|
PatentVote.com: Vote on your favourite invention!
|
|
 |
Title: Refractory abrasive body containing chromium oxide and method of producing it
Description: Description:
In Applicants' previously filed application, Ser. No. 642,704 filed June 1, 1967 and now abandoned in favor of continuation application Ser. No. 063,998, filed June 18, 1970, now U.S. Pat. No. 3,734,767 issued May 22, 1973, of which the present application is a continuation-in-part, Applicants disclosed the process of treating underfired porous partially vitrified relatively soft refractory ceramic which comprises the steps of shaping an underfired partially vitrified relatively soft refractory ceramic into a predetermined shape, impregnating the shaped ceramic with phosphoric acid and curing the impregnated ceramic at temperatures of at least 600.degree. F., but below vitrification temperatures for a time sufficient to drive out the moisture and produce a hard ceramic. Also disclosed was a process of producing a chemically hardened ceramic body which comprised the steps of providing a structure of a porous underfired partially vitrified substantially pure machinable refractory ceramic oxide, impregnating the core with a metal compound capable of being converted to an oxide and curing the impregnated core at temperatures of 600.degree. F. and above for a time sufficient to convert the impregnant to an oxide to harden the ceramic. Ceramic materials normally undergo substantial dimensional changes during the usual firing or vitrification steps. Thus, it has heretofore been extremely difficult to produce precision parts or intricate shapes from ceramics. Precision parts had to be shaped slightly oversize before firing. After firing, the parts required further machining with diamond cutting wheels or by using lapping methods. Many intricate shapes were just not available since thin sections of parts would crack during firing. In accordance with the invention, it has been found that underfired or so-called machinable grade refractory ceramics can be shaped while in the relatively soft state and then impregnated and heat treated to produce a ceramic having all the characteristics of a vitrified ceramic without the usual change in dimensions. The process of the instant invention appears to be useful in the treatment of such refractory ceramic materials as the oxides of aluminum, beryllium, zirconium, titanium, magnesium and the like. These materials in the commercially available machinable grade are quite soft and easily broken. Also, in the soft state, they can be readily cut with carbide cutting tools, drilled, filed, sanded and otherwise formed to practically any desired shape. One such aluminum and beryllium oxide material is available from Coors Poreclain Company of Golden, Colo. When the machinable ceramics are treated by the method of this invention, they become very hard, approximating highly vitrified ceramic and, in addition, will retain the original machined and pre-treated dimensions. The treated material becomes so hard that the only practical method to do further machining is with diamond cutting wheels or by using lapping techniques. The commercial value of the instant invention is readily seen when it is recognized that close tolerances on many intricate vitrified ceramic parts can only be obtained by machining with diamond cutting methods after firing. This is the case since there is considerable shrinkage which occurs during the firing. Also, there are many desired shapes which cannot be economically cast or molded during the firing process. In addition, it is often not feasible to construct molding dies for small quantities of a particular part. The method of the present invention in contrast thereto permits easy machining of parts to exact tolerances and then hardening the part without change in original dimensions. It has now been found that the hardening process may be equally applied to the hardening of non-sintered bodies. It has been found that the base refractory material can be prepared in a powdered form (such as ball-milled aluminum oxide) and simply pressed, molded, slip cast, extruded or otherwise processed so that the base oxide particles are packed into close proximity to provide a porous body. The hardening of the non-sintered bodies is essentially the same method as applied to the porous, partially sintered materials. The hardening is accomplished by impregnating the porous body with a metal compound, which may be in solution, which compound is capable of being converted to the metal oxide in situ at a temperature below sintering temperature in the range of from about 600.degree. F. to about 1500.degree. F. and heating the body to convert the compound to its oxide. The impregnation and cure cycle must be repeated at least for two cycles to provide any usable hardening. It has further been found that other finely divided materials, such as a powdered metal, oxide mixtures and the like will serve as the base material which may also contain additives such as glass or metal fibers or abrasive grains to provide special characteristics in the finished product. It is, therefore, the principal object of this invention to provide an improved low temperature process for the forming and treating and shaping and treating of relatively soft porous bodies which avoids one or more of the disadvantages of prior art methods of producing close tolerance hardened shaped parts. A further object of the present invention is to provide an improved low temperature process of producing hardened articles of manufacture of predetermined shapes, of predetermined characteristics and of predetermined dimensions. Another object is to provide an improved low temperature method of producing articles of manufacture in close tolerance shapes of selected hardness, porosity and surface characteristics. A still further object of the invention is to provide an improved process for the production of ceramic bearings capable of use with or without lubricants under unfavorable conditions. A further object of the invention is to provide an improved process for the application of a refractory oxide coating to a substrate and/or the hardening of the oxide coating applied thereto. A further object of the invention is to provide an improved low temperature process for the production of improved abrasive or polishing stones and grinding wheels which may include abrasive grain additives. A further object of the invention is to provide a process for the production of a refractory ceramic oxide material having a negative temperature coefficient of electrical and heat conduction. For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken with the drawings, and its scope will be pointed out in the appended claims. FIGS. 1, 2 and 3 constitute a series of photographs of a pressed body of alumina powder with an increasing number of impregnation-cure cycles according to the present invention; FIG. 4 is a photograph showing a grid at the same magnification as FIGS. 1-3; FIG. 5 is a metallographic photograph at 200X magnification of a pressed body of Alcoa T-61(-325 mesh) alumina ball milled 48 hours and chemically hardened; FIG. 6 is a 200X metallographic photograph of a pressed body of Alcoa T-61(-325 mesh) alumina ball milled 96 hours and chemically hardened; FIG. 7 is a 200X metallographic photograph of a pressed body of beryllium oxide powder which has been chemically hardened; FIG. 8 is a 200X metallographic photograph of a pressed body of chromium oxide powder which has been chemically hardened; FIG. 9 is a 200X metallographic photograph of a pressed body of Alcoa T-61(-325 mesh) ball milled 96 hours with aluminum fibers added and chemically hardened; FIG. 10 is a 200X metallographic photograph of a refractory oxide painted on a metal substrate and chemically hardened; FIG. 11 is a photograph of a commercial sintered grinding wheel side by side with a grinding wheel made by the process of this invention; FIG. 12 is a 300X metallographic photograph of a cross section through a commercial plasm sprayed chromia coating prior to treatment according to this invention; and, FIG. 13 is a 300X photograph of the cross section of FIG. 12 after treatment. This invention is directed to a process and product involving new types of materials that are formed by multiple chemical impregnations of a relatively soft porous body of finely divided refractory oxide base materials, each followed by a low temperature cure to convert the impregnant to an oxide. The resulting ceramic structure formed in this manner has been shown to exhibit extreme hardness, a high compressive strength and a dimensionally stable material over a wide temperature range. In addition, a number of these new ceramic materials show an inherently small coefficient of friction coupled with a very low wear rate characteristic. Parts can be economically fabricated of this new material in a wide variety of intricate shapes and sizes. This is most easily accomplished by providing the base refractory material in a powdered form and packing the powder particles into close proximity by suitable means to provide a porous body of predetermined shape. The shaped pieces are then repeatedly chemically treated and cured at a temperature substantially below that used for normal ceramic vitrification. One of the unique features of this chemical treatment and hardening method is that virtually no change occurs in the original dimensions of the shaped part during the hardening process. Therefore, expensive diamond machining of the finished hardened part is eliminated. These new ceramic materials will withstand repeated water quenching from 1000.degree. F. as well as prolonged exposure to temperature extremes of 2000.degree. to -300.degree. F. Mohs scale hardness is in excess of 9, normally being about equal to that of silicon carbide. Rockwell hardness can be as high as A-85 to A-90, with associated compressive strengths in excess of 125,000 psi. In addition to their use for the manufacture of precision parts, many of these ceramics exhibit excellent characteristics for low friction and low wear rate bearing and seal applications; in particular, journal bearings, thrust bearings and sliding type bearings and seals. When used in this manner, lubrication may be by means of a wide variety of conventional and non-conventional lubricants. Among those successfully tested to date include: tap water, sea water, alcohol, kerosene, polyethylene glycol, trichlorethylene, lubricating oils, silicone fluids and liquid metals. Solid lubricants have been used with good results at temperatures up to about 2000.degree. F. In addition, lightly loaded bearings have been operated for limited periods at high speed without lubrication. Life tests of sleeve-type bearings have been and still are currently in progress. However, to date wear has been too low to obtain quantitative data, even after many months' time. Rub-shoe type wear rate tests have consequently been conducted and have shown exceptionally low wear rate characteristics. For example, a ceramic shoe of this invention riding on a ceramic wheel of the same material exhibited many times less wear than a bearing bronze shoe riding against a steel wheel using oil as the lubricating media. Also, unlike a conventional bronze-steel bearing combination, very heavy loads can be applied to many of the ceramic-to-ceramic material bearings without their showing any tendency toward galling, even when running with such poor lubricants as alcohol or water. A special variation in treatment of this invention has also been found that will produce a honing or finishing material that appears to be superior in several respects to both natural and artifically produced grinding stones. For example, one such ceramic will remove metal far more rapidly than will an Arkansas stone, while at the same time producing a finer and more highly polished finish. Another ceramic material of this invention displays a wide variation in electrical and heat conduction with relatively small changes in temperature. The basic method employed for producing the new ceramic materials consists of chemically impregnating a porous, refractory oxide structure followed by a low temperature cure. The porous refractory acts as the skeletal framework around which the final ceramic structure is formed. The simplest chemical hardening method consists of impregnating the shaped porous body with a solution of chromic acid. The thoroughly impregnated material is then cured in an oven with the final temperature reaching at least 600.degree.- 1000.degree. F. or higher. The impregnation and curing cycle is repeated several times. With a suitable refractory base material, this simple acid treatment will produce a hard ceramic body having numerous uses. The finally divided base material may be mixed with a binder, such as kaolin and the like, before shaping or the impregnant may serve as the binder after the first cure. This also may be accomplished by impregnation of the porous structure with a water solution of a soluble metal compound convertible to an oxide and subsequently converting same to the oxide by simply elevating the temperature to the required conversion point. The metal compound is selected so that the oxide conversion will normally take place at a temperature less than about 1500.degree. F. X-ray diffraction tests indicate that these chemical treatment methods form a new microcystalline structure or at least a very close bond between the added oxides, and phosphoric acid and the porous refractory skeletal structure. As mentioned previously, the ceramic material is built around a porous refractory base material that functions as the skeletal structure. The types of such materials that are suitable for use in the present invention include various grades of alumina, titania, beryllia, magnesia, magnesium silicate and stabilized zirconia. Some materials were obtained from the manufacturer in an "underfired" or "machinable" form. In this condition, these materials were normally found to be soft enough to allow machining by conventional means, and exhibited a relatively high effective porosity (10 to 50%) to allow for subsequent chemical treatment by the process of this invention. Table 1 lists the major type designation, manufacturer, hardness, porosity and fabrication method for each of the skeletal refractory materials tested. The addresses of the manufacturers referred to in Table 1 are as follows: American Lava Corp., Chattanooga, Tenn.; Amerisil, Inc., Hillside, N.J.; Coors, Golden, Colo.; and, Du-Co Ceramics, Saxonburg, Pa.
TABLE I
__________________________________________________________________________
UNDERFIRED, POROUS REFRACTORY BASE MATERIALS
Manufacturer's Mohs
Base Type Major Other Sintering
Effective
Hard-
Material
Designation
Manufacturer
Oxide Oxides Temp. Porosity
ness
Remarks
__________________________________________________________________________
Alumina
AHP-99 Coors 99% Al.sub.2 O.sub.3
0.5% SiO.sub.2
2670.degree. F.
45.7%
2-3 isostatic
0.2% CaO Pressed
0.2% MgO
Alumina
AP-99-L3 Coors 99% Al.sub.2 O.sub.3
2570.degree. F.
42.4%
2-3 Extruded
Alumina
Al-99-l1 Coors 99% Al.sub.2 O.sub.3
1700.degree. F.
0-1 Extruded
Alumina
AP-99-l2 Coors 99% Al.sub.2 O.sub.3
2130.degree. F.
1 Extruded
Alumina
AP-99-L1 Coors 99% Al.sub.2 O.sub.3
2642.degree. F.
Extruded
Alumina
AP-99-L2 Coors 99% Al.sub.2 O.sub.3
2670.degree. F.
5-6 Extruded
Alumina
AP-99C-L1
Coors 99% Al.sub.2 O.sub.3
2642.degree. F.
4-5 Cast
Alumina
AP-99c-l2
Coors 99% Al.sub.2 O.sub.3
2130.degree. F.
Cast
Alumina
AP-99C-L3
Coors 99% Al.sub. 2 O.sub.3
2570.degree. F.
Cast
Alumina
AlSiMag 614
Am.Lava Corp.
96% Al.sub.2 O.sub.3
SiO 2000.degree. F.
1-2 ordered green,
(green) MgO fired for
CaO 20 min. at
2000.degree. F.
Extruded rod
Alumina
AlSiMag 393
Am.Lava Corp.
90% Al.sub.2 O.sub.3 4-6
Alumina
AlSiMag 548
Am.Lava Corp.
99.8% Al.sub.2 O.sub.3
Beryllia
BP-96-i1 Coors 96% BeO 1700.degree. F.
1-2 Extruded
Magnesia
187E4 Du-Co Ceramics
89% MgO
SiO.sub.2
2000.degree. F.
1-2
Magnesia
187E77 Du-Co Ceramics
96% MgO
SiO.sub.2
2000.degree. F.
1-2
Magnesium
AlSiMag 222
Am.Lava Corp.
MgO.SiO.sub.2 2-3
Silicate
Silica
No. 3 Porosity
Amersil, Inc.
99% SiO.sub.2 2-3 Hot Pressed
Zirconia
172H20 Du-Co Ceramics
95% ZrO.sub.2
5% CaO 1-2 Made from ZCA
Type F Coarse
Grain Zirconia-
(CaO stabilized)
Titania
AlSiMag 192
Am.Lava Corp.
98% TiO.sub.2
SiO.sub.2
2000.degree. F.
2-3 Ordered Green
MgO fired 20 min.
(Underfired) CaO at 2000.degree. F.
Alumina
AP-995-L3
Coors 99.5% Al.sub.2 O.sub.3
2570.degree. F.
Extruded
Alumina
AP-997-L3
Coors 99.7% Al.sub.2 O.sub.3
2570.degree. F.
Cast
Alumina
AP-94-l1 Coors 94% Al.sub.2 O.sub.3
3.75% SiO.sub.2
33.1%
2-3 Extruded
0.9% CaO
0.75% MgO
1700.degree. F.
0.5% ZrO.sub.2
0.1% Fe.sub.2 O.sub.3
Alumina
AP-94-l2 Coors 94% Al.sub.2 O.sub.3
3.75% SiO.sub.2
2130.degree. F.
33.0%
2-3 Extruded
0.9% CaO
0.75% MgO
0.5% ZrO.sub.2
0.1% Fe.sub.2 O.sub.3
Alumina
AP-94-l2 Coors 94% Al.sub.2 O.sub.3
0.1% Fe.sub.2 O.sub.3
2130.degree. F.
44.1%
2-3 isostatic
(isostatic) Pressed
Alumina
AP-85-l1 Coors 85% Al.sub.2 O.sub.3
10% SiO.sub.2
1700.degree. F.
33.4%
2-3 Extruded
2.75% MgO
1.25% CaO
0.75% BaO
0.25% Fe.sub.2 O.sub.3
Alumina
AlSiMag 614
Am.Lava Corp.
96% Al.sub.2 O.sub.3
>2000.degree. F.
6-7 Too hard for
(underfired) easy
__________________________________________________________________________
machining
These materials are fabricated by one or more of several commercially used methods such as powder pressing, extrusion, isostatic forming or slip casting. The important factor, however, is that the formed or pressed oxide be only partially sintered since optimum sintering will result in a dense body with insufficient porosity to be usable in the chemical treatment method of this invention. In addition to the alumina, beryllia, magnesia, titania and zirconia materials, it is anticipated that many of the other partially sintered refractory oxides would make applicable skeletal structures for the improved ceramic material. Among these would be the oxides of Barium, Calcium, Cerium, Chromium, Cobalt, Gallium, Hafnium, Lanthanum, Manganese, Nickel, Niobium, Tantalum, Thorium, Tin, Uranium, Vanadium, Yttrium and Zinc. Also, many of the complex-refractory oxides should be suitable base materials. Of the complex-refractories, only the magnesium silicate has been tested to date. Other complex-refractories that may be suitable if produced in a porous, partially sintered (underfired) form are Aluminum silicate, Aluminum titanate, Barium aluminate, Barium silicate, Barium zirconate, Beryllium aluminate, Beryllium silicate, Beryllium titanate, Beryllium zirconate, Calcium chromite, Calcium phosphate, Calcium silicate, Calcium titanate, Calcium zirconate, Cobalt aluminate, Magnesium aluminate, Magnesium chromite, Magnesium ferrite, Magnesium lanthanate, Magnesium silicate, Magnesium titanate, Magnesium zirconate, Magnesium zirconium silicate, Nickel aluminate, Potassium aluminum silicate, Strontium aluminate, Strontium phosphate, Strontium zirconate, Thorium zirconate, Zinc aluminate, Zinc zirconium silicate and Zirconium silicate. The novel process according to the invention is particularly adapted to the treating of porous, partially vitrified refractory ceramics such as the oxides of Aluminum, Barium, Beryllium, Calcium, Cerium, Chromium, Cobalt, Gallium, Hafnium, Lanthanum, Magnesium, Manganese, Nickel, Niobium, Tantalum, Thorium, Tin, Titanium, Uranium, Vanadium, Yttrium, Zinc and Zirconium and mixtures thereof. The oxides may be substantially pure or may contain or have amounts of impurities or additives, such as an oxide of a metal other than that of the body such as Cadmium, Chromium, Cobalt, Copper, Iron, Magnesium, Manganese, Nickel, Titanium and the like and/or other salts of such metals which ultimately will convert to oxides at least during the final curing step. The process of this invention also contemplates the addition of amounts of additives such as a salt of a metal other than that of the body and convertible to an oxide such as the acetates, chlorides, nitrates and oxalates of Aluminum, Beryllium, Cadmium, Calcium, Cerium, Chromium, Cobalt, Copper, Iron, Lanthanum, Lithium, Magnesium, Molybdenum, Nickel, Strontium, Thorium, Tin, Tungsten, Zinc and Zirconium which are added to the ceramic during treatment. The process of this invention may comprise the forming of a partially sintered untreated ceramic into a predetermined shape or the forming thereof from a powder and a binder. It will be understood that, while precast machinable stock may be used, it is possible to precast to intricate shapes and prefire to an underfired condition before the ceramic is subjected to Applicants' process. The ceramic, either stock or formed, is usually quite porous. The simplest method of chemically hardening the porous refractory structure is with a phosphoric acid treatment; however, this precludes multiple treatments as the reaction seems to go to completion in one treatment. The ceramic is impregnated with a concentrated phosphoric acid solution, usually of 85% concentration. The ceramic can be evacuated in a vacuum before immersion in the acid to hasten the impregnation or, as has been found to be particularly effective, the ceramic can be heated to from about 300.degree. to about 600.degree. F. and then immersed in the phosphoric acid solution. The heating causes a vacuum to be produced within the voids of the ceramic and the phosphoric acid is drawn all through the ceramic upon immersion. While a considerably longer time is required, the ceramic also can be just immersed in the acid solution for a length of time sufficient for complete impregnation. Greater uniformity is achieved by using the vacuum or heating impregnation techniques. When the part is thoroughly impregnated with acid, it is removed from the solution, excess acid on the surface is drained or wiped off. Next, Applicants' novel process comprises the controlled heat curing of the acid impregnated ceramic. The heating cycle is usually started around 150.degree. and ends at about at least about 900.degree. F. The ceramic pieces are preferably placed in powdered asbestos, and the like, to minimize shock during the heating and cooling cycle. The powdered asbestos also serves to absorb liquid driven out of the ceramic as the temperature is raised. The excess liquid, if not absorbed, would be likely to craze the surface of the ceramic. As pointed out, one of the unique features of the method of the invention is that virtually no dimensional changes occur in the machined piece during the hardening process. Therefore, expensive diamond-type machining of a hardened part is eliminated. The property of physical hardness has been used as the primary means of determining effects of varying the underfired base materials, chemical treatment and curing methods. Table 11 below sets forth the hardness measurements for various materials which have been given a simple acid treatment.
TABLE II
__________________________________________________________________________
HARDNESS MEASUREMENTS FOR SIMPLE ACID TREATMENT
Sample
Base Type Major H.sub.3 PO.sub.4
Mohs Rockwell
No. Material
Designation
Manufacturer
Oxide Impregnation
Hardness
Hardness
Remarks
__________________________________________________________________________
21E Alumina
AP-85-l1 Coors 85% Al.sub.2 O.sub.3
85% 8-9 A-66.5
22E Alumina
AP-94-l1 Coors 94% Al.sub.2 O.sub.3
85% 6-7 A-69.5
23E Alumina
AP-94-l2 Coors 94% Al.sub.2 O.sub.3
85% 6-7 A-71.0
24E Alumina
AP-94-l2 Coors 94% Al.sub.2 O.sub.3
85% 6-7 A-57.5
(isostatic)
25E Alumina
AP-99-L3 Coors 99% Al.sub.2 O.sub.3
85% 8-9 A-70.5
20E Alumina
AHP-99 Coors 99% Al.sub.2 O.sub.3
85% 6-7 A-52.5
A7 Alumina
AlSiMag 614
Am.Lava Corp.
96% Al.sub.2 O.sub.3
85% 8-9 A-73.7
(underfired)
30E Alumina
AlSiMag 393
Am.Lava Corp.
90% Al.sub.2 O.sub.3
85% 8-9 fractured
29E Alumina
AlSiMag 548
Am.Lava Corp.
99.8% Al.sub.2 O.sub.3
85% 6-7 fractured
26E Beryilia
BP-96-11 Coors 96% BeO 85% 6-7 fractured
a-1 Magnesia
187E4 Du-Co Ceramics
89% MgO 85% 4-5 fractured
6-1 Magnesia
187E77 Du-Co Ceramics
96% MgO 85% 4-5 A-37.0
28-E Magnesium
AlSiMag 222
Am.Lava Corp.
MgO . SiO.sub.2
85%
Silicate
27-E Silica
No. 3 Porosity
Amersil, Inc.
99% SiO.sub.2
85% Fractured
56-T Titania
AlSiMag 192
Am.Lava Corp.
TiO.sub.2
85% 4-6 Fractured
(underfired)
Z-1 Zirconia
172H20 Du-Co Ceramics
95% ZrO.sub.2
85% 8-9 A-54.0
44T Alumina
AlSiMag Am.Lava Corp.
MgO . SiO.sub.2
85% 5-6 A-65.5
(2000.degree. F.)
C60 Alumina
AP-99C-l2
Coors 99% Al.sub.2 O.sub.3
85%
146 Alumina
AP-99C-L1
Coors 99% Al.sub.2 O.sub.3
85% A-66.4
__________________________________________________________________________
Several significant differences in the final product are achieved by the variation of portions of the treating process. While a pure or nearly pure ceramic material can be significantly hardened by a phosphoric acid treatment, prior multiple impregnations of the ceramic with a solution of a salt convertible to an oxide and cures converting same to the oxide will produce an increase in the hardness of the ceramic and the further acid treatment which may be given if desired usually produces an even harder end product. Where the ceramic material is impregnated with a high concentration of phosphoric acid and heat treated, a good bearing material is produced and two pieces of this same material will slide against one another with a low coefficient of friction. After such pieces are worn in for a short while, a shiny surface film is produced which remains shiny even at elevated temperatures. Where the more concentrated phosphoric acid is used, the resulting product is more dense with smaller unfilled pores. Where a relatively pure ceramic oxide is treated, the addition thereto of another oxide during treatment substantially increases the hardness of the finished product. While it is not completely known what occurs in the treating process, the pores of the underfired ceramic are believed to be filled or partially filled with a reaction product of the ceramic and the additive, if any, with the acid, probably a complex metal phosphate. Where the ceramic material is impregnated with a high concentration of phosphoric acid having dissolved therein aluminum phosphate crystals until saturated at from 250.degree.- 400.degree. F. and is then heat treated, a material is produced which cannot be polished to more than a dull finish, is quite porous and makes an excellent polishing and sharpening stone. This characteristic is also produced where the treatment with phosphoric acid is carried out with dilute acid solutions. It is believed that less reaction product is available to fill the pores, providing a more open and abrasive surface. Here again, the addition of another oxide during treatment substantially increases the hardness of the final product. The starting porous aluminum oxide grades have ranged from about 25 to about 60% effective porosity and, when subjected to a starved acid treatment, remain quite porous which may account for the excellent polishing and sharpening characteristics of the thus treated material. The heat treating of the acid impregnated ceramic should be initiated at about 150.degree. to 350.degree. F. for a short period of time to drive out excess moisture and then the temperature is raised in steps for a series of time intervals until the final cure is accomplished at at least 500.degree.- 600.degree. F. and preferably at at least 850.degree.- 900.degree. F. The ceramic will become quite hard at 500.degree.- 600.degree. F., but good electrical resistivity is not achieved until the ceramic is subjected to a temperature of 850.degree. F. or higher. Temperatures above 1000.degree. and as high as 3000.degree. F. have been used with good success. It is found that, once the heat treatment has been carried to above 850.degree. F., the temperature may be increased to well above the normal vitrifying temperatures (e.g. 3000.degree. F.) without producing any shrinkage or change in the original physical dimensions. Further, the high temperatures do not appear to affect the hardness of the material from that of the material heated to 850.degree. F. While the mechanism of Applicants' process is not completely understood, it is believed that aluminum phosphate may be formed and deposited in the crystal lattice structure of the aluminum oxide as well as within the voids of the porous ceramic. Further, the phosphates of the impurities and/or additives may be formed and possibly as part of the lattice structure. As pointed out above, the ceramic materials which are chemically treated and hardened according to one embodiment of the present process display the unique characteristic of exhibiting a low coefficient of friction when sliding against themselves. The coefficient of friction between identical pieces of the material is considerably less than when used in contact with any dissimilar ceramic or metal tested to date. Although these materials may be operated dry where they are lightly loaded for limited periods of time, the starting friction is considerably higher than when a lubricating material is present. Lubrication may be by a number of different liquids such as tap water, sea water, kerosene, trichlorethylene, lubricating oils, silicone fluids and liquid metals. Dry lubricants such as molybdenum di-sulfide, graphite, wax and the like are also suitable. It is possible also to form the lubricant in situ within the pore structure of the bearing. The bearings can be easily and economically fabricated in a wide variety of shapes and sizes. The untreated ceramic material in the form of partially fired bars or plates is machined to size and shape using conventional high speed steel or carbide tooling. The machined pieces are then chemically treated and hardened at temperatures substantially below normal vitrification temperatures. The hardening occurs with substantially no change in dimensions, thus avoiding expensive diamond machining of the finished part. The ceramic bearing being fairly porous may be used as the lubricant reservoir analogous to that of sintered bronze bearings. In other instances, the bearing can be operated partially or totally submerged in the lubricant or the non-rotating member can be connected to an external lubricant reservoir. Typical bearings fabricated of ceramic according to the present invention can withstand repeated water quenching from at least 1000.degree. F., as well as prolonged exposure to temperatures as high as 2000.degree. F. and as low as -300.degree. F. The compressive strength is on the order of about 125,000 psi or better, and the hardness on the Mohs scale is between 9- 10 or on the order of about A-80 -A90 on the Rockwell scale. The ceramic materials of Table 1 was subjected to several slightly different treatments according to this invention, which are: (1) impregnation in phosphoric acid alone; (2) one or more oxide impregnations followed by a single phosphoric acid treatment; or, (3) one or more oxide impregnations alone. A typical acid impregnation process according to the present invention comprises heating the ceramic piece to about 300.degree.- 600.degree. F. for about 20 minutes, the piece is then immersed in an 85% phosphoric acid solution while hot for about 40 minutes. The piece is then placed in an oven and progressively heated from 150.degree. to about 1000.degree. F. over a period of about 120 minutes. The piece is then cooled to room temperature. A typical combination salt and acid impregnation process comprises heating the ceramic piece to about 250.degree.- 450.degree. F. for about 20 minutes. The heated piece is then immersed in the salt solution for about 40 minutes. The piece is removed from the salt solution and cured progressively from 150.degree. to 1000.degree. F. over a period of 120 minutes. The previous steps can be repeated if desired. The piece is then cooled to about 600.degree. F. and immersed in an 85% phosphoric acid solution for about 40 minutes. The piece is then placed in an oven and cured over a temperature range of from 150.degree. to 1000.degree. F. over a period of about 120 minutes and subsequently cooled to ambient temperature in about 15 minutes. Fully hardened samples were prepared according to the above treatments from the materials of Table 1. As previously stated, impurities existing in the base material appear to have an effect on the resultant hardness of the treated piece. Therefore, it was decided to artificially add refractory oxides to the porous base structure prior to treating with the acid. This was accomplished by impregnating the refractory base material with a nitrate, chloride, acetate or other highly water soluble salt or an acid of the oxide desired, and then converting to the metal oxide by heating slowly to an elevated temperature. Following the oxide impregnation (which may consist of one or more salt or acid treatments) the body was then treated with phosphoric acid. Tables III, IV, V and VI show the effect of added oxides to Coors alumina products AP-94-11, AP-85-11, AP-99-L3 and AHP-99, respectively. In these tests, three impregnations of the saturated salt were used (to assure ample "loading" with the desired oxide), followed by the 85% phosphoric acid treatment. It is interesting to note that these tables show a wide variation hardness depending on the oxide treatment. In some cases, the hardness is considerably increased over that of the same base material treated with acid only, while in others, the increase is not so marked. The hardness that is obtained with the acid treatment only (no oxide impregnation) is listed for comparison purposes. The Cr.sub.2 O.sub.3 treatment is of special interest in that, when used with the 99, 94 and 85% Al.sub.2 O.sub.3 base structures, the resulting ceramic is exceptionally high in hardness as compared to all other oxide impregnations tested. The Cr.sub.2 O.sub.3 may be added as a solution of a soluble salt or preferably as a concentrated solution of chromic acid. These four tables also show that the AHP-99 material (99% Al.sub.2 O.sub.3) is the poorest choice for the base structure of these four types. However, since the AP-99-L3 is also a 99% alumina composition, it must be assumed that the hardness is not a factor of the refractory purity alone, but that other factors such as difference in effective pore size is probably responsible for some or all of the noted differences. Tables VII, VIII and IX show the same type of data using aluminum oxides secured from the American Lava Corporation as their types 614 (underfired), 393 and 548. These are 96, 90 and 99.8% Al.sub.2 O.sub.3 compositions, respectively. Hardness measurements obtained with Coors 96% beryllium oxide for four different salt impregnations is shown in Table X. It is interesting that this base material produces results about equal to the best alumina material tested (Coors AP-99), indicating that refractory skeletal structures other than alumina are definite candidates for the ceramic fabrication method. Tables XI and XII show hardness results for oxide impregnated magnesia material. While the hardness values are quite low as compared to the alumina or the beryllia, this is to be expected since magnesia, even in its fully fired stated, is not a particularly hard material Mohs 5-1/2). Tables XIII and XIV cover "AlSiMAG" No. 222 magnesium silicate and "Amersil" 99% silica, respectively. For reasons not fully understood, refractory base materials containing a high percentage of silica do not appear to respond well to the phosphoric acid hardening method. Even in these two tests, however, the chromic oxide impregnation provided noticeably better results than the other impregnations used. Table XV lists results obtained with a partially sintered, zirconia refractory base material. This particular underfired zirconia was fabricated from a calcia stabilized but coarse grain material. It is anticipated that a fine grained zirconia, and possibly a magnesium oxide stabilized type, would provide better results. Nevertheless, the zirconia also reacts to the chemical hardening method in the same general manner as does the alumina, magnesia and beryllia and, to a leser extent, the magnesia silicate and silica materials. Table XVA lists results obtained with aluminum oxide material and Table XVB lists results obtained wih titanium dioxide material. With regard to the effect of pore size, it would be noted that the AHP-99 Coors material has quite large pores, compared to the other Coors material, being on the order of less than one micron compared with 2 to 3 microns for the AHP-99 materials. It would appear that the pore size would preferably be less than 2 microns and substantially uniform in size.
TABLE III
__________________________________________________________________________
HARDNESS MEASUREMENTS FOR VARIOUS OXIDE IMPREGNATIONS
USING COORS AP-94-l1 ALUMINA REFRACTORY BASE MATERIAL
(Acid Treated Hardness Mohs 8-9, Rockwell, 70.7)
Sample Oxide Salt No. Salt
H.sub.3 PO.sub.4
Mohs
Rockwell
No. Formed Impregnation
Impreg.
Impregnation
Hardness
Hardness
Cracks
Remarks
__________________________________________________________________________
1 Al.sub.2 O.sub.3
Al(NO.sub.3).sub.2
3x 85% 9-10
A-71.5
None
7-1 BeO BeCl.sub.2
3x 85% 9-10
A-74.4
None
5 CaO Ca(NO.sub.3).sub.2
3x 85% 8-9 A-55 None
3 CaO Cd(NO.sub.3).sub.2
3x 85% 8-9 A-63 None
C-1 CeO.sub.2
Ce(NO.sub.3).sub.2
3x 85% 9-10
A-71.1
None
9 CaO Co(NO.sub.3).sub.2
3x 85% 8-9 A-74.8
None
L-4 Cr.sub.2 O.sub.3
CrO.sub.3
3x 85% 9-10
A-81.5
None
7-3 CuO Cu(NO.sub.3).sub.2
3x 85% 9-10
A-61.0
None
7 Fe.sub.2 O.sub.3
FeCl.sub.3
3x 85% 8-9 A-72.5
None
7-5 La.sub.2 O.sub.3
La(NO.sub.3).sub.2
3x 85% 8-9 A-53.5
Yes
7-7 Li.sub.2 O
LiC.sub.2 H.sub.3 O.sub.2
3x 85% 8-9 A-48.2
Yes
11 MgO Mg(C.sub.2 H.sub.3 O.sub.2).sub.2
3x 85% 9-10
Fractured
Yes
D-5 MgCr.sub.2 O.sub.4
MgCrO.sub.4
3x 85% 9-10
A-73.8
None
13 NiO Ni(NO.sub.3).sub.2
3x 85% 9-10
A-75.6
None
D-1 SnO SnCl.sub.2
3x 85% 9-10
A-71.7
None
15 SrO Sr(NO.sub.3).sub.2
3x 85% 8-9 Fractured
Yes
7-9 ThO.sub.2
Th(NO.sub.3).sub.4
3x 85% 9-10
A-73.5
None
17 TiO.sub.2
Ti.sub.2 (C.sub.2 O.sub.4).sub.3
3x 85% 9-10
A-73.5
None
9-X WO.sub.3
H.sub.4 SiW.sub.16 O.sub.40
3x 85% 9-10
A-72.1
None
Zn94 ZnO ZnCl.sub.2
3x 85% 8-9 A-73.8
None
D-3 ZrO.sub.2
ZrOCl.sub.2
3x 85% 9-10
A-76.1
None
l-A Fe.sub.2 O.sub.3.Cr.sub.2 O.sub.3
(1)FeCl.sub.3+
3x 85% 9-10
A-77 None
(1)CrO.sub.3
__________________________________________________________________________
Other Refs: Primary Examiner: Arnold, Donald J. Assistant Examiner: Attorney: Wymore; Max L. |
|