- Email: [email protected]
Al+Y codeposition using EB-PVD method for improvement of high-temperature oxidation resistance of TiAl
Surface and Coatings Technology 112 (1999) 91–97
Al+Y codeposition using EB-PVD method for improvement of high-temperature oxidation resistance of TiAl J.P. Kim *, H.G. Jung, K.Y. Kim Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, South Korea
Abstract An ‘‘Al+Y codeposition’’ on TiAl by the single EB-PVD method has been developed to improve the oxidation resistance of TiAl. To determine the optimum codeposition condition, the Al+Y codepositions with various ratios of Al and Y are evaluated through the isothermal and cyclic oxidation tests. The oxidation resistance of TiAl can be improved extensively by the Al+Y codeposition due to the formation of a gradient coating of Al and Y. The Al+Y codeposition with the ratio of Al:2Y for evaporation source material is proved to be the best. With a proper ratio of Al:Y, the Al+Y codeposition forms two distinctive layers of the oxides during high-temperature oxidation; Al O in the inner layer and (Y, Al )O type oxide in the outer layer. In 2 3 addition to the inner Al O layer, the formation of the outer (Y, Al )O type oxide layer further improves the stability of the 2 3 coatings. The stability of the Al+Y codeposition greatly depends upon the alloying element of TiAl substrate or oxidation resistance of the TiAl substrate alloy. The non-alloyed TiAl shows a poor coating stability, whereas TiAl–2.8Nb and Alloy K5 show a good coating stability under severe thermal stresses during cyclic oxidation since a stable Al O can form on the surface 2 3 of these alloys. © 1999 Elsevier Science S.A. All rights reserved. Keywords: TiAl; High-temperature oxidation; Al+Y codeposition; Gradient coating; Cyclic oxidation; Thermal stress
1. Introduction The TiAl intermetallic compound is considered as a good candidate for high-temperature structural material because of its high strength-to-density ratio at high temperatures [1]. However, it has critical limitations for commercialization due to its poor room-temperature formability and poor high-temperature oxidation resistance. To overcome such limitations, extensive studies are conducted for alloy design and surface coating. The TiAl contains about 50 a/w of Al, but it cannot form a stable protective Al O because of preferential 2 3 formation of porous TiO due to its thermodynamic 2 nature [2,3]. Ternary alloys such as Nb, Mo and W have been reported to exhibit beneficial effects by promoting the formation of dense Al O [4–6 ]. However, 2 3 alloying of these elements is not effective at temperatures above 900 °C and results in adverse effects on the ductility at room temperature. Therefore, a proper surface treatment is the most desirable to improve the
* Corresponding author. Tel: +82 562 2792134; Fax: +82 562 2794499; e-mail: [email protected]
oxidation resistance since it does not have any adverse effects on the mechanical properties of base metals. In this study, a new coating process of Al+Y codeposition is developed to improve the high-temperature oxidation resistance of TiAl. A gradient coating of Al and Y can be expected since the vapor pressure of Al is much higher than that of Y. The nature of the coating structure is analyzed in terms of the distribution of Al and Y across the coating layer and the barrier effect against diffusion of the oxidants. The optimum coating condition is determined through both isothermal and cyclic oxidation tests. To investigate the effect of base metals on Al+Y codeposition, three different TiAl base metals are used: non-alloyed TiAl, TiAl–2.8Nb and Alloy K5 ( Ti–46.5Al–3Nb–2Cr–0.2W ) [4].
2. Experimental Al+Y codeposition was performed using a single EB-PVD process. The substrate materials used for coating were non-alloyed TiAl, TiAl–2.8Nb and Alloy K5. The pure TiAl has a single c-TiAl structure, whereas
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 78 7 - 7
92
J.P. Kim et al. / Surface and Coatings Technology 112 (1999) 91–97
the TiAl–2.8Nb and Alloy K-5 has a duplex structure consisting of c-grains and lamellar grains of alternating layers of c and a2 platelets. However, the grain size of Alloy K5 is much smaller than that of TiAl–2.8Nb. Three different compositions of Al and Y were used as evaporation sources for Al+Y codeposition. The single Al coating was used for comparing with Al+Y codeposition. Table 1 shows the coating conditions. The electron beam power was 1.6 kW, with an acceleration voltage of 8 kV, and the vacuum level was kept at 10−5 Torr at 400 °C. A pellet-type evaporation source material was used, and the size of the pellet was 5–8 mm for Al and 2–5 mm for Y. They were mixed well before melting them. The homogeneous mixing of the melt was obtained by using the sweeping function of electron gun, and melting was conducted for more than 10 min. The isothermal and cyclic oxidation tests were conducted for evaluation of oxidation resistance of the coated specimens. The detailed experimental procedures for oxidation test are described elsewhere [7]. Isothermal oxidation tests were performed using a Cahn 1000 thermogravimetric electrobalance apparatus. The cyclic oxidation test was performed in a tube furnace equipped with a program controller providing a stable thermal cycle. For each cycle, the specimens were exposed to the test temperature for 45 min and cooled to 100 °C for 15 min. After every 20 cycles, the specimen was taken out to measure the weight change. After the oxidation test, specimens were examined using SEM, EPMA and X-ray diffraction to analyze the morphology and composition of the oxides.
3. Results 3.1. Al+Y codeposition The behavior of the Al+Y codeposition was observed with the source materials of three different mixing ratios, and the TiAl–2.8Nb was used as the substrate. Table 2 presents the coating conditions and EDS analyses on the coating layers. Since the vapor pressure of Al is significantly higher than that of Y, a gradient coating of Al+Y is expected with a high Al content in the inner layer and relatively high Y content in the outer layer. The gradient coating is clearly observed from both ‘‘codeposition B and C’’, but no gradient coating is
obtained from ‘‘codeposition A’’. This result indicates that a proper ratio of evaporation source materials should be maintained to secure the gradient coating. 3.2. Isothermal oxidation test An isothermal oxidation test was performed at 950 °C for 20 h to investigate the initial oxidation behavior of each coated specimen. Fig. 1 shows the results of the isothermal oxidation test by plotting the square of weight change per unit area vs. oxidation time. The bare TiAl–2.8Nb shows a typical parabolic oxidation behavior. However, the oxidation rate is greatly reduced by both the single Al deposition and Al+Y codeposition. The specimens of ‘‘single Al deposition’’ and ‘‘codeposition A’’ show an extremely low oxidation rate. On both specimens, a stable Al O is formed. For the ‘‘codeposi2 3 tion A’’ specimen, the Y content is extremely low, as shown in Table 2, and thus no significant layer of Y oxide can be formed. In the case of ‘‘codeposition B and C’’, a rapid oxidation rate is observed in the initial stage. This is due to oxidation of yttrium, which has a high atomic weight as well as a high reactivity with oxygen. The ‘‘codeposition C’’ specimen has a higher oxidation rate than that of the ‘‘codeposition B’’ specimen, simply because of the higher Y content in the ‘‘codeposition C’’ specimen. For both specimens, after initial rapid oxidation, the oxidation rate is reduced by the formation of stable oxides of Al and Y. Fig. 2 shows the line profile of each specimen after the isothermal oxidation test at 950 °C for 200 h. The line profile of ‘‘codeposition A’’ suggests that the oxide is composed of Al O alone since the Y level is negligible 2 3 [Fig. 2(a)]. In the case of ‘‘codeposition B’’, the yttrium distribution is uniform across the oxide layer. This indicates that the oxide layer is a uniform mixture of Al O and Y O [Fig. 2(b)]. The ‘‘codeposition C’’ 2 3 2 3 specimen shows two maxima in Y line profile in the outer layer and an Al peak in the inner layer. These line profiles indicate that a mixture of yttrium oxides forms in the outer layer and Al O forms in the inner layer 2 3 [Fig. 2(c)]. To identify the phases developed in the ‘‘codeposition C’’ specimen after the oxidation test at 950 °C for 200 h, the specimen surface was analyzed by X-ray diffraction. The result is shown in Fig. 3 along with the cross-sectional view of this specimen. The peak
Table 1 Composition of evaporation source and coating time
Single Al deposition codeposition A codeposition B codeposition C
Ratio of Al:Y (wt ratio)
Al (g)
Y (g)
Coating time (min)
1:0 2:1 1:1 1:2
25 25 15 12.5
0 12.5 15 25
14 15 17 19
93
J.P. Kim et al. / Surface and Coatings Technology 112 (1999) 91–97 Table 2 EDS analysis on the lower and upper parts of Al+Y codeposition layer Coating condition
Single Al deposition codeposition A codeposition B codeposition C
Semi-quantitive EDS analysis
Weight ratio of source (Al:Y )
Y in source (at.%)
Lower part Y (at.%)
Upper part Y (at.%)
1:0 2:1 1:1 1:2
0 13.2 23.4 37.8
0 1 8 10
0 1 11 25
intensities for Y O , Y A O ( YAG), and YAlO are 2 3 3 5 12 3 strong, whereas the peak intensity for Al O is relatively 2 3 weak. This XRD analysis suggests that the outer layer is composed of a mixture of Y O , Y A O ( YAG), 2 3 3 5 12 and YAlO and the inner layer is composed of Al O . 3, 2 3 Since the mixture of yttrium oxides cannot be identified individually, it will be called hereafter an ( Y, Al )O-type oxide. Therefore, the oxide developed in the ‘‘codeposition C’’ specimen can be identified, as shown in Fig. 3(b). 3.3. Cyclic oxidation test A cyclic oxidation test was performed to evaluate the stability of oxide formed on the coating specimens under cyclic thermal stresses. Fig. 4(a) shows a plot of the weight change per unit area of the specimen for the test performed at 950 °C. The bare TiAl–2.8Nb specimen shows the rapid decrease in weight after the initial few cycles of the test. This is due to formation of the nonprotective TiO , and it is easily spalled under thermal 2 stress. The single Al-coated specimen shows a relative stability up to 120 cycles, after which it shows a gradual decrease in weight due to spallation of Al O . 2 3 The coating specimens of Al+Y codeposition show a better cyclic oxidation resistance than the single Al-coated specimen. Although the Y content is below 1% in ‘‘codeposition A’’, it shows a stable weight change up to about 500 cycles and then a gradual decrease in weight. With increasing Y content, the beneficial effect
Fig. 1. Plot of square of weight change per unit area vs. time for the isothermal oxidation test at 950 °C.
of Y on the stability of the oxide layer seems to be increased. Of the three Al+Y codeposition specimens, the ‘‘codeposition C’’ specimen shows the best cyclic oxidation resistance. As shown in Fig. 3, this specimen develops an ( Y, Al )O-type oxide in the outer layer and Al O in the inner layer. The inner Al O seems to act 2 3 2 3 as a diffusion barrier, and the outer ( Y, Al )O type oxide seems to reduce the thermal stress by modifying the grain structure of the oxide layer. It has been reported that the Y O distributed in Al O greatly 2 3 2 3 refines the grain size of Al O layer, and thus this grain2 3 refined oxide layer can easily release plastic deformation [7–11]. The effects of oxygen-active elements on the morphology of alumina scale are twofold: one is a decrease in scale convolutions and the other is grain refinement [9]. Grain boundary sliding is expected to be the major deformation mechanism in the scale, and it has been suggested that an oxide with fine grains can accommodate growth and thermally induced stresses by grain boundary sliding more easily than an oxide with large grains [10]. Therefore, it is expected that the Al+Y codeposition can substantially increase the stability of the coating layer under severe thermal fluctuations. To investigate the effect of substrate alloys on the coating stability, three different alloys of non-alloyed TiAl, TiAl–2.8Nb and Alloy K5 were coated with the same condition as that of ‘‘codeposition C’’. The cyclic oxidation test was conducted for these three specimens at 1000 °C. Fig. 4(b) shows a plot of the weight change per unit area of specimen vs. number of cycles. The non-alloyed TiAl shows a rapid weight loss during the initial stage of cyclic oxidation. This is attributed to the fact that the non-alloyed TiAl forms a non-protective TiO instead of forming a stable Al O . However, both 2 2 3 the TiAl–2.8Nb and Alloy K5 specimens show a good stability of the coating layer up to 250 cycles. This result clearly indicates that the oxidation resistance of the substrate alloy is also critical to the stability of the coating layer formed by the Al+Y codeposition. For oxidation of the TiAl substrate, Nb and W ( less than 5%), when added individually, have a beneficial effect, whereas Cr and Ta ( less than 5%) have a detrimental effect [12,13]. However, the oxidation resistance of TiAl can be improved more by the addition of a combination
94
J.P. Kim et al. / Surface and Coatings Technology 112 (1999) 91–97
Fig. 3. X-ray diffraction spectra and cross-sectional micrograph of ‘‘codeposition C’’ specimen after the isothermal oxidation test at 950 °C for 200 h: (a) X-ray diffraction spectra; and (b) cross-sectional micrograph.
exposed to high temperature. This stable alumina formed on the substrate can adhere well with the alumina forming out of the inner layer of the codeposition of Al and Y.
Fig. 2. Line profiles across the scale formed on the coatings after the isothermal oxidation test at 950 °C for 200 h: (a) codeposition A; (b) codeposition B; and (c) codeposition C.
of Nb and Cr than by addition of the individual element [12]. For a ternary alloy of Ti–48Al–X and a quaternary alloy of Ti–48Al–X–Y, increasing concentrations of X and Y will result in a decrease in the Ti activity in the alloy and an increasing tendency to form protective alumina scales on oxidation. Therefore, the specimens of TiAl–2.8Nb and Alloy K5 with ‘‘codeposition C’’ are expected to form a stable alumina scale at the interface between the substrate and coating when
4. Discussion The single Al deposition significantly reduces the oxidation rate of TiAl under isothermal oxidation conditions (Fig. 1). However, during the cyclic oxidation test, the single Al deposition does not show a good stability of the oxide [Fig. 4(a)]. A similar behavior of cyclic oxidation is observed with the ‘‘codeposition A’’ specimen on which only Al O is formed due to a lack of Y 2 3 content in the codeposition layer [Fig. 2(a), Fig. 4(a)]. This means that the Al O layer alone can act as a good 2 3 diffusion barrier against oxidants, but cannot absorb a sufficient amount of thermal stress due to its brittle
J.P. Kim et al. / Surface and Coatings Technology 112 (1999) 91–97
Fig. 4. Plot of weight change per unit area vs. number of cycles for cyclic oxidation test: (a) 950 °C; and (b) 1000 °C.
nature. According to the cyclic oxidation test result, ‘‘codeposition C’’ shows the best oxidation resistance. The proper amount of Y in the Al+Y codeposition does lead to a good improvement in oxide stability. This is because Y and/or Y O modifies the nucleation and 2 3 growth mechanism of the alumina forming in the coating layer of Al+Y codeposition. Many studies have been performed to understand the effect of oxygen active elements (OAE ) on the oxidation behavior of the alumina forming alloys, but mostly with superalloys of Niand Co-base alloys [14–18]. However, since the alumina is forming from the Al+Y codeposited layer on the TiAl alloys, it is relevant to examine the mechanisms suggested in the literature to account for the effect of the oxygen active elements. The most frequently mentioned mechanisms are as follows: (1) Oxide pegging or mechanical keying mechanism [15]. (2) Vacancy sink mechanism [16 ].
95
(3) Modification of nucleation and growth mechanism [14,17]. (4) Enhanced scale plasticity [7–11]. (5) Prevention of the segregation of impurities [18]. By understanding the effect of Y on the alumina forming alloys, the role of Y in the Al+Y codeposition may be described as follows. During oxidation at high temperatures, the Y in the Al+Y codeposition is segregated as an ( Y, Al )O-type oxide or YAG because the solubility of Y in Al O is below 1–2%. As discussed previously 2 3 ( Fig. 3), with a proper ratio of Al to Y in the Al+Y codeposition, the outer layer is oxidized to an ( Y, Al )Otype oxide and the inner layer to an Y-rich Al O . It is 2 3 very likely that the segregation of Y or Y O in Al O 2 3 2 3 refines the grain size of Al O , which can easily release 2 3 the plastic deformation developed under thermal stress during cyclic oxidation. Therefore, the coherence of the Al O layer on the substrate can be improved. It has 2 3 also been reported that YAG in the grain boundary of Al O can prevent propagation of the crack in the 2 3 Al O oxide layer [19]. The Y segregation can also 2 3 inhibit the count current diffusions of oxygen and Ti through the rapid diffusion path of the grain boundaries in Al O . 2 3 In the cyclic oxidation test, ‘‘codeposition C’’ specimen, which has the outer ( Y, Al )O type layer and the inner Al O layer, shows better cyclic oxidation resis2 3 tance than ‘‘codeposition B’’ specimen which has a single Al O layer (Figs. 2–4). This is attributed to the 2 3 role of the outer ( Y, Al )O type oxide layer developed only in ‘‘codeposition C’’ specimen in addition to the inner Al O layer which is common to both ‘‘codeposi2 3 tion B and C’’ specimens. The thermal stress developed in the coating layer during cyclic oxidation can be estimated by the following equation [20]: E DT(a −a ) ox ox m , E t ox ox 1+2 E t m m where E is Young’s modulus, a is the coefficient of thermal expansion, and t is the thickness. In the equation, ‘‘ox’’ refers to the oxide, and ‘‘m’’ refers to the base metal. This equation has been derived to evaluate the magnitude of thermal stress in the two-layer system. On cooling, the thermal stress is developed because of the difference in the coefficients of thermal expansion between the two contacting layers. For a simple evaluation of the stress, however, the three-layer system can be divided into two systems of two layers with the intermediate layer being common to both systems. The overall stress is determined by simple summation of the two. It is assumed that the oxides are defect-free and uniform. For the ‘‘codeposition C’’ specimen, it is further assumed that the outer layer consists mainly of YAG, s = ox
A BA B
96
J.P. Kim et al. / Surface and Coatings Technology 112 (1999) 91–97
Table 3 Physical property data of various materials
Thermal expansion coefficient (10−6 °C−1) Young’s modulus (GPa) Thickness layer (mm)
TiAl
Al O 2 3
YAG
12.0a [21] 176 [24] 2000
9.0 [22] 379 [25] 6 for coating B 3 for coating C
8.0 [23] 152a [25] 4 for coating C
a Approximated data.
and the inner Al O layer does not contain yttrium. 2 3 Table 3 lists the physical properties of Al O and YAG. 2 3 Fig. 5 shows schematic illustrations of thermal stresses that may develop in ‘‘codeposition B and C’’ specimens. The states of tensile and compressive stress are indicated, respectively. In ‘‘codeposition B’’, a compressive stress of 951 MPa developed between Al O and the base 2 3 metal. In ‘‘codeposition C’’, the compressive stress developed between Al O and the base metal is 957 MPa, 2 3 whereas the tensile stress developed between YAG and Al O is 68 MPa. Therefore, the net stress of ‘‘codeposi2 3 tion C’’ becomes 889 MPa of compressive stress, which is less than that of ‘‘codeposition B’’.
5. Conclusion (1) The oxidation resistance of TiAl–2.8Nb can be improved extensively by the Al+Y codeposition, which can be obtained by a gradient coating of Al and Y using the EB-PVD method. The Al+Y codeposition with the ratio of Al:2Y for evaporation source material is proved to be the best. (2) With the proper ratio of Al:Y, the Al+Y codeposition forms two distinctive layers of the oxides during high-temperature oxidation; Al O in the inner layer 2 3
and ( Y, Al )O type oxide in the outer layer. In addition to the inner Al O layer, formation of the 2 3 outer ( Y, Al )O-type oxide layer further improves the stability of the coatings. (3) The easy plastic deformation of the duplex layer of Al O and ( Y, Al )O type oxide is responsible for 2 3 improvement of the coating stability during cyclic oxidation. This easy plastic deformation may be possible because of grain refinement of Al O by 2 3 segregation of yttrium oxides in the Al O . 2 3 (4) The stability of the Al+Y codeposition greatly depends upon the alloying element of the TiAl substrate or the oxidation resistance of the TiAl substrate alloy. The non-alloyed TiAl shows a poor coating stability, whereas TiAl–2.8Nb and Alloy K5 show a good coating stability under severe thermal stresses during cyclic oxidation since a stable Al O can form on the surface of these alloys. 2 3
Acknowledgement This study is supported by the academic research fund of the Ministry of Education of Education, Republic of Korea.
References
Fig. 5. Schematic illustration of thermal stresses developed in the Al+Y codeposition layer during cyclic oxidation: (a) codeposition B; and (b) codeposition C.
[1] S. Zhang, D.E. Mikkola, Scripta Metall. Mater. 24 (1990) 1099. [2] A. Rahmel, W.I. Quadakkers, M. Schutze, Mater. Corr. 46 (1995) 271. [3] H.A. Lispsitt, D. Shechtman, R.E. Schafrik, Metall. Trans. A 11 (1980) 1369. [4] Y.W. Kim, J. Metal. 41 (1989) 24. [5] Y. Shida, H. Anada, Oxid. Met. 45 (1996) 197. [6 ] M.P. Brady, I.E. Locci, JOM 48 (1996) 46. [7] H.G. Jung, K.Y. Kim, Oxid. Met. 46 (1996) 147. [8] K.Y. Kim, S.H. Kim, K.W. Kwon, I.H. Kim, Oxid. Met. 41 (1994) 179. [9] J.K. Tien, F.S. Pettit, Met. Trans. A 3 (1972) 1587. [10] A.H. Heuer, N.T. Tighe, R.M. Cannon, J. Am. Ceram. Soc. 63 (1980) 53. [11] D.J. Kuenzly, D.L. Douglass, Oxid. Met. 8 (1974) 139.
J.P. Kim et al. / Surface and Coatings Technology 112 (1999) 91–97 [12] D.W. McKee, S.C. Huang, Corros. Sci. 33 (1992) 1899. [13] Y. Shida, H. Anada, Oxid. Met. 45 (1996) 197. [14] A.M. Huntz, in: E. Lang (Ed.), The Role of Active Elements in the Oxidation Behavior of High Temperature Metals and Alloys, Elsevier Applied Science, London, 1989, p. 81. [15] P. Kofstad, High Temperature Corrosion, Elsevier Applied Science, London, 1988, p. 411. [16 ] F.H. Stott, G.C. Wood, Mater. Sci. Eng. 87 (1987) 267. [17] P.Y. Hou, J. Stringer, J. Electrochem. Soc. 134 (1987) 1836. [18] J.G. Smeggil, Mater. Sci. Eng. 87 (1987) 574. [19] K.T. Faber, A.G. Evans, Acta Met. 31 (1983) 565.
97
[20] G.D. Oxx, Prod. Eng. 29 (1958) 61. [21] R. Wagner et al., in: Y.W. Kim, R. Wagner, M. Yamaguchi ( Eds.), Gamma Titanium Aluminides, TMS, Warrendale, PA, 1995, p. 387. [22] D.J. Whittemore Jr., N.N. Ault, J. Am. Ceram. Soc. 39 (1956) 443. [23] T.K. Gupta, J. Valentch, J. Am. Ceram. Soc. 54 (1971) 355. [24] R.A. Varin, Z. Metallkde 81 (1990) 373. [25] Engineering Property Data on Selected Ceramics, Vol. III: Single Oxides, Metals and Ceramics Information Center, Battelle, OH, 1981.
Al+Y codeposition using EB-PVD method for improvement of high-temperature oxidation resistance of TiAl J.P. Kim *, H.G. Jung, K.Y. Kim Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, South Korea
Abstract An ‘‘Al+Y codeposition’’ on TiAl by the single EB-PVD method has been developed to improve the oxidation resistance of TiAl. To determine the optimum codeposition condition, the Al+Y codepositions with various ratios of Al and Y are evaluated through the isothermal and cyclic oxidation tests. The oxidation resistance of TiAl can be improved extensively by the Al+Y codeposition due to the formation of a gradient coating of Al and Y. The Al+Y codeposition with the ratio of Al:2Y for evaporation source material is proved to be the best. With a proper ratio of Al:Y, the Al+Y codeposition forms two distinctive layers of the oxides during high-temperature oxidation; Al O in the inner layer and (Y, Al )O type oxide in the outer layer. In 2 3 addition to the inner Al O layer, the formation of the outer (Y, Al )O type oxide layer further improves the stability of the 2 3 coatings. The stability of the Al+Y codeposition greatly depends upon the alloying element of TiAl substrate or oxidation resistance of the TiAl substrate alloy. The non-alloyed TiAl shows a poor coating stability, whereas TiAl–2.8Nb and Alloy K5 show a good coating stability under severe thermal stresses during cyclic oxidation since a stable Al O can form on the surface 2 3 of these alloys. © 1999 Elsevier Science S.A. All rights reserved. Keywords: TiAl; High-temperature oxidation; Al+Y codeposition; Gradient coating; Cyclic oxidation; Thermal stress
1. Introduction The TiAl intermetallic compound is considered as a good candidate for high-temperature structural material because of its high strength-to-density ratio at high temperatures [1]. However, it has critical limitations for commercialization due to its poor room-temperature formability and poor high-temperature oxidation resistance. To overcome such limitations, extensive studies are conducted for alloy design and surface coating. The TiAl contains about 50 a/w of Al, but it cannot form a stable protective Al O because of preferential 2 3 formation of porous TiO due to its thermodynamic 2 nature [2,3]. Ternary alloys such as Nb, Mo and W have been reported to exhibit beneficial effects by promoting the formation of dense Al O [4–6 ]. However, 2 3 alloying of these elements is not effective at temperatures above 900 °C and results in adverse effects on the ductility at room temperature. Therefore, a proper surface treatment is the most desirable to improve the
* Corresponding author. Tel: +82 562 2792134; Fax: +82 562 2794499; e-mail: [email protected]
oxidation resistance since it does not have any adverse effects on the mechanical properties of base metals. In this study, a new coating process of Al+Y codeposition is developed to improve the high-temperature oxidation resistance of TiAl. A gradient coating of Al and Y can be expected since the vapor pressure of Al is much higher than that of Y. The nature of the coating structure is analyzed in terms of the distribution of Al and Y across the coating layer and the barrier effect against diffusion of the oxidants. The optimum coating condition is determined through both isothermal and cyclic oxidation tests. To investigate the effect of base metals on Al+Y codeposition, three different TiAl base metals are used: non-alloyed TiAl, TiAl–2.8Nb and Alloy K5 ( Ti–46.5Al–3Nb–2Cr–0.2W ) [4].
2. Experimental Al+Y codeposition was performed using a single EB-PVD process. The substrate materials used for coating were non-alloyed TiAl, TiAl–2.8Nb and Alloy K5. The pure TiAl has a single c-TiAl structure, whereas
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 78 7 - 7
92
J.P. Kim et al. / Surface and Coatings Technology 112 (1999) 91–97
the TiAl–2.8Nb and Alloy K-5 has a duplex structure consisting of c-grains and lamellar grains of alternating layers of c and a2 platelets. However, the grain size of Alloy K5 is much smaller than that of TiAl–2.8Nb. Three different compositions of Al and Y were used as evaporation sources for Al+Y codeposition. The single Al coating was used for comparing with Al+Y codeposition. Table 1 shows the coating conditions. The electron beam power was 1.6 kW, with an acceleration voltage of 8 kV, and the vacuum level was kept at 10−5 Torr at 400 °C. A pellet-type evaporation source material was used, and the size of the pellet was 5–8 mm for Al and 2–5 mm for Y. They were mixed well before melting them. The homogeneous mixing of the melt was obtained by using the sweeping function of electron gun, and melting was conducted for more than 10 min. The isothermal and cyclic oxidation tests were conducted for evaluation of oxidation resistance of the coated specimens. The detailed experimental procedures for oxidation test are described elsewhere [7]. Isothermal oxidation tests were performed using a Cahn 1000 thermogravimetric electrobalance apparatus. The cyclic oxidation test was performed in a tube furnace equipped with a program controller providing a stable thermal cycle. For each cycle, the specimens were exposed to the test temperature for 45 min and cooled to 100 °C for 15 min. After every 20 cycles, the specimen was taken out to measure the weight change. After the oxidation test, specimens were examined using SEM, EPMA and X-ray diffraction to analyze the morphology and composition of the oxides.
3. Results 3.1. Al+Y codeposition The behavior of the Al+Y codeposition was observed with the source materials of three different mixing ratios, and the TiAl–2.8Nb was used as the substrate. Table 2 presents the coating conditions and EDS analyses on the coating layers. Since the vapor pressure of Al is significantly higher than that of Y, a gradient coating of Al+Y is expected with a high Al content in the inner layer and relatively high Y content in the outer layer. The gradient coating is clearly observed from both ‘‘codeposition B and C’’, but no gradient coating is
obtained from ‘‘codeposition A’’. This result indicates that a proper ratio of evaporation source materials should be maintained to secure the gradient coating. 3.2. Isothermal oxidation test An isothermal oxidation test was performed at 950 °C for 20 h to investigate the initial oxidation behavior of each coated specimen. Fig. 1 shows the results of the isothermal oxidation test by plotting the square of weight change per unit area vs. oxidation time. The bare TiAl–2.8Nb shows a typical parabolic oxidation behavior. However, the oxidation rate is greatly reduced by both the single Al deposition and Al+Y codeposition. The specimens of ‘‘single Al deposition’’ and ‘‘codeposition A’’ show an extremely low oxidation rate. On both specimens, a stable Al O is formed. For the ‘‘codeposi2 3 tion A’’ specimen, the Y content is extremely low, as shown in Table 2, and thus no significant layer of Y oxide can be formed. In the case of ‘‘codeposition B and C’’, a rapid oxidation rate is observed in the initial stage. This is due to oxidation of yttrium, which has a high atomic weight as well as a high reactivity with oxygen. The ‘‘codeposition C’’ specimen has a higher oxidation rate than that of the ‘‘codeposition B’’ specimen, simply because of the higher Y content in the ‘‘codeposition C’’ specimen. For both specimens, after initial rapid oxidation, the oxidation rate is reduced by the formation of stable oxides of Al and Y. Fig. 2 shows the line profile of each specimen after the isothermal oxidation test at 950 °C for 200 h. The line profile of ‘‘codeposition A’’ suggests that the oxide is composed of Al O alone since the Y level is negligible 2 3 [Fig. 2(a)]. In the case of ‘‘codeposition B’’, the yttrium distribution is uniform across the oxide layer. This indicates that the oxide layer is a uniform mixture of Al O and Y O [Fig. 2(b)]. The ‘‘codeposition C’’ 2 3 2 3 specimen shows two maxima in Y line profile in the outer layer and an Al peak in the inner layer. These line profiles indicate that a mixture of yttrium oxides forms in the outer layer and Al O forms in the inner layer 2 3 [Fig. 2(c)]. To identify the phases developed in the ‘‘codeposition C’’ specimen after the oxidation test at 950 °C for 200 h, the specimen surface was analyzed by X-ray diffraction. The result is shown in Fig. 3 along with the cross-sectional view of this specimen. The peak
Table 1 Composition of evaporation source and coating time
Single Al deposition codeposition A codeposition B codeposition C
Ratio of Al:Y (wt ratio)
Al (g)
Y (g)
Coating time (min)
1:0 2:1 1:1 1:2
25 25 15 12.5
0 12.5 15 25
14 15 17 19
93
J.P. Kim et al. / Surface and Coatings Technology 112 (1999) 91–97 Table 2 EDS analysis on the lower and upper parts of Al+Y codeposition layer Coating condition
Single Al deposition codeposition A codeposition B codeposition C
Semi-quantitive EDS analysis
Weight ratio of source (Al:Y )
Y in source (at.%)
Lower part Y (at.%)
Upper part Y (at.%)
1:0 2:1 1:1 1:2
0 13.2 23.4 37.8
0 1 8 10
0 1 11 25
intensities for Y O , Y A O ( YAG), and YAlO are 2 3 3 5 12 3 strong, whereas the peak intensity for Al O is relatively 2 3 weak. This XRD analysis suggests that the outer layer is composed of a mixture of Y O , Y A O ( YAG), 2 3 3 5 12 and YAlO and the inner layer is composed of Al O . 3, 2 3 Since the mixture of yttrium oxides cannot be identified individually, it will be called hereafter an ( Y, Al )O-type oxide. Therefore, the oxide developed in the ‘‘codeposition C’’ specimen can be identified, as shown in Fig. 3(b). 3.3. Cyclic oxidation test A cyclic oxidation test was performed to evaluate the stability of oxide formed on the coating specimens under cyclic thermal stresses. Fig. 4(a) shows a plot of the weight change per unit area of the specimen for the test performed at 950 °C. The bare TiAl–2.8Nb specimen shows the rapid decrease in weight after the initial few cycles of the test. This is due to formation of the nonprotective TiO , and it is easily spalled under thermal 2 stress. The single Al-coated specimen shows a relative stability up to 120 cycles, after which it shows a gradual decrease in weight due to spallation of Al O . 2 3 The coating specimens of Al+Y codeposition show a better cyclic oxidation resistance than the single Al-coated specimen. Although the Y content is below 1% in ‘‘codeposition A’’, it shows a stable weight change up to about 500 cycles and then a gradual decrease in weight. With increasing Y content, the beneficial effect
Fig. 1. Plot of square of weight change per unit area vs. time for the isothermal oxidation test at 950 °C.
of Y on the stability of the oxide layer seems to be increased. Of the three Al+Y codeposition specimens, the ‘‘codeposition C’’ specimen shows the best cyclic oxidation resistance. As shown in Fig. 3, this specimen develops an ( Y, Al )O-type oxide in the outer layer and Al O in the inner layer. The inner Al O seems to act 2 3 2 3 as a diffusion barrier, and the outer ( Y, Al )O type oxide seems to reduce the thermal stress by modifying the grain structure of the oxide layer. It has been reported that the Y O distributed in Al O greatly 2 3 2 3 refines the grain size of Al O layer, and thus this grain2 3 refined oxide layer can easily release plastic deformation [7–11]. The effects of oxygen-active elements on the morphology of alumina scale are twofold: one is a decrease in scale convolutions and the other is grain refinement [9]. Grain boundary sliding is expected to be the major deformation mechanism in the scale, and it has been suggested that an oxide with fine grains can accommodate growth and thermally induced stresses by grain boundary sliding more easily than an oxide with large grains [10]. Therefore, it is expected that the Al+Y codeposition can substantially increase the stability of the coating layer under severe thermal fluctuations. To investigate the effect of substrate alloys on the coating stability, three different alloys of non-alloyed TiAl, TiAl–2.8Nb and Alloy K5 were coated with the same condition as that of ‘‘codeposition C’’. The cyclic oxidation test was conducted for these three specimens at 1000 °C. Fig. 4(b) shows a plot of the weight change per unit area of specimen vs. number of cycles. The non-alloyed TiAl shows a rapid weight loss during the initial stage of cyclic oxidation. This is attributed to the fact that the non-alloyed TiAl forms a non-protective TiO instead of forming a stable Al O . However, both 2 2 3 the TiAl–2.8Nb and Alloy K5 specimens show a good stability of the coating layer up to 250 cycles. This result clearly indicates that the oxidation resistance of the substrate alloy is also critical to the stability of the coating layer formed by the Al+Y codeposition. For oxidation of the TiAl substrate, Nb and W ( less than 5%), when added individually, have a beneficial effect, whereas Cr and Ta ( less than 5%) have a detrimental effect [12,13]. However, the oxidation resistance of TiAl can be improved more by the addition of a combination
94
J.P. Kim et al. / Surface and Coatings Technology 112 (1999) 91–97
Fig. 3. X-ray diffraction spectra and cross-sectional micrograph of ‘‘codeposition C’’ specimen after the isothermal oxidation test at 950 °C for 200 h: (a) X-ray diffraction spectra; and (b) cross-sectional micrograph.
exposed to high temperature. This stable alumina formed on the substrate can adhere well with the alumina forming out of the inner layer of the codeposition of Al and Y.
Fig. 2. Line profiles across the scale formed on the coatings after the isothermal oxidation test at 950 °C for 200 h: (a) codeposition A; (b) codeposition B; and (c) codeposition C.
of Nb and Cr than by addition of the individual element [12]. For a ternary alloy of Ti–48Al–X and a quaternary alloy of Ti–48Al–X–Y, increasing concentrations of X and Y will result in a decrease in the Ti activity in the alloy and an increasing tendency to form protective alumina scales on oxidation. Therefore, the specimens of TiAl–2.8Nb and Alloy K5 with ‘‘codeposition C’’ are expected to form a stable alumina scale at the interface between the substrate and coating when
4. Discussion The single Al deposition significantly reduces the oxidation rate of TiAl under isothermal oxidation conditions (Fig. 1). However, during the cyclic oxidation test, the single Al deposition does not show a good stability of the oxide [Fig. 4(a)]. A similar behavior of cyclic oxidation is observed with the ‘‘codeposition A’’ specimen on which only Al O is formed due to a lack of Y 2 3 content in the codeposition layer [Fig. 2(a), Fig. 4(a)]. This means that the Al O layer alone can act as a good 2 3 diffusion barrier against oxidants, but cannot absorb a sufficient amount of thermal stress due to its brittle
J.P. Kim et al. / Surface and Coatings Technology 112 (1999) 91–97
Fig. 4. Plot of weight change per unit area vs. number of cycles for cyclic oxidation test: (a) 950 °C; and (b) 1000 °C.
nature. According to the cyclic oxidation test result, ‘‘codeposition C’’ shows the best oxidation resistance. The proper amount of Y in the Al+Y codeposition does lead to a good improvement in oxide stability. This is because Y and/or Y O modifies the nucleation and 2 3 growth mechanism of the alumina forming in the coating layer of Al+Y codeposition. Many studies have been performed to understand the effect of oxygen active elements (OAE ) on the oxidation behavior of the alumina forming alloys, but mostly with superalloys of Niand Co-base alloys [14–18]. However, since the alumina is forming from the Al+Y codeposited layer on the TiAl alloys, it is relevant to examine the mechanisms suggested in the literature to account for the effect of the oxygen active elements. The most frequently mentioned mechanisms are as follows: (1) Oxide pegging or mechanical keying mechanism [15]. (2) Vacancy sink mechanism [16 ].
95
(3) Modification of nucleation and growth mechanism [14,17]. (4) Enhanced scale plasticity [7–11]. (5) Prevention of the segregation of impurities [18]. By understanding the effect of Y on the alumina forming alloys, the role of Y in the Al+Y codeposition may be described as follows. During oxidation at high temperatures, the Y in the Al+Y codeposition is segregated as an ( Y, Al )O-type oxide or YAG because the solubility of Y in Al O is below 1–2%. As discussed previously 2 3 ( Fig. 3), with a proper ratio of Al to Y in the Al+Y codeposition, the outer layer is oxidized to an ( Y, Al )Otype oxide and the inner layer to an Y-rich Al O . It is 2 3 very likely that the segregation of Y or Y O in Al O 2 3 2 3 refines the grain size of Al O , which can easily release 2 3 the plastic deformation developed under thermal stress during cyclic oxidation. Therefore, the coherence of the Al O layer on the substrate can be improved. It has 2 3 also been reported that YAG in the grain boundary of Al O can prevent propagation of the crack in the 2 3 Al O oxide layer [19]. The Y segregation can also 2 3 inhibit the count current diffusions of oxygen and Ti through the rapid diffusion path of the grain boundaries in Al O . 2 3 In the cyclic oxidation test, ‘‘codeposition C’’ specimen, which has the outer ( Y, Al )O type layer and the inner Al O layer, shows better cyclic oxidation resis2 3 tance than ‘‘codeposition B’’ specimen which has a single Al O layer (Figs. 2–4). This is attributed to the 2 3 role of the outer ( Y, Al )O type oxide layer developed only in ‘‘codeposition C’’ specimen in addition to the inner Al O layer which is common to both ‘‘codeposi2 3 tion B and C’’ specimens. The thermal stress developed in the coating layer during cyclic oxidation can be estimated by the following equation [20]: E DT(a −a ) ox ox m , E t ox ox 1+2 E t m m where E is Young’s modulus, a is the coefficient of thermal expansion, and t is the thickness. In the equation, ‘‘ox’’ refers to the oxide, and ‘‘m’’ refers to the base metal. This equation has been derived to evaluate the magnitude of thermal stress in the two-layer system. On cooling, the thermal stress is developed because of the difference in the coefficients of thermal expansion between the two contacting layers. For a simple evaluation of the stress, however, the three-layer system can be divided into two systems of two layers with the intermediate layer being common to both systems. The overall stress is determined by simple summation of the two. It is assumed that the oxides are defect-free and uniform. For the ‘‘codeposition C’’ specimen, it is further assumed that the outer layer consists mainly of YAG, s = ox
A BA B
96
J.P. Kim et al. / Surface and Coatings Technology 112 (1999) 91–97
Table 3 Physical property data of various materials
Thermal expansion coefficient (10−6 °C−1) Young’s modulus (GPa) Thickness layer (mm)
TiAl
Al O 2 3
YAG
12.0a [21] 176 [24] 2000
9.0 [22] 379 [25] 6 for coating B 3 for coating C
8.0 [23] 152a [25] 4 for coating C
a Approximated data.
and the inner Al O layer does not contain yttrium. 2 3 Table 3 lists the physical properties of Al O and YAG. 2 3 Fig. 5 shows schematic illustrations of thermal stresses that may develop in ‘‘codeposition B and C’’ specimens. The states of tensile and compressive stress are indicated, respectively. In ‘‘codeposition B’’, a compressive stress of 951 MPa developed between Al O and the base 2 3 metal. In ‘‘codeposition C’’, the compressive stress developed between Al O and the base metal is 957 MPa, 2 3 whereas the tensile stress developed between YAG and Al O is 68 MPa. Therefore, the net stress of ‘‘codeposi2 3 tion C’’ becomes 889 MPa of compressive stress, which is less than that of ‘‘codeposition B’’.
5. Conclusion (1) The oxidation resistance of TiAl–2.8Nb can be improved extensively by the Al+Y codeposition, which can be obtained by a gradient coating of Al and Y using the EB-PVD method. The Al+Y codeposition with the ratio of Al:2Y for evaporation source material is proved to be the best. (2) With the proper ratio of Al:Y, the Al+Y codeposition forms two distinctive layers of the oxides during high-temperature oxidation; Al O in the inner layer 2 3
and ( Y, Al )O type oxide in the outer layer. In addition to the inner Al O layer, formation of the 2 3 outer ( Y, Al )O-type oxide layer further improves the stability of the coatings. (3) The easy plastic deformation of the duplex layer of Al O and ( Y, Al )O type oxide is responsible for 2 3 improvement of the coating stability during cyclic oxidation. This easy plastic deformation may be possible because of grain refinement of Al O by 2 3 segregation of yttrium oxides in the Al O . 2 3 (4) The stability of the Al+Y codeposition greatly depends upon the alloying element of the TiAl substrate or the oxidation resistance of the TiAl substrate alloy. The non-alloyed TiAl shows a poor coating stability, whereas TiAl–2.8Nb and Alloy K5 show a good coating stability under severe thermal stresses during cyclic oxidation since a stable Al O can form on the surface of these alloys. 2 3
Acknowledgement This study is supported by the academic research fund of the Ministry of Education of Education, Republic of Korea.
References
Fig. 5. Schematic illustration of thermal stresses developed in the Al+Y codeposition layer during cyclic oxidation: (a) codeposition B; and (b) codeposition C.
[1] S. Zhang, D.E. Mikkola, Scripta Metall. Mater. 24 (1990) 1099. [2] A. Rahmel, W.I. Quadakkers, M. Schutze, Mater. Corr. 46 (1995) 271. [3] H.A. Lispsitt, D. Shechtman, R.E. Schafrik, Metall. Trans. A 11 (1980) 1369. [4] Y.W. Kim, J. Metal. 41 (1989) 24. [5] Y. Shida, H. Anada, Oxid. Met. 45 (1996) 197. [6 ] M.P. Brady, I.E. Locci, JOM 48 (1996) 46. [7] H.G. Jung, K.Y. Kim, Oxid. Met. 46 (1996) 147. [8] K.Y. Kim, S.H. Kim, K.W. Kwon, I.H. Kim, Oxid. Met. 41 (1994) 179. [9] J.K. Tien, F.S. Pettit, Met. Trans. A 3 (1972) 1587. [10] A.H. Heuer, N.T. Tighe, R.M. Cannon, J. Am. Ceram. Soc. 63 (1980) 53. [11] D.J. Kuenzly, D.L. Douglass, Oxid. Met. 8 (1974) 139.
J.P. Kim et al. / Surface and Coatings Technology 112 (1999) 91–97 [12] D.W. McKee, S.C. Huang, Corros. Sci. 33 (1992) 1899. [13] Y. Shida, H. Anada, Oxid. Met. 45 (1996) 197. [14] A.M. Huntz, in: E. Lang (Ed.), The Role of Active Elements in the Oxidation Behavior of High Temperature Metals and Alloys, Elsevier Applied Science, London, 1989, p. 81. [15] P. Kofstad, High Temperature Corrosion, Elsevier Applied Science, London, 1988, p. 411. [16 ] F.H. Stott, G.C. Wood, Mater. Sci. Eng. 87 (1987) 267. [17] P.Y. Hou, J. Stringer, J. Electrochem. Soc. 134 (1987) 1836. [18] J.G. Smeggil, Mater. Sci. Eng. 87 (1987) 574. [19] K.T. Faber, A.G. Evans, Acta Met. 31 (1983) 565.
97
[20] G.D. Oxx, Prod. Eng. 29 (1958) 61. [21] R. Wagner et al., in: Y.W. Kim, R. Wagner, M. Yamaguchi ( Eds.), Gamma Titanium Aluminides, TMS, Warrendale, PA, 1995, p. 387. [22] D.J. Whittemore Jr., N.N. Ault, J. Am. Ceram. Soc. 39 (1956) 443. [23] T.K. Gupta, J. Valentch, J. Am. Ceram. Soc. 54 (1971) 355. [24] R.A. Varin, Z. Metallkde 81 (1990) 373. [25] Engineering Property Data on Selected Ceramics, Vol. III: Single Oxides, Metals and Ceramics Information Center, Battelle, OH, 1981.