STUDY OF THE EFFECT OF THIN HARD COATINGS ON THE RESISTANCE OF HIGH STRENGTH-TO-MASS RATIO COMPONENTS

The rising need of lightening mechanical components by means of broader and broader use of light alloys is becoming crucial for the coming future. In the automotive and aerospace industries, for example, the request for more power with lower gas emissions is expected to acquire more and more importance. The fatigue behaviour of light alloys, however, is often worse than the one of traditional construction steels. The wear resistance, surface hardness and load bearing capacity can represent weak points for the former materials too. As a result, suitable surface treatments – such as thin hard coating deposition – may be decisive for improving their properties and make such properties be at least similar to the ones shown by the steels. Thin hard coatings deposited by means of physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques are, for the time being, wide spread in an increasing number of technological applications. The broad use of thin hard coatings is mainly due to the improvements achieved in the tribological and corrosion behaviour of the coated components and has concurred to consolidate the deposition techniques and justify the growing interest of researchers. Recent research studies, both experimental and numerical, have also demonstrated that thin hard coatings can prove effective in increasing the fatigue resistance of mechanical components and machine elements. The application of such coatings gives rise to surface modifications in the substrate which have been widely investigated with respect to the corrosion and wear resistance. As far as the fatigue resistance is concerned, in case of compressive residual stresses induced by the deposition process in correspondence of the surface layers of the base material, an increase of the fatigue limit, therefore of the number of load cycles until failure, is possible. The developed doctoral research activity has tried to go in depth through the fatigue and contact/rolling contact fatigue behaviour of thin hard-coated mechanical components. Experimental, numerical and statistical methods were used to study different bulk materials and coatings. Particular attention has been paid to those applications where increasing the fatigue limit of mechanical components can represent a decisive factor for performance elevation. Also, theoretical-numerical models enabling a designer to foresee the performance of coated components under fatigue and contact/rolling contact fatigue conditions were applied and proposed with good results. These procedures enable reliable fatigue life prediction of thin hard-coated components and were applied, in particular, to gears. The number of cycles necessary to achieve specified crack depths, until final failure, in coated and uncoated steel and titanium transmission spur gears was evaluated with one of the aforementioned previsional procedures. CrN and TiN coatings PVD-deposited on, respectively, steel and titanium base material were analyzed in this case and measurements of surface micro-hardness and residual stress fields were used in the calculations. As far as the fatigue tests on coated and uncoated specimens are concerned, several rotating bending experiments were carried out on both steel and light alloys. Different PVD and CVD coatings were deposited and the fatigue behaviour of the coated base materials was compared with the one shown without coating. The influence of the presence of both coating and a notch on the fatigue behaviour of coated titanium alloy specimens was also studied both numerically and experimentally. The contact and rolling contact fatigue behaviour of coated components was investigated with experimental, numerical and statistical methods. CrN PVD-coated and uncoated case hardened automotive transmission spur gears were investigated by means of numerical models and experimental contact fatigue tests. A suitable theoretical-numerical procedure able to predict the number of cycles necessary to promote initial fatigue damage at the contact area was developed and the results were compared with the experimental ones. Furthermore, this procedure was used to collect the data necessary to apply an advanced statistical method – namely Design and Analysis of Computer Experiments – to achieve the optimization of parameters influencing the fatigue and the rolling contact fatigue behaviour of coated components. Such parameters were related to both coating and base material. The presence of the self-equilibrated residual stresses induced by the coating deposition process was accurately simulated in the numerical models.