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Abstract Theoretical investigations on the ZrxTa1-xO1+xN1-x system predict that some of its phases are likely to possess relatively high hardness values.(1) Such materials may be suitable for industrial application as cutting tools. The motive of the project was to investigate the best synthesis route and a method for obtaining well sintered, dense oxynitride phases with a nominal composition TaON (x=0), Ta0.8Zr0.2O1.2N0.8 (x=0.2) and Ta0.3Zr0.7O1.7N0.3 (x=0.7). This was achieved through three main steps, i.e. synthesis of the oxynitride powders, high pressure sintering and evaluation of mechanical properties. A sol gel method was used to obtain the precursor oxide powders. TaCl5 and 70wt% zirconium propoxide were used as the starting materials. Oxide gels were formed by dissolving precursor materials in absolute ethanol for 15minutes with continuous stirring, followed by subsequent hydrolysis to form gels which were aged for 24hrs at 800C. The gels were dried in air at 1000C for 12hrs in a drying oven followed by calcinations in a muffle furnace at 6000C for 6hrs to remove the alkyls and chloride ions. High surface area amorphous powders were obtained (~6.60 ± 0.02 m2/g in the case of Ta2O5) after milling with 4mm steel balls for 4hrs in a planetary mill. The respective oxynitrides were obtained by thermal nitridation using an ammonia (99.99%) flow method. A temperature of 9000C maintained for 4hrs in the presence of water vapour at an ammonia flow rate of 50cm3/min were found to be the optimum nitridation conditions. The water vapour pressure was realised by bubbling the ammonia through a water bath at room temperature prior to supply to the furnace. The water vapour pressure of such a set up was approximated to be ~3.1*103Pa. This nitridation process was carried out in a tube furnace using a silica tube of length 1200mm and external diameter of 40mm and an alumina boat as the holding vessel. Approximately 2g of oxide powder were used for each run. The dependency of nitridation on temperature and ammonia flow rate iii was investigated for the formation of TaON. Pure TaON formation was found to be more favoured by temperatures of 9000C with a heating rate of 200C/min and by an ammonia flow rate range of 40-50cm3/min. These conditions were also used for the mixed Ta-Zr oxynitrides. Ta0.3Zr0.7O1.7N0.3 formation was found to be dependent on the heating rate with ZrO2 forming beside the oxynitride solid solution above a heating rate of 100C/min. In the present work the phenomenon has been found to be dependent on the kinetics of the crystallisation reactions. At higher heating rates crystallisation of the separate phases is favoured leading to the formation of separate phases. On the other hand with an optimum heating rate the solid solution is maintained to the final nitridation temperature. The powders were found to be thermally stable in air above 6000C with TaON being the most stable with a weight change occurring at a temperature of ~6900C. The powders were stable in pure nitrogen well above 10000C. Sintering in a hot press in the temperature range of 900-14000C at a heating rate of 500C/min and a pressure range of 50-85MPa using previously heat treated h-BN crucibles in argon resulted in porous, partially densified materials. A maximum % theoretical density of 81.6% was obtained for TaON at 10000C and 85MPa pressure applied for 1hr. TaON oxidised to Ta2O5 above 10000C with an oxide phase transition being observed above 13000C. High pressure sintering was carried out in the temperature and pressure regime of 920- 12000C and 3-5.5GPa respectively in the case of TaON. The mixed Ta-Zr oxynitrides were sintered at 3GPa at a temperature of 11000C. No phase transitions were observed in all cases. An average hardness value of ~16.8GPa and fracture toughness of ~3.4MPam1/2 were obtained for the TaON phase. Ta0.3Zr0.7O1.7N0.3 and Ta0.8Zr0.2O1.2N0.8 were found to possess hardness values of 13.4GPa and 13.02GPa respectively under the same sintering conditions. It was observed that the hardness values obtained for TaON are higher than those for ZrO2 or HfO2 ceramics, due to the stronger covalent bonding in nitrogen present in TaON. On the other hand the fracture toughness values are as low as those of fully stabilised ZrO2 materials due to lack of phase transformation toughening.

In the past, hardness measurements on nanocrystalline metals were limited to Vickers micro-hardness and nano-indentation tests, mainly due to sample size/thickness limitations. On the other hand, most industries require hardness values on the Rockwell scale and make extensive use of hardness conversion relationships for various hardness scales. However, hardness conversions currently do not exist for nanocrystalline metals. With recent advances in electrodeposition technology, thicker specimens with a wide range of grain sizes can now be produced. In this study, the relationships between Vickers and Rockwell hardness scales have been developed for such materials. In addition, hardness indentations were used to gain further insight into the work hardening of nanocrystalline and polycrystalline nickel. Vickers microhardness and nano-indentation profiles below large Rockwell indentations showed that polycrystalline nickel exhibited considerable strain hardening, as expected. On the other hand, for nanocrystalline nickel the micro-Vickers and nano-indentations hardness profile showed low strain hardening capacity.

In the past, hardness measurements on nanocrystalline metals were limited to Vickers micro-hardness and nano-indentation tests, mainly due to sample size/thickness limitations. On the other hand, most industries require hardness values on the Rockwell scale and make extensive use of hardness conversion relationships for various hardness scales. However, hardness conversions currently do not exist for nanocrystalline metals. With recent advances in electrodeposition technology, thicker specimens with a wide range of grain sizes can now be produced. In this study, the relationships between Vickers and Rockwell hardness scales have been developed for such materials. In addition, hardness indentations were used to gain further insight into the work hardening of nanocrystalline and polycrystalline nickel. Vickers microhardness and nano-indentation profiles below large Rockwell indentations showed that polycrystalline nickel exhibited considerable strain hardening, as expected. On the other hand, for nanocrystalline nickel the micro-Vickers and nano-indentations hardness profile showed low strain hardening capacity.

Proefschrift--Technische Hogeschool, Deltf. 1951. / "Stellingen": [3] p. inserted. eContent provider-neutral record in process. Description based on print version record. Bibliographical footnotes.

Research into determining what intrinsic characteristics cause materials to be hard is imperative if one would like to design future materials with a hardness exceeding that of diamond. Measuring the hardness of materials in order to obtain a fundamental quantity independent of extrinsic factors is difficult, if not impossible. However, many theories have been proposed pertaining to the quantification of hardness as a fundamental property. While it is clear that the hardness of a material will strongly depend on its crystal structure, another fundamental quantity, the electronic band gap, has also been linked to the intrinsic hardness of materials. The electronic band gap is a seemingly simple quantity, but is difficult to de- termine for novel or complicated materials. Core-level spectroscopy techniques that probe the occupied and unoccupied density of states separately allow for an indirect determination of the electronic band gap. These methods have several advantages over conventional tech- niques in that they do not strongly depend on the extrinsic material properties such as defects and impurities. The electronic band gap has been determined in this way for several novel materials. These include group 14 nitrides with spinel structure that were recently studied over the last decade. The electronic band gap of three synthesized binary spinel nitrides γ-Si3N4, γ-Ge3N4 and γ-Sn3N4 are determined using core-level spectroscopy to be 4.8 ± 0.2 eV, 3.5 ± 0.2 and 1.6 ± 0.2 eV, respectively. These measurements agree with the calculated values of 4.97, 3.59 and 1.61 eV for γ-Si3N4, γ-Ge3N4 and γ-Sn3N4, respectively. We have also extended these measurements and calculations to include the solid solutions γ-(GexSi1−x)3N4 and γ-(SnxGe1−x)3N4 showing these spinel-structured nitrides form a multi-functional class of semiconductors. This band gap measurement technique has also been applied successfully to the phosphor converting light emitting diode material Ba3Si6O12N2 and the novel semicon- ductor MnNCN. This shows that using core-level spectroscopy is a very effective method to determine the electronic band gap where there are no other feasible techniques. Aside from the electronic band gap, core-level spectroscopy is also a complementary technique to deter- mine the crystal structure, which is also an important parameter with regard to hardness. The crystal structure, particularly aspects such as anion ordering and vacancy ordering, have been determined for the spinel-structured oxonitride Ga3O3N and a novel phase of calcium nitride Ca3N2. These results show that core-level spectroscopy is a powerful technique to determine the anion ordering in oxonitrides and was further applied to the material class β-sialons, allowing for the determination of the electronic band gap as well as ascertaining both the anion and cation ordering. Combining all of these aspects we show that the electronic band gap is not only useful for predicting the hardness of materials, but in some cases can be used to predict the existence of certain materials. We use theoretical methods, combined with experimental measurements, to calculate the hardness and electronic band gap of all possible binary and ternary group 14 spinel-structured nitrides. Through the correlation of the hardness and electronic band gap we show that only the three already synthesized binary group 14 spinel-structured nitrides are stable along with their solid solutions and that the elusive spinel-structured carbon nitride γ-C3N4 will be never synthesized.

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