Research on the physical properties of sputter deposited Al-C thin films discovered by combinatorial approaches

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 exceptional 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, 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 the Al-C film and showed that regions with ~6at% carbon possess a good balance of mechanical and electrical properties (2.8 GPa and $142 \Omega \cdot nm$). Micro-tensile experiments of freestanding Al, Al-C 6.4at%, and Al-C 10.3at% films were conducted to acquire tensile mechanical 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 fall in 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 demonstrate that C impurities suppress stress-induced grain growth, which leads to improved microstructural stability during deformation. The annealing and passivation effects are analyzed to acquire the deep understanding on the Al-C thin films. This paper demonstrates that carbon addition by co-sputter deposition is a viable approach for improving the mechanical properties of Al thin films without loss of its lightweight characteristic.