The structure and tribological properties of gradient layers prepared by plasma-based ion implantation on 2024 Al alloy

Using plasma-based ion implantation, two types of gradient layers have been prepared on 2024 Al alloy. One is prepared by N-implantation then C-deposition, the other adds an interlayer composed of a Ti layer and a Ti-N layer between N-implantation and C-deposition. C-deposition is carried out at various implanting voltages or C₂H₂/H₂ ratios. The composition depth profiles of these layers were characterized by x-ray photoelectron spectroscopy. The structure, morphologies and microstructure of the C layers were studied using Raman spectroscopy, atomic force microscope and transmission electron microscope, respectively. The surface hardness was measured with a Knoop tester and a mechanical property microprobe. The dry ball-on-disc wear tests were performed in ambient air. The gradient layer without interlayer is composed of an N-implanted layer rich in AlN and a diamond-like carbon (DLC) layer (film), and the two layers are connected with a C-Al transition layer containing Al₄C₃. The Ti layer rich in α-Ti and the N-implanted layer are connected by a Ti-Al transition layer containing TiAl₃, while the Ti-N layer rich in TiN and the DLC film are connected by a C-Ti transition layer containing TiC, TiCN, etc. Thus, the gradient layer with interlayers has optimized the gradient structure. DLC films are compact and amorphous, contain high sp³/sp² ratios and depend on the implanting voltage and the C₂H ₂/H₂ ratio. Similarly, these gradient layers exhibit significant improvement in morphologies, surface hardness and tribological properties; the interlayer, the implanting voltage and the C₂H ₂/H₂ ratio all have prominent effects on these properties.