Amostras de ferros fundidos nodulares ligados com Cu, Cu-Ni ou Cu-Ni-Mo foram austemperadas, boretadas e boroaustemperadas e caracterizadas quanto à dureza e o comportamento ao desgaste microadesivo, tendo sido também estudada a cinética de formação da camada. O método de boretação utilizado foi via líquida em banho de bórax fundido, com tempos de permanência de 2 e 4 horas nas temperaturas de 850, 900 e 950ºC. Procedeu-se o tratamento direto de austêmpera, a partir dessa temperatura, em banhos de sal fundidos nas temperaturas de 240, 300 e 360ºC com tempos de permanência de 4 horas (boroaustêmpera). Realizou-se também, para fins de comparação, tratamento convencional de austêmpera. Microscopias óptica e eletrônica de varredura, EDS por raios-X, testes de dureza Brinell (substrato) e Vickers (revestimento) foram realizados, bem como ensaios de desgaste microadesivo do tipo esfera presa. A boretação resultou na formação de camadas de elevadas durezas, na faixa de 1300 a 1700 HV, e elevadas resistências ao desgaste. As resistências ao desgaste das amostras boretadas ou boroaustemperadas foram aumentadas em até 40x em relação às amostras brutas de fundição ou apenas austemperadas, indicando a grande eficácia do tratamento neste tipo de propriedade. / Samples of ductile cast iron alloyed with Cu, Cu-Ni or Cu-Ni-Mo were austempered, borided and boroaustempered and afterwards characterized for hardness and micro-adhesive wear behavior. The kinetics of layer formation were also studied. The boriding method used was liquid molten borax bath, in periods of 2 and 4 hours at temperatures of 850, 900 and 950°C. The direct austempering treatment was performed from the borax bath temperature using molten salt baths at temperatures of 240, 300 and 360°C for 4 hours (boroaustempered). For comparative purposes, the conventional austempering treatment was also conducted. Optical microscopy, scanning electron microscopy, EDX, Brinell hardness measurements (substrate) and Vickers (coating) were performed, as were the tests for micro-adhesive wear. The boriding treatment resulted in the formation of layers with high hardness, in the range of 1300 to 1700 HV and high wear resistance. The wear resistance of borided or boroaustempered samples were increased by 40 times when compared to cast irons or austempered samples, indicating the high efficiency of this type of treatment in increasing the wear resistance of this material.

Intermetallic borides are characterized by a great variety of crystal structures and bonding interactions, however, a comprehensive rationalisation of the electronic structure is missing. A more general interpretation will be targeted towards comparing several boride phases of one particular transition metal on one hand side, but also isostructural borides of various metals at the other side. Finally, a concise model should result from a detailed analysis of excellent data both from experimental charge density analysis and quantum chemical methods. Ultimate target is a transferability model based on typical building blocks. Experimental investigations of the electron density derived from diffraction data are very rare for intermetallic compounds. One of the main reasons is that the suitability of such compounds for charge density analysis is estimated to be relatively low as compared to organic compounds. In the present work, X-ray single crystal diffraction measurements up to high resolution were carried out for TiB2, VB2, V3B4, and VB crystals. The respective experimental electron densities were reconstructed using the multipole model introduced by Hansen and Coppens [1]. The topological aspects of the experimental electron density were analysed on the basis of the multipole parameters using Bader’s Quantum Theory, Atoms in Molecules [2] and compared with theoretical calculations. References [1] Hansen, N.K.; Coppens, P. Acta Crystallogr. 1978, A34, 909 [2] Bader, R.F.W. Atoms in Molecules─A Quantum Theory; Oxford University Press: Oxford, 1990

No description available.

This thesis present research on synthesis, microstructure, and properties of carbide- and boride- based thin films. The films have been synthesized by dc magnetron sputtering, and their microstructures have been characterized mainly by X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and transmission electron microscopy.  One of the main objectives with this research has been to evaluate the thin films potential as materials for sliding electrical contact applications and this have influenced, which properties that have been evaluated. Co-sputtered Nb-C films have a microstructure comprising of nanocrystalline NbCx  (nc-NbCx) grains embedded in a matrix of amorphous C (a-C). A thinner a-C matrix form in the Nb-C films compared to the well-studied Ti-C system. As a consequence, the Nb-C films have a higher hardness and conductivity than previously studied Ti-C sputtered under similar conditions. The promising electrical contact properties are attained for reactively sputtered Nb-C films under industrial conditions, at deposition rates two orders of magnitude higher. A reduction in crystallinity is seen when Si is added to the Nb-C films and amorphous films forms at Si content > 25 at.%. The alloying of Si was however not beneficial for the electrical contact properties. Substoichiometric CrB2-x (B/Cr = 1.5) and NbB2-x (B/Nb = 1.8) films are achieved when deposited from MeB2 targets. Boron segregates to grain boundaries forming a B-rich tissue phase. This result in superhardness for the NbB2-x films (42 ± 4 GPa) as well as a low friction attributed to the formation of a boric acid film. Carbon forms a solid solution in the MeB2 grains as well as segregating to grain boundaries forming an amorphous BCx (a-BCx) phase when alloyed to CrB2-x and NbB2-x films. The formation of the a-BCx phase drastically improves the electrical contact resistance of the NbB2-x films. However, the mechanical properties are degraded, which result in a high friction and wear rate. It was in TEM studies of the metastable amorphous structures for the Nb-Si-C films found that the electron beam induces crystallization. Hence, great care is required when studying these types of metastable structures.

Zirconium- and tantalum-based diborides, and diboride/carbide composites are of interest for ultra-high temperature applications requiring improved thermomechanical and thermochemical stability. This thesis focuses on the synthesis, processing and sintering of nanocrystalline powders with Zr- and Ta-based diboride/carbide/silicide compositions. A solution-based processing method was developed to prepare reactive mixtures that were precursors for ZrB₂-based powders. The precursors reacted to form the ceramic powders after suitable pyrolysis and borothermal/carbothermal reduction heat treatments. Single-phase ZrB₂ powders were prepared with initial composition of C/Zr = 4.8 and B/Zr = 3.0. ZrB₂-based composite powders with ZrC, ZrO₂, TaB₂, TaC, SiC, TaSi₂ and B₄C were prepared with particle sizes of 10-500 nm for different phases based SEM micrographs. The composite powders were highly sinterable with proper processing methods developed to avoid and remove oxide impurities. The relative densities of ZrB₂/B₄C, ZrB₂/TaB₂, ZrB₂/TaB₂/B4C, ZrB₂/TaSi₂ were in the range of 91%-97% after pressureless sintering at 2020 ℃ for 1 h or 30 min.

Neste trabalho, camadas de elevadas durezas e resistentes ao desgaste, constituídas de NbC, VC ou Fe-B, foram produzidas nos aços AISI H13 e D2, por meio de tratamentos termo-reativos em banhos de bórax, com adição de Fe-Nb, Fe-V, Fe-Ti e Al. As amostras foram caracterizadas por meio de microscopias ótica e eletrônica de varredura com EDS (espectroscopia de dispersão de energia), difração de raios-X, GDOS (“Glow discharge optical spectroscopy”), microdureza Vickers, e ensaios de adesividade e desgaste em micro-escala. Para os banhos contendo Fe-Nb/Al, foram obtidas camadas de NbC nos dois aços, com espessuras médias de 6,2 mícrons, quando produzida no aço AISI H13, e 15,6 mícrons para o aço AISI D2. No caso do banho com adição de Fe-V/Al, produziram-se camadas de VC, com espessuras médias de 5,8 e 14,2 mícrons, para os aços AISI H13 e D2, respectivamente. As durezas das camadas de NbC e VC situaram-se na faixa de 2338 a 2471HV. No caso dos banhos contendo Fe-Ti e Al ou apenas Al, foram produzidas camadas de boretos, principalmente FeB e Fe2B, com espessuras variando de 30 a 130 mícrons, e durezas médias na faixa de 1583 a 1742HV. Os ensaios de adesividade evidenciaram as excelentes aderências das camadas com os substratos. Todas as camadas apresentaram resistências ao desgaste muito superiores as dos substratos, destacando-se as camadas de NbC e VC. Os ensaios de desgaste em micro-escala foram realizados sob carga elevada e sem uso de abrasivo, simulando condições de desgaste adesivo-abrasivo, uma vez que inexistem dados a respeito desse comportamento. Com o decorrer do ensaio, o modo de desgaste passou de adesivo para abrasivo, o que foi registrado pela presença de sulcos paralelos e de superfícies polidas nas calotas de desgaste. / In the present work, hard and wear resistant layers, consisting of NbC, VC or Fe-B, were produced on AISI H13 and D2 steels, by thermo-reactive treatments in borax baths, added with Fe-Nb, Fe-V, Fe-Ti and Al. Samples were characterized by optical and scanning electron microscopy, energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), Glow discharge optical spectroscopy (GDOS), Vickers microhardness, and adhesion and micro-scale wear tests. For the baths containing Fe-Nb/Al, the layers obtained on both steels consisted of NbC, with an average thickness of 6.2 micrometers, when formed on AISI H13 steel, and 15.6 micrometers for the AISI D2 steel. In the case of borax bath added with Fe-V/Al, the produced layers contained VC, which had average thicknesses of 5.8 and 14.2 micrometers, for the AISI H13 and D2 steels, respectively. The hardness of the NbC and VC layers varied from 2338 to 2471HV. In the case of the borax bath containing Fe-Ti and Al or only Al, boride layers were produced, mainly FeB and Fe2B, with depths ranging from 30 to 130 micrometers, and average hardnesses varying from 1583 to 1742HV. The obtained layers presented excellent adhesion to the substrates. All layers presented wear resistance superior to the substrates, mainly the NbC and VC layers. The micro-scale wear tests were performed under high applied loads and without addition of abrasive, thus simulating adhesive-abrasive wear conditions, as there is no existing data concerning this wear behavior. During the micro-scale wear test, the wear mode changed from adhesive wear to abrasive wear, which was verified by the presence of parallel grooves and smooth surfaces in the wear craters.

Amostras de ferros fundidos nodulares ligados com Cu, Cu-Ni ou Cu-Ni-Mo foram austemperadas, boretadas e boroaustemperadas e caracterizadas quanto à dureza e o comportamento ao desgaste microadesivo, tendo sido também estudada a cinética de formação da camada. O método de boretação utilizado foi via líquida em banho de bórax fundido, com tempos de permanência de 2 e 4 horas nas temperaturas de 850, 900 e 950ºC. Procedeu-se o tratamento direto de austêmpera, a partir dessa temperatura, em banhos de sal fundidos nas temperaturas de 240, 300 e 360ºC com tempos de permanência de 4 horas (boroaustêmpera). Realizou-se também, para fins de comparação, tratamento convencional de austêmpera. Microscopias óptica e eletrônica de varredura, EDS por raios-X, testes de dureza Brinell (substrato) e Vickers (revestimento) foram realizados, bem como ensaios de desgaste microadesivo do tipo esfera presa. A boretação resultou na formação de camadas de elevadas durezas, na faixa de 1300 a 1700 HV, e elevadas resistências ao desgaste. As resistências ao desgaste das amostras boretadas ou boroaustemperadas foram aumentadas em até 40x em relação às amostras brutas de fundição ou apenas austemperadas, indicando a grande eficácia do tratamento neste tipo de propriedade. / Samples of ductile cast iron alloyed with Cu, Cu-Ni or Cu-Ni-Mo were austempered, borided and boroaustempered and afterwards characterized for hardness and micro-adhesive wear behavior. The kinetics of layer formation were also studied. The boriding method used was liquid molten borax bath, in periods of 2 and 4 hours at temperatures of 850, 900 and 950°C. The direct austempering treatment was performed from the borax bath temperature using molten salt baths at temperatures of 240, 300 and 360°C for 4 hours (boroaustempered). For comparative purposes, the conventional austempering treatment was also conducted. Optical microscopy, scanning electron microscopy, EDX, Brinell hardness measurements (substrate) and Vickers (coating) were performed, as were the tests for micro-adhesive wear. The boriding treatment resulted in the formation of layers with high hardness, in the range of 1300 to 1700 HV and high wear resistance. The wear resistance of borided or boroaustempered samples were increased by 40 times when compared to cast irons or austempered samples, indicating the high efficiency of this type of treatment in increasing the wear resistance of this material.

No description available.

No description available.

Intermetallic borides are characterized by a great variety of crystal structures and bonding interactions, however, a comprehensive rationalisation of the electronic structure is missing. A more general interpretation will be targeted towards comparing several boride phases of one particular transition metal on one hand side, but also isostructural borides of various metals at the other side. Finally, a concise model should result from a detailed analysis of excellent data both from experimental charge density analysis and quantum chemical methods. Ultimate target is a transferability model based on typical building blocks. Experimental investigations of the electron density derived from diffraction data are very rare for intermetallic compounds. One of the main reasons is that the suitability of such compounds for charge density analysis is estimated to be relatively low as compared to organic compounds. In the present work, X-ray single crystal diffraction measurements up to high resolution were carried out for TiB2, VB2, V3B4, and VB crystals. The respective experimental electron densities were reconstructed using the multipole model introduced by Hansen and Coppens [1]. The topological aspects of the experimental electron density were analysed on the basis of the multipole parameters using Bader’s Quantum Theory, Atoms in Molecules [2] and compared with theoretical calculations. References [1] Hansen, N.K.; Coppens, P. Acta Crystallogr. 1978, A34, 909 [2] Bader, R.F.W. Atoms in Molecules─A Quantum Theory; Oxford University Press: Oxford, 1990