Microstructure and high temperature oxidation resistance of Si-Y co-deposition coatings prepared on TiAl alloy by pack cementation process

In order to improve the high temperature oxidation resistance of TiAl alloy, Y modified silicide coatings were prepared by pack cementation process at 1030, 1080 and 1130 °C, respectively, for 5 h. The microstructures, phase constitutions and oxidation behavior of these coatings were studied. The results show that the coating prepared by co-depositing Si?Y at 1080 °C for 5 h has a multiple layer structure: a superficial zone consisting of Al-rich (Ti,Nb)5Si4 and (Ti,Nb)5Si3, an out layer consisting of (Ti,Nb)Si2, a middle layer consisting of (Ti,Nb)5Si4 and (Ti,Nb)5Si3, and aγ-TiAl inner layer. Co-deposition temperature imposes strong influences on the coating structure. The coating prepared by Si?Y co-depositing at 1080 °C for 5 h shows relatively good oxidation resistance at 1000 °C in air, and the oxidation rate constant of the coating is about two orders of magnitude lower than that of the bare TiAl alloy.

Effects of slurry pack cementation temperature on microstructure and wear resistance of Ti-Al co-deposited coating on copper plated nickel layer

In order to improve the wear resistance properties of copper substrate, a layer of electroplated nickel was firstly deposited on copper substrate, subsequently these electroplated specimens were treated by slurry pack cementation process with a slurry pack cementation mixture composed of TiO2 as titanizing source, pure Al powder as aluminzing source and also a reducer for titanizing, an activator of NH4Cl and albumen （egg white） as cohesive agent. The Ti-Al coating was fabricated on the surface of electro-deposited nickel layer on copper matrix followed by the slurry pack cementation process. The effects of slurry pack cementation temperature on the microstructures and wear resistance of Ti-Al coating were studied. The results show that the microstructure of the coating changed from NiAl＋Ni3（Ti,Al） to NiAl ＋Ni3（Ti,Al）＋Ni4Ti3 to Ni4Ti3＋NiAl, and to NiAl＋Ni3（Ti,Al）＋NiTi with slurry pack cementation temperature ranging from 800 ℃ to 950 ℃ in 12 h. The friction coefficient of Ti-Al coating decreased and the hardness increased with increasing the slurry pack cementation temperature. The minimum friction coefficient was 1/3 and the minimum hardness was 5 times larger than that of pure copper.

A STUDY ON ALUMINIDE COATINGS ON TiAl ALLOYS BY PACKCEMENTATION METHOD

The halide-activated pack cementation method is utilized to deposit aluminide coat- ings on TiAl alloys. Emphasis is placed on the effect of alloying elements on the aluminizing behavior of TiAl alloy. The addition of a small amount of Nb or Cr in the TiAl improves significantly the aluminizing kinetics of TiAl alloys by increasing the solid-state division of Al through the formation of stable TiAl3 layer. The TiAl3 layer formed on the TiAl alloyed with Nb or Cr has better toughness than the TiAl3 formed on the non-alloyed TiAl. The reason for better toughness of the coating formed on TiAl is that partial TiAl3 with tetragonal structure was changed to high symmetry cubic L12 structure since Nb or Cr was dissolved into TiAl3. The TiAl3 layer formed on the TiAl alloyed with Nb or Cr has much better oxidation resistance than the TiAl3 layer formed on the non-alloyed TiAl. It is attributed to change in the crystal structure of TiAl3 from the brittle tetragonal DO22 to the ductile cubic L12 by addition of small amount of Nb or Cr.

Microstructures and wear resistance of chromium coatings on P110 steel fabricated by pack cementation

In order to obtain a high-performance surface on P110 steel that can meet the requirements in oil/gas field environment, the chromium coatings were fabricated by pack cementation. The chromium coatings differed in with/without the addition of La2O3. Scanning electron microscope （SEM）, energy dispersive X-ray spectrometer （EDS）, X-ray diffractometer （XRD） and microhardness tester were employed to investigate the surface morphologies, surface element distributions, microstructures, phase constitutions and microhardness of the coatings. Friction-wear tests of the P110 steel substrate and the coatings were conducted in air at ambient temperature and humidity. The results show that ＇uniform and continuous coatings are formed on P110 steel regardless of adding La2O3 or not. The chromium coatings consist of Cr23C6, Cr7C3, and （Cr, Fe）7C3. The La2O3-added chromium coating is more beneficial in terms of surface morphology, microstructure, thickness and microharduess as compared with the coating without adding La2O3. Chromizing treatment significantly improves the surface hardness and wear resistance of the P110 steel. The wear resistance of the tested samples can be sorted in the following sequence： La2O3-coating 〉 no RE-coating 〉bare P110 steel.

CODEPOSITION OF ALUMINUM AND SILICON ON PURE MOLYBDENUM SUBSTRATE USING HALIDE ACTIVATED PACK CEMENTATION TREATMENTS

A multi-component diffusion coating has been developed to protect Mo-based alloys from high temperature environmental attack. Aluminum addition was made during the coating process to improve the oxidation resistance by developing hexagonal Mo(Si, Al)2 through the development of the halide activated pack cementation coating process on pure Mo substrate. The results show that Mo(Si, Al)2 formed as a main phase on the surface and a little amount of Mo5Si3 also formed. The total thickness of coating is tens ofμm at 1373K. During the cyclic oxidation test at high temperature(at about 1323K in air), mullite (3Al2O3.2SiO2) and some SiO2 formed. The addition of Al is beneficial for MoSi2 coating and the Al-doped coating exhibited only a small weight gain and protected the Mo substrate, while the MoSi2 coating without Al suffered a significant weight loss, indicating a loss of volatile MoO3 after cycles.

DEPOSITION OF ALUMINIDE AND CHROMIUM-MODIFIED ALUMINIDE COATINGS ON TiAl ALLOYS USING THE HALIDE-ACTIVATED PACK CEMENTATION METHOD

The halide activated pack cementation method is utilized to deposit aluminide and chromium modified aluminide coatings on TiAl alloys. Emphasis is placed on the effect of alloying elements on the aluminizing kinetics as well as on the study of formation of chromium modified aluminide coating. The addition of a small amount of Nb and Cr in the TiAl improves significantly the aluminizing kinetics of TiAl alloys by increasing the reaction rate of active Al atoms with substrate through grain refinement as well as by increasing the solid state diffusion of Al through the formation of stable TiAl 3 layer. Cr and Al are simultaneously co deposited by diffusion into K 5 alloy, by a single step, pack cementation process. The morphologies of the coating formed on K 5 substrate, i.e., an external layer with L1 2 structure and an underlying interdiffusion zone, are presented. The mechanism for the formation of Cr modified aluminide coating is discussed.

Improving hot corrosion resistance of aluminized TiAl alloy by anodization and pre-oxidation

To shield TiAl alloy from hot corrosion attack,a compact protective coating was fabricated by the combination of aluminizing,anodization and pre-oxidation.The hot corrosion behavior of the coated-TiAl specimen was investigated in the mixture salt consisting of 75 wt.%Na2SO4 and 25 wt.%NaCl at 700°C.Results indicated that the anodization and pre-oxidation were beneficial to the generation of Al2O3 layer,which could act as a diffusion barrier to prevent the molten salts and oxygen from diffusing into the alloy during exposure to a hot corrosion environment while the aluminizing coating could provide sufficient aluminum source to support the continuous formation of Al2O3 layer.Moreover,the internal stress of the coating was reduced due to the formation of a gradient coating consisting of TiAl3 and TiAl2.

Ablation resistance and mechanism of SiC/ZrC-ZrB2 double layer coating for C/C composites under plasma flame

To improve the ablation resistance of carbon/carbon(C/C)composites,a SiC/ZrC-ZrB2 double layer coating was fabricated by pack cementation and slurry-sintering method.The ablation resistance of the SiC/ZrC-ZrB2 coating was tested under plasma flame above 2300°C.The results indicate that the SiC/ZrC-ZrB2 double layer coating exhibits superior ablation resistance than the ZrC-ZrB2 single layer coating.After being ablated under the plasma flame for 20 s,the mass and linear ablation rates of the ZrC-ZrB2 coating are 0.89 mg/s and 15.3μm/s,while those for SiC/ZrC-ZrB2 coating are 0.09 mg/s and 24.15μm/s,respectively.During ablation,the SiC inner layer can generate SiO2 glass and result in the formation of ZrO2-SiO2 molten film.Compared with the ZrO2 molten film formed on the ZrC-ZrB2 coating surface,the ZrO2-SiO2 molten film with lower oxygen diffusion rate and viscosity enables the SiC/ZrC-ZrB2 coating to have better self-healing ability.Therefore,the enhanced ablation resistance of the SiC/ZrC-ZrB2 coating can be attributed to the formation of dense ZrO2-SiO2 molten film under the plasma flame.

Effect of Substrate Characteristics on Interdiffusion Coefficients of Ni and Al Atoms in β-NiAl Phase of Aluminide Coatings

Interdiffusion coefficients at 950℃ and 1050℃ are calculated by Wagner analysis method as a function of composition of β-NiAI phase. The β-NiAI phase is formed by pack cementation on surface of superalloy. Results of the calculation show that interdiffusion coefficients in β-NiAI phase strongly depend on the compositions and vary over several orders of magnitude. Compared with the interdiffusion coefficients in the stoichiometric β-NiAI phase, the interdiffusion coefficients in β-NiAI phase formed on superalloy is obviously small, probably due to the composition, complicated microstructure and precipitates. However, it could be seen clearly that the shapes of the diffusivity curves are very similar to each other. The similarity of the diffusion curves and the difference between interdiffusion coefficients imply that the compositions, microstructures and precipitates of superalloy have a distinctly adverse effect on the interdiffusion of Ni and Al atoms during aluminization, but do not change the essential characteristics of β-NiAI phase.

Microstructure and Growth Kinetics of Silicide Coatings for TiAl Alloy

In order to improve the oxidation resistance of Ti Al alloy, silicide coatings were prepared by pack cementation method at 1273, 1323, and 1373 K for 1-3 hours. Scanning electron microscopy（SEM）, energy dispersive spectrometry（EDS） and X-ray diffraction（XRD） were employed to investigate the microstructures and phase constitutions of the coatings. The experimental results show that all silicon deposition coatings have multi-layer structure. The microstructure and composition of silicide coatings strongly depend on siliconizing temperatures. In order to investigate the rate controlling step of pack siliconizing on Ti Al alloy, coating growth kinetics was analyzed by measuring the mass gains per unit area of silicided samples as a function of time and temperature. The results showed that the rate controlling step was gas-phase diffusion step and the growth rate constant（k） ranged from 1.53 mg^2/（cm^4·h^2） to 2.3 mg^2/（cm^4·h^2）. Activation energy（Q） for the process was calculated as 109 k J/mol, determined by Arrhenius＇ equation： k = k0 exp[–Q/（RT）].

Low-temperature Formation of Aluminide Coatings on Ni-base Superalloys by Pack Cementation Process

This article investigates the low-temperature formation of aluminide coatings on a Ni-base superalloy by pack cementation process. The pack cemented coatings characteristic of high density and homogeneity possess a two-layer structure. The top layer mainly consists of Al3Ni2 and Al3Ni,while the bottom layer of Al3Ni2. Great efforts are made to elucidate the effects of different experimental parameters on the microstructure and the constituent distribution of the coatings. The results show that all the parameters exclusive of the pack activator （NH4Cl） content produce effect on the coating thickness,but do not on the microstructure and the constituent distribution. The pack activator （NH4Cl） content affects neither the coating thickness nor structure and constituent distribution. The parabolic relationship between the coating thickness and the deposition time suggests that the process is diffusion-controlled. Furthermore,the article demonstrates a linear relationship between the coating thickness and the re-ciprocal deposition temperature.

Influence of adding various rare earths on microstructures and corrosion resistance of chromizing coatings prepared via pack cementation on P110 steel

In order to improve the corrosion resistance and increase the service lifetime of P110 steel during operation,four chromizing coatings were formed onto its surface with/without addition of rare earths via pack cementation process.The surface morphologies and microstructures of the chromizing coatings were observed using scanning electron microscopy(SEM),and the phase constitutions were investigated by X-ray diffraction(XRD).Electrochemical corrosion behavior of the chromizing coatings in simulated oilfield ...

Influence of process parameters on thickness and wear resistance of rare earth modified chromium coatings on P110 steel synthesized by pack cementation

The pack cementation was employed to produce rare earth modified chromium coatings on P110 steel aiming at improving its performance and increasing the usage lifetime during operation. The orthogonal array design （OAD） was applied to set experiments. Contents of NH4Cl, types of RE, contents of RE, test temperature and soaking time were the main factors, and each factor was endowed with four levels. While the range analysis and analysis of variance were used to investigate the results of OAD tests on thickness and wear resistance. The results indicated that for a promising coating with higher thickness value and excellent anti-wear property, the test temperature was the most significant process factor. The potential promising conditions for chromizing treatment were： adding 1% NH4Cl and 1% LaCl3, maintaining the test temperature at 1000 oC for 8 h. The results of verification showed that the coating formed under the optimal process parameter had a valid thickness of 28 μm and a reduction of 0.32 mg in wear resistance test.

Electrochemical corrosion resistance of CeO_2-Cr/Ti coatings on 304 stainless steel via pack cementation

The pack cementation was employed to improve the electrochemical corrosion resistance of 304 stainless steel via CeO2- Cr modified Ti coatings. Continuous coatings were formed on 304 stainless steel surface by this method. A series of electrochemical experiments were carried out to investigate the corrosion resistance of 304 stainless steel, Ti coating and CeO2-Cr/Ti coatings. The sample surface was investigated by scanning electron microscopy （SEM）. The phases of sample surface were detected by X-ray diffraction （XRD）. It was concluded from all the outcomes that the Corrosion resistance of the samples could be sorted in the following sequence： CeO2-Cr/Ti coatings〉Ti coating〉304 stainless steel.

Zn-Fe and Y-modified Zn-Fe coatings on 42CrMo steel via pack cementation

Zn-Fe and Y-modified Zn-Fe coatings were prepared on 42 CrMo steel through pack cementation processes at 370,390 and 410℃ for 4 h.Y modification was achieved through the co-deposition of Zn and Y_(2)O_(3).The effects of Y modification on microstructure,formation and corrosion behavior of the Zn-Fe coating were investigated.The coating thickness increased with an increase in temperature.The Y-modified Zn-Fe coating was thicker than the plain Zn-Fe coating.Both the Zn-Fe coating and Y-modified Zn-Fe coating showed single-layered structures,but the overall microstructure was improved by Y modification.The activation energies for Zn-Fe coating and Y-modified Zn-Fe coating were 113.15 and80.65 kJ·mol^(-1),respectively.The Zn-Fe coating consisted of FeZn_(13) and Fe11Zn40 phases.The effects of Y modification on the corrosion behavior of the Zn-Fe coating were evaluated through an immersion test and polarization measurements.The results showed that the corrosion resistance was improved by Y modification.

Preparation of a tantalum-based MoSi_(2)–Mo coating resistant to ultra-high-temperature thermal shock by a new two-step process

To develop an ultra-high-temperature resistant coating for a reusable thermal protection system,the preparation of a tantalum-based MoSi_(2)-Mo coating by a new two-step process of multi-arc ion plating and halide activated pack cementation is presented.The coating has a dense structure and is well compatible with the tantalum substrate,which can be thermally shocked from room temperature to 1750℃ for 360 cycles without failure.The mechanism of the coating’s excellent resistance to high-temperature thermal shocks is that a strong-binding gradient interface and a dense SiO_(2) oxide scale with good oxygen resistance are formed by the high-temperature self-diffusion of Si.

Microstructure and Oxidation Behavior of Modified Aluminide Coating on Ni3Al-based Single Crystal Superalloy

A1, A1-Si and Cr-A1-Si coatings are prepared onto IC20 alloy by powder pack cementation to improve the oxidation resistance of the alloy. The isothermal oxidation behaviors of the coatings and the uncoated IC20 alloy at 1 100 ℃ are comparatively in- vestigated. For the coatings, less weight gains are obtained compared to the uncoated alloy after 100 h thermal exposure. The A1-Si coating exhibits the best oxidation resistance, while the addition of Cr accelerates the scale spallation. Phase transforma- tion from β-NiA1 to y and y＇ occurs in the coating after oxidation for extended periods. The oxidation protection and degradation mechanisms for these coatings are discussed.

Effect of Deposition Time on Thickness and Corrosion Behavior of Zn-Fe Coating

Zn-Fe coatings on Q235 steel are prepared by pack cementation process at 390 ℃ for 2,4,6,8 and10 h to investigate the effects of the deposition time on the thickness and corrosion behavior of the Zn-Fe coatings.The thickness of the coating increases with the increase of the deposition time.The coating is composed of a thick outer layer and a thin inner layer.The formation of the coating depends on the inward diffusion of Zn atoms and the outward diffusion of Fe atoms.The outer layer is composed of Fe11 Zn40 and FeZn10 phases.The corrosion behavior of the Zn-Fe coatings is evaluated by immersion test and polarization test.The results show that the Zn-Fe coatings can effectively prevent the Q235 steel from corrosion.The corrosion resistance of the coating is proportional to the deposition time.

Hot Corrosion Behavior of a Cr-Modified Aluminide Coating on a Ni-Based Superalloy

A Cr-modified aluminide coating is prepared on a Ni-based superalloy using arc ion plating and subsequent pack cementation aluminizing.Hot corrosion behavior of the Cr-modified aluminide coating exposed to molten Na2SO4/K2SO4（3：1） or Na2SO4/NaCl（3：1） salts at 900 °C in static air are evaluated as well as the aluminide coating.The results indicate that compared with the aluminide coating,the anti-corrosion properties of the Cr-modified aluminide coating in the both salts are improved,which should be attributed to the beneficial effect of the Cr in the coating.The corrosion mechanism of the Cr-modified aluminide coating,especially the role of Cr in the mixture salt corrosion,is discussed.

Mechanical behaviors and energy absorption properties of Y/Cr and Ce/Cr coated open-cell nickel-based alloy foams

In this study, Y-and Ce-modified Cr coatings applied by pack cementation method were prepared on the surface of open-cell nickel-based alloy foam. The morphologies and microstructures of Y- and Ce-modified Cr coatings with various Y and Ce contents were investigated in detail. Then, the effects of Y and Ce addition on the mechanical properties of open-cell nickel-based alloy foams were analyzed and compared. Simultaneously, the energy absorption capacity and energy absorption efficiency of the Y- and Ce-modified Cr coated alloy foams were discussed and compared at the room and high temperatures. The results show that Cr coatings containing minor amounts of rare earth element （Y and Ce） are well adhered to the nickel-based foam struts. Especially, the microstructure of the 2 wt% Ce-modified Cr coating is denser and uniform. In addition, the compressive strength and plateau stress of Y- and Ce-modified Cr coated alloy foams firstly increase and then decrease by increasing the Y and Ce contents at room and high temperatures. The energy absorption capacity of Y/Cr and Ce/Cr coated alloy foams increases linearly with the strains increasing. The Ce/Cr coated alloy foams can absorb more energy than Y/Cr coated alloy foams in the plateau and densification regions at room temperature. Compared to those at room temperature, the Y- and Ce-modified Cr coated alloy foams show higher energy absorption efficiency when deforma- tion within 10%-30% at high temperature.