DE2036972A1 - Deforming ceramic oxide materials - to increase hardness and creep resistance - Google Patents

Abstract

Ceramic oxide materials are deformed at a temp. at which the temperature-fracture stress curve markedly rises, under a mechanical stress between the stress value at which the displacement-deformation starts, and that at which the material is damaged (by separation of grains, fissures or fracture), with the proviso that the extension (or compression) remains below 0.5%. The applied stress should be gradually increased to the final value within 1-2 mins. to ensure the predominance of the sliding displacements. In the case of sintered alumina, the hardness can be increased by 30%, and the creep resistance by a factor of 10.

Description

Process to improve the hardness and creep resistance of oxide ceramics Materials through deformation The identification concerns a process for improvement the hard and creep strength of oxide ceramic materials through deformation, in particular to improve the mechanical properties of Al2O3 sintered bodies.

Solidification processes as a result of plastic deformation, which with a Increase in mechanical properties are associated with depriving even with ceramic Substances partly on such sliding processes, which are preferred by the movement run off from Veretngen. The deformation itself creates further dislocations generated whose reactions among each other contribute to solidification; with advancing Elongation increases the hardening.

The possibility of plastic deformation is, however, with oxide ceramic materials are severely restricted. This is because the number the functional sliding systems in oxide ceramic materials is low. One A mixed crystal is homogeneous, plastic deformation in any direction only possible if there are at least five independent gliding systems owns. This condition does not apply to any of the high-melting temperatures up to temperatures of 10000 0 oxide ceramic materials met. For example, aluminum oxide (Al2O3) only has two sliding systems.

For this reason, many oxide ceramic materials are brittle, i.e. that required to move a dislocation mechanical tension is higher than the breaking stress of the resin. Measures must therefore be taken evaluate to decrease the stress required to move dislocations, to increase the hardness and creep strength of oxide ceramic materials through deformation to be able to improve.

This task is performed according to the invention in the case of oxide-ceramic materials solved in that the material at that temperature Tv with a mechanical Stress is stressed at which for the first time significant plastic elongation occurs.

This deformation temperature TV according to the invention can be derived from the "stress-strain curve" [das iet a curve in which the dependence of the elongation (epsilon) on the applied Voltage at different temperatures -draëe £ -tellt is] taken from ground.

The material is subjected to a mechanical tension as long as between that mechanical stress at which deformations due to dislocations take place, and the mechanical tension at which damage to the material (separation of grain boundaries, cracks or breakage) occur, lies, exposed It becomes clear that the elongation (or compression) of the substance is less than 0.5 ».

In the temperature-elongation curve, the elongation value is epsilon, which leads to breakage of the material, shown as a function of the temperature T. At low temperatures, epsilon is small (the material is brittle) and independent on the temperature. Above a certain temperature there is a significant increase in elongation. The "ductility range" begins at this temperature value Tv, at its initial temperature TV according to the invention, the deformation should take place. Of course, this can also be done with one higher temperature, but only part of the increase in hardness or the increase in creep resistance is achieved as at the temperature Tv, there - assuming the same rate of deformation - recovery processes in stronger Counteract the degree of solidification.

The elongation should through a mechanical tension leads between those mechanical stress at which deformations take place by dislocation, and that mechanical stress at which Damage emerge, l:; egt0 It is advantageous to increase the applied mechanical Voltage evenly but relatively quickly, i.e. in comparatively short times, z .. within one to two minutes, on the 'nawert, on which ie then until Remains constant at the end of the stretching. bine too slow increase in the mechanical tension is not desirable, otherwise deformation without significant Participation of dislocations could take place, -which the desired solidification of the material would not be achieved, a particularly high level of solidification is then achieved when the applied mechanical stress is closer to the upper limit value, at what damage to the material occurs; the deformation then runs accordingly faster.

When using the oxide ceramic treated according to the invention Material ind temperatures and stresses above those used according to the invention to avoid.

The temperature at which the deformation according to the invention takes place, can be used not only in the temperature-elongation curve, but also in the curve in the dependence of the strain on the applied stress at different temperatures is shown. With different temperatures as parameters a curve Echar is obtained, with each curve at that value of the elongation Epeilon ends at which the workpiece breaks. The individual curves are all the bigger Elongation values shifted the higher the temperature. With lower values the temperature occurs break at the end of a straight curve piece, which one referred to as the "Hooke's straight line". At higher temperatures occur before entry of the fracture larger deviations from Hooke's straight line and the temperature at which these deviations begin to occur is the lower limit the defined temperature Tv s according to the invention be used should, identical.

The most important example of a high-melting oxide material are Al2O3 sintered bodies. This material has the characteristic of up to the highest Temperatures only have two functional sliding systems, causing deformation with the preferred participation of dislocation movements is made very difficult.

@an uses sintered Al2O3 moldings with grain sizes between 1 u unu about 35 / u. The density is 95% of the theoretical density, the deformation temperature TV is selected according to the invention in such a way that ie in the lower part of the ductile region which is caused by a sharp drop in the inconsistency and a sharp increase the elongation at break is characterized. The transition from brittle to ductile behavior can be performed in hot bending, hot compression or hot tensile tests for the respective material In the case of aluminum oxide, according to the invention, the temperature TV is between 14500 C and 1530 ° C.

The applied external mechanical tension is also essential. You must reach or exceed a certain lower limit value, because at smaller stresses the deformation mainly according to the Nabarro-Herring mechanism, i.e.

without substantial involvement of dislocations. With bending deformation of Al2 03 the lower limit is about 6 kp / mm² (tensile load) for 1480 ° C. The strengthening effect increases with increasing tension, with the upper limit due to the breaking strength of the material according to the specific stress given is. The optimal value of the applied voltage can be found in creep tests be determined. The load is applied evenly in about 1 to 2 minutes and kept constant during the deformation. In this way it is achieved that plastic elongation is mainly based on such sliding processes, which in Dislocation movements exist.

In the case of polycrystalline magnesium oxide with a grain size of less than 10 µm the temperature Tv is 1450 ° C and a bending load of about 4.5 kgf / mm².

The corresponding values for polycrystalline beryllium oxide with a Grain sizes of less than 10 mm are Tv = 12000 0 and a compressive stress of 4.5 kp / mm². Another example is uranium oxide. The polycrystalline material with a grain size of about 15 - 20 mm is at least 1400 ° C with more than 4.5 kp / mm² loaded on bending.

in the case of aluminum oxide (Al2O3) it has been shown that elongations of 0.1 to 0.2% is completely sufficient to reduce the hardness at room temperature by about 30% to increase; A reduction in the creep speed was achieved on Al2O3 sintered bodies by a factor of 10.

The type of stress during the deformation has no influence das Brgebni £ 0 One can apply pressure, bending, and tensile loads or also the so-called process of hot pressing of solid or powdery starting products choose, the numerical value is then different.

The drawing shows Al2O3 sintered bodies in double logarithmic form Representation of the temperature-compensated, steady-state creep speed as a function on the applied bending tensile stress. Curve 1 shows the dependency of this Creep speed of the applied bending tensile stress without treatment according to the invention, whereas curves 2 and 3 treated according to the invention at a tensile stress of more than 6 kp / mm² Affect samples with reduced creep speed.

It can be seen that the process according to the invention results in a reduction the creep speed is achieved by a factor of about 10.

Claims (3)

Expectations 1. Methods of improving hardness and creep resistance oxide ceramic materials by deformation, characterized in that the material at the temperature Tv at which the temperature-elongation curve is clear increases as long as a mechanical tension that is between that mechanical Stress at which deformations occur due to dislocations, and that mechanical stress at which damage to the material (grain boundary separation, Cracks or fractures) occur, lies, is exposed to the elongation (or compression) of the material is below 0.5% 2. The method according to claim 1, characterized in that that the applied mechanical tension is within 1 to 2 minutes to the final value is increased evenly and remains constant until the end of the deformation. 3. The method according to claim 1 or 2, characterized in that the applied mechanical stress is closer to the upper limit at which Damage to the material occurs, 4, method according to claims 1 to 3, thereby characterized in that the deformation occurs at a higher temperature than the lowest possible takes place.