Microstructure and creep characteristics of experimental SiCf–YMAS composites

A unidirectional SiC_f –YMAS glass–ceramic composite has been developed by Céramiques-Composites (Bazet) and ONERA (Establishment of Palaiseau) in France. The matrix is totally crystalline and consists essentially of two main phases, cordierite and yttrium disilicate, with some minor phases, mullite, spinel, zirconium and titanium oxides. Image analysis methods have been used to characterize the homogeneity of the composite plates and to obtain granulometric information on the different matrix phases. Different interphase layers formed during the process by reaction between the matrix and the Nicalon NLM 202 fibres have been studied by using HREM and EDX. Their chemical composition has been determined by stepping the probe (8 nm) across the fibre–matrix interface. Two distinct nanoscale sublayers have been imaged. The sublayer on the matrix side has a light contrast in the TEM. The microstructure of this layer (≈ 80 nm) is typical of a turbostratic carbon. The carbon layer also contains Al, O, Mg and Si. The silicon content is low in the carbon layer. The sublayer on the fibre side (≈ 100 nm thick) has a dark contrast in the TEM. Profiles have been taken across this sublayer also. Tensile creep tests in air have been performed to investigate the tensile creep behaviour at 1223 K. They have been conducted in the 50–200 MPa stress range. Tensile creep results indicate that creep rates are of the same order of magnitude as for other glass–ceramic composites. Optical micrographs and SEM observations have revealed the damage in the composite. Changes occurring in the interface region have been studied at a finer scale by TEM and HREM at the surface of the sample and in the core. These observations enable us to explain the mechanical behaviour of the composite observed on a macroscopic scale.     

SiCf–SiBC composites: microstructural investigations of the as-received material and creep tested composites under an oxidative environment

SiC_f–SiBC composites fabricated by Snecma Propulsion Solide (St Médard en Jalles, France) were investigated by SEM and HRTEM in the as‐received state and after creep tests performed in air, in a temperature range 1423–1573 K, under 170 and 200 MPa. These composites are reinforced by Hi‐Nicalon fibres (Nippon Carbon). A pyrocarbon interphase was first deposited on the fibres. The matrix was then deposited on the fibrous preform by several chemical vapour infiltrations (CVI). As a result the matrix is multilayered and based on the Si–B–C ternary system. This matrix is self‐sealing: this is due to the presence of boron inducing the formation of a sealant glass if the material is heated in an oxidative environment. This glass will protect fibres and fibre/matrix interphases against oxidation. Hi‐Nicalon fibres as well as the different matrix layers were studied by HRTEM and EDX. Some investigations were carried out on the creep‐tested specimens in order to characterize modifications observed in the different constituents of the composites, particularly at the interfaces between the matrix layers and at the fibre/matrix interface. It was shown that several matrix layers crystallized during the creep tests. Moreover, a thin silica layer was observed at the pyrocarbon/matrix interfaces. Differences between the behaviour of the same type of material creep tested under neutral atmosphere are discussed.     

Microstructural characterization in diffusion-bonded SiC/Ti–6Al–4V composites

The microstructural evolution during the diffusion bonding consolidation of a Ti–6Al–4V/SiC fibre composite was investigated by optical, scanning and transmission electron microscopy. The effects of processing parameters, particularly temperature, on the microstructures of the matrix and the fibre and their bonding were considered. Processing at too high a temperature can result in growth of SiC crystals in the fibre in addition to rapid interfacial reaction, while interfacial bonding cannot be established if the temperature is too low. Various defects can be caused by inadequate fabrication practices. These include micro-pores, matrix-cracking, cracking, bending and impingement of fibres, and heterogeneous fibre distribution. Methods for avoiding these are discussed. A defect-free and uniformly distributed fibre composite can only be achieved by optimizing the processing parameters (such as temperature, pressure, time and cooling rate) and adequately combining fibre spacing and matrix thickness with accurate fibre alignment.     

Interfacial reactions and silicate formation in highly dispersed Lu2O3– SiO2 system

Solid state interface reactions in highly dispersed Lu₂O₃– SiO₂ binary oxide system were studied at 600–1100 °C with X‐ray powder diffraction (XRD), high‐resolution transmission electron microscopy (HRTEM) and Fourier Transform Infrared spectroscopy (FTIR). The results show that at 600–900 °C an amorphous, nanometer thick Lu‐O‐Si layer covering SiO₂ particles exists in the system. At higher temperatures the breakage of the layer into amorphous islands occurs and crystalline silicates with various structures are formed. In particular, Lu₄[Si₃O₁₀][SiO₄] silicate, analogue of B‐type Dy – Tm disilicates, forms at 1000 °C.     

Friction and wear properties of CN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/SiC in water lubrication

Amorphous carbon nitride coatings (a-CN_x) were deposited on SiC disk by ion beam assisted deposition (IBAD). The tribological behavior of a-CN_x coating sliding against SiC ball in water was investigated and compared with that of SiC/SiC system at room temperature. The influences of testing conditions on friction coefficient and specific wear rate of both kinds of tribopairs were studied. The worn surfaces on disks were observed by scanning electron microscope (SEM). The results indicate that the running-in period of a-CN_x/SiC was shorter than that of SiC/SiC system in water. At a sliding velocity of 120 mm/s, the mean steady-state friction coefficients of SiC/SiC (0.096) was higher than that of a-CN_x/SiC (0.05), while at 160 mm/s, lower friction coefficient (0.01) was obtained for SiC/SiC in water. With an increment of normal load, the mean steady-state friction coefficients after running-in first decreased, reaching a minimum value, and then increased. For self-mated SiC, the specific wear rate of SiC ball was a little higher than that of SiC disk, while for a-CN_x/SiC, the specific wear rate of SiC ball were 10 times smaller than that of a-CN_x coating. Furthermore, the specific wear rate of SiC ball sliding against a-CN_x coating was reduced by a factor up to 100~1000 in comparison to that against SiC in water. The wear mechanism of SiC/SiC system in water is related to micro-fracture of ceramic and instability of tribochemical reaction layer. Conversely, wear mechanism for a-CN_x/SiC is related to formation and transfer of easy-shear friction layer.     

Ceramic particulate composites in the system SiC–TiC–TiB2 sliding against SiC and Al2O3 under water

The tribological behaviour of SiC, SiC–TiC and SiC–TiC–TiB₂ was determined in oscillating sliding against SiC and α-Al₂O₃ in water at room temperature. The tribo-systems with the composite materials containing TiC and TiB₂ differ significantly from the systems with the single phase SiC: The wear is reduced and the friction is increased. The wear reduction up to a factor of 10 is mainly due to the formation of an oxide film containing titanium oxides which is soft, stable in water and well adhering to the bulk material. This oxide film is transferred to the alumina ball but not to the silicon carbide ball.