Jackson M.J. Micro and Nanomanufacturing

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352 Micro- and Nanomanufacturing

3 10 20 30

Negative Bias Time (minute)

(a)

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Fig. 7.7. (a) nucleation density of diamond by BEN; (b) SEM of nucleation den-

sity on substrate

Diamond Microcutting Tools 353

2100

I 1800

1500

1900-'C

1700''C

5 10 15 20

Filament distance (mm)

Fig. 7.8. Deposition temperature against filament position

Deposition temperature can also influence the diamond film

thickness in terms of substrate and filament position. Analysis of

temperature distribution along the coiled filament showed that the

temperature is highest at the center of the filament with a rapid de-

crease toward the edges (Fig. 7.8). This suggests that position A is

the hottest followed by position B and C on the bur. Generally,

higher substrate temperatures increase diamond film growth rate and

the crystallite size. At the bottom of the filament coil temperature is

lower; therefore, the part of the bur parallel to the coil at this tem-

perature will be coated with the diamond film at a lower growth rate.

Therefore, it can be expected that at these regions the film will be

thinner. The thermal gradient gives variations in the film thickness

and crystal sizes as evident from figure. Generally, with columnar

growth, the average crystallite size increases as the films become

thicker. The films were thicker and the crystallite size was larger at

position A compared to position C.

354 Micro- and Nanomanufacturing

Filament voltage

CH4/H2

Micro tool

Bias

voltage

Fig. 7.9. Schematic diagram of

micro-tool assembled with filament

Analysis of temperature distribution along the coiled filament

showed that the temperature is highest at the center of the filament

with a rapid decrease toward the edges. The bur substrate and fila-

ment temperature have been measured parallel to the positions A, B,

and C respectively (Fig. 7.9). This suggests that position A is the

hottest followed by position B and C on the bur. Variations in the

film thickness and crystal sizes are mainly due to thermal gradients

at various positions on the bur.

Diamond Microcutting Tools 355

Diamond

Coiled nianicnt

Cross-section of

ihc tool

Cutting teeth

g[^;^^j Ctitling area

Fig. 7.10. Schematic diagram of micro-tool cutting edges

356 Micro- and Nanomanufacturing

7.9 Deposition on Three-Dimensional

Substrates

7.9.1 Diamond Deposition on Metallic (Molybdenum)

Wire

It is difficult to deposit CVD diamond onto cutting tools, which gen-

erally have a 3-D shape and possess complex geometry and sharp

edges,

using a single step growth process [76]. The cylindrical shape

wire,

which has complex geometry, can be used as a model applica-

tion for deposition of diamond on cutting tools such as microdrills

and micro-tools. The molybdenum (Mo) wires are deposited with

CVD diamond by modified vertical filament approach. After deposi-

tion time of

hours is continuous films of

5 |Lim

thick CVD diamond

were obtained (Fig. 7.11). The film morphology showed that it has

good uniformity and high purity of diamond. The Raman spectros-

copy confirmed that sp^ diamond peak at wave number 1332.6 cm"^

as shown on Fig. 7.12.

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Uniform growth of (111) faceted

octahedral diamond film on

Fig. 7.11. Diamond film on molybdenum

Diamond Microcutting Tools 357

7.9.2 Deposition on WC-Co IVIicro-Drill

Deposition of diamond on wires can be readily extended to mi-

crodrill used for machining tool, NEMS, and MEMS devices. The

uniform and adherent coating are essential in order to obtain an im-

proved performance. Figure 7.13 (a) shows an SEM micrograph of

an uncoated microdrill. The WC-Co cutting edges are welded onto

the steel shaft (Fe-Cr). The cutting tip is 4 mm in length and 0.8 mm

in diameter. The microdrill has six sharp cutting edges, which is

clearly visible in Fig. 7.13 (b).

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Fig. 7.12. Raman spectra of CVD diamond on molybdenum wire

358 Micro-and Nanomanufacturing

Figure 7.14 shows the SEM micrographs and the corresponding

EDX spectra of the WC-Co microdrill before and after the chemical

etching process. Before etching, the EDS spectrum (Figure 7.14 (a))

show the peaks for cobalt (Co), carbon (C), and tungsten (W). High

cobalt content inhibits diamond deposition, resulting generally in

graphitic phases, which degrade the coating adhesion. The Co

dif-

fuses to the surface regions, preventing effective bonding between

the substrate surface and the film coating. To improve the coating

adhesion of diamond on WC-Co tools, several approaches can be

employed. For example, first, the use of interlayer material such as

chromium can act as a barrier against cobalt diffusion during dia-

mond CVD. Second, the cobalt from the tool surface can be etched

using either chemical or plasma methods. Third, the cobalt can be

converted into stable intermediate interlayer cobalt compounds.

These can act as a barrier to cobalt diffusion from the substrate dur-

ing film growth [77]. Murakami solution followed by H2SO4/H2O2

etch can be used to chemically remove the cobalt from the bur sur-

face.

The EDX spectrum shows that the Co peak has disappeared

after etching. This will prove to be beneficial in enhancing the coat-

ing adhesion. Comparison of the SEM micrograph in Figs. 7.14 (a)

and (b) shows that the surface topography is significantly altered af-

ter etching in Murakami and H2SO4/H2O2 solutions. The etching

process makes the surface much rougher with a significant amount

of etch pits, which act as low-energy nucleation sites for diamond

crystal growth.

An SEM micrograph of a diamond-coated WC microdrill is

shown in Fig. 7.15. Six cutting edges of the microdrill tip were

coated with a polycrystalline diamond film using the modified verti-

cal HFCVD method. Analysis of the SEM picture shows that the

coating uniformly covered the cutting edges as well as the nearby

regions in which the placement of the microdrill within the coils of

the filament, ensuring uniform deposition. The diamond crystal

structure and morphology are uniform and adherent, as shown in

Figs.

7.15 (a) and (b). It is also shows a close-up view of the dia-

mond-coated region of the microdrill in Fig. 7.15 (c).

Diamond Microcutting Tools 359

Fig. 7.13. Tip and cutting edge of micro-drill

Typically the crystallite sizes are of the order of 5-8 |Lim. The

visibly adherent diamond coatings on the WC-Co microdrills consist

of mainly (111) faceted diamond crystals. The design of the filament

and substrate in the reactor offer the possibility of uniformly coating

even larger diameter cylindrical substrates.

Raman analysis was performed in order to evaluate the diamond

carbon-phase quality and film stress in the deposited films. The

Raman spectrum in Fig. 7.16 shows a single peak at 1335 cm"^ for

the tip of the diamond-coated microdrill. The Raman spectrum also

gives information about the stress in the diamond coatings. The dia-

mond peak is shifted to a higher wave number of 1335 cm"^ than that

of natural diamond peak 1332 cm"^ indicating that stress, which is

compressive in nature, exists in the resultant coatings [78].

360 Micro- and Nanomanufacturing

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10 11 12 t3

(a)

(b)

Fig. 7.14. (a) SEM and EDX of WC-Co micro-drill before etching; (b) WC-Co

micro-drill after etching

7.9.3 Diamond Deposition on Tungsten Carbide (WC-Co)

IVIicro-Tool

Laboratory grade tungsten carbide (WC-Co) micro-tools are

shown in Fig. 7.17 (a) and (b) (AT23 LR) with fine WC grain sizes

(1 |j,m) 20 -30 mm in length and 1.0 -1.5 mm in diameter [supplied

by Metrodent Ltd, UK] that are used for the CVD diamond deposi-

tion process.

Diamond Microcutting Tools 361

(a)

(c)

Fig. 7.15. (a) cutting edge of micro-drill after depositing with CVD diamond; (b)

cutting edge of micro-drill uniformly coated with diamond; (c) SEM of micro-drill

after coating with diamond