Coating Materials News Vol 7 Issue 2
April - June, 1997
Wear Resistant, High Temperature Coatings
Introduction:
This issue serves as an introduction to the subject of wear resistant, high temperature coatings. The reader is referred to the literature cited for in-depth reviews [1] and recent advances.
The need for hard coatings spans both optical and mechanical industries. Coatings that exhibit high mechanical hardness often require simultaneously high temperature and corrosion resistance. This overlap is exploited in our discussion.
Applications of tribological coatings include high speed cutting tools, thermal/corrosion barriers on engine parts such as combustor walls, turbine blades, exhaust linings, etc. The science of hard coating deposition is a complicated one involving not only the appropriate coating material choice, but also substrate/coating chemical, mechanical, and thermal interactions. Adhesion, interface strain, internal stresses, film layer ductility, strength, etc., are important parameters. Finally, interaction with the environment, including the abrasive surface contact where the coefficient of sliding friction comes into play, must be considered. Layer thicknesses are 2 to 10 µm, requiring high rate deposition techniques be used.
Hard Coating Materials:
Coating materials for abrasive wear/high temperature applications fall into three groups as determined by their chemical bond character: Transition Metal Oxides, Transition Metal Borides/Carbides/Nitrides and Covalent Coatings. Table I lists several common materials and their properties [2,3] including Vickers microhardness (HV) and coefficient of thermal expansion (TCE) data. The data in Table I should be considered as a guideline and does not necessarily contain accurate values because the parameters mentioned above affect these properties. Current technology employs multilayer structures where thin layers of covalent material are placed intermediate to the metal substrate and another hard layer(s) such as AlN, BN, etc. As many as 13 alternating layers are used. Composite hardnesses as high as 5000 HV have been achieved. [4].
Table I. Properties of Some Hard Coating Materials
A. Transition Metal Oxides (ionic) | |||
Coating | Hardness (HV) | TCE | Melting/Decompos. (°C) |
Al2O3 | 2100 | 8 | 2100 |
Al2TiO3 | - | 0.8 | 1900 |
BeO | 1500 | 9 | 2550 |
HfO2 | 780 | 6.5 | 2900 |
MgO | 750 | 13 | 2830 |
Nb2O5 | >1500 | - | 1400 |
TiO2 | 1100 | 9 | 1870 |
Y2O3 | ~700? | - | 2400 |
ZrO2 | 1200 | 11 | 2680 |
B. Transition Metal Borides/Carbides/Nitrides | |||
Coating | Hardness (HV) | TCE | Melting/Decompos. (°C) |
LaB | 2530 | 6 | 2770 |
LaB6 | 2530 | 6 | 2770 |
TiB2 | 3000 | 8 | 3225 |
W2B2 | 2700 | 8 | 2360 |
HfC | 2700 | 6 | 3930 |
TaC | 1500±500 | 6 | 3980 |
TiC | 2800 | 8 | 3070 |
VC | 2900 | 7 | 2650 |
WC | 2200±100 | 4 | 2770 |
ZrC | 2560 | 7 | 3440 |
TiN | 2100 | 9 | 2950 |
ZrN | 1600 | 7 | 2980 |
C. Covalent Coatings | |||
Coating | Hardness (HV) | TCE | Melting/Decompos. (°C) |
B4C | 3-4000 | 5 | 2450 |
BN (cubic) | 4-5000 | - | 2730 |
C (diamond) | 7000 | 1 | ~1000 |
SiC | 2600 | 5 | 2760 |
Si3N4 | 1720 | 2.5 | 1900 |
Notes: All of these materials are generally deposited by magnetron sputtering. Ion beam sputtering and ion beam assisted deposition (IAD) are sometimes used to optimize particular properties, but at the price of lower deposition rates. Zirconia-yttria mixtures and TiNx and TiC compounds are deposited by plasma spray to several µm thicknesses, or can be deposited by activated reactive evaporation. On surfaces not required for contact service, such as turbine blades, the coating can be highly textured - as plasma spraying produces. CVD can be used for some materials, but requires very high substrate temperatures. Diamond and DLC are included to show comparative hardness, neither is stable at high temperatures. A non-cubic BN material provides very durable coatings, and is easier to obtain than the cubic form. Wear Resistant Coating Composition: Substrate temperature during film growth is an important parameter in film hardness. We discussed the structure zone model relating substrate temperature to film microstructure in CMN Volume 2, Issue 3 (July-Sept., 1992). For metals, higher temperatures produce larger grain sizes which possess low strength and hardness. With materials such as alumina, zirconia, and TiN, higher temperatures generally lead to harder films. The explanation does not seem to be completely attributable to grain size, but perhaps to intergrain binding forces. If impurities are present, grain bonds will be weakened. Hard Films for Optics: Aluminum Oxide Films: The stress level in the coating plays a significant role in determining wear resistance, and in some cases is related to thickness. During the sputter deposition of alumina on steel, the internal stress decreases by a factor of 3 from highly compressive to a constant value at thicknesses greater than ~1.5 µm. Adhesion strength doubles between 1.5 µm thickness and 3 µm. During high temperature service, decreases in hardness values of nearly 25% are observed for some of the nitrides and carbides between room temperature and 800° C [5]. The effect is less for sputtered and ion plated coatings than for CVD coatings of TiN, HfN, and TiAlN, for example. Cemented carbide cutting tools are coated with these materials to extend wear life. The typical temperature of the cutting edge when steels or carbon iron are machined at high speed is near 800° C. HfN and alumina are more stable and harder than TiC or TiN under these conditions. Silicon Nitride: Titanium Carbonitride, TiN, and TiC: Summary: Future articles will cover requirements for managing hazardous waste prior to shipping off-site, along with treatment and disposal options for typical waste materials generated in coating applications. If there are any other specific topics involving hazardous waste management that you are interested in seeing addressed in this newsletter, feel free to contact us via e-mail (marketing@cerac.com) or call us directly at (414) 289-9800. Special Section: Focus on Evaporation: Reactive Ion Plating: In the technique of reactive ion plating (RIP), a high current (50 A) low voltage (50 V) argon plasma is created and couples to an e-beam evaporator, which is electrically isolated from the chamber and acts as an anode. The plasma ionizes the reactive gas, oxygen or nitrogen, whose +ions are attracted to the substrate by a -10 V bias. The substrates are in contact with the plasma sheath and the accelerated metal and reactant ions condense to form a thin film layer of the metal compound with relatively high energy. One outstanding advantage this technique has over Ion Assisted Deposition (IAD) is that the plasma volume nearly fills the chamber, thus producing a more uniform reaction volume at the substrates. IAD uses ion sources that are directional and provide sufficient particle density only over a small area. The starting materials must be metals or suboxides so as to form electrically conducting melts. Reaction forms nitride or oxide compounds of bulk-like density and hardness that exhibit amorphous, glassy microstructure. The abundance of ionized species promotes complete reaction, resulting in low optical absorbing films. A disadvantage of the RIP process is that the films generally are under high stress. Many of the compounds listed in the table can be produced by RIP. References:
Dr. Ervin Colton, Editor CERAC, inc. P.O. Box 1178 | Milwaukee, WI 53201 Phone: 414-289-9800 | FAX: 414-289-9805 e-mail: marketing@cerac.com Samuel Pellicori, Principal Contributor |
|
back to top | ©Copyright 1997, CERAC, inc.
All printed, graphic and pictorial materials made available on this website are owned by CERAC and protected by Federal Copyright laws. None of the materials, in whole or in part, may be reprinted and distributed or otherwise made available to others for any purpose without CERAC's prior written consent. Phone: 414-289-9800 / FAX: 414-289-9805 / info@cerac.com |