Method of engraving pre-sintered ceramic articles

Abstract: A method of engraving pre-sintered ceramic articles involves providing a source of laser energy for forming a polyester mask having a predetermined spiral pattern etched therein. To form the polyester mask, means are provided for computationally communicating a predetermined engraving pattern to the source of laser energy. Once the polyester mask is formed, it is overlaid onto the ceramic article forming a laminate and the laminate is bombarded with a hard alumina grit to form the spiral pattern in the ceramic article. After the ceramic article is engraved with the predetermined spiral pattern, the polyester mask is removed exposing an engraved ceramic article.

Description: Technical Field This invention relates generally to a method of engraving ceramic articles, and more particularly, the invention concerns a method of engraving air channels in pre-sintered hydrodynamic hearing assemblages which are more durable and wear resistant, and can be cost effectively manufactured. Background of the Invention Air-lubricated hydrodynamic bearings are known to be promising candidates for various high precision applications such as rotary scanners and VTR head assemblies for high recording densities, and flying spot laser sensitometers for high precision sensitometry. There are two different types of gas-lubricated bearings: externally pressurized (hydrostatic), and self-acting (hydrodynamic). Hydrostatic bearings can carry higher loads, but need external pressure supply. In hydrostatic bearings the pressure is generated through the motion of bearing surfaces. These bearings are most suitable for applications with light loads and high speeds. Advantages that air lubricated hydrodynamic bearings have over other type of bearings generally include their simplicity of design, high accuracy, low flutter (speed variation), high stiffness, low vibration, low wear, long life (even at high speeds), contamination-free from lubricants, and wide use temperatures. Conventional air bearings, however, have several undesirable shortcomings. It is well known that they require high precision manufacturing which increases the product cost. Moreover, these bearings take up more room than conventional ball bearings. Notwithstanding the stringent specification requirements imposed by product designers, air bearings are nonetheless still required in specific applications. It is also known that conventional air bearings require impact resistance to eliminate catastrophic failures and corrosion, and wear and abrasion resistance to improve service life and productivity. Thus, conventional air bearings made of, for instance, metals or alloys, most commonly steel, meet some of these requirements but have considerable limitations of requiring frequent maintenance due to wear. Experience indicates that ytrria-doped tetragonal zirconia polycrystal (Y-TZP) ceramic materials offer many advantages over conventional materials, including many other ceramics. Y-TZP is one of the toughest ceramics. The toughness is achieved at the expense of hardness and strength. Tetragonal zirconia alloy-alumina composite, that is, the product of sintering a particulate mixture of zirconia alloy and alumina, is another tough and relatively soft structural ceramic composite. Y-TZP ceramic and zirconia-alumina composites have tribological properties that are not as attractive as other high performance structural ceramics like SiC and Si 3 N 4 . An example of a material having good hardness and strength is monolithic cubic spinel, however, this material is highly brittle and is unusable for structural applications. It is further known that a bearing assemblage having a bearing surface in rotating contact with a shaft has a longer service life and better performance if made with a hard material and the mating surfaces have low coefficient of friction. An alternative approach is taught by U.S. Patent No. 5,358,913: In that approach, a tetragonal zirconia alloy article, which can be near net-shape, is compacted and then sintered in the presence of an MgO, CaO, Y 2 O 3 , Sc 2 O 3 , Ce 2 O 3 , or other rare earth oxide dopant to produce an article having a tetragonal core and a cubic case. The dopant can be provided in a number of different forms such as a solid plate, a powder, or a layer produced by decomposition of an organo-metallic precursor film. In U.S. Patent 5,677,072 a method is described for producing articles having a tetragonal zirconia alloy core and a monoclinic case. U.S. Patent 4,263,085 discloses a method of engraving a ceramic article comprising the steps of: providing a mask having a predetermined engraved pattern therein, overlying said mask onto said ceramic article; bombarding said mask overlying said ceramic article with hard ceramic grits; and removing the mask from said ceramic article. The use of this technique is not effective for engraving on a ceramic articles made from zirconia and zirconia because the mask generally deteriorates before an engraving of useful depth is achieved. There is a need therefore for an improved method of engraving ceramic articles such as hydrodynamic bearings. The need is met by the method described in claim 1. Brief Description of the Drawings Fig. 1 is a schematic flow diagram of a method of making a ceramic article engravable by the method of the invention; Fig. 2 is a schematic of a wet bag isostatic pressing system useful in the method of making the ceramic article; Figure 3 is a perspective view of the hydrodynamic bearing made with the method of the invention; and, Figure 4 is a plan end view of a typical spiraled shaft formed with the mask made by the method of invention; Figure 5 is a schematic of a device for making masks for engraving the bearing flange made using the method of the invention; Figure 6 is a schematic view of a preferred bead blasting operation useful in the invention. Detailed Description of the Invention Broadly illustrated in Fig. 1 (Steps A-F), a ceramic article, in particular an air lubricated hydrodynamic ceramic bearing, may be engraved using the method of the invention, as described fully below. The ceramic article includes a ceramic powder comprising primarily zirconium oxide, or a first concentration of particulate aluminum oxide and a second concentration of particulate zirconium oxide alloy. A typical hydrodynamic ceramic bearing 116 made in accordance with the method of the invention is illustrated in Figures 1 (Step F) and 3. The zirconium oxide alloy consists essentially of zirconium oxide and a secondary oxide selected from the group consisting of MgO, CaO, Y 2 O 3 , Sc 2 O 3 , and rare earth oxides. Moreover, the zirconium oxide alloy has a concentration of the secondary oxide of, in the case of Y 2 O 3 , about 0.5 to about 5 mole percent; in the case of MgO, about 0.1 to about 1.0 mole percent, in the case of CeO 2 , about 0.5 to about 15 mole percent, in the case of Sc 2 O 3 , about 0.5 to about 7.0 mole percent and in the case of CaO from about 0.5 to about 5 mole percent, relative to the total of said zirconium oxide alloy, said compacting further comprising forming a blank. A mold is provided for receiving and processing the ceramic powder. The ceramic powder is then compacted (as described below) in the mold provided to form a ceramic billet. The ceramic billet is then shaped or green-machined so as to form a near net-shaped green ceramic bearing having opposing flange portions and a body portion between the opposing flange portions. In this embodiment of the invention, after the initial shaping, the green ceramic bearing is sintered thereby forming a sintered net-shape ceramic bearing, as described more fully below. The ceramic bearing is then further machined or shaped, as indicated below. After shaping the ceramic bearing, at least two air inlet ports are formed in the body portion of the bearing in fluid communications with air channels, having preferably a spiral pattern, formed in at least one flange portion of the bearing. Ceramic Powder Material Mixing According to Fig. 1, step "A" diagrammatically illustrates the alloying process. Zirconia powder 100 is combined with one or more secondary oxide powders 102 to provide zirconia alloy powder 104. The preparation of zirconia alloys is well known to those skilled in the art and zirconia alloys are available commercially. For example, particulate zirconia alloy having 3 mole percent Y 2 O 3 is marketed by Z-TECH Corporation, Bow, New Hampshire, as "SYP-ULTRA 5.2 Yttria Stabilized Zirconia". More particularly, we prefer using tetragonal zirconia ceramic material for manufacturing a hydrodynamic bearing assemblage comprising either a stationary bearing/rotating shaft or a rotating bearing/stationary shaft in a cost effective way. The most preferred material which we prefer using is essentially zirconia having 100% tetragonal crystal structure. We developed this 100% tetragonal zirconia by alloying zirconia with a number of secondary oxides as described in U.S. Patent No. 5,336,282 and 5,358,913. The preferred ceramic powder mixture most preferred in the method of making zirconia-alumina composites of the invention includes a particulate alumina and particulate alloys of ZrO 2 and additional "secondary oxide" selected from: MgO, CaO, Y 2 O 3 , Sc 2 O 3 and Ce 2 O 3 and other rare earth oxides (also referred to herein as "Mg-Ca-Y-Sc-rare earth oxides"). Zirconia alloys useful in the methods of the invention have a metastable tetragonal crystal structure in the temperature and pressure ranges at which the ceramic article produced will be used. For example, at temperatures up to about 200°C and pressures up to about 1000 MPa, zirconia alloys having, wherein zirconium oxide alloy has a concentration of said secondary oxide of, in the case of Y 2 O 3 , about 0.5 to about 5 mole percent; in the case of MgO, about 0.1 to about 1.0 mole percent, in the case of CeO 2 , about 0.5 to about 15 mole percent, in the case of Sc 2 O 3 , about 0.5 to about 7.0 mole percent and in the case of CaO from about 0.5 to about 5 mole percent, relative to the total of said zirconium oxide alloy, said compacting further comprising forming a blank, exhibit a tetragonal structure. Preferred oxides for alloying with zirconia are Y 2 O 3 , MgO, CaO, Ce 2 O 3 and combinations of these oxides. It is preferred that the zirconia powder have high purity, greater than about 99.9 percent. Specific examples of useful zirconia alloys include: tetragonal structure zirconia alloys having from about 2 to about 5 mole percent Y 2 O 3 , or more preferably about 3 mole percent Y 2 O 3 . Examples of tetragonal structure zirconia alloys useful in the methods of the invention are disclosed in U.S. Patent No. 5,290,332. Such zirconia alloys are described in that patent as being useful to provide a "net-shape" ceramic article: a ceramic article that is dimensionally true after sintering and therefore does not necessitate further machining prior to use in its intended working environment. Referring again to Fig. 1, Step A' diagrammatically illustrates an alternative mixture of particulate zirconia alloy 104 and a particulate aluminum oxide 103 . This alternative mixture can be achieved by mixing mechanically or chemically, for example, mixing by co-precipitation. The particulate mixture formed is from about 75 to 100 percent by weight (weight/total weight of particulate mixture) Al 2 O 3 , and preferably is from about 80 to about 99 percent by weight Al 2 O 3 , or more preferably is from about 80 to about 95 percent by weight Al 2 O 3 . The grain and agglomeration sizes and distributions, moisture contents, and binders (if any) can be varied in both the alumina and the zirconia alloy, in a manner known to those skilled in the art. "Grain" is defined as an individual crystal, which may be within a particle, having a spatial orientation that is distinct from that of adjacent grains. "Agglomerate" is defined as an aggregation of individual particles, each of which may comprise multiple grains. In a particular embodiment of the invention, the grain and agglomeration sizes and distributions, and moisture contents of the alumina and the zirconia alloy are substantially the same and are selected as if the zirconia alloy was not going to be mixed with the alumina, that is in a manner known to the art to be suitable for the preparation of a zirconia alloy article. An example of convenient particulate characteristics for a particular embodiment of the invention is the following. Purity of ZrO 2 is preferably well controlled at 99.9 to 99.99 percent, that is, impurities are no more than about 0.1 to 0.01 percent. The grain size is from about 0.1 micrometers to about 0.6 micrometers. The average grain size is 0.3 micrometers. The distribution of grain sizes is 5-15 percent less than 0.1 micrometers, 40-60 percent less than 0.3 micrometers, and 85-95 percent less than 0.6 micrometers. The surface area of each individual grain ranges from about 10 to about 15 m 2 /gram or is preferably 14 m 2 /gram. Agglomerate size is from about 30 to about 60 micrometers and average agglomerate size is 40-60 micrometers. Moisture content is about 0.2 to 1.0 percent by volume of blank and is preferably 0.5 percent. The mixture of particulates is compacted in the presence of an organic binder. Referring once again to Fig. 1, Step B, binders such as gelatin or polyvinyl ionomer or more preferably polyvinyl alcohol, are added to and mixed with the particulate mixture Y-TZP, 104 or a composite mixture of Y-TZP and alumina, 103 , both illustrated in Step A and A', respectively. This can be achieved preferably by spray drying or ball milling prior to placement of the mixture in a compacting device. Further, Step "B" also illustrates an alternative mixing process known to those who are proficient in the art as "compounding" in which the particulate mixture or mixtures are mixed with greater than 20% by weight of an organic binder such as paraffin at a temperature higher than the glass transition temperature of such binder for subsequent injection molding process, illustrated as Step C'. Compacting Turning now to compacting and more particularly to the process illustrated in Fig. 2, the particulate mixture 106 is cold compacted using preferably an isostatic press 150 to provide an unsintered blank 108 (Fig. 1). Unsintered blank 108 is alternatively referred to herein as a "green preform". It should be apparent to skilled artisans that a particular method of compacting the powder is not critical. The terms "cold compaction" and the like refer to compression of the particulate mixture at a temperature below glass transition or decomposition temperature of the organic binder. The green preform can be produced by such methods as cold uniaxial pressing, cold isostatic pressing, or cold extrusion. The particulate mixture is preferably subjected to uniform compacting forces in order to provide a unsintered blank which has a uniform density. Alternatively, near net-shape green blank 110' is generated using injection molding process. In a preferred method of the invention, the powder is cold compacted to provide a "green preform", which has a "green" density that is substantially less than the final sintered density of the ceramic article 116 . It is preferred that the green density be between about 40 and about 65 percent of the final sintered density, or more preferably be about 60 percent of the final sintered density. Referring to Fig. 2, the cold isostatic pressing of the powder was done by filling rubber mold 152 with powder 106 and keeping the mold 152 sealed by plate 154 in autoclave 156 of cold isostatic press 150 . Mold 152 is then pressurized to about 25,000 pounds per square inch. Seal plate 154 may either be a metallic material, such as aluminum or steel or a hard rubber. Thus, in accordance with Fig. 1, Step D, near-net-shape bearing 110 is formed by green machining (as discussed below) of the blank 108 using carbide tools. In Fig. 3, spiral grooves 114 defining air channels are formed in flange portion 115 of net-shaped hydrodynamic bearing 110. Then the bearing 110 is sintered to full density using preferably sintering schedules described in U.S. Patent No. 5,336,282 and 5,358,913, and final precision machining were made to tight tolerances using diamond tools (Fig. 4). Referring to Fig. 3, again, the air channels 114 on the flange portion 115 of the bearing assemblage (for stationary bearing/rotating shaft the air channels are either on two opposing bearing faces or on one bearing face; for rotating bearing/stationary shaft the air channels are on the shaft body) were formed or engraved in the green ceramic near-net-shaped bearing 110 . While various methods of forming air channels in the sintered ceramic bearing may be used, we prefer a novel technique which we developed comprising bead blasting through a polyester mask (prepared for example by the technique illustrated in Fig. 5), as described in details below. Sintering Referring to Fig. 1, Step F, sintering of the green machined air bearing 112 is performed in a temperature range from about 1400°C to about 1600°C, or more preferably about 1500°C. Preferable sintering times is in the range from about 1 hour to about 3 hours, or more preferably, about 2 hours. In a particular embodiment of the methods of the invention, the sintering peak temperature is 1500°C and that temperature is maintained for about 2 hours. It is preferred that the pre-sintered bearing 112 be slowly heated to sintering temperature and slowly cooled so as to avoid undesirable dimensional changes, distortions and crack development. In an embodiment of the invention having a preferred sintering temperature of 1500°C, preferred temperature ramps during heating are: about 0.3°C/minute for room temperature to about 300°C, about 0.1°C/minute for about 300°C to about 400°C, about 0.4°C/minute for about 400°C to about 600°C, and about 1.5°C/minute for about 600°C to about 1500°C. Preferred temperature ramps during cooling are: about 2°C/minute for about 1500°C to about 800°C and about 1.6°C/minute for about 800°C to room temperature. Alternatively, sintering may be achieved in the presence of a dopant selected from: MgO, FeO, ZnO, NiO, and MnO, and combination thereof, as discussed below in detail. The resulting alumina-zirconia ceramic article of the invention has a core of α-alumina or α-alumina and tetragonal zirconia alloy and a case of cubic spinel or cubic spinel along with cubic structure or cubic and monoclinic structure of zirconia alloy. In the sintering of the dopant oxide selected from: MgO, FeO, ZnO, CoO, NiO, and MnO, and combination thereof, is in contact with the blank. It is preferred that the sintering result in a ceramic article 116 having a "full" or nearly theoretical density, and it is more preferred that the density of the ceramic article 116 be from about 99.5 to about 99.9 percent of theoretical density. Sintering is conducted in air or other oxygen containing atmosphere. The methods of the invention are not limited to any particular sintering pressure and temperature conditions. Sintering can be performed at atmospheric pressure or alternatively a higher pressure can be used during all or part of the sintering to reduce porosity. The sintering is continued for a sufficient time period for the case of the article being sintered to reach a thermodynamic equilibrium structure. An example of a useful range of elevated sintering pressures is from about 69 MPa to about 207 MPa, or more preferably about 100-103 MPa. The exact manner in which the dopant is in contact with the blank during sintering is not critical, however, the "case", as that term is used herein, is limited to those areas of the blank in contact with the dopant during sintering. For example, a cubic spinel and tetragonal zirconia case can be readily produced by the methods of the invention on a portion of the overall surface of an article. It is not critical that the dopant be in contact with the blank during initial sintering, that is, sintering which does not result in an increase in density to full density. Prior to observing the results of the Examples, the inventors had thought that they would be able to provide an explanation for conversion methods having any relative percentages of zirconia alloy and alumina. The inventors had expected results to be in accord with the concepts that the formation of cubic spinel is highly favored thermodynamically over the conversion of tetragonal zirconia to cubic zirconia and that the mechanism of action follows alumina concentration. What has been discovered by the inventors is that, surprisingly, if the concentration of alumina in the blank 108 is from about 5 weight percent (relative to the total weight of zirconia and alumina) to about 50 weight percent, then the method of the invention produces an article having a case that is cubic spinel and tetragonal zirconia and a core that is α-alumina and tetragonal zirconia. During sintering, dopant, in effect, diffuses past tetragonal zirconia until all of the dopant has contacted and reacted, i.e. "partitioned", with alumina. In contrast, if the concentration of alumina in the blank is less than about 5 weight percent or greater than about 75 weight percent, then the method of the invention produces an article that has a case that is substantially composed of cubic spinel and cubic zirconia or cubic and monoclinic zirconia and a core that is α-alumina and tetragonal zirconia. During sintering, dopant does not, in effect, diffuse past tetragonal zirconia until all of the dopant has contacted and reacted with alumina; but rather reacts with alumina and tetragonal zirconia in the same vicinity, leaving alumina deeper within the blank unreacted. These results are not compatible with a simple mechanism of action based on concentration alone. The results seen are compatible with a mechanism of action based upon an unpredictable alignment of several competing factors, such as rate of diffusion of dopant during sintering. Shaping/Machining It is known that ceramic parts can be fabricated to net-shape by the compaction processes such as dry pressing, injection molding, slip casting, and cold isostatic accompanied by green machining (Fig. 1, Step D). Green machining refers to the process of machining the ceramic particulate compact prior to densification. (For more general information refer to David W. Richardson, Modern Ceramic Engineering: Properties, Processes and Use in Design, 2nd Edition (1992). In this process, it is important that care be exercised to avoid overstressing the fragile material and producing chips, cracks, breakage, or poor surface. For instance, it is important that the ceramic billet is held rigidly, but with no distortion or stress concentration, during green machining. The part can be rigidly held by one of a numerous ways, including by simple mechanical gripping, by bonding or potting with a combination of beeswax and precision metal fixtures, the latter being preferred by the inventors. Once the ceramic billet is secured rigidly in a fixture, green machining can be accomplished in a variety of methods, including: turning, milling, drilling, form wheel grinding, and profile grinding. We prefer turning and profile grinding the billet during green machining to achieve the best results. Machining can be either dry or wet, depending on the binder present and whether or not the part has been bisque fired, i.e., fired at a high enough temperature to form bonds at particle-particle contact points, but not at a high enough temperature to produce densification. Apart from green machining, a further precision machining step E, according to Fig. 1, of some of the surfaces is required to meet dimensional tolerances, achieve improved surface finish or remove surface flaws. Maintaining dimensional tolerances to the extent of few millionths of (an inch) a centimeter or achieving surface finish to less than (10 microinches) 25 microns is not possible unless final machining after sintering is undertaken. We accomplished dimensional tolerances to the extent of ± (100 millionth of an inch) 25 x 10 -2 microns using dry pressing (uniaxial pressing) for parts of simple geometry and controlling the shrinkage by our patented sintering process. Precision machining is required when the part demands tolerances in certain dimensions less than (100 millionth of an inch) 25 x 10 -2 microns and also to meet some dimensional tolerances such as roundness, perpendicularity, parallelness, etc. As contrasted with green machining, final precision machining of the sintered ceramic bearing requires diamond tooling and also sophisticated machines. Skilled artisans would know that milling, grinding, lapping and polishing are some effective procedures which are used in precision machining. Hydrodynamic Bearing 116 In Fig. 3, hydrodynamic bearing 116 is shown having spiral grooves 114 defining air channels arranged in at least one flange portion 115 in fluid communication with air inlet ports 121 formed in the body portion. Preferably, the air channels 114 are formed on a flange portion 115 of the ceramic bearing 116 or, alternatively, air channels 134 may be formed on a circumferential portion 132 of ceramic shaft 130 (Fig. 4). For stationary bearing/rotating shaft, the air channels are either on opposing faces of the bearing or on one bearing face as shown in Fig. 3. Alternatively, for rotating bearing/stationary shaft, the air channels 134 are on the shaft body 132 , as illustrated in Fig. 4. Air channels provide air flow across the bearing surface which lubricates the bearing-shaft assembly. While the number of air inlet ports 121 may depend on the application, generally we prefer minimally two air inlet ports 121 in the bearing. Polyester Mask Forming According to the device illustrated schematically in the Fig. 5, data processing equipment, such as personal computer 160 , is used to drive a CO 2 laser 162 . In our invention, we preferred using an empirical mathematical model for generating the database profile for forming our preferred predetermined spiral engraved pattern. Thus, polar coordinates r and Θ for the engraved spiral pattern in the ceramic article conform with the below empirical mathematical equation in which various values of x, y, r and Θ were used, as illustrated in Table 1. According to the equation which defines the position of the leading edge E 1 of a spiral portion, Θ = 153.0649 + 176.4505 In (r); wherein Θ defines the position of a spirally engraved portion relative to the r; and, r defines the distance from the center of the x, y coordinates to the leading edge of a spirally engraved portion comprising the spiral pattern. According to our invention, a corresponding trailing edge E t of a spirally engraved portion is rotatably about 11.43° from its corresponding leading edge E 1 . Thus, according to the above equation, a preferred engraved spiral pattern useful in a ceramic hydrodynamic bearing assemblage is illustrated in Figs. 3 and 5. Coordinate No. x y r Θ 1 0.420000 0.000000 0.420000 0.0000 2 0.430504 0.037956 0.432174 5.0386 3 0.437682 0.076680 0.444348 9.9372 4 0.441572 0.115872 0.456522 14.7034 5 0.442236 0.155251 0.468696 19.3441 6 0.439753 0.194558 0.480870 23.8659 7 0.434217 0.233553 0.493043 28.2746 8 0.425737 0.272016 0.505217 32.5757 9 0.414430 0.309745 0.517391 36.7744 10 0.400422 0.346557 0.529565 40.8755 11 0.383847 0.382287 0.541739 44.8833 12 0.364841 0.416786 0.553913 48.8022 13 0.343547 0.449922 0.566087 52.6357 14 0.320107 0.481577 0.578261 56.3878 15 0.294667 0.511649 0.590435 60.0616 16 0.267371 0.540046 0.602609 63.6605 17 0.238363 0.566693 0.614783 67.1874 18 0.207785 0.591523 0.626957 70.6451 19 0.175779 0.614483 0.639130 74.0363 20 0.142482 0.635528 0.651304 77.3635 21 0.108030 0.654624 0.663478 80.6292 22 0.072555 0.671745 0.675652 83.8354 23 0.036185 0.686874 0.687826 86.9844 24 0.000954 0.699999 0.700000 90.0781 The database profile for the preferred spiral pattern, as illustrated above, is inputted into a data processing system, such as personal computer 160 , which communicates with the source of laser energy 162 . Laser beams 163 from the source of laser energy 162 is then directed onto the polyester sheet 170 for engraving, in accordance with the database profile, the predetermined spiral pattern therein, thereby forming the polyester mask 164 . Polyester mask 164 is then arranged in intimate contact using spray adhesive 166 (Fig. 6) with the ceramic article ( or bearing) so as to avoid distortions in the spiral pattern imparted to the ceramic article. Alumina grits (in a conventional sand blaster) are blasted onto the masked ceramic at (90 psi) 62 x 10 4 Pa For most effective results, the blasting nozzle is held at about (14 to 16 inches) 35 to 41 cm from the workpiece. The process of sand blasting to abrade hard surfaces is common to those who are familiar with the art and the machines are available commercially. The hard alumina grits, upon impinging on relatively more brittle surface than the plastic mask, removes the ceramic preferentially and thus forms the pattern. The plastic mask is so chosen such that it survives the very aggressive bead blasting process. We prefer either polyester or polyurethane sheets, thickness of which ranges from (5 to 10 mil) 12,7 to 25,4 x 10 -3 cm. Other plastics like polycarbonate, mylar, etc. did not survive the rigor of grit blasting. Engraving We, however, prefer engraving the green ceramic bearing 110 by a method involving bead blasting through a specially formed polyester mask from a polyester sheet using the device illustrated schematically in Fig. 6. According to Fig. 6, the apparatus 180 for bead blasting includes a source 182 of ceramic grits 184 , preferably alumina, a polyester mask 164 containing the spiral pattern 190 , as described above. A plug 192 , either metal or plastic, is arranged in a central opening (not shown) in the mask 164 and through the flange portion 115 and bearing body 117, as illustrated in Fig. 6. The apparatus 180 is substantially enclosed, with preferably a plastic shroud 119, to protect portions of the bearing 116 that are not being engraved by the alumina grits 184 . This invention is further clarified through the following examples: EXAMPLES Working Example 1: Zirconia ceramic powder (prealloyed with secondary oxides described in U.S. Patent No. 5,336,282 and 5,358,913) were packed and sealed in molds made of rubber/polyurethane of 55 to 70 durometers of Shore hardness A (preferably 65 durometers). These molds were cold isostatically pressed in an autoclave at 15 to 30 kpsi (preferably 25 kilo pounds per square inch) to obtain billets of appropriate sizes in diameter and length. Working Example 2: Same as in working Example 1, except that the zirconia alloy powder is premixed with polyvinyl alcohol binder. Working Example 3: Same as in working Example 1, except that the zirconia alloy powder is premixed with acrylic binder. Working Example 4: Preformed blanks produced by cold isostatically pressing are machined in their green state (i.e. before sintering) using carbide tools in lathe and milling machines. The cutting speeds in green machining of zirconia billets were maintained between 2800 and 3400 rpm (preferably at 32,000 rpm). In some cases, depending on the design, air inlet ports were drilled on the body of the bearings. Working Example 5: Grooves defining air channels in the desired locations of the air bearing assembly were engraved by bead blasting through masks. The procedure for mask generation is illustrated in Fig. 5. Various plastic and metal masks were used to generate spiral pattern. Polyester and polyurethane masks performed the best to generate uniform spiral patterns on ceramic articles. Working Example 6: The green machined and air channel engraved near-net-shaped air bearings are sintered following schedule described in U.S. Patent Nos. 5,336,282 and 5,358,913. After sintering the bearings achieved full theoretical density of 6.05 gms/cc for yttria stabilized zirconia. Working Example 7: The final precision machining of the bearing assembly and lapping of the flange portion containing the engraved spiral pattern were carried out using diamond tools. The surface finish of the bearing assembly including the air channels were < 0.1 microns.

Claims: A method of engraving a ceramic article (116), comprising the steps of providing a mask (164) having a predetermined engraved pattern (190) therein, overlying said mask (164) onto said ceramic article (116); bombarding said mask overlying said ceramic article with hard ceramic grits (184); and removing the mask from said ceramic article, characterized by : forming a green ceramic article (110) from a ceramic powder (100), (102) and binder; overlaying the mask (164) onto the green ceramic article (110); bombarding the mask (164) overlaying the green ceramic article (110) with the hard ceramic grits (184); removing the mask (164) from said green ceramic article (110) exposing an engraved green ceramic article (112); and sintering the engraved green ceramic article (112) to produce an engraved ceramic article (116). The method recited in claim 1, wherein said mask (164) is formed by the steps comprising: providing a polyester sheet (170); providing a source of laser energy (162); providing means (160) for computationally communicating a predetermined pattern to said source of laser energy (162), said source of laser energy (162) being responsive thereto; directing said source of laser energy toward said polyester sheet (170); bombarding said polyester sheet (170) with said source of laser energy until said pre-determined pattern is engraved therein thereby forming an engraved polyester mask (164). The method recited in claim 1, wherein said engraved green ceramic article (112) is sintered at a temperature in the range of from about 1400 degrees C to about 1600 degrees C. The method recited in claim 3, wherein the sintering time is in the range from about 1 hour to about 3 hours. The method recited in claim 1, wherein said green ceramic article (110) comprises a ceramic powder comprising particulate zirconium oxide alloy, said zirconium oxide alloy consisting essentially of zirconium oxide (100) and a secondary oxide (102) selected from the group consisting of MgO, CaO, Y 2 O 3 , Sc 2 O 3 , and rare earth oxides, said zirconium oxide alloy has a concentration of said secondary oxide in the case of Y 2 O 3 , about 0.5 to about 5 mole percent; in the case of MgO, about 0.1 to about 1.0 mole percent, in the case of CeO 2 , about 0.5 to about 15 mole percent, in the case of Sc 2 O 3 , about 0.5 to about 7.0 mole percent and in the case of CaO from about 0.5 to about 5 mole percent, relative to the total of said zirconium oxide alloy, said compacting further comprising forming a blank. The method recited in claim 1, wherein said green ceramic article (110) comprises a mixture of particulate zirconium oxide alloy and particulate aluminum oxide. The method recited in claim 1, wherein said green ceramic article (110) comprises a composite ceramic comprising a mixture of particulate zirconium alloy and particulate aluminum oxide, and wherein said mixture comprises from about 75 to 100 percent by weight alumina. The method recited in claim 1 wherein said hard ceramic grits (184) is selected from the group consisting of: (a) alumina; (b) silicon carbide; (c) diamond: and, (d) mixture thereof

Inventors: CHATTERJEE DILIP K, GHOSH SYAMAL K, NARAYAN BADHRI, CHATTERJEE, DILIP K., GHOSH, SYAMAL K., NARAYAN, BADHRI, Chatterjee, Dilip K., Eastman Kodak Company, Ghosh, Syamal K., Eastman Kodak Company, Narayan, Badhri, Eastman Kodak Company
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1997
Main classification: B23K26
IPC codes: B23K26, B24C1, C04B35, F16C33