Opening lecture of the 4th Mediterranean Conference on Heat Treatment and Surface Engineering

ABSTRACT

Hot forming, especially hot forging is a very demanding forming application requiring tool material with high strength and toughness and above all good wear resistance at elevated temperatures. Prevailing wear mechanisms in hot forming are abrasive wear and galling, with abrasive wear leading to dimensions mismatch and galling to unstable friction and poor surface quality of the forgings. In order to reduce tool wear and improve forming process different wear resistant coatings are used, often combined with solid lubricants to prevent work material adhesion.

Hard coatings, like chromium nitride (CrN) and diamond-like carbon coatings (DLC), are considered as one of the best candidates for wear protection in many different forming application where adhesive wear and galling dominate. However, low stability of DLC coatings at high temperatures and relatively high coefficient of friction of hard nitride coatings against soft light-weight alloys hinders their applicability, still requiring the use of different lubricants in the most demanding applications. Self-lubricating claddings and coatings with the capability of in-situ forming of sulphur or oxide based lubricious compounds at elevated temperatures, on the other hand, constantly attract interest. This includes different Ag-MoS2 containing claddings, metal-doped DLC coatings, as well as V containing PVD coatings.

The aim of the presented work was to compare and study self-lubricating and anti-galling properties of Ni-Ag-MoS2 based claddings as well as some PVD coatings (W-DLC, CrN, CrVN) coatings at elevated temperatures when used in contact against different light-weight alloys. Ni-based laser cladding with the incorporation of 5 wt% silver and 10 wt% MoS2 as solid lubricant precursors was compared to some DLC coatings and CrN coatings doped with V in concentrations from 15 to 30 %. To simulate hot forming process of forging, wire drawing and extrusion, tests were done at elevated temperatures (RT, 150°C, 300°C and 600°C) against typical light-weight alloys, including structural steel for forgings, AISI 316L stainless steel, 6xxx series Al alloy and Ti6Al4V Ti alloy and results evaluated in terms of coefficient of friction vs. load (single and multi-cycle mode), critical load for galling initiation, volume of adhered material and wear track surface analysis. Surface analysis was focused on the formation of self-lubricating surface layers and phases depending on the coating type, temperature and work material.