Surface and subsurface damage of reaction-bonded silicon carbide induced by electrical discharge diamond grinding

TLDR: In this paper, surface and subsurface damages induced by the interactions between EDM and diamond grinding during the EDDG of reaction-bonded silicon carbide (RB-SiC) were examined.

Abstract: Reaction-bonded silicon carbide (RB-SiC) ceramic, one of the best candidates for large optical mirrors, is difficult to machine because of its high hardness and brittleness. A hybrid process called electrical discharge diamond grinding (EDDG) exhibits potential for improving the machinability of RB-SiC by combining electrical discharge machining (EDM) and diamond grinding. However, this hybrid process leads to damages that differ from those in conventional processes owing to the simultaneous actions of EDM and diamond grinding. In the present study, surface and subsurface damages induced by the interactions between EDM and diamond grinding during the EDDG of RB-SiC were examined. The effect of the discharge energy was considered. The surface and subsurface topographies and microstructures were characterized via scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. The EDM and grinding zones exhibited distinctive surface topographies and different dominant material removal mechanisms. An increase in the discharge energy facilitated ductile removal of the material and decomposition of SiC. Thus, a thinner subsurface damage layer was obtained compared with that in the less-thermally affected zone. The decomposed C and material migration tended to increase with the discharge energy. Owing to the interactions between EDM and diamond grinding, the subsurface was a mixture of amorphous/crystalline C, polycrystalline/nanocrystalline SiC, and a crystalline SiC matrix.

Figures

Fig. 1 Schematic of the experimental setup. During EDDG of the RB-SiC, the iron-bonded diamond grinding wheel was connected to the positive pole of the power supply by a carbon brush fastened on the cover,

Fig. 12 Subsurface damage at different conditions. (a) In the less-thermally affected zone and at discharge energy of (b) U = 70 V, Pulseon = 10 μs, (c) U = 100 V, Pulseon = 50 μs, and (d) U = 140 V, Pulseon = 90 μs. However, an excessive discharge energy led to a large amount of thermal damage, along

Table 2 EDDG conditions

Fig. 14 STEM and EDS images of the cross-sectional specimen at different dischargeenergy levels. (a) STEM image at U = 70 V, Pulseon = 10 μs, (b) STEM image at U = 100 V, Pulseon = 50 μs, and (c) STEM image at U = 140 V, Pulseon = 90 μs; (a1)–(a4) distributions of C, Si, O, and Fe elements for (a), (b1)–(b4) distributions of C, Si, O, and Fe elements for (b), and (c1)–(c4) distributions of C, Si, O, and Fe elements for (c). At the middle discharge energy, high concentrations of C and O and low concentration of Si were observed in the damage layer, as shown in Fig. 14 (b1)–(b3). It indicates that

Fig. 16 HRTEM image and FFT patterns of region B. The nanostructure of the carbonaceous component at location 3 was surrounded by the nanostructures of the SiC phases at locations 1, 2, 4, and 5. The magnified HRTEM image shows C ribbons at

Fig. 8 3D surface morphology at different discharge-energy levels: (a) U = 70 V, Pulseon = 10 μs, (b) U = 100 V, Pulseon = 50 μs, and (c) U = 140 V, Pulseon = 90 μs, and (d)

Full text

This is a peer-reviewed, accepted author manuscript of the following research article: Rao, X., Zhang, F.,
Lu, Y., Luo, X., & Chen, F. (Accepted/In press). Surface and subsurface damage of reaction-
bonded silicon carbide induced by electrical discharge diamond grinding.
International Journal
of Machine Tools and Manufacture
. https://doi.org/10.1016/j.ijmachtools.2020.103564
1
Surface and subsurface damage of reaction-bonded silicon carbide
induced by electrical discharge diamond grinding
Abstract: Reaction-bonded silicon carbide (RB-SiC) ceramic, one of the best
candidates for large optical mirrors, is difficult to machine because of its high hardness
and brittleness. A hybrid process called electrical discharge diamond grinding (EDDG)
exhibits potential for improving the machinability of RB-SiC by combining electrical
discharge machining (EDM) and diamond grinding. However, this hybrid process leads
to damages that differ from those in conventional processes owing to the simultaneous
actions of EDM and diamond grinding. In the present study, surface and subsurface
damages induced by the interactions between EDM and diamond grinding during the
EDDG of RB-SiC were examined. The effect of the discharge energy was considered.
The surface and subsurface topographies and microstructures were characterized via
scanning electron microscopy, Raman spectroscopy, and transmission electron
microscopy. The EDM and grinding zones exhibited distinctive surface topographies
and different dominant material removal mechanisms. An increase in the discharge
energy facilitated ductile removal of the material and decomposition of SiC. Thus, a
thinner subsurface damage layer was obtained compared with that in the less-thermally
affected zone. The decomposed C and material migration tended to increase with the
discharge energy. Owing to the interactions between EDM and diamond grinding, the
subsurface was a mixture of amorphous/crystalline C, polycrystalline/nanocrystalline

This is a peer-reviewed, accepted author manuscript of the following research article: Rao, X., Zhang, F.,
Lu, Y., Luo, X., & Chen, F. (Accepted/In press). Surface and subsurface damage of reaction-
bonded silicon carbide induced by electrical discharge diamond grinding.
International Journal
of Machine Tools and Manufacture
. https://doi.org/10.1016/j.ijmachtools.2020.103564
2
SiC, and a crystalline SiC matrix.
Keywords: subsurface damage; electrical discharge diamond grinding; reaction-
bonded silicon carbide; phase transformation; material migration
1. Introduction
Reaction-bonded silicon carbide (RB-SiC) is one of the best candidate materials for the
fabrication of large-scale optical mirrors owing to its high specific stiffness and surface
shape stability [1]. However, its hardness makes it difficult to machine RB-SiC into the
desired shapes, e.g., planar, spherical, aspheric, and free-form surfaces [2], with high
efficiency via the conventional grinding process. Additionally, efficient grinding of
large-scale RB-SiC mirrors faces the problems of serious clogging and wear of the
metal-bonded grinding wheel [3,4]. To solve these problems, a hybrid process called
electrical discharge diamond grinding (EDDG) has been employed for grinding RB-
SiC by introducing electrical discharge machining (EDM) [5]. Owing to the effects of
the discharge-induced erosion on the metal bonds of the grinding wheel and the
workpiece, this process has the advantages of reduced grinding force [6], improved
material removal rate [7,8], and effective declogging of the wheel surface [9]. However,
EDM can generate a temperature of up to 8000-10000 ℃ at the center of the plasma
[10,11], causing melting/vaporization of the material and affecting the diamond
grinding of this process via thermal transmission. EDDG allows material removal
through the interaction between EDM and diamond grinding. Therefore, the surface
and subsurface damage is significantly different from that resulting from the individual

This is a peer-reviewed, accepted author manuscript of the following research article: Rao, X., Zhang, F.,
Lu, Y., Luo, X., & Chen, F. (Accepted/In press). Surface and subsurface damage of reaction-
bonded silicon carbide induced by electrical discharge diamond grinding.
International Journal
of Machine Tools and Manufacture
. https://doi.org/10.1016/j.ijmachtools.2020.103564
3
processes of EDM and diamond grinding.
Surface and subsurface damage has been widely investigated in the machining of SiC
materials, as it is directly related to the surface integrity, particularly for EDM. Melting
and evaporating the workpiece material allows EDM to be employed for drilling [12],
milling [13,14], cutting [15], and slicing [16] of SiC irrespective of its hardness.
Although an increasing machining rate and reduced tool wear have been achieved via
parameter optimization [15,17,18], surface and subsurface damage is inevitable in
EDM of SiC, owing to the discharge-induced thermal energy. Thermal energy was
suggested to be a main material removal mechanisms in EDM of SiC, and it causes
surface damages such as cracks, pores, and rectangular pits [15]. EDM predominantly
decomposed SiC and formed Si, C, and cracks [16]. An extra layer over the top of the
original surface and a porous layer with plenty of voids underneath was formed through
EDM of RB-SiC [10]. Three regions of subsurface damage were also identified as a
resolidified layer, heat-affected zone and microcrack region in EDM of 4H-SiC [19].
Furthermore, the material migration from the tool electrode to the workpiece surface
resulted in the contamination of the machined surface [20]. This surface and subsurface
damage is harmful to the integrity of the machined surface, which should be reversed
through subsequent processes.
The EDDG process offers the possibility to reduce the damage induced by EDM
through simultaneous diamond grinding. However, the surface and subsurface damage
is influenced by the discharge energy. A high discharge energy leads to poor grinding

This is a peer-reviewed, accepted author manuscript of the following research article: Rao, X., Zhang, F.,
Lu, Y., Luo, X., & Chen, F. (Accepted/In press). Surface and subsurface damage of reaction-
bonded silicon carbide induced by electrical discharge diamond grinding.
International Journal
of Machine Tools and Manufacture
. https://doi.org/10.1016/j.ijmachtools.2020.103564
4
performance of the diamond wheel because of the premature pullout of grits [9]. The
deeper discharge craters generated at high voltage are not completely removed by
grinding [21]. The main damage types in the pure grinding process, such as
pulverization and microcracks [22,23], change owing to the discharge-induced thermal
softening. Therefore, understanding the effects of EDM and diamond grinding on the
surface and subsurface damage is essential for quality control in EDDG of RB-SiC.
However, limited literature can be found regarding the surface and subsurface damage
induced by the interaction between EDM and diamond grinding during EDDG.
This study focused on the analysis of the surface and subsurface damage in EDDG of
RB-SiC. The effects of EDM and diamond grinding on the surface and subsurface
damage were investigated at three different discharge-energy levels. The grinding
tracks and phase transformation on the ground surface were characterized via scanning
electron microscopy (SEM) and Raman spectroscopy, respectively. Transmission
electron microscopy (TEM) was conducted to examine the microstructure and detect
the subsurface damage. According to the fast Fourier transformation (FFT) patterns of
the phases on the subsurface, the nanostructure changes at different locations under the
interactions between EDM and diamond grinding were determined.
2. Experimental procedures
2.1 Material
Commercial RB-SiC provided by Goodfellow Cambridge Ltd. (UK) was used in the
experiments. To facilitate the subsequent detection and characterization, the received

This is a peer-reviewed, accepted author manuscript of the following research article: Rao, X., Zhang, F.,
Lu, Y., Luo, X., & Chen, F. (Accepted/In press). Surface and subsurface damage of reaction-
bonded silicon carbide induced by electrical discharge diamond grinding.
International Journal
of Machine Tools and Manufacture
. https://doi.org/10.1016/j.ijmachtools.2020.103564
5
RB-SiC was cut into small blocks with dimensions of 10 mm × 10 mm × 6 mm using
wire EDM. The as-received RB-SiC contained approximately 10 vol.% free Si.
Additionally, a pristine RB-SiC sample was polished with a diamond slurry having a
particle size of 0.25 μm until a surface finish of 20 nm (Sa) was obtained, which was
used to determine the subsurface damage in the EDDG process through comparison. In
a previous study [24], X-ray diffraction analysis revealed that an RB-SiC sample
primarily consisted of 6H-SiC phases. The main properties of the RB-SiC supplied by
Goodfellow Cambridge Ltd. are listed in Table 1.
Table 1 Properties of the RB-SiC
Property
Value
Density (g/cm
3
)
3.10
Tensile modulus (GPa)
410
Vickers hardness (kgf/mm
2
)
2500–3500
Fracture toughness (MPa·m
1/2
)
4.0
Electrical resistivity (Ω·cm)
10
2
–10
3
Thermal conductivity [W/(m·K)]
150–200
Coefficient of thermal expansion (×10 K
-1
)
4.3–4.6
Specific heat [J/(K·kg)]
1100
Melting point (K)
3000
Free Si content (vol.%)
10
Apparent porosity ( % )
0

Frequently asked questions

Q1. What have the authors contributed in "Surface and subsurface damage of reaction-bonded silicon carbide induced by electrical discharge diamond grinding" ?

In the present study, surface and subsurface damages induced by the interactions between EDM and diamond grinding during the EDDG of RB-SiC were examined. This is a peer-reviewed, accepted author manuscript of the following research article: Rao, X., Zhang, F., Lu, Y., Luo, X., & Chen, F. ( Accepted/In press ). 

Q2. What was the effect of the discharge energy on the grinding track?

As the discharge energy increased, the grinding track transitioned from brittle fracture to plastic deformation and then to oxidation. 

Q3. What was used to examine the subsurface structure of the specimens?

TEM (FEI Talos F200x, FEI Co., USA) was used to examine the subsurface structure of the specimens at an acceleration voltage of 200 kV. 

Q4. What is the effect of the discharge energy on the damage layer?

Owing to the facilitated ductile removal of the material and the dressing effect induced by EDM, the thickness of the damage layer decreased with the increasing discharge energy. 

Q5. What is the effect of discharge on the surface of the RB-SiC ceramics?

The RB-SiC material melted by discharge sparks was rapidly cooled by the dielectric fluid, resultingbonded silicon carbide induced by electrical discharge diamond grinding. 

Q6. What is the amorphous peak of Si at the highest discharge energy in the EDM?

Owing to the lack of sufficient grinding actions, an amorphous peak of Si at 464.1 cm-1 was observed at the highest discharge energy in the EDM zone. 

Q7. What is the spectral signature of the sintering process?

It reveals stacking faults in the SiC grains, and dislocation in the SiC grain with phase boundary generated by the sintering process. 

Q8. What is the reason for the graphite sheets at location 6?

the mechanical pressure induced by diamond grinding is responsible for the formation of graphite sheets at location 6. 

Q9. What is the relationship between the discharge energy and the crater?

If the discharge energy transferred to the material is utilized for formation of the crater, the relationship between crater and discharge energy can be expressed by a proportionality constant 𝐾 ′,𝑉𝑐 = 𝐾 ′(𝜂𝐸𝑄) (4)where 𝐸𝑄 is the discharge energy induced by a single discharge spark, which can be given by𝐸𝑄 = 𝑈𝐼𝜏 (5)where U is the discharge/breakdown voltage, The authoris the mean peak current, and τ is the pulse-on time. 

Q10. What caused the thickness of the damage layer to increase?

The thickness increased again at the highest discharge energy owing to the heavy thermal damage and the excessive dressing of the grinding wheel. 

Q11. What is the phase distribution in the tested area?

https://doi.org/10.1016/j.ijmachtools.2020.103564distributions of phase in the tested area: (b) SiC phase, (c) C phase, and (d) Si phase. 

Q12. What was the purpose of the comparative tests?

To provide evidence for the subsurface damage and microstructure changes induced by the EDDG process, comparative tests were performed on a pristine RB-SiC sample.