Thick nitride coatings as hard chrome alternative: pertinence of the supercritical fluid chemical deposition process – ALTERCHROM

The ALTERCHROM project is a three years collaborative project of applied research between one academic partner the “Institut de Chimie de la Matière Condensée de Bordeaux” and one industrial partner SAFRAN. This project address the development of hard chrome plating alternatives. Hard chrome plating is used by Original Equipment Manufacturer to provide sealing surface for hydraulic seals, wear resistance to moving parts and corrosion protection to a wide variety of components for civil and military aircraft such as landing gear components, steering covers, piston heads…. The primary alternative in use today for aircrafts is HVOF thermal spray. SAFRAN will take care during the whole project of the scalability towards industry of the different choices. The project ALTERCHROM is in the TRL range between 2 and 4.
This project concerns the development of a new sustainable and efficient technology which is the semi-continuous supercritical fluid chemical deposition (SFCD) process. It aims to validate the feasibility of at least few tens thickness nitride coatings as an alternative to vacuum coatings (PVD and CVD) which are developed for extremely hard and wear resistant materials but limited to thin coating, and to thermal spray coating solutions which allow thick coatings but without wear resistance compared with that of traditional surface treatments as PVD.
The main objectives of the ALTERCHROM project are: 1) To develop a new cold-wall supercritical fluid chemical deposition reactor working in semi-continuous mode to address the formation of thick nitride coatings, 2) To validate the feasibility of at least few tens thickness nitride coatings up to 100 µm on flat and more complex ring shape samples, 3) To establish relationships between process parameters, microstructure and properties, and 4) To check the properties of these coatings in term of adhesion to the substrate, hardness, roughness, wear and corrosion resistance. Obtaining properties close to (or better than) hard chromium properties means: 1) Hardness from 800-1100 HV, 2) Average roughness after machining of 1.2 µm, 3) Friction coefficient against stainless steel between 0.3 and 0.5 and 4) Corrosion resistance of 96 hours in salt spray test. Successful nitrides will be TaN and TiN selected for their compatibility with a lot of metallic substrates as well as for their good hardness, wear and corrosion properties. For both, the challenges will be to succeed: 1) to increase the coating thickness until ten few microns up to 100 µm preserving a good adhesion on substrate and controlling the final surface roughness for application on complex shape structures, 2) to control the coating microstructure as its nanostructuration, thanks to the understanding of its correlation with the process parameters, 3) to improve the properties (hardness, wear and corrosion).
The project is organized in six tasks in strong interactions and will be conducted in three steps. First, the precursors to realize TiN and TaN coatings will be selected based on a study on thin films. Second, the new semi-continuous SCF film deposition reactor will be developed and thin films characterized. Finally, an optimization of the process parameters will be attempt in order to reach at least few tens thickness nitride coatings with a controlled composition and microstructure adapted to wear resistant film requirements.
This proposal falls within the framework of Theme 6 “Matériaux, Chimie et énergie”. Specifically it deals with both “Matériaux multifonctionnels de surface” and “Chimie des systèmes anticorrosion et traitements de surface et de protection associés; Nouvelles voies de synthèses respectueuses de l’environnement » as far as a SFCD reactor involving chemical transformations of precursors into nitrides is concerned for depositing protective coatings.