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Annealing effects on plasma-sprayed Ni: An XRPD study
Surface & Coatings Technology 203 (2008) 345–349
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Annealing effects on plasma-sprayed Ni: An XRPD study Magdalena Lassinantti Gualtieri a,⁎, Maria Prudenziati a, Alessandro F. Gualtieri b a b
Dipartimento di Fisica, Università di Modena e Reggio Emilia, Via G. Campi 213/A, I-41100 Modena, Italy Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, S. Eufemia 19, I-41100 Modena, Italy
a r t i c l e
i n f o
Article history: Received 13 February 2008 Accepted in revised form 8 September 2008 Available online 15 September 2008 PACS: 61.10.-i 61.72.Lk 81.40.Ef Keywords: (X) Annealing (B) X-ray diffraction (C) Thermal spraying (B) Dislocations (D) Nickel
a b s t r a c t The variation of the size of coherently-diffracting domains and strain due to annealing at moderate temperature (500 °C) has been estimated for plasma-sprayed Ni using X-ray Powder Diffraction (XRPD) and line broadening analysis in conjunction with classical and modified Williamson–Hall methods. It was found that annealing provokes a narrowing of Ni diffraction peaks which was basically associated to a decrease in dislocations present in the as-sprayed material. The evolution of the microstructure with temperature of plasma-sprayed Ni was studied by in situ X-ray Powder Diffraction (XRPD). It was found that the breadth of the Ni profiles continuously decreased with heating up to 500 °C, mainly due to healing of dislocations. These results were used to explain the irreversible decrease in electrical resistance of plasma-sprayed Ni resistors after annealing which was previously observed in our laboratory. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Thermal spray allows for rapid and inexpensive deposition of a wide variety of materials. The feedstock, generally a powder, is forced through a plasma flame and propelled towards a surface. The molten droplets, formed in the plasma flame, rapidly solidify and coalesce to form a coating typically 10–1000 μm thick. Coatings deposited by thermal spray exhibit a complex, anisotropic microstructure with elongated brick-like structures, called “splats,” which are formed from the rapid flattening and solidification of molten droplets of the sprayed material [1]. Thermally sprayed materials are mainly used as protective coatings against wear, heat, corrosion and oxidation. In particular, coatings of Ni and Ni alloys are widely used for reclamation (re-work and repair of damaged peaces) and bond coats. Recently, thermally sprayed metals have been proposed as possible candidates as heaters [2–5] and sensors [6,7]. In these applications, it is of paramount importance to acquire detailed knowledge of the electrical properties (such as resistance and its temperature dependence) of the materials. This recently prompted a thorough investigation of the electrical properties of metal resistors prepared by thermal spray [8] whose outcome evidenced that the electrical resistance of as-sprayed Ni resistors irreversibly decreased after thermal treatment at moderate temperatures. It was suggested that this phenomenon could be related to healing of structural defects. A ⁎ Corresponding author. Tel.: +39 059 2055576; fax: +39 059 2055235. E-mail address: [email protected] (M. Lassinantti Gualtieri). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.09.010
decrease in resistance after annealing has also been reported for thin films [9] and quenched bulk materials [10]. In the former case, the phenomenon was suggested to be related to grain growth [9]. In the latter case, the phenomenon has been explained by temperatureinduced healing of defects [10]. Suggested methods to study materials microstructure in terms of grain size and/or microstrain are X-ray Powder Diffraction (XRPD) and Line Broadening Analysis (LBA). In fact, small dimensions of coherently diffracting domains and the presence of structure defects (i.e. microstrain) result in broadening of the sample profiles. The most common approach to LBA is pattern decomposition, which involves the fitting of analytical reflection profile functions to the various identified Bragg reflections without reference to a crystal structure model. Frequently, the experimental profiles are modeled as Voigtian, i.e. the shape is an intermediate between Lorentzian and Gaussian. The Voigt, the pseudo-Voigt and the Pearson VII functions belong to this category. Profile fitting gives the position, intensity, width and shape of individual reflections. However, the observed profile broadening must be corrected for the contribution from the instrument. In fact, the observed diffraction line profile is the result of the convolution of the instrumental profile (which includes aberrations introduced by the diffractometer and wavelength dispersion) and the sample profile in addition to the background. The instrumental profile is fixed for a particular experimental setup and can be determined using data collected from a standard material with no sample broadening. In the second step of LBA, the information extracted from pattern decomposition is used to estimate the crystallite size and shape as well as the
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M. Lassinantti Gualtieri et al. / Surface & Coatings Technology 203 (2008) 345–349
strain. In many cases, both size and strain effects occur simultaneously. The two effects can be separated on the basis of the different diffractionorder dependence of peak broadening. Two classical methods have evolved during the past five decades: the Williamson–Hall [11] and the Warren–Averbach methods [12]. The first is based on the integral breadths (β) whereas the second is based on the Fourier coefficients of the profiles. Modifications of these methods have been proposed for the specific case of dislocation strain broadening [13]. A recently emerged method for LBA is Whole Powder Pattern Modeling (WPPM) which adopts models based on physical parameters which are directly related to the microstructural properties that influence the shape and width of the diffraction profiles [14]. In this work, the microstructure of as-received and annealed plasma-sprayed Ni was investigated by XRPD and LBA in order to separate the contribution of size and microstrain to the observed line broadening in as-sprayed and annealed plasma-sprayed Ni. Hence, the contribution of size and strain effects to the observed decrease in resistance of plasma-sprayed Ni can be modeled. In addition, the narrowing of the diffraction peaks was followed in temperature using XRPD data collected in situ. These data were compared with previously observed changes in electrical resistance of plasmasprayed Ni [8]. 2. Experimental 2.1. Sample preparation and instrumentation A ca 0.3 mm thick Ni coating was deposited on a steel support by means of Atmospheric Plasma Spray (APS) using standard conditions. The starting powder was Metco 56FNS with a grain size of −45 + 11 μm. Following deposition, the coating was detached from the steel substrate and manually ground in an agate mortar in order to obtain a fine powder which was used for subsequent analyses. X-ray Powder Diffraction data (XRPD) were collected using a θ/θ diffractometer (PANalytical, CuKα radiation), equipped with an Anton Paar HTK 16 resistance heating chamber and a Real Time Multiple Strip (RTMS) detector [15]. Divergence slit (0.25°) and anti-scattering slit (0.25°) as well as a soller slit (0.04 rad) were mounted in the incident beam pathway. The diffracted beam pathway includes a Ni filter, soller slit (0.04 rad) and antiscatter blade (5 mm). An integrated step scan of the RTMS detector of 0.0167°2θ was used. Initial ex situ experiments were performed on as-sprayed and annealed plasma-sprayed Ni in order to confirm the presence of sample broadening and investigate its nature (i.e. size and/or microstrain). Annealing was performed in a furnace using a temperature program equal to the one used for the in situ XRPD experiment (see below). The ex situ XRPD data was collected using a θ/θ scan carried out in the region 40–100°2θ using high counting statistics (300 s/step). The samples for the ex situ XRPD investigation were ground in an agate mortar, and side-loaded in an aluminum sample holder. Data for Si standard (NIST640c) were collected using the same instrumental setup in order to determine the instrumental broadening. Si was chosen as standard as this material possesses peaks of relative high intensity close to the Ni peaks under investigation. Hence, a good estimation of the instrumental broadening in the critical 2θ ranges could be obtained. The microstructure variations of plasma-sprayed Ni were followed in temperature using in situ X-ray Powder Diffraction data (XRPD). Powder of as-sprayed Ni, ground in an agate mortar, was mixed with a Si standard powder (NIST640c) and placed on the Pt heating strip. The silicon standard was added so that the instrumental contribution to the observed profile broadening of Ni (see next paragraph) could be determined at each investigated temperature. However, it was found that the breadth of the profiles of the standard was identical throughout the experiment. Hence, the room temperature (RT) data for the instrumental broadening was
used for all calculations. Data were collected at regular temperature intervals, both during heating up to 500 °C and subsequent cooling down to 20 °C. The heating/cooling rate was 50 °C/min. The acquisition time was 20 s/ step, and the investigated angular range was 40–54°2θ. Prior to the experiment, the temperature of the heating strip was calibrated using known phase transitions/ transformations of standard materials. 2.2. Data evaluation The microstructure of plasma-sprayed Ni was investigated by means of pattern decomposition. Before pattern decomposition, the Kα2 contribution was stripped from the raw data using X'Pert Highscore Plus (PANalytical, version 2.1) and the modified Rachinger method [16] implemented in the software. All peaks of the Si standard were also fitted and the profile parameters were used to obtain the instrumental profile function (i.e. angular dependence of instrumental broadening). The peaks under investigation were modeled with a pseudo-Voigt (pV) analytical profile function using the software ProFit (Philips Electronics N.V., version 1.0c). The peak position, the Full Width at Half Maximum (FWHM), the maximum intensity and the shape parameter (η, also called the mixing parameter) for the pseudo-Voigt function were refined as well as background parameters. The refined values of the integral breadth (β, the width of a rectangle having the same area and height as the line profile) and the mixing parameter (η) were used in a subsequent analysis. The integral breadth of the Lorentzian (βL) and Gaussian (βG) components of the Voigt function corresponding to the pV function can be determined according to the following empirical formulas [17]: βL ¼ 0:017475 þ 1:500484η−0:534156η2 βpV βG ¼ 0:184446 þ 0:812692ð1−0:998497ηÞ1=2 −0:659603η βpV þ 0:445542η2
ð1Þ
ð2Þ
Applying the above equations to the powder pattern collected from the sample, h(2θ), and from the Si standard, g(2θ), the line broadening due to the microstructure of the sample, f(2θ), can be calculated due to the additive property of the breadths of the Lorentzian functions and the squares of the breadths of Gaussian functions: βfL ¼ βhL −βgL
ð3Þ
2 2 g 2 βfG ¼ βhG − βG
ð4Þ
The total integral breadth of the f(2θ) function (βf) corresponding to the given peak was calculated according to the following empirical formula given by de Keijser et al. [18]; βfG βf
1=2 1 1 ¼ − kπ1=2 þ 4 þ k2 π −0:234k expð−2:176kÞ 2 2
where k ¼
ð5Þ
β fL βfG π1=2
A multi-line microstructure analysis was performed on as-sprayed and annealed Ni using ex situ data. Such analyses could not be performed using in situ data as the investigated 2θ range was compromised in order to have a reasonable counting statistics and time resolution. In addition, the (220) Ni profile overlaps with the (331) profile of the Si internal standard and could therefore not be used.
M. Lassinantti Gualtieri et al. / Surface & Coatings Technology 203 (2008) 345–349
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f
The integral breadth in reciprocal units, βf ⁎ ¼ β cosθ λ , was plotted versus d⁎ ¼ 2 sinθ in a classical Williamson–Hall plot [11], where λ is the λ wavelength (Å) and θ is the Bragg angle. The microstructure-related integral breadth (i.e. βf) is expressed in radians. In the plot, the reciprocal of the intercept gives an estimate of the apparent size 〈L〉ν of coherently-diffracting domains and the slope is a measure of microstrain ɛ as shown in the following equation: βf ⁎ ¼
1 þ 2ed⁎ hLiv
ð6Þ
The plot can be used to give a qualitative indication of the sample microstructure [19]. However, this method is not appropriate in case of anisotropic peak broadening due to dislocations [20]. In this case, modified Williamson–Hall methods can be used which takes into account the dependence of the strain fields caused by lattice defects on the crystallographic direction [20]. If dislocations are the main source of microstrain, Eq. (6) can be modified so that [21]; βf ⁎ ¼
1=2 1 þ kd⁎ ρC hkl þO d⁎2 C hkl hLiv
ð7Þ
where ρ is the dislocation density, k is a constant depending on the burgers vector b_ and on the effective outer cut-off radius of the dislocations (Re), Chkl is the average contrast factor and the function O _ (d⁎2Chkl) accounts for higher-order terms related to dislocation correlation. It should be noted that the use of average contrast factors is only valid if the dislocations randomly populate the different slip systems [22]. The average contrast factor for cubic materials can be written as:
Fig. 2. The Full Width at Half Maximum (FWHM) versus 2θ for the profiles of as-sprayed and annealed Ni as well as for Si NIST640c standard. Data were collected at room temperature.
The temperature-induced microstructure development was followed by plotting βf⁎ for the (111) and (200) peaks as a function of temperature using in situ XRPD data. 3. Results and discussion 3.1. Temperature-induced microstructure change
_ where _ Ch00 = A and q is −B/A. Ch00 as a function of the elastic constants in cubic materials has been determined [23]. The factor q is related to the nature of the dislocations (screw and/or edge) and has been determined as a function of the elastic constants for pure edge and pure screw dislocations [23]. In the present work, Eq. (9) and the functions _ _ relating Ch00 and q to the elastic constants was used to calculate Chkl. The elastic constants for Ni (c11 = 243.6, c12 = 149.4 and c44 = 119.6 GPa) were taken from [24].
Fig. 1 shows ex situ XRPD data collected from as-sprayed and annealed Ni samples. From the figure, it is observed that the peaks become narrower as a consequence of annealing, indicating growth of coherently-diffracting domains and/or healing of structural defects. The presence of sample broadening becomes evident by plotting the Full Width at Half Maximum (FWHM) versus 2θ for the sample profiles as well as the profiles from a standard reference material (which shows no sample broadening and thus represents the instrumental broadening). If the instrumental broadening is sufficiently small, so that the sample broadening due to size and/or microstrain is a significant part of the total, information about the microstructure can be extracted. Fig. 2 shows FWHM versus 2θ for the standard Si powder as well as plasma-sprayed Ni before and after annealing (ex situ data). From the figure it is clear that the as-sprayed Ni sample shows a significant sample broadening, indicating that size and/or strain effects are present. On the other hand, the sample broadening in the Ni powder is greatly reduced by the thermal treatment. It should be mentioned here that preliminary ex situ XRPD experiments showed that the line broadening of the as-sprayed Ni coating before and after grinding was the same. Hence, it was concluded that the grinding procedure did not affect the sample microstructure.
Fig. 1. Indexed XRPD patterns of plasma-sprayed Ni before (a) and after (b) annealing at 500 °C (ex situ data). The (200) peak for the patterns collected before (a) and after (b) annealing is displayed in the inset, showing a change in broadening due to annealing.
Fig. 3. Classical Williamson–Hall plots for as-sprayed and annealed Ni sample.
C hkl ¼ A þ BH ¼ A þ B
h2 k2 þ h2 l2 þ k2 l2 2
ðh2 þ k2 þ l2 Þ
! ð8Þ
The factors A and B can be calculated from the elastic constants for screw and edge dislocations. Ungár et al. [23] also proposed the following notation; C hkl ¼ C h00 1−qH2
ð9Þ
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Classical Williamson–Hall plots were drawn in order to evaluate the nature of the observed sample broadening (i.e. size and/or strain) in plasma-sprayed Ni. Fig. 3 shows the plots for as-sprayed and annealed Ni powder (ex situ data). For the as-sprayed sample (i) an overall increase in βf⁎ with d⁎ is observed which is indicative of microstrain; (ii) a considerable scatter of the data points is observed which is indicative of anisotropic peak broadening, possibly due to dislocations [20]; (iii) a non-zero intercept is observed, indicating the presence of broadening due to size. Qualitatively, the data scattering is in concert with the one observed for highly deformed Ni powder prepared by ball milling [14,20] and Ni foils produced by electrodeposition [23,25] where the major strain contribution was attributed to dislocations. The calculated isotropic size and microstrain (see Eq. (6)) for the as-sprayed sample were 810 ± 500 Å and 0.24 ± 0.13%, respectively. Corresponding data for the annealed sample were 700 ± 290 Å and 0.05 ± 0.07%. In contrast to the classical Williamson–Hall plot, the so-called modified Williamson–Hall methods take in consideration strain anisotropy due to dislocations. Fig. 4 shows modified Williamson– Hall plots for as-sprayed and annealed Ni sample (see Eq. (7)). The factor q (used to calculate C ̄hkl according to Eq. (9)) was determined assuming screw dislocations as this gave the best fit of the experimental data. A significant decrease in the slope of the linear curve after annealing is observed, which is indicative of a decrease in microstrain (i.e. dislocation density). The intercept of the linear curve yields a Dv of 117 ± 41 nm and 137 ± 33 nm for the as-sprayed and annealed Ni sample, respectively. Although the size is larger after annealing, as could be expected, the difference in size before and after annealing is within the error. The microstructure development of plasma-sprayed Ni powder was studied as a function of temperature using XRPD data collected in situ. Fig. 5 shows the integral breadth of the sample profiles (βf) (111) and (200) as a function of temperature. As discussed in the Experimental section, a multi-line analysis was not possible due to the limited 2θ range of the collected data. In addition, single-line profile analyses were not attempted as such analyses require highquality data [18] and relatively low counting statistics were applied for the in situ data collection. In Fig. 5, it is clearly observed that the integral breadth of both peaks decreases during heating (left side of plot), mainly up to 400 °C. During cooling down to RT (right side of the plot), no further change in sample broadening is observed. 3.2. Microstructure and its impact on electrical resistance In our previous work, as-sprayed Ni showed an irreversible decrease in resistance after thermal treatment at moderate temperature [8]. In fact, resistors deposited by the same laboratory using the same instrumental setup as the Ni investigated here showed a 17 ± 6% decrease in resistance after thermal treatment at 500 °C [8]. Furthermore, it was shown that the resistance measured at RT
Fig. 4. Modified Williamson–Hall plots for as-sprayed and annealed Ni sample.
Fig. 5. Integral breadth (βf) of the sample profiles (111) and (200) as a function of temperature.
continuously decreased after subjecting the resistor to 2 h annealing at various temperatures [8]. Fig. 6 shows the sample integral breadth (βf) of the (111) Ni peak as a function of temperature, together with the electrical resistance (R) measured at RT after 2 h annealing at various temperatures (data taken from ref. [8]). The trend in resistance decreases and diffraction peak narrowing is rather similar, indicating a connection between the two phenomena. The presence of dislocations in APS nickel coatings has been verified by TEM analyses [26] which is not surprising considering the very high level of mechanical stresses measured in thermally sprayed coatings. A close correlation between electrical resistance and structural defects have already been reported for massive quenched materials [10]. In fact, the change in electrical resistance after annealing has become a powerful tool to study healing of structural defects [10]. In addition, Kazi et al. found that the resistance of sputtered nicrome films decreased after annealing which was attributed to removal of defects in the film [27]. Based on previous reports and the results obtained from the present study, it can be postulated that the temperature-induced decrease in resistance of plasma-sprayed Ni observed in our laboratory [8] is likely to be correlated to the change in microstructure of the resistor. Here, it was found that broadening of Ni diffraction profiles in as-sprayed as well as in annealed Ni is due to crystallite size and microstrain (dislocations). However, a considerable decrease in microstrain is observed after annealing whereas the change in size of coherently-diffracting domains is less than the estimated error. Based on these results, it is postulated that the major contribution to the resistance decrease in plasma-sprayed resistor after annealing is related to healing of structure defects.
Fig. 6. Integral breadth (βf) of profile (111) of plasma-sprayed Ni as a function of temperature (in situ data). For comparison, the electrical resistance (R, measured at room temperature) of plasma-sprayed Ni is displayed as a function of annealing temperature (see text for details).
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4. Conclusions
References
In a recent work, it was found that plasma-sprayed Ni resistors exhibited a irreversible decrease in electrical resistance after annealing at moderate temperatures. The phenomenon was suggested to be connected to changes in microstructure of the resistor. In order investigate this further, the microstructure of plasma-sprayed Ni was studied in the present work by X-ray Powder Diffraction and Line Profile analyses in combination with classical and modified Williamson–Hall plots. It was found that annealing of as-sprayed Ni annealing provoked a narrowing of the diffraction peaks. The decrease in sample broadening with temperature was mainly related to healing of the structure, i.e. decrease in the amount of dislocations. The evolution of the microstructure with temperature of plasma-sprayed Ni was also studied using in situ X-ray Powder Diffraction (XRPD) data. A clear correlation was found between temperature-induced decrease in electrical resistance and narrowing of diffraction peaks, suggesting that the two phenomena are related.
[1] L. Pawłowski, The Science and Engineering of Thermal Spray Coatings, Wiley, Chichester, 1995. [2] R. Gadow, A. Killinger, C. Li, Intl. J. Appl. Ceram. Technol. 2 (2005) 493. [3] T. Tong, J. Li, Q. Chen, J.P. Longtin, S. Tankiewicz, S. Sampath, Sens. Actuators A 114 (2004) 102. [4] D. Michels, J. Hadeler, J.H. Lienhard V, Exp. Heat Transf. 11 (1998) 341. [5] M. Prudenziati, G. Cirri, P. Dal Bo, J. Therm. Spray Technol. 15 (2006) 329. [6] J. Longtin, S. Sampath, R.J. Gambino, S. Tankiewicz, R. Greenlaw, IEEE Sens. J. 4 (2004) 118. [7] R.J. Gambino, M. Manivel Raja, S. Sampath, R. Greenlaw, IEEE Sens. J. 4 (2004) 764. [8] M. Prudenziati, M. Lassinantti, J. Gualtieri, Therm. Spray Technol. 17 (2008) 385. [9] J.L. Vosson, W. Kern, Thin Films Processes, Academic Press, New York NY, 1987. [10] R.W. Cahn, P. Haasen, Physical Metallurgy, 3rd ed.Elsevier Sci. Publ., Amsterdam, 1983 part II. [11] G.K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22. [12] B.E. Warren, B.L.J. Averbach, Appl. Phys. 21 (1950) 595. [13] T. Ungár, A. Borbély, Appl. Phys. Lett. 69 (1996) 3173. [14] P. Scardi, M. Leoni, Acta Cryst. A58 (2002) 190. [15] C.A. Reiss, CPD Newslett. 27 (2002) 21. [16] R. Delhez, E.J. Mittemeijer, J. Appl. Cryst. 8 (1975) 609. [17] Th.H. de Keijser, E.J. Mittemeijer, H.C.F. Rozendaal, J. Appl. Cryst. 16 (1983) 309. [18] Th. H. Keijser, J.I. Langford, E.J. Mittemeijer, A.B.P. Vogels, J. Appl. Cryst. 15 (1982) 308. [19] J.I. Langford, A. Boultif, J.P. Auffréic, D. Louër, J. Appl. Cryst. 26 (1993) 22. [20] P. Scardi, M. Leoni, R. Delhez, J. Appl. Cryst. 37 (2004) 381. [21] T. Ungár, S. Ott, P.G. Sanders, A. Borbély, J.R. Weertman, Acta Mater. 46 (1998) 3693. [22] T. Ungár, G. Dichy, Phys. Status Solidi 171 (1999) 425. [23] T. Ungár, I. Dragomir, Á. Révész, A. Borbély, J. Appl. Cryst. 32 (1999) 992. [24] R.F.S. Hearmon, Landolt-Börnstein 1 (1966) 1. [25] T. Ungár, Á. Révés, A. Borbély, J. Appl. Cryst. 31 (1998) 554. [26] S. Sampath, H. Herman, J. Therm. Spray Technol. 5 (1996) 445. [27] I.H. Kazi, P.M. Wild, T.N. Moore, M. Sayer, Thin Solid Films 433 (2003) 337.
Acknowledgements The authors are grateful to Centro Sviluppo Materiali (CSM, Rome, Italy) for financially supporting this project. C. Rossi and M. Tonelli, members of the staff of the Centro Interdipartimentale Grandi Strumenti, Università di Modena e Reggio Emilia, are kindly acknowledged for their help with the XRPD data collection.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Annealing effects on plasma-sprayed Ni: An XRPD study Magdalena Lassinantti Gualtieri a,⁎, Maria Prudenziati a, Alessandro F. Gualtieri b a b
Dipartimento di Fisica, Università di Modena e Reggio Emilia, Via G. Campi 213/A, I-41100 Modena, Italy Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, S. Eufemia 19, I-41100 Modena, Italy
a r t i c l e
i n f o
Article history: Received 13 February 2008 Accepted in revised form 8 September 2008 Available online 15 September 2008 PACS: 61.10.-i 61.72.Lk 81.40.Ef Keywords: (X) Annealing (B) X-ray diffraction (C) Thermal spraying (B) Dislocations (D) Nickel
a b s t r a c t The variation of the size of coherently-diffracting domains and strain due to annealing at moderate temperature (500 °C) has been estimated for plasma-sprayed Ni using X-ray Powder Diffraction (XRPD) and line broadening analysis in conjunction with classical and modified Williamson–Hall methods. It was found that annealing provokes a narrowing of Ni diffraction peaks which was basically associated to a decrease in dislocations present in the as-sprayed material. The evolution of the microstructure with temperature of plasma-sprayed Ni was studied by in situ X-ray Powder Diffraction (XRPD). It was found that the breadth of the Ni profiles continuously decreased with heating up to 500 °C, mainly due to healing of dislocations. These results were used to explain the irreversible decrease in electrical resistance of plasma-sprayed Ni resistors after annealing which was previously observed in our laboratory. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Thermal spray allows for rapid and inexpensive deposition of a wide variety of materials. The feedstock, generally a powder, is forced through a plasma flame and propelled towards a surface. The molten droplets, formed in the plasma flame, rapidly solidify and coalesce to form a coating typically 10–1000 μm thick. Coatings deposited by thermal spray exhibit a complex, anisotropic microstructure with elongated brick-like structures, called “splats,” which are formed from the rapid flattening and solidification of molten droplets of the sprayed material [1]. Thermally sprayed materials are mainly used as protective coatings against wear, heat, corrosion and oxidation. In particular, coatings of Ni and Ni alloys are widely used for reclamation (re-work and repair of damaged peaces) and bond coats. Recently, thermally sprayed metals have been proposed as possible candidates as heaters [2–5] and sensors [6,7]. In these applications, it is of paramount importance to acquire detailed knowledge of the electrical properties (such as resistance and its temperature dependence) of the materials. This recently prompted a thorough investigation of the electrical properties of metal resistors prepared by thermal spray [8] whose outcome evidenced that the electrical resistance of as-sprayed Ni resistors irreversibly decreased after thermal treatment at moderate temperatures. It was suggested that this phenomenon could be related to healing of structural defects. A ⁎ Corresponding author. Tel.: +39 059 2055576; fax: +39 059 2055235. E-mail address: [email protected] (M. Lassinantti Gualtieri). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.09.010
decrease in resistance after annealing has also been reported for thin films [9] and quenched bulk materials [10]. In the former case, the phenomenon was suggested to be related to grain growth [9]. In the latter case, the phenomenon has been explained by temperatureinduced healing of defects [10]. Suggested methods to study materials microstructure in terms of grain size and/or microstrain are X-ray Powder Diffraction (XRPD) and Line Broadening Analysis (LBA). In fact, small dimensions of coherently diffracting domains and the presence of structure defects (i.e. microstrain) result in broadening of the sample profiles. The most common approach to LBA is pattern decomposition, which involves the fitting of analytical reflection profile functions to the various identified Bragg reflections without reference to a crystal structure model. Frequently, the experimental profiles are modeled as Voigtian, i.e. the shape is an intermediate between Lorentzian and Gaussian. The Voigt, the pseudo-Voigt and the Pearson VII functions belong to this category. Profile fitting gives the position, intensity, width and shape of individual reflections. However, the observed profile broadening must be corrected for the contribution from the instrument. In fact, the observed diffraction line profile is the result of the convolution of the instrumental profile (which includes aberrations introduced by the diffractometer and wavelength dispersion) and the sample profile in addition to the background. The instrumental profile is fixed for a particular experimental setup and can be determined using data collected from a standard material with no sample broadening. In the second step of LBA, the information extracted from pattern decomposition is used to estimate the crystallite size and shape as well as the
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M. Lassinantti Gualtieri et al. / Surface & Coatings Technology 203 (2008) 345–349
strain. In many cases, both size and strain effects occur simultaneously. The two effects can be separated on the basis of the different diffractionorder dependence of peak broadening. Two classical methods have evolved during the past five decades: the Williamson–Hall [11] and the Warren–Averbach methods [12]. The first is based on the integral breadths (β) whereas the second is based on the Fourier coefficients of the profiles. Modifications of these methods have been proposed for the specific case of dislocation strain broadening [13]. A recently emerged method for LBA is Whole Powder Pattern Modeling (WPPM) which adopts models based on physical parameters which are directly related to the microstructural properties that influence the shape and width of the diffraction profiles [14]. In this work, the microstructure of as-received and annealed plasma-sprayed Ni was investigated by XRPD and LBA in order to separate the contribution of size and microstrain to the observed line broadening in as-sprayed and annealed plasma-sprayed Ni. Hence, the contribution of size and strain effects to the observed decrease in resistance of plasma-sprayed Ni can be modeled. In addition, the narrowing of the diffraction peaks was followed in temperature using XRPD data collected in situ. These data were compared with previously observed changes in electrical resistance of plasmasprayed Ni [8]. 2. Experimental 2.1. Sample preparation and instrumentation A ca 0.3 mm thick Ni coating was deposited on a steel support by means of Atmospheric Plasma Spray (APS) using standard conditions. The starting powder was Metco 56FNS with a grain size of −45 + 11 μm. Following deposition, the coating was detached from the steel substrate and manually ground in an agate mortar in order to obtain a fine powder which was used for subsequent analyses. X-ray Powder Diffraction data (XRPD) were collected using a θ/θ diffractometer (PANalytical, CuKα radiation), equipped with an Anton Paar HTK 16 resistance heating chamber and a Real Time Multiple Strip (RTMS) detector [15]. Divergence slit (0.25°) and anti-scattering slit (0.25°) as well as a soller slit (0.04 rad) were mounted in the incident beam pathway. The diffracted beam pathway includes a Ni filter, soller slit (0.04 rad) and antiscatter blade (5 mm). An integrated step scan of the RTMS detector of 0.0167°2θ was used. Initial ex situ experiments were performed on as-sprayed and annealed plasma-sprayed Ni in order to confirm the presence of sample broadening and investigate its nature (i.e. size and/or microstrain). Annealing was performed in a furnace using a temperature program equal to the one used for the in situ XRPD experiment (see below). The ex situ XRPD data was collected using a θ/θ scan carried out in the region 40–100°2θ using high counting statistics (300 s/step). The samples for the ex situ XRPD investigation were ground in an agate mortar, and side-loaded in an aluminum sample holder. Data for Si standard (NIST640c) were collected using the same instrumental setup in order to determine the instrumental broadening. Si was chosen as standard as this material possesses peaks of relative high intensity close to the Ni peaks under investigation. Hence, a good estimation of the instrumental broadening in the critical 2θ ranges could be obtained. The microstructure variations of plasma-sprayed Ni were followed in temperature using in situ X-ray Powder Diffraction data (XRPD). Powder of as-sprayed Ni, ground in an agate mortar, was mixed with a Si standard powder (NIST640c) and placed on the Pt heating strip. The silicon standard was added so that the instrumental contribution to the observed profile broadening of Ni (see next paragraph) could be determined at each investigated temperature. However, it was found that the breadth of the profiles of the standard was identical throughout the experiment. Hence, the room temperature (RT) data for the instrumental broadening was
used for all calculations. Data were collected at regular temperature intervals, both during heating up to 500 °C and subsequent cooling down to 20 °C. The heating/cooling rate was 50 °C/min. The acquisition time was 20 s/ step, and the investigated angular range was 40–54°2θ. Prior to the experiment, the temperature of the heating strip was calibrated using known phase transitions/ transformations of standard materials. 2.2. Data evaluation The microstructure of plasma-sprayed Ni was investigated by means of pattern decomposition. Before pattern decomposition, the Kα2 contribution was stripped from the raw data using X'Pert Highscore Plus (PANalytical, version 2.1) and the modified Rachinger method [16] implemented in the software. All peaks of the Si standard were also fitted and the profile parameters were used to obtain the instrumental profile function (i.e. angular dependence of instrumental broadening). The peaks under investigation were modeled with a pseudo-Voigt (pV) analytical profile function using the software ProFit (Philips Electronics N.V., version 1.0c). The peak position, the Full Width at Half Maximum (FWHM), the maximum intensity and the shape parameter (η, also called the mixing parameter) for the pseudo-Voigt function were refined as well as background parameters. The refined values of the integral breadth (β, the width of a rectangle having the same area and height as the line profile) and the mixing parameter (η) were used in a subsequent analysis. The integral breadth of the Lorentzian (βL) and Gaussian (βG) components of the Voigt function corresponding to the pV function can be determined according to the following empirical formulas [17]: βL ¼ 0:017475 þ 1:500484η−0:534156η2 βpV βG ¼ 0:184446 þ 0:812692ð1−0:998497ηÞ1=2 −0:659603η βpV þ 0:445542η2
ð1Þ
ð2Þ
Applying the above equations to the powder pattern collected from the sample, h(2θ), and from the Si standard, g(2θ), the line broadening due to the microstructure of the sample, f(2θ), can be calculated due to the additive property of the breadths of the Lorentzian functions and the squares of the breadths of Gaussian functions: βfL ¼ βhL −βgL
ð3Þ
2 2 g 2 βfG ¼ βhG − βG
ð4Þ
The total integral breadth of the f(2θ) function (βf) corresponding to the given peak was calculated according to the following empirical formula given by de Keijser et al. [18]; βfG βf
1=2 1 1 ¼ − kπ1=2 þ 4 þ k2 π −0:234k expð−2:176kÞ 2 2
where k ¼
ð5Þ
β fL βfG π1=2
A multi-line microstructure analysis was performed on as-sprayed and annealed Ni using ex situ data. Such analyses could not be performed using in situ data as the investigated 2θ range was compromised in order to have a reasonable counting statistics and time resolution. In addition, the (220) Ni profile overlaps with the (331) profile of the Si internal standard and could therefore not be used.
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f
The integral breadth in reciprocal units, βf ⁎ ¼ β cosθ λ , was plotted versus d⁎ ¼ 2 sinθ in a classical Williamson–Hall plot [11], where λ is the λ wavelength (Å) and θ is the Bragg angle. The microstructure-related integral breadth (i.e. βf) is expressed in radians. In the plot, the reciprocal of the intercept gives an estimate of the apparent size 〈L〉ν of coherently-diffracting domains and the slope is a measure of microstrain ɛ as shown in the following equation: βf ⁎ ¼
1 þ 2ed⁎ hLiv
ð6Þ
The plot can be used to give a qualitative indication of the sample microstructure [19]. However, this method is not appropriate in case of anisotropic peak broadening due to dislocations [20]. In this case, modified Williamson–Hall methods can be used which takes into account the dependence of the strain fields caused by lattice defects on the crystallographic direction [20]. If dislocations are the main source of microstrain, Eq. (6) can be modified so that [21]; βf ⁎ ¼
1=2 1 þ kd⁎ ρC hkl þO d⁎2 C hkl hLiv
ð7Þ
where ρ is the dislocation density, k is a constant depending on the burgers vector b_ and on the effective outer cut-off radius of the dislocations (Re), Chkl is the average contrast factor and the function O _ (d⁎2Chkl) accounts for higher-order terms related to dislocation correlation. It should be noted that the use of average contrast factors is only valid if the dislocations randomly populate the different slip systems [22]. The average contrast factor for cubic materials can be written as:
Fig. 2. The Full Width at Half Maximum (FWHM) versus 2θ for the profiles of as-sprayed and annealed Ni as well as for Si NIST640c standard. Data were collected at room temperature.
The temperature-induced microstructure development was followed by plotting βf⁎ for the (111) and (200) peaks as a function of temperature using in situ XRPD data. 3. Results and discussion 3.1. Temperature-induced microstructure change
_ where _ Ch00 = A and q is −B/A. Ch00 as a function of the elastic constants in cubic materials has been determined [23]. The factor q is related to the nature of the dislocations (screw and/or edge) and has been determined as a function of the elastic constants for pure edge and pure screw dislocations [23]. In the present work, Eq. (9) and the functions _ _ relating Ch00 and q to the elastic constants was used to calculate Chkl. The elastic constants for Ni (c11 = 243.6, c12 = 149.4 and c44 = 119.6 GPa) were taken from [24].
Fig. 1 shows ex situ XRPD data collected from as-sprayed and annealed Ni samples. From the figure, it is observed that the peaks become narrower as a consequence of annealing, indicating growth of coherently-diffracting domains and/or healing of structural defects. The presence of sample broadening becomes evident by plotting the Full Width at Half Maximum (FWHM) versus 2θ for the sample profiles as well as the profiles from a standard reference material (which shows no sample broadening and thus represents the instrumental broadening). If the instrumental broadening is sufficiently small, so that the sample broadening due to size and/or microstrain is a significant part of the total, information about the microstructure can be extracted. Fig. 2 shows FWHM versus 2θ for the standard Si powder as well as plasma-sprayed Ni before and after annealing (ex situ data). From the figure it is clear that the as-sprayed Ni sample shows a significant sample broadening, indicating that size and/or strain effects are present. On the other hand, the sample broadening in the Ni powder is greatly reduced by the thermal treatment. It should be mentioned here that preliminary ex situ XRPD experiments showed that the line broadening of the as-sprayed Ni coating before and after grinding was the same. Hence, it was concluded that the grinding procedure did not affect the sample microstructure.
Fig. 1. Indexed XRPD patterns of plasma-sprayed Ni before (a) and after (b) annealing at 500 °C (ex situ data). The (200) peak for the patterns collected before (a) and after (b) annealing is displayed in the inset, showing a change in broadening due to annealing.
Fig. 3. Classical Williamson–Hall plots for as-sprayed and annealed Ni sample.
C hkl ¼ A þ BH ¼ A þ B
h2 k2 þ h2 l2 þ k2 l2 2
ðh2 þ k2 þ l2 Þ
! ð8Þ
The factors A and B can be calculated from the elastic constants for screw and edge dislocations. Ungár et al. [23] also proposed the following notation; C hkl ¼ C h00 1−qH2
ð9Þ
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Classical Williamson–Hall plots were drawn in order to evaluate the nature of the observed sample broadening (i.e. size and/or strain) in plasma-sprayed Ni. Fig. 3 shows the plots for as-sprayed and annealed Ni powder (ex situ data). For the as-sprayed sample (i) an overall increase in βf⁎ with d⁎ is observed which is indicative of microstrain; (ii) a considerable scatter of the data points is observed which is indicative of anisotropic peak broadening, possibly due to dislocations [20]; (iii) a non-zero intercept is observed, indicating the presence of broadening due to size. Qualitatively, the data scattering is in concert with the one observed for highly deformed Ni powder prepared by ball milling [14,20] and Ni foils produced by electrodeposition [23,25] where the major strain contribution was attributed to dislocations. The calculated isotropic size and microstrain (see Eq. (6)) for the as-sprayed sample were 810 ± 500 Å and 0.24 ± 0.13%, respectively. Corresponding data for the annealed sample were 700 ± 290 Å and 0.05 ± 0.07%. In contrast to the classical Williamson–Hall plot, the so-called modified Williamson–Hall methods take in consideration strain anisotropy due to dislocations. Fig. 4 shows modified Williamson– Hall plots for as-sprayed and annealed Ni sample (see Eq. (7)). The factor q (used to calculate C ̄hkl according to Eq. (9)) was determined assuming screw dislocations as this gave the best fit of the experimental data. A significant decrease in the slope of the linear curve after annealing is observed, which is indicative of a decrease in microstrain (i.e. dislocation density). The intercept of the linear curve yields a Dv of 117 ± 41 nm and 137 ± 33 nm for the as-sprayed and annealed Ni sample, respectively. Although the size is larger after annealing, as could be expected, the difference in size before and after annealing is within the error. The microstructure development of plasma-sprayed Ni powder was studied as a function of temperature using XRPD data collected in situ. Fig. 5 shows the integral breadth of the sample profiles (βf) (111) and (200) as a function of temperature. As discussed in the Experimental section, a multi-line analysis was not possible due to the limited 2θ range of the collected data. In addition, single-line profile analyses were not attempted as such analyses require highquality data [18] and relatively low counting statistics were applied for the in situ data collection. In Fig. 5, it is clearly observed that the integral breadth of both peaks decreases during heating (left side of plot), mainly up to 400 °C. During cooling down to RT (right side of the plot), no further change in sample broadening is observed. 3.2. Microstructure and its impact on electrical resistance In our previous work, as-sprayed Ni showed an irreversible decrease in resistance after thermal treatment at moderate temperature [8]. In fact, resistors deposited by the same laboratory using the same instrumental setup as the Ni investigated here showed a 17 ± 6% decrease in resistance after thermal treatment at 500 °C [8]. Furthermore, it was shown that the resistance measured at RT
Fig. 4. Modified Williamson–Hall plots for as-sprayed and annealed Ni sample.
Fig. 5. Integral breadth (βf) of the sample profiles (111) and (200) as a function of temperature.
continuously decreased after subjecting the resistor to 2 h annealing at various temperatures [8]. Fig. 6 shows the sample integral breadth (βf) of the (111) Ni peak as a function of temperature, together with the electrical resistance (R) measured at RT after 2 h annealing at various temperatures (data taken from ref. [8]). The trend in resistance decreases and diffraction peak narrowing is rather similar, indicating a connection between the two phenomena. The presence of dislocations in APS nickel coatings has been verified by TEM analyses [26] which is not surprising considering the very high level of mechanical stresses measured in thermally sprayed coatings. A close correlation between electrical resistance and structural defects have already been reported for massive quenched materials [10]. In fact, the change in electrical resistance after annealing has become a powerful tool to study healing of structural defects [10]. In addition, Kazi et al. found that the resistance of sputtered nicrome films decreased after annealing which was attributed to removal of defects in the film [27]. Based on previous reports and the results obtained from the present study, it can be postulated that the temperature-induced decrease in resistance of plasma-sprayed Ni observed in our laboratory [8] is likely to be correlated to the change in microstructure of the resistor. Here, it was found that broadening of Ni diffraction profiles in as-sprayed as well as in annealed Ni is due to crystallite size and microstrain (dislocations). However, a considerable decrease in microstrain is observed after annealing whereas the change in size of coherently-diffracting domains is less than the estimated error. Based on these results, it is postulated that the major contribution to the resistance decrease in plasma-sprayed resistor after annealing is related to healing of structure defects.
Fig. 6. Integral breadth (βf) of profile (111) of plasma-sprayed Ni as a function of temperature (in situ data). For comparison, the electrical resistance (R, measured at room temperature) of plasma-sprayed Ni is displayed as a function of annealing temperature (see text for details).
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4. Conclusions
References
In a recent work, it was found that plasma-sprayed Ni resistors exhibited a irreversible decrease in electrical resistance after annealing at moderate temperatures. The phenomenon was suggested to be connected to changes in microstructure of the resistor. In order investigate this further, the microstructure of plasma-sprayed Ni was studied in the present work by X-ray Powder Diffraction and Line Profile analyses in combination with classical and modified Williamson–Hall plots. It was found that annealing of as-sprayed Ni annealing provoked a narrowing of the diffraction peaks. The decrease in sample broadening with temperature was mainly related to healing of the structure, i.e. decrease in the amount of dislocations. The evolution of the microstructure with temperature of plasma-sprayed Ni was also studied using in situ X-ray Powder Diffraction (XRPD) data. A clear correlation was found between temperature-induced decrease in electrical resistance and narrowing of diffraction peaks, suggesting that the two phenomena are related.
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Acknowledgements The authors are grateful to Centro Sviluppo Materiali (CSM, Rome, Italy) for financially supporting this project. C. Rossi and M. Tonelli, members of the staff of the Centro Interdipartimentale Grandi Strumenti, Università di Modena e Reggio Emilia, are kindly acknowledged for their help with the XRPD data collection.