Cavitational Strain Contribution to Tensile Creep in Vitreous Bonded Ceramics

Abstract

Analysis of the role of cavitation during uniaxial creepThe contribution of cavitation to tensile strain can be quantideformation in vitreous bonded ceramics reveals that the tatively determined by separating true creep strain from the cavity volume contributes only to the strain in the direction total tensile strain actually measured during creep tests. The parallel to the tensile stress regardless of the shape and basic method for such separation was suggested by Raj 23 from orientation of cavities. Creep asymmetry results from the the analysis of the diagonalized strain tensor expressed in terms fact that cavitation preferentially contributes to axial tensile of the principal strains ε 1 , ε 2 , and ε 3 . Generally, deformation in strain while the strain observed under the same conditions solids consists of a combination of volume change and change in compression is produced only by volume-conserving in shape. Dilatational strain corresponding to the volume mechanisms. The contribution of cavitational strain in the change is defined by axial tensile strain is equal to the volume fraction of cavities ⌬ ϭ (1 ϩ ε 1 )(1 ϩ ε 2 )(1 ϩ ε 3 ) Ϫ 1 and proportional to the difference between tensile and compressive strains in the axial direction. The density change which gives ⌬ Ϸ ε 1 ϩ ε 2 ϩ ε 3 for small strains. Distortional method and a newly proposed method based on the differstrain, ε e , representing the shape change, can be determined by ence in the axial strains were used for separating the cavitasubtracting one third of the dilatational strain from each of the tional from the true tensile strain in self-reinforced silicon strain components. Such a strain tensor transformation in the nitride. Both methods consistently revealed more than 90%case of the uniaxially deformed cylindrical specimen led to contribution of cavitation to the total tensile strain. Cavitathe expression 23 tion is concluded to be the dominant mechanism of tensile creep deformation in vitreous bonded ceramics because the ε e ϭ ε 1 Ϫ 1 3 |⌬| reported volume fractions of cavities during their deformation are usually in the range of 70-90% of tensile strain.This relation was used for the estimation of the cavitational strain contribution during tensile creep assuming that ⌬ is I. Introduction equivalent to the density change due to cavitation and ε e to the true creep strain at constant volume. 10 For a volume change N UMEROUS creep studies carried out on vitreous bonded equal to ϳ90% of total tensile strain measured in silicon nitride ceramics, 1-20 such as silicon nitride, siliconized silicon carit was concluded that cavitation is not an important part of bide, and alumina, have revealed that intensive creep cavitation tensile strain. 10 However, the volume change ⌬ results from the is intrinsic to tensile creep deformation. 12,20-26 Creep cavities strain tensor transformation and it does not reflect the contribuform predominantly inside the glassy phases at the two-grain tion of the additional volume created by cavitation. Dilatational and mul...

Creep mechanism and microstructure evolution in silicon nitride ceramics

“…However, in compression it contributes to the radial strains, which are typically not measured. Thus, the axial compressive strains result only from non-cavitation mechanisms whereas tensile strains are composed from cavitation and non-cavitation contributions (Lofaj et al, 1997a;Lofaj, 2004).…”

“…However, at temperatures over 1350°C, a prolonged, slightly accelerated stage appeared. Figure 3 shows creep curves obtained on button-head specimens at 1400 C at different tensile stresses as well as under compressive stress of 160 MPa (Lofaj et al, 1997a). The maximum failure strains in tension in the studied stress range were less than 3.5%.…”

“…The studied material, SN 88, is a commercial gas-pressuresintered silicon nitride produced by NGK Insulators, Ltd., Nagoya, Japan, for the hot stage components of ceramic gas turbines. This material represents a generation of SiC nano-composite silicon nitride ceramics from the beginning of the 1990s, which was extensively studied by Luecke et al (1995), Lofaj et al (1997aLofaj et al ( , 1997bLofaj et al ( , 1999aLofaj et al ( , 2000Lofaj et al ( , 2003aLofaj et al ( , 2003b, Luecke and Wiederhorn (1999), Lofaj (2000), Wiederhorn (2000) and Yoon et al (2000). Its microstructure consists of large -silicon nitride grains (up to 50-80 m long, diameter 5-10 m) embedded in a fine -Si 3 N 4 matrix.…”

“…Tensile creep behaviour was studied on the dog bone (4 2.5 mm 2 and 2.5 2.5 mm 2 , gauge lengths of 18 mm and 20 mm) and button-head (diameter 6 mm, gauge 35 mm) samples which were described earlier (French et al, 1996;Luecke and Wiederhorn, 1997;Lofaj et al, 1997a). Creep curves obtained below 1300°C consist of a primary and prolonged secondary-like creep with an abrupt failure.…”

“…The effects of the phase changes in the secondary phase are not understood, yet. Therefore, the objectives of the current work are to summarise the results of earlier tensile creep studies (Lofaj et al, 1997a(Lofaj et al, , 1997b(Lofaj et al, , 1999a(Lofaj et al, , 2003a(Lofaj et al, , 2003bYoon et al, 2000;Lofaj, 2000Lofaj, , 2004 performed on SN 88 commercial silicon nitride, and studies of the microstructure changes, to understand the effects of environment on cavitation and creep mechanism during long-term creep deformation. Section 2 addresses the role of cavitation in tensile deformation and possible cavitation creep mechanism.…”