US3436306A - Suction box cover - Google Patents

Description

April 1, 1969 R. R. HABERECHT ETAL SUCTION BOX COVER Sheet Filed Dec.

. INVENTORS RONALD c. BRACKEN ROLF R. HABERE T JAMES w. MCCR BY W ATTORNEY April 1, 1969 R. R. HABERECHT ETA!- 374369306 SUCTION BOX COVER Filed Dec. 5. 1965 Sheet 2 of 2 CURRENT SOURCE INVENTORS RONALD C. BRAKEN ROLF R. HABERECHT JAMES W. M CRARY BY 27. m

ATTORNEY United States Patent 3,436,306 SUCTION BOX COVER Rolf R. Haberecht, Ronald C. Bracken, and James W.

McCrary, Richardson, Tex., assignors to Texas Instruments Incorporated, Dallas, Tex, a corporation of Delaware Filed Dec. 3, 1965, Ser. No. 511,546 Int. Cl. D21f N50 US. Cl. 162-374 4 Claims ABSTRACT OF THE DISCLOSURE A method for depositing silicon carbide on a silicon substrate to form a composite suction box cover for Fourdrinier paper-making machines. A suction box cover comprising a cast silicon blank and having an impervious, dense coating of silicon carbide deposited thereon.

This invention relates to suction box covers and more particularly to an improved suction box cover for use with apparatus for the manufacture of paper.

The principal method of making paper involves the use of an apparatus known as the Fourdrinier machine. In such a machine a slurry of wood pulp and water is fed onto a rapidly moving endless wire screen. The slurry deposited on the wire screen generally contains less than about 1 percent wood pulp. The wire provides support for the fiber while the fibers are matted and water drained off leaving a paper mat. A sidewise shaking motion is usually imparted to the moving wire to help orient the fibers and give better felting action. A large portion of the water normally drains through the wire screen. However, to increase the speed and efficiency of drainage, the wire is drawn over a series of suction boxes. These suction boxes have perforated covers which contact the underside of the wire. A vacuum maintained in the suction boxes draws water from the partially felted sheet into the boxes through the perforations in the covers. In order to reduce wire abrasion and drag load on the wire, the wire-contacting surface of the suction box covers should have a smooth bearing surface.

A recognized problem in the paper making industry has been the excessive wear of the wires used in the Fourdinier machine. One source of this problem is sand and pulpstone grit which is present in the Wood pulp slurry. As the slurry is carried over the suction box covers by the Fourdinier wire, the grit abraids the suction box covers. Some of the grit may even become embedded in the wire-bearing surface and, as the bearing surface smoothness diminishes, wire wear increases.

In the past, the suction box covers have been made from igneous materials such as granite and marble, glass, enamel and other hard materials in attempts to provide surfaces with hardness sufficient to resist the embedding and brasion by the sand and pulpstone grit. However, for best results the hardness of the suction box cover bearing surfaces must be at least equal to the hardness of the grit in the pulp slurry.

Suction box covers made of hard, dense ceramic materials such as self-bonded silicon carbide have generally been found unsatisfactory. Such hard ceramic materials are expensive, difiicult to machine, brittle, and cannot be obtained in sufiicient densities to prevent the embedding of foreign material in the surfaces thereof. Furthermore, the brittle materials frequently break when subjected to the stress of high vacuums maintained in the suction boxes.

It is therefore an object of the present invention to provide an inexpensive suction box cover which has a smooth, high density surface. Another object is to provide a suction box cover which may be cast in suitable forms, thus minimizing expensive polishing, milling and machining 3,436,306 Patented Apr. 1, 1969 "ice steps in the fabrication thereof, and a method of producing same. A further object is to provide a strong, durable, inexpensive suction box cover which is capable of withstanding high vacuums and stresses when used in'a Fourdrinier machine.

A particular advantage of the suction box cover of this invention is that it is comprised of a composite material; the substrate being an inexpensive material which may be cast in any desired shape or form upon which a coating of silicon carbide is formed. The silicon carbide coating provides the required high density, smooth, hard surface necessary for a successful suction box cover, and the cast substrate substantially reduces the undesirable and expensive steps of milling, grinding, machining and polishing inherent in the manufacture of previous suction box covers. Other objects, features and advantages of the invention will become more readily understood from the following detailed description when read in conjunction with the appended claims and attached drawing in which:

FIGURE 1 is a perspective view illustrating the general arrangement of a paper making machine;

FIGURE 2 is a top plan view of a suction box cover segment;

FIGURE 3 is a sectional view of a suction box cover in place on a suction box; and

FIGURE 4 is a schematic drawing of a process system illustrating the process of producing the suction box of the present invention.

In accordance with the invention, a suction box cover having a surface film or continuous coating of siliocn carbide is provided. The suction box cover is of a com.- posite structure, however, the substrate being cast silicon and having a smooth, continuous coating of beta silicon carbide thereon.

The general arrangement of the Fourdrinier section of a paper making machine is shown in FIGURE 1. A Four drinier wire 11 carrying the pulp slurry 10 is drawn over table rolls 14, suction boxes 15, and rolls 12 and 13, one of which is driven. A vacuum is maintained in each of suction boxes 15 as hereinabove described. However, the covers of each of suction boxes 15 are comprised of perforated cover segments 20 such as that shown in FIGURE 2. A plurality of the segments 20 are generally arranged adjacent each other to form a complete suction box cover. As shown in FIGURE 2, each segment of the suction box cover has perforations therein through which water is drawn from the Fourdrinier wire into the suction box. The wire contacting surface 25 of the suction box cover 20 must be impervious-and of sufficient hardness to resist abrasion by the pulpstone grit and other foreign matter.

The composite suction box cover ofthis invention is shown in place on a suction box in FIGURE 3. The suction box is comprised of a suction box cover 20 and a base member 21. The cover 20 is conveniently attached to the base member by any suitable means such as clamps 22 and 23, which firmly secure the cover in place on the base member 21. Each suction box is connected to a suitable vacuum source (not shown) by a means such as a suction line 24.

The suction box cover 20 is generally made up of a plurality of segments such as that shown in FIGURE 2. Each segment is provided with a plurality of openings or perforations 26 whereby the vacuum maintained in the suction box draws water from the slurry 10 through the screen and into the suction box base member 21.

In accordance with the invention, each suction box cover segment 20 is comprised of a cast silicon substrate 28 having a hard, impervious, continuous coating of silicon car-bide 25 thereon as shown in FIGURE 3. The silicon carbide coating 25 completely covers the Wire bearing surface of the suction box cover 20, thus providing a smooth bearing surface of a hardness exceeding that of the grit in the wood pulp slurry. The substrate 28 may be conveniently cast in a suitable mold. Any suitable material, such as boron nitride, may be used for the casting mold.- The blank segment may be cast by pouring molten silicon in a mold or form of the blank configuration desired. The perforations 26 can be formed in the casting step, thus eliminating the need for drilling perforations in the blank segment. Furthermore, under proper casting conditions, mirror-like surfaces can be achieved on the cast blank, thus eliminating expensive machining and polishing steps. Even when polishing is required, silicon is much easier to polish than other materials previously used for suction box covers. The blank is then coated with silicon carbide.

The silicon carbide coating 25 may be formed on the surface of the cast silicon blank 28 by a process carried out in the system illustrated schematically in FIGURE 4. The system is comprised of a reaction chamber 100 which may be provided with suitable insulation (not illustrated) in order to efficiently and conveniently maintain a high and uniform temperature within the chamber. The chamber 100 may also be provided with some suitable access means such as a bolted flange connection between the top and bottom halves of the chamber. A suitable support means 102 having a plurality of relatively sharp spikes 104 support the cast silicon blank 28. The spikes 104 contact a minimum area of the blank and space the remainder of the support means 102 away from the lower surface of the cover blank. The support means 102 has a shaft 106 which extends through and is rotatably journaled in the bottom of the chamber 100. A suitable drive is connected to the shaft 106 for rotating the shaft and therefore the blank 28.

A resistive heating element 108 is disposed beneath and axially spaced from the silicon blank 28. The resistive heating element 108 is connected to a suitable variable current source 110. The heating element 108 preferably has a configuration corresponding to that of the blank 28 so as to promote uniform heating of the substrate.

A system for injecting reactants into the chamber 100 includes a hydrogen pressure tank 112 and a material container 114. A conduit 116 interconnects the hydrogen tank 112 and a proportioning valve 118. A second conduit 120 extends from the valve 118 to a point within the material container 114. The end of the conduit 120 within the container 114 extends below the surface of the liquid in the container so as to bubble the hydrogen through the material and entrain vapors of the material in the hydrogen. A conduit 122 extends from the container 114 to a second valve 124. A bypass conduit 126 extends from the proportioning valve 118 to the conduit 122. A pair of fan shaped nozzles 128 and 132 are connected to valve 124 by conduits 130 and 134, respectively. The nozzle 128 is so positioned as to direct the process stream onto the top surface of the blank as the blank is rotated about its longitudinal axis. The nozzle 132, on the other hand, is so positioned as to direct the process stream through the perforations 26 of the blank 28. Additional nozzles may be provided as desired to direct the process stream across the surface of the substrate body. An exhaust conduit 136 having a valve 138 removes the process stream from the chamber 100.

The process of the present invention is an improvement over the process described in US. application, Ser. No. 68,767, entitled, Novel Vapor Deposition Process and Product, filed by William A. Santini, Jr., on Nov. 14, 1960. In the present process, a gaseous stream containing hydrogen, silicon and carbon in an appropriate ratio is introduced into the reaction zone in which is located a heated substrate. The carrier gas of the process stream is hydrogen and the flow conditions and geometry of the reaction zone are chosen with reference to the heated substrate such that the process stream flows about the heated substrate to form a minimum thickness quiescent zone through which a relatively high rate of diffusion occurs to produce the rapid codeposition of silicon and carbon atoms onto the surface of the heated substrate. The proportion of atoms of silicon and carbon that are deposited can be controlled to yield a material which is substantially stoichiometric silicon carbide or may be silicon carbide having either carbon or silicon atoms as a second phase. The process provides a diffusion controlled, surface reaction in which molecules of the reactants move across the thin quiescent zone existing adjacent the surface of the heated substrate by virtue of a relatively high diffusion gradient. The molecules, upon reaching the surface of the substrate, are degraded to yield free silicon and carbon atoms which subsequently react to form a coat of silicon carbide. In the reaction, hydrogen favors the formation of silicon atoms and this can be employed to control the proportion of silicon and carbon atoms formed.

In one specific embodiment of the invention, methyltrichlorosilane is used to supply the silicon and carbon atoms in the hydrogen carrier gas and form the process stream. The heating element 108 is resistively heated by current from the source 110. The segment blank 28 is then rotated by the drive means connected to the shaft 106 at a relatively slow rate of from about 0.5 to 10 rpm. As the segment blank 28 is rotated, it is uniformly heated by radiant energy from the heating element 108 to a temperature of from about 900 C. o about 1400" C. A suitable heat sensing means such as an optical pyrometer (not illustrated) may be used to determine the temperature of the blank 28 and automatically control the current to the heating element to maintain a present temperature.

After the silicon blank has been uniformly heated to the desired temperature, some hydrogen from the pressure tank 112 is passed through the proportioning valve 118 into the container 114 where methyltrichlorosilane vapor is entrained in the hydrogen. The hydrogen-methyltrichlorosilane vapors then pass through the conduit 122 and are reunited before the valve 124 with pure hydrogen bypassed through the conduit 126 by the proportioning valve 118 Thus the valve 118 provides a means for controlling the ratio of hydrogen to methyltrichlorosilane in the process stream. The mol ratio of the hydrogen to the methyltrichlorosilane should be from about 50:1 0 about 4:1. The ratios between the hydrogen and methyltrichlorosilane may be determined by a thermal conductivity cell. The flow rate of the process stream may be monitored by suitable flow meters and controlled by the valve 124. As the process stream leaves the valve 124, it is divided and passes through the nozzles 128 and 132 and is directed into the reaction chamber 100 onto the hot segment blank 28. The process stream is injected through nozzle 128, and silicon carbide is deposited on the surface of the blank as the blank is rotated under the nozzle. Similarly, the process stream of the nozzle 132 is directed at the undersurface of the blank 28 as the blank is rotated. The process materials are exhausted through the conduit 136 and valve 138.

In order to afford a better understanding of the Silicon carbide coating, specific properties of silicon carbide coatings produced by the present process will now be presented. A relatively Wide range of process variables including the mol ratios, process stream flow rate and therefore flow velocities, temperature of the substrate, and duration of the run will vary the thickness and physical and chemical properties of the silicon carbide coat within the following ranges. Runs have been conducted using hydrogen flow rates of 20 liters per minute to 100 liters per minute depending upon the system. Duration of the runs have been varied from 0.05 to 8.7 hours at temperatures from 1180 C. to 1400 C. Silicon carbide coatings having thicknesses from 3 to 115 mils have been produced. The silicon carbide coatings have beta crystalline structure, are very dense and are essentially fluidimpervious. The silicon carbide coatings have ranged from stoichiometrically pure silicon carbide to silicon carbide having as much as 0.89% free carbon or as much as 36.5% free silicon as a second phase element, depending upon the excess materials present and the amount. The compressive strength of the materials produced by the process ranges from about 31x10 to about 55x10 psi. The modulus of elasticity ranges from about 45 10 to about 50x10 psi. The coefficient of thermal expansion ranges from about 4.0 10 to about 5.4x l0- in./in./ C. when tested in the temperature range of to 810 C. The Knoop hardness of the material determined by utilizing a 1000 gram load ranges from about 988 to about 2900. The resistivity of the material ranges from 0.005 to about 4 ohm-centimeters. The density determined by water displacement ranges from 2.59 to 3.28 grams per cc. The material was checked for thermal shock and no appreciable adverse effect resulted when material at 1000 C. was plunged into water at room temperature.

Atlhough methyltrichlorosilane was specified in the above described example, it will be appreciated that various other materials can be employed to furnish the silicon and carbon without departing from the teachings of the invention. For example, the silicon carbide source may be single compounds, such as dimethyldichorosilane, trimethylchlorosilane, tetramethylsilane and other aliphatic and aromatic substituted halogenated silanes. Also, the silicon atoms and carbon atoms may be supplied in separate compounds. For example, the carbon atoms may be supplied by compounds such as methane, ethane, propane, benzene, toluene, xylene, ethylene, propylene, and other aliphatic and aromatic hydrocarbons, and the silicon atoms may be supplied by compounds, for example, such as silicon tetrachloride, silicon tetrabromide, silicon tetraiodide, or any one or more of mono-, di-, and trichloro-, bromo-, and iodo-silane.

The silicon carbide coating prepared as described above is a uniform, continuous, reproduction of the surface of the cast substrate. Since silicon can be cast in a suitable blank having the perforations therein and having a smooth mirror-like surface; milling, machining and polishing steps in the process of making suction box cover segments are substantially reduced. The segment blank 28 can be cast from molten low quality silicon thus substantially lowering the cost of suction box covers. The silicon carbide coating is applied directly to the casting surfaces by the process described hereinabove. Therefore, since the surface of the casting is smooth, the silicon carbide film surface is likewise smooth, thus eliminating or substantially reducing the amount of grinding and polishing necessary to produce a smooth bearing surface for suction box covers.

As an alternate method of producing the composite suction box cover of this invention, the surface of the silicon blank can be carbonized to produce a coating of silicon carbide utilizing the silicon blank as the silicon course. For example, the silicon surface can be carbonized by heating the silicon blank 28 as shown in the apparatus of FIGURE 4. In this method of forming the silicon carbide coating, the silicon blank is heated to a temperature of about 700 C. to about 1400 C. A gase ous carbon containing compound, such as the organic compounds listed above, is directed at the hot silicon surface. The organic compound is decomposed, yielding free carbon to react with the hot silicon surface and form silicon carbide. The composition of the coating formed by this method varies with temperature. The composition formed at lower temperatures will be silicon carbide having carbon as a second phase. Beta silicon carbide is formed at the higher temperatures.

Coating silicon with silicon carbide results in an integral composite structure combining the desirable light weight and strength of silicon with the wear resistant qualities of silicon carbide. Thus the suction box cover of this invention advantageously combines the desirable qualities of both materials in a strong monolithic structure having an extremely hard surface.

The use of silicon casting for the suction box cover segment blank provides other advantages. For example, silicon is lighter than silicon carbide and stronger than graphite. Polycrystalline silicon has a breaking strength of 10,000 to 35,000 psi. while industrial graphite varies in a tensile strength from about 400 to 800 p.s.i. and a crushing strength between 1900 and 8500 p.s.i. Silicon is easily finished to a high polished finish and may be cast in any suitable form, while graphite and silicon carbide cannot be cast. Furthermore, the coefiicient of thermal expansion of silicon closely approximates that of silicon carbide. Thus, coatings of silicon carbide on silicon are not subject to cracking from thermal shock as are silicon carbide coatings on graphite, and the composite structure is strong and durable and will not break under high vacuums or heavy stress.

It will be understood that although the invention has been described with particular reference to suction box covers comprising a plurality of segments, the invention is equally applicable to suction box covers of unitary construction. It is to be further understood that the above described apparatus and methods of using it are merely illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.

What is claimed is:

1. In a paper making machine, a suction box cover comprising a cast silicon blank having perforations therein and having a continuous coating on the surface thereof, said coating being characterized by:

(a) silicon carbide having a beta crystalline structure containing therein free carbon in an amount less than about 0.89 percent by weight and free silicon in an amount less than about 36.5 percent by weight; and

(b) a. hardness of about 988 to about 2900 on the Knoop scale.

2. The method of making a composite suction box cover segment comprising a silicon substrate with a silicon carbide coating on the surface thereof comprising the steps of:

(a) casting a silicon suction box cover blank; and

(b) heating said cover to a temperature of about 900 C. to about 1400" C. and directing a fluid process stream of hydrogen, silicon-containing compounds, and carbon-containing compounds against the cover whereby the surface of said cover is coated with a continuous coating of silicon carbide.

3. The method of claim 2 wherein said compounds are in the form of methyltrichlorosilane, said methyltrichlorosilane being degraded to form silicon and carbon atoms which combine to form said continuous coating of silicon carbide.

4. The method of claim 2 wherein said coating is formed by heating said cast silicon suction box cover segment blank to a temperature of about 700 C. to about 1400 C. and directing a fluid process stream comprising a gaseous carbon bearing compound against the cast silicon suction box cover segment blank whereby said carbon bearing compound is degraded to yield carbon atoms which react with silicon atoms in said cast silicon suction box cover to form said continuous coating of silicon carbide.

References Cited UNITED STATES PATENTS 3,'067,816 12/ 1962 Gould 16 2-374 DONALL H. SYLVESTER, Primary Examiner.

A. C. HODGSON, Assistant Examiner.

US. Cl. X.R.

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