Jackson Mark. Machining with Abrasives

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cracking evidence. This can be explained by the possibility of microcracking due to

the pile-up of dislocations on an active slip plane. The threshold effective resolve

shear stress (t

) for cracking could be determined by [24]:

ð1  nÞL



(5.2)

where g is fracture surface energy, m is shear modulus, n is Poisson’s ratio, and L is

the length of pile-up. Considering the length of pile-up of 150–500 nm (i.e., pene-

tration depth of dislocation in grinding), we can see that for such a small pile-up

length the nucleation of microcracks does not usually occur. It is also worth noting

that the interaction of different slip systems was a rear event here and could not

create microcracks.

In short, the investigation above on grinding alumina enables us to conclude that

the ductile-mode of material removal is possible due to the initiation of more than

ﬁve independent slip and twin systems, that the subsurface structure of alumina

after ductile-mode grindin g is composed of a layer with high dislocation density in

the surface vicinity followed by a layer with a much lower dislocation density, and

that the absence of microcracks in the immediate subsurface is the small length of

pile-ups that cannot create sufﬁcient stress for microcracking.

5.2.3 Grinding of Composites

Composites have been widely used in industry and are a class of most important

materials across a broad range of disciplines. This is because composites have

excellent properties, such as the low thermal expansion, superior damping capacity,

and high speciﬁc strength and speciﬁc modulus.

Generally speaking, a composite contains at least two different materials whose

properties are usually very different. Some typical examples include the carbon

Fig. 5.10 Subsurface structure in alumina after ductile-mode grinding with the table speed¼1m/min

5 Surface Integrity of Materials Induced by Grinding 255

ﬁber reinforced polymer matrix composite [25] and the ceramic particle reinforced

aluminium matrix composite [26].

To obtain a high quality surface, including surface integrity and dimensional

accuracy, grinding is often an essential process in producing composite compo-

nents. There have been a number of studies on the machinability of carbon ﬁber

reinforced plastics (CFRP), focusing on the cutting, turning and drilling processes

[27–34], as well as grinding [35–38]. As generally known, different machining

processes have different effects on the surface integrity of a CFRP component. In

a ground CFRP, surface imperfections can be the cracking origins. Furthermore, a

variety of surface and subsurfa ce damages, such as macro- and micro-cracks,

de-laminations and ﬁber-matrix de-bonding, can occur.

In this section of the chapter, we will focus on the characterization of the

subsurface damage of ground components and the mechanism of defect formation

under various grinding conditions. The specimens used had a geometry of 45  15

 4 mm, cut from the CFRP laminates with unidirectional carbon ﬁbers of about

7–8 mm in diameter. The laminates were fabricated from a com mercial resin system

of F593 prepregs, cured under the constant pressure of 0.6 MPa at the temperature

of 177



C for 2 h. The mechanical properties of the composites are as follows:

tensile strength ¼ 1,331 MPa, tensile modulus ¼ 120 GPa, compressive strength

¼ 1,655 MPa and compressive modulus ¼ 115 GPa. All samples were pre-ground

using a ﬁner grinding condition to minimize the possible subsurface damage, so that

the characterization of experimental results will be reliable. Figure 5.11 shows the

cross-section image of a pre-ground specimen with the ﬁber orientation of 150



.It

can be clearly seen that the specimen surface is smooth and the subsurface damage

is negligible.

The grinding tests were conducted on a MININI M286 CN surface-grindi ng

machine with an aluminium oxide grinding wheel BWA36HVAA. Down-

grinding under a dry condition was employed as shown in Fig. 5.12. The grinding

wheel was dressed regularly with a single point diamond dresser at the dressing

Fig. 5.11 Surface and subsurface quality of a specimen before grinding

256 L.C. Zhang

depth of 50 mm each time, dressing feed of 200 mm/min and wheel peripheral speed

of 20 m/s. The ﬁber orientations of the specimens were y ¼ 0



,30



,60



,90



,120



150



and 180



(equivalent to 0



), deﬁned clockwise from the ground surface to the

direction of the ﬁbers as illustrated in Fig. 5.12. The wheel speeds used were 16, 19,

22, 25 and 27 m/s, respectively, and the table speeds were 1, 2, 3, 4 and 5 m/min,

respectively. The surface integrity of a ground specimen was assessed in terms of

surface roughness, surface damage and de-lamination in the subsurface, using a

surface roughness tester, Surftest-402 and Surftest Analyzer, an optical microscope,

Leica LEITZ DMRXE, and a scanning electronic microscope (SEM), Philips XL-30.

5.2.3.1 Inﬂuence of Fiber Orientation

Fiber orientation has a signiﬁcant effect on the surface quality of a ground CFRP

component. Figure 5.13 shows the relationship between the longitudinal surface

roughness of the ground specimens and the ﬁber orientations under different

grinding depths. The results indicate that the roughness is dependent on the ﬁber

orientation remarkably. The ground components with the ﬁber orientation in the

range of 120–180



have much rougher surfaces. Under relatively small grinding

depths close to the diameter of the carbon ﬁbers, such as 10 mm and 20 mm, the

worst surface roughness occurs at y ¼ 0



. The reason is that the rupture of the ﬁbers

and the ﬁber/matrix interface de-bonding occurred at the ﬁber orientation of 0



Partial matrix in the machining zone was pulled off with the ruptured ﬁbers during

the grinding process, leaving some arc-shaped grooves where the ﬁbers origina lly

located. This makes the ground surface at y ¼ 0



rougher. However, when a large

grinding depth (e.g., 50 mm) is used, the roughest surface appears at y ¼ 150



with the roughness about 2–3 times that at y ¼ 0



and at least ten times that at

y ¼ 90



. The results in the transverse direction are similar.

Figure 5.14 shows a typical subsurface defect, whe re some cracks are more than

100 mm in length. These defects manifest that the fracture of ﬁbers and matrix is

Fig. 5.12 Illustration of surface down-grinding

5 Surface Integrity of Materials Induced by Grinding 257

below the cutting point of grits, which may be caused by ﬁber buckling and

shearing. There is an inherent weakness of ﬁber-reinforced composites in the

transverse direction due to the weak bonding of ﬁber/matrix interfaces [39]. At

the ﬁber orientation of y ¼ 150



, the serious buckling is prone to causing partial de-

bonding and micro-cracks in ﬁber/matrix interfaces along ﬁber direction. As there

are severe subsurface damages on ground CFRP, and these damages just resemble

pre-service cracks and edge de-lamination on CFRP components, it would not

be surprised that these components would have poor mechanical properties and

low fracture toughness. Figur e 5.15 shows typical cavitations caused by grinding.

Fig. 5.14 Serious subsurface damage and cracks when y ¼ 150



Fig. 5.13 Inﬂuence of ﬁber orientation on surface roughness

258 L.C. Zhang

5.2.3.2 Inﬂuence of Grinding Depth

The grinding depth has an obvious effect when y ¼ 150



. As shown in Fig. 5.16,at

y ¼ 150



the increase in grinding depth results in a sharp rise of surface roughness

until the depth reaches 100 mm. For other y values, however, the effect is trivial. It is

worth noting that the ground surface at y ¼ 150



shows a typical saw-toothed

surface morphology as shown in Figs. 5.14, 5.17 and 5.18. This again implies

that the chip fracture in grinding is often below the cutting edge of the active

abrasive grits. The mechanism of material removal seems to be the same as that in

single point cutting, which was investigated by Zhang et al. [40]. In other words,

Fig. 5.15 Damages on ground surface when y ¼ 150



Fig. 5.16 Inﬂuence of grinding depth on surface roughness

5 Surface Integrity of Materials Induced by Grinding 259

under the cutting of an abrasive grit, ﬁber/matrix de-bonding occurs ﬁrst along the

ﬁber/matrix interface and then ﬁber fractures when the tensile stress at the ﬁber

surface becomes great enough to initiate the cracking.

By comparing the results in Figs. 5.17 and 5.18 corresponding to different

grinding depths, we can see that the subsurface damage at a shallow grinding

depth is less severe. This is because at a large grinding depth, an active grit

would exert a greater axial force on a ﬁber, which shifts the cracking point of the

ﬁber towards the deeper substrate, because ﬁber cracking needs a sufﬁciently large

tensile stress. As a result, a deeper saw-toothed morphology is produced.

Fig. 5.17 Subsurface damage at a shallow grinding depth (grinding depth ¼ 20 mm, y ¼ 150



)

Fig. 5.18 Subsurface damage at a deep grinding depth (grinding depth ¼ 100 mm, y ¼ 150



)

260 L.C. Zhang

5.2.3.3 Inﬂuence of Grinding Wheel Speed and Table Speed

Grinding wheel speed and table speed are generally inﬂuential parameters.

However, their effects are not obvious in the range of grinding conditions used in

this section except the case of y ¼ 150



. This may be due to the matrix smearing

occurred in the grinding of a CFRP component. The smearing brings about a

ﬁctitiously small surface roughness that may conceal the effects of wheel speed

and table speed. At y ¼ 150



the surface ﬁnish is much poor so that the smearing

may not cover the surface so easily because of the much deeper surface damage.

Figure 5.19 shows the comparison of the matrix smearing on the ground surfaces

with different ﬁber orientations.

The above discussion brings about the following understanding:

1. A ﬁber orientation y in the range of 120–180



has a greater effect on surface

integrity. The most serious surface and subsurfa ce damages occur at y ¼ 150



2. Fiber fracture in grinding often takes place at a distance below the cutting edge.

This becomes more severe when the depth of grinding becomes larger. How-

ever, the smearing of the matrix on a ground surface may conceal the effects of

table and wheel speeds.

3. In order to obtain a satisfactory surface integrity in the grinding of CFRP

components, one needs to take a smaller grinding depth or avoid the ﬁber

orientation in the range of 120–180



5.2.4 Grinding of Monocrystalline Silicon

As the last example of materials in this chapter, this sectio n discusses the grinding-

induced surface integrity in a semiconductor, monocrystalline silicon. Monocrys-

talline silicon is an important material to the production of micro-electronics and

micro-mechanical components. It has also been used widely as an etalon material.

These applications require that the surface ﬁnishing must be high quality and

Fig. 5.19 Ground surfaces with matrix smearing

5 Surface Integrity of Materials Induced by Grinding 261

subsurface structure be damage-free [41]. To achieve the requirements the mecha-

nism of subsur face formation during surface processing, such as indentation

[42–45], turning [46], grinding [47, 48] and polishing [41, 48–50], must be fully

understood. These have led to extensive theoretical and experimental studies

[51–56]. The theoretical analysis by means of molecular dynamic simulations has

provided the evidence of amorphous phase change in indentation, sliding and

polishing, and has established a criterion for predictin g its initiation based on the

octahedral shearing stress [41, 53, 55, 56]. Direct and indirect experimental studies

have conﬁrmed the amorphous transformation in ultra-precision grinding or turn-

ing. Some traces of the amorphous phase were also admitted inside the residual

indentation marks [57–60]. In the following, we will discuss the subsurface damage

in silicon due to grinding.

All experiments were conducted on precisely polished (100) surfaces of

monocrystalline silicon with Rms roughness of 2 nm. Before a grinding experiment,

the subsurface structure of specimens was examined to guarantee that a specimen

was damage-free in its initial state. The grinding experiments were done on the

same surface grinder as that used for ceramics, metals and composites (Minini

Junior 90 CF CNC M286) discussed in the previous sections. The grinding para-

meters are listed in Table 5.1 below.

The subsurface structure of the specimens after grinding was investigated by

means of TEM using CM12 and a scanning transmission electron microscope

(STEM) using VG HB601. The cross-sectional view TEM samples were prepared

using the method developed by Zarudi and Zhang [42].

The subsurface structure of silicon after grinding at a table speed of 1 m/min is

shown in Fig. 5.20, featuring an amorphous transformation layer developed in the

immediate vicinity of the subsurface region. Below this amorphou s layer, some

dislocations (two dislocation systems) appear. By recalling the experimental and

theoretical studies aforementioned, it is clear that during the grinding of silicon,

amorphous silicon was always the ﬁrst subsurface damage to emerge, followed by

the dislocations. This observation is further conﬁrmed in Fig. 5.21 where the table

speed was reduced to 0.02 m/min so that there is only a single dislocation, because

at this lower table speed, the grindin g force was much smaller.

The above results show that in the grinding of monocrystalline silicon, the most

possible types of subsurface damages are the amorphous phase transformation and

dislocations. To avoid the emergence of the amorphous phase, one must control the

Table 5.1 Grinding parameters

Grinding conditions Depth of cut (nm) 100

Grinding wheel SD4000L75BPF Coolant Syntilo 3

Wheel diameter (mm) 305 Wheel dressing conditions

Wheel speed (m/s) 27 Type of dresser Multipoint diamond

Grinding width (mm) 5 Wheel speed (m/s) 10

Table speed (m/min) 0.02, 0.1, 0.2, 0.5, 1 Dressing cross-feed rate

(mm/rev)

0.01

262 L.C. Zhang

grinding parameters, such as the depth of cut and table speed, to make the grinding

stresses be lower than the initiation stress (or threshold) of the phase transformation.

The reader can ﬁnd more details in [41, 50 , 53].

5.3 Summary

The discussions in this chapter have demonstrated that in the grinding of different

workpiece materials, the material removal mechanisms vary, which in turn leads to

diverse types of subsurface damages.

With a metal workpiece, the material removal mechanism is via dislocations.

Due to the ductility of the materials, cracking in a workpiece during grinding

Fig. 5.20 Dislocations induced by grinding at a table speed of 1 m/min. Note that two dislocation

systems appear

Fig. 5.21 The subsurface structure of silicon after grinding with a table speed of 0.02 m/min. Note

that only one dislocation system appears

5 Surface Integrity of Materials Induced by Grinding 263

is usually not a problem. Tensile residual stresses can occur under certain

combinations of thermal deformation and phase transformation.

In the case of a ceramics workpiece, to avoid macroscopic cracking, ductile

regime grinding is necessary. The ductile-mode of material removal is achieved by

the activation of sufﬁcient number of independent slip systems. If a polycrystalline

ceramics contains surface pores, it is impossible to eliminate the micro-cracking at

the pore edge.

When grinding composites (ﬁber reinforced plastics in this chapter), ﬁber

breakage, de-lamination and cavitations are the major sources of subsurface

damages. To minimize these defects, smaller depth of abrasive cut is reco m-

mended. Meanwhile, it will be very helpful to obtain high surface integrity if the

ﬁber orientation relative to the grinding direction can be controlled or adjuste d.

In the grinding of monocrystalline silic on, amorphous phase transformation and

dislocations are the most critical events to avoid. It is possible to obtain damage-

free silicon surface (or perfect surface integrity) if critical stresses in grinding can

be controlled [41, 50].

The discussion in this chapter has been limited to the surface integrity of

workpiece materials under given grinding conditions. Nevertheless, there are

many other inﬂuential factors as outlined in Sect. 5.1 of the chapter. A change of

these will contribute to the achievable level of surface integrity. For a production

aiming at an optimal grinding process, a comprehensive consideration is necessary

to incorporate other factors such as properties of grinding wheels [61, 62] and

variations of wheel-workpiece contact details [63, 64].

Acknowledgments The continuous support of Australian Research Council to this work is

appreciated.

References

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grinding, Manufacturing Review, 5, 261–273.

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problems in surface grinding, International Journal of Machine Tools and Manufacture,

42, 905–913.

3. Mahdi M, Zhang LC (1994) Correlation between grinding conditions and phase transforma-

tion of an alloy steel, in: Advanced Computational Methods in Heat Transfer III, edited by

LC Wrobel, et al, Computational Mechanics Publications, Southampton, pp. 193–200.

4. Mahdi M, Zhang LC (1995) The ﬁnite element thermal analysis of grinding processes by

ADINA, Computer & Structure, 56, 313–320.

5. Zhang LC, Mahdi M (1995) Applied mechanics in grinding, Part IV: the mechanism of

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264 L.C. Zhang