Abstract:
MoO 2  is used as an accelerant for transformation of zirconia (ZrO 2 ) from a tetragonal to a monoclinic crystal phase. A ZrO 2  -MoO 2  alloy having an increased content of monoclinic ZrO 2  is prepared by mixing MoO 2  with ZrO 2  followed by heat treatment. In another aspect of the invention, the surface of an alumina-zirconia composite is strengthened by heating the composite in the presence of MoO 2  -ZrO 2  mixed powder.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of strengthening the surface of alumina-zirconia composites with MoO 2  as an accelerant for phase transformation of ZrO 2  from tetragonal to monoclinic. More particularly, the present invention relates to a method of using MoO 2  for accelerating the phase transformation of zirconia from tetragonal to monoclinic to strengthen the surface of alumina-zirconia composites. 
     2. Description of the Prior Art 
     Zirconia (ZrO 2 ) is a ceramic material which is widely used as structural parts, chemical sensors, and so on, attributing to its unique useful, mechanical and electromagnetic properties. It has been reported that ZrO 2  shows three polymorphisms such as cubic, tetragonal and monoclinic phases depending on the temperatures under ambient pressure. 
     Among various physical properties intrinsic to ZrO 2 , probably the most important is its phase transformability from a tetragonal to a monoclinic phase which is featured by a martensitic transformation. During the above phase transformation, there are neither heat emission nor absorption and no atomic diffusion, while a certain expansion of volume is observed. This property effects an important role to enhance the fracture toughness of polycrystalline ZrO 2  or ZrO 2  -reinforced composites. In particular, when a crack forms in a partially stabilized ZrO 2  or a tetragonal ZrO 2  polycrystal, the tetragonal ZrO 2  around the crack tip is transformed to a monoclinic phase to increase its volume, whereby further development of the crack is prohibited. Similarly, dispersion of fine particles of ZrO 2  in a ceramic matrix results in an increase in the fracture toughness of the composite. The elastic strain energy formed on the surface during the development of cleavage is reduced by the phase transformation of ZrO 2  from a tetragonal to a monoclinic phase or by the action of the residual stress formed around the ZrO 2  particles in the matrix. 
     Thus, it is important to control the phase transformation of ZrO 2  in order to afford desired properties to polycrystalline ZrO 2  or ZrO 2  -reinforced composites. Accordingly, a number of extensive studies on this problem have heretofore been made. Such studies can be categorized into two types: one for controlling the particle size of ZrO 2 , and the other for changing the structural stability of ZrO 2  itself by alloying ZrO 2  with suitable solute atoms such as stabilizers or destabilizers (accelerants of the phase transformation). 
     Various oxides including Y 2  O 3 , CaO and MgO have been known as the stabilizer to suppress the phase transformation of ZrO 2  from tetragonal to monoclinic. However, no accelerants have been known for promoting the phase transformation except HfO 2 . 
     It has been reported by Claussen et al. that the transformation of a tetragonal phase of ZrO 2  to a monoclinic phase can be accelerated by preparing a solid solution of ZrO 2  with 30 to 60 mol % of HfO 2  in an Al 2  O 3  matrix. [See, Claussen et al., Advances in Ceramics, vol. 3, p164, (1981).]However, this method has serious drawbacks that an exceedingly large amount of HfO 2  should be used in order to attain the desired acceleration of phase transformation. 
     It has also been reported by Green that upon diffusing and extracting Y 2  O 3  over the surface of an Al 2  O 3  -(Y 2  O 3  doped) ZrO 2  composite by heating the composite in the presence of pure zirconia, compressive residual stresses can be introduced on the surface of the composite, thereby resulting in an improvement in the surface strength. [See, D. J. Green, &#34;A Technique for Introducing Surface Compression into Zirconia Ceramics,&#34; J. Am. Cer. Soc., 66[9], C-178, (1983).]However, this method has shortcomings that the amount of ZrO 2  transformation from tetragonal to monoclinic is limited by the size of ZrO 2  grains employed and that complicated and time consuming procedures should be carried out in order to remove Y 2  O 3 . 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a simple and economically viable method of accelerating the transformation of ZrO 2  from a tetragonal to a monoclinic phase, whereby a desired degree of transformation can be achieved at an increased rate even if a small amount of the accelerant is used. 
     It is another object of the invention to provide a ZrO 2  -MoO 2  alloy where the content of monoclinic ZrO 2  in ceramic matrices is increased. 
     It is still another object of the invention to provide a method of strengthening the surface of an Al 2  O 3  -ZrO 2  composite. 
     It is still a further object of the invention to provide an alumina-zirconia composite having a strengthened surface. 
     Any additional objects of the invention will become apparent through reading the remainder of the specification. 
     DETAILED DESCRIPTION OF THE INVENTION 
     We, the inventors of the present invention, have intensively conducted a wide range of experiments in order to develop an improved method for accelerating the phase transformation of ZrO 2  from a tetragonal to a monoclinic phase. As a result, it has been surprisingly discovered that by adding MoO 2   in a trace amount, for example, several hundred of ppm, ZrO 2  contained in a ceramic composite can be readily transformed from the tetragonal to the monoclinic phase and that the degree of the phase transformation can be enhanced with the increase of the amount of MoO 2  added. 
     According to the invention, the transformation of tetragonal ZrO 2  to monoclinic ZrO 2  in an Al 2  O 3  matrix can be accelerated by admixing 0.02 to 2.0% by weight of MoO 2  with ZrO 2  and subjecting the resulting mixture to heat-treatment. The heat-treatment can be carried out at a temperature ranging from about 1,000° C. to about 1,400° C. for less than 4 hours under a nitrogen atmosphere. The heat-treatment allows MoO 2  to dissolve into ZrO 2  in solid state. The ZrO 2  composite thus prepared contains a large amount of monoclinic ZrO 2 . The content of monoclinic ZrO 2  in the composite can be further increased by increasing the amount of MoO 2  used. 
     In another aspect, the present invention provides a method of strengthening the surface of an Al 2  O 3  -ZrO 2  composite comprising heating an Al 2  O 3  -ZrO 2  composite at a temperature of 1,000° to 1,400° C. for 30 min. to 4 hours in the presence of 0.3 to 10% by weight of MoO 2  in a mixture of ZrO 2  and MoO 2 . 
     In this method, an Al 2  O 3  -ZrO 2  powder mixture is isostatically pressed into compacts and the resulting compacts are sintered at about 1,600° C. in air. The sintered material is sufficiently ground and heated in the presence of a ZrO 2  -MoO 2  powder mixture. 
     There are no limits to the temperatures and time for the heat-treatment, unless they adversely affect negative effects on the control of the diffusion concentration of MoO 2 . For example, preferred physical properties can be obtained by heating the composite at a temperature of about 1,000 to about 1,400° C. for less than 3 hours in the presence of MoO 2  powder. 
     The Al 2  O 3  -ZrO 2  composite which has been subject to the above heat-treatment shows an increased content of monoclinic ZrO 2  on the surface of the composite, as high as above 20% by volume, compared with that of an untreated composite. The bending strength of the heat-treated Al 2  O 3  -ZrO 2  composite is higher by about 15 to 30% than that of an untreated composite. 
     Accordingly, the method according to the present invention enables us to produce an Al 2  O 3  -ZrO 2  composite having the strengthened surface without surface defects by adjusting the temperatures and the duration of the heat-treatment. Furthermore, according to the invention, the magnitude of the residual stress and the thickness of the stress layer can be easily controlled, and an ideal stress distribution curve which is determined by the concentration of MoO 2  can be obtained. In addition, according to the invention, it is possible to reduce the particle size of the monoclinic ZrO 2  and to increase the degree of the phase transformation under the action of MoO 2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will be illustrated in greater detail by way of the following examples. The examples are presented for illustrative purpose only and should not be construed as limiting the invention which is properly delineated in the claims. 
     EXAMPLE 1 
     Four groups of sintered specimens were prepared by the following procedures. MoO 2  powder was mixed with ZrO 2  powder at the concentration of 0.02%, 0.1%, 1%, and 2% by weight, respectively, to give four mixture of ZrO 2  powders and MoO 2 . Each powder mixture was heated at 1,300° C. for 2 hours under a nitrogen atmosphere. The resulting ZrO 2  -MoO 2  solid solution powder was mixed with Al 2  O 3  powder to give a mixture of Al 2  O 3  -ZrO 2  powder containing 15% by volume of ZrO 2 . The powder mixture was subject to cold isostatic pressing, while maintaining the relative density of the powder at 65%, and then sintered at 1,600° C. for an hour under atmospheric pressure in air to give a specimen to be tested. 
     For the purpose of comparison, a standard Al 2  O 3  -15 vol. % ZrO 2  specimen without MoO 2  was prepared by repeating the same procedures as described above, except that pure ZrO 2  powder was mixed with Al 2  O 3 . 
     The amount of the transformed monoclinic ZrO 2  was determined by an X-ray diffraction technique. The results are shown in Table 1 below. 
     
                       TABLE 1______________________________________Content of Monoclinic ZrO.sub.2 in pressurelesssintered Al.sub.2 O.sub.3 -15 vol. % ZrO.sub.2Amount of MoO.sub.2 used          Content of monoclinic ZrO.sub.2(wt %)         (vol %)______________________________________0              29.00.02           83.40.1            93.21              95.22              97.1______________________________________ 
    
     As can be seen from Table 1, the standard specimen shows 29.0% by volume of transformability from a tetragonal to monoclinic phase, while the specimen in which MoO 2  is used in an amount of 0.02% by weight shows above 80% by volume of transformability. The data in Table 1 also shows that the amount of ZrO 2  transformed to a monoclinic phase increases as the amount of MoO 2  used increases. 
     EXAMPLE 2 
     Two specimens containing 0.02 and 0.04% by weight, respectively, of MoO 2  according to the invention, and a standard specimen without MoO 2  were prepared by repeating the same procedures as in Example 1, except that the sintering was carried out under high pressure of 30 MPa. The results of an X-ray diffraction of the specimens are listed in Table 2 below. 
     
                       TABLE 2______________________________________Content of Monoclinic ZrO.sub.2 in hot-pressedAl.sub.2 O.sub.3 -15 vol. % ZrO.sub.2 specimensAmount of MoO.sub.2 used          Content of monoclinic ZrO.sub.2(wt %)         (vol %)______________________________________0              11.50.02           40.00.04           43.6______________________________________ 
    
     From Table 2 above, it is noted that a substantial increase, namely, about 4-fold higher than that of the standard specimen, in the transformability to a monoclinic phase can be obtained by adding MoO 2  to ZrO 2 . The transformability increases with the increase in the amount of MoO 2  used. 
     EXAMPLE 3 
     A mixture of Al 2  O 3  -15 vol. % pure ZrO 2  powder was compacted in a rod-shaped die of 2 mm×20 mm in dimension. The resulting compact was isostatically pressed and then sintered in a vertical tube furnace at 1,600 ° C. for 30 minutes in air. The sintered material was polished to 1 μm finish and heated at 1,350° C. for 30 minutes within a ZrO 2  -MoO 2  mixed powder bed containing MoO 2  powder at the concentration of 0.3, 1, 3, and 10% by weight, respectively, to give four specimens that are to be used in testing the bending strength thereof. 
     For the purpose of comparison, a standard specimen which was not subject to the heat-treatment in the presence of the ZrO 2  -MoO 2  mixed powder was prepared in the same manner as described above. 
     The four-point bending strength and the amount of ZrO 2  transformed to a monoclinic phase of each specimen were measured. The results are shown in Table 3 below. 
     
                       TABLE 3______________________________________Content of monoclinic ZrO.sub.2 at the surface of variousAl.sub.2 O.sub.3 -15 vol. % ZrO.sub.2 specimens and their bendingstrengthAmount of MoO.sub.2contained in   Content of Monoclinic                         BendingZrO.sub.2 --MoO.sub.2 mixed          ZrO.sub.2      Strengthpowder (wt %)  (vol %)        (MPa)______________________________________0              30             4550.3            49             5181              53             5363              56             56910             62             586______________________________________ 
    
     As can be seen from Table 3 above, the content of the monoclinic ZrO 2  is increased by above 60% and the bending strength by above 15% after the heat-treatment of the specimen in a ZrO 2  -MoO 2  powder mixture. As the amount of MoO 2  contained in the ZrO 2  -MoO 2  mixed powder increases, the content of monoclinic ZrO 2  and the bending strength also increase.