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Ion beam sputter deposition of thin insulating layers for applications in highly loaded contacts
Thin Solid Films, 109 (1983) 19-25 METALLURGICAL
AND
PROTECTIVE
19
COATINGS
ION BEAM SPUTTER DEPOSITION OF THIN INSULATING FOR APPLICATIONS IN HIGHLY LOADED CONTACTS M. E. KBNIGER
LAYERS
AND G. REITHMEIER
Messerschmitt-Biilkow-Blohm
G.m.b.H.,
Postfach 80 II 49,8012 Ottobrunn
(F.R.G.)
M. SIMON Forschungsstelle (Received
f& Zahnriider und Getriebebau.
March
16,1983;
accepted
Technische Universik2 Miinchen, Munich (F. R.G.)
June 10,1983)
In highly loaded lubricated rolling and sliding contacts which occur in different kinds of machines and machine elements the experimental determination of the temperature or pressure distribution is desirable. Measurements can be accomplished with thin film transducers which must be electrically insulated from the test specimens by a thin insulating layer. This layer has to withstand wear under extremely severe conditions. Ion beam sputter deposition was used to deposit insulating layers of Al,O, between 0.6 and 4 urn thick onto steel discs. The durability of the coated discs was tested in a twin-disc machine under various working conditions. The insulating layers showed good durability under conditions of high hertzian pressure, high shear stress or high temperature stress caused by high slip values. Owing to their excellent mechanical properties the insulating layers are qualified for use as base layers for thin film transducers.
1.
INTRODUCTION
Highly loaded lubricated rolling and sliding contacts occur in many different kinds of machines and machine elements. Typical examples are roller bearings, gears, cams and transmission elements in traction drives. These contraform contacts are subjected to high loadsand, as a result, undergo high local elastic deformations. In spite of the locally high contact pressures, the two surfaces can be completely separated by a lubricating film in adequate speed ratio ranges. The hertzian pressure in the contacts can be as high as 3 GPa with a lubricating film thickness between 0 and 5 urn. For sliding conditions, additional shear stresses of up to 0.3 GPa and flash temperatures up to 400 “C can occur. In the evaluation of machine elements which are supposed to work properly under these conditions, the exact knowledge of the stress conditions is necessary. This problem is dealt with in many reports (see for example refs. l-4) on the theory of elastohydrodynamics. In addition, a great deal of experimental work has been done in the field to help to clarify the stress conditions. However, the high load, the small dimensions of the contact areas (contact widths less than 1 mm) and the short contact times (as short as 20 ps) make it very difficult. 0040~6090/83/%3.00
0 Elsevier Sequoia/Ptinted
in The Netherlands
20
M. E. KijNIGER,
G. REITHMEIER,
M. SIMON
The experimental determination of the temperature or pressure distribution in elastohydrodynamic contacts can be carried out using thin film transducers. This is accomplished by measuring the change in electrical resistance of the contacts (under load). For example, the resistivity of manganin changes as a function of pressure and that of titanium as a function of temperature. In refs. 5 and 6, measurements using thin film transducers are reported. The thin film transducers must be electrically insulated from the test specimens by a thin insulating layer. This layer must be very thin to prevent the signal from being influenced by the differences in the heat conduction and Young’s modulus etc. between the test specimen and the insulating material. Furthermore, this layer has to withstand wear in rolling and sliding contacts under extremely severe conditions. In the present paper the manufacture of these insulating layers is dealt with and practical tests of their durability are given. 2.
ION BEAM SPUTTER
DEPOSITION
2. I. General remarks Physical vapour deposition techniques are widely used for the deposition of various kinds of thin films. Amongst the number of different deposition techniques, sputtering7*8 and ion plating’, lo are particularly suitable for the deposition of dense and adherent thin films. Commonly practised d.c. or r.f. sputtering is carried out in a glow discharge environment. The glow discharge is an inexpensive large-area source for producing high ion densities at the target. In the present investigation another sputter technique, called ion beam sputtering rlsl’ has been found to be appropriate for the following reasons. (1) Sputteiing can be performed at comparatively low gas pressures, typically at (l-5) x lop2 Pa. Therefore pure films containing few gas inclusions can be obtained. (2) An exact and independent control of the ion beam energy and the ion flux to the target is possible. (3) The ion beam is produced in a separate vacuum chamber and extracted to the deposition chamber. By this means, only a slight heating of the substrates takes place. (4) Substrates of various shapes, e.g. planar or cylindrical, can be coated in front of the target, whereas in glow discharge sputtering the shape of the substrate influences the glow discharge and only more or less planar substrates can be coated. 2.2. Zon beam sputtering apparatus In Fig. 1 a schematic drawing of the ion beam sputter system used for the experiments is shown. An ion beam is generated in a separate chamber of the system. A low pressure gas discharge of the Kaufman type13v14 is used as the ion source (Veeco Instruments Inc.). It incorporates the following features: a hot-filament electron-emitting cathode, a molybdenum cylinder as the anode and a solenoidal electromagnet outside the chamber. Argon gas is fed into the chamber through an automatically controlled valve. An argon ion beam approximately 50mm in diameter is extracted and accelerated through a grid assembly towards the coating chamber. The grids also produce a pressure difference between the two chambers, i.e.
DEPOSITION
OF INSULATING
LAYERS
FOR LOADED
CONTACTS
21
a higher pressure in the ion beam chamber to maintain a stable gas discharge and a lower pressure in the coating chamber, and thus provide a sufficiently large mean free path for the ion beam. A neutralizing filament is located directly beneath the grid system. Electrons emitted from this filament neutralize the positive space charge of the ion beam and also eliminate surface charge when insulating materials are sputtered. A shutter shields the sputter target from the ion beam during the initial beam adjustment and is also used to measure the ion beam intensity (in milliamperes per square centimetre). If the shutter is in an open position, the ion beam is accelerated directly towards the sputter target assembly, situated roughly at the centre of the coating chamber. This assembly consists of three different sputter targets, each water cooled. One target at a time is in a sputtering position. Each sputter target has a circular shape with a diameter of 125 mm. The ion beam is incident on a target at an angle of approximately 45”.
aon beam
chamber
to cryopump
Fig. 1. Schematic
drawing of the ion beam sputtering
apparatus.
The substrates (steel discs), which are coated on their outer rims only, are positioned in front of the target so as to rotate about an axis perpendicular to the plane of the diagram. The substrates can be swung closer to the target if an additional ion bombardment during sputter deposition is desired. Immediately before film deposition the substrates are swung into the ion beam and precleaned by ion bombardment. The chamber has a diameter of approximately 400 mm and can be evacuated with a 400 1s-l cryopump to a pressure of 1O-4 Pa. 3:
EXPERIMENTAL
PROCEDURES
3.1. Coating of substrates Mainly insulating layers of Al,O, were deposited onto steel discs (16 MnCr 5) (diameter, 80 mm; width, 20 mm). The discs were case hardened and their surfaces were ground and polished to a surface roughness of about 0.07 urn centre-line average (c.1.a.). The discs were degreased and cleaned in different ultrasonic baths. Immediately before deposition a further cleaning by backsputtering ofmaterial from
22
M. E. KijNIGER,
G. REITHMEIER,
M. SIMON
the disc surfaces was performed (beam energy, 1OOOeV; beam current density, 1 mA cm-‘). Most of the Al,O, layers were deposited with an ion beam energy of 1000 eV at current densities of 1 mA cm-’ . (In a few cases a beam energy of 500 eV was used.) Layers of thicknesses between 0.6 and 4 urn were produced. The average growth rate on non-rotating substrates was approximately 100 A mini. During deposition the geometrical arrangement of substrate, target and ion beam shown in Fig. 2 was chosen. In case a the substrate is non-rotating. Only a small part of the substrate surface opposite the target is coated. In case b the substrate and target are in the same position as in case a but the substrate rotates during deposition. A uniform layer is deposited onto the whole rolling surface. In case c the substrate rotates during deposition. The substrate centre is positioned closer to the target. Thus the substrate is bombarded continuously by the argon beam during deposition.
III
Ar beam
””Y
boating (4
Fig. 2. The different geometrical
(‘4
positions
(4
of the target and the substrate
during sputter deposition.
a
Fig. 3. Cross section of the twin-disc
machine
for the load tests with coated discs.
3.2. Load tests with coated discs The durability of the coated discs was examined in a twin-disc machine under various working conditions. The twin-disc machine is shown in Fig. 3. On a test rig the coated cylindrical disc runs against a crowned disc (radius I of curvature, 40mm). It is possible to choose four different tracks for contact on the wider disc and thus by axial motion of the smaller disc four different test conditions can be applied to a single test disc. The discs are driven independently of each other, so all possible slide-to-roll ratios can be realized. The circumferential velocities and the slip between discs are measured
DEPOSITION
OF INSULATING
LAYERS
FOR LOADED
23
CONTACTS
electronically. Lubricating oil is usually injected into the contact zone and can be adjusted for temperature. The discs are loaded after they have reached their full speed ratios. The lower disc is mounted in a sled which is connected by spring suspension, allowing the direct measurement of the traction force by means of a load cell. A voltage is applied to the test discs to monitor that they are electrically insulated from each other. A voltage breakdown during the test indicates the destruction of the sputtered insulating layer. The conditions for the load tests are listed in Table I. TABLE
I
CONDITIONS
FOR THE LOAD
Test conditions Circumferential w
{(v, - v,)h)
TESTS
velocily v1 (m s-l) x loo (%)
Traction coe$icient p = FF/FN Hertzian pressure (GPa) Mean shear stress (GPa) Specific tractionpower ( x l+ W me’)
A
B
C
4
12.5
0.42
2
40
0.105
0.024
1.9
1.9
2 0.18-0.45 1.9
0.132
0.032
0.5 (max)
10.5
156
6.5 (max)
For condition A a high shear stress is transmitted in the contact by the use of a traction lubricant with a high coefficient of traction. According to the low slip values the temperature stress of the layer is not significant. For condition B the lubricant is a mineral oil (JSO VG 100). The shear stress that has to be transmitted is very small. With significant slip, considerable traction power Pt = &o,s can be transmitted in the contact, which leads to a high temperature stress in the contact. This is a typical stress condition as it occurs in gears with admittedly smaller pressures. The oil injection temperature for conditions A and B was 60 “C. Because of the good results for conditions A and B, test condition C was performed without lubrication. The discs were cleaned thoroughly with a solvent before the test was started. The circumferential velocity and slip were kept low to prevent scoring. With the formation of reaction layers according to the duration of the test, the coefficient of friction p = FF/FN increased to a level of 0.45. The tests were considered successful when there was no damage after 300 s. 4.
RESULTS
AND DISCUSSION
The Al,O, layers deposited under the conditions described in Section 3.1 typically exhibited an extremely smooth surface topography; a typical example is shown in Fig. 4. In the same figure the morphology of a fractured layer 3 urn thick can be seen as well. It shows a dense and smooth structure characteristic of zone T growth in Thornton’s growth modells. For any of the load tests A, B and C of Table I, the minimum required load to destroy the layers on the discs was already as high as a normal force of 2000 N, or equivalently 1.9 GPa hertzian pressure. A typical damage track as a result of test condition C is shown in Fig. 5. Basically the discs, which were coated on their entire outer rims (Section 3.1,
24
M. E. KijNIGER,
Fig. 4. Scanning
electron
micrograph
of a typical AI,O,
Fig. 5. Scanning
electron
micrograph
of a damage
G. REITHMEIER,
M. SIMON
layer 3 pm thick.
track as a result of test condition
C.
cases b and c), exhibited improved layer durability compared with the partially coated discs (Section 3.1, case a) under all the applied test conditions. The partially coated discs were mainly damaged in a transition zone between the coated and noncoated area. This can be attributed to the fact that the coating particles which are deposited in this area are incident extremely obliquely or have lost kinetic energy due to scattering processes in the gas. The ion bombardment during layer deposition in case c certainly has a beneficial effect on the layer adhesion but no large improvement in comparison with case b was discovered. More detailed investi. gations would be necessary to distinguish between cases b and c. All the layers passed test condition A without damage. Under test conditions with a high slip (condition B) all layers with a thickness greater than 1.5 urn were destroyed (with one exception). This shows that under working conditions entailing high temperature stress, the thinner layers are more resistant than the thicker layers.
DEPOSITION OF INSULATING LAYERS FOR LOADED CONTACTS
25
Referring to the above exception, it was observed that one disc with a layer 4 urn thick did not fail under condition B. A possible explanation for this could be that the surface of this particular disc had a lower roughness value (about 0.04 c.1.a.)than the other discs. Thus the smooth surface has a positive influence on the results. Under test condition C the inferior adhesion characteristics of the partially coated discs were demonstrated. The transmitted shear stress was between 0.15 and 0.2 GPa. The layers of completely coated discs were not destroyed even at a mean shear stress of about 0.5 GPa in the contact. No influence of the layer thickness (OS-4 urn) on adhesion was found under test condition C. In summary it was shown by the above investigations that ion beam sputtering is a valuable technique for the deposition of insulating layers which show good durability in highly loaded lubricated sliding contacts. At present, thin film transducers are deposited on top of the insulating layers and the operation of the transducers in loaded contacts is investigated. Results of this will be reported in another paper. REFERENCES 1 A. N. Grubin and J. E. Vinogradova, Investigation of the contact machine components, Rep. 30, 1949 (Central Scientific Research Institute of Technology, Mechanics and Engineering, Moscow). 2 D. Dowson and G. R. Higginson, Elasrohydrodynamic Lubrication, Pergamon, Oxford, 1977. 3 H. S. Cheng and B. Sternlicht, J. Basic Eng., 87 (1965) 695. 4 P. Oster, Beanspruchung der Zahnflanken unter Bedingungen der Elastohydrodynamik, Thesis, Technical University of Munich, Munich, 1982. 5 J. W. Kannel and T. A. Dow, Trans. ASLE, 23 (1980) 262. 6 H. Peeken and A. Kiihler, Konstruktion, 33 (1981) 175. 7 L. J. Maissel, in L. J. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGrawHill, New York, 1970, Chap. 4. 8 J. L. Vossen, J. Vat. Sci. Technol., 8 (1971) 12. 9 D. M. Mattox, J. Vat. Sci. Technol., 10 (1973) 47. 10 D. G. Teer, J. Adhes., 8(1977) 289. 11 J. M. E. Harper, in J. L. Vossen (ed.), Thin Film Processes, Academic Press, London, 1978, p. 175. 12 Chr. Weissmantel, J. Vat. Sci. Technol., 18 (1981) 179. 13 H. R. Kaufm&, Adv. Electron. Electron Phys., 36 (1974) 265. 14 P. D. Reader and H. R. Kaufmaki, J. Vat. Sci. Technol., 12 (1975) 1344. 15 J. A. Thornton, in J. L. Vossen (ed.), Thin Film Processes, Academic Press, London, 1978, p. 106.
AND
PROTECTIVE
19
COATINGS
ION BEAM SPUTTER DEPOSITION OF THIN INSULATING FOR APPLICATIONS IN HIGHLY LOADED CONTACTS M. E. KBNIGER
LAYERS
AND G. REITHMEIER
Messerschmitt-Biilkow-Blohm
G.m.b.H.,
Postfach 80 II 49,8012 Ottobrunn
(F.R.G.)
M. SIMON Forschungsstelle (Received
f& Zahnriider und Getriebebau.
March
16,1983;
accepted
Technische Universik2 Miinchen, Munich (F. R.G.)
June 10,1983)
In highly loaded lubricated rolling and sliding contacts which occur in different kinds of machines and machine elements the experimental determination of the temperature or pressure distribution is desirable. Measurements can be accomplished with thin film transducers which must be electrically insulated from the test specimens by a thin insulating layer. This layer has to withstand wear under extremely severe conditions. Ion beam sputter deposition was used to deposit insulating layers of Al,O, between 0.6 and 4 urn thick onto steel discs. The durability of the coated discs was tested in a twin-disc machine under various working conditions. The insulating layers showed good durability under conditions of high hertzian pressure, high shear stress or high temperature stress caused by high slip values. Owing to their excellent mechanical properties the insulating layers are qualified for use as base layers for thin film transducers.
1.
INTRODUCTION
Highly loaded lubricated rolling and sliding contacts occur in many different kinds of machines and machine elements. Typical examples are roller bearings, gears, cams and transmission elements in traction drives. These contraform contacts are subjected to high loadsand, as a result, undergo high local elastic deformations. In spite of the locally high contact pressures, the two surfaces can be completely separated by a lubricating film in adequate speed ratio ranges. The hertzian pressure in the contacts can be as high as 3 GPa with a lubricating film thickness between 0 and 5 urn. For sliding conditions, additional shear stresses of up to 0.3 GPa and flash temperatures up to 400 “C can occur. In the evaluation of machine elements which are supposed to work properly under these conditions, the exact knowledge of the stress conditions is necessary. This problem is dealt with in many reports (see for example refs. l-4) on the theory of elastohydrodynamics. In addition, a great deal of experimental work has been done in the field to help to clarify the stress conditions. However, the high load, the small dimensions of the contact areas (contact widths less than 1 mm) and the short contact times (as short as 20 ps) make it very difficult. 0040~6090/83/%3.00
0 Elsevier Sequoia/Ptinted
in The Netherlands
20
M. E. KijNIGER,
G. REITHMEIER,
M. SIMON
The experimental determination of the temperature or pressure distribution in elastohydrodynamic contacts can be carried out using thin film transducers. This is accomplished by measuring the change in electrical resistance of the contacts (under load). For example, the resistivity of manganin changes as a function of pressure and that of titanium as a function of temperature. In refs. 5 and 6, measurements using thin film transducers are reported. The thin film transducers must be electrically insulated from the test specimens by a thin insulating layer. This layer must be very thin to prevent the signal from being influenced by the differences in the heat conduction and Young’s modulus etc. between the test specimen and the insulating material. Furthermore, this layer has to withstand wear in rolling and sliding contacts under extremely severe conditions. In the present paper the manufacture of these insulating layers is dealt with and practical tests of their durability are given. 2.
ION BEAM SPUTTER
DEPOSITION
2. I. General remarks Physical vapour deposition techniques are widely used for the deposition of various kinds of thin films. Amongst the number of different deposition techniques, sputtering7*8 and ion plating’, lo are particularly suitable for the deposition of dense and adherent thin films. Commonly practised d.c. or r.f. sputtering is carried out in a glow discharge environment. The glow discharge is an inexpensive large-area source for producing high ion densities at the target. In the present investigation another sputter technique, called ion beam sputtering rlsl’ has been found to be appropriate for the following reasons. (1) Sputteiing can be performed at comparatively low gas pressures, typically at (l-5) x lop2 Pa. Therefore pure films containing few gas inclusions can be obtained. (2) An exact and independent control of the ion beam energy and the ion flux to the target is possible. (3) The ion beam is produced in a separate vacuum chamber and extracted to the deposition chamber. By this means, only a slight heating of the substrates takes place. (4) Substrates of various shapes, e.g. planar or cylindrical, can be coated in front of the target, whereas in glow discharge sputtering the shape of the substrate influences the glow discharge and only more or less planar substrates can be coated. 2.2. Zon beam sputtering apparatus In Fig. 1 a schematic drawing of the ion beam sputter system used for the experiments is shown. An ion beam is generated in a separate chamber of the system. A low pressure gas discharge of the Kaufman type13v14 is used as the ion source (Veeco Instruments Inc.). It incorporates the following features: a hot-filament electron-emitting cathode, a molybdenum cylinder as the anode and a solenoidal electromagnet outside the chamber. Argon gas is fed into the chamber through an automatically controlled valve. An argon ion beam approximately 50mm in diameter is extracted and accelerated through a grid assembly towards the coating chamber. The grids also produce a pressure difference between the two chambers, i.e.
DEPOSITION
OF INSULATING
LAYERS
FOR LOADED
CONTACTS
21
a higher pressure in the ion beam chamber to maintain a stable gas discharge and a lower pressure in the coating chamber, and thus provide a sufficiently large mean free path for the ion beam. A neutralizing filament is located directly beneath the grid system. Electrons emitted from this filament neutralize the positive space charge of the ion beam and also eliminate surface charge when insulating materials are sputtered. A shutter shields the sputter target from the ion beam during the initial beam adjustment and is also used to measure the ion beam intensity (in milliamperes per square centimetre). If the shutter is in an open position, the ion beam is accelerated directly towards the sputter target assembly, situated roughly at the centre of the coating chamber. This assembly consists of three different sputter targets, each water cooled. One target at a time is in a sputtering position. Each sputter target has a circular shape with a diameter of 125 mm. The ion beam is incident on a target at an angle of approximately 45”.
aon beam
chamber
to cryopump
Fig. 1. Schematic
drawing of the ion beam sputtering
apparatus.
The substrates (steel discs), which are coated on their outer rims only, are positioned in front of the target so as to rotate about an axis perpendicular to the plane of the diagram. The substrates can be swung closer to the target if an additional ion bombardment during sputter deposition is desired. Immediately before film deposition the substrates are swung into the ion beam and precleaned by ion bombardment. The chamber has a diameter of approximately 400 mm and can be evacuated with a 400 1s-l cryopump to a pressure of 1O-4 Pa. 3:
EXPERIMENTAL
PROCEDURES
3.1. Coating of substrates Mainly insulating layers of Al,O, were deposited onto steel discs (16 MnCr 5) (diameter, 80 mm; width, 20 mm). The discs were case hardened and their surfaces were ground and polished to a surface roughness of about 0.07 urn centre-line average (c.1.a.). The discs were degreased and cleaned in different ultrasonic baths. Immediately before deposition a further cleaning by backsputtering ofmaterial from
22
M. E. KijNIGER,
G. REITHMEIER,
M. SIMON
the disc surfaces was performed (beam energy, 1OOOeV; beam current density, 1 mA cm-‘). Most of the Al,O, layers were deposited with an ion beam energy of 1000 eV at current densities of 1 mA cm-’ . (In a few cases a beam energy of 500 eV was used.) Layers of thicknesses between 0.6 and 4 urn were produced. The average growth rate on non-rotating substrates was approximately 100 A mini. During deposition the geometrical arrangement of substrate, target and ion beam shown in Fig. 2 was chosen. In case a the substrate is non-rotating. Only a small part of the substrate surface opposite the target is coated. In case b the substrate and target are in the same position as in case a but the substrate rotates during deposition. A uniform layer is deposited onto the whole rolling surface. In case c the substrate rotates during deposition. The substrate centre is positioned closer to the target. Thus the substrate is bombarded continuously by the argon beam during deposition.
III
Ar beam
””Y
boating (4
Fig. 2. The different geometrical
(‘4
positions
(4
of the target and the substrate
during sputter deposition.
a
Fig. 3. Cross section of the twin-disc
machine
for the load tests with coated discs.
3.2. Load tests with coated discs The durability of the coated discs was examined in a twin-disc machine under various working conditions. The twin-disc machine is shown in Fig. 3. On a test rig the coated cylindrical disc runs against a crowned disc (radius I of curvature, 40mm). It is possible to choose four different tracks for contact on the wider disc and thus by axial motion of the smaller disc four different test conditions can be applied to a single test disc. The discs are driven independently of each other, so all possible slide-to-roll ratios can be realized. The circumferential velocities and the slip between discs are measured
DEPOSITION
OF INSULATING
LAYERS
FOR LOADED
23
CONTACTS
electronically. Lubricating oil is usually injected into the contact zone and can be adjusted for temperature. The discs are loaded after they have reached their full speed ratios. The lower disc is mounted in a sled which is connected by spring suspension, allowing the direct measurement of the traction force by means of a load cell. A voltage is applied to the test discs to monitor that they are electrically insulated from each other. A voltage breakdown during the test indicates the destruction of the sputtered insulating layer. The conditions for the load tests are listed in Table I. TABLE
I
CONDITIONS
FOR THE LOAD
Test conditions Circumferential w
{(v, - v,)h)
TESTS
velocily v1 (m s-l) x loo (%)
Traction coe$icient p = FF/FN Hertzian pressure (GPa) Mean shear stress (GPa) Specific tractionpower ( x l+ W me’)
A
B
C
4
12.5
0.42
2
40
0.105
0.024
1.9
1.9
2 0.18-0.45 1.9
0.132
0.032
0.5 (max)
10.5
156
6.5 (max)
For condition A a high shear stress is transmitted in the contact by the use of a traction lubricant with a high coefficient of traction. According to the low slip values the temperature stress of the layer is not significant. For condition B the lubricant is a mineral oil (JSO VG 100). The shear stress that has to be transmitted is very small. With significant slip, considerable traction power Pt = &o,s can be transmitted in the contact, which leads to a high temperature stress in the contact. This is a typical stress condition as it occurs in gears with admittedly smaller pressures. The oil injection temperature for conditions A and B was 60 “C. Because of the good results for conditions A and B, test condition C was performed without lubrication. The discs were cleaned thoroughly with a solvent before the test was started. The circumferential velocity and slip were kept low to prevent scoring. With the formation of reaction layers according to the duration of the test, the coefficient of friction p = FF/FN increased to a level of 0.45. The tests were considered successful when there was no damage after 300 s. 4.
RESULTS
AND DISCUSSION
The Al,O, layers deposited under the conditions described in Section 3.1 typically exhibited an extremely smooth surface topography; a typical example is shown in Fig. 4. In the same figure the morphology of a fractured layer 3 urn thick can be seen as well. It shows a dense and smooth structure characteristic of zone T growth in Thornton’s growth modells. For any of the load tests A, B and C of Table I, the minimum required load to destroy the layers on the discs was already as high as a normal force of 2000 N, or equivalently 1.9 GPa hertzian pressure. A typical damage track as a result of test condition C is shown in Fig. 5. Basically the discs, which were coated on their entire outer rims (Section 3.1,
24
M. E. KijNIGER,
Fig. 4. Scanning
electron
micrograph
of a typical AI,O,
Fig. 5. Scanning
electron
micrograph
of a damage
G. REITHMEIER,
M. SIMON
layer 3 pm thick.
track as a result of test condition
C.
cases b and c), exhibited improved layer durability compared with the partially coated discs (Section 3.1, case a) under all the applied test conditions. The partially coated discs were mainly damaged in a transition zone between the coated and noncoated area. This can be attributed to the fact that the coating particles which are deposited in this area are incident extremely obliquely or have lost kinetic energy due to scattering processes in the gas. The ion bombardment during layer deposition in case c certainly has a beneficial effect on the layer adhesion but no large improvement in comparison with case b was discovered. More detailed investi. gations would be necessary to distinguish between cases b and c. All the layers passed test condition A without damage. Under test conditions with a high slip (condition B) all layers with a thickness greater than 1.5 urn were destroyed (with one exception). This shows that under working conditions entailing high temperature stress, the thinner layers are more resistant than the thicker layers.
DEPOSITION OF INSULATING LAYERS FOR LOADED CONTACTS
25
Referring to the above exception, it was observed that one disc with a layer 4 urn thick did not fail under condition B. A possible explanation for this could be that the surface of this particular disc had a lower roughness value (about 0.04 c.1.a.)than the other discs. Thus the smooth surface has a positive influence on the results. Under test condition C the inferior adhesion characteristics of the partially coated discs were demonstrated. The transmitted shear stress was between 0.15 and 0.2 GPa. The layers of completely coated discs were not destroyed even at a mean shear stress of about 0.5 GPa in the contact. No influence of the layer thickness (OS-4 urn) on adhesion was found under test condition C. In summary it was shown by the above investigations that ion beam sputtering is a valuable technique for the deposition of insulating layers which show good durability in highly loaded lubricated sliding contacts. At present, thin film transducers are deposited on top of the insulating layers and the operation of the transducers in loaded contacts is investigated. Results of this will be reported in another paper. REFERENCES 1 A. N. Grubin and J. E. Vinogradova, Investigation of the contact machine components, Rep. 30, 1949 (Central Scientific Research Institute of Technology, Mechanics and Engineering, Moscow). 2 D. Dowson and G. R. Higginson, Elasrohydrodynamic Lubrication, Pergamon, Oxford, 1977. 3 H. S. Cheng and B. Sternlicht, J. Basic Eng., 87 (1965) 695. 4 P. Oster, Beanspruchung der Zahnflanken unter Bedingungen der Elastohydrodynamik, Thesis, Technical University of Munich, Munich, 1982. 5 J. W. Kannel and T. A. Dow, Trans. ASLE, 23 (1980) 262. 6 H. Peeken and A. Kiihler, Konstruktion, 33 (1981) 175. 7 L. J. Maissel, in L. J. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGrawHill, New York, 1970, Chap. 4. 8 J. L. Vossen, J. Vat. Sci. Technol., 8 (1971) 12. 9 D. M. Mattox, J. Vat. Sci. Technol., 10 (1973) 47. 10 D. G. Teer, J. Adhes., 8(1977) 289. 11 J. M. E. Harper, in J. L. Vossen (ed.), Thin Film Processes, Academic Press, London, 1978, p. 175. 12 Chr. Weissmantel, J. Vat. Sci. Technol., 18 (1981) 179. 13 H. R. Kaufm&, Adv. Electron. Electron Phys., 36 (1974) 265. 14 P. D. Reader and H. R. Kaufmaki, J. Vat. Sci. Technol., 12 (1975) 1344. 15 J. A. Thornton, in J. L. Vossen (ed.), Thin Film Processes, Academic Press, London, 1978, p. 106.