Studies of thermal stability of a-c:h:si coatings produced by radio-frequency plasma assisted chemical vapour deposition (RF-PACVD) method

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1 Inżynieria Materiałowa 4 (212) (2016) DOI / Copyright SIGMA-NOT MATERIALS ENGINEERING Studies of thermal stability of a-c:h:si coatings produced by radio-frequency plasma assisted chemical vapour deposition (RF-PACVD) method Damian Batory, Anna Jędrzejczak *, Anna Sobczyk-Guzenda, Witold Szymański, Piotr Niedzielski Institute of Materials Science and Engineering, Lodz University of Technology, Lodz, Poland, * anna.jedrzejczak@p.lodz.pl In this work the thermal stability of silicon-doped diamond-like carbon (DLC) films was investigated. The studied coatings were produced by radio-frequency plasma assisted chemical vapour deposition (RF-PACVD) method with use of tetramethylsilane (TMS) as a silicon precursor. As-deposited Si-DLC coatings with three different silicon concentrations were annealed at 400 C, 500 C, and 600 C for 1 hour in air atmosphere. For comparison DLC coatings were also examined. It has been shown that the level of disorder of Si-DLC increases with the increase of silicon concentration. Silicon admixture improves the thermal stability of Si-DLC coatings by slowing down and delaying the graphitization processes compared to the undoped DLC films. Furthermore, an increase in hardness of the Si-DLC coatings annealed at the temperature of 400 C has been observed. The DLC and Si-DLC coatings with the lowest Si concentration annealed at 500 C, and all of the coatings annealed at 600 C have been completely degraded. The coatings with the highest concentration of silicon that have stood the annealing process at 500 C have demonstrated a high degree of graphitization and degradation, manifesting itself in the lowest mechanical properties and a significant reduction in their. Key words: thermal stability, silicon doping, DLC films. 1. INTRODUCTION Diamond-like carbon (DLC) films are characterized by many unique properties. These include high biocompatibility [1], high hardness [2], low coefficient of friction, [3, 4] good anti-wear [4] and anti-corrosion [5] characteristics. Hence, these films are commonly used as protective coatings in automotive industry, magnetic data storage, machining tools and biomedical applications [1 3, 6]. However, DLC coatings have several known disadvantages. One of them, severely limiting their large scale application possibilities is the low thermal stability at high working temperature. It has been reported that DLC coatings maintain stable properties up to approximately 400 C. Above this temperature the graphitization process starts [7]. Graphitization consists in the conversion of sp 3 hybridized carbon bonds to the sp 2 hybridized ones, which is revealed by the increasing I(D)/I(G) intensity ratio of the Raman spectra and it manifests itself in a decrease of the mechanical properties. It has been well recognized that DLC coatings doped with various elements possess improved properties. Recently in the literature more and more common reports of excellent properties of silicon-doped carbon coatings (Si-DLC), such as improved corrosion resistance [8], biocompatibility [9] and lower coefficient of friction [10] can be found. A review of the literature shows that silicon introduced into the DLC coating structure increases the sp 3 - hybridized carbon content [11, 12]. Moreover the stabilization of sp 3 -hybridized carbon caused by the presence of silicon, delays the graphitization process. In this work the influence of silicon admixture on the thermal stability of diamond-like carbon films was investigated. Si-DLC coatings with three different concentrations of silicon were produced by radio-frequency plasma assisted chemical vapour deposition (RF-PACVD) method with the use of tetramethylsilane (TMS) as a silicon precursor. Subsequently the obtained coatings were annealed at various temperature in air atmosphere. 2. EXPERIMENT 2.1. The substrate material As a substrates, one-side polished monocrystalline silicon <100> wafers with a of 500 μm were used. Furthermore, for the purpose of FTIR analysis mirror polished, 500 μm thick, p-type Si wafers of <111> orientation were applied. Directly before the synthesis, the substrates were washed in an ultrasonic cleaner in acetone for 10 minutes. Such prepared substrates were placed in the reactor chamber, directly on the water-cooled RF electrode Synthesis process of Si-DLC coatings s with a es of ~200 were prepared using RF-PACVD method. Prior to the synthesis process substrates were etched in argon plasma for 10 minutes, under negative self-bias of 1100 V. Si-DLC films were obtained from CH 4 /TMS gas mixture with the flow ratios of 18/4, 16/12 and 14/21 (further denoted as Si-DLC1, Si-DLC2 and Si-DLC3, respectively), with the use of negative self-bias of 800 V. Table 1 presents the basic parameters Table 1. Parameters of Si-DLC coatings deposition Tabela 1. Parametry syntezy powłok Si-DLC type Gas atmosphere CH 4 /TMS Bias V Pressure Pa Deposition time s Si % at. DLC 20 sccm CH Si-DLC1 18/ Si-DLC2 17/ Si-DLC3 16/ INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

2 of Si-DLC coatings deposition together with the obtained silicon concentration determined by means of X-ray photoelectron spectroscopy method (XPS). Subsequently, in order to examine the thermal stability, deposited coatings were annealed at 400 C, 500 C, and 600 C for 1 hour in air atmosphere. For the full evaluation of the silicon admixture effect on the properties of Si-DLC coatings at higher temperature, the study includes the uodified DLC coating as well Research methodology The DLC and Si-DLC1 coatings annealed at 500 C, as well as annealed at 600 C have completely degraded. The remaining samples have been investigated as follows. Chemical structure was examined using the Raman spectroscopy (RS) method. A Raman spectrometer invia Renishaw working with 532 wavelength was applied. Measurements were carried out in 3 different points. The power of the laser on the surface of the studied samples was 2.9 mw and the laser spot size was 500. All measurements were carried out in ambient conditions at room temperature. The Raman data processing was made using PeakFit software. Additionally, the investigation of the chemical structure of the deposited coatings was performed using Fourier transform infrared spectroscopy (FTIR) technique. The FTIR spectra were measured in the range of cm 1 by a Nicolet is50 spectrophotometer (Thermo Scientific, USA) working in an absorbance mode with the use of diffuse reflectance (DRIFT) cell (Harrick Scientific). A DTGS KBr beamsplitter was used. The number of scans per measurement cycle was 64. The analysis was performed with the use of the X-ray reflectivity method (XRR), which uses the measurement of a period of oscillations caused by the X-ray interference (Kiessig fringes) [13]. The XRR studies were conducted on an Empyrean diffractometer (Panalytical), working with Co Kα radiation (λ = ). For a better interpretation of the of the analysed layers the Fourier transform was applied to each spectra. Hardness and Young s modulus of the coatings were investigated using nanoindentation technique. A Nano Indenter G200 (Agilent Technologies, USA) modular system with a diamond Berkovich tip (Micro Star Technologies, USA) working in the continuous stiffness measurement (CSM) mode was used. 3. RESULTS AND DISCUSSION 3.1. Raman spectroscopy The evolution of the Raman spectra of DLC and Si-DLC coatings annealed at 400 C is shown in Figure 1. Wide asymmetric slopes, which are characteristic for amorphous carbon are visible [14]. The spectra were fitted using standard Gaussian peak shape. Deconvolution procedure allowed to obtain two peaks: D and G. The presence of the D-band (~1350 cm 1 ) is attributed to the existence of sp 2 -hybridized carbon atoms vibrations only in the aromatic rings, while the presence of the G-band (~1550 cm 1 ) indicates the presence of stretching vibration of the sp 2 -hybridized carbon atoms both in chains and aromatic rings [15, 16]. Based on the results of deconvolution following parameters were determined: the G-peak position, full width at half maximum (FWHM) of the G peak and the I(D)/I(G) intensity ratio. Since all the parameters describe the sp 2 -hybridized carbon they also give the indirect information on the quality and quantity of the sp 3 -hybridized one [14 16]. Table 2 summarizes the results of deconvolution of Raman spectra recorded before and after the annealing processes. In the case of the as deposited coatings a decrease of the I(D)/I(G) ratio with an increasing flow rate of the silicon precursor was observed. The reason for the decrease of the I(D)/I(G) intensity ratio is the increasing disorder in the chemical structure of the coatings Fig. 1. Raman spectra of the samples annealed at 400 C Rys. 1. Widma Ramana próbek wygrzewanych w temperaturze 400 C Table 2. Comparison of the Raman spectra parameters of the examined coatings before and after the annealing processes Tabela 2. Zestawienie wyników dekonwolucji widm Ramana badanych powłok przed i po wygrzewaniu type DLC Annleaning temperature C G peak position cm 1 FWHM G I(D)/I(G) Si-DLC As-deposited Si-DLC Si-DLC DLC Si-DLC Si-DLC Si-DLC DLC Si-DLC1 500 Si-DLC Si-DLC caused by the presence of silicon. Authors of works [14 16] identify the decrease in the I(D)/I(G) value with a reduction of size of sp 2 -hybridized carbon clusters and an increase of the sp 3 -hybridized carbon concentration in the chemical structure of coatings. It can be concluded that the higher is the Si content, the greater is the content of the sp 3 -hybridized carbon in the coating. The full width at half maximum of the G band (FWHM G) provides information about the size of sp 2 -hybridized carbon clusters in the amorphous carbon matrix [16]. Considering the silicon concentration in these coatings, the rise of its content resulted in an increase of FWHM. This represents an increase of disorder in chemical structure and reduction in the content of sp 2 -hybridized carbon, which is consistent with the results of ref. [17]. The analysis of the G band position also allows determining the structural changes occurring in the coatings. A shift in the G band position toward lower wavenumbers with increasing Si content was observed. This phenomenon may indicate a decrease in the residual stress level in the coating as a result of Si incorporation [14, 15]. In the case of NR 4/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 179

3 coatings annealed at 400 C similar trends with increasing Si content can be observed. But when comparing with the parameters of Raman spectra of the as-deposited coatings, the influence of silicon admixture on the behaviour of the coatings at high temperature is revealed. DLC and Si-DLC coatings with the lowest concentration of silicon, which were annealed at 400 C show a greater increase in the I(D)/I(G) ratio (1.52 and 1.14, respectively) and thus a higher degree of graphitization than Si-DLC2 and Si-DLC3 coatings, wherein the value of I(D)/I(G) intensity ratio is 0.65 and 0.5, respectively. For comparison, authors of work [18] examined the thermal stability of Si-DLC coatings produced using silane (SiH 4 ) as a silicon precursor. They revealed a noticeable increase in I(D)/I(G) ratio for DLC and Si-DLC (with silicon addition up to 19.2% at.) coatings annealed at a temperature of 300 C and 400 C, respectively. As for the FWHM of the G band the lower value of this parameter compared to as-deposited coatings also indicates progressive graphitization which is the result of exposure to high temperature. However, in the case of those with the highest concentration of Si decrease in this parameter is slight. When analyzing the changes in position of the G peak, a shift to higher wavenumbers in the spectra of annealed coatings, compared to the as deposited, was observed. Based on the model proposed by Ferrari [15], a band shift toward higher wavenumbers may indicate the formation of sp 2 -hybridized carbon clusters. The analysis of the G-band position shows that the coatings with the highest concentration of silicon are characterized by the smaller blue-shift of the G-band and thus the lowest degree of graphitization. In the case of coatings which have stood the annealing processes at 500 C, the I(D)/I(G) ratios, the G-band positions and the values of FWHM of the G band indicate an advanced graphitization of their chemical structure. It is worth to emphasize, that in case of the Si-DLC3 coating annealed at 500 C, the Raman analysis showed a lower degree of graphitization comparing to the DLC coating annealed at 400 C Fourier transform infrared spectroscopy Summary of the characteristic vibrations for functional groups present in the examined coatings is listed in Table 3. Across all of FTIR spectra of the studied coatings the presence of CH x groups in the range of cm 1 was observed [19]. These groups are characteristic for the hydrogenated amorphous carbon coatings (a-c:h). Within the range between cm 1 the presence of bands derived from the vibration modes of C=O and C=C groups was revealed [19, 20]. In the case of Si-DLC coatings it was also observed the presence of peaks derived from combinations of Si C, Si O and Si H bands. Figure 2 shows the effect of annealing process on the chemical structure of the examined samples. Figure 2a displays the influence of annealing at 400 C on structural changes in the DLC coating. The changes which were observed in the presented FTIR spectra are apparent in the range between cm 1, wherein are located vibrations of C=O bonds. This phenomenon is related to the oxidation of the coating at high temperature. As visible, the content of CH x groups is also reduced. Figure 2b shows the spectra of Si-DLC1 coatings. At a temperature of 400 C, in the range between cm 1 which is assigned to the Si O group vibrations, there are two additional maxima at 1130 cm 1 and 1070 cm 1. These peaks originate from Si O (CH 2 ) n and Si O Si groups respectively, with the angle of 144 between the bonds [21]. Such position of the maximum absorption indicates the crystalline form of structure. An additional band originating from C=O groups has emerged at approximately 1770 cm 1. Furthermore, the reduction in the number of CH n and Si H groups has been also observed in the case of the films with the highest content of Si (Si-DLC2 and Si-DLC3). The peak intensities derived from CH x groups are comparable for both coatings annealed at 400 C and as-deposited. It means that the coating is stable and the changes concern only the Table 3. Summary of the characteristic vibrations for functional groups present in the examined coatings before and after annealing process Tabela 3. Zestawienie charakterystycznych drgań grup funkcyjnych występujących w strukturze badanych powłok przed i po wygrzewaniu Wavenumber cm 1 Functional group (vibrations) Ref CH 3 asymmetric C=O and Si O combinations, the number of which increased. In the case of the coatings annealed at 500 C, an intense sharp peak originating from the crystalline and a peak with a lower maximum which is assigned to the Si O (CH 2 ) n groups have emerged. Simultaneously, the content of CH x groups unbound with Si atoms has been decreased. Furthermore, at this temperature, a peak originating from the Si C bonds (800 cm 1 ) has also been observed X-ray reflectometry type DLC Si-DLC CH 2 asymmetric + + [19] 2875 CH 3 symmetric CH 2 symmetric Si H [21] C=O, C=C [19, 20] CH 2 bending + + [19] 1420 CH 3 bending asymmetric Si CH Si O (CH 2 ) n Si O asym. stretching Si C stretchning in SiMe 2 [21, 22] Si C stretchning in SiMe Si C in Si C non-hydrogenated Si C in Si C non-hydrogenated C=C [19, 20] + The XRR spectra for as-deposited Si-DLC3 sample as well as annealed at 400 C and 500 C are presented in Figure 3. Since we observed the same behaviour for all analysed coatings the changes in the XRR spectra will be discussed based on this example. The spectra of sample Si-DLC3 clearly resemble the one characteristic for a system composed of only one layer and the substrate. There is one type of oscillation visible which makes it easy to determine the. Moreover after the Fourier transform of the XRR spectra no additional peaks were observed despite the fact that usually the silicon substrates are covered by a thin (c.a. 2 3 ) layer of silicon oxide. The annealing at 400 C resulted in a slight decrease in the intensity of the oscillations and at the same time the spectra show the slight bend on the oscillation curve which suggests appearance on an additional layer in the system. This can be either the formation of on the surface of the Si-DLC3 layer or changed density contrast between the layer and thin silicon oxide on the surface of the wafer, which crystallized as the result of annealing. However, since we did not observe this effect for the DLC layer annealed at 400 C, the first hypothesis is more probable. The FTIR results also proved that the is crystalline. The final annealing procedure most strongly affected the XRR spectra. Additional oscillations are clearly visible at the same time causing more complex fringes. In this case the Fourier transform revealed the appearance of thick additional layer accompanied by Si-DLC3 coating thinner than as-deposited one. In Table 4 are presented results of a calculation of of all examined samples before and after the annealing. Uodified DLC coating seems to be invulnerable to the 180 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

4 a) b) c) d) Fig. 2. FTIR spectra of the following coatings: a) DLC, b) Si-DLC1, c) Si-DLC2, d) Si-DLC3 before and after annealing process Rys. 2. Widma FTIR powłok: a) DLC, b) Si-DLC1, c) Si-DLC2, d) Si-DLC3 przed oraz po wygrzewaniu Table 4. The comparison of all analysed samples before and after the annealing procedure Tabela 4. Porównanie grubości wszystkich badanych powłok przed oraz po wygrzewaniu As-deposited Annealed at 400 C Annealed at 500 C type DLC Si-DLC Si-DLC Si-DLC Fig. 3. The XRR spectra of as-deposited and annealed Si-DLC3 sample Rys. 3. Widma XRR powłok Si-DLC3 przed i po wygrzewaniu temperature of 400 C. After the annealing there was no change in the. For the other three coatings a minor decrease in the coating was observed. Moreover, in each case we noticed the formation of an additional layer as also confirmed by FTIR results. Interestingly, the of the layer decreases with the concentration of silicon in the coating. Further increase in the temperature for samples Si-DLC2 and Si-DLC3 resulted in progressing destruction of the layer and advancing formation of. Nevertheless, still we observed the opposite trend in formation of with regard to the concentration of silicon Nanoindentation Figure 4 presents the variation of measured values of hardness for DLC and Si-DLC layers. In the case of as deposited films a slight increase in the hardness for Si-DLC coatings (up to 15.7 GPa) compared to pure DLC (15 GPa) was observed, which is consistent with the work [23]. As a result of annealing at 400 C, the mechanical properties of DLC coatings deteriorated and hardness decreased to 12.7 GPa, as expected. This is the result of progressive graphitization of the coating, as demonstrated by Raman spectroscopy. A surprisingly high hardness characterized the Si-DLC coatings annealed at this temperature (up to 16.6 GPa). The increase of Si-DLC coatings hardness observed at 400 C can be explained as follows. Annealing of the hydrogenated DLC films at elevated temperature causes hydrogen desorption in the film. As is well known, the existence of hydrogen increases the proportion of sp 3 bonds [24]. NR 4/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 181

5 withstood the annealing process at 500 C have shown a high degree of graphitization and partial degradation, evidenced by a decrease in concentration of CH n groups and a drastic decrease in their and mechanical properties, as demonstrated by the XRR and nanoindentation analysis. ACKNOWLEDGEMENT This work was supported by research project: Determine the relationship between the content of impurities of silicon and titanium atoms and the effectiveness modification of self-assembled compounds on tribological properties of the carbon coatings, UMO- 2014/13/B/ST8/03114, financed by the National Science Centre. Fig. 4. Hardness of DLC and Si-DLC coatings as-deposited and annealed Rys. 4. Twardość powłok DLC i Si-DLC przed i po wygrzewaniu As shown by the Raman spectroscopy results the share of sp 3 -hybridized carbon in Si-DLC coatings does not decrease so significantly as in case of DLC. For Si-DLC coatings the proportion of sp 3 -hybridized carbon and its stabilization results not only due to the presence of hydrogen but also and mainly due to silicon. Therefore it can be concluded that less pronounced decrease in sp 3 concentration in Si-DLC results rather from the beneficial influence of silicon than hydrogen. Hydrogen desorption manifested in decreased concentration of Si H and CH n functional groups revealed hidden bands for Si C and thin crystalline Si O layer or inclusions (as confirmed by FTIR and XRR). This in turn may be the reason of increased hardness of coatings annealied at 400 C. The lowest hardness (2 GPa) was registered for Si-DLC2 and Si-DLC3 coatings annealed at 500 C. As shown by Raman spectroscopy, these coatings display the highest degree of graphitization. Taking into account a decrease in their (demonstrated by means of XRR analysis), immense decrease in the content of CH x groups (confirmed by FTIR spectroscopy), and the influence of the substrate, it can be stated that the reason for this is a partial degradation of these films which is reflected in low mechanical properties. In this case also the hardness promoting Si O based structures degraded and thus we did not observe their positive effect on this parameter. 4. CONCLUSIONS The thermal stability of the silicon-incorporated DLC films was investigated. The DLC and Si-DLC1 coatings annealed at 500 C, and all of the coatings annealed at 600 C have completely degraded. It was observed that concentration of silicon admixture equal and higher than 10% at. raise the stability of carbon coatings up to 400 C. Raman spectroscopy analysis has revealed that the presence of silicon in the DLC matrix promotes the formation of the sp 3 -hybridized carbon bonds. The higher the silicon concentration, the higher the degree of structure disorder observed. As a result, the structure of Si-doped layers has turned out to be more stable at temperature of 400 C (this was confirmed by FTIR analysis), and at the same time the graphitization of Si-DLC coatings annealed at the temperature of 500 C was slowed down. However, it should be noted that the coating with the lowest silicon content has shown a similar behaviour to the undoped DLC coating. This indicates that only sufficiently high concentration of Si leads to ensure the stability of carbon coatings at high temperature. FTIR analysis has demonstrated the hydrogen effusion from the coatings (reduction in the number of CH n and Si H groups) and oxidation of the structure, as evidenced by the increase in the content of SiO and C=O groups. Annealing of Si-DLC coatings at 400 C increases their hardness. 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6 Inżynieria Materiałowa 4 (212) (2016) DOI / Badania stabilności termicznej powłok a-c:h:si wytwarzanych metodą chemicznego osadzania z fazy gazowej wspomaganego plazmą częstotliwości radiowej (RF PACVD) Damian Batory, Anna Jędrzejczak *, Anna Sobczyk-Guzenda, Witold Szymański, Piotr Niedzielski Instytut Inżynierii Materiałowej, Politechnika Łódzka, * anna.jedrzejczak@p.lodz.pl Słowa kluczowe: stabilność termiczna, domieszkowanie krzemem, powłoki DLC. Copyright SIGMA-NOT MATERIALS ENGINEERING 1. CEL PRACY Diamentopodobne warstwy węglowe (DLC) charakteryzują się szeregiem unikatowych właściwości, do których można zaliczyć m.in. wysoką biokompatybilność, dużą twardość, mały współczynnik tarcia oraz odporność na korozję. Dlatego powłoki DLC są powszechnie stosowane w przemyśle motoryzacyjnym, maszynowym, a także medycznym. Pomimo wielu korzystnych właściwości wykazują one niską stabilność termiczną, co ogranicza ich zastosowanie. Jednym z możliwych rozwiązań powodujących zwiększenie temperatury pracy powłok węglowych jest włączenie do ich struktury atomów domieszki. Celem pracy było zbadanie stabilności termicznej powłok (DLC) domieszkowanych krzemem. Powłoki wytworzono metodą chemicznego osadzania z fazy gazowej wspomaganego plazmą częstotliwości radiowej (RF PACVD). Jako prekursor krzemu zastosowano tetrametylosilan (TMS). Powłoki Si-DLC o różnej zawartości Si były wygrzewane w trzech różnych temperaturach w atmosferze powietrza. Dla celów porównawczych stabilność temperaturową badano również dla niedomieszkowanych powłok DLC. 2. MATERIAŁ I METODYKA BADAŃ Jako podłoże zastosowano jednostronnie polerowane płytki krzemowe <100> o grubości 500 µm. Ponadto dla celów analizy spektroskopii w podczerwieni z transformatą Fouriera (FTIR) zastosowano obustronnie polerowane podłoże krzemowe typu p o orientacji <111>. Powłoki Si-DLC oraz DLC o grubości ~200 zostały wytworzone za pomocą metody RF PACVD z wykorzystaniem mieszaniny gazów CH 4 /TMS o trzech różnych proporcjach prędkości przepływu: 18/4, 16/12 oraz 14/21 (oznaczone, odpowiednio, jako Si-DLC1, Si-DLC2 i Si-DLC3), z zastosowaniem ujemnego potencjału autopolaryzacji 800 V. W tabeli 1 zamieszczono parametry wytwarzania powłok wraz z uzyskaną koncentracją krzemu wyznaczoną za pomocą spektroskopii fotoelektronów XPS. W celu zbadania stabilności termicznej powłoki były wygrzewane w 400 C, 500 C i 600 C w atmosferze powietrza przez 1 godzinę. Strukturę chemiczna powłok przed i po wygrzewaniu badano za pomocą spektroskopii ramanowskiej (RS) oraz spektroskopii FTIR. Grubość powłok wyznaczono za pomocą reflektometrii rentgenowskiej XRR. Twardość oraz moduł Younga wyznaczono z wykorzystaniem nanoindentacji. 3. WYNIKI I ICH DYSKUSJA Powłoki DLC oraz Si-DLC1 wygrzewane w temperaturze 500 C oraz wszystkie powłoki wygrzewane w temperaturze 600 C uległy całkowitej degradacji. Analiza za pomocą spektroskopii Ramana (tab. 2) wykazała, iż w przypadku powłok niewygrzewanych wraz ze wzrostem zawartości Si obserwuje się zmniejszenie ilorazu I(D)/I(G) oraz zwiększenie szerokości połówkowej pasma G (FWHM G). Świadczy to o większym stopniu nieuporządkowania ich struktury chemicznej oraz wiąże się ze wzrostem udziału wiązań węgla o hybrydyzacji sp 3. Po wygrzewaniu w 400 C oraz 500 C dla rosnącej zawartości krzemu w powłokach obserwowano również tendencję spadkową ilorazu I(D)/I(G) oraz wzrost szerokości połówkowej pasma G. Niemniej jednak równocześnie zaobserwowano zjawisko postępującej grafityzacji ich struktury. W porównaniu z próbkami wyjściowymi wygrzewane powłoki charakteryzowały się większym ilorazem I(D)/I(G) oraz przesunięciem pasma G w kierunku większych liczb falowych. Największy stopień zaawansowania procesu grafityzacji zaobserwowano w przypadku powłok nie zawierających krzemu oraz o najmniejszej jego koncentracji (DLC oraz DLC1). Analiza FTIR wykazała obecność grup CH 2 oraz CH 3 charakterystycznych dla powłok uwodornionego amorficznego węgla. Ponadto w przypadku powłok domieszkowanych zaobserwowano drgania pochodzące od grup Si H, Si CH 3, Si C oraz Si O (tab. 3). Wraz ze wzrostem temperatury wygrzewania powłok zaobserwowano zjawisko desorpcji wodoru przejawiające się zmniejszeniem zawartości grup CH n oraz Si H, któremu towarzyszył wzrost udziału grup krystalicznego Si O. Analiza XRR wykazała zmniejszenie grubości powłok wraz ze wzrostem temperatury wygrzewania, co świadczy o stopniowej ich degradacji. Stwierdzono zwiększenie twardości powłok domieszkowanych krzemem wygrzewanych w temperaturze 400 C (rys. 4). Może to być wynikiem desorpcji wodoru, a tym samym zwiększeniem udziału wiązań Si C oraz krystalicznego Si O w amorficznej strukturze powłok. 4. PODSUMOWANIE Zbadano stabilność termiczną powłok węglowych domieszkowanych krzemem, wytwarzanych metodą RF PACVD z wykorzystaniem TMS. Powłoki DLC, powłoki o najmniejszej zawartości krzemu wygrzewane w temperaturze 500 C oraz wszystkie powłoki wygrzewane w temperaturze 600 C uległy całkowitej degradacji. Zaobserwowano, iż obecność krzemu w strukturze powłok DLC promuje tworzenie się obszarów węgla o hybrydyzacji sp 3. Procesowi wygrzewania powłok DLC oraz Si-DLC towarzyszy desorpcja wodoru. Zaobserwowano wzrost twardości powłok o największych zawartościach krzemu wygrzewanych w temperaturze 400 C. Wykazano również, że zawartość krzemu równa lub większa niż 10% at. poprawia stabilność termiczną powłok w temperaturze 400 C. NR 4/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 183