MRS Online Proceedings Library (OPL)

Induction coupled plasma deposition (ICPD) is currently being explored to produce metal matrix composite (MMC) materials in the aircraft engine/aerospace industry. This paper addresses the development and integration of process knowledge, process simulation, process sensors and control strategy, as part of the intelligent processing of materials (IPM) initiative for the orderly transfer of processing knowledge from laboratory to production. The driving force for IPM is the stringent quality constraints on the ICPD process. In order to meet these constraints, an intelligent process control strategy had to be developed, which involves two key elements – a process simulator and process sensors. The objectives of the process simulator were to establish control strategy algorithms through sensitivity studies on the process parameters, to provide a computer-aided tool to reduce the process development cycle time, and to provide further understanding of and improvement in the ICPD manufacturing process. The objectives of the process sensors were to provide experimental data for on-line process control, as well as validation of the process simulator.

During the course of this work, we have developed an innovative, fast-acting process simulator, which maintains fidelity to the process physics and retains all of the control variables, yet operates in a responsive, interactive manner. We have defined and demonstrated a process sensor system for measuring the deposit surface temperature, and identified several plasma transmission window regions for the ICPD process. We have developed a preliminary control strategy for the intelligent processing of MMC monotapes.

Ti-6Al-4V alloy test coupons were ion implanted using methane Plasma Source Ion Implanter at an energy of 30 KeV. Multi-energy ion implantation, carbon film deposition as well as ion beam mixing were involved in this process. The resulted carbon profile is flat-top near the surface which forms a TiC layer. The implanted layer has demonstrated high load capacity and long life time under pin on disk wear test condition.

Microlaminated metal matrix (M3 ) composite coatings of MCrAlY/Al2O3 were fabricated using coordinated two-gun low pressure plasma deposition rocessing. These coatings are shown to hold promise as durable thermal barrier coatings (TBC's) for aircraft engine applications. The properties of analogous M3 coatings of MCrAlY/Zr)2 Y2O3 also were evaluated and found to be significantly less durable when thermplly cycled.

Microstructural analysis of the M3 coatings revealed that no interaction occurred between the alumina and metal lamellae in the MCrAlY/Al2O3 TBC's. However, alumina and chromia layers formed at the metal/ceramic interfaces in the ZrO2-containing TBC's, and grew into the metal layers of the ZrO2-containing TBC's during air-cycling. This growth may contribute to TBC cracking and spallation by lowering the overall coating ductility.

The combination of broad materials flexibility and rapid solidification rates achievable during controlled thermal deposition processes provide material designers with exciting new opportunities. Titanium and aluminum alloys have been melted and deposited as dense (90%) sheet and foil products in a controlled low pressure, inert atmosphere chamber. A high frequency (rf) plasma torch is used for a wide range of powder feed sizes. Conversions of powder to deposit have exceeded 90% yields. The deposition chamber accommodates a 1.2 m diameter by 1.2 m wide rotating mandrel. The mandrel drive system and torches are controlled to achieve uniform deposit thickness and effective heat extraction.

To produce composite products, the programmable mandrel drive has been coupled with a continuous filament feed system to achieve precise spacing of fiber reinforcements. Initial fiber winding and matrix deposition trials utilized surrogate metal fibers, IN909 and stainless steel, until suitable high strength ceramic fibers became available. The spectrum of materials included metal/metal composites and particulate reinforced matrices. Deposits were characterized with respect to density, composition and metallurgical structure. Aluminum deposits were hot rolled to full density. Preliminary mechanical properties were determined. An overview of Alcoa work to date will be presented and some future composite materials synthesis opportunities will be described.

Two titanium metal matrix composites (MMC's) have been successfully fabricated from low pressure induction plasma sprayed monotape and their mechanical behavior has been characterized. Powders of Ti6Al-4V (Ti6–4) and Ti6Al-2Sn-4Zr-2Mo (Ti6-2-4-2) were used as matrix sources and the reinforcement was Textron Specialty Materials (TSM) SCS-6 silicon carbide fiber.

The importance of process control to minimize interstitial (O, N, H and C) contamination effects is discussed. Oxygen pick-ups were reduced to typically less than 200 ppm and total interstitial pick-up levels were in the range of 200–500 ppm.

Uniaxially reinforced composite panels of 4-ply construction were fabricated from these monotape materials by the HIP process and tension tests in the fiber direction were performed. Tensile strengths and elastic moduli averaged 1565 MPa (227 KSI) and 182 GPa (26.4 MSI) for the SCS-6/Ti6-4; and 1531 MPa (222 KSI) and 184 GPa (26.7 MSI) for the SCS-6/Ti6-2-4-2 for composite fiber volume fractions of 0.27 – 0.28 and 0.29 – 0.30, respectively. The results compared favorably with other fabrication approaches for these composite systems and with rule-of-mixture (ROM) predictions. It was concluded that SCS-6/titanium composites fabricated from plasma sprayed monotapes exhibit properties consistent with state-of-the-art MMC fabrication technology.

Alloys based on intermetallic compounds such as Ti3Al (alpha-two), offer excellent elevated temperature strength and creep resistance. These same properties, however, combined with the susceptibility of the materials to interstitial element contamination, limit the use of “conventional” thermomechanical processing (TMP) in the production of sheet product forms. Plasma spraying of intermetallic alloys offers an alternative, near net shape processing path to produce homogeneous sheet products, having fine grained, equiaxed microstructures. This paper details the microstructural evolution and associted mechanical properties of Ti-24A1-11Nb (at.%) processed to sheet form using RF plasma spraying. The response of the microstructure/tensile properties of the plasma sprayed alpha-two to various controlled heat treatments is discussed.

It has long been recognized that grain size has a marked effect on the subsequent processing characteristics of metallic materials. Plasma cold hearth melting is a promising technique for the production of reactive and refractory metal ingots. A study has been conducted to determine the effect of melt parameters on the subsequent grain size and shape in plasma cold hearth withdrawal of Ti-6A1-4V ingots. In the study, a number of ingots were made in Retech's cold hqarth development furnace and sectioned for examination. The relative importance of operating parameters such as surface heat distribution and flux and of stirring on grain size and shape are reported.

Plasma spraying of two yttria powders with different morphology (agglomerated and porous vs fused and dense) was investigated. Coatings were produced on steel and TZM (Ti-Zr-Mo alloy) substrates. Coatings microstructure, both degree of melting of particles and coating density, were measured by metallographic techniques and successfully related to plasma processing parameters. Microstructure were explained by particles behavior in the plasma jet. Yttria coatings were deposited on graphite using molybdenum as an interlayer to match thermal expansion differences and interfaces have been characterized by SEM observation.

A mathematical model for the analysis and design of RF, and hybrid DC/RF plasma torches for the thermal spraying of materials under conditions of high particle loading is presented. The model is based upon a numerical solution of Maxwell's equations for the electromagnetic field, the turbulent Navier/Stokes equations for the plasma velocity field, and the thermal energy balance equation for the temperature field. The interaction between the plasma and the injected particles is calculated using the Particle Source in Cell technique. The trajectories and thermal histories of nickel particles injected into a hybrid torch are compared to those of particles injected into a conventional RF torch. It is demonstrated that hybrid torches possess some superior characteristics over conventional plasma spray systems and may constitute a viable future technology for the spraying of advanced materials.

Yttria stabilized zirconia powder compacts were rapidly sintered in rf plasmas. Final sintered densities varied with plasma gas composition and plasma pressure. The final densities obtained at low pressures were comparable to those obtained at high pressures. This fact can be explained by plasma chemical effects which enhanced sintering at low pressures. Specimen exposure time in the plasma has a minor effect on the final sintered density. The experimental results demonstrates that the powder processing history has a strong influence on the sintering process. Sintered densities exceeding 97% of the theoretical value were obtained in less than 5 minutes in mixed plasmas. X-ray analysis of the sintered specimens shows the formation of single phased cubic solid solutions. SEM analysis of the high density samples shows uniformly fine-grained microstructures with some isolated intragranular micropores. Low density specimens show large open pores along the grain boundaries and at the triple points indicating sintering stops at an early stage.

Zirconium carbide has been synthesized using (RO)4Zr, a liquid organometallic precursor in a newly developed Triple Torch Plasma Reactor. Thermodynamic equilibrium simulations indicate that in the temperature range of 1,800 - 3,800 K, zirconium carbide can be formed by plasma pyrolysis. The calculation results also suggest that by adding a certain amount of CO2 into the plasma, the excess carbon can be removed. The product is characterized with X-ray powder diffraction, SEM and BET. X-ray powder diffraction profiles support the results predicted by the equilibrium calculations. The product powder is porous and spherical. The specific area of the powder measured by BET is 140 m2/gram.

Fine powders of aluminate, ferrite, and chromite spinels were successfully produced by a novel Counter-Flow Liquid Injection Plasma Synthesis (CF LIPS) method using a DC plasma jet. CF LIPS has been demonstrated to be an excellent method to produce fine ceramic powders with a narrow particle size distribution. The spinels produced by this techniques, both magnetic and non-magnetic, have a very similar particle size distribution. The powders produced were spherical, non-agglomerating and dispersable with average sizes of 1 μm.

Optical emission spectroscopy and laser Doppler velocimetry were done for an Rf inductively coupled plasma at atmospheric pressure in order to elucidate the super-rapid coofling of the thermal plasma during the growth of diamond thin film. Attention was given to the vicinity of the water-cooled substrate located 20 mm beneath the RF coil. It was found that at 1.5 mm above the substrate, the temperature of plasma was still high. At the temperature, high concentration of hydrogen atoms exist, which may take a important role in diamond growth.

Metallic coatings can be fabricated using the intense plasma generated by the metal vapor vacuum arc. We have made and tested an embodiment of vacuum arc plasma source that operates in a pulsed mode, thereby acquiring precise control over the plasma flux and so also over the deposition rate, and that is in the form of a miniature plasma gun, thereby allowing deposition of metallic thin films to be carried out in confined spaces and also allowing a number of such guns to be clustered together.

The plasma is created at the cathode spots on the metallic cathode surface, and is highly ionized and of directed energy a few tens of electron volts. Adhesion of the film to the substrate is thus good. Virtually all of the solid metals of the Periodic Table can be used, including highly refractory metals like tantalum and tungsten. Films, including multilayer thin films, can be fabricated of thickness from Angstroms to microns. We have carried out preliminary experiments using several different versions of miniature, pulsed, metal vapor vacuum arc plasma guns to fabricate metallic thin films and multilayers.

Here we describe the plasma guns and their operation in this application, and present examples of some of the thin film structures we have fabricated, including yttrium and platinum films of thicknesses from a few hundred Angstroms up to 1 micron and an yttrium-cobalt multilayer structure of layer thickness about 100 Angstroms.

By combining pentaborane (B5H9) and decarborane (B10H14) with methanein a plasma reactor, a variety of boron-carbides can be made over a wide range of compositions. The resulting thin films have uniform composition and appear to be polycrystalline.

Si3N4 films have been deposited on silicon substrates at high temperatures (800–1000° C) in a plasma CVD hot wall reactor using SiF4 and NH3 as primary reactant gases. In this range of temperatures the activation energy is 35.9 Kcal mol−1 grad−1. The effect of an RF plasma induced either in the up or in the down stream configuration has been evaluated. The results show that in the 200–400 w range the reaction rate increases linearly with the RF power. The addition of hydrogen to the above gas mixture also produces an enhancement of the deposition reaction, probably as a consequence of the inhibition of the etching effect of the fluorine atoms on the Si3N4 deposited layers.

Y-Ba-Cu-O superconducting films were prepared on polycrystalline CaO stabilized ZrO2 substrates using RF thermal plasma chemical vapor deposition. The important feature of this work is that superconducting films can be deposited at a high rate of ˜0.1 μm/min in the as-deposited state without post-oxidation. Mixed aqueous nitrate solutions of yttrium, barium and copper were used. The liquid precursors were atomized by an ultrasonic nebulizer and introduced into the RF thermal plasma using oxygen as a carrier gas. Different substrate temperatures of 450 °C, 500 °C and 600 °C were used during deposition. The substrate temperatures were controlled by varying the feed rate of cooling gas to the substrate holder. X-ray diffraction, SEM and transition temperature measurements were used to characterize the as-deposited films. The X-ray diffraction patterns show that the as-deposited films were preferentially oriented with the c-axis perpendicular to the plane of the films.

Soda-lime-silica glasses were treated by an argon plasma, that was generated by an inductively-coupled rf power supply. The surface composition of the treated glasses were profiled using SIMS, and the glass structure was probed using diffuse reflectance FTIR spectroscopy. A Buehler Micromet II (micro hardness tester) was used to measure hardness. The effects of various process parameters such as temperature, gas pressure and treatment time on glass surface composition, structure and properties are discussed. The results show that the surfaces of the treated samples were dealkalized to some depths as great as 0.5 μ. The surface structure of the treated glasses is close to that of pure silica glass and surface hardness is improved after plasma treatment.

The detection and quantification of the diamond cubic, graphite, and amorphous carbon phases in plasma assisted chemical vapor deposition of diamond films is difficult by diffraction techniques. We have employed thermogravimetric analysis (TGA) to analyze these phases by comparison of the oxidation rates, both in the as deposited films and in physical mixtures of powders of the phases. The oxidation rates of the three allotropes of carbon differ enough to allow a separation of peaks in the differential of the weight loss versus time/temperature plots. TGA scans were run between 2.5 and 100 degrees per minute to achieve optimum separation of the oxidation rates as indicated by the weight loss curves. The peak of the differential of the weight loss versus temperature curves indicates the temperature of maximum oxidation rate. The appearance of multiple peaks appears to be a sensitive indicator of the presence of mixed phases in the PACVD deposites on silicon.

Polycrystalline diamond coatings have been deposited on metal substrates using a 50 kW atmospheric pressure inductively coupled plasma torch. The argon-hydrogen-methane plasma generated has a free stream active area of 35 cm2 and a temperature of approximately 4500 K. Growth rates are of the order of 10 μm/hour. The growth morphology is found to vary significantly with reactor processing conditions as well as gasdynamic effects near the substrate surface. In this work, we explore the effects of varying the parameters controlling both the surface kinetics (surface temperature and near surface flowfield) and gas phase chemistry (initial gas feed composition and plasma temperature). Stagnation point and flat plate boundary layer flows are investigated. Scanning electron microscopy indicates that well facetted crystals are obtained with growth along the 100 and 111 planes. Nearly continuous films are also formed and found to be of lower quality. Raman scattering data is used to/compare the bonding structure to that obtained by other various deposition techniques.