Hydrogen incorporation in Ti-based metal–matrix composites fabricated by vacuum plasma spraying and vacuum hot pressing

In this paper we investigate the incorporation of hydrogen as a temporary alloying element during vacuum plasma spraying (VPS) of Ti-based metal–matrix composites and show that it has probable beneficial effects for secondary processing by vacuum hot pressing (VHP).
The composites investigated included both commercial purity Ti and Ti–6Al–4V matrices, which were sprayed onto DRA Sigma SiC fibres. The as-sprayed matrix microstructures were investigated by transmission electron microscopy and X-ray diffractometry, and were found to consist in both cases of a fine mixture of (1) hexagonal α'-Ti martensite plates, with an increased lattice parameter relative to the usual value, presumably due to dissolved hydrogen; and (2) an increased proportion of equiaxed b.c.c. β grains, probably stabilized by dissolved hydrogen. Both phases, but particularlyβ, contained needle-shaped b.c.t. hydride precipitates.
It has been found that monotapes produced by VHP can be consolidated more easily by secondary VHP than those produced by electric arc spraying, leading to reduced adverse interfacial reactions between fibre and matrix. This improved processability is believed to be due to the combination of a greater proportion of the more ductile β phase and the very small grain sizes inherent in VPS. However, the dissolved hydrogen and the potentially embrittling hydride precipitates are removed during VHP. The presence of hydrogen after primary VPS may therefore have an overall beneficial effect.     

The compatibility of TiB2 protective coatings with SiC fibre and Ti-6Al-4V

TiB₂ coatings have been studied as prospective protective layers to inhibit the interfacial reaction between SiC fibres and Ti-alloy matrices. This protective coating has been deposited onto SiC monofilament fibres using a chemical vapour deposition (CVD) technique. The fibre-matrix compatibility of these TiB₂-coated SiC fibres in Ti-6Al-4V composites was evaluated by incorporating the coated fibres into Ti-6Al-4V using a diffusion bonding technique. The interfaces of this composite were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalysis, to evaluate the interfacial microstructures, chemical stability and the efficiency of TiB₂ as a protective coating for SiC fibres in Ti-alloy matrices, and to study the effects of deposition temperature on the interface of the coated fibre. Results show that stoichiometric TiB₂ coatings are stable chemically to both SiC fibres and Ti-6Al-4V and hinder the deleterious fibre-matrix reactions effectively. Boron-rich TiB₂ coatings should be avoided, as they lead to the formation of a needle-like TiB phase at the fibre–matrix interface. These findings provide promising evidence for the value of further exploration of the use of stoichiometric TiB₂ as a protective coating for SiC fibre in Ti-based composites.     

The study on the properties of TiCxN1−x coatings processed by cathodic arc physical vapour deposition

Titanium carbonitride (TiCN) is a popular hard coating for carbide cutting tools in various applications. The properties of TiCN are within its composition and can be controlled by maintaining the C–N ratio within the coating to a certain level. This paper studied the influence of carbon content and coating composition within TiC_xN_1?x coatings with regard to their mechanical properties. The substrate used was tungsten carbide (WC-6Co), which was prepared in-house through a powder metallurgy process, while the TiC_xN_1?x coatings were deposited in-house using cathodic arc physical vapour deposition (CAPVD). TiC_xN_1?x coatings improved the mechanical properties of carbide inserts. An increase in carbon content within TiC_xN_1?x coatings improved surface lubricity, reduced coefficient of friction, improved surface microhardness and increased Young's modulus, but reduced thermal conductivity of carbide inserts. The colour of TiC_xN_1?x coatings also changed with carbon content.     

Ion microprobe studies of reactions in squeeze-cast aluminium alloy matrix composites

An ion microprobe with high lateral resolution has been used to study the chemical reactions at the fibre/matrix interface of metal–matrix composites. During the squeeze-casting process, the Al–Si–Mg matrix reacts with the preform made of Saffil fibres (96% Al₂O₃, 4% SiO₂). The reaction occurs mainly between the silica binder and Mg from the matrix according to SiO₂ + 2Mg = 2MgO + Si. A continuous layer of MgO was formed around the fibres, even on surfaces that were not covered by the silica binder. Possible reasons are discussed for the formation of MgO in areas where binder coating was missing. In such areas, Mg reduces SiO₂ that is contained in the fibre. However, the fibres (A1₂O₃) are not attacked by Mg. In the isolated case of fibres that were completely uncoated, no reaction products were observed at the interface. The presence of silica binder seems to be an essential requirement for this reaction to occur. When squeeze-casting is performed with sufficiently high melt temperature, Al from the matrix also reduces silica.     

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.     

The microstructure of spray-formed Ti-6Al-4V/SiCf metal-matrix composites

Spray-forming is a possible manufacturing route for the fabrication of Ti alloy fibre-reinforced metal-matrix composites (MMCs) because high rates of alloy-droplet cooling on impact with the fibres prevent excessive fibre-matrix reaction. Ti–6Al–4V matrix MMC monotapes containing TiB₂-coated SiC fibres have been manufactured by electric-arc spray-forming, and the key MMC microstructural characteristics in the as-sprayed monotapes have been investigated by optical and scanning electron microscopy. Fibre infiltration increases with decreasing spraying distance, decreasing atomizing gas pressure and increasing arc current, because of higher temperatures in the Ti alloy spray droplets on impact with the fibres. Too much binder in the fibre preform leads to poor fibre–matrix contact, while removing the binder leads to the fibres becoming misaligned during spraying.     

The microstructure of silicon carbide (Nicalon) fibre reinforced silicon carbide ceramic-matrix composites heat treated in vacuo and in pure oxygen

Silicon carbide (Nicalon) fibre reinforced SiC composites have been heat treated in vacuo and in pure oxygen environments at 1400°C for 100 h. The response of the microstructure and, in particular, of the interface between fibre, carbon interlayer and SiC matrix components has been studied. Microstructural modifications were observed by transmission electron microscopy, using imaging, electron energy-loss spectroscopy and electron diffraction techniques, and fibre stoichiometries were determined using a scanning Auger microprobe. Recrystallization of Nicalon fibres within composites heat treated in vacuo was found to result from decomposition of the metastable silicon oxy-carbide phase found in the fibres. No significant changes to the pyrolytic carbon interlayer were observed. Fibre recrystallization was considered to embrittle the composite. Samples heat treated in oxygen showed no appreciable fibre recrystallization. Study of interlayers in such aged samples often revealed decohesions, holes and narrow silica layers. In the most extreme cases, complete displacement of carbon by SiO₂ was found and such interfaces were identified as silica and α-cristobalite. Interfacial modifications were considered to be responsible for the retention of the small β-SiC grain size in Nicalon fibres and were also considered to be deleterious to the mechanical properties.     

The microstructure of experimental SiC fibre-reinforced yttrium magnesium aluminosilicate (SiCf-YMAS) materials

Two experimental SiC fibre-reinforced yttrium magnesium aluminosilicate (SiCf-YMAS)-type ceramic-matrix composite (CMC) materials fabricated (i) by the glass process and (ii) by chemical precursor infiltration have been studied by light microscopy, transmission electron microscopy (TEM), high-resolution electron microscopy (HREM) and energy-dispersive X-ray spectroscopy (EDS). The distribution of the fibres inside the composite as well as the average diameter of fibres have been determined by image analysis. The microstructure of the YMAS matrices has been characterized by TEM observations. YMAS matrices are formed of two main phases, cordierite and β-yttrium silicate (Y₂Si₂O₇). Two minor phases (mullite and spinel) have been found to crystallize inside the cordierite and the yttrium silicate crystals. Fibre-matrix interfaces have been observed in HREM. A thin turbostratic carbon layer (20–30 nm) has been imaged in both composites at the fibre-matrix interface. It crystallizes along the matrix interface and grows inside the fibre, forming a diffuse interphase. The carbon layer is believed to be the consequence of reaction between oxygen in the matrix and SiC nanocrystals of the Nicalon fibres.     

Wear resistant multilayer nanocomposite WC1−x/C coating on Ti–6Al–4V titanium alloy

A significant improvement of tribological properties on Ti–6Al–4V has been achieved by developed in this study multilayer treatment method for the titanium alloys. This treatment consists of an intermediate 2 μm thick TiC_xN_y layer which has been deposited by the reactive arc evaporation onto a diffusion hardened material with interstitial O or N atoms by glow discharge plasma in the atmosphere of Ar+O₂ or Ar+N₂. Subsequently, an external 0.3 μm thin nanocomposite carbon-based WC_1−x/C coating has been deposited by a reactive magnetron sputtering of graphite and tungsten targets. The morphology, microstructure, chemical and phase compositions of the substrate material after treatment and coating deposition have been investigated with use of AFM, SEM, EDX, XRD, 3D profilometry and followed by tribological investigation of wear and friction analysis. An increase of hardness in the diffusion treated near-surface zone of the Ti–6Al–4V substrate has been achieved. In addition, a good adhesion between the intermediate gradient TiC_xN_y coating and the Ti–6Al–4V substrate as well as with the external nanocomposite coating has been obtained. Significant increase in wear resistance of up to 94% when compared to uncoated Ti–6Al–4V was reported. The proposed multilayer system deposited on the Ti–6Al–4V substrate is a promising method to significantly increase wear resistance of titanium alloys.     

Silicon carbide/SRBSN composites

Ceramic-matrix composites have been produced using unidirectionally aligned Textron SCS-6 fibres in a sintered reaction-bonded matrix. A tape-casting technique was used to produce a prepreg sheet that could be cut and stacked to form a layup. Al₂O₃ and Y₂O₃ were used as sintering aids, final sintering being carried out in a hot press at 1700°C. Matrix, fibre and interfacial microstructure has been characterized using analytical microscopical techniques. X-ray mapping of the carbon and silicon distribution at the fibre–matrix interface was carried out, and evidence of reaction between the outer carbon-rich layer of the fibre and the matrix was found. Micromechanical behaviour of interfaces has been investigated and compared with interfacial microstructure and macromechanical properties.     

Microstructural observations of reaction zones in Ti-based metal-matrix composites

The interaction between Ti-alloy matrices and ceramic particulate and fibres in Ti-based metal-matrix composites (MMCs) is reviewed. It is concluded that the stability of surface-treated SiC fibres is adequate for some applications of MMCs, but no other fibres have yet been developed which are useful additions to Ti alloys. Although there is less interest at this stage in particulate-reinforced Ti MMCs, there are two ways in which useful particulate MMCs could be developed. First, the approach used in fibre-reinforced composites, of using special coating techniques to stabilize the particulate against any reaction with the matrix. Second, either C or B additions could be used, both of which give rise to particulate phases which are stable in the presence of a Ti matrix, and which can therefore be manufactured using a wider range of process routes than is possible for other composites. The significance of these conclusions with regard to the ability to shape particulate composites into final products is briefly discussed.     

X-ray tomographic imaging of Ti/SiC composites

In this paper, high‐resolution tomographic synchrotron X‐ray imaging is applied to study the occurrence and evolution of damage in Ti‐6Al‐4V/SCS6 SiC fibre composite materials. Three composite morphologies of increasing complexity have been studied, namely single fibre, single‐ply and multi‐ply composites. The single fibre composite was strained to full fibre fragmentation and the progressive introduction of damage monitored. For the single‐ply composite, damage was introduced deliberately by laser drilling to establish the effect of damaged fibres on their neighbours, whereas for the multi‐ply composite the morphology of a fibre bridging fatigue crack was studied. In addition to traditional mode I fibre fractures, subsequent fibre wedge cracks were observed presumably nucleating from damage introduced into the fibre surface by the first fracture event. In addition to these crack morphologies, spiral defects were observed for the single ply during failure. Finally, for the multi‐ply composite, the matrix crack front showed a number of characteristic features, including advancement in fibre‐free regions, crack bifurcation near fibres and different crack plane heights either side of a fibre.     

Microstructure and thermo-mechanical stability of a low-oxygen Nicalon fibre

A new Nicalon SiC-based fibre, characterized by a low oxygen content (0–5% wt) has been studied. The absence in this fibre of a continuous Si–C–O phase, which characterized the previous NLM 202 series of fibres, induces larger mean sizes for the constituents: the fibre is composed of β-SiC grains 5–20 run in diameter and turbostratic aggregates of carbon 2–5 nm in diameter. The fibre is seen to be suffer at room temperature (E = 300 GPa) and stronger due to a reduction in critical defects thanks to improvements in processing conditions. The Young's modulus remains almost stable up to 1473 K in air and above this temperature the core of the fibre exhibits continuous grain growth up to 1773 K, but without the degradation that occurred in the previous generation of fibres. Fibre strength was seen to be lowered when compared to room temperature values even when exposed in air to temperatures of 1073 K. A comparable fall is not seen with the NLM 202 fibres until 1273 K and this difference is attributed to the oxidation of the carbon-rich surface of the new fibre. SiC is oxidized at higher temperatures, inducing, above 1473 K, the growth of a silica layer on the surface, with defects at the glass/ceramic interface. The large discrepancies between the good thermo-mechanical characteristics in inert atmosphere and the behaviour in air may be reduced if a coating resistant to oxidation could be applied to the fibre.     

Comparison of interfaces in Ti composites reinforced with uncoated and TiB2/C-coated SiC fibres

Interfaces play an important role in determining the mechanical properties of composite materials. The interfaces established between a titanium-alloy matrix (Ti-6Al-4V) and uncoated and TiB₂/C-coated SiC fibres are analysed by scanning electron microscopy, transmission electron microscopy and X-ray techniques. Emphasis is placed upon the interfacial morphology and microstructure, identification of reaction products, and the stability of the coating layer. Complex multi-reaction layers are observed frequently in the interfacial zones. Previous, often contradictory, reports about the interlayers are reviewed. Experimental observation demonstrates that the type and distribution of interlayers vary in a given system, due to prolonged treatment of the samples at temperature. The formation and distribution of the interlayers are discussed further, with respect to these and previous findings. Methods of reducing interfacial reactivity 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.     

Structure and tribological behavior of nanocomposite C-Ti-WSe x coatings

Nanocomposite thin C-Ti-WSe _x coatings, which contain antifriction (WSe _x and amorphous carbon a-C) and hard (β-WC and TiC) components, are produced by pulsed laser deposition. In order to improve the tribological characteristics of the WSe _x phase and to transform the structural state of the a-C matrix, alloying with titanium is used. The characteristics of the coatings are determined using the sliding of a steel ball at an increased humidity. A comparison of the characteristics of a composite C-Ti-WSe _x coating and a carbon coating alloyed with titanium (a-C(Ti)) shows that the modification of the a-C matrix by introducing the WSe _x phase leads to an increase in the coefficient of friction from 0.05 to 0.2; the wear of the composite coating is approximately six times higher than that of the a-C(Ti) coating. Possible solutions for improving the tribological characteristics of the composite C-Ti-WSe _x coating in humid air are considered.     

Microstructural investigations in Cf–SiC composites in as-sintered state and after creep experiments

The high interest in ceramic matrix composites during the last decade has led to a considerable number of studies devoted to their thermomechanical properties and damage processes. Despite their sensitivity to oxygen partial pressure, carbon fibres appear to possess higher stability and better mechanical properties if they are treated under protective atmospheres than other ceramic fibres (especially classical silicon carbide fibres). The aim of this investigation is to characterize at the nanoscale the main microstructural parameters of C_f–SiC composites provided by the SEP (Division of SNECMA, Bordeaux, France). This material was fabricated from a 2.5D preform made of high strength polyacrylonitrile (PAN)-based carbon fibres densified according to the chemical vapour infiltration process. A pyrocarbon (PyC) interphase was deposited on the fibre prior to the β-SiC matrix infiltration. A careful high resolution electron microscopy (HREM) microstructural investigation focused on the fibre microstructure as well as on the different interfaces in the material: pyrocarbon/fibre and matrix/pyrocarbon interfaces. All these observations have been realized in longitudinal and transverse sections of the specimen. These observations are found in good agreement with Guigon's model for high strength ex-PAN carbon fibres. The PyC interphase texture was strongly anisotropic at the fibre/interphase and interphase/matrix interfaces over a mean thickness of 8–15 nm. Tensile creep tests were performed under partial pressure of argon between 1273 and 1673 K for stress levels ranging from 110 to 220 MPa. Scanning electron microscopy and high resolution electron microscopy were used to study the microstructural modifications inside the fibres and at the different interfaces. A discussion of the possible creep mechanisms based on the microstructural investigation and the creep results is presented.     

On the use of the geodesic metric in image analysis

Let X be a phase in a specimen. Given two arbitrary points x and y of X, let us define the number d_x(x, y) as follows: d_x(x, y) is the greatest lower bound of the lengths of the arcs in X ending at points x and y, if such arcs exist, and + ∞ if not. The function dX is a distance function, called ‘geodesic distance’. Note that if x and y belong to two disjoint connected components of X, d_x(x, y) = + ∞. In other words, d_x seems to be an appropriate distance function to deal with connectivity problems. In the metric space (X, d_x), all the classical morphological transformations (dilation, erosion, skeletonizations, etc.) can be defined. The geodesic distance d_x also provides rigourous definitions of topological transformations, which can be performed by automatic image analysers with the help of iterative algorithms. All these notions are illustrated with several examples (definition of the length of a fibre; automatic detection of cells having at least one nucleus, or having exactly a single nucleus; definitions of the geodesic centre and of the ends of a particle without holes, etc.). As an application, a general problem of segmentation is treated (automatic separation of balls in a polished section).     

In Situ Studies of TiC<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Hard Coating Tribology

TiC_1−x N _x hard coatings present time-dependent tribological behavior with an initial running-in period (500–2000 cycles) marked by an elevated friction coefficient, followed by >10000 cycles with low-friction and wear at room temperature (RT) in ambient air. The mechanisms behind this behavior are not completely understood. Tribological tests performed at RT and at different relative humidity (RH) levels revealed that a minimum value between 15 and 25% RH is needed to trigger the low-friction regime at a sliding speed of 100 mm s⁻¹. By in situ observations of transfer film growth, it could be observed that third body material is formed during this running-in period by plowing of the coating and shearing of the removed material. The appearance and thickening of the transfer film marks the beginning of the steady-state low-friction regime where the velocity is accommodated by interfacial sliding. At this stage in the tribological test, the recorded Raman spectra indicated the presence of C–H bonds in the wear track. Use of in situ analytical tools during wear tests provided insights with respect to tribological phenomena that were not available by conventional, post-mortem analysis methods.     

Wear behaviour of iron boride coatings produced by VPS technique on carbon steels

The vacuum plasma spray (VPS) technique is a useful tool for designing the characteristics of the coatings and, thus, the tribological properties of coated components. In the present paper, the wear properties of iron boride coatings produced by means of VPS technique on AISI 1040 steel samples were evaluated as a function of their microstructural characteristics. One coating type was obtained by using Fe₂B pure powder, the other with differentiated FeB + α-Fe blends, with the FeB content increasing and α-Fe content decreasing from the matrix to the surface. Wear tests were performed by means of a tribometer in block-on-ring configuration, without lubricant and in air, by using 40- and 60-N coupling loads and 0.8- and 1.6-m s⁻¹ sliding velocities. On Fe₂B coated samples, wear is essentially oxidative until the failure of the coating, the fragments of which cause a third body abrasion. On the FeB + α-Fe coated samples the wear mechanism is mainly oxidative and the coating totally wears out without spalling as a consequence of its graded structure, which succeeds in both improving the adhesion of the coating to the substrate and reducing the residual stress at the coating–substrate interface.