Papers by Keyword: Contact Damage

Abstract: Tools for rolling steels and super alloys, which are nowadays in general made from steel or cemented carbides, suffer from wear and/or from surface cracking caused by thermal fatigue. New tools made from silicon nitride show improved performance in respect to thermal shock loading and wear. But their low toughness manifests also a high risk of brittle failure. Nevertheless the successful use of silicon nitride rolls with having more than a manifold lifetime (compared to the conventional solutions) has been reported in the last years [1 -3]. In this paper earlier work of the Institut für Struktur- und Funktionskeramik at Montanuniversität Leoben on highly loaded silicon nitride rolls is summarized, where the limits of the Application of silicon nitride rolling tools are discussed. On the extreme example of rolls for super alloy wire rolling the behaviour of small surface cracks in the roll track is discussed. It is shown that – for the investigated conditions - rolling high strength steel wires is manageable but rolling of super alloy wires will cause the growth of fatigue cracks, which may destroy the rolls after some tons of rolled wire. A not trivial problem to be solved is the connection of the ceramic tool with the metal parts of the roll stand. Thermal strains of the metal parts can be several times larger than those of the silicon nitride ceramic and can therefore cause very high thermal misfit strains, even if the heating of metal parts seems to be modest. This case is discussed on the example of a catastrophically failed ceramic tool. This clearly shows that not only the tool but also the joint of the tool to the rest of the machinery has to be designed carefully. In summary this work demonstrates that a successful use of silicon nitride ceramic tools for cold and hot forming of metals and alloys is possible.

Abstract: Contact damage of curved multi-layered systems, consisting of a brittle zirconia (3Y-TZP)/Alumina (Al2O3) coating – prepared by slip casting – on a compliant polymeric substrate from indentation by a hard tungsten carbide sphere is investigated. The essence of this study is to compare the structural performance of layered ceramics, and monolithic ceramic in the brittle coating and compliant substrate of dental crown-like systems. The specimens are loaded at the axis of symmetry. The failure evolution to initiate cracking and final failure patterns in curved multi-layered specimens are compared to those of the single layer specimens of the same thickness and the same study are conducted again with the flat specimens. The onset of fracture is observed in situ using a video camera. It is demonstrated that in all specimens, cone cracking occurs prior to radial cracking, with the latter being defined as the primary mode of catastrophic failure. The multi-layered ZTA (Zirconia Toughened Alumina) specimens prepared by slip casting possessed a distinct final failure pattern in comparison with a single layer specimen. The results of this study provide useful guidelines for building brittle multi-layered coating systems with the functionally graded feature, and complex shapes with geometrical uniformity.

Abstract: The effects of the introduction of a buffer layer between the bond and top coats on the indentation stress-strain behavior and the contact damage were investigated in air-plasma sprayed (APS) zirconia (ZrO2)–based thermal barrier coatings (TBCs). The microstructure is relatively continuous in the TBC system with the buffer layer, showing Zr, Ni, Cr, and Mg elements between the top and bond coats, whereas the Zr element suddenly disappears by passing the interface between the top and bond coats. The TBC system with the buffer layer shows less strain than that without the buffer layer in the higher stress regions above about 1.3 GPa, while both TBC systems become soft by forming the top coat in the lower stress regions compared with the substrate. The stress–strain curve in both TBC systems is dependent on the dwell time of thermal exposure condition. The TBC system with the buffer layer shows the lower stress-strain curves than that without the buffer layer in thermal cycles with the relatively short dwell time of 1 h, showing the reverse trend with the relatively long dwell time of 10 h. Subsurface damage in substrate is reduced at both indentation loads of P = 500 N and P = 2000 N by introducing the buffer layer, independent of thermal exposure. Therefore, the TBC system with the buffer layer is more efficient in protecting the substrate from contact environments than that without the buffer layer, showing cracking or delamination between the top coat and the buffer layer in the TBC system with the buffer layer.

Abstract: For hot rolling wires of high-alloyed steels or superalloys tools are nowadays made of ce¬mented carbides. In service they suffer from roughening of the surfaces and severe wear, which de¬teriorates the surface quality of the wires and restricts the lifetime of the tool. Due to their high hard¬ness and good high-temperature properties, improvements in tool behaviour can be expected by the use of silicon nitride tools. Experiments with several types of rollers were performed in commercial rolling mills. At modest and medium severe loaded positions (e.g. in the case of guidance rolls) silicon nitride rolls show superior performance to conventional steel or cemented carbide rolls. At the most severe loaded positions silicon nitride rolls were also superior to conventional rolls when rolling high strength steel wires. But for rolling superalloy wires, cracks, which limited further applications of the rolls, appeared in the roll surface profile (calibre). Cracks in the surface of the rollers are in general caused by Hertzian contact stresses, which can reach several hundred MPa. These cracks come into existence if a limiting load is exceeded. Then small flaws can quickly extend to a length of more then one millimetre, and then they stop again (pop in behaviour). Popped in cracks can slowly extend by cyclic fatigue up to a length where breaking out of large fragments of the rollers occurs. The critical load depends on the flow curve of the rolled materials and on the design of the rolls. For the analysed design it is exceeded when rolling superalloy wires, but it is not exceeded when rolling materials having a lower flow curve.

Abstract: A study is made of the damage resistance and strength degradation of nitrided pressureless sintered (NPS) silicon nitride ceramics. The silicon nitride is prepared by cost-effective NPS process combining by nitridation and consecutive pressureless sintering. Contact testing with spherical indenters is used to characterize the damage response. Examination of the indentation sites indicates a quasi-plastic damage modes are observed. Bend tests on specimens containing quasi-plastic contact damages reveal those materials to be not susceptible to strength degradation.

Abstract: Natural human tooth consists of multiple layered quasi-brittle biomaterials, which make dental restorations experience a complex stress state under masticatory contact loading. As such, many restorations are prone to failure and a constant effort is made to improve the mechanical characteristics of the restorative materials. Clinical observations have shown that improved strengths and fracture toughness in ceramic materials do not necessarily lead to an anticipated higher functional longevity of the restoration. While substantial experimental investigations have been carried out to identify the contact induced fracture in such multi-layer material systems, numerical modelling of this event was largely unexplored. This paper presents a new numerical method to account for micro-damage driven fracture in various multi-layered biomaterial structures. In this study, a Rankine constitutive model is adopted and the crack initiation and propagation are automatically implemented in an explicit finite element (FE) framework. The effects of indenter radius, surface curvature and thickness of layered biomaterials on the cracking patterns are investigated. The results show good agreement with the experimental studies in literature.

Abstract: Fretting is a major cause of surface damage and fretting fatigue crack initiation at the interface between contact materials subjected to small oscillatory movements. In the present paper, a multi-layered model is developed to analyze fretting fatigue of functionally graded materials (FGMs) with arbitrarily varying shear modulus under plane strain-state deformation. Based on the fact that an arbitrary curve can be approached by a series of continuous but piecewise linear curves, the FGM is divided into several sub-layers and in each sub-layers the shear modulus is assumed to be a linear function while the Poisson’s ratio is assumed to be a constant. With the model, the problem of fretting contact of two similar functionally graded coated cylinders is investigated. By using the transfer matrix method and Fourier integral transform technique, the problem is reduced to two uncoupled Cauchy singular integral equations. The tangential contact pressures in the slip and stick zones are calculated by solving the equations numerically. The results show that appropriate gradual variation of the shear modulus can significantly alter the pressures in the contact zone. This may lead to suppression of fretting fatigue cracks at the edges of the contact zone and thus modify the fretting contact damage.

Abstract: 4H SiC high power photoconductive semiconductor switching devices were fabricated. A highly doped n+-GaN subcontact epilayer was grown on SiC by organometallic vapor phase epitaxy in order to improve ohmic contact and avoid contact damage or degradation due to current filamentation, under high power operation. With an n+-GaN subcontact layer, the contact resistance was reduced and current crowding alleviated. Therefore the electrodes were not damaged or degraded at high power operation. Photocurrent up to 200 A and breakdown voltage up to 2900 V have been observed for the devices.

Abstract: The effects of thermal fatigue conditions on the mechanical and contact damage behavior in thermal barrier coatings (TBCs) are investigated as functions of the bond coat thickness and the preparation method. Three kinds of TBCs with different thickness in the bond coat are prepared by two different methods of APS and HVOF. The static and cyclic thermal fatigues for the TBCs are conducted at temperatures of 950
and 1100
with different dwell times of 10 and 100 hr and 10 cycles at each temperature. Mechanical properties, hardness H and modulus E, in each condition and component are measured by nano-indentation. The contact damage behaviors are investigated by Hertzian indentation, including the cyclic fatigue behavior on the surface of the TBCs. The TGO formation is dependent on both temperature tested and time exposed, showing a similar effect with the cyclic thermal fatigues. The mechanical properties of the TBCs are increased due to the re-sintering of the top coating and the composition change of the bond coat during the thermal fatigues. The contact damage behaviors are affected by the thermal fatigue conditions and the fabrication method, independent of the bond coat thickness.