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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:
It has been found that strength and hardness of a wear resistant coating can be improved by combining different materials. The components must be mutually soluble so that strong, stable interfaces are produced. The multilayer can be constructed with the material that forms the strongest bond to the substrate deposited first. A series of intermediate layers whose functions are to provide the proper ratio of toughness to hardness and adjustment for grain size and shape differences, are then terminated by layer(s) that provide low surface friction. Coatings on tool bits are often of this construction. Thus, combinations of TiC and TiB2 or TiN form stable interfaces, as does TiCxNy.

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:
Yttria, zirconia, alumina, india, and recently niobia, are materials of special interest for the optical industry because of their great hardness and transparency. Studies with yttria demonstrate that both grain size and microhardness increase with increasing temperature [5]. An interesting aspect of the growth microstructure of zirconia and yttria is that more than one crystal phase can be deposited depending on the evaporation source parameters and substrate temperature. Transformation to the cubic phase can occur at high service temperatures, with accompanying mechanical instability caused by internal stresses. It has been found that these high temperature oxides can be stabilized to the high temperature tetragonal or cubic phases by mixing with one of the following: SiO2, MgO, In2O3, Y2O3, Al2O3, SnO2, Sc2O3, and others [6,7]. The additive lowers the transition temperature between tetragonal and monoclinic states from 1200° C to ~565° C. Thus mixtures of ZrO2 with ~5-15% of the second material results in improved optical homogeneity and mechanical stability; specifically, greater packing densities, higher refractive indices, lower intrinsic stresses, and greater hardnesses are obtained. Control of the crystal state can improve laser damage hardness by eliminating the presence of low temperature phases. Those phases could transition to high temperature states at the high temperatures generated during irradiation, with the consequence of stress increases. The mixtures, available from CERAC, can be deposited from prepared sputter targets or mixed pellets for e-beam evaporation. CERAC's IRX® has been reported to possess high temperature durability properties.

Aluminum Oxide Films:
Alumina films provide protection from chemical corrosion and abrasive wear, and can be deposited at relatively high rates by electron beam. Coating adhesion increases with temperature, as does hardness up to ~700° C. Decreasing adhesion to metals follows the order: Monel 400, molybdenum, 304 stainless, 316 stainless. While there is a relatively large TCE difference between metals and alumina, (stainless TCE ~14), the coating is resistant to thermal shock and adhesion is quite good in moderate temperature applications.

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:
Silicon nitride is a material that possesses high wear resistance and chemical stability and can be produced by reactive or R.F. sputtering or CVD. Its main use in the microelectronics industry is as an insulator, but it has also been used in abrasive wear applications. To achieve maximum hardness, the hydrogen content of the film must be minimized. Deposition techniques have been modified to accomplish this. Silicon nitride's transparency has found wide application in the optical and semiconductor industry.

Titanium Carbonitride, TiN, and TiC:
These materials are widely used as protective coatings on cemented carbide cutting tools. These ceramics are brittle, and the deposition method, substrate type and preparation, layer thickness, type of wear, etc., must be considered to optimize the tribological properties. TiCxNy is harder than TiN, making it useable as a single material coating or as an interlayer between TiN and TiC layers. The coating is deposited by CVD, and until recently the required process temperatures reaching 1000° C were found to affect the properties of the steel tool. Lower temperature processes now in use include: plasma assisted CVD, cathodic arc, reactive sputtering, and ion plating. The latter processes result in lower chlorine content than CVD, and film layers do not suffer from the lower adhesion and strength that high chlorine containing film do.

Summary:
The reader has been introduced to the vast subject of high temperature tribological wear resistant coatings for mechanical wear resistance and optical applications. Materials and deposition processes play a significant role in advancing the technology.

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:
It has become evident in the history of vacuum deposition technology that the energy of the evaporant determines such qualities of the growing film as packing density (and therefore refractive index and atmospheric stability), hardness, adhesion, stress, and absorption (for optical films). Good quality hard films are deposited by the various sputtering techniques, but deposition rate, packing density, etc. are sometimes less than ideal. The ion plating process was first invented by D. Mattox in 1964, and refined and developed into a commercial product by Balzers researchers in the mid-80's.

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:

  1. M. G. Hocking, Surf. and Coatings Techn., 62, 460 (1995), and M. G. Hocking, V. V. Vasantasree and P. S. Sidky, Metallic and Ceramic Coatings-Production, High Temperature Properties and Applications, Longman, London, 1989. Has over 2000 references.
  2. H. Holleck, J.V.S.T. A4 (6), 2661 (Nov/Dec 1986).
  3. J. E. Sundgren and H. T. G. Hentzell, J.V.S.T. A4 (5), 2661 (Sept. /Oct. 1986).
  4. H. Holleck and V. Schier, Surf. and Coatings Techn., 76, 328 (1995).
  5. Dennis Quinto, George Wolfe, and Prem C. Jindal, Thin solid Films 153, 19 (1987).
  6. M. Colen and R. F. Bunshah, J.V.S.T. 13 (10), 536 (1976).
  7. S. B. Quardri, E. F. Skelton, M. Z. Harford, C. Kim, and P. Lubitz, Surface and Coating Tech., 63, 155 (1994).
  8. F. Tcheliebou, A. Boyer, and L. Martin, Thin Solid Films, 249, 86 (1994).
  9. R. F. Bunshah, Thin Solid Films, 80, 255 (1981).
If there are any other specific topics 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.


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
Pellicori Optical Consulting
P.O. Box 60723 | Santa Barbara, CA 93160
Phone/FAX: 805-682-1922

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