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The oxidation at 700-1300°C of the AIN-TiB2 equimolar composite in flowing oxygen is accompanied by aluminium borates formation with entrapment of a great amount of boron oxide by alumina, the latter reducing the volatilisation of B2O3 comparison with pure TiB2. Such a composite proved to have good thermomechanical characteristics and, by heating up to 1000°C, it was not possible to detect any creep deformation.
The structure of AIN-based coatings (mainly, AIN-TiB2 films obtained by r.f. magnetron sputtering on sapphire and silicon substrates) as well as their corrosion behaviour were also studied using various physical methods : DTA, XRD, EPMA, SEM, EDS...
It was shown that the AIN-ZrB2, ceramics with 2.5% Fe additive are corrosion-resistant materials up to 1400°C owing to the formation on the samples of a dense and adherent 3Al2O3.4B2O3 surface layer In this borate AI3+-ions were partially substituted for Fe3+-ions which stabilize its lattice.
In the case of AIN-SiC ceramics with a fine-grain structure it was ascertained that a three-stage oxidation mechanism took place, with formation of Al10N8O2, ß-SiO2, -Al2O3 (or ß-SiAION) in the inner scale layer and 3Al2O3.2SiO2 mullite in the outer one, the latter ensuring the extremely high protective properties of oxide films forming up to 1550°C. As a result of high-temperature creep tests, it was established that the AIN-SiC ceramics have a high deformation potential up to 1400°C. At 1400°C, their creep resistance is higher than that of Si3N4 ceramics. Long-term corrosion tests of a 50% AIN-50% SiC composite were conducted at 1200°C in both dry air and two combustion gas atmospheres (Kerosene Jet A1 and Marine/Industrial Diesel). They revealed its high corrosion resistance in dry air up to 50 h due to the formation of a mullite-containing protective surface scale while, in the burner rig tests, high rates of corrosion attack were observed for exposures up to 240 h.
Concerning AIN-TiN ceramics tested in air up to 1400°C, one can conclude that the maximum temperature of their possible long-term exploitation increases as a function of the AIN content as follows : 1) AIN:3TiN - 1050°C, 2) AIN:TiN - 1150°C, and 3) 3AIN:TiN - 1350°C.
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