Summary



On the last five years surface coating technology has spread widely. Different achievements have appeared on material surface properties about: competitiveness, quality production, and new coating technologies developing coating technique adaptability, adherence properties, availability, productivity and cost. Nowadays deep drawing dies are liable to the following service problems: measures change, wear, impacts,  strain, and fracture due to high shear stresses on the sheet-die sliding area. However, dies with premature corner wear and possible cracks, can be recovered.

Worldwide this problem has been studied but the problem is really complex. Quality and performance efficiency for operating conditions on dies are the result objectives for surface modification techniques including thermal treatments, surface coatings and crack or wear regeneration. On the current research flame thermal spraying was studied as mending technique, as it is key to recover deep drawing dies independently of their own fabrication. In this way, the final function of the die is guaranteed before it is rejected either from cold or hot work. Flame thermal spraying is a costless alternative, specially apt for growing ceramic layers on worn dies. The main requirement on this tecnique is a good adherence between coating and substrate. This is a difficult enough task considering the ceramic coating nature and short application times. For quantifying adherence different samples were tested under shear stress, for creating a shear strain on the interface between coating and substrate.

It is important to point out that throughout the research statistics have played an important role, first for test design planning, and later for result analysis. Input variables considered were temperature, or thermal process, rugosity and its originating coating. Output variables were: shear stress, sample rigidity, microhardness and porosity. In order to analyze strain and microcracking processes maintaining coating/substrate adherence, acoustic emission techniques were used, obtaining information on elastic energy emission that can be correlated with microstrain originated when samples are tested to shear stress step by step until breaking point is reached. Strain and microcracking prior to fracture is evaluated through the correlation between accumulated impacts on acoustic emission and indicators from shear loading measuring coating adherence. It was also demonstrated, through acoustic emission, the different causes conditioning microjumps on the evolution and value change of the indicators.

In much the same way, the tendency for adhesive tribology was evaluated through the pin on disk test for coatings nickel-oxides, and cermets on steel sheet, stainless steel and aluminium. Each pair was evaluated for four different loads combined with five different rotating speeds. Similarly, the research on the adherence on low carbon steel (F-1110) dies was performed. The analysis through acoustic emission of nickel-oxide, and cermets (WC/Ni, Al2O3/TiO2) coatings applied through flame thermal spraying was achieved, as well as the determination of adhesive performance on deep drawing metallic alloys (low carbon steel DC01, stainless steel AISI 304, and aluminium AA1060). Researched coatings perform according to the expectations of application and service for the die, allowing later recovery of damaged surfaces on used dies. The statistical analysis and correlations obtained, determined that the best coating was WC/Ni and the best base alloy for the die varied on different applications. The best adherence properties for WC/Ni alloys was obtained for small porosity and low adhesion on lubrication.

Keywords: Thermal Spraying, Adherence, Rugosity, Ceramic Coating, Acoustic Emission, Fracture, Shear Stress, Nickel, Aluminium Oxide, Titanium Oxide, Tungsten Carbide, Tribology, Friction, Adhesion.