Peter Tatarko

Toughened and machinable glass matrix composites reinforced with graphene and graphene-oxide nano platelets

Abstract The processing conditions for preparing well dispersed silica–graphene nanoplatelets and silica–graphene oxide nanoplatelets (GONP) composites were optimized using powder and colloidal processing routes. Fully dense silica–GONP composites with up to 2.5 vol% loading were consolidated using spark plasma sintering. The GONP aligned perpendicularly to the applied pressure during sintering. The fracture toughness of the composites increased linearly with increasing concentration of GONP and reached a value of ∼0.9 MPa m1/2 for 2.5 vol% loading. Various toughening mechanisms including GONP necking, GONP pull-out, crack bridging, crack deflection and crack branching were observed. GONP decreased the hardness and brittleness index (BI) of the composites by ∼30 and ∼50% respectively. The decrease in BI makes silica–GONP composites machinable compared to pure silica. When compared to silica–Carbon nanotube composites, silica–GONP composites show better process-ability and enhanced mechanical properties.

Scratch behaviour of graphene alumina nanocomposites

The scratch resistance behaviour of alumina-graphene nanoplatelet (GNP) (0.5, 2 and 5 vol.-%) composites was investigated using a Rockwell indenter with normal applied loads ranging from 1 to 200 N. The alumina-GNP composites behaved differently during scratch testing depending on the normal applied load. The coefficient of friction of the composites did not change much at low normal loads but increased with increasing amount of GNP in the alumina matrix for high normal loads. The addition of GNP contributed to improved scratch resistance of alumina nanocomposites only for low loads below ∼97 N. This correlates with the mechanical properties of the composites. As the applied load increased, the scratch resistance of the GNP composites decreased due to the presence of weakly bonded grain boundaries in the alumina matrix, which enhanced chipping of material.

Boron nitride nanosheets reinforced glass matrix composites

The effect of reinforcing boron nitride nanosheets (BNNSs) on the mechanical properties of an amorphous borosilicate glass (BS) matrix was studied. The BNNSs were prepared using liquid exfoliation method and characterised by transmission electron microscopy, scanning electron microscopy and X-ray diffraction (XRD) analysis. The average length was ∼0.5 μm, and thickness of the nanosheets was between 4 and 30 layers. These BNNSs were used to prepare BS-BNNS composite with different loading concentrations of 1, 2.5 and 5 mass-% (i.e. 1.395, 3.705 and 7.32 vol.-%). Spark plasma sintering (SPS) was used to densify these composites to avoid structural damages to the BNNSs and/or crystallisation within the composite sample during high temperature processing. The BNNSs were found to be evenly distributed in the composites matrix and were found to be aligned in an orientation perpendicular to the direction of the applied force in SPS. The mechanical properties including fracture toughness, flexural strength and elastic modulus were measured. Both fracture toughness and flexural strength increased linearly with increasing concentration of BNNSs in BS glass. There was an enhancement of ∼45% in the fracture toughness (1.10 MPa.m1/2) as well as flexural strength (118.82 MPa) with the addition of only 5 mass-% loading of BNNSs compared to BS glass (0.76 MPa.m1/2; 82.16 MPa). The toughening mechanisms developed in the composites because of the reinforcement of BNNSs were thoroughly investigated.

Fracture Toughness of Si3N4 Based Ceramics with Rare-Earth Oxide Sintering Additives

Fracture toughness of hot-pressed silicon nitride and Si3N4+SiC nanocomposites prepared with different rare-earth oxides (La2O3, Sm2O3, Y2O3, Yb2O3, Lu2O3) sintering additives have been investigated by Chevron Notched Beam, Indentation Strength and Indentation Fracture techniques. The fracture toughness values of composites were lower due to the finer microstructures and the lack of toughening mechanisms. In the Si3N4 with higher aspect ratio (Lu or Yb additives) crack deflection occurred more frequently compared to the Si3N4 doped with La or Y, which was responsible for the higher fracture toughness.

Failure mechanisms of ceramic nanocomposites

Abstract: This chapter first builds a basic understanding of structural failure and its determining critical factors. It describes typical fracture origins and modes of crack propagation. It then deals with the concept of reinforcing ceramic nanocomposites. Different strategies for preventing failures are discussed, and the influence of microstructure and secondary nanometric phases on friction and wear properties of some ceramic nanocomposites is described.

Microstructure, Fracture and Damage Mechanisms in Rare-Earth Doped Silicon Nitride Ceramics

Influence of rare-earth oxide additives on the strength, fracture toughness and tribological behaviour of hot-pressed Si3N4 and Si3N4/SiC micro/nano-composites has been investigated. Four-point bending mode and ball on disc methods have been used for strength and wear tests and Single-Edge V-Notched Beam, Chevron Notched Beam, Indentation Strength and Indentation Fracture techniques for fracture toughness measurement. Fractography has been used to characterize strength limiting defects, fracture micromechanisms and damage mechanisms during the wear test. The strength values were strongly influenced by the present processing flaws. Wear behavior is significantly influenced by the chemical composition and by the microstructure of the materials.

Wear Behavior of Stoichiometric and Nonstoichiometric Zirconia

Zirconium oxide samples were sintered at two different temperatures (1250°C and 1450°C) using SPS method with maximum holding time 10 min. to obtain nonstoichiometric composition. Some of the samples were subsequently annealed for 6 hours at 1100°C to obtain stoichiometric composition. Wear rate and friction coefficient were obtained from tribological tests carried out on the UMT3 equipment (Bruker) using two different static partners (zirconia and alumina ball). Main aim of this work is to study the influence of composition and sintering temperature on the wear behavior, friction coefficient and basic mechanical properties.

Corrosion Behavior of Human Teeth Measured by Nanoindentation Method

The decrease of the mechanical properties – hardness and reduced elastic modulus after corrosion in white wine was measured. Under static corrosion conditions no significant decrease was observed up to 8 hours of corrosion. Dynamic corrosion conditions cause detrimental decrease of properties (one order of magnitude) compare to the results of static corrosion test. This is due to the removal of the harder outer layer of the enamel during polishing. To obtain a relevant data concerning corrosion test, natural surface of a tooth should be investigated and tested.

Microstructure and Mechanical Properties of Rare-Earth Doped Si3N4 and Si3N4/SiC Ceramics

Microstructure and mechanical properties of Si3N4 and Si3N4 + SiC nanocomposites sintered with rare-earth oxide additives (La2O3, Y2O3, Yb2O3 and Lu2O3) have been investigated. The composites exhibited smaller grain diameter compared to that of monolithic materials. The aspect ratio of β-Si3N4 grains increased with a decreasing ionic radius of rare-earth elements in the Si3N4 monoliths as well as in the Si3N4-SiC nanocomposites. The hardness of both systems increased with a decreasing ionic radius of rare-earth element. The fracture toughness of the materials with coarser microstructure and higher aspect ratio was higher due to the more frequent toughening mechanisms. No significant difference between strength values of monoliths and composites was observed and the strength in the composites was determined mainly by the present processing flaws. Significantly improved creep resistance was observed in the case of composites and for materials with smaller ionic radius of RE3+.

Microstructure and Mechanical Properties of Rare-Earth Doped Si3N4/SiC Nanocomposites

Influence of various rare-earth oxide sintering additives (La2O3, Y2O3, Yb2O3 and Lu2O3) on the microstructure and mechanical properties of hot-pressed Si3N4/SiC nanocomposites have been investigated. Fracture toughness of investigated materials slightly decreased with increasing ionic radius of rare-earth element (RE). Aspect ratio of the Si3N4 grains increased with decreasing ionic radius of rare-earth both in the studied Si3N4/SiC nanocomposites. Materials with higher aspect ratio of the Si3N4 grains exhibited crack deflection more frequently compared to the Si3N4 doped with lower aspect ratio, which was responsible for the higher fracture toughness. Flexural strength of the investigated materials increased with decreasing ionic radius of rare-earth elements. Significantly improved creep resistance was observed in case of nancomposite materials with smaller ionic radius of RE3+.