On the interaction mechanisms between sources of hydrogen and Al-Si coated or bare high strength steels during the hot stamping process

The economic and ecological challenges of the automotive industry coupled to safety considerations involve lightweight design based on the development of new ultra high strength materials. The hot stamping process of Al-Si coated steels ensures these strength levels and, moreover, allows an easy forming of the material without the necessity of a controlled atmosphere. At high temperatures, the coating is hydrogen-permeable, while at low temperatures (below 120°C), it becomes tight to hydrogen. In this context, the aim of this work is to understand how hydrogen enters in the material during the hot stamping process with respect to different atmospheric sources. An important aspect of the problem is the continuous evolution of the material during the hot stamping process. At ambient temperature, the coating is indeed a solid Al-Si alloy, which has weakly reacted with iron during deposition. As temperature increases, this coating first melts and then forms ternary intermetallic compounds Al-Fe-Si. These different subsystems of the material have been studied and compared to bare steel. The influence of the different hydrogen sources on bare and coated steels were also scrutinised: water vapour, which is probably the active source in the industrial process and dihydrogen, a more fundamental study case. Heavy water vapour (D2O) is also used to ensure a controlled atmosphere in the furnace, while it allows to avoid misinterpretation that could arise with the dissociation of water into hydrogen

Subcritical crack growth in freestanding silicon nitride and silicon dioxide thin films

Introduction/Purpose: Thin film materials are the main building blocks in many fields of application like flexible electronics, microelectromechanical /nanoelectromechanical systems (MEMS/NEMS) and functional coatings. However, the determination of the intrinsic fracture toughness of these materials, which is the key property controlling the failure resistance remains a challenge. Methods: In this work, a new on-chip testing method for freestanding films is developed. The technique consists of two long actuators beams pulling on a notched specimen by exploiting the residual stress inside the actuators. The residual stress upon release by chemical etching leads to the actuator contraction, hence pulling on the central notched specimen. A crack is initiated at the notch tip, propagates and finally stops when the energy release rate has decreased down to its critical value. This crack arrest measurement avoids the problem of introducing a sufficiently sharp precrack. Further subcritical crack growth can be investigated by this fracture on-chip testing technique through repeating crack length measurement over time, without monopolizing any test equipment. Results: The understanding of the mechanisms causing the subcritical crack growth is crucial to define the reliability of devices made with thin films that are showing time-dependent failure. Low-pressure chemical vapor deposition (LPCVD) silicon nitride (SiN) and silicon dioxide (SiO2) films deposited by electron beam-evaporation technique were studied with a variety of thicknesses. The specimens were tested in laboratory air and dry nitrogen environments under various temperature conditions. Conclusions: Based on both experimental data and finite element simulation results (FE), the stress intensity factor (K) and the crack velocity (v); K-v curve in different environments is determined, following classical exponential law

Graphene effect on mechanical response of metal substrate

Introduction/Purpose This work investigates the effect of a single layer graphene on the development of the contact plasticity inside a copper underlying substrate. Methods A copper film deposited on a Si wafer has been used as the substrate in the chemical vapor deposition process for graphene production. Hence, there is no need for transferring graphene which avoids many possible artifacts. Nanoindentation was performed on the Cu-film with and without graphene. In order to understand the root causes of these effects of the presence of graphene on the plastic flow, transmission electron microscopy is used to compare samples after nanoindentation in terms of dislocation structures. 3D discrete dislocation dynamics simulations are also performed to analyze the long-range back stress that are generated by the dislocation arrangements. To further extend this research and investigate the known effect of hardening by graphene insertion into metals, another system has been addressed which involves the deposition of a Cu film on top of the graphene layer, lying itself on top of the annealed Cu substrate (Copper-Graphene-Copper). Results The analysis of the force-displacement curves indicates that the presence of graphene modifies the onset of plasticity which appears in the form of a pop-in. The first pop-in occurs at lower loads and the pop-in lengths are smaller with graphene in comparison to the Cu film without graphene. The magnitude of the effect of the presence of a graphene cap layer varies also with respect to the orientation of the indented Cu grain. In addition, the presence of graphene caused marked effect on the indentation response in the second configuration, even larger than the first configuration. Conclusions Graphene shows a profound effect on the indentation response of copper film in the both case when graphene lying on the top of copper film also placing between two copper film