Coatings to Resist Wear at High Temperatures
Wednesday, May 2, 2001 1:30 PM in Room Royal Palm Salon 1-3
A2-1-1 New PVD Coatings Applied to Dies for Extrusion of Aluminium
T. Björk (Karlstad University, Sweden); M. Berger, R. Westergård, S. Hogmark (Uppsala University, Sweden); J. Bergström (Karlstad University, Sweden)
The bearing surface of dies for extrusion of aluminium is subjected to very difficult wear conditions. To minimise wear and thereby getting a long service life these dies are typically made of hot work tool steel and surface treated by nitriding.@Paragraph@ In the present study both commercial and potential, experimental PVD coatings are evaluated in a device which simulates the bearing wear in an extrusion die. The tested surfaces include hot work tool steel coated with PVD TiN, CrN, (Ti,Al)N and TiB@sub2@, respectively, and for comparison, uncoated tool steel. To simulate the extruded material an aluminium cylinder heated to 550@super o@C is used as counter body in a block-on-ring configuration. Prior to wear testing, the test surfaces were characterised by roughness, hardness, thickness and intrinsic stress measurements.@paragraph@ The simulation shows that coatings of CrN, (Ti,Al)N and TiB@sub 2@ effectively reduce the wear in extrusion dies. These coatings are mechanically, thermally and chemically stable when sliding against hot aluminium, in contrast with uncoated steel. It was also observed that the mechanical strength of the TiN coating severely degenerated during this test, probably due to a poor chemical inertness when exposed to hot aluminium.
A2-1-2 Influence of Deposition Temperature and Time During PVD Coating of CrN on Corrosive Wear in Liquid Aluminium
A. Persson, J. Bergström, C. Burman (Karlstad University, Sweden); S. Hogmark (Uppsala University, Sweden)
Erosive and corrosive wear are two major life-limiting factors in die casting dies. To resist the corrosive and erosive attack from molten metal flow the tool surface needs to be hard and chemically inert. It has been indicated that local coating defects rather than intrinsic deficiencies limit the potential gain of coatings on hot work tool steel exposed to liquid aluminium. This motivates a search for suitable protective coatings on the dies. A number of ceramic coatings are of interest. @paragraph@ In this work, physically vapour deposited (PVD) CrN coatings were applied on hot work tool steel specimens and treated in an aluminium melt. Substrate temperature and time during deposition were varied to give coatings with individual properties. Type and density of defects as well as thickness of the coatings were characterised for all coatings. The influence of substrate temperature and time during deposition on coating characteristics and corrosion resistance was studied as well as the mechanism of corrosion damage.@paragraph@ It is clearly demonstrated that liquid aluminium corrosion of CrN coated tool steel is initiated at defects which penetrate through the coating, and localised corrosion pits are formed. Subsequently, the pits coalesce and the corrosive attack aggravates. Consequently, the corrosion resistance is improved by reducing the density of defects through the coating, which, for PVD CrN, was achieved by increasing the coating thickness.
A2-1-3 Development and Implementation of Plasma Sprayed Nanostructured Ceramic Coatings
M. Gell, E. Jordan, L. Shaw (University of Connecticut); Y.H. Sohn (University of Connecticut "now with" University of Central Florida); T.D. Xiao (Inframat Corporation)
A broad overview of the science and technology leading to the development and implementation of the first nanostructured coating is described in this paper. Nanostructured alumina and titania powders were blended and reconstituted to sprayable size. Thermal spray process diagnostics, modeling and Taguchi design of experiments were used to define the optimum plasma spray conditions to produce nanostructured alumina-titania coatings. It was found that the microstructure and properties of these coatings could be related to a critical process spray parameter (CPSP), defined as the gun power divided by the primary gas flow rate. Optimum properties were determined at intermediate values of CPSP. These conditions produce limited melting of the powder and retained nanostructure in the coatings. A broad range of mechanical properties of the nanostructured alumina-titania coatings was evaluated and compared to the Metco 130 commercial baseline. It was found that the nanostructured alumina-titania coatings exhibited superior wear resistance, adhesion, toughness, and spallation resistance. The technology for plasma spraying these nanostructured coatings was transferred to the U.S. Navy and one of their approved coating suppliers. They confirmed the superior properties of the nanostructured alumina-titania coatings and qualified them for use in a number of shipboard and submarine applications.
A2-1-5 New Coatings for Continuous Casting Rolls
A. Sanz (Danieli & C. Centro Research and Development, Italy); J.-P. Bernadou (Ecole Nationale Sup@aa e@rieure de l'A@aa e@ronautique et de l'Espace, France)
Rolls in steel industry withstand very high loads, thermal cycling (and thermal fatigue) and severe environmental aggression. Wear is the major single cause of failure in bare rolls (and in some coated applications). In the case of coated rolls, it has been observed that under high load the coating might fail by rolling fatigue process instead of wear. As any fatigue damage, the process is hardly predictable leading to sudden delamination caused by fatigue surface cracking. Poorly bonded coatings and/or not dense coating as well as the presence of tensile residual stresses can translate into spalling of the deposit. The fatigue behavior of coatings is not only depending on the Hertzian contact pressure alone but it is highly influenced by the machine characteristics (tribological duty cycles, vibrations, geometrical features of the contact, eventual debris formation, etc). Rolls are mainly exposed to: a) thermal fatigue due to high temperature thermal cycling (this is the most critical parameter), b) mechanical stresses due to the high bending stresses associated with the slab weight, c) corrosion-oxidation, d) Wear due to abrasive solid mold fluxes and oxide scale on the slab surface. The reciprocating tribological test allows characterizing failures due to the thermal and mechanical stresses. The test stroke must be equal or higher than 2 mm to effectively evacuate the eventual debris formed during the test (to keep out of the fretting field). A disk coated with the layer to be characterized was the static partner and a WC-Co ball will be used as counterpart. The test cycle enhances the thermal stresses due to the difference of temperature and casting speed. The test is divided in two main phases that are repeated until maximum test time is reached. Phase 1 has a duration of 60 minutes at 400 °C, 10Hz, 30N (Hertzian pressure 1300 MPa) and phase 2 lasts 15 minutes at 900 °C, 1Hz, 30 N (Hertzian pressure 1300 MPa). The cycle was repeated until the end of the coating lifetime or interrupted after 8 hours. A second test for a shorter period of time (50% of the total test duration) was carried out. All tests are in ambient air without any lubricant. The coatings were also characterized by abrasive wear tests. This measurement is performed with abrasive slurry (SiC particles in water). The test is a three-body wear system (the coated substrate, a rotating sphere and the abrasive slurry) with a preselected load. The position of the sphere relative to the sample and the contact load must be constant. The sphere made in 100Cr6 steel, the test load and speed are 0,4 N and 10 cm/sec respectively (so as to ensure a hydrodynamic regime between sphere and the substrate). Abrasive tests last 1 minute. Several coating obtained with different coating techniques (laser cladding, thermal spray + chemical slurry and welding) were characterized to validate The objective of this work is design coatings able to cast at least 2,000,000 tons with a coated roll. This aimed average life is double as compare with a bare roll, 50 % more than a roll with an AISI 420 welded coating or in other terms, two years of thin slab casting production.
A2-1-7 Tribological Behaviour of Coatings for Continuous Casting of Steel
A. Sanz (Danieli & C. Centro Research and Development, Italy)
There is a large number of steel making processes in which great demands are made on the surface behavior of several components that come to direct contact with steel under various conditions. Continuous casting is mainly a heat-extraction process. The mold must rapidly transit heat from the steel to the cooling water. In continuous casting, steel solidification starts when it gets in contact with the mold liner’s inside surface. The key job of every mold consists in cooling the molten steel in a controlled way. The mold is a major element in the overall economics of a continuous casting plant which explains the number of innovative approaches to increase the working life (length of time during which the mold shows acceptable dimensional stability to meet the quality standards) or to satisfy the new demands to be met by the mold liners. The mold inner coating is a firmly established practice, in particular with electrolytic surface modification treatments to cope with the various operating needs including low wettability, high hardness, good wear resistance and low cost. Several pin-on-disk tests have been carried out to determine the friction and wear behavior of different coatings. The friction partner for all coatings was a K30 (WC-Co 9%) chip, the sliding speed was 6 m/min at a temperature of 250 °C and a load of 5N. The sliding distance was fixed as 200 hours ( 720,000 revolutions for a radius of friction of 16 mm). Additional tests for shorter times allow verifying the morphological evolution of the wear track. All coatings were also evaluated by scratch test. This test allows introducing stresses at the interface between the coating and the substrate as the sample is displaced at constant speed. The critical load (Lc) recorded from the scratch test translates the complex intrinsic properties of a specific coating into a very reproducible figure of great practical significance. The paper presents a tribological characterization of conventional electrolytic coatings, bare copper alloys and some new surface solutions for steel continuous casting molds.
A2-1-8 High-Rate Sputter Deposited Tantalum Coatings on Steel for Wear and Erosion
S.L. Lee, D. Windover, M. Audino (US Army ARDEC, Benet Laboratory); D.W. Matson, E.D. McClanahan (Pacific Northwest National Laboratory)
A rifled 4340 steel liner of 20mm inside diameter was plated with tantalum coatings at 13-microns/hr using krypton gas and a triode sputtering deposition system. The liner was subjected to 1500 cycles of high temperature and high pressure firing tests and appeared almost intact. The analysis of the structural properties of the coatings after firing is presented. X-ray diffraction of the land and groove areas showed fairly uniform bcc phase surface tantalum across the length of the coatings. Surface zinc oxide, firing debris from the zinc phosphate process of the projectiles, was observed with increasing intensities towards the muzzle end. The cross section analysis showed large areas of soft all bcc-phase tantalum, and areas with surface bcc-tantalum and tetragonal phase tantalum at the coatings and steel substrate interface. X-ray intensities on the coatings surface compared with tantalum powder showed near random orientation in major parts of the coatings, although pole figures from a piece cut from the muzzle end showed preferred (211) orientation and texture. Residual stress analysis showed surface stress distribution ranging from -280 MPa to -700 MPa. Previously electrochemically deposited tantalum liners in molten salt appeared intact after 1097 and 5034 cycles of firing tests@footnote 1@. This work is the first to demonstrate that sputtered tantalum coatings can be successfully deposited on the inside of steel gun barrels for the mitigation of wear and erosion. However, optimization of the sputtering conditions and parameters is necessary to further improve the coatings properties. Structural properties important to protective coatings, such as phase, orientation, density, porosity, and stress are discussed. @FootnoteText@ @footnote 1@Analysis of magnetron-sputtered tantalum coatings versus electrochemically deposited tantalum from molten salt, S.L. Lee et al, Surface and Coatings Technology 120-121 (1999) 44-52.
A2-1-9 Laser Pulse Heating of Protective Coatings for Large Caliber Gun Bores
P.J. Cote, G. Kendall, M.E. Todaro (US Army Armament Research, Development and Engineering Center, Benet Laboratories)
The authors have developed laser pulse heating (LPH) as a tool for studying erosion processes affecting the bores of large caliber guns and for evaluating newer, emergent coatings for bore use. LPH isolates heating from other contributors to bore erosion, including chemical attack by propellant gases, mechanical wear from projectile passage, and mechanical loading from gas pressurization. Bore coatings typically experience brief, intense periods of heating as short as one to five milliseconds. This heating can cause melting, metallurgical transformations, transformational stresses, and surface cracking.@paragraph@In LPH, a sample is heated with a pulsed laser to duplicate the heating of gun firing and then examined under a microscope to determine the effects of such heating. LPH can be used on bore materials without the difficulty and expense of testing them in an actual gun.@paragraph@In an effort to validate the method, as well as provide insight into the erosion process, LPH was applied to so-called high-contractile electroplated chromium, which has long been used to protect large caliber gun bores. In this coating, LPH was shown to produce the same sorts of thermal damage that actual gun firing does: recrystallization and grain growth in the chromium, development of major cracks in the chromium, a heat-affected zone in the substrate steel, and rapid oxidation of the steel at the tips of the chromium cracks.@paragraph@LPH was also applied to bare steel as well as a number of other coatings, including low-contractile electroplated chromium and magnetron-sputtered tantalum. In all cases, LPH offered new insights into the degradation of bore materials due to repetitive heating.@paragraph@.
A2-1-10 High Temperature Erosion Protective Coatings for Turbomachinery Use
W. Tabakoff, V. Shanov, G. Drenski (University of Cincinnati)
Because of the serious consequences of turbomachinery erosion on performance and life, it is important to have reliable methods for predicting erosion when solid particles are ingested with the incoming flow. Several studies, which are essential in predicting blade surface erosion intensity and pattern, have been conducted at the University of Cincinnati Turbomachinery Erosion Laboratory. This paper will present experimental erosion results for a number of coated blade materials due to high velocity solid particle impacts at elevated temperature. The testing was conducted in a high temperature erosion wind tunnel, which simulates the aerodynamic conditions on the blades. Some of the coatings included: rhodium platinum aluminized, TiC, TiN, and aluminum oxide. In addition, the erosion rates at high temperature for Alanx Ceramics (75% SiC, 15% Al2O3, and 10 %Al-metal) was investigated. The velocity of the particle laden flow varied from 180 to 305 m/s, an the temperature from ambient to 815? C. The impingement angle changed from 15 to 90 degrees. The erosive wear of the coated samples and of the solid ceramics were investigated experimentally.
A2-1-11 Erosion Testing of Coatings for V-22 Aircraft Applications
G.Y. Richardson, C.S. Lei (Naval Air Station); W. Tabakoff (University of Cincinnati)
High velocity (600ft/s) sand erosion testing utilizing a wind-tunnel facility was conducted to evaluate developmental coatings from 3 separate companies under Navy phase I SBIR program funding. This sbir solicitation addressed a particular problem the V-22 (Osprey) helicopter was having with regards to ingestion of sand particles by a titanium impeller that was associated with the aircraft environmental control system. The coatings, deposited on titanium substrates and erosion tested included: (1) WC/TaC/TiC processed by electro-spark deposition, (2) SiC/DLC multilayers deposited by CVD and (3) polymer/ceramic mixtures via aqueous synthesis. Results from erosion testing will be presented as well as other factors considered in the final coating downselect for this application.