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A method for elastic modulus measurements of magnetron sputtered thin films dedicated to mechanical applications
Thin Solid Films 270 ( 1995) 270-274
A method for elastic modulus measurements of magnetron sputtered thin films dedicated to mechanical applications Andre Rouzaud *, E. Barbier, J. Ernoult, E. Quesnel CEA/DTA/CEREM/DEM/SGSA, CEN-G 38054, Grenoble Cede-x 9, France
Abstract Mechanical properties of thin films may highly differ from those of chemically identical bulk materials. Their knowledge is of paramount importance for coatings manufacturers. In this paper, attention is paid on Young’s modulus measurements of PVD magnetron sputtered thin films on metallic substrates. The method is based on the three-point bending test of partially coated stainless steel or titanium alloy substrates. After derivation of the corresponding equations, the method and its accuracy according to qualification tests are first described. Results are then presented for coatings as varied as C, W, Cr( C), and TiN with thicknesses spanning OS-10 pm, and deposited under different sputtering conditions. It is first seen that the Young’s modulus highly depends on the deposition parameters, varying from the bulk values down to the fourth of this value. In the case of reactive sputtering, the evolution of the modulus with the gas flow is also presented and a correlation with phases or microstructure is observed. It is finally concluded that this kind of test, owing to its simplicity and its reliability, may help on a scientific point of view towards understanding the influence of morphology on mechanical properties, and on a practical basis for ensuring a control quality reference when performed systematically after deposition. Some foreseen extensions of this work to adhesion measurements will be mentioned shortly. Keywords: Elastic properties; Physical vapour deposition; Sputtering
1. Introduction
2. Derivation
The measurement of mechanical properties of thin films is nowadays of paramount importance for understanding the correlation between the microstructure of the deposited material and the final mechanical behaviour of the coating/substrate composite. Moreover, simple on-line measurements may improve the control of industrial coating technologies. Among the prime mechanical properties to estimate, the elastic modulus is the most important. For instance, mechanical resistance to a thermal shock depends linearly on this key parameter. A lot of techniques, generally complex to deal with, are proposed in the literature: bending test of symmetrically coated substrates [ 11, derivation from XRD stress measurement [ 11, ultrasonic surface wave [ 21, speed of bulk wave [ 31, nanoindentation [ 41. In this paper, we analyse the capabilities of a simpler bending test and the accuracy of its results applied to magnetron sputtered thin films of different materials (C, W, W(C), Cr, TIN) with different thicknesses ranging from 0.5 to 10 p.m, that are classically encountered in metallurgical applications. The influence of deposition parameters is also investigated.
We consider the classical three-point bending configuration applied to the case of a flat rectangular beam of dimensions L X A X e,, whose upper surface is coated by a perfectly adherent thin film of thickness e,. It is also supposed that: 1. the substrate is highly thicker than the coating:
* Corresponding author. 0040.6090/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDIOO40-6090(95)06921-6
of the leading equation
e, -=Z< e,
(I)
2. the aspect ratio of the beam is very large: e,-=ZL
3. The Poisson’s together: V,= vr
(2)
coefficient
of both materials
are close
(3)
Under the previous assumptions, the structure can be regarded as a thin composite beam, and related classical equations can be handled [ 5,6] if low bending is also considered. According to the elastic theory, and to the reference frame of Fig. 1, the strain in the composite may be written as:
A. Rouzaud et al. /Thin Solid Films 270 (1995) 270-274
271
video camera
(7)
(8) imposed displacement
(9)
t
coating
F, ( F2) being the force required to bend the substrate (the composite) up to a same deflection 6. According to the initial assumption of a thin coating on a thick substrate, and noticing that:
central point acoustic detector
Fig. 1. Schematic
e
experimental
Force detection
z,=z, (
1+3e’ e, 1
set-up and notations
and 1 Fxz (4)
&==2ig
The subscripts s, f and c are standing for the substrate, the film and the composite (substrate + film), respectively, E for the Young’s modulus and Z for the inertia momentum (I= e3h/12). If the film is thin enough, compared to the substrate, the xx component of the stress field in the coating will be given by: 1 E. Fxz (T =->--.r.r 2 EC Z, withe,/2
(6)
with: A
eJ2
L/2
II I 2
0
-es/2
a,&,
dx dy dz
0
and
where U is the energy consumed for bending the coating/ substrate composite, UC the energy for bending the coating, US the energy for bending the substrate, and W is the work done by the applied force needed to bend the composite. A straightforward calculation leads to:
F,L3 ~=-z48E,Z,
F2L3 48EJ,
we are lead to our central equation:
4 esF2-F1 -=_4 3ef F,
( 10)
F, and F2 being the forces to bend the substrate only and the composite, respectively, to the same given deflection 6. This equation suggests some comments: a similar but simpler calculation applied to the pure tensile test leads to a very close equation:
4 e,F2--Ft _=_Es e, Ft
(11)
It is thus observed that a tensile test performed on the same composite sample will lead to a lower (A F/F) compared to the bending test ((AFIF),,,,,=3(AFFIF),,,i,,). The accuracy of the bending test is thus three times higher. a similar equation has been recently derived [7] by an other method for the cantilever beam configuration and has been used to estimate the Young’s modulus of a coated micromachined silicon cantilever beam ( 17 X 200 pm). the validity range of Eq. ( 10) has been studied numerically by finite element modeling, by means of the Castem 2000 software [ lo]. First computations have shown that a large number of high precision elements (QS) are mandatory to get accurate numerical results ( 12 000 elements). Once the mesh was accurate enough to get the exact solution at e,< e, (2/200 I*m), new elements were added to simulate the influence of larger coatings. It was numerically found an excellent agreement up to a ratio e,/ er of ten.
A. Rouzaud et al. /Thin Solid Films 270 (1995) 270-274
212
4. due to superposition theorem, Eq. ( 10) remains perfectly correct even in the case of coatings with internal intrinsic stresses, as generally observed. 5. Eq. (10) shows that the lower substrate modulus, the higher the ratio ( AFIF) in a given geometry. Accurate selection of the substrate nature will thus improve the results accuracy.
3. Device description
and experimental
protocol
A schematic drawing of the bending device is presented in Fig. 1. The central point for load application is kept fixed whereas the programmable displacement (O-600 mm min-‘) is imposed by t h e ends points, in order to maintain the central part of the sample fixed in the laboratory frame. An optical microscope coupled with a video camera allows to monitor this area of the coating where the stress field is maximal (see Eq. (5) ) , and where thus the first cracks will appear as soon as the critical stress is exceeded. A piezo electric force measurement system (Sensotec, nominal range O-50 N) is located just below the central bending point. An acoustic emission detector (Bruel and Kjaer 8313) is also stuck to this central point in order to detect acoustic burst occurring at the onset of the cracks. Depending on the nature of the substrate, the coating and the geometry, cracks may occur either during the elastic or plastic deformation of the substrate. The knowledge of the imposed critical deflection (and therefore the critical stress) allows estimation of the adhesion of the coating according to classical mechanical models of interfacial adhesion energy [ 8,9]. However, such experiments are beyond the scope of this paper which focuses on the determination of coating elastic modulus far away from the conditions for cracking or for plastic deformation. In order to perform such experiments, commercially available standard beams (L = 70 mm, h = 4 mm, e, = 0.25,0.2 or 0.1 mm) of different substrates (stainless steel, TA6V) have been coated on one side with different materials excepted for a small region at the end of the beam (the last 10 mm), used to measure the reference force F, needed to impose a given deflection 6 to the substrate only. This offers a further advantage to determine the Young’s modulus of the substrate via the corresponding equation. The beam is then moved to locate a part of the coated zone between the two 8 mm spaced end points. In this way, many measurements may be run on the same beam at different locations, allowing to check the reproducibility of the experiments. Such reproducible measurements also allow to quantify the accuracy of the measurements, i.e. the errors bars, according to classical statistical theorems [ 131. Moreover, about ten beams have been inserted in the PVD reactor for each deposition experiment. Similar results have been obtained. Modulus measurements are then performed by computing the S, and S, slopes dFldz obtained from the uncoated and coated zones in the elastic domain (basically up to l&15 N
50 CI
Fig. 2. Bending results (force-deflection curves) and acoustic emission in a Cr coating on a stainless steel substrate. Associated optical view of cracks.
for stainless steel 200-250 pm thick substrates), Eq. ( 10) under the modified form:
Ef e, h-S1 -=Es 3ef Sl
and using
(10’)
The film integrity is always monitored by means of acoustic emission. Large bending of the samples are sometimes achieved after determination of the Young’s modulus. Reproducible results give the critical deflection (force) leading to the onset of cracks in the coating. Fig. 2 illustrates first cracks detection in a Cr coating on a stainless steel substrate at two different positions along the beam. Working far below this critical point in the linear part of the force-deflection curve ensures that the assumption of elastic behavior is fully satisfied.
4. Experimental
results
4.1. Validation First experiments have been performed on magnetron DC sputtered dense Cr coatings elaborated on similar conditions in terms of argon pressure, applied power and substrate temperature, but with different sputtering durations in order to get four different coatings thicknesses, 1, 3, 6 and 10 pm. Results have shown a fairly linear dependence of the AFIF
A. Rouzaud et al. /Thin Solid Films 270 (1995) 270-274
Table 1 Young’s modulus measurements
A-
400,
,
MODULUS.GPa
,
!
,
for low pressure DLC
Substrate
Stainless steel
Substrate thickness (mm) Substrate width (mm) Coating thickness (pm) Coating modulus ( GPa) Standard deviation (GPa) Number of experiments Sample ID # Accuracy range ( GPa)
0.1 4.21 2.40 207 22 10 #l f 13.6
TA6V
0.1 4.14 2.40 200 21 10 #2 +13
0.1 4.18 2.54 215 I9 10 #3 rt 6.2
0.25 4.02 2.54 185 33 10 #4 * 20.5
term as a function of the coating thickness. They lead to a Young’s modulus of 292 f 18 GPa, close to the bulk value. 4.2. Carbon coatings Two diamond like carbon (DLC) coatings elaborated at two different argon pressures (0.25 and 2 Pa, respectively) leading to two different morphologies (dense/cellular) have been deposited on both 250 pm thick TA6V and 100 pm thick stainless steel substrates. Experimental procedures and morphologies are given in Ref. [ 111, where it is shown that, basically, the coating morphology (dense or columnar) is lead by the plasma pressure. For the low pressure range (typically < 0.8 Pa), the mean free path of the sputtered carbon atoms is higher than the substrate-target distance and they impinge the substrate with their initial energy: this results in dense coatings. On the other hand, for the largepressurerange ( > 0.8 Pa), sputtered atoms loose a part of their energy during the collisions they undergo before reaching the substrate: this leads to columnar morphologies. The major results for low pressure DLC are summarized in Table 1. The mean Young’s modulus value obtained for such coatings is therefore about 210 GPa. The results obtained on TA6V substrates exhibit a standard deviation systematically higher than that for stainless steel substrates, about 50 percent higher. This seems to be related to the lower adhesion of the DLC on titanium alloys, since local delamination appears after cracking. Such measurements could thus be a simple way to estimate the relative adhesion of a given coating to its substrate. However, this explanation needs to be investigated more fully before final conclusions can be drawn. Further experiments are now in progress. Table 2 Young’s modulus measurements
213
for W coatings elaborated
Substrate
TA6V
Beam thickness (mm) Beam width (mm) Coating thickness (km) Coating modulus ( GPa) Standard deviation ( GPa) Number of experiments Sample ID number Accuracy range ( GPa)
0.25 4.02 5 387 58 10 #l *36
at low pressure
360
-
320
-
280: 240
-
200+0
P(CH4)/P(Ar) Fig. 3. Plot of the evolution of the Young’s modulus of Cr(C) a function of the reactive gas pressure.
coatings
as
The corresponding results for high pressure (columnar) DLC on identical stainless steel substrates give coating modulus values of about 59.0 f 8.7 GPa. Some basic conclusions may thus been drawn in the frame of this study on DLC: At a given deposition pressure, according to the error bars range, the Young’s modulus of the coatings are identical, independent of the substrate nature. The two different morphologies are leading to very different mechanical properties. It must be pointed out that this difference in behaviour has also been emphasized on internal pin/disk tribological studies. Concerning more specifically the Young’s modulus values, it can be thought, as discussed by Bull and Rickerby for TIN coatings [ 121, that, although the modulus of individual columns remains the same for both coatings at their local scale, the effective modulus is decreasing as the columnar morphology is more markedly observed. Such an evolution of Young’ modulus values as a function of the coatings morphology is in good agreement with recent results in the literature obtained with another technique [ 21.
4.3. W coatings W dense coatings have been deposited at low Ar pressure onto TA6V and stainless steel substrates. The results, summarized in Table 2, show the same trends: 1. The Young’s modulus is independent of the nature of the substrate. 2. Again, larger standard deviation for titanium alloy substrate is certainly related to lower adhesion. 3. The modulus of dense coatings correspond to bulk values given in the literature.
Stainless steel 0.25 4.02 5 374 19 10 #2 *49
0.215 4.11 5 417 35 10 #3 +21.7
0.215 4.10 5 392 40 10 #4 5 24.8
Similar values have been obtained for a 0.5 km thick W coating deposited onto a 100 p,rn thick stainless steel substrate. 4.4. Cr(C) coatings Cr(C) solid solution coatings have been deposited with different CH4 partial pressures. Our main investigations led
274
A. Rouzaud et al. /Thin Solid Films 270 (199.5) 270-274
the modulus evolution plotted on Fig. 3. It is observed that the modulus remains roughly constant excepted for the last ratio, where an increase in the order of 100 GPa P ~C-4~IP~Ar~ is observed. XRD analysis performed on same samples show that this phenomenon is related to a phase change in the coating, as demonstrated in Fig. 4. Carbide phases are identified for the highest pressures ratio, while other samples show a solid solution of C in the Cr textured phase. to
properties are known and are not too close together. According to the right choice of the substrate nature and geometry, even very low moduli of columnar coatings may be estimated (30 GPa). The uncertainty range is about k 10% of the modulus, which seems a good compromise for most applications. Some points need to be investigated more fully by this technique: The modulus values obtained in such a way give a macroscopic description for the mechanical behaviour of a large number of crystallites, due to the size of the analyzed zone (8 X 4 mm). Comparison with results obtained on same samples with the nanoindentation technique, which measures on a more local scale, is now in progress and will help to understand the influence of microstructure on mechanical properties. The influence of poor adhesion on the higher scattering of the results is presently under investigation. If this experimental fact was confirmed and understood, this test could be a simple non-destructive way to characterize, at least qualitatively, the adhesion of the coating to a given substrate.
4.5. TiN coatings 4 pm thick TIN, coatings have been deposited onto stainless steel substrates by reactive sputtering of a titanium target in an Ar + N, plasma, with different N, pressures. Here again, a large variation of the Young’s modulus is observed as a function of the reactive gas pressure inside the reactor. As an example, a 133 k 13 GPa modulus is obtained for low N? pressures corresponding to nitrogen inserted into a titanium phase. On the other hand, a value of about 269 + 26 GPa is measured in the case of an over stoichiometricallydark brown TiN, coating.
5. Discussion Results obtained in the frame of this study show that the presented bending test gives accurate values of the Young’s modulus for a wide variety of metallurgical coatings. Measurements simply require to insert thin beams in the coating device, a part of the beam being masked in order to get the pure substrate reference measurements. Depending on the selected process for coating deposition, mechanical properties of the substrate may be modified and the preliminary measurement of substrate modulus ensures the reliability of that of the the coating. It has been shown that phase changes may be detected, provided that the corresponding mechanical
Acknowledgements The authors would like to thank Mr. Petit for his precious help in X-ray analysis, Mr. Ignat (Enseeg/Ltpcm) for fruitful discussions on mechanical properties of thin films and Mr. Le Gallo for advice on numerical analysis.
References [ 11 H. Hasada, Y. Kishi and Y. Hirose, Thin Solid Films, 236 ( 1993) 247. [ 21 D. Schneider, H.J. Scheibe, T. Schwarz and P. Hess, Diamond Related Mater., 2 (1993) 1396. [3] S. Takeshita, S. Taki and K. Matsushige. Mater. Eval., (1990) 1473. [4] R.B. King, In!. J. SolidStruct., 23(12) (1987) 1657. [ 51 L. Landau and E. Lifchitz, Theorie de l’&lasticitP,MIR, Moscow, 1967. [6] S.P. Timoshenko and J.N. Goodier, Theory of Elasticity, McGrawHill, New York, NY, 1951. [7] J.A. Scheitz, MRSBull. XVII, 7 ( 1992) 34. [81 T.S. Chow, CA. Liu and R.C. Penwell, Adhesion Science and Technology, Plenum, New York, NY, 1975, p. 687. [9] T.S. Chow, J. Appl. Phys., 46 (1975) 219. [ 101 CASTEM 2000, Manuel d’utilisation, Rapport CEA/DRN/DMT93/ 006. [ 111 E. Mounier, E. Quesnel, P. Juliet and Y. Pauleau, Vlllth Cimtec, Florence, June 1994, in press. [ 121 S.J. Bull and D.S. Rickerby, Mater. Res. Sot. Symp. Proc., 188 ( 1990) 337. [ 131 G. Saporta, Theories et Methodes de la Statistique TECHNIP, 1978.
A method for elastic modulus measurements of magnetron sputtered thin films dedicated to mechanical applications Andre Rouzaud *, E. Barbier, J. Ernoult, E. Quesnel CEA/DTA/CEREM/DEM/SGSA, CEN-G 38054, Grenoble Cede-x 9, France
Abstract Mechanical properties of thin films may highly differ from those of chemically identical bulk materials. Their knowledge is of paramount importance for coatings manufacturers. In this paper, attention is paid on Young’s modulus measurements of PVD magnetron sputtered thin films on metallic substrates. The method is based on the three-point bending test of partially coated stainless steel or titanium alloy substrates. After derivation of the corresponding equations, the method and its accuracy according to qualification tests are first described. Results are then presented for coatings as varied as C, W, Cr( C), and TiN with thicknesses spanning OS-10 pm, and deposited under different sputtering conditions. It is first seen that the Young’s modulus highly depends on the deposition parameters, varying from the bulk values down to the fourth of this value. In the case of reactive sputtering, the evolution of the modulus with the gas flow is also presented and a correlation with phases or microstructure is observed. It is finally concluded that this kind of test, owing to its simplicity and its reliability, may help on a scientific point of view towards understanding the influence of morphology on mechanical properties, and on a practical basis for ensuring a control quality reference when performed systematically after deposition. Some foreseen extensions of this work to adhesion measurements will be mentioned shortly. Keywords: Elastic properties; Physical vapour deposition; Sputtering
1. Introduction
2. Derivation
The measurement of mechanical properties of thin films is nowadays of paramount importance for understanding the correlation between the microstructure of the deposited material and the final mechanical behaviour of the coating/substrate composite. Moreover, simple on-line measurements may improve the control of industrial coating technologies. Among the prime mechanical properties to estimate, the elastic modulus is the most important. For instance, mechanical resistance to a thermal shock depends linearly on this key parameter. A lot of techniques, generally complex to deal with, are proposed in the literature: bending test of symmetrically coated substrates [ 11, derivation from XRD stress measurement [ 11, ultrasonic surface wave [ 21, speed of bulk wave [ 31, nanoindentation [ 41. In this paper, we analyse the capabilities of a simpler bending test and the accuracy of its results applied to magnetron sputtered thin films of different materials (C, W, W(C), Cr, TIN) with different thicknesses ranging from 0.5 to 10 p.m, that are classically encountered in metallurgical applications. The influence of deposition parameters is also investigated.
We consider the classical three-point bending configuration applied to the case of a flat rectangular beam of dimensions L X A X e,, whose upper surface is coated by a perfectly adherent thin film of thickness e,. It is also supposed that: 1. the substrate is highly thicker than the coating:
* Corresponding author. 0040.6090/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDIOO40-6090(95)06921-6
of the leading equation
e, -=Z< e,
(I)
2. the aspect ratio of the beam is very large: e,-=ZL
3. The Poisson’s together: V,= vr
(2)
coefficient
of both materials
are close
(3)
Under the previous assumptions, the structure can be regarded as a thin composite beam, and related classical equations can be handled [ 5,6] if low bending is also considered. According to the elastic theory, and to the reference frame of Fig. 1, the strain in the composite may be written as:
A. Rouzaud et al. /Thin Solid Films 270 (1995) 270-274
271
video camera
(7)
(8) imposed displacement
(9)
t
coating
F, ( F2) being the force required to bend the substrate (the composite) up to a same deflection 6. According to the initial assumption of a thin coating on a thick substrate, and noticing that:
central point acoustic detector
Fig. 1. Schematic
e
experimental
Force detection
z,=z, (
1+3e’ e, 1
set-up and notations
and 1 Fxz (4)
&==2ig
The subscripts s, f and c are standing for the substrate, the film and the composite (substrate + film), respectively, E for the Young’s modulus and Z for the inertia momentum (I= e3h/12). If the film is thin enough, compared to the substrate, the xx component of the stress field in the coating will be given by: 1 E. Fxz (T =->--.r.r 2 EC Z, withe,/2
(6)
with: A
eJ2
L/2
II I 2
0
-es/2
a,&,
dx dy dz
0
and
where U is the energy consumed for bending the coating/ substrate composite, UC the energy for bending the coating, US the energy for bending the substrate, and W is the work done by the applied force needed to bend the composite. A straightforward calculation leads to:
F,L3 ~=-z48E,Z,
F2L3 48EJ,
we are lead to our central equation:
4 esF2-F1 -=_4 3ef F,
( 10)
F, and F2 being the forces to bend the substrate only and the composite, respectively, to the same given deflection 6. This equation suggests some comments: a similar but simpler calculation applied to the pure tensile test leads to a very close equation:
4 e,F2--Ft _=_Es e, Ft
(11)
It is thus observed that a tensile test performed on the same composite sample will lead to a lower (A F/F) compared to the bending test ((AFIF),,,,,=3(AFFIF),,,i,,). The accuracy of the bending test is thus three times higher. a similar equation has been recently derived [7] by an other method for the cantilever beam configuration and has been used to estimate the Young’s modulus of a coated micromachined silicon cantilever beam ( 17 X 200 pm). the validity range of Eq. ( 10) has been studied numerically by finite element modeling, by means of the Castem 2000 software [ lo]. First computations have shown that a large number of high precision elements (QS) are mandatory to get accurate numerical results ( 12 000 elements). Once the mesh was accurate enough to get the exact solution at e,< e, (2/200 I*m), new elements were added to simulate the influence of larger coatings. It was numerically found an excellent agreement up to a ratio e,/ er of ten.
A. Rouzaud et al. /Thin Solid Films 270 (1995) 270-274
212
4. due to superposition theorem, Eq. ( 10) remains perfectly correct even in the case of coatings with internal intrinsic stresses, as generally observed. 5. Eq. (10) shows that the lower substrate modulus, the higher the ratio ( AFIF) in a given geometry. Accurate selection of the substrate nature will thus improve the results accuracy.
3. Device description
and experimental
protocol
A schematic drawing of the bending device is presented in Fig. 1. The central point for load application is kept fixed whereas the programmable displacement (O-600 mm min-‘) is imposed by t h e ends points, in order to maintain the central part of the sample fixed in the laboratory frame. An optical microscope coupled with a video camera allows to monitor this area of the coating where the stress field is maximal (see Eq. (5) ) , and where thus the first cracks will appear as soon as the critical stress is exceeded. A piezo electric force measurement system (Sensotec, nominal range O-50 N) is located just below the central bending point. An acoustic emission detector (Bruel and Kjaer 8313) is also stuck to this central point in order to detect acoustic burst occurring at the onset of the cracks. Depending on the nature of the substrate, the coating and the geometry, cracks may occur either during the elastic or plastic deformation of the substrate. The knowledge of the imposed critical deflection (and therefore the critical stress) allows estimation of the adhesion of the coating according to classical mechanical models of interfacial adhesion energy [ 8,9]. However, such experiments are beyond the scope of this paper which focuses on the determination of coating elastic modulus far away from the conditions for cracking or for plastic deformation. In order to perform such experiments, commercially available standard beams (L = 70 mm, h = 4 mm, e, = 0.25,0.2 or 0.1 mm) of different substrates (stainless steel, TA6V) have been coated on one side with different materials excepted for a small region at the end of the beam (the last 10 mm), used to measure the reference force F, needed to impose a given deflection 6 to the substrate only. This offers a further advantage to determine the Young’s modulus of the substrate via the corresponding equation. The beam is then moved to locate a part of the coated zone between the two 8 mm spaced end points. In this way, many measurements may be run on the same beam at different locations, allowing to check the reproducibility of the experiments. Such reproducible measurements also allow to quantify the accuracy of the measurements, i.e. the errors bars, according to classical statistical theorems [ 131. Moreover, about ten beams have been inserted in the PVD reactor for each deposition experiment. Similar results have been obtained. Modulus measurements are then performed by computing the S, and S, slopes dFldz obtained from the uncoated and coated zones in the elastic domain (basically up to l&15 N
50 CI
Fig. 2. Bending results (force-deflection curves) and acoustic emission in a Cr coating on a stainless steel substrate. Associated optical view of cracks.
for stainless steel 200-250 pm thick substrates), Eq. ( 10) under the modified form:
Ef e, h-S1 -=Es 3ef Sl
and using
(10’)
The film integrity is always monitored by means of acoustic emission. Large bending of the samples are sometimes achieved after determination of the Young’s modulus. Reproducible results give the critical deflection (force) leading to the onset of cracks in the coating. Fig. 2 illustrates first cracks detection in a Cr coating on a stainless steel substrate at two different positions along the beam. Working far below this critical point in the linear part of the force-deflection curve ensures that the assumption of elastic behavior is fully satisfied.
4. Experimental
results
4.1. Validation First experiments have been performed on magnetron DC sputtered dense Cr coatings elaborated on similar conditions in terms of argon pressure, applied power and substrate temperature, but with different sputtering durations in order to get four different coatings thicknesses, 1, 3, 6 and 10 pm. Results have shown a fairly linear dependence of the AFIF
A. Rouzaud et al. /Thin Solid Films 270 (1995) 270-274
Table 1 Young’s modulus measurements
A-
400,
,
MODULUS.GPa
,
!
,
for low pressure DLC
Substrate
Stainless steel
Substrate thickness (mm) Substrate width (mm) Coating thickness (pm) Coating modulus ( GPa) Standard deviation (GPa) Number of experiments Sample ID # Accuracy range ( GPa)
0.1 4.21 2.40 207 22 10 #l f 13.6
TA6V
0.1 4.14 2.40 200 21 10 #2 +13
0.1 4.18 2.54 215 I9 10 #3 rt 6.2
0.25 4.02 2.54 185 33 10 #4 * 20.5
term as a function of the coating thickness. They lead to a Young’s modulus of 292 f 18 GPa, close to the bulk value. 4.2. Carbon coatings Two diamond like carbon (DLC) coatings elaborated at two different argon pressures (0.25 and 2 Pa, respectively) leading to two different morphologies (dense/cellular) have been deposited on both 250 pm thick TA6V and 100 pm thick stainless steel substrates. Experimental procedures and morphologies are given in Ref. [ 111, where it is shown that, basically, the coating morphology (dense or columnar) is lead by the plasma pressure. For the low pressure range (typically < 0.8 Pa), the mean free path of the sputtered carbon atoms is higher than the substrate-target distance and they impinge the substrate with their initial energy: this results in dense coatings. On the other hand, for the largepressurerange ( > 0.8 Pa), sputtered atoms loose a part of their energy during the collisions they undergo before reaching the substrate: this leads to columnar morphologies. The major results for low pressure DLC are summarized in Table 1. The mean Young’s modulus value obtained for such coatings is therefore about 210 GPa. The results obtained on TA6V substrates exhibit a standard deviation systematically higher than that for stainless steel substrates, about 50 percent higher. This seems to be related to the lower adhesion of the DLC on titanium alloys, since local delamination appears after cracking. Such measurements could thus be a simple way to estimate the relative adhesion of a given coating to its substrate. However, this explanation needs to be investigated more fully before final conclusions can be drawn. Further experiments are now in progress. Table 2 Young’s modulus measurements
213
for W coatings elaborated
Substrate
TA6V
Beam thickness (mm) Beam width (mm) Coating thickness (km) Coating modulus ( GPa) Standard deviation ( GPa) Number of experiments Sample ID number Accuracy range ( GPa)
0.25 4.02 5 387 58 10 #l *36
at low pressure
360
-
320
-
280: 240
-
200+0
P(CH4)/P(Ar) Fig. 3. Plot of the evolution of the Young’s modulus of Cr(C) a function of the reactive gas pressure.
coatings
as
The corresponding results for high pressure (columnar) DLC on identical stainless steel substrates give coating modulus values of about 59.0 f 8.7 GPa. Some basic conclusions may thus been drawn in the frame of this study on DLC: At a given deposition pressure, according to the error bars range, the Young’s modulus of the coatings are identical, independent of the substrate nature. The two different morphologies are leading to very different mechanical properties. It must be pointed out that this difference in behaviour has also been emphasized on internal pin/disk tribological studies. Concerning more specifically the Young’s modulus values, it can be thought, as discussed by Bull and Rickerby for TIN coatings [ 121, that, although the modulus of individual columns remains the same for both coatings at their local scale, the effective modulus is decreasing as the columnar morphology is more markedly observed. Such an evolution of Young’ modulus values as a function of the coatings morphology is in good agreement with recent results in the literature obtained with another technique [ 21.
4.3. W coatings W dense coatings have been deposited at low Ar pressure onto TA6V and stainless steel substrates. The results, summarized in Table 2, show the same trends: 1. The Young’s modulus is independent of the nature of the substrate. 2. Again, larger standard deviation for titanium alloy substrate is certainly related to lower adhesion. 3. The modulus of dense coatings correspond to bulk values given in the literature.
Stainless steel 0.25 4.02 5 374 19 10 #2 *49
0.215 4.11 5 417 35 10 #3 +21.7
0.215 4.10 5 392 40 10 #4 5 24.8
Similar values have been obtained for a 0.5 km thick W coating deposited onto a 100 p,rn thick stainless steel substrate. 4.4. Cr(C) coatings Cr(C) solid solution coatings have been deposited with different CH4 partial pressures. Our main investigations led
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the modulus evolution plotted on Fig. 3. It is observed that the modulus remains roughly constant excepted for the last ratio, where an increase in the order of 100 GPa P ~C-4~IP~Ar~ is observed. XRD analysis performed on same samples show that this phenomenon is related to a phase change in the coating, as demonstrated in Fig. 4. Carbide phases are identified for the highest pressures ratio, while other samples show a solid solution of C in the Cr textured phase. to
properties are known and are not too close together. According to the right choice of the substrate nature and geometry, even very low moduli of columnar coatings may be estimated (30 GPa). The uncertainty range is about k 10% of the modulus, which seems a good compromise for most applications. Some points need to be investigated more fully by this technique: The modulus values obtained in such a way give a macroscopic description for the mechanical behaviour of a large number of crystallites, due to the size of the analyzed zone (8 X 4 mm). Comparison with results obtained on same samples with the nanoindentation technique, which measures on a more local scale, is now in progress and will help to understand the influence of microstructure on mechanical properties. The influence of poor adhesion on the higher scattering of the results is presently under investigation. If this experimental fact was confirmed and understood, this test could be a simple non-destructive way to characterize, at least qualitatively, the adhesion of the coating to a given substrate.
4.5. TiN coatings 4 pm thick TIN, coatings have been deposited onto stainless steel substrates by reactive sputtering of a titanium target in an Ar + N, plasma, with different N, pressures. Here again, a large variation of the Young’s modulus is observed as a function of the reactive gas pressure inside the reactor. As an example, a 133 k 13 GPa modulus is obtained for low N? pressures corresponding to nitrogen inserted into a titanium phase. On the other hand, a value of about 269 + 26 GPa is measured in the case of an over stoichiometricallydark brown TiN, coating.
5. Discussion Results obtained in the frame of this study show that the presented bending test gives accurate values of the Young’s modulus for a wide variety of metallurgical coatings. Measurements simply require to insert thin beams in the coating device, a part of the beam being masked in order to get the pure substrate reference measurements. Depending on the selected process for coating deposition, mechanical properties of the substrate may be modified and the preliminary measurement of substrate modulus ensures the reliability of that of the the coating. It has been shown that phase changes may be detected, provided that the corresponding mechanical
Acknowledgements The authors would like to thank Mr. Petit for his precious help in X-ray analysis, Mr. Ignat (Enseeg/Ltpcm) for fruitful discussions on mechanical properties of thin films and Mr. Le Gallo for advice on numerical analysis.
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