Silicon nitrides whisker reinforcement

S. R. Choi, et al., Dynamic FatigueProperty of Silicon Carbide Whisker-Reinforced Silicon Nitride, Ceram. Eng. Sci. Proc., 12[7-8], 1524-1536 (1991). 

A low cost-high performance silicon nitride-BAS composite can be fabricated via pressureless sintering. In this composite, the BAS glass-ceramic serves as an effective liquid-phase-sintering aid, to attain full densification and complete the a-(3 Si,iN4 phase transformation, and remains as a structural matrix that is reinforced by the silicon nitride whiskers, Si3N4 whiskers grow in random directions in a near completely crystallized matrix of hex-acelsian BAS. 

If this carbothermal process is brought to only partial completion (Equation 11a and 11b), a homogeneous mixture of silicon carbide whiskers and silicon nitride powder [10] is obtained which can be fired directly to yield whisker reinforced ceramics. Silicon carbide reinforced alumina composites and silicon carbide whisker reinforced zirconia composites [31] are also products of the "chemical mixing process". The whisker growth rate in the zirconia process can be accelerated by adding metal particle catalysts such as cobalt chloride, thus potentially facilitating a VLS phase transformation. 

Silicon nitride whiskers n. Very fine fibers of Si3N4 prepared by vapor-phase reaction of silicon and a silicate in nitrogen and hydrogen at 1400°C. Density is 3.2g/cm, ultimate strength is 4.8 GPa (700kpsi), and modulus is 276 GPa. They are used as reinforcements in specialty laminates. 

Other ceramic cutting-tool materials include alumina, Si-Al-0-N, alumina-carbide composites and, more recently, a composite of silicon nitride reinforced with silicon carbide whiskers. This last material can be produced by chemical-vapor infiltration (CVI) and has high strength and toughness as shown in Table 18.3.Cl... 

The whiskers that are commonly added to silicon nitride to form composites are Si3N4 and SiC (common nomenclature is to add the suffix (w) to denote whiskers and this will be used where appropriate). Although other ceramic and metallic whiskers are available (e.g. BN, TaC, TiC, B4C and Fe), Si3N4 and SiC each have properties that make them prime candidates as reinforcements, not least of which is that they can sustain the high temperatures and reactivity of the sintering process without being degraded. 

Pugh, M.D. and Gavoret, L. Nitridation of whisker reinforced reaction bonded silicon nitride ceramics , J. Mat. Sci., 35 (2000) 3257-3262. 

Dogan, C.P. and Hawk, J.A. Influence of whisker reinforcement on the abrasive wear behavior of silicon nitride- and alumina-based composites , Wear, 203-204 (1997) 267-277. 

Nixon, R.D., Koester, D.A., Chevacharoenkul, S. and Davis, R.F. Steady-state creep of hot-pressed SiC whisker-reinforced silicon nitride , Composites Sci. Tech., 37 (1990) 313-328. 

In order to elucidate the effect of the whisker orientation on the erosion behavior of material SN-C, erosion tests were carried out in directions both parallel and perpendicular to the whisker orientation. It is apparent that in the highly directional whisker-reinforced silicon nitride material, solid particle erosion in the direction parallel to the whisker orientation resulted in a faster rate of material removal compared to that in the perpendicular direction (Fig. 20.4). 

The highly directional whisker-reinforced silicon nitride material, SN-C, showed a lower erosion resistance than the finer grained self-reinforced silicon nitride material, SN-F, and the Ca a-sialon material, CA2613. This can be attributed to substantial pullout of the reinforcing whiskers during the erosion of material SN-C. The significant pullout of the whiskers is a direct... 

In addition to the initial work in the alumina and mullite matrix systems previously mentioned, SiC whiskers have also been used to reinforce other ceramic matrices such as silicon nitride,9-13 glass,14 15 magnesia-alumina spinel,16 cordierite,17 zirconia,18 alumina/zirconia,18 19 mullite/zirconia,18-21 and boron carbide.22 A summary of the effect of SiC whisker additions on the mechanical properties of various ceramics is given in Table 2.1. As shown, the addition of whiskers increases the fracture toughness of the ceramics in all cases as compared to the same monolithic materials. In many instances, improvements in the flexural strengths were also observed. Also important is the fact that these improvements over the monolithic materials are retained at elevated temperatures in many cases. 

---