Controlled Superficial Cracking of the Substrate to Enhance Adhesion of Plasma Sprayed Coatings

摘要

Deliberate cracking of the substrate surface was shown to be beneficial for improving the coating adhesion of thermally sprayed coatings. This results from a better mechanical anchoring of the coating from cracks which act as efficient pegging sites. Controlled cracking was obtained though heat treatment of the surface prior or simultaneously to spraying. Cracking process and effects on adhesion were studied for two coating/ substrate systems, i.e. stainless steel onto an acrylate resin and tin onto lithium fluoride, for which adhesion is known to be conventionally very limited. Coatings were achieved using a CAPS-ATC ("Controlled Atmosphere/Temperature Plasma Spraying") unit. SEM, EDS, EPMA, FTIR, QIA, and confocal microscopy were particularly employed to study microstructures and interfaces, coupled with a finite element analysis of adhesion due to cracks. One of the prominent advantages of the cracking process, e.g. compared to conventional sand blasting, is that substrate roughness can be kept (except for crack sites) very low (up to a mirror-like polished surface). This corresponds to a quasi flat coating/substrate interface except for the pinning cracks. Due to this unique adhesion process (patented for some bonding applications), thermal spray could be successfully applied to a new route for rapid prototyping of molds and to new materials for optical pyrometric measurements. The superficial substrate cracking concept can be extended to coatings other than thermally sprayed coatings, primarily to those of polymers and minerals as in the two examples which are discussed in this contribution.