Content uploaded by Michail Vardavoulias
Author content
All content in this area was uploaded by Michail Vardavoulias on Feb 12, 2015
Content may be subject to copyright.
C.V.D. Coating of Alumina Reinforcing Particles to Improve the Wear Resistance of H.S.S. Based Composites
Materials and Manufacturing Processes
Abstract
C.V.D. coating of the reinforcing ceramic particles used in particulate metal matrix composites allows the control of reactivity at the particle/matrix interface. Wear resistant high speed steel-based composites containing uncoated A1203, uncoated TiC and C.V.D. coated A1203 were liquid phase sintered, then characterized using “pin-on-disc” wear testing. TiC or TiN C.V.D. coatings of A1203 were tested to determine die increase in reactivity of the particles with the liquid phases formed during sintering. This resulted in a porosity decrease at the particle/matrix interface in addition to a better ceramic/metal cohesion due to improved wettability. Reactivity and wettability were studied using differential thermal analysis, electron microprobe analysis, transmission electron microscopy, and image analysis. Results from pin-on-disc wear testing illustrated the role of the C.V.D. coating on the wear behavior of the studied materials. Lower wear rates were obtained with the composites containing TiC or TiN-coated Al203. These results showed that there is a relation between wettability of ceramic particles by the metallic phases and wear resistance of the composites.
... Winkelmann et al. [15] demonstrated that such kind of interlayer on AlZrO-particles consisting of titanium leads to metallurgical bonding between hard-particles and steel matrix. In [16] it was shown by Jouanny-Trésy et al. that titanium nitride (TiN) can also be used as interlayer for the production of alumina reinforced M3/2 high-speed steels with Cu3P addition. These additions leads to the formation of low melting phosphide phase which wets the coated ceramic and thus leading to a lower porosity at the interface and improved particle-matrix bonding resulting in an improved wear-resistance. ...
Economic and political driving forces are leading to an ambitious search for substitutes for fused tungsten carbide (FTC) in ultra-high wear-resistant metal matrix composites (MMC), which are used for mining applications. In the presented paper, possible substitutes such as alumina (Al2O3), zirconia (ZrO2) and silicon carbide (SiC) are discussed. To enhance the wettability of oxides (e. g. Al2O3, ZrO2) by Fe-base melts or to counteract strong dissolution of metastable covalent bonded hard-particles (e.g. SiC) it is proposed to coat the particles with a thin titanium nitride (TiN) layer by means of chemical vapor deposition (CVD). For this reason a CVD-apparatus for particle coating was constructed and is shown in this paper. In addition, it is demonstrated that such a TiN coating on the oxide particles can increase the wettability and therefore improve the embedding behavior of the particles into a Fe-base matrix. In addition, it is shown that TiN coatings on covalent bonded hard-particle SiC can be used as a diffusion barrier coating, thus counteracting a dissolution of the hard-particles during processing by sintering techniques. However, due to the difference in linear thermal expansion coefficients the coating tends to delaminate, partially.
A Fluidized Bed Metal Organic Chemical Vapour Deposition (FB-MOCVD) process has been successfully applied to coat Fe particles with Al. N2 inert gas was used to transport the organometallic Al precursor (liquid triethylaluminium) inside the fluidized bed reactor whose temperature is 350 °C. XPS analyses and TEM observations have revealed the presence of a thin alumina layer surrounding the Al coating. Oxidation treatments, performed in the temperature range 350–500 °C, demonstrate that this multi-scale coating constitutes an efficient barrier to protect iron particles against oxidation. Such a treatment may be used to perform environmental barrier coatings around magnetic powders.
Aluminum coatings were created onto glass beads by chemical vapor deposition in a fluidized bed reactor at different temperatures. Two different routes were examined. First, tri-isobutyl-aluminum (TIBA) vapor was enriched in nitrogen and thermally decomposed inside the fluidized bed to deposit elemental aluminum. On the other hand, liquid injection of TIBA via a two-fluid nozzle directly into the fluidized bed was tested. To ensure homogeneous coating on the bed material, the fluidizing conditions necessary to avoid agglomeration were investigated for a broad range of temperatures. Also, different glass types and pretreatments of the substrate surface were investigated to elucidate the influence of the surface chemistry on the growth and morphology of the layers deposited.
Aluminum coatings were created onto glass beads by chemical vapor deposition in a fluidized bed reactor at different temperatures. Nitrogen was enriched with Triisobutylaluminum (TIBA) vapor and the latter was thermally decomposed inside the fluidized bed to deposit the elemental aluminum. To ensure homogeneous coating on the bed material, the fluidizing conditions necessary to avoid agglomeration were investigated for a broad range of temperatures. The deposition reaction was modeled on the basis of a discrete particle simulation to gain insight into homogeneity and thickness of the coating throughout the bed material. In particular, the take-up of aluminum was traced for selected particles that exhibited a large mass of deposited aluminum.
For better understanding the process of particle coating by chemical vapor deposition (CVD) in the fluidized bed, the simulation of the deposition process was combined with a discrete particle model (DPM). Based on the experimental results of the thermal decomposition of tri-isobutyl-aluminum (TIBA) to produce aluminum onto glass beads, mechanisms on the micro-scale were investigated by single particle tracking. Zones of excessive growth as well as zones of insufficient mixing were identified. In particular, the take-up of aluminum was traced for selected particles that exhibited a large mass of deposited aluminum what in turn provides insight into the homogeneity and quantity of the coating throughout the bed material.
Powder metallurgy techniques were used to manufacture metal matrix composites. M3/2 high-speed steel was used as the matrix, and NbC and TaC, in different percentages, as reinforcements. Graphite was added to the M3/2 powders, to compensate carbon losses during sintering, and copper–phosphorous (Cu–P) to promote liquid phase sintering. The conventional powder metallurgy (P/M) route consists of dry mixing, uniaxially compacting (at 700 MPa) and vacuum sintering (at 1190°C and 1230°C) for 30 min. Their characterisation included a broad microstructural study by optical and scanning electron microscopy (SEM). The wear behaviour of all the sintered materials was studied. Wear tests were carried out on polished samples through a pin-on-disc test, using alumina as countermaterial. Wear tracks were analysed by SEM to clarify wear mechanisms. Wear is abrasive in all the materials and both NbC and TaC withstand wear. Only some TaC particles have been detached and spread across the wear track. NbC composites show a wear track clean of carbide particles.
Attempts were made to reduce the sintering temperature required to density two different grades of high-speed steel by the introduction of copper-phosphorus to promote liquid-phase sintering and by adding carbon to lower the solidus temperature. The use of nitrogen-atmosphere sintering in an effort to lower the sintering temperature through absorption of interstitial nitrogen into the steel did not succeed. Copper-phosphorus combined with carbon additions was successful in reducing sintering temperatures by approximately 100 C but was more effective in the M3/2 (molybdenum) than in the BT6 (tungsten grade) steels. Densification via a liquid-phase sintering mechanism occurs in a number of stages as different liquid phases are produced by interaction between the copper-phosphorus and the steel. Growth of carbides by solution and reprecipitation due to the transport of alloying elements through these liquid phases is suggested as a major densification process.
Reactive chemical vapour deposition (RCVD) has been used for the coating of the single filaments of a PAN-based carbon yarn by SiC. A continuous process has been developped, it consists to react a mixture SiCl4-H2 with the fiber, at T 1000°C, under, atmospheric pressure. By the aid of thermodynamic calculations and experimentation, the conditions of RCVD (gaz composition, temperature, reaction time) are optimized to elabore a coating without degrading the fiber. It is shown that a SiC-β coating, 0.05 micron thick, modified slighty the characteristics and is sufficient to reduce the rate of combustion of the fibers by a factor closed to 100.
Iron powders have been coated with a number of metallic coatings containing copper, nickel, and lead. An atomised iron powder was used as the substrate on which these metallic coatings were deposited using three 'wet' techniques, namely: (i) electrolysis; (ii) electroless (or chemical) methods; and (iii) chemical displacement methods. A liquid based fluidised bed reactor was used, which enabled multilayered deposits to be laid down on the iron particles. The results show that it is possible to produce composite coatings such as Cu-Sn, Cu-Pb, for example, and coherent coatings are formed, giving an even thickness around each particle. The powders take up the properties of the coating once a critical thickness has been reached and this coating controls the mechanical behaviour of the powders. Copper coatings produced on iron particles by chemical displacement gave greater green strength to the compacts than coatings of copper produced by the other methods.
Attempts have been made to develop ceramic/metal composites based on nickel/alumina and copper/alumina particulate systems. The uniform distribution of metal phase along the alumina grain boundary was accomplished by electroless coating of the alumina powders. Up to 68 weight % of nickel and 32 weight % of copper could be deposited on the random shaped alumina particles. The coated particles were hot-pressed and sintered for densification. Preliminary results suggest that this may be a reasonable route to oxide/metal composites.
This paper describes the preparation of CVD coatings (Ta, TiC, TiN, TiNC and SiC) on carbon fibers and the effect of the deposition parameters on the mechanical properties of the fibers. Ta does not influence the fiber strength, whereas for the other coating materials the deposition conditions must be optimized in order to retain the original fiber properties. In view of the application of such coated carbon fibers in composites, their wetting behavior with liquid aluminum as well as the behavior of the coatings as diffusion barriers were studied. The coatings are instantly wetted only at temperatures so high that a partial penetration of aluminum through the thin coatings and hence a reaction at the surface of the carbon fiber leading to the formation of aluminum carbide become inevitable. The fiber strength of high performance fibers is then considerably impaired although low strength fibers are hardly affected. Wetting by liquid aluminum below 800°C can be achieved by using additional thin nickel layers. In this way the infiltration of coated carbon fiber bundles can be realized and theoretical fiber strength yields in accordance with the rule of mixtures can be obtained. This improved wetting behavior was found not to depend on the type of coating. The barrier effect of the coatings allows heat treatments at up to 600°C for short times with high fiber strength yields. Heat treatments of the order of 100 h, however, gradually decrease the composite strength. In order to retain high fiber strength during long heat treatments at high temperatures, a minimum coating thickness of some microns is needed. The preparation of such thicker coatings is discussed in detail. With such a procedure, one can obtain reinforcing elements comparable with commercially available boron or silicon carbide monofilaments.
The cemented structure of ceramic-metal composites is examined as a means of effective toughening in ceramics; Ni-TiC was identified as a favorable binder for Al{sub 2}O{sub 3}. A new process using plasma CVD was developed that could deposit TiC on fine Al{sub 2}O{sub 3} particles. The TiC-coated alumina powders were consolidated with nickel-based binders to produce the cemented structure. An indentation crack resistance test showed that TiC-coated alumina cermets impeded crack propagation effectively, improving substantially the toughness compared to conventionally processed alumina cermets.
Tribological properties are considered as system properties and not as intrinsic properties of materials. The great number of parameters which can influence the wear resistance does not permit one to simply characterize a material as good'' or bad'' for low-wear applications. However, the wear behavior of materials is strongly dependent on microstructural parameters: type and mechanical properties of the matrix, second phase distribution, presence of hard particles, solid lubricants or pores. High speed steels are conventional cutting tool materials with a high wear resistance. The purpose of the present work is the investigation of the microstructure of a powder metallurgical M3 class 2 high speed steel before and after tempering at 600[degree]C, by means of the development of Quantitative Image Analysis (QIA) techniques for analyzing complex microstructures such as those exhibited by high speed steels. In particular, the effect of tempering on the mean size and distribution of the primary M[sub 6]C and MC carbides, as well as on the porosity, is studied. These microstructural features influence to a significant degree the general wear behavior of high speed steels. Therefore, the results of Quantitative Image Analysis are discussed in association with the role of primary carbides and porosity on the tribological properties of the studied material.
Cu–P alloy powder additions to M3 class 2 high speed steels reduce the sintering temperature required to produce near full density by ∼100 K. Densification was found to occur in distinct stages, due to the successive formation of a series of liquid phases at various sintering temperatures. These phases, each of which gave rise to an identifiable change in densification rate, were identified as the ferrite-carbide-phosphide eutectic at ∼1050°C, melting of residual copper at 1090°C, and an austenite-carbide eutectic at 1150°C. An activated sintering model for densification by the formation of a phosphide rich grain boundary liquid was developed. This concluded that sintering occurs by rapid grain boundary migration due to solutionprecipitation across an unstable grain boundary liquid film. PM/0512