KR100588097B1 - Injection molding of structural zirconia-based materials by an aqueous process - Google Patents

Abstract

To form a net- or nearly net-form product, a casting compound consisting essentially of 50-100 wt% ZrO ₂ (Y ₂ O ₃ ) and 0-50 wt.% Al ₂ O ₃ is used. The compound containing ceramic powder with an average particle size of 1 μm or less can be mixed with the liquid carrier, gel forming binder, and processing additives to be cast at relatively low pressures in conventional injection casting machines.

Injection Casting, Casting Compound, Ball Mill, Ceramic Suspension, Casting

Description

INJECTION MOLDING OF STRUCTURAL ZIRCONIA-BASED MATERIALS BY AN AQUEOUS PROCESS}

The present invention relates to a process for forming a ceramic part from powder and to a casting composition used therein. More specifically, the present invention relates to a casting process and a molding composition for forming a net-shaped and almost net-shaped composite parts in a ZrO ₂ -based structural material that can be manufactured with high density and high strength.

When cooled from an elevated temperature, usually from a sintering temperature, ZrO ₂ undergoes a martensite transformation from tetragonal crystal structure to monoclinic crystal structure. The transformation results in changes in volume and anisotropic form. Under controlled conditions, the tetragonal phase is maintained at room temperature and transformed only when the crack intersects the grain.

The addition of a small amount of stabilizer such as Y ₂ O ₃ can have a great effect on the stabilization of the tetragonal phase. For example, pure yttrium-stabilized tetragonal polycrystalline zirconia (Y-TZP) materials may be used for Y ₂ O ₃ concentration, grain size, and sintering (eg, pressureless vs. HIP'ing). As a result, it can be sintered with high strength or high toughness. While the instability of the tetragonal phase (depending on the Y ₂ O ₃ concentration) determines toughness, the fine grain size in the Y-TZP material provides a high strength material. Such materials are disclosed by Masaki & Shingo in US 4,742,030, Cassidy et al in US 4,866,014, and Ghoshid et al in US 5,336,282.

One of the major drawbacks of Y-TZP materials is their environmental degradation. Particularly when exposed to a humid environment and a temperature range of 150-300 ° C, the tetragonal phase spontaneously transforms into a monoclinic with a sharp decrease in strength. For details on this behavior, see J. Europ. Ceram. Environmental degradation of zirconia ceramics by S. Lawson of Soc. Vol. 15, pp. 485-502 (1995). The addition of particulate alumina to Y-TZP increases both the strength and environmental stability of the Y-TZP material. Similarly, heat treatment at elevated temperatures also improves environmental stability.

ZrO ₂ -based ceramics are widely used for metal forming tools, automobiles, textiles, and consumers such as knives, scissors and golf clubs. Ceramic components used in most of these applications are made by techniques for pressing or slip cast forming powders.

One purpose of any forming method is to be sintered in a non-fired form of dimensional stability, reproducible and free from defects, so that certain densities and particle packing (hereinafter referred to as "green" parts, forming, Density, etc.). During green-forming and sintering, cracks, warpage and other defects can occur due to shrinkage associated with the particle strengthening process. In general, this defect-causing process is mitigated by making a uniform green body with a suitable green strength.

Another purpose of the shape-forming method is to eliminate or eliminate downstream operations, such as a machine, to obtain the dimensions of the final part, thus having a net shape. To produce. Dry pressing relates to the compaction of the powder in the die. Dry pressing, among other various form-forming methods, adds further cutting downstream processing and diamond grinding in order to obtain complex shapes, non-symmetrical geometries and small tolerances. Require. In slip-casting, a liquid suspension of ceramic powder is "de-watered" in a porous mold to produce a powder cake in the form designated as the mold. Slip casting contributes to the production of net shaped parts, but the method is considered relatively poor for the manufacture of bulky and complex parts.

Injection molding is considered the best molding method for complex, ceramic forms. It is possible to quickly produce complex parts in the form of nets with high volumes, thus providing a significant advantage over other forming methods. First, injection molding involves mixing a ceramic powder with a dispersant and a thermosetting organic binder of various compositions. The molten powder / binder mixture was heated during the injection casting and injected into a relatively cold mold. After solidification, the part was injected in a similar fashion to the plastic part. The binder was then removed and the part was densified by high temperature heat treatment. There have been a number of critical stages in this process, including mixing the initial powder and binder, injecting the mixture into a mold, and removing the organic matrix material. One of the major advantages of the initial powder injection casting (PIM) process is the removal of organic vehicles. Currently, the cross section limit for fine particle size in the PIM process is 0.5-0.7 inches. If the particle size exceeds that limit, the binder removal process will cause defects, pinholes, cracks, blisters and the like. Removal of the binder occurs by slow heat treatment which takes several weeks. During removal of the binder at elevated temperatures, the binder becomes a liquid that can cause distortion of the green part due to capillary force. Another advantage of the PIM process is that relatively high molecular weight organics tend to degrade through the green body, causing internal or external defects. The use of solvent extraction, in which a portion of the organism is removed using an organic or supercritical liquid, sometimes minimizes defect formation. However, the solvent extraction process results in the formation of porosity over that portion, resulting in the removal of the remaining organisms. During the binder removal, if the green density / strength is not high enough, part slumping causes problems, especially for larger particle sizes.

As such, PIM offers specific advantages in net form, high dimensional control and high volume automation of complex parts, but the very long binder removal time coupled with the limitation of part size and their environmental impacts makes use of this technique. It does not bring about the expected growth.

Several improvements have been applied to early PIM processes, such as the use of water based binder systems. Hens et al. Have developed a water leachable binder system that allows water to pass through (US Pat. No. 5,332,537). Injection molding feed-stock with finely refined particle size distribution (to control rheology), PVA based multiple binders, and coating for each of the binder particles Is prepared). During casting, this coating forms a neck that provides part stiffness. There is water de-bind that lasts several hours after injection molding. After the remaining binder is cross-linked by UV or chemical methods, the portion undergoes thermal dissociation that takes 8 to 12 hours for the same portion as the golf club head. Other aqueous-based binders contain polyethylene glycol, PVA copolymers, or COOH-containing polymers. BASF has developed a polyacetal based system in which the binder is removed by heat treatment with gaseous formic or nitric acid and then cast at a moderately high temperature. Since low temperatures exclude the formation of liquid phases, the green part's deflection causes viscous flow. The gaseous catalyst does not penetrate the polymer, and only decomposition occurs at the interface of the gas and the binder, thus preventing the formation of internal defects. These improvements have limitations that require separate binder furnaces and time, depending on the part size.

Powder injection molding for ready moldable feed-stock, containing the right proportions of ceramic powder, the necessary binders, liquid carriers, and other additives in ready-to-use forms in commercially available injection molding techniques. Is required.

The present invention provides a format useful for producing a low cost manufacturing of ceramic articles by aqueous zirconia-based casting compounds, their component materials into a homogeneous mixture, and injection molding. The term "zirconia-based" as used herein means a composition containing 50-100 wt.% Zirconium oxide in fired ceramic. The casting compound of the present invention advantageously contains, as a homogeneous mixture, (i) inredients essential to the form-forming part by injection molding and (ii) a yielding zirconia-based ceramic material after firing. . Generally, according to the present invention, there is provided a molding compound comprising essentially a ceramic precursor, zirconium oxide, yttrium oxide, and alumina in a form suitable for a manufactured product by injection casting.

Advantageously, the instant moldable zirconia-based compounds of the present invention eliminate the need for high casting pressures and special binders. The casting compound of the present invention can be cast at low instrument pressures of about 1000 psi or less using water as the liquid carrier. Moreover, the cast parts are dried before sintering by evaporation of water, eliminating the polymer-based casting system, which is a long and complex dissociation step. After firing, ZrO ₂ material gains high density and high strength.

Hereinafter, the present invention will be described in more detail.

According to the process of the invention, the ceramic powder is first mixed with a gel-forming powder and a solvent for the gel-forming material. In normal practice, zirconia ceramics require a stabilizing additive to prevent large breakage of the product due to transition of the monoclinic phase when cooled from the sintering temperature. Any stabilizer known to those skilled in the art of manufacturing zirconia ceramics can be used in this process. Common stabilizers include compounds such as carbonates, nitrates, oxylates, etc., produced with elements of Y, Ce, Ca and Mg or oxides of those elements during high temperature processes. The amount of stabilizer can be selected according to the preparation of tetragonal, cubic or monoclinic or mixture of phases. Preferred stabilizer is yttria. Alumina provides specifically required effects such as improvements in environmental stability. In order to densify the material to high density and achieve high strength, the average particle size should be 1 μm or less. Preferably, the average particle size is in the range of 0.1-0.9 μm, more preferably 0.3-0.5 μm. The term 'particle size' as used herein refers to the diameter of an equivalent sphere.

The present invention is essentially a ceramic composed mainly of zirconium oxide and consisting of a small amount of other metal inorganic compounds, water, a binder (selected from the group of polysaccharides) and trace amounts of other additives for improving the processability of the cast feed-stock. Provide casting compounds. The present invention further provides a method for producing a feed-stock that is readily moldable from ceramic powders, binders, carriers, and other processing tools of the composition.

Regardless of the actual phase present after firing, it is customary to express the ceramic components of the fired ceramic body in terms of the constituent metal oxide compounds. When using this custom, the ceramic component of the casting composition disclosed herein has the formula [ZrO ₂ ] _a [Y ₂ O ₃ ] _b [Al ₂ O ₃ ] _c , wherein a is 50-95 wt%, b is 4- 6wt% and c is 0-45wt%. In the present invention, preferred casting compounds in view of the constituent metal oxides are a = about 85.8 wt%, b = about 4.3 wt%, and c = about 14.3 wt%. A second example of the preferred casting compound in terms of initial ceramic powder contains about 95 wt% zirconium oxide and 5 wt% yttrium oxide.

Generally, the amount of powder in the mixture is 50-95% by weight of the mixture. Preferably, the powder is 75-90% by weight of the mixture, more preferably 83-86% by weight of the mixture. The preferred and more preferred amounts are very useful for the production of injection molded parts in the form of nets and near nets.

The casting compound provides a binder that provides a mechanism to cure the flowable material in a mold to be removed as a self-supporting structure. In the present invention, this role is explained by the compounds resulting from the classification of polysaccharides known as agaroids. Agaroids are defined as gum-like Agars, but do not meet all of their properties (HH Selby et al., "Agar", Industrial Gums, Academic Press, New York, NY, 2nd edition, 1973, 3 Chapter, p. 29). However, agaroid as used herein means not only any gum-like agar, but also their derivatives such as agar agarose. Agaroid is used because it is a component that rapidly gelatinizes within a narrow temperature range and rapidly increases the production rate of the product. Preferred gel-forming materials are water soluble and include agar, agarose, or carrageenan, and more preferred gel-forming materials consist of agar, agarose, and mixtures thereof.

The gel-forming material is present in the mixture in an amount of 0.2 to 6 wt%, based on solids. At least about 6 wt% of the gel-forming material in the mixture may be used. Although such amounts may lead to some reduced benefits than would have been made with conventional compositions, it is believed that larger amounts do not have a process adverse effect. More preferably, the gel-forming material may be 1 to 4% by weight of solids in the mixture.

The casting compound also provides a liquid carrier that facilitates moving the casting compound to a mold along the barrel of an injection casting machine. Water is the most preferred liquid carrier in the casting compound because it is ideally used for two purposes: the solvent for the gel forming binder and the liquid carrier for the solid component in the mixture. Moreover, the boiling point is low, so that it is easily removed from the molded part before and / or during firing. The amount of water is chosen to be such that it has the necessary flow properties for the casting compound for proper behavior in the injection casting machine. The appropriate amount of water is 10-30 wt% of the mixture and preferably 15-20 wt%.

The casting compound also contains various additives that can serve many useful purposes. Additives known to be very useful in the casting compounds of the present invention include dispersants, pH adjusters, biocides, and gel strength enhancers (eg, metal borate compounds such as calcium borate, magnesium borate, and zinc borate). Biocides may be used in casting compounds to inhibit the growth of bacteria, especially when stored for long periods of time.

 The use of dispersants and pH adjusting agents is known to greatly improve the flowability and processability of ceramic suspensions. In the case of the present invention, dispersants based on polyacrylate and polymethylmethacrylate polymer backbones are known to be useful for improving the processability of aluminum oxide-based compositions, the amount of dispersant in the casting compound being 0.2-1 wt% relative to the ceramic powder. And preferably 0.2 to 0.8 wt%. Similarly, tetramethylammonium hydroxide is known to be useful for adjusting the pH of the suspension, with a useful pH range of 8.8-11 and preferably 9.5-10.5.

The casting compounds of the present invention are combined with the processing tools of ceramic powders, liquid carriers, binders, and readily-formable shapes. In view of the constituent compounds, the preferred composition is 66.90 wt% zirconium oxide, 4 wt% yttrium oxide, 11.7 wt% aluminum oxide, 2.5 wt% agar, 0.33 wt% dispersant, 0.53 wt% tetramethylammonium hydroxide, 0.02 wt% biocide And 14 wt% water (the dispersant is added TMA as 40% aqueous solution and 25% aqueous solution)

The present invention also provides a method of combining all the various components of the casting compound into a homogeneous mixture to produce a uniform cast body that can be fired without cracking and other bonding. Raw ceramic powders are often very agglomerated and need to be unwound before they are made into useful ceramic products free of defects, warpage, and other defects. Among various possible methods, ball milling is known to be convenient and useful for preparing aqueous-based casting compounds, which are powders homogenized and simultaneously homogenized in the aqueous media disclosed herein. The useful concentration range for the ball milling ceramic powder is 50 to 85 wt%, and the preferred range is 65 to 80 wt%.

The compounding of the ceramic suspension with the binder can be done in a number of efficient mixers, such as, for example, sigma mixers or planetary type mixers. The biocide may optionally be mixed into the composition at the compounding stage of the process or near the end of the ball milling cycle. During the mixing, the mixture is heated for 15 to 120 minutes, preferably for 30 to 60 minutes, in the temperature range of 75 to 95 ° C, preferably 80 to 90 ° C.

The casting compound is in a form suitable for injection into an injection casting machine. In the present invention, the combined homogeneous mixture is cooled below the gelling point of the gel-forming agent and removed from the blender. Thereafter, it is broken into particulates using a rotary cutter blade typically used in food processing. The crushed particles are put directly into the hopper of the injection casting machine. The broken feed-stock can be dried to a specific casting solid by evaporation until the desired humidity level is achieved by exposing the material to the atmosphere. Useful solids levels in the casting compound range from 75 to 88 wt%, preferably 83 to 86 wt%.

A very wide range of casting pressures can be used. Generally, the casting pressure is in the range of 20 to 3500 psi. More preferably, the casting pressure is in the range of 40 ~ 1500psi. The casting temperature, of course, must be lower than the gelation point of the gelling material to produce the self supporting body. Suitable casting temperatures can be made before, during or after the mixture is fed into the mold. Usually, the casting temperature is maintained at a temperature of less than 40 ℃, preferably 15 ~ 25 ℃ temperature range.

After the part is cast and cooled to a temperature below the gelling point of the gel-forming material, the body is removed from the mold. The green body is typically sufficiently capable of self-support that does not require special treatment during removal from the mold. After removal from the mold, the part is dried. Similar to the drying of slip-cast parts, it is necessary to control the drying behavior. Depending on the size and complexity of the part, fast drying can cause cracking. In that case, the part can be dried in a controlled humidity environment.

After the part has dried, the body is sintered at elevated temperature to produce the final product. The sintering time and temperature are regulated depending on the powder material used to form the part. Preferably, the elevated temperature at which the body is sintered is at least 1250 ° C., more preferably 1300-1550 ° C., and most preferably 1350-1500 ° C. Preferably, the sintering time at maximum temperature is less than 4 hours, more preferably 1 to 3 hours, and most preferably 1 to 2 hours.

Thus, the present invention can be used to form complex and thick net- or nearly net-shaped bodies of zirconia-based materials having excellent strength properties and environmental stability. Containing 20 vol.% Alumina called AS280 The physical properties of the densified ceramics obtained from the preferred casting compounds, as summarized in Table 1, have been shown to be excellent in various structural applications.

Property unit exam value color － － Off-white density g / cm 3 ASTM C20-83 5.63 Bending strength MPA (ksi) 3-point 970 (140) Bending strength MPA (ksi) 4-point 820 (118) Hardness kg / mm2 Knoop (100 g) 1518 Fracture toughness MPa.m ^1/2 Indentation 5.6 Modulus GPA (10 ⁶ psi) ASTM C623 239 (34.7) Shear rate GPA (10 ⁶ psi) ASTM C623 92 (13.4) Poisson's Ratio － ASTM C623 0.3 CTE ppm / ℃ ASTM E 228 (Theta Dilatometer) 50 ℃ 8.9 250 ℃ 9.42 500 ℃ 9.98 750 ℃ 10.31 1000 ℃ 10.5

The following examples are provided to provide a more complete understanding of the present invention. Specific techniques, conditions, substances, ratios, and reporting materials are described by explaining the principles, and the experiments of the present invention are illustrative and should not limit the scope of the present invention.

With reference to the accompanying drawings and preferred embodiments of the present invention described in detail below, the present invention will be more fully understood and further advantages will become apparent.

1 is a representative diagram depicting the basic steps of an embodiment of the invention.

(Example 1)

2314.27 g of HSY-3 zirconia powder and 384.74 g of Alcan C-901 alumina powder were weighed into a 1.6 gallon Abbethane ball-mill jar. 10.6 kg of 3/8 "zirconia medium was added. The mixture was weighed with 889.2 g of deionized water, 10.8 g of Darvan 821A ammonium polyacrylate (40% solution Vanderbilt Laboratories), and 17.5 g of TMA (25 wt% solution, Alfa Inorganics). The slips were ball milled for 24 hours, 3200 g were obtained and transferred to a sigma mixer, while stirring in a sigma mixer, 72 g of agar (S-100, Frutarom Meer Corp.), methyl-p-hydroxy benzoate ( 0.62 g of Penta Mfg) and 0.45 g of propyl-p-hydroxy benzoate (Penta Mfg) were further mixed in. The sigma mixer was heated to 190 F for 45 minutes, after which the temperature was lowered to 170 F and another 45 minutes of mixing After the material was cooled to room temperature, it was filtered using a # 5 sieve to shred it using a food processor (Kitchen Aid KSM90) and remove any large and fine debris.

Prior to casting, the broken feed-stock was dried to the desired solid level by exposing a loose bed of material to the atmosphere. The load of the solid was determined using a humidity scale (Ohaus Corp).

Plates were cast on Boy 15s and 22M. The plates were dried slowly in the bench for several hours after drying in a vacuum oven @ 100 ° C. After the plate was dried, it was densified at @ 1450 ° C. for 2 hours. Standard 3- and 4-point bars were cut out (military type B) and the bending strengths were determined to be 0.97 and 0.82 GPa, respectively.

Example 2

As in Example 1, a cast feed-stock was prepared and various types of molds such as "3-hole sensors" were used. The fired part is cylindrical in shape with three holes, nominally 0.85 "in length, and nominally 0.1" in diameter in the longitudinal direction. The steps divided each part into larger diameter shoulders of "0.45" OD x 0.35 "length and smaller diameter shoulders of 0.35" OD x 0.5 "length. The parts were dried under ambient conditions and dried at 1450 ° C. for 2 hours. Casting was carried out at 85 wt% after firing at. The average density after firing was 5.59 ± 0.012 g / cm 3. Their average dimensions were 0.407 "± 0.001" for large diameters and 0.358 "± 0.0011" for small diameters. And the length was 0.7404 ± 0.002 ". The average shrinkage for these three dimensions was 21.6 ± 0.2%, 22.2 ± 0.2%, and 19.7 ± 0.2%, respectively.

Another batch of parts, hereinafter referred to as "half shell", was cast. The part was in the form of a half cylinder with several steps and grooves on the flat side. The part was dried under ambient conditions and calcined at 1450 ° C. for 2 hours before casting at 86 wt%. After firing, the average density was 5.6 ± 0.01 g / cm 3. The fired length was nominally 0.9 "in 0.4" width. Within 84 green parts, the average length was 0.949 ± 0.005 "and the average diameter was 0.496 ± 0.003". After firing, the average shrinkage was 21.1 ± 0.5% and 21.4 ± 0.5%, respectively, for the length and diameter.

Example 3

This example shows the scale-up of the casting compound preparation described in Example 1. Prepared with 38.4 kg HSY-3 zirconia / yttria, 6.24 kg aluminum oxide, 14.62 kg deionized water, 0.179 kg ammonium polyacrylate and adjusted to pH 11 with TMA. After ball milling, 55 kg of slip was transferred to a terrestrial blender mixed with 1.24 kg agar, 0.011 kg methyl-p-hydroxy benzoate and 0.0077 kg propyl-p-benzoate, stirred and heated. Mixing was continued for 1 hour after the blender reached a final temperature of 95 ° C. The material was broken up and injected into the feed-stock.

By describing the present invention in a more detailed description, this detailed description is not necessarily fixed, and more changes and modifications are made by those skilled in the art while falling within the scope of the invention as defined by the appended claims. It will be appreciated that it can be presented.

According to the present invention, if the injection casting method of the zirconia-based structural material by the aqueous process is applied, it is possible to form parts of the ZrO ₂ -based structural material of high density and high strength, and to manufacture the ceramic product at low cost. There is.

Claims (13)

A ceramic powder, a dispersant, a liquid carrier, a gel-forming agent and an additive selected from the group consisting of pH adjusting agents, biocides, gel strength enhancers and mixtures thereof, The ceramic powder is composed of 13.98-45wt.% Al ₂ O ₃ , 50-82.02wt.% ZrO ₂ , and 4-6wt.% Y ₂ O ₃ . Casting compositions for forming net-shape or nearly net-shaped articles According to claim 1, wherein the ceramic powder is characterized in that it comprises a composition comprising 13.98 ~ 14.30wt.% Al ₂ O ₃ , 79.7 ~ 82.02wt.% ZrO ₂ , and 4 ~ 6wt.% Y ₂ O ₃ . Casting composition delete The casting composition of claim 1, wherein the casting composition further contains 0.2 to 1 wt.% Of a dispersant based on a polyacrylate or polymethyl methacrylate polymer backbone. delete Mixing together a ceramic powder comprising 13.98-45 wt.% Al ₂ O ₃ , 50-82.02 wt.% ZrO ₂ , and 4-6 wt.% Y ₂ O ₃ ; And Ball milling the ceramic powder in the presence of an aqueous medium to produce a ceramic suspension; 7. The method of claim 6, further comprising optionally mixing a biocide with a binder in the ceramic suspension to produce a combined homogeneous mixture. The method of claim 6, wherein during the mixing, the suspension is heated for 15 to 120 minutes in a temperature range of 75 to 95 ℃. The method of claim 8, wherein the temperature is 80 ~ 90 ℃ and the time is 30 ~ 60 minutes the production method of the casting composition. 8. The method of claim 7, wherein the mixture comprises a gel-forming agent, and the method of preparation comprises cooling the mixture at a temperature below the gelling point of the gel-forming agent and removing it from the blender. Method for producing a casting composition characterized in that it further comprises The method of claim 6, further comprising shredding the mixture to form particulate materials. 12. The method of claim 11, further comprising drying the mixture until the mixture exhibits a solids level of 75-88 wt%. 13. The method of claim 12, wherein the particulate material is dried until a solids level of 84 to 86 wt% is obtained.

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