Development of ZrO2/Al2O3 and ZrO2/Al2O3-NbC using conventional and non-conventional sintering

High performance ceramic composites have been the subject of frequent studies in recent decades, aiming at improving mechanical properties and increasing their range of applications in technological products. This work consisted in studying the preparation, the conventional and non-conventional sintering and the mechanical properties resulting from two t-ZrO2 matrix composites: the t-ZrO2/Al2O3 system and the t-ZrO2/Al2O3-NbC system. In the t-ZrO2/Al2O3 system, the compositions of 0, 5 and 15% by volume of Al2O3 using commercial powders were studied, while in the t-ZrO2/Al2O3-NbC system, an Al2O3-NbC nanocrystalline powder obtained by high energy reactive milling, deagglomerated, leached in HCl and added in the proportion of 5% by volume to the t-ZrO2 matrix. The obtained powders were uniaxially and isostatically pressed and sintered in conventional furnace and using flash sintering (t-ZrO2/Al2O3) and spark plasma sintering (SPS) (t-ZrO2/Al2O3-NbC). Conventionally sintered t-ZrO2/Al2O3 and conventionally sintered t-ZrO2/Al2O3-NbC composites were characterized by measurements of apparent density, dilatometry, SEM, and mechanical properties: hardness, Young's modulus and fracture toughness. The t-ZrO2/Al2O3 composites sintered by FS were characterized by measurements of apparent density, in situ dilatometry and SEM. t-ZrO2/Al2O3-NbC nanocomposites were also characterized for wear strength by the ball-in-disc method, using Al2O3 and WC-6%Co beads as countermaterials. The results showed that the high energy reactive milling was complete and effective in obtaining nanometric powders of Al2O3-NbC, with crystallite sizes equal to 9.1 and 9.7 nm, for Al2O3 and NbC, respectively. The deagglomeration after high energy reactive milling was effective in reducing the size of agglomerates. Conventionally sintered t-ZrO2/Al2O3 and t-ZrO2/Al2O3-NbC composites and SPS-sintered t-ZrO2/Al2O3-NbC showed high densification (> 97% TD), good dispersion of the inclusions in the matrix and good mechanical properties. The t-ZrO2/Al2O3 nanocomposites sintered by FS presented an ultrafast densification (97% DT e boas propriedades mecânicas. Os nanocompósitos de t-ZrO2/Al2O3 sinterizados por FS apresentaram uma densificação ultrarrápida (< 1 min) com retração linear superior às amostras sinterizadas em forno convencional, ocorrente a temperaturas inferiores a 1000°C, com densidades relativas superiores a 90% DT em algumas composições. Os nanocompósitos de t-ZrO2/Al2O3-NbC apresentaram propriedades competitivas entre os compósitos sinterizados convencionalmente e por SPS, com dureza e tenacidade à fratura superiores às da t-ZrO2 monolítica. A resistência ao desgaste desses nanocompósitos sinterizados convencionalmente, no entanto, foi notadamente superior à dos sinterizados por SPS. A oxidação do NbC nos compósitos sinterizados convencionalmente influiu negativamente nas propriedades, levando à sugestão de uma “janela” de temperaturas em que a sinterização do nanocompósito de t-ZrO2/Al2O3-NbC seja interessante sem a degradação das propriedades mecânicas. Os resultados permitiram concluir que os materiais estudados apresentam potencial para aplicações industriais que requerem cerâmicas de alto desempenho mecânico e de resistência ao desgaste