Investigation of microstructure, mechanical properties and cell viability of YSZ toughened alumina composites reinforced with graphene nano plates

Bioceramic term is used to name ceramics used for the repair and reconstruction of diseased or damaged parts of the skeletal system. This class of materials can be bioinert (alumina, zirconia), resorbable (tricalcium phosphate), bioactive (hydroxyapatite, bioactive glasses, and glass-ceramics), or porous for tissue ingrowth (hydroxyapatite-coated metals, alumina). Hip, knees, teeth, tendons, and ligaments replacements and repair for periodontal disease, maxillofacial reconstruction, augmentation and stabilization of the jaw bone, spinal fusion and bone fillers after tumor surgery are some of the application fields. Despite having comparingly low fracture toughness then metallic materials, ceramics are the materials that mimic the bone tissue the best. In last decades patients have become more demanding regarding esthetic and biocompatibility aspects of their dental restorations, ceramic material has become a main object of scientific interest especially from the material point of view. The alumina-zirconia composites are one of the relatively good and promising candidate for biomaterials application, due to biocompatibility and their mechanical properties that combines high flexural strength with a high toughness. Aluminium oxide is one of the most widely used ceramic oxides. α-Al2O3, which is the most stable form. The crystalline structure of α-Al2O3 is rhombohedral.Alumina is extremely stable in the human body and cannot be dissolved by strong acids or bases. Alumina has been widely used in the acetabulum and femur head and articulations digitorum manus. Alumina are attractive materials because of their high hardness, low wear rates and excellent biocompatibility, which make alumina an alternative to the common metal femoral heads articulating against an acetabular cup of polyethylene or to metal–metal bearing devices. Chemical and thermal stability, relatively good strenght and biocompatibility, thermal and electrical insulation characteristics combined with avalibility in abundance have made aluminium oxide, Al2O3 or alümina attractive for biomedical and engineering applications. Beside all its good properties as high hardness, chemical inertness, etc. low fracture toughness limits applications of alumina. One way to overcome low fracture toughness is to produce these materials as composites. For this purpose, a variety of reinforcing nano and submicron particles, such as MgO, SiC, ZrB2, ZrO2 have been incorporated to make alumina based composites. These particles are located on the grain boundaries of the matrix grains, which inhibits abnormal grain growth and enhances the strength of the materials. Zirconium oxide (Zr2O) is one of the most important components which increases the fracture toughness of alumina. Introduction of zirconia in the aluminium matrix develops a class of material (zirconia toughened alumina) with better mechanical properties. Alumina and zirconia based bioceramics have found their wide application in the field of load bearing orthopaedics (total hip and knee replacement) and dental implants. However the long-term use of zirconia in orthopaedics has had controversial results. A major drawback of zircona ceramics is their strenght reduction, due to an unfavorable tetragonal (t) to monoclinic (m) martensitic phase transformation , with time when they are in contact with physiological fluids. The transformation occurs between tetragonal and monoclinic phases is a reversible martensitic transformation, associated with a large temperature change (around 200oC), a volume change around 4-5%, and a large shear strain (14-15 %) which leads to crumbling of the sintered part made of pure zirconia during cooling. Phase transformation is useful when it occurs in the vicinity of a progressive fracture, but this transformation reduces the strength of the material as it causes microcracks and surface roughening when it occurs on the surface of the implant in contact with body fluids. The additions of cubic oxides such as MgO, CaO, Y2O3, CeO2 and other rare earth oxides stabilize the high temperature cubic phase all the way back to room temperature. They also tend to decrease the transformation temperature. Yttria (Y2O3) is the most popular stabilizer used for zirconia ceramics for its excellent mechanical and wear properties, and a good effect on tetragonal phase transfomability. Zirconia (YSZ) ceramics stabilized with alumina and yttria combined with their superior properties are preferred to monolithic alumina and zirconia ceramics in many structural applications. Alumina, zirconia or alumina–zirconia-based composite bioinert ceramics are currently used as femoral heads, acetabular cups for hip replacement, and dental implants. Nano-structured bioinert ceramics with significantly improved toughness and stability are desirable for future clinical needs. Recently, many studies have been carried out on the use of various carbon sources as reinforcing materials for composites. These studies have shown that carbon sources increase fracture toughness of Al2O3-YSZ composites. Two dimensional graphene plates are thought to provide matrix-reinforcement interface interactions that are not provided by dimensionless nanoparticles and one-dimensional carbon nanotubes (CNTs). In the recent studies, mechanical and electrical improvements were observed in graphene-doped nanocomposites. Graphene, the elementary structure of graphite, is an atomically thick sheet composed of sp2 carbon atoms arranged in a flat honeycomb structure. It possesses very high mechanical strength, surface area and thermal and electrical conductivity In this study, the effects of GNP addition on densification behavior, microstructure, mechanical properties and biocompatibility of Al2O3-YSZ composites were investigated by adding GNP (1,3,5,7% volume) as reinforcing material. Compositions containing 70% alumina ,25% yttria stabilized zirconia and 5% GNP by volume had the highest fracture toughness values, and the increased GNP ratio resulted in a decrease in hardness values. The effect of varying amounts of YSZ (10-20-30 % volume) additions in Al2O3 were investigated. SPS system was used in experimental studies. To compare the effects of carbon types on microstructure and mechanical properties, the same amounts (2 wt%) of carbon black (CB), carbon nanotubes (CNTs) and graphene nano plates (GNPs) were used as reinforcement additives for the sample containing 70 % alumina and 30 % YSZ. In the study, it was observed that as matrix reinforcement material, all the carbon sources increased the fracture toughness and the largest increase in fracture toughness was achieved with 2% wt graphene nanoplates (GNP). The cell viability test (XTT) was applied to investigate the biocompatibility of the composites examined in vitro for their mechanical properties. Human osteoblast cells (hOB) were plated on samples for 7 days to check the viability of these cells. It was determined that composites containing CNT, CB and GNP did not cause any toxic effect during this period and that the cells survived on composite materials.