F.A. Almeida

High resolution study of the strong diamond/silicon nitride interface

Silicon nitride (Si3N4) is known to offer the required adhesion to chemically vapor deposited diamond coatings for demanding mechanical solicitations but the nature of their strong interface is not well know. Focused ion beam preparation preserved such thin layer for high resolution transmission electron microscopy. Contrarily to earlier suppositions, SiC interlinking particles were not found. Instead, the interface shows diamondlike carbon interlayers (approximately 3nm in thickness) intercalated with regions of directly-grown nanomicrometric and submicrometric-diamond crystals. A grain-to-grain epitaxial relationship of the type ⟨111⟩Dia∥⟨010⟩Si3N4 and {111}Dia∥{1¯20}Si3N4 is observed, concomitant with a 7:1 match arrangement, which assists on the interface strength.

Interfaces in nano-/microcrystalline multigrade CVD diamond coatings.

The interfaces of multilayered CVD diamond films grown by the hot-filament technique were characterized with high detail using HRTEM, STEM-EDX, and EELS. The results show that at the transition from micro- (MCD) to nanocrystalline diamond (NCD), a thin precursor graphitic film is formed, irrespectively of the NCD gas chemistry used (with or without argon). On the contrary, the transition of the NCD to MCD grade is free of carbon structures other than diamond, the result of a higher substrate temperature and more abundant atomic H in the gas chemistry. At those transitions WC nanoparticles could be found due to contamination from the filament, being also present at the first interface of the MCD layer with the silicon nitride substrate.

Electroconductive Ceramic Composites for Cutting Tools

The addition of titanium nitride (TiN) particles to a Si3N4 matrix reduces the intrinsic electric resistivity of this ceramic allowing it to be machined by EDM in cutting tools manufacturing. Gains can be expected given the cost reduction by the increase of productivity when shaping these hard to machine ceramic materials. Si3N4 ceramic matrix composites (CMC’s) with 0- 30vol.% of TiN sub-micrometric particles were produced by uniaxial hot pressing (HP) and pressureless sintering (PS). For the PS samples, EDM tests showed that machining of the composites is possible when they contain at least 23vol.% TiN particles what corresponds to a resistivity of 7.5cm. For HP samples at least 30vol.% of TiN is required to get an electroconductive material for EDM machining. This difference is due to the lower temperatures used in the HP process that delay the formation of a conductive network between the TiN particles.

CVD diamond/substrate interface FIB preparation of cutting tools

CVD diamond coated cutting tools are used for machining of abrasive and hard materials such as Al-Si alloys and tungsten carbide. The knowledge of the mechanisms governing the diamond/substrate interfacial strength is crucial in cutting tools design. The most adequate substrate material for maximizing the adhesion of diamond films is the silicon nitride (Si3N4) ceramic that possesses a thermal expansion coefficient similar to that of diamond. Buchkremer-Hermanns and co-workers consider the formation of a SiC interlayer between diamond and Si3N4, which may favour chemical bonding to diamond, although they could not detect it by glazing incidence X-Ray diffraction. They believed that insufficient detection sensitivity for very thin films, texture effects or presence of amorphous layers are possible reasons. In the case of TiN substrates, a graded interlayer of amorphous TiCN of only 8 A was suggested by Contreras, as observed by HRTEM images and EDS measurements. Due to the difficulty in the detection of such layers, which can be in the order of a few angstrons, a definite evidence of their nature is yet to be demonstrated.

From Micro to Nanometric Grain Size CVD Diamond Tools

CVD diamond coated tools are developed for applications as different as turning of cemented carbides and bone drilling. The diamond films are deposited by Hot Filament Chemical Vapor Deposition (HFCVD), with grain sizes varying from conventional micrometric (12 urn) to nanometric (< 100 nm) and film thickness up to 50 jam. Silicon nitride (Si3N4) ceramics are chosen for the base material in order to guarantee maximal adhesion. Both the micrometric and nanometric CVD diamond grades endure the cemented carbide turning showing slight cratering, having flank wear as the main wear mode. However, nanocrystalline diamond present the best behavior regarding cutting forces (<150 N) and tool wear (KM=30 urn, KT=2 um and VB=110 urn) and workpiece surface finishing (Ra=0.2 um). In the case of the dental drilling experiments, a polymeric laminated test block is used to simulate the human mandible and maxilla. The temperature rise during drilling is monitored to prevent overheating above 42- 47 °C that is known to cause tissue death and implant failure. It is possible to drill with a CVD diamond SisN4 coated tool with significantly lower forces (fourfold smaller), lower rise in temperature (4°C less), lower spindle speeds (100 rpm) and higher infeed rates (30 mm/min), when compared to the commercial steel (AISI420) drill bits.

Effect of La2O3-Rich Glass Addition on Sintering of Zirconia Based Ceramics

In this study, the influence of La-rich glass addition and sintering conditions on the densification and mechanical properties of 3 mol.%Y2O3-stabilized tetragonal zirconia polycrystals (3Y-TZP) ceramics were evaluated. High-purity tetragonal ZrO2 powder stabilized with 3 mol.% Y2O3 and La2O3-Rich glass were used as starting powders. Two compositions, ZrO2 containing 5 and 10 wt.% of a La2O3-rich glass were studied in this work. The starting powders were mixed/milled by planetary milling, dried at 90°C for 24 hours, sieved through a 60 mesh screen and uniaxially cold pressed under 80 MPa. The samples were sintered in air at 1200, 1300 and 1400°C for 60min, and at 1450°C for 120min, with heating and cooling rates of 10°C/min. Sintered samples were characterized by relative density, X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). The mechanical properties, hardness and fracture toughness, were obtained by Vickers indentation method. Dense sintered samples were obtained for all conditions. Samples sintered at 1300°C for 60 min presented the optimal mechanical properties with hardness of 1170 kgf.mm-2 and fracture toughness of 8.3 MPam1/2.

Creep Behaviour of Si3N4 Ceramics Sintered with RE2O3

The objective of this work was to evaluate the creep behaviour of Si3N4 based ceramics obtained by uniaxial hot-pressing. As sintering additive, an yttrium-rare earth oxide solid solution, designed RE2O3, that shows similar characteristics to pure Y2O3, was used. Samples were sintered using high-purity α-Si3N4 powder, with additive mixtures based on RE2O3/Al2O3 or RE2O3/AlN, at 5 and 20 vol.%, respectively. The sintered samples were characterized by X-ray diffractometry, scanning electron microscopy and density. Specimens of 3x3x6 mm3 were submitted to creep tests, under compressive stresses between 100 and 350 MPa at temperatures ranging from 1250 to 13750C in air. Samples with RE2O3/Al2O3 showed β-Si3N4 as crystalline phase, with grains of high aspect ratio, and a relative density around 99% of the theoretical density. The Si3N4/RE2O3/AlN samples presented α-Si3N4 solid solution, designed α-SiAlON, with a more equiaxed microstructure and slightly lower relative density (96-98%). The results of creep tests indicated that these ceramics containing α-SiAlON are the more creep resistant, with steady-state creep rates around 10-4 h-1, with stress exponents (n) in the range 0.67-2.53, indicating grain boundary sliding as the main creep mechanism.