Pack Cementation Coatings for High-Temperature Oxidation Resistance of AISI 304 Stainless Steel

Aluminum and titanium are deposited on the surface of steel by the pack cementation method to improve its hot-corrosion and high-temperature oxidation resistance. In this research, coatings of aluminum and titanium and a two-step coating of aluminum and titanium were applied on an AISI 304 stainless steel substrate. The coating layers were examined by carrying out scanning electron microscopy (SEM) and x-ray diffraction (XRD). The SEM results showed that the aluminized coating consisted of two layers with a thickness of 450???m each, the titanized coating consisted of two layers with a thickness of 100???m each, and the two-step coatings of Al and Ti consisted of three layers with a thickness of 200???m each. The XRD investigation of the coatings showed that the aluminized coating consisted of Al₂O₃, AlCr₂, FeAl, and Fe₃Al phases; the titanized layers contained TiO₂, Ni₃Ti, FeNi, and Fe₂TiO₅ phases; and the two-step coating contained AlNi, Ti₃Al, and FeAl phases. The uncoated and coated specimens were subjected to isothermal oxidation at 1050?°C for 100?h. The oxidation results revealed that the application of a coating layer increased the oxidation resistance of the coated AISI 304 samples as opposed to the uncoated ones.     

Tribological properties of titanium aluminides coatings produced on pure Ti by laser surface alloying

Titanium aluminides coatings were in-situ synthesized on a pure Ti substrate with a preplaced Al powder layer by laser surface alloying. The friction and wear properties of the titanium aluminides coatings at different normal loads and sliding speeds were investigated. It was found that the hardness of the titanium aluminides coatings was in the following order: Ti₃Al coating > TiAl coating > TiAl₃ coating. Friction and wear tests revealed that, at a given sliding speed of 0.10 m/s, the wear volume of pure Ti and the titanium aluminum coatings all increased with increasing normal load. At a given normal load of 2 N, for pure Ti, its wear volume increased with increasing sliding speed; for the titanium aluminides coatings, the wear volume of Ti₃Al coating and TiAl coating first increased and then decreased, while the wear volume of TiAl₃ coating first decreased and then increased with increasing sliding speed. In addition, the friction coefficients of pure Ti and the titanium aluminides coating decreased drastically with increasing sliding speed. Under the same dry sliding test conditions, the wear resistance of the titanium aluminium coatings was in the following order: Ti₃Al coating > TiAl coating > TiAl₃ coating.     

Oxidation Resistant Ce-modified Silicide Coatings Prepared on Ti–6Al–4V Alloy by Pack Cementation Process

Cerium-modified silicide coatings were prepared on Ti–6Al–4V by pack cementation. The effects of different kinds of activators (NaCl, AlF₃, AlCl_3, and NH₄Cl) and pack CeO₂ concentrations (1, 3, and 5 wt%) on the coating structures were studied. The results show that the coatings were mainly composed of a TiSi₂ outer layer, a TiSi middle layer, a Ti₅Si₄ inner layer and a 1–2 μm thick Ti₅Si₃ interdiffusion zone. NH₄Cl was a more suitable activator for preparing the Ce-modified silicide coating on Ti–6Al–4V, based on the coating microstructure and growth rate. The coating thickness decreased with increasing CeO₂ concentration in the pack. Oxidation tests at 800 °C in air showed that the Ce-modified silicide coating showed improved oxidation resistance compared to both the uncoated alloy and the pure silicide coating. A dense, but thick oxide scale formed that was composed of a TiO₂ outer layer and a SiO₂ inner layer.     

Protection of titanium aluminides by FeCrAlY Coatings

The effectiveness of Fe+15%Cr+5%Al+0.3%Y coatings. produced by sputter ion plating, in improving the oxidation behaviour of γ-2 Ti₃Al at 800°C and γ-TiAl at 900° and 1000°C. during cyclic exposures in air of up to 1000 hours duration, has been studied. The 45-135 μm coatings were tested in both the as-coated condition and following densification by peening. The kinetics of attack and of spallation were followed gravimetrically, while the reaction product compositions were examined by a range of surface analytical techniques. Oxidation protection was afforded by the formation on the coating surface of a γ-Al₂O₃ oxide scale but chemical reactions at the coating-substrate interface with accompanying voidage development, eventually caused the coating to fail mechanically. To surmount this problem the potential of several ceramic interlayers produced by the same coating route, and interposed as diffusion barriers between the FeCrAlY overlayers and the Ti₃Al substrate was examined in 1000 hour oxidation tests at 800°C.     

The Effect of Water Vapor on the Oxidation Behavior of CVD Iron-Aluminide Coatings

The oxidation behavior of iron-aluminide coatings, Fe₃Al or (Fe,Ni)₃Al, produced by chemical-vapor deposition (CVD) was studied in the temperature range of 700–800°C in air + 10 vol.% H₂O. A typical ferritic steel, Fe–9Cr–1Mo, and an austenitic stainless steel, 304L, were coated. For both substrates, the as-deposited coating consisted of a thin (<5μm), Al-rich outer layer above a thicker (30–50 μm), lower-Al-content inner layer. In addition to coated and uncoated Fe–9Cr–1Mo and 304L, cast Fe–Al model alloys with similar Al contents (13–20 at.%) to the CVD coatings were included in the oxidation exposures for comparison. The specimens were cycled to 1000 1 hr cycles at 700°C and 500 1 hr cycles at 800°C, respectively. The CVD coating specimens showed excellent performance in the water-vapor environment at both temperatures, while the uncoated alloys were severely attacked. These results suggest that an aluminide coating can substantially improve resistance to water-vapor attack under these conditions.     

Diffusion aluminide coatings for TiAl intermetallic turbine blades

Alloys based on intermetallic phases of a Ti–Al system are materials that, thanks to their resistance characteristics, can be widely used in automotive and aerospace applications. The main restriction for the use of Ti–Al materials is their insufficient oxidation resistance above 850 °C. Oxidation parameters might be improved by aluminide coatings based on TiAl₂ and TiAl₃ phases, which could induce the creation of an Al₂O₃ scale in the oxidation process. This type of aluminide could be deposited on the surface of TiAl alloys by various methods such as pack cementation, plasma spraying or magnetron sputtering. This article presents a new method of aluminide coating deposition on TiAl intermetallic alloys: out of pack technology. The investigated coating was deposited on turbine blades made of a Ti45Al5Nb intermetallic alloy. The surface morphology, structure, phase and chemical composition have been investigated using XRD phase analysis, SEM and EDS. The phase analysis showed that TiAl₃ and TiAl₂ were the main components of the deposited coating. An isothermal oxidation test of the TiAl turbine blades was conducted as well. After 1000 h of testing at 950 °C, the scale formed on the surface of the uncoated blades underwent spallation. The scale on the turbine blade with deposited aluminide coatings was very thin and no spallation was observed.     

TEM and DTA study on the stability of Al5Ti3- and h-Al2Ti-superstructures in aluminium-rich TiAl alloys

The phases Al₅Ti₃ and h-Al₂Ti, which are superstructures of the L1₀ TiAl structure, are frequently observed in as-cast and low-temperature-annealed aluminium-rich TiAl alloys. The strong decrease of the solubility of aluminium in TiAl with decreasing temperature leads to a supersaturation of the solid solution with aluminium during cooling. The decomposition of the supersaturated TiAl results in the precipitation of the superstructure phases at low temperatures. The evolution of the Al₅Ti₃ and h-Al₂Ti phases and the resulting microstructures were studied as a function of time, temperature, and composition by TEM and DTA investigations on Ti–Al alloys with 55 to 64 at.% Al. Both superstructures were found not to be equilibrium phases. Al₅Ti₃ is metastable below a composition-dependent critical temperature in the range of about 750–900°C with a maximum value reached near the stoichiometric composition. Above this temperature, Al₅Ti₃ rapidly dissolves. Extended lamellar Al₅Ti₃+TiAl microstructures have been found in a Ti–60 at.% Al alloy after low-temperature annealing, whereas in Ti–62 at.% Al large single-phase domains of Al₅Ti₃ have grown. h-Al₂Ti is a metastable phase at least up to 1200°C. It slowly transforms into the equilibrium phase r-Al₂Ti during annealing.     

Indirect evaluation of the long-term oxidation properties of Al−21Ti−23Cr and Al−37Ti−12Cr coating materials for TiAl alloy

The most pertinent coating materials in the Al−Ti−Cr alloy system to improve the high temperature oxidation resistance of a TiAl alloy, with respect to oxidation properties, resistance to thermal stress, and chemical compatibility, are the two-phase alloys of Al−21Ti−23Cr (L1₂+Cr₂Al) and Al−37Ti−12Cr (γ+TiAlCr). In this study, cyclic oxidation tests at 1000 °C and 1200 °C were performed for the specimens coated with both materials of 10 im thickness. Furthermore, breakaway oxidation caused by the formation of a rutile TiO₂ scale was observed, though both bulk alloys showed very stable oxidation behavior. This phenomenon was resulted from the depleted Al content in the coating layer due to Al₂O₃ oxide growth and interdiffusion with the substrate. Considering the decrease of Al content due to oxide growth, the Al−21Ti−23Cr coating with the initial higher Al content was more effective for long-term oxidation protection of the TiAl alloy. On the other hand, when the Al content changes due to the interdiffusion with the substrate, the Al−37Ti−12Cr coating with a smaller compositional gradient with the TiAl substrate was more effective than the Al−21Ti−23Cr coating. Cyclic oxidation tests at 1000 °C and 1200 °C confirmed that for the longer lifetime of coating materials the initial Al content was more important than the smaller compositional gradient with the substrate. Consequently, the Al−21Ti−23Cr coating was considered as more effective one for the long-term oxidation resistance of TiAl alloys.     

NiCoCrAlY coatings with and without an Al2O3/Al interlayer on an orthorhombic Ti2AlNb-based alloy: Oxidation and interdiffusion behaviors

The mechanism of oxidation protection of NiCoCrAlY overlay coatings on the orthorhombic Ti₂AlNb-based alloy (O alloy) Ti–22Al–26Nb (at.%) is described. While the bare alloy exhibited poor oxidation resistance at 800 °C, adding NiCoCrAlY coatings significantly improved the oxidation resistance. However, serious interdiffusion between the coatings and the substrate resulted in rapid degradation of the coating system. Several reaction layers were formed at the coating/substrate interface by interdiffusion, and non-protective scales mainly of Cr₂O₃ and TiO₂ were formed due to the degradation of the coating. In order to solve this problem, an Al₂O₃/Al interlayer was sandwiched into the coating system as a diffusion barrier. The isothermal and cyclic oxidation protection of the multilayer coating system on the Ti–22Al–26Nb substrate was evaluated at 800 and 900 °C. The results indicated that the interdiffusion was much suppressed, and the duplex coating system demonstrated improved oxidation resistance on the Ti–22Al–26Nb substrate, with a thin and adherent protective α-Al₂O₃ scale forming on the surface.     

Si-modified aluminide coatings deposited on Ti46Al7Nb alloy by slurry method

Ti46Al7Nb alloy has been used as the research substrate material for the deposition of water-based slurries containing Al and Si powders. The diffusion treatment has been carried out at 950 °C for 4 h in Ar atmosphere. The structure of the silicon-modified aluminide coatings 40 μm thick is as follows: (a) an outer zone consisting of TiAl₃ phase and titanium silicides formed on the matrix grain boundaries composed of TiAl₃–type Ti₅Si₃; (b) a middle zone containing the same phase components with the matrix TiAl₃ and the silicides Ti₅Si₃, which formed columnar grains; (c) an inner zone, 2 μm thick, consisting of TiAl₂ phase. Cyclic oxidation tests were conducted in 30 cycles (690 h at high temperature) and showed a remarkably higher oxidation resistance of the Ti46Al7Nb alloy with the protective coating in comparison with the uncoated sample.     

The improvement of high temperature oxidation of Ti–50Al by sputtering Al film and subsequent interdiffusion treatment

The high temperature oxidation resistance of Ti–50Al can be improved by sputtering an Al film and subsequent interdiffusion treatment at 600 °C for 24 h in high vacuum. In these conditions, a TiAl₃ layer is formed on the surface, which exhibits good adhesion with Ti–50Al substrate and provides high oxidation resistance. Cyclic and isothermal oxidation tests show that the Ti–50Al with 3–5 μm Al film can dramatically reduce the oxidation at 900 °C in air, at which the parabolic oxidation rate constant K_p of specimen with 5 μm Al film is only about 1/15,000 of that of bare Ti–50Al. XRD and SEM results indicate that the TiAl₃ layer can promote the formation of a protective Al₂O₃ scale on the surface as well as react with γ-TiAl to form TiAl₂ during the oxidation. Simultaneously, layers of Al₂O₃/TiAl₂/Al-enriched γ-TiAl/Ti–50Al are also formed on specimens. The TiAl₂ layer thickness will decrease gradually with increasing the oxidation time. After oxidation at 900 °C for 300 h, there is a clearly discontinuous thin layer of Ti₃₇Al₅₃O₁₀ compound observed in between Al₂O₃ and TiAl₂.     

Dry sliding wear behavior of PVD TiN,Ti55Al45N,and Ti35Al65N coatings at temperatures up to 600 °C

Three PVD nitride coatings (TiN, Ti₅₅Al₄₅N, and Ti₃₅Al₆₅N) with different Al content were deposited on the cemented carbides by cathode arc-evaporation technique. Microstructural and fundamental properties of these nitride coatings were examined. The friction and wear behavior of these coatings were evaluated at temperatures up to 600 °C. The wear surface features of the test samples were examined by scanning electron microscopy. Results showed that the friction coefficient of these nitride coatings is different depending on the temperature. The friction coefficient of TiN coating increased with the increase of test temperature; while the friction coefficient of Ti₅₅Al₄₅N and Ti₃₅Al₆₅N coatings with the addition of Al decreased with the increase of test temperature. The Ti₅₅Al₄₅N and Ti₃₅Al₆₅N coatings exhibited higher wear resistance over the one without Al (TiN coating). The wear resistance of these nitride coatings at high temperature wear tests is significantly dependent on their tribological oxidation behavior. The Ti₅₅Al₄₅N and Ti₃₅Al₆₅N coatings with the addition of Al exhibited improved wear resistance as compared to the TiN coating, which was attributed to that their tribo-chemically formed Al₂O₃ exhibited better tribological properties than the TiO₂ of the latter.     

Thermal and thermo-mechanical properties of Ti–Al–N and Cr–Al–N coatings

Metastable Ti–Al–N and Cr–Al–N coatings have been proven to be an effective wear protection due to their outstanding mechanical and thermal properties. Here, a comparative investigation of mechanical and thermal properties, for Ti–Al–N and Cr–Al–N coatings deposited by cathodic arc evaporation with the compositions (c-Ti_0.52Al_0.48N, c/w-Ti_0.34Al_0.66N and c-Cr_0.32Al_0.68N) widely used in industry, has been performed in detail. The hardness of Ti_0.52Al_0.48N and Ti_0.34Al_0.66N coatings during thermal annealing, after initially increasing to the maximum value of ~ 34.1 and 38.7 GPa with T_a up to 900 °C due to the precipitation of cubic Al-rich and Ti-rich domains, decreases with further elevated T_a, as the formation of w-AlN and coarsening of precipitated phases. A transformation to Cr₂N and finally Cr via N-loss in addition to w-AlN formation during annealing of the Cr_0.32Al_0.68N coating occurs, and thus results in a continuous decrease in hardness. Among our coatings, the mixed cubic-wurtzite Ti_0.34Al_0.66N coating exhibits the highest thermal hardness, but the worst oxidation resistance. The Cr_0.32Al_0.68N coating shows the best oxidation resistance due to the formation of dense protective α-Al₂O₃-rich and Cr₂O₃-rich layers, with only ~ 1.4 μm oxide scale thickness, after thermal exposure for 10 h at 1050 °C in ambient air, whereas Ti–Al–N coatings are already completely oxidized at 950 °C.     

Joining of Cf/SiC Composite and TC4 Using Ag-Al-Ti Active Brazing Alloy

Carbon fiber reinforced SiC (C_f/SiC) composite was successfully joined to TC4 with Ag-Al-Ti alloy powder by brazing. Microstructures of the brazed joints were investigated by scanning electron microscope, energy dispersive spectrometer, and x-ray diffraction. The mechanical properties of the brazed joints were measured by mechanical testing machine. The results showed that the brazed joint mainly consists of TiC, Ti₃SiC₂, Ti₅Si₃, Ag, TiAl, and Ti₃Al reaction products. TiC + Ti₃SiC₂/Ti₅Si₃ + TiAl reaction layers are formed near C_f/SiC composite while TiAl/Ti₃Al/Ti + Ti₃Al reaction layers are formed near TC4. The thickness of reaction layers of the brazed joint increases with the increased brazing temperature or holding time. The maximum room temperature and 500 °C shear strengths of the joints brazed at brazing temperature 930 °C for holding time 20 min are 84 and 40 MPa, respectively.     

The Effect of Hf on the Oxidation and Corrosion Behavior of Ti0.7Al0.3N Coating Prepared by Arc-Ion Plating

Ti_0.7Al_0.3N and Ti_0.68Al_0.30Hf_0.02N coatings were deposited on 1Cr–11Ni–2W–2Mo–V stainless steel by arc-ion plating (AIP), and their oxidation and corrosion performance were characterized using TGA, TEM, SEM/EDS, EPMA and XRD. The oxidation behavior of the coatings at 800 °C for up to 100 h was investigated, and the results showed that the introduction of hafnium into the Ti_0.7Al_0.3N coating dramatically improved the oxidation-resistance of the coating in air. Compared to the Ti_0.7Al_0.3N coating, the presence of Hf in the nitride coating promoted the outward diffusion of Al, and suppressed the outward diffusion of Ti and inward diffusion of O. The Ti_0.7Al_0.3N coating was completely oxidized and formed a layered scale after oxidation at 800 °C for 20 h in air. Meanwhile, local serious oxidation of the substrate occurred. Corrosion tests of the coatings with a NaCl deposit in wet oxygen at 650 °C for 10 h were also conducted, and the results showed that the Ti_0.7Al_0.3N coating suffered serious local corrosion, while a thin and dense scale formed on the surface of the Ti_0.68Al_0.30Hf_0.02N coating, and its anti-corrosion performance was remarkably enhanced.     

Microstructures and mechanical properties of TiAl alloy prepared by spark plasma sintering

A fine-grained TiAl alloy with a composition of Ti-47%Al(mole fraction) was prepared by double mechanical milling(DMM) and spark plasma sintering(SPS). The relationship among sintering temperature, microstructure and mechanical properties of Ti-47%Al alloy was studied by X-ray diffractometry(XRD), scanning electron microscopy(SEM) and mechanical testing. The results show that the morphology of double mechanical milling powder is regular with size of 20?40 μm. The main phase TiAl and few phases Ti₃Al and Ti₂Al were observed in the SPS bulk samples. For samples sintered at 1000 °C, the equiaxed crystal grain was achieved with size of 100?250 nm. The samples exhibited compressive and bending properties at room temperature with compressive strength of 2013 MPa, compression ratio of 4.6% and bending strength of 896 MPa. For samples sintered at 1100 °C, the size of equiaxed crystal grain was obviously increased. The SPS bulk samples exhibited uniform microstructures, with equiaxed TiAl phase and lamellar Ti₃Al phase were observed. The samples exhibited compressive and bending properties at room temperature with compressive strength of 1990 MPa, compression ratio of 6.0% and bending strength of 705 MPa. The micro-hardness of the SPS bulk samples sintered at 1000 °C is obviously higher than that of the samples sintered at 1100 °C. The compression fracture mode of the SPS TiAl alloy samples is intergranular fracture and the bending fracture mode of the SPS TiAl alloy samples is intergranular rupture and cleavage fracture.     

Effects of Al and Ni doping on oxidation and corrosion resistance of electrophoretic deposited YSZ coatings

Yttria-stablized zirconia (YSZ)/(Ni,Al) coatings were deposited on Inconel 600 alloy substrate by the electrophoretic deposition combined with vacuum sintering technique. The effects of isothermal oxidation at 1100 °C on the composition of the coatings and the crack healing were investigated, and the corrosion resistance of the coatings in 3.5% NaCl (mass fraction) solution was also studied. The results showed that the cracks on the coating gradually healed up with the increase of the isothermal oxidation time. During isothermal oxidation process, the coating composed of Ni₃Al was transformed to α-Al₂O₃ particulates. The α-Al₂O₃ particulates can seal the defects such as pores and cracks, and meanwhile prevent the oxygen diffusion into the coatings. The polarization curves and EIS results indicated that the coatings oxidized for 40 h had a more positive corrosion potential, higher breakdown potential, higher impedance module at low frequency and much lower corrosion current density compared with YSZ coated and uncoated Inconel 600 alloys.     

Boro-aluminising of P91 steel by pack cementation for protection against steam oxidation

Abstract

High performance alloys are often the materials used for various components exposed to high temperature environments. In many cases, protective coatings are applied in these alloys, providing higher corrosion and oxidation resistance, compared to the base material. This study investigates the feasibility to apply boro-aluminising treatment on P91 steel by pack cementation process, to increase the steel high temperature properties in oxidising and corrosive environments. Packs activated by AlCl₃, NH₄Cl and KBF₄ were used to carry out the coating deposition at a temperature of 715°C for 6 h. The coating formed was analysed by means of SEM and XRD, and the compounds formed were identified. Cyclic steam oxidation for a total of 1008 h at 650°C revealed an oxide scale of 50 μm for the uncoated P91 steel, while the coated steel shows practically no oxidation effect.     

Influence of thermodynamic activities of different masteralloys in pack powder mixtures to produce low activity aluminide coatings on TiAl alloys

Titanium aluminides are interesting high temperature materials, but show insufficient oxidation resistance as well as embrittlement at higher temperatures (>750 °C). Al-enriched coatings can be manufactured by pack cementation on many high temperature alloys to promote the formation of a protective alumina layer at high temperatures, which not only protects the alloy from oxidation but is also expected to impede embrittlement of TiAl at high temperatures. One drawback of such coatings is that Al-rich phases are very brittle. Therefore the major intermetallic aluminide phase in the coating plays a critical role for the protection behavior. Based on thermodynamic calculations different masteralloys were chosen to control the pack cementation process. Particular attention is given to the gradient between the aluminum activity of the different masteralloy powders and the aluminum activity of the substrate surface (alloy TNM^®-B1) in order to control the deposited phase at the surface. It is revealed that powder pack with Al as masteralloy provides a high Al activity and produces thick multi-layered coatings consisting of brittle TiAl₃ and TiAl₂ phase and aluminum-rich TiAl. By using different chromium aluminides as masteralloys, thinner, low-activity coatings could be produced, consisting of a bi-layer of brittle TiAl₂ phase and aluminum-rich TiAl or just the targeted pure aluminum-rich TiAl, which is known to have much better mechanical properties.     

EFFECT OF CoCrAl COATING ON OXIDATION RESISTANCE OF INTERMETALLIC COMPOUND TiA1

Effect of a sputtered Co-30Cr-5Al microcrystalline coating on oxidation resistance ofintermetallic compound TiAl and oxidation behavior of the bare TiAl were investigatedat 900—1000℃ in static air.The oxidation kinetics for the TiAl alloy seems toapproximately follow a linear rate law.Poor oxidation resistance of TiAl is due to theformation of the mixed Al_2O_3+TiO_2 scale which is loosely packed and easily spalledoff.but not of dense and adherent pure Al_2O_3.A sputtered Co-30Cr-5Al coating,30μm thickness,can remarkably improve the oxidation resistance of TiAl owing to theformation of adherent Al_2O_3 protective scale.However,many Kirkendall voids wereformed between coating and substrate.