NEW Si3N4 CERAMIC CUTTING TOOL: DEVELOPMENT AND APPLICATION
ON GREY CAST IRON
J.V.C. Souza1, C. Santos, 2, M. V.Ribeiro3, J.P.B. Machado1, M. C. A.Nono1, O.M.M.
Silva4
1
INPE - Av. dos Astronautas, 1.758, S. J. Campo s - SP, CEP. 12245-970, Brazil
2
FAENQUIL-DEMAR – Polo Urbo Industrial, Gleba AI-6, s/n, Lorena - SP, CEP. 12600-000, Brazil
FEG-UNESP – Av.Ariberto Ferreira da Cunha, 333, Guaratinguetá – SP, CEP. 12516-410, Brazil
3
4
CTA-IAE/AMR - Pça. Marechal do Ar Eduardo Gomes, 50, S. J. Campo s - SP, CEP. 12228-904, Brazil
vitor@las.inpe.br
Abstract
Machining, a major manufacturing process, plays a key role in overall manufacturing
costs. The development of new ceramic cutting tools with unique chemical and mechanical
properties is essential for increasing metal removal rates, extending tool life and machining
exceptionally hard materials. When properly utilized, these tools could reduce machining
costs and increase productivity. In this work, Si3N4 ceramic cutting tool inserts were made
using powder additives that were pressed and sintered at 1850oC. Physical and mechanical
properties were evaluated and subsequent machining studies were conducted on a grey cast
iron workpiece to evaluate the tool’s performance.
Keywords: Si3N4; Ceramics; New cutting tool; Flank wear; Roughness.
* Corresponding author. Tel.: +55 (12) 3945-6679, 39456989, FAX: +55 (12) 3945-6717
E-mail address: vitor@las.inpe.br (J.V.C. Souza)
1. Introduction
It is widely recognized that machining operation performance, as measured by toollife time, forces and surface finish, must be improved to lower the overall costs of
machining operations [1]. Through research and development, including the investigation
of new wear-resistant materials and tool geometries, researchers have sought to continually
improve the technological performance of machining operations [2]. The advance of
ceramic tool materials, which are increasingly used in machining operations, is one of the
most important developments in the past decades [3]. While ceramic tool materials can
significantly increase tool-life time and hence reduce production times and costs, their
extremely low fracture toughness has limited their applications [4].
Advances in ceramic composites have resulted in the emergence of newer materials
[5]. Control of microstructure has led to the development of ceramic composite cutting tool
materials like titanium-carbide-added alumina, zirconia-toughened alumina and siliconcarbide whisker-reinforced alumina that are successfully used for cutting tool applications
[6 and 7]. Apart from binary ceramic–ceramic composites, it is possible to create advanced
ternary ceramic–ceramic composites [8]. Aluminum, yttrium and cerium oxide are good
candidates for composite materials since they have exceptionally good properties (e.g., high
hardness, chemical inertness, high melting point and wear resistance [9, 10, 11 and 12]).
Recently Si3N4-coated ceramic cutting tools have been said to significantly lengthen
tool-life time, enabling components to be machined at higher “economic” speeds and
reducing the forces and power involved due to the lower frictional coefficients on the rake
face [13].
The present work investigates a new development in ceramic cutting tool inserts with
Si3N4-based ceramic. Two different Si3N4-based compositions, SNYA1 and SNYA2, were
fabricated via N2 pressing technology to obtain different compositions. After characterizing
the new ceramic-tool materials, cutting performance was tested.
2. Heat generation in machining
In the metal-cutting process, the tool cuts by overcoming the shear strength of the
workpiece material. This generates a large amount of heat in the workpiece, resulting in
highly localized thermo mechanically-coupled deformation in the shear zone. High
temperatures in the cutting zone considerably affect the stress–strain relationship, the
fracture and flow of the workpiece material. Generally, increasing temperature decreases
the strength of the workpiece material and thus increases its ductility. It is now assumed
that nearly all of the work done by the tool and the energy input during the machining
process are converted into heat. The heat is generated in three main regions during the
cutting process: a) Heat is generated in the primary deformation zone due to plastic work
done on the shear plane, resulting in very high temperatures and thereby softening the
material and allowing greater deformation. b) Heat is generated in the secondary
deformation zone due to work done by deforming the chip and in overcoming the sliding
friction at the tool/chip interface. c) Heat is generated in the tertiary deformation zone, at
the tool/workpiece interface, due to the work done to overcome friction that occurs because
of rubbing between the tool flank face and the newly machined surface of the workpiece.
Heat generation and temperatures in the primary and secondary zones highly depend on the
cutting conditions, while heat generation in the tertiary zone is strongly influenced by the
tool flank wear. In summary, the power consumption and heat generation in metal-cutting
processes are dependent on a combination of the physical and chemical properties of the
workpiece material, the cutting-tool materials and geometry and the cutting conditions [14].
3. Grey cast iron properties
Although the relative composition of the components of grey cast iron can be
variable, the mechanical properties of the workpiece should be stable [15] to avoid
problems during the subsequent machining process. Usually, the foundryman chooses the
chemical composition based on the needs of his client and the nature of his particular
foundry [16 and 17]. Grey cast iron is an important material for the automotive industry,
and its microstructure usually consists of flake graphite and a matrix of pearlite and/or
ferrite, which result in its mechanical properties and machining performance [18]. Grey cast
iron has a pearlite matrix, and its chemical composition and mechanical properties are
given in Table 1.
4. Wear behavior
The main wear mechanisms in ceramic cutting tools are abrasion, adhesion, diffusion,
plastic deformation and fracture. Cutting tools are subjected to highly localized stresses,
high temperatures, sliding of the chip along the tool rake face and sliding of the machined
surface along the tool flank. These conditions induce tool wear, which, in turn, adversely
affects the tool’s life span, the quality of the machined surface and dimensional accuracy.
5. Experimental procedure
Two different Si3N4-based compositions were prepared using the following high
purity starting powders (wt. %) to obtain different ceramic cutting tools: SNYA1, 85.0 αSi3N4– 5.0 Y2O3–10.0 AlN (H. C. Starck) and SNYA2, 80.0 α-Si3N4–8.0 Y2O3–12.0 AlN
(H. C. Starck. The starting powders were weighed and milled in water-free isopropyl
alcohol for 8 hr in an agate jar using agate milling media. The mixed powders were dried
and sieved. The green bodies were fabricated by uniaxial pressing under 50 MPa pressure
and subsequent isostatic pressing under 300 MPa pressure. After compaction, samples were
15.86 x 15.86 x 6.5 mm with a green density of 60 % by geometric methods. Before
sintering, 70 % Si3N4 + 30 % BN were assembled as a powder bed, and the samples were
placed in a furnace with a graphite heating element (Thermal Technology Inc. type 10004560-FP20) in a nitrogen atmosphere. The heating rate was 25 oC/min to a maximum
sintering temperature of 1850 oC, with a holding time of 2 hours. The cooling rate was
identical to the heating rate. The density of the samples was measured via the Archimedes
method, and the weight loss was determined before and after sintering measurements. The
surfaces of the sintered samples were removed (at least 2 mm), and then the phase
composition was analyzed by X-ray diffraction using Cu-K radiation and a scanning
speed of 0.02 deg/s. Microstructure characterization was performed with a scanning
electron microscope (SEM). The smoothed and polished samples for SEM were chemically
etched in a NaOH : KOH mixture (1:1 at 500 oC/10 minutes) to reveal the microstructure.
Hardness was determined using Vicker´s indentations under an applied load of 20 N
for 30 s. For statistical reasons, 20 indentations were made in each sample. The fracture
toughness was determined by the measurement of crack length created by indentations.
Fracture toughness values were calculated using a relation proposed by Evans et al., valid
for Palmqvist-type cracks [20 and 21]:
KIC = 0.16(E/H)1/2.F.b-3/2
(1)
Where: KIC = fracture toughness [MPa.m1/2]; E = Young’s modulus of the material
[GPa]; HV = Vickers hardness [GPa]; b = crack size [m] and F = applied load [N].
6. Cutting performance
All experiments were carried out on a computer numerically controlled (CNC) lathe
(Romi, Mod. Centur 30D) under dry cutting conditions. The sintered Si3N4 cutting tools
were cut and ground to make SNGN120408 (12.7 mm×12.7 mm, 4.76 mm thickness,
0.8 mm nose radius and 0.2 mm×20° chamfer). A tool holder of CSRNR 2525 M 12CEA
type (offset shank with 15° [75°] side cutting-edge angle, 0° insert normal clearance and 25
mm×25 mm×150 mm) was used for the cutting experiments. Cutting performance was
evaluated by machining grey cast iron (iron composition with the best mechanical
properties).
Cutting tests were performed at cutting speeds of 150, 230, 400 m/min with a feed
rate of 0.30 mm/rev and a depth of cut of 1.0, 2.0 and 3.0 mm. The dimension of the work
material was about 105 mm in diameter and 300 mm in length. The wear of the tools was
determined by measuring the wear depth on the flank face by using a scanning electron
microscope (LEO-1450 VP). More than four points of the flank face were measured, and
the average was taken as a nominal flank wear depth. Flank wear of 0.3 mm (ISO 3685)
and the abrupt variation of Ra and Ry were used as the end of tool-life criteria. An infrared
pyrometer measured the tool/chip temperature without physical contact. The workpiece
surface roughness was measured using a micrometer (Mitutoyo Surftest 402 series 178).
7. Results and discussion
7.1. Sintering
The particle size distributions of both powder batches, SNYA1 and SNYA2, showed
that the average particle size found was approximately 0.48 – 0.62 μm, resulting in very
similar green densities in both green samples. The SNYA2 sample had 98.75 ± 0.16 %
relative density, and the SNYA1 sample had 97.92 ± 0.22 % relative density. The mass
losses during the sintering were about 2 % in both samples. The similar behavior in both
samples demonstrates that Y2O3/AlN sintering additives are good choices; they promote
similar sintering activity with slightly different concentrations. Thus, the sintering
parameters applied are adequate to produce high density ceramic cutting-tools.
7.2. X-ray difractometry (XRD)
β-Si3N4 was the main matrix phase present in sintered SNYA1 (Fig.1). The randomly
distributed grains of Si3N4 ceramic generated during N2-pressure sintering led to isotropic
mechanical and physical properties [22]. A Y2Si3O3N4 phase, identified in XRD spectra,
was formed by the following reaction:
Y2O3+Si3N4→Y2Si3O3N4
(2)
The X-ray diffraction pattern of SNYA2 (N2-pressure sintering at 1850 °C) is shown
in Fig. 2. As seen in Fig. 2, a majority α-SiAlON phase was identified, suggesting the
existence of a sintering-complete system. Fig. 2 shows the relationship between the phase
formation and the amount of additive reflected in an increase of mechanical properties as
the Y2Si3O3N4 phase disappeared. A higher sintering temperature (1850oC) and 20 %
additive effectively improved the mechanical properties and reduced the intergranular
phase. Therefore, samples with 15 and 20 % Y2O3/AlN sintering additives were completely
densified, which is in accord with the results presented by Santos et al.[9].
7.3. Microstructure and mechanical properties
As shown in Fig. 3, the microstructure of SNYA1 consisted of elongated β-Si3N4
grains randomly distributed in a matrix. The elongated β-Si3N4 grains were well developed
and had an irregular cross-section perpendicular to the growth direction. The β-Si3N4 grains
had an average grain size of about 4.5 μm and aspect ratios higher than 5.1 μm. This
microstructure demonstrates the efficiency of the gas-pressure sintering process [23 and
24]. The intergranular phase located between the grains was revealed after etching the
grain-boundary phase.
The effects of the amount of additives on the microstructure and morphology of a
Si3N4 ceramic cutting tool are shown in the micrograph of SNYA2 (sintered at
temperatures of 1850°C) in Fig 4. With less additive, most of the α-SiAlON grains were
equiaxed and small (Fig. 3), confirming the results of Fig. 1. As the amount of additive
increased, the volume fraction and the average quantity of α-SiAlON grains also increased.
In a previous report, the formation of the elongated α-SiAlON grains was attributed to a
reduction of the driving force for the nucleation [9]. These studies show that the formation
of elongated grains was directly related to the sintering parameters (i.e., time, temperature
and isothermal holding time) and to the characteristics of the starting powders [25].
In Fig.4, the average grain size changed to 3.8 μm with aspect ratios higher than 4.2
μm, thus confirming the formation of α-SiAlON with an elongated microstructure.
Next, we evaluated the relationship between material density and hardness. The
SNYA1 and SNYA2 compositions showed variations in hardness with changes in density.
As expected, the hardness of SNYA2 increased with increasing relative density. The
highest value for hardness (21.52 GPa) was found in the sample with the highest relative
density (98.75 %). Similar results for the variation of hardness with density were observed
in the SNYA1 sample, which showed decreases in hardness with decreases in relative
density.
It is interesting to note that elongated β-Si3N4 grains grew in the SNYA1
composition, suggesting that this composition self-reinforces and may display enhanced
toughness. In addition to composition, the amount of additive had an equally important role
in determining the growth and volume fraction of elongated grains. The SNYA2
composition showed similar changes in average grain aspect ratio and grain volume
fraction with 15 % and 20% additive (sintered at 1850°C); for both samples, the grain
aspect ratio decreased and the volume fraction of the grains increased.
Both the grain aspect ratio and elongated grain volume fraction affected fracture
toughness. In contrast, no definite relationship existed between fracture toughness and grain
diameter (grain width). However, the SNYA1 sample has a fracture toughness of
5.92 MPa m1/2, and the SNYA2 sample has a fracture toughness of 5.45 MPa m1/2. Fig. 3
and Fig. 4 show the changes in fracture toughness as a function of average grain diameter
(grain width). The high fracture toughness caused by a high degree of crack deflection is
consistent with large grains and high aspect ratios.
Despite the clear difference in grain morphology and toughness values, the SNYA2
sample displayed high hardness (>21 GPa). This high hardness resulted from the α-SiAlON
ceramic crystal structure in which the longer atomic stacking sequence (ABCD) increased
the slip resistance of the dislocations. β-Si3N4 and β-SiAlON had relatively low hardnesses
(15–16 GPa) due to their shorter atomic stacking sequence (AB).
7.4. Machining performance of α-SiAlON cutting tool
We analyzed the effect of wear on tool life at different cutting speeds. Wear generally
affects tool life, quality of the machined surface and dimensional accuracy, thereby
influencing the economics of cutting operations [26, 2]. In this work, we looked at different
types of wear at different cutting speeds. High cutting speeds had a decisive advantage
because the layer of graphite coating the surface of the tool resulted in an extremely low
friction coefficient and greater stability. It is well-known that reducing cutting forces is a
key way to improve tool life-time and wear behavior. When machining grey cast iron, the
low friction coefficient values started at elevated temperatures. During the cutting process,
the chip slid against the tool rake face at high speed, creating high cutting temperatures that
lubricated the surfaces with graphite and improved the tool performance and wear
resistance. As concluded by Sliney, H.E. (1982), graphite is a high-temperature lubricant
due to its low shear strength, reducing friction while the matrix endures the load [27]. The
microstructure of the α-SiAlON cutting tool was unaffected by the dissolution of the
graphite phase, which was uniformly distributed throughout the matrix (see Fig. 5
(micrograph)). This resulted in a prolonged tool life. As the friction coefficient value
dropped, cutting forces decreased and heat was continually generated at the tool/workpiece
interface, resulting in an overall reduction of wear. Machining conditions, therefore,
controlled friction with positive feedback. In synergy with the others coolants used during
some machining processes, a lubricant layer can significantly increase tool life time. We
concluded that the lubricant/protective graphite film on the tool surface insulated the
workpiece from the cutting tool surface. Thus, dry machining of grey cast iron was quite
efficient at high cutting speeds. Possibly, the coated tool surface enhanced mass transfer at
the start of the running-in stage that resulted in quick generation of a tribo-layer with a
suitable composition and microstructure. These self-adaptive features make α-SiAlON
ceramic tools attractive for high-temperature applications. As seen in Fig. 6 and 7 (Surface
roughness), the cutting speed clearly affected the friction coefficients at the tool/chip
interface. As discussed above, the friction coefficients showed a downward trend as cutting
speed increased due to the presence of graphite as a solid lubricant. Future studies of
cutting tools for grey cast iron should be based on an understanding of the friction-triggered
synergistic action of alloying elements present in the workpiece. Understanding the
complexity of friction, including an in-depth understanding of the self-organization
processes leading to better machining performance, is important for future dry machining
advances.
7.5. Wear behavior
The fractures in Fig 8 and Fig 9 were related to low speed and cutting depth, which
led to a variety of thermomechanical and thermochemical events. During turning, many
micro-constituents were transferred on the contacting surfaces. During low speed operation,
the high slope of the wear rate versus machining time plot indicated that cutting speed only
marginally influenced the wear rate in dry conditions due to severe cracking and the loss of
proper contact of the cutting tool with the workpiece material. As seen in Fig. 10 (Vb x Lc),
the stability of the α-SiAlON ceramic cutting tool was not favored during turning of grey
cast iron at low speeds. This was related to the cutting temperature, which was relatively
low at low cutting speeds (Fig. 11). Under these conditions, the graphite material was not
softened and released, and there was no reduction in friction coefficients at the tool/chip
interface. Therefore, lower cutting speeds resulted in an increased wear rate and possible
vibrations that could be attributed to the severity of the surface damage. Fig. 12 (v=400
m/min, ap=2.0 mm), Fig 13 (v=400, ap=3.0 mm) and Fig. 10 (Vb x Lc) again prove that as
the cutting speed increased, the cutting temperature increased monotonically (Fig. 11
(temperature)) and the graphite was softened, released and smeared on the tool rake and
flank face, decreasing the friction coefficient. High cutting speed led to reductions in wear
rate, reducing the possibility of tool cracking, therefore encouraged the lubricating phase
(graphite flakes) present within the workpiece to smear and form a stable lubricant film. As
seen in Fig. 8, Fig. 9 and Fig. 13, dark patches appeared on the rake face surface of the αSiAlON ceramic cutting tool. This tribo-induced layer formed in such cases was also
stabilized and contributed to improvements in wear performance and tool life-time. In Fig.
12 and Fig. 13, fracture and a slight flank wear was observed without any chipping. This
indicated that abrasive wear was the dominant wear mechanism. In Fig. 12 and 13, the
development of grooves and ridges in the direction of tool sliding against a newly machined
surface of the work piece is evident. The abrasive wear was due to crack development and
intersection caused by wear particles acting as small indenters on the tool face. The cutting
speed (v=400 m/min) on the flank surface, VB, remained fairly consistent over the course
of machining time, indicating that there were no wear transitions and the wear mechanism
was the same throughout the cutting process, as seen in Fig. 10. In agreement with
previous work, our results with new α-SiAlON cutting tools are unique for the machining
of grey cast iron under these conditions.
7.6. Surface finish
Similar behavior was observed during the machining tests, which showed a change
in average surface roughness using α-SiAlON ceramic tools under different cutting
conditions. The Ra and Ry values showed only a short increase at the beginning of the cut
(first 1,44 min parts to Vc=150 m/min), then started to decrease evenly below the initial
value recorded with a new tool. Fig. 6 and Fig. 7 present the changes in average surface
roughness for wear trials under different conditions. A micrometer (Mitutoyo Surftest 402
series 178) revealed a difference in slope versus the time machined. This was explained by
the layer of graphite coating the surface of the tool during machining, leading to an
extremely low friction coefficient. When higher cutting speeds were used, wear did not
appear as fast or as steeply as for the Vc=150 m/min test. This phenomenon was attributed
to the temperature of the chips, which was higher for higher speeds, leading to the softening
of the chip-edge sawing action. The four types of cutting conditions displayed different
effects on the workpiece surface roughness. Ra and Ry decreased almost constantly over the
course of machining, with a tendency to reach the plateau stage towards the end of the tool
life at higher cutting speeds. Irregular behavior was observed for Vc=150 m/min. Fig. 6 and
Fig. 7 show irregular changes in roughness at 14.33 min of machining. In this case, the tool
broke on the rake face, demonstrating that the increases of Ra and Ry could have been due
to the adhesion of micro-particles from the α-SiAlON ceramic tool. It is possible that after
the tool broke, the cutting edge was still fresh, and the topography of the machined surface
was close to the geometrical shape of the turning process. After a few minutes of cutting, a
rapid decrease of Ra and Ry was recorded. This decrease was explained by the new
accommodation of the flank face and rake face. The irregularities of the cutting edge
caused an uneven distribution of peaks and valleys in the surface profile. This behavior was
confirmed by a repeated test at Vc= 400 m/min; ap=2.00 mm and Vc= 400 m/min; ap=3.00
mm, which presented lower surface roughness.
8. Conclusion
A new ceramic cutting tool insert has been developed in our laboratory that displayed
promising machining performance on a grey cast iron workpiece:
1- The α-SiAlON ceramic tool was clearly advantageous at high cutting speeds. A
reduction in the wear rate at high cutting speeds suppressed cracking tendencies in the
cutting tool due to a layer of graphite coating the tool surface that produced extremely low
friction coefficients. This led to better stability between the cutting tool and the workpiece.
2- The performance of the α-SiAlON ceramic tool used for machining grey cast iron
should be based on the understanding of the friction-triggered synergistic action of alloying
elements that are present in the workpiece.
3- When machining at low speeds, a high wear rate was observed, indicating severe
cracking. This resulted in the loss of proper contact between the cutting tool and the
workpiece material, causing irregular behavior of Ra and Ry. The tool breaking allowed
micro particles to adhere at the tool/workpiece interface.
4- These results were attributed to properties of the α-SiAlON ceramic cutting tool
insert and to the machining conditions.
Acknowledgement
The authors would like to thank CAPES and FAPESP for their financial support.
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List of captions for illustrations
Fig. 1. X-ray diffraction patterns of SNYA1.
Fig. 2. X-ray diffraction patterns of SNYA2.
Fig. 3. SEM micrograph of the sintered SNYA1 sample.
Fig. 4. SEM micrograph of the sintered SNYA2 sample.
Fig. 5. TEM micrograph of grey cast iron.
Fig. 6. Surface roughness (Ra) vs. machining time (t).
Fig. 7. Surface roughness (Ry) vs. machining time (t).
Fig .8. SEM micrographs of the wear profile for cutting speed v=150 m/min, depth of cut
ap=1.0 mm, feed rate f=0.30 mm/r, cutting time t=17 min.
Fig .9. SEM micrographs of the wear profile for cutting speed v=230 m/min, depth of cut
ap=2.0 mm, feed rate f=0.30 mm/r, cutting time t=10 min.
Fig. 10. Flank wear (Vb) vs. machining time (t).
Fig. 11. Temperature (T) vs. machining time (t).
Fig .12. SEM micrographs of the wear profile for cutting speed v=400 m/min, depth of cut
ap=2.0 mm, feed rate f=0.30 mm/r, cutting time t=13 min.
Fig .13. SEM micrographs of the wear profile for cutting speed v=400 m/min, depth of cut
ap=3.0 mm, feed rate f=0.30 mm/r, cutting time t=17 min.
Table 1. Chemical composition and mechanical properties of grey cast iron.