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Friction characteristics of coated tungsten carbide cutting tools
353
Wear, 49 (1978) 353 - 357 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
FRICTION CHARACTERISTICS CUTTING TOOLS
S. B. RAO*, K. V. KUMAR** Carnegie-Mellon
OF COATED
TUNGSTEN
CARBIDE
and M. C. SHAW**
University, Pittsburgh, Pa. 15213 (U.S.A.)
(Received January 5, 1978)
Summary A series of experiments has been carried out to explain the lower frictional characteristics of carbide tools coated with TiN relative to those coated with Tic. The reason for the higher friction of TiC appears to lie in the tendency for C to diffuse from the TiC coating into the thin layer of steel that transfers to the tool surface, thus strengthening it. No such strengthening mechanism is evident when TiN is the coating.
1. Introduction A number of compounds are in use today for coating tungsten carbide inserts, the most widely used being titanium carbide (Tic) followed by titanium nitride (TiN) with combinations of the two also being in use. The superiority of one coating material over another has been discussed in qualitative terms, but until now conclusive laboratory data have not been presented to support the claims made [l] . It is generally reported that TiN-coated tools give rise to a lower coefficient of friction than Tic-coated tools when cutting steel. This paper presents frictional data for Tic- and TiN-coated inserts and an experimentally based mechanism that explains the difference in the friction characteristics of the two coatings.
2. Experimental
part
The experiments
performed
may be classified into the following
three
*Present address: Department of Mech. Eng., University of Wisconsin at Madison, Wise. 53706. U.S.A. **Present address: Engineering Science Department, Arizona State University, Tempe, Ariz. 85281, U.S.A.
354
uncoted
(a)
Conical
Pin
(b)
Fig. 1. The schematic arrangements for (a) the rubbing test and (b) the diffusion test.
0
100
200
300
400
500
Electron Energy
600
700
000
900
IC IO
(eV)
Fig. 2. The Auger electron spectroscopic trace for the work material before the diffusion test: Ep, 1.5 kV; I,, 20 PA; VMO,,, 2 V; VmT, 800 V; RC, 0.1 s; SENS, 0.5 mV.
categories: (1) tool rubbing tests; (2) static diffusion tests; (3) Auger electron spectroscopy (AES). In the tool rubbing tests [2] , coated inserts clamped in a tool holder mounted in a lathe dynamometer were loaded against a freshly machined steel workpiece rotating at 450 ft mine1 (Fig. l(a)). The tool was mounted so as not to produce a chip. A normal load Fp was applied and the feed rate set at 0.008 in rev-l so that the insert continuously rubbed a fresh surface. Plastic deformation of the work material occurred as the surface was burnished. The two force components Fv and Fp were recorded and the ratio F,/F, is the coefficient of friction (f = F,/F,).
355
0
100
200
300
400 Electron
500 Energy
600 (eV)
700
600
900
1000
Fig. 3. The Auger electron spectroscopic trace for the surface of a Tic-coated tungsten carbide tool after the diffusion test: E,, 1.5 kV; iP, 20 PA; VMOD 2 V; VmT, 800 V; RC, 0.1 s; SENS, 0.5 mV.
In the static diffusion test (Fig. l(b)) a conical steel pin machined from the workpiece material of the rubbing test, with a flat at its tip, was kept in contact with the coated insert under a lo’ad of 11 lbf and heated to 800 “C in a vacuum of 8 FmHg for 1 h. At the end of this time the surfaces were separated without cooling which generally resulted in a spot of steel adhering to the insert in the area of contact. The purpose of these two tests was to simulate under controlled conditions the fundamental processes that occur on the rake face of the hard metal inserts during cutting. The tool rubbing tests were intended to simulate the flow of a chip across the rake face and the static diffusion test was intended to simulate the diffusion processes that occur between the chip and the insert. AES was used to study the nature of the surfaces after diffusion. Before making an AES determination the surfaces were sputter etched to a depth of 25 8. The following AES profiles were obtained: (1) for the surface of the pin material (Fig. 2); (2) for a Tic-coated insert at the contact area (Fig. 3); (3) for a TiN-coated insert at the contact area (Fig. 4). 3. Results and discussion From force measurements in the rub tests a coefficient of friction of 1.32 was obtained for Tic-coated tools and 1.18 for TiN-coated tools. These are average values of ten determinations and the range of values was less
356
r
dt
100
200
300
400 Electron
500 600 Energy (eV)
700
800
900
I
10
Fig. 4. The Auger electron spectroscopic trace for the surface of a TiN-coated tungsten carbide tool after the diffusion test: E,, 1.5 kV; Ip, 20 PA; VMOD, 2 V; V-T, 800 V; RC, 0.1 s; SENS, 0.5 mV.
than 5% of the average. These values are in agreement with the generally reported results of cutting tests. A study of the wear scar indicated the retention of a considerably larger amount of transferred steel on the TiCcoated inserts than on the TiN-coated inserts. The AES results were studied to understand the friction results. A comparison of Figs. 3 and 4 shows Ti peaks that are the same in the two cases. No nitrogen was present in the case of Fig. 3. The similarity of the Ti peaks suggests the absence of substantial nitrogen in the case of Fig. 4 since the presence of nitrogen would be expected to alter the character of the Ti peak observed. While nitrogen and titanium have characteristic Auger peaks that are close to each other, it has been reported [3, 41 that plain carbon steels do not absorb nitrogen unless special elements such as chromium and aluminum are present. Since these elements were not present in the steel used it may be concluded that an appreciable amount of nitrogen was not present in the steel adhering to the TiN-coated insert. A comparison of Figs. 2 and 4 shows that the carbon in the steel adhering to the TiN-coated inserts was at about the same level as in the bulk material. In contrast, Fig. 3 shows that the carbon concentration in the steel adhering to the Tic-coated inserts was about twice as great as in the bulk material. From these observations it may be concluded that a significant amount of iron carbide resulted in the metal adhering to the tool in the case of a Tic-coated tool, while very little or no iron nitride was present in the steel adhering to a TiN-coated insert. The increase of iron carbide would of course result in a dispersionstrengthening action on the metal adhering to the tool and hence an increase
357
in the friction force. In contrast, a corresponding nitride strengthening by the formation of Fe4N, or Fe,N, would not be expected since both of these nitrides are unstable at temperatures above 600 “F [5] . As a result, the stress required to shear the layer of metal adhering to a TiN-coated tool would be expected to remain unaltered. The increased shear stress in the steel adhering to the Tic-coated insert should result in a higher value of Fv (Fig. l(a)) for the same F, and consequently should result in a higher coefficient of friction.
4. Concluding
remarks
The mechanism suggested here for galling should hold whenever two metals of differing carbon content are in sliding contact. Diffusion of carbon from the material of higher carbon content into the surface of lower carbon content should account for an increase in the strength of the softer material of lower content and hence result in an increase in the wear rate. The solution to this problem appears to lie in the use of an effective diffusion barrier to both surfaces, but particularly to the surface of higher carbon content.
Acknowledgments The authors wish to acknowledge a grant from the Office of Naval Research that has made this investigation possible. In particular they wish to acknowledge the encouragement of Drs. R. Miller and K. Ellingsworth in connection with this study.
References 1 M. S. Kalish, An evaluation of Kalpakjian (ed.), Symp. at IIT tions, IIT, Chicago, 1977. 2 M. C. Shaw and K. Nakayama, (1967) 48. 3 A. Sauveur, The Metallography New York, 1956. 4 G. M. Enos and W. E. Fontane, 5 F. W. Habbord and J. W. Hall, London, 1918.
coatings on cemented on New Developments Machining
high strength
and Heat Treatment Elements Metallurgy
carbides in machining. In S. in Tool Materials and Applicamaterials,
Ann. CIRP, 15
of Iron and Steel, McGraw-Hill,
of Heat Treating, Wiley, New York, 1953. of Steel, Vol. 2, Griffin Metallurgical Series,
Wear, 49 (1978) 353 - 357 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
FRICTION CHARACTERISTICS CUTTING TOOLS
S. B. RAO*, K. V. KUMAR** Carnegie-Mellon
OF COATED
TUNGSTEN
CARBIDE
and M. C. SHAW**
University, Pittsburgh, Pa. 15213 (U.S.A.)
(Received January 5, 1978)
Summary A series of experiments has been carried out to explain the lower frictional characteristics of carbide tools coated with TiN relative to those coated with Tic. The reason for the higher friction of TiC appears to lie in the tendency for C to diffuse from the TiC coating into the thin layer of steel that transfers to the tool surface, thus strengthening it. No such strengthening mechanism is evident when TiN is the coating.
1. Introduction A number of compounds are in use today for coating tungsten carbide inserts, the most widely used being titanium carbide (Tic) followed by titanium nitride (TiN) with combinations of the two also being in use. The superiority of one coating material over another has been discussed in qualitative terms, but until now conclusive laboratory data have not been presented to support the claims made [l] . It is generally reported that TiN-coated tools give rise to a lower coefficient of friction than Tic-coated tools when cutting steel. This paper presents frictional data for Tic- and TiN-coated inserts and an experimentally based mechanism that explains the difference in the friction characteristics of the two coatings.
2. Experimental
part
The experiments
performed
may be classified into the following
three
*Present address: Department of Mech. Eng., University of Wisconsin at Madison, Wise. 53706. U.S.A. **Present address: Engineering Science Department, Arizona State University, Tempe, Ariz. 85281, U.S.A.
354
uncoted
(a)
Conical
Pin
(b)
Fig. 1. The schematic arrangements for (a) the rubbing test and (b) the diffusion test.
0
100
200
300
400
500
Electron Energy
600
700
000
900
IC IO
(eV)
Fig. 2. The Auger electron spectroscopic trace for the work material before the diffusion test: Ep, 1.5 kV; I,, 20 PA; VMO,,, 2 V; VmT, 800 V; RC, 0.1 s; SENS, 0.5 mV.
categories: (1) tool rubbing tests; (2) static diffusion tests; (3) Auger electron spectroscopy (AES). In the tool rubbing tests [2] , coated inserts clamped in a tool holder mounted in a lathe dynamometer were loaded against a freshly machined steel workpiece rotating at 450 ft mine1 (Fig. l(a)). The tool was mounted so as not to produce a chip. A normal load Fp was applied and the feed rate set at 0.008 in rev-l so that the insert continuously rubbed a fresh surface. Plastic deformation of the work material occurred as the surface was burnished. The two force components Fv and Fp were recorded and the ratio F,/F, is the coefficient of friction (f = F,/F,).
355
0
100
200
300
400 Electron
500 Energy
600 (eV)
700
600
900
1000
Fig. 3. The Auger electron spectroscopic trace for the surface of a Tic-coated tungsten carbide tool after the diffusion test: E,, 1.5 kV; iP, 20 PA; VMOD 2 V; VmT, 800 V; RC, 0.1 s; SENS, 0.5 mV.
In the static diffusion test (Fig. l(b)) a conical steel pin machined from the workpiece material of the rubbing test, with a flat at its tip, was kept in contact with the coated insert under a lo’ad of 11 lbf and heated to 800 “C in a vacuum of 8 FmHg for 1 h. At the end of this time the surfaces were separated without cooling which generally resulted in a spot of steel adhering to the insert in the area of contact. The purpose of these two tests was to simulate under controlled conditions the fundamental processes that occur on the rake face of the hard metal inserts during cutting. The tool rubbing tests were intended to simulate the flow of a chip across the rake face and the static diffusion test was intended to simulate the diffusion processes that occur between the chip and the insert. AES was used to study the nature of the surfaces after diffusion. Before making an AES determination the surfaces were sputter etched to a depth of 25 8. The following AES profiles were obtained: (1) for the surface of the pin material (Fig. 2); (2) for a Tic-coated insert at the contact area (Fig. 3); (3) for a TiN-coated insert at the contact area (Fig. 4). 3. Results and discussion From force measurements in the rub tests a coefficient of friction of 1.32 was obtained for Tic-coated tools and 1.18 for TiN-coated tools. These are average values of ten determinations and the range of values was less
356
r
dt
100
200
300
400 Electron
500 600 Energy (eV)
700
800
900
I
10
Fig. 4. The Auger electron spectroscopic trace for the surface of a TiN-coated tungsten carbide tool after the diffusion test: E,, 1.5 kV; Ip, 20 PA; VMOD, 2 V; V-T, 800 V; RC, 0.1 s; SENS, 0.5 mV.
than 5% of the average. These values are in agreement with the generally reported results of cutting tests. A study of the wear scar indicated the retention of a considerably larger amount of transferred steel on the TiCcoated inserts than on the TiN-coated inserts. The AES results were studied to understand the friction results. A comparison of Figs. 3 and 4 shows Ti peaks that are the same in the two cases. No nitrogen was present in the case of Fig. 3. The similarity of the Ti peaks suggests the absence of substantial nitrogen in the case of Fig. 4 since the presence of nitrogen would be expected to alter the character of the Ti peak observed. While nitrogen and titanium have characteristic Auger peaks that are close to each other, it has been reported [3, 41 that plain carbon steels do not absorb nitrogen unless special elements such as chromium and aluminum are present. Since these elements were not present in the steel used it may be concluded that an appreciable amount of nitrogen was not present in the steel adhering to the TiN-coated insert. A comparison of Figs. 2 and 4 shows that the carbon in the steel adhering to the TiN-coated inserts was at about the same level as in the bulk material. In contrast, Fig. 3 shows that the carbon concentration in the steel adhering to the Tic-coated inserts was about twice as great as in the bulk material. From these observations it may be concluded that a significant amount of iron carbide resulted in the metal adhering to the tool in the case of a Tic-coated tool, while very little or no iron nitride was present in the steel adhering to a TiN-coated insert. The increase of iron carbide would of course result in a dispersionstrengthening action on the metal adhering to the tool and hence an increase
357
in the friction force. In contrast, a corresponding nitride strengthening by the formation of Fe4N, or Fe,N, would not be expected since both of these nitrides are unstable at temperatures above 600 “F [5] . As a result, the stress required to shear the layer of metal adhering to a TiN-coated tool would be expected to remain unaltered. The increased shear stress in the steel adhering to the Tic-coated insert should result in a higher value of Fv (Fig. l(a)) for the same F, and consequently should result in a higher coefficient of friction.
4. Concluding
remarks
The mechanism suggested here for galling should hold whenever two metals of differing carbon content are in sliding contact. Diffusion of carbon from the material of higher carbon content into the surface of lower carbon content should account for an increase in the strength of the softer material of lower content and hence result in an increase in the wear rate. The solution to this problem appears to lie in the use of an effective diffusion barrier to both surfaces, but particularly to the surface of higher carbon content.
Acknowledgments The authors wish to acknowledge a grant from the Office of Naval Research that has made this investigation possible. In particular they wish to acknowledge the encouragement of Drs. R. Miller and K. Ellingsworth in connection with this study.
References 1 M. S. Kalish, An evaluation of Kalpakjian (ed.), Symp. at IIT tions, IIT, Chicago, 1977. 2 M. C. Shaw and K. Nakayama, (1967) 48. 3 A. Sauveur, The Metallography New York, 1956. 4 G. M. Enos and W. E. Fontane, 5 F. W. Habbord and J. W. Hall, London, 1918.
coatings on cemented on New Developments Machining
high strength
and Heat Treatment Elements Metallurgy
carbides in machining. In S. in Tool Materials and Applicamaterials,
Ann. CIRP, 15
of Iron and Steel, McGraw-Hill,
of Heat Treating, Wiley, New York, 1953. of Steel, Vol. 2, Griffin Metallurgical Series,