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Novel morphologies and growth mechanism of Cr2O3 oxide formed on stainless steel surface via Nd: YAG pulsed laser oxidation
Journal of Alloys and Compounds 635 (2015) 101–106
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Novel morphologies and growth mechanism of Cr2O3 oxide formed on stainless steel surface via Nd: YAG pulsed laser oxidation C.Y. Cui a,⇑, C.D. Xia a, X.G. Cui a,b,⇑, J.Z. Zhou a, X.D. Ren a, Y.M. Wang c a
School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, PR China State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China c Laser Institute of Jilin, Qianjin Street No. 1244, Changchun 130012, PR China b
a r t i c l e
i n f o
Article history: Received 19 December 2014 Received in revised form 30 January 2015 Accepted 6 February 2015 Available online 13 February 2015 Keywords: Chromium sesquioxide (Cr2O3) Novel morphologies Stainless steel Nd: YAG pulsed laser oxidation Growth mechanism
a b s t r a c t Novel morphologies of chromium sesquioxide (Cr2O3) were successfully formed on AISI 304 stainless steel surface by Nd: YAG pulsed laser oxidation in air. The obtained Cr2O3 presented mainly dendritic, gear-like, flower-like and regular hexagonal morphologies with sizes from hundred nanometers to several micrometers. These various morphologies chiefly originated from two groups of diverse growth 1 0i and h0 1 1 0i directions. The preferential growth along directions within (0 0 0 1) basal plane, i.e., h2 1 different directions determined the final Cr2O3 morphologies. And the crystallographic analyses basically accorded with the experimental results. Moreover, the growth mechanism of the hexagonal Cr2O3 was analyzed in detail from its morphology evolution, and a corresponding growth model was reasonably proposed. Ó 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Chromium sesquioxide (Cr2O3) is one of the most attractive metal oxides due to its useful and important properties required in industrial applications, such as high hardness (about 29.5 GPa), high melting point (about 2400 °C), high wear resistance, high temperature oxidation resistance and antiferromagnetic properties [1–5]. Therefore, various methods have been developed to prepare Cr2O3. At present, the primary preparation method is chemical synthesis. Khamlich et al. have successfully synthesized monodispersed spherical a-Cr2O3 particles from a diluted solution of KCr(SO4)212H2O using Aqueous Chemical Growth (ACG) technique [6]. Bai et al. have fabricated cylinderand cake-like Cr2O3 with a rhombohedral structure using hydrothermal and microemulsion methods, respectively [7]. And Liang et al. have prepared Cr2O3 green pigments by calcining two types of CrOOH (a-CrOOH and c-CrOOH) [8]. However, these chemical synthesis methods have a certain environmental pollution and component uncertainty. Therefore, it is necessary to exploit and adopt new environment-friendly preparation methods to produce Cr2O3. Recently, laser surface treatment (LST) has been used
to fabricate novel Cr2O3 structures. Dong et al. have successfully prepared Cr2O3 on Cr film by Nd-YAG laser oxidation process and analyzed its morphologies [9]. Lin et al. have fabricated a-Cr2O3 single-crystal nanocondensates on Cr plate (99.9% pure) by pulsed laser ablation in air and discussed the shape-dependent local internal stress of the anisotropic crystal [10]. The obtained Cr2O3 possesses different morphologies such as nano-particle, leaf-like, pyramid-like, nutshell-like [9], truncated rhombohedral and hexagonal shapes [10]. The above studies are emphasized on Cr film or Cr plate with high Cr content, while the oxidation states and morphologies of other materials with a relatively low Cr content, such as the stainless steel (Cr content can be up to about 30 wt.%), are rarely referred. In this respect, we have carried out some researches on stainless steel surface oxidation induced by Nd: YAG pulsed laser and reported the regular hexagonal structures, oxidation mechanism and surface properties [11–13]. However, other morphologies, such as dendritic, flower-like and gear-like, are rarely reported. And their growth mechanisms are also little discussed. In this paper, the novel Cr2O3 morphologies formed on AISI 304 stainless steel surface after Nd: YAG pulsed laser oxidation are investigated in detail. And a possible growth mechanism is proposed to explain their formation process.
⇑ Corresponding authors at: School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, PR China (X.G. Cui). Tel.: +86 511 88797898; fax: +86 511 88780241. E-mail addresses: [email protected] (C.Y. Cui), [email protected] (X.G. Cui). http://dx.doi.org/10.1016/j.jallcom.2015.02.053 0925-8388/Ó 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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2. Experimental procedures The investigated material was AISI 304 austenite stainless steel with chemical composition (wt.%) of 0.049 C, 18.20 Cr, 8.66 Ni, 0.58 Si, 1.04 Mn, 0.021 P, 0.007S and Fe balance. The surfaces of stainless steel samples were first ground and polished, and then cleaned in deionized water, followed by ultrasonically degreasing in ethanol. Subsequently, the samples were mounted on a computer-driven X–Y stage. Finally, laser oxidation was conducted in air using Nd: YAG pulsed laser (1064 nm). The surface morphology was characterized by field emission scanning electron microscope (FESEM, Model JSM-6700F, Japan). The microstructure of the oxidation layer was observed by transmission electron microscope (TEM, Model JEM-2100F, Japan). For TEM observation, the thin slices were cut from the most outside of the treated layer of the samples and then mechanically polished, followed by dimpling and ion-milling to make the foil electron transparent.
3. Results and discussion 3.1. Morphology of Cr2O3 According to our previous investigations [11,13], it can be known that there mainly forms Cr2O3 in the edge of one laser spot on the stainless steel surface after laser oxidation treatments [14]. However, the latest findings show that the formed Cr2O3 on stainless steel surface can present different dominant morphologies, which can be divided into four types and designated as type I–IV. Fig. 1 shows the representative FESEM images of these four types of Cr2O3 morphologies. Type I mainly exhibits dendritic Cr2O3 morphology with six dendrite arms (shown in Fig. 1(a)). This may be an irregular hexagon induced by incomplete growth, because the interspaces between adjacent dendrites cannot be completely filled out by ambient grains. The six divergent dendrite arms have three angles close to 90° and the other three angles close to 30°, as marked in Fig. 1(a). Such angles match well with their divergent 1 0i and h0 1 1 0i directions within (0 0 0 1) plane, growths along h2 1 respectively. Type II and III are mainly the flower-like and gear-like Cr2O3 morphologies with six petals and gear teeth, respectively (shown in Fig. 1(b) and (c)). By comparison, it can be deduced that types I–III morphologies are basically the evolution of hexagons.
Different from type I–III, type IV shows regular hexagonal Cr2O3 morphology (shown in Fig. 1(d)). It can be found that the hexagonal Cr2O3 has general characteristics of regular hexagon, such as equal sides (length of about 0.5 lm) and equal angles (an inner angle of about 120°), reflecting the perfect growth state. These hexagons are comparatively evenly distributed on (0 0 0 1) base plane and exhibit clear flat-like structure, suggesting a preferential growth direction perpendicular to c-axis ([0 0 0 1] direction), e.g., 1 0i and/or h0 1 1 0i directions. That is to say, the thickalong h2 1 ness direction of the hexagon should be the [0 0 0 1] orientation. However, the thickness of the flat-like hexagons is very small, indicating that the crystal growth along [0 0 0 1] direction is severely suppressed by the rapid heating and cooling process of the laser oxidation treatment. Therefore, the hexagons can only grow sideways in the form of thin plates. Another noteworthy is that the Cr2O3 patterns are smaller in overall size, from about hundred nanometers to several micrometers. This can be explained by the classical solidification theory [15]. Under laser treatment, the cooling rate is higher, thereby inducing the larger undercooling. The larger the undercooling is, the higher the nucleation rate is. Once a large amount of nuclei are induced, Cr2O3 growth will be restrained by adjacent ones, thus forming Cr2O3 patterns with smaller sizes. The formation of the above different Cr2O3 morphologies mainly depends on the crystal growth direction, i.e., preferred growth, which is mainly determined by the crystal structure of Cr2O3. Most researchers agree that Cr2O3 has the same corundum-type structure as a-Al2O3 (space group R3c) with hexagonal close-packed (HCP) (0 0 0 1) layers of O atoms and Cr atoms occupying two thirds of the octahedral gaps [16–18]. Structurally, Cr2O3 has three types 1 0i (±½2 1 1 0; ±½1 21 0; ±½1 1 2 0), of rapid growth directions: h2 1 h0 1 1 0i (±½0 1 1 0; ±½1 0 1 0; ±½1 1 0 0) and ±[0 0 0 1]. In theory, 1 0i directions are the close-packed plane (0 0 0 1) plane and h2 1 and close-packed crystal directions for HCP crystal structure, respectively. Generally, there are the lowest energies along the close-packed planes and close-packed crystal directions [19–21].
Fig. 1. FESEM images of different Cr2O3 morphologies formed on stainless steel surface after laser oxidation: (a) dendritic Cr2O3 (type I); (b) flower-like Cr2O3 (type II); (c) gear-like Cr2O3 (type III); (d) regular hexagonal Cr2O3 (type IV).
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Fig. 2. Schematic illustration for the formation of different Cr2O3 morphologies: (a) atoms arrangement and growth directions within (0 0 0 1) plane of the HCP structure; (b– g) illustrations for the formations of dendritic, gear-like, flower-like, regular hexagonal, triangular and truncated triangular morphologies.
Moreover, the crystal is always trying to be in the lowest free energy state. Therefore, if Cr2O3 follows the normal growth behavior, it should grow along the close-packed plane and directions, eventually growing into a regular hexagonal morphology (shown in Fig. 1(d)). However, the crystal growth behavior can be changed by altering the external growth conditions. Thus, the final morphology of Cr2O3 is not entirely determined by the minimization of energy, but rather the actual growth directions. The flat-like regular hexagons in Fig. 1(d) indicate the restriction of the crystal growth along the ±[0 0 0 1] direction. Therefore, the discussion on
1 0i the growth state of Cr2O3 is primarily concentrated on h2 1 and h0 1 1 0i directions. To facilitate the discussions, the main atoms arrangement and growth directions within (0 0 0 1) plane of the HCP structure are shown in Fig. 2(a). If the crystal grows 1 0i directions (½2 1 1 0; ½1 21 0 and ½1 1 2 0) and along three h2 1 three h0 1 1 0i directions (½0 1 1 0; ½1 0 1 0 and ½1 1 0 0), the dendritic Cr2O3 will be formed, as shown in Fig. 2(b). This schematic morphology is basically the same as the as-obtained FESEM morphology in Fig. 1(a). When the crystal grows along other crystal directions, the formed morphology will be also changed. For exam-
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Fig. 3. TEM image of the hexagonal Cr2O3.
1 0i (±½2 1 1 0; ±½1 21 0; ±½1 1 2 0) ple, when it grows along six h2 1 and six h0 1 1 0i (±½0 1 1 0; ±½1 0 1 0; ±½1 1 0 0) directions (shown in
Fig. 2(c)), the gear-like Cr2O3 can be obtained similar to FESEM 0i directions morphology in Fig. 1(c). Among them, the six h0 1 1 contribute to the formation of the small regular hexagons in the center. The six small triangles around the six vertexes of the small 1 0i directions. The small hexagon are formed by growing along h2 1 hexagon in the center and the peripheral six small triangles together form the gear-like morphology. For the flower-like Cr2O3, its 0i (±½0 1 1 0; ±½1 0 1 0; ±½1 1 0 0) directions growth is along six h0 1 1 (shown in Fig. 2(d)). However, when the growth directions are fur1 0i (±½2 1 1 0; ±½1 21 0; ±½1 1 2 0) ther changed to six preferred h2 1 directions, the well-defined regular hexagons can be formed (shown in Fig. 2(e)). Overall, the formed Cr2O3 via laser oxidation can exhibit various novel morphologies by changing the preferred growth directions. And the morphologies obtained from the crystallographic analyses basically accord with those in Fig. 1. To further investigate the characteristics of the aforementioned regular hexagonal Cr2O3 (shown in Fig. 1(d)), TEM image is shown in Fig. 3. It can be found that a large amount of nanoscale hexagonal structures can be observed. The side length of the hexagons may vary within a certain range. Although each side length is different, most polygons possess well-defined hexagonal structure with sharp edges especially. And the thickness of the hexagon is
Fig. 4. FESEM images of Cr2O3 obtained at different growth stages: (a) slight agglomerated Cr2O3 nano-particles; (b) Cr2O3 particles with growth steps; (c) appearance of prototype hexagons with the hollow truncated triangular composed of nano-rods and nano-particles; (d) continuous growth towards regular hexagon; (e) the final compact and regular hexagons on the stainless steel surface.
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Fig. 5. Schematic growth model of the regular hexagons.
very small, indicating that the growth along the longitudinal direction, e.g., h0 0 0 1i crystal direction, is not obvious. This feature also implies that the (0 0 0 1) plane has the lowest surface energy. 3.2. Growth mechanism of hexagonal Cr2O3 The novel morphologies of Cr2O3 have been discussed based on the crystallographic analyses. However, the exact growth mechanisms governing the formation of these different morphologies are not clear. To this end, we take the regular hexagonal Cr2O3 as an example to analyze its growth process and mechanism in detail. In order to better understand the growth process and mechanism of hexagonal Cr2O3, we have researched the morphology changes of the regular hexagons with respect to different growth stages, as shown in Fig. 4. In the initial stage, Cr2O3 nucleates and forms nano-particles with grains size of 200–300 nm. And then the nano-particles agglomerate together (shown in Fig. 4(a)). After a period of time, the agglomerations are increasingly apparent (shown in Fig. 4(b)), and the partial nano-particles grow and connect into some growing steps (shown in the lower-left inset of Fig. 4(b)), finally presenting rod-like morphologies. Over time, Cr2O3 morphologies are significantly changed (shown in Fig. 4(c)). The Cr2O3 nano-particles almost disappear, and the rod-like structure evolves to the complex micro/nano structure. Many hollow submicron flat-like polygons are formed by the assembly of submicron-rods and nano-particles. Most submicron polygons are not quite hexagons in shape. They are triangles or truncated triangles constituted by three or six submicron-rods, as indicated in Fig. 4(c). The schematic formation of triangular and truncated triangular morphologies along different directions is illustrated in Fig. 2(f) and (g). Different from the dendritic, flower-like and gear-like morphologies, the formation of special trian1 0i gular morphology can be realized by growing along three h2 1 directions (½2 1 1 0; ½1 2 1 0 and ½1 1 2 0) or three h0 1 1 0 i directions 1 0; ½1 0 1 0, ½1 1 0 0). In addition, if there are other directions (½0 1 1 0; ½1 21 0 and to participate in the growth process, such as (½2 1 1 2 0) or (½0 1 1 0; ½1 0 1 0 and ½1 1 0 0), the truncated triangular ½1 morphology can be formed (shown in Fig. 2(g)) due to the different growth rates along these directions. Such triangles or truncated triangles with hollows can truly be regarded as the appearance of prototype hexagons. They gradually grow to the hexagon at the cost of submicron-rods and nano-particles. Compared Fig. 4(d) with (c), the hexagons grow larger and the hollows in their center
(indicated by arrows) become smaller. They are no longer a simple aggregation of the submicron-rods and nano-particles, but grow into a whole. This phenomenon indicates that the growth of hexagon is clearly visible from submicron-rods and nano-particles, which may be mainly controlled by the dissolution and then recrystallization process. Therefore, the growth mechanism of hexagonal Cr2O3 may be a typical nucleation–assembly–dissolution– recrystallization–growth process. Finally, the regular hexagonal Cr2O3 morphologies are formed through the growth process, as shown in Fig. 4(e). The hexagons of (0 0 0 1) planes can be clearly seen, and the surfaces of the formed hexagons are relatively smooth. On the basis of the above growth process of hexagonal Cr2O3 morphology, the growth mechanism of the regular hexagons can be reasonably proposed step by step and a corresponding schematic growth model is displayed in Fig. 5. It can be seen from Fig. 5(a) and (b) that the fine Cr2O3 particles are aggregated firstly along the 1 0i and ½1 1 0 0 to realize the close-packed crystal directions h2 1 initial growth of Cr2O3. Then the surrounding particles gradually approach the aggregates along the nearest directions to form triangular or truncated triangular morphologies, as shown in Fig. 5(c) and (d). Under the dissolution and recrystallization process, these morphologies grow up quickly and show a prototype of hexagonal Cr2O3 (shown in Fig. 5(e) and (f)). Finally, the prototype grows into the regular hexagon, as shown in Fig. 5(g).
4. Conclusions Novel Cr2O3 morphologies, including dendritic, gear-like, flower-like and regular hexagonal morphologies, are formed on AISI 304 stainless steel surface via Nd: YAG pulsed laser oxidation in air. The formation of these different Cr2O3 morphologies essentially results from the growth in (0 0 0 1) plane along different directions. 1 0i and h0 1 1 0i directions. The denCr2O3 mainly grows along h2 1 1 0i direcdritic Cr2O3 can be formed by growing along three h2 1 0i directions. Similarly, Cr2O3 can also grow tions and three h0 1 1 0i directions. into the flower-like morphology along six h0 1 1 1 0i and six h0 1 1 0i Moreover, the preferred growth along six h2 1 directions can result in the formation of gear-like Cr2O3. Furthermore, the well-defined regular hexagonal Cr2O3 can be formed 1 0i directions. Based on the morwhen it grows along six h2 1 phology evolution of hexagonal Cr2O3, the growth mechanism of
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Cr2O3 is analyzed in detail and a reasonable growth model is proposed to explain the formation of the representative regular hexagons. This work provides a new insight into the oxidation of stainless steel surface to fabricate novel Cr2O3 morphologies under laser irradiation. Acknowledgements The authors are grateful for the supports from Specialized Research Fund for the Doctoral Program of Higher Education (No. 20123227120023), Open Fund from the Key Laboratory of Automobile Materials (Jilin University), Ministry of Education (No. 11450060445348), Open Fund of Jiangsu Province Key Laboratory of Tribology (No. Kjsmcx2012002), Open Foundation of State Key Laboratory of Silicon Materials, Zhejiang University, China (No. SKL2013-9). References [1] Y. Zhang, J. Li, J. Huang, C. Ding, Mechanical and tribological properties of plasma-sprayed Cr3C2–NiCr, WC–Co, and Cr2O3 coatings, J. Therm. Spray Technol. 7 (1998) 242–246. [2] P. Gibot, L. Vidal, Original synthesis of chromium (III) oxide nanoparticles, J. Eur. Ceram. Soc. 30 (2010) 911–915. [3] D.C. Dube, D. Agrawal, S. Agrawal, R. Roy, High temperature dielectric study of Cr2O3 in microwave region, Appl. Phys. Lett. 90 (2007). 124105-1-3. [4] E. Celik, C. Tekmen, I. Ozdemir, H. Cetinel, Y. Karakas, S.C. Okumus, Effects on performance of Cr2O3 layers produced on Mo/cast-iron materials, Surf. Coat. Technol. 174–175 (2003) 1074–1081. [5] S.A. Makhlouf, Magnetic properties of Cr2O3 nanoparticles, J. Magn. Magn. Mater. 272–276 (2004) 1530–1532. [6] S. Khamlich, E. Manikandan, B.D. Ngom, J. Sithole, O. Nemraoui, I. Zorkani, R. McCrindle, N. Cingo, M. Maaza, Synthesis, characterization, and growth mechanism of a-Cr2O3 monodispersed particles, J. Phys. Chem. Solids 72 (2011) 714–718. [7] G.M. Bai, H.X. Dai, Y.X. Liu, K.M. Ji, X.W. Li, S.H. Xie, Preparation and catalytic performance of cylinder- and cake-like Cr2O3 for toluene combustion, Catal. Commun. 36 (2013) 43–47.
[8] S.T. Liang, H.L. Zhang, M.T. Luo, K.J. Luo, P. Li, H.B. Xu, Y. Zhang, Colour performance investigation of a Cr2O3 green pigment prepared via the thermal decomposition of CrOOH, Ceram. Int. 40 (2014) 4367–4373. [9] Q.Z. Dong, J.D. Hu, Z.X. Guo, J.S. Lian, J.W. Chen, B. Chen, Surface morphology study on chromium oxide growth on Cr films by Nd-YAG laser oxidation process, Appl. Surf. Sci. 202 (2002) 114–119. [10] C.H. Lin, S.Y. Chen, N.J. Ho, D. Gan, P. Shen, Shape-dependent local internal stress of a-Cr2O3 nanocrystal fabricated by pulsed laser ablation, J. Phys. Chem. Solids 70 (2009) 1505–1510. [11] C.Y. Cui, X.G. Cui, Y.K. Zhang, K.Y. Luo, Q. Zhao, J.D. Hu, Z. Liu, Y.M. Wang, Microstructure and microhardness analysis of the hexagonal oxides formed on the surface of the AISI 304 stainless steel after Nd:YAG pulsed laser surface melting, Appl. Surf. Sci. 256 (2010) 6782–6786. [12] C.Y. Cui, J.D. Hu, Y.H. Liu, Z.X. Guo, Microstructure and mechanical properties of stainless steel under Nd:YAG pulsed laser irradiation, Mater. Sci. Technol. 24 (2008) 964–968. [13] C.Y. Cui, X.G. Cui, Y.K. Zhang, Q. Zhao, J.Z. Lu, J.D. Hu, Y.M. Wang, Microstructure and corrosion behavior of the AISI 304 stainless steel after Nd:YAG pulsed laser surface melting, Surf. Coat. Technol. 206 (2011) 1146– 1154. [14] PDF Cards #38-1479, #30-0820PCPDFWIN, Version 2.02, JCPDS-ICDD, 1999. [15] H.C. Liao, J.J. Bi, M. Zhang, K. Ding, Y.F. Jiang, M.D. Cai, Effect of strontium and solidification rate on eutectic grain structure in an Al–13 wt% Si alloy, China Foundry 6 (2009) 226–231. [16] C. Rehbein, N.M. Harrison, A. Wander, Structure of the a-Cr2O3 (0 0 0 1) surface. An ab initio total-energy study, Phys. Rev. B 54 (1996) 14066–14070. [17] J.Z. Sun, T. Stirner, Ab initio supercell calculations of the (0 0 0 1) a-Cr2O3 surface with a partially or totally Al-substituted external layer, Thin Solid Films 517 (2009) 5512–5515. [18] J.S. Park, S.K. Hong, I.H. Im, J.S. Ha, H.J. Lee, S.H. Park, J.H. Chang, M.W. Cho, T. Yao, Growth of high-quality ZnO films on Al2O3 (0 0 0 1) by plasma-assisted molecular beam epitaxy, J. Cryst. Growth 311 (2009) 2163–2166. [19] J.Z. Sun, T. Stirner, A. Matthews, Structure and surface energy of low-index surfaces of stoichiometric a-Al2O3 and a-Cr2O3, Surf. Coat. Technol. 201 (2006) 4205–4208. [20] J.J. Zhu, C.X. Kan, H.C. Li, Y.L. Cao, X.L. Ding, J.G. Wan, Synthesis and growth mechanism of gold nanoplates with novel shapes, J. Cryst. Growth 321 (2011) 124–130. [21] J.Z. Sun, T. Stirner, A. Matthews, Structure and electronic properties calculation of ultrathin a-Al2O3 films on (0 0 0 1) a-Cr2O3 templates, Surf. Sci. 601 (2007) 5050–5056.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Novel morphologies and growth mechanism of Cr2O3 oxide formed on stainless steel surface via Nd: YAG pulsed laser oxidation C.Y. Cui a,⇑, C.D. Xia a, X.G. Cui a,b,⇑, J.Z. Zhou a, X.D. Ren a, Y.M. Wang c a
School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, PR China State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China c Laser Institute of Jilin, Qianjin Street No. 1244, Changchun 130012, PR China b
a r t i c l e
i n f o
Article history: Received 19 December 2014 Received in revised form 30 January 2015 Accepted 6 February 2015 Available online 13 February 2015 Keywords: Chromium sesquioxide (Cr2O3) Novel morphologies Stainless steel Nd: YAG pulsed laser oxidation Growth mechanism
a b s t r a c t Novel morphologies of chromium sesquioxide (Cr2O3) were successfully formed on AISI 304 stainless steel surface by Nd: YAG pulsed laser oxidation in air. The obtained Cr2O3 presented mainly dendritic, gear-like, flower-like and regular hexagonal morphologies with sizes from hundred nanometers to several micrometers. These various morphologies chiefly originated from two groups of diverse growth 1 0i and h0 1 1 0i directions. The preferential growth along directions within (0 0 0 1) basal plane, i.e., h2 1 different directions determined the final Cr2O3 morphologies. And the crystallographic analyses basically accorded with the experimental results. Moreover, the growth mechanism of the hexagonal Cr2O3 was analyzed in detail from its morphology evolution, and a corresponding growth model was reasonably proposed. Ó 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Chromium sesquioxide (Cr2O3) is one of the most attractive metal oxides due to its useful and important properties required in industrial applications, such as high hardness (about 29.5 GPa), high melting point (about 2400 °C), high wear resistance, high temperature oxidation resistance and antiferromagnetic properties [1–5]. Therefore, various methods have been developed to prepare Cr2O3. At present, the primary preparation method is chemical synthesis. Khamlich et al. have successfully synthesized monodispersed spherical a-Cr2O3 particles from a diluted solution of KCr(SO4)212H2O using Aqueous Chemical Growth (ACG) technique [6]. Bai et al. have fabricated cylinderand cake-like Cr2O3 with a rhombohedral structure using hydrothermal and microemulsion methods, respectively [7]. And Liang et al. have prepared Cr2O3 green pigments by calcining two types of CrOOH (a-CrOOH and c-CrOOH) [8]. However, these chemical synthesis methods have a certain environmental pollution and component uncertainty. Therefore, it is necessary to exploit and adopt new environment-friendly preparation methods to produce Cr2O3. Recently, laser surface treatment (LST) has been used
to fabricate novel Cr2O3 structures. Dong et al. have successfully prepared Cr2O3 on Cr film by Nd-YAG laser oxidation process and analyzed its morphologies [9]. Lin et al. have fabricated a-Cr2O3 single-crystal nanocondensates on Cr plate (99.9% pure) by pulsed laser ablation in air and discussed the shape-dependent local internal stress of the anisotropic crystal [10]. The obtained Cr2O3 possesses different morphologies such as nano-particle, leaf-like, pyramid-like, nutshell-like [9], truncated rhombohedral and hexagonal shapes [10]. The above studies are emphasized on Cr film or Cr plate with high Cr content, while the oxidation states and morphologies of other materials with a relatively low Cr content, such as the stainless steel (Cr content can be up to about 30 wt.%), are rarely referred. In this respect, we have carried out some researches on stainless steel surface oxidation induced by Nd: YAG pulsed laser and reported the regular hexagonal structures, oxidation mechanism and surface properties [11–13]. However, other morphologies, such as dendritic, flower-like and gear-like, are rarely reported. And their growth mechanisms are also little discussed. In this paper, the novel Cr2O3 morphologies formed on AISI 304 stainless steel surface after Nd: YAG pulsed laser oxidation are investigated in detail. And a possible growth mechanism is proposed to explain their formation process.
⇑ Corresponding authors at: School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, PR China (X.G. Cui). Tel.: +86 511 88797898; fax: +86 511 88780241. E-mail addresses: [email protected] (C.Y. Cui), [email protected] (X.G. Cui). http://dx.doi.org/10.1016/j.jallcom.2015.02.053 0925-8388/Ó 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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2. Experimental procedures The investigated material was AISI 304 austenite stainless steel with chemical composition (wt.%) of 0.049 C, 18.20 Cr, 8.66 Ni, 0.58 Si, 1.04 Mn, 0.021 P, 0.007S and Fe balance. The surfaces of stainless steel samples were first ground and polished, and then cleaned in deionized water, followed by ultrasonically degreasing in ethanol. Subsequently, the samples were mounted on a computer-driven X–Y stage. Finally, laser oxidation was conducted in air using Nd: YAG pulsed laser (1064 nm). The surface morphology was characterized by field emission scanning electron microscope (FESEM, Model JSM-6700F, Japan). The microstructure of the oxidation layer was observed by transmission electron microscope (TEM, Model JEM-2100F, Japan). For TEM observation, the thin slices were cut from the most outside of the treated layer of the samples and then mechanically polished, followed by dimpling and ion-milling to make the foil electron transparent.
3. Results and discussion 3.1. Morphology of Cr2O3 According to our previous investigations [11,13], it can be known that there mainly forms Cr2O3 in the edge of one laser spot on the stainless steel surface after laser oxidation treatments [14]. However, the latest findings show that the formed Cr2O3 on stainless steel surface can present different dominant morphologies, which can be divided into four types and designated as type I–IV. Fig. 1 shows the representative FESEM images of these four types of Cr2O3 morphologies. Type I mainly exhibits dendritic Cr2O3 morphology with six dendrite arms (shown in Fig. 1(a)). This may be an irregular hexagon induced by incomplete growth, because the interspaces between adjacent dendrites cannot be completely filled out by ambient grains. The six divergent dendrite arms have three angles close to 90° and the other three angles close to 30°, as marked in Fig. 1(a). Such angles match well with their divergent 1 0i and h0 1 1 0i directions within (0 0 0 1) plane, growths along h2 1 respectively. Type II and III are mainly the flower-like and gear-like Cr2O3 morphologies with six petals and gear teeth, respectively (shown in Fig. 1(b) and (c)). By comparison, it can be deduced that types I–III morphologies are basically the evolution of hexagons.
Different from type I–III, type IV shows regular hexagonal Cr2O3 morphology (shown in Fig. 1(d)). It can be found that the hexagonal Cr2O3 has general characteristics of regular hexagon, such as equal sides (length of about 0.5 lm) and equal angles (an inner angle of about 120°), reflecting the perfect growth state. These hexagons are comparatively evenly distributed on (0 0 0 1) base plane and exhibit clear flat-like structure, suggesting a preferential growth direction perpendicular to c-axis ([0 0 0 1] direction), e.g., 1 0i and/or h0 1 1 0i directions. That is to say, the thickalong h2 1 ness direction of the hexagon should be the [0 0 0 1] orientation. However, the thickness of the flat-like hexagons is very small, indicating that the crystal growth along [0 0 0 1] direction is severely suppressed by the rapid heating and cooling process of the laser oxidation treatment. Therefore, the hexagons can only grow sideways in the form of thin plates. Another noteworthy is that the Cr2O3 patterns are smaller in overall size, from about hundred nanometers to several micrometers. This can be explained by the classical solidification theory [15]. Under laser treatment, the cooling rate is higher, thereby inducing the larger undercooling. The larger the undercooling is, the higher the nucleation rate is. Once a large amount of nuclei are induced, Cr2O3 growth will be restrained by adjacent ones, thus forming Cr2O3 patterns with smaller sizes. The formation of the above different Cr2O3 morphologies mainly depends on the crystal growth direction, i.e., preferred growth, which is mainly determined by the crystal structure of Cr2O3. Most researchers agree that Cr2O3 has the same corundum-type structure as a-Al2O3 (space group R3c) with hexagonal close-packed (HCP) (0 0 0 1) layers of O atoms and Cr atoms occupying two thirds of the octahedral gaps [16–18]. Structurally, Cr2O3 has three types 1 0i (±½2 1 1 0; ±½1 21 0; ±½1 1 2 0), of rapid growth directions: h2 1 h0 1 1 0i (±½0 1 1 0; ±½1 0 1 0; ±½1 1 0 0) and ±[0 0 0 1]. In theory, 1 0i directions are the close-packed plane (0 0 0 1) plane and h2 1 and close-packed crystal directions for HCP crystal structure, respectively. Generally, there are the lowest energies along the close-packed planes and close-packed crystal directions [19–21].
Fig. 1. FESEM images of different Cr2O3 morphologies formed on stainless steel surface after laser oxidation: (a) dendritic Cr2O3 (type I); (b) flower-like Cr2O3 (type II); (c) gear-like Cr2O3 (type III); (d) regular hexagonal Cr2O3 (type IV).
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Fig. 2. Schematic illustration for the formation of different Cr2O3 morphologies: (a) atoms arrangement and growth directions within (0 0 0 1) plane of the HCP structure; (b– g) illustrations for the formations of dendritic, gear-like, flower-like, regular hexagonal, triangular and truncated triangular morphologies.
Moreover, the crystal is always trying to be in the lowest free energy state. Therefore, if Cr2O3 follows the normal growth behavior, it should grow along the close-packed plane and directions, eventually growing into a regular hexagonal morphology (shown in Fig. 1(d)). However, the crystal growth behavior can be changed by altering the external growth conditions. Thus, the final morphology of Cr2O3 is not entirely determined by the minimization of energy, but rather the actual growth directions. The flat-like regular hexagons in Fig. 1(d) indicate the restriction of the crystal growth along the ±[0 0 0 1] direction. Therefore, the discussion on
1 0i the growth state of Cr2O3 is primarily concentrated on h2 1 and h0 1 1 0i directions. To facilitate the discussions, the main atoms arrangement and growth directions within (0 0 0 1) plane of the HCP structure are shown in Fig. 2(a). If the crystal grows 1 0i directions (½2 1 1 0; ½1 21 0 and ½1 1 2 0) and along three h2 1 three h0 1 1 0i directions (½0 1 1 0; ½1 0 1 0 and ½1 1 0 0), the dendritic Cr2O3 will be formed, as shown in Fig. 2(b). This schematic morphology is basically the same as the as-obtained FESEM morphology in Fig. 1(a). When the crystal grows along other crystal directions, the formed morphology will be also changed. For exam-
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Fig. 3. TEM image of the hexagonal Cr2O3.
1 0i (±½2 1 1 0; ±½1 21 0; ±½1 1 2 0) ple, when it grows along six h2 1 and six h0 1 1 0i (±½0 1 1 0; ±½1 0 1 0; ±½1 1 0 0) directions (shown in
Fig. 2(c)), the gear-like Cr2O3 can be obtained similar to FESEM 0i directions morphology in Fig. 1(c). Among them, the six h0 1 1 contribute to the formation of the small regular hexagons in the center. The six small triangles around the six vertexes of the small 1 0i directions. The small hexagon are formed by growing along h2 1 hexagon in the center and the peripheral six small triangles together form the gear-like morphology. For the flower-like Cr2O3, its 0i (±½0 1 1 0; ±½1 0 1 0; ±½1 1 0 0) directions growth is along six h0 1 1 (shown in Fig. 2(d)). However, when the growth directions are fur1 0i (±½2 1 1 0; ±½1 21 0; ±½1 1 2 0) ther changed to six preferred h2 1 directions, the well-defined regular hexagons can be formed (shown in Fig. 2(e)). Overall, the formed Cr2O3 via laser oxidation can exhibit various novel morphologies by changing the preferred growth directions. And the morphologies obtained from the crystallographic analyses basically accord with those in Fig. 1. To further investigate the characteristics of the aforementioned regular hexagonal Cr2O3 (shown in Fig. 1(d)), TEM image is shown in Fig. 3. It can be found that a large amount of nanoscale hexagonal structures can be observed. The side length of the hexagons may vary within a certain range. Although each side length is different, most polygons possess well-defined hexagonal structure with sharp edges especially. And the thickness of the hexagon is
Fig. 4. FESEM images of Cr2O3 obtained at different growth stages: (a) slight agglomerated Cr2O3 nano-particles; (b) Cr2O3 particles with growth steps; (c) appearance of prototype hexagons with the hollow truncated triangular composed of nano-rods and nano-particles; (d) continuous growth towards regular hexagon; (e) the final compact and regular hexagons on the stainless steel surface.
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Fig. 5. Schematic growth model of the regular hexagons.
very small, indicating that the growth along the longitudinal direction, e.g., h0 0 0 1i crystal direction, is not obvious. This feature also implies that the (0 0 0 1) plane has the lowest surface energy. 3.2. Growth mechanism of hexagonal Cr2O3 The novel morphologies of Cr2O3 have been discussed based on the crystallographic analyses. However, the exact growth mechanisms governing the formation of these different morphologies are not clear. To this end, we take the regular hexagonal Cr2O3 as an example to analyze its growth process and mechanism in detail. In order to better understand the growth process and mechanism of hexagonal Cr2O3, we have researched the morphology changes of the regular hexagons with respect to different growth stages, as shown in Fig. 4. In the initial stage, Cr2O3 nucleates and forms nano-particles with grains size of 200–300 nm. And then the nano-particles agglomerate together (shown in Fig. 4(a)). After a period of time, the agglomerations are increasingly apparent (shown in Fig. 4(b)), and the partial nano-particles grow and connect into some growing steps (shown in the lower-left inset of Fig. 4(b)), finally presenting rod-like morphologies. Over time, Cr2O3 morphologies are significantly changed (shown in Fig. 4(c)). The Cr2O3 nano-particles almost disappear, and the rod-like structure evolves to the complex micro/nano structure. Many hollow submicron flat-like polygons are formed by the assembly of submicron-rods and nano-particles. Most submicron polygons are not quite hexagons in shape. They are triangles or truncated triangles constituted by three or six submicron-rods, as indicated in Fig. 4(c). The schematic formation of triangular and truncated triangular morphologies along different directions is illustrated in Fig. 2(f) and (g). Different from the dendritic, flower-like and gear-like morphologies, the formation of special trian1 0i gular morphology can be realized by growing along three h2 1 directions (½2 1 1 0; ½1 2 1 0 and ½1 1 2 0) or three h0 1 1 0 i directions 1 0; ½1 0 1 0, ½1 1 0 0). In addition, if there are other directions (½0 1 1 0; ½1 21 0 and to participate in the growth process, such as (½2 1 1 2 0) or (½0 1 1 0; ½1 0 1 0 and ½1 1 0 0), the truncated triangular ½1 morphology can be formed (shown in Fig. 2(g)) due to the different growth rates along these directions. Such triangles or truncated triangles with hollows can truly be regarded as the appearance of prototype hexagons. They gradually grow to the hexagon at the cost of submicron-rods and nano-particles. Compared Fig. 4(d) with (c), the hexagons grow larger and the hollows in their center
(indicated by arrows) become smaller. They are no longer a simple aggregation of the submicron-rods and nano-particles, but grow into a whole. This phenomenon indicates that the growth of hexagon is clearly visible from submicron-rods and nano-particles, which may be mainly controlled by the dissolution and then recrystallization process. Therefore, the growth mechanism of hexagonal Cr2O3 may be a typical nucleation–assembly–dissolution– recrystallization–growth process. Finally, the regular hexagonal Cr2O3 morphologies are formed through the growth process, as shown in Fig. 4(e). The hexagons of (0 0 0 1) planes can be clearly seen, and the surfaces of the formed hexagons are relatively smooth. On the basis of the above growth process of hexagonal Cr2O3 morphology, the growth mechanism of the regular hexagons can be reasonably proposed step by step and a corresponding schematic growth model is displayed in Fig. 5. It can be seen from Fig. 5(a) and (b) that the fine Cr2O3 particles are aggregated firstly along the 1 0i and ½1 1 0 0 to realize the close-packed crystal directions h2 1 initial growth of Cr2O3. Then the surrounding particles gradually approach the aggregates along the nearest directions to form triangular or truncated triangular morphologies, as shown in Fig. 5(c) and (d). Under the dissolution and recrystallization process, these morphologies grow up quickly and show a prototype of hexagonal Cr2O3 (shown in Fig. 5(e) and (f)). Finally, the prototype grows into the regular hexagon, as shown in Fig. 5(g).
4. Conclusions Novel Cr2O3 morphologies, including dendritic, gear-like, flower-like and regular hexagonal morphologies, are formed on AISI 304 stainless steel surface via Nd: YAG pulsed laser oxidation in air. The formation of these different Cr2O3 morphologies essentially results from the growth in (0 0 0 1) plane along different directions. 1 0i and h0 1 1 0i directions. The denCr2O3 mainly grows along h2 1 1 0i direcdritic Cr2O3 can be formed by growing along three h2 1 0i directions. Similarly, Cr2O3 can also grow tions and three h0 1 1 0i directions. into the flower-like morphology along six h0 1 1 1 0i and six h0 1 1 0i Moreover, the preferred growth along six h2 1 directions can result in the formation of gear-like Cr2O3. Furthermore, the well-defined regular hexagonal Cr2O3 can be formed 1 0i directions. Based on the morwhen it grows along six h2 1 phology evolution of hexagonal Cr2O3, the growth mechanism of
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Cr2O3 is analyzed in detail and a reasonable growth model is proposed to explain the formation of the representative regular hexagons. This work provides a new insight into the oxidation of stainless steel surface to fabricate novel Cr2O3 morphologies under laser irradiation. Acknowledgements The authors are grateful for the supports from Specialized Research Fund for the Doctoral Program of Higher Education (No. 20123227120023), Open Fund from the Key Laboratory of Automobile Materials (Jilin University), Ministry of Education (No. 11450060445348), Open Fund of Jiangsu Province Key Laboratory of Tribology (No. Kjsmcx2012002), Open Foundation of State Key Laboratory of Silicon Materials, Zhejiang University, China (No. SKL2013-9). References [1] Y. Zhang, J. Li, J. Huang, C. Ding, Mechanical and tribological properties of plasma-sprayed Cr3C2–NiCr, WC–Co, and Cr2O3 coatings, J. Therm. Spray Technol. 7 (1998) 242–246. [2] P. Gibot, L. Vidal, Original synthesis of chromium (III) oxide nanoparticles, J. Eur. Ceram. Soc. 30 (2010) 911–915. [3] D.C. Dube, D. Agrawal, S. Agrawal, R. Roy, High temperature dielectric study of Cr2O3 in microwave region, Appl. Phys. Lett. 90 (2007). 124105-1-3. [4] E. Celik, C. Tekmen, I. Ozdemir, H. Cetinel, Y. Karakas, S.C. Okumus, Effects on performance of Cr2O3 layers produced on Mo/cast-iron materials, Surf. Coat. Technol. 174–175 (2003) 1074–1081. [5] S.A. Makhlouf, Magnetic properties of Cr2O3 nanoparticles, J. Magn. Magn. Mater. 272–276 (2004) 1530–1532. [6] S. Khamlich, E. Manikandan, B.D. Ngom, J. Sithole, O. Nemraoui, I. Zorkani, R. McCrindle, N. Cingo, M. Maaza, Synthesis, characterization, and growth mechanism of a-Cr2O3 monodispersed particles, J. Phys. Chem. Solids 72 (2011) 714–718. [7] G.M. Bai, H.X. Dai, Y.X. Liu, K.M. Ji, X.W. Li, S.H. Xie, Preparation and catalytic performance of cylinder- and cake-like Cr2O3 for toluene combustion, Catal. Commun. 36 (2013) 43–47.
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