Development of a process chain for grinding and subsequent laser beam polishing of quartz glass components.

Hecht, Kerstin ; Bliedtner, Jens ; Mueller, Hartmut 等

1. INTRODUCTION

From state of the art methods of polishing surfaces with laser radiation are already known. So it's possible e.g. to reduce processing time of metallic injection moulding moulds from 30min/[cm.sup.2] to a few seconds per [cm.sup.2] (Fraunhofer-ILT 2008). For the current study a C[O.sub.2]-laser beam is used as a polishing tool to finish quartz glass surfaces. This method could save time and money, if it is possible to optimize the various process parameters. To reach a high-class surface special requirements and preparations are necessary. The result of the study should be a process chain, which includes specifications for grinding, polishing and the surface analyses, to bring quartz glass blanks to reflective parts.

Former analysis have shown, that specific roughness shall be kept to polish a surface in just one machining step. So the grinding process itself includes the optimization of lots of parameters as well as the polishing procedure does. To check the influence of changing parameters the surface is examined with a stylus instrument, an atomic force microscope (AFM) and a scanning electron microscope (SEM).

2. EXPERIMENTS AND RESULTS OF GRINDING

Fig. 1 shows the planned process chain from the start with a quartz glass blank (size: 25x25x3 [mm.sup.3] or 50x50x3 [mm.sup.3]) to the final polished part with rougness Rms up to 5 nm and less.

[FIGURE 1 OMITTED]

The grinding is carried out as a process that uses an abrasive, geometric undefined wheel as cutting tool on swing frame grinding machine. It is also possible to use a grinding tool on a 3-axis milling machine to grind not just surfaces on--but also outlines into--quartz glass blanks. This second step has to fallow after finishing the current study. The grinding process depends on different machining parameters.

To check the significance of these values DoE (Design of Experiments) was used. Tab. 1 shows which parameters were changed and checked.

Figure 2 compares the 9 different parts, which were grinded on basis of the [2.sub.3] Factorial Design, by means of the roughness Rms and the waviness Wq. These values were measured with a stylus instrument.

[FIGURE 2 OMITTED]

It gets obvious that the parameters--used for grinding the 2nd part--lead to the best surface roughness results. In the next step these parameters (from part 2) were kept constant and roughing infeed as well as rotational speed were changed. But their influence is (in analysed limits) not that significant.

It is possible to manufacture parts with a roughness Rms of 0,53 [micro]m, this is one result of the grinding experiments. The analysis gives us fields of parameters to grind quartz glass blanks with defined and selected surface qualities.

In further experiments we will explore the possibility of grinding geometries and outlines with mounted points on a 3-axis milling machine.

3. EXPERIMENTS AND RESULTS OF LASER BEAM POLISHING

The laser beam polishing is a process that not just depends on a multitude of parameters, these parameters also interact with each other strongly. The principle of laser polishing seems simple at first. A scanner system moves a defocused C[O.sub.2]-laser beam with a defined high velocity over the glass surface and the feed rate is realized with an additional x-axis. By doing this so much (nearly 100% of) energy is inserted in the material, that the surface melts and that's why unevenness and roughness were smoothed. This polishing process is associated with just a light stock removal--the smoothing happens because of the material tensions in the melted surface layer.

[FIGURE 3 OMITTED]

The challenge of this polishing method is the great deal of parameters--which have to be adapted--to reach a perfect glass surface. In doing so much depends on the inserted energy so it's basically important to keep this value constant. But also with a constant application of energy defocusing, feed-rate and beam velocity are important. Several series of tests took place in which the mentioned parameters were checked concerning their influence on each other and the surface quality. Tab. 2 shows the parameters and the range of their variation.

To optimize the polishing process itself a lot of experiments were carried out on different quartz glass blanks. Finally parameter combinations were found to reach excellent surface quality. So it was possible to finish the grinded parts too. During and after the polishing-tests the surfaces were analyzed with a stylus instrument as long as the roughness was big enough. But as Rms reaches 5 nm and less is was necessary to check the measured values with a proper measuring method--the AFM.

[FIGURE 4 OMITTED]

In Figure 4 you can see the afm-picture and the associated roughness of one of the best polished surfaces. Consider this picture in particular it get's obvious, that there is a kind of surface structure on the quartz glass part. It seems that there a smooth hills and valleys. This structure is considered as a result of the surface melting but it is so weak, that we are able to reach optical surface quality with this new method of laser beam polishing. This gets even more obvious if you consider Figure 5 too. There is a diagram that includes the psd-function of a laser and a mechanically polished quartz glass part as well as a lapped one. The grinded part was manufactured on a ultrasonic supported milling machine. A psd-function includes the spectrum of the spatial frequencies of the surface roughness measured in reciprocal units of length. It allows a complete description of the surface quality and it is qualified for description of high polished surfaces. (DIN ISO 10110-8) The equation 1 describes the functional relation of spectral power density and spatial frequency.

2D-isotropic-PSD=P/(2[pi]f([DELTA]f)) (1)

P Power under a part of surface (in nm2)

[DELTA]f change of frequency

f frequency which equates to a defined surface size (in [nm.sup.-1]) (Biedtner & Graefe, 2008)

[FIGURE 5 OMITTED]

Figure 5 we can demonstrates that the laser beam polishing method permits better surface qualities as the classical and time-consuming mechanical polishing method does. With this new process machining times of 6 [cm.sup.2]/s and less are possible.

4. FUTURE PROSPECTS

With the results of the grinding and polishing analyses it is possible to develop a complete process chain to manufacture quartz glass parts with a high-class surface. With known basic values and optimized machining parameters it is possible to finish a glass surface with requested quality.

In further investigations it is planned to grind profiles with ultrasonic support and to polish them. Furthermore there is still a problem with impurities on the hot surface while polishing. They lead to micro-fine areas of polluted glass. These areas behave different while cooling and they peel off, which leads to micro-defects on the surface. This was detected during SEM-measurements. So it is necessary to reduce foreign components in the polishing area--to keep the space around the interacting zone of the laser beam clean

5. REFERENCES

Bliedtner, J. & Greafe, G. (2008). Optiktechnologie, Carl Hanser Verlag, ISBN: 978-3-446-40896-8, Germany

DIN ISO 10110-8. Optics and optical instruments--Preparation of drawings for optical elements and systems--Part 8: Surface texture. 2000-02

*** (2008). http://www.ilt.fraunhofer.de--Laser-Beam Polishing of Injection Molded Tools, Fraunhofer-ILT, Accessed on: 2008-05-06

Tab. 1. Grinding parameters

 checked with the [2.sup.3] Factorial Design

contact width 0,1 ... 3 mm
feed rate 7,4 ... 17,4 m/s limits
finishing infeed 0,005 ... 0,02 mm

 changed, no significant influence

cutting speed 25 m/s; 35 m/s 4
sparking out 0, 2, 4 steps

roughing infeed
rotational speed kept constant
wheel grit (64)

Tab. 2. Polishing parameters

[V.sub.b][mm/s] [V.sub.f] [mm/min] P [W] Rms [[micro]m]

400 ... 800 11,9 ... 50 490 ... 830 0,2 ... 0,8

influenced values: intensity, line spacing, polishing time