ICMCTF2018 Session G4: Pre-/Post-Treatment and Duplex Technology

Electroplating is a coating process which is used to increase the durability of workpieces or to change their appearance. To form a high adhesive bonding between the coating and the workpiece, its surface has to meet certain requirements achieved by a comprehensive pretreatment. The conventional pretreatment process chain consists of different steps to increase the roughness and the surface tension of the workpieces. For this process chain chemicals are used, which are harmful to the environment and health, besides their high costs.

The complexity of the pretreatment and coating process chain differs depending on the used workpiece material. Particularly challenging to coat are Aluminium-Silicon alloys. During the conventional pretreatment of these casting alloys, the Silicon phase enriches on the surface of the workpieces and has to be removed in an additional process step using nitric acid.

This study compares alternative mechanical pretreatment processes for AlSi12‑workpieces prior to electroplating. The investigated processes are conventional grinding, double face grinding with planetary kinematics, lapping and dry ice blasting. Subsequent to the structuring, the workpieces are cleaned by carbon dioxide snow blasting. The investigation of the different surface structures and their properties, like roughness, surface tension and wetting behaviour, allowed the determination of their impact on the adhesive strength of the coating by empirical modelling.

By structuring the surfaces of the workpieces they could be coated by electroplating. As a result, the chemicals used in the conventional pretreatment could be substituted. Especially the creation of surfaces, which could be wetted homogenously, showed a satisfactory adhesive strength of the applied coatings. Concluding the study, a comparison of the economic performance of the mechanical and conventional pretreatment processes is created, which confirms the profitability of the mechanical pretreatment for small batch sizes.

Surface texturation at micro and meso scales play an important role in applications where cosmetic, aesthetic and clean functionalities are specified. We are hence dealing with a multiscale surface, in which texturing and texture have a larger influence because they are scaled differently. In this research paper, an experimental method is illustrated to highlight the important effect on the anti-fingerprinting performance (i.e. surface hydrophobicity) rated in term of surface wettability. We examine first, in detail, the wetting response of surfaces textured on aluminum alloy 6063 plates using nano-peening with a range of processing parameters. Roughness was measured by atomic force microscopy (AFM) over a 100 x 100 µm² surface. In addition, the surface wettability was quantified by measuring the contact angles of different liquids using the sessile drop method according to the norm AFNOR EN 828. The calculation takes into account the wetting behavior of the textured surfaces at different scales. A correlation was made between the micro-scale roughness and the macro-scale apparent solid surface energy.

The technology that achieves abrasion resistance and low friction is essential for improving the product features and the added value of slide members, which are used under severe conditions such as in a high-temperature and high-speed environment. Typically, abrasion resistance and low friction are achieved with the use of the deposition technique, which involves coating of a material surface with a hard coating, and/or the modification technique, which hardens the material surface itself.

A piston ring is an engine component fitted into the groove cut in a piston (piston groove), and moves in a reciprocating motion with the piston inside a cylinder. Specifically, a piston ring has two different surfaces: a surface that contacts the cylinder (peripheral surface), and a surface that contacts the piston (slide faces, or inside surface). These surfaces contact different materials, and different functions are required for these surfaces. It is accordingly very important to have possession of not one but many surface treatment techniques.

The deposition technique used for piston rings often uses chrome plating by a wet process, and CrN, and TiN coating by a dry process (Arc Ion Plating, AIP). Gas nitriding and salt bath nitriding are two processes commonly used for surface modification. Though the deposition and surface modification techniques are often used by themselves, these techniques are also used in combination depending on engine specifications.

In this lecture, we introduce a technology based on both deposition and surface modification, along with the recent composite technology.