Jackson M.J. Micro and Nanomanufacturing

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Diamond Nanogrinding 563

301

-I

'•"•.

\ >

Soaking time (h)

—I—

- - 1200 C (model)

- - - -1250 C (model)

1300 C (model)

1350 C (model)

— 1400 C (model)

• Authors' experimental data (1200 C)

-•—Authors' experimental data (1250 C)

• Authors' experimental data (1300 C)

Fig.

10.21.

Effect of time on residual quartz content of a sintering bonding system

according to Jackson and Mills's model [11] and compared with the authors' ex-

perimental data

564 Micro-and Nanomanufacturing

Table 10.5. Residual quartz content for different soaking times at 1300°C for a

sintering bonding system composed of 40 wt.% kaolin, 40 wt.% quartz, and 20

wt.% feldspar (Lundin's mixture number M21 [6]) compared with other dissolu-

tion models.

Time

(hours)

~ 0

0.5

120

190

258

Lundin's

exp.

data [6]

41.9

35.9

32.8

29.2

23.2

19.5

13.3

10.7

6.9

3.6

2.7

2.0

Jander

[14]

4L9

41.72

41.54

41.19

40.49

39.11

35.72

33.74

26.18

7.85

0.00

Krause

and Keet-

man [15]

0.00

22.55

20.76

18.97

17.18

15.39

13.30

12.56

10.77

8.96

7.22

6.43

Monshi

[16]

41.9

34.61

31.97

28.58

24.39

19.49

13.30

11.13

6.43

2.17

1.00

0.54

Jackson and

Mills [11]

4L9

34.76

32.12

28.72

24.51

19.59

13.36

11.19

6.51

2.17

1.01

0.55

The results obtained using Monshi's model [16] are in much

better agreement compared to Lundin's data. However, the results

obtained using Jackson and Mills' model [11] is more accurate at

predicting the mass fraction of quartz remaining owing to the differ-

ences in the density of quartz. After long periods of heat treatment,

the model predicts lower magnitudes of mass fractions of quartz

when compared to Lundin's experimental results [6].

10.6.5 Fusible Bonding Systems - Verification and

Comparison of Dissolution Models for Quartz

The constants, A and B, for the fusible bonding system were de-

termined using time and temperature couples at 2 hours and 10 hours

and were calculated to be, -5.2 x 10

and -33205, respectively. The

dissolution equation then becomes,

10.06 exp

-33205

(10.50)

Diamond Nanogrinding 565

Equation (10.50) is used to compare the experimentally de-

termined mass fraction of quartz remaining after heat treatment with

the predicted values. The calculated mass fraction of quartz remain-

ing after a period of heat treatment is calculated using the equation

derived by Jackson and Mills [11]. The results of the dissolution

model compare well with the experimental data over short periods of

time.

However, over longer periods of heat treatment the model

tends to become less accurate (Fig. 10.22). A comparison was made

with published dissolution models. The equations shown were com-

pared with experimental data at 1050°C.

The equations shown were compared with experimental data

at 1050°C. According to the transformed equations, the mass frac-

tion of quartz can be calculated as follows:

Jander's model [14]

m =

10.1.(l

- {3.44xl0"

i})

3/2

(10.51)

Krause and Keetman's model [15]

m = 10.1-(0.59.1nf) (10.52)

Monshi's model [16]

l0.l.e-

64xl0

(10.53)

Jackson and Mills' model [11]

\0.06.e'

(10.54)

566 Micro-and Nanomanufacturing

12i

Soaking time (h)

-•—Authors' experimental data (950 C)

- —950 C (model)

• Authors' experimental data (1000 C)

- - 1000 C (model)

-A—Authors' experimental data (1050 C)

- - 1050 C (model)

Fig. 10.22. Effect of time on residual quartz content of a fusible bonding system

according to Jackson and Mills's model [11] and compared with the author's ex-

perimental data

Diamond Nanogrinding 567

With reference to Table 10.6, it can be shown that the mass

fraction of quartz obtained using the equations derived by Jander

[14] and Krause and Keetman [15], did not agree with the experi-

mental results at 1050°C. The results obtained using Monshi's

model [16] are in much better agreement compared to the experi-

mental data. However, the results obtained from Jackson and

Mills's model are more accurate at predicting the mass fraction of

quartz remaining owing to the differences in the density of quartz.

After long periods of heat treatment, the model predicts slightly

lower magnitudes of mass fractions of quartz when compared to the

experimental results.

Table 10.6. Residual quartz content for different soaking times at 1050°C for a

fusible bonding system compared with other dissolution models

Time

(h)

Exp.

Data

10.1

6.84

5.79

5.13

4.7

4.28

3.2

1.1

Jander

model [14]

10.1

9.91

9.72

9.54

9.36

9.18

8.28

6.6

3.62

Krause and

Keetman

model

[15]

5.23

4.82

4.58

4.41

4.28

3.87

3.46

3.04

Monshi's

model

[16]

10.1

6.88

5.87

5.21

4.7

4.28

2.99

1.81

0.89

Jackson and

Mills'

model

[11]

10.1

6.86

5.86

5.19

4.68

4.28

1.82

0.89

The dissolution of quartz during heat treatment has a signifi-

cant effect on the wear of vitrified grinding wheels. Figure 10.23

shows the effect of using a high and a low quartz content bonding

system on the wear of vitrified corundum grinding wheels grinding a

large number of tool steel materials [17]. The classification of tool

steels is in the form of an abrasive hardness number, which is a

weighted average of the number of carbides contained within the

tool material. As shown in Fig.

10.23,

the grinding ratio, or G-ratio,

is a measure of the efficiency of the grinding wheel. It is the quo-

tient of the volume of workpiece material removed and the volume

568 Micro-and Nanomanufacturing

of the wheel material removed. The figure demonstrates the effec-

tiveness of reducing the quartz content of the bonding system of po-

rous nanogrinding tools.

60 i

50 J

40 H

'•6

20 A

10 J

600

800

1000

1200

Abrasive Number

•

High quartz content Low quartz content

Fig.

10.23.

Effect of the abrasive number on the grinding ratio for a high-quartz

content and a low quartz-content bonding system

Diamond Nanogrinding 569

10.7 Laser Dressing of Nanogrinding Tools

The processing of abrasive materials by locally restructuring

their surface using a high power laser allows one to provide a local-

ized heat source at high speed that produces surface features that are

able to remove material at the nanoscale. The depth of thermal

processing of the surface that allows the material to melt, solidify

and cool is controlled by parameters such as fluence (energy per unit

area),

laser beam scanning speed, and input power. Previous studies

on laser dressing of grinding wheels have resulted in the localized

modification of the morphology of corundum abrasive materials [19-

21].

The grinding wheel is initially made by sintering corundum

grains with a low melting point binding material, which is subse-

quently melted, vaporized, and selectively removed using a concen-

trated beam of optical energy that changes surface topography and

composition. The refinement of grain size, densification of grinding

layer, and evolution of competitive grains with sharp vertices is

shown to occur during laser dressing [19]. These changes result in

an increase in grinding ratios, especially when grinding tool steels

[19].

X-ray diffraction (XRD) of the laser dressed surface indicated

that crystallographic texture occurs on the surface of the grinding

wheel, and the identification of planar texture is considered impor-

tant as it controls the formation of multifaceted grains that take part

in removing workpiece material. Pole figure analysis of laser

dressed corundum composite materials indicates that crystallo-

graphic texture is preferred along the (110) plane [19-21]. Grinding

properties are particularly sensitive to preferred orientation and as

such variations in grinding are apparent. To predict the performance

of these materials, it is important to identify crystallographic texture

using XRD techniques. Orientation imaging microscopy (OIM) is

used to characterize micro-texture [22] as it gives phase specific

crystallographic texture. The OIM technique uses Kikuchi patterns

to predict crystallographic texture at various points on the surface of

the laser dressed material. Microstructural variations are observed

on the laser dressed surface owing to temperature gradients and

variations in cooling rates. Corundum, or alumina, has poor thermal

570 Micro-and Nanomanufacturing

conductivity compared to metals that allows thermal energy to be re-

tained in the surface for a longer period of time compared to the

workpiece material. The occurrence of porosity between grains also

reduces thermal conductivity, which results in the retardation of heat

lost from the surface of the abrasive material [23]. In addition to the

effects of porosity, consolidation of the corundum abrasive material

with the bonding material changes the thermal properties during la-

ser dressing [24-26]. Therefore, the combined thermophysical prop-

erties of the composite material must be used. Table 10.7 shows the

thermophysical properties of the abrasive material. It is important to

quantify the cooling rate in order to understand how the microstruc-

ture evolves into preferred orientation during laser dressing. Many

models have been generated that predict how microstructures form

under rapid solidification conditions [27-29]. The chapter presents

an understanding of how cooling rates affect the formation of multi-

ple cutting points on the surface of the abrasive material and how it

affects grindability of a hypereutectoid steel such as AISI 52100

bearing steel. This is achieved by observing cooling rates using a

two-color pyrometer, by characterizing the microstructural texture

using OIM, and by determining grinding ratios when grinding AISI

52100 bearing steel.

Table 10.7. Thermophysical properties of vitrified corundum abrasive material

Thermophysical property Numerical value of property**

Density, p 2.7xl0

kg/m

Melting temperature, T

, 2045 K

Latent heat of fusion, AH

, 1.03 x 106 J/mol

Specific heat, Cp, 1137 J/kg K

Thermal diffusivity, D, 1.08 x 10"

"Source: Vitrified Technologies Inc., Central Research and Development Labo-

ratory, P. O. Box 3724, West Lafayette, IN 47996

Diamond Nanogrinding 571

10.7.1 Experimental Procedures

Vitrified corundum abrasive products were supplied by Vitrified

Technologies Inc., and were composed of corundum grains doped

with 0.25% wt. chromia that were bonded together using a bonding

system composed of borosilicate glass and orthoclase feldspar. The

chemical composition of the abrasive grain material is 99.52%

0.18% Na

0, 0.05% Fe

, and 0.25% Cr

. A 2.5kW

Hobart continuous-wave Nd:YAG laser equipped with a fiber optic

beam delivery system was used for dressing the surface of the grind-

ing wheel. Laser power was varied from 400 W to 1000 W, whilst

the overlap between dressed layers was less than 20%. The laser

beam was scanned at a linear speed of 250 cm/min on the surface of

the grinding wheel. Full experimental details of the dressing proce-

dure are provided in references [19,21]. Laser dressed grinding

wheels were finally compared to conventionally dressed grinding

wheels grinding a hypereutectoid steel typically used for bearing ap-

plications that is capable of being superfinished.

Measurement of Dressing Temperatures

A two-color optical pyrometer was used to measure the temperature

of the abrasive surface during dressing. The temperature range of

the pyrometer is 600°C to 1500°C with a response time of 0.5 ms.

The pyrometer is calibrated against National Institute standards to

within 10°C over the range 800°C to 3000°C. A specially con-

structed telescope with filter was constructed in order to measure the

laser beam temperature at the surface, and a program was written to

directly record temperature against processing time. When the laser

beam dresses the surface, a large plume of light is emitted that inter-

feres with the intensity of the light collected by the telescope. To

overcome the large emission of light, a delay was programmed in

the software program to prevent the erroneous measurement of tem-

perature. As the plume of light fades away, the pyrometer records

the temperature. The temperature is measured at 400 W and 500 W

of laser power, but at very high laser power such as 1000 W the tem-

perature is almost impossible to record because of the intensity of

572 Micro-and Nanomanufacturing

light. Therefore, data are presented at the lower values of laser

power because the plume lasts for less than

second.

Orientation Imaging Microscopy

To gain high-quality Kikuchi patterns from laser dressed grinding

wheels, samples were cross-sectioned by polishing with a water so-

lution on SiC abrasive papers then followed by polishing with a

diamond paste to achieve a fine surface. The samples were then

carbon coated with a uniform thickness of

nm. Electron backscat-

tered diffraction patterns (EBSD), or backscattered Kikuchi patterns,

were generated using a Philips FEG SEM equipped with an orienta-

tion-imaging microscope [30-32]. OIM analysis was conducted at

Oak Ridge National Laboratory. To produce crystallographic orien-

tation maps, the electron beam is scanned over a selected surface

area and the resulting EBSD patterns are automatically indexed and

the local orientation is determined [33-34]. For the present study, a

sample processed at 1000 W was used in order to provide a crystal-

lographic map of the whole re-solidified layer. From XRD and en-

ergy dispersive analysis (EDS), it was found that the primary phase

created was oc-alumina [19-20] and so the hexagonal crystal struc-

ture of alumina was selected to index the EBSD Kikuchi patterns ob-

tained during the analysis. For OIM analysis, the 1000W laser

dressed surface was used whereas the lower power processed sam-

ples were used to measure cooling rates. This does not affect solidi-

fication behavior as all samples follow the same morphological fea-

tures during solidification, the only variable being the depth of the

re-solidified layer.

Nanogrinding Experiments

To characterize the differences between conventional abrasives, sol-

gel abrasives, monocrystalline alumina abrasives and laser dressed

chromium-doped alumina, a series of grinding experiments were

conducted in order to confirm results gained in field trials. In order

to duplicate the field trials, a series of controlled experiments were

conducted under increasing feed rates. The experiments were termi-

nated when a condition of severe burn, chatter, or grinding wheel