Development of PcBN composites by spark plasma sintering technique

• Autor(a) principal: Mineiro, Ricardo Miguel Caixa
• Data de Publicação: 2022
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/36011
• topic: Spark plasma sintering; Polycrystaline cubic boron nitride (PcBN); cBN-TiCN composites; Nickel

PcBN (Polycrystalline cubic Boron Nitride) is a material densified by HPHT techniques and processes a high economic cost. In this work it was intended to study the behavior during sintering of the cBN-TiCN-Ni and the possibility of densification by Spark Plasma Sintering technique (kindly made available by the Institute of Ceramic and Glass – ICV). Additionally, the dispersion of constituents was studies by two mixing techniques: ball milling and colloidal processing. With this purpose it was used two types of cBN powder: a cBN powder with particle size of 57 μm (D₅₀) and a coating of Ti₂N, and another with particle size of 1,5 μm (D₅₀) without coating. For each powder it was prepared 2 compositions with TiCN and variated the addition of 5% vol. of Ni to see its effect in the sintering of the composite. Additionally, it was implemented the use of thermodynamic tools, namely Thermocalc, to help predicting the resulting phases for the composite and therefor adjust this one to try to obtain more interesting properties for the desired application. The sintered samples where structurally characterized by X-Ray Diffraction Technique (XRD) and Raman Spectroscopy, microstructurally characterized by Scanning Electron Microscopy (SEM), chemically characterized by Energy-Dispersive X-ray Spectroscopy (EDS), and physically by the Archimedes principle. For these systems it was observed that the addition of Ni increased the densification of the composite (Liquid Phase Sintering) and that this one is reactive with BN which led to the formation of Ni₃B phase. The BN phase transformation from cubic to hexagonal occurred during all the sintering temperature range of 1400 – 2000 °C, having completely transformed at 1700 °C for the coarser particles (57 μm) while it was still verified the presence of cubic phase at 1800 °C for a finer particle size (1,2 μm) although in reduced amount. For the composition with the course coated cBN (69% vol.) an open porosity of 0% was only obtained at 1700 °C, where the BN volume expansion from the transformation favoured the removal of porosity. For the composition with the finer cBN (50% vol.) it was also achieved 0% of open porosity but with an expensive BN phase transformation. It was verified that the finer cBN powder does not have a tendency to agglomerate for pH in the 2 – 12 interval and that the use of dispersant (PEI) is effective at 0,5% (%m/m) both for the Short form and Long form. With the mixing conditions used, both the ball milling and the colloidal processing were not capable of achieving a good homogenization, resulting in a microstructure containing several defects.