- Email: [email protected]
Behaviour of CVD and PVD coatings under impact load
Surface and Coatings Technology, 68/69 (1994) 253—258
253
Behaviour of CVD and PVD coatings under impact load 0. Knotek, E. Lugscheider*, F. Löffler, A. Schrey, B. Bosserhoff Materials Science Institute, Aachen University of Technology, D-52056 Aachen, Germany
Abstract The impact test is a new test method to determine the impact load of coated substrates. Chemical (CVD) and physical vapour deposition (PYD) coatings on cemented carbide substrates have been tested. The description of the stress distribution in the coating—substrate compound gives important information for understanding the test results. The different kinds of coating damage can be explained by different stress models. Under impact load PVD and CYD coatings show totally different behaviours. While PVD coatings are damaged suddenly, CVD coatings show a continuously increasing wear mark. Based on stress analysis, a model has been developed which can explain the different behaviours of PVD and CYD coatings in the impact test. Practical wear tests (interrupted cutting test) show the relevance of the impact test. A comparison with other test methods, e.g. the scratch test, shows the delimitation of the test.
1. Introduction
2. Experimental
In practical interrupted cutting applications, e.g. milling, physical vapour deposition (PVD) coatings show better performance than chemical vapour deposition (CVD) coatings up to 350mm min’ cutting speed [1]. This is displayed in Fig. 1. Fig. 2 shows the causes of wear in interrupted cutting operations. A main factor of the applied load is fatigue of the coating—substrate compound and the bulk material. The resistance against fatigue can be determined with the coating impact test. Other factors are adhesion, abrasion and thermal effects. Thus it should be possible to obtain information with the coating impact test about the different behaviours of PVD and CVD coatings in interrupted cutting operations. Generally the dynamic loadability of a coating—substrate composite is influenced by the mechanical behaviour of the coating and the substrate material as well as the adhesion between the coating and the substrate. The impact tester is a model test to investigate the fatigue strength of the composite. The loading configuration chosen for this test is the ball—plane system, which permits application of a high load with a simple geometry by means of hertzian stress, simulating a wide range of tribological systems. Like fatigue strength diagrams for bulk materials, impact test results are presented in the form of a curve showing the loading of the coating—substrate composite as a function of the number of stress cycles endured.
In order to test the impact strength, reactive magnetron sputter ion plating was used to deposit TiN, TiC, Ti(C,N) and (Ti,V)(C,N) films. Sintered titanium (Ti>99.9%) and titanium—vanadium (70 at.% Ti, 30 at.% V) targets were sputtered in a low pressure argon plasma (PAr = 1 Pa), with admission of nitrogen 3
VOl StilpTumlngTest
x10
90
,
Cuthng Parameters: f -0,2mm
a~- 2,5mm 80
Work Material: SAE 4140
‘
~ c~70 ~ ~ 60 .~
-
R~-980l~Wmm2 Sub~ate:P20/30
Arc-TICN
-
SDHN 120312 -
50
-
~ ~
-
~
.
AIC—TIN
-
~ ~
.
-
nCVD—TIN/TjCN
Z 20
.Uncoated
10 0 0
-
_______________________________ 100 200 300m/mIn400
Cuthng Speed v~ *
Corresponding author,
SSDI 0257-8972(94)08110-K
Fig. 1. Performance of PVD and CVD coatings in interrupted cutting operations [1].
Elsevier Science S.A.
254
0. Knotek et al.
/
CVD and PVD coatings under impact load
thermal shot
bending strain
cutting speed Fig. 2. Wear mechanisms in interrupted cutting operations [1].
or nitrogen and methane as reactive gases. The process has been described by Haefer [2]. The substrate materials were cemented carbides. The hardness of the coatings was determined according to the Vickers method (HV 0.05). Titanium nitride and carbide films achieved a hardness of 1.5—2.0 GPa, Ti(C,N) films about 2.3 GPa and (Ti,V)(C,N) about 2.8 GPa. The investigated CVDcoated cemented carbides were a TiN/TiC multilayer coating. The coatings have been produced in an industrial deposition process and the thickness was around 12 Jim. The impact strength was investigated using a newly developed impact tester. The surface of a solid body is stressed periodically at a defined point by impacts of a hard test body. The impact force, impact frequency and number of impacts are controlled via a stored programme control (SPC) system. Fig. 3 shows front and side views of the impact tester, which has been developed in cooperation with CemeCon GmbH, Aachen. The tester consists of the actual impact unit, a specimen holder and a solid granite baseplate. Fig. 4 illustrates the working principle, The force is exerted as a linear impact on the test surface by a cemented carbide ball (R = 2.5 mm; 7.5% TiC, 7.5% Co, 85% WC), which is fixed by a nut. The force of impact is registered by a force sensor with strain gauges and is fed to the SPC closed loop as an actual value. The impact motion is part of a spring—mass oscillating system comprising a freely prestressed spring with compliance c and the carbide ball, force sensor and a cylindrical coil as the sprung mass. The oscillation is excited by magnetic forces created between the permanent magnet and the cylindrical coil. The frequency f can be varied from 0 to 50Hz via the coil current. The SPC system controls the force by reference to the actual
impact force ‘value. The number of impacts can be selected prior to the test. Up to io~impacts can be chosen. At high force amplitudes (greater than 1000 N) the cemented carbide ball has to be changed after roughly 106 impacts owing to flattening caused by plastic deformation at the impact point. In order to minimize adhesion as a wear mechanism and to isolate surface degradation as the main form of wear, oil was applied as a separating medium. The stress distribution, i.e. the maximum shear stress tmax, the radial stress Yr and the in-plane stress ~ in the contact zone is shown in Fig. 5. This distribution is the result of a finite element method (FEM) analysis of the internal stresses in a coated substrate loaded by a ball [3]. The most critical load, the maximum tensile stress, appears at the edge of the contact zone. The maximum shear stresses are present in the interface zone. A stereomicroscope with a magnification of 40 x was used to evaluate the wear between cycles. All points under investigation were then characterized by optical and scanning electron microscopy. Complete spalling of the coating, i.e. exposure of the substrate material, was selected as the failure criterion.
3. Results Under continuous loading, failure occurred suddenly in all PVD coatings. All CYD coatings showed no spalling of the coating but a continuous enlargement of the wear mark caused by the contact between the cemented carbide ball and the sample surface. This has been pointed out in a previous paper [4]. The failure criterion, similarly to the PVD coatings, is the exposure
0. Knotek et al.
/
CVD and PVD coatings under impact load
255
Ceme Cou~
Fig. 3. The coating impact tester (dimensions in millimetres).
/
N —
3.1. Behaviour ofPVD-coated composites
//,‘~‘.lz,,.’z.. ,
SPRING COIL
I
PERMANENT MAGNET
I f =0-50 Hz
I
MAGNET COIL I STRAIN GAUGE I ___________________
o~5oON
I
CARBIDE BALL 2,5mm
The typical behaviour of TiN, TiC and Ti(C,N) PVD coatings on cemented carbide in the impact test is shown in Fig. 6. The impact number range in which the coating fails can be determined with relative accuracy. A more or less sudden spalling of the coating occurs. It is difficult to decide whether the interface or the bulk material fails, because it is impossible to detect the exact moment at which the spalling occurs and to stop the test. After this moment all further impacts will destroy the wear mark. The microhardness of the coatings (TiN, 1500 HV 0.05; TiC, 1950 HV 0.05; Ti(C,N), 2350 HV 0.05) correlates with the impact test results. Better hardness values seem to provide better fatigue resistance. Complex PVD coatings in, the system Ti—V—C—N
FILM
have been deposited varioustest. process parameters and investigated with with the impact An improvement
SUBSTRATE Fig. 4. Working principle of the coating impact tester.
in fatigue strength was possible by varying the process gas composition (Fig. 7). The coatings have been sputtered with a Ti70V30 target and various gas compositions. The (Ti,V)(C,N)-coated samples show similar wear
of the substrate material. Thus it is difficult to compare the behaviours of PVD and CVD coatings in the impact test.
behaviour, but a nitrogen-to-methane partial pressure ratio of 4.5/4.5 leads to very good wear resistance. The microhardness and critical load of these coatings were also determined (Table 1). In the case of (Ti,V)(C,N)
256
0. Knotek et al.
/
CVD and PVD coatings under impact load
0,0
-0,4
-
-
-
X -1,2
or/PG
D tmax/PO
-1,6 -1,5
-1,0
I -0,5 Gr/PO
____________
0,0
0,5
Fig. 5. Distribution of stresses for a ball—plate contact in a coating substrate composite [3].
2O~
1OE5
10E6
1OE7
number of impacts n Fig. 6. Coating impact test results of PVD coatings in the system Ti—C—N.
coatings there is no clear relationship between microhardness, critical load and impact test results. In addition, pin-on-disk tests were carried out to detect the wear resistance of the deposited films. In this test a coated specimen was pressed on a rotating silicon carbide paper (P 1000). Fig. 8 displays the results of the test using (Ti,V)(C,N) films. The coatings are deposited from a Ti70V3Q target with various nitrogen-to-methane pressure ratios (7.0/2.0, 4.5/4.5 and 6.0/3.0). There is an opposite behaviour of the coatings in the coating impact test to that in the pin-on-disk test. In a milling test only coatings with good abrasion resistance and high fatigue strength showed good performance. Thus it is important to have information about the main wear components (Fig. 2) present in interrupted cutting operations.
3.2. Comparison ofCVD and PVD coatings under impact load All CVD coatings showed no spalling of the coating but a continuous enlargement of the wear mark caused by the contact between the cemented carbide ball and the sample surface. This agrees with the behaviour of an uncoated cemented carbide [4]. As noted, the failure criterion is the exposure of the substrate material. Fig. 9 gives an overview of the different behaviours of a CVD TiN/TiC coating and a PVD Ti(C,N) coating. At high loads and fewer impacts the CVD coating shows better performance. The PVD coating has a higher fatigue strength at a high number of impacts than does the CVD coating. Possible reasons are the different internal stress situations in the coatings. Because of the
0. Knotek et al.
/
CVD and PVD coatings under impact load
~TT
1500-
_______
L
[N] I
~----
—s--
_
-
—~?~--
3I6p~ICH4 ~
Th\
I7/2prnIj~i~
300-
-
target: Ti7QV~ 400W U. :-60V
120-~------
~,900-
257
~-—--
-
-
:
___
~ 6,5/6,5 ~/p~~4
,
~2,
—~--__
-~_~___[_
JE+03
/2,Sp~p,
___
.
0-
-
:~u
_
JE+04
JE+05
1E+06
number of impacts N
Fig. 7. Coating impact test results of PYD coatings in the system Ti—V—C—N (target Ti10V30, nitrogen-to-methane pressure ratios 7.0/2.0, 4.5/4.5, 3.0/6.0). Table 1
PVD coating has excellent performance in this cutting
Microhardness and critical load of PVD coatings in the system
test. CVD coatings become better at high cutting speeds, which result in high impact forces. This behaviour
—
Nitrogen/methane partial pressure
7.0/2.0
4.5/4.5
3.0/6.0
2.5/2.5
6.5/6.5
Microhardness (HV 0.05) Critical force F (N)
1920
2660
2690
2260
2880
30
35
40
38
28
correlates with the impact test results, so that a prediction of material performance in interrupted cutting operations with the coating impact test is possible.
4. Summary higher deposition temperature in CVD processes and the different thermal expansions of coating and substrate (cemented carbide), there are tensile stresses in CVD coatings. The ion bombardment due to the substrate bias and the low diffusion rates at low temperatures cause compressive stresses in PYD coatings [5]. Therefore in hard thin coatings the external applied load causes a maximum tensile stress around the contact zone of the ball as is to be expected in the case of hard and brittle materials. Because of the compressive stress in PVD coatings, the reaction of the external load is compensated by the internal compressive stress. This is not possible for CVD coatings, because tensile stress is present. Results of a previous interrupted cutting test with CVD and PVD coatings were published earlier [1]. The situation is similar to the impact test results (Fig. 9). At low cutting speed, which is due to a low impact force, a
The impact tester represents a model test capable of reproducibly determining the dynamic loadability of thin films. The study demonstrates that clear differences can be established between different coating—substrate cornbinations investigated. PVD-coated substrates failed suddenly in the impact test owing to spalling of the coating, while CVD-coated cemented carbides suffered surface damage due to continuous detachment of small particles, similarly to an uncoated cemented carbide. The results could be interpreted using stress distribution theory for this load case. Failure of the material occurred in the peak tensile stress zone as is to be expected in the case of hard and brittle materials. The comparison between results of the coating impact test with PVD and CVD coatings from the system Ti—C—N and interrupted cutting tests shows that the coating impact test is a suitable test to characterize coated tools and to choose the best coating—substrate combination.
258
0. Knotek et al.
2
/
CVD and PVD coatings under impact load
____
+TjN
T,~. 550°C
[mgJ ~ T17030;7.0/2.0
~
Bias
~
~Ti70V30; 4.5/4.5
1,5
~Ti70V30;
3.0/6.0
Cl) 0)
x
.2
x
~-)< <
<
0
100
200
400
600
800
1000
1200
1400
1600
1800
cycles Fig. 8. Pin-on-disk test results of PVD coatings in the system Ti—V—C—N.
I
[N] 1400
—
200
—
/Ti(C~N)~ PVD
10E5
—
—
I
0
I
I
I
10E6
10E7
number of impacts n Fig. 9. Coating impact test results of PVD and CVD coatings in the system Ti—C—N.
However, in most cases it will be necessary to obtain further information e.g. about wear or thermal stability.
References [1] 0. Knotek, F. Löffler and G. Kramer, Surf. Coat. Technol., 54—55 (1992) 241.
[2] R. Haefer, Oberfiachen- und Dunnschicht-Technologie, Part I, Springer, Berlin, 1987. [3] H. Djabella and RD. Arnell, Thin Solid Films, 213 (1992) 205. [4] 0. Knotek, B. Bosserhoff, A. Schrey, T. Leyendecker, 0. Lemmer and S. Esser, Surf. Coat. Technol., 54—55 (1992) 102. [5] J. Birkholzer and V. Hauk, Hart-Tech. Mitteil., 48 (1993) 25.
253
Behaviour of CVD and PVD coatings under impact load 0. Knotek, E. Lugscheider*, F. Löffler, A. Schrey, B. Bosserhoff Materials Science Institute, Aachen University of Technology, D-52056 Aachen, Germany
Abstract The impact test is a new test method to determine the impact load of coated substrates. Chemical (CVD) and physical vapour deposition (PYD) coatings on cemented carbide substrates have been tested. The description of the stress distribution in the coating—substrate compound gives important information for understanding the test results. The different kinds of coating damage can be explained by different stress models. Under impact load PVD and CYD coatings show totally different behaviours. While PVD coatings are damaged suddenly, CVD coatings show a continuously increasing wear mark. Based on stress analysis, a model has been developed which can explain the different behaviours of PVD and CYD coatings in the impact test. Practical wear tests (interrupted cutting test) show the relevance of the impact test. A comparison with other test methods, e.g. the scratch test, shows the delimitation of the test.
1. Introduction
2. Experimental
In practical interrupted cutting applications, e.g. milling, physical vapour deposition (PVD) coatings show better performance than chemical vapour deposition (CVD) coatings up to 350mm min’ cutting speed [1]. This is displayed in Fig. 1. Fig. 2 shows the causes of wear in interrupted cutting operations. A main factor of the applied load is fatigue of the coating—substrate compound and the bulk material. The resistance against fatigue can be determined with the coating impact test. Other factors are adhesion, abrasion and thermal effects. Thus it should be possible to obtain information with the coating impact test about the different behaviours of PVD and CVD coatings in interrupted cutting operations. Generally the dynamic loadability of a coating—substrate composite is influenced by the mechanical behaviour of the coating and the substrate material as well as the adhesion between the coating and the substrate. The impact tester is a model test to investigate the fatigue strength of the composite. The loading configuration chosen for this test is the ball—plane system, which permits application of a high load with a simple geometry by means of hertzian stress, simulating a wide range of tribological systems. Like fatigue strength diagrams for bulk materials, impact test results are presented in the form of a curve showing the loading of the coating—substrate composite as a function of the number of stress cycles endured.
In order to test the impact strength, reactive magnetron sputter ion plating was used to deposit TiN, TiC, Ti(C,N) and (Ti,V)(C,N) films. Sintered titanium (Ti>99.9%) and titanium—vanadium (70 at.% Ti, 30 at.% V) targets were sputtered in a low pressure argon plasma (PAr = 1 Pa), with admission of nitrogen 3
VOl StilpTumlngTest
x10
90
,
Cuthng Parameters: f -0,2mm
a~- 2,5mm 80
Work Material: SAE 4140
‘
~ c~70 ~ ~ 60 .~
-
R~-980l~Wmm2 Sub~ate:P20/30
Arc-TICN
-
SDHN 120312 -
50
-
~ ~
-
~
.
AIC—TIN
-
~ ~
.
-
nCVD—TIN/TjCN
Z 20
.Uncoated
10 0 0
-
_______________________________ 100 200 300m/mIn400
Cuthng Speed v~ *
Corresponding author,
SSDI 0257-8972(94)08110-K
Fig. 1. Performance of PVD and CVD coatings in interrupted cutting operations [1].
Elsevier Science S.A.
254
0. Knotek et al.
/
CVD and PVD coatings under impact load
thermal shot
bending strain
cutting speed Fig. 2. Wear mechanisms in interrupted cutting operations [1].
or nitrogen and methane as reactive gases. The process has been described by Haefer [2]. The substrate materials were cemented carbides. The hardness of the coatings was determined according to the Vickers method (HV 0.05). Titanium nitride and carbide films achieved a hardness of 1.5—2.0 GPa, Ti(C,N) films about 2.3 GPa and (Ti,V)(C,N) about 2.8 GPa. The investigated CVDcoated cemented carbides were a TiN/TiC multilayer coating. The coatings have been produced in an industrial deposition process and the thickness was around 12 Jim. The impact strength was investigated using a newly developed impact tester. The surface of a solid body is stressed periodically at a defined point by impacts of a hard test body. The impact force, impact frequency and number of impacts are controlled via a stored programme control (SPC) system. Fig. 3 shows front and side views of the impact tester, which has been developed in cooperation with CemeCon GmbH, Aachen. The tester consists of the actual impact unit, a specimen holder and a solid granite baseplate. Fig. 4 illustrates the working principle, The force is exerted as a linear impact on the test surface by a cemented carbide ball (R = 2.5 mm; 7.5% TiC, 7.5% Co, 85% WC), which is fixed by a nut. The force of impact is registered by a force sensor with strain gauges and is fed to the SPC closed loop as an actual value. The impact motion is part of a spring—mass oscillating system comprising a freely prestressed spring with compliance c and the carbide ball, force sensor and a cylindrical coil as the sprung mass. The oscillation is excited by magnetic forces created between the permanent magnet and the cylindrical coil. The frequency f can be varied from 0 to 50Hz via the coil current. The SPC system controls the force by reference to the actual
impact force ‘value. The number of impacts can be selected prior to the test. Up to io~impacts can be chosen. At high force amplitudes (greater than 1000 N) the cemented carbide ball has to be changed after roughly 106 impacts owing to flattening caused by plastic deformation at the impact point. In order to minimize adhesion as a wear mechanism and to isolate surface degradation as the main form of wear, oil was applied as a separating medium. The stress distribution, i.e. the maximum shear stress tmax, the radial stress Yr and the in-plane stress ~ in the contact zone is shown in Fig. 5. This distribution is the result of a finite element method (FEM) analysis of the internal stresses in a coated substrate loaded by a ball [3]. The most critical load, the maximum tensile stress, appears at the edge of the contact zone. The maximum shear stresses are present in the interface zone. A stereomicroscope with a magnification of 40 x was used to evaluate the wear between cycles. All points under investigation were then characterized by optical and scanning electron microscopy. Complete spalling of the coating, i.e. exposure of the substrate material, was selected as the failure criterion.
3. Results Under continuous loading, failure occurred suddenly in all PVD coatings. All CYD coatings showed no spalling of the coating but a continuous enlargement of the wear mark caused by the contact between the cemented carbide ball and the sample surface. This has been pointed out in a previous paper [4]. The failure criterion, similarly to the PVD coatings, is the exposure
0. Knotek et al.
/
CVD and PVD coatings under impact load
255
Ceme Cou~
Fig. 3. The coating impact tester (dimensions in millimetres).
/
N —
3.1. Behaviour ofPVD-coated composites
//,‘~‘.lz,,.’z.. ,
SPRING COIL
I
PERMANENT MAGNET
I f =0-50 Hz
I
MAGNET COIL I STRAIN GAUGE I ___________________
o~5oON
I
CARBIDE BALL 2,5mm
The typical behaviour of TiN, TiC and Ti(C,N) PVD coatings on cemented carbide in the impact test is shown in Fig. 6. The impact number range in which the coating fails can be determined with relative accuracy. A more or less sudden spalling of the coating occurs. It is difficult to decide whether the interface or the bulk material fails, because it is impossible to detect the exact moment at which the spalling occurs and to stop the test. After this moment all further impacts will destroy the wear mark. The microhardness of the coatings (TiN, 1500 HV 0.05; TiC, 1950 HV 0.05; Ti(C,N), 2350 HV 0.05) correlates with the impact test results. Better hardness values seem to provide better fatigue resistance. Complex PVD coatings in, the system Ti—V—C—N
FILM
have been deposited varioustest. process parameters and investigated with with the impact An improvement
SUBSTRATE Fig. 4. Working principle of the coating impact tester.
in fatigue strength was possible by varying the process gas composition (Fig. 7). The coatings have been sputtered with a Ti70V30 target and various gas compositions. The (Ti,V)(C,N)-coated samples show similar wear
of the substrate material. Thus it is difficult to compare the behaviours of PVD and CVD coatings in the impact test.
behaviour, but a nitrogen-to-methane partial pressure ratio of 4.5/4.5 leads to very good wear resistance. The microhardness and critical load of these coatings were also determined (Table 1). In the case of (Ti,V)(C,N)
256
0. Knotek et al.
/
CVD and PVD coatings under impact load
0,0
-0,4
-
-
-
X -1,2
or/PG
D tmax/PO
-1,6 -1,5
-1,0
I -0,5 Gr/PO
____________
0,0
0,5
Fig. 5. Distribution of stresses for a ball—plate contact in a coating substrate composite [3].
2O~
1OE5
10E6
1OE7
number of impacts n Fig. 6. Coating impact test results of PVD coatings in the system Ti—C—N.
coatings there is no clear relationship between microhardness, critical load and impact test results. In addition, pin-on-disk tests were carried out to detect the wear resistance of the deposited films. In this test a coated specimen was pressed on a rotating silicon carbide paper (P 1000). Fig. 8 displays the results of the test using (Ti,V)(C,N) films. The coatings are deposited from a Ti70V3Q target with various nitrogen-to-methane pressure ratios (7.0/2.0, 4.5/4.5 and 6.0/3.0). There is an opposite behaviour of the coatings in the coating impact test to that in the pin-on-disk test. In a milling test only coatings with good abrasion resistance and high fatigue strength showed good performance. Thus it is important to have information about the main wear components (Fig. 2) present in interrupted cutting operations.
3.2. Comparison ofCVD and PVD coatings under impact load All CVD coatings showed no spalling of the coating but a continuous enlargement of the wear mark caused by the contact between the cemented carbide ball and the sample surface. This agrees with the behaviour of an uncoated cemented carbide [4]. As noted, the failure criterion is the exposure of the substrate material. Fig. 9 gives an overview of the different behaviours of a CVD TiN/TiC coating and a PVD Ti(C,N) coating. At high loads and fewer impacts the CVD coating shows better performance. The PVD coating has a higher fatigue strength at a high number of impacts than does the CVD coating. Possible reasons are the different internal stress situations in the coatings. Because of the
0. Knotek et al.
/
CVD and PVD coatings under impact load
~TT
1500-
_______
L
[N] I
~----
—s--
_
-
—~?~--
3I6p~ICH4 ~
Th\
I7/2prnIj~i~
300-
-
target: Ti7QV~ 400W U. :-60V
120-~------
~,900-
257
~-—--
-
-
:
___
~ 6,5/6,5 ~/p~~4
,
~2,
—~--__
-~_~___[_
JE+03
/2,Sp~p,
___
.
0-
-
:~u
_
JE+04
JE+05
1E+06
number of impacts N
Fig. 7. Coating impact test results of PYD coatings in the system Ti—V—C—N (target Ti10V30, nitrogen-to-methane pressure ratios 7.0/2.0, 4.5/4.5, 3.0/6.0). Table 1
PVD coating has excellent performance in this cutting
Microhardness and critical load of PVD coatings in the system
test. CVD coatings become better at high cutting speeds, which result in high impact forces. This behaviour
—
Nitrogen/methane partial pressure
7.0/2.0
4.5/4.5
3.0/6.0
2.5/2.5
6.5/6.5
Microhardness (HV 0.05) Critical force F (N)
1920
2660
2690
2260
2880
30
35
40
38
28
correlates with the impact test results, so that a prediction of material performance in interrupted cutting operations with the coating impact test is possible.
4. Summary higher deposition temperature in CVD processes and the different thermal expansions of coating and substrate (cemented carbide), there are tensile stresses in CVD coatings. The ion bombardment due to the substrate bias and the low diffusion rates at low temperatures cause compressive stresses in PYD coatings [5]. Therefore in hard thin coatings the external applied load causes a maximum tensile stress around the contact zone of the ball as is to be expected in the case of hard and brittle materials. Because of the compressive stress in PVD coatings, the reaction of the external load is compensated by the internal compressive stress. This is not possible for CVD coatings, because tensile stress is present. Results of a previous interrupted cutting test with CVD and PVD coatings were published earlier [1]. The situation is similar to the impact test results (Fig. 9). At low cutting speed, which is due to a low impact force, a
The impact tester represents a model test capable of reproducibly determining the dynamic loadability of thin films. The study demonstrates that clear differences can be established between different coating—substrate cornbinations investigated. PVD-coated substrates failed suddenly in the impact test owing to spalling of the coating, while CVD-coated cemented carbides suffered surface damage due to continuous detachment of small particles, similarly to an uncoated cemented carbide. The results could be interpreted using stress distribution theory for this load case. Failure of the material occurred in the peak tensile stress zone as is to be expected in the case of hard and brittle materials. The comparison between results of the coating impact test with PVD and CVD coatings from the system Ti—C—N and interrupted cutting tests shows that the coating impact test is a suitable test to characterize coated tools and to choose the best coating—substrate combination.
258
0. Knotek et al.
2
/
CVD and PVD coatings under impact load
____
+TjN
T,~. 550°C
[mgJ ~ T17030;7.0/2.0
~
Bias
~
~Ti70V30; 4.5/4.5
1,5
~Ti70V30;
3.0/6.0
Cl) 0)
x
.2
x
~-)< <
<
0
100
200
400
600
800
1000
1200
1400
1600
1800
cycles Fig. 8. Pin-on-disk test results of PVD coatings in the system Ti—V—C—N.
I
[N] 1400
—
200
—
/Ti(C~N)~ PVD
10E5
—
—
I
0
I
I
I
10E6
10E7
number of impacts n Fig. 9. Coating impact test results of PVD and CVD coatings in the system Ti—C—N.
However, in most cases it will be necessary to obtain further information e.g. about wear or thermal stability.
References [1] 0. Knotek, F. Löffler and G. Kramer, Surf. Coat. Technol., 54—55 (1992) 241.
[2] R. Haefer, Oberfiachen- und Dunnschicht-Technologie, Part I, Springer, Berlin, 1987. [3] H. Djabella and RD. Arnell, Thin Solid Films, 213 (1992) 205. [4] 0. Knotek, B. Bosserhoff, A. Schrey, T. Leyendecker, 0. Lemmer and S. Esser, Surf. Coat. Technol., 54—55 (1992) 102. [5] J. Birkholzer and V. Hauk, Hart-Tech. Mitteil., 48 (1993) 25.