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Laser surface nitriding of yttria stabilized tetragonal zirconia
Surface & Coatings Technology 201 (2007) 5865 – 5869 www.elsevier.com/locate/surfcoat
Laser surface nitriding of yttria stabilized tetragonal zirconia Y.P. Kathuria ⁎,1 Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai Mumbai-400076, India Received 29 August 2006; accepted in revised form 23 October 2006 Available online 4 December 2006
Abstract Surface nitriding of yttria (Y2O3) stabilized tetragonal zirconia (t-ZrO2) has been performed using the pulsed Nd-YAG laser irradiation in the nitrogen atmosphere. The results show modifications of the crystallographic structure between the original tetragonal zirconia and the treated one. Nitrogen atoms enter in the zirconia substituting to oxygen atoms in the network. X-ray diffraction and Raman spectra show an efficient transformation of the t-ZrO2 at the surface that exhibit the typical yellow-gold color of ZrN with high hardness and wear resistance. The hardness measured with Vickers indenter bears a high value of about 1409 HV. Nitriding is accomplished by the formation of a few micron thick gradient layer of ZrN structure on the top and a partial transformation of t-ZrO2 into cubic zirconia (c-ZrO2) beneath deep in the bulk due to nitrogen concentration gradient. © 2006 Published by Elsevier B.V. Keywords: Laser; t-ZrO2; ZrN; Raman microscopy
1. Introduction Crystallographic structure of zirconia and its transformation into various phases are of considerable interest [1]. Besides that stabilization of pure zirconia (ZrO2) has been an important issue to conserve its cubic or tetragonal high-temperature phases down to room temperature. Therefore it is usually doped with 2–3 mol% of yttria (Y2O3). Surface treatment of such materials i.e. yttria stabilized tetragonal zirconia (t-ZrO2) by means of high energetic beam in the reactive/dissociative atmosphere of nitrogen is an attractive technique to enhance the surface features such as corrosion, wear resistance and hardness. Nitrogen being very stable and inert under normal conditions reacts with such elements forcing a tangible compound such as nitrides (ZrN). They possess interesting properties e.g. high melting point, high wear and corrosion resistance and are much beneficial to tool industries especially in coating applications. Besides its use in decorative marking, it can also find application in superconducting devices such as Josephson's
⁎ Tel.: +22 2576 7615; fax: +22 2572 3480. E-mail address: [email protected]. 1 Part of this work was done when the author was with Ritsumeikan University and Laser X Co. Ltd. Japan. 0257-8972/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.10.041
junction (ZrN/Zr3N4/ZrN) etc., and as a diffusion barrier in IC technology due to lowest electrical resistivity (13.6 μΩ cm) and better thermal stability (ΔH = − 87.3 kcal/mol). Several techniques such as plasma nitriding, physical vapor deposition (PVD), sol–gel, magnetron sputtering or ion implantation technique etc., are in use to modify the surface of zirconia so as to develop a nitride coat on it or any other substrate material [2–7]. Besides this, various kinds of lasers have also been used effectively to modify the surface characteristics of various materials for nitriding in the reactive atmosphere [8–10]. However, using the pulsed Nd-YAG laser has emerged as an attractive technique [11] in nitriding the tetragonal zirconia (t-ZrO2) to a stable nitride (ZrN) phase, demonstrating the efficient nitrogen incorporation and formation of thick nitrided layer with superior tribological properties. On the contrary [2], in this case hydrogen is not required to remove oxygen, but it is transformed via dissociative process: 2ZrO2 + N2 = 2ZrN + 2O2. The technique is relatively much simple and works under ambient conditions. The present work deals with such a study and explores the possibility of analyzing the structural properties of the nitrided sample with X-ray diffraction and Raman microscopy. It opens new challenges to obtain structural information of the nitrided (ZrN) and bare tetragonal zirconia (t-ZrO2) sample. Example of the decorative marking applications is well illustrated.
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2. Experimental
Table 1 Laser processing parameters
The laser-nitriding experiments were performed with a pulsed Nd-YAG laser (Lumonics: Model: JK701) provided with beam bending optics, coaxial nozzle etc. [Fig. 1]. The beam was focused by a lens of focal length f = 80 mm so as to produce a spot size of 300–500 μm onto the specimen plate. The typical process parameters are given in Table 1. Initially, the study was carried out for decorative marking of stabilized tetragonal zirconia doped with 3 mol% of yttria, and was procured commercially in the form of small square plate (10 mm × 10 mm) having a thickness d = 2 mm. The plates were cleaned ultrasonically with acetone. While laser processing, each plate was mounted in turn on a fixture. Nitrogen gas was used coaxially to assist decorative marking, nitriding as well as in cooling in order to reduce/avoid thermal damage to the specimen. Prior to marking/ nitriding, CAD data of the desired letter/word i.e., “LASER” is generated which is then fed to the laser processing system. Laser beam is then scanned to mark/nitride the desired word/letter/area onto the t-ZrO2 plate. Fig. 2 shows the top and a partial magnified view of the laser-scanned sample with about 10 μm thick gradient layer of ZrN structure. It exhibits a few microcracks, possibly due to rapid cooling and or due to stress generated in the treated sample. However, these cracks can be reduced by optimized process parameters such as laser energy, pulse width, pulse repetition rate and beam dwell time etc. Finally the scanned and non-scanned areas are characterized for the X-ray diffraction (XRD), Raman spectra and microhardness respectively. The X-ray diffraction (XRD) patterns of the yttria stabilized tetragonal zirconia (t-ZrO2) before and after the laser nitriding were obtained via conventional powder diffractometry (Model: EXPERT-PRO, PANanalytical Philips Research Laboratory, Eindhoven, the Netherlands) using Cu-Kα incident radiation (at a wavelength of λ = 1.54056 Å), and a take-off angle of 3°. The generator settings were 40 kV and 30 mA. The diffraction data were collected from nitrided surface (character E) as well as from
Wavelength Average power Pulse repetition rate Pulse width Processing speed Energy Assist gas N2 Gas flow rate
Fig. 1. Block diagram of Nd-YAG laser marking/nitriding of yttria stabilized tetragonal zirconia (t-ZrO2).
(λ) = 1.06 μm (P) = 50 W (PRF) = 100 Hz (tp) = 0.5 ms (V) = 20 mm/min (E) = 0.5 J (Pressure) = 5.5 kg/cm2 =3.5 l/min
untreated white surface over a 2θ range of 10–100°, with a step width of 0.02° and a counting time of 5 s per step. A Confocal micro Raman Spectrometer (Model: Jobin-Yvon HR800 UV) with Olympus microscopic attachment was used in backscatter mode to obtain Raman spectra of yttria stabilized tetragonal zirconia and subject to laser nitriding. Argon ion laser (20 mW) operating at 514 nm was used for excitation of Raman signals. Power of 2–3 mW was made incident onto the samples in a few μm diameter spots through a standard 10× and 20× microscope objectives. A Leco (Model: LM300AT) microhardness testing machine fitted with the Vicker indenter was used to measure the microhardness at various locations on the laser treated surface (character E) and untreated white surface. It resulted an average value of 1409 HV for the scanned surface i.e. ZrN and 1219 HV for the un-scanned surface i.e. t-ZrO2. 3. Results and discussion 3.1. X-ray diffraction analysis Fig. 3 shows the experimental X-ray diffraction patterns of the yttria stabilized tetragonal zirconia (t-ZrO2) before and after the laser-nitriding process. The results show modifications of the crystallographic structure between the original tetragonal zirconia and the treated one. Firstly, the new diffraction lines [c-ZrN(200)] that appear on the nitrided sample of Fig. 3B show the characteristic NaCl-type structure indicating the formation of ZrN. Secondly, one observe small changes in the t-ZrO2 structure after laser nitriding. As can be seen, in the doublet lines such as t-(0 0 2) and t-(2 0 0), t-(2 0 2) and t-(2 2 0) or t-(1 1 3) and t-(3 1 1), the lowest diffraction angle lines decreases [2]. The diminishing of these lines indicates that the nitriding process also transforms the tetragonal structure of zirconia into cubic (c-ZrO2), where nitrogen acts as a stabilizer that adds to yttria to retain the cubic form of ZrO2. This structure is usually formed beneath deep in the bulk, when there is no sufficient nitrogen inside the zirconia to transform into the zirconium nitride structure [12]. This may be due to the nitrogen concentration gradient along the z-axis. The crystallographic modifications involved by the nitriding process on t-ZrO2 are therefore changing from t-ZrO2 → c-ZrO2 → ZrN with increasing nitrogen concentration. From a thermodynamic point of view, this can be understood by the fact that ZrO2 is more stable than ZrN (Gibbs free energy for standard conditions: ΔGf0 (ZrN) ∼
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Fig. 2. A) Photographic view of laser scan logo: LASER (ZrN: yellow-gold); White un-scanned area is yttria stabilized tetragonal zirconia (t-ZrO2). B) A partial and magnified view of scan area from the above character — E (ZrN: yellow-gold).
− 315 kJ/mol and ΔGf0 (ZrO2) ∼ − 1020 kJ/mol [13]); so to create ZrN, one has to remove the oxygen through nitrogen incorporation via the dissociative process: 2ZrO2 + N2 = 2ZrN + 2O2.
The operating mechanism in the laser surface nitriding process is the formation of vacancies in the unit-cell of zirconia that are induced by a high-temperature treatment in the presence of nitrogen. This element stabilizes the cubic phase of
Fig. 3. A) X-ray diffraction spectra of non-treated yttria stabilized tetragonal zirconia (t-ZrO2). B) X-ray diffraction spectra of laser treated yttria stabilized tetragonal zirconia (t-ZrO2) i.e. ZrN. C) Inset of XRD peak positions of non-treated yttria stabilized tetragonal zirconia (t-ZrO2). D) Inset of XRD peak positions of laser treated yttria stabilized tetragonal zirconia (t-ZrO2) i.e. ZrN.
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sub-lattice due to charge compensation (N3− versus O2− ), which is also believed to contribute further alteration in the dspacing or the lattice parameter [5]. This in turn leads to the lattice microstrains and may result in a residual stress build up during nitriding. 3.2. Raman spectra analysis
Fig. 4. A) Raman spectra of non-treated yttria stabilized tetragonal zirconia (t-ZrO2). B) Raman spectra of laser treated yttria stabilized tetragonal zirconia (t-ZrO2) i.e. ZrN.
zirconia (c-ZrO2) by virtue of a partial substitution of oxygen ions by nitrogen ions, causing the desired vacancies [14]. As the binding energy of ZrN coincides well with Y2O3 and ZrO2, there is no obvious difference in shape and peak position [4]. However, there is a slight lateral shift for the curves with nitriding. The peak shifting after laser nitriding suggests the modification of the lattice parameters. The X-ray diffraction pattern after the nitriding process [Fig. 3D] exhibits a change in peaks shifting with respect to the spectrum of the nonnitrided sample [Fig. 3C]. This shift in peak is associated with alteration in d-spacing or lattice parameter [3,5,7]. Interestingly cubic line [c-ZrN(111)] is shifted to higher d111 value of 2.6592 Å with a cubic lattice parameter of 4.6057 Å which have a characteristic feature close to Zr3N4 [15,16]. The transformations that take place during nitriding as well as the nitrogen concentration gradient may cause cell or lattice dilation. In this sense, it should be recalled that nitrogen in yttria stabilized tetragonal zirconia (t-ZrO2) forms nitrogen ions which substitute oxygen ones [14,17]. The alteration in the d-spacing or lattice parameter and hence cell dilation would indicate that N atoms enter in the zirconia substituting to the O atoms in the network [3]. As the ionic size of O 2− (140 pm) is less than that of N3− (171 pm), the substitution of O by N leads to creation of additional vacancies in the anion
Fig. 4A shows the Raman spectra of an yttria stabilized tetragonal zirconia (t-ZrO2) sample well away from the nitrided surface and corresponds to well-known Raman spectra of the tetragonal zirconia as reported by several authors [1], whereas a typical Raman spectrum from ZrN exhibits two pronounced bands [Fig. 4B], which are related to the acoustic part and the optical part of the phonon spectrum respectively [18,19]. ZrN has a NaCl-type cubic structure in which N atoms occupy all the four octahedral sites of the f.c.c. metal lattice. It is highly conductive owing to finite density of states at the Fermi level from residual electrons contributed by each atom in the metal d band [20] and it has no first order Raman spectra, therefore one expects to observe only broad spectral features due to secondorder processes. From the Raman spectra of nitrided sample i.e. ZrN, it is observed that the dispersion curves lead to a group of ‘lines’ (bands) due to acoustic transitions in the 150–260 cm− 1 region (TA: Transverse Acoustic and LA: Longitudinal Acoustic) and another set of lines due to optic modes in the 450–750 cm− 1 region (TO: Transverse Optic and LO: Longitudinal Optic) [6]. As can be seen from Fig. 4B, transverse and longitudinal branches for Zr vibrations in the acoustic range (178, 233 cm− 1) as well as higher frequency nitrogen band due to optic modes (499 cm− 1) are well resolved [19]. The bands due to acoustic modes appear to be more intense than those of the optic modes, which suggest that the nitrogen ions must be contributing in the acoustic range (second-order spectra) of the acoustic modes. The low frequency scattering is caused by acoustic phonons, and the high frequency scattering is due to optical phonons. The two peaks in the acoustic range as well as the high frequency optical branch are in close agreement with phonon density of states obtained from neutron scattering data in ZrN [19,21], and higher order, frequency spectral density around 700 cm− 1 arises via second-order transitions (A + O, 2O etc.). Clearly, there is a high probability of strong overlap between the optic modes of the crystalline lattice and overtones [6]. 4. Conclusion The analysis of yttria stabilized tetragonal zirconia (t-ZrO2) substrate in the nitrogen atmosphere by means of pulsed Nd-YAG laser has revealed the formation of nitrides (ZrN), disclosing its correlation among surface morphology, crystallographic structure and degree of laser spot overlap to achieve good surface quality and high surface hardness. The nitriding was achieved effectively at the surface with a yellow-gold color of ZrN. X-ray diffraction and Raman spectroscopy have been used to study the effect of laser nitriding in yttria stabilized zirconia. The X-ray diffraction results evidenced the alteration and shift in peaks as well as
Y.P. Kathuria / Surface & Coatings Technology 201 (2007) 5865–5869
change in d-spacing or lattice parameter of the nitrided surface, whereas the Raman spectra revealed the two characteristic bands related to acoustic part of low frequency and optical part at high frequency values. Finally it was shown that it has a good application in decorative marking. References [1] Gianluca Deghenghi, Tai Joo Chung, Valter Serg, J. Am. Ceram. Soc. 86 (1) (Jan. 2003) 169. [2] T. Delachaux, Ch. Hollenstein, F. Levy, C. Verdon, Thin Solid Films 425 (2003) 113. [3] R. Caruso, B.J. Gomez, O. de Sanctis, J. Feugeas, A. Diaz-Parralejo, F. Sanchez-Bajo, Thin Solids Films 468 (1-2) (1 December 2004) 142. [4] Q.G. Zhou, X.D. Bai, X.Y. Xue, Y.H. Ling, X.w. Chen, j. Xu, D.R. Wang, Vacuum 76 (2004) 517. [5] A.L. Ortiz, A. Diaz-Parralejo, O. Borrero-Lopez, F. Guiberteau, Appl. Surf. Sci. 252 (2006) 6018. [6] C.P. Constable, J. Yarwood, W.D. Muenz, Surf. Coat. Technol. 116–119 (September 1999) 155. [7] H.M. Benia, M. Guemmaz, G. Schmerber, A. Mosser, J.C. Parlebas, Appl. Surf. Sci. 200 (2002) 231. [8] A.L. Thomann, E. Sicard, C. Boulmer-Leborgne, C. Vivien, J. Hermann, C. Andreazza-Vignolle, P. Andreazz, Surf. Coat. Technol. 200 (1997) 448.
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[9] E. Carpene, M. Shinn, P. Schaaf, Appl. Surf. Sci. 247 (2005) 307. [10] P. Schaaf, Prog. Mater. Sci. 47 (2002) 1. [11] S. Fujitsu, M. Sawai, K. Kawamura, H. Hosono, MRS Proc. 397 (1996) 561. [12] Y. Chang, D.F. Thompson, J. Amer. Ceram. Soc. 76 (3) (1993) 683. [13] R.C. West (Ed.), Handbook of chemistry and physics, CRC Press Inc., Ohio U. S. A., 1977–1978, p. 1977. [14] N. Clausen, R. Wagner, L.J. Gauckler, G. Petzow, J. Amer. Ceram. Soc. 61 (1978) 369. [15] Jinwang Li, Dmytro Dzivenko, Andreas Zerr, Claudia Fasel, Yanping Zhou, Ralf Riedel, Z. Anorg. Allg. Chem. 631 (2005) 1449. [16] K. Schwarz, A.R. Williams, J.J. Cuomo, J.H.E. Harper, H.T.G. Hentzell, Phys. Rev. B 32 (15 December 1995) 8312. [17] A. Feder, J. Alcala, L. Llanes, M. Anglada, J. Eur. Ceram. Soc. 23 (15) (2003) 2955. [18] A. Cassinese, M. Iavarone, R. Vaglio, M. Grimsditch, S. Uran, Phys. Rev. B 62 (2000) 13915. [19] Xiao Jia Chen, Viktor V. Struzhkin, Simon Kung, Ho-kwang Mao, Russell J. Hemley, Axel Norlund Christensen, Physical Rev. B 70 (2004) 014501. [20] Manish Chhowalla, H. Emrah Unalan, Nature Materials 4 (April 2005) 317. [21] W. Spengler, R. Kaiser, Solid State Commun. 18 (1976) 881.
Laser surface nitriding of yttria stabilized tetragonal zirconia Y.P. Kathuria ⁎,1 Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai Mumbai-400076, India Received 29 August 2006; accepted in revised form 23 October 2006 Available online 4 December 2006
Abstract Surface nitriding of yttria (Y2O3) stabilized tetragonal zirconia (t-ZrO2) has been performed using the pulsed Nd-YAG laser irradiation in the nitrogen atmosphere. The results show modifications of the crystallographic structure between the original tetragonal zirconia and the treated one. Nitrogen atoms enter in the zirconia substituting to oxygen atoms in the network. X-ray diffraction and Raman spectra show an efficient transformation of the t-ZrO2 at the surface that exhibit the typical yellow-gold color of ZrN with high hardness and wear resistance. The hardness measured with Vickers indenter bears a high value of about 1409 HV. Nitriding is accomplished by the formation of a few micron thick gradient layer of ZrN structure on the top and a partial transformation of t-ZrO2 into cubic zirconia (c-ZrO2) beneath deep in the bulk due to nitrogen concentration gradient. © 2006 Published by Elsevier B.V. Keywords: Laser; t-ZrO2; ZrN; Raman microscopy
1. Introduction Crystallographic structure of zirconia and its transformation into various phases are of considerable interest [1]. Besides that stabilization of pure zirconia (ZrO2) has been an important issue to conserve its cubic or tetragonal high-temperature phases down to room temperature. Therefore it is usually doped with 2–3 mol% of yttria (Y2O3). Surface treatment of such materials i.e. yttria stabilized tetragonal zirconia (t-ZrO2) by means of high energetic beam in the reactive/dissociative atmosphere of nitrogen is an attractive technique to enhance the surface features such as corrosion, wear resistance and hardness. Nitrogen being very stable and inert under normal conditions reacts with such elements forcing a tangible compound such as nitrides (ZrN). They possess interesting properties e.g. high melting point, high wear and corrosion resistance and are much beneficial to tool industries especially in coating applications. Besides its use in decorative marking, it can also find application in superconducting devices such as Josephson's
⁎ Tel.: +22 2576 7615; fax: +22 2572 3480. E-mail address: [email protected]. 1 Part of this work was done when the author was with Ritsumeikan University and Laser X Co. Ltd. Japan. 0257-8972/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.10.041
junction (ZrN/Zr3N4/ZrN) etc., and as a diffusion barrier in IC technology due to lowest electrical resistivity (13.6 μΩ cm) and better thermal stability (ΔH = − 87.3 kcal/mol). Several techniques such as plasma nitriding, physical vapor deposition (PVD), sol–gel, magnetron sputtering or ion implantation technique etc., are in use to modify the surface of zirconia so as to develop a nitride coat on it or any other substrate material [2–7]. Besides this, various kinds of lasers have also been used effectively to modify the surface characteristics of various materials for nitriding in the reactive atmosphere [8–10]. However, using the pulsed Nd-YAG laser has emerged as an attractive technique [11] in nitriding the tetragonal zirconia (t-ZrO2) to a stable nitride (ZrN) phase, demonstrating the efficient nitrogen incorporation and formation of thick nitrided layer with superior tribological properties. On the contrary [2], in this case hydrogen is not required to remove oxygen, but it is transformed via dissociative process: 2ZrO2 + N2 = 2ZrN + 2O2. The technique is relatively much simple and works under ambient conditions. The present work deals with such a study and explores the possibility of analyzing the structural properties of the nitrided sample with X-ray diffraction and Raman microscopy. It opens new challenges to obtain structural information of the nitrided (ZrN) and bare tetragonal zirconia (t-ZrO2) sample. Example of the decorative marking applications is well illustrated.
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2. Experimental
Table 1 Laser processing parameters
The laser-nitriding experiments were performed with a pulsed Nd-YAG laser (Lumonics: Model: JK701) provided with beam bending optics, coaxial nozzle etc. [Fig. 1]. The beam was focused by a lens of focal length f = 80 mm so as to produce a spot size of 300–500 μm onto the specimen plate. The typical process parameters are given in Table 1. Initially, the study was carried out for decorative marking of stabilized tetragonal zirconia doped with 3 mol% of yttria, and was procured commercially in the form of small square plate (10 mm × 10 mm) having a thickness d = 2 mm. The plates were cleaned ultrasonically with acetone. While laser processing, each plate was mounted in turn on a fixture. Nitrogen gas was used coaxially to assist decorative marking, nitriding as well as in cooling in order to reduce/avoid thermal damage to the specimen. Prior to marking/ nitriding, CAD data of the desired letter/word i.e., “LASER” is generated which is then fed to the laser processing system. Laser beam is then scanned to mark/nitride the desired word/letter/area onto the t-ZrO2 plate. Fig. 2 shows the top and a partial magnified view of the laser-scanned sample with about 10 μm thick gradient layer of ZrN structure. It exhibits a few microcracks, possibly due to rapid cooling and or due to stress generated in the treated sample. However, these cracks can be reduced by optimized process parameters such as laser energy, pulse width, pulse repetition rate and beam dwell time etc. Finally the scanned and non-scanned areas are characterized for the X-ray diffraction (XRD), Raman spectra and microhardness respectively. The X-ray diffraction (XRD) patterns of the yttria stabilized tetragonal zirconia (t-ZrO2) before and after the laser nitriding were obtained via conventional powder diffractometry (Model: EXPERT-PRO, PANanalytical Philips Research Laboratory, Eindhoven, the Netherlands) using Cu-Kα incident radiation (at a wavelength of λ = 1.54056 Å), and a take-off angle of 3°. The generator settings were 40 kV and 30 mA. The diffraction data were collected from nitrided surface (character E) as well as from
Wavelength Average power Pulse repetition rate Pulse width Processing speed Energy Assist gas N2 Gas flow rate
Fig. 1. Block diagram of Nd-YAG laser marking/nitriding of yttria stabilized tetragonal zirconia (t-ZrO2).
(λ) = 1.06 μm (P) = 50 W (PRF) = 100 Hz (tp) = 0.5 ms (V) = 20 mm/min (E) = 0.5 J (Pressure) = 5.5 kg/cm2 =3.5 l/min
untreated white surface over a 2θ range of 10–100°, with a step width of 0.02° and a counting time of 5 s per step. A Confocal micro Raman Spectrometer (Model: Jobin-Yvon HR800 UV) with Olympus microscopic attachment was used in backscatter mode to obtain Raman spectra of yttria stabilized tetragonal zirconia and subject to laser nitriding. Argon ion laser (20 mW) operating at 514 nm was used for excitation of Raman signals. Power of 2–3 mW was made incident onto the samples in a few μm diameter spots through a standard 10× and 20× microscope objectives. A Leco (Model: LM300AT) microhardness testing machine fitted with the Vicker indenter was used to measure the microhardness at various locations on the laser treated surface (character E) and untreated white surface. It resulted an average value of 1409 HV for the scanned surface i.e. ZrN and 1219 HV for the un-scanned surface i.e. t-ZrO2. 3. Results and discussion 3.1. X-ray diffraction analysis Fig. 3 shows the experimental X-ray diffraction patterns of the yttria stabilized tetragonal zirconia (t-ZrO2) before and after the laser-nitriding process. The results show modifications of the crystallographic structure between the original tetragonal zirconia and the treated one. Firstly, the new diffraction lines [c-ZrN(200)] that appear on the nitrided sample of Fig. 3B show the characteristic NaCl-type structure indicating the formation of ZrN. Secondly, one observe small changes in the t-ZrO2 structure after laser nitriding. As can be seen, in the doublet lines such as t-(0 0 2) and t-(2 0 0), t-(2 0 2) and t-(2 2 0) or t-(1 1 3) and t-(3 1 1), the lowest diffraction angle lines decreases [2]. The diminishing of these lines indicates that the nitriding process also transforms the tetragonal structure of zirconia into cubic (c-ZrO2), where nitrogen acts as a stabilizer that adds to yttria to retain the cubic form of ZrO2. This structure is usually formed beneath deep in the bulk, when there is no sufficient nitrogen inside the zirconia to transform into the zirconium nitride structure [12]. This may be due to the nitrogen concentration gradient along the z-axis. The crystallographic modifications involved by the nitriding process on t-ZrO2 are therefore changing from t-ZrO2 → c-ZrO2 → ZrN with increasing nitrogen concentration. From a thermodynamic point of view, this can be understood by the fact that ZrO2 is more stable than ZrN (Gibbs free energy for standard conditions: ΔGf0 (ZrN) ∼
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Fig. 2. A) Photographic view of laser scan logo: LASER (ZrN: yellow-gold); White un-scanned area is yttria stabilized tetragonal zirconia (t-ZrO2). B) A partial and magnified view of scan area from the above character — E (ZrN: yellow-gold).
− 315 kJ/mol and ΔGf0 (ZrO2) ∼ − 1020 kJ/mol [13]); so to create ZrN, one has to remove the oxygen through nitrogen incorporation via the dissociative process: 2ZrO2 + N2 = 2ZrN + 2O2.
The operating mechanism in the laser surface nitriding process is the formation of vacancies in the unit-cell of zirconia that are induced by a high-temperature treatment in the presence of nitrogen. This element stabilizes the cubic phase of
Fig. 3. A) X-ray diffraction spectra of non-treated yttria stabilized tetragonal zirconia (t-ZrO2). B) X-ray diffraction spectra of laser treated yttria stabilized tetragonal zirconia (t-ZrO2) i.e. ZrN. C) Inset of XRD peak positions of non-treated yttria stabilized tetragonal zirconia (t-ZrO2). D) Inset of XRD peak positions of laser treated yttria stabilized tetragonal zirconia (t-ZrO2) i.e. ZrN.
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sub-lattice due to charge compensation (N3− versus O2− ), which is also believed to contribute further alteration in the dspacing or the lattice parameter [5]. This in turn leads to the lattice microstrains and may result in a residual stress build up during nitriding. 3.2. Raman spectra analysis
Fig. 4. A) Raman spectra of non-treated yttria stabilized tetragonal zirconia (t-ZrO2). B) Raman spectra of laser treated yttria stabilized tetragonal zirconia (t-ZrO2) i.e. ZrN.
zirconia (c-ZrO2) by virtue of a partial substitution of oxygen ions by nitrogen ions, causing the desired vacancies [14]. As the binding energy of ZrN coincides well with Y2O3 and ZrO2, there is no obvious difference in shape and peak position [4]. However, there is a slight lateral shift for the curves with nitriding. The peak shifting after laser nitriding suggests the modification of the lattice parameters. The X-ray diffraction pattern after the nitriding process [Fig. 3D] exhibits a change in peaks shifting with respect to the spectrum of the nonnitrided sample [Fig. 3C]. This shift in peak is associated with alteration in d-spacing or lattice parameter [3,5,7]. Interestingly cubic line [c-ZrN(111)] is shifted to higher d111 value of 2.6592 Å with a cubic lattice parameter of 4.6057 Å which have a characteristic feature close to Zr3N4 [15,16]. The transformations that take place during nitriding as well as the nitrogen concentration gradient may cause cell or lattice dilation. In this sense, it should be recalled that nitrogen in yttria stabilized tetragonal zirconia (t-ZrO2) forms nitrogen ions which substitute oxygen ones [14,17]. The alteration in the d-spacing or lattice parameter and hence cell dilation would indicate that N atoms enter in the zirconia substituting to the O atoms in the network [3]. As the ionic size of O 2− (140 pm) is less than that of N3− (171 pm), the substitution of O by N leads to creation of additional vacancies in the anion
Fig. 4A shows the Raman spectra of an yttria stabilized tetragonal zirconia (t-ZrO2) sample well away from the nitrided surface and corresponds to well-known Raman spectra of the tetragonal zirconia as reported by several authors [1], whereas a typical Raman spectrum from ZrN exhibits two pronounced bands [Fig. 4B], which are related to the acoustic part and the optical part of the phonon spectrum respectively [18,19]. ZrN has a NaCl-type cubic structure in which N atoms occupy all the four octahedral sites of the f.c.c. metal lattice. It is highly conductive owing to finite density of states at the Fermi level from residual electrons contributed by each atom in the metal d band [20] and it has no first order Raman spectra, therefore one expects to observe only broad spectral features due to secondorder processes. From the Raman spectra of nitrided sample i.e. ZrN, it is observed that the dispersion curves lead to a group of ‘lines’ (bands) due to acoustic transitions in the 150–260 cm− 1 region (TA: Transverse Acoustic and LA: Longitudinal Acoustic) and another set of lines due to optic modes in the 450–750 cm− 1 region (TO: Transverse Optic and LO: Longitudinal Optic) [6]. As can be seen from Fig. 4B, transverse and longitudinal branches for Zr vibrations in the acoustic range (178, 233 cm− 1) as well as higher frequency nitrogen band due to optic modes (499 cm− 1) are well resolved [19]. The bands due to acoustic modes appear to be more intense than those of the optic modes, which suggest that the nitrogen ions must be contributing in the acoustic range (second-order spectra) of the acoustic modes. The low frequency scattering is caused by acoustic phonons, and the high frequency scattering is due to optical phonons. The two peaks in the acoustic range as well as the high frequency optical branch are in close agreement with phonon density of states obtained from neutron scattering data in ZrN [19,21], and higher order, frequency spectral density around 700 cm− 1 arises via second-order transitions (A + O, 2O etc.). Clearly, there is a high probability of strong overlap between the optic modes of the crystalline lattice and overtones [6]. 4. Conclusion The analysis of yttria stabilized tetragonal zirconia (t-ZrO2) substrate in the nitrogen atmosphere by means of pulsed Nd-YAG laser has revealed the formation of nitrides (ZrN), disclosing its correlation among surface morphology, crystallographic structure and degree of laser spot overlap to achieve good surface quality and high surface hardness. The nitriding was achieved effectively at the surface with a yellow-gold color of ZrN. X-ray diffraction and Raman spectroscopy have been used to study the effect of laser nitriding in yttria stabilized zirconia. The X-ray diffraction results evidenced the alteration and shift in peaks as well as
Y.P. Kathuria / Surface & Coatings Technology 201 (2007) 5865–5869
change in d-spacing or lattice parameter of the nitrided surface, whereas the Raman spectra revealed the two characteristic bands related to acoustic part of low frequency and optical part at high frequency values. Finally it was shown that it has a good application in decorative marking. References [1] Gianluca Deghenghi, Tai Joo Chung, Valter Serg, J. Am. Ceram. Soc. 86 (1) (Jan. 2003) 169. [2] T. Delachaux, Ch. Hollenstein, F. Levy, C. Verdon, Thin Solid Films 425 (2003) 113. [3] R. Caruso, B.J. Gomez, O. de Sanctis, J. Feugeas, A. Diaz-Parralejo, F. Sanchez-Bajo, Thin Solids Films 468 (1-2) (1 December 2004) 142. [4] Q.G. Zhou, X.D. Bai, X.Y. Xue, Y.H. Ling, X.w. Chen, j. Xu, D.R. Wang, Vacuum 76 (2004) 517. [5] A.L. Ortiz, A. Diaz-Parralejo, O. Borrero-Lopez, F. Guiberteau, Appl. Surf. Sci. 252 (2006) 6018. [6] C.P. Constable, J. Yarwood, W.D. Muenz, Surf. Coat. Technol. 116–119 (September 1999) 155. [7] H.M. Benia, M. Guemmaz, G. Schmerber, A. Mosser, J.C. Parlebas, Appl. Surf. Sci. 200 (2002) 231. [8] A.L. Thomann, E. Sicard, C. Boulmer-Leborgne, C. Vivien, J. Hermann, C. Andreazza-Vignolle, P. Andreazz, Surf. Coat. Technol. 200 (1997) 448.
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