Migration of Oxides in a Composite Coating and Its Effect on Inhibiting Coating Degradation

Based on the fact that the dispersed rare earth oxide particles in an electrolytic co-depositedNi-R_2O_3 layer migrated and accumulated at the substrate/coating interface immediately afteraluminizing,the mechanism of migration of the oxides and its effect on the behaviour of degradationof the coatings oxidized at 1100℃ were studied in this paper.The Ni-R_2O_3 layers were treated invacuum at 900℃ and aluminized at 1100℃ separately.The results indicated that the migration andaccumulation of R_2O_3 particles were favorable to the gathering of R_2O_3 at higher temperatures,andthe small size oxides were easier to migrate and gather.The drawing effect of grain boundary wasused to explain this behaviour.The microstructures of the coatings oxidized at 1100℃ were ob-served.It was found that the coating degradations were related to the amount and distribution of theaccumulated R_2O_3 at the alloy/coating interface.Three typical kinds of the accumulated layerswere identified.The continuous oxide layers acted as diffusion barrier which could inhibit the coat-ing degradation effectively.     

STUDY OF HIGH Cr-NiCrAlY AND CoCrAlY COATING ON SUPERALLOY BY HRS

Structure and performance of high Cr-NiCrAlY and CoCrAlY high temperature protective coating on Ni-base superalloy K3 by High Rate Sputtering (HRS) have been studied. The results indicate that these coatings consist of β-phase and γ-phase mainly. They have a out layer of β+γ-phase with riching Al, also. There are a rich Cr(W. Mo)-σ-phase in out layer and lots of mass M_(23)C_6-phase in diffusion layer in high Cr-NiCrAlY on K3 alloy. These two coatings have excellent high temperature oxidation resistance and hot corrosion resistance and do not affect the mechanical properties of the underlying alloy K3.     

Designing multilayer diamond like carbon coatings for improved mechanical properties

New multilayer coatings were produced by incorporating alternating soft and hard DLC layers enabled by varying the bias voltage during deposition process while maintaining a constant hard-to-soft layer thickness ratio.These coatings were deposited onto a Cr/Cr Cxgraded layer by closed field unbalanced magnetron sputtering(CFUBMS).The cross-sectional analysis of the coatings showed that the multilayer coatings possess sharp interfaces between the soft and hard layers with the hard to soft layer thickness ratio(1:1.33)constant in all the coatings.Raman analysis uncovered the increasing sp³character of the DLC coatings as a result of decreasing ID/IGratio and increasing full width at half maximum(FWHM)values of the G band peak induced supposedly by an increase in bias voltage during hard layer deposition.Nanoindentation tests showed an increase in hardness of the DLC coatings which can be correlated with the increase in the sp³content of the coatings as well as decreasing sp²-C cluster size,as calculated from the ID/IGratio.Furthermore,the coatings exhibited excellent plastic deformation resistance and adhesion strength upon microindentation and scratch testing,respectively.Although further investigations are required to assess coating durability,the multilayer design could offer the DLC coatings with a rare opportunity to combine the high hardness with damage resistance with a constant bilayer thickness and without the need to introduce complex multilayer system.     

Oxygen Permeation Behaviors and Hardening Effect of Titanium Alloys at High Temperature

In order to improve the surface hardness and wear resistance of Ti and Ti alloy components, an oxygen permeationtreatment (OPT) was developed. The oxygen permeation behaviors of three Ti alloys, TA2, TB5 and TC11, treatedin air with O-P medium at high temperature have been studied. The results show that the O-P treatment cansignificantly improve the surface hardness of Ti alloys. The oxidation mass-gain of β-Ti alloy (TB5) is much higherthan α-Ti alloy (TA2) under the same condition, while α β Ti alloy (TC11) is the lowest. All the Ti alloys treatedat this condition produce two surface layers: the outer layer consists mainly of TiO2, as well as trace of other oxides,and the inner layer consists of a Ti-O interstitial solid solution formed by the diffusion of oxygen in α crystal latticeThick scales of β Ti alloy (TB5) are easily formed depending mainly on the poor solid solution content of oxygen,while deep solution layer can be formed since partial β phase has been transformed into α phase. The scales of α-βTi alloy (TC11) are very thin and compact. Aluminum-rich zone, as well as deficient zone, is found in oxide layerrs.A crystallographic characterization of oxygen solution layer has been performed and evaluated by crystallographiclattice constant.     

Combined Effect of Aluminum Content and Layer Structure on the Oxidation Performance of Ti1-xAlxN Based Coatings

TiN,Ti1-xAIxN single layer coatings and TiN/Ti1-xAIxN multilayer coatings were deposited on SKH51 tool steel substrate by arc ion plating.The coatings were annealed in air to study the effect of aluminum and film structure on the oxidation performance.The surface morphology and structure were characterized by scanning electron microscopy and X-ray diffraction.The element distribution on the cross section was analyzed by electron probe microscopy.It is found that the oxidation resistance of these Ti1-xAlxN based coatings is mainly attributed to aluminum content in them.In comparison with the Ti1-xAlxN single layer coating,the TiN layer inserting into the Ti1-xAlxN in a multilayer coating increases the tendency of Ti diffusion toward the surface and forms a Ti-enriched top surface oxide layer,thus degrades the oxidation resistance.As far as the oxidation resistance is concerned in this study,Ti0.33Al0.67N single layer coating performs the best among all coatings.The kinetic of oxidation behavior of all coatings presents two definite stages.One is a slow oxidation growth which conforms to parabolic law,and the other presents severe mass gain with oxidation duration.The annealing time for severe oxidation initiation is responsible to Fe2O3formation in the oxide scale.     

Surface Microstructure and High Temperature Oxidation Resistance of Thermal Sprayed NiCoCrAlY Bond-Coat Modified by Cathode Plasma Electrolysis

The surface properties of the air-plasma sprayed bond-coat have been modified by cathode plasma electrolysis(CPE). After modification, a re-melted layer without obvious pores and oxide stringers is formed,the gain size of re-melted layer is approximately 80–120 nm. It is shown, from cyclic oxidation at 1100℃,that a thin oxide scale mainly composed of α-Al_2O_3 has been formed on the modified bond-coat and the oxidation resistance of the modified bond-coat has been significantly improved. Such beneficial result can be attributed to following effects: during CPE process, the plasma discharges with high temperature take place on the bond-coat surface. With plasma discharge treatment, the surface is melted and quickly re-solidified, the grain size decreases, and the pores and oxide stringers disappear. During cyclic oxidation, owing to the above modification of surface properties, the critical content of Al for selective oxidation is significantly decreased. Therefore, a continuous Al_2O_3 scale is formed.     

Influence of Re on the Properties of a NiCoCrAlY Coating Alloy

MCrAlY can serve as stand-alone overlay coatings or bond coats in thermal barrier coating systems, and its properties play a vital role in determining the performance of these coating systems. In order to further understand the behavior of MCrAlY coatings, several NiCoCrAlY model alloys with different levels of Re (0.3 wt%, 6 wt%, and 9 wt%) were investigated. Microstructural observation showed the addition of Re promoted the precipitation of Cr-rich phases, such as α-Cr and σ. The presence of α-Cr lowered the coecient of thermal expansion (CTE) of the coating alloys, which could reduce the CTE mismatch at the scale-metal interface. The solid solution strengthening effect of Re is responsible for an increase in Rockwell hardness of the coating alloys. But the isothermal oxidation resistance at 1100oC was deteriorated due to the precipitation of brittle α-Cr phase, a phase of inferior oxidation resistance compared with β-NiAl and γ-Ni.     

Development of Al356e-Al2O3 Nanocomposite Coatings by High Velocity Oxy-fuel Technique

In this research,development of Al356e-Al2O3 nanocomposite coatings has been investigated.Al356e-Al2O3 composite powders were prepared by mechanical milling of Al356 powder and 5 vol.% micro and nanoscaled alumina particles.The milled powders were used as feedstock to deposit composite coatings on A356e T6 aluminum alloy substrate using high velocity oxy-fuel(HVOF) process.X-ray diffractometry,optical and scanning electron microscopy,microhardness and wear tests were used to characterize the composite powders and coatings.The hardness of composite coatings containing micro and nanosized Al2O3 were 114.1 5.9 HV and 138.4 6.9 HV,respectively which were higher than those for substrate(79.2 1.1 HV).Nano and microcomposite coatings revealed low friction coefficients and wear rates,which were significantly lower than those obtained for Al356e T6 substrate.Addition of 5 vol.% micro and nanoscaled alumina particles improved the wear resistance by an average of 85% and 91%,respectively.This is mainly caused by the presence of Al2O3 in matrix and nanocrystalline structure of matrix.Scanning electron microscopy tests revealed different wear mechanisms on the surface of the wear test specimens.     

Dynamic evolution of oxide scale on the surfaces of feed stock particles from cracking and segmenting to peel-off while cold spraying copper powder having a high oxygen content

The oxide scale present on the feedstock particles is critical for inter-particle bond formation in the cold spray(CS)coating process,therefore,oxide scale break-up is a prerequisite for clean metallic contact which greatly improves the quality of inter-particle bonding within the deposited coating.In general,a spray powder which contains a thicker oxide scale on its surface(i.e.,powders having high oxygen content)requires a higher critical particle velocity for coating formation,which also lowers the deposition efficiency(DE)making the whole process a challenging task.In this work,it is reported for the first time that an artificially oxidized copper(Cu)powder containing a high oxygen content of 0.81 wt.%with a thick surface oxide scale of 0.71μm.,can help achieve an astonishing increment in DE.A transition of surficial oxide scale evolution starting with crack initiations followed by segmenting to peeling-off was observed during the high velocity particle impact of the particles,which helps in achieving an astounding increment in DE.Single-particle deposit observations revealed that the thick oxide scale peels off from most of the sprayed powder surfaces during the high-velocity impact,which leaves a clean metallic surface on the deposited particle.This makes the successive particles to bond easily and thus leads to a higher DE.Further,owning to the peeling-off of the oxide scale from the feedstock particles,very few discontinuous oxide scale segments are retained at inter-particle boundaries ensuring a high electrical conductivity within the resulting deposit.Dependency of the oxide scale threshold thickness for peeling-off during the high velocity particle impact was also investigated.     

Oxidation of a Co-base Superalloy

The oxidation behavior of a cast polycrystalline Co-base superalloy was studied at temperatures from 900 to 1050℃ and analyzed by thermogravimetric analysis （TGA）, X-ray diffraction （XRD） and scanning electron microscopy （SEM）. The results indicate that a cast Co-base superalloy follows the subparabolic oxidation kinetics at 900 and 1000℃, which are controlled by the growth of the inner Cr-rich layer, and that after oxidation at 1050℃ for 200 h, it almost exhibits the linear oxidation kinetics possible due to the volatility of Cr-rich oxide. A mixed scale forms on the alloy after prolonged oxidation. The oxide scale formed at 900 and 1000℃ is composed of an outer layer of spinel and an inner continuous Cr-rich layer and at I050℃ is composed of a very discontinuous Cr-rich layer.     

Effects of yttria content on the CMAS infiltration resistance of yttria stabilized thermal barrier coatings system

The effects of YO_(1.5) doping in yttria-zirconia based thermal barrier coatings(TBCs) against CMAS interaction/infiltration are discussed. The TBCs with an YO_(1.5) content ranging from 43–67 mol.%(balance Zr O_2) were produced by electron beam physical vapor deposition(EB-PVD) techniques. The results reveal a trend of higher apatite formation probability with the higher free YO_(1.5) available in the yttriazirconia system. Additionally, the infiltration resistance and amount of consumed coating appears to be strongly dependent on the YO_(1.5) content in the coating. The thinnest reaction layer and lowest infiltration was found for the highest produced 67 YO_(1.5) coating. Complementary XRD experiments with volcanic ash/YO_(1.5) powder mixtures with higher yttria contents than in the coatings(80 YO_(1.5) and pure YO_(1.5)) also showed higher apatite formation with respect to increasing yttria content. The threshold composition to promote apatite-based reaction products was found to be around 50 YO_(1.5) in zirconia which was proved in the coatings and XRD powder experiments. An YO_(1.5)-ZrO_2-Fe O-TiO_2 bearing zirconolite-type phase was formed as a reaction product for all the coating compositions which implicates that TiO_2 in the melt acts as a trigger for zirconolite formation. This phase could be detrimental for CMAS/volcanic ash infiltration resistance since it can be formed alongside with apatite which controls or limits the amount of Y~(3+) available for glass crystallization. The Fe rich garnet phase containing all the possible elements exhibited a slower nucleation compared to apatite and its growth was enhanced with slow cooling rates.The implications of phase stability and heat treatment effects on the reaction products are discussed for tests performed at 1250°C.     

Influence of Lithium Oxide Addition on the Sintering Behavior and Electrical Conductivity of Gadolinia Doped Ceria

Ceria-based electrolytes have been widely researched in intermediate-temperature solid oxide fuel cell (SOFC), which might be operated at 500-600?C. Sintering behavior with lithium oxide as sintering additive and electrical conductivity of gadolinia doped ceria (Gd0.1Ce0.9O2δ, GDC10) electrolyte was studied in this paper by X-ray di?raction (XRD), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). As the results, the fully dense GDC10 electrolytes are obtained at a low temperature of 800?C with 2.5 mol% Li2O as sintering additive (called 5LiGDC800). During sintering process, lithium oxides adsorbed by around GDC10 surface help to sinter at 800?C and are kept at the grain boundary of GDC10 in the end. The fine grains of 100-400 nm and high electrical conductivity of 0.014 S/cm at 6000C in 5LiGDC800 were achieved, which contributed to the lower sintering temperature and enhanced grain boundary conductivity, respectively. Lithium, staying at grain boundary, reduces the depletion of oxygen vacancies in the space charge layers and increases the oxygen vacancy concentration in the grain boundary, which leads to improve the total electrical conductivity of 5LiGDC800.     

Analysis of Surface Oxide Films Formed in Hydrogenated Primary Water on Alloy 690TT Samples With Different Surface States

Oxidation of Alloy 690 TT samples either manually ground to 400 and 1500 grit, mechanically polished, or electropolished was performed in a solution of 1500 10 6B and 2.3 10 6Li with 2.5 10 6dissolved H2, at 325℃ and 15.6 MPa for 60 days. The oxide films grown on samples with different surface states were analyzed using various techniques. Results show that a triple-layered structure was formed after immersion： an outermost layer with large scattered oxide particles rich in Fe and Ni, an intermediate layer with small compact oxide particles rich in Cr and Fe for the ground surfaces and loose needle-like oxides rich in Ni for the polished surfaces, and an inner layer with continuous Cr-rich oxides. The surface state was found to affect not only the surface morphology, but also the corrosion rate. Grinding accelerated the growth of protective oxide films such that the ground samples showed a lower oxidation rate than the polished ones.Samples of ground Alloy 690 TT showed superior resistance to intergranular attack（IGA）.     

Effects of rejuvenation heat treatment on microstructure and creep property of a Ni-based single crystal superalloy

Both surface and internal microstructures of a second-generation Ni-based single crystal(SX) superalloy were studied after creep and rejuvenation heat treatment(RHT).It is indicated that the microstructures,such as the dislocation network,the γ phase and the γ' phase,can be recovered to those after the standard heat treatment(SHT).It is found that RHT affected zone(RAZ) formed at the surface is composed of theγ'-free layer,the transition layer and the recrystallization(RX),which are less than 20 μm in depth totally.Such depth of the RAZ doesn't affect the properties of the superalloy.The morphology of γ' phase at the RAZ is related to the composition of the elements.The average creep life after RHT is close to the average life after SHT.It is concluded that RHT could effectively repair SX parts and increase the total life of the sample after a damage by creep.     

Failure processes within ceramic coatings at high temperatures

Plasma sprayed coatings have a complex structure which is produced by the overlaying of many molten or semi-molten particles in the diameter range of 20 to 120 μm. There is a need to characterize the failure behaviour of coatings and this has been carried out by using acoustic emission (AE) methodology. Coatings of NiCrAIY bond coat with a zirconia-12 wt% yttria overlay were applied to discshaped specimens of U-700 alloy. A waveguide of 1 mm diameter platinum was TIG welded to the specimen and allowed it to be suspended in a tubular furnace. The specimen was thermally cycled to 1150° C and the AE monitored. One method of examining the AE is from the viewpoint of the accumulative count data. It is also convenient to establish the temperatures for “initial” AE and “significant” AE (i.e., the temperature at which 100 counts is exceeded) so that coatings may be compared. Several other analyses have been carried out with the aim of establishing parameters which are related to the crack size and crack population. These studies have been used to postulate types of cracking mechanisms which may occur in plasma sprayed coatings during thermal cycling. It is shown that microcracking gave rise to a large amount of AE. However, this coating still survived more thermal cycles than a coating which exhibited macrocracking events. Data of this nature will be presented and the results discussed.     

Polycrystalline diamond compact with enhanced thermal stability

Polycrystalline diamond compacts(PDC), which are composed of diamond and WC/Co substrate, and synthesized at high pressure and high temperature(HPHT), are widely applied as the tooth of drilling bit. However, the thermal stability of PDC will be reduced when diamond transforms into graphite due to cobalt in PDC acting as a catalyst during the drilling work. In this study, a new three-layer structured PDC with enhanced thermal stability has been successfully synthesized at pressures of 5.5–7.0 GPa and temperatures of 1650–1750?C. In this structure, the diamond-Si C composite acts as the working layer,and the diamond-Si C-Co composite and WC/Co cements are as the intermediate layer and substrate,respectively. It is found that the initial oxidizing temperature of the three-layered PDC is enhanced up to820?C, which is significantly higher than that(～780?C) of the conventional PDC counterpart.     

Structural Thermal Stability of Graphene Oxide-Doped Copper–Cobalt Oxide Coatings as a Solar Selective Surface

3d transition metal oxides based thin film coatings such as copper–cobalt oxides exhibit high absorption in the visible region and low emittance in the infrared to far-infrared region of the solar spectrum which is favourable for use as potential selective surface materials in photothermal devices. These materials have the potential to minimize heating while increasing absorption in the operative spectrum range and therefore achieve higher solar selectivity. A series of mixed copper–cobalt metal spinel oxides(Cu_xCo_yO_z)doped with graphene oxide thin films were deposited on commercial grade aluminium substrates using a sol-gel dip-coating technique at an annealing temperature of 500 °C in air for 1 h. Characterizations of the synthesized films were carried out by high temperature synchrotron radiation X-ray Diffraction(SRXRD), UV–Vis, Fourier Transform infrared spectroscopy(FTIR) and X-ray photoelectron microscopy(XPS)techniques. High thermal stability of coatings with multiple phases, binary and ternary metal oxides, was defined through SR-XRD study. FTIR analysis shows moderate(80%) to high(up to 99%) reflectance in the infrared region while the UV–Vis investigations demonstrate that, in the visible region, solar absorption increases gradually(up to 95%) with the addition of graphene oxide to the Cu_x Co_y O_z coatings.With the incorporation of 1.5 wt.% of graphene oxide to the copper–cobalt oxide coatings, a high solar selectivity of 29.01(the ratio of the average solar absorptance in visible and the average thermal emittance in infrared to far infrared region; α/ε) was achieved.     

Improvement on the Oxidation Resistance of a Ti3Al Based Alloy by Cr1-xAlxN （0≤x≤0.47） Coatings

Cr1-xAlxN coatings have been deposited on a Ti3Al based alloy by reactive sputtering method. The results of the isothermal oxidation test at 800-900℃ showed that Cr1-xAlxN coatings could remarkably reduce the oxidation rate of the alloy owing to the formation of Al2O3＋Cr2O3 mixture oxide scale on the surface of the coatings. No spallation of the coatings or oxide scales took place during the cyclic oxidation at 800℃. Ti was observed to diffuse into the coatings, the diffusion distance of which was very short, and the diffusion ability of it was proportional to the AI content in the coatings. Compared to Ti, Nb can diffuse much more easily through the whole coatings and oxide scales.     

Temperature Dependent Work-hardening Behaviour in an Fe-Mn-Si-Cr-Ni Shape Memory Alloy

The work-hardening behaviour in an Fe-Mn-Si-Cr-Ni alloy has been investigated using tensile test at different temperatures and TEM observation. It was found that besides the intersection of εmartensite, the intersections of ε martensite with stacking fault and the cross-slip of dislocation which is difficult to occur in the alloy with low stacking fault energy are also important factors to the temperature dependent work-hardening behaviour.     

Influence of Additive Silica on the Laser Melting of the Ceramic Coatings

The influence of additive silica on the microstructure of plasma sprayed Al2O3 and Al2O3 13 wt pct TiO2 ceramic coatings at laser melting has been investigated in this study ,At the laser melting ,additive silica in Al2O3ceramic coating can reduce the stress of cooling shrinkage generated during solidification ,Moreover,silica can render finer size of grains of the melting layer and form continuous glassy matter around the grain boundaries so as to reduce further the cooling stresses and to suppress the formation and spreading of cracks ,On the other hand,at the laser melting,TiO2 reacts with Al2O3 and transforms into TiAl2O5,The latter new phase has great and anisotropic coefficients of thermal expansion leading to big and asymmetrical stresses and thus to form cracks in the melting layer of Al2O3 13 wt pct TiO2 coating ,Due to the fact that the influence of additive silica on the suppression of the formation of cracks is rather limited and cannot counterbalance the negative effect of TiAl2O5,thus the melting layer of Al2O3 13 wt pct TiO2 coating doped with 3 wt pct SiO2 cracks also ,Nevertheless,TiO2 can greatly develop the wear resistance of the ceramic coating as sprayed or laser melted.