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Performance of silicon nitride cutting tool inserts prepared by gas pressure sintering
Refractory Metals & Hard Materials 11 (1992) 121-126
Performance of Silicon Nitride Cutting Tool Inserts Prepared by Gas Pressure Sintering Elisabeth Besenyei Research Institute for Technical Physics of HAS, Budapest, Hungary
& Reuven Porat I S C A R Ltd., M a a l o t , Israel (Received 14 M a y 1992; accepted 6 July 1992)
Abstract: Hot-pressed silicon nitride inserts have been used for several years in
order to obtain the best results in machining cast iron. Nevertheless, extensive research is directed towards the development of new, less expensive preparation methods of silicon nitride base materials. One of the most promising new methods for the preparation of ceramic materials of high performance is gas pressure sintering, by which inserts with complicated shapes can be prepared. Cold-pressed and gas pressure sintered cutting tool inserts were prepared from silicon nitride and additives. The effects of composition, oxide content, structure and sintering parameters on the mechanical properties of gas pressure sintered silicon nitride materials were examined. Cutting tool inserts prepared either by hot pressing, pressureless or gas pressure sintering were tested in the turning and milling of gray cast iron. The performance of gas pressure sintered inserts was comparable with that of hot pressed inserts.
INTRODUCTION
individually from the desnified bulk material, in the latter case encapsulation must be used which increases costs. Pressureless sintering, using a processing route similar to the manufacturing of hardmetal cutting inserts, has been developed in order to avoid postcutting or the expense of encapsulation. It involves cold pressing to net shape with subsequent dewaxing and sintering. If sufficient amount of sintering aids are present, densification can be achieved by pressureless sintering. The additives are used to promote the formation of a liquid phase with low viscosity. In this case, silicon nitride ceramics can be sintered at a relatively low temperature where no decomposition occurs. The presence of additives leads to the formation of grain boundary phases which may affect the mechanical properties, especially at elevated temperatures. 2 In recent years new manufacturing methods have been developed for advanced ceramics. Of these, gas
Due to the unique combination of the chemical, physical and mechanical properties of silicon nitride based ceramics, they are among the most promising material with a wide range of applications at elevated temperatures. To overcome the difficulties arising from the covalent character of the Si-N bond and the decomposition of the material at high temperature, several pressure-applying processing methods have been developed for manufacturing silicon nitride ceramics, x Hot-pressed silicon nitride inserts have been used for several years in machining gray cast iron. Hot isostatic pressing (HIP) of encapsulated green compacts is an alternative processing route which ensures the hot-pressed material properties. However from an economic point of view they are not ideal processes since complicated shapes are difficult and expensive to manufacture. In the former case they have to be cut 121
Refractory Metals & Hard Materials 02634368/92 $05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain.
E. Besenyei, R. Porat
122
-'x ~
/Pressure gradient
sented in this paper. A comparison of the properties of cutting inserts prepared by different manufacturing methods, including hot pressing, gas pressure and pressureless sintering, as well as their performance in cutting and milling are shown. EXPERIMENTAL PROCEDURE
T i _
,
~
/
/
r
l
\!
IJ
-
~ _ _-L__d
LDewax~ng
--~' I~
~
~'7
Presintering
,
,Fast c o q l i n g
: l ~ "~x"
~ ~--Venting
Cooling "
Fig. 1. A typical time-temperature-pressure schedule for the new sintering processes?
pressure sintering is one of the most suitable for the preparation of silicon nitride? In HIP treatments a pressure of 100-200 MPa is necessary, while in gas pressure sintering pressures as low as 50 MPa or less are enough to achieve a high density. In the literature several different names such as gas pressure sintering, sinter-HIP and overpressure sintering are used for the same process. This involves a combination of sintering at appropriate temperature and low pressure, and the subsequent application of isostatic gas pressure in one cycle in the same equipment. During the process the material is pre-sintered at low pressure until a closed porosity is reached, after which treatment of high pressure results in complete densification. In this case no encapsulation is needed. In the second part of the process, at high pressure, the residual macropores are eliminated either by a mixed flow of both the liquid phase and the grains or by the flow of the liquid phase alone. The latter case may result in soft spots and inhomogeneous microstructure 4 (indicated by lower flexural strength). The nature of the flow can be influenced by the composition and the sintering schedule. The time-temperaturepressure relation during the process is illustrated in Fig. 1.~ As materials with different properties can be obtained depending on the time-temperaturepressure schedule, it is very important to reveal the relationship between composition and sintering parameters as well as between the properties and the performance of gas pressure sintered material in machining tests. The results of gas pressure sintered silicon nitride base ceramics of different compositions are pre-
Sample preparation
The gas pressure and pressureless sintered samples were prepared in the Research Institute for Technical Physics of HAS, while the hot-pressed samples were prepared by Iscar Ltd. For gas pressure and pressureless sintering, the samples were compacted by dry cold pressing from different powder mixtures containing 76-90 wt % silicon nitride and additives. Gas pressure sintering was carried out in an A B R A SHIRP 8-16-200 apparatus in high purity nitrogen of 10-20 MPa. The details of pressureless sintering are given in Ref. (6). T h e ' I S C A N I T E ' type hot-pressed silicon nitride inserts were prepared by the dry milling and dry pressing of discs of 20 m m diameter. A reaction bonded process was then followed by hot pressing in an induction furnace. Density was determined by the Archimedes method. The modulus of elasticity and the 4 point flexural strength were measured at room temperature with a span of 40 x 20 mm. In several cases 3 point flexural strength with a span of 30 m m were also measured. Hardness and fracture toughness (K~c) were determined using a Vicker's diamond indenter on a polished surface. Microstructure of the fractured surface was investigated by scanning electron microscopy. X-ray diffraction analysis was also carried out to determine the alpha prime phase content of silicon nitride using the Gazzara and Messier method. 7 Machining test
Turning and milling were selected for the machining tests as they cover a wide range of applications and usually require different cutting tool grades. Inserts prepared by the different methods were surface ground, sliced (if necessary), periphery ground and edge chamfered. Turning tests were carried out on cast iron at a speed of 4 0 0 m / m i n , which is considered an economical high cutting speed. It is double the cutting speed used for coated cemented carbide. The depth of the cut was 2 m m and a 0-2 m m / r e v feed was used. Feritic Perlitic Gray Cast Iron with
Silicon nitride inserts prepared by gas pressure sintering
1000-
Table 1. Properlies after gas pressure sintering (pressure is
applied before (B) or after (A) pore closure) ©
Material
B A
Density g/cm a
3"09 3-24
Flexural strength MPa
Modulus q/ elasticity GPa
Hardness GPa
457 763
230 266
10"7 15"4
£1_ 900 Z 800x_
123
-20.5
.... \~rl@ss
-18.5 ~ 0
[3_ -16.5 0
4~ Oh C Co L
L/3
700
4.5
(/3
-12.5
600-
% a composition of 3.21 C, 2.63 Si, 0"7 Mn, 0.2 P, 0"08 S was machined in the turning test. The milling tests were carried out at the cutting speed of 1176 m/rain, four times higher than the speed used for coated cemented carbide cutting tools. Low chromium gray iron with a composition of 3"25 C, 2"2 Si, 0"3 Cr, 0"7 Mn, 0-12 P, 0"15 S and with the hardness of 220 H B N was machined. The feed and depth of the cut were 0.33 r a m / t o o t h and 1-5 ram, respectively. The tool geometry was S N G 433 in both machining tests. RESULTS AND DISCUSSION In gas pressure sintering the initial densification behaviour of a powder mixture determines the time-temperature-pressure schedule necessary to achieve dense, homogeneous microstructure. The results of two different experiments are shown in Table 1. The sintering was conducted at 1730°C for 1 hour at 20 MPa in both cases. If the pressure was applied before the closed porosity state was reached it resulted in low density and low strength (B). If the same material is sintered in a proper schedule (A), high density and much better mechanical properties
x Co LL
u~ co C ~D L
©
I 500
400
-10.5
70
8'O
9'O
Silicon n[tride (%)
8.5 1O0
Fig. 3. The dependence of hardness and flexural strength on composition for materials sintered at 1800°C, 1/2 h, 20 MPa. are obtained. The microstructure of both materials can be seen in Fig. 2. In the first case several pores of different sizes can be seen which were closed during sintering, however the material did not densify properly due to the high pressure gas remaining in the structure. It resulted in low density and therefore low strength. In the second case a dense and homogeneous microstructure developed and the strength of the material was higher. It is important to apply the high pressure after pore closure, but the high density in itself does not ensure the required properties. The extent to which the hardness and strength of gas pressure sintered silicon nitride base ceramics depends on the silicon nitride content is shown in Fig. 3. The sintering was conducted at 1800°C for 0"5 h at 20 MPa, and for every composition a high density was obtained. Increasing the a m o u n t of the liquid phase (decreasing the silicon nitride content) resulted in
e
J
Lm
(a)
(b)
Fig. 2. The microstructure of gas pressure sintered materials (pressure is applied (a) after pore closure (b) before pore closure).
124
E. Besenyei, R. Porat
Table 2. The properties of silicon nitride materials with different composition after gas pressure sintering
Silicon nitride %
Flexural strength MPa
Hardness GPa
Fracture toughness MPa*m 1/2
~t/(~ + fl) %
90 87 84 76-5
786 782 586 561
16'2 16.1 18'4 19.9
5'0 4-9 4.3 4.6
1 6 26 71
Table 3. The properties of the silicon nitride inserts prepared by three different methods Properties
Hotpressed
Density g/cm3 Hardness GPa Flexural strength MPa
3-30 16 800
0.5
Gas pressure- Pressureless sintered sintered
3"24 16.2 786
3'15 17"9 547
0.4
PL
0.4
0.3
OP
r-..O.3
E Eo.2
E E '-~o.2 >.
>
0.1
0.1
0.0
f
4-
8 Time
112 (min)
116
20
Fig, 4, The flank wear of silicon nitride cutting tool inserts
0.0
o
1'0
Number
2'0
of
3'0
40
Posses
prepared by pressureless sintering (PL), gas pressure sintering (GP) and hot-pressing (HP). Turning conditions: cutting speed 400 m/min, depth of cut 2 mm, feed 0'2 mm/rev.
Fig. 5. The flank wear of the major cutting edge (Vbh)of inserts prepared by pressureless sintering (PL), gas pressure sintering (GP) and hot-pressing (HP). Milling conditions: cutting speed 1176 m/min, feed 0-33 mm/tooth, depth of cut 1'5 mm.
lower strength and higher hardness. The higher alpha sialon content found in the latter case may be responsible for the higher hardness. The properties of the material are influenced by varying the time-temperature-pressure schedule. In Table 2 the characteristics achieved for the different compositions are shown. The time-temperature-pressure schedules were chosen in accordance with the compositions. The flexural strength o f the material can be increased considerably if the sintering parameters are chosen properly. As can be seen from the data presented, either high strength with moderate hardness or high hardness with moderate strength go together. In most cases there is a compromise between these characteristics. In Table 3 the hardness, the fractural strength and the density o f silicon nitride inserts prepared by the three different methods are presented. Similar properties were achieved for the hot-pressed and the gas pressure sintered material, while the pressureless sintered material has lower strength, lower density and higher hardness. The fracture toughness, measured by 'the single edge notch in bending' method 8 was 6"5 M P a * m 1/2 for
the hot-pressed material. In indentation fracture toughness measurement we found almost identical values (within 0"5 M P a * m 1/2) for the three different materials. In cutting applications the cost for the total operation of material removal depends on the efficiency of the tool (the ratio of the removed material and the total tool cost). The hardness of the tool is considered decisive regarding wear. On the other hand, the time during which the tool can operate without breakage is also important. The breakage o f different modes depends on the strength and the fracture toughness, which is especially critical for dynamic loading. Inserts with low wear and long tool life can be prepared by compromising between characteristics. This compromise depends on the mode of cutting (turning or milling) as well as on the parameters of machining (speed, feed etc.). The performance of the inserts prepared by the three different methods is shown in Fig. 4, Fig. 5 and Fig. 6. As can be seen in Fig. 4, the cutting tool inserts prepared by hot-pressing showed the slightest wear, while the highest wear was found in the pressureless
Silicon nitride inserts prepared by gas pressure sintering 0.5
Pj
0.4
E 8 gO.2 :> 01
0.0
1'0
Nbmber
2'0
40
50
of Posses
Fig. 6. The flank wear of the secondary cutting edge (V~,,) of inserts prepared by pressureless sintering (PL), gas pressure sintering (GP) and hot-pressing (HP). Milling conditions: cutting speed 1176 m/rain, feed 0-033 mm/tooth, depth of cut 1.5 mm.
125
fracture toughness properties. Both the very high cutting speed and the large feed used in milling represent such unfavourable conditions, that the materials which are able to withstand them should be considered excellent cutting tool materials. A similar order in wear was found in the turning test. The lowest belonged to the hot-pressed material and the highest wear to the pressureless sintered material. There was a significant difference between V~,h and V,~., especially for the inserts produced by pressureless sintering. Though the hardness of this material was relatively high it has the potential to become soft at the test temperature, due to its high glassy phase content. The wear of the gas pressure sintered inserts was close to that of the hot-pressed ones, showing that a similar performance can be achieved by a less expensive manufacturing method. CONCLUSION
F radial
F axial
Fig. 7. Definition of V.. and V,,h. V,,. is the wear of the major cutting edge parallel to the cutter axis, V~,,l is the wear of the secondary cutting edge.
sintered ones. This may be due to the higher amount of glassy phase. Though the wear of the inserts prepared by gas pressure sintering is slightly higher than that of the hot-pressed ones, it is acceptable for an economical cutting tool material. The results of the milling tests are shown in Figs 5 and 6. The definition of the quantities Vu, and Vb, (which are characteristic of wear) can be seen in Fig. 7. Vbh is the wear effected by the radial forces, while Vb,, is effected by the axial forces. V,,1 is usually higher than V~,h because of the severe plastic deformation underneath the insert, the consequence of which is a local temperature increase. The high Vb,, value points to a material in which the glassy phase becomes soft at the machining temperature. All the inserts (independent of the processing technology) fully ran the 30 passes without breakage, giving a total tool life of 45 minutes. This means that all of the material shows high dynamic
Silicon nitride inserts were prepared by three different methods. The dependence of the properties on the composition, as well as on the processing parameters of gas pressure sintering was investigated. It was shown that the flexural strength of the material could be increased considerably through careful choice of sintering parameters. The performance of the silicon nitride inserts prepared by pressureless sintering and by gas pressure sintering, was similar to that of the hotpressed inserts both in turning and milling, although a characteristic order in wear was observed. The lowest wear belonged to the hot-pressed inserts, the highest to the pressureless sintered inserts. There was only a slight difference in wear between the gas pressure sintered and the hot-pressed inserts, indicating the advantage of gas-pressure sintering. All the silicon nitride inserts examined in the machining tests were able to withstand highly unfavourable conditions without breakage, thus highlighting their excellent quality in terms of cutting tool materials. ACKNOWLEDGEMENTS The authors are especially grateful to P. Arat6 for critical discussions and valuable comments throughout the preparation of this paper. The experimental work of P. Arat6, F. Weber and A. Kele from the Research Institute of Technical Physics is greatly appreciated. We are also grateful for the technical assistance and dedication received from the machining centers at Iscar Ltd., Tefen, Israel and from the faculty of engineering at the Israel Institute
E. Besenyei, R. Porat
126
of Technology. Finally we would like to thank l~. Hajdu, M. M6szfiros and J. Zsoldos for their technical assistance.
REFERENCES 1. Ziegler, G., Heinrich, J. & W6tting, G., J. Mat. Sci. 22 (1987) 3041-86. 2. Layyous, A. A., Greil, P. & Petzow, G., Proc. 12th Int. Plansee Seminar, eds. H. Bildsteinn & H . M . Ortner. Reutte, 1989, 63745.
3. Agranov, D., Proc. 12th Int. Plansee Seminar, eds. H. Bildsteinn & H. M. Ortner. Reutte, 1989, 647-659. 4. Fritsch, A., Kaysser, W. A. & Petzow, G., Proc. of P M '90. The Institute of Metals, London, 1990, 237 50. 5. 'The strive cost reductions-all about sinter-HIP': Powder Met. Int., 20 (1988) 32. 6. Arat6, P., Besenyei, E., Kele, A. & W6ber, F., Science of Sintering: New Directions for Materials Processing and Microstructural control. Eds. P. Uskokovic, H. Palmour I I I & R. M. Spriggs. Plenum Publishing Corp., New York, 429-37. 7. Gazzara, C. P. & Messier, D. R., Ceram. Bull., 56 (1977) 777-80. 8. Anderson, R. M., Adv. Mat. Proc., 135 N3 (1989) 31-6.
Performance of Silicon Nitride Cutting Tool Inserts Prepared by Gas Pressure Sintering Elisabeth Besenyei Research Institute for Technical Physics of HAS, Budapest, Hungary
& Reuven Porat I S C A R Ltd., M a a l o t , Israel (Received 14 M a y 1992; accepted 6 July 1992)
Abstract: Hot-pressed silicon nitride inserts have been used for several years in
order to obtain the best results in machining cast iron. Nevertheless, extensive research is directed towards the development of new, less expensive preparation methods of silicon nitride base materials. One of the most promising new methods for the preparation of ceramic materials of high performance is gas pressure sintering, by which inserts with complicated shapes can be prepared. Cold-pressed and gas pressure sintered cutting tool inserts were prepared from silicon nitride and additives. The effects of composition, oxide content, structure and sintering parameters on the mechanical properties of gas pressure sintered silicon nitride materials were examined. Cutting tool inserts prepared either by hot pressing, pressureless or gas pressure sintering were tested in the turning and milling of gray cast iron. The performance of gas pressure sintered inserts was comparable with that of hot pressed inserts.
INTRODUCTION
individually from the desnified bulk material, in the latter case encapsulation must be used which increases costs. Pressureless sintering, using a processing route similar to the manufacturing of hardmetal cutting inserts, has been developed in order to avoid postcutting or the expense of encapsulation. It involves cold pressing to net shape with subsequent dewaxing and sintering. If sufficient amount of sintering aids are present, densification can be achieved by pressureless sintering. The additives are used to promote the formation of a liquid phase with low viscosity. In this case, silicon nitride ceramics can be sintered at a relatively low temperature where no decomposition occurs. The presence of additives leads to the formation of grain boundary phases which may affect the mechanical properties, especially at elevated temperatures. 2 In recent years new manufacturing methods have been developed for advanced ceramics. Of these, gas
Due to the unique combination of the chemical, physical and mechanical properties of silicon nitride based ceramics, they are among the most promising material with a wide range of applications at elevated temperatures. To overcome the difficulties arising from the covalent character of the Si-N bond and the decomposition of the material at high temperature, several pressure-applying processing methods have been developed for manufacturing silicon nitride ceramics, x Hot-pressed silicon nitride inserts have been used for several years in machining gray cast iron. Hot isostatic pressing (HIP) of encapsulated green compacts is an alternative processing route which ensures the hot-pressed material properties. However from an economic point of view they are not ideal processes since complicated shapes are difficult and expensive to manufacture. In the former case they have to be cut 121
Refractory Metals & Hard Materials 02634368/92 $05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain.
E. Besenyei, R. Porat
122
-'x ~
/Pressure gradient
sented in this paper. A comparison of the properties of cutting inserts prepared by different manufacturing methods, including hot pressing, gas pressure and pressureless sintering, as well as their performance in cutting and milling are shown. EXPERIMENTAL PROCEDURE
T i _
,
~
/
/
r
l
\!
IJ
-
~ _ _-L__d
LDewax~ng
--~' I~
~
~'7
Presintering
,
,Fast c o q l i n g
: l ~ "~x"
~ ~--Venting
Cooling "
Fig. 1. A typical time-temperature-pressure schedule for the new sintering processes?
pressure sintering is one of the most suitable for the preparation of silicon nitride? In HIP treatments a pressure of 100-200 MPa is necessary, while in gas pressure sintering pressures as low as 50 MPa or less are enough to achieve a high density. In the literature several different names such as gas pressure sintering, sinter-HIP and overpressure sintering are used for the same process. This involves a combination of sintering at appropriate temperature and low pressure, and the subsequent application of isostatic gas pressure in one cycle in the same equipment. During the process the material is pre-sintered at low pressure until a closed porosity is reached, after which treatment of high pressure results in complete densification. In this case no encapsulation is needed. In the second part of the process, at high pressure, the residual macropores are eliminated either by a mixed flow of both the liquid phase and the grains or by the flow of the liquid phase alone. The latter case may result in soft spots and inhomogeneous microstructure 4 (indicated by lower flexural strength). The nature of the flow can be influenced by the composition and the sintering schedule. The time-temperaturepressure relation during the process is illustrated in Fig. 1.~ As materials with different properties can be obtained depending on the time-temperaturepressure schedule, it is very important to reveal the relationship between composition and sintering parameters as well as between the properties and the performance of gas pressure sintered material in machining tests. The results of gas pressure sintered silicon nitride base ceramics of different compositions are pre-
Sample preparation
The gas pressure and pressureless sintered samples were prepared in the Research Institute for Technical Physics of HAS, while the hot-pressed samples were prepared by Iscar Ltd. For gas pressure and pressureless sintering, the samples were compacted by dry cold pressing from different powder mixtures containing 76-90 wt % silicon nitride and additives. Gas pressure sintering was carried out in an A B R A SHIRP 8-16-200 apparatus in high purity nitrogen of 10-20 MPa. The details of pressureless sintering are given in Ref. (6). T h e ' I S C A N I T E ' type hot-pressed silicon nitride inserts were prepared by the dry milling and dry pressing of discs of 20 m m diameter. A reaction bonded process was then followed by hot pressing in an induction furnace. Density was determined by the Archimedes method. The modulus of elasticity and the 4 point flexural strength were measured at room temperature with a span of 40 x 20 mm. In several cases 3 point flexural strength with a span of 30 m m were also measured. Hardness and fracture toughness (K~c) were determined using a Vicker's diamond indenter on a polished surface. Microstructure of the fractured surface was investigated by scanning electron microscopy. X-ray diffraction analysis was also carried out to determine the alpha prime phase content of silicon nitride using the Gazzara and Messier method. 7 Machining test
Turning and milling were selected for the machining tests as they cover a wide range of applications and usually require different cutting tool grades. Inserts prepared by the different methods were surface ground, sliced (if necessary), periphery ground and edge chamfered. Turning tests were carried out on cast iron at a speed of 4 0 0 m / m i n , which is considered an economical high cutting speed. It is double the cutting speed used for coated cemented carbide. The depth of the cut was 2 m m and a 0-2 m m / r e v feed was used. Feritic Perlitic Gray Cast Iron with
Silicon nitride inserts prepared by gas pressure sintering
1000-
Table 1. Properlies after gas pressure sintering (pressure is
applied before (B) or after (A) pore closure) ©
Material
B A
Density g/cm a
3"09 3-24
Flexural strength MPa
Modulus q/ elasticity GPa
Hardness GPa
457 763
230 266
10"7 15"4
£1_ 900 Z 800x_
123
-20.5
.... \~rl@ss
-18.5 ~ 0
[3_ -16.5 0
4~ Oh C Co L
L/3
700
4.5
(/3
-12.5
600-
% a composition of 3.21 C, 2.63 Si, 0"7 Mn, 0.2 P, 0"08 S was machined in the turning test. The milling tests were carried out at the cutting speed of 1176 m/rain, four times higher than the speed used for coated cemented carbide cutting tools. Low chromium gray iron with a composition of 3"25 C, 2"2 Si, 0"3 Cr, 0"7 Mn, 0-12 P, 0"15 S and with the hardness of 220 H B N was machined. The feed and depth of the cut were 0.33 r a m / t o o t h and 1-5 ram, respectively. The tool geometry was S N G 433 in both machining tests. RESULTS AND DISCUSSION In gas pressure sintering the initial densification behaviour of a powder mixture determines the time-temperature-pressure schedule necessary to achieve dense, homogeneous microstructure. The results of two different experiments are shown in Table 1. The sintering was conducted at 1730°C for 1 hour at 20 MPa in both cases. If the pressure was applied before the closed porosity state was reached it resulted in low density and low strength (B). If the same material is sintered in a proper schedule (A), high density and much better mechanical properties
x Co LL
u~ co C ~D L
©
I 500
400
-10.5
70
8'O
9'O
Silicon n[tride (%)
8.5 1O0
Fig. 3. The dependence of hardness and flexural strength on composition for materials sintered at 1800°C, 1/2 h, 20 MPa. are obtained. The microstructure of both materials can be seen in Fig. 2. In the first case several pores of different sizes can be seen which were closed during sintering, however the material did not densify properly due to the high pressure gas remaining in the structure. It resulted in low density and therefore low strength. In the second case a dense and homogeneous microstructure developed and the strength of the material was higher. It is important to apply the high pressure after pore closure, but the high density in itself does not ensure the required properties. The extent to which the hardness and strength of gas pressure sintered silicon nitride base ceramics depends on the silicon nitride content is shown in Fig. 3. The sintering was conducted at 1800°C for 0"5 h at 20 MPa, and for every composition a high density was obtained. Increasing the a m o u n t of the liquid phase (decreasing the silicon nitride content) resulted in
e
J
Lm
(a)
(b)
Fig. 2. The microstructure of gas pressure sintered materials (pressure is applied (a) after pore closure (b) before pore closure).
124
E. Besenyei, R. Porat
Table 2. The properties of silicon nitride materials with different composition after gas pressure sintering
Silicon nitride %
Flexural strength MPa
Hardness GPa
Fracture toughness MPa*m 1/2
~t/(~ + fl) %
90 87 84 76-5
786 782 586 561
16'2 16.1 18'4 19.9
5'0 4-9 4.3 4.6
1 6 26 71
Table 3. The properties of the silicon nitride inserts prepared by three different methods Properties
Hotpressed
Density g/cm3 Hardness GPa Flexural strength MPa
3-30 16 800
0.5
Gas pressure- Pressureless sintered sintered
3"24 16.2 786
3'15 17"9 547
0.4
PL
0.4
0.3
OP
r-..O.3
E Eo.2
E E '-~o.2 >.
>
0.1
0.1
0.0
f
4-
8 Time
112 (min)
116
20
Fig, 4, The flank wear of silicon nitride cutting tool inserts
0.0
o
1'0
Number
2'0
of
3'0
40
Posses
prepared by pressureless sintering (PL), gas pressure sintering (GP) and hot-pressing (HP). Turning conditions: cutting speed 400 m/min, depth of cut 2 mm, feed 0'2 mm/rev.
Fig. 5. The flank wear of the major cutting edge (Vbh)of inserts prepared by pressureless sintering (PL), gas pressure sintering (GP) and hot-pressing (HP). Milling conditions: cutting speed 1176 m/min, feed 0-33 mm/tooth, depth of cut 1'5 mm.
lower strength and higher hardness. The higher alpha sialon content found in the latter case may be responsible for the higher hardness. The properties of the material are influenced by varying the time-temperature-pressure schedule. In Table 2 the characteristics achieved for the different compositions are shown. The time-temperature-pressure schedules were chosen in accordance with the compositions. The flexural strength o f the material can be increased considerably if the sintering parameters are chosen properly. As can be seen from the data presented, either high strength with moderate hardness or high hardness with moderate strength go together. In most cases there is a compromise between these characteristics. In Table 3 the hardness, the fractural strength and the density o f silicon nitride inserts prepared by the three different methods are presented. Similar properties were achieved for the hot-pressed and the gas pressure sintered material, while the pressureless sintered material has lower strength, lower density and higher hardness. The fracture toughness, measured by 'the single edge notch in bending' method 8 was 6"5 M P a * m 1/2 for
the hot-pressed material. In indentation fracture toughness measurement we found almost identical values (within 0"5 M P a * m 1/2) for the three different materials. In cutting applications the cost for the total operation of material removal depends on the efficiency of the tool (the ratio of the removed material and the total tool cost). The hardness of the tool is considered decisive regarding wear. On the other hand, the time during which the tool can operate without breakage is also important. The breakage o f different modes depends on the strength and the fracture toughness, which is especially critical for dynamic loading. Inserts with low wear and long tool life can be prepared by compromising between characteristics. This compromise depends on the mode of cutting (turning or milling) as well as on the parameters of machining (speed, feed etc.). The performance of the inserts prepared by the three different methods is shown in Fig. 4, Fig. 5 and Fig. 6. As can be seen in Fig. 4, the cutting tool inserts prepared by hot-pressing showed the slightest wear, while the highest wear was found in the pressureless
Silicon nitride inserts prepared by gas pressure sintering 0.5
Pj
0.4
E 8 gO.2 :> 01
0.0
1'0
Nbmber
2'0
40
50
of Posses
Fig. 6. The flank wear of the secondary cutting edge (V~,,) of inserts prepared by pressureless sintering (PL), gas pressure sintering (GP) and hot-pressing (HP). Milling conditions: cutting speed 1176 m/rain, feed 0-033 mm/tooth, depth of cut 1.5 mm.
125
fracture toughness properties. Both the very high cutting speed and the large feed used in milling represent such unfavourable conditions, that the materials which are able to withstand them should be considered excellent cutting tool materials. A similar order in wear was found in the turning test. The lowest belonged to the hot-pressed material and the highest wear to the pressureless sintered material. There was a significant difference between V~,h and V,~., especially for the inserts produced by pressureless sintering. Though the hardness of this material was relatively high it has the potential to become soft at the test temperature, due to its high glassy phase content. The wear of the gas pressure sintered inserts was close to that of the hot-pressed ones, showing that a similar performance can be achieved by a less expensive manufacturing method. CONCLUSION
F radial
F axial
Fig. 7. Definition of V.. and V,,h. V,,. is the wear of the major cutting edge parallel to the cutter axis, V~,,l is the wear of the secondary cutting edge.
sintered ones. This may be due to the higher amount of glassy phase. Though the wear of the inserts prepared by gas pressure sintering is slightly higher than that of the hot-pressed ones, it is acceptable for an economical cutting tool material. The results of the milling tests are shown in Figs 5 and 6. The definition of the quantities Vu, and Vb, (which are characteristic of wear) can be seen in Fig. 7. Vbh is the wear effected by the radial forces, while Vb,, is effected by the axial forces. V,,1 is usually higher than V~,h because of the severe plastic deformation underneath the insert, the consequence of which is a local temperature increase. The high Vb,, value points to a material in which the glassy phase becomes soft at the machining temperature. All the inserts (independent of the processing technology) fully ran the 30 passes without breakage, giving a total tool life of 45 minutes. This means that all of the material shows high dynamic
Silicon nitride inserts were prepared by three different methods. The dependence of the properties on the composition, as well as on the processing parameters of gas pressure sintering was investigated. It was shown that the flexural strength of the material could be increased considerably through careful choice of sintering parameters. The performance of the silicon nitride inserts prepared by pressureless sintering and by gas pressure sintering, was similar to that of the hotpressed inserts both in turning and milling, although a characteristic order in wear was observed. The lowest wear belonged to the hot-pressed inserts, the highest to the pressureless sintered inserts. There was only a slight difference in wear between the gas pressure sintered and the hot-pressed inserts, indicating the advantage of gas-pressure sintering. All the silicon nitride inserts examined in the machining tests were able to withstand highly unfavourable conditions without breakage, thus highlighting their excellent quality in terms of cutting tool materials. ACKNOWLEDGEMENTS The authors are especially grateful to P. Arat6 for critical discussions and valuable comments throughout the preparation of this paper. The experimental work of P. Arat6, F. Weber and A. Kele from the Research Institute of Technical Physics is greatly appreciated. We are also grateful for the technical assistance and dedication received from the machining centers at Iscar Ltd., Tefen, Israel and from the faculty of engineering at the Israel Institute
E. Besenyei, R. Porat
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of Technology. Finally we would like to thank l~. Hajdu, M. M6szfiros and J. Zsoldos for their technical assistance.
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