Characterization of fretting products between austenitic and martensitic stainless steels using Mössbauer and X-ray techniques

Wear 300 (2013) 90–95

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Characterization of fretting products between austenitic and martensitic ¨ stainless steels using Mossbauer and X-ray techniques K. Szyman´ski a, W. Olszewski a,n, D. Satu"a a, K. Rec´ko a, J. Waliszewski a, B. Kalska-Szostko b, J.R. Da˛browski c, J. Sidun c, E. Kulesza c a

Faculty of Physics, University of Bialystok, Lipowa 41, 15-424 Bia!ystok, Poland Institute of Chemistry, University of Bialystok, Hurtowa 1, 15-399 Bia!ystok, Poland c Faculty of Mechanical Engineering, Bialystok University of Technology, Wiejska 45a, 15-351 Bia!ystok, Poland b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 May 2012 Received in revised form 5 November 2012 Accepted 22 January 2013 Available online 8 February 2013

Fretting experiments with an austenitic pin on a martensitic disc were performed. X-ray diffraction revealed that a part of the austenite pin was transformed into a martensite layer. The martensite is ¨ covered by a layer of Fe–Cr–Ni oxides, detected by the Mossbauer spectroscopy. Martensite and Fe-oxide, originating from the spinel structure, were identified in the debris powder. Obtained results indicate that the crystal structures of the Fe-oxides produced during fretting are highly defected. This is the cause of the observed superparamagnetic properties, broad diffraction maxima, and differences between predicted and observed Bragg spot intensities. & 2013 Elsevier B.V. All rights reserved.

Keywords: Fretting Wear Stainless steel Ferrite ¨ Mossbauer spectroscopy

1. Introduction Austenitic stainless steel is widely used in many industrial fields as well as in the reparation or replacement of human body parts. Its biocompatibility, corrosion resistance, good mechanical and tribological properties are among the important parameters determining the suitability of the material for biomedical applications. In order to better understand the processes occurring during friction, many investigations have been performed until today [1–4]. Fretting is a type of friction process with small amplitudes of shifts between wearing parts. Fretting in orthopaedic implants disrupts the protective passive layer on materials that are usually highly resistant to corrosion, leading to the release of heavy metal ions into the surrounding tissue [5]. ¨ The Mossbauer technique is a particularly important tool for investigating processes of steel degradation, because local properties on an atomic scale can be investigated [4,6,7]. The detection of conversion electrons emitted during the recoilless process allows investigation of the near surface region [8]. In this work, the results of a laboratory experiment with fretting between 316 L and H18N9T steels are presented. The

n

Corresponding author. Tel.: þ48 85 7457242; fax: þ48 85 7457223. E-mail address: [email protected] (W. Olszewski).

0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2013.01.116

structure of produced debris and deposit formed on the wearing surface of the 316L steel was identified.

2. The experiment and spectroscopic techniques A 316L steel pin with a diameter of 5 mm on an H18N9T steel disc was subjected to fretting process with an oscillation amplitude of about 100 mm and a load of 14 N under ambient conditions. This friction pair (pin-disc) facilitates the determination of constant pressures at the level of 14 MPa. Studies were conducted in the atmosphere of air with an ambient temperature of about 283 K and an average relative humidity of 30%. Scheme of the measurement system pin-on-disc is shown in Fig. 1. Spark analysis revealed content of the main components for 316L and H18N9T steels (Table 1). Chemical composition analysis of the tested materials was performed using the ARL QUANTRIS optical emission spectrometer. Debris produced during the fretting process (Fig. 2) was collected for spectroscopic analysis. ¨ Mossbauer measurements were performed in constant acceleration mode with a 57Co source in a Cr matrix. The velocity scale was calibrated using a-Fe standard foil at room temperature. For low temperature transmission measurements, a closed cycle He refrigerator was used. The He flow detector [9] was used in conversion ¨ electron Mossbauer spectroscopy (CEMS) investigations of the pin

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Fig. 1. Scheme of the pin-on-disc measurement system.

Table 1 Chemical composition analysis results in at% of the pin and disc subjected to fretting process. Other elements are at a level below 0.3 at%. Element

Pin

Disc

Fe Cr Ni Mn

72.6(1) 18.55(4) 6.57(4) 1.14(2)

70.2(1) 18.80(2) 7.7(1) 1.477(2)

Fig. 3. (a) CEMS spectrum of deposit formed on the wearing surface of the 316L steel pin (parameters of fitted doublet: cs¼0.33(1) mm/s, qs¼ 0.79(1) mm/s). Transmission spectrum of debris measured at: (b) room temperature and (c) T¼30 K.

Fig. 2. Debris produced during the wearing experiments, visible on the image obtained from the scanning electron microscope.

surface. The spectra were analyzed using the commercially available NORMOS package. X-ray diffraction measurements of flat surfaces of the pin were performed on an HZG4 diffractometer working in Bragg–Brentano geometry with unfiltered Ag-Ka radiation. Other measurements of powder diffraction were performed on the SuperNova four circle diffractometer from Agilent Technologies, with filtered Mo-Ka radiation.

3. Experimental results ¨ The Mossbauer spectrum of the deposit formed on the pin surface during fretting process is shown in Fig. 3a. Results of X-ray analysis performed on the same pin surface are shown in Fig. 4a. It was also possible to take a small grain of the deposit and perform X-ray diffraction in the microbeam (Fig. 4b). ¨ The Mossbauer spectra of debris measured at room and at low temperature are shown in Fig. 3b and c, together with the results of analysis, the numerical parameters of which are presented in Table 2. The doublet is the dominating component at room temperature. A decrease in temperature results in a gradual change of the doublet into a magnetically split component

(spectra not shown) indicating superparamagnetic behaviour. At T¼30 K the doublet almost disappears, and a hyperfine magnetic field distribution with average values typical for magnetite is formed (Fig. 3c). The X-ray diffraction spectrum of a grain of debris is shown in Fig. 4b. For easier analysis of X-ray diffraction spectra, hematite and magnetite (Fig. 4d and e), as well as martensite and austenite (Fig. 4f), were measured. The intensities ¨ measured in the X-ray scattering and Mossbauer experiments have the Poisson distribution and their statistical uncertainties are equal to the square root of the number of counts, as shown by the error bars in Figs. 3 and 5. Some of them are smaller than the symbol size. In Fig. 4a, uncertainties are given by the amplitude of the noise, while in Fig. 4b–f, error bars are smaller than the thickness of the lines. Pieces of the steels were also ground into powders that served as a reference material in spectroscopic analyses. Spectra of the H18N9T powder (Fig. 5a and b) show a weak change with temperature typical for martensitic steel; see hyperfine fields in the last row of Table 2. Because these spectra are rather complicated, in order to be convinced of a correct interpretation, an additional measurement in an external magnetic field was performed. In this case, the magnetic moments of iron are aligned, and relative line intensities change substantially, while their positions remain practically unchanged (spectrum not shown). Spectra of another reference, 316 L steel measured on a clear metallic surface (inset Fig. 5), as well as on the powder in transmission mode (Fig. 5c), has the shape of a singlet at room

92

´ ski et al. / Wear 300 (2013) 90–95 K. Szyman

Fig. 4. X-ray diffraction performed at room temperature on: (a) the surface of the 316L steel pin, (b) the grain taken from the surface of the 316L steel pin, (c) debris powder, (d) hematite, (e) magnetite and (f) powder of H18N9T steel. Miller indices of the Bragg maxima are given for hematite (triangle), magnetite (box), martensite (diamond) and austenite (disk) phases shown in (d–f). Some Bragg maxima originate from the scotch tape used for sample preparation and are indicated by a star symbol. The main crystallographic phases contributing to the Bragg maxima are indicated in the spectra of fretting products (a–c).

temperature. The spectrum does not show any magnetic splitting at the temperature of T ¼30 K (spectrum not shown). This behaviour is typical for austenitic steel. Because the structure of austenitic steel is sensitive to plastic deformation, the powder of the 316L steel (particle size about several dozen mm) was milled in a mortar, and during the process, changes of material proper¨ ties were monitored using Mossbauer spectroscopy. It is clear that powder subjected to 24 h grinding partially transforms to martensite (Fig. 5d).

4. Discussion In order to proceed further to the interpretation of the results, one has to discuss the properties of mixed Fe1  xCrx oxides formation during fretting with an accepted reasonable x fraction between 0.2 and 0.4, since these elements are major constituents of the investigated steels. Corundum and spinel structures are

stable low temperature phases in the Fe–O system and their Cr doping has been shortly reviewed below. Fe2O3 and Cr2O3 form a solid solution within the corundum structure in the entire concentration range. The average hyperfine magnetic field of (Fe1  xCrx)2O3 measured at room temperature decreases with x almost linearly in the concentration range 0rx r0.5 [10,11]. For x between 0.2 and 0.4, the magnetic transition temperature decreases from 800 to 600 K [11], the average hyperfine field decreases from 50 to 47 T (at room temperature), and from 53 to 52 T (at T¼30 K) [10]. Bulk Cr doped hematite was not found as a component in the measured spectra. In the (Fe1  xCrx)3O4 spinel, hyperfine field distribution changes in a characteristic manner: for a small x, a so-called A site subspectrum formed by Fe(III) remains as a sharp sextet while the B subspectrum originating from a less-defined oxidation state of iron shows substantial broadening [12]. However, the average values of the hyperfine magnetic field do not change significantly with x at room temperature up to x¼0.17, as confirmed by measurements on bulk samples prepared using different methods of synthesis [12–14]. Reduction of remanent and saturation magnetization by a factor of 2.8 when x changes from 0 to 0.31 for 700–800 nm thick (Fe1  xCrx)3O4 film was reported in Ref. [15]. For a composition as large as x ¼0.33 (Fe2CrO4), the magnetic transition temperature decreases to ¨ 303 K [16], while the Mossbauer spectrum for x¼0.5 consists of Fe(II) and Fe(III) doublets covering a velocity range from 0 to 0.4 mm/s [17]. In the case of the reported experiment, xE 0.2, and it is also unlikely that the (Fe1  xCrx)3O4 spinel in bulk form is a dominant component of the wearing deposit. ¨ Mossbauer spectra of mixed Fe–Cr oxides are sensitive to the size of particles. (Fe1  xCrx)2O3 samples prepared by means of the sol–gel method in the concentration range 0rx r2 show only a doublet with center shift (cs) in a range between 0.32 and 0.36 mm/s, while quadrupole splitting (qs) is between 0.60 and 0.70 mm/s [18]. These samples, measured at room temperature after calcination at T¼873 K, show magnetic splitting of spectra components. Somewhat similar tendencies were reported for hydrothermally synthesized (Fe1  xCrx)2O3 with particle sizes in the range of 19–87 nm [19]. Two crystallographic phases—Cr poor and Cr rich, were observed in a certain concentration range along with the presence of a doublet and a sextet in some spectra. For the as-prepared sample with x ¼0.3, the parameters of the doublet were qs¼0.7 mm/s, cs ¼0.15 mm/s [19]. In the Fe2CrO4 spinel with particle sizes between 6 and 35 nm, no magnetic splitting was observed down to 16 K. In Ref. [20], ¨ Mossbauer measurements of nanosized iron-rich chromium oxide Fe2CrO4 are reported. A quadrupole doublet down to T¼ 16 K is observed for particle sizes between 6 and 35 nm. Only Fe(III) is observed with doublet parameters at RT: qs ¼0.51–0.56 mm/s, cs¼0.28–0.31 mm/s. The authors of Refs. [18,19] interpret the presence of the doublet as a result of superparamagnetic behaviour. Mixed Fe–Cr oxides are sensitive to nonequilibrium cation distribution, as demonstrated by different annealing procedures [11]. A sample of (Fe1 xCrx)2O3 with x¼0.8 annealed at T¼773 K was magnetically ordered, while a sample of the same composition annealed at T¼1273 K was paramagnetic. The authors ruled out the possibility of superparamagnetic behaviour and argued that the changes in cation distribution are responsible for the difference in the magnetic transition temperature. A general trend can be found by analyzing published spectra of Fe1  xCrx oxides: magnetically split spectra of the spinel structure exhibit rather broad distribution [12–14], while the corundum structure exhibits a single sextet with rather narrow lines [10,11,19].

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Table 2 Hyperfine parameters of components obtained from fits to the spectra measured at room and low temperatures, shown in Figs. 2b–c and 3a–b. Debris T¼ 298 K (Fig. 2b)

A [%]

T ¼30 K (Fig. 2c)

mt Sextet

mt Sextet

aust Singlet

sp Doublet

21

19

4

56

mt Sextet

mt Sextet 19

aust Singlet

Sp Sextet

2

32

22 cs

 0.04

 0.17

qs

0.18

–

 0.01

–

cs

33.3

mt Sextet

mt Sextet

aust Singlet

82

3

15

 0.02

–

 1.14

48.7

44.5 40.2

mt Sextet

Mt Sextet

81

3

0.09

0.12

0.02

0.00

 0.06

26.6

33.5

 0.04

–

24.6

–

– 32.5

aust Singlet 16

 0.12

 0.04 B

0.05

T ¼30 K (Fig. 3a)

0.06 qs

–

26.2

T¼ 298 K (Fig. 3b)

0.98

0.15

–

32.3 H18N9T steel

A [%]

0.44

0.00

25.6

9

0.44

0.82

 0.01 B

 0.17

0.12

0.03

sp Sextet

16

0.35

0.08

sp Sextet

–

The parameters are: A—relative spectral area, cs—center shift in mm/s relative to a-Fe at room temperature, qs—quadrupole splitting in mm/s, B— hyperfine magnetic field in T, mt—martensite, aust—austenite, sp—spinel. The uncertainties correspond to 7 1 at the position of the last significant digit.

Fig. 5. Transmission spectrum of H18N9T steel powder measured at: (a) T¼ 30 K and (b) room temperature. Room temperature spectrum of 316L steel absorbers made of (c) powder (cs¼  0.09(1) mm/s) and (d) powder milled at ambient conditions for 24 h. Inset: CEMS spectrum of 316L clear surface measured at room temperature (cs of the singlet:  0.09(1) mm/s).

Literature data indicates that Ni, which is the third main component of the investigated steels, does not change the temperature of magnetic transition of mixed Fe–Ni oxides. The first observation derived from the results of analysis is that only a doublet is recorded in CEMS measurements on the

surface of 316L steel subjected to fretting (Fig. 3a). The same component is seen in the spectrum of the debris (Fig. 3b). The relative spectral area of the austenitic singlet in the spectrum of the debris is not more than 4%. This component is present in the spectra measured on the 316L clear surface (Fig. 5, inset) as well

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Table 3 Results of X-ray phase analysis of the fretting products. Product

Grain from surface Debris

Magnetite

Martensite

Hematite

A

˚ a [A]

A

˚ a [A]

A

˚ a [A]

˚ c [A]

0.60 0.30

8.40(6) 8.40(6)

0.10 0.70

2.86(1) 2.88(1)

0.30 –

5.11(5) –

13.3(2) –

The parameters are: A—relative spectral area, a and c—lattice parameters. Uncertainties of spectral areas are about 20%.

as in those measured on the powder (Fig. 5c). It can thus be concluded that 316L steel subjected to fretting is covered by an oxide layer with a thickness greater than the depth characteristic for the CEMS experiment, with this depth being of the order of 100 nm [8,9,21]. The results of debris analysis shown in Fig. 3c and Table 2 show that over 50% of iron forms a doublet which splits into the broad components at a low temperature, with hyperfine fields in a range between 40 and 49 T, related to the magnetite compound type. In the case of a corundum structure, one would expect a more narrow hyperfine field distribution. ¨ Mossbauer spectra of debris acquired during friction are similar to those presented in Ref. [4], obtained in ball-on-flat friction experiments with 316L steel performed in water and in alcohol. The authors observed martensite and a doublet with cs ¼0.34(2) mm/s and qs¼0.83(1) mm/s in their room temperature spectra. The doublet was interpreted as being a result of the presence of ferrihydrite Fe5O7(OH)  4H2O. However, the spectra of small hematite particles are similar to those of ferrihydrite, as discussed in Ref. [22]. Because the processes described in this work were performed in air at ambient conditions, formation of small amounts of hydroxides cannot be excluded. In this experiment, the fraction of Fe þNi in the steel subjected to fretting is about 0.8, so the mixed Fe–Cr–Ni oxides in corundum as well as in the spinel structure should exhibit magnetic splitting at room temperature. The observation of a doublet at room temperature and of magnetite-like distribution of the hyperfine field at T ¼30 K, indicates that the oxide structure strongly deviates from the bulk, equilibrium crystals. As has been shown, their properties resemble the behaviour of mixed oxides of small particles. At the present stage of investigations, it is not clear whether the solid deposit on the 316L steel surface, as well as the main constituent of the debris produced during the fretting process, is formed by small particles or by mixed oxides with a high density of structural defects. All Bragg peaks in the X-ray diffraction spectra of fretting products can be indexed with three crystallographic phases: magnetite, martensite and hematite (Fig. 4a–c). No separate peak of austenite was detected. This is particularly interesting in the case of the investigated surface of the austenitic pin (Fig. 4a). It is clear that the top layer of the austenitic pin was transformed to martensite, preventing X-ray penetration into the austenite. One may estimate the low limit of thickness of the martensitic layer considering the attenuation of the Ag-Ka radiation scattered under Bragg angles of austenite. Because the position of the (1 1 1) Bragg peak of austenite is close to the position of the (1 1 0) peak of martensite, the other intense peak of austenit, (2 0 0) is considered. The attenuation of the scattered radiation I by the martensite layer of thickness d may be approximated by the formula: I ¼ I0 eð2d=siny200 Þrm ,

ð1Þ

where I0 is the initial intensity, y200 is the Bragg angle of austenite, while r and m signify the density and mass absorption coefficient of martensite, respectively. Arbitrarily assuming that

the absence of (2 0 0) Bragg peak of austenite in Fig. 4a was caused by absorption I/I0 ¼0.05, the lower limit of d¼ 15 mm can be estimated from Eq. (1). Almost all of the positions of hematite Bragg peaks coincide with the positions of magnetite in Fig. 4b. In fact, only the hematite maximum with Miller indices (0 0 6)/(1 1 3) in Fig. 4b is separated and unambiguously indicates the presence of this structure. From their intensities one may conclude that the amount of the hematite phase is not dominant. Because of the broadening of the Bragg peaks (Fig. 4b and c), the accuracy of estimation of the lattice constant is poor, see Table 3. The martensite phase exhibits a small tetragonal distortion with c/a ¼1.01(3). The lattice parameter of detected hematite (Table 3) differs slightly from the reported values for pure ˚ c¼13.740 A˚ [23] and a ¼5.035 A, ˚ c¼ hematite a ¼5.034 A, 13.748 A˚ [10]. The lattice parameter of magnetite is close to the values reported in Ref. [12]. The amplitudes of Bragg maxima shown in Fig. 4a–c deviate from the prediction of the Rietveld analysis, indicating that crystal structures formed during fretting depart from the well crystallized oxides, most probably because of a large amount of structural defects. This coincides well with the superparamag¨ netic behaviour observed in the Mossbauer spectroscopy. Because we could not decompose the diffraction patterns shown in Fig. 4b and c, neither to the combination of experimental patterns shown in Fig. 4d–f, nor to the shapes generated in the Rietveld package, the estimation of the phase composition is particularly difficult. Therefore, we have made only a rough estimation of the spectral areas of the phases shown in Fig. 4. The dominating phase of debris is magnetite, which corresponds ¨ well with the results of Mossbauer spectroscopy.

5. Conclusions Debris collected during fretting between austenitic and mar¨ tensitic stainless steels was characterized by Mossbauer and X-ray techniques. An overall agreement between the results of both techniques was achieved. Fretting of the austenitic pin causes transformation of the near-surface part to martensite. Xray scattering indicates that the thickness of the martensitic layer is greater than 15 mm. During fretting, a Fe–Cr–Ni magnetite layer is formed on the 316L steel pin. This layer was detected in the CEMS experiment, which is sensitive to the thickness of a few hundred nanometres. The main debris component is Fe–Cr–Ni mixed oxide originating from the spinel structure. Its magnetic and structural properties do not correspond to the equilibrium bulk phase. Superparamagnetic behaviour was demonstrated in particular. The stainless steel phase found in the debris mostly ¨ corresponds to martensite, as shown by the Mossbauer experiment and X-ray diffraction. A small fraction of iron, about 4%, forms an austenitic phase detected in the debris, while this amount was not detected in the X-ray experiment. A quantitative estimation of the phases was difficult, because Bragg intensities are strongly influenced by structural defects.

Acknowledgements The work was partially supported by EU founds under contract number POPW.01.03.00-20-034/09-00 and by the National Centre for Research and Development under the research project no. N R15 0117 10/NCBR.

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