ABSTRACT
The study focuses on obtaining Inconel 625-NbC composites for high-temperature applications, e.g., jet engines, waste-to-energy combusting systems or gas engine turbines, and characterizing them in terms of their microstructure and hardness improvement. Synthesis was performed utilizing Spark Plasma Sintering (SPS) at 1150 °C under the load of 45 MPa in medium vacuum (under 10-3 MPa) for a total time of 60 min. Four sets of samples with different Inconel 625 to NbC weight ratios were prepared (5, 10, 20, and 30 wt.%), followed by a reference sample containing no ceramic reinforcement. Obtained materials were hot-rolled at 1150 °C with a 10% reduction step and later cut and polished to perform characterization utilizing scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) module and microhardness testing device equipped with Vickers indenter. Hardness was improved proportionally to NbC addition achieving an increase of up to 20% of reference values. Additional heat treatment was conducted on the hot-rolled samples at 1200 °C in an argon atmosphere to further observe the interaction between reinforcement and alloy. Their microstructure revealed the coarsening of precipitates within the metal matrix and partial reinforcement dissolution, which proved to be crucial to obtaining the highest quality composites with homogenous hardness improvement.
ABSTRACT
Analysis of dense Potassium Sodium Niobate (KNN) ceramic obtained by hot pressing (HP) method at 1100 °C are presented in this paper. The synthesis of KNN-based piezoelectrics meets the following challenges-low density of material, uncontrolled K/Na ratio, multiphase composition and formation of different KNN structures. The classical hot pressing approach results in contamination by carbon originating from graphite molds. The proposed hexagonal Boron Carbide (h-BN) layer between green sample and graphite mold could protect samples from carbon contamination. Additionally, the presence of h-BN may decrease the formation of oxygen vacancies, which allows us to maintain the semiconductor features of the KNN structure. Remaining issues were addressed with the addition of excess Na and Er2O3 doping. The results showed that excess Na addition allowed us to compensate evaporation of sodium during the synthesis and sintering. Er2O3 was added as sintering aid to limit abnormal grain growth caused by h-BN addition. The modification of amount of Na and Er2O3 addition resulted in high purity KNN samples with tetragonal structure and apparent density higher than 97%. Finally, piezoelectric features of prepared dense samples were measured and presented.
ABSTRACT
In this study, the influence of the addition of rare earth oxides on the phase composition and density of KNN piezoelectric ceramics was investigated. The initial powders of Na2CO3 and K2CO3 were dried at 150 °C for 2 h. Then, a powder mixture for synthesis was prepared by adding a stoichiometric amount of Nb2O5 and 5 and 10 wt % overabundance of Na2CO3. All powders were mixed by ball-milling for 24 h and synthesized at 950 °C. The phase composition of the reaction bed was checked by means of X-ray diffraction (XRD). It had an appearance of tetragonal and monoclinic K0.5Na0.5NbO3 (KNN) phases. Then, 1 and 2 wt % of Er2O3 and Yb2O3, were added to the mixture. Green samples of 25 mm diameter and 3 mm thickness were prepared and sintered by hot pressing at 1000 °C for 2 h under 25 MPa pressure. The final samples were investigated via scanning electron microscopy (SEM)-energy-dispersive X-ray spectroscopy (EDS), XRD, Rietveld, and ultrasonic methods. Phase analysis showed tetragonal and orthorhombic KNN phases, and a contamination of (K2CO3·1.5H2O) was present. The obtained KNN polycrystals had a relative density above 95%. Texturing of the material was confirmed as a result of hot pressing.
