H. Oettel

Cyclic deformation and near surface microstructures of shot peened or deep rolled austenitic stainless steel AISI 304

Cylindrical specimens of the austenitic stainless steel AISI 304 were shot peened or deep rolled with different peening intensities, and rolling pressures, respectively. The resulting near surface properties were characterized by cross sectioning transmission electron microscopy (TEM), residual stress and phase analysis as well as interference line half-width and microhardness measurements. Cyclic deformation curves were obtained by hysteresis measurements under stress control with zero mean stress. The microstructural alterations in the fatigued surface regions were again characterized by the above mentioned methods. The investigations revealed that both shot peening and deep rolling lead to a complex near surface microstructure, consisting of nanocrystalline regions, deformation bands and strain induced martensitic twin lamellae with high dislocation densities in the austenitic matrix. These microstructural changes severely influence the cyclic deformation behaviour: Plastic strain amplitudes and cyclic creep were drastically decreased by shot peening and especially by deep rolling. Both surface finishing methods were found to decrease crack initiation and propagation rate. Remarkably, the initial residual stress profile and surface strain hardening were not completely eliminated even by applying high cyclic stress amplitudes. This is due to the fact that during cyclic loading dislocation cell structures were only formed in greater depths whereas the nanocrystalline layer remained stable. In the case of deep rolled surfaces, the martensitic layer was even increased by fatigue-induced martensite formation.

Residual stresses in PVD hard coatings

Abstract High macroscopic residual stresses in PVD hard coatings up to about −10 GPa are caused by both the thermoelastic effects and grown-in defects, generated by fast particles during deposition. Defect recovery, relaxation by plastic deformation or cracking limit the residual stresses. The mechanical behaviour of coated materials can be explained by the combination of the residual stresses and the exterior stresses (bending, hardness, critical load, erosion, abrasive wear). Owing to the lack of generalized knowledge about these relations a prediction of the performance of tools or components has to be sketchy.

Structure and mechanical properties of amorphous Ti-B-N coatings

Abstract In this investigation Ti–B–N-coatings were prepared on high speed steel and silicon substrates by reactive magnetron sputtering from a TiB 2 target with a stepwise increase of the nitrogen flow rate. Some samples were vacuum-annealed up to 600°C. All Ti–B–N layers had a dense structureless morphology, whereas the pure TiB 2 coating was crystalline and had a strong [001] fibre texture. The composition of the films was characterized by a nearly constant B/Ti ratio of 2.6 and a variable N/Ti ratio from 0 to 6.3. Consequently, the samples were situated in two different ternary phase regions. With the increasing nitrogen flow rate the measured compressive residual stresses and the hardness of the Ti–B–N coatings decreased up to 50% in comparison with the TiB 2 , whereas the critical load of the adhesive failure was not influenced. It could be detected that the amorphous state and the chemical composition were stable during annealing up to 600°C. The compressive residual stresses had been reduced by thermally activated relaxation processes. The TiB 2 coatings got tensile stresses, whereas in the Ti–B–N films the compressive stresses were still present. Probably, ordering occurs in the amorphous coatings, because their hardness increases by up to 40%. The results of scratch tests were not influenced by annealing.

Cyclic deformation and near surface microstructures of normalized shot peened steel SAE 1045

Cylindrical specimens of the normalized plain carbon steel SAE 1045 were shot peened and cyclically deformed under stress control. A special cross-sectioning technique was applied to prepare near surface regions for transmission electron microscopy. The resulting properties of these regions were characterized by X-ray diffraction and microhardness measurements. The cyclic deformation curves are affected characteristically by dislocation arrangements induced by shot peening. The stability of these dislocation arrangements and the depth profile obtained by residual stress and interference line half-width value measurements depend strongly on the stress amplitude. High stress amplitudes lead to the formation of dislocation cell structures in the shot peened layers, accompanied by the total disappearance of compressive stresses and the decrease of interference line half-width values down to the initial level. Thus, a clear correlation between stress relaxation, cyclic deformation behaviour and microstructural alterations in shot peened regions is revealed. Experiments carried out on shot peened hollow specimens indicate that the cyclic deformation behaviour is not merely a function of dislocation arrangement and density but also of the thickness of strain hardened near surface material in relation to the diameter of the specimen.

Mechanical Properties of TiN Coatings

It is difficult to measure the mechanical properties of physical vapor deposition coatings owing to their small layer thickness. This report c describes a special four-point bending device that makes it possible to determine quantitative critical tensile or compressive failure stresses and shear stresses in thin coatings. The bending test was also used to carry out measurements of hardness and critical loads (scratch test) under stress, excluding other influences such as microstructure changes. The material parameters determined form the basis of a finite element model of the indentation process, and this offers new possibilities for mechanical characterization of coated materials.

Microstructural gradients in thin hard coatings—tailor-made

Microstructural modifications resulting from time dependent variations of the bias voltage during the deposition of thin hard coatings are discussed. TiN-coatings are produced by reactive magnetron sputtering in several modes: (a) stepwise increase of the bias voltage during the deposition, (b) alternating sputtering with and without substrate voltage and (c) pulsed bias voltage. On the basis of X-ray diffraction measurements, it is demonstrated that residual stress gradients and texture gradients can be designed tailor-made. Furthermore, results of microhardness measurements and scratch tests indicate an improvement of the mechanical properties, especially for the application of a pulsed bias voltage.

Influence of the material composition on the nitriding result of steels by plasma immersion ion implantation

Abstract The influence of different alloying elements and their concentration on the nitriding behaviour of steel by plasma immersion ion implantation (PI 3 ) was examined. Treatments were performed at 300°C using different high voltage (HV) repetition rates of 100, 200 and 300 Hz. The steels were alloyed either with 0.09, 0.45 and 0.9 wt.% C, 0.11, 0.3 and 0.93 wt.% Al or 0.93, 3.6 and 13.2 wt.% Cr. The nitride formation for the various alloys and parameters was studied using X-ray diffraction (XRD) and metallographic cross-sections in scanning electron microscopy (SEM), showed a nitride formation that is strongly influenced by the material composition. Additional information is obtained by glow discharge optical emission spectroscopy (GDOS) depth profiles and instrumented microhardness measurements. The effect of the HV repetition rate is not very clear, as on the one hand the implanted dose is increased, but on the other nitride formation seemed to be disturbed by higher repetition rates. The results will be discussed under the aspect of improving the nitriding efficiency of PI 3 .

Oxide dispersion-strengthened silver: manufacturing and properties

Novel oxide dispersion-strengthened (ODS) silver alloys were synthesised by mechanical alloying and subsequent consolidation via hot pressing in protective atmosphere. The effect of dispersoid size and volume fraction on the electrical and mechanical properties is exemplified with Cr 2 O 3 , CaO and Y 2 O 3 as model candidates for a systematic study of the principles of particle selection. The results for the development of the microstructure under different processing conditions show that control of the impurity level is of primary importance for achieving high-quality, fully dense material. Hence, a modified cryo-milling technique at liquid-N 2 temperature was applied. Data at ambient temperature are presented, revealing that the mechanical and electrical properties can be tailored within a wide range as a function of the microstructure. These results are explained by microstructural concepts for room temperature yielding and electrical conductivity.

Thermal stability of PI3 nitrided surface layers on ferritic steels

Abstract Ferritic alloys with different contents of carbon, aluminum and chromium are nitrided by plasma immersion ion implantation at 300 °C for 3 h. The thermal stability of the nitrided surface layers is evaluated by annealing the specimens in vacuum and in air both at 400 °C. The composition and the structure of the surface layers is studied by means of GDOES-spectroscopy and X-ray diffraction. In comparison to the nitrided reference specimens the nitrogen concentration decreases after heat treatment at 400 °C in air as well as under vacuum condition. The possibly main reasons are outward (degassing) and inward diffusion of nitrogen at elevated temperature. The e-nitride layer of a Fe–Cr alloy with high chromium content transforms to a stable Fe–Cr–N solid solution.

The effect of HV in the nitriding of ferritic steels by plasma immersion ion implantation

Abstract Nitride formation was studied in various steels with different alloying elements. Results of PI 3 nitriding, r.f. nitriding with self bias and DC voltage at 300 and 400°C were compared. It was found that nitriding of ferritic steels depends strongly on the formation of surface nitride layers. The habit of the nitrides and the supersaturation of nitrogen is strongly influenced by the HV pulsing during PI 3 and thus also the treatment depth and layer properties.