Respuesta mecánica bajo solicitaciones de contacto esférico en aceros recubiertos

Resumen

In recent years hard coatings have acquired a significant importance in the microstructural and micromechanical design of forming tools. Among the wide range of surface modification techniques, physical and chemical vapour deposition are the most suitable routes to meet the demanding requirements of surface finish and dimensional tolerances that need some specific tools, such as those for the automotive sector. However, although thin films tend to reduce the impact of tribological service conditions, improved resistance against contact fatigue failure still remains a challenge in the forming tools. The latter has been emphasized in drawing, die cutting and punching processes, where the introduction of advanced high strength steels requires higher impact pressures than those used in conventional metal sheets. In those cases, the load is mainly supported by the substrate. Thus, if any subsurface damage occurs in coated tools, then a premature detachment of the layer could be produced accelerating the wear damage mechanism. However, standardized experimental techniques designed to evaluate the contact fatigue behaviour of coated systems are scarce. Following the above ideas, the aim of the present work was to conduct a systematic study of the contact response and damage mechanisms, under both monotonic and cyclic loading, of conventional thin films deposited onto cold-work tool steels. In doing so, the spherical indentation technique was used to simulate a typical ¿blunt¿ in-service condition and to identify damage evolution associated with increasing load or number of cycles using a small dimension size of samples. Experimental procedure was based on determining critical applied loads and pressures for emergence and evolution of distinct damage modes: circumferential cracking, cohesive spallation and interfacial decohesion. The materials were selected following the criteria of coated systems currently used or potentially applicable as forming tools in industry. The study was divided into two parts: 1) substrate used as a variable to determine the influence of three microstructurally distinct cold work tool steels (1.2379, Universal and HWS); and then 2) coating employed as a variable selecting hard coatings such as TiN and TiC, and a low friction coating such as W-C:H. Experimental results showed that microstructural characteristics of the substrate as well as chemical and mechanical properties of the coating have an important influence in the overall coated system behavior. Concerning substrate influence, differences are clearly evidenced by considering coating detachment under contact fatigue as the critical damage mechanism, specifically for a TiN coating deposited by PVD. On the other hand, conventional scratch tests and even tests under monotonic spherical indentation, were not able to discriminate differences on the mechanical response of the three coated steels studied. In this regard, microstructural effects at the substrate level were rationalized on the basis of crack nucleation resistance of the existing primary carbides as the main reason for the distinct adhesive fatigue response observed. Hence, the optimum intrinsic strength/toughness of primary carbides in HWS and Universal steels as well as their finer and more regular morphology, as compared to the 1.2379 steel, were suggested as key factors for the improved contact fatigue strength exhibited by their corresponding coated systems. This was particularly true for the PM tool steel, where adhesive fatigue failure was completely suppressed as a consequence of their finer and homogeneously distributed primary carbides. Regarding coating properties, droplets were discerned to be detrimental seeds for crack nucleation and cohesive failure at the film surface. Delamination resistance of TiN deposited on a steel substrate Universal under cyclic contact loads increased significantly by using a process of low-pressure chemical vapour deposition.