Availableonlineatwww.sciencedirectcom ScienceDirect E噩≈RS ELSEVIER Joumal of the European Ceramic Society 27(2007)2103-2110 www.elsevier.comlocate/jeurceramsoc Influence of individual thermal shock parameters on stress generated in silicon nitride and its prediction Ernest gondar , Miroslav rosko a jan Slovak University of Technology, Faculry of Mechanical Engineering, Pionierska 15, 83102 Bratislava, Slovak Republic Received 19 February 2006: received in revised form 12 July 2006; accepted 21 July 2006 Available online 2 October 200 Influence of repeated thermal shock on the stress generated in silicon nitride, was determined by a new testing method. This method allows verification of temperature and stress progress obtained from puter simulation. Input parameters were temperature, temperature difference, heating and cooling time Output parameters were the mean stress and stress peaks of specific cycles. Two methods were used to compare the influence-a newly defined parameter of infuence(PI)and a least square method. The results show a dominant influence of the temperature value which is higher than the influence of temperature difference. The least squares method was also used to predict the value of stress, with coefficient of determination higher than 0.95 o 2006 Elsevier Ltd. All rights reserved. Keywords: Thermal shock resistance; Testing: Si3N4 1. Introduction lute values of heating and cooling temperature and the heating and cooling time. The aim of our research is also the prediction Silicon nitride is known for its outstanding properties such of stress values, based on statistical analysis as hardness, abrasion resistance, chemical stability, creep resis- tance, high-temperature strength(up to 1400C)and its resis- 2. Experimental tance to thermal shocks There are several methods used to test the resistance of silicon nitride to thermal shocks, among Silicon nitride specimens were prepared by cold pressing and which the indentation-quench method is one of the most widely then hot pressing in nitrogen atmosphere. Activating densifica- used for technical ceramics with high resistance to thermal tion aids for silicon nitride were AlO3 and Y2O3 with a mass shocks--since 1996 to the present time. 2. A number of mod- ratio corresponding to 10% of YAG. The hot pressing of the ifications of this method have been introduced to lower the experimental material was performed on a laboratory hot press time consumption especially when dealing with repeated ther- with a special construction for the heating body.(Fig. 1).This mal shocks.In the Department of Materials and Technologies graphite body consists of two sections(1)and their electrical of the Faculty of Mechanical Engineering of the Slovak Univer- inputs(2)end with semicircular segments(3).These segments sity of Technology, a new testing method has been developed provide shielding of main electrical inputs(4) against heat radi and optimized' together with a computer simulation of the ated from the body temperature and stress conditions in the tested material.This The prepared specimens were of 2 mm thickness with 8mm simulation makes it possible to evaluate the influence of ther- diameter. In the middle of the specimens, cracks were initiated mal shock parameters on the stress, generated in the specimen. using a Vicker's indentor. These cracks were used to determine These parameters include the temperature difference, the abso- the fracture toughness and were also important for the testing initiated using a hemispherical indentor. o Sed also for indents, method itself. A similar method is being Corresponding author. Tel. +421 244455087: fax: +421 2 4445509 The depth profile of cracks was determined by horizont E-mail address: ernest gondar@stub.sk(E Gondar) erial sectioning of the material as well as from the observation 0955-2219/S-see front matter o 2006 Elsevier Ltd. All rights reserved. doi: 10. 1016/j-jeurceramsoc. 2006.07.024Journal of the European Ceramic Society 27 (2007) 2103–2110 Influence of individual thermal shock parameters on stress generated in silicon nitride and its prediction Ernest Gondar a,∗, Miroslav Rosko a, Jan Somorcik b a Slovak University of Technology, Faculty of Mechanical Engineering, Pionierska 15, 831 02 Bratislava, Slovak Republic b Comenius University, Faculty of Mathematics, Physics and Informatics, Mlynska dolina, 842 48 Bratislava, Slovak Republic Received 19 February 2006; received in revised form 12 July 2006; accepted 21 July 2006 Available online 2 October 2006 Abstract Influence of repeated thermal shock on the stress generated in silicon nitride, was determined by a new testing method. This method allows verification of temperature and stress progress obtained from a computer simulation. Input parameters were temperature, temperature difference, heating and cooling time. Output parameters were the mean stress and stress peaks of specific cycles. Two methods were used to compare the influence—a newly defined parameter of influence (PI) and a least square method. The results show a dominant influence of the temperature values, which is higher than the influence of temperature difference. The least squares method was also used to predict the value of stress, with coefficient of determination higher than 0.95. © 2006 Elsevier Ltd. All rights reserved. Keywords: Thermal shock resistance; Testing; Si3N4 1. Introduction Silicon nitride is known for its outstanding properties such as hardness, abrasion resistance, chemical stability, creep resis￾tance, high-temperature strength (up to 1400 ◦C) and its resis￾tance to thermal shocks. There are several methods used to test the resistance of silicon nitride to thermal shocks, among which the indentation-quench method is one of the most widely used for technical ceramics with high resistance to thermal shocks—since 19961 to the present time.2,3 A number of mod￾ifications of this method have been introduced to lower the time consumption especially when dealing with repeated ther￾mal shocks.4–6 In the Department of Materials and Technologies of the Faculty of Mechanical Engineering of the Slovak Univer￾sity of Technology, a new testing method has been developed and optimized7 together with a computer simulation of the temperature and stress conditions in the tested material.8 This simulation makes it possible to evaluate the influence of ther￾mal shock parameters on the stress, generated in the specimen. These parameters include the temperature difference, the abso- ∗ Corresponding author. Tel.: +421 2 44455087; fax: +421 2 44455091. E-mail address: ernest.gondar@stuba.sk (E. Gondar). lute values of heating and cooling temperature and the heating and cooling time. The aim of our research is also the prediction of stress values, based on statistical analysis. 2. Experimental Silicon nitride specimens were prepared by cold pressing and then hot pressing in nitrogen atmosphere. Activating densifica￾tion aids for silicon nitride were Al2O3 and Y2O3 with a mass ratio corresponding to 10% of YAG. The hot pressing of the experimental material was performed on a laboratory hot press with a special construction for the heating body9 (Fig. 1). This graphite body consists of two sections (1) and their electrical inputs (2) end with semicircular segments (3). These segments provide shielding of main electrical inputs (4) against heat radi￾ated from the body. The prepared specimens were of 2 mm thickness with 8 mm diameter. In the middle of the specimens, cracks were initiated using a Vicker’s indentor. These cracks were used to determine the fracture toughness and were also important for the testing method itself. A similar method is being used also for indents, initiated using a hemispherical indentor.10 The depth profile of cracks was determined by horizontal serial sectioning of the material, as well as from the observation 0955-2219/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2006.07.024