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Development and evaluation of PACVD coated cermet tools
Surfaceand Coatings Technology93 (1997) 119-127
Development and evaluation of PACVD coated cermet tools H.K. Tiinshoff *, C. Blawit Institutfiir
Fertimgstecknik
md Spunende T~/elkrelrgmaschinen
(IF?V,,, UnicersitBt Hunnorer,
Hcmnocer, Germnq
Abstract Cermetsare the link betweenthe hard ‘but brittle ceramiccutting tools and the tough but lesswear resistantcementedcarbides. As Cermetsare lesstough than WC-basedcementedcarbides,limitations are given concerningthe feed rate. In order to avoid theselimit PACVD-coated Cermetswith different layer systemshave beendeveloped.Apart from TIN and Ti(C,N)-singlelayers, multilayers consistingof TiN, Ti(C,N) and TiC layers were depositedin different layer thicknessesand sequences. This report describesthe structure and the coating adhesionby scratch testsof the layer systems.Milling testswith PACVD-coated Cermet cutting tools werecarried out in temx of performanceand wear behaviour of thesetools. 0 1997ElsevierScienceS.A. Keywords:
PACVD; Cermet;Coatings;Tool wear
1. Introduction Today’s coating technology has contributed significantly to the advancement of materials for the metalcutting industries. Various coating techniques, especially PACVD (plasma assisted chemical vapour deposition) and PVD (physical vapour deposition), have been found to enhance tool life and increase cutting speed and feed rate, resulting in higher productivity [ 11. This has been proven beyond doubt for the coated tungsten carbide tools and it has since been the practice to use coated tools whenever possible to get the best performance from a cutting tool. Such a remarkable break through by coated tungsten carbide tools has promoted the market to be flooded with various kinds of coated tools and among the newer variants is coated cermet. Due to their properties, cermets are the link between the hard but brittle ceramic cutting tools and the tough but less wear-resistant cemented carbides. As cermets are less tough than WC-based cemented carbides, limitafions are given con-
cerning the feed rate [2,3]. In order to avoid this limit PACVD-coated cermets have been developed. The object of the studies by the IFW are to determine
the coating adhesion and the cutting performance of these coated cutting toois. These investigations will be * Corresponding author. 0257~8972/97/917.00 0 1997 Elsevier Science S.A. All rights reserved. PII SO257897297000234
done by the use of different measurement techniques like those shown in Fig. 1, e.g. scratch tests, residual stress measurements, and thermal wave analysis. Parallel work will be done by finite-element calculation. Modelling of different multilayer compositions and/or different film thicknesses will be used to optimise the residual stresses in coated tools. Cutting temperature and cutting forces will be measured and used for the simulation to achieve the dynamic temperature and
stress distribution
in the coating-substrate
composition.
Table 1 Data for cermet substrate material IS0 Classification Chemical composition TiCjTiN 18 Mechanical properties Young’s modulus E (GPa) Hardness HV 0.5 (-) Fracture toughness Thermal properties Thermal expansion rate Thermal conductivity Surface roughness Rake face Clearence Face
P25 WC 23
MoC 6
Ni 7
500 1570 K,, (MPam”‘) ci (10’6K”) ?, (W/mK) R, % R, 8
(v-6 (w3 WI (w)
Co TaC NbC 10 14 22 12
7.2 40 0.047 0.266
0.058 0.362
H, K. TiinshofS, C. Blawit
/ Surface
and Coatings
Technology
coating adhesion by scratch and 1 rockwell-test
93 (1997)
119-127
I influence on surface integrity
thermal wave characterisation of coated cuttina tools
residual stresses by GI D-X-ray diffraction
tribological investigations finite-element simulation
Fig. 1. Characterisation of coatings.
carbide-
ni.
Ta. Nb. wl
c I carbide
PI0
cermet
shell a"
1core a'
(77,MO) C
X
2,03 16,5 1560 IO,6 1700
binder phase w
average grain size content of binder phase hardness density flexural strength fracture toughness thermal conductivity thermal expansion rate
IJm Q! HV30 g/cm3 MPa MPa.rn12 W/mK 1O*K-’
I,82 14 1760 61 1500 597 W f32
CO
binder phase p Co, Ni 310/16780c0
Fig. 2. Comparison of carbide (HW) and cerrnet (HT) tools.
IFW
H.K.
EnshofL
C. Blawir
1 Surface
and
After characteriztion of these coated cermet tools cutting experiments will show the machinability performance of these tools. In order to determine the thermal and mechanical wear resistance turning and milling experiments have been carried out.
2. Development of cernlet tools Cermets are, technically, ceramic particles bonded with metal; hence “cer” for ceramic and “met” for metal. Consequently, they have the same structure as conventional cemented carbides (Fig. 2). The ingredient that sets a cermet apart from a cemented carbide is titanium nitride. TiN, in combination with Tic, forms the hard phase of the tool. The binder phase-consists mainly of nickel. The advantages of cermets are the high hot hardness which allows high cutting speeds and the chemical stability which effects high wear resistance as well as good surface quality of the workpiece. In contrast to WC-based cemented carbides the use of cermet tools is limited by the feed rate. This behaviour can be directly attributed to the lower toughness of the cermet tools. Hence, tendencies in the development of new cermet tools are focused on this matter (Fig. 3). A first possibility for improving the flexural strength is to increase the amount of binder phase. As a result a reduced hot hardness is likely. A second possibility, which is now realized, is to reduce the grain size, similarly to what has been done for tungsten-carbide tools. Parallel to this work, investigations concerning coating of cermet tools were carried out. Up to now, only PVD-coated cermet tools with single layers of TiN or TiCN are available.
3. Experimental
details
For the investigation a cermet insert designed for turning in interrupted cutting or milling with the grade SNGN 120412T01020 was chosen (Table 1). These inserts, when attached in a tool-holder, give the following effective cutting geometry: radial rake angle yI = - 8”30’ axial rake angle ya = - 8”30’ and tool cutting edge angle K = 5”. A standard cutting speed of v,=200 m*min-l and a feed rate of fi =0.2 mm were used in the milling tests while for the turning test the cutting conditions were v, = 250 rn. min-’ and feed rate of f=0.25 mm. In both cases a depth of cut of q, = 2.0 mm was selected. The workpiece material used for the current investigations was heat-treatable steel DIN Ck 45 (corresponds with AISI 1045). The test specimens for milling experiments were bars of 120 mm width, 50 mm height and 1000 mm length. For turning, specimen 100 mm in diameter and 300 mm in length were used. As there
Coatings
Technology
93 (1997)
119-127
121
already exist a lot of test results which describe the wear behaviour of conventional cutting tools when machining Ck 45, evaluation of the performance of coated cermets can be done easily by machining this steel. For cutting experiments in face milling, a Heller PFVI CNC vertical milling machine was used. The nominal power of the machine tool is P, = 30 kW. The maximum number of revolutions is limited to n,,,=5000 min-‘, feed speed can be varied in the range yf= 1- 10000 mm/min. The machine used for the turning test was a VDF Boehringer CNC lathe with power P= 25 kW and center distance I= 1000 mm. The machining tests were carried out according to the relevant IS0 standard. The tool-life criterion was either the average width of flank wear land (E&=0.3 mm) or breakage, whichever was reached first. Flank wear was monitored at suitable intervals, using a special microscopeassisted device. Commercial coatings are titanium carbide (TIC), titanium nitride (TIN), titanium carbonitride (TiCN), titanium aluminide (TiAIN) and aluminium oxide (A1203). Multi layer composition offers coatings with different possibilities. CVD (chemical vapor deposition) and PVD (physical vapor deposition) are the famous coating techniques. Lower coating temperature is an advantage of PVD technology compared to CVD usage. Special requirements concerning the geometry of the substrate is a limiting item when using PVD techniques. When using CVD the coating temperature is 900 to 1200°C so that problems with the substrate material will occur. The coating technique which was applied is plasma assisted chemical vapor deposition (PACVD). This facility combines the advantages of CVD and PVD (Fig. 4). Operating at a pressure of 200 Pa, it converts the coating gas into the plasma state under high d.c. current at temperatures of 450 to 650”. A process computer controls and monitors temperature, pressure, plasma parameters, gas composition and the entire operation of the facility. The equipment consists of a reactor with auxiliary heating, a gas supply device, a vacuum pump system and a pulsed d.c. power supply. A detailed description of the experimental set-up is given elsewhere [4]. The coating tests and the development of the process parameters were carried out at the Institut fur Oberflachentechnik und plasmatechnische Werkstoffentwicklung, TU Braunschweig, Germany. Different layer systems containing TIN-, Ti(C,N)- and Tic-layers were deposited on the cermet substrate. Multilayers of these hard coatings were deposited which can be divided according to the combination of the layers and the thicknesses of each layer in the system. Variation in the thicknesses of the single layers is due to different deposition times. Table 2 shows the chosen deposition parameters, the layer systems and the thicknesses of each layer.
H.K. Tiinsho$ C. Blawit /Surface and Coatings Tecimoiogy 93 (1997) 119-127
Fig. 3. Tendencies
in the development
of cermet
tools.
HW without reduction of toughness
substrate Differences
temperature
in deposition Fig. 4. Coating
technics.
3~ techniques
I
H.K.
T6nshqfL
C. Blair,it
1 Surface
and Coatings
4. Evaluation of PACVD coated certnets
The scratch test (according to DIN V ENV 1071 Tei12) has been applied to determine the adhesive and/or cohesive strength of the coatings depos,ited on the cermet substrates. A diamond cone (r=200 urn; cone angle 120’) is used to scratch the surface of the coated cermet specimen at a constant speed and under a continuously increasing speed. This load leads to deformations which causes stresses resulting in flaking (adhesive failure) or chipping (cohesive failure) of the coating. The smallest load at which the coating is damaged, called the critical load, L,, is determined by SEM and frictional force measurement, i.e. by a sudden increase in the driving force. Scratch tests were carried out parallel and perpendicular to the grinding tracks. Fig. 5 shows a typical SEM
Preliminary tests were carried out to determine suitable machining processes, feed rates and cutting speeds. The influence of the machining process on tool life is shown in Fig. 6. As mentioned above, a cermet substrate corresponding to IS0 classification P 25 was selected for the coating tests. Therefore, the substrate is based on its material date suitability for milling processes, performed in down milling. Shorter tool life in turning can be directly attributed to the reduced hot hardness of the cermet. For both machining operations an improved tool life was obtained when TiN-coated substrate materials were used. Based on these results, the following tests with the developed PACVD-coated cermet tools were performed in down milling. The material removed when machining steel Cl< 45 with a cermet, coated with a titanium nitride and a titanium carbonitride layer, yields an outstanding performance for the coated cermet with increasing value of critical load (Fig. 7). With respect to coated carbides, whereas values of 50 to 70 N are normal, the value of the critical load for all three cermets is always less than 30 N. The difference in the rate of the removed material between cermet HT-C8 and HT-C7 can be explained by the more constant film adhesion of cermet HT-C& which is expressed by the variation in the values of the critical load in Table 3. Compared to the uncoated substrate material, the cermet HT-C, shows a decrease in the wear behaviour so that an influence of the coating process itself on the substrate material could not be excluded. A linear behaviour between deposition time and film thickness of each layer can not be established
PACVD coating parameters
Type 1: TiN-Ti(C,N) Type 2: Ti(C,N)-TiN Type 3: TiN-Ti (C,N)-TiN Type 4: TiN,Ti (C,N)-TiC Gas composition Hydrogen H, (vol.%) Argon Ar (vol.%) Titanium tetrachloride Tic& (vol.%)
500 200
200 600
Ti(C,N): TiN: TiN:
1.2-1.5 1.3-2.0 0.5-1.5
Ti(C,N): Tic:
0.7-3.8
53
15 0.3-l .3
Thickness (!-M 1.8-6.5 3.2-6.0 3.6-5.1
2.6-7.2
1.2-2.3 TiN: nitrogjen Nz Tic: methane CH, Ti(C,N): nitrogen Nz methane CH,
I23
I1 9-127
4.2. Machining tests
Table 2 Deposition parameters of layer systemsand thicknesses of single layers
Thickness of single layer (urn) TiN: 1.0-3.5 Ti(C.N): 0.9-4.0 Ti(C,N): 1.5-3.0 TiN: 1.4-3.0 TiN: 0.8-1.8
93 (1997)
photograph of a scratch obtained for a TiNPACVD coated cermet. Range II characterizes the first area where the coating is removed at the surface of the ground substrate by adhesive failure (flaking), Additionally, areas were found were the failure of the coating occurred inside the substrate material itself. An example is shown on the right-hand side of Fig. 5, described by range III The results obtained for the reported PACVD-coated cermets are listed in Table 3. A discussion of these results will be presented with the results of the machining test in the next Section.
4.1. ScK!tch tests
Discharge voltage u (VI Pressure p (Pa) Substrate temperature r, (“Cl Pulse duration td (us) Pulse pause rp (us) Layer systems
Technology
31 vol.%
10 vol.% 28 ~01% 4~vol’o
Table 3 Critical load L, of coated specimen Critical load L,
Cermet
Scratch (N) Parallel Perpendicular
HT-C3 15-20
HT-C7 17-26
3-6
15-21
HT-C8
HT-C9
XT-Cl0
HT-Cl1
I-IT-Cl2
HTC13
23-28 28-32
34-41 45-50
45-49 37-42
31-43 20-42
12-14 12-16
20-28 25-28
H.K.
T&Lv%oJ
substrate coating coating thickness diamond tip
: :
scratch speed
:
cermet PACVD - TIN 3-4pm 120” 0.2 mm radius v, = 0.167 mm/s
124
C. Bla\r*it J Surface
and Coatings
Technology
93 (1997)
119-127
range I
Fig. 5. Scratch of the PACVD-coated cermet HT-TiN-9.
due to the different growing rate depending on the bottom layer for the films. When coating the cermet substrate with a TiCNTiN layer (Fig. 8),~ the value for the critical load rises to 49 N (HT-ClO). Both specimens (I-IT-C9, HT-ClO) reached a high volume of removed material up to the power of 1.8 compared to the uncoated substrate. Cermet HT-Cll, which was coated with a multi layer of TiN-TiCN-TiN, reached similar values in critical load but only 66% of the volume of the removed material compared to the substrate. An explanation for this low value could be the less regular film adhesion. Scratching parallel to the grinding tracks leads to high values of L,, while a perpendicular scratch is characterized by high deviations in L,. The influence of the growing rate of the films can be discussed by considering cermets HT-C7 and HT-C9. The deposition time for each film is constant while the order of the TiN-TiCN layers was changed. The results show that the growing rate for a film deposit on a cermet is less compared to that when a film is deposited on a second one. The results for the TiN-TiCN-TiC mulitlayer coatings are shown in Fig. 9. The results obtained are close to those received with the TiN-TiCN-coated cermets in Fig. 7. The values of the critical load are again less than 30 N and the volume of removed material is equal
or more than that for the uncoated substrate. Compared with cermet HT-Cl1 the values of L, for the inserts HT-Cl2 and HT-C13 in parallel and perpendicular directions of scratch are of similar size, which characterizes a more regular coating adhesion. Due to this fact, an increase in the volume of removed material can be observed. 5. Conclusions
The capabilities of cermets can be influenced over a wide range by PACVD coatings. By coating this cermet with high toughness the lifetime in milling experiments could be increased up to the power of 1.8. The wear behaviour of the coated cermets exhibited an almost linear increase in the width of flank wear land. The coating adhesion was determined by scratch test. By scratching parallel and perpendicular to grinding tracks more detailed information about the coating adhesion can be established. Titanium carbonitride as a first layer on the cermet substrate leads to the highest values of L,. Furthermore, the test points out that the high hardness of the TiCN top layer made these cermets less vulnerable to wear, so that higher surface qualities of the workpiece can be reached.
H.K.
ZGnshoff,
C. Blawit
1 Surface
and
Coatings
Technology
93 (1997)
125
119-127
8 7
film thickness
[urn]
6
1 0
Substrat
HT-Cl1
tool material tool grade Yr
-8”30’
Ya -7”
HT-Cl2
: Cermet, coated Cermet : SNGN 120412 T 01020 E
Kr
90”
75”
rc
1,2mm
HT-Cl3
workpiece material cutting speed feed rate per tooth depth of cut cutting edges
: Ck 45 : vc = 200 m.min-‘l : fi = 0,20 mm : ap = 2,0 mm :z=l 36/13916c
(4
tool material tool geometry
-8”30’
: Cermet, coated Cermet : SNGN 120412 T 01020
Y
E
K
-7”
90”
75”
rr. 1,2mm
chamfer O,lmm.20”
workpiece cutting speed feed rate depth of cut tool-life criterion
:Ck45 : vc = 250 m.min-I : fi = 0,25 mm : ap = 2,0 mm : VB, = 0,3 mm XVI
(b L-l Fig. 6. Removed material versus machining process.
SBUYC
62 IkW
0 IFW
126
H.K
Tiinshofi
C. Blawit
/
Surface and Coatings
Tecknolog~
93 (1997)
119-1.27
KI crit. Load [x10’ N]
tool material tool grade
: Cermet, coated Cermet : SNGN 120412 T 01020
Yr
Ya
E
Kr
rc
-8”30’
-7”
90”
75”
1,2mm
workpiece material cutting speed feed rate per tooth depth of cut cutting edges
: Ck 45 : vc = 200 m.min-I : fz = 0,20 mm : an = 2,0 mm : z= 1 36/13917c 8 Il=W
Fig. 7. Capabilities of TiN-TiCN-PACVD-coated
cermets.
8
6
1 0
HTC9
Subs&at
HT-10
tool material : Cermet, coated Cermet tool grade : SNGN 120412 T 01020 Yr
-8”30’ L
Ya
E
9
rr
-7“
90”
75”
1,2mm
I-IT-Cl 1
workpiece material cutting speed feed rate per tooth depth of cut cutting edges
: Ck 45 : vc = 200 meminI : fz = 0,20 mm : an = 2,0 mm :z=l 36/i 3918~ 0 IFW
Fig. 8. Capabilities of TiCN-TiN-PACVD-coated
cermets.
127
coating /
HI-Cl 1
Sub&at tool material tool grade Yr -8”30’
Ya -7”
HT-Cl2
: Cermet, coated Cermet : SNGN 120412 T 01020 & 90”
%
r.
75”
1,2mm
HI-Cl 3
workpiece material cutting speed feed rate per tooth depih of cut cutting edges
: Ck 45 : vc = 200 m,mirl : fi = 0,20 mm : ap = 2,0 mm :z=l 36113916c
Fig. 9. Capabilities of TiCN-PACVD-coated
References [ l] H.K. TGnshoff, Spanen, Springer, Berlin, Heidelberg. 1995. [2] N. Reiter, Neue Werkstoffe-und Bearbeitungstechnologien Werkstatttechnik 79 (1989) 513-516 [3] H.K. T6nshoff, C. Cassel, Machining of heat-treatable steel and
StiUCtUR?
Q IFW
cermets.
stainless steel with cermet tools, Production engineering, research and development in Germany, Ann. German Academic Sot. Production Eng. 1 ( 1993) pp. 81-84. [4] K.-T. Rie, U. Kiinig, Hard coatings on steel and hard metals by pulsed plasma CVD process, in: Proc. 6th European Conference on Chemical Vapour Deposition, EURO CVD 6, Jerusalem 1987, Proceedings, S. pp. 311-315.
Development and evaluation of PACVD coated cermet tools H.K. Tiinshoff *, C. Blawit Institutfiir
Fertimgstecknik
md Spunende T~/elkrelrgmaschinen
(IF?V,,, UnicersitBt Hunnorer,
Hcmnocer, Germnq
Abstract Cermetsare the link betweenthe hard ‘but brittle ceramiccutting tools and the tough but lesswear resistantcementedcarbides. As Cermetsare lesstough than WC-basedcementedcarbides,limitations are given concerningthe feed rate. In order to avoid theselimit PACVD-coated Cermetswith different layer systemshave beendeveloped.Apart from TIN and Ti(C,N)-singlelayers, multilayers consistingof TiN, Ti(C,N) and TiC layers were depositedin different layer thicknessesand sequences. This report describesthe structure and the coating adhesionby scratch testsof the layer systems.Milling testswith PACVD-coated Cermet cutting tools werecarried out in temx of performanceand wear behaviour of thesetools. 0 1997ElsevierScienceS.A. Keywords:
PACVD; Cermet;Coatings;Tool wear
1. Introduction Today’s coating technology has contributed significantly to the advancement of materials for the metalcutting industries. Various coating techniques, especially PACVD (plasma assisted chemical vapour deposition) and PVD (physical vapour deposition), have been found to enhance tool life and increase cutting speed and feed rate, resulting in higher productivity [ 11. This has been proven beyond doubt for the coated tungsten carbide tools and it has since been the practice to use coated tools whenever possible to get the best performance from a cutting tool. Such a remarkable break through by coated tungsten carbide tools has promoted the market to be flooded with various kinds of coated tools and among the newer variants is coated cermet. Due to their properties, cermets are the link between the hard but brittle ceramic cutting tools and the tough but less wear-resistant cemented carbides. As cermets are less tough than WC-based cemented carbides, limitafions are given con-
cerning the feed rate [2,3]. In order to avoid this limit PACVD-coated cermets have been developed. The object of the studies by the IFW are to determine
the coating adhesion and the cutting performance of these coated cutting toois. These investigations will be * Corresponding author. 0257~8972/97/917.00 0 1997 Elsevier Science S.A. All rights reserved. PII SO257897297000234
done by the use of different measurement techniques like those shown in Fig. 1, e.g. scratch tests, residual stress measurements, and thermal wave analysis. Parallel work will be done by finite-element calculation. Modelling of different multilayer compositions and/or different film thicknesses will be used to optimise the residual stresses in coated tools. Cutting temperature and cutting forces will be measured and used for the simulation to achieve the dynamic temperature and
stress distribution
in the coating-substrate
composition.
Table 1 Data for cermet substrate material IS0 Classification Chemical composition TiCjTiN 18 Mechanical properties Young’s modulus E (GPa) Hardness HV 0.5 (-) Fracture toughness Thermal properties Thermal expansion rate Thermal conductivity Surface roughness Rake face Clearence Face
P25 WC 23
MoC 6
Ni 7
500 1570 K,, (MPam”‘) ci (10’6K”) ?, (W/mK) R, % R, 8
(v-6 (w3 WI (w)
Co TaC NbC 10 14 22 12
7.2 40 0.047 0.266
0.058 0.362
H, K. TiinshofS, C. Blawit
/ Surface
and Coatings
Technology
coating adhesion by scratch and 1 rockwell-test
93 (1997)
119-127
I influence on surface integrity
thermal wave characterisation of coated cuttina tools
residual stresses by GI D-X-ray diffraction
tribological investigations finite-element simulation
Fig. 1. Characterisation of coatings.
carbide-
ni.
Ta. Nb. wl
c I carbide
PI0
cermet
shell a"
1core a'
(77,MO) C
X
2,03 16,5 1560 IO,6 1700
binder phase w
average grain size content of binder phase hardness density flexural strength fracture toughness thermal conductivity thermal expansion rate
IJm Q! HV30 g/cm3 MPa MPa.rn12 W/mK 1O*K-’
I,82 14 1760 61 1500 597 W f32
CO
binder phase p Co, Ni 310/16780c0
Fig. 2. Comparison of carbide (HW) and cerrnet (HT) tools.
IFW
H.K.
EnshofL
C. Blawir
1 Surface
and
After characteriztion of these coated cermet tools cutting experiments will show the machinability performance of these tools. In order to determine the thermal and mechanical wear resistance turning and milling experiments have been carried out.
2. Development of cernlet tools Cermets are, technically, ceramic particles bonded with metal; hence “cer” for ceramic and “met” for metal. Consequently, they have the same structure as conventional cemented carbides (Fig. 2). The ingredient that sets a cermet apart from a cemented carbide is titanium nitride. TiN, in combination with Tic, forms the hard phase of the tool. The binder phase-consists mainly of nickel. The advantages of cermets are the high hot hardness which allows high cutting speeds and the chemical stability which effects high wear resistance as well as good surface quality of the workpiece. In contrast to WC-based cemented carbides the use of cermet tools is limited by the feed rate. This behaviour can be directly attributed to the lower toughness of the cermet tools. Hence, tendencies in the development of new cermet tools are focused on this matter (Fig. 3). A first possibility for improving the flexural strength is to increase the amount of binder phase. As a result a reduced hot hardness is likely. A second possibility, which is now realized, is to reduce the grain size, similarly to what has been done for tungsten-carbide tools. Parallel to this work, investigations concerning coating of cermet tools were carried out. Up to now, only PVD-coated cermet tools with single layers of TiN or TiCN are available.
3. Experimental
details
For the investigation a cermet insert designed for turning in interrupted cutting or milling with the grade SNGN 120412T01020 was chosen (Table 1). These inserts, when attached in a tool-holder, give the following effective cutting geometry: radial rake angle yI = - 8”30’ axial rake angle ya = - 8”30’ and tool cutting edge angle K = 5”. A standard cutting speed of v,=200 m*min-l and a feed rate of fi =0.2 mm were used in the milling tests while for the turning test the cutting conditions were v, = 250 rn. min-’ and feed rate of f=0.25 mm. In both cases a depth of cut of q, = 2.0 mm was selected. The workpiece material used for the current investigations was heat-treatable steel DIN Ck 45 (corresponds with AISI 1045). The test specimens for milling experiments were bars of 120 mm width, 50 mm height and 1000 mm length. For turning, specimen 100 mm in diameter and 300 mm in length were used. As there
Coatings
Technology
93 (1997)
119-127
121
already exist a lot of test results which describe the wear behaviour of conventional cutting tools when machining Ck 45, evaluation of the performance of coated cermets can be done easily by machining this steel. For cutting experiments in face milling, a Heller PFVI CNC vertical milling machine was used. The nominal power of the machine tool is P, = 30 kW. The maximum number of revolutions is limited to n,,,=5000 min-‘, feed speed can be varied in the range yf= 1- 10000 mm/min. The machine used for the turning test was a VDF Boehringer CNC lathe with power P= 25 kW and center distance I= 1000 mm. The machining tests were carried out according to the relevant IS0 standard. The tool-life criterion was either the average width of flank wear land (E&=0.3 mm) or breakage, whichever was reached first. Flank wear was monitored at suitable intervals, using a special microscopeassisted device. Commercial coatings are titanium carbide (TIC), titanium nitride (TIN), titanium carbonitride (TiCN), titanium aluminide (TiAIN) and aluminium oxide (A1203). Multi layer composition offers coatings with different possibilities. CVD (chemical vapor deposition) and PVD (physical vapor deposition) are the famous coating techniques. Lower coating temperature is an advantage of PVD technology compared to CVD usage. Special requirements concerning the geometry of the substrate is a limiting item when using PVD techniques. When using CVD the coating temperature is 900 to 1200°C so that problems with the substrate material will occur. The coating technique which was applied is plasma assisted chemical vapor deposition (PACVD). This facility combines the advantages of CVD and PVD (Fig. 4). Operating at a pressure of 200 Pa, it converts the coating gas into the plasma state under high d.c. current at temperatures of 450 to 650”. A process computer controls and monitors temperature, pressure, plasma parameters, gas composition and the entire operation of the facility. The equipment consists of a reactor with auxiliary heating, a gas supply device, a vacuum pump system and a pulsed d.c. power supply. A detailed description of the experimental set-up is given elsewhere [4]. The coating tests and the development of the process parameters were carried out at the Institut fur Oberflachentechnik und plasmatechnische Werkstoffentwicklung, TU Braunschweig, Germany. Different layer systems containing TIN-, Ti(C,N)- and Tic-layers were deposited on the cermet substrate. Multilayers of these hard coatings were deposited which can be divided according to the combination of the layers and the thicknesses of each layer in the system. Variation in the thicknesses of the single layers is due to different deposition times. Table 2 shows the chosen deposition parameters, the layer systems and the thicknesses of each layer.
H.K. Tiinsho$ C. Blawit /Surface and Coatings Tecimoiogy 93 (1997) 119-127
Fig. 3. Tendencies
in the development
of cermet
tools.
HW without reduction of toughness
substrate Differences
temperature
in deposition Fig. 4. Coating
technics.
3~ techniques
I
H.K.
T6nshqfL
C. Blair,it
1 Surface
and Coatings
4. Evaluation of PACVD coated certnets
The scratch test (according to DIN V ENV 1071 Tei12) has been applied to determine the adhesive and/or cohesive strength of the coatings depos,ited on the cermet substrates. A diamond cone (r=200 urn; cone angle 120’) is used to scratch the surface of the coated cermet specimen at a constant speed and under a continuously increasing speed. This load leads to deformations which causes stresses resulting in flaking (adhesive failure) or chipping (cohesive failure) of the coating. The smallest load at which the coating is damaged, called the critical load, L,, is determined by SEM and frictional force measurement, i.e. by a sudden increase in the driving force. Scratch tests were carried out parallel and perpendicular to the grinding tracks. Fig. 5 shows a typical SEM
Preliminary tests were carried out to determine suitable machining processes, feed rates and cutting speeds. The influence of the machining process on tool life is shown in Fig. 6. As mentioned above, a cermet substrate corresponding to IS0 classification P 25 was selected for the coating tests. Therefore, the substrate is based on its material date suitability for milling processes, performed in down milling. Shorter tool life in turning can be directly attributed to the reduced hot hardness of the cermet. For both machining operations an improved tool life was obtained when TiN-coated substrate materials were used. Based on these results, the following tests with the developed PACVD-coated cermet tools were performed in down milling. The material removed when machining steel Cl< 45 with a cermet, coated with a titanium nitride and a titanium carbonitride layer, yields an outstanding performance for the coated cermet with increasing value of critical load (Fig. 7). With respect to coated carbides, whereas values of 50 to 70 N are normal, the value of the critical load for all three cermets is always less than 30 N. The difference in the rate of the removed material between cermet HT-C8 and HT-C7 can be explained by the more constant film adhesion of cermet HT-C& which is expressed by the variation in the values of the critical load in Table 3. Compared to the uncoated substrate material, the cermet HT-C, shows a decrease in the wear behaviour so that an influence of the coating process itself on the substrate material could not be excluded. A linear behaviour between deposition time and film thickness of each layer can not be established
PACVD coating parameters
Type 1: TiN-Ti(C,N) Type 2: Ti(C,N)-TiN Type 3: TiN-Ti (C,N)-TiN Type 4: TiN,Ti (C,N)-TiC Gas composition Hydrogen H, (vol.%) Argon Ar (vol.%) Titanium tetrachloride Tic& (vol.%)
500 200
200 600
Ti(C,N): TiN: TiN:
1.2-1.5 1.3-2.0 0.5-1.5
Ti(C,N): Tic:
0.7-3.8
53
15 0.3-l .3
Thickness (!-M 1.8-6.5 3.2-6.0 3.6-5.1
2.6-7.2
1.2-2.3 TiN: nitrogjen Nz Tic: methane CH, Ti(C,N): nitrogen Nz methane CH,
I23
I1 9-127
4.2. Machining tests
Table 2 Deposition parameters of layer systemsand thicknesses of single layers
Thickness of single layer (urn) TiN: 1.0-3.5 Ti(C.N): 0.9-4.0 Ti(C,N): 1.5-3.0 TiN: 1.4-3.0 TiN: 0.8-1.8
93 (1997)
photograph of a scratch obtained for a TiNPACVD coated cermet. Range II characterizes the first area where the coating is removed at the surface of the ground substrate by adhesive failure (flaking), Additionally, areas were found were the failure of the coating occurred inside the substrate material itself. An example is shown on the right-hand side of Fig. 5, described by range III The results obtained for the reported PACVD-coated cermets are listed in Table 3. A discussion of these results will be presented with the results of the machining test in the next Section.
4.1. ScK!tch tests
Discharge voltage u (VI Pressure p (Pa) Substrate temperature r, (“Cl Pulse duration td (us) Pulse pause rp (us) Layer systems
Technology
31 vol.%
10 vol.% 28 ~01% 4~vol’o
Table 3 Critical load L, of coated specimen Critical load L,
Cermet
Scratch (N) Parallel Perpendicular
HT-C3 15-20
HT-C7 17-26
3-6
15-21
HT-C8
HT-C9
XT-Cl0
HT-Cl1
I-IT-Cl2
HTC13
23-28 28-32
34-41 45-50
45-49 37-42
31-43 20-42
12-14 12-16
20-28 25-28
H.K.
T&Lv%oJ
substrate coating coating thickness diamond tip
: :
scratch speed
:
cermet PACVD - TIN 3-4pm 120” 0.2 mm radius v, = 0.167 mm/s
124
C. Bla\r*it J Surface
and Coatings
Technology
93 (1997)
119-127
range I
Fig. 5. Scratch of the PACVD-coated cermet HT-TiN-9.
due to the different growing rate depending on the bottom layer for the films. When coating the cermet substrate with a TiCNTiN layer (Fig. 8),~ the value for the critical load rises to 49 N (HT-ClO). Both specimens (I-IT-C9, HT-ClO) reached a high volume of removed material up to the power of 1.8 compared to the uncoated substrate. Cermet HT-Cll, which was coated with a multi layer of TiN-TiCN-TiN, reached similar values in critical load but only 66% of the volume of the removed material compared to the substrate. An explanation for this low value could be the less regular film adhesion. Scratching parallel to the grinding tracks leads to high values of L,, while a perpendicular scratch is characterized by high deviations in L,. The influence of the growing rate of the films can be discussed by considering cermets HT-C7 and HT-C9. The deposition time for each film is constant while the order of the TiN-TiCN layers was changed. The results show that the growing rate for a film deposit on a cermet is less compared to that when a film is deposited on a second one. The results for the TiN-TiCN-TiC mulitlayer coatings are shown in Fig. 9. The results obtained are close to those received with the TiN-TiCN-coated cermets in Fig. 7. The values of the critical load are again less than 30 N and the volume of removed material is equal
or more than that for the uncoated substrate. Compared with cermet HT-Cl1 the values of L, for the inserts HT-Cl2 and HT-C13 in parallel and perpendicular directions of scratch are of similar size, which characterizes a more regular coating adhesion. Due to this fact, an increase in the volume of removed material can be observed. 5. Conclusions
The capabilities of cermets can be influenced over a wide range by PACVD coatings. By coating this cermet with high toughness the lifetime in milling experiments could be increased up to the power of 1.8. The wear behaviour of the coated cermets exhibited an almost linear increase in the width of flank wear land. The coating adhesion was determined by scratch test. By scratching parallel and perpendicular to grinding tracks more detailed information about the coating adhesion can be established. Titanium carbonitride as a first layer on the cermet substrate leads to the highest values of L,. Furthermore, the test points out that the high hardness of the TiCN top layer made these cermets less vulnerable to wear, so that higher surface qualities of the workpiece can be reached.
H.K.
ZGnshoff,
C. Blawit
1 Surface
and
Coatings
Technology
93 (1997)
125
119-127
8 7
film thickness
[urn]
6
1 0
Substrat
HT-Cl1
tool material tool grade Yr
-8”30’
Ya -7”
HT-Cl2
: Cermet, coated Cermet : SNGN 120412 T 01020 E
Kr
90”
75”
rc
1,2mm
HT-Cl3
workpiece material cutting speed feed rate per tooth depth of cut cutting edges
: Ck 45 : vc = 200 m.min-‘l : fi = 0,20 mm : ap = 2,0 mm :z=l 36/13916c
(4
tool material tool geometry
-8”30’
: Cermet, coated Cermet : SNGN 120412 T 01020
Y
E
K
-7”
90”
75”
rr. 1,2mm
chamfer O,lmm.20”
workpiece cutting speed feed rate depth of cut tool-life criterion
:Ck45 : vc = 250 m.min-I : fi = 0,25 mm : ap = 2,0 mm : VB, = 0,3 mm XVI
(b L-l Fig. 6. Removed material versus machining process.
SBUYC
62 IkW
0 IFW
126
H.K
Tiinshofi
C. Blawit
/
Surface and Coatings
Tecknolog~
93 (1997)
119-1.27
KI crit. Load [x10’ N]
tool material tool grade
: Cermet, coated Cermet : SNGN 120412 T 01020
Yr
Ya
E
Kr
rc
-8”30’
-7”
90”
75”
1,2mm
workpiece material cutting speed feed rate per tooth depth of cut cutting edges
: Ck 45 : vc = 200 m.min-I : fz = 0,20 mm : an = 2,0 mm : z= 1 36/13917c 8 Il=W
Fig. 7. Capabilities of TiN-TiCN-PACVD-coated
cermets.
8
6
1 0
HTC9
Subs&at
HT-10
tool material : Cermet, coated Cermet tool grade : SNGN 120412 T 01020 Yr
-8”30’ L
Ya
E
9
rr
-7“
90”
75”
1,2mm
I-IT-Cl 1
workpiece material cutting speed feed rate per tooth depth of cut cutting edges
: Ck 45 : vc = 200 meminI : fz = 0,20 mm : an = 2,0 mm :z=l 36/i 3918~ 0 IFW
Fig. 8. Capabilities of TiCN-TiN-PACVD-coated
cermets.
127
coating /
HI-Cl 1
Sub&at tool material tool grade Yr -8”30’
Ya -7”
HT-Cl2
: Cermet, coated Cermet : SNGN 120412 T 01020 & 90”
%
r.
75”
1,2mm
HI-Cl 3
workpiece material cutting speed feed rate per tooth depih of cut cutting edges
: Ck 45 : vc = 200 m,mirl : fi = 0,20 mm : ap = 2,0 mm :z=l 36113916c
Fig. 9. Capabilities of TiCN-PACVD-coated
References [ l] H.K. TGnshoff, Spanen, Springer, Berlin, Heidelberg. 1995. [2] N. Reiter, Neue Werkstoffe-und Bearbeitungstechnologien Werkstatttechnik 79 (1989) 513-516 [3] H.K. T6nshoff, C. Cassel, Machining of heat-treatable steel and
StiUCtUR?
Q IFW
cermets.
stainless steel with cermet tools, Production engineering, research and development in Germany, Ann. German Academic Sot. Production Eng. 1 ( 1993) pp. 81-84. [4] K.-T. Rie, U. Kiinig, Hard coatings on steel and hard metals by pulsed plasma CVD process, in: Proc. 6th European Conference on Chemical Vapour Deposition, EURO CVD 6, Jerusalem 1987, Proceedings, S. pp. 311-315.