WEAR RESISTANT CERAMIC Field of the Invention The present invention relates generally to ceramic materials, and more particularly, to a ceria/zirconia ceramic material with improved wear resistance.

Background of the Invention Ceramic materials find advantageous use in many industrial and commercial applications. In some applications it is highly desirable to utilize a very hard, wear resistant ceramic material. It is well established that wear resistance of relatively soft materials (metals) is directly proportional to their hardnesses. However, the wear resistance of a brittle ceramic is mostly related to its fracture toughness, and hardness plays a much smaller role.

The present invention relates to a ceria/zirconia ceramic having superior wear resistance.

Summary of the Invention In accordance with a preferred embodiment of the present invention, there is provided a sintered ceramic component comprised of a mixture of about 70 wt% to about 85 wt% of zirconia, about 15 wt% to about 30 wt% of ceria and about 0. 1 wt% to about 0. 4 wt% of manganese oxide.

In accordance with another aspect of the present invention, there is provided a Zr02 based ceramic material, comprised of Zr02 grains containing Ce02 as a stabilizer and having an average grain size of about 1 um or less. At least 95% of the Zur02 grains have a tetragonal phase, and the ceramic material contains at least 15 wt% cerum oxide and 0. 1 wt% to 0. 4 wt% manganese oxide.

In accordance with another aspect of the present invention, there is provided a method of forming a Zur02 based ceramic, comprising the steps of a) mixing a first particulate constituent, a second particulate constituent, a third particulate constituent, a deflocculant and water to form a solid-in-water suspension wherein, the first particulate constituent is comprised of Zr02 having an average particle size of less than 0. 8 um and forming between 70 wt% and 85 wt% of the solids in the suspension, the second particulate constituent is comprised of Ce02 having an average particle size of less than 0. 8 um and forming between 15 wt% and 30 wt% of the solids in the suspension and the third particulate constituent is comprised of MnO and forming

between 0. 1 wt% and 0. 4 wt% of the solids in the suspension, b) forming a green component having a desired shape from the solids-in-water suspension, and c) sintering the green component at a temperature between 1300°C and 1475°C, until at least 95% of said Zr02 based ceramic is tetragonal phase.

It is an object of the present invention to provide a ceramic material having superior wear resistance.

It is another object of the present invention to provide a ceramic material as described above that is comprised of a zirconia, ceria and manganese oxide.

It is another object of the present invention to provide a ceramic material as described above having a Vickers hardness (at a 50 N load) of at least 10 GPa.

It is still further another object of the present invention to provide a ceramic material as described above having a fracture toughness of at least 7 MPa 4.

A still further object of the present invention is to provide a method of forming a ceramic as described above.

These and other objects will become apparent from the following description of a preferred embodiment taken together with the accompanying drawings and the appended claims.

Brief Description of the Drawings The invention may take physical form in certain parts and arrangement of parts, a preferred embodiment of which will be described in detail in the specification and illustrated in the accompanying drawings which form a part hereof, and wherein : FIG. 1 is a photograph at 6000 magnification showing a Ce-TZP (77. 0 wt% Zr02 + 22. 4 wt% Ce02) + 0. 2 wt% MnO fired at 1400°C for four hours illustrating a preferred embodiment of the present invention ; FIG. 2 is a photograph at 6000 magnification showing a Ce-TZP (77. 0 wt% ZrO2 + 22. 4 wt% Ce02) + 0. 2 wt% MnO fired at 1550°C for two hours ; FIG. 3 is a photograph at 6000 magnification showing a Ce-TZP (78. 0 wt% Zr02 and 22. 0 wt% Ce02) fired at 1400°C for four hours ; FIG. 4 is a photograph at 6000 magnification showing a Ce-TZP (80. 3 wt% Zur02 and 19. 2 wt% Ce02) fired at 1520°C for four hours ; and FIG. 5 is a photograph showing a component formed in accordance with the present invention after a one-hour wear test.

Detailed Description of Preferred Embodiment The present invention relates to a ceria-stabilized zirconia ceramic material and a method of forming the same.

Broadly stated, the ceramic material is comprised of zirconia (ZrO2), ceria (Ce02), and manganese oxide (MnO). More specifically, the ceramic material is comprised of 70 wt% to 85 wt% zirconia, 15 wt% to 30 wt% ceria and 0. 10 wt% to 0. 40 wt% manganese oxide. In a preferred embodiment, as will be described below, the ceria content exceeds 20 wt% and the zirconia content is less than 80 wt%. A ceramic material formed in accordance with the present invention is at least 95% tetragonal phase, and has an average grain size of 1 um or less. Further, the ceramic material exhibits a Vickers hardness (at a 50 N load) of at least 10 GPa, and a fracture toughness of at least 7 MPa nu, where"m"is meters.

The physical properties of the ceramic material are based in part upon the composition of the material, and in part upon the method of forming the same, as shall hereinafter be described.

In accordance with one method of forming a Zr02 based ceramic according to the present invention, a particulate-in-water suspension is formed. The suspension includes a first particulate constituent comprised of zirconia (Zr02), a second particulate constituent comprised of ceria (Ce02) and a third particulate constituent comprised of manganese oxide (MnO).

The zirconia (Zr02) particulate preferably has an average particle size of less than 0. 8 um, and more preferably an average particle size of about 0. 4 m to about 0. 6 um. (particle sizes are measured by a Leeds Northrup model FRA Microtrac analyzer). The ceria (Ce02) particulate preferably has an average particle size of less than 0. 8 u. m, and more preferably an average particle size of about 0. 4 um to about 0. 6 um. The manganese oxide (MnO) particulate preferably has an average particle size of less than 0. 8 u. m, and more preferably an average particle size between about 0. 4 um to about 0. 6 u. m.

The above-identified particulate constituents are mixed with water to form a solid-in-water suspension, wherein the particulate constituents form the solids of the suspension. The water is preferably de-ionized and the particulate constituents forming the solids are added in the following proportions :

about 70 wt% to about 85 wt% of zirconia (Zr02) ; about 15 wt% to about 30 wt% of ceria (Ce02) ; and about 0. 1 wt% to about 0. 4 wt% manganese oxide (MnO).

The foregoing particulate constituents are mixed with water to form a slurry- like suspension. Preferably, the solids in the suspension (i. e., the particulate constituents) comprise about 60 wt% to about 70 wt% of the slurry suspension.

To facilitate wetting of the particles and mixing of the particulate constituents, a deflocculant is added, as is conventionally known. The amount of deflocculant necessary will vary based upon the type of deflocculant used. Preferably, the amount of deflocculant necessary to facilitate the mixing of the constituents and water is less than 0. 5 wt% of the resultant slurry-like suspension, and more preferably about 0. 3 wt% of the resultant slurry. As the addition of a deflocculant will vary the pH of the slurry-like suspension, it is preferable that the composition and amount of the deflocculant be such that the pH of the mixture be maintained between about 6. 8 and about 9.

The particulate constituents are preferably mixed together one constituent at a time. For example, one of the major constituents, i. e., zirconia particulate or the ceria particulate, is first mixed with water to form a slurry. A portion of the total deflocculant is added to the mixture to facilitate mixing. The other major constituent is then added to the existing slurry, together with the remaining deflocculant. Lastly, the manganese oxide is added and mixed until a slurry-like suspension is obtained.

The resultant particulate-in-water suspension preferably has a viscosity in a range of about 130 to about 250 CPS (measured by a Brookfield Viscometer model RVT, #4 Spindle, at 50 rpm) and a moisture content from about 35 to about 57, and more preferably about 35 measured with Computrac (Arizona Instruments) model MAX-20.

Once the water suspension has been prepared, it may be poured into a mold to form a component of any desired shape, by conventional molding techniques, or may be formed into shape by other known techniques. The shape of the component formed in and of itself forms no part of the present invention, and does not affect the resulting material or the properties thereof, as shall be described in greater detail below. The component is then dried forming a"green component."As used herein, the term

"green component"refers to any unfired shape cast or otherwise formed from the water suspension heretofore described.

The green component is then sintered, at a temperature below 1475°C for a period of time sufficient to form a Zur02 based ceramic material having a grain size of 1 Fm or less and being composed of at least 95% tetragonal phase. Preferably, the green components are fired at temperatures between 1300°C and 1475°C, and more preferably between 1350°C and 1450°C. As will be appreciated, at lower firing temperatures a longer firing period is required to produce the desired material.

The invention shall now further be described by the following Example wherein a first ceramic (designated SAMPLE I), prepared as heretofore described is contrasted with three other ceramics (designated SAMPLE II, SAMPLE III AND SAMPLE IV), that have a different composition and/or are fired at a different temperature.

EXAMPLE Table I shows the composition of the"particulate constituents"in each respective sample, as well as the firing time and temperature for each of SAMPLES I through IV.

TABLE I SAMPLE I SAMPLE II SAMPLE III SAMPLE IV (fired at 1400°C (fired at 1550°C (fired at 1400°C (fired at 1520°C for four hours) for two hours) for four hours) for four hours) Zirconia (Zr02) 77. 0 wt% 77. 0 wt% 78. 0 wt% 80. 3 wt% Ceria (CeO2) 22. 4 wt% 22. 4 wt% 22. 0 wt% 19. 2 wt% Manganese Oxide (MnO) 0. 2 wt% 0. 2 wt% Deflocculant 0.2 wt% 0.2 wt% 0.2 wt% 0.2 wt%

Each sample is prepared by mechanical mixing (as described above) in a horizontal 20 liter Netzsch mill. The average particle size in the slurry of each sample is about 0. 55 um. Green components are formed from the slurry of each sample and dried.

As indicated in TABLE I, SAMPLE I ceramic is fired at about 1400°C for four hours. SAMPLE II ceramic is fired at about 1550°C for two hours. SAMPLE III ceramic is fired at about 1400°C for four hours. SAMPLE IV ceramic is fired at about 1520°C for four hours.

The following characteristics of each sample ceramic are measured : density, grain size, crush strength, hardness, fracture toughness and wear rate. The hardness and fracture toughness of each sample ceramic is measured on polished sections.

Hardness (Hv) is calculated as : HV = 18. 54 * (P ! a2) where P = 50 N, (indentor load) and a is the residual impression diagonal size.

Fracture toughness is calculated as : <BR> <BR> <BR> <BR> 0.026 # #E # #P # a<BR> <BR> lc =<BR> <BR> <BR> <BR> <BR> <BR> (c)3/2<BR> t/ where E is modulus of elasticity, and c is the crack length from the center of the residual impression.

The wear rate of the respective sample components are measured using a Laboratory Mill (Netzsch LMZO. 3E) under conditions simulating hydraulic packing for one hour. Wear rate is determined as percentage of the total weight loss from the components after one hour.

FIGS. 1-4 show the grain structures of the respective ceramic samples at the same magnification, i. e., 6000 magnification. As can be seen by contrasting FIG. 1 with FIGS. 2-4, a ceramic formed in accordance with the present invention has a significantly smaller grain size. The grain sizes of the respective ceramic samples is set forth in TABLE II.

Table II shows a summary of the properties of components formed from the three above-described sample compositions.

TABLE II SAMPLE III SAMPLE IV SAMPLE I SAMPLE II (fired at (fired at Property/ (fired at 1400°C (fired at 1550°C 1400°C for 1520°C for Characteristic Units for four hours) for two hours) four hours) four hours) Density g/cm3 625 6. 25 b. 25 6. 1 Grain Size µm 0.8 4.2 1.0 2.44 Fracture MPa 8. 25 > 10 7. 2 9. 2 toughness, KIC Hardness (Hv) GPa 10.59 9.58 10.46 9.80 Crush Strength Ibs. force 114 55 106 94 (Newtons) (51. 7) (25) (48. 1) (42. 7) Bead Wear Rate (1 hour under Hydraulic wt% 0. 11 7. 14 0. 59 11. 77 Packing) Monoclinic after Crush Test % <2 16 <2 27

TABLE II shows that SAMPLE I, formed in accordance with the present invention, is significantly more wear resistant than the other samples. In this respect, the same composition, when fired at a higher temperature (about 1, 550°C) for a shorter period of time (2 hours), had a wear rate that is almost seven times greater (contrast the 0. 11% wear for SAMPLE I with the 7. 14% wear for SAMPLE II).

SAMPLE III (without MnO) shows that a composition similar to SAMPLE I, fired at the same temperature for the same period of time, will have a wear rate over five times as great (contrast 0. 11% wear rate for SAMPLE I with 0. 59% wear rate for SAMPLE III).

The present invention thus provides a ceramic material that may be formed into a variety of shapes and is extremely wear resistant.

The foregoing description is a specific embodiment of the present invention. It should be appreciated that this embodiment is described for purposes of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.