Zirconia mullite/boron nitride composites

A hot-pressed ceramic composite having good resistance to thermal shock and good erosion/corrosion resistance in molten alloys, and a method for producing said composite which comprises the blending of fused zirconia mullite with boron nitride.

 

What is claimed is:

1. A method for the manufacture of a ceramic composite having good resistance to thermal shock and good erosion/corrosion in molten alloys comprising:

(a) blending a mixture comprising from 10 to 74 weight percent particulate fused zirconia mullite and from 26 to 90 weight percent particulate boron nitride, wherein said fused zirconia mullite comprises from 25 to 45 weight percent zirconia and from 55 to 75 weight percent mullite based on the weight of the fused zirconia mullite;

(b) heating and compressing the blended mixture of step (a) in a mold under an inert atmosphere at a temperature between about 1680.degree. C. and about 1710.degree. C. at a pressure between 1800 and 2500 pounds per square inch for a time period until the composite stops shrinkage; and

(c) cooling the composite.

2. The method of claim 1 wherein in step (a) the blended mixture comprises from 40 to 72 weight percent particulate fused zirconia mullite and from 28 to 60 weight percent particulate boron nitride.

3. The method of claim 1 wherein in step (b) the temperature is between about 1690.degree. C. and 1705.degree. C. and the pressure is between 2100 and 2300 pounds per square inch.

EXAMPLE I

Various amounts of boron nitride and fused zirconia mullite were mixed for about two hours with a twin shell V-blender with an intensifier bar. The blended materials were then prepressed into fine-grain graphite molds each having an inside diameter of 2.0 inches (5cm), an outside diameter of 3.75 inches (9.5 cm) and a length of 18 inches (45 7 cm). Each mold was lined with a layer of Grafoil (Trade Mark of Union Carbide Corporation for flexible graphite) to keep the composite from sticking to the mold when processed.

The molds and contents were loaded into a 4 inch (10cm) diameter resistance heated tube furnace, and heated for two hours at various temperatures under a pressure of 2200 psi provided by a hydraulic ram in a nitrogen atmosphere. The heating and pressurizing rates were 330.degree. C./hr and 500 psi/hr, respectively. The molds were allowed to cool and then two-inch diameter billets were removed from the molds. The amounts of the blend, proportions of the blend, and temperature of the heating step for each sample billet produced are shown in Table 1. In addition, for comparison, a boron nitride billet was produced in a similar manner at a temperature of 1775.degree. C. as shown in Table 1.

Sample bars were cut from each billet parallel to the pressing direction and then the percent theoretical density was obtained for each sample. The data obtained are shown in Table 1. A sample bar from each billet measuring 0.75 inch (1.9 cm) diameter by 3 inches (7.6 cm) long was rotated at 60 revolutions per minute (rpm) in molten 304 stainless steel at a temperature of 1575.degree. C. for four hours. The reduction in the diameter of each sample was then measured as a percent reduction per hour and the data obtained are shown in Table 1. On all samples except Sample No. 1, a reaction layer was observed. It is postulated that this reaction layer protects the composite from further degradation. This reaction layer is believed to be Al.sub.2 O.sub.3 and ZrO.sub.2. The reduction in the diameter of the sample is an indication to the erosion/corrosion resistance characteristics of the sample.

A sample bar from each billet was subject to a thermal shock test which consisted of dunking a room-temperature sample bar measuring 0.75 inch (1.9 cm) diameter by 3 inches (7.6 cm) long into a 1575.degree. C. stainless steel melt and then air quenching. The visual results observed are shown in Table 1.

As can be seen in Table 1, a 50/50 weight percent composite of zirconia mullite and boron nitride possesses equivalent thermal shock resistance to plain boron nitride and significantly improved erosion/corrosion resistance (Compare Sample 1 with Samples 3, 5, 15 and 16).

                                      TABLE I
    __________________________________________________________________________
                                                       Compatibility Test
         w/o ZrO.sub.2
               w/o* w/o**
                        Temp.
                             % Theoretical
                                          Thermal Shock
                                                       Results (% Reduction
    Sample
         Mullite
               BN   BN  (.degree.C.)
                             Density      Test Results in Diameter/Hour)
    __________________________________________________________________________
     1    0    100   0  1775 90.0         No Cracking or Spalling
                                                       16.0
     2   50    50    0  1640 73.3         Catastrophically Failed
                                                       --
     3   50    0    50  1700 90.7         No Cracking of Spalling
                                                       0.065 (w/Reaction
                                                       Layer Attached)
     4   50    0    50  1740 84.7         --           --
                             (Temperature too High;
                             Billet Bonded to Mold)
     5   50    50    0  1700 92.0         No Cracking or Spalling
                                                       0.330 (w/Reaction
                             (Slight Bonding to Mold)  Layer Attached)
     6   50    0    50  1640 76.7         Catastrophically Failed
                                                       --
     7   90    10    0  1700 --           --           --
                             (Complete Melting)
     8   90    0    10  1640 98.0         Too Hard to Machine
                                                       --
     9   75    0    25  1675 88.8         Slight Cracking
                                                       --
    10   75    0    25  1640 88.1         No Cracking or Spalling
                                                       0.64 (w/o Reaction
                                                       Layer Attached)
    11   75    12.5   12.5
                        1700 92.9         No Cracking or Spalling
                                                       --
                             (melting)
    12   75    25    0  1640 86.9         Catastrophically Failed
                                                       --
    13   50    0    50  1700 82.9         --           --
    14   50    0    50  1705 90.3         --           --
    15   50    0    50  1710 90.9         --           1.67 (w/o Reaction
                                                       Layer Attached)
    16   50    50   0   1710 88.4         --           0.94 (w/o Reaction
                                                       Layer Attached)
    17   50    50    0  1705 86.4         --           --
    18   60    0    40  1645 78.6         No Cracking or Spalling
                                                       --
    19   60    0    40  1710 92.0         No Cracking or Spalling
                                                       --
                             (Slight Melting)
    20   60    0    40  1690 91.0         No Cracking or Spalling
                                                       1.71 (w/o Reaction
                             (Slight Melting)          Layer Attached)
    22   60    0    40  1680 85.5         No Cracking or Spalling
                                                       --
    23   60    0    40  1685 85.6         --           --
    __________________________________________________________________________
     *Commerical BN containing relatively high impurities
     **Union Carbide Corporation Grade HCP BN that was temperature purified.

EXAMPLE II

Samples 14, 17 and 22 from Example 1 were examined and their properties measured. Specifically, the density, room-temperature flexural strength, 1500.degree. C. flexural strength, coefficient of thermal expansion (CTE), Rockwell R hardness and sonic modulus were measured for each of the sample bars.

The flexural strengths, at 1500.degree. C., and at room temperature, were determined using the ASTM D-690 three point loading method.

The sonic modulus for each sample bar was determined using the sonic resonance technique as disclosed in ASTM C747-74.

The CTE was measured by placing the sample bar on support pins and inserting lengthwise into a graphite tube furnace. An optical extensometer was used which comprised two parallel telescopic sights mounted so that the distance between the axis of the sights could be adjusted. A caliper was used to measure the distance between the sights. The sights were focused on the ends of the sample bar through the sight tubes in the furnace. The furnace was heated under an inert atmosphere, and at appropriate temperature intervals the length of the sample was measured with the caliper. The CTE was calculated as the fractional change of the length of the sample divided by the rise in temperature in degrees centigrade.

The data obtained from these tests are shown in Table II.

As evidenced from the data shown in Tables 1 and 2, ceramic composites made in accordance with this invention exhibit high abrasion resistance, high thermal shock resistance, good high temperature strength and will provide longer service life than conventional boron nitride when subjected to molten metal applications.

                                      TABLE II
    __________________________________________________________________________
                     Sonic
                          Room Temp.
                                 1500.degree. C.
               %     Modulus
                          Flexural
                                 Flexural
                                      Rockwell
                                           1500.degree. C. CTE
                                                  %
    Sample
          Density
               Theoretical
                     .times. 10.sup.6
                          Strength
                                 Strength
                                      Hardness
                                           (in/in/.degree.C.)
                                                  Permanent
    (diameter)
          (g/cm.sup.3)
               Density
                     (psi)
                          (psi)  (psi)
                                      (R-Scale)
                                           .times. 10.sup.-6
                                                  Expansion
    __________________________________________________________________________
    14(2")
          2.585
               92.85 1.82 5,879  2,667
                                      115.9
                                           --     --
    14(6")
          2.431
               87.30 1.49 4,386  2,652
                                      111.0
                                           9.17   0.112
    17(2")
          2.395
               86.03 5.73 10,254 1,517
                                      116.5
                                           --     --
    22(2")
          2.484
               84.99 2.04 3,263  2,174
                                      102.3
                                           12.0   1.18
    __________________________________________________________________________

While this invention has been described with reference to certain specific embodiments and examples, it will be recognized by those skilled in the art that many variations are possible without departing from the scope and spirit of the invention, and that the invention, as described by the claims, is intended to cover all changes and modifications of the invention which do not depart from the spirit of the invention.