Fretting wear behaviour and frictional force mapping of Al2O3 based thermal barrier coatings

Abstract

Efficiency and structural stability of gas-turbine engines were acheived with compliant thermal barrier coatings (TBCs) on structural components. In TBCs, top ceramic coatings encountered with other structural parts in a relative motion (amplitude of 100–500 μm and frequency of 1–5 Hz) was leading to failure, namely fretting wear. So, a high hardness material, Al2O3 based ceramic matrix composite (CMC) coatings were utilized to improve the tribological properties of the engineered surface. At this juncture, fretting wear behaviour of atmospheric plasma sprayed Al2O3–3 and 8 mol% Y2O3 stabilized ZrO2 (3YSZ and 8YSZ)‑carbon nanotube (CNT) based CMC coatings were investigated. The fretting wear rate of Al2O3 based CMC coatings against WC ball was observed as 0.97–1.87 × 10−6 mm3N−1 m−1. The least in wear resistance of Al2O3-CNT (1.87 × 10−6 mm3N−1 m−1) among the composite coating is attributed to low hardness and high Hertzian contact pressure (1.65 GPa) on the coatings. Among the composite coatings, Al2O3-3YSZ and Al2O3-8YSZ-CNT showed an improved wear resistance (~40.1% and ~34.5%, respectively) with wear rate of 0.97 × 10−6 mm3N−1 m−1 and 1.06 × 10−6 mm3N−1 m−1, respectively. It is attributed to improved mechanical properties, altered surface by reciprocating motion and the presence of wear debris, which reduces the wear rate by acting as a third body. So, there is an overestimation of wear rate (4.1–6.6 times) compared to that of experimentally calculated wear rate. Further, the Al2O3-YSZ-CNT composite showed fatigue crack on the fretting wear surface due to low plasticity index (62.0–109.3 MPa) of the coating. Also, the gradual change of running-in regime to the steady-state regime is achieved in the CNT reinforced composite coatings due to high resistance by the coatings which possess commendable mechanical properties. The frictional force mapping during the reciprocating motion confirms the rapid and gradual transition of two different regimes in the composite coatings.