Combinatorial experiments for discovering Al-C thin films with high strength and ductility

Abstract

Aluminum (Al) is one of the most widely used structural material in various technologically relevant applications due to its light weight and formability. However, these advantages have been overshadowed by its relatively poor mechanical properties. In this paper, we report theexceptional mechanical properties of aluminum-carbon (Al-C) thin films deposited by multiple source co-sputtering. In the first set of experiments, a high-throughput combinatorial approach was taken to investigate mechanical and electrical properties of a blanket of Al-C thin film deposited with a compositional gradient. Hardness increased along with carbon addition, but at the cost of reduced electrical conductivity. These measurements allowed rapid screening of Al-C films and showed that regions with ∼ 6at% carbon possess a good balance of mechanical and electrical properties (2.8 GPa and 142 Ω·nm). Micro-tensile experiments of freestanding Al, Al-C 6.4at%, and Al-C 10.3at% films were conducted to acquire tensile properties and understand the strengthening mechanism. Al-C films with 6.4at% and 10.3at% carbon both exhibited significantly increased yield stress over 300 MPa while retaining ductility. Unexpectedly, an upper yield followed by a fall in the stress was observed, which resembles the mechanical behavior of low-carbon steel. The unusual mechanical behavior of Al-C is attributed to formation of the Cottrell atmosphere. We also demonstrated that C impurities suppress stress-induced grain growth, which leads to improved microstructural stability during deformation. This paper demonstrates that carbon addition by co-sputter deposition is a viable approach for improving the strength and microstructural stability of Al thin films without loss in ductility.