Silicon creep performance

J. R. Porter and A. Chokshi, Creep Performance of Silicon Carbide Whisker-Reinforced Alumina in Ceramic Microstructures 86 The Role of Interfaces, eds. J. Pask and A. Evans, Plenum Press, New York, NY, 1987, p. 919. 

Figure 13.2 Comparison ofthe creep performance in different grades of silicon nitride ceramics at 150 MPa [21],...

Fig. 9.3 Influence of silicon on the creep performance of Ti-6242. After Seagle et at. (see BiaMKiNSOP IBle85]).

PEI polymers exhibit minimal creep under load. For example, unreinforced Ultem 1000 maintains part dimensions over thousands of hours at room temperature at a loading of 34 N/mm2 (4930 psi). PEI resins are available with HDTs ranging to 220°C (Ultem 5000). PEI copolymers with silicone mbber allow for flame-retardant, high temperature applications such as plenum wire coatings (Sfltem). Reinforcement of PEI resins improves their temperature performance. Table 13 compares unreinforced and 20% glass-reinforced Ultem resins. 

Creep testing was performed on a Clash-Berg Torsion Stiffness Tester. Silicone oil was used as the heat transfer medium. Temperatures were held constant to within + 1... 

As high performance ceramic matrix composite systems, such as Melt Infiltrated (MI) SiC/SiC, are being considered for advanced gas turbine engine applications, the characterization of the material becomes more important. A series of tests were conducted where Pt and Ni sheathed Pt thermocouples were used to monitor temperature for short and long duration fast fracture, fatigue and creep tests. While it is known that Si forms eutectics with Pt and Ni, this was initially not considered an issue. But since Ml SiC/SiC composite achieves much of its performance from the infiltrated phase of Silicon (for high conductivity and low porosity), it was felt that further study of possible interactions of the Si phase has to be considered. 

Exploratory creep tests were performed on tensile specimens of NT-154 silicon nitride at 1300 and 1370 C, These specimens were made from the same lot of the material, designated as CP-serles, used In the cyclic fatigue tests discussed In the last progress report, Creep strain was measured using the laser diffraction strain extensometer described elsewhere,2 Figure 7 shows the creep curves of specimens CP-28 and CP-10 tested at 1300 C under applied stress of 160 and 180 MPa, respectively. Both tests were shutdown Inadvertently due to equipment adjustments. The problems have been corrected since then. Therefore, the results of the tests do not represent the creep life of the specimens. However, there were sufficient data points to delineate the essential features of the creep behavior up to the steady-state phase which was clearly definable. Both curves showed a reversed Inflection at the transition between the primary and secondary creep phases, resulting In forming a bump In the otherwise smooth creep curves. 

The creep/fatigue studies Involving a high performance silicon nitride were also initiated. In particular, the creep strain associated with both button-head tensile samples and four-point flexure samples was measured as a function of time and stress at 1370 C, In the case of the button-head tensile samples, a direct-contact capacitance extensometer was used to monitor the displacement during constant load tests. The deflection of the specimen midspan in the flexure samples was measured with respect to the Inner load points using a three-probe LVDT extensometer. Additional details of the experimental procedure are given elsewhere. 

To illustrate the dependence of composite performance on the properties of the fibres and matrices, creep data comparisons and creep curve shape analyses have been undertaken for three SiC-matrix products reinforced with either silicon carbide or alumina fibres. These materials were selected to assess the effects of... 

---