Coatings Technology Handbook Episode 2 Part 1 potx

35-6 Coatings Technology Handbook, Third EditionCoatings used for high-temperature applications require high thermal stability.. Typical applications for these coatings include rocket no

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35-6 Coatings Technology Handbook, Third Edition

Coatings used for high-temperature applications require high thermal stability Refractory compounds having low vapor pressure and high decomposition temperature are generally suitable in these cases, depending on service environment Other properties, such as abrasion resistance, oxidation resistance, thermal shock resistance, and compatible thermal expansion characteristics, are also important Thus, typical coatings used in these applications include certain refractory metals, Al2O3, B4C, SiC, Si3N4, SiO2, and ZrO2, and refractory metal silicides Composite coatings such as Al2O3+ ZrO2 and Al2O3+ Y2O3

have also been studied Most of these coatings can be deposited by CVD Typical applications for these coatings include rocket nozzles, reentry cones, ceramic heat exchanger components, afterburner parts in rocket engines, and gas turbine and automotive engine components Another well-known example of a protective refractory coating is the SiC-coated hardware used in the microelectronics field for

manufac-FIGURE 35.4 Photography showing a 17–4 PH stainless steel compressor blade coated with a tungsten carbide coating in a MTCVD process The blade is first coated with an interlayer of nickel by electrolytic or electroless plating techniques to protect it from the corrosive action of hydrofluoric acid gas generated during the deposition reaction.

FIGURE 35.5 Photography showing cemented tungsten carbide cutting tool inserts coated with TiC and TiN coating

in a conventional CVD process These coatings impart improved wear resistance to the carbide tools, allowing them

to run at higher speeds and chip loads in the machining of various materials.

DK4036_book.fm Page 6 Monday, April 25, 2005 12:18 PM

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Chemical Vapor Deposition 35-7

FIGURE 35.6 Steady state erosive wear rate of ultrafine-grained CVD tungsten–carbon (CM 500L) and SiC (CM 4000) coatings and other hardfacing materials, coatings, and ceramics The eroding medium is 200-micron SiC particles impinging at a velocity of 30 ms –1 at room temperature [Data from Hickey et al., Thin Solid Films, vol 118,

p 321 (1984) Reprinted with permission from Elsevier Sequoia, S.A., Switzerland.]

0 0.1 0.2 0.3 0.4

30 ° 90 ° 30 ° 90 ° 30 ° 90 ° 30 ° 90 ° 30 ° 90 ° 30 ° 90 ° 90 ° 90 ° 90 ° 30 ° 90 °

0.011 0.011 0.047 0.053 0.0114 0.0322 0.038 0.068 0.092 0.246 0.071 0.201 0.053 0.059 0.098 0.381 0.203

3 /g)

San Fernando Labs CM500L, CNTD Tungsten Carbide Heat Treated 1 hr., 600

Union Carbide LW-15 Tungsten Carbide

San Fernando Labs CM4000 CNTD Silicon Carbide

Kennametal K701 Silicon Carbide

Norton NC-203 Hot Pressed Silicon Carbide

Norton NC-132 Hot Pressed Silicon Nitride

Braze Coat Flame Spray

1403 VHN

1400 VHN

3266 VHN

439 VHN

542 VHN

150 VHN

1090 VHN

2747 VHN

1791 VHN

525 VHN

AMS 4777 87% Ni 7% Cr 4% Si 3% Fe 3% B

Angle of Impingement

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Chemical Vapor Deposition 35-9

In recent years, advances in the technology of carbon–carbon composites have led to the fabrication

of components out of these materials, which are then coated by CVD or the new technology of chemical

vapor infiltration (CVI) with various refractory compound coatings, most notably SiC Other ceramic

fiber composites based on alumina and silica have also been coated in a similar manner for high

temperature service Figure 35.9 illustrates one of the techniques used for coating of porous fiber preforms

by CVI

The more exotic CVD techniques that were mentioned earlier, such as PACVD and LCVD, have found

tions is the deposition of diamond films by PACVD The diamond films have unique properties and

application potential ranging from wear-resistant coatings for cutting tools to coatings for laser mirrors,

deposited by the LCVD technique find applications in laser photolithography, repair of VLSIC masks,

laser metallization, and laser evaporation deposition

35.4 Summary

The chief characteristics of CVD may be summarized as follows:

1 The solid is deposited by means of a vapor phase chemical reaction between precursor compounds

in gaseous form at moderate to high temperatures

2 The process can be carried out at atmospheric pressure as well as at low pressures

3 Use of plasma and laser activation allows significant energization of chemical reactions, permitting

deposition at very low temperatures

4 Chemical composition of the coating can be varied to obtain graded deposits or mixtures of

coatings

FIGURE 35.9 Schematic diagram showing a technique of chemical vapor infiltration of porous fiber preforms, in

which a coating of a protective material such as SiC is deposited In this method, a thermal gradient across the

preform allows diffusion of the reactive gas mixture progressively from the hot surface to the cold surface, uniformly

coating the preform [Data from Stinton et al., Ceramic Bulletin, vol 65, p 347 (1986) Reprinted with permission

from The American Ceramic Society.]

Hot Zone

1200 ° C Exhaust Gas

Heating Element Retaining Ring

Water-Cooled Holder

Coating Gas

Cold Surface

Fibrous Preform

Infiltrated Composite

Hot Surface DK4036_book.fm Page 9 Monday, April 25, 2005 12:18 PM

important applications for the deposition of new types of coatings One of the most interesting

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Chemical Vapor Deposition 35-11

Holzl, R A., “Chemical vapor deposition techniques,” Techniques of Materials Preparation and Handling

— Part 3 (Techniques of Metals Research Series, vol 2) R F Bunshah, Ed New York: Interscience

Publishers, 1968, p 1377

Pierson, H O (Ed.), Chemically Vapor Deposited Coatings Columbus, OH: American Ceramic Society,

1980

Powell, C F., J H Oxley, and J M Blocher, Jr (Eds.), Vapor Deposition New York: John Wiley & Sons,

1966

Yee, K K., International Metals Reviews, Review No 226 (1978).

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36

Solvent Vapor Emission Control

36.1 Regulatory Background 36-1 36.2 Alternative Control Processes for Volatile

Organic Compounds 36-2 36.3 Vapor Oxidation 36-3 36.4 Solvent Recovery 36-5

For business operations that include the wet coating of a surface, followed by drying, the amount of volatile organic compound (VOC) released to the atmosphere is important Increased awareness of ambient air quality, and various regulations affecting solvent vapor emissions do not change the need to make a business economically profitable

36.1 Regulatory Background

For a perspective on the VOC regulations, the government now monitors ambient air quality to measure

is associated with “smog” and volatile organics in the air; it is most noticeable on hot summer days and

in metropolitan areas Industrial coating operations are important point sources that may emit tons of VOC Automotive traffic and refueling release much more VOC, but the thousands of smaller sources are not as easy to control

The federal Clean Air Act of 1961 promulgated an important set of regulations that establish limits and also require the states to act to meet ambient air quality standards State regulations may be more stringent than federal regulations, but not less Also, local regulations, such as county, municipal, or regional authority, may be more stringent In some areas, the state or local authorities are judged by some to be too lenient toward emissions and by others to be antibusiness in enforcement of regulations

In many areas, the industrial emissions have been reasonably well controlled, but the ambient ozone standard of 0.12 ppm ozone has not been attained (This is unrelated to the “ozone depletion” problem

at high altitudes.) The federal government now discriminates between “attainment areas” and “nonattainment areas.” Regulations also discriminate between New Sources and Existing Sources New source performance standards may be based on a cost–benefit analysis, but in some nonattainment areas, a more stringent LAER (lowest achievable emission rate) may be required, to be negotiated on a case-by-case basis Existing sources and some new sources may be subject to RACT (reasonable available control technology)

Richard Rathmell

Consultant, Londonderry, NH

Safety • Operating Costs

Carbon Adsorption • Direct Vapor Condensation

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However, if the vapor concentration is maintained close to 40% LEL or above, the solvent vapor can supply substantially all the energy required At lower concentrations, it becomes increasingly necessary

to supply auxiliary fuel or to provide more air–air heat transfer to preheat the vapor laden air For example, one cubic foot of toluene vapor diluted with more or less air in the exhaust flow to be incinerated will be as shown in Table 36.1

From Table 36.1, it can be appreciated that a reduction in airflow (for a given flow of solvent vapor) will proportionately reduce the size of the vapor incinerator, but the size of heat exchanger or the amount

of added fuel required is affected to a much greater degree

It is theoretically possible to provide enough heat exchanger capacity to obviate the need for additional fuel for normal operation In practice, an auxiliary fuel burner is needed for start-up, and it must be kept ignited and ready to heat the air when the vapor concentration decreases

Heat exchangers for vapor thermal oxidizers usually are the shell-and-tube type, using stainless steel tubes, or ceramic beds Some metal plate–plate exchangers also are used, but in every case, it is important

to prevent leakage or short-circuiting of vapor-laden air to the exhaust gases, or bypassing the combustion zone Such leakage or bypassing can generate objectionable odors from partially oxidized organics The ceramic bed heat exchangers operate by periodically reversing the flow direction through at least two or more beds, which are alternately heated and cooled Outgoing hot combustion gases flow through

a bed until the ceramic pieces reach a set temperature, then the flow is reversed and vapor-laden gases are heated so they flow through the hot bed into the combustion zone There is no problem if the vapor-laden gases ignite in the bed prior to the combustion space, but before flows are switched back, it is desirable to first purge vapor-laden gases from the cooking bed into the combustion zone Nonoxidized vapors should not be pushed out with exhaust flow With relatively large beds, it is practical (but not inexpensive) to provide the high heat transfer area needed to accommodate relatively dilute vapor flows The bed size required can be minimized by a high frequency of flow switching; the airtight dampers may be switched every few minutes The ceramic pieces must be selected to tolerate frequent temperature changes and to accommodate the thermal expansion–contraction cycle that occurs If dust is released

by thermal movements or abrasion, it may prevent direct usage of the residual hot gases in the dryers and ovens

Metal surface heat exchangers, with hot combustion gases in one side and the cooler vapor-laden gases

on the other side, operate continuously, without flow reversal or switching dampers Thermal expan-sion–contraction can be a problem, leading to torn welds or fractures and to leakage of the higher pressure vapor-laden air into the lower pressure oxidized discharge flow Such leakage can generate objectionable odors by the scorching of the vapors

TABLE 36.1 Operating Variables a for Thermal and Catalytic Incinerators

Variable

Type of Incinerator

Temperature rise resulting from vapor oxidation 1160 725 290 290 145 Temperature rise required from preheater or auxiliary fuel 40 745 910 510 655 Requirement from preheater or other fuel, Btu × 10 3 /h 9 170 820 460 1180 Available temperature differential across heat exchanger

(with no other fuel used)

Ratio of heat exchanger area b required (to avoid auxiliary

fuel consumption)

a All temperatures in degrees Fahrenheit.

b Assuming equal coefficient; A=Q/ ∆T, where A= the heat transfer area, Q= heat flow, and ∆T= temperature differential.

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The sources of inert (low oxygen) gas required include the flue gas of a gas-fired steam boiler and purchased liquid nitrogen or carbon dioxide Where flue gases are used, the gas burner must be of the type that can maintain a low ratio of excess air to fuel for various fuel firing rates A compressor and pressurized storage tank can provide the ready reservoir for last start-up and fail-safe shutdowns, or a tank of liquid nitrogen with vaporization facilities can be used

In some important respects, the operation of an inerted airtight dryer is inherently safer than a conventional air-swept dryer In an air-swept dryer there is a transition zone between a flammable wet interface and a nonflammable exhaust, and there is the potential for a temporary excess solvent loading into the dryer to produce a large volume of combustible mixture In an inerted dryer, there is no flammable interface, and any temporary excess solvent loading will not make a combustible mixture When the coating process and wet web is stopped for any reason, there is no tendency for outside air to exchange with the atmosphere contained in the dryer, except as air may be drawn in to replace the volume

of vapor condensed, or to make up for gas volume contraction as the contained gas cools down In the Wolverine systems, the normal operating vapor concentration in the dryer is designed to prevent the condensation due to vapor volume of any unsafe air inhalation into the dryer Normally, the vapor condenser temperature is selected to draw the vapor concentration below the organic LEL level when

and a safe organic-in-air LEL level

When a dryer is shut down overnight or for a weekend, it is not necessary or desirable to purge the contained atmosphere to the outside atmosphere

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37

Surface Treatment

of Plastics

37.1 Introduction 37-1 37.2 Functions of Surface Preparation 37-1 37.3 Factors Impacting Preparation Intensity 37-2 37.4 Surface Preparation Techniques 37-3

37.5 Evaluation of Surface Preparation 37-6 Bibliography 37-7

37.1 Introduction

No single step in the coating process has more impact on film adhesion than surface preparation Film adhesion to a plastic is primarily a surface phenomenon and requires intimate contact between the substrate surface and the coating However, intimate contact of that plastic surface is not possible without appropriate conditioning and cleansing

Plastic surfaces present a number of unique problems for the coater Many plastics, such as polyethylene

or the fluorinated polymers, have a low surface energy Low surface energy often means that few materials will readily adhere to the surface Plastic materials often are blends of one or more polymer types or have various quantities of inorganic fillers added to achieve specific properties The coefficient of thermal expansion is usually quite high for plastic compounds, but it can vary widely depending on polymer blend, filler content, and filler type Finally, the flexibility of plastic materials puts more stress on the coating, and significant problems can develop if film adhesion is low due to poor surface preparation

37.2 Functions of Surface Preparation

Treatment of the plastic surface performs a great many functions depending on the individual polymer type involved

William F Harrington, Jr

Uniroyal Adhesives and Sealants Company, Inc.

Removal of Contamination • Control of Surface Roughness •

Solvent Cleaning • Detergent Cleaning • Mechanical Matching of Surface to Adhesive • Providing a Boundary Layer •

Treatment • Chemical Treatment • Other Treatments

Control of Oxide Formation • Control of Absorbed Water Type of Plastic • Surface Contamination • Initial and Ultimate Strength Requirements • Service Environment • Time • Component Size • Cost 2

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Surface Treatment of Plastics 37-5

Virtually all chemical etch procedures require water rinsing (once or twice), and an elevated temper-ature drying is recommended With active ingredient treatments, it is imperative that solution strength

be monitored and renewed at appropriate intervals

37.4.4.1 Sulfuric Acid–Dichromate Etch

By far the most commonly recommended chemical treatment for plastic parts, the sulfuric acid–dichro-mate etch is used on acrylonitrile–butadieme–styrene (ABS), acetal, melamine or urea, polyolefins, polyphenylene oxide, polystyrene, polysulfone, and styrene–acrylonitrile (SAN) For each plastic, a dif-ferent ingredient ratio and immersion temperature and time may be recommended

The following list is offered as a guide to a possible range of parameters:

While the ranges are extremely wide, experimental trials coupled with test results will allow the user

to identify the most appropriate values for a given plastic

37.4.4.2 Sodium Etch

For truly difficult surfaces to coat, such as the various fluoroplastics and some thermoplastic polyesters, highly reactive materials must be used Metallic sodium (2 to 4 parts) is dispersed in a mixture of naphthalene (10 to 12 parts) and tetrahydrofuran (85 to 87 parts)

Immersion time is approximately 15 min at ambient temperatures, followed by thorough rinsing with solvent (ketone) before water rinsing

37.4.4.3 Sodium Hydroxide

A mixture of 20 parts by weight of sodium hydroxide and 80 parts of water is an effective treatment of

immers-ing for 2 to 10 min is appropriate

37.4.4.4 Satinizing

Satinizing is a process developed by DuPont for their homopolymer grade of acetal (U.S Patent 3,235,426) Parts are dipped in a heated solution of dioxane, paratoluene sulfonic acid, perchloroethylene, and a thickening agent After the dip cycle, parts are heat treated, rinsed, and dried according to a prescribed procedure

37.4.4.5 Phenol

Nylon is often etched with an 80% solution of phenol in water Generally, the treatment is conducted at

37.4.4.6 Sodium Hypochlorite

A number of plastics, particularly the thermoplastic types and the newer thermoplastic rubbers, can be chlorinated on the surface by applying a solution of the following ingredients (parts by weight): Water: 95 to 97

Sodium hypochlorite, 15%: 2 to 3

Concentrated hydrochloric acid: 1 to 2

Parts can be immersed for 5 to 10 min at room temperature, or the solution can be brushed onto the surface for the same period

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