Carbide formation in titanium coatings deposited on carbon steel. Y. BENARIOUA, N. BOUAOUADJA*, B. WENDLER$. Institut Mécanique, Centre Universitaire ...
J O U R N A L OF M A T E R I A L S S C I E N C E L E T T E R S 15 (1996) 1067
1069
Carbide formation in titanium coatings deposited on carbon steel Y. BENARIOUA, N. BOUAOUADJA*, B. WENDLER$ Institut Mécanique, Centre Universitaire de M'Sila-Algeria, * Laboratorie Matériaux, I.O.M.R, Université de Sétif 19000 Algeria, +Irrst. Mater. Eng., Technical University of Lodz, Poland
During the last few years, the use of coatings as a material treatment, in particular for cutting tools, has shown extensive development. The prime interest of this technique is that the treated surfaces exhibit great mechanical and chemical resistance [1~4]. Most of the transition-metal carbides and nitrides of the fourth to sixth group o f the periodic table have extremely high melting points, great hardness, very good chemical and thermal stability and typically metallic electrical, magnetic and optical properties [5]. These unusual properties make transition-metal carbides and nitrides both interesting and useful. The aim of the present work is to describe the results of a study concerning carbide phase formation and characterization of titanium coatings on high carbon steel substrates. The material used for the substrate is in the form of discs (15 mm diameter, 10 mm thick) cut from plain carbon steel rods. The chemical composition in weight is: 1.34% C, 0.11% Si, 0.21% Mn, 0.06% Cr, 0.10% Ni, 0.02% A1, 0.10% Cu. The specimens were polished with emery paper and finished with alumina pastes (0.25#m). The discs were then ultrasonically degreased and cleaned with trichloroethylene, acetone and methanol for 10min each. Prior to titanium deposition, the substrates were sputter etched using an argon pressure o f 1 Pa, 1 0 0 m A current and - 1 0 0 0 V voltage for 15 min and a base pressure of 10 -4 Pa. During deposition, the experimental conditions were as follows: applied power to magnetron 2000 W; magnetic field 0.05 T; target polarization - 4 0 0 V;
current intensity 5 A; maximum temperature 150 °C; base pressure 0.3 Pa. After deposition, the samples were exposed for 1 h to vacuum annealing in the temperature range from 500 to 1100 °C in steps of 100 °C (every specimen was annealed only once). The rate of heating to the desired temperature and the rate o f cooling after annealing was about 20 °C/min. The structure of the layers before and after annealing was then investigated using a HZG-3 powder diffractometer with Bragg-Brentano focusing and monochromatic Co (Ka) radiation, and compared with that after deposition. The titanium films were deposited to a thickness of about 4 #m. The morphology of the sample surface was studied by a Neophot 21 optical microscope. The surface hardness was measured with a Zwick 3212 microhardness tester at a load of 50g. Finally, the adhesion is evaluated using a scratch tester similar to the Revetest-LSRH; the radius of curvature of the diamond tip was about 0.1 mm. The data in Table I show that in the as-prepared state only the reflections from polycrystalline a-Tl films were obtained. The spectra o f samples annealed at 600 and 700°C for 1 h are nearly identical with those of samples as-deposited. After annealing at higher temperatures (above 700 °C) a new phase gradually appears and is assigned to a TiC phase (Fig. 1). Simultaneously the a-Ti compound becomes unstable and changes completely to TiC phase at 1100 °C and all their reflections disappear. The growth and stability o f the TiC
TABLE I Results of the X-ray analysis obtained for different annealing temperatures Annealing temperature (°C)
Phases
As-prepared 500
a-Ti a-Ti
600
a-Ti
700
a-Ti TiC a-Ti TiC a-Ti TiC a-Ti TiC TiC
800 900 1000 1100
0261-8028
© 1996 Chapman & Hall
a-Ti parameters
TiC parameters
(hkl)
I
0°
(1 0 1) (002) (1 0 1) (0 0 2) (1 0 1) (00 2) (1 0 1) (1 0 1)
0.875 0.625 0.975 0.750 0.825 0.375 1.00 0.750
23.3 22.1 23.2 22.1 23.1 22.1 23.2 23.2
(1 0 1)
0.700
23.2
(1 0 1)
0.175
23.2
-
(hkl)
I
0°
(1 1 1)
0.095
21.0
(1 1 I)
0.120
21.0
(I 1 1) (200) (1 1 1) (2 0 o) (1 1 1) (2 0 o)
0.250 0.125 0.725 0.625 0.875 0.870
21.0 24.5 21.0 24.5 21.0 .~24.5
1067
0
(c)
I--
I-
ó"
LL
0
(b)
ô I-
o
i=
(a)
o ._~ I..Q
g o LL
ô
v
O
o_
bI
- - ~
31
29
27
25
23
I.........
21
i .......
F
19
17
grains, see Fig. 2). After treatment, the microhardness and adhesion measurements were determined as a function of annealing temperature. From Fig. 3, it appears that the microhardness Hv increases with increase of annealing temperature in the range 500 to 1100 °C. The microhardness reaches the value of 2720 kg/mm 2 at 1100 °C, which corresponds to those of TiC phase. This result is in good agreement with that obtained by Wendler and Jakubowski [1 ]. On the other hand, Shikama et al. [8, 9] found a very high microhardness of a TiC phase deposited by magnetron sputtering on molybdenium specimens. In our case, this significant change in microhardness is certainly due to the diffusion of carbon atoms from steel substrate into the layer and their reaction with titanium atoms. The results of the adhesion measurements show that the critical load (Pc) values increase when the temperature rises from 500 to 1100 °C. Titanium films of 4 # m thickness deposited by magnetron sputtering on high carbon steel substrates, were subjected to 1 h vacuum annealing in the temperature range 500-1100 °C. The effects of this annealing on the structure, chemical composition, microhardness and adhesion were studied. It was found that no reaction occurred at temperatures below 700 °C. However, between 700 and 1100 °C, one TiC stable phase was detected. After annealing at 1100 °C, only the TiC phase was observed and the atomic ratio C:Ti was approximately equal to 1. Moreover, it has been shown that there is a significant increase in microhardness and adhesion of the films with increase of annealing temperature. The results of this study can be divided into three
Diffraction angle (e °)
Figure ] X-ray diffraction spectra of 4 ffm Ti film on carbon steel substrates subjected to various stages of annealing: (a) 700 °C; (b) 900 °C; (c) 1100 °C.
compound is not only favoured by its lower free energy of formation ( A H f ( T i C ) = - 4 2 k c a l / m o l e [6]), but also by its simple crystal structure. Fig. 1 shows X-ray diffraction spectra of Ti film on carbon steel substrates subjected to various stages of annealing. It can be seen that TiC phase formation becomes more. distinct beyond 900 °C. The results of X-ray microprobe analysis for a sample annealed at 1100 °C for 1 h show that the Ti:C ratio is approximately equal to unity, i.e. the composition of the titanium carbide film was found to be close to stoichiometric. However, the presence of a-iron and some traces of other elements impurities were detected in the monocarbide film. Similar traces have been reported by Wendler and Benarioua [7] in the case of Ti films deposited on steel by PVD. After annealing at 700 °C, the TiC phase appears near the interface as extremely fine grains. With increasing annealing temperature, this phase develops with columnar structure perpendicular to the surface substrate. When the annealing temperature reaches 1100°C, the cc-Ti phase is transformed completely to TiC grains (with coarse equiaxed 1068
Figure 2 Micrograph oftitanium film deposited on steel substrate after annealing at 1100 °C (x 720).
3000
I ooo []
2500 /
"Il ~ z
800
,~
ooo
2000
/
/,
B 1500 /
1000 500
. . . . . . . . . . . . .
~
400
,.~~E~ ~ "E]~ ....-+~"
200
I I I I I I I I I I I 0 100 200 300 400 500 600 700 800 900 100011001200
T(°C)
Figure 3 Variations of Pc and Hv versus annealing temperatures up to 1100 °C: [] Hv; + Pc-
annealing temperature ranges: (1) (500-600 °C) the deposited films keep their single-phase character aTi. The mechanical resistance (microhardness and adhesion) increases weakly. (2) (700-1000 °C) the TiC phase appears and grows with annealing temperture increase. The carbon atoms are provided by the high carbon steel substrate and transported by diffusion to the titanium layer. The resistance characteristics increase sensibly. At the same time, the a-Ti phase disappears gradually. (3) (10001100 °C) The TiC phase is fully developed. The microhardness increases about five-fold compared to that of the substrate, and the critical load is nearly seven times greater than that of the as-prepared specimens.
References 1.
B. WENDLER and K. J A K U B O W S K I , J. Vac. Sci. Technol. A6 (1988) 93.
2. 3. 4.
5. 6. 7. 8.
9.
C . R . G U A R N I E R I , F. M. D ' H E U R L E , J. J. CUOMO and S. J. W H I T E H A I R , Appl. Sutf. Sci. 53 (1991) 115. M. Y. A L - J A R O U D I , H. T. G. H E N T Z E L L , S. GONG and A. BENGSTON, Thin Solid Films 195 (1991) 63. G. H A K A N S S O N , L. HULTMAN, J. E. SUNDGREN, J. E. GREENE and W. D. MUNZ, Suff Coating Technol. 48 (1991) 51. L. E. TOTH, "Transition metal carbides and nitrides" (Academic Press, New York, 1979). H. L. SCHICK, "Thermodynamics of certain refractory compounds" (Academic Press, New York, 1966) p. 775. B. W E N D L E R and Y. BENARIOUA, XV Conference on Applied Crystalography, Cieszyn, Poland, August 9-12, 1992. T. S H I K A M A , H. SHINNO, M. FUKUTOMI, M. FUJITSUKA, M. KATAJIMA and M. OKADA, Thin SoIid Films 101 (1983) 233. T. S H I K A M A , H. ARAKI, M. F U J I T S U K A , M. FUKUTOMI, H. SHINNO and M. OKADA, ibid. 106 (1983) 185.
Received 10 August 1995 and accepted 17 January 1996
1069
