Patent Number: 
Section: description

Manufacture of a Composite Material According to the Invention 90.9 g of hafnium diboride powder with a mean particle size less than 50 xcexcm and 9.1 g of hafnium dioxide powder with a mean particle size less than 20 xcexcm were mixed, being 90% by volume of hafnium diboride and 10% by volume of hafnium dioxide. The mixing was carried out to be as homogeneous as possible by the application of ultrasound to a slip made up of the two powders dispersed in ethanol. The mixture was then dried in an oven heated to 80xc2x0 C. and then sieved with a sieve having a mesh size of 60 microns. A mass of 8 g of the mixture was put into a graphite mold 9.5 mm in diameter. Two graphite pistons closed off the ends of the mold previously lined with a sheet of graphite and allowed pressure to be applied to the powder. The mixture and mold assembly was brought to a temperature of 1900xc2x0 C. under a pressure of 83 MPa for one hour in an oven maintained under a dynamic vacuum. Pellets were obtained 9.5 mm in diameter and 12 mm high. The cortical area of these pellets had fine fissures due to a chemical reaction between HfO2 present in the mixture and the graphite of the mold. This cortical area was removed by machining using diamond tipped tools over a thickness of 750 microns for the cylindrical surface and over a thickness of 1000 microns for the ends. The pellets obtained had a final density equal to 95% of the theoretical density, namely a density of 10590 kg/m3. Influence of the Quantity of Hafnium Dioxide on the Sintering Temperature of a Mixture of HfB2 and HfO2 According to the Invention Under the same conditions as those in Example 1, different mixtures of hafnium diboride and hafnium dioxide powders comprising 0, 5, 10 and 20% by volume of hafnium dioxide were sintered and the density of the composite material obtained was measured as a function of the sintering temperature of these different mixtures. For each mixture, a relative density (rd) of the composite material obtained after sintering, was calculated as a % by calculating the ratio of the measured density and the theoretical density after sintering. Table 1 below brings together the results from this example. In FIG. 1, the values in Table 1 are reported in such a way that a graph is constructed of the relative density (%) of the composite material as a function of the temperature for a concentration of 0% HfO2 by volume in the mixture curve reference number 2, for 5% HfO2 in the mixture:curve reference number 4, for 10% HfO2 in the mixture curve reference number 6, for 20% HfO2 in the mixture:curve reference number 8. This Figure reveals the influence of the HfO2 content on the sintering temperature of a mixture of HfB2 and HfO2 powders according to the invention. The results from this example show in particular that a pure HfB2 powder requires, in order to be sintered, a temperature about 200xc2x0 C. higher than that necessary for a mixture of hafnium diboride and 10% by volume of hafnium dioxide. These results show in a more general way that when one increases the HfO2 content in a mixture of HfB2 and HfO2 powders, one lowers the sintering temperature. Measurements have shown that, depending on the sintering parameters used, that is to say the pressure, the temperature and the duration, the pellets formed can have a relative density which varies between 80 and 99% of the theoretical density of the starting mixture. Corrosion Kinetics of a Material According to the Invention Corrosion tests were carried out under conditions representative of those for the water of a primary medium in a PWR type reactor, that is to say at a temperature of 345xc2x0 C. and a pressure of 155 bars. These tests were carried out in an autoclave, on composite materials according to the invention and comprising 0% HfO2 by volume for the tests designated below tests 1, 10% HfO2 by volume for the tests designated below tests 2, 20% HfO2 by volume for the tests designated below tests 3. Table 2 below brings together the results of the measurements obtained in this example. FIG. 2 is a graphical representation of the results in Table 2. In this FIG. 2, the tests 1 correspond to curve reference number 10, the tests 2 correspond to curve reference number 20 the tests 3 correspond to curve reference number 30. These tests show that in the case of the composite material according to the invention, that is to say comprising a HfB2 and HfO2 mixture, there is a quasi-zero dissolution of boron in the water. The HfB2/HfO2 composite according to the invention there shows better corrosion behavior in the water than pure HfB2. Measurement of the Toughness of a Material According to the Invention The toughness is the macroscopic value that characterizes the resistance to the propagation of the fissures in a material. It is concerned with the critical value of the calculated stress intensity factor at a pre-fissure introduced into the material being investigated (creating in this way a critical defect of a size much greater than that of the other defects existing naturally in the test piece). The double torsion test chosen in our case to measure the toughness consists of making a 3 mm notch in a rectangular plate (length 35 mmxc3x97width 17 mmxc3x97thickness 2 mm) and refining the center of this notch by successive Knoop indentations along the long median axis of the test piece. The characteristics of the defect created in this way must be as close as possible to those of a natural defect, and in particular, the radius of curvature at the bottom of the notch must be very small. The fissure is initiated under the point of application of the load from the pre-fissure and is propagated along the axis of the test piece. The test then consists of applying three point bending forces to the end of the plate in a way that causes the opening of the fissure to proceed in order to bring about its propagation while controlling the stress applied and recording the displacement under the end of the notch. The plate rests on four point supports and the load is transmitted by a point at the end of the pre-fissure. The test procedure consists of applying to the test piece, a deflection which increases at a constant rate (5 xcexcm/min.). The measurement of the critical load Pc that causes propagation of the fissure then allows one to calculate the toughness. These results show greater toughness for the composite material HfB2/HfO2 than for a pure HfB2 material.