One well-known prior art method for preparing sintered zirconia parts involves forming sheets by means of a doctor blade. According to this method, sintered zirconia parts are prepared by furnishing a slurry composition comprising zirconia powder, an organic binder, and a solvent, applying the slurry onto a carrier film to a predetermined thickness by means of a doctor blade, drying the coating by evaporating off the solvent, thereby solidifying the coating into a green sheet, working the green sheet into members of desired shape as by cutting or punching, and firing the members. It is critical for this method that green sheets possess sufficient flexibility and strength to withstand stresses applied during working steps such as cutting and punching and handling steps such as roll-up and raveling. It is also important that parts resulting from binder removal and firing of such green sheets are free of distortion, deflection or cracking and have a minimal shrinkage factor, a high density, and dimensional stability.
Several organic binders are known in the art to impart flexibility to ceramic molding compositions containing zirconia powder and be susceptible to burning off and sintering. Exemplary of such known binders are acrylic polymers, acrylic polymer emulsions, ethylene oxide polymers, hydroxyethyl cellulose, methyl cellulose, polyvinyl alcohol, isocyanates, wax lubricants, aqueous urethanes, methacrylate salts, wax emulsions, and ethylene-vinyl acetate copolymer emulsions.
Although sheet-forming compositions were prepared by adding these organic binders to zirconia powder along with solvents, it was very difficult to find a compromise among the above-mentioned requirements including flexibility, binder removal, sinterability, and dimensional stability. More particularly, if compositions were specially formulated to form green sheets having improved flexibility, binder removal and sinterability would be poor. If compositions were specially formulated to form green sheets having improved dimensional stability, the sheets suffered from flexibility. In addition, the optimum binder content largely varies with the type of zirconia powder.
In summary, sheet-forming slurry compositions should meet the requirements that green sheets formed therefrom are of uniform thickness and have appropriate flexibility and tensile strength, and that parts resulting from binder removal and firing of such green sheets are free of distortion, deflection or cracking and have a minimal shrinkage factor, a high density, and dimensional stability.
Although prior art sheet-forming slurry compositions were formulated so as to improve one or more factors among the flexibility, binder removal and sintering capabilities, and dimensional stability of green sheets as mentioned above, it was very difficult to find a good compromise among these factors insofar as conventional types of zirconia powder were used. Some factors were improved fully, but the remainders improved to only limited extents. No satisfactory sintered parts were obtained.