2104 E Gondar et al Joumal of the European Ceramic Society 27(2007)2103-2110 ew method of testin resistance of silicon nitride to repeated ther cks. (1)Specimen, (2 nt,(3)crack, (4)punch, (5) diameterof the punch(5 mm) is equal to the hole diameter of the sealing. This way, the only mechanical loading of the specimen is shear acting under the perimeter of the punch. Mechanical loading acting on the indent and the cracks is negligible. The Fig.1.Graphite heating body of the laboratory hot press. (1)Sections of the type of mechanical loading can be changed by modifying the body.(2)electrical input to the heating body. (3)semicircular segments and (4) punch diameter. The method is described more in detail in Refs main electrical input [14, 15]. Test chamber of the device is made of steel, the transpar ent front wall, which allows observation, is made of temperature of fracture areas. The sectioning took place in direction parallel resistant glass Rear wall, through which leads the induction coil with the surface of the specimen, contrary to a previously used is made of thermally insulating material based on aramide fibers method,2 where the direction of sectioning was perpendicular Because the punch is made of molybdenum, the chamber must be to the surface Low speed of grinding discs, as well as rich lubri- evacuated before the test. During testing, protective gas(argon) ation ensure that the crack size does not increase during the is passing through the test chamber. Simulation and verification ectioning. The depth profile is needed to specify the critical of temperature and stress progress has been introduced in Ref point for crack growth, which is located on the crack, under the [15] The principle of the new testing method(protected by a parameters of the test. The reference parameters are as or deformed zone(Fig. 2) The new testing method allows modification of various input patent)is shown in Fig 3. Specimens of circular cross-section lows: heating temperature Th=1100C, cooling temperature (1)are being used in this test, with cracks (3) formerly initiated Tc=500C, heating time th=16s and cooling time tc=6 by Vicker's indentor. The specimen is placed with its damaged These values provide the most intense thermal shock that can be side(2)facing downwards and this side is constantly(without achieved using our available equipment. For simplicity, a new interruptions) cooled by water. Its opposite side is cyclically method of description of input parameters has been introduced heated using a molybdenum punch(4). The punch is heated by Using this method, the reference parameters would be described an induction coil and is loaded by a mass of 6 kg. Its weight pre- as 1100/500-16/6 vents the cooling water from leaking from under the sealing The The stress progress for these parameters for a specimen with thickness of 20 mm(in the critical point--Fig. 2)is shown in Fig. 4. It can be seen, that the amplitude even in the first cycle is low, hence we can use the mean stress as representative for the entire stress progress. Thus, the first step was the evaluation of the influence of input parameters on the mean stress in a 20 mm thick specimen In Ref [15], specimens with thickness of 2 mm were subject of a more thorough research and verification. The stress progress for this specimen thickness is shown in Fig. 5. It can be seen that even after 200 cycles the stress is not stabilized. This fact deformed zone makes the evaluation of parameter influence more complicated. To facilitate the reproducibility of the results, it is necessary to study not only the mean stress, but also the value of stress in Fig. 2. Depth profile of the crack concrete cycles. This case is more common when dealing with2104 E. Gondar et al. / Journal of the European Ceramic Society 27 (2007) 2103–2110 Fig. 1. Graphite heating body of the laboratory hot press. (1) Sections of the body, (2) electrical input to the heating body, (3) semicircular segments and (4) main electrical input. of fracture areas. The sectioning took place in direction parallel with the surface of the specimen,11 contrary to a previously used method,12 where the direction of sectioning was perpendicular to the surface. Low speed of grinding discs, as well as rich lubri￾cation ensure that the crack size does not increase during the sectioning. The depth profile is needed to specify the critical point for crack growth, which is located on the crack, under the deformed zone (Fig. 2). The principle of the new testing method (protected by a patent13) is shown in Fig. 3. Specimens of circular cross-section (1) are being used in this test, with cracks (3) formerly initiated by Vicker’s indentor. The specimen is placed with its damaged side (2) facing downwards and this side is constantly (without interruptions) cooled by water. Its opposite side is cyclically heated using a molybdenum punch (4). The punch is heated by an induction coil and is loaded by a mass of 6 kg. Its weight pre￾vents the cooling water from leaking from under the sealing. The Fig. 2. Depth profile of the crack. Fig. 3. Principle of the new method of testing the resistance of silicon nitride to repeated thermal shocks. (1) Specimen, (2) indent, (3) crack, (4) punch, (5) sealing, and (6) support. diameter of the punch (5 mm) is equal to the hole diameter of the sealing. This way, the only mechanical loading of the specimen is shear acting under the perimeter of the punch. Mechanical loading acting on the indent and the cracks is negligible. The type of mechanical loading can be changed by modifying the punch diameter. The method is described more in detail in Refs. [14,15]. Test chamber of the device is made of steel, the transpar￾ent front wall, which allows observation, is made of temperature resistant glass. Rear wall, through which leads the induction coil, is made of thermally insulating material based on aramide fibers. Because the punch is made of molybdenum, the chamber must be evacuated before the test. During testing, protective gas (argon) is passing through the test chamber. Simulation and verification of temperature and stress progress has been introduced in Ref. [15]. The new testing method allows modification of various input parameters of the test. The reference parameters are as fol￾lows: heating temperature Th = 1100 ◦C, cooling temperature Tc = 500 ◦C, heating time th = 16 s and cooling time tc = 6 s. These values provide the most intense thermal shock that can be achieved using our available equipment. For simplicity, a new method of description of input parameters has been introduced. Using this method, the reference parameters would be described as 1100/500-16/6. The stress progress for these parameters for a specimen with thickness of 20 mm (in the critical point15—Fig. 2) is shown in Fig. 4. It can be seen, that the amplitude even in the first cycle is low, hence we can use the mean stress as representative for the entire stress progress. Thus, the first step was the evaluation of the influence of input parameters on the mean stress in a 20 mm thick specimen. In Ref. [15], specimens with thickness of 2 mm were subject of a more thorough research and verification. The stress progress for this specimen thickness is shown in Fig. 5. It can be seen that even after 200 cycles the stress is not stabilized. This fact makes the evaluation of parameter influence more complicated. To facilitate the reproducibility of the results, it is necessary to study not only the mean stress, but also the value of stress in concrete cycles. This case is more common when dealing with