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Structure and mechanical properties of silica doped zirconia thin films
Thin Solid Films 527 (2013) 200–204
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Structure and mechanical properties of silica doped zirconia thin films Ina Uhlmann a,⁎, Dominik Hawelka b, Erwin Hildebrandt a, Jens Pradella c, Jürgen Rödel a a b c
Institute of Materials Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany Fraunhofer Institute for Laser Technology ILT, 52074 Aachen, Germany Merck KGaA Darmstadt, 64293 Darmstadt, Germany
a r t i c l e
i n f o
Article history: Received 19 May 2011 Received in revised form 3 August 2012 Accepted 3 August 2012 Available online 11 August 2012 Keywords: Sol–gel ZrO2 SiO2 Wear Hardness Crystal structure Micro abrasion Ball cratering
a b s t r a c t Sol–gel based wear resistant coatings are presented as an alternative to existing vapor deposition coatings. The films consist of zirconia which has been doped with 8 wt.% silica. Crack-free single as well as multilayer coatings with thicknesses of 80 and 150 nm, respectively, could be produced after sintering at 1000 °C. The evolution of layer thickness, optical, chemical and mechanical properties during film annealing was investigated by ellipsometry, scanning electron microscopy, thermal gravimetric analysis, Fourier transform infrared spectroscopy, X-ray diffraction, nanoindentation and micro-abrasion. Micro-abrasion has been established as an easy and powerful tool to achieve first comparative abrasion data which could be correlated to hardness, Young's modulus and structure of the films. Above 600 °C a tetragonal, oxide coating with a Young's modulus ranging from 80 to 90 GPa, a hardness from 7 to 8 GPa and an increased abrasion resistance was obtained. The film density reached 4.64 g/cm3 with the mean refractive index n550 nm lying between 1.88 and 1.93. © 2012 Published by Elsevier B.V.
1. Introduction Reduction of energy and material losses in industrial processes due to friction and wear becomes more and more important due to economical and environmental reasons. Rising prices and shortage of raw materials require lifetime enhancement of machine and engine parts [1]. Such parts often are surface modified and protected by wear resistant coatings, such as chemical and physical vapor deposition coatings [2–5]. They show excellent mechanical properties, but are very expensive and not feasible for in-line applications. Here, sol–gel coatings could be a promising alternative [6,7]. The sol–gel process avoids costly ultra high vacuum technology as it is a wet chemistry method and can be easily implemented in a production line. A huge variety of metal oxides can be produced in this manner. Especially sol–gel derived zirconia coatings are already used as thermal barriers, corrosion and wear resistant coatings [8–10]. Unfortunately such ZrO2 films treated at 400–450 °C have a low critical layer thickness between 50 and 150 nm [11–13]. Above this critical thickness cracks can be observed in the coating [14–16]. Cracking occurs due to stress built-up in the films during heat treatment. In the literature different approaches were investigated to enhance the critical layer thickness of sol–gel films. One method is the addition of organic thickening agents such as polyethyleneglycol [16,17]. Another method is the use of inorganic–
⁎ Corresponding author. Tel.: +49 6151 72 7939; fax: +49 6151 16 6314. E-mail address: [email protected] (I. Uhlmann). 0040-6090/$ – see front matter © 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.tsf.2012.08.007
organic copolymers [18]. The latter approach carries the drawback that the high organic content of these coatings degrades hardness. In this work silica as dopant in conjunction with zirconia nanoparticles are used to increase critical layer thickness without decreasing hardness and Young's modulus. A further enhancement of film thickness is achieved by preparation of multilayer coatings. A number of publications deal already with chemical properties of silica–zirconia sol–gel coatings [19–24]. In this work several complementary characterization methods were applied, such as thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray diffraction (XRD). These afforded salient correlation of structural properties of the particular system ZrO2–8 wt.% SiO2 with mechanical characteristics such as hardness, Young's modulus and abrasion resistance. Therefore, the micro abrasion technique was implemented for sol–gel coatings [25]. This method is a simple and powerful tool not only for measuring wear rates of coatings in the micron range, but also for comparative studies of abrasion resistance of ultra thin films. The micro abrasion technique uses a rotating steel ball that causes a crater with a certain diameter in the film. Comparing these crater diameters leads to information about the abrasion resistance of the films. 2. Experimental details Sols were provided by Merck KGaA, Darmstadt, Germany and contained a zirconia-precursor, a silica-precursor, amorphous, acetic acid stabilised zirconia nanoparticles with a diameter of 10 nm and
I. Uhlmann et al. / Thin Solid Films 527 (2013) 200–204
demineralised water in a methanol–butanol mixture. The solid content of the sols was 18.5 wt.% (25 wt.% for SEM and comparative ellipsometry investigations). Films were prepared by spincoating (Spincoater, Headway Research Inc., Garland, USA) the sol on cleaned silicon wafers at a velocity of 3000 rpm (4000 rpm for SEM and comparative ellipsometry investigations) for 30 s. Cleaning procedure for the silicon wafers contained a 30 min ultrasonic cleaning in ethanol, followed by cleaning in bath 1 (H2O2 35%:NH3 32%:demineralised H2O = 1:1:5) and bath 2 (H2O2 35%:HCl 37%: demineralised H2O = 1:1:6) at 70 °C for 10 min each. Between the cleaning steps silicon wafers were washed in demineralised water three times. Finally, wafers were rinsed with isopropanol and dried with nitrogen. All chemicals were provided by Merck KGaA, Darmstadt, Germany. Coated samples were dried on a hot plate at 100 °C for 5 min and annealed in a preheated tube furnace (RO 7/75, Heraeus Instruments, Hanau, Germany) under 5 l/min nitrogen flux for 15 min subsequently. 2.1. Powder analysis The sol was dried at 100 °C in a vacuum drying cabinet (Thermo Scientific Heraeus, Thermo Fisher Scientific Inc., Waltham, USA). The powder was examined via TGA (Q5000, TA Instruments, New Castle, USA) under nitrogen while annealing up to 1000 °C with a heating rate of 10 K/min. Additionally, the powder was studied in transmission configuration using an X-Ray diffractometer (StadiP 611 Combi diffractometer with imaging plate, position sensitive detector and glass capillar furnace, STOE & Cie GmbH, Darmstadt, Germany) with CuKα radiation. Measurement was carried out in situ while heating the powder from room temperature to 900 °C with a rate of 10 K/min. Settling time at each temperature step was 15 min. Furthermore, the dried sol was treated in a tube furnace under 5 l/min nitrogen flux for 30 min at temperatures in the range of 200 °C and 1000 °C. Infrared absorption spectra of the annealed powders were recorded with a Bruker Optics Alpha FT-IR spectrometer (Alpha FT-IR, Bruker Optics Inc., Billerica, USA) at room temperature, at a spectral resolution of 4 cm−1, with an average of 24 scans. 2.2. Film analysis Samples were analyzed for the existence of cracking under an optical microscope (BX51, Olympus Corporation, Tokyo, Japan). Refractive index and film thickness were determined with spectral ellipsometry within a range of wavelengths from 350 to 850 nm (SE800, Sentech Instruments GmbH, Berlin, Germany). Psi and delta curves were measured at 50, 60 and 70° incident beam angle and simultaneously fitted assuming that the refractive index n follows the Cauchy model [26] and the extinction coefficient k is zero due to transparency of the films. Layer thickness was double checked by studying the cross sections under a scanning electron microscope (SUPRA 35, Carl Zeiss NTS GmbH). Therefore, specimens were broken and sputtered with a 4 nm Pt layer (Sputter Coater 108/auto, Cressington Scientific Instruments Ltd., Watford, UK). A relative density was calculated from relative mass loss (obtained via TGA) divided through relative thickness shrinkage (obtained via spectral ellipsometry) at a certain temperature. The absolute density of the films dried at 100 °C was experimentally determined. Therefore, 10 silicon wafers were weighed before and after coating and drying at 100 °C (Ultra Micro Balance SE2, Sartorius AG, Goettingen, Germany, reproducibility 0.25 μg, readability 0.0001 mg). For area investigation all wafers were photographed and encircled. Layer thickness was measured with spectral ellipsometry. All calculated density values of 100 °C dried films were averaged. Density values for films treated at higher temperatures were estimated by normalizing the
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Fig. 1. Cross sections of ZrO2–8 wt.% SiO2 films annealed at a) 200 °C, b) 400 °C, c) 600 °C, and d) 800 °C highlighting a decrease of film thickness with increasing annealing temperature.
relative density values to the absolute value of the coating dried at 100 °C. A theoretical density for the ZrO2–8 wt.% SiO2 coatings was calculated from literature values of the pure materials. Structural analysis was carried out in an in-plane configuration using a thin film X-ray diffractometer (SmartLab, 9 kW rotating anode, Rigaku Corporation, Tokyo, Japan) with CuKα radiation. Micro-abrasion measurements [25] were performed using a ball cratering apparatus (kaloMAX NT, BAQ GmbH, Braunschweig, Germany). Specimens were fixed at an angle of 45°. On top a rotating 100Cr6 steel ball (Ø 30 mm, m=110 g, v=60 rpm) was placed, which is driven by an external shaft at constant speed. The contact was unlubricated and each test lasted 30 s. Eight tests were performed on every sample, three identically annealed samples were measured. Horizontal crater diameters were determined using optical micrographs. Reduced modulus and hardness were measured at ILT Aachen with a nanoindenter with a Vickers probe (PicoDentor HM500, Helmut Fischer GmbH, Sindelfingen-Maichingen, Germany). Each indentation cycle consisted of 20 s loading, 10 s holding at constant force and 20 s unloading. The maximum force applied was 0.05 mN, with 35–40 indentations being performed on each sample. For each annealing temperature one specimen was measured. Curve analysis was carried out on the unloading segment of the curve (30–100% of maximum force) using the Oliver and Pharr method [27]. Special multilayer coatings were prepared in order to increase film thickness. Preparation consisted of five spin coating steps at 3000 rpm for 30 s. In between these spin coating steps samples were dried at 400 °C (except one sample which was dried at 200 °C) for 5 min on a hot plate. After application of five layers, samples were subsequently annealed in a preheated tube furnace under 5 l/min nitrogen flux for 15 min at temperatures of
Fig. 2. Thickness of ZrO2–8 wt.% SiO2 films annealed at different temperatures provided using SEM (triangles) and ellipsometry (squares) derived results for comparison.
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Fig. 3. a) TGA measurement and b) FTIR measurement of ZrO2–8 wt.%SiO2 dried gel showing a complete release of organic matter at 600 °C.
Fig. 5. In situ powder X-ray diffractograms of a) ZrO2 dried gel and b) ZrO2–8 wt.% SiO2 dried gel.
The evolution of layer thickness during the heating process was measured with SEM and spectral ellipsometry. Fig. 1 provides cross sections of the silica doped zirconia films after treating them at different temperatures. A decrease in film thickness and a change in microstructure with temperature can clearly be discerned. During the sintering process the spin coated layers shrink to about 30% of their initial film thickness. A treatment of the films at 800 °C lead to crack-free coatings with a thickness of 80 nm. Thickness results obtained with SEM and ellipsometry are in good agreement above 200 °C. In the lower temperature region ellipsometry data deviates from SEM results (Fig. 2). As the coatings treated at low temperatures are not fully inorganic, it is suggested that the Cauchy model is not fully applicable in this instance.
To study the material composition in more detail, TGA and FTIR measurements were made. The organic content of the coatings decreases with rising annealing temperature. Above 500 °C the films are completely inorganic. TGA results (Fig. 3a) delineate four main steps of mass loss at temperatures of 80, 200, 400 and 500 °C. The first step at 80 °C can be related to the release of residual alcoholic solvent. The other mass loss steps can be explained using FTIR spectroscopy. The IR spectra obtained from powders treated at 100 °C and 200 °C show a broad OH-band around 3400 cm − 1 and small CH3- and CH2-bands at 2965 cm − 1 and 2925 cm − 1, respectively (Fig. 3b). The disappearance of these bands in the spectrum obtained at 400 °C suggests that the second mass loss step in the TGA curve occurs according to the release of alcohol that is formed during the condensation process. The C_O\bands at 1582 cm−1 and 1518 cm−1 disappear in the spectrum obtained at 600 °C indicating that the third and the fourth mass loss step in the TGA curve occur according to decomposition of organic stabilization ligands. Above 400 °C a broad inorganic band forms around 950 cm−1. This band can be related to either Zr\O\Si-bonds [19–22] or Si\O−groups [23,24].
Fig. 4. Mean refractive index at 550 nm (squares) and density (triangles) of ZrO2–8 wt.% SiO2 films as a function of temperature.
Fig. 6. X-ray diffractogram of ZrO2–8 wt.% SiO2 coatings after annealing at 400, 600 and 800 °C.
600–1000 °C. One specimen each, dried at 200 °C and 400 °C, respectively, was measured without heat treatment in the tube furnace. 3. Results and discussion
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Fig. 7. a) Hardness (triangles) and Young's modulus (squares) of ZrO2–8 wt.% SiO2 films annealed at different temperatures and b) indentation depth (hollow bars) in relation to layer thickness (hatched bars). Error in the indentation depth was estimated at 0.6–1.6 nm (3–12%).
The density for the film dried at 100 °C, obtained with the weighing method, is 2.1 ±0.3 g/cm 3. Density as well as mean refractive index at 550 nm (n550 nm) rise during the annealing process and saturate above 600 °C (Fig. 4a). After tempering to at least 600 °C n550 nm reaches values around 1.93. With no obvious reason the n550 nm values measured after annealing at 800 °C (n550 nm = 1.886) and 900 °C (n550 nm = 1.884) are a bit low as compared to the trend. The highest measured density is 4.64 g/cm3. Assuming a theoretical density of 5.24 g/cm3 (5.86 g/cm3 for t-ZrO2 [28] and 2.36 g/cm3 for SiO2 [29,30]), the coatings have a relative density of 88.6% after treatment at 1000 °C. XRD measurements demonstrate that pure zirconia powder is amorphous below 500 °C (not shown). Above this temperature a tetragonal phase forms. A tetragonal to monoclinic phase transition takes place above 700 °C. A mixture of monoclinic and tetragonal phase (ICDD Pdf No. 00-037-1484 and ICDD Pdf No. 00-042-1164) can be identified at 900 °C (Fig. 5a). This is in good agreement with already published structural data of zirconia thin films fabricated via the sol–gel route [10,15,37].
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Several explanations for the presence of a low temperature tetragonal zirconia phase can be found in the literature. A widely spread explanation uses the crystallite size effect [31]. Garvie predicted, that below a critical crystallite size of 30 nm for pure zirconia the tetragonal phase becomes more stable than the thermodynamically predicted [32] monoclinic phase. At small crystallite sizes the surface to bulk ratio is high and therefore surface effects dominate. The lower surface energy of the tetragonal phase compared to that of the monoclinic phase rationalizes their metastability at room temperature for small crystallites. If the crystallites grow during sintering, surface effects can be neglected and the thermodynamically stable monoclinic phase forms. In contrast to the structure formation in pure zirconia powder, the tetragonal to monoclinic phase transition in ZrO2–8 wt.% SiO2 powder is shifted to higher temperatures [33–35]. At 900 °C only the tetragonal phase (ICDD Pdf No. 040-10-3279) is present (Fig. 5b). In silica doped zirconia systems lattice deformations could be due to the presence of Zr\O\Si-bonds, which cause the tetragonal-monoclinic-transition temperature to rise [20–22]. Del Monte et al. suggest another model based on homogeneously distributed Zr- and Si-atoms after drying [36]. During heating Zr-atoms interdiffuse to form small crystallites. These zirconia crystallites are surrounded by an amorphous silica layer. Hence, diffusion and crystallite growth are suppressed and the tetragonal-monoclinic phase transition is shifted to higher temperatures. Indeed, X-ray diffraction revealed wider reflections indicating smaller grain size in the silica-doped powder. Quantification with SEM, however, was not accessible with the resolution available. Gaudon et al. examined the mixed oxide Zr0.3Si0.7O2 and observed an amorphous phase separation even before crystallisation. The nucleation of metastable tetragonal zirconia during annealing starts then in the zirconia-rich domains. Therefore the size and the structure of the ZrO2 crystals in the amorphous SiO2 matrix are determined by the initial phase separation morphology [37]. The structure formation in the ZrO2–8 wt.% SiO2 films is comparable to the formation in the powders, but the amorphous-tetragonal phase transition is shifted to slightly higher temperatures. The reflections of the tetragonal phase are apparent at 800 °C and are slightly shifted to higher 2θ values (Fig. 6). In the following the results of the structural analysis as a function of annealing temperature are correlated with mechanical properties such as hardness, Young's modulus and abrasion resistance. Fig. 7a provides hardness and modulus results for multilayer coatings treated at six different temperatures. Both, hardness and Young's modulus, increase with rising annealing temperatures and saturate above 600 °C. The highest obtained values were 7–8 GPa for hardness and 80–90 GPa for Young's modulus. This is comparable to previously published data for pure sol–gel derived zirconia coatings [38,39]. These results show that small additions of silica do not reduce hardness and Young's modulus significantly. For high temperatures the
Fig. 8. Optical micrographs of craters in ZrO2 8 wt.% SiO2 films annealed at a) 100 °C on a hot plate and b) 900 °C in a furnace under nitrogen flux. Crater diameter is pictured by a black bar.
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studies of sol–gel films micro-abrasion has been established as a powerful tool and correlated to structural and mechanical properties such as hardness and Young's modulus. Acknowledgments This work was financially supported by the BMBF (Bundesministerium für Bildung und Forschung, Support Code 13N9661). We acknowledge the use of the SEM, powder XRD, ellipsometry and TGA facilities at Merck KGaA Darmstadt and thank Stefan Koehl, Clemens Kuehn, Bernd Fiebranz, and Susanne Rudolph for helping with the measurements and Oliver Guillon from TU Darmstadt for the scientific discussion. Fig. 9. Inner crater (squares) diameter in ZrO2–8 wt.% SiO2 films annealed at different temperatures. Hardness values (triangles) are plotted for better comparison. The hatched area visualizes the presence of the tetragonal phase.
standard deviation of hardness and Young's modulus is high, due to the low indentation depths, which are in the range of the surface roughness of the samples. The indentation depths are below 10% of the film thickness (Fig. 7b). The micro-abrasion results are in good agreement with hardness data. The craters shown in the micrographs (Fig. 8) depend on annealing temperature. After the heat treatment at 900 °C the coating is more abrasion resistant, the crater diameter is smaller and there is no chipping compared to the coating dried at 100 °C. The scratches in the craters indicate the abrasion direction (top to bottom). Fig. 9 provides crater diameters for different annealing temperatures, whereas hardness values are added for better comparison. The crater diameter decreases and therefore abrasion resistance increases with rising annealing temperatures. At the same time hardness is also increasing. Both values reach a plateau above 600 °C. The improvement in hardness and abrasion resistance in the temperature region above 600 °C can be attributed to the elevated density and the presence of a tetragonal phase as demonstrated by XRD. 4. Conclusions We investigated sol–gel derived silica doped zirconia thin films and obtained crack-free ZrO2 8 wt.% SiO2 coatings with a thickness of approximately 80 nm after heat treatment at 800 °C. Furthermore, it was possible to produce crack-free multilayer coatings with a thickness of approximately 250 nm. All measurements indicate a phase transition at around 600 °C. Below this temperature an amorphous phase with high organic content and low density is present. This material is not abrasion resistant and with 1–2 GPa hardness too soft for wear application. Above the amorphous-tetragonal phase transition temperature the coating becomes completely inorganic, crystalline and due to densification and sintering 2–3 times harder. Hardness after annealing at 800 °C is between 7 and 8 GPa, and Young's modulus is between 70 and 80 GPa. This is in good agreement with published data for pure sol–gel derived ZrO2 coatings [38]. Besides hardness and Young's modulus, abrasion resistance increases significantly with annealing temperature. Due to the low film thickness of the coatings, it was not possible to calculate wear rates. However, by comparison of the crater diameter we obtained relative information about abrasion resistance. For comparative
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Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Structure and mechanical properties of silica doped zirconia thin films Ina Uhlmann a,⁎, Dominik Hawelka b, Erwin Hildebrandt a, Jens Pradella c, Jürgen Rödel a a b c
Institute of Materials Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany Fraunhofer Institute for Laser Technology ILT, 52074 Aachen, Germany Merck KGaA Darmstadt, 64293 Darmstadt, Germany
a r t i c l e
i n f o
Article history: Received 19 May 2011 Received in revised form 3 August 2012 Accepted 3 August 2012 Available online 11 August 2012 Keywords: Sol–gel ZrO2 SiO2 Wear Hardness Crystal structure Micro abrasion Ball cratering
a b s t r a c t Sol–gel based wear resistant coatings are presented as an alternative to existing vapor deposition coatings. The films consist of zirconia which has been doped with 8 wt.% silica. Crack-free single as well as multilayer coatings with thicknesses of 80 and 150 nm, respectively, could be produced after sintering at 1000 °C. The evolution of layer thickness, optical, chemical and mechanical properties during film annealing was investigated by ellipsometry, scanning electron microscopy, thermal gravimetric analysis, Fourier transform infrared spectroscopy, X-ray diffraction, nanoindentation and micro-abrasion. Micro-abrasion has been established as an easy and powerful tool to achieve first comparative abrasion data which could be correlated to hardness, Young's modulus and structure of the films. Above 600 °C a tetragonal, oxide coating with a Young's modulus ranging from 80 to 90 GPa, a hardness from 7 to 8 GPa and an increased abrasion resistance was obtained. The film density reached 4.64 g/cm3 with the mean refractive index n550 nm lying between 1.88 and 1.93. © 2012 Published by Elsevier B.V.
1. Introduction Reduction of energy and material losses in industrial processes due to friction and wear becomes more and more important due to economical and environmental reasons. Rising prices and shortage of raw materials require lifetime enhancement of machine and engine parts [1]. Such parts often are surface modified and protected by wear resistant coatings, such as chemical and physical vapor deposition coatings [2–5]. They show excellent mechanical properties, but are very expensive and not feasible for in-line applications. Here, sol–gel coatings could be a promising alternative [6,7]. The sol–gel process avoids costly ultra high vacuum technology as it is a wet chemistry method and can be easily implemented in a production line. A huge variety of metal oxides can be produced in this manner. Especially sol–gel derived zirconia coatings are already used as thermal barriers, corrosion and wear resistant coatings [8–10]. Unfortunately such ZrO2 films treated at 400–450 °C have a low critical layer thickness between 50 and 150 nm [11–13]. Above this critical thickness cracks can be observed in the coating [14–16]. Cracking occurs due to stress built-up in the films during heat treatment. In the literature different approaches were investigated to enhance the critical layer thickness of sol–gel films. One method is the addition of organic thickening agents such as polyethyleneglycol [16,17]. Another method is the use of inorganic–
⁎ Corresponding author. Tel.: +49 6151 72 7939; fax: +49 6151 16 6314. E-mail address: [email protected] (I. Uhlmann). 0040-6090/$ – see front matter © 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.tsf.2012.08.007
organic copolymers [18]. The latter approach carries the drawback that the high organic content of these coatings degrades hardness. In this work silica as dopant in conjunction with zirconia nanoparticles are used to increase critical layer thickness without decreasing hardness and Young's modulus. A further enhancement of film thickness is achieved by preparation of multilayer coatings. A number of publications deal already with chemical properties of silica–zirconia sol–gel coatings [19–24]. In this work several complementary characterization methods were applied, such as thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray diffraction (XRD). These afforded salient correlation of structural properties of the particular system ZrO2–8 wt.% SiO2 with mechanical characteristics such as hardness, Young's modulus and abrasion resistance. Therefore, the micro abrasion technique was implemented for sol–gel coatings [25]. This method is a simple and powerful tool not only for measuring wear rates of coatings in the micron range, but also for comparative studies of abrasion resistance of ultra thin films. The micro abrasion technique uses a rotating steel ball that causes a crater with a certain diameter in the film. Comparing these crater diameters leads to information about the abrasion resistance of the films. 2. Experimental details Sols were provided by Merck KGaA, Darmstadt, Germany and contained a zirconia-precursor, a silica-precursor, amorphous, acetic acid stabilised zirconia nanoparticles with a diameter of 10 nm and
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demineralised water in a methanol–butanol mixture. The solid content of the sols was 18.5 wt.% (25 wt.% for SEM and comparative ellipsometry investigations). Films were prepared by spincoating (Spincoater, Headway Research Inc., Garland, USA) the sol on cleaned silicon wafers at a velocity of 3000 rpm (4000 rpm for SEM and comparative ellipsometry investigations) for 30 s. Cleaning procedure for the silicon wafers contained a 30 min ultrasonic cleaning in ethanol, followed by cleaning in bath 1 (H2O2 35%:NH3 32%:demineralised H2O = 1:1:5) and bath 2 (H2O2 35%:HCl 37%: demineralised H2O = 1:1:6) at 70 °C for 10 min each. Between the cleaning steps silicon wafers were washed in demineralised water three times. Finally, wafers were rinsed with isopropanol and dried with nitrogen. All chemicals were provided by Merck KGaA, Darmstadt, Germany. Coated samples were dried on a hot plate at 100 °C for 5 min and annealed in a preheated tube furnace (RO 7/75, Heraeus Instruments, Hanau, Germany) under 5 l/min nitrogen flux for 15 min subsequently. 2.1. Powder analysis The sol was dried at 100 °C in a vacuum drying cabinet (Thermo Scientific Heraeus, Thermo Fisher Scientific Inc., Waltham, USA). The powder was examined via TGA (Q5000, TA Instruments, New Castle, USA) under nitrogen while annealing up to 1000 °C with a heating rate of 10 K/min. Additionally, the powder was studied in transmission configuration using an X-Ray diffractometer (StadiP 611 Combi diffractometer with imaging plate, position sensitive detector and glass capillar furnace, STOE & Cie GmbH, Darmstadt, Germany) with CuKα radiation. Measurement was carried out in situ while heating the powder from room temperature to 900 °C with a rate of 10 K/min. Settling time at each temperature step was 15 min. Furthermore, the dried sol was treated in a tube furnace under 5 l/min nitrogen flux for 30 min at temperatures in the range of 200 °C and 1000 °C. Infrared absorption spectra of the annealed powders were recorded with a Bruker Optics Alpha FT-IR spectrometer (Alpha FT-IR, Bruker Optics Inc., Billerica, USA) at room temperature, at a spectral resolution of 4 cm−1, with an average of 24 scans. 2.2. Film analysis Samples were analyzed for the existence of cracking under an optical microscope (BX51, Olympus Corporation, Tokyo, Japan). Refractive index and film thickness were determined with spectral ellipsometry within a range of wavelengths from 350 to 850 nm (SE800, Sentech Instruments GmbH, Berlin, Germany). Psi and delta curves were measured at 50, 60 and 70° incident beam angle and simultaneously fitted assuming that the refractive index n follows the Cauchy model [26] and the extinction coefficient k is zero due to transparency of the films. Layer thickness was double checked by studying the cross sections under a scanning electron microscope (SUPRA 35, Carl Zeiss NTS GmbH). Therefore, specimens were broken and sputtered with a 4 nm Pt layer (Sputter Coater 108/auto, Cressington Scientific Instruments Ltd., Watford, UK). A relative density was calculated from relative mass loss (obtained via TGA) divided through relative thickness shrinkage (obtained via spectral ellipsometry) at a certain temperature. The absolute density of the films dried at 100 °C was experimentally determined. Therefore, 10 silicon wafers were weighed before and after coating and drying at 100 °C (Ultra Micro Balance SE2, Sartorius AG, Goettingen, Germany, reproducibility 0.25 μg, readability 0.0001 mg). For area investigation all wafers were photographed and encircled. Layer thickness was measured with spectral ellipsometry. All calculated density values of 100 °C dried films were averaged. Density values for films treated at higher temperatures were estimated by normalizing the
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Fig. 1. Cross sections of ZrO2–8 wt.% SiO2 films annealed at a) 200 °C, b) 400 °C, c) 600 °C, and d) 800 °C highlighting a decrease of film thickness with increasing annealing temperature.
relative density values to the absolute value of the coating dried at 100 °C. A theoretical density for the ZrO2–8 wt.% SiO2 coatings was calculated from literature values of the pure materials. Structural analysis was carried out in an in-plane configuration using a thin film X-ray diffractometer (SmartLab, 9 kW rotating anode, Rigaku Corporation, Tokyo, Japan) with CuKα radiation. Micro-abrasion measurements [25] were performed using a ball cratering apparatus (kaloMAX NT, BAQ GmbH, Braunschweig, Germany). Specimens were fixed at an angle of 45°. On top a rotating 100Cr6 steel ball (Ø 30 mm, m=110 g, v=60 rpm) was placed, which is driven by an external shaft at constant speed. The contact was unlubricated and each test lasted 30 s. Eight tests were performed on every sample, three identically annealed samples were measured. Horizontal crater diameters were determined using optical micrographs. Reduced modulus and hardness were measured at ILT Aachen with a nanoindenter with a Vickers probe (PicoDentor HM500, Helmut Fischer GmbH, Sindelfingen-Maichingen, Germany). Each indentation cycle consisted of 20 s loading, 10 s holding at constant force and 20 s unloading. The maximum force applied was 0.05 mN, with 35–40 indentations being performed on each sample. For each annealing temperature one specimen was measured. Curve analysis was carried out on the unloading segment of the curve (30–100% of maximum force) using the Oliver and Pharr method [27]. Special multilayer coatings were prepared in order to increase film thickness. Preparation consisted of five spin coating steps at 3000 rpm for 30 s. In between these spin coating steps samples were dried at 400 °C (except one sample which was dried at 200 °C) for 5 min on a hot plate. After application of five layers, samples were subsequently annealed in a preheated tube furnace under 5 l/min nitrogen flux for 15 min at temperatures of
Fig. 2. Thickness of ZrO2–8 wt.% SiO2 films annealed at different temperatures provided using SEM (triangles) and ellipsometry (squares) derived results for comparison.
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Fig. 3. a) TGA measurement and b) FTIR measurement of ZrO2–8 wt.%SiO2 dried gel showing a complete release of organic matter at 600 °C.
Fig. 5. In situ powder X-ray diffractograms of a) ZrO2 dried gel and b) ZrO2–8 wt.% SiO2 dried gel.
The evolution of layer thickness during the heating process was measured with SEM and spectral ellipsometry. Fig. 1 provides cross sections of the silica doped zirconia films after treating them at different temperatures. A decrease in film thickness and a change in microstructure with temperature can clearly be discerned. During the sintering process the spin coated layers shrink to about 30% of their initial film thickness. A treatment of the films at 800 °C lead to crack-free coatings with a thickness of 80 nm. Thickness results obtained with SEM and ellipsometry are in good agreement above 200 °C. In the lower temperature region ellipsometry data deviates from SEM results (Fig. 2). As the coatings treated at low temperatures are not fully inorganic, it is suggested that the Cauchy model is not fully applicable in this instance.
To study the material composition in more detail, TGA and FTIR measurements were made. The organic content of the coatings decreases with rising annealing temperature. Above 500 °C the films are completely inorganic. TGA results (Fig. 3a) delineate four main steps of mass loss at temperatures of 80, 200, 400 and 500 °C. The first step at 80 °C can be related to the release of residual alcoholic solvent. The other mass loss steps can be explained using FTIR spectroscopy. The IR spectra obtained from powders treated at 100 °C and 200 °C show a broad OH-band around 3400 cm − 1 and small CH3- and CH2-bands at 2965 cm − 1 and 2925 cm − 1, respectively (Fig. 3b). The disappearance of these bands in the spectrum obtained at 400 °C suggests that the second mass loss step in the TGA curve occurs according to the release of alcohol that is formed during the condensation process. The C_O\bands at 1582 cm−1 and 1518 cm−1 disappear in the spectrum obtained at 600 °C indicating that the third and the fourth mass loss step in the TGA curve occur according to decomposition of organic stabilization ligands. Above 400 °C a broad inorganic band forms around 950 cm−1. This band can be related to either Zr\O\Si-bonds [19–22] or Si\O−groups [23,24].
Fig. 4. Mean refractive index at 550 nm (squares) and density (triangles) of ZrO2–8 wt.% SiO2 films as a function of temperature.
Fig. 6. X-ray diffractogram of ZrO2–8 wt.% SiO2 coatings after annealing at 400, 600 and 800 °C.
600–1000 °C. One specimen each, dried at 200 °C and 400 °C, respectively, was measured without heat treatment in the tube furnace. 3. Results and discussion
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Fig. 7. a) Hardness (triangles) and Young's modulus (squares) of ZrO2–8 wt.% SiO2 films annealed at different temperatures and b) indentation depth (hollow bars) in relation to layer thickness (hatched bars). Error in the indentation depth was estimated at 0.6–1.6 nm (3–12%).
The density for the film dried at 100 °C, obtained with the weighing method, is 2.1 ±0.3 g/cm 3. Density as well as mean refractive index at 550 nm (n550 nm) rise during the annealing process and saturate above 600 °C (Fig. 4a). After tempering to at least 600 °C n550 nm reaches values around 1.93. With no obvious reason the n550 nm values measured after annealing at 800 °C (n550 nm = 1.886) and 900 °C (n550 nm = 1.884) are a bit low as compared to the trend. The highest measured density is 4.64 g/cm3. Assuming a theoretical density of 5.24 g/cm3 (5.86 g/cm3 for t-ZrO2 [28] and 2.36 g/cm3 for SiO2 [29,30]), the coatings have a relative density of 88.6% after treatment at 1000 °C. XRD measurements demonstrate that pure zirconia powder is amorphous below 500 °C (not shown). Above this temperature a tetragonal phase forms. A tetragonal to monoclinic phase transition takes place above 700 °C. A mixture of monoclinic and tetragonal phase (ICDD Pdf No. 00-037-1484 and ICDD Pdf No. 00-042-1164) can be identified at 900 °C (Fig. 5a). This is in good agreement with already published structural data of zirconia thin films fabricated via the sol–gel route [10,15,37].
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Several explanations for the presence of a low temperature tetragonal zirconia phase can be found in the literature. A widely spread explanation uses the crystallite size effect [31]. Garvie predicted, that below a critical crystallite size of 30 nm for pure zirconia the tetragonal phase becomes more stable than the thermodynamically predicted [32] monoclinic phase. At small crystallite sizes the surface to bulk ratio is high and therefore surface effects dominate. The lower surface energy of the tetragonal phase compared to that of the monoclinic phase rationalizes their metastability at room temperature for small crystallites. If the crystallites grow during sintering, surface effects can be neglected and the thermodynamically stable monoclinic phase forms. In contrast to the structure formation in pure zirconia powder, the tetragonal to monoclinic phase transition in ZrO2–8 wt.% SiO2 powder is shifted to higher temperatures [33–35]. At 900 °C only the tetragonal phase (ICDD Pdf No. 040-10-3279) is present (Fig. 5b). In silica doped zirconia systems lattice deformations could be due to the presence of Zr\O\Si-bonds, which cause the tetragonal-monoclinic-transition temperature to rise [20–22]. Del Monte et al. suggest another model based on homogeneously distributed Zr- and Si-atoms after drying [36]. During heating Zr-atoms interdiffuse to form small crystallites. These zirconia crystallites are surrounded by an amorphous silica layer. Hence, diffusion and crystallite growth are suppressed and the tetragonal-monoclinic phase transition is shifted to higher temperatures. Indeed, X-ray diffraction revealed wider reflections indicating smaller grain size in the silica-doped powder. Quantification with SEM, however, was not accessible with the resolution available. Gaudon et al. examined the mixed oxide Zr0.3Si0.7O2 and observed an amorphous phase separation even before crystallisation. The nucleation of metastable tetragonal zirconia during annealing starts then in the zirconia-rich domains. Therefore the size and the structure of the ZrO2 crystals in the amorphous SiO2 matrix are determined by the initial phase separation morphology [37]. The structure formation in the ZrO2–8 wt.% SiO2 films is comparable to the formation in the powders, but the amorphous-tetragonal phase transition is shifted to slightly higher temperatures. The reflections of the tetragonal phase are apparent at 800 °C and are slightly shifted to higher 2θ values (Fig. 6). In the following the results of the structural analysis as a function of annealing temperature are correlated with mechanical properties such as hardness, Young's modulus and abrasion resistance. Fig. 7a provides hardness and modulus results for multilayer coatings treated at six different temperatures. Both, hardness and Young's modulus, increase with rising annealing temperatures and saturate above 600 °C. The highest obtained values were 7–8 GPa for hardness and 80–90 GPa for Young's modulus. This is comparable to previously published data for pure sol–gel derived zirconia coatings [38,39]. These results show that small additions of silica do not reduce hardness and Young's modulus significantly. For high temperatures the
Fig. 8. Optical micrographs of craters in ZrO2 8 wt.% SiO2 films annealed at a) 100 °C on a hot plate and b) 900 °C in a furnace under nitrogen flux. Crater diameter is pictured by a black bar.
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studies of sol–gel films micro-abrasion has been established as a powerful tool and correlated to structural and mechanical properties such as hardness and Young's modulus. Acknowledgments This work was financially supported by the BMBF (Bundesministerium für Bildung und Forschung, Support Code 13N9661). We acknowledge the use of the SEM, powder XRD, ellipsometry and TGA facilities at Merck KGaA Darmstadt and thank Stefan Koehl, Clemens Kuehn, Bernd Fiebranz, and Susanne Rudolph for helping with the measurements and Oliver Guillon from TU Darmstadt for the scientific discussion. Fig. 9. Inner crater (squares) diameter in ZrO2–8 wt.% SiO2 films annealed at different temperatures. Hardness values (triangles) are plotted for better comparison. The hatched area visualizes the presence of the tetragonal phase.
standard deviation of hardness and Young's modulus is high, due to the low indentation depths, which are in the range of the surface roughness of the samples. The indentation depths are below 10% of the film thickness (Fig. 7b). The micro-abrasion results are in good agreement with hardness data. The craters shown in the micrographs (Fig. 8) depend on annealing temperature. After the heat treatment at 900 °C the coating is more abrasion resistant, the crater diameter is smaller and there is no chipping compared to the coating dried at 100 °C. The scratches in the craters indicate the abrasion direction (top to bottom). Fig. 9 provides crater diameters for different annealing temperatures, whereas hardness values are added for better comparison. The crater diameter decreases and therefore abrasion resistance increases with rising annealing temperatures. At the same time hardness is also increasing. Both values reach a plateau above 600 °C. The improvement in hardness and abrasion resistance in the temperature region above 600 °C can be attributed to the elevated density and the presence of a tetragonal phase as demonstrated by XRD. 4. Conclusions We investigated sol–gel derived silica doped zirconia thin films and obtained crack-free ZrO2 8 wt.% SiO2 coatings with a thickness of approximately 80 nm after heat treatment at 800 °C. Furthermore, it was possible to produce crack-free multilayer coatings with a thickness of approximately 250 nm. All measurements indicate a phase transition at around 600 °C. Below this temperature an amorphous phase with high organic content and low density is present. This material is not abrasion resistant and with 1–2 GPa hardness too soft for wear application. Above the amorphous-tetragonal phase transition temperature the coating becomes completely inorganic, crystalline and due to densification and sintering 2–3 times harder. Hardness after annealing at 800 °C is between 7 and 8 GPa, and Young's modulus is between 70 and 80 GPa. This is in good agreement with published data for pure sol–gel derived ZrO2 coatings [38]. Besides hardness and Young's modulus, abrasion resistance increases significantly with annealing temperature. Due to the low film thickness of the coatings, it was not possible to calculate wear rates. However, by comparison of the crater diameter we obtained relative information about abrasion resistance. For comparative
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