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ecent Progress in Silicon Nitride and Silicon Carbide Ceramics Mamoru Mitomo and GCinter Petzow, Guest Editors We know from experience that ceramic materials are brittle and easily broken. This is one reason why ceramics have not been used as engineering materials. Fracture is the result of crack growth through the microstructure. It was Griffith who proposed that ceramics have intrinsic cracks which grow under applied stress.1"2 The concentration of the applied stress at the crack tip decreases the strength to a level of about 1% or less of the theoretical strength. If the crack starts to grow, strength decreases so sharply that a catastrophic fracture occurs. In spite of the brittle nature of ceramics, their application as engineering materials was proposed in the 1960s3 because ceramic materials made of silicon nitride or carbide have higher strength at high temperatures than metals and oxide ceramics. Non-oxide ceramics have lower thermalexpansion-coefficients than oxides, resulting in better thermal shock resistance, which is one of the most important requirements for engineering ceramics.4 Silicon nitride and silicon carbide are intrinsically difficult to sinter because of their basically covalent bonding and low self-diffusion coefficients. Therefore, the heating of a pure powder compact of Si3N4 and SiC at high temperatures shows no shrinkage at all and results in neck growth on account of surface diffusion. An important step in the development of non-oxide ceramics was the success of pressureless sintering of SiC.5 The preparation of a fine, high-purity powder and the addition of B and C as sintering aids allow the SiC to densify by accelerating grain boundary diffusion. Pressureless6 and gas-pressure sintering7 of silicon nitride was developed a little later. These methods enabled the

MRS BULLETIN/FEBRUARY 1995

Figure 1. Ceramic turbocharger rotor.

Durability Testing of Rotor (900°C)

^.350 a 300 ^250 200 in

D

-Instantaneous Failure

O

O

a

loga = - - U o g t • c

.

E 150 I— 100"/o i nn "i Rated Stress

100 10"

10"

!

10"' Time

to

10 Failure

102

103

10'

(H)

Figure 2. Relation between time to failure t of rotor and centrifugal stress a. The subcritical crack-growth exponent is denoted by n, and c /s a constant. The minimum lifetime of a survived component is certificated by a spin test.

19

Recent Progress in Silicon Nitride and Silicon Carbide Ceramics

Figure 3. Gas-pressure sintered, high-strength Si3N4 valves for application in an automobile engine.

Figure 4. Washers and sliding rings with various geometries made of pressureless-sintered silicon carbide, containing boron and carbon as sintering additives.

structural development has been a main subject of research. It has been shown that highly elongated grains develop by gas-pressure sintering at high temperatures and increase fracture toughness to about 9-11 MPa m1/2. Tremendous improvement in fracture resistance provides future prospects for reliable and flaw-tolerant ceramics.1112 Elongated grains grow in a fine, uniform matrix du