Ceramics tend to be weak in tension, but strong in compression. For a metal, the compressive strength is near that of the tensile strength, while for a ceramic, the compressive strength may be 10 times the tensile strength. Alumina, for example, has a tensile strength of 20,000 psi 1138 MPa), while the compressive strength is 350,000 psi (2400 MPa).
The discrepancy between tensile and compressive strengths is in part due to the brittle nature of ceramics. When subjected to a tensile load, ceramics, unlike metals, are unable to yield and relieve the stress. Another important factor is the presence of internal flaws from which cracks can propagate in tension, but not in compression.
More important than the quantity of flaws is the flaw size. According to the Griffith relationship for brittle materials: the strength; (where E is the elastic modulus, M is the fracture surface energy, and C is the length of the hole or flaw). Thus, as the flaw size increases, the strength decreases.
Ceramics containing a single phase are usually stronger than those with several phases. When a part cools after sintering, the different phases contract unequal amounts causing localized stresses in the material. And unlike metals, ceramics are unable to relieve the localized stress.
Grain size also affects ceramics strength, Increasing the average grain size weakens the ceramic. This weakening may occur because larger grain sizes usually bring larger flaws.
Machining can introduce flaws into a part. Sintering to net shape not only saves time and labor, it produces stronger parts. Air can be entrapped during processing. Isostatic pressing under high pressure forces out the trapped air particles, which would weaken the part.
Materials and methods
Single-crystal sapphire can be grown relatively free from internal flaws. After flame polishing to create a flaw-free surface, the sapphire’s flexural strength may exceed 1,450,000 psi (10,000 MPa), more than 20 times that of steel.
A more common and less expensive material than sapphire is cement. Ordinary cement consists mainly of calcium silicate with a small quantity of calcium aluminate. Due to entrapment of air and incomplete packing of particles, cement is weak in tension and bending. Another calcium-based ceramic, mother of pearl (CaCO3), has a flexural strength ten times that of ordinary cement, and a fracture energy in excess of 5.5 in.lb/in. (1000 J/m), 50 times greater than cement.
Sapphire and mother of pearl are strong, but not practical structural materials. By using two different sizes of cement powder to give greater packing, and adding polyacrylamide gel to prevent the entrapment of air, macro-defect-free (MDF) cement is produced: The flexural strength of MDF cement is 14,000 psi (100 MPa) compared to only 2500 psi (17 MPa) for ordinary cement.
Cement, sand, stones, and water are mixed together to form concrete. The range of aggregate sizes, from fine sand particles to small to larger stones, allows denser packing and minimal air entrapment leading to greater strength.
The water/cement ratio also is important; more water weakens the concrete. A water/cement ratio of 0.7 produces a concrete with a compressive strength of 4100 psi (28 MPa), while a water/cement ratio of 0.4 in a concrete with over twice the strength, 8800 psi (60.5 MPa). However, less water makes the concrete less workable, so low-water concrete requires mechanical compacting or vibration equipment.
To improve the tensile strength of concrete, steel rods can be added. The steel carries tensile loads. Unfortunately, as the steel stretches, the brittle concrete is pulled with it, fracturing well before the steel does.
Prestressing the concrete allows greater tensile loading before failure. Steel rods are initially placed in tension, then released, exerting a compressive force on the concrete, When the concrete is then subjected to a tensile load in service, the effect is unload the precompression.
The idea of prestressing to improve subsequent tensile and flexural properties applies to other ceramics. Quenching alumina in silicone oil increases the flexural strength. The average strength after quenching is 128,000 psi (880 MPa) compared to 85,000 psi (590 MPa) for the unquenched control. Quenching causes a compressive surface layer to form on the alumina.
Another method of prestressing is ion implantation. At the surface of a ceran3ic, one set of ions, such as potassium, is substituted for another set of ions, such as sodium. The larger potassium ions crowd the surface, placing it in compression. The precompressed ceramic can better withstand tensile loading.
Glass fibers are very strong in tension, with tensile strengths up to 624,000 psi (4300 MPa), compared to only 58,000 psi (400 MPa) for ordinary steel. The glass fibers commonly reinforce polymer-matrix composites. They also can give tensile strength to ceramic-matrix composites.
Unlike metals, which are most often tested in tension, ceramics are usually tested in compression. When tensile tests for ceramics are desired, care must be taken to align the grips accurately and apply the load axially to avoid an additional bending or torsional stress. Due to the brittle nature of ceramics, a small bending or torsional stress could significantly alter the results.
|Compressive strength, psi||Property Tensile strength, psi||Flexural strength, psi|
|3% 1/2 03 PSZ*||430,000||-||-|
|9% MgO PSZ*||270,000|
|Reaction bonded SiC||100,000||20,000||37,000|
|Pressureless sintered SiC||560,000||25,000||80,000|
|Sintered SiC with free silicon||150,000||24,000||47,000|
|Sintered SiC with graphite||60,000||5,000||8,000|