Coatings to Resist Wear at High Temperatures
Wednesday, April 12, 2000 1:30 PM in Room Royal Palm Salon 1-3
A2-1 CVD Coatings for Erosion Protection at Elevated Temperatures
V. Shanov (University of Cincinnati)
Chemical vapor deposition (CVD) has been an important technique significantly impacting the aerospace, cutting tool and other industries, where erosion, wear, and corrosion protection play crucial roles. While in usage for many years, this technique will remain one of the main driving forces behind surface engineering in the next few decades because of its versatility, and ability to be easily scaled up for industrial applications at a reasonable price. The review will provide data demonstrating promising results on CVD coating for erosion protection at elevated temperatures. In addition, conventional and advanced Electron Cyclotron Resonance (RCR) Micro-Wave CVD facilities plus related thin film processing of hard coatings will be discussed.@paragraph@Small solid particles entrained in a gas-fluid flow can cause degradation of the component surfaces containing the flow by erosion and corrosion processes. Some of the systems with operations involving this problem are: heat exchangers, steam and gas turbines, aircraft engine flow inlets and many others. Different coatings have been introduced recently to control the resulting metal wastage to tolerable levels. The objective of the reported research conducted in the last few years were to develop CVD erosion barriers for surface protection of metal and ceramic materials like: Ni-based superalloys, stainless steel, cemented tungsten carbide, and mullite at high temperatures, and to study the erosion behavior of the coatings. Thermal and micro-wave plasma CVD techniques were utilized to apply titanium carbide (TiC), titanium nitride (TiN), alumina (Al@sub 2@O@sub 3@), and polycrystalline diamond (PCD) on different substrates, frequently used for manufacturing of turbine and rocket components, cutting tools, and high-temperature parts. The erosion behavior of the coated specimens were investigated experimentally by exposing them to abrasive particle-laden flow in a high temperature wind tunnel. The eroded surfaces were then examined by scanning electron microscopy (SEM). The obtained results reveal the effects of velocity, temperature and impingement angle on the erosion rate. In addition, further data show the variation of the coating erosion rate with the quantity of the abrasive particles used. It was found that the CVD ceramic coatings protect better the base materials at elevated temperatures. The coated alloys reveal one order of magnitude less wear compared to some commercial non-CVD coating on the same substrates. The conducted study demonstrates that the tested CVD coating provide an excellent erosion resistance to MAR 234, Waspaloy, INCO 718, stainless steel 410, WC-Co, and mullite-based ceramic, when exposed to high temperature in particulate flow environment.@paragraph@In this paper, special attention is given to CVD polycrystalline diamond coatings for wear protection. New experimental data at elevated temperatures are obtained and presented, which demonstrate that PCD is emerging as a promising erosion protective coating for high temperature applications.
A2-3 High Temperature Erosion of Thermal Spray Carbide Coatings
M. Dorfman (Sulzer Metco); W. Tabakoff (University of Cincinatti); K. Laul (Sulzer Metco (US) Inc.)
Chrome Carbide-Nickel Chrome coatings are used in aerospace and industrial applications for their good corrosion resistance, erosion and sliding wear properties at high temperatures. Typical applications of these coatings include hard-chrome replacement, boiler tubes, ball valves, and exhaust stacks and gas turbine engine components. Chromium carbide based powders obtained by various manufacturing methods - pre-alloyed, plasma densified, and chemical clad and simple blends - have been studied. In this paper, coating erosion properties and their relationship to powder manufacturing methods are reviewed. The properties particularly reviewed include room temperature and high Temperature erosion. The effect of erosion variables - abrasives type, erosion temperature and erosion angle - will be discussed. Coating microstructure and characteristics will be related to coating performance. Studies specifically show the improved performance of recently developed prealloyed carbide materials. Results indicate deposition efficiencies up to 50% higher than those of currently available carbide powders via the HVOF equipment do. In addition, high carbon retention and low oxidation while spraying has been measured. This results in excellent surface finish of the superfinished coatings. In addition, due to the high carbide retention, the erosion and wear properties of the coatings are reported to be good.
A2-4 Erosion Study of Magnetron-sputtered TiAlCrYN Coating for Turbo-machinery Application
W. Tabakoff (University of Cincinatti); V. Shanov (University of Cincinnati); W.-D. Munz, P.Eh. Hovsepian (Sheffield Hallam University, United Kingdom); I.J. Smith (Bodycote SHU Coatings, LTD)
Turbomachinery operated in particulated environment are exposed to erosion, which decreases the engine performance and life. This paper describes an experimental investigation on erosion behaviour of magnetron-sputtered TiAlCrYN coating for use in turbomachinery and other systems, exposed to high-temperature erosion environment. The coating is tested in a high temperature erosion wind tunnel using solid abrasive particles.
A2-5 Innovative Solutions to Prevent Erosion/Corrosion Degradation in Water Cooled Ducts for EAF Fume Extraction
A. Sanz (Danieli & SpA. Centro Research and Development (CRD), Italy)
Water Cooled Ducts (WCD) in the fume Direct Extraction Systems (DES) are generally identified as erosion-prone areas. The introduction of several contaminants in the scrap or during the operating practice have introduced corrosion to the already erosive environment. The protection of the WCD components to erosion/corrosion conditions is essential to minimize maintenance costs and the loss of available production time. Advanced protection solutions for fumes WCD is a relatively new field. Several solutions supplied or under study in the steel industry are reviewed and compared in those used in the field of fuel boilers. Enhanced longevity and reliability are the main followed objectives aimed at attaining a significant reduction of downtimes at the scheduled outages. New advanced steel production methods and more rigorous operating conditions are also opening the possibilities for new coating techniques and materials.
A2-7 Reduction of High Temperature Oxidation in Titanium Diboride Thin films by Silicon Doping
S. Mollica, M.K. Ghantasala, D.K. Sood (RMIT University, Australia)
Titanium di boride is an attractive material for protective coatings due to its high hardness and resistance to wear. However, many of its high temperature applications are limited because of its poor oxidation behaviour at temperatures beyond 700@super o@C. This paper presents a novel approach to improving the coatings' oxidation resistance at 700@super o@C and above by doping with silicon. Titanium di boride films were deposited onto Si(100) substrates using a DC magnetron sputtering system. Silicon doping in certain films was achieved using a ‘mosaic sputtering target’ having silicon pieces pasted on to the TiB@sub 2@ target. Size of the silicon pieces was decided by the required silicon composition in the film. Films were deposited with three different silicon dopant concentrations, 5%, 10% and 20% and two different thicknesses 200 and 800 nm. These samples were vacuum annealed at 700@super o@ C and 800@super o@C at 5x10@super -7@Torr for four hours to ensure that they were crystalline. Samples were then oxidised in air at 700@super o@C and 800@super o@C for four hours to investigate their oxidation resistance. These films were analysed using Rutherford Backscattering Spectroscopy (RBS), X-ray diffraction (XRD) and Secondary Ion Mass Spectrometry (SIMS) techniques. Sheet resistance of these films was measured using a four probe measurement setup. Sheet resistance of the films varied in the range 9 ohms/ÿ to 112 ohms/ÿ depending on the silicon content in the films. Films were stoichiometric and turned completely crystalline after annealing in vacuum. SIMS analysis indicated that the doping is uniform throughout the thickness of the films. Pure TiB @sub 2@ films after annealing in air at both temperatures are oxidised forming TiO@sub 2@ and B@sub 2@O@sub 3@. As silicon content increased to around 20%, we observed that the films didn’t form any of the above oxides. Silicon doping has clearly improved the oxidation resistance of these films at both the temperatures investigated.
A2-8 Plasma Nitridng Behaviours of Simultaneously Al-Cr Diffusion Treated Alloy Steels
S.Y. Lee, J.H. Yang, J.S. Lee (Hankuk Aviation University, Korea); J.H. Lee (Korea Institute of Machinery and Metals, Korea); S.C. Yang (SamHwa Brazing Alloy Co. Ltd., Korea)
Plasma nitriding on simultaneously Al-Cr diffusion-treated AISI H13 and STS 403 steel was performed and analysis of microstructure, microhardness, cyclic oxidation behaviors and high temperature wear resistance of these duplex-treated steels were made. The results were compared with those from steels treated by single process of either simultaneous aluminizing-chromizing or plasma nitriding. Both steels showed a FeAl compound layer of approximately 350µm thickness on the surface after simultaneous diffusion coating and nitrided layer of approximately 70~80µm formed after the subsequent plasma nitriding process. The microhardness was improved much more by the duplex surface treatment than only by plasma nitriding. In addition the duplex treated specimens showed an improved high temperature wear resistance. Results from the cyclic oxidation test at 900@super o@C showed that much improved oxidation resistance could be observed from the duplex treated specimens and this could be attributed to formation of a Cr@sub 2@O@sub 3@ layer as an additional protective layer on the surface of the multi-element duplex-treated specimen. Detailed experimental results will be presented.
A2-9 Influence of Powder Manufacturing on Performance of Chromium Carbide Coatings
K. Laul, M. Dorfman (Sulzer Metco (US) Inc.); S. Zimmermann (Sulzer Metco, Germany); P. Heimgartner (Sulzer Innotec, Switzerland)
Thermally sprayed coatings, particularly chromium carbide and tungsten carbide coatings applied by High Velocity Oxy-fuel Spraying (HVOF) are widely used for hard chrome replacement applications. Typical applications of these coatings include industrial valves, boiler components, steel mill, paper mill and printing rolls, and gas and land based turbine components. Coatings of Cr3C2-NiCr are preferred when the application requires the excellent corrosion resistance of the NiCr alloys and the reasonable wear and erosion resistance of the Cr3C2 at temperatures up to 900@super o@C.@paragarph@In the present study the influence of powder manufacture process, carbide content and carbide size of the powder on microstructure, wear resistance, and corrosion behaviour of HVOF sprayed chromium carbide coating was examined. The experiments performed comprise electrochemical tests, salt spray test, immersion tests, and various wear tests. The influence of the spray parameters on oxidation and decarburisation of the powder and on the properties of the coatings was examined, as well.
A2-10 New Generation of Coatings for Aluminum Die Casting Industry: Solving the Soldering Problem
G.B. DeMaggio, R.R. Aharonov, S. Chellapilla, B. Janoss (Multi-Arc Inc.)
A new generation of coatings to resist corrosion and soldering of molten aluminum has been developed. The H13 core pins were coated with the new ternary coatings. The tests were performed in molten A380 with the pins stationary and rotating. The performance by means of weight loss measurements of the coated pins seem to be at least an order of magnitude better then the current industry standard. Also the new coatings offer better protection for features such as sharp corners. Preliminary testing simulating thermal fatigue showed dramatic improvement as well. Field tests were performed by commercial die casters. Improvement of performance over other commercially available treatments of at least two to three times have been reported. Properties of these new materials are discussed in the paper as well.
A2-11 Local Thermal Characterization of Refractory Metallic Coatings Aging of a Gun Tube Inner Coating Sudy
C. Gervaise, C. Novals (ENSICA, France); O. Gagliano, J.-J. Serra (DCE/CTA/LOT, France); S. Serror (DCE/CTA/SMP, France)
Thermomechanical stresses generated in a large caliber gun tube during a shot are such that the internal surface has to be protected by a hard and refractory coating. Modelling the behavior and selecting new materials for coatings necessitate the knowledge of their thermophysical parameters, generally very different of bulk materials ones. The microstructure of these layers, originating properties gradients or anisotropies, evolves following the thermal cycling suffered by the tube, making evolve at the same time the whole of their characteristics. The identification of these parameters, at the scale of the coating thickness, and the study of their evolution in course of time constitutes an important research topic in our labs. In the present work, a photothermal microanalysis method has been used to measure the local thermal diffusivity within a coating. These results have been compared with microhardness measurements undertaken on the same sample. The experimental method is derived from that introduced by Rosencwaig et al. in 1986 . It concerns a photothermal method whose periodic excitation is localized on a micron-scale spot. When modulation frequencies are higher than 10 kHz, the thermally excited volume does not exceed some µm3. In this volume, the temperature field is characterized by the amplitude and the phase - compared to the heating reference - of its periodic component. Local properties estimations are deduced from the temperature evolution in a micron-scale zone as well . This identification method by thermal transfer function has been applied on the study of an aged chromium coating on which several local diffusivity measurements have been undertaken along the layer depth and the steel used as substrate. The diffusivity distribution shows a gradient along the depth. The diffusivity is higher near the surface than to the interior of the coating that has undergone less important heatings during the gun lifetime. This behavior is comparable to the microhardness evolution along the coating depth. The coating microstructure evolution seems to be the cause of the two phenomena. After several shots, the coating is partially restored, perhaps recristallized; large grains can be observed near the surface. This evolution, being accompanied by a structure rearrangement and a decrease of the crystal defect density, involves the progressive return to equilibrium state of the material. This local evolution of the matter can explain the hardness decrease and the thermal diffusivity increase.  Rosencwaig A., Opsal J., Smith W.L., Willenborg D.L., Detection of thermal waves through modulated optical transmittance and modulated optical scattering, J.Appl.Phys, (1986), 1392-1394  Milcent E., Benet S., Gervaise C., Nouals C., Serra J-J., Microdiffusivity Measurement on Refractory Metal Coatings by Photothermal Microscopy, High Temperatures High Pressures, (1998), 85-90