Presence Of High Compressive Stresses Research Articles

Nanomechanical properties and fracture toughness of hard ceramic layer produced by gas boriding of Inconel 600 alloy

Abstract Gas-boriding in N2−H2−BCl3 atmosphere resulted in the formation of a thick layer on Inconel 600 alloy. The microstructure of layer produced at 920 °C for 2 h consisted of a mixture of chromium borides and nickel borides. The objective of investigations was to determine the influence of the chemical and phase compositions of borided layer on its mechanical properties. The nanoindentation was carried out using Berkovich diamond tip under a load of 50 mN. The gas-borided layer was characterized by high indentation hardness HIT from 1542.6 HV to 2228.7 HV and high elastic modulus EIT from 226.9 to 296.4 GPa. It was found that the mixture with higher percentage of chromium borides was the reason for the increase in HIT and EIT values. The fracture toughness (KC) was measured using Vickers microindentation technique under a load of 0.98 N. The presence of high compressive stresses in normal direction to the top surface caused the strong anisotropy of the borided layer, in respect of fracture toughness. The high difference between the lowest (0.5763 MPa·m1/2) and the highest (4.5794 MPa·m1/2) fracture toughness was obtained. This situation was caused by the differences in chemical and phase compositions of tested areas, presence of porosity and residual stresses. Generally, the higher KC values were obtained in areas with lower chromium content.

Structural and phase transformations in Hadfield steel upon frictional loading in liquid nitrogen

Structural transformations that occur in 110G13 steel (Hadfield) upon sliding friction in liquid nitrogen (–196°С) have been investigated by metallographic, electron-microscopic, and X-ray diffraction methods. The frictional action was performed through the reciprocating sliding of a cylindrical indenter of quenched 110G13 steel over a plate of the studied steel. A like friction pair was immersed into a bath with liquid nitrogen. It has been shown that the Hadfield steel quenched from 1100°С under the given temperature conditions of frictional loading retains the austenitic structure completely. The frictional action forms in a surface layer up to 10 μm thick the nanocrystalline structure with austenite grains 10–50 nm in size and a hardness 6 GPa. Upon subsequent low-temperature friction, the tempering of steel at 400°С (3 h) and at 600°С (5 min and 5 h) brings about the formation of a large amount (tens of vol %) of e (hcp) martensite in steel. The formation of this phase under friction is supposedly a consequence of the reduction in the stacking fault energy of Hadfield steel, which is achieved due to the combined action of the following factors: low-temperature cooling, a decrease in the carbon content in the austenite upon tempering, and the presence of high compressive stresses in the friction-contact zone.

Microstructural and Mechanical Properties of TiAlN and Ti<sub>3</sub>AlN Films Deposited by Reactive Magnetron Sputtering

Two series of TiAlN and Ti3AlN films have been deposited on Si (100) substrates by reactive magnetron sputtering TiAl and Ti3Al targets in Ar/N2 mixture. The effects of incoming ions energies controlled by Vb on the microstructure, morphologies, residual stress and hardness have been explored by XRD, SEM, AFM, surface profiler and nanoindentation. The results showed that single phase cubic Ti-Al-N solid solubility formed by Al atoms replacing some Ti atoms in the cubic TiN lattice occured in both TiAlN and Ti3AlN films. As substrate bias increased, the preferred orientation firstly changed from (111) to (200), and then returned to (111) at higher substrate bias. At the same time, high substrate bias promoted the densification of films and presence of high compressive stress, which is benefit for improvement of hardness.

A combinatorial X-ray sub-micron diffraction study of microstructure, residual stress and phase stability in TiAlN coatings

The understanding of the relationship between structure and properties of thin coatings is one of the key requirements for their further improvement. Especially for metastable TiAlN, commonly used as a protective coating for cutting applications, detailed knowledge about the apparent microstructure, the resulting residual stresses and the accompanied decomposition behavior at elevated temperatures are of vital importance, as the subsequent high-temperature formation of the thermally stable wurtzite phase results in deteriorated mechanical properties. In this study, the recently introduced cross-sectional scanning X-ray diffraction method is applied as a combinatorial tool to investigate the depth evolution of the local microstructure and residual stresses and their effect on phase stability of a TiAlN coating synthesized by cathodic arc evaporation at stepwise increased bias voltages. The decomposition behavior of the coating during annealing is obviously given by the initial microstructure and stress state, which control the local stress relaxation as well as the formation of precipitates. Thus, the phase transformation is favored in the coating regions that developed under moderate ion-irradiation conditions, characterized by low stress and well-developed columnar grains, and retarded under the presence of high compressive stress. The observed non-linear dependency of phase decomposition on the initial microstructure and stress is interpreted by ab initio calculations.

Crystallographic Effects on Microscale Machining of Polycrystalline Brittle Materials

This paper studies the effects of crystallography on the microscale machining characteristics of polycrystalline brittle materials on a quantitative basis. It is believed that during micromachining of brittle materials, plastic deformation can occur at the tool-workpiece interface due to the presence of high compressive stresses which leads to chip formation as opposed to crack propagation. The process parameters for such a machining process are comparable to the size of the grains, and hence crystallography assumes importance. The crystallographic effects include grain size, grain boundaries (GB), and crystallographic orientation (CO) for polycrystalline materials. The size of grains (crystals), whose distribution is analyzed as a log-normal curve, has an effect on the yield stress of a material as described by the Hall–Petch equation. The effects of grain boundary and orientation have been considered using the principles of dislocation theory. The microstructural anisotropy in a deformed polycrystalline material is influenced by geometrically necessary boundaries (GNB) and incidental dislocation boundaries (IDB). The dislocation theory takes both types of dislocations into account and relates the material flow stress to the dislocation density. The proposed analysis is compared with previously reported experimental data on polycrystalline germanium (p-Ge). This paper aims to provide a deeper physical insight into the microstructural aspects of polycrystalline brittle materials during precision microscale machining.

Early detection and monitoring of fatigue in high strength steels with MWM-Arrays

Fatigue monitoring of cyclically loaded shot peened high-strength steel components can be accomplished via magnetic permeability measurements during laboratory tests or in service. These measurements can be performed either continuously using permanently mounted Meandering Winding Magnetometer Arrays (MWM ®-Arrays) or intermittently with scanning MWM-Arrays. The results obtained to date suggest that MWM-Array permeability measurements can provide early detection of fatigue damage in steels before conventional methods can detect any changes. This has been demonstrated to be particularly significant in the presence of high compressive stresses introduced by shot peening. One of the fatigue tests was suspended when accelerating changes in local permeability were detected. Examination of the fatigue specimen in a scanning electron microscope detected only a few relatively small cracks, e.g. 50–200 μm long at the surface. Fractography, however, revealed significantly longer cracks. For the same specimen, conventional eddy current and ultrasonic testing failed to provide any indications of cracks, and fluorescent liquid penetrant detected only an inconclusive spot indication. This paper provides a comparison of the permeability changes and fractography data with a fatigue crack growth curve based on a FASTRAN analysis accounting for residual stresses from shot peening. A comparison of the experimental data and crack growth analysis results suggested that MWM-Array magnetic permeability measurements may detect cracks in the compressive stress field when they are about 50 μm deep.

Diamond-like carbon films with extremely low stress

We in this paper report different ways to realise thick diamond-like carbon (DLC) films with stress values lower than 0.5 GPa, Thick DLC films grown by conventional r.f. self bias technique often delaminate from the substrates due to the presence of high compressive stresses of the order of 4–7 GPa. We have made an in-depth study of the delamination problem of DLC films at NPL and found that only for substrates kept away from the plasma (plume) it is possible to grow thick DLC films. This goes to show the heating of the substrates, when m contact with the plasma, appears to be one of the most important factors giving rise to the high stress values. Techniques that have produced consistently low stress values (0.2–0.5 GPa) in this laboratory are pulse plasma PECVD and the one using dc saddle field fast atom beam source. Electronic properties of the materials so produced have been estimated by evaluating Urbach energy using photothermal deflection spectroscopy (PDS) measurements. A correlation between the unbound hydrogen in these films, as measured by a nuclear technique (ERDA), and the stress levels has been found. Deposition rate, room temperature conductivity, optical bandgap and refractive index have also been measured for these films.

Modelling of silicon oxidation based on stress relaxation

Abstract This paper deals with the modelling of thermal oxidation of silicon based on stress relaxation by viscous flow. An analysis of the anomalously high rate of dry oxidation occurring at low thickness is given. This initial regime is attributed to a reduced diffusivity in the vicinity of the Si-SiO2, interface due to the presence of high compressive stress. A model of oxidation based on stress-dependent diffusivity, namely the stress-state model, is then described. The model makes it possible to obtain the oxidation kinetics over a wide range of temperature. Moreover, the influence of stress both on the parabolic and linear kinetic constants is discussed. Finally, the effects of intrinsic oxidation stress on the structural properties of thermal oxides are also described. In particular, the role of growth temperature on densification phenomena is emphasized.