AlN Solid Solution Research Articles

Effect of sintering additives on mechanical properties of C f/SiC composites

The C f/SiC composites were prepared by precursor pyrolysis-hot pressing with AlN and B as sintering additives, and the effect of sintering additives on the microstructure and mechanical properties of the composites was investigated. In the composites, a SiC–AlN solid solution was formed, which was considered to be mainly due to the reactions between PCS and AlN during the pyrolysis of PCS instead of the inter-diffusion between original SiC and AlN. Although it was beneficial to the microstructure and mechanical properties of the matrix because of the formation of SiC–AlN solid solution that could strengthen the grain boundaries and suppress the coarsening of SiC grains in the matrix, AlN exhibited a small effect on the fiber/matrix interfacial bonding strength. As a result, AlN had little effect on the density and mechanical properties of the composites. In contrast, the density and mechanical properties of the composites were improved significantly by the introduction of a trace of B, and the density and strength increased with the amount of B. However, the maximum of fracture toughness was obtained in the composite with 0.5 wt.% B, which was mainly due to the desirable fiber/matrix interfacial bonding. In the case of composites with 1 wt.% B, the fracture toughness decreased in spite of the enhancement in the density and strength, which was mainly due to the strong fiber/matrix interfacial bonding.

Microstructures of a carbon-fiber-reinforced silicon-carbide composite produced by precursor pyrolysis and hot pressing

The microstructures of a 1D-C f/SiC composite, prepared by polymeric precursor pyrolysis and hot pressing, with polycarbosilane (PCS) as precursor and AlN–Y 2O 3 as sintering additives, were investigated by using XRD, HRTEM, and SEM. During sintering, Y 2O 3 reacted with the pyrolysis products of the PCS and the oxides on the surfaces of the AlN and SiC grains, forming a liquid-phase which assisted in the densification. Owing to the formation of a carbon-rich fiber/matrix interphase with a certain number of SiC–AlN solid solution grains, a desirable level of fiber/matrix interfacial bonding could be obtained which facilitated the debonding and pull-out of the fibers. The composite therefore exhibited better mechanical properties and the average flexural strength and fracture toughness values were 691.6 MPa and 20.7 MPa m 1/2, respectively.

Columnar inter-grains and toughening of SiC–AlN ceramics induced by addition of α-SiC

The fabrication of SiC–AlN solid solutions with columnar inter-grains using the addition of small amounts of α-SiC to an AlN–β-SiC mix was studied by hot-pressing under 2030°C. The appreciable amounts of α-SiC content for a reaction of AlN and β-SiC affected the SiC–AlN phase relationships and the microstructures by stabilizing a 2H phase. It could develop columnar inter-grains of 4H phase and the stable 2H phase with a uniform composition and grain size distributions. The α-SiC containing samples showed fracture toughness of 5.1–5.4 MPa m 1/2. The fracture toughness was related to the amounts of 4H present. The enhanced fracture toughness was due to the reinforcement of the columnar inter-grains of 4H phase in the 2H phase induced by appreciable amounts of α-SiC.

Effect of AlN and Al2O3 additions on the phase relationships and morphology of SiC Part II: Microstructural observations

Additions of AlN and Al2O3 to β-SiC hot pressed at 2100°C strongly effect the β- to α-SiC phase transformation and the resultant α-SiC polytypes which are formed. Scanning and transmission electron microscopy were utilized to investigate the microstructural changes occurring in SiC due to these additions and to correlate these observations to their mechanical properties. The results suggest that Al2O3 additions stabilize the formation of the 6H-polytype of α-SiC which grows rapidly into an elongated plate-like morphology, while AlN additions stabilize the 2H-polytype of α-SiC resulting in fine equiaxed 2H-SiC: AlN solid solution grains. It is speculated that the elongated growth of 6H-SiC with Al2O3 additions can be controlled through the simultaneous addition of AlN. The formation of 2H-SiC : AlN solid solution grains inhibits the growth of the 6H-SiC grains since AlN(2H) will not go into solid solution in the SiC(6H) structure, effectively pinning the growth of the 6H-SiC grains.

Study of the oxidation of aluminium nitride coatings at high temperature

Thin films of aluminium nitride have been deposited on silicon or alumina substrates by plasma enhanced chemical vapour deposition. The oxidation behavior of these coatings is studied and compared with the behavior of aluminium nitride thick films. We have analyzed the evolution of their optical properties at high temperature by laser spectrophotometry. The complementary characterization methods were high temperature X-ray diffractometry, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses. The oxidation of these coatings involves the formation of a solid solution of oxygen in the aluminium nitride in excess of 25 at.% up to a temperature of 900 °C, then of a biphasic AlN solid solution + alumina-α until the complete transformation into α-alumina at 1200 °C. The methods we have used underline a supplementary phenomenon of interaction with the substrate in the case of thin films on silicon. The results obtained led to a coherent presentation of the behavior of this material under oxidation.

Phase formation and characterization of hard coatings in the TiAlN system prepared by the cathodic arc ion plating method

Hard Ti x N and (Ti 1− x Al x )N films were prepared using the cathodic arc ion plating method. When the ratio of the partial pressure of N 2 to the evaporation rate of titanium was reduced, the resulting phase in the TiN system changed from cubic TiN to tetragonal Ti 2N, and then to h.c.p. α-Ti. When the AIN content in the TiNAlN solid solution was increased, the films, from TiN up to (Ti 0.3Al 0.7)N, exhibited a cubic structure. As the AlN content increased, the lattice parameter monotonically decreased from 4.26 Å for TiN, to 4.16 Å for (Ti 0.3Al 0.7)N. With a further increase in AlN content, (Ti 0.15Al 0.85)N films were found to have a wurtzite structure. It was concluded that the crystal structure of the ternary TiAlN system along with the wide-ranging solubility of AlN in TiN is typical for the isostructural group of cubic titanium-based nitrides. The (Ti,Al)N coatings were stable in air at a temperature of 800°C, whereas the TiN films began to oxidize at a temperature of 550°C. It was found that protective layers of amorphous aluminum oxides formed on top of the (Ti,Al)N films during elevated temperature oxidation tests which protected the film from further oxidation. The crystallized TiO 2 that was formed on the TiN film did not give the same protection. The nicrohardness and wear resistance of the (Ti,Al)N films were also investigated.

High‐Temperature Behavior of the Aluminum Oxycarbide Al2OC in the System Al2O3–Al4C3 and with Additions of Aluminum Nitride

A large number of high‐temperature experiments involving binary mixtures of alumina and aluminum carbide (Al4C3) were performed to clarify the behavior of the aluminum oxycarbide Al2OC. It is first shown from heat treatments on the molar composition 55Al2O3·45 Al4 C3 that, in conditions of stable equilibrium, Al2OC decomposes to Al4O4 C and Al4 C3 at 1715°C according to the reaction 4Al2 OC → Al4 O4 C+Al4 C3. However, Al2 OC can be obtained at room temperature from compositions of lower carbide contents (<20 mol%) and by rapid cooling. In that case Al2 OC is solidified in a metastable state, and, when subsequently annealed to temperatures above 1200°C, its lattice reorganizes and progressively transforms into the lattice of Al4 O4 C according to the reaction Al2 O3+xAl2 OC → (1—x)Al2 O3+xAl4 O4 C. This transformation is described in terms of TTT curves and is accompanied by a decrease in hardness and wear resistance. Additions of AlN to Al2 O3 and Al4 C3 have been found to create a solid‐solution Al2 OC—AlN in an unexpected region of the ternary system Al2 O3—Al4 C3—AlN. As a result the quasi‐binary section Al2 OC—AIN of the ternary diagram Al2 O3—Al4 C3—AlN was extensively investigated. We report the solubility limit of AlN in Al2OC, the improved high‐temperature stability of the Al2OC—AlN solid solution compared with pure Al2OC, and eventually the existence of a possible intermediate ternary compound of formula Al10O3C3N4.

Solid solutions and composites in the SiCAlN and SiCBN systems

The effects of AlN and BN additives on hot-pressed SiC were investigated. SiCAlN diffusion couples were also studied. Analytical scanning transmission electron microscopy was used to determine the phases in contact, the local compositions and the composition gradients. Approximate diffusion depths range from 0.5 μm at 1950°C to 15 μm at 2300°C (both for 1 h), and the corresponding diffusion coefficients are of the order of from 1.5 × 10 −12 to 6 × 10 −10 cm 2s −1. The slow diffusion rates and the added β-to-α nucleation barrier cause considerable inhomogeneities in the high temperature hot-pressed SiCAlN solid solutions. The diffusion rates at the lower temperatures, although small, may lead to dissolution of small particles in an SiCAlN composite. The solubility of BN in βSiC under conventional hot-pressing conditions is small and was not directly measured. BN stabilizes βSiC and low hexagonality polytypes.