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Effects of lanthanum ion-implantation on microstructure of oxide film formed on Co-Cr alloy
JOURNAL OF RARE EARTHS, Vol. 26, No. 3, Jun. 2008, p. 406
Effects of lanthanum ion-implantation on microstructure of oxide film formed on Co-Cr alloy JIN Huiming (靳惠明)1, ZHOU Xiaowei (周小卫)1, ZHANG Linnan (张林楠)2 (1. Materials Research Center, College of Mechanical Engineering, Yangzhou University, Yangzhou 225009, China; 2. College of Environmental Science and Engineering, Peking University, Beijing 100015, China) Received 25 June 2007; revised 12 November 2007
Abstract: Isothermal and cyclic oxidizing behavior of Co-40Cr alloy and its lanthanum ion-implanted samples were studied at 1000 °C in the air by thermal-gravimetric analysis (TGA). Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) were used to examine the morphology and structure of oxide film after oxidation. Secondary ion mass spectrum (SIMS) method was used to examine the binding energy change of chromium caused by La-doping and its influence on the formation of Cr2O3 film. Laser Raman spectrum was used to examine the stress changes within the oxide film. It was found that lanthanum implantation remarkably reduced isothermal oxidizing rate of Co-40Cr and improved anti-cracking and anti-spalling properties of Cr2O3 oxide film. The reasons for the improvement were mainly that the implanted lanthanum reduced the grain size and internal stress of Cr2O3 oxide and increased high temperature plasticity of the oxide film. Lanthanum mainly existed on the outer surface of Cr2O3 oxide film in the form of fine La2O3 and LaCrO3 spinel particles. Keywords: ion implantation; lanthanum; chromium oxide; raman spectrum; SIMS; rare earths
The resistance of high temperature alloys to oxidizing environment depends on the formation of slowly growing and adhered oxide films. There are intrinsic growing stress and external thermal stress formed in oxides which can lead to cracking and spalling of oxides[1,2]. Usually, the addition of a small amount of rare earth elements to alloys can greatly improve their anti-oxidation property. In this study, high temperature oxidation behavior and effects of rare earth on basic Co-Cr binary system was examined by several methods.
1 Experimental Co-40Cr alloy was wire-cut into 10 mm×10 mm×1 mm samples which were ultimately polished by 0.2 µm Al2O3 abrasive paste. After being ultrasonically cleaned in acetone and alcohol, some specimens were ion-implanted with 3×1017La+/cm2 lanthanum using MEVVE-8010 ion-implantation machine at an accelerating voltage of 50 keV. Isothermal oxidation and cyclic oxidation (55 min heating +5 min air-cooling) were carried out at 1000 °C in air in M25DV thermal balance to study the oxidation behavior of Co-40Cr and La-implanted Co-40Cr alloy. Scanning electronic microscopy (SEM) and transmission electronic mi-
croscopy (TEM) were used to examine the surface morphology and phase composition of oxide films. In TEM sample preparation, oxidized specimen was dipped in Br2/formaldehyde solution (20 wt.%) to make the metal thoroughly dissolved and the oxide film peeled off, then the oxide film was Ar+ ion-beam thinned at 8 degrees to get the final TEM sample. Secondary ion mass spectrum (SIMS) technique was used to analyze the lanthanum depth profile in substrate Co-40Cr before oxidation and in Cr2O3 oxide film after oxidation, respectively, and was also used to measure the binding energy changes of element Cr caused by La-implantation. Laser Raman spectrum was used to examine the internal stress change within oxide films.
2 Results and discussion 2. 1 Growing behavior of oxide film The isothermal oxidation mass-gain curves of Co-40Cr and La-implanted Co-40Cr are shown in Fig.1. It can be seen that lanthanum implantation remarkably reduces the oxidizing rate of Co-40Cr alloy. Mass change curves of Co-40Cr and La-implanted Co-40Cr during cyclic oxidation are shown in Fig.2; it can
Foundation item: Project supported by the National Natural Science Foundation of China (29231011) and the Natural Science Foundation of Jiangsu Province (07KJD430246) Corresponding author: JIN Huiming (E-mail: [email protected]; Tel.: +86-514-2077110)
JIN H M et al., Effects of lanthanum ion-implantation on microstructure of oxide film formed on Co-Cr alloy
407
Fig.1 Isothermal oxidizing mass gain curves of Co-40Cr and Laimplanted Co-40Cr
be seen that severe oxide spallation (i.e., weight loss) occurs in La-free Co-40Cr after 20 h cyclic oxidation, while La-doped Co-40Cr shows no obvious spallation during the 90 h cyclic oxidation stage. Fig.3 show the SEM morphologies of the oxide films formed on Co-40Cr and its La-implanted samples after 50 h isothermal oxidation. We can find that lanthanum ion-implantation remarkably reduces the grain size of oxide film, and ridge character has apparently arisen in the La-containing oxide film (Fig.3(b)). Usually the maximum depth of ion-implantation is less than 100 nm and the implantation process can introduce large amounts of dislocations in alloy surface[2]. During the initial oxidizing stage, the implanted lanthanum with high local concentration and high chemical activity would act as Cr2O3 crystal forming site and promote the formation of fine-grained Cr2O3 oxide[2,3]. 2. 2 Chromium binding energy test In SIMS experiment, there is linear relationship[3] between element binding energy change (∆EB) and retarding voltage change (∆UR), that is, ∆EB=–e∆UR, in which UR can
Fig.2 Mass change curves of Co-40Cr and La-implanted Co-40Cr during cyclic oxidation
Fig.3 SEM morphology of oxide films formed on Co-40Cr (a) and La-implanted Co-40Cr (b) after 50 h isothermal oxidation
be obtained by extrapolating the linear part of the secondary ion intensity curve until it intersects with the abscissa as shown in Fig.4. Fig.4(a) shows chromium secondary ion intensity curve of La-free and La-implanted Co-40Cr before oxidation, and Fig.4(b) shows chromium secondary ion intensity curve of two kinds of samples after 1 h of isothermal oxidation. Fig.4(c) shows the lanthanum depth profile along oxide/substrate cross-section after 10 h isothermal oxidation. When Cr content exceeds 35 at.% for Co-Cr or Ni-Cr systems, the oxide film will be pure Cr2O3 without CoO or NiO[4], so the position of the oxide/alloy interface in Fig.4(c) can be determined by detecting secondary Co2+ ion intensity during Ar+ ion-beam sputtering process. La-implantation reduces Cr binding energy before oxidation (Fig.4(a)). This effect means that lanthanum can increase Cr2O3 crystal formation rate and finally refines Cr2O3 oxide grain size. In addition, lanthanum would inhibit [Cr3+] ion diffusion within Cr2O3 oxide film[3], which can be partially confirmed by the increase of the Cr binding energy after a long time of oxidation (e.g., 1 h in the experiment as shown in Fig.4(b)), and change the oxidizing rate-controlling step from predominant [Cr3+] cation outwards diffusion to predominant [O2-] anion inward diffusion[5]. Studies related with ion diffusion within oxide film has been investigated by some researchers[6] using O16/O18 tracing element. Predominant [Cr3+] cation’s outwards diffusion also can be partially
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confirmed by the SIMS experiment, in which the final position of lanthanum after 10 h oxidation is located near the outer surface of Cr2O3 oxide film as shown in Fig.4(c). 2. 3 Stress and micro-structure of oxide film In Raman experiment, pure fine grain-sized Cr2O3 powder is annealed at 900 °C for 20 h and is considered as the standard stress-free status as shown in Fig.5(a). The Raman peaks of Cr2O3 oxides formed on La-free and La-implanted sample are shown in Fig.5(b) and (c), respectively. According to Birnie’s study[7], mono-chromatic laser light is inelastically scattered to give the Raman spectrum corresponding to particular molecular and lattice vibrations of solid. When the sample is stressed, the vibration frequencies of the Raman peaks shift to higher frequencies for compressive stress and to lower frequencies for tensile stress. So it
can be used in stress measurement and there is a linear relationship between stress and its Raman wave-number shift[5,7]. From Fig.5, La-implantation reduces the internal compressive stress level in Cr2O3 oxide film. Fig.6 shows the TEM bright field images of La-free and La-doped Cr2O3 oxide films, showing their phase compositions. La implantation remarkably reduces the grain size of Cr2O3. In Fig.6(a), some internal voids form along Cr2O3 grain boundaries, which indicates relatively faster outward diffusion of Cr3+ cation in La-free oxide film compared with La-doped film. These voids are results of vacancy condensation left by outward diffusion of Cr3+. Some researchers[5,8] even believe that chromium can diffuse outward in the form of vapor at a temperature exceeding 1050 °C. Additionally, some inter-granular cracks develop along the Cr2O3 grain
Fig.4 Secondary Cr3+ ion intensity curves of Co-40Cr and La-implanted Co-40Cr before (a) and after (b) oxidation and La depth profile along cross-section (c)
Fig.5 Raman peaks of standard Cr2O3 powder (a), Cr2O3 oxides formed on Co-40Cr (b) and La-implanted Co-40Cr (c)
Fig.6 TEM bright field images of La-free (a) and La-doped (b) Cr2O3 oxide films
JIN H M et al., Effects of lanthanum ion-implantation on microstructure of oxide film formed on Co-Cr alloy
boundaries and these cracks are evidence of transient stress relieving in La-free oxide film. In Fig.6(b), small nano-sized La2O3 particles exist mainly at Cr2O3 grain boundaries, and some smaller La2O3 particles even exist inside Cr2O3 grains. Some co-existing LaCrO3 spinel particles exist at chromia grain boundaries which can be identified by the diffraction patterns in TEM; No obvious voids and cracks are observed in the La-doped film. Lanthanum implantation reduces the internal compressive stress level in the Cr2O3 film and promotes the formation of fine-grained Cr2O3 oxide. This kind of fine-grained oxide film has better high temperature plasticity and creep property[8,9], which means that Cr2O3 oxide film could relieve parts of internal compressive stress by means of high temperature creep rather than by means of transient cracking as in the case of Fig.6(a). This type of stress relief via oxide’s creep could be experimentally confirmed by the ridge character of La-doped oxide film in Fig.3(b).
3 Conclusions 1. Lanthanum doping remarkably reduced the isothermal oxidizing rate of Co-40Cr at 1000 °C; Meanwhile, the anti-cracking and anti-spalling properties of the surface oxide film were greatly improved. 2. The effects of rare earth were mainly attributed to lower internal stress level, grain-size refining and improved high temperature plasticity and creep property of the oxide film. Lanthanum mainly existed in the outer surface of
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Cr2O3 oxide film in the form of fine La2O3 and LaCrO3 spinel particles; these particles might inhibit cation diffusion within oxide films.
References: [1] Evans A G, Cannon R. Stress and decohesion of oxide scales. Materials Science Forum, 1989, 43(3): 243. [2] Evans H E. Cracking and spalling of protective oxide layers. Materials Science and Engineering, 1989, A120(10): 139. [3] Rahmel A, Schutze M. Mechanical aspects of rare earth effects. Oxidation of Metals, 1992, 38(2): 314. [4] Jin H M, Liu X J, Zhang L N. Influence of nanometric ceria coating on oxidation behavior of chromium at 900 °C. Journal of Rare Earths, 2007, 25(1): 63. [5] Hou P U, Stringer J. The influence of ion-implanted yttrium on the selective oxidation of chromium in Co-25Cr. Oxidation of Metals, 1988, 29(8): 45. [6] Przybilla W, Schutze M. Role of growth stresses on the structure of oxide scales on nickel at 800 and 900 °C. Oxidation of Metals, 2002, 58(1): 103. [7] Birnie J, Cragga C. Ex situ and in situ determination of stress distribution in chromium oxide films by Raman microscopy. Corrosion Science, 1992, 33(2): 1. [8] Ul-Hamid A. Study of the effect of Y on the scale microstructures of Cr2O3- and Al2O3-forming alloys. Oxidation of Metals, 2002, 58(1): 23. [9] Jin H M, Felix A C, Aroyave M H. Rare earth effect yttrium on formation and property of Cr2O3 oxide film formed on Co-Cr binary alloy. Progress in Natural Science, 2006, 16(1): 72.
Effects of lanthanum ion-implantation on microstructure of oxide film formed on Co-Cr alloy JIN Huiming (靳惠明)1, ZHOU Xiaowei (周小卫)1, ZHANG Linnan (张林楠)2 (1. Materials Research Center, College of Mechanical Engineering, Yangzhou University, Yangzhou 225009, China; 2. College of Environmental Science and Engineering, Peking University, Beijing 100015, China) Received 25 June 2007; revised 12 November 2007
Abstract: Isothermal and cyclic oxidizing behavior of Co-40Cr alloy and its lanthanum ion-implanted samples were studied at 1000 °C in the air by thermal-gravimetric analysis (TGA). Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) were used to examine the morphology and structure of oxide film after oxidation. Secondary ion mass spectrum (SIMS) method was used to examine the binding energy change of chromium caused by La-doping and its influence on the formation of Cr2O3 film. Laser Raman spectrum was used to examine the stress changes within the oxide film. It was found that lanthanum implantation remarkably reduced isothermal oxidizing rate of Co-40Cr and improved anti-cracking and anti-spalling properties of Cr2O3 oxide film. The reasons for the improvement were mainly that the implanted lanthanum reduced the grain size and internal stress of Cr2O3 oxide and increased high temperature plasticity of the oxide film. Lanthanum mainly existed on the outer surface of Cr2O3 oxide film in the form of fine La2O3 and LaCrO3 spinel particles. Keywords: ion implantation; lanthanum; chromium oxide; raman spectrum; SIMS; rare earths
The resistance of high temperature alloys to oxidizing environment depends on the formation of slowly growing and adhered oxide films. There are intrinsic growing stress and external thermal stress formed in oxides which can lead to cracking and spalling of oxides[1,2]. Usually, the addition of a small amount of rare earth elements to alloys can greatly improve their anti-oxidation property. In this study, high temperature oxidation behavior and effects of rare earth on basic Co-Cr binary system was examined by several methods.
1 Experimental Co-40Cr alloy was wire-cut into 10 mm×10 mm×1 mm samples which were ultimately polished by 0.2 µm Al2O3 abrasive paste. After being ultrasonically cleaned in acetone and alcohol, some specimens were ion-implanted with 3×1017La+/cm2 lanthanum using MEVVE-8010 ion-implantation machine at an accelerating voltage of 50 keV. Isothermal oxidation and cyclic oxidation (55 min heating +5 min air-cooling) were carried out at 1000 °C in air in M25DV thermal balance to study the oxidation behavior of Co-40Cr and La-implanted Co-40Cr alloy. Scanning electronic microscopy (SEM) and transmission electronic mi-
croscopy (TEM) were used to examine the surface morphology and phase composition of oxide films. In TEM sample preparation, oxidized specimen was dipped in Br2/formaldehyde solution (20 wt.%) to make the metal thoroughly dissolved and the oxide film peeled off, then the oxide film was Ar+ ion-beam thinned at 8 degrees to get the final TEM sample. Secondary ion mass spectrum (SIMS) technique was used to analyze the lanthanum depth profile in substrate Co-40Cr before oxidation and in Cr2O3 oxide film after oxidation, respectively, and was also used to measure the binding energy changes of element Cr caused by La-implantation. Laser Raman spectrum was used to examine the internal stress change within oxide films.
2 Results and discussion 2. 1 Growing behavior of oxide film The isothermal oxidation mass-gain curves of Co-40Cr and La-implanted Co-40Cr are shown in Fig.1. It can be seen that lanthanum implantation remarkably reduces the oxidizing rate of Co-40Cr alloy. Mass change curves of Co-40Cr and La-implanted Co-40Cr during cyclic oxidation are shown in Fig.2; it can
Foundation item: Project supported by the National Natural Science Foundation of China (29231011) and the Natural Science Foundation of Jiangsu Province (07KJD430246) Corresponding author: JIN Huiming (E-mail: [email protected]; Tel.: +86-514-2077110)
JIN H M et al., Effects of lanthanum ion-implantation on microstructure of oxide film formed on Co-Cr alloy
407
Fig.1 Isothermal oxidizing mass gain curves of Co-40Cr and Laimplanted Co-40Cr
be seen that severe oxide spallation (i.e., weight loss) occurs in La-free Co-40Cr after 20 h cyclic oxidation, while La-doped Co-40Cr shows no obvious spallation during the 90 h cyclic oxidation stage. Fig.3 show the SEM morphologies of the oxide films formed on Co-40Cr and its La-implanted samples after 50 h isothermal oxidation. We can find that lanthanum ion-implantation remarkably reduces the grain size of oxide film, and ridge character has apparently arisen in the La-containing oxide film (Fig.3(b)). Usually the maximum depth of ion-implantation is less than 100 nm and the implantation process can introduce large amounts of dislocations in alloy surface[2]. During the initial oxidizing stage, the implanted lanthanum with high local concentration and high chemical activity would act as Cr2O3 crystal forming site and promote the formation of fine-grained Cr2O3 oxide[2,3]. 2. 2 Chromium binding energy test In SIMS experiment, there is linear relationship[3] between element binding energy change (∆EB) and retarding voltage change (∆UR), that is, ∆EB=–e∆UR, in which UR can
Fig.2 Mass change curves of Co-40Cr and La-implanted Co-40Cr during cyclic oxidation
Fig.3 SEM morphology of oxide films formed on Co-40Cr (a) and La-implanted Co-40Cr (b) after 50 h isothermal oxidation
be obtained by extrapolating the linear part of the secondary ion intensity curve until it intersects with the abscissa as shown in Fig.4. Fig.4(a) shows chromium secondary ion intensity curve of La-free and La-implanted Co-40Cr before oxidation, and Fig.4(b) shows chromium secondary ion intensity curve of two kinds of samples after 1 h of isothermal oxidation. Fig.4(c) shows the lanthanum depth profile along oxide/substrate cross-section after 10 h isothermal oxidation. When Cr content exceeds 35 at.% for Co-Cr or Ni-Cr systems, the oxide film will be pure Cr2O3 without CoO or NiO[4], so the position of the oxide/alloy interface in Fig.4(c) can be determined by detecting secondary Co2+ ion intensity during Ar+ ion-beam sputtering process. La-implantation reduces Cr binding energy before oxidation (Fig.4(a)). This effect means that lanthanum can increase Cr2O3 crystal formation rate and finally refines Cr2O3 oxide grain size. In addition, lanthanum would inhibit [Cr3+] ion diffusion within Cr2O3 oxide film[3], which can be partially confirmed by the increase of the Cr binding energy after a long time of oxidation (e.g., 1 h in the experiment as shown in Fig.4(b)), and change the oxidizing rate-controlling step from predominant [Cr3+] cation outwards diffusion to predominant [O2-] anion inward diffusion[5]. Studies related with ion diffusion within oxide film has been investigated by some researchers[6] using O16/O18 tracing element. Predominant [Cr3+] cation’s outwards diffusion also can be partially
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JOURNAL OF RARE EARTHS, Vol. 26, No. 3, Jun. 2008
confirmed by the SIMS experiment, in which the final position of lanthanum after 10 h oxidation is located near the outer surface of Cr2O3 oxide film as shown in Fig.4(c). 2. 3 Stress and micro-structure of oxide film In Raman experiment, pure fine grain-sized Cr2O3 powder is annealed at 900 °C for 20 h and is considered as the standard stress-free status as shown in Fig.5(a). The Raman peaks of Cr2O3 oxides formed on La-free and La-implanted sample are shown in Fig.5(b) and (c), respectively. According to Birnie’s study[7], mono-chromatic laser light is inelastically scattered to give the Raman spectrum corresponding to particular molecular and lattice vibrations of solid. When the sample is stressed, the vibration frequencies of the Raman peaks shift to higher frequencies for compressive stress and to lower frequencies for tensile stress. So it
can be used in stress measurement and there is a linear relationship between stress and its Raman wave-number shift[5,7]. From Fig.5, La-implantation reduces the internal compressive stress level in Cr2O3 oxide film. Fig.6 shows the TEM bright field images of La-free and La-doped Cr2O3 oxide films, showing their phase compositions. La implantation remarkably reduces the grain size of Cr2O3. In Fig.6(a), some internal voids form along Cr2O3 grain boundaries, which indicates relatively faster outward diffusion of Cr3+ cation in La-free oxide film compared with La-doped film. These voids are results of vacancy condensation left by outward diffusion of Cr3+. Some researchers[5,8] even believe that chromium can diffuse outward in the form of vapor at a temperature exceeding 1050 °C. Additionally, some inter-granular cracks develop along the Cr2O3 grain
Fig.4 Secondary Cr3+ ion intensity curves of Co-40Cr and La-implanted Co-40Cr before (a) and after (b) oxidation and La depth profile along cross-section (c)
Fig.5 Raman peaks of standard Cr2O3 powder (a), Cr2O3 oxides formed on Co-40Cr (b) and La-implanted Co-40Cr (c)
Fig.6 TEM bright field images of La-free (a) and La-doped (b) Cr2O3 oxide films
JIN H M et al., Effects of lanthanum ion-implantation on microstructure of oxide film formed on Co-Cr alloy
boundaries and these cracks are evidence of transient stress relieving in La-free oxide film. In Fig.6(b), small nano-sized La2O3 particles exist mainly at Cr2O3 grain boundaries, and some smaller La2O3 particles even exist inside Cr2O3 grains. Some co-existing LaCrO3 spinel particles exist at chromia grain boundaries which can be identified by the diffraction patterns in TEM; No obvious voids and cracks are observed in the La-doped film. Lanthanum implantation reduces the internal compressive stress level in the Cr2O3 film and promotes the formation of fine-grained Cr2O3 oxide. This kind of fine-grained oxide film has better high temperature plasticity and creep property[8,9], which means that Cr2O3 oxide film could relieve parts of internal compressive stress by means of high temperature creep rather than by means of transient cracking as in the case of Fig.6(a). This type of stress relief via oxide’s creep could be experimentally confirmed by the ridge character of La-doped oxide film in Fig.3(b).
3 Conclusions 1. Lanthanum doping remarkably reduced the isothermal oxidizing rate of Co-40Cr at 1000 °C; Meanwhile, the anti-cracking and anti-spalling properties of the surface oxide film were greatly improved. 2. The effects of rare earth were mainly attributed to lower internal stress level, grain-size refining and improved high temperature plasticity and creep property of the oxide film. Lanthanum mainly existed in the outer surface of
409
Cr2O3 oxide film in the form of fine La2O3 and LaCrO3 spinel particles; these particles might inhibit cation diffusion within oxide films.
References: [1] Evans A G, Cannon R. Stress and decohesion of oxide scales. Materials Science Forum, 1989, 43(3): 243. [2] Evans H E. Cracking and spalling of protective oxide layers. Materials Science and Engineering, 1989, A120(10): 139. [3] Rahmel A, Schutze M. Mechanical aspects of rare earth effects. Oxidation of Metals, 1992, 38(2): 314. [4] Jin H M, Liu X J, Zhang L N. Influence of nanometric ceria coating on oxidation behavior of chromium at 900 °C. Journal of Rare Earths, 2007, 25(1): 63. [5] Hou P U, Stringer J. The influence of ion-implanted yttrium on the selective oxidation of chromium in Co-25Cr. Oxidation of Metals, 1988, 29(8): 45. [6] Przybilla W, Schutze M. Role of growth stresses on the structure of oxide scales on nickel at 800 and 900 °C. Oxidation of Metals, 2002, 58(1): 103. [7] Birnie J, Cragga C. Ex situ and in situ determination of stress distribution in chromium oxide films by Raman microscopy. Corrosion Science, 1992, 33(2): 1. [8] Ul-Hamid A. Study of the effect of Y on the scale microstructures of Cr2O3- and Al2O3-forming alloys. Oxidation of Metals, 2002, 58(1): 23. [9] Jin H M, Felix A C, Aroyave M H. Rare earth effect yttrium on formation and property of Cr2O3 oxide film formed on Co-Cr binary alloy. Progress in Natural Science, 2006, 16(1): 72.