 h oxidation at 1000 °C were plotted. In addition, the phase constitution of alloy powder and the distribution of β phase were analyzed by SEM and EDS. Furthermore, the effect of Si-addition on the melting temperature and oxidation resistance of the alloy powder were investigated by DSC-TG from the room temperature to 1400 °C. And the element concentrations at the grain boundary of alloy powder with Si addition of 2% were also analyzed. The results show that the melting temperature of alloy powder decreases as increasing Si content, which indicates that adding Si element could influence on the selective oxidation of Al and Cr elements in the alloy system, and improve the oxidation resistance of CoCrAlY powder. In addition, the weight gain of powder with Si addition of 2% is lowest. And Si element has a enrich tendency in the grain boundary. Therefore, the higher Si content would have a negative effect on the high temperature oxidation resistance of powder.