Depth-Sensing Indentation on REBa2Cu3O(7-\delta) Single Crystals obtained from Xenotime Mineral

A natural mixture of heavy rare earths oxides extracted from xenotime mineral have been used to prepare large single crystals of high-temperature REBa2Cu3O(7-\delta) superconductor grown using the CuO-BaO self-flux method. Its mechanical properties along the ab-plane were characterized using instrumented indentation. Hardness and elastic modulus were obtained by the Oliver and Pharr method and corresponds to 7.4 \pm 0.2 GPa and in range 135-175 GPa at small depths, respectively. Increasing the load promotes the nucleation of lateral cracks that causes a decrease in hardness and the measured elastic modulus by instrumented indentation at higher loads. The indentation fracture toughness was estimated by measuring the radial crack length from cube-corner indentations at various loads and was 0.8 \pm 0.2 MPa.m1/2. The observed slip systems of REBa2Cu3O(7-\delta) single crystals were [100](001) and [010](001), the same as for YBa2Cu3O(7-\delta) single crystals. The initial stages of deformation and fracture in the indentation process were investigated. The hardness and elastic modulus were not strongly modified by the crystallographic orientation in the ab-plane. This was interpreted in terms of the resolved shear stresses in the active slip systems. Evidence of cracking along the {100} and {110} planes on the ab-plane was observed. As a conclusion, the mechanical properties of REBa2Cu3O(7-\delta) single crystals prepared from xenotime are equivalent to those of YBa2Cu3O(7-\delta) single crystals produced by conventional rare earths oxides.Comment: The paper will appear in Volume 42 (2012) of the Brazilian Journal of Physic

Estudo de propriedades mecânicas de cristais de Lif submetidos a implantaçao iônica e radiaçoes (X-Gama)

Orientador: Vicente Roberto DumkeDissertaçao (mestrado) - Universidade Federal do ParanáResumo: O presente trabalho teve por objetivo a análise do comportamento plástico de amostras de LiF submetidas à implantação iônica e radiações (Raios-X e Raios-Gama). O estudo foi realizado através da associação entre as técnicas de dureza Vickers modificada (Hv*, fotoelasticidade e ataque químico. Esta combinação revelou-se muito útil em função da maior quantidade de informações obtidas, principalmente, quando o uso de uma única técnica é de certo modo limitada nas medidas de variação da plasticidade de amostras cristalinas. Desta combinação, foi possível obter-se a relação gama= l/d onde l é a medida do alcance do padrão de distribuição das discordâncias e d é a diagonal de impressão, que é muito mais ampla nas informações sobre o comportamento plástico das amostras estudadas. A partir dos resultados obtidos, verifica-se que existe uma alteração bastante significativa na plasticidade das amostras implantadas e das irradiadas. Esta modificação no comportamento plástico produz na superfície das amostras de LiF implantadas, uma fragilização de caráter anisotrópico. Entretanto , nas irradiadas esta não foi verificada nas doses usadas. Nas amostras implantadas com N+2 é verificada a validade da relação l alfaP0,5 dentro dos limites de dose usadas(1x10^13 íons/cm2 a 1x10^17 íons/cm2 ). Podemos ainda relacionar nas mesmas a mobilidade das discordâncias com a dose recebida, através da relação ln D alfa l. Para as amostras irradiadas também foi possível verificar a validade da segunda relação.Abstract: The present works objective is the analysis of plastic behaviour of LiF samples subject to ionic implantation, X and gama irradiation. Modified Vickers hardness (Hv*p), photoelasticity and chemical etching tecniques were employed to develop this study. This combination, itself, reveals to be useful due to the larger amount of information obtained, mainly because when only one technique is used, the measurement of variations in plastic behaviour of crystal samples is limited. From this combinations, it's possible to attaim the gama = l/d relation, that offers more extensive informations about the plasticity of the studies samples, where l. is the measure of the dislocation range and d is the measure of the piramidal edge. This work results a significant change in the implanted samples plasticity and this also happens in the irradiated samples. These modifications in the plastic behaviour render the samples fragile, although anisotropic, only for LiF. This does not happen in the irradiated samples in the used doses. In the samples implanted from 10^13 ions/cm2 to 10^17 ions/cm2 of N2+ the validity of the l alfa P^0,5 relation is verified and a new relation of ln D alfa l is established

Nanomechanical properties of rough surfaces

The nanoindentation technique allows the determination of mechanical properties at nanometric scale. Hardness (H) and elastic modulus (E) profiles are usually determined by using the Oliver-Pharr method from the load/unload curves. This approach is valid only for flat surfaces, or at least, when a very low degree of asperity is present (lower than 30 nm). The basic statement is the determination of the zero tip-surface contact point. If a rough surface is present, errors can occur in determining this contact point and, as a consequence, the surface hardness and elastic modulus profiles are drastically altered resulting in under evaluated values. Surfaces with different roughness were produced by controlled nitrogen glow discharge process on titanium. The changed nitriding parameters were different N2/H2 atmospheres and temperatures (600 °C-900 °C). The most correct H and E profiles were obtained by using the contact stiffness analysis method, proposed here, that overcomes the surface roughness. The obtained results were compared with available literature data

A comparative study of mechanical and tribological properties of AISI-304 and AISI-316 submitted to glow discharge nitriding

Mechanical and tribological properties os AISI 304 and AISI 316 stainless steels submited to glow discharge ion nitriding are reported.The atmosphere was 20:80 - N2:H2 with substrate temperatures ranging from 300 to 500 °C. Treatment at 300 °C produced expanded austenite (γN) in both steels. Increasing the temperature, the phases γ´-Fe4N and - Fe2+xN were present and the latter is the major phase for AISI 304. At 500 °C, the CrN phase was also identified in both steels. Hardnesses of about 13-14 GPa at near surface regions were obtained in both steels. Moreover, AISI 316 nitrided at 500 °C has the deepest hard layer. Tribological tests showed that wear can be reduced by up to a factor of six after the nitriding processes, even for a working temperature of 300 °C. The profiles during and after nanoscratch tests did not reveal significant differences after nitriding processes in both steels