An investigation of the contact mechanisms between Inconel 718 blades and a NiCrAl-bentonite abradable system

This work has focused on contact mechanisms between Inconel 718 blades and a NiCrAl-bentonite abradable – a combination commonly used in the hot end of aero-engine compressors, where a transition to high forces and blade wear has been observed in certain contact conditions. In the first set of tests the effects of blade length and rig arrangement were investigated through testing on two different rigs. The purpose was twofold: establishing a connection to the previous research performed on the slower of the two rigs, and developing a general understanding of the effects of these two parameters on results. It was demonstrated that both rig stiffness and blade length in the considered range did not have a strong effect on the likelihood of transition of a test to the aforementioned high wear regime. The higher speed rig was then used to investigate the progression to high contact forces and blade wear in more detail by performing tests at speeds of 200 m/s and 280 m/s, with three incursion rates considered at each speed. Test to test variability was similarly investigated by performing five repeats for each test condition. Two distinct contact modes were observed – one where forces remained low and no blade wear occurred, and another where forces progressively increased until blade wear initiated and forces stabilised at significantly higher values than in the case of low force tests. These contact modes were explained through interaction between the incursion rate and rate of abradable fracture. The results have shown that an increase in incursion rate has increased the likelihood of the high-force contact mode, and an increase in blade tip speed decreased it. The inherent randomness of the abradable spraying process was demonstrated to lead to variability in material properties for nominally similar samples, and in turn the transition in contact mode was in essence probabilistic in nature. This variability also highlighted the importance of performing repeats when contacts with sprayed abradable materials are considered.