William Rafaniello

Structures and properties of disordered boron carbide coatings generated by magnetron sputtering

Disordered boron carbide coatings with their high hardness, high lubricity, and low surface friction have become the coatings of choice to enhance the wear performance of many existing products. These coatings have been successfully commercialized using a magnetron sputtering process. In this paper, the effects of one of the critical process parameters, bias voltage, on the chemistry, microstructure, and the properties of the coatings are discussed. In combination with microstructure examination, special emphasis was made on nanoscopic level chemical analyses in order to explain the effects of this process parameter. The substrate bias was found to have strong effects on the hardness and the stress of the coating, but it has little influence on the frictional characteristics of the coating. The results of the examination and the analyses of the coating using FTIR, XPS, TEM, PEELS, and SIMS revealed that the morphology of the coating changed from a columnar structure to a continuous solid structure as the substrate bias voltage increased from 0 to 200 V. Oxide species were found in between the columns, while the columns mainly consisted of boron carbide with a boron to carbon atomic ratio of about 4. The atomic ratio of boron to carbon appeared to be independent of the substrate bias.

Room temperature tensile and flexural strength of ceramics in AlN-SiC system

Abstract Tensile and bend strength were obtained for four compositions in the aluminum nitride-silicon carbide system: AlN, 75%AlN-25%SiC particulate, 50%AlN-50%SiC solid solution and SiC. Room temperature tensile strength, four-point bend strength, and the Weibull parameters for each of the tested materials are presented. Optical and SEM fractography were employed to identify failure initiating flaws for the fractured tensile specimens. The four-point bend strength is predicted using the Weibull tensile parameters and compared to measured values. Accuracy of the predicted results are rationalized based on observed microstructures and failure initiating flaws.

Sintering behavior of direct nitrided AlN powder

Abstract Classified direct nitrided (DN) aluminum nitride (AlN) powders were examined by dilatometry to study the effect of particle size on sinterability. The shrinkage behavior of three DN powders was compared to that of a commercial, carbothermally produced, AlN powder. Yttrium oxide was added to AlN powder as a sintering aid. Two distinct features of the resulting shrinkage curves, corresponded to particle rearrangement coincident with yttrium aluminate formation and high temperature sintering. The presence of coarse particles in the original DN powder, greater than 8 μm, distinguished it from the other powders studied and appeared to have the greatest influence on the sintering behavior. For fine powders without coarse particles (>8 μm), the synthesis method did not seem to affect the shrinkage. The grain size distribution of the sintered parts mirrored the particle size distribution of the four powders studied. Thermal conductivity (TC) of the sintered AlN bodies was strongly dependent on the oxygen content of the starting powders.

Microstructures and properties in aluminum nitride–titanium nitride composite ceramics

Abstract Aluminum nitride–titanium nitride (AlN–TiN) composites (TiN content; 0, 5 and 10 vol.%) were prepared using mixtures of fine AlN and TiN powders. Pressureless sintering of the AlN–TiN powder mixtures was done at 1850°C for 1 to 20 h, using yttrium oxide powder as a sintering aid. Sintered samples achieved almost full density. While AlN grains grew isotropically with sintering time, TiN particles in the composite maintained their small and nearly round shape. The TiN grains were present at both grain boundary and within AlN grains. Heat treatment of these composites caused both microstructural change and improvements in thermal diffusivity. Inhibition of AlN grain growth was greatest for the 10 vol.% TiN–AlN composites.

Fabrication, thermal treatment and microstructure development in SiC-AIN-Al2OC ceramics

Samples containing equimolar amounts of SiC, AIN and Al2OC were fabricated by hot pressing a mixture of SiC, AIN, Al2O3 and Al4C3. The predominant constituent in the hot pressed material was of 2H crystal type. The samples were subsequently annealed at a temperature up to 2050‡ C for up to 153 h in an atmosphere of nitrogen. Samples made with IbigawaΒ-SiC exhibited formation of needle-type precipitates. Carbon deficient specimens contained substantial amounts of Al3O3N. Principal characterization techniques employed consisted of optical microscopy, electron microscopy, X-ray diffraction and chemical analysis.

Non-Oxide Materials: Applications and Engineering

The advanced ceramics of yesteryear, specifically the non-oxide materials, have become fully commercialized in several applications where long life, improved performance, unique design capability and cost advantages have been explicitly utilized. Exploiting the advantages of their unique properties combined with the availability of complex shapes, several industries such as mining and metallurgical, paper and pulp, commodity and specialty chemicals, mechanical machinery, food processing, electronics, semiconductor processing, aerospace, defense and medical device manufacturing have discovered the unique benefits of these technical ceramics (RCG/Hagler, Bailly, Inc., 1990; Schwartz, 1992).

Microstructural development in AlN composite ceramics

AlN-BN composite ceramics were fabricated by sintering a mixture of fine AlN and BN powders containing Y2O3 as the sintering aid at 1850°C in nitrogen atmosphere. BN contents up to 15 vol% were studied. The heat treatment of sintered AlN-BN composite was performed at 1800 °C in nitrogen atmosphere. BN particles in the composite showed large anisotropic grain growth during sintering, and the shape of grown BN particles appeared to be plate-like. Moreover, the heat treatment gave very interesting microstructures of the composite, in which many more interfaces between AlN grains were observed and most of the secondary phase was located at triple points of AlN grains. The resultant microstructural change is thought to be due to the change in the ratio of interfacial energy (between grain and liquid) and grain boundary energy (between grains).

Critical Powder Characteristics

The growth of advanced non-oxide ceramics can be directly traced to the availability of high quality powders. Particle size reduction has been the primary powder feature that has allowed this technological development. Controlling phase and overall chemical purity have also been important.

Fatigue crack propagation in aluminum nitride ceramics under cyclic compression

Room temperature fatigue crack growth characteristics under cyclic compressive loads were investigated in pure and 3 wt % yttria doped hot pressed aluminum nitride ceramics. A single edge-notch specimen geometry was used to induce a stable Mode I fatigue crack under cyclic compressive loads. The fatigue crack growth occurred in three stages, where the first stage is dominated by microcrack nucleation, coalescence and slow growth within the notch root. During the second stage, the crack growth is accelerated and finally, the crack growth deceleration and arrest occurred in third stage. The fatigue crack growth occurred predominantly by intergranular fracture. Insights gained from the experimental results and microscopic observations are discussed.