Oxidation of Ni-toughened nc-tin/a-sin x nanocomposite thin films

Transcription

1 Oxidation of Ni-toughened nc-tin/a-sin x nanocomposite thin films Sam Zhang a) and Deen Sun School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore Xianting Zeng Singapore Institute of Manufacturing Technology, Singapore (Received 28 February 2005; accepted 6 July 2005) Oxidation behavior of Ni-toughened reactively sputtered composite thin films of nanocrystalline TiN and amorphous SiN x [denoted as nc-tin/a-sin x (Ni)] was explored to understand the oxidation mechanism. The films were deposited on silicon substrate using a magnetron sputtering technique. Oxidation was carried out from 450 C up to 1000 C. The nature of the oxidation was determined using x-ray photoelectron spectroscopy. The microstructure of the oxidized films was studied using grazing incidence x-ray diffraction. The topography was characterized using atomic force microscopy. It was determined that the oxidation of the nc-tin/a-sin x (Ni) thin film proceeds primarily through a diffusion process, in which nickel atoms diffuse outward and oxygen ions inward. The oxidation takes place by progressive replacement of nitrogen with diffused oxygen. Five regions were identified in the oxidized layer from surface into the film. For films doped with 2.1 at.% Ni, a threshold temperature of 850 C was determined, below which, excellent oxidation resistance prevails but above which, oxidation takes place at exponential rate, accompanied by abrupt increase of surface roughness. I. INTRODUCTION The use of hard coatings in engines, machines, tools, and other wear resistant components has achieved a high level of commercial success. 1 Nanocomposite hard films have received much attention recently because they have improved mechanical 2 4 properties due to the size effect; 5,6 thus they become prime candidates for use in dry machining and related applications. Veprek et al. 7 reported that films of nanocrystalline TiN and TiSi 2 imbedded in amorphous Si 3 N 4 and amorphous TiSi 2 achieved a hardness of 105 GPa via chemical vapor deposition. Zhang et al. 8,9 prepared nc-tin/a-sin x nanocomposite thin films with hardness about 40 GPa through reactive magnetron sputtering. However, for practical industrial applications, the highest hardness is not the primary goal. Instead, a combination of hardness, toughness and, in many cases, oxidation resistance, is a) Address all correspondence to this author. msyzhang@ntu.edu.sg This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to mrs.org/publications/jmr/policy.html. DOI: /JMR sought after. To overcome the brittleness of bulk ceramics, usually a second ductile phase is incorporated or nanometer-sized metal particles are added 13,14 to improve toughness. This also applies to thin films and coatings. Musil and co-workers embedded crystalline nitrides in metallic Cu, 15,16 Ni, and Y, 20 etc., for improved toughness. It is obvious that there are two aspects of reasons for choosing Ni: (i) oxidation resistance 21 Ni is the base material for superalloys used in aerospace engines that operate at high temperatures and (ii) toughening effect addition of Ni brings about higher toughness, as illustrated by Irie et al. 22 in TiN Ni nanocomposite thin films deposited via cathodic arc ion plating. Recently, we studied the toughening effect of Ni in nc- TiN/a-SiN x. 23 The microstructure of the film is shown in Fig. 1 together with the selected area diffraction patterns of the nanocrystalline phase and amorphous matrix (SiN x ). The high-resolution transmission electron microscopy (HRTEM) characteristics of the Ni-doped film are completely identical to those without doping, as we reported recently. 7 The maximum grain size is less than 15 nm. Analysis of the selected area diffraction patterns shows that these crystallites are polycrystalline TiN and the matrix is amorphous. X-ray photoelectron spectroscopy (XPS) confirms that the amorphous matrix is of 2754 J. Mater. Res., Vol. 20, No. 10, Oct Materials Research Society

2 FIG. 1. HRTEM bright-field micrograph of nc-tin/a-sin x (2.1 at.% Ni) nanocomposite thin film including selected area diffraction patterns of the crystallite phase and amorphous matrix. 23 SiN x with metallic Ni. In view of the high-temperature application prospect of hard and yet tough nanocomposite films (as coatings for cutting tools for instance), this work was intended to study the oxidation behavior of these Ni-toughened nc-tin/a-sin x nanocomposite films. II. EXPERIMENTAL A. Film preparation and oxidation The films were prepared through co-sputtering of Ti (99.99%), TiNi (atomic ratio of 50/50, 99.99%), and Si 3 N 4 (99.999%) targets in an Ar/N 2 atmosphere in E303A magnetron sputtering system (Penta Vacuum, Singapore). The system was equipped with a cryogenic ultrahigh vacuum (UHV) and a high-performance rotary backing pump. The deposition chamber was pumped to a base pressure of Pa; during deposition the gas pressure was 0.67 Pa. The gas flow rates for both Ar and N 2 were 15 sccm. Radio frequency (rf) power was applied to Si 3 N 4 target, and direct current (dc) power was applied to Ti and TiNi targets. The target power density for different samples was listed in Table I. Silicon (100) wafers with a diameter of cm and thickness of 450 m were used as substrates, which were ultrasonically rinsed in acetone before deposition. During the deposition, the substrate temperature was kept at 450 C, and the substrate holder was rotated at 15 rpm for uniformity. The substrate-to-target distance was 100 mm. The deposition time was controlled to achieve film thickness of 0.7 m or about 5.8 nm/min. The film crosssectional thickness was determined using scanning electron microscopy (SEM; JEOL JSM-5600LV). To examine the oxidation resistance, as-deposited films with 2.1 at.% Ni and 13.0 at.% Ni were oxidized TABLE I. Deposition conditions and XPS determined composition of the as-deposited films (at.%) Sample code P1 P2 P3 Target power density (W/cm 2 ) Si 3 N TiNi Ti XPS chemical composition (at.%) Ni nil Ti Si N together with the un-doped films in open air (relative humidity 65%) in an Elite Furnace (BRF14/5-2416, Elite Thermal Systems, Ltd., UK) at fixed temperatures between 450 and 1000 C. The samples were placed in the furnace at room temperature before heating up to the desired temperature in 10 min. After reaching the temperature, the samples were kept for 15 min before cooling down to around 300 C in 20 min. Then the samples were taken out from the furnace and ambient air cooled to room temperature. B. Film characterization The elemental composition of the films was obtained by XPS analysis using a Kratos-Axis spectrometer with monochromatic Al K ( ev) x-ray radiation (15 kv and 10 ma) and a hemispherical electron energy analyzer (Kratos, UK). The base vacuum of the XPS chamber was Pa. The survey spectra in the range of ev were recorded in 1 ev step for each sample, followed by high-resolution spectra over selected element peaks in 0.1 ev steps. The area under these peaks is related to the amount of each element J. Mater. Res., Vol. 20, No. 10, Oct

3 present. By measuring the peak areas and correcting them for the appropriate instrumental factors, the percentage of each element detected is determined 24 and thus tabulated in Table I. Curve fitting was performed after a Shirley background subtraction by nonlinear least square fitting using a mixed Gauss/Lorentz function. In the least square fitting analysis of Ti 2p spectra, the area ratio of the 2p 3/2 to 2p 1/2 envelope was kept constant at 2 with a constant energy difference of 5.8 ev. The parameters used in fitting the Ti 2p,Si2p, and Ni 2p spectra are listed in Table II. Also listed are binding energies available from the literature. Sputter depth profiles of the oxidized films were obtained by recording the XPS spectra after Ar sputtering with an accelerating voltage of 4 kev. The bombardment was performed at an angle of incidence of 45 with respect to the surface normal. The spectra were referenced to the C 1s line of ev. 25 Sputtering conditions gave a sputtering rate of 4.5 nm/ min determined by measurement of a standardized layer of SiO 2 (30 nm thickness) on Si. Taking half of the intensity until the oxygen plateau as the measure of the oxide layer thickness, the thickness of oxide layer was calculated and listed in Table III. Phase information of the oxidized films was obtained using a Rigaku (Japan) X-ray Diffractometer MAX 2000 with Cu K radiation ( nm) at a scan rate of 2 /min. The scanning was conducted with different incident angle at 0.1, 0.5, 1.0, and 1.5. The lattice parameter of the nanocrystalline TiN was calculated according to peak positions or the distance between the crystal planes. 38 Surface morphology of the oxidized film was characterized using an atomic force microscope (AFM; Shimadzu SPM-9500J2, Japan). The measurement was conducted in ambient atmosphere and contact mode with a Si 3 N 4 tip. The scan size was 8 8 m at resolution of pixels; set point V and scan rate Hz. Table III lists the roughness after the oxidation treatment of films with and without Ni. III. RESULTS AND ANALYSIS A, Oxidation variation with depth 1. Chemical state of Ti, Si, and Ni Figure 2 shows XPS depth profile of the nanocomposite thin film oxidized at 850 C. Judging from the oxygen and nitrogen concentration, the profile in Fig. 2 can be divided into 5 regions. Detailed analysis of the chemical state of Ti (Fig. 3) gives rise to composition evolution from TiO 2 to TiN x O y to TiN as entering into the oxidation layer from depth of the film. Figure 3(a) plots Ti 2p core level spectra in the binding energy range from 452 to 468 ev for all the 5 regions. Sampling for Region I is at the surface, for Region II V in the middle of each region. Figure 3(b) is the quantitative deconvolution result of relative concentration of Ti in TiO 2, TiN x O y, and TiN. In Fig. 3(a), Ti 2p peaks of the oxidized film consist of three doubles: Ti 2p 3/2 at 459.0, 457.6, ev and Ti 2p 1/2 at ( ), ( ), and ( ) ev. The pair at and ( ) ev is assigned to TiO 2 ; the pair at and ( ) ev is ascribed to oxynitride TiN x O y, and the pair at and ( ) ev is for TiN (compare with Table II). From Fig. 3(a), it is obvious that deep inside the film (Region V), the main composition is still TiN (the nanocrystalline TiN in the nanocomposite film); moving more toward the surface (Regions IV and III), the amount of TiN decreases while TiN x O y increases. At the same time, TiO 2 appears. As the oxidation degree becomes even more severe (Regions II and I), both TiN x O y and TiN decrease to give way to formation of more and more TiO 2. At the surface, TiN and TiN x O y completely disappear while TiO 2 prevails [Fig. 3(b)]. As seen also from Fig. 2, the nitrogen content drastically drops from its bulk composition of about 50 at.% in Region V to <2 at.% in the oxidation layer (Regions II and I). This is in agreement with earlier analysis of the TABLE II. Ti 2p, Si 2p, and Ni 2p binding energy positions (ev) from literature. Ni 2p Element Ti 2p 3/2 Ref. Si 2p Ref. Ni 2p 1/2 Satellite peak Ni 2p 3/2 Ref. Position (ev) ± ± a ± a a State TiO 2 TiN x O y TiN SiO 2 Si 3 N 4 Si Metallic Ni a This work J. Mater. Res., Vol. 20, No. 10, Oct 2005

4 TABLE III. Film characteristics after 15 min of oxidation. Temperature ( C) Roughness R a (nm) Without Ni at.% Ni at.% Ni Oxide thickness (nm) Without Ni at.% Ni >620 FIG. 2. XPS depth profile of nc-tin/a-sin x (2.1 at.%ni) oxidized at 850 C in air for 15 min. evolution of the compounds as depth varies in the oxide layer. Since the total amount of N is low and it is responsible for TiN, TiN x O y, and Si 3 N 4 (to be discussed later), it is reasonable to assume that x in TiN x O y is very small while y is large (oxygen content is >65 at.% in Region II). From the shape of the oxygen and nitrogen profiles, it is obvious that during the oxidation process, oxygen diffuses inward and nitrogen diffuses outward. It is interesting to note that nickel diffuses toward the surface from the depth and builds up close to the surface: in Fig. 2, from 100 nm deep toward the surface, Ni content increases from the bulk composition of 2.1 at.% to about 15 at.% at the surface. (The drop of the relative amount at the very surface comes from the calculation involving surface adsorbed C). Compare to the XPS depth profile of the film without doping of Ni but underwent the same oxidation treatment (compare with Fig. 4), a 420-nmthick layer of oxide resulted in the undoped film while for film doped 2.1 at.% Ni, the oxide thickness is only 315 nm (compare with Fig. 2). This 25% reduction in oxide thickness is due to only about 2 at.% inclusion of Ni. Figure 5(a) plots Si 2p core level spectra in different region of the oxide layer. Si 2p spectrum has three possible states: 99.6 ev for atomic silicon (Si o ); ev corresponds to Si N bond (stoichiometric Si 3 N 4 ); ev for Si O bond (stoichiometric SiO 2 ). Some authors reported Si Ti bonding at 98.8 ev or existence of titanium silicide. 41 In this experiment no Si Ti bonding FIG. 3. (a) Ti 2p core level XPS spectra for nc-tin/a-sin x (2.1 at.% Ni) oxidized at 850 C in air for 15 min and (b) relative concentration of TiO 2, TiN x O y, and TiN. was observed. Figure 5(b) is the quantitative deconvolution result of relative concentration of Si in SiO 2,Si 3 N 4, and Si o. Going from deep in the film outward to the surface of the oxide layer [from Region V down to Region I in Fig. 5(a)], it is worth noting: (i) The amorphous silicon nitride matrix is actually prominently a-si 3 N 4 with a very small amount of free silicon [<10 at.% of all silicon in the film; see Fig. 5(b)]. The existence of free Si is due to J. Mater. Res., Vol. 20, No. 10, Oct

5 FIG. 4. XPS depth profile of the nc-tin/a-sin x nanocomposite thin film (without Ni) oxidized at 850 C in air for 15 min. deficit in nitrogen source compared with Si source during deposition process. (ii) Moving toward the surface, the amount of the free silicon decreases to zero, the amount of Si in the state of Si 3 N 4 drops from about 80 to 15 at.% while Si in SiO 2 increases from about 10 to 85 at.%. In other words, the free silicon and some of the Si 3 N 4 become oxidized into SiO 2. Gogotsi and Porz 39,40 have noted that Si 3 N 4 is more oxidation resistant than TiN. In fact, this is also noticed by comparing surface oxidation state of Ti and Si from this experiment: at the surface, TiN is completely oxidized into TiO 2 [compare Region I in Fig. 3(b)]; while also at the surface of the same sample, there is still about 15 at.% Si in the form of Si 3 N 4 free from oxidation [compare Region I in Fig. 5(b)]. Figure 6 shows Ni 2p core level spectra. Metallic nickel (Ni o ) has a binding energy value of ev (2p 3/2 ) and ev (2p 1/2 ) (compare with Table II). The peak at ev is the satellite peak probably due to the consequence of sputter-damaged crystallites. 32 A few points worth noticing in Fig. 6 include the following: (i) There is no peak shift for Ni 2p, which indicates that the Ni does not react with oxygen at 850 C. (ii) From deep in the film towards the surface, Ni peak intensities increase from Region V to Region I, indicating Ni diffusion towards sample surface, as also illustrated in Fig Phase identification Figure 7 shows grazing incident x-ray diffraction (GIXRD) patterns with different incident angle for the nanocomposite thin film oxidized at 550 C for 15 min. No x-ray diffraction (XRD) peaks are observed for crystalline Si 3 N 4, SiO 2, and Ni; however, XPS analysis indicated the existence of Si 3 N 4, SiO 2 [compare with Fig. 5(b)] and Ni (compare with Fig. 6), so these phases must thus be amorphous. This is also evident from Fig. 7; FIG. 5. (a) Si 2p core level XPS spectra for nc-tin/a-sin x (2.1 at.% Ni) nanocomposite film oxidized at 850 C in air for 15 min; (b) relative concentration of Si o, SiO 2, and Si 3 N 4. even at incident angle increases of 1.5, no crystalline peaks of Si 3 N 4, SiO 2, and Ni are observed. At low incident angle (such as 0.1 ), the presence of wellcrystallized TiO 2 (rutile) and TiN are observed. At higher incident angle, the intensity of TiO 2 peaks decreases while the intensity of TiN peaks increases, indicating reduction in oxidation with depth. (X-ray penetration depth increases as incident angle increases). This agrees well with the XPS analysis [compare with Fig. 3(b)]. B. Oxidation variation with temperature 1. Chemical composition Figure 8 shows the surface composition of the thin films oxidized at C determined by XPS. With increasing oxidation temperature, the oxygen content increases slightly, silicon decreases slightly, and titanium remains constant, while nitrogen decreases sharply. At 450 C nitrogen is about 25 at.%. A hundred degree 2758 J. Mater. Res., Vol. 20, No. 10, Oct 2005

6 FIG. 6. Ni 2p XPS spectra in the oxidation layer for Region I through V for nc-tin/a-sin x (2.1 at.% Ni) oxidized at 850 C in air for 15 min. FIG. 8. Surface elemental concentration of nc-tin/a-sin x (2.1 at.% Ni) at various oxidation temperatures. FIG. 7. GIXRD patterns with different incident angle for nc-tin/a- SiN x (2.1 at.% Ni) oxidized at 550 C in air for 15 min. increase to 550 C brings about a significant decrease to 7 at.%. With further increase of temperature to 625 C, N decreases to <4 at.%. The significant decrease in N comes from the oxidation of Ti from TiN and the oxidation of Si from Si 3 N 4. These reactions deplete N through formation of N 2 (see details in discussion). The fact that nitrogen still persists in the surface layer indicates that Si 3 N 4 still exists even at 1000 C. Also note from Fig. 8, as the oxidation temperature increases from 450 to 900 C, surface nickel increases from 1 to 9 at.%; i.e., higher temperature promotes outward diffusion of nickel. With further increase to 1000 C, however, surface nickel drastically decreases to 3 at.%. The reason is still unclear. Figure 9 shows the GIXRD patterns of the nc-tin/a- SiN x (Ni) films oxidized at elevated temperature of 550, 625, 700, 750, 850, and 950 C. Since an incident angle of 0.5 is low enough to observe both TiN and TiO 2 FIG. 9. GIXRD patterns of nc-tin/a-sin x (2.1 at.% Ni) oxidized in air for 15 min under various temperatures. peaks (compare with Fig. 7), 0.5 is chosen for all the samples in Fig. 9. As temperature increases, the intensity of the TiO 2 peaks increases and that of TiN peaks decreases. At 850 C and above, the number and intensity of TiO 2 peaks increase significantly, signaling the total collapse of the film s oxidation resistance. Based on the GIXRD pattern analysis for film oxidized at 950 C, taken , and , respectively, calculated lattice parameter of the TiN nanocrystal is nm, which agrees with the standard value of TiN (a JCPDF nm) very well (as stipulated by Ref. 41, 41 an error of nm can be considered as perfect match). This indicates that there exists no interstitial or substitutional solid solution after annealing treatment. It is interesting to note that in the as-deposited films, the TiN crystals are substitutional solid solution with Si and Ni replacing some of its Ti atoms, forming (Ti,Si,Ni)N with reduced lattice constant. 23 At high temperature, these invaded atoms (Si, J. Mater. Res., Vol. 20, No. 10, Oct

7 Ni) diffuse out of the TiN lattice, resulting in the resumption of the TiN lattice constant. This contributes to relaxation of part of the residual stress. 2. Topography Figure 10 shows the roughness change of nc-tin/a- SiN x (Ni) films (where Ni contents are 0, 2.1, and 13.0 at.%) oxidized at 450 C through 1000 C for 15 min. The roughness data are also tabulated in Table III. For films without Ni, at 800 C, R a is 2 nm, but increases to 6 nm at 850 C and sharply to 25 nm at 950 C. For the film with only 2.1 at.% Ni, the roughness increases slightly from 1 to 4 nm as the temperature increases from 450 to 850 C; at 950 C, it increases to 14 nm. As the temperature further increases to 1000 C, roughness increases drastically to 41 nm. For the film with 13 at.% Ni, at 850 C, the roughness is only 3 nm, and it becomes 10 at 950 C and further increases to 32 nm at 1000 C. Roughness increase with temperature can be attributed to (i) grain growth or grain conglomeration, (ii) formation of surface oxides, and (iii) the growth of these oxides. Since the growth of oxides is a diffusion process, 41 roughness should also be related to the diffusion process. To better understand the temperature influence, Fig. 11 shows a plot roughness as a function of 1/T. For 2.1 at.% Ni, two segments of straight line (a and c) are seen: a is from 450 to 750 C, and c is for 850 to 950 C. To compare the Ni content effect at high temperatures, that without Ni and with 13.0 at.% Ni are plotted (curve b and d). Clearly, line a and c converge at about 850 C, and lines b, c, and d have different slopes. The change of slope for line a to line c indicates a threshold temperature at about 850 C. In studying oxidation, the oxidation layer thickness is considered proportional to the square FIG. 11. Roughness of nc-tin/a-sin x (Ni) as a function of reciprocal temperature at different Ni content: (a) 2.1 at.% Ni from 450 to 750 C, (b) 850 to 950 C without Ni, (c) 850 to 950 C with 2.1 at.% Ni, and (d) 850 to 950 C with 13.0 at.% Ni. root of (Dt) where D is the diffusion coefficient and t the oxidation time. The slope of layer thickness versus 1/T is the diffusion activation energy. 41 In the case of roughness, similarly, the slope of R a versus 1/T qualitatively gives rise to diffusion activation energy. The change of slope for lines b, c, and d signals change of activation energy: from 0, to 2.1 to 13.0 at.% Ni, the activation energy increases, and thus at higher Ni content, less roughness is observed. (Quantitative determination of the diffusion activation energy from the roughness data needs explicit knowledge of the relationship between roughness and diffusion coefficient). IV. DISCUSSION The experiment suggests that the oxidation of the Nidoped nitride is mainly a diffusional process; nickel atoms diffuse outward and oxygen ions diffuse inward (compare with Fig. 2). Oxidation proceeds by a progressive replacement of nitrogen with diffused oxygen. The oxidation of titanium nitride to rutile or titanium dioxide (TiO 2 ) starts at 450 C according to TiN s + O 2 g TiO 2 s N 2 g. (1) FIG. 10. Surface roughness of nc-tin/a-sin x (with different Ni) films at various oxidation temperatures. This mechanism is in agreement with the general theory for the oxidation of titanium nitride, which considers that the process is controlled by anionic diffusion The oxidation of silicon nitride to stoichiometric SiO 2 starts at 450 C according to this reaction: Si 3 N 4 s + 3O 2 g 3SiO 2 s + 2N 2 g. (2) This oxidation is the result of the inward diffusion of oxygen through the oxide layer. 47,48 Five regions can be 2760 J. Mater. Res., Vol. 20, No. 10, Oct 2005

8 along random direction occur, and roughness of the surface increases drastically. FIG. 12. Schematic representation of the phase distribution after oxidation of the nc-tin/a-sin x (Ni) nanocomposite thin films; the five regions correspond to those in Fig. 2. distinguished in the oxidized layer (compare with Fig. 2) consisting of the following six components: TiO 2, TiN x O y, TiN, SiO 2,Si 3 N 4, and Ni (compare with Figs. 3, 5, and 6). The TiO 2 and TiN are crystalline phases while Si 3 N 4, SiO 2 are amorphous phases (compare with Fig. 7). Following XPS depth profile (Fig. 2) and GIXRD result at different incident angles (Fig. 7) of the oxidized film, a schematic representation of the phase distribution in the oxidized layer is proposed in Fig. 12 where white cycles represent crystalline TiO 2, gray cycles represent crystalline TiN x O y, black cycles represent crystalline TiN, small solid cycles represent metallic Ni, and the background is amorphous Si 3 N 4 with amorphous SiO 2. From top to the core of the film, or from left to the right in Fig. 12, Region I is composed of mainly crystalline TiO 2, amorphous SiO 2, and metallic Ni in the amorphous matrix of Si 3 N 4. In Region II, the extent of oxidation is lessened (less amount TiO 2 and lots of TiN x O y ). In Region III, more TiN presents while TiN x O y reduces; In Region IV, even less TiN crystalline is oxidized (reduced amount of TiO 2 and TiN x O y ). In Region V, though there are still TiN x O y, basically no TiO 2 exists. The presence of a Ni-rich zone at the top of the oxide layer effectively blocked the inward diffusion of O. As oxidation temperature increases, the oxidation process becomes faster, presumably by the increase of the diffusivity of O 2 and N 2. Combining the components information at the oxide area from XPS (compare with Fig. 8), GIXRD (compare with Fig. 9) and the topographical information from AFM observations (compare with Fig. 10) allows us to assume there is a threshold temperature, in this case about 850 C, below which the nc-tin/a-sin x (Ni) forms a stable nickel rich layer. The number of diffusion paths (grain-boundaries, defects) would considerably decrease, leading to the passivation regime, limiting oxygen diffusion thus increase the oxidation resistance. Above the threshold temperature, the barrier effect of metallic nickel can no longer prevent oxygen diffusion. Consequently, oxide thickness increases significantly, the rutile nucleation and growth V. CONCLUSIONS (1) Doping of Ni results in reduced oxide thickness and high temperature roughness of nc-tin/a-sin x nanocomposite film. (2) Ni does not participate in the oxidation process within the temperature studied but exists in the metallic state. Ni tends to accumulate towards the surface of the film. (3) Both TiN nanocrystals and SiN x amorphous matrix participate in the oxidation. The oxidation of the nc-tin results in formation of crystalline TiO 2 at the top layer followed by a transient layer of TiN x O y. The oxidation of the a-sin x matrix gives rise to amorphous SiO 2. (4) There exists a threshold temperature above which oxidation takes place at much increased rate. ACKNOWLEDGMENT This work was supported by Nanyang Technological University s research Grant No. RG12/02. REFERENCES 1. T. Cselle and A. Barimani: Today s applications and future developments of coatings for drills and rotating tools. Surf. Coat. Technol , 712 (1995). 2. S. Veprek and S. Reiprich: A concept for the design of novel superhard coatings. Thin Solid Films 268, 64 (1995). 3. S. Zhang, D. Sun, Y. Fu, and H. Du: Recent advances of superhard nanocomposite coatings: a review. Surf. Coat. Technol. 167, 113 (2003). 4. S. Zhang, D. Sun, and Y. Fu: Superhard nanocomposite coatings. J. Mater. Sci. Technol. 18, 485 (2002). 5. A.I. Gusev: Effects of the nanocrystalline state in solids. Physics- Uspekhi. 41(1), 49 (1998). 6. B. Cantor, C.M. Allen, R. Dunin-Burkowski, M.H. Green, J.L. Hutchinson, K.A.Q. O Reilly, A.K. Petfor-Long, P. Schumacher, J. Sloan, and P.J. Warren: Applications of nanocomposites. Scripta Mater. 44, 2055 (2001). 7. S. Veprek, A. Niederhofer, K. Moto, T. Bolom, H-D. Mannling, P. Nesladek, G. Dollinger, and A. Bergmaier: Composition, nanostructure and origin of ultrahardness in nc-tin/a-si 3 N 4 /a- and nc-tisi 2 nanocomposite with H v 80 to 105 GPa. Surf. Coat. Technol , 152 (2000). 8. S. Zhang, D. Sun, Y. Fu, and H. Du: Effect of sputtering target power on microstructure and mechanical properties of nanocomposite nc-tin/a-sin x thin films. Thin Solid Films , 462 (2004). 9. S. Zhang, D. Sun, Y. Fu, H. Du, and Q. Zhang: Effect of sputtering target power density on topography and residual stress during growth of nanocomposite nc-tin/a-sinx thin films. Diamond Relat. Mater. 13, 1777 (2004). 10. M. Ruhle and A.G. Evans: High toughness ceramics and ceramic composites. Prog. Mater. Sci. 33, 85 (1989). J. Mater. Res., Vol. 20, No. 10, Oct

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