Appl. Phys. A (2014) 115:1469–1477 DOI 10.1007/s00339-013-8064-x

Chromium oxide dissolution in steels via short pulse laser processing Evgeny Kharanzhevskiy · Sergey Reshetnikov

Received: 5 July 2013 / Accepted: 11 October 2013 / Published online: 5 November 2013 © Springer-Verlag Berlin Heidelberg 2013

Abstract Changes of microstructure, chemical and phase compositions in thin surface layers of low carbon steel saturated by chromium oxide have been studied by TEM, XPS and XRD methods. Ultrafine chromium oxide powder was spread on a steel surface and subjected to laser processing with nanosecond pulses. It was found that such conditions of processing as overheating of small volume of metal, high temperature gradient, rapid solidification and laser-induced plasma formation lead to dissolution of chromium oxide in the metal matrix. As a result of laser processing the surface layers contain chromium oxide, chrome-spinel FeO·Cr2 O3 and chromium in metal state dispersed in alpha and gamma iron. The processing technique allows to obtain surface layers whose chemical composition might be equivalent to the composition of stainless steels.

1 Introduction Surface saturation of low carbon alloy-free steels by different elements is of some interest because it leads to changing of material functional properties. It is well known that properties of metals and alloys including absorption, electrochemical and corrosive properties are mainly determined by structure and chemical composition of surface [1]. For example, high corrosion resistance of iron–chromium alloys is explained by formation of passive films containing chromium oxide (+3), combined iron oxides in oxidation state +2 and +3 and dense spinel structures with compositions FeO·Cr2 O3 and Fe2 O3 ·Cr2 O3 [2,3]. Therefore, surface saturation of steels by E. Kharanzhevskiy (B) · S. Reshetnikov Udmurt State University, Universitetskaya st., 1, 426034 Izhevsk, Russia e-mail: [email protected]

chromium oxide might have significant interest in forming protective properties of alloy-free steels. In addition to a good corrosion behavior many materials require good wear resistance, and it was suggested that a chromium oxide coatings lead to these important properties for steel or Ni-base alloys [4]. Laser oxidation of chromium thin films deposited by a PVD process has been studied during the last two decades [5] but there is no extensive literature on the subject. Laser oxidation induces fast oxidation kinetics, especially at the beginning of oxidation, without modifying the oxide nature (Cr2 O3 ) and morphology [5]. Tuominen et al. [6] studied laser remelting of high chromium coatings obtained by a thermal spray process. Laser remelting resulted in the formation of a very thin and dense oxide layer on top of the coating, giving good corrosion resistance in an aqueous chloride solution and excellent resistance to oxidation at 800 ◦ C in the air. Thin chromium oxide films deposited on steels by PVD methods have important industrial applications, for instance in catalysis [7] and solar thermal energy collectors [8]. Pulsed laser deposition as one of the PVD processes is a technique that uses laser energy to remove material from a chromium oxide target and film growth occurs on a substrate as a result of vapor deposition. Structural and chemical composition studies [9] showed that the films consist of a mixture of amorphous chromium oxides exhibiting different stoichiometry depending on the processing parameters, where nanocrystals of mainly Cr2 O3 are dispersed. A novel technique that can provide the formation of chromium oxide solutions and thin chromium oxide surface films might be based on direct short pulse laser processing of ultrafine chromium oxide powder deposited on the surface of a sample. In works devoted to high speed laser sintering of high dispersive powders [10,11] it was found that high heating and cooling rates lead to rapid solidification in strong

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nonequilibrium conditions accompanied by metastable phase formation. In this context the paper aims to study the structure and chemical composition of low carbon steel saturated by chromium oxide using high speed laser processing with nanosecond pulses.

2 Experimentation 2.1 Experimental set-up The experimental facility consists of the LDesigner F1 ytterbium fiber laser with maximum power 50 W and a process chamber where atmosphere can be controlled. The chamber is initially evacuated by a backing pump and then filled with high purity argon to pressure 1.05 bar. The power of the laser was set to 20 W operating in short pulse mode with wavelength 1.065 µm. 2.2 Energy characteristic and scanning strategy of laser processing The laser chosen for laser processing has pulse time about 10−8 s which is three to four times of magnitude longer than relaxation time for energy transfer from gas of valence electrons which absorbs laser energy to lattice of metal parts. Thus action of laser energy on chromium oxide powder can be described mainly by a thermal mechanism but the pulse time is short enough to ensure fast heating (about 107 K/s) and high temperature gradient (about 108 K/m) in the treated zone [10]. The pulse frequency is 80 kHz, scanning speed of the laser beam—900 mm/s, pulse energy—0.2 mJ and instantaneous power—2 kW. The laser beam was focused into a spot with diameter 30 µm, so instantaneous power density of the laser radiation was 0.3 × 1013 W/m2 . By computer simulation of heat transport in porous media [11] it was established that such energy characteristics lead to overheating of a local area of the powder bed greater than the melting point of the chromium oxide (2,435 ◦ C). Calculation area included two subdomains, the first one for the steel matrix and the second one for the porous layer of ultra-fine chromium oxide powder. Computer simulation results have shown that simultaneously with the melting of chromium oxide powder a thin layer of the steel substrate with a thickness about 3 µm is melted only because of the high temperature gradient in the irradiated area. Active mixing in the molten zone results in chromium oxide dissolution and chemical interaction between chromium oxide and iron. Kruth et al. [12] found that both the appropriate process parameters adjustment and the application of special scanning strategy make influence on the resulting roughness, microstructure, density and mechanical properties. Our pre-

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Fig. 1 The scanning strategy. Bold arrow indicates the resulting transverse speed of laser motion along a track 100 µm width

liminary study has shown that the best result under those terms was observed when a Z-type scanning strategy which is shown in Fig. 1 was applied. The laser beam was moved over the samples surface with the use of a system that consists of two mirrors as it is shown in Fig. 1. Computer controlled tilting of the mirrors allows to perform beam moving along any complex track. Very low weight of the mirrors make it possible to stop the movement at the edges of x-axis almost immediately and start the movement in the opposite direction so the velocity is almost constant both in x- and y-directions. The scanning speed 900 mm/s is in the fact the velocity of the beam movement along the x-axis in Fig. 1, giving an average laser-beam overlap ratio 60 % in x-direction. The resulting transversal speed along the y-axis is 300 mm/s. Actually, the laser track is a set of triangles so the beam overlap ratio in y-direction depends on the current position of the laser beam and ranged from the minimum value of 20 % to the maximum 100 % at angular points where the beam reverses the movement. 2.3 Preparation of Cr2 O3 powder The condition of laser processing characterized by high temperature gradient imposes the following requirements for powder preparation: the chromium oxide powder should be spread on the surface of the substrate as a thin layer, not more than 10 µm in thickness; the powder layer should be smooth and dense. To fulfill these requirements we used ultrafine powder of chromium oxide in an agglomerated state. To break the agglomeration the chromium oxide powder was

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mechanically milled in a high-energy planetary mill AGO2S together with a hexane. Duration of the milling stage is 2 min. Obtained powder-hexane suspension was spread on the surface of the steel substrate (Fe-0.2 wt% C) as a thin layer, normally 10 µm in thickness by a spraying technique. After drying in the air samples were placed into the process chamber. At the end of the laser processing, samples were evacuated from the chamber, washed in alcohol and dried in the air. 2.4 Microstructure characterization Prismatic shaped samples 10 × 10 × 4 mm were prepared to study microstructure, phase and chemical composition. Xray diffraction study was performed using DRON-6 diffractometer with Co-Kα radiation picking intensity during 5 s at each 2ϑ point with step 0.02◦ . TEM investigations were performed on EM-125 microscope with accelerating voltage 100 kV. The TEM samples were prepared by electropolishing of foils in an aqueous solution of glacial acetic acid and chromic anhydride. Surface of a sample after laser processing was prevented from the polishing by a stainless steel plate, so the TEM samples were produced directly from the surface layers with modified structure. The thickness of the TEM samples was about 50 nm. XPS-measurements of samples were done using a SPECS electron spectrometer with MgKα radiation. Etching by flow of Ar+ ions with energy 4 keV and current density 10 µA/sm2 has allowed performing layer-bylayer analysis of chemical state of elements. Etching rate in these conditions is 2.7 nm/min. Spectra resolution and semiquantitative concentration determination were done using reference 2p spectra of pure chromium and iron. Roughness of samples was measured by atomic force microscopy in contact mode using Solver P47 NT-MDT device.

3 Results An optical image of the surface within width of a single laser track is shown in Fig. 2. It can be seen that the Zscanning strategy results in the non homogeneous surface of a sample: at the edges of the track (area I in Fig. 2) melted areas of chromium oxide are clearly visible indicating its melting and dissolution in the metal substrate. Because of the fact that at the center of a track (area II in Fig. 2) the temperature is higher than at the edges and the chromium oxide is almost completely dissolved in the steel substrate. As it was mentioned above the chromium oxide powder is heated to high temperatures greater than its melting point during laser processing. However, because of high temperature gradient the substrate is melted to thickness about 3 µm only. High temperature leads to emergence of stable plasma

Fig. 2 An optical image of steel surface after short pulse laser processing in the presence of the ultrafine chromium oxide ×1,000. Arrow indicates a direction of the resulting traverse speed of laser beam motion (along the y-axis in Fig.1)

torch (argon and chromium oxide plasma) in the area of laser irradiation. The laser-induced plasma formation provides for intense reaction of chromium oxide with the molten steel and makes the surface smoother. Laser processing in such conditions generally leads to visible decrement in roughness except for laser treatment of polished samples where slight increase of root-mean-squared roughness (Rq) from the value 136 to value 138 nm in average was observed. X-ray study of samples after laser processing (Fig. 3) shows presence of α-Fe (ferrite or martensite) and γ -Fe (residual austenite) indicated by lines (111) and (210) in Fig. 3. Diffraction pattern of a sample without laser treatment shows presence of peaks that belong to α-Fe only which is usual for low carbon steels. Presence of the austenite phase in processed samples indicates an occurrence of α → γ phase transformation for a few microns in depth, as this is an essential requirement for a phase registration by the ϑ → 2ϑ method. Oxide and other phases were not detected by XRD method. Results of layer-by-layer elemental analysis using XPS method are shown in Fig. 4 and Table 1. Spectra of the films in area of 2p electrons binding energy of chromium is shown in Fig. 4a and for iron in Fig. 4b versus etching time. Both total concentrations of metals and separation on oxide and metal state in four outermost columns are shown in Table 1. On the surface of the sample chromium is presented only in oxidized state (+3). After etching during 0.5 min together with the main oxide component the metal chromium component is clearly visible at the left part of the spectra (573.8 eV). Increasing the etching time leads to detection both the component corresponding to the metal state of chromium and component of chromium in connection with carbon (Cr–C). In samples at different depths of etching simultaneously with

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Fig. 3 Diffraction pattern of a sample after laser processing

Fig. 4 Results of a depth profile XPS-measurements of a sample after laser processing. Characters at right of spectral curves correspond to the time of etching by argon ions flow. a Spectra of chromium, b spectra of iron

metal chromium iron appears in oxidized state (Fe2+ , Fe3+ ). As it is shown in Fig. 4b iron in different oxidation numbers presents even at depth of about 38 nm after etching during 14 min. Chromium 2p electrons spectra are diffused to the right that can be explained by the presence of different states of chromium, contributing to the area 574–575 eV, for example, chromium carbides.

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For detailed study of microstructure TEM investigation of thin films was performed after laser processing. In aims to avoid an artifact appearance, we carefully investigated the microstructure of steel Fe-0.2 wt% C in state of delivery which was used for samples producing. As it is shown in Fig. 5 the microstructure of the initial steel consists of deformed ferrite and pearlite.

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Table 1 Surface atomic concentrations of elements by its chemical state after etching with the argon ions flow Time of etching (min)

Depth (nm)

Elements concentration (at.%) O

C

Fe (total)

Cr (total)

Fe-ox. (+2, +3)

Fe-met. (0)

Cr-ox. (+3)

Cr-met. (0)

0

0

46.0

48.7

0.7

4.6

0.7

0.0

4.6

0.5

1

58.4

6.6

22.8

12.2

22.8

0.0

12.2

0.0 0.0

1

3

58.9

6.2

20.5

14.5

18.7

1.8

14.5

0.0

4

11

54.6

4.6

21.7

19.1

9.6

12.1

17.4

1.7

9

24

42.1

3.3

37.5

17.1

11.7

25.7

15.0

2.1

14

38

36.5

3.8

45.4

14.3

12.5

32.9

12.3

2.0

Oxidation numbers are shown in brackets

in Fig. 2. Electron diffraction pattern of the area is shown in Fig. 6b which contains reflexes of Cr2 O3 , chrome-spinel FeO·Cr2 O3 (card N4-752 in JCPDS database) and oxidized iron (+2). A formation of shell structures around the inclusions is observed which for visual presentation is indicated by arrows in Fig. 6a. Dark-field image in reflex (210) of the chrome-spinel FeO·Cr2 O3 (Fig. 6c) shows that the space among inclusions is enriched by this phase. Shell structures and presence of chrome-spinel evidence for active chemical interaction of chromium oxide with iron in the molten zone. As it is well seen in Fig. 6a an average inclusions size is about 40 nm and the smallest ones have size about 5 nm. Inclusions with smaller size are not registered so the size 5 nm can be considered as a limit of chromium oxide inclusions stability in the metal matrix with iron base. Inclusions with smaller size become unstable and completely dissolved in steel. 3.2 Area B: an area of completely dissolved chromium oxide

Fig. 5 TEM image of the steel in state of delivery and electron diffraction pattern of ferrite (inset)

Microstructure of surface films after laser processing is not homogeneous and depends on an area wherein the TEM image was obtained. It might be explained by dissimilarity in solidification conditions of various layers of a sample under high speed laser processing conditions. In fact gradient by chemical and phase composition films are formed as a result of the laser processing. 3.1 Area A: inclusions of chromium oxide In Fig. 6a TEM image of a film that contains large number of chromium oxide inclusions is shown and we can conclude that the film was taken from the place indicated by “I”

Microstructure in this area is homogeneous and does not have any visible inclusions and phase boundaries (Fig. 7a). TEM image of the area shows only signs of stratifying of liquid phase prior to the solidification process. Electron diffraction patterns of this area contain reflexes of α- and γ -Fe, iron oxides, chrome-spinel and even bcc-phase of chromium but there are no visible traces of chromium oxide (+3) so it is completely dissolved in the metal matrix. All these lead to the conclusion that this area was formed in the centre of the track of laser beam scanning motion (an area II in Fig. 2) where temperature is greater and exposure under high temperatures is longer. As it was mentioned the registration of the bcc-phase of chromium was observed in the electrons diffraction patterns of this area. Sometimes, as it is shown in Fig. 7b, metal chromium is the only registered phase. Repeated registration of the Cr bcc-phase excludes its appearance as an “artifact”. It is interesting to note that the electron diffraction pattern in

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Fig. 6 TEM image of microstructure in area where Cr2 O3 is not completely dissolved during laser processing. a Bright-field image of microstructure. b Electron diffraction pattern containing mainly reflexes

E. Kharanzhevskiy, S. Reshetnikov

of FeO·Cr2 O3 spinel. c Dark-field image in the reflex (210) of the chrome-spinel that is shown by the arrow in the electron diffraction pattern

Fig. 7 TEM image of microstructure in area where chromium oxide is completely dissolved. a Bright-field image of microstructure. b Electron ¯ of bcc-Cr that is shown by the arrow in the diffraction pattern containing reflexes of bcc-Cr phase only. c Dark-field image in the reflex (011) electron diffraction pattern

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Fig. 8 TEM image of microstructure supposedly formed in an area of a former pearlite grain. a Bright-field image of microstructure. b Electron diffraction pattern containing reflexes of α-Fe phase (indexed) and

chromium carbide Cr23 C6 . c Dark-field image in the reflex of chromium carbide Cr23 C6 that is shown by an arrow in the electron diffraction pattern

Fig. 7b corresponds to the one obtained from a monocrystal, but actual reflexes are “comets-like”. At that tails of “comets” are formed by many separate reflections. Such type of the electron diffraction pattern might be observed from a mosaic monocrystal with an angular divergence of blocks about 14◦ . In the dark-field image (Fig. 7c) which was obtained in the reflex (011) of bcc-Cr one can observe many separate crystallites of metal chromium in form of inclusions. So we can conclude that all chromium crystallites have an orientational relationship whose nature will be discussed in the next section.

Unlike the metal state of chromium the chromium carbide inclusions do not have an orientation relationship which is seen in Fig. 8c where reflexes of α-Fe phase are marked by indexes, other reflexes mostly belong to the Cr23 C6 phase.

3.3 Area C: chromium carbides This area is in general similar to the area B, but chromium is detected in form of a chromium carbide phase Cr23 C6 . It is possible that this area is formed in a place of a former pearlite grain in the area II in Fig. 2 where the carbon was presented in state of cementite Fe3 C (low-temperature phase). Heating by laser beam leads to α → γ phase transformation of iron, decomposition of cementite and subsequent melting of iron. As a result the carbon dissolved in liquid can react with the chromium. Figure 8a shows bright-field image of microstructure in the area. Dark spotted inclusions in the image are inclusions of the chromium carbide which is confirmed by the dark-field image in the reflex of Cr23 C6 phase (Fig. 8b).

4 Discussion As it was shown in the previous section reduced chromium in the metal state has been detected in samples after laser processing by two independent methods. Firstly by XPSmethod after etching by argon flow during 4 min and longer. Secondly the bcc-Cr has been detected by TEM investigations. Etching by argon ions flow cannot reduce chromium oxide so the films containing bcc-Cr are formed as a result of the short pulse laser processing. A model of thin films composition on different depths after laser processing has been suggested based on XPS and TEM investigations. On a free surface there is very thin layer containing chromium and iron oxides transforming in depth into Fex Cr(2−x) O4 spinel with variable composition, chromium and iron in metal state and chromium carbides. Presence of iron in the metal state (α- and γ -Fe) might be explained by sufficiently lower affinity for oxygen in compare to chromium. At the same time presence of metal chromium having sufficient amount of oxygen in films is not

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clear enough. In this case metal chromium must be in the form of inclusions separately from any oxygen phases. Two aspects should be noted after TEM-images analyzing: (a) separate crystallites of metal chromium have the same orientation giving a mosaic monocrystal structure; (b) inclusions of metal chromium are formed in the matrix on iron base. According to the equilibrium Fe–Cr phase diagram inclusions of metal chromium cannot be formed because chromium forms solid solutions with iron at high and low temperatures. Both aspects can be explained applying a concept of a model of fast solidification in strong nonequilibrium conditions. Surface is heated up to the melting point of chromium oxide and higher during laser processing. Laser induced plasma formation might lead to the initiation of chromium oxide reduction process in films near to the surface of a sample. Since the time of a laser pulse is very short the liquid begins to cool fast because of effective heat transfer into the sample which leads to rapid solidification. Solidification under laser processing conditions starts from the walls and bottom of the molten pool where concentration of metal chromium is low due to insufficient time for its equilibrium distribution throughout the melted volume. Therefore, in contrast to an equilibrium system Fe–Cr where the high temperature phase is α-phase, solidification in samples starts from growing of γ -Fe crystallites which is confirmed by XRD analyses. As it is well investigated, under conditions of rapid crystallization during laser processing of surface a phenomenon of structural heredity is occurred whose nature has been carefully investigated by Sadovskiy et al. and Sthaslivtsev et al. (see for example, [13,14]). The essence of this phenomenon lies in reconstruction of an initial structure of steels after high speed laser quenching, including quenching from the liquid phase [15]. At that, there is a crystallographic orientation relationship between initial and forming crystallites during solidification process. Certainly, such memory of the preceding structure is a result of solidification on a substrate. An additional factor, strengthening the structural heredity, is extremely high rate of solidification which under used processing characteristics is greater than the velocity of absolute stability of the solid–liquid interface for an alloy Fe–C [16]. Such speed of solidification makes stable a planar type of the phase interface and contributes to reconstruction of a preceding austenite grain. Thus because of the fact that in γ -Fe can be dissolved not more than 13 % of chromium its excesses might be separated in form of inclusions which have a crystallographic orientation relationship with the forming austenite grain. Actuality of chromium reduction process can be confirmed by XPS registration of iron oxides in different states of oxidation (+2, +3) in layers which lie deep from sample surface (see Table 1). Laser processing in an inert environment without deposited chromium oxide does not lead to iron oxidation at these depths. Spectra of corresponding samples

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do not contain oxidized iron after first few minutes of etching. Thus the only source for iron oxidizing in samples could be a result of chromium oxide reduction. Thus the results of this experimental study indicate the reduction process of chromium oxide by iron. But simple thermodynamic calculation of the equilibrium 3Fe + Cr2 O3 = 3FeO + 2Cr

(1)

in volume of the substance shows that the only product of this reaction has to be Cr2 O3 and there should not be neither iron oxide nor metal chromium phases. However such conclusion contradicts not only to results of this work but also to set of experimental data in works devoted to study of chemical composition of passive films in stainless steels (see references in [17]). Andreev et al. [17] have developed a thermodynamic model of passive films on a surface of chromium stainless steels which can help explaining appearance of chromium in the metal state in samples after laser processing. The model takes into account a contribution of the surface energy G S into Gibbs energy change of the reaction (1). It was shown that the contribution G S results in a negative adsorption of chromium by surface of stainless steels which displaced the moving equilibrium (1) towards iron oxides formation. Statements of the thermodynamic model of passive films in stainless steels can be used to explain the phenomena of chromium reducing by iron via short pulse laser processing. Due to high temperature of the treated surface there might be an appearance of a phase boundary between liquid chromium oxide and iron phases, so the contribution of the surface energy G S on phase boundary might make conditions to shift equilibrium (1) to the right, towards metal chromium formation.

5 Conclusion An opportunity of surface saturation of low-carbon steel by chromium oxide using short-pulse laser processing has been shown. As a result of the short pulse laser processing and laser-induced plasma formation layers with gradient chemical and phase composition are formed. Characterization of microstructure by TEM and XPS methods allows to conclude that chromium oxide efficiently dissolving in the steel matrix which forms chrome-spinel, chromium carbide and inclusions of bcc-phase of chromium. There is a general similarity in chemical and phase compositions of passive films forming on surface of stainless steels and films formed as a result of short pulse laser processing of steel in presence of chromium oxide. Thus this technique might be used to protect structural components from corrosion. The uniformity of films can be improved by applying of different scanning strategies and processing parameters so further investigations should be done.

Chromium oxide dissolution in steels Acknowledgments This study is financially supported by the program 07.08 “Applied Research in Education” of the Ministry of Education and Science of Russia (2.947.2011).

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