CA1140165A - Supporting members in paper or paperboard making machinery - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A supporting member, for example, the screen table supporting table, foil, suction box cover or tubular suction apparatus, for positioning in engagement with the screen or felt of a paper or cardboard machine, which supporting member is capable of supporting, or assisting in the supporting of a screen, for drainage and for sheet forming, and which has a part or parts as a wear element or elements that are in contact with the screen or felt and consist of one or more ceramics components of hard, dense sintered aluminium oxide having an Al2O3 content of at least 95% by weight, the or each ceramics component having the following features:
(1) a content of Al2O3 of from 97% to 98.5% by weight, a content of MgO of 0.5 + 0.3% by weight, and a content of SiO2 of from 1% to 2.5% by weight;
a total of not more than 0.2 weight % impurities being present (2) a Synder-Graff grain size of 5/µm + 2µm;
(3) a Vickers hardness of more than 1600 daN/mm2 at a testing load of 0.2 daN/mm2;
(4) a bending strength of at least 300 N/mm2;
(5) a transcrystalline fracture proportion of more than 70%; and (6) a surface characteristic specified by (a) a roughness of Ra < 0.2µm, measured with a feeler needle having a radius of 3µm on a polished surface, (b) a proportion of pores in the surface of less than 10%, and (c) a mean (median) pore size < 5µm.

Description

The invention relates to supporting members, such as, for example, the screen table, supporting table, foil, suction box cover and tubular suction apparatus, which in use are posi-tioned in engagement with the screen or felt of a paper-or card-board-making machine, which supporting members in particular serve for the support of the screen, for drainage and for sheet forming, and in which at least the parts iA contact with the screen or felt consist of ceramics components made of hard, dense, sintered aluminium oxide having an A12O3 content of at least 95%. It also relates to ceramic components themselves, a process for the manufacture of the components, and to paper-making and cardboard-making machines incorporating one or more such components.
Supporting members having ceramics components made of sintered hard materials, especially of hard, dense, sintered aluminium oxide, have, in the last 15 years, substantially superseded the conventional materials for such supporting members, such as wood or plastics materials, because they have the ad-vantage of a substantially greater resistance to wear and an almost unlimited life span. In addition, it has been found possible considerably to increase the service life of the screen by using supporting members having ceramics components of sintered aluminium oxide. Supporting members may consist entirely of ceramics components, of which several are arranged next to one another, until the full width of the paper machine is covered.

Alternatively, only those parts that are as such in direct contact with the screen or felt and are subjected to a particularly high degree of stress may consist of ceramics. Eor example, the surfaces facing the screen in the case of suction box covers may be made of ceramics, or the sharp leading edges and the transition points from the horizontal to the inclined surface may be so made in the case of foils in which the ceramics components are incor-porated as so-called inserts.
Such supporting members are described, for example, in DE-PS 12 27 326. It was also already known at that time that the surface characteristic of the supporting members of sintered aluminium oxide is of quite considerable importance. The technical teaching of DE-PS 12 27 326 is more especially concerned with having as small a supporting area as possible and a rela-tively large proportion of pores in the surface of the ceramics material, in order to avoid a surface contact over the whole of the surface and thereby a high frictional resistance, and, by the intentional inclusion of pores, to produce a film of water by which the screen or the felt sliding thereover is supported.
In addition, a content of A12O3 of at least 95% was considered necessary, because, with a relatively large proportion of impurities, these are comparatively easily dissolved out at the grain boundaries and then lead to destruction of the entire grain structure and thus to abrasion. Because of the abrasive character of the loose grains, ~14`~165 the abrasion is not confined to the supporting member itself, but the screen sliding thereover is also especially heavily affected once individual grains have broken away.
A first aim in the further development of these supporting members of ceramics components made of sintered aluminium oxide was to strive for increasingly higher contents of ~1203, in the expectation that the hardness, density and abrasion-resistance of the suppor-ting members would be thereby further increasedO
Even though the supporting members having ceramicscomponents made of sintered A1203 have proved ~
success in paper machines, difficulties have occasionally still been encountered, including an unexpectedly higher rate of abrasion of the screen. The rapid abrasion of the screen can be attributed in part to the fàct that, owing to the relatively low friction of the ceramics components of sintered aluminium oxide, the op rat1ng speed of the paper machines in question has been increased. It has been found, however, that the aforesaid difficulties occur whenever work is being perormed with suspensions of fibrous material having an especially high content of fillers, those suspensions being especiallv common in the manufacture of very light-weight papers, and in the printing of newspapers and illustrations (for .~
example, illustrated magazines). In those instances it was noticed particularly that the individual fillers, dependil~g on their origin, had widely varying compositions and the highly abrasive constituents, such as alpha quartz and feldspar, were present in a very high proportion in some of the fillers used, and, for example, in some fillers used in Japan, amounted to up to 70% by weight of the filler. The average grain size of these quartz portions generally lies between 10 and 50Jum.
Investigations under standardised conditions showed that even the relatively low proportion of from 5 to 10% by weight of large grains of such abrasive constituents, such as alpha quartz, is sufficient to cause a dramatic increase both in the abrasion of the cover, that is to say, of the ceramics components, and also of the screen.
This influence of the fillers on the abrasion is parti-cularly great in the case of the plastics screens which are gaining increasingly in popularity, and which, in contrast to the bronze screens, are so-called "weft runners" in their type of weave, that is to say, the weft wire, which extends longi-tudinally with respect to the direction of movement of the screen, comes into contact with the cover and, when there are defects in the cover, becomes particularly badly worn. In addition, the abrasive filler particles are pushed across the cover of the supporting members by the weft threads and, at the high speed of the screen, in turn cause a higher abrasion of the cover than was the case with bronze screens, in which the warp threads run in the direction of movement of the screen.

114~165 This disclosure is there~ore based on the real~sation that there is a close connection between the parameters of the screen, the desirable properties of the cover of the supporting members and the proportion of filler in the fibrous material suspension, and it is an object further to improve the supporting members having ceramics components made of sintered aluminium oxide, increasing their resistance to abrasion, and reducing their abra-sive action on the screen or the ~elt sliding thereover.
More particularly, in accordance with one aspect of the invention, there is provided a paper machine screen or felt wear element comprising:
(1) about 97 to 98.5 ~ by weight A12O3, about 0.5 - 0.3 by weight MgO, and about 1 to 2.5 % by weight SiO2 and not more than about 0.2 ~ by weight impurities; and having

(2) a Synder-Graff grain size of 5 ~um - 2 ~m;

(3) a Vickers hardness of more than 1600 daN/mm at a testing load of 0.2 daN/mm ;

(4) a bending strength of at least 300 N/mm i - (5) a transcrystalline fracture proportion of more than 70~; and (6) a surface characteristic determined by (a) a center line average roughness Ra <0.2 ~um, measured with a feeler needle radius of 3 ,um on a polished surface,

5 --:~14~.65 (b) a proportion of pores in the surface of less than 10 %, and ~e a~
(c) a mcli~r. pore size~5 ~m.
By keepiny to these quite specific requirements as regaeds the composition and the physical properties of the ceramics material and the surface characteristic thereof, the abrasion factor for these supporting members can be reduced to a low value that was not previously thought possible.
Thus, supporting members according to DE-PS
12 27 326~having an A1203 content of 97 % had an abra-sion factor of 1000. By raising the A1203 content to 99.5 %, the abrasion factor could be reduced to a valae between 30 and 400. On the other hand, ceramics components according to the invention having a content of A1203 of 98 % and having the other specified features in accordance with the invention have an abrasion actor of less than~l.
The abrasion factor is a value that is determined as follows:
The abrasion is ascertained usir.g a ring-to-ring process. In that process, a 1 mm wide friction wheel having a diameter of 66 mm and made of hardened steel having a Rockwell hardness RC 65 and a ground surface of a roughness Ra = 4~m i5 driven under a specified contact force at a constant circumferential speed and with a given degree of slip against the ceramics test piece to be investigated, which has a diameter of 40 mm ~14~165 , and a ground and polished surface having a roughness Ra~
0.2~m. As a result of the test conditions, a specific Hertzian surface pressure is obtained. The quantitative evaluation of the abrasion is undertaken by measuring the run-in groove on the test body. The abrasios~ factor Vz is defined as the quotient of abraded volume and glide path given by the equation V~ = V/W ~mr,l3.104)~m.
The numerical values referred to above for the abrasion factor were ascertained under the following o?erating Io conditions:
Contact force 86.6 N (~.6S kp) Circumferential speed of the friction wheel 60 m/min Circumferential speed of the test body56.4 m/min _ Slip 6 %
Temperature 30C
Running time 24 h Running path5200 m.
20 (It will be noted that 60x60x24x,oo=5rl84).
It should be noted that this immense increase in the serviceability of the ceramics components according to the invention is only achieved when all of the specified conditions are satisfied. It should be noted particularly that the invention is-based on the interaction of al] those conditions and also that in some cases one condition entails another.

It was wholly unexpected that an A1203 content of from 97 to 98 %~ compared with a higher percentage A1203 ceramics material, would bring such a great technical advance. In general, the principle that the important properties, such as inter alia hardness and resistance to abrasion, are improved as the A1203 content increases, is true. The fact that a ceramics -material having a lower ~1203 content is better suited to the intended purposes provided for here is probably accounted for by the fact that the use of additions of specific amounts of magnesium oxide and silicon oxide enables a whole series of further properties to~be achieved which could not be realised with a higher A1203 content.
Thus, these defined additions render possible the requisite high binding force between grains, the significance of which is explained hereinafter. They also allow the sinter temperature to be kept re~ tively low, advantageously below ]70GC, preferably, at a temperature within the ranse of from 1620C to 1650C.
As a result, it becomes possible to observe the further, absolutely necessary, relatively small mean grain size of < 5 ~ m according to Synder-Graff, which is of special importance because, in general, the strellgth of a ceramics material is better, the smaller is the grain.
A small grain size is necessary, however, because the size of those pores that are still present at the surface - ~14~)~65 - can thereby be kept small, the importance of that being discussed in dctail hereinafter.
It is a known fact with sintered hard materials of ~1203 that the hardness is appreciably reduced with a decreasing A1203 con~ent and an increase in impurities. As a result, however, of the additions of specific amounts of magnesium oxide and silicon oxide, that undesirable effect can be limited to a large extent and a Vickers hardness of more than 1600, preferably 1~00 + 50 daN/mm2 at a testing load of 0.2 daN/mm2 can be achieved. It is absolutely essential to comply with this minimum hardness, because otherwise t~here would not be an adequate resistance to abrasion even with a low surface stress.
` An absolutely essential, further important feature of the ceramics components according to the invention for ~ these supporting members is a bending strength of at least 305 N/mm2 ~hat is absolutely vital because many OL
these supporting members, such as the screen table and foils, have sharp leading edges and are especially exposed to the risk of fracture at those sharp leading edges. In addition, the mechanical stress to which these supporting members are subjected mere;y as a result of the high machine operating speeds of up to 1000 m/min is quite considerable. The bending strength is measured on columnar test bodies having a length of 60 mm and a square cross-section having an edse length of 4 mm, the test body having a finely ground surface.

g _ 114~)165 The high bending strength is also closely connected with the other specified features, such as the defined composition with quite specific additives and the small grain sizes of the ceramics components.
One of the most important influencing factors on the extremely low abrasion factor is that the transcrystalline fracture proportion is more than 70 %.
Preferably, it is at least 80 ~ or more than 80 %. Such a high transcrystalline fracture proportion, which again is substantially dependent on the sintering conditions, especially on the relatively low sintering temperature, the small grain size and in particular, the very pure starting materials and speciEied additions, results in a very high binding force between grains. Explained simply, such ceramics components are distingulshed by _ the fact that, when exposed to breaking stresses, the fracture occurs not at the boundaries between grains, but within the grain itself. Crumbling at the grain boundar es, as .cquently occurred previously, is responsible for the wear of the cover itself, because the grain structure of the ceramics component is loosened by crumbling grains.
Analysis of the transcrystalline fracture proportion is effected by measuring the fractures on a scanning elec-tron photograph or by projecting the scanning electron image onto a screen and measuring the fractures.
In that process, the transcrystalline fractures, which can be clearly distinguished visually by shape and \
1~4~)~65 surface structure from intcrcrystalline fractures are expressed as an area by adding together the areas of the broken crystalline areas and then expressed as a percentage of the observed total surface area. Moreover, it is possible to determine the proportion of fractures with automatic structure analysis using appropriate apparatus.
The property of the high transcrystalline fracture proportion is again occasioned by the fact that the purity of the A1203 is intentionally kept below the highest purity, namely in the rar.ge of from 97 to 98.S %, and that specified additions of magnesium oxide anc~ silicon oxide are made. As a result, an increase is achieved as regards the transcrystalline fracture proportion and therewith a combination of properties that is much more significant than the greater hardness that can be achieved with the use of extremely pure A1203 sintered articles.
As already stated, the surface characteristics of the ceramics components for the supporting members are also of considerable irnportance. The first of the D characteristics is the roughness, which is less than ~ , when measured with a feeler needle radius of 3~m on a polished surface, and it has a considerable influence on the wear of the screen sliding over the supporting members.
~ he pore area proportion of less than 10 ~ is of specially important because, purely qualitatively, the 6S~

possibility of sm~ll filler particles becoming fixed in the ceramics surface is thereby reduced. The risk of individual grains breaking out of the crystalline lattice is also reduced when the pore area proportion is smaller.
Two factors contribute to the pore area proportion :
firstly, the number and sizes of the pores cut at the surface, that is to say, which were present in the sintered aeticle before the surface in question was formed, and, secondly, the pores that are additionally formed in the surface as a resu]t of treatment of the surface.
As already stated, the size of the pores is of very considerable importance, and the pore size has a median value of less than 5~m, preferably, less than 4~m. That small pore size, which is closely linked to the small mean grain size, is of such critical importance because small -- - particles of filler having a size of more than 5~m in the f krous ma erial suspensivn, which are especially responsible for the abrasion on the screen and the cover, are unable to lodge in these small pores.
~e~
D The expression "mcdian-value" shall be understood to mean the following :
The particle size distribution of the filler, and the pore size distribution of the ceramics cover are normally represented by a summation distribution such that the quantity parameters are plotted against the percentage frequency up to 100 ~. Corresponding summation distribution curves are normally characterised -`' 114V165 - in simplified form by specifying the median value. The med.ian val.ue is defined as the poi.nt of intersection of the summation curve with the 50 % line of the frequency function and therefore denotes the median size (particle size or pore size~ for which 50 % are larger than it and 50 % are smaller than it.
However, it is not only desirable for the median value to be low; especially preferred ar~ those ceramics components with which care has been taken to ensure that, if possible, no pores, or only a few pores, have a size of more than 5~m. A ceramics material is therefore preferred in which 90 % of all pores are less t~han 5~m in size, and especially preferably less than 4~m in size.
The importance of the relatively smal]. pore area proportion of less than 10 % and above all of the median size of less than 5~m, will be obvious from the following -- comparison with the A1203 ceramics material according to DE-PS 12 2~ 326. That matoria~. had a pore are~ ~ro-portior. of 20 ~ and a mean ~median) pore size of ~um.
Tests were performed to investigate the effect of kaolin as a filler on the strength of a plastics screen having weft runners. In those experiments, the ceramics components of DE-PS 12 27 326 suffered a loss of strength in the case of the screen of 84 %.
~iS G~ o5 l~re ' ~ Ceramics components according-~o t~o i~ ntio~
having a pore area proportion of 9 % and a median pore size of less than 4~m, on the other hand, exhibit a loss in streng~h of the screen of only 27 ~.

~ ' .

4~)165 Even greater is the influence of the small mean pore diameter in the case of CaC03 illers, which are to be particularly feared on account of their strong abrading action on the cover material and es@ecially on the screen.
The measurement of the screen strength is carried out in accordance with the following method :
The infl~ence of filler and cover surface of the aluminium oxide ceramics material on the screen wear was ascertained using a special testing device. In ~hat test, several strips of screen approximately 20 mm wide were placed around a 150 mm wide ceramics roll~er of a diameter of 115 mm with a contact angle of 180, and the roller was caused to rotate about its axis at various specific circumferential speeds against the stationary strips of screen, which were subjected to a defined tension. The tension was applied by hanging 1300 g weights ^n ea_--h strip. The arrangement was placed in a container that was filled witll a filler SUSpenSiGn ~ept in motion with a stirring device. After carrying out the abrasion test with constant operating parameters, the residual tensile strength of the screen was ascertained, the residual tensile strength being regarded as a measure of the abrasion on the screen. The residual tensile strength was determined by a rupture test on the screen strip.
I'he density for the ceramics components of these supporting members is advantageously more than 96 % of the theoretical density and is approximately 3.80, whilst the theoretical density of the claimed composition ~ould be around 3.92. Also with this property of the density it is not, therefore, the optimal density that is required, on the contrary, in conjunction with the other properties, such as the defined additions and the high binding force between grains, the given values are adequate.
It should be es~ecially emphasised that the corrosion resistance of these ceramics components both in acidic ~nd in alkaline media and over a long period of being subjected to the media, even at elevated tem-peratures of from 60 C to 70 C, is excellent. Thus, for example, heating for 100 hours in 2N hydrochloric acid at a temperature of 100C produces a loss in strength of less than 10 % relative to the initial strength, and generally it is even below 5 %. The bending strength is t~ken as the measu.e ~f this loss in strength.
This compleLe-iy unexpected behaviour in view of the relatively low proportion of A1203 and the relatively low density, can probably again be attributed to the precisely defined additions to the A1203 and the large binding forces between grains that stem therefrom. In general, the corrosion commences at the grain boundaries and the glass-like portions of binders that are usually present there. If the latter are present in the orm of conventiona] impurities, then they can be especially easily dissolved out, the crystalline structure opens up and individual grains break away. In contrast to that, the use of very pure A12O3 as starting material, and magnesium oxide and silicon oxide, likewise in pure form, as the defined additions, produces the high binding force between grains required according to the invention, so that even corrosive liquids are virtually no longer able to attack the grain boundaries.
The achievement of the combination of properties required according to this disclosure is closely linked with the process according to which the ceramics components for the supporting members are manufactured.
In accordance with a second aspect of the invention, there is provided a process for the manufacture of a paper machine screen or felt wear element which comprises the steps of preparing a mixture of from 97 % to 98.5 % by weight of pure, pulverulent A12O3 and 0.5 + 0.3 % by weight of pure, pulverulent MgO and from 1 to 2.5 - 0.5 % by weight of pure, pulverulent SiO2 of a mean grain size of from 1.0 to 2.5 ,um, not more than 0.2 % by weight . .
impurities being present, with the addition of additives to facil-itate compression, compressing the prepared mixture at a pressure of between 400 and 1200 daN/cm, to form a green, shaped article, and sintering the green article at a temperature within the range of from 1600C to 1700C to give a dense sintered article. The starting materials used are pure, so that only traces of contaminating constituents such as Na2O, K2O, Fe2O3, inter alia, are present, and do not amount in total to more than 0.2 % by weight.
The starting powders are preferably already used in a relatively small mean grain size, which is below 4~ m and preferably below 2.5 ~m. By grinding these powders for a period ranging from 6 to 10 hours in a vibratory mill, they are brought to a mean grain size of from 1.0 to 2.5 ~m.
After grinding, the homogeneously blended powder is advantageously made into an aqueous suspension by the addition of 1 % by weight of polyvinyl alcohol, and then spray-dried, and the spray-dried powder is compressed to form a green, shaped article.
Heating to the sintering temperature within the range of from 1600C to 1700C is preferably carried out at a heating-up rate of approximately 5~C!h, and the sintering temperature within the range of from 1600C to 1700C is advantageously maintained for a period ranging from 2 to 8 hours. The preferred sir.t2ring temperature is around 1620C to 1650C.
The following example illustrates the invention.
97.7 ~ by weight of aluminium oxide powder having a purity of 99.9 % was ground with the addition of 0.5 %
by weight of magnesium oxide powder of high purity and 1.7 % by weight of silicon dioxide powder of high purity, the grinding being effected in a vibratory mill for 5 hours. The starting powders had a mean particle size of from approximately 3.5 to 3.8~m. After grinding, the mean particle size was 1.8~1n. The ground powder 11~0165 homogeneously mixed with the additives was brought into aqueous suspens;on by the addition of 1 ~ by weight of polyvinyl alcohol, and then spray-dried.
The spray-dried powder was compressed at a pressure of 1000 da~/cm2 to form a suction box cover measuring 180 x 400 x 20 mm. After a mechanical treatment and application of the desired geometric contours, the compacted shaped article was heated at a heating-up rate of 50C/h to a temperature of 1630C, and maintained at that temperature for a period of ~ hours. The adjustment of ~he desired surface characteristic was effected by grinding and polishing operations using diamond tools. The suction box cover produccd in this manner had a Vickers hardness of 1800 daN/mm2 àt a testing load of 0.2 daN/mm2, a bending strength of 360 N/mm2, and a transcrystalline fracture proportion of more than 82 %. ~`he surface roughness, measured with a feel-r needle radius of 3~m on the polished surface was ~a = O.llt~m. The median pore size was 4.0~m. mhe density was around 3.80.

_. ,

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A papermachine screen or felt wear element comprising:
(1) about 97 to 98.5 % by weight Al2O3, about 0.5 ? 0.3 %
by weight MgO, and about 1 to 2,5 % by weight SiO2 and not more than about 0.2 % by weight impurities; and having (2) a Synder-Graff grain size of 5 µm - 2 µm;
(3) a Vickers hardness of more than 1600 daN/mm2 at a testing load of 0.2 daN/mm2;
(4) a bending strength of at least 300 N/mm2;
(5) a transcrystalline fracture proportion of more than 70%; and (6) a surface characteristic determined by (a) a center line average roughness Ra< 0.2 µm, measured with a feeler needle radius of 3 µm on a polished surface, (b) a proportion of pores in the surface of less than 10%, and (c) a mean pore size< 5 µm.

2. A wear element as defined in claim 1 having a Vickers hardness of 1800 ? 50.

3. A wear element as defined in claim 1 having a trans-crystalline fracture proportion of more than 80%.

4. A wear element as defined in claim 1, 2 or 3 having a mean pore size of <4 µm.

5. A wear element as defined in claim 1, 2 or 3, wherein at least 90 % of the pores have a size not greater than 5 µm.

6. A process for the manufacture of a paper machine screen or felt wear element which comprises the steps of preparing a mixture of from 97 % to 98.5 % by weight of pure, pulverulent Al2O3 and 0.5 % ? 0.3 % by weight of pure, pulverulent MgO and from 1 to 2.5 % by weight of pure, pulverulent SiO2 having a mean grain size within the range of from 1.0 to 2.5 µm, not more than about 0.2 % by weight impurities being present, with the addition of additives to facilitate compression, compressing the prepared mixture at a pressure within the range of from 400 to 1200 daN/cm2 to form a green, shaped article, and sintering such green article at a temperature within the range of from 1600°C to 1700°C to give a dense, sintered article.

7. A process as defined in claim 6, wherein the pulveru-lent starting mixture is brought to a mean grain size within the range of from 1.0 to 2.5 µm by a grinding operation of from 6 to 10 hours' duration in a vibratory mill.

8. A process as defined in claim 6, wherein the temperature of the green, shaped article is increased at a rate of approximately 50°C per hour to the sintering temperature within the range of from 1600°C to 1700°C and is thereafter maintained at a temperature within that range for a period within the range of from 2 to 8 hours.

9. A process as defined in claims 6, 7 or 8 wherein the sintering temperature is within the range of from 1620C to 1650C.