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DOI: http://dx.doi.org/10.20534/ESR-17-1.2-43-46
Saydakhmedov Ravshan Khalhodjaevich,
Professor,
State Unitary Enterprise at Tashkent State Technical University,
E-mail: [email protected] Kadirbekova Kutpiniso Karimovna, Associate professor Tashkent State Technical University, Saidakhemedova Gulira'no Ravshanovna,
student
Turin Polytechnic University in Tashkent, E-mail: [email protected] Bakhadirov Kudratkhon Gayratovich, Senior teacher, Tashkent State Technical University, E-mail: [email protected] Umarov Erkin Adilovich, Tashkent State Technical University, Associate professor, the Faculty of Mechanics and Machine building
P-T-C diagrams for Ti — C base coatings obtained by Physical Vapour Deposition methods
Abstract: The thermodynamic method for predicting phase and chemical composition of coatings based on titanium carbide formed by the method of ion plasma has been investigated. For experimental verification of theoretical data used the phase and chemical analysis methods.
Keywords: TiC hard coatings, composition of non-stoichiometric coatings, diamond-like clusters, AES, XPS, HREELS.
Introduction. In physical vapour deposition (PVD) processes, the coating is deposited in vacuum by condensation from a flux of neutral or ionized atoms of metals. Several PVD techniques are available for deposition of hard coatings. Among them, cathode arc vapour (plasma or arc ion plating) deposition, magnetron sputtering (or sputter ion plating), and combined magnetron and arc processes [1] are the most widely used techniques to deposit various hard coatings. These PVD processes differ with respect to the type of evaporation of the metallic components and the plasma conditions employed during the deposition process. The transition of the metallic component (to be deposited) from a solid to a vapour phase (in which metal atoms are ionized in different ways) may be performed by heating of an evaporation source (as in cathode arc) or by sputtering of a target (as in magnetron sputtering).
In paper [2] (CAD-cathode arc deposition) on a copper substrate obtained by coating a titanium carbide (TiC) stoichiom-etry vacuum — arc method. We have studied the adhesion of the composition and structure of TiC coatings with a thickness of 1 to 3 microns. As the reaction gas used in the synthesis of titanium carbide pair C6H6 benzene. It is noted that the titanium carbide has a wide range of homogeneity (TiC048-TiC0 ). However, in the article a non-stoichiometric composition of titanium carbide and the effect of titanium and carbon ratio on the phase composition and properties of the coatings have not been studied. To improve adhesion of the coating to the substrate pre-coated pure titanium, followed by a subsequent change in the gas supply chamber is adjusted by the thickness ratio of the titanium and carbon.
Carbides of transition metals have high melting points, sublimation specific heats, elasticity moduli, hardness etc. These properties
are playing the decisive role for application of these compounds as wear resistant coatings on tool steels. There should be reached the optimal combination of these properties: hardness, heat expansion coefficient, elasticity modulus, heat conductivity and others.
The range of coatings properties could be significantly expanded due to high homogeneity limits for transition metals carbides with defects in carbon sublattice. So, titanium carbides of non-stoichiometric composition (TiC0.60) have high heat expansion coefficient close to one for M15 (AISI) steel, and stoichiometric carbides (TiC ) have high elasticity modulus, hardness and heat conductivity [3]. So the investigations and prediction of phase and chemical composition of carbide coatings built by ion plasma method is very important.
The aim of present work is prediction of phase and chemical composition of PVD coatings in dependence from technology parameters of the ion plasma process.
Objects and methods of research. Thermodynamic method of phase equilibria based on entropy maximum principle for isolated system [4] was used for calculations. Chemical elements concentrations in system and two thermodynamic parameters: reaction gas (acetylene) pressure P and substrate temperature T — were used as input data.
Acetylene (C2H2) mass flow was determined as: qC2H2=1.27.10-3-P, kg-m-2-s-1 [3-6]. Metal mass flow is equal qMe=K-v-yMexCy, kg-m-2-s-1, where v is the coating deposition velocity, YMexCy is density of carbide, K is the coefficient dependent from molecular mass of metals (Kri= 0.8) [8; 9]. Thermodynamic properties of TiCx in wide range of temperatures [10] were used for phase composition prediction.
Before the coating application the substrate was firstly polished mechanically, then degreased in a ultrasonic rinse of ethanol and finally sputter-etched with argon "in situ" (current density j = 50 ^A/cm 2, dose D = 10 17 ions/cm2, accelerating voltage U = 30 kV).
Chemical composition and electronic structure of coatings were investigated by AES and XPS methods on Electron spectrometer ESCALAB MK-2 (VG, UK), using electron gun LEG200, monochromated X-ray source Al Ka (hv=1486.6 eV) and low energy electrons source EMU-50. The spectra of valence band had low intensity and were recorded at 0.04 eV/s with 24 scans. The spectrometer was adjusted using Au reference sample. The spectrometer tunings provided energy resolution better than 0.6 eV. HREEL spectra were recorded with the same spectrometer settings at primary beam energy 7.0 eV, 50o angle of incidence with respect to the surface normal. The nominal resolution of the spectrometer was 17 meV with using the graphite standard. We were
able to record spectra with 33-35 meV full width at half maximum (FWHM), depending on the state of the sample. HREEL spectra were recorded at 0.002 eV/sec, 5 scans. Typical count rates were 2x104 s-1 in the elastic channel for the PVD coatings.
The AES quantitative analysis of chemical composition was made using standard approach [7; 8].
The samples were cleaned by high purity acetone and than by Ar ions in the spectrometer preparation chamber during 20 min. at pressure about 1-10-4 Pa, accelerating voltage 10 kV, ion beam 20 ^A. Than the samples were moved to the work chamber at vacuum 10-8 na. These measures allow to avoid influence of occasional contamination.
Results and their analysis. The results of calculations of PVD coatings phase compositions are presented as P-T-C diagrams (Fig. 1). These results shows that acetylene pressure, temperature, and deposition velocity strongly influence on chemical and phase composition of Ti-C coatings.
Figure 1. P-T-C diagrams with phase composition of PVD coatings on base of titanium carbides (top) * — AES experimental data.
These coatings can contain carbides of non-stoichiometric composition and free carbon inclusions. Comparison of calculated phase composition of PVD coatings (P-T-C diagrams) with equilibrium diagrams Me-C (where Me is Ti) allows to conclude the identity of phase compositions in both cases.
The calculated phase and chemical compositions of coatings were checked experimentally.
According to quantitative Auger analysis, the composition of surface is the following. The TiCx coating applied on the M15 high speed steel substrate at acetylene pressure 1.1 Pa has 33% at. TiC and 67% at. of free carbon that corresponds to 70% mass. TiC and 30% mass. C. These data are obtained from assumption on formation of TiC-type carbide and Auger data where the relations of C
C C 2
and Ti atomic concentrations are — = 3.05 or-= —.
Ti TiC 1
Figure 2 show 1s core-level spectra from TiCx PVD coatings
made at various pressures of reactive gas (0.004 and 1.06 Pa). Complex composition of coating leads to doublet due to C-C and C-Ti bonds. The position of the individual lines correspond to kinetic energies -1205.0 eV and 1201.6 eV.
The estimated C1s core-level shifts with respect to the kinetic energy of the graphite standard (1202.3 eV) were +1.7 eV and -0.7 eV. These shifts are similar to those reported for TiC, graphite and diamond [9; 10]. The intensity of free state carbon line increases with increasing the concentration (pressure) of reactive gas.
Figs. 3 present XPS valence band spectra of TiC PVD coating and TiC standard sample. Fig. 3 also shows the model density of states (DOS) for TiC. The electronic structure of valence band has 2 groups of levels. One group has high binding energy (1 a1g, 1t1u, 1eg) filled mainly by s-electrons of non-metal and partly by s- and p-electrons of metal. Other group is separated from the first by wide energy gap and has low binding energies with main input of p-electrons of non-metal and d-electrons of metal.
Free carbon in the TiC-based coating makes input in structure of valence band. To separate this input the (2) spectrum was sub-stractid from (1) with preliminary normalization on TiC content (33% at.). The line 3 shows the result tied with free C: PVD coating has high DOS at 9...14 eV, medium one at 15...19 eV and low one at 0.. .9 eV. Energy state and relationship between intense peaks conforms with the theoretical DOS of diamond [9]. It is known that the
characteristic feature of the energy-band structure of diamond is the absence of n - lectrons. However, in the sample under examination, the low DOS is found not only in the ct- states region of 5.. .8 eV, but also in the states region of 0.5 eV. At present it is rather tied with a small quantity of carbyne in PVD coating [10; 11]. Thus, this study of the outer shells of atoms in the surface coating has confirmed it complex phase structure. The coating contains TiC and carbon in two states diamond-like and carbyne.
S 3
scs
^ TiC
c J 1 Y
TiC
c / 2
I.I.I / i.ii
1194 1198 1202 1206 1210 1214 Kinetic energy, eV
Figure 2. C1s photoelectron spectrum of PVD TiC films applied at various pressures (P) of reactive gas. 1 - P = 0.004 Pa; 2 - P = 1.06 Pa
s
-Q
es
S3
a*
i A / \ 7 \ J \
A (/i)d
2 - (Ti)d\ 1 " (Qs X/V / x(C)p
3 A J. i i i X 2 i i . H
20 16 12 8 4 0 Binding energy, eV
Figure 3. Valence band photoelectron spectra of PVD TiC coating (1), TiC standard (2) and differential spectrum (3)
Figure 4. Vibrational spectra of TiC PVD coating, fabricated at pressure of reactive gas of 1.06 Pa before (1) and after Ar+ etching during 900 s (2)
Figure 4. (top and bottom) presents vibrational spectra of the PVD TiC coating with 30% of diamond-like and amorphous carbon, before ion etching and after 900 sec. Ar+ etching. The interpretation of these spactra contains in Table 1.
The ion etching allowed to investigate the peculiarities of pho-non spectra in coatings at different depths. The upper spectrum includes high number of peaks tied with carbon out of carbides. The peaks (2-4) at 107, 127 and 130 meV correspond to stretching vibrational modes for =C=C= bonds. Thus the carbyne clusters we observed is probably polycumulene which is consistent with the chemical bonding ofbeta-carbyne reported in the literature [11, 12]. The peak at 164.0 meV is tied with diamond and other more broad one at 189 meV is the amorphous carbon. The analogous Raman spectrum of the a-C magnetron sputtered films was presented in [14]. The intense and broad peak (7) at 375 meV corresponds to the stretching frequency of CmHn hydrocarbons [13].
The 900 s ion etching has changed the spectrum significantly. The argon etching allowed to reach more deep layers of the coating. The peaks 1 and 5 at 78 and 165 meV are clearly resolved. Its tied with TiC and diamond-like carbon with sp3 character ofhybridization. Evidently that these features are tied with decomposition of carbyne and decreasing ofcarbon sp2 fraction, and therefore increase in the number of four-fold coordinated carbon atoms (sp3) and TiC concentration in PVD coating. These data are in good agreement with investigations of other PVD nanocomposite TiC — a-C: H coatings [16].
Conclusions. 1. The coatings based on titanium and zirconium carbides include stoichiometric and non- stoichiometric phases, diamond-like carbon and carbyne dependent from deposition conditions.
2. Basing on P-T-C diagrams, one can determine optimal parameters of deposition: reaction gas pressure P and substrate temperature T. Such description of process parameters could be applied to widest class of PVD units.
3. The calculated and experimentally determined compositions cation of calculated P-T-C could be used to get coating with definite
of coatings at ion plasma deposition are enough close. So the appli- composition and properties.
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