Protective coatings for Al press tools used in automotive industry

Abstract: The application of the thermally sprayed WC-Co and Cr 3C2-NiCr coatings have been widely used in industry for its superior sliding, abrasive and erosive wear properties. The Al2O3 coating possessing high hardness can be utilized to improve the wear resistance of the metallic surface. Replacing the stamping tool material from steel to lightweight aluminium with protective coatings offers significant gains such as reduction in power costs and increasing operational efficiency during manufacturing of sheet automobile parts. In this study, the WC-CoCr and Cr3C2-NiCr coatings were sprayed with High Velocity Air-Fuel (HVAF) process and Al2O3 coating was deposited using Atmosphere Plasma Spraying (APS) process. The coatings were evaluated and compared based on the results of roughness tests, hardness tests, adhesion tests, quantitative imaging analysis and microstructure analysis. The experimental results revealed that the roughness value varied in a lower range which implies of a high density of the coatings. It was found that coatings have an extremely high hardness value as tested by the Vickers hardness test. The WC-Co coating was found to be the hardest (1215.2HV). The adhesion test was implemented according to the ASTM C633-79 standard. The result showed that WC-Co and Cr3C2-NiCr coatings without a bond coat had high tensile strength, higher than the respective glue strength (63.54MPa, 75.89MPa). Whereas, the Al2O3 coating has a much lower tensile strength (15.2MPa, 25.68MPa, with and without bond) than others and using a bond coat layer does not contribute to an increase in adhesion strength of the cermet coatings. The coating thickness was evaluated using Light Optical Microscopy (LOM) and microstructure analysis was carried out using Scanning Electron Microscopy (SEM). From the microstructure analysis, it was observed that all the coatings have a dense microstructure, very low porosity and low oxide inclusions in top coat. The WC and Cr3C2 grains retain a large volume fraction of finely dispersed in matrix. The alumina coating was found to have low levels of un-melted or the re-solidified particles in the coating. All of the aforementioned analysis and results reveal that WC/CoCr and Cr3C2-NiCr coating show promising potential for press tool applications. However, abrasive resistance test still remained to be done and will be performed in the future. The result of it can reveal the actual wear resistance between two coatings in reality and will be helpful in determining the better protective coating for aluminium press tools