Method for producing a coated component of transparent or opaque fused silica

Abstract

A method for producing a coated component consisting of transparent or opaque fused silica comprises a method step in which a SiO.sub.2 granulation layer is applied to a coating surface of a substrate, which in the area of the free surface has a relatively great granulation fine fraction. Starting from this, in order to achieve a smooth, preferably also dense surface layer, it is suggested according to the invention that the application of the SiO.sub.2 granulation layer comprises (i) providing a dispersion containing a dispersion liquid and amorphous SiO.sub.2 particles which form a coarse fraction with particle sizes ranging between 1 m and 50 m and a fine fraction of SiO.sub.2 nanoparticles having particle sizes of less than 100 nm, wherein the solids content of the dispersion is between 70 and 80 wt.-%, and of which between 2 wt.-% and 15 wt.-% are the SiO.sub.2 nanoparticles, (ii) applying the dispersion to the coating surface by casting or spraying it thereonto so as to form a slurry layer having a layer thickness of at least 0.3 mm; and (iii) drying the slurry layer by removing the dispersion liquid at a rate and in a direction such that under the action of the dispersion liquid being removed the fine fraction is enriched in the outer portion of the granulation layer, thereby forming a casting skin.

Claims

1. A method for producing a coated component consisting of transparent or opaque fused silica, the method comprising: (a) providing a substrate of transparent or of opaque fused silica that comprises a coating surface; (b) applying a single SiO.sub.2 granulation layer to the coating surface, said granulation layer having an inner portion adjoining the coating surface and having a first granulation fine fraction, and an outer portion adjoining a free surface of the granulation layer and having a second granulation fine fraction, wherein the second granulation fine fraction is higher than the first granulation fine fraction; (c) sintering the granulation layer so as to form a dense SiO.sub.2 surface layer, wherein the applying the SiO.sub.2 granulation layer according to method step (b) comprises: (I) providing a dispersion containing a dispersion liquid and a solids content that comprises amorphous SiO.sub.2 particles, said amorphous SiO.sub.2 particles comprising a coarse fraction with particle sizes ranging between 1 m and 50 m and a fine fraction of SiO.sub.2 nanoparticles having particle sizes of less than 100 nm, wherein the solids content constitutes between 70 and 80 wt.-% of the dispersion, and wherein the SiO.sub.2 nanoparticles constitute between 2 wt.-% and 15 wt.-% of the dispersion; (II) applying the dispersion to the coating surface as a continuous stream not divided into individual drops thereonto so as to form a slurry layer having a layer thickness of at least 0.3 mm; and (III) drying the slurry layer by removing the dispersion liquid at a rate and in a direction such that the removing enriches the fine fraction in the outer portion of the granulation layer, and a casting skin is formed, said casting skin having a fine fraction of SiO.sub.2 nanoparticles with particle sizes of less than 100 nm that constitutes a volume fraction of more than 70% thereof.

2. The method according to claim 1, wherein a measure is provided that slows the drying of the slurry layer.

3. The method according to claim 2, wherein the measure comprises moisturizing the coating surface prior to the applying of the dispersion according to method step (II).

4. The method according to claim 1, wherein the slurry layer is mechanically densified.

5. The method according to claim 4, wherein the slurry layer is densified with a treatment of the slurry layer by spreading with a doctor blade.

6. The method according to claim 1, wherein the layer thickness of the slurry layer is not more than 3 mm.

7. The method according to claim 1, wherein the SiO.sub.2 nanoparticles in the dispersion constitute not more than 10% by weight of the dispersion.

8. The method according to claim 1, wherein the coarse fraction consists of splintery amorphous SiO.sub.2 granules with a grain size distribution having a D.sub.50 value between 3 m and 30 m.

9. The method according to claim 1, wherein the dispersion liquid consists of an aqueous base.

10. The method according to claim 1, wherein the solids content of the dispersion is in the range between 74 wt.-% and 78 wt.-%.

11. The method according to claim 1, wherein the dispersion is free of binders.

12. The method according to claim 1, wherein the casting skin has a thickness ranging from 3 m to 15 m.

13. The method according to claim 1, wherein the layer thickness of the slurry layer is not more than 1.5 mm.

14. The method according to claim 1, wherein the casting skin has a thickness ranging from 5 m to 10 m.

15. The method according to claim 1, wherein the fine fraction of SiO.sub.2 nanoparticles with particle sizes of less than 100 nm accounts for a volume fraction of the casting skin of more than 80%.

Description

PREFERRED EMBODIMENTS

(1) The invention will now be described in more detail with reference to embodiments and a drawing. In detail,

(2) FIGS. 1 to 5 show photos of green layers of different samples with the same magnification;

(3) FIG. 6 is a diagram with measurement results of the mean roughness of surface layers of different samples;

(4) FIG. 7 is a high-resolution computed tomography scan (micro CT scan) of a green layer produced according to the method of the invention, in a side view;

(5) FIG. 8 is a micro CT scan of a green layer produced by spraying on and drying a slurry layer, in a side view; and

(6) FIG. 9 is a scanning electron micrograph showing a breaking edge in a green layer according to the invention.

PRODUCING A SIO2 SLURRY

(7) In a dispersion liquid, amorphous quartz glass granules of natural raw material with grain sizes in the range between 250 m and 650 m are mixed into a drum mill lined with quartz glass. The quartz glass granules were previously cleaned in a hot chlorination method; attention is paid that the cristobalite content is below 1% by volume.

(8) This mixture is ground by means of grinding balls of quartz glass on a roller block at 23 rpm for a period of 3 days to such an extent that a homogeneous slurry is formed. During grinding the pH is lowered to about 4 due to the dissolving SiO.sub.2.

(9) The SiO.sub.2 granulation particles obtained after grinding the quartz glass granules are of a splintery type and have a particle size distribution which is distinguished by a D.sub.50 value of about 8 m and by a D.sub.90 value of about 40 m. SiO.sub.2 nanoparticles with diameters of about 40 nm (pyrogenic silica) is added to the homogeneous slurry. After further homogenization one obtains a binder-free SiO.sub.2 slurry.

(10) With SiO.sub.2 slurries of a different, but similar composition, coating samples were produced on a different substrate and with the help of different application techniques. The composition of the respective slurry and the coating results achieved thereby are shown in Table 1:

Sample 1

Comparative Example

(11) The SiO.sub.2 slurry has a low viscosity and can per se directly be used as a spray slurry. In a first test this slurry was used for producing a coating on a porous plate. The plate consists of absorbent opaque quartz glass with an open porosity.

(12) For coating purposes the quartz glass plate was introduced in horizontal orientation into a spray chamber and the upper side was successively provided by spraying on the slurry with a carrying SiO.sub.2 slurry layer having a thickness of about 0.7 mm. To this end a spray gun was used that was continuously fed with the spray slurry.

(13) In the subsequent partial drying process in air, a rough and rugged surface layer is formed within one minute on the thus successively applied slurry layer. This result is at any rate partly due to the fact that the slurry layer was dried because of the porous substrate so rapidly that a segregation of the fine fraction in the upper portion of the slurry layer was not possible, so that no dense and closed casting skin could be formed.

(14) Then the further drying took place at a slow pace in that the slurry layer was allowed to stand in air for eight hours. The complete drying takes place using an IR radiator in air for 4 hours.

(15) This yields a rough and cracked inhomogeneous surface layer of opaque porous quartz glass, which has the appearance shown in FIG. 1.

(16) The dried green layer is subsequently sintered in a sintering furnace at a temperature of about 1400 C. into an opaque surface layer having a density of about 1.9 g/cm.sup.3.

Sample 2

Comparative Example

(17) To exclude the effect of the porous substrate on the drying process, a quartz glass plate with a dense smooth surface was used in a further test instead of the porous quartz glass plate. Since the slurry layer runs off easily in this process, a slightly higher solids content than in Sample 1 was set and the final thickness of the slurry layer was here only 0.4 mm. Otherwise, the manufacturing parameters were maintained as in Sample 1.

(18) On the whole, after the slurry layer had been partially dried in air, a surface layer was obtained with an appearance as shown in FIG. 2 in a top view. It is slightly less rough and inhomogeneous than that of Sample 1. The improvement is however comparatively small.

(19) The reason why the improvement turns out to be so small can be explained by the fact that the initial drying rate of the slurry layers in Samples 1 and 2 does not significantly differ despite the non-absorbent substrate in Sample 2. This can only be ascribed to the application technique itself. The reason is that during spraying fine drops of the slurry are produced that already lose their moisture during their flight phase. Moreover, the slurry layer is successively built up in several layers. The individual layers are thin and immediately dry in air. It is only with the thin spray slurry that it is possible to achieve a layer structure of an adequate thickness at all. As a consequence, however, there is no sufficiently large reservoir of SiO.sub.2 nanoparticles for a segregation of the fine fraction on the surface of the spray layer.

(20) The further drying and sintering takes place, as has been described above with reference to Sample 1.

Sample 3

(21) Like in Sample 1, a SiO.sub.2 surface layer with a thickness of 2 mm is to be produced on a flat plate of absorbent opaque quartz glass.

(22) In contrast to Sample 1, the slurry layer is produced by doctor blade spreading (also called casting on). To this end a SiO.sub.2 slurry layer with a thickness of about 4 mm is applied by doctor blade to the horizontally supported quartz glass plate and a pressure is applied directly thereafter by means of the doctor blade device to the slurry layer such that it is densified to a thickness of about 0.8 mm.

(23) A thin liquid film is formed on the slurry layer applied and densified in this way, and a homogeneous and closed surface layer is formed during the subsequent partial drying in air. A high segregated fine fraction can be seen under the microscope. This means that within the casting skin the fraction of fine SiO.sub.2 particles and particularly of SiO.sub.2 nanoparticles is significantly higher than in the remainder of the slurry layer.

(24) The way of applying the complete layer thickness in one operation provides, on the one hand, an adequately large reservoir of SiO.sub.2 nanoparticles at once, which is suited for segregation on the surface, and prevents on the other hand an excessively rapid drying of the layer in air, which would otherwise counteract segregation and casting-skin formation. Therefore, despite an initially slightly lower solids content, and otherwise under similar process parameters as in Sample 1, a slower drying of about 3 to 5 min and a consolidation of the slurry layer into the carrying layer, which permits the formation of a substantially smooth casting skin, is achieved in Sample 3.

(25) During casting the slurry layer gets its final shape under the action of a tool, such as a doctor blade, a brush, a spatula, or an outlet nozzle from which during application a continuous slurry jet exits. Due to the spreading action of the work tool the layer surface gets slightly more liquid, which facilitates the enrichment of SiO.sub.2 nanoparticles also in the case of a relatively low liquid content. This result, i.e. no significant reduction of the liquid content of the slurry, can also be expected in the case of other application techniques (e.g. injection), in which the slurry layer in its total thickness is produced at once and without division into fine drops of less than 1 mm.

(26) The slurry layer produced thereby is dried within 3 minutes into a carrying layer and is subsequently dried at a still slow pace in that it is allowed to stand in air for 1 hour. The casting skin thereby gets a wax-like appearance. The complete drying is carried out by using an IR radiator in air for 4 to 8 hours, whereupon it has the appearance shown in FIG. 3.

(27) The surface seems to be substantially smooth. The smooth surface portions are formed by SiO.sub.2 fine fraction, i.e. by SiO.sub.2 nanoparticles and their agglomerates and aggregates. The rough surface portions are produced by SiO.sub.2 coarse fraction (see also FIG. 9 in this connection). An evaluation of the surface texture by means of image analysis shows a smooth fraction of about 85% in the total surface. When looking at the surface layer in a lateral section, one can see a skin layer (casting skin) with a thickness of about 9 m (see FIG. 9)though with a weak structure.

(28) The SiO.sub.2 nanoparticles enriched in the surface portion of the dried slurry layer show a high sintering activity and improve the densification of the layer. During sintering of the dried green layer in a sintering furnace at a temperature of about 1400 C., the layer becomes first dense and then closed-porous. One obtains a crack-free and substantially smooth surface layer of opaque quartz glass with a density of about 2.1 g/cm.sup.3 and thus a porosity of 5%.

Sample 4

(29) A further test was carried out as described with reference to Sample 3, wherein, instead of the porous quartz glass plate as the substrate, a quartz glass plate with a dense smooth surface was used. To counteract the running off of the slurry layer, the contents of SiO.sub.2 nanoparticles and solid were increased on the whole.

(30) As a result, after the partial drying of the slurry layer in air a surface layer was obtained having a roughness still lower than that of Sample 3, which layer, however, contained cracks, as shown in FIG. 4. This result, which as such is surprising, can be explained in that due to the non-absorbing substrate the partial drying into the carrying layer took longer here. This leads to an enhanced enrichment of the SiO.sub.2 nanoparticles in the upper region of the slurry layer, i.e. to a comparatively thick casting skin within the meaning of the invention. This effect can lead to strong drying shrinkage and thus to crack formation. When the surface layer is viewed in a lateral section one can seethough with a weak structurea skin layer (casting skin) with a thickness of about 6 m. Coarse-grained SiO.sub.2 particles are almost completely embedded in a mass consisting of finely divided SiO.sub.2, so that they do not penetrate the surface, which explains the particularly smooth surface layer of Sample 4. The evaluation of the surface texture by means of image analysis shows a smooth percentage of almost 100% on the total surface.

(31) The crack formation shows, however, that for the achievement of an optimal result with respect to the surface quality the initial drying speed is an important parameter for the formation of an optimally thick casting skin. An initial drying duration until the achievement of a carrying layer of about 2 to not more than 5 minutes turns out to be optimal. In this respect the absorbing capacity of the substrate is a decisive parameter again.

(32) Drying and sintering are carried out as described above with reference to Sample 3. A dense surface was obtained. This demonstrates that the cracks obtained after drying were restricted to the near-surface region, possibly to the casting skin itself.

Sample 5

(33) In a further test, the result achieved in Sample 3 should be optimized. On a flat plate of absorbent opaque quartz glass with an open porosity, a SiO.sub.2 surface layer is to be produced with a thickness of 2 mm.

(34) In contrast to Sample 3, the quartz glass plate was first soaked for 5 minutes in an ultrasonic bath in horizontal orientation, so that the outwardly open pores were largely filled with water. The quartz glass plate is subsequently lifted in the ultrasonic bath, so that its top side projects beyond the liquid level of the bath.

(35) With the ultrasonic vibration being switched on, a SiO.sub.2 slurry layer with a thickness of about 0.8 mm is applied by doctor blade to the still horizontally positioned quartz glass plate. A pressure which is as high as possible is exerted by means of the doctor blade device, so that the slurry layer is densified to a thickness of about 0.7 mm.

(36) A thin liquid film is formed on the slurry layer applied and densified in this way, and a homogeneous and closed surface layer is produced during subsequent partial drying in air. The layer remains substantially smooth also after the further drying as described with reference to Sample 3, and resembles in its appearance that of Sample 3, as shown in FIG. 5. Here, the image analysis also reveals a high smooth percentage in the range of about 75% on the total surface.

(37) In a microscopic view of the surface layer in lateral section, one can see a skin layer (casting layer)though with a weak structure, the skin layer having a thickness of about 4 m. Within this skin layer the fraction of SiO.sub.2 nanoparticles is significantly higher than in the remaining green layer and accounts for much more than 70% of the volume of the casting skin.

(38) The SiO.sub.2 nanoparticles enriched in the surface portion of the dried slurry layer show a high sintering activity and improve the densification of the layer in the subsequent sintering process. One obtains a crack-free homogeneous layer of opaque quartz glass with a density of about 2.1 g/cm.sup.3, corresponding to a porosity of 5%.

Sample 6

(39) In a further test it was to be checked whether the result achieved with Sample 5 can also be achieved in the case of a different absorbent substrate. For this purpose a surface layer of porous SiO.sub.2 with a thickness of 1.5 mm was first produced on a plate of quartz glass. The porous surface layer was produced by spraying a slurry layer thereonto and by subsequently drying the slurry layer and by sintering according to Sample 2. The volume of this surface layer and thus its suction effect for water are slightly smaller than in Sample 5.

(40) A slurry layer with the slurry of Sample 5 and the process technique explained with reference to this sample was produced during drying and sintering on the surface layer pretreated in this way. A significant difference of the surface layer obtained thereby in comparison with that of Sample 5 was not detected. The diagram of FIG. 6 shows the surface roughness (R.sub.a value) of the green layer samples 1 to 5, namely maxima, minima and mean values for the Ra value (mean surface roughness), each time measured at different measurement points.

(41) It follows that after drying the sprayed-on slurry layer shows a comparatively high mean roughness in comparison with surface layers produced by doctor blade spreading. The lowest surface roughness is achieved in Sample 4, but shows cracks. These cracks, however, have no significant impact on the R.sub.a value. A component with this surface of the sample can be used in applications where a smooth, but not a dense, surface is required. Sample no. 5 achieves the second lowest value of the surface roughness.

(42) The micro CT scans of FIGS. 7 and 8, each with the same magnification (about 50 times) shows cross sections of the green layers of Samples 1 and 5. Hence, Sample 1 (FIG. 8) shows a rugged and irregular surface, whereas the surface of Sample 5 (FIG. 7) is substantially smooth. At a closer look one can see that a thin portion of the surface slightly stands out, which is due to the casting skin.

(43) This surface layer can be better seen in a view on a breaking edge in Sample 5 according to FIG. 9. Here, a casting skin 1 with a particularly fine-grained structure and with a thickness d of about 9 m clearly stands out from the rest of the layer 2 with a more coarse-grained structure. Within the casting skin 1, coarse-grained particles are almost completely embedded in a mass which consists of finely divided SiO.sub.2, which is particularly formed by SiO.sub.2 nanoparticles and agglomerates thereof. The volume percentage of this mass in the casting skin having a thickness of about 9 m is more than 75%. One can also see that coarse-grained SiO.sub.2 particles can hardly penetrate the surface, so that a substantially smooth and dense surface layer is obtained, as is also shown for Samples 3 and 6.

(44) Table 1 summarizes the characteristic production parameters and the measurement results of Samples 1 to 5.

(45) TABLE-US-00001 TABLE 1 F Soot Ra No. Liquid (wt. %.) (wt. %) Technique Substrate [m] Cracks Q 1 DI 74 5 spraying porous/ 24.2 no no absorbent 2 DI 77 9 spraying dense/ 27.1 no no smooth 3 DI 76 4 spreading porous/ 1.8 no yes by doctor absorbent blade 4 DI:ET = 70:30 78 7.5 spreading dense/ 1.2 yes yes only by doctor smooth to a limited blade extent 5 DI:ET = 90:10 78 2.5 spreading porous/ 1.6 no yes by doctor non- blade absorbent 6 DI:ET = 90:10 76 2.5 spreading porous/ 1.8 no yes by doctor not very blade absorbent Meanings as follows: DI: deionized water (as part of the dispersion liquid) ET: ethanol (as part of the dispersion liquid) F: weight percentage of the solids in the total weight of the dispersion Soot: weight percentage of the SiO.sub.2 nanoparticles in the solids content of the Dispersion Technique: Technique for application of the slurry layer R.sub.a: mean value of the surface roughness of the surface after sintering Q: suitable surface after sintering?

(46) A comparison of Samples 1, 3, 5 and 6 on the one hand and of Samples 2 and 4 on the other hand shows that the roughness of the surface is substantially independent of the type of substrate; rather, the application technique is here of decisive importance. The drying duration of the slurry layer until the formation of a carrying layer, and the available segregation time, respectively, are here again of importance. The drying duration follows from the interaction of the moisture content of the dispersion, the application technique and the thickness of the slurry layer. As a rule, a long drying period results in the formation of a dense casting skin and is of advantage if a smooth surface is of importance. With very long drying periods, however, a very thick casting skin may be formed in a corresponding manner, resulting in the formation of cracks during drying, as shown by Sample 4.