Weitao Wu

Effect of a sputtered TiAlCr coating on the oxidation resistance of TiAl intermetallic compound

The effect of a sputtered TiAlCr coating on the oxidation resistance of TiAl intermetallic compound was investigated in static air. The bare TiAl alloy exhibited poor isothermal and cyclic-oxidation resistance at 800–1000°C due to the formation of TiO2-base scales which tend to spall during cooling. A sputtered Ti-50Al-10Cr coating remarkably improved the oxidation resistance of TiAl, due to the formation of an adherent Al2O3 scale at 800–1000°C. After long-term oxidation (at 900°C for 1000 hr), TiAlCr coating still provided excellent protection for the TiAl alloy. Minor interdiffusion occurred due to the inward diffusion of Cr, while no Kirkendall voids were found at the coating/ substrate interface. In contrast, NiCrAlY and CoCrAlY coatings reacted extensively with the TiAl alloys. Moreover, the TiAlCr coating alloy is based on γ-TiAl and TiAlCr Laves phases, which may offer improved mechanical properties. The TiAlCr coating exhibited a better combination of oxidation resistance and substrate compatibility than conventional aluminide and MCrAlY coatings.

Reactive sputter deposition of alumina films on superalloys and their high-temperature corrosion resistance

Abstract The deposition of stoichiometric alumina films by means of d.c. reactive sputtering was investigated. The oxidation and hot corrosion performance of the Al2O3 films on Ni-base superalloys were evaluated. The oxygen partial pressure (PO2) could dramatically affect the chemical compositions and microstructures of the deposited films, and two PO2 thresholds were revealed, showing close relations with the increase or decrease of PO2 during sputtering. When PO2 varied through the PO2 thresholds, the total gas pressure, the sputtering voltage and the depositing rate exhibited abrupt changes. If PO2 was lower than the PO2 thresholds, the sputtered films were mainly composed of metallic aluminum with little alumina. When PO2 was higher than the PO2 thresholds the films were composed only of amorphous alumina. The Al2O3 films deposited on superalloys can reduce their oxidation rates by 3–8 times. It was observed that Cr2O3 grew both in the cracks within the Al2O3 films and at the film-substrate interface at 1273 K or below. At 1373 K, the Al2O3 film was gradually displaced by a mixed layer of Al2O3 and Cr2O3. It is proposed that the oxidation of the specimens coated with Al2O3 is controlled predominantly by inward diffusion of anions through the Al2O3 films at 1273 K or below, and by outward diffusion of cations through the (Al, Cr)2O3 layers formed during the degradiation of the Al2O3 film at the higher temperature, 1373 K. The Al2O3 films showed beneficial effects on the hot-corrosion resistance of superalloys GH30 and K38G. The Al2O3 films on specimens of GH30 eliminated internal sulfidation and oxidation, while those on K38G alleviated internal sulfidation and oxidation.

The mechanism of scale adhesion on sputtered microcrystallized CoCrAl films

The oxidation mechanisms of sputtered microcrystalline Co−30Cr−5Al coatings were investigated by an acoustic emission technique, scatch test, transmission electron microscope (TEM), which was compared with CoCrAlY alloy. The results indicated that the beneficial effects of microcrystallization on the scale adhesion of Co−30Cr−5Al alloy are as follows: (1) The sputtered CoCrAl coating possesses a columnar structure, and oxidation along the columnar grains may form many micropegs which can anchor the scale to the metallic substrate, enhancing bonding of the scale. (2) The grain size of the sputtered coating is several orders of magnitude smaller than that of the cast alloy, and the grain size of oxide scales formed on the former is finer than that of the latter. The finer oxide scale may relieve the growth stresses during isothermal oxidation and partial thermal stresses during cooling by plastic deformation through grain sliding. The microcrystalline coating is more plastic than the cast alloy, which may relieve a certain amount of thermal stresses of the oxide scales. On the basis of oxide adhesion and plasticity, microcrystallization is more beneficial than the addition of reactive elements.

The effects of several coatings on cyclic oxidation resistance of TiAl intermetallics

The behaviours of Al2O3, aluminide, CoCrAlY and TiAlCr coatings on the cyclic oxidation resistance of a TiAl alloy were investigated. Al2O3 coating exhibited limited improvement of the cyclic oxidation of TiAl due to spallation. Aluminide and CoCrAlY coatings remarkably improved the cyclic oxidation kinetics of TiAl due to the formation of adherent Al2O3 scales, but the two coatings exhibited a poor coating-substrate compatibility, especially cracks occurred in two coatings after cyclic oxidation. These coatings exhibited a poor substrate-coating compatibility in comparison with TiAlCr coating

Effect of La2O3 particles on the oxidation of electrodeposited nickel films

Electrodeposited Ni-La2O3composite films with nanometer-sizeLa2O3 oxide inclusions werefabricated by the codeposition of nickel withLa2O3 particles. The comparativeoxidation behavior in air at 900 and 1000°C of nickel coated with theNi-La2O3 composite and films withand without nickel-plating was studied by TGA, AE,SEM/EDX, EPMA, and TEM/EDX. In general, theNi-La2O3 composite-coated nickelhad the slowest rate and the best resistance tothermal cycling. AE tests revealed that cracking eventsin NiO scales on Ni-La2O3composite-coated nickel was significantly reduced incomparison to that of the scale on nickel-coated nickel during thermalcycling at 900°C. SEM investigation showed that theLa2O3-free NiO scale was composedof outer coarse columnar grains and inner equiaxed ones.By contrast, the scale on the Ni-La2O3composite-coated nickel consisted of only fine equiaxedNiO grains. The scale on theLa2O3-free samples wascharacterized by cracks that originated at thescale-substrate interface and spanned the scale thickness. By contrast,no scale cracks formed at theLa2O3-doped NiO scale-substrateinterface, but small voids were created at the triplepoints of the grain boundaries of NiO. In the La2O3-doped NiOscale, segregation of La ions to the NiO grainboundaries near the scale-surface was observed by EDXmicroanalyses in the TEM. It is believed that the Laions segregated at the grain boundaries of NiO led to an increase in thecohesion between nickel oxides and in a reduction of thescaling rate and the formation of scale with fineequiaxed crystal structure by blocking the outward and lateral growth of scale. The latter was dueto the predominant outward diffusion of nickel along NiOgrain boundaries being inhibited effectively by thesegregated La ions. The mechanism of the effect of the added La2O3particles on the nickel electrodeposits is discussed indetail.

Effect of a sputtered TiAlCr coating on hot corrosion resistance of gamma-TiAl

The effect of a sputtered Ti-50Al-10Cr coating on hot corrosion Behaviour of gamma-TiAl was investigated in (Na,K)(2)SO4 and Na2SO4 + NaCl melts. The TiAlCr coating was effective in improving the hot corrosion resistance of TiAl in (Na,K)(2)SO4 melts due to the formation of a continuous Al2O3 scale. In Na2SO4 + NaCl melts, however, the TiAlCr coating had little effect on the hot corrosion resistance of TiAl because it suffered from severe corrosion due to the formation of an Al2O3 + TiO2 mixed scale which spalled easily

Effect of chromium on the oxidation resistance of TiAl intermetallics

The effect of 10 at.%Cr on the oxidation resistance of TiAl intermetallic compound at 800–1100°C in air was investigated. The results indicated that 10 at.%Cr equally substituting for Ti and Al in TiAl alloy had duplex effects on the isothermal kinetics of DAL At lower temperatures (800–900°C), Cr increased the oxidation rates as a result of the doping effect of Cr in the scale and at higher temperatures (1000–1100°C), especially at 1100°C, Cr significantly reduced the oxidation rates as a result of the formation of a continuous Al2O3 film on the surface. 10 at.%Cr only substituting for Ti in TiAl alloy remarkably reduced the oxidation rates at all temperatures by about two orders of magnitude. Moreover, 10 at%Cr significantly improved the cyclic-oxidation rsistance of TiAl alloy.

Role of platinum in the Na2SO4-Induced hot corrosion resistance of aluminum duffusion coatings

Electrochemical corrosion measurements have been carried out with Pt-containing and Pt-free Al-diffusion coatings on IN 738 LC in a 90Na2SO4+ 10K2SO4 (mol%) melt at 1173 K. Pt improves the resistance to basic fluxing while there are no significant differences between both coating types in their resistance to acidic fluxing. The corrosion resistance of the Pt-containing coating is also higher in the passive potential region where protective scales rich in Al2O3 are formed. The reason for the different behavior of both coating types appears to be related to the high corrosion resistance of the Pt-rich surface layer of the coating and an increased Al2O3 content in the scale of the Pt-containing type.

Synergistic effect of NaCl and water vapor on the corrosion of 1Cr-11Ni-2W-2Mo-V steel at 500-700°C

The corrosion behavior of 1Cr-11Ni-2W-2Mo-Vsteel was studied in the presence of a NaCl deposit andwater vapor at 500-700°C. Results indicated thatcorrosion was not obvious without an NaCl deposit, while corrosion was accelerated remarkably by thesimultaneous presence of NaCl and water vapor at anytemperature. Optical microscopy (OM), X-ray diffraction(XRD), scanning electron microscopy (SEM), andelectron-probe microanalysis (EPMA) were used to analyze thecorrosion products. A mechanism of the synergisticeffect of water vapor and NaCl on corrosion of the steelis proposed.

The oxidation resistance of a sputtered, microcrystalline TiAl-intermetallic-compound film

The oxidation behavior of a cast TiAl intermetallic compound and its sputtered microcrystalline film was investigated at 700–900°C in static air. At 700°C, both the cast alloy and its sputtered microcrystalline film exhibited excellent oxidation resistance. No scale spallation was observed. However, at 800–900°C, the oxidation kinetics for the cast TiAl alloy followed approximately a linear rate law, which indicates that it has poor oxidation resistance over this temperature range. The poor oxidation resistance of TiAl was due to the formation of an Al2O3+TiO2 scale which spalled extensively during cooling. Nevertheless, the sputtered, TiAl-microcrystalline film exhibited very good oxidation resistance. The oxidation kinetics followed approximately the parabolic rate law at all temperatures. Although the composition of the scales was the same as that of scales formed on the cast alloy, the scales formed on the sputtered microcrystalline-TiAl film are adherent strongly to the substrate. No scale spallation was found at 700–850°C, while a small amount of spallation was observed only at 900°C. This indicates that microcrystallization can improve the oxidation resistance of the TiAl alloy.