New reaction paths for advanced SiAlON/TiN composites
This thesis demonstrates how selected ceramic additives, including titanium nitride (TiN), impact upon the “chemistry ↔ microstructure ↔ properties” relationship as it applies to composites in the generic Sialon-TiN composite field. Examination and optimisation of this feedback loop enables control of industrially important thermal, electrical and engineering properties of β-Sialon based ceramics. The effects of a range of additives on the nitridation and sintering of β-Sialon composite bodies have been studied and the chemical and mechanical properties of the sintered bodies have been measured. The additives can be divided in three groups: nitridation additives which improve the yield and the rate of the reaction; sintering aids; and additives that improve resistance to thermal shock. A suite of additives consisting of a mixture of calcium aluminate cement, yttrium aluminium garnet and boron nitride was found to deliver an optimum set of mechanical properties with a fracture toughness achieved of over 4 MPa.m-1/2. This thesis also reports a new reaction path for the formation of a β-Sialon/TiN composite by the reaction bonding of aluminium powder coated with nanosized titania. In this novel technique, the aluminium reacts under an inert atmosphere with titania to form alumina and a TixAly intermediate which is then nitrided to form aluminium nitride and titanium nitride. The addition of a suitable silicon phase enables the formation of a β-Sialon phase under nitrogen at high temperature. The TiN was added in the range 1 to 10 wt% (0.6 to 6 vol%). The effects of milling time on the aluminium powder particle size distribution and reactivity have been studied, with a minimum of two days milling time required to modify the particle shape and reduce melting coagulation during firing. Firing parameters have been optimised, using XRD and MAS-NMR to monitor the samples’ composition and SEM to observe their microstructure. The reduction of titania by aluminium was completed at 900 ºC for 4 hours in an argon atmosphere and the nitridation of the titanium aluminide at 1400 ºC for 3 hours in a nitrogen flow. The nitridation and sintering of the β-Sialon/TiN composite were both performed in nitrogen at 1400 ºC and 1600 ºC, respectively. A low level of addition of TiN (1 wt%) has shifted the composition toward the AlN corner of the Sialon behaviour diagram, forming α-Sialon and AlN polytypes. Other levels of addition in the studied range formed a dense β-Sialon/TiN composite. The TiN inclusions are found at the grain boundaries but are of insufficient volume fraction to form a continuous network in the Sialon matrix. Mechanical and electrical properties of the newly fabricated β-Sialon/TiN composites have been measured. These properties were generally improved by the highest levels of TiN addition: Young’s modulus (up to 210 GPa), hardness (up to 17.7 GPa), fracture toughness (up to 3.3 MPa.m-1/2) and compressive strength (up to 188 MPa). However the presence of TiN had no impact on the resistance to thermal shock and electrical conductivity of the β−Sialon. Finally, the oxidation process for β-Sialon/TiN composites has been observed by a combination of XRD, SEM and Ion Beam Analysis techniques. The results show early enrichment of yttrium and titanium in the first 0.1 μm of the samples’ surface; replacement of nitrogen by oxygen to form crystalline phases on the surface and in the glassy phase up to 1.5 μm deep; and, major crystalline and chemical changes in an outer layer of about 100 μm thickness at 1200 ºC. The partial depletion of SiO species from the external sample surface during sintering firing leaves this surface zone more vulnerable to oxidation than the protected body of the ceramic. The oxidation of TiN forms a TiO₂ skin which acts as a protection from further oxidation. The outcome of this research is a novel reaction path to fabricate new advanced Sialon composites and an improved understanding of the effect of a broad range of additives on the nitridation and sintering behaviour of β-Sialon and β-Sialon/TiN composites.