The addition of polycrystalline metastable tetragonal Y - partially stabilized zirconia
(Y-PSZ) aggregates to a fine - grained alumina matrix results in so - called “duplex” structures (see Figure 7.32), with a high fracture strength > 700 MPa and fracture toughness up to 12 MPa.m1/2. Since Y - PSZ has a lower elastic modulus (ca. 210 GPa) than alumina (350 – 400 GPa), cracks introduced by external loads tend to move towards the zirconia particle aggregates. The crack energy will then be
Dissipated by forcing the t - ZrO2 to transform to m - ZrO2. This mechanism will be augmented by the compressive stress generated, that tends to counteract crack movement.
The considerably improved mechanical properties of such duplex ceramics are currently being exploited in novel developments in the field of femoral heads for hip endoprostheses that rely on high - purity alumina (BIOLOX R delta), with the addition of 17 vol% tetragonally stabilized zirconia and 1.4 vol% chromia particles.
Whilst the former addition provides mechanical strengthening by transformation toughening (see Section 7.2.4), the latter addition acts as a reinforcement by dissipating crack energy via the deflection of crack paths. By applying these mechanisms, the four - point bending strength is increased three - to fourfold compared to that of unalloyed alumina (Table 7.1 ), to reach 1400 MPa, while the fracture toughness is increased to 6.5 MPa.m1/2 .
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
الرجوع الى لوحة التحكم
|