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Multilayer diamond-like carbon coatings
Diamond and Related Materials, 1 (1992) 543-545
543
Elsevier Science Publishers B,V., Amsterdam
Multilayer diamond-like carbon coatings D. V. Fedoseev Institute of Physical Chemistry of the USSR Academy of Seienees, Moscow (Russia)
S. N. Dub, I. N. Lupich and B. A. Maslyuk lnstitute lor Superhard Materials of the Ukrainian S.S.R. Academy olScienees, 252153 Kier (Ukraine)
1. Introduction
TABLE 1, Properties of single-layer coatings (P
The unique properties of diamond-like carbon coatings make these coatings of particular interest to many researchers. The methods for producing coatings with predetermined properties have been extensively developed. This paper will discuss the possibility of depositing multilayer diamond-like coatings having carbon layers with different properties obtained by changing the synthesis conditions. The temperature of a substrate is one of the most effective deposition parameters in controlling the conditions for coating formation, affecting both the structure and adhesion properties of the coating.
Layer
2. Experimental
Diamond-like carbon coatings were deposited onto polished molybdenum substrate by low-temperature low-frequency discharge plasma CVD. Deposition was carried out using a diode-circuit type installation. The chamber pressure was maintained at 0.6 tort and the discharge current was 0.5 mA. Layers were deposited for 60 rain. In order to obtain layered structures, each layer was deposited at a definite substrate temperature within the range of 160-260 °C. The characterisation of each layer required a number of single layer deposition tests. The thickness of coatings was determined by gravimetry. Microhardness was measured using an IIMT-3 device with a Knoop indenter under 0.1 N. Vickers indenter scratching tests were carried out at different loads to study and assess qualitatively the details of coating fracture.
Substrate
temperature
{ 'CI
a b c d
160
190 220
260
0.6 torr, 1 - 0 . 5 mA)
Properties of layer Thickness
Microhardness {G Pal
1.5 1.5 1.0
13.1
5 x I(/~
17.4
4 x 1( f
15.5 10.0
1 xlO 7 5 x 106
0-tm)
2.0
The results of gravimetric measurement of coating thickness have shown that the highest growth rate corresponds to 190 °C under otherwise equal conditions. For other deposition temperatures the growth rate may be assumed to reach its peak at different levels of the process parameters (deposition current and chamber pressure). Vickers indenter scratching tests performed at different loads revealed that at 0.1 N the coatings undergo no fracture (Fig. 1). Crack formation occurs in a scratch at
3. Results and discussion
Table 1 gives the synthesis conditions and some characteristics of singleqayer coatings.
0925 9635/92/$5.00 ;ci'~Elsevier Science Publishers B.V. All rights reserved
Spccilic electrical resistance (ohm/cm)
Fig. I. A scratch on the single-layer coating at 0.1 N.
544
D. V. Fedoseet, et al. / Muhilayer diamond-like carbon coatings
0.2 N; this represents the chain of circumferential Hertzian cracks (Fig. 2). At 0.5 N chips appear from the scratch side (Fig. 3). Single-layer coating is peeled off the molybdenum substrate under the load of 1 N (Fig. 4). The fact that beyond the scratch, the coating is not peeled off suggested a high adhesion strength of singlelayer coatings. The figures show layer (b) since its fracture mechanism is typical for the remaining single-layer coatings. The characteristics of multilayer coatings are summarised in Table 2. In Vickers indenter scratching tests, the fracture of multilayer coatings initiated at higher loads than in the case of single-layer coatings. Two-layer coatings (a-b, c-a) and three-layer coatings (c b-a) did not fracture under 0.2 N. Scratch-initiated cracking occurred at the load of 0.5 N. Complete peeling of the coating in the scratch was observed at 2 N. In
Fig. 4. A scratch on the single-layer coating at l N. TABLE 2. Properties of multilayer coatings ( P - 0 . 6 tort, I - 0 . 5 mA) Layer
a-b a-d c a d-b a-d-b c-b a
Fig. 2. A scratch on the single-layer coating at 0.2 N.
Fig. 3. A scratch on the single-layer coating at 0.5 N.
Substrate temperature
Properties of layer
('c)
Thickness (/am)
Microhardness (G Pa)
Specific electrical resistance (ohm/cm)
160-t90 160 260 220-160 260 190 160 260-190 220 190-160
3.0 1.5 2.5 2.0 3.5 4.0
19.5 18.0 19.5 19.0 20.0 21.0
3 × [Os 5 × 107 4 x 107 3 x 107 1 x l0 ~ Ixl0 s
scratching tests of the above coatings the layers did not separate from each other. However, in coatings of (ad b) and (d-b) types, the layer (d) caused the adhesion degradation. Figure 5 shows the fracture behaviour of a
Fig. 5. A scratch on the two-layer d - b coating at 1 N.
D. V. Fedoseev et al. / Multilayer di¢mumd-like carbon coatin~,,s
545
two-layer (d-b) coating: the peeling takes place beyond a scratch. Single-layer (d) and (b) coatings did not show such fracture pattern. When scratching the three-layer (a-d b) coating at an indenter load of 0.5 N, layers (d) and (b) break away from layer (a) (Fig. 6). Based on these data one can conclude that the substrate temperature difference of >60 C in the process of deposition deteriorates adhesion strength between coating layers. Electrical characteristics of single-layer and multilayer coating have been studied. The specific electrical resistance of single-layer coatings depends on the substrate temperature. Thus, the coatings deposited at 160 C and 260 'C have t)=5 x l0 s ohm/cm and ~ = 5 x 10~' ohm/cm, respectively. For multilayer coatings a determining layer is that of the highest specific electrical resistance. Fig. 6. A scratch on the three-layer a- d b coating at 0.5 N.
543
Elsevier Science Publishers B,V., Amsterdam
Multilayer diamond-like carbon coatings D. V. Fedoseev Institute of Physical Chemistry of the USSR Academy of Seienees, Moscow (Russia)
S. N. Dub, I. N. Lupich and B. A. Maslyuk lnstitute lor Superhard Materials of the Ukrainian S.S.R. Academy olScienees, 252153 Kier (Ukraine)
1. Introduction
TABLE 1, Properties of single-layer coatings (P
The unique properties of diamond-like carbon coatings make these coatings of particular interest to many researchers. The methods for producing coatings with predetermined properties have been extensively developed. This paper will discuss the possibility of depositing multilayer diamond-like coatings having carbon layers with different properties obtained by changing the synthesis conditions. The temperature of a substrate is one of the most effective deposition parameters in controlling the conditions for coating formation, affecting both the structure and adhesion properties of the coating.
Layer
2. Experimental
Diamond-like carbon coatings were deposited onto polished molybdenum substrate by low-temperature low-frequency discharge plasma CVD. Deposition was carried out using a diode-circuit type installation. The chamber pressure was maintained at 0.6 tort and the discharge current was 0.5 mA. Layers were deposited for 60 rain. In order to obtain layered structures, each layer was deposited at a definite substrate temperature within the range of 160-260 °C. The characterisation of each layer required a number of single layer deposition tests. The thickness of coatings was determined by gravimetry. Microhardness was measured using an IIMT-3 device with a Knoop indenter under 0.1 N. Vickers indenter scratching tests were carried out at different loads to study and assess qualitatively the details of coating fracture.
Substrate
temperature
{ 'CI
a b c d
160
190 220
260
0.6 torr, 1 - 0 . 5 mA)
Properties of layer Thickness
Microhardness {G Pal
1.5 1.5 1.0
13.1
5 x I(/~
17.4
4 x 1( f
15.5 10.0
1 xlO 7 5 x 106
0-tm)
2.0
The results of gravimetric measurement of coating thickness have shown that the highest growth rate corresponds to 190 °C under otherwise equal conditions. For other deposition temperatures the growth rate may be assumed to reach its peak at different levels of the process parameters (deposition current and chamber pressure). Vickers indenter scratching tests performed at different loads revealed that at 0.1 N the coatings undergo no fracture (Fig. 1). Crack formation occurs in a scratch at
3. Results and discussion
Table 1 gives the synthesis conditions and some characteristics of singleqayer coatings.
0925 9635/92/$5.00 ;ci'~Elsevier Science Publishers B.V. All rights reserved
Spccilic electrical resistance (ohm/cm)
Fig. I. A scratch on the single-layer coating at 0.1 N.
544
D. V. Fedoseet, et al. / Muhilayer diamond-like carbon coatings
0.2 N; this represents the chain of circumferential Hertzian cracks (Fig. 2). At 0.5 N chips appear from the scratch side (Fig. 3). Single-layer coating is peeled off the molybdenum substrate under the load of 1 N (Fig. 4). The fact that beyond the scratch, the coating is not peeled off suggested a high adhesion strength of singlelayer coatings. The figures show layer (b) since its fracture mechanism is typical for the remaining single-layer coatings. The characteristics of multilayer coatings are summarised in Table 2. In Vickers indenter scratching tests, the fracture of multilayer coatings initiated at higher loads than in the case of single-layer coatings. Two-layer coatings (a-b, c-a) and three-layer coatings (c b-a) did not fracture under 0.2 N. Scratch-initiated cracking occurred at the load of 0.5 N. Complete peeling of the coating in the scratch was observed at 2 N. In
Fig. 4. A scratch on the single-layer coating at l N. TABLE 2. Properties of multilayer coatings ( P - 0 . 6 tort, I - 0 . 5 mA) Layer
a-b a-d c a d-b a-d-b c-b a
Fig. 2. A scratch on the single-layer coating at 0.2 N.
Fig. 3. A scratch on the single-layer coating at 0.5 N.
Substrate temperature
Properties of layer
('c)
Thickness (/am)
Microhardness (G Pa)
Specific electrical resistance (ohm/cm)
160-t90 160 260 220-160 260 190 160 260-190 220 190-160
3.0 1.5 2.5 2.0 3.5 4.0
19.5 18.0 19.5 19.0 20.0 21.0
3 × [Os 5 × 107 4 x 107 3 x 107 1 x l0 ~ Ixl0 s
scratching tests of the above coatings the layers did not separate from each other. However, in coatings of (ad b) and (d-b) types, the layer (d) caused the adhesion degradation. Figure 5 shows the fracture behaviour of a
Fig. 5. A scratch on the two-layer d - b coating at 1 N.
D. V. Fedoseev et al. / Multilayer di¢mumd-like carbon coatin~,,s
545
two-layer (d-b) coating: the peeling takes place beyond a scratch. Single-layer (d) and (b) coatings did not show such fracture pattern. When scratching the three-layer (a-d b) coating at an indenter load of 0.5 N, layers (d) and (b) break away from layer (a) (Fig. 6). Based on these data one can conclude that the substrate temperature difference of >60 C in the process of deposition deteriorates adhesion strength between coating layers. Electrical characteristics of single-layer and multilayer coating have been studied. The specific electrical resistance of single-layer coatings depends on the substrate temperature. Thus, the coatings deposited at 160 C and 260 'C have t)=5 x l0 s ohm/cm and ~ = 5 x 10~' ohm/cm, respectively. For multilayer coatings a determining layer is that of the highest specific electrical resistance. Fig. 6. A scratch on the three-layer a- d b coating at 0.5 N.