Investigation and characterisation of the wear mechanisms of abradable compressor linings

• Main Author: Fois, Nicola
• Other Authors: Marshall, Matthew B. ; Dwyer-Joyce, Rob
• Published: University of Sheffield 2015
• Subjects: 621
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.675062

During aero-engine operation, rotor misalignment, thermal and centrifugal dilatations, and unbalanced parts lead to contact between the rotating blade and the abradable lining of the surrounding casing. Observation of aero-engine service has highlighted undesirable issues and relative wear mechanisms that have created problems during operation, with significant reductions in aero-engine performance. Examples of issues observed were adhesive transfer of abradable material to the blade tip, along with blade wear. These observations have highlighted the final result of the contact between the blade and abradable; however, the complexity of introducing instruments to the aero-engine has led to a gap in knowledge with respect to the parameters that influence these wear mechanisms. This issue has provided the motivation for this research, where wear mechanism between a Ti-6Al-4V rotating blade and abradable material AlSi-hBN has been investigated on a scaled test platform. AlSi-hBN has been chosen as it represents the most common technology used in the compressor stage of the engine. Alternative approaches that characterised the wear mechanism in real time were introduced in order to try to explore the nature of the contact. The introduction of an innovative stroboscopic imaging technique allowed the progression of adhesive transfer / blade wear to be investigated in real time during a test, revealing that the standard practice of performing analysis of adhered material at the end of a test does not necessarily characterise the overall mechanics of adhesive transfer satisfactorily. In addition, the measurement of contact force and the calculation of the efficiency of cut and force ratio, highlighted different material behaviour in relation to the incursion rate and hardness. It was shown that it was difficult to dislocate the material at low incursion rate, with consolidation evident, whereas at higher incursion rates the material was well fractured. The measurement of the coating temperature highlighted the heat generated in the contact, along with the effect of changing coating hardness on the thermal properties of the abradable, leading to markedly different thermal behaviour at low hardness, and in particular at low incursion rate. The wear mechanisms observed, adhesive transfer, blade wear and cutting, were mainly incursion rate dependent, with a thermal wear mechanism at low incursion rate and a well cut mechanism at high incursion rate. At low incursion rate, different wear mechanisms were observed in relation to the hardness, with adhesive transfer on the blade tip from the hard coating and blade wear in the test performed against the soft coating, with similar results observed at all blade speeds. A wear map was generated in relation to the input parameters, such as the incursion rate, speed and coating hardness, and also the reflected different thermal properties of the coating. This indicated that it is better to have a high thermally conductivity coating; therefore, the use of a hard AlSi-hBN is a better option, because less thermal damage was observed. The wear map with the highlighted wear regimes is a useful design tool with respect to planning running and handling manoeuvres for the engine, where the manufacturer has the option to control incursion parameters. As these represent the most significant incursion performed, this finding is of particular benefit.