Microstructure and mechanical properties of silicon carbide-titanium diboride ceramic composites

The microstructure, hardness, fracture toughness, Young\u27s modulus, strength and Weibull modulus of silicon carbide-titanium diboride (SiC-TiB₂) ceramics were studied. First, SiC-TiB₂ ceramics with 15 vol.% TiB₂ particles were processed using two green processing methods, spray drying (ST) and ball milling (SiC-15TiB₂). In addition, SiC-TiB2 ceramics with TiB₂ contents ranging from 0 to 100 vol.% were produced to determine a TiB₂ content that produced the best combination of mechanical properties. From spray drying, segregation of the TiB₂ particles in ST led to a granule-like microstructure and spontaneous microcracking in the final ceramic. In ceramics containing 20 and 40 vol.% TiB₂, the TiB₂ particle sizes were also large enough to allow for spontaneous microcracking. Spontaneous microcracking decreased the hardness from 28 GPa for SiC to 24 GPa for SiC-TiB₂ with TiB₂ contents of 80 vol.% or higher. In contrast, fracture toughness increased from 2 MPa·m1/2 for SiC to ~6 MPa·m1/2 for SiC containing 40 vol.% TiB₂ or more. Using a two-parameter Weibull analysis, SiC with 20 vol.% TiB₂ had the highest average strength (522 MPa), followed by SiC-15TiB₂ (500 MPa), then SiC with 40 vol.% TiB₂ (420 MPa), and ST (380 MPa). While microcracking in ST lowered the strength, hardness, and elastic modulus compared to SiC-15TiB₂, the granule-like microstructure combined with microcracking, narrowed the flaw size distribution of ST and boosted the Weibull modulus of ST to 21 compared to 12 for SiC-15TiB₂, which had a uniform distribution of TiB₂ particles. The Weibull moduli of SiC containing 20 and 40 vol.% TiB₂ was also boosted to 17, compared to 12 for a TiB₂ content of 15 vol.%. To maximize each property, TiB₂ particle sizes should be kept just below the spontaneous microcracking threshold to prevent spontaneous flaw formation --Abstract, page iv