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CVD diamond coated cemented carbide cutting tools
MATERIAlS SCIEMCE& E_IMEERIMG ELSEVIER
Materials Science an-I Engineering A209 (1996) 405-413
A
CVD diamond coatee cemented carbide cutting tools J. Karner a , M. Pedrazzini'\ I Reineck b , M.E. Sjostrand b , E. BergmannC "Sal:ers Lt, .. FL-9496 Sal:ers. LiechrellSrein hAS Sandrik c.-romanl. S-126 80 Srockholm. S1tHlen "Ecole d'lng,'nieur de Genh·e. I, Roure de la Prairie. CH-/202 Gel1l?t:e, S1t'ir:erland
Abstract The development of CVD diamond coated cuttin~ tools has faced several challenges since the commencement of the low pressure synthesis of diamond coatings from a vapor IT ixture of hydrogen and a carbon containing gas based on the original work by Derjaguin and Fedoseev. The reason for the fairly slow progress can be attriluted to a number of problems. For example, obtaining a satisfactory and consistent adhesion between the diamond coating an( the substrate has proven to be difficult. Further, the problems faced in expanding any of the existing CVD diamond laborat( ry processes into a large scale production technique have also turned out to be greater than exprected. The recent development of a scaled-up process wit) a high current density plasma beam will be described. This new CVD diamond process, the high current DC-arc process (I-i COCA), which is based on a high current DC discharge arc with a long discharge column, has been successful and high qualit) diamond coated cemented carbide inserts can be manufactured with good reproducibility and large productivity. Diamond coated tools are compared with both PC[ tools and uncoated carbide tools. The main advantage compared to PCD tools are the possibilities of using multiple edges per tc JI insert and advanced chip breaker technology. Compared to an uncoated carbide tool, the CVD diamond coated insert shows, iJ I addition to a much larger abrasive wear resistance, less built-up edge and a lower cutting forces resulting in a much improved ~ urface finish to the workpiece material. Several results from field tests at end user machint shops in the turning of AI-Si alloys, particularly in wheel turning, are presented. In the tests diamond coated carbide inserts perform typically 3-5 times better than uncoated tools but peak values of 10-20 times improvement can be obtained. An exam{:le of machining of Cu is reported where the diamond coated tool is found to outperform a PCD tool. Key1twds: CVD diamond coating; Cutting tools; HCDCA
1. Introduction The development of CVD diamond thin filr1s has faced several challenges since the commencement of the low pressure synthesis of diamond coatings f 'om a vapor mixture of hydrogen and a carbon containi ng gas based on the original work by Derjaguin and Fe joseev [1]. By now several years have passed since lage research efforts were initiated in Europe and the USA and great expectations were raised on low p'essure diamond coated cutting tools which were predi( ted to become one of the first large-scale commercial a 1plications for CVD diamond [2,3]. Several laboratory techniques have also been developed for growing diamond coatings of good quality on a wide range of substrates notably cemented c, rbides 0921-5093/96/$15.00 © 1996
SSDl 0921-5093(95)10140-3
Elsevier Science S.A. All rig lts reserved
and silicon nitrides. These different deposition methods include, for example hot filament CVD, microwave-, RF and DC plasmas, DC and RF arc jets and combustion torches [2.4]. The great interest for diamond as a tool material is because of its unique hardness which is twice that of cubic boron nitride (cBN) and about four times harder than TiC. In addition, diamond also exhibits the highest thermal conductivity known for any solid, which for a diamond tool means that the heat generated in a cutting operation will be efficiently dissipated. Diamond tools, CVD- or PCD (polycrystalline diamond)-based, can successfully be used in the machining of alloys such as aluminium, copper, magnesium and also of composite materials such as fibre reinforced polymers in printed circuit boards (PCB) for example.
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1. Karner et al. / Materials Science and Engineering A209 (1996) 405-413
Diamond tools can also be applied successfully in the machining of wood, semi-sintered ceramics, hard rubbers, graphite, etc. One serious limitation on the application of diamond tools in metal machining is the high chemical reactivity (solubility) of carbon to ferrous materials at elevated temperatures. Also the thermal degradation of diamond starts at temperatures around 700-800°C. Hence, steels and most cast irons are excluded from the workpiece materials that can be machined and, consequently, in the forseeable future the majority of all metal machining operations will be performed with coated tools where the most common coatings are TiC, TiCN, TiN and AI 2 0 3 or combinations thereof. Using diamond tools in the machining of cast iron might be possible when using very low cutting speeds but that would probably result in a mediocre productivity which counteracts the benefit of a long tool life [5]. Polycrystalline diamond (PCD) has been used for cutting tools in the manufacturing industry for many years. The PCD "chip" is being made by sintering diamond grit together with a binder of a metallic or ceramic material, at a very high pressure and temperature and, subsequently, brazing the diamond chip (thickness :::::: 500-1000 ,Um) onto a cemented carbide substrate usually limited to one cutting edge per tool insert. Hence, the possibilities of using multiple edges per tool (indexable inserts) and advanced chipbreaker technology for the insert design are the main advantages of CVD diamond coated tools compared to PCD tools. Further, the diamond coated shank type tools should be easier to manufacture than PCD brazed shank type tools [6]. It is not a priori obvious that a CVD diamond coated tool should always compete with the established PCD tools with respect to performance and cost. Compared to an uncoated cemented carbide tool, a CVD diamond coated insert shows [5], in addition to a much larger abrasive wear resistance which sometimes results in up to ten times longer tool life for reasonable coating thicknesses, also less tendency for build-up edge formation and lower cutting forces which yields a better surface finish on the workpiece material. No major tool manufacturers introduced CVD diamond coated tools on to the market until 1994. The reason for the fairly slow progress in establishing reliable techniques for the deposition of a thin film of diamond on suitable substrate materials for tool applications can be attributed to a number of problems. For example, obtaining a satisfactory and consistent adhesion between the diamond coating and the substrate, notably cemented carbides, has proven to be difficult though great progress has been made on different methods of mechanical and chemical substrate surface pretreatments. Another challenge has been the design of appropriate laboratory machining methods for dia-
mond coated tools, methods which can demonstrate pure failure mechanisms such as flaking or abrasive wear of the coating [5]. Such methods should simultaneously correlate well with field tests being carried out at end user machine shops. The difficulties in expanding any of the existing CVD diamond laboratory processes into a large scale production technique have also turned out to be greater than expected. However, the development of a scaled-up process based on a high current density plasma beam has been successful and high quality diamond coated cemented carbide inserts can be manufactured with good reproducibility and large productivity.
2. Deposition technique A number of different coating techniques for depositing thin and thick films of diamond exist. The methods are distinguished from each other essentially by the choice of energy source applied to dissociate the reaction gas and, in particular, converting the hydrogen molecules into atomic hydrogen [7]: H 2 ....... H
+ H( -
104.2 kcal mol- ')
Some of the deposition methods, notably the DC and RF arc jets and the combustion flame, are capable of deposition rates ranging from 10 to I000 ~m, h - , [8 -II] thus enabling the production of thick, and if so desired free-standing, films which can be mechanically processed and brazed onto a cemented carbide substrate resulting in a tool very similar to the PCD tool. However, the work presented in this article will deal with the deposition of thin films of diamond (6--15 ,Um) directly onto indexable inserts of cemented carbides with a new CVD deposition process based on a high current arc discharge with a long discharge column. 2.1. The high current DC-arc (HCDCA) process
The high current DC-arc (HCDCA) process is a new type of CVD diamond deposition method. The reactor lay-out principle is seen in Fig. I. The process is based on a high current arc discharge with a long discharge column. The plasma density in the discharge column is extremely high, so that the substrates can be positioned at a relatively low plasma density at a large distance from the intense discharge column. Therefore fluctuations in the discharge column have a relatively small effect on the plasma conditions at the substrate positions. It is possible in this way to generate a large surface area with uniform conditions for the deposition of high quality diamond films. The substrates are exposed to a high flow of atomic hydrogen, produced in the discharge column and heated mainly by hydrogen recombination on the sur-
1. Karner et ai.
the substrate using traditional arc jet methods is considerable, generating the necessity for cooling of the substrates. In comparison to the DC-arc discharges described by Buck et al. [12,13] or in [14], the HCDCA-process works with a much longer discharge column and much higher discharge currents. Instead of some 10 mm discharge length, plasma column lengths of 700 mm are realized. The new process is a further development of
CATHODE
SUBSTRATES DISCHARGE COLUMN
+
407
Materiais Science and Enl{ineerinl{ A209 (i996) 405-413
[15]. To achieve this for large scale production, uniformity and stability of the discharge column is required, also the pressure is reduced in comparison to Refs. [12-14]. This means that the HCDCA plasma is a non-equilibrium plasma. where the mechanism of hydrogen dissociation is dominated by joint vibroelectronic excitation [16]. The HCDCA process gives a very uniform coating quality on a large number of indexable inserts with arbitrary shapes, The coating uniformity is illustrated in Fig. 2 where the diamond coating morphology at different reactor positions is shown. The diamond coating quality is further discussed below. Figs. 3 and 4 show the substrate temperatures and the growth rate respectively at different positions including the extreme positions at the top and bottom of the reactor. The minor variations in temperature and growth rate further verify the uniformity of the process conditions.
Fig. 1. Principle of reactor lay-out.
face. The atomic hydrogen flow is adjusted so the t the rate of substrate heating is compensted by an :qual cooling rate by radiation. Hence, no additional cc oling is necessary, which saves difficult design work and expenses since this is difficult to accomplish at a temperature of ~ 800°C. This new method of diamond deposition cc n be distinguished from the more established DC-arc nethods by the fact that the substrates are located i 1 the diffusion region of the plasma whereas in conven ional arc jet synthesis methods a plasma beam orgir ating from stacked electrodes is directed onto the subs rates with high velocity [8-11]. The heat load transfen ed to
k:" .~ ~ ...
'
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••••. ,
"," • •
.oL<', 'P9" ,
'I.
4
.,.
-~,' -~
:C'.... -;;
-
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.. .....
--~
.~-
'
z
.,.CoL. ..... .
,
.A
mnl
'"
...
,
._--_.~._~
Fig. 2. Coating morphol. gy as a function of different reactor positions.
408
J. Karner et at. / Materials Science and Engineering A209 (1996) 405-413
Process temperature distribution 880 - I I
860
i
840 +i
U 820 0
CD
800
~
780
~
..
Co
E 760
!
740 720 700 top
bottom
centre
reactor position
Fig. 3. Substrate temperatures as a function of different reactor positions.
coating thickness distribution 8
---------------
3. Machining tests
7
E 3III III CD
nanometer sized diamond to adhere to the substrate and these fragments may act as growth sites. Such fragments have been seen in, for example, high resolution scanning electron microscope (HRSEM) studies [17]. It is also likely that the substrate surface is roughened on a nanometer scale which enhances the formation of diamond nuclei. The conditions used for the diamond deposition process are shown in Table I. The diamond coatings resulting from the deposition show good diamond quality as analysed by Raman spectroscopy and scanning electron microscopy (SEM). A typical Raman spectrum of the produced films is shown in Fig. 5 where the characteristic diamond peak at 1332 cm - I indicates a good diamond quality. Fig. 6(a) and (b) show SEM micrographs of diamond coated cutting tool edges from a cross section view (Fig. 6(a)) and from a top view (Fig. 6(b)). The cross section picture shows a diamond film with a thickness of about 15 ,um.
6
3.1. Laboratory machining tests
5
r::
.... (J
4
Cl
3
:s r::
..
';l
0
2
(J
0 bottom
centre
top
reactor position
Fig. 4. Substrate weight increases as a function of different reactor positions.
2.2. Diamond coating procedure and coating characteristics
Prior to the deposition process, the carbide tool substrates are subjected to a standard seeding treatment in order to enhance the nucleation rate during the initial deposition phase. The inserts are treated in an ultrasonic bath consisting of diamond grit powder (size I ,um) suspended in oil. After the seeding the tools are ultrasonically cleaned in alcohol and then quickly dried. The seeding most likely causes very small fragments of Table I Process conditions for diamond deposition Process gases
Ratio CH 4 /H 2
Substrate temperature
Growth rate
The development of laboratory machining methods has been directed towards finding test operations where either pure abrasive wear or pure flaking of the diamond coating limits the edge life. A selective flaking test is the most difficult to design but also the most important since it is used to assess the adhesion preparation technique being applied to the substrate surface. A series of different laboratory machining tests have been tried such as longitudinal turning in A390, dry and wet, at different cutting speeds. Further, an accelerated test in the form of a facing operation in a slotted workpiece in A390 material was also developed in order to shorten the tool life of the diamond coated insert [5]. However, none of the above machining tests fulfilled the requirements of being a selective flaking test during which the coating flaked off within a reasonable time scale. Finally a milling operation in an age-hardened AI4.5%Cu-0.8%Si alloy was developed from the experience gained from a field test in this material. This field test was a facing operation (turning) which included both roughing and fine machining of shell casings. Good correlations in the flaking behaviour of the diamond coating was found between the laboratory milling test and the facing operation. The milling operation, which is schematically illustrated in Fig. 7, is a pure flaking test where the intermittent conditions accelerate fatiguing of the coating or the coating-substrate bond. The feed rate is increased in intervals of 0.05 mm rev 1 starting with 10 passes over
1. Karner el al.
1550
1500
1550
Marerial.\ Science and Enj;ineerinj; A209 (/996) 405 4/3
1500
\j
50
1400
1350
1300
1250
409
Wavenumber
Fig. 6. SEM micrographs of: (a) the cross section of a diamon j coated cutting tool and: (b) a diamond coatcd cutting tool edge seen from a top view.
the workpiece at 0.10 mm rev - I and then 20 [asses at each 0.15, 0.20, ... etc up to 0.50 mm rev - I The test can be further accelerated by increasing the steps between feed rates. The cutter is equipped with only one, single i lsert. The life of a diamond coated cemented carbidf cutting edge in this test is about I h for a su :cessful diamond deposition including a good adr esion technique. Fig. 8 shows an SEM micrograph of a diamond coated cutting edge of insert style SPK N l203-EDR which has failed by excessive flaking
3.2. Field tests of cutting tools
Field test at end user machine shops have been performed concentrating mainly on turning operations. In this first step of developing a diamond coated product for machining applications, turning has proven to be more suitable for optimization than milling because of a more complex set-up of cutting parameters in the milling operation. A selection of tests are listed in Table 2, which are examples of well working operations when using a diamond coated cutting tool. Comparison is mainly made with uncoated cemented carbide as a
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J. Karner et al. / Materials Science and Engineering A209 (1996) 405-413
TURNING TEST no 1
Material: Operation: Coolant: Component:
AI-7-9%Si -T6 Roughing None Truck wheel
Cutting data: Cutting speed (m/min): Depth of cut (mm): Feed (mm/rev):
Fig. 7. Schematics of a laboratory machining test, milling of a slotted workpiece of age-hardened AI-4.5%Cu-0.8%Si, wet cutting; v = 1500 m min - 1, a = 1.0 mm, S = 0.10-0.50 mm rev. - 1
#1 1850 - 2500 0.5 - 3.0 0.25 - 0.8
#2
Insert: VCGX 160412-AL
VCGX-AL
RIO
6~m diamond coated CC
Pieces per edge:
7
140
Wear type:
even abrasion
Grade:
Results:
flaking
Fig. 9. Parameters and results for turning field test no. I, continuous turning of the wheel bed of truck wheels in age hardened Al-7%-9'Yr, Si.
-----------""""""""""""""",
-""-"""""-",,,,
Fig. 8. SEM micrograph of a diamond coated cutting edge applied in the machining test described in Fig. 7. The edge has failed by severe flaking of the diamond coating.
reference tool material, but the thin film diamond is also found to be competitive in comparison with peD as shown in one of the cases below. The details of the operations are given in the figures to which reference is Table 2 Summary of field tests Test no.
2 4
3
Description
Continuous wheel bed turning, AI-7'YoSi Intermittent facing of wheel front, AI-7%Si Longitudinal, intermittent turning of electrical motor armature, Cu Continuous wheel bed turning, AI-II %Si
Parameters in Fig.
Wear pattern in Fig.
9
10
II
12 a,b
15
14
13
Fig. 10. Diamond coated cutting tool after use in turning field no. I (scanning electron micrograph). The failure mechanism flaking.
made in Table 2. The table also gives the reference to the figures of the wear patterns for the used inserts. The results seen in Figs. 11 and 13 with an improvement in the cutting tool lives for the turning of Al
1. Karner et at.
/\ilaterial.\ Science and Engineerillg A209 (/996) 405 4/3
TURNING TEST no 2
Material: Operation: Coolant: Component:
r~
AI-7%Si-T6 Roughing Emulsion Wheel
Cutting data: Cutting speed (m/min) RPM: Depth of cut (mm): Feed (mm/rev):
~l_
#1 <2150 1800 0.8 - 1-5 0.5
#2
HIO
61lm diamond coated CC
Insert:
411
The major failure mechanism of the diamond coated tools is flaking of the diamond coating as can be seen in Fig. 10 and Fig. 14. However, cases are found where the extension of the flaking is very small as in Fig. 12(a). In this case the failure mode is rather flank wear, following the same pattern as is the case for the reference tool. the uncoated carbide, insert shown in Fig. 12(b). No straight forward correlation can be found between the results in terms of tool lives or wear patterns, such as the extension of the area of flaking, and the specific machining parameters, such as continuous or intermittent cutting or the cutting speed, or the choices of workpiece material. It is more likely that the stability of the specific operation plays a dominant role since the failure mechanism in terms of flaking is sensitive to the stability conditions.
NI51.4-800-60-AL
Grade:
Results: Pieces per edge:
600
2250
Wear type:
even abrasion
even
Fig. II. Parameters and results for turning field test no. 2, intermittent facing of the wheel front of car wheels in age hardened AI-7";:,si.
alloys by a factor of 4 may be regarded as typical but as in the case of Fig. 9 (test no. I), the improvement factor may be as large as 20 times.
4. Conclusions A diamond coating process well suited for large scale coating of cutting tool inserts has been presented. The process is based on a high current density arc plasma CVD method which gives a high degree of uniformity of conditions such as deposition rate and diamond quality over large areas. Cemented carbide cutting tools can be coated with diamond of good quality by using the above described method. The coated tools have been found to perform well in laboratory tests as well as in field tests at end users, in operations covering the machining of AI-Si alloys and copper. The adhesion of the diamond coatings is well assessed in the specially designed laboratory milling test.
Fig. 12. (a) Diamond coated cutting tool after use in turning field test no. 2 (scanning electron micrograph). The failure mechanism is even, abrasive wear accompanied with a slight flaking. (b) Uncoated cemented carbide cutting tool after use in field test no. 2 (scanning electron micrograph). Failure mechanism: even, abrasive wear.
412
J. Karner et al. / Materials Science and Engineering A209 (1996) 405-413
TURNING TEST no 3 TURNING TEST no 4 Material: Operation: Coolant: Component:
AI-ll%Si Rough +finish Emulsion Wheel
Cutting data: Cutting speed (m/min): RPM: Depth of cut (mm): Feed (mm/rev):
#1 2100 - 2600 2000 0.5 - 1.0 0.35 - 0.65
#2
Cutting data: Cutting speed (m/min): Depth of cut (mm): Feed (mm/rev):
Insert:
#2
PCD
6/-tm diamond coated CC
2
4
Insert:
RCGX 0803MO-AL Grade:
#1 300 1.0 0.15
CCGX 120408-AL K15
6ltm diamond coated CC
Results:
Grade:
Results:
Pieces per edge:
200
800
Wear type:
even abrasion
flaking
Fig. 13. Parameters and results for turning field test no. 3, continuous turning of the wheel bed of car wheels in AI-II %Si.
Pieces per edge: Wear type: Comment:
even burr formation
no burr
Fig. 15. Parameters and results for turning field test no. 4, longitudinal, intermittent turning of electrical motor armature in Cu.
Research. Dr. S. Lindholm and Dr. C. Broman at the Institute of Geology, University of Stockholm are acknowledged for carrying out the Raman analyses.
References
Fig. 14. Diamond coated cutting tool after u~e in turning field test no. 3 (scanning electron micrograph). The failure mechanism is flaking.
Acknowledgements
The authors wish to acknowledge Dr. H. Kaufmann, Dr. A.E. Thelin and Dr. S. Soderberg for their interest and support in this work. The work was also partly financed by the Swiss Priority Program of Materials
[I] B.V. Derjaguin and D.V. Fedoseev, Growth of Diamond and Graphite from the Gas Phase, Nauka, Moscow (1977) in Russian; English translation, Elsevier, (1989). [2] P.K. Bachmann and R. Messier, Chem. Eng. News, 15 May (1989) 24. [3] B. Lux and R. Haubner, Proc. 12th Plansee Seminar, 615, Metallwerk Plansee, Reutte, Austria (1989). [4] K.E. Spear, J. Am. Ceram. Soc., 72 (1989) 71. [5] 1. Reineck, S. Soderberg and M.E. Sjostrand, CVD-diamantbes-
chichtete Zerspanwerkzeuge, VDI-geselischaft Produktionstechnik, "Neuentwicklungen in der Zerspantechnik ", Diisseldorf, September 1993, pp. 55-70. [6] K. Kanda, S. Takehana, S. Yoshida, F. Shikamura and K. Ishigane in M. Yoshikawa, M. Murakawa, Y. Tzeng, W.A. Yarbrough (Eds.), 2nd Int. Con! on the Application of Diamond Films and Relat. Mater. MYU, Tokyo, 1993, p. 565. [7] J.e. Angus and e.e. Hayman, Science 241 (1988) 913. [8] K. Kurihara, K. Sasaki, M. Kawarada and N. Koshino, Appl. Phys. Lett., 52(6) (1988) 437.
1. Karner et al.
Materials Science and Engineering A209 (1996) 405-413
[9] N. Ohtake and M . Yoshikawa, J. Electrochem. Soc.. 137 (1990) 717. [10] T.G. Owano, C.H. Kruger and M.A. Capelli, Mar. Res. Soc. Symp. Proc. Vol. 190 (1991) 131. [II] S. Matsumoto, H. Hino and T. Kobayashi, Appl. PhI's. Leu .. 51
(1987) 737. [12] V. Biihm, V. Buck, M. Liesenfeld and T. Naubert, 1. Zeng, Diamond Relar. Mater., 4 (1995) 33.
413
[13] V. Buck, K. Piirschke and M. Pauser, 1. Zeng, Proc. CIMTEC 28 June 4 July, Florence, Italy. [14] US patent 4851254 (1989). [15] US patent 4749587 (1988). [16] M. Capitelli, and E. Molinari, in F.L. Bloschke (Ed.), Topics in Currellt Chemistry, Vol. 90, p. 61. [17] T. Okubo, S. Ikari, K. Kusakabe and S. Morooka, 1. Mat. Sci. Leuer, JJ (1992).
Materials Science an-I Engineering A209 (1996) 405-413
A
CVD diamond coatee cemented carbide cutting tools J. Karner a , M. Pedrazzini'\ I Reineck b , M.E. Sjostrand b , E. BergmannC "Sal:ers Lt, .. FL-9496 Sal:ers. LiechrellSrein hAS Sandrik c.-romanl. S-126 80 Srockholm. S1tHlen "Ecole d'lng,'nieur de Genh·e. I, Roure de la Prairie. CH-/202 Gel1l?t:e, S1t'ir:erland
Abstract The development of CVD diamond coated cuttin~ tools has faced several challenges since the commencement of the low pressure synthesis of diamond coatings from a vapor IT ixture of hydrogen and a carbon containing gas based on the original work by Derjaguin and Fedoseev. The reason for the fairly slow progress can be attriluted to a number of problems. For example, obtaining a satisfactory and consistent adhesion between the diamond coating an( the substrate has proven to be difficult. Further, the problems faced in expanding any of the existing CVD diamond laborat( ry processes into a large scale production technique have also turned out to be greater than exprected. The recent development of a scaled-up process wit) a high current density plasma beam will be described. This new CVD diamond process, the high current DC-arc process (I-i COCA), which is based on a high current DC discharge arc with a long discharge column, has been successful and high qualit) diamond coated cemented carbide inserts can be manufactured with good reproducibility and large productivity. Diamond coated tools are compared with both PC[ tools and uncoated carbide tools. The main advantage compared to PCD tools are the possibilities of using multiple edges per tc JI insert and advanced chip breaker technology. Compared to an uncoated carbide tool, the CVD diamond coated insert shows, iJ I addition to a much larger abrasive wear resistance, less built-up edge and a lower cutting forces resulting in a much improved ~ urface finish to the workpiece material. Several results from field tests at end user machint shops in the turning of AI-Si alloys, particularly in wheel turning, are presented. In the tests diamond coated carbide inserts perform typically 3-5 times better than uncoated tools but peak values of 10-20 times improvement can be obtained. An exam{:le of machining of Cu is reported where the diamond coated tool is found to outperform a PCD tool. Key1twds: CVD diamond coating; Cutting tools; HCDCA
1. Introduction The development of CVD diamond thin filr1s has faced several challenges since the commencement of the low pressure synthesis of diamond coatings f 'om a vapor mixture of hydrogen and a carbon containi ng gas based on the original work by Derjaguin and Fe joseev [1]. By now several years have passed since lage research efforts were initiated in Europe and the USA and great expectations were raised on low p'essure diamond coated cutting tools which were predi( ted to become one of the first large-scale commercial a 1plications for CVD diamond [2,3]. Several laboratory techniques have also been developed for growing diamond coatings of good quality on a wide range of substrates notably cemented c, rbides 0921-5093/96/$15.00 © 1996
SSDl 0921-5093(95)10140-3
Elsevier Science S.A. All rig lts reserved
and silicon nitrides. These different deposition methods include, for example hot filament CVD, microwave-, RF and DC plasmas, DC and RF arc jets and combustion torches [2.4]. The great interest for diamond as a tool material is because of its unique hardness which is twice that of cubic boron nitride (cBN) and about four times harder than TiC. In addition, diamond also exhibits the highest thermal conductivity known for any solid, which for a diamond tool means that the heat generated in a cutting operation will be efficiently dissipated. Diamond tools, CVD- or PCD (polycrystalline diamond)-based, can successfully be used in the machining of alloys such as aluminium, copper, magnesium and also of composite materials such as fibre reinforced polymers in printed circuit boards (PCB) for example.
406
1. Karner et al. / Materials Science and Engineering A209 (1996) 405-413
Diamond tools can also be applied successfully in the machining of wood, semi-sintered ceramics, hard rubbers, graphite, etc. One serious limitation on the application of diamond tools in metal machining is the high chemical reactivity (solubility) of carbon to ferrous materials at elevated temperatures. Also the thermal degradation of diamond starts at temperatures around 700-800°C. Hence, steels and most cast irons are excluded from the workpiece materials that can be machined and, consequently, in the forseeable future the majority of all metal machining operations will be performed with coated tools where the most common coatings are TiC, TiCN, TiN and AI 2 0 3 or combinations thereof. Using diamond tools in the machining of cast iron might be possible when using very low cutting speeds but that would probably result in a mediocre productivity which counteracts the benefit of a long tool life [5]. Polycrystalline diamond (PCD) has been used for cutting tools in the manufacturing industry for many years. The PCD "chip" is being made by sintering diamond grit together with a binder of a metallic or ceramic material, at a very high pressure and temperature and, subsequently, brazing the diamond chip (thickness :::::: 500-1000 ,Um) onto a cemented carbide substrate usually limited to one cutting edge per tool insert. Hence, the possibilities of using multiple edges per tool (indexable inserts) and advanced chipbreaker technology for the insert design are the main advantages of CVD diamond coated tools compared to PCD tools. Further, the diamond coated shank type tools should be easier to manufacture than PCD brazed shank type tools [6]. It is not a priori obvious that a CVD diamond coated tool should always compete with the established PCD tools with respect to performance and cost. Compared to an uncoated cemented carbide tool, a CVD diamond coated insert shows [5], in addition to a much larger abrasive wear resistance which sometimes results in up to ten times longer tool life for reasonable coating thicknesses, also less tendency for build-up edge formation and lower cutting forces which yields a better surface finish on the workpiece material. No major tool manufacturers introduced CVD diamond coated tools on to the market until 1994. The reason for the fairly slow progress in establishing reliable techniques for the deposition of a thin film of diamond on suitable substrate materials for tool applications can be attributed to a number of problems. For example, obtaining a satisfactory and consistent adhesion between the diamond coating and the substrate, notably cemented carbides, has proven to be difficult though great progress has been made on different methods of mechanical and chemical substrate surface pretreatments. Another challenge has been the design of appropriate laboratory machining methods for dia-
mond coated tools, methods which can demonstrate pure failure mechanisms such as flaking or abrasive wear of the coating [5]. Such methods should simultaneously correlate well with field tests being carried out at end user machine shops. The difficulties in expanding any of the existing CVD diamond laboratory processes into a large scale production technique have also turned out to be greater than expected. However, the development of a scaled-up process based on a high current density plasma beam has been successful and high quality diamond coated cemented carbide inserts can be manufactured with good reproducibility and large productivity.
2. Deposition technique A number of different coating techniques for depositing thin and thick films of diamond exist. The methods are distinguished from each other essentially by the choice of energy source applied to dissociate the reaction gas and, in particular, converting the hydrogen molecules into atomic hydrogen [7]: H 2 ....... H
+ H( -
104.2 kcal mol- ')
Some of the deposition methods, notably the DC and RF arc jets and the combustion flame, are capable of deposition rates ranging from 10 to I000 ~m, h - , [8 -II] thus enabling the production of thick, and if so desired free-standing, films which can be mechanically processed and brazed onto a cemented carbide substrate resulting in a tool very similar to the PCD tool. However, the work presented in this article will deal with the deposition of thin films of diamond (6--15 ,Um) directly onto indexable inserts of cemented carbides with a new CVD deposition process based on a high current arc discharge with a long discharge column. 2.1. The high current DC-arc (HCDCA) process
The high current DC-arc (HCDCA) process is a new type of CVD diamond deposition method. The reactor lay-out principle is seen in Fig. I. The process is based on a high current arc discharge with a long discharge column. The plasma density in the discharge column is extremely high, so that the substrates can be positioned at a relatively low plasma density at a large distance from the intense discharge column. Therefore fluctuations in the discharge column have a relatively small effect on the plasma conditions at the substrate positions. It is possible in this way to generate a large surface area with uniform conditions for the deposition of high quality diamond films. The substrates are exposed to a high flow of atomic hydrogen, produced in the discharge column and heated mainly by hydrogen recombination on the sur-
1. Karner et ai.
the substrate using traditional arc jet methods is considerable, generating the necessity for cooling of the substrates. In comparison to the DC-arc discharges described by Buck et al. [12,13] or in [14], the HCDCA-process works with a much longer discharge column and much higher discharge currents. Instead of some 10 mm discharge length, plasma column lengths of 700 mm are realized. The new process is a further development of
CATHODE
SUBSTRATES DISCHARGE COLUMN
+
407
Materiais Science and Enl{ineerinl{ A209 (i996) 405-413
[15]. To achieve this for large scale production, uniformity and stability of the discharge column is required, also the pressure is reduced in comparison to Refs. [12-14]. This means that the HCDCA plasma is a non-equilibrium plasma. where the mechanism of hydrogen dissociation is dominated by joint vibroelectronic excitation [16]. The HCDCA process gives a very uniform coating quality on a large number of indexable inserts with arbitrary shapes, The coating uniformity is illustrated in Fig. 2 where the diamond coating morphology at different reactor positions is shown. The diamond coating quality is further discussed below. Figs. 3 and 4 show the substrate temperatures and the growth rate respectively at different positions including the extreme positions at the top and bottom of the reactor. The minor variations in temperature and growth rate further verify the uniformity of the process conditions.
Fig. 1. Principle of reactor lay-out.
face. The atomic hydrogen flow is adjusted so the t the rate of substrate heating is compensted by an :qual cooling rate by radiation. Hence, no additional cc oling is necessary, which saves difficult design work and expenses since this is difficult to accomplish at a temperature of ~ 800°C. This new method of diamond deposition cc n be distinguished from the more established DC-arc nethods by the fact that the substrates are located i 1 the diffusion region of the plasma whereas in conven ional arc jet synthesis methods a plasma beam orgir ating from stacked electrodes is directed onto the subs rates with high velocity [8-11]. The heat load transfen ed to
k:" .~ ~ ...
'
•
••••. ,
"," • •
.oL<', 'P9" ,
'I.
4
.,.
-~,' -~
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-
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.. .....
--~
.~-
'
z
.,.CoL. ..... .
,
.A
mnl
'"
...
,
._--_.~._~
Fig. 2. Coating morphol. gy as a function of different reactor positions.
408
J. Karner et at. / Materials Science and Engineering A209 (1996) 405-413
Process temperature distribution 880 - I I
860
i
840 +i
U 820 0
CD
800
~
780
~
..
Co
E 760
!
740 720 700 top
bottom
centre
reactor position
Fig. 3. Substrate temperatures as a function of different reactor positions.
coating thickness distribution 8
---------------
3. Machining tests
7
E 3III III CD
nanometer sized diamond to adhere to the substrate and these fragments may act as growth sites. Such fragments have been seen in, for example, high resolution scanning electron microscope (HRSEM) studies [17]. It is also likely that the substrate surface is roughened on a nanometer scale which enhances the formation of diamond nuclei. The conditions used for the diamond deposition process are shown in Table I. The diamond coatings resulting from the deposition show good diamond quality as analysed by Raman spectroscopy and scanning electron microscopy (SEM). A typical Raman spectrum of the produced films is shown in Fig. 5 where the characteristic diamond peak at 1332 cm - I indicates a good diamond quality. Fig. 6(a) and (b) show SEM micrographs of diamond coated cutting tool edges from a cross section view (Fig. 6(a)) and from a top view (Fig. 6(b)). The cross section picture shows a diamond film with a thickness of about 15 ,um.
6
3.1. Laboratory machining tests
5
r::
.... (J
4
Cl
3
:s r::
..
';l
0
2
(J
0 bottom
centre
top
reactor position
Fig. 4. Substrate weight increases as a function of different reactor positions.
2.2. Diamond coating procedure and coating characteristics
Prior to the deposition process, the carbide tool substrates are subjected to a standard seeding treatment in order to enhance the nucleation rate during the initial deposition phase. The inserts are treated in an ultrasonic bath consisting of diamond grit powder (size I ,um) suspended in oil. After the seeding the tools are ultrasonically cleaned in alcohol and then quickly dried. The seeding most likely causes very small fragments of Table I Process conditions for diamond deposition Process gases
Ratio CH 4 /H 2
Substrate temperature
Growth rate
The development of laboratory machining methods has been directed towards finding test operations where either pure abrasive wear or pure flaking of the diamond coating limits the edge life. A selective flaking test is the most difficult to design but also the most important since it is used to assess the adhesion preparation technique being applied to the substrate surface. A series of different laboratory machining tests have been tried such as longitudinal turning in A390, dry and wet, at different cutting speeds. Further, an accelerated test in the form of a facing operation in a slotted workpiece in A390 material was also developed in order to shorten the tool life of the diamond coated insert [5]. However, none of the above machining tests fulfilled the requirements of being a selective flaking test during which the coating flaked off within a reasonable time scale. Finally a milling operation in an age-hardened AI4.5%Cu-0.8%Si alloy was developed from the experience gained from a field test in this material. This field test was a facing operation (turning) which included both roughing and fine machining of shell casings. Good correlations in the flaking behaviour of the diamond coating was found between the laboratory milling test and the facing operation. The milling operation, which is schematically illustrated in Fig. 7, is a pure flaking test where the intermittent conditions accelerate fatiguing of the coating or the coating-substrate bond. The feed rate is increased in intervals of 0.05 mm rev 1 starting with 10 passes over
1. Karner el al.
1550
1500
1550
Marerial.\ Science and Enj;ineerinj; A209 (/996) 405 4/3
1500
\j
50
1400
1350
1300
1250
409
Wavenumber
Fig. 6. SEM micrographs of: (a) the cross section of a diamon j coated cutting tool and: (b) a diamond coatcd cutting tool edge seen from a top view.
the workpiece at 0.10 mm rev - I and then 20 [asses at each 0.15, 0.20, ... etc up to 0.50 mm rev - I The test can be further accelerated by increasing the steps between feed rates. The cutter is equipped with only one, single i lsert. The life of a diamond coated cemented carbidf cutting edge in this test is about I h for a su :cessful diamond deposition including a good adr esion technique. Fig. 8 shows an SEM micrograph of a diamond coated cutting edge of insert style SPK N l203-EDR which has failed by excessive flaking
3.2. Field tests of cutting tools
Field test at end user machine shops have been performed concentrating mainly on turning operations. In this first step of developing a diamond coated product for machining applications, turning has proven to be more suitable for optimization than milling because of a more complex set-up of cutting parameters in the milling operation. A selection of tests are listed in Table 2, which are examples of well working operations when using a diamond coated cutting tool. Comparison is mainly made with uncoated cemented carbide as a
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J. Karner et al. / Materials Science and Engineering A209 (1996) 405-413
TURNING TEST no 1
Material: Operation: Coolant: Component:
AI-7-9%Si -T6 Roughing None Truck wheel
Cutting data: Cutting speed (m/min): Depth of cut (mm): Feed (mm/rev):
Fig. 7. Schematics of a laboratory machining test, milling of a slotted workpiece of age-hardened AI-4.5%Cu-0.8%Si, wet cutting; v = 1500 m min - 1, a = 1.0 mm, S = 0.10-0.50 mm rev. - 1
#1 1850 - 2500 0.5 - 3.0 0.25 - 0.8
#2
Insert: VCGX 160412-AL
VCGX-AL
RIO
6~m diamond coated CC
Pieces per edge:
7
140
Wear type:
even abrasion
Grade:
Results:
flaking
Fig. 9. Parameters and results for turning field test no. I, continuous turning of the wheel bed of truck wheels in age hardened Al-7%-9'Yr, Si.
-----------""""""""""""""",
-""-"""""-",,,,
Fig. 8. SEM micrograph of a diamond coated cutting edge applied in the machining test described in Fig. 7. The edge has failed by severe flaking of the diamond coating.
reference tool material, but the thin film diamond is also found to be competitive in comparison with peD as shown in one of the cases below. The details of the operations are given in the figures to which reference is Table 2 Summary of field tests Test no.
2 4
3
Description
Continuous wheel bed turning, AI-7'YoSi Intermittent facing of wheel front, AI-7%Si Longitudinal, intermittent turning of electrical motor armature, Cu Continuous wheel bed turning, AI-II %Si
Parameters in Fig.
Wear pattern in Fig.
9
10
II
12 a,b
15
14
13
Fig. 10. Diamond coated cutting tool after use in turning field no. I (scanning electron micrograph). The failure mechanism flaking.
made in Table 2. The table also gives the reference to the figures of the wear patterns for the used inserts. The results seen in Figs. 11 and 13 with an improvement in the cutting tool lives for the turning of Al
1. Karner et at.
/\ilaterial.\ Science and Engineerillg A209 (/996) 405 4/3
TURNING TEST no 2
Material: Operation: Coolant: Component:
r~
AI-7%Si-T6 Roughing Emulsion Wheel
Cutting data: Cutting speed (m/min) RPM: Depth of cut (mm): Feed (mm/rev):
~l_
#1 <2150 1800 0.8 - 1-5 0.5
#2
HIO
61lm diamond coated CC
Insert:
411
The major failure mechanism of the diamond coated tools is flaking of the diamond coating as can be seen in Fig. 10 and Fig. 14. However, cases are found where the extension of the flaking is very small as in Fig. 12(a). In this case the failure mode is rather flank wear, following the same pattern as is the case for the reference tool. the uncoated carbide, insert shown in Fig. 12(b). No straight forward correlation can be found between the results in terms of tool lives or wear patterns, such as the extension of the area of flaking, and the specific machining parameters, such as continuous or intermittent cutting or the cutting speed, or the choices of workpiece material. It is more likely that the stability of the specific operation plays a dominant role since the failure mechanism in terms of flaking is sensitive to the stability conditions.
NI51.4-800-60-AL
Grade:
Results: Pieces per edge:
600
2250
Wear type:
even abrasion
even
Fig. II. Parameters and results for turning field test no. 2, intermittent facing of the wheel front of car wheels in age hardened AI-7";:,si.
alloys by a factor of 4 may be regarded as typical but as in the case of Fig. 9 (test no. I), the improvement factor may be as large as 20 times.
4. Conclusions A diamond coating process well suited for large scale coating of cutting tool inserts has been presented. The process is based on a high current density arc plasma CVD method which gives a high degree of uniformity of conditions such as deposition rate and diamond quality over large areas. Cemented carbide cutting tools can be coated with diamond of good quality by using the above described method. The coated tools have been found to perform well in laboratory tests as well as in field tests at end users, in operations covering the machining of AI-Si alloys and copper. The adhesion of the diamond coatings is well assessed in the specially designed laboratory milling test.
Fig. 12. (a) Diamond coated cutting tool after use in turning field test no. 2 (scanning electron micrograph). The failure mechanism is even, abrasive wear accompanied with a slight flaking. (b) Uncoated cemented carbide cutting tool after use in field test no. 2 (scanning electron micrograph). Failure mechanism: even, abrasive wear.
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J. Karner et al. / Materials Science and Engineering A209 (1996) 405-413
TURNING TEST no 3 TURNING TEST no 4 Material: Operation: Coolant: Component:
AI-ll%Si Rough +finish Emulsion Wheel
Cutting data: Cutting speed (m/min): RPM: Depth of cut (mm): Feed (mm/rev):
#1 2100 - 2600 2000 0.5 - 1.0 0.35 - 0.65
#2
Cutting data: Cutting speed (m/min): Depth of cut (mm): Feed (mm/rev):
Insert:
#2
PCD
6/-tm diamond coated CC
2
4
Insert:
RCGX 0803MO-AL Grade:
#1 300 1.0 0.15
CCGX 120408-AL K15
6ltm diamond coated CC
Results:
Grade:
Results:
Pieces per edge:
200
800
Wear type:
even abrasion
flaking
Fig. 13. Parameters and results for turning field test no. 3, continuous turning of the wheel bed of car wheels in AI-II %Si.
Pieces per edge: Wear type: Comment:
even burr formation
no burr
Fig. 15. Parameters and results for turning field test no. 4, longitudinal, intermittent turning of electrical motor armature in Cu.
Research. Dr. S. Lindholm and Dr. C. Broman at the Institute of Geology, University of Stockholm are acknowledged for carrying out the Raman analyses.
References
Fig. 14. Diamond coated cutting tool after u~e in turning field test no. 3 (scanning electron micrograph). The failure mechanism is flaking.
Acknowledgements
The authors wish to acknowledge Dr. H. Kaufmann, Dr. A.E. Thelin and Dr. S. Soderberg for their interest and support in this work. The work was also partly financed by the Swiss Priority Program of Materials
[I] B.V. Derjaguin and D.V. Fedoseev, Growth of Diamond and Graphite from the Gas Phase, Nauka, Moscow (1977) in Russian; English translation, Elsevier, (1989). [2] P.K. Bachmann and R. Messier, Chem. Eng. News, 15 May (1989) 24. [3] B. Lux and R. Haubner, Proc. 12th Plansee Seminar, 615, Metallwerk Plansee, Reutte, Austria (1989). [4] K.E. Spear, J. Am. Ceram. Soc., 72 (1989) 71. [5] 1. Reineck, S. Soderberg and M.E. Sjostrand, CVD-diamantbes-
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Materials Science and Engineering A209 (1996) 405-413
[9] N. Ohtake and M . Yoshikawa, J. Electrochem. Soc.. 137 (1990) 717. [10] T.G. Owano, C.H. Kruger and M.A. Capelli, Mar. Res. Soc. Symp. Proc. Vol. 190 (1991) 131. [II] S. Matsumoto, H. Hino and T. Kobayashi, Appl. PhI's. Leu .. 51
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[13] V. Buck, K. Piirschke and M. Pauser, 1. Zeng, Proc. CIMTEC 28 June 4 July, Florence, Italy. [14] US patent 4851254 (1989). [15] US patent 4749587 (1988). [16] M. Capitelli, and E. Molinari, in F.L. Bloschke (Ed.), Topics in Currellt Chemistry, Vol. 90, p. 61. [17] T. Okubo, S. Ikari, K. Kusakabe and S. Morooka, 1. Mat. Sci. Leuer, JJ (1992).