Abstract:
The reduction of noise and emissions is becoming increasingly important in civil aircraft jet
engines as well as requirements for reduced fuel consumption and improved efficiency. This
has resulted in the drive towards increasing turbine entry temperatures and the development
of thermal barrier coatings (TBCs). Due to the effectiveness of the platinum-modified nickel
aluminides currently used as bond coat layers for Ni-based superalloy TBCs, higher
temperature ruthenium-containing bond coat layers are being examined as a possible low
cost alternative to platinum.
Rolls Royce have a patented process, whereby precious metal layers directly react with
single crystal substrate alloys to form an aluminium containing surface coating. The
aluminium is sourced from the single crystal alloy and the coating so formed has a +
structure, but contains other intermetallic phases due to the reaction between the coating and
the single crystal substrate. This bond coat layer acts as a diffusion barrier, which limits
interdiffusion between the coating and the substrate.
The aim of this research was to examine the stability of various phases within platinum and
ruthenium-containing multilayer systems formed during the above reaction process and to
determine the most stable intermetallics for inclusion in future coating systems. Foil samples
were manufactured using multilayer sputter coating methods and the exothermic formation
of these phases was examined using differential scanning calorimetry. The identification of
the phases formed was carried out using X-ray diffraction.
It was found that the interdiffusion between the initial multi-layers had been incomplete
during the samples heat treatment, and so more intermetallic phases formed in some samples
than aimed for. Hence, from the large number of samples studied it was shown that, as a
result of kinetic factors, the reaction onset (or trigger) temperature was not related to the
enthalpy of the intermetallic phases formed or the sample compositions within a target phase
field.
For the β-phase (NiAl) type intermetallic systems, the samples that produced the highest
enthalpy values (i.e. the most stable intermetallic compounds) were those with the nominal
compositions (in atomic %) of; ‘47Ni53Al’, ‘48Ni6Pt46Al’ and ‘51Ni7Ru42Al’.
For the γ΄-phase (Ni3Al) type intermetallic systems, the highest enthalpy values were from
samples with nominal compositions of ‘60Ni16Pt24Al’ and ‘74Ni5Ru24Al’
