Fabrication And Mechanical Properties Of Bioinspired Nacre-like Ceramic-polymer Composites Matching Human Tooth

Dentures,also known as artificial teeth,are restorations of human teeth that are missing and are essential for ensuring the normal oral functions.Zirconia ceramics are widely used as dental materials.However,zirconia dental materials are limited by the fact that they have markedly higher hardness and Young’s modulus than human teeth,exhibit apparent brittleness,poor machinability,and tend to accelerate the abrasion of opposing teeth.Therefore,the development of new dental materials with high strength and good toughness along with moderate hardness and Young’s modulus matching human teeth is of great importance for improving the clinical service performance and prolonging the service life of dental materials.Dental materials have to withstand cyclic masticatory forces under the condition of saliva lubrication in service.So,it is necessary to study the compression-compression fatigue properties and damage behavior.Besides,efficient mass production is essential for ensuring the practical oral applications of new dental materials.In this thesis,new ceramic-polymer(zirconia-polymethyl methacrylate(PMMA))composites with nacre-like lamellar and brick-and-mortar architectures were fabricated by mimicking the microstructure of natural nacre and developing mechanistic models of Young’s modulus to guide the microstructural designs.The bioinspired composites exhibited moderate hardness and Young’s modulus,that are respectively comparable to those of human dentin and enamel.The composites showed high fracture toughness,especially for the composite with brick-and-mortar architecture(～8.3 MPa m1/2),which was superior compared to most commercial dental materials.Toughening mechanisms such as crack deflection were promoted by the bioinspired architectures to inhibit the crack propagation.Besides,the composites exhibited good machinability that was suitable for processing by computer-aided design and manufacturing(CAD/CAM)method.Furthermore,the composites showed a lower coefficient of friction than zirconia and could alleviate the abrasion to antagonistic teeth.On this basis,the composites were non-cytotoxic to meet the requirements for clinical oral applications.The combination of these properties demonstrated a good potential of bioinspired composites as new-generation dental materials for applications in dental restorative field.Besides,the compression-compression fatigue properties and damage behavior of bioinspired composites with nacre-like lamellar and brick-and-mortar architectures were further investigated under the condition of saliva lubrication,with cyclic loads applied parallel and perpendicular to the layers.Bioinspired composites exhibited good fatigue properties,with the fatigue limits(or the corresponding maximum compressive stress)for the brick-and-mortar architecture with loading parallel and perpendicular to the layers and for the lamellar architecture with loading parallel to the layers all exceeding the maximum stress in human teeth during normal clenching and mastication.The fatigue damage behavior of bioinspired composites was associated with their architectural types and orientations.Specifically,nacre-like composites failed by splitting along interfaces between ceramic and polymer phases when loaded parallel to the layers,causing a relatively small fracture angle of less than 25°.With the increase in the applied stress amplitude,kink(shear)gradually became a major factor for fatigue failure,leading to increased fracture angles of composites with brick-and-mortar architecture.In comparison,the bioinspired composites showed different fatigue damage behavior when loaded perpendicular to the layers.The fatigue damage behavior was dominated by shearing with a fracture angle of around 45° for the lamellar architecture.Additionally,the composites showed strain accumulation effects,i.e.,the strain was gradually accumulated with the increase of fatigue cycles,which may help avoid catastrophic failure.By contrast,the bioinspired composites with brick-andmortar architecture demonstrated a staircase-like fracture damage behavior,which was strongly dependent on the applied stress amplitude.An increase in the stress amplitude resulted in a transition in the cracking mode from the fracture of ceramic bricks to separation along the inter-brick polymer phase.Furthermore,a new processing technique by accumulative rolling of ceramic green body was developed for constructing nacre-like structures.Bioinspired alumina-PMMA composites with good mechanical properties were fabricated using this technique.The densities of nano-scale mineral bridges and asperities were adjusted by changing the adding amounts of alumina nano-particles,thereby enabling an effective manipulation of the micro-to nano-scale structures.In addition,the strength and fracture toughness of the composites showed an increasing trend and then decreased with the increase of the adding amounts of alumina nano-particles.At an adding amount of 4%,the composites had good flexural strength and fracture toughness with values of～192 MPa and～7.07 MPa m1/2,respectively;this was accompanied with a good matching of hardness and Young’s modulus with human enamel which was superior to most other commercial dental materials.On this basis,the materials fabricated by accumulative rolling technique were virtually unlimited in dimension and could be processed in a short time,thereby may significantly shorten the production cycle of dentures and the waiting time of patients.The bioinspired nacre-like ceramic-polymer composites matching human teeth may address the limitations of zirconia dental materials and demonstrate a good potential for dental applications.First,bioinspired composites with comparable hardness and Young’s modulus with human teeth,good fracture toughness,machinability and diminished abrasion to antagonistic teeth were successfully fabricated by mimicking the microstructure of nacre and developing mechanistic models of Young’s modulus.Second,this study characterized the compressioncompression fatigue properties of the composites and clarified the effects of architectural types and orientations on the fatigue properties and damage behavior,thereby may provide key data support for their oral applications and offer guidance for the design of new fatigue-resistant materials.Third,this study developed a new processing technique for efficiently fabricating bioinspired composites and manipulating their micro-to nano-scale structures,which was helpful to promote the mass production of new dental materials for applications.