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Fracture toughness and adhesion of thermally grown titanium oxide on medical grade pure titanium
Surface & Coatings Technology 201 (2007) 6325 – 6331 www.elsevier.com/locate/surfcoat
Fracture toughness and adhesion of thermally grown titanium oxide on medical grade pure titanium B.A. Latella a,⁎, B.K. Gan b , H. Li a a
Institute of Materials and Engineering Science, Australian Nuclear Science and Technology Organisation, Private Mail Bag 1, Menai, New South Wales 2234, Australia b School of Physics, University of Sydney, New South Wales 2006, Australia Received 20 September 2006; accepted in revised form 29 November 2006 Available online 25 January 2007
Abstract The mechanical properties and adhesion characteristics of a thin thermally grown titanium oxide film on commercially pure titanium were examined. Tensile tests were used to introduce controlled strains in the thermally grown oxide (TGO), through the titanium substrate, to study the damage evolution and to quantitatively evaluate the intrinsic strength and fracture toughness of the TGO layer. Details of the TGO adhesion behaviour were explored. Nanoindentation was used to determine the Young's modulus and hardness of the TGO. Tensile loading resulted in multiple cracking to occur in the TGO layer along with distinctive inclined cracking driven by shear band deformation in the titanium substrate. The fracture toughness of the TGO was determined to be 1.8 ± 0.6 MPa m1/2. Localised delamination of the TGO was observed but only when the titanium substrate was strained well within the plastic deformation region by more than about 2.5%. © 2007 Elsevier B.V. All rights reserved. Keywords: Titanium oxide; Thermal oxide; Adhesion; Biomaterial
1. Introduction Titanium metal and its alloys are used extensively in many fields including the aerospace and biomedical industries [1]. The key advantages of using titanium as medical implants such as hip protheses and dental posts are its corrosion-resistance, the high strength to weight ratio and biocompatibility to bond with bone. The excellent corrosion-resistance is provided by the stable native oxide film on titanium, which is less than about 10 nm in thickness. The thickness of this protective oxide film can be increased by using a variety of methods, including thermal oxidation, anodisation or deposition [2]. The methods used can influence the corrosion properties and stability based on the composition and phase assemblage of the oxide film. The influence of film structure, composition and thickness of titanium oxide films formed on titanium on corrosion and biocompatibility have been widely studied [1–3]. In contrast little is known about the mechanical reliability and adhesion of ⁎ Corresponding author. E-mail address: [email protected] (B.A. Latella). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.11.037
the protective oxide film when subject to applied loading. The oxide scale or film is essentially brittle compared to the ductile titanium substrate and this inherent brittleness can limit its lifetime under localised contact stresses and external loading [4]. Furthermore, engineering design with these materials requires basic knowledge of the strength, toughness and adhesion behaviour, for improving the reliability and lifetime characteristics. Determining the fracture properties of ultra-thin films and coatings (≪ 1 μm) is of tremendous interest to the materials community. Techniques are available for ascertaining film toughness such as nanoindentation but this becomes problematic for film thicknesses in the nanometre range due to substrate effects and initiating film cracking in a reproducible manner. Accordingly, the aim of this work is to examine the fracture properties and adhesion characteristics of the TGO scale on commercially pure titanium. The titanium was subjected to a simple heat-treatment to grow the oxide film. Tensile tests were then performed on dogbone samples whilst viewing the damage evolution in-situ using optical microscopy. The strength, fracture toughness and interfacial shear strength
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were determined using fracture mechanics relations. Nanoindentation was used determine the hardness and Young's modulus of the TGO. 2. Experimental The material used in this study was a 0.9 mm thick sheet of commercially pure (cp) titanium (Titanium International). The nominal composition of the cp titanium is 99.67 wt.% Ti, 0.08 wt.% C, 0.03 wt.% Fe, 0.03 wt.% N, 0.18 wt.% O and 0.01 wt.% H. Specimens were cut from the sheet into disks of diameter 25 mm and tensile test “dogbones” of dimensions 33 mm total length with 12 mm gauge length and width 3 mm on the gauge. Five dogbones were prepared for the tensile tests. The samples were glued to a metal disk with crystalbond and then routinely ground and polished with diamond paste to a 1 μm surface finish. Each specimen was then cleaned in acetone to remove any trace of the glue, dried and then cleaned again with pure ethanol. The polished specimens were placed in a resistance-heated furnace and heated at a rate of 300 °C/h to a maximum of 600 °C for a dwell of 5 h and then cooled at 300 °C/h to ambient. The average surface roughness (Ra) was measured by stylus profilometry using a TENCOR Alpha-Step IQ system. At least ten scans at random locations on the test pieces were conducted. Spectroscopic ellipsometry (Woolam M2000 V) was performed to determine the thickness and optical constants of the TGO over a wavelength range of 500 nm to 1000 nm. Ellipsometric data were collected at 60°, 65°, 70° and 75° angle of incidence. A model consisting of a Cauchy layer with absorption on top of titanium was used to fit the data. The thickness and optical constants associated with the best fits of the model to the data were then obtained. The crystal structure of the TGO layer was determined by grazing incidence X-ray diffraction (XRD). Diffraction data were collected with a Siemens D500 instrument using the following operating conditions: fixed incident angle of 5°, CoKα radiation (λ = 1.7902 Å), 2θ range 20°–100°, step size = 0.05°, counting time of 10 s/step. The phases were identified by search-match analysis using the International Centre for Diffraction (ICDD) database. Samples of unstrained and strained polished titanium and TGO on titanium were examined using a scanning electron microscope (SEM) operating at 15 kV and equipped with an energy dispersive spectrometer (EDS). The EDS spectra showed only Ti and O peaks in the analysis of the TGO layer and the polished cp titanium. Nanoindentation was performed using an Ultra Micro Indentation System (UMIS 2000, CSIRO, Australia). Indentations were made with a Berkovich diamond tipped indenter. An effective tip radius profile over a range of penetration depths for the indenter was calculated by calibration tests on fused silica glass, single-crystal silicon and single-crystal sapphire at numerous loads. Measurements of the load and depth penetration on the TGO were recorded simultaneously using a load– partial unload procedure. Peak contact loads, P, of 1, 3, 5, 10 and 20 mN were used with a 50% unload for each increment (performed 20 times) to the load maximum. Ten indents were
Fig. 1. Schematic diagram showing the dogbone tensile sample positioned under the objective lens of an optical microscope.
made for each load at fixed intervals at random locations on the surface. All the load–displacement data at each load was used in the analysis to determine the hardness and composite Young's modulus [5]. The hardness value reported is the average obtained from the plateau region. The Young's modulus of the TGO film, Ef, was obtained by extrapolation to zero penetration by force fitting a curve to the measured composite modulus data as a function of penetration. The fracture strength and toughness of the TGO and the TGOtitanium substrate interfacial adhesion were investigated using in-situ tensile tests, as shown schematically in Fig. 1. The tests were conducted using a small in-house built mechanical tester equipped with a 2500 N capacity load cell [6]. The samples were fixed in place on the device and pulled at a rate of 3 μm/s with the mechanical testing device positioned directly under the objective lens of an optical microscope (Zeiss, Axioplan) at a fixed magnification of 500×. Load and displacement were recorded every second along with corresponding images of the TGO surface using an analogue camera. The combination of the images captured and the stress–strain data calculated from the experimental measurement of the load and displacement are key to the calculation of the mechanical properties. The intrinsic fracture properties of the TGO layer were determined from the in-situ tensile testing based on the image when first cracking occurred and the associated strain calculated from load–displacement data corresponding to this image: (i) fracture strength (σc): rc ¼ ðeexp þ er ÞEf
ð1Þ
where Ef is the Young's modulus of the coating, εexp is the strain for first cracking observed in the tensile test and εr is the calculated residual strain in the film (see Eq. (3)). (ii) fracture toughness (KIC) [7]: " KIC ¼
r2c t
rc kFðaD Þ þ pffiffiffi 3ry
#!1=2 ð2Þ
where t is the layer thickness, F(αD) is a function of the elastic contrast (αD) between film and substrate [8] and σy is the yield stress of the titanium substrate obtained experimentally (see Fig. 5).The residual thermal stress in the TGO was calculated using the relation: rr ¼ Ef DT ðaf −as Þ
ð3Þ
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where Ef is the Young's modulus of the TGO film, ΔT is the difference between the treatment temperature and ambient and α is the thermal expansion coefficient of the film (αf = 9.4 × 10− 6 °C− 1) and substrate (αs = 9.4 × 10− 6 °C− 1). The crack spacing in the TGO was monitored until saturation at a strain, εso. The interfacial shear strength, τ, was determined from [9,10]: s¼
rc t kexp
ð4Þ
where λexp is the mean saturation intercracking distance from SEM images. The shear strength provides a measure of interfacial adhesion. 3. Results The average surface roughness of the polished titanium samples was Ra = 6.9 ± 1.2 nm. After the thermal treatment the roughness of the oxide layer grown from the polished titanium surface was found to be much higher, Ra = 18.8 ± 1.8 nm. The grazing incidence XRD pattern of the TGO layer is given in Fig. 2. The XRD spectrum shows anatase (JCPDS pattern no. 21-1272) and rutile (JCPDS pattern no. 21-1276) peaks as the constituent crystalline phases of the layer with the other peaks coming from the underlying titanium substrate (JCPDS pattern no. 05-0682). The cp titanium substrate has a hexagonal closepacked structure, commonly referred to as α-phase. The diffraction results are in agreement with the findings from other studies at treatment temperatures of 600 °C [1,2]. Fig. 3(a) shows a representative ellipsometric experimental Ψ data as a function of wavelength for the four incident angles and the best fits achieved using the Cauchy layer. Fig. 3(b) shows the refractive index (n) and extinction coefficient (k) as a function of wavelength for the TGO layer. From the modelling,
Fig. 2. Grazing incidence X-ray diffraction pattern of TGO layer. The peaks are labelled either A (anatase), R (rutile) or Ti (titanium substrate).
Fig. 3. (a) Ellipometric Ψ data from TGO on titanium as a function of wavelength. Symbols indicate experimental data for the four angles of incidence and solid lines are best fits using a Cauchy layer model with absorption. (b) Optical constants for the TGO layer as a function of wavelength.
the thickness of the TGO was determined to be 180 ± 2 nm and the refractive index at λ = 550 nm is n = 2.52, which is comparable to the refractive index of rutile (n = 2.75 at λ = 550 nm) and anatase (n = 2.54 at λ = 550 nm) [11]. The differences in the refractive index are due to density of the layer material and the crystalline structure and phase assemblage. Fig. 4(a) and (b) show hardness and Young's modulus, respectively, as a function of penetration depth for the TGOtitanium system. The hardness data (Fig. 4(a)) displays an increase to a plateau then subsequent drop-off with increasing penetration depth [12]. The TGO layer hardness was determined as the average value of the data points within the plateau region for an indentation depth between 0.05 and 0.07 μm (≈ 28 to 39% of the film thickness), i.e. H = 13.9 ± 0.2 GPa. For comparison the hardness of cp titanium from Vickers microindentation is ≈ 2 GPa. The Young's modulus (Fig. 4(b)) shows a slight increase at low penetration depth and then falls away
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The grey rectangular boxes denote the range in applied strains that first cracking, crack saturation and first debonding occurs in the TGO layer from the optical microscopy observations. Cracking and debonding damage in the TGO layer occur within the plateau region of the stress–strain data corresponding to plastic deformation of the titanium. Fig. 6 shows optical images of the surface damage evolution in the TGO film with increasing tensile strain. The tensile axis is vertical in the plane of the images. Clear differences can be seen in the optical micrographs of cracking in the TGO film as follows: (a) At 0.5% strain showing the TGO surface with no signs of damage during elastic loading of the titanium substrate. (b) At 1.7% strain, initial cracks in the TGO layer are apparent (arrows) as long, parallel cracks transverse to the loading direction. Also observable are the titanium grains and the initial effects of shear banding due to plastic deformation of the substrate (cf. Fig. 5). (c) At 3.5% strain, the transverse cracks are more welldefined and greater in extent, and shear deformation of the titanium has resulted in further cracking of the TGO but at inclined angles of 45°. Debonding and buckling of the TGO has commenced but is not apparent in the image. (d) At 10% strain, which corresponded to the end of the test, the cracks have saturated and the inclined cracking is quite extensive and film buckling is apparent between parallel cracks as darkened areas and along the inclined cracks which now cover the entire field of view.
Fig. 4. (a) Hardness and (b) Young's modulus as a function of penetration depth from nanoindentation with Bekovich indenter. Measured data based on load– partial unload tests for maximum loads ranging from 1 to 20 mN. Solid line in (b) is exponential force fit to the data.
slowly tending towards the substrate modulus (E = 116 GPa) at higher indentation depths, as would be expected for a high modulus film on a lower modulus substrate. The solid line is an exponential fit to the data with the TGO Young's modulus value taken as the vertical axis intercept (i.e. zero depth, ht = 0) so that Ef = 259 GPa. The surface roughness of the TGO layer (≈ 19 nm) has an effect on the data obtained at low indentation loads with the calculated hardness and Young's modulus exhibiting greater variability particularly at penetration depths less than 0.05 μm [13]. However, the overall trends in the data indicate that the measurements are reliable and the hardness and Young's modulus of the TGO are similar to results obtained for deposited amorphous and crystalline TiO2 films [14,15]. Fig. 5 shows a stress–strain curve obtained from a tensile test of the TGO film on the cp titanium substrate. The yield stress of the titanium substrate is about 180 MPa and as expected the TGO has no discernible influence on the stress–strain response.
Post-mortem SEM examinations were made of the surface damage in the TGO layer and typical examples of the cracking and delamination are shown in Fig. 7 (equivalent regions to the optical image in Fig. 6(d)). Fig. 7(a) shows many inclined cracks between successive transverse cracks and small regions where delamination of the TGO has occurred. The transverse
Fig. 5. Stress–strain curve of the cp titanium with TGO layer. Grey boxes indicate strain ranges for initial cracking, crack saturation and initial debonding of the TGO.
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Fig. 6. Optical micrographs showing surface views of damage evolution in the TGO during tensile loading at applied strains of: (a) 0.5%, (b) 1.7%, (c) 3.5% and (d) 10%. Loading axis is vertical. Arrows in (b) point to the first cracks observed in the TGO layer.
cracks are slightly wavy and there are instances of small cracks up to about 25 μm in length between the well-defined longer cracks. The saturation intercracking distance is approximately 15 μm. Fig. 7(b) shows at higher magnification an area in which extensive debonding and buckling of the TGO from the substrate is evident. The TGO has de-adhered from the surface and fragments are clearly visible. Fig. 8(a) and (b) show SEM images of the shear bands in the polished titanium and the corresponding inclined cracks in the TGO–Ti system, respectively. The base titanium sample was strained to 10% in accord with tests on the TGO materials. The plastic deformation of the titanium substrate has a dramatic effect on the cracking in such thin layers as has been observed previously for other types of coatings [5,16–18]. From the combined experimental strains and the observations of cracking events, the strength and fracture toughness of the TGO layer were determined using Eqs. (1) and (2). The determination of the intrinsic layer properties using these equations is based on the following assumptions: (i) the layer is brittle and deforms elastically to failure and (ii) perfect adhesion between TGO and Ti substrate exists. From the experimental results of the in-situ tensile tests, the strength of the 180 nm TGO layer was found to be 1340 ± 360 MPa. The fracture toughness of the TGO was determined to be 1.8 ± 0.6 MPa m1/2. This toughness is essentially quite reasonable and is slightly lower than the fracture toughness of bulk crystalline titanium oxide 2.5 MPa m1/2 but higher than amorphous titanium oxide thin film, 0.26–0.37 MPa m1/2 [19]. The large variability in the fracture properties is due to differences in the defect size in the TGO layer of each sample tested.
Fig. 7. Scanning electron micrographs of damage in the TGO: (a) overview of damage and (b) high magnification view of heavily debonded region. Sample strained to 10% in vertical direction of the images.
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Fig. 8. Scanning electron micrographs showing (a) shear bands in a polished cp titanium and (b) cracking in TGO driven by shear bands in the titanium substrate. Sample strained to 10% in vertical direction of the images.
From the mean saturation intercracking distance λexp ≈ 15 μm, the shear strength of the interface was evaluated using Eq. (4) yielding τ = 16 ± 4 MPa. For comparison, the interfacial shear strength of an amorphous titania film on polycarbonate is similar, ≈ 19 MPa [19] but for a TiN coating on stainless steel τ = 32 MPa [5]. 4. Discussion In-situ tensile testing has been used to investigate the cracking damage and adhesion behaviour of a TGO layer on cp titanium. Typical brittle cracking occurs in the TGO layer, transverse to the loading direction. These cracks develop throughout the gauge section of the dogbone samples and secondary cracks appear between primary ones quite rapidly to a saturation point above which no further transverse cracking is observed. At the same time, inclined cracking of the TGO occurs, driven by shear band formation in the cp titanium. Debonding and delamination of the TGO layer develops after the through-thickness cracking events have ceased, predominantly between successive parallel cracks and at localised regions around inclined cracks. The delamination buckling and film fragments observed is caused by compressive stresses perpendicular to the loading direction as a result of Poisson contraction of the substrate.
By following the evolution of damage in the TGO using optical microscopy (Fig. 6) and the stress–strain data logged (Fig. 5) during tensile loading provided the necessary information for quantitative assessment of the mechanical stability of the 180 nm TGO on the cp titanium. The work has demonstrated that by applying simple uniaxial tensile stress to the system important information can be gauged on the cracking behaviour of the TGO layer as well as its fracture properties and delamination susceptibility. The strength and fracture toughness of the TGO were found be 1340 ± 360 MPa and 1.8 ± 0.6 MPa m1/2, respectively. The considerably large variability in these properties is suggested to be due to the effective flaw state developed in the TGO films during the thermal treatment given that the polished sample surfaces were essentially the same — it is known that defects or scratches in the substrate can act as stress raisers and accelerate cracking in films [20,21]. An estimate of the mean defect size, c, using the relation KIC = σc (πc)1/2 gives c = 0.57 μm. This defect size is considerably larger than the thickness of the TGO indicating that the defects may originate from asperities or impurities on the substrate surface and/or from pores in and extending along the layer due to local variations during growth. Despite the high level of strains achieved in the controlled tensile tests, the degree of film debonding was not severe with the majority of the layer remaining attached to the substrate indicating relatively good adhesion. The experiments provide a basis from which useful information can be extracted in terms of the upper limits of strain that the system can attain before film cracking and buckling are activated. The implications of this work relate to the design, performance and life prediction of such systems in medical implants. The salient features governing the modes of damage in the TGO and the loss of adhesion that occurs under tensile stress have been identified. Future work will explore the adhesion of titania on cp titanium and a titanium alloy (Ti–6Al– 4V) substrate by anodisation as well as more complex systems which consist of a deposited coating such as hydroxyapatite on the anodised layer. Preliminary fracture and wear studies, as shown elsewhere [16,18], of these complex film-substrate systems provide crucial information for materials design, processing and selection. 5. Conclusions In-situ tensile tests under an optical microscope of a 180 nm thick TGO layer on commercially pure titanium revealed the evolution of cracking and delamination. The TGO produced at 600 °C is crystalline with XRD showing both rutile and anatase. The refractive index of the TGO at λ = 550 nm is n = 2.52, typical of TiO2. A hardness of ≈ 14 GPa and Young's modulus of 259 GPa were obtained from nanoindentation. The fracture properties and adhesion of the TGO–Ti system when pulled in tension can be categorised by the events: (i) transverse cracking of the TGO occurs above a critical strain; (ii) cracks multiply to a saturation level; and (iii) localised debonding and delamination of the TGO occurs. From these events, the intrinsic properties of the TGO and adhesion to the titanium were
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determined. The TGO strength and fracture toughness were found to be 1340 ± 360 MPa and 1.8 ± 0.6 MPa m1/2, respectively, and the apparent interfacial shear strength was 16 MPa.
[6] [7] [8] [9] [10]
Acknowledgements Thanks to ANSTO colleagues Graham Smith for polishing the titanium specimens and Ken Short for assistance with the grazing incidence XRD. Thanks also to Florian Aitelli an ANSTO intern student from UTC Compiègne for the schematic illustration of the tensile test. References
[11] [12] [13] [14] [15] [16] [17] [18]
[1] D. Velten, V. Biehl, F. Aubertin, B. Valeske, W. Possart, J. Breme, J. Biomed. Materi. Res. 59 (2002) 18. [2] J.R. Birch, T.D. Burleigh, Corrosion 56 (2000) 1233. [3] J. Pouilleau, D. Devilliers, F. Garrido, S. Durand-Vidal, E. Mahé, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 47 (1997) 235. [4] B.A. Latella, M. Ignat, C.J. Barbé, D.J. Cassidy, H. Li, J. Sol-Gel Sci. Technol. 31 (2004) 143. [5] B.A. Latella, B.K. Gan, K.E. Davies, D.R. McKenzie, D.G. McCulloch, Surf. Coat. Technol. 200 (2006) 3605.
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Fracture toughness and adhesion of thermally grown titanium oxide on medical grade pure titanium B.A. Latella a,⁎, B.K. Gan b , H. Li a a
Institute of Materials and Engineering Science, Australian Nuclear Science and Technology Organisation, Private Mail Bag 1, Menai, New South Wales 2234, Australia b School of Physics, University of Sydney, New South Wales 2006, Australia Received 20 September 2006; accepted in revised form 29 November 2006 Available online 25 January 2007
Abstract The mechanical properties and adhesion characteristics of a thin thermally grown titanium oxide film on commercially pure titanium were examined. Tensile tests were used to introduce controlled strains in the thermally grown oxide (TGO), through the titanium substrate, to study the damage evolution and to quantitatively evaluate the intrinsic strength and fracture toughness of the TGO layer. Details of the TGO adhesion behaviour were explored. Nanoindentation was used to determine the Young's modulus and hardness of the TGO. Tensile loading resulted in multiple cracking to occur in the TGO layer along with distinctive inclined cracking driven by shear band deformation in the titanium substrate. The fracture toughness of the TGO was determined to be 1.8 ± 0.6 MPa m1/2. Localised delamination of the TGO was observed but only when the titanium substrate was strained well within the plastic deformation region by more than about 2.5%. © 2007 Elsevier B.V. All rights reserved. Keywords: Titanium oxide; Thermal oxide; Adhesion; Biomaterial
1. Introduction Titanium metal and its alloys are used extensively in many fields including the aerospace and biomedical industries [1]. The key advantages of using titanium as medical implants such as hip protheses and dental posts are its corrosion-resistance, the high strength to weight ratio and biocompatibility to bond with bone. The excellent corrosion-resistance is provided by the stable native oxide film on titanium, which is less than about 10 nm in thickness. The thickness of this protective oxide film can be increased by using a variety of methods, including thermal oxidation, anodisation or deposition [2]. The methods used can influence the corrosion properties and stability based on the composition and phase assemblage of the oxide film. The influence of film structure, composition and thickness of titanium oxide films formed on titanium on corrosion and biocompatibility have been widely studied [1–3]. In contrast little is known about the mechanical reliability and adhesion of ⁎ Corresponding author. E-mail address: [email protected] (B.A. Latella). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.11.037
the protective oxide film when subject to applied loading. The oxide scale or film is essentially brittle compared to the ductile titanium substrate and this inherent brittleness can limit its lifetime under localised contact stresses and external loading [4]. Furthermore, engineering design with these materials requires basic knowledge of the strength, toughness and adhesion behaviour, for improving the reliability and lifetime characteristics. Determining the fracture properties of ultra-thin films and coatings (≪ 1 μm) is of tremendous interest to the materials community. Techniques are available for ascertaining film toughness such as nanoindentation but this becomes problematic for film thicknesses in the nanometre range due to substrate effects and initiating film cracking in a reproducible manner. Accordingly, the aim of this work is to examine the fracture properties and adhesion characteristics of the TGO scale on commercially pure titanium. The titanium was subjected to a simple heat-treatment to grow the oxide film. Tensile tests were then performed on dogbone samples whilst viewing the damage evolution in-situ using optical microscopy. The strength, fracture toughness and interfacial shear strength
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were determined using fracture mechanics relations. Nanoindentation was used determine the hardness and Young's modulus of the TGO. 2. Experimental The material used in this study was a 0.9 mm thick sheet of commercially pure (cp) titanium (Titanium International). The nominal composition of the cp titanium is 99.67 wt.% Ti, 0.08 wt.% C, 0.03 wt.% Fe, 0.03 wt.% N, 0.18 wt.% O and 0.01 wt.% H. Specimens were cut from the sheet into disks of diameter 25 mm and tensile test “dogbones” of dimensions 33 mm total length with 12 mm gauge length and width 3 mm on the gauge. Five dogbones were prepared for the tensile tests. The samples were glued to a metal disk with crystalbond and then routinely ground and polished with diamond paste to a 1 μm surface finish. Each specimen was then cleaned in acetone to remove any trace of the glue, dried and then cleaned again with pure ethanol. The polished specimens were placed in a resistance-heated furnace and heated at a rate of 300 °C/h to a maximum of 600 °C for a dwell of 5 h and then cooled at 300 °C/h to ambient. The average surface roughness (Ra) was measured by stylus profilometry using a TENCOR Alpha-Step IQ system. At least ten scans at random locations on the test pieces were conducted. Spectroscopic ellipsometry (Woolam M2000 V) was performed to determine the thickness and optical constants of the TGO over a wavelength range of 500 nm to 1000 nm. Ellipsometric data were collected at 60°, 65°, 70° and 75° angle of incidence. A model consisting of a Cauchy layer with absorption on top of titanium was used to fit the data. The thickness and optical constants associated with the best fits of the model to the data were then obtained. The crystal structure of the TGO layer was determined by grazing incidence X-ray diffraction (XRD). Diffraction data were collected with a Siemens D500 instrument using the following operating conditions: fixed incident angle of 5°, CoKα radiation (λ = 1.7902 Å), 2θ range 20°–100°, step size = 0.05°, counting time of 10 s/step. The phases were identified by search-match analysis using the International Centre for Diffraction (ICDD) database. Samples of unstrained and strained polished titanium and TGO on titanium were examined using a scanning electron microscope (SEM) operating at 15 kV and equipped with an energy dispersive spectrometer (EDS). The EDS spectra showed only Ti and O peaks in the analysis of the TGO layer and the polished cp titanium. Nanoindentation was performed using an Ultra Micro Indentation System (UMIS 2000, CSIRO, Australia). Indentations were made with a Berkovich diamond tipped indenter. An effective tip radius profile over a range of penetration depths for the indenter was calculated by calibration tests on fused silica glass, single-crystal silicon and single-crystal sapphire at numerous loads. Measurements of the load and depth penetration on the TGO were recorded simultaneously using a load– partial unload procedure. Peak contact loads, P, of 1, 3, 5, 10 and 20 mN were used with a 50% unload for each increment (performed 20 times) to the load maximum. Ten indents were
Fig. 1. Schematic diagram showing the dogbone tensile sample positioned under the objective lens of an optical microscope.
made for each load at fixed intervals at random locations on the surface. All the load–displacement data at each load was used in the analysis to determine the hardness and composite Young's modulus [5]. The hardness value reported is the average obtained from the plateau region. The Young's modulus of the TGO film, Ef, was obtained by extrapolation to zero penetration by force fitting a curve to the measured composite modulus data as a function of penetration. The fracture strength and toughness of the TGO and the TGOtitanium substrate interfacial adhesion were investigated using in-situ tensile tests, as shown schematically in Fig. 1. The tests were conducted using a small in-house built mechanical tester equipped with a 2500 N capacity load cell [6]. The samples were fixed in place on the device and pulled at a rate of 3 μm/s with the mechanical testing device positioned directly under the objective lens of an optical microscope (Zeiss, Axioplan) at a fixed magnification of 500×. Load and displacement were recorded every second along with corresponding images of the TGO surface using an analogue camera. The combination of the images captured and the stress–strain data calculated from the experimental measurement of the load and displacement are key to the calculation of the mechanical properties. The intrinsic fracture properties of the TGO layer were determined from the in-situ tensile testing based on the image when first cracking occurred and the associated strain calculated from load–displacement data corresponding to this image: (i) fracture strength (σc): rc ¼ ðeexp þ er ÞEf
ð1Þ
where Ef is the Young's modulus of the coating, εexp is the strain for first cracking observed in the tensile test and εr is the calculated residual strain in the film (see Eq. (3)). (ii) fracture toughness (KIC) [7]: " KIC ¼
r2c t
rc kFðaD Þ þ pffiffiffi 3ry
#!1=2 ð2Þ
where t is the layer thickness, F(αD) is a function of the elastic contrast (αD) between film and substrate [8] and σy is the yield stress of the titanium substrate obtained experimentally (see Fig. 5).The residual thermal stress in the TGO was calculated using the relation: rr ¼ Ef DT ðaf −as Þ
ð3Þ
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where Ef is the Young's modulus of the TGO film, ΔT is the difference between the treatment temperature and ambient and α is the thermal expansion coefficient of the film (αf = 9.4 × 10− 6 °C− 1) and substrate (αs = 9.4 × 10− 6 °C− 1). The crack spacing in the TGO was monitored until saturation at a strain, εso. The interfacial shear strength, τ, was determined from [9,10]: s¼
rc t kexp
ð4Þ
where λexp is the mean saturation intercracking distance from SEM images. The shear strength provides a measure of interfacial adhesion. 3. Results The average surface roughness of the polished titanium samples was Ra = 6.9 ± 1.2 nm. After the thermal treatment the roughness of the oxide layer grown from the polished titanium surface was found to be much higher, Ra = 18.8 ± 1.8 nm. The grazing incidence XRD pattern of the TGO layer is given in Fig. 2. The XRD spectrum shows anatase (JCPDS pattern no. 21-1272) and rutile (JCPDS pattern no. 21-1276) peaks as the constituent crystalline phases of the layer with the other peaks coming from the underlying titanium substrate (JCPDS pattern no. 05-0682). The cp titanium substrate has a hexagonal closepacked structure, commonly referred to as α-phase. The diffraction results are in agreement with the findings from other studies at treatment temperatures of 600 °C [1,2]. Fig. 3(a) shows a representative ellipsometric experimental Ψ data as a function of wavelength for the four incident angles and the best fits achieved using the Cauchy layer. Fig. 3(b) shows the refractive index (n) and extinction coefficient (k) as a function of wavelength for the TGO layer. From the modelling,
Fig. 2. Grazing incidence X-ray diffraction pattern of TGO layer. The peaks are labelled either A (anatase), R (rutile) or Ti (titanium substrate).
Fig. 3. (a) Ellipometric Ψ data from TGO on titanium as a function of wavelength. Symbols indicate experimental data for the four angles of incidence and solid lines are best fits using a Cauchy layer model with absorption. (b) Optical constants for the TGO layer as a function of wavelength.
the thickness of the TGO was determined to be 180 ± 2 nm and the refractive index at λ = 550 nm is n = 2.52, which is comparable to the refractive index of rutile (n = 2.75 at λ = 550 nm) and anatase (n = 2.54 at λ = 550 nm) [11]. The differences in the refractive index are due to density of the layer material and the crystalline structure and phase assemblage. Fig. 4(a) and (b) show hardness and Young's modulus, respectively, as a function of penetration depth for the TGOtitanium system. The hardness data (Fig. 4(a)) displays an increase to a plateau then subsequent drop-off with increasing penetration depth [12]. The TGO layer hardness was determined as the average value of the data points within the plateau region for an indentation depth between 0.05 and 0.07 μm (≈ 28 to 39% of the film thickness), i.e. H = 13.9 ± 0.2 GPa. For comparison the hardness of cp titanium from Vickers microindentation is ≈ 2 GPa. The Young's modulus (Fig. 4(b)) shows a slight increase at low penetration depth and then falls away
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The grey rectangular boxes denote the range in applied strains that first cracking, crack saturation and first debonding occurs in the TGO layer from the optical microscopy observations. Cracking and debonding damage in the TGO layer occur within the plateau region of the stress–strain data corresponding to plastic deformation of the titanium. Fig. 6 shows optical images of the surface damage evolution in the TGO film with increasing tensile strain. The tensile axis is vertical in the plane of the images. Clear differences can be seen in the optical micrographs of cracking in the TGO film as follows: (a) At 0.5% strain showing the TGO surface with no signs of damage during elastic loading of the titanium substrate. (b) At 1.7% strain, initial cracks in the TGO layer are apparent (arrows) as long, parallel cracks transverse to the loading direction. Also observable are the titanium grains and the initial effects of shear banding due to plastic deformation of the substrate (cf. Fig. 5). (c) At 3.5% strain, the transverse cracks are more welldefined and greater in extent, and shear deformation of the titanium has resulted in further cracking of the TGO but at inclined angles of 45°. Debonding and buckling of the TGO has commenced but is not apparent in the image. (d) At 10% strain, which corresponded to the end of the test, the cracks have saturated and the inclined cracking is quite extensive and film buckling is apparent between parallel cracks as darkened areas and along the inclined cracks which now cover the entire field of view.
Fig. 4. (a) Hardness and (b) Young's modulus as a function of penetration depth from nanoindentation with Bekovich indenter. Measured data based on load– partial unload tests for maximum loads ranging from 1 to 20 mN. Solid line in (b) is exponential force fit to the data.
slowly tending towards the substrate modulus (E = 116 GPa) at higher indentation depths, as would be expected for a high modulus film on a lower modulus substrate. The solid line is an exponential fit to the data with the TGO Young's modulus value taken as the vertical axis intercept (i.e. zero depth, ht = 0) so that Ef = 259 GPa. The surface roughness of the TGO layer (≈ 19 nm) has an effect on the data obtained at low indentation loads with the calculated hardness and Young's modulus exhibiting greater variability particularly at penetration depths less than 0.05 μm [13]. However, the overall trends in the data indicate that the measurements are reliable and the hardness and Young's modulus of the TGO are similar to results obtained for deposited amorphous and crystalline TiO2 films [14,15]. Fig. 5 shows a stress–strain curve obtained from a tensile test of the TGO film on the cp titanium substrate. The yield stress of the titanium substrate is about 180 MPa and as expected the TGO has no discernible influence on the stress–strain response.
Post-mortem SEM examinations were made of the surface damage in the TGO layer and typical examples of the cracking and delamination are shown in Fig. 7 (equivalent regions to the optical image in Fig. 6(d)). Fig. 7(a) shows many inclined cracks between successive transverse cracks and small regions where delamination of the TGO has occurred. The transverse
Fig. 5. Stress–strain curve of the cp titanium with TGO layer. Grey boxes indicate strain ranges for initial cracking, crack saturation and initial debonding of the TGO.
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Fig. 6. Optical micrographs showing surface views of damage evolution in the TGO during tensile loading at applied strains of: (a) 0.5%, (b) 1.7%, (c) 3.5% and (d) 10%. Loading axis is vertical. Arrows in (b) point to the first cracks observed in the TGO layer.
cracks are slightly wavy and there are instances of small cracks up to about 25 μm in length between the well-defined longer cracks. The saturation intercracking distance is approximately 15 μm. Fig. 7(b) shows at higher magnification an area in which extensive debonding and buckling of the TGO from the substrate is evident. The TGO has de-adhered from the surface and fragments are clearly visible. Fig. 8(a) and (b) show SEM images of the shear bands in the polished titanium and the corresponding inclined cracks in the TGO–Ti system, respectively. The base titanium sample was strained to 10% in accord with tests on the TGO materials. The plastic deformation of the titanium substrate has a dramatic effect on the cracking in such thin layers as has been observed previously for other types of coatings [5,16–18]. From the combined experimental strains and the observations of cracking events, the strength and fracture toughness of the TGO layer were determined using Eqs. (1) and (2). The determination of the intrinsic layer properties using these equations is based on the following assumptions: (i) the layer is brittle and deforms elastically to failure and (ii) perfect adhesion between TGO and Ti substrate exists. From the experimental results of the in-situ tensile tests, the strength of the 180 nm TGO layer was found to be 1340 ± 360 MPa. The fracture toughness of the TGO was determined to be 1.8 ± 0.6 MPa m1/2. This toughness is essentially quite reasonable and is slightly lower than the fracture toughness of bulk crystalline titanium oxide 2.5 MPa m1/2 but higher than amorphous titanium oxide thin film, 0.26–0.37 MPa m1/2 [19]. The large variability in the fracture properties is due to differences in the defect size in the TGO layer of each sample tested.
Fig. 7. Scanning electron micrographs of damage in the TGO: (a) overview of damage and (b) high magnification view of heavily debonded region. Sample strained to 10% in vertical direction of the images.
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Fig. 8. Scanning electron micrographs showing (a) shear bands in a polished cp titanium and (b) cracking in TGO driven by shear bands in the titanium substrate. Sample strained to 10% in vertical direction of the images.
From the mean saturation intercracking distance λexp ≈ 15 μm, the shear strength of the interface was evaluated using Eq. (4) yielding τ = 16 ± 4 MPa. For comparison, the interfacial shear strength of an amorphous titania film on polycarbonate is similar, ≈ 19 MPa [19] but for a TiN coating on stainless steel τ = 32 MPa [5]. 4. Discussion In-situ tensile testing has been used to investigate the cracking damage and adhesion behaviour of a TGO layer on cp titanium. Typical brittle cracking occurs in the TGO layer, transverse to the loading direction. These cracks develop throughout the gauge section of the dogbone samples and secondary cracks appear between primary ones quite rapidly to a saturation point above which no further transverse cracking is observed. At the same time, inclined cracking of the TGO occurs, driven by shear band formation in the cp titanium. Debonding and delamination of the TGO layer develops after the through-thickness cracking events have ceased, predominantly between successive parallel cracks and at localised regions around inclined cracks. The delamination buckling and film fragments observed is caused by compressive stresses perpendicular to the loading direction as a result of Poisson contraction of the substrate.
By following the evolution of damage in the TGO using optical microscopy (Fig. 6) and the stress–strain data logged (Fig. 5) during tensile loading provided the necessary information for quantitative assessment of the mechanical stability of the 180 nm TGO on the cp titanium. The work has demonstrated that by applying simple uniaxial tensile stress to the system important information can be gauged on the cracking behaviour of the TGO layer as well as its fracture properties and delamination susceptibility. The strength and fracture toughness of the TGO were found be 1340 ± 360 MPa and 1.8 ± 0.6 MPa m1/2, respectively. The considerably large variability in these properties is suggested to be due to the effective flaw state developed in the TGO films during the thermal treatment given that the polished sample surfaces were essentially the same — it is known that defects or scratches in the substrate can act as stress raisers and accelerate cracking in films [20,21]. An estimate of the mean defect size, c, using the relation KIC = σc (πc)1/2 gives c = 0.57 μm. This defect size is considerably larger than the thickness of the TGO indicating that the defects may originate from asperities or impurities on the substrate surface and/or from pores in and extending along the layer due to local variations during growth. Despite the high level of strains achieved in the controlled tensile tests, the degree of film debonding was not severe with the majority of the layer remaining attached to the substrate indicating relatively good adhesion. The experiments provide a basis from which useful information can be extracted in terms of the upper limits of strain that the system can attain before film cracking and buckling are activated. The implications of this work relate to the design, performance and life prediction of such systems in medical implants. The salient features governing the modes of damage in the TGO and the loss of adhesion that occurs under tensile stress have been identified. Future work will explore the adhesion of titania on cp titanium and a titanium alloy (Ti–6Al– 4V) substrate by anodisation as well as more complex systems which consist of a deposited coating such as hydroxyapatite on the anodised layer. Preliminary fracture and wear studies, as shown elsewhere [16,18], of these complex film-substrate systems provide crucial information for materials design, processing and selection. 5. Conclusions In-situ tensile tests under an optical microscope of a 180 nm thick TGO layer on commercially pure titanium revealed the evolution of cracking and delamination. The TGO produced at 600 °C is crystalline with XRD showing both rutile and anatase. The refractive index of the TGO at λ = 550 nm is n = 2.52, typical of TiO2. A hardness of ≈ 14 GPa and Young's modulus of 259 GPa were obtained from nanoindentation. The fracture properties and adhesion of the TGO–Ti system when pulled in tension can be categorised by the events: (i) transverse cracking of the TGO occurs above a critical strain; (ii) cracks multiply to a saturation level; and (iii) localised debonding and delamination of the TGO occurs. From these events, the intrinsic properties of the TGO and adhesion to the titanium were
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determined. The TGO strength and fracture toughness were found to be 1340 ± 360 MPa and 1.8 ± 0.6 MPa m1/2, respectively, and the apparent interfacial shear strength was 16 MPa.
[6] [7] [8] [9] [10]
Acknowledgements Thanks to ANSTO colleagues Graham Smith for polishing the titanium specimens and Ken Short for assistance with the grazing incidence XRD. Thanks also to Florian Aitelli an ANSTO intern student from UTC Compiègne for the schematic illustration of the tensile test. References
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