1、_ SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising there
2、from, is the sole responsibility of the user.” SAE reviews each technical report at least every five years at which time it may be revised, reaffirmed, stabilized, or cancelled. SAE invites your written comments and suggestions. Copyright 2013 SAE International All rights reserved. No part of this p
3、ublication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: +1 724-776-497
4、0 (outside USA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.orgSAE values your input. To provide feedback on this Technical Report, please visit http:/www.sae.org/technical/standards/ARP5935AEROSPACERECOMMENDED PRACTICEARP5935Issued 2007-02 Reaffirmed 2013-10 Use o
5、f HVOF Thermal Spray Coatings for Hard Chrome Replacement in Landing Gear Applications RATIONALE ARP5935 has been reaffirmed to comply with the SAE five-year review policy. 1. SCOPE Electrolytically deposited chrome plate is the current standard surface treatment for landing gear component interface
6、 surfaces that require good wear resistance and corrosion protection. Chrome plated components are typically plagued by a slight debit in fatigue performance, detrimental mud cracking surface pattern, susceptibility to scoring, wear, and seal leakage. In addition, recent changes in environmental com
7、pliance standards place further restrictions on the use of electrolytically deposited chromium. Some commercial applications have already eliminated the use of chrome plate on current and future products. As a result, a substitute for electrolytically deposited chrome plate has been sought for sever
8、al years. High Velocity Oxygenated Fuel (HVOF) thermal spray coatings have been developed to the point where they are being implemented as an alternative to hard chrome plate on high strength low alloy steels for external surfaces on landing gear applications. Some of the characteristics of HVOF coa
9、tings, such as fatigue and wear resistance are superior to hard chrome plate. Yet, performance characteristics, such as adhesion under very high strains, are clearly different. This document is intended to outline and describe the design and usage guidelines for HVOF tungsten carbide coatings to ass
10、ure adequate quality and performance of the landing gear equipment. This document is not intended to act as a detailed process specification for applying HVOF coatings. Also, the recommendations in this document do not necessarily apply to certain proprietary spraying systems (i.e., D-Gung165). The
11、information in this document pertains to HVOF tungsten carbide based coatings applied to high strength steel alloys only, and is not necessarily applicable to other substrates. 2. REFERENCES 2.1 Applicable Documents The following publications form a part of this document to the extent specified here
12、in. The latest issue of SAE publications shall apply. The applicable issue of other publications shall be the issue in effect on the date of the purchase order. In the event of conflict between the text of this document and references cited herein, the text of this document takes precedence. Nothing
13、 in this document, however, supersedes applicable laws and regulations unless a specific exemption has been obtained. 2.1.1 SAE Publications Available from SAE International, 400 Commonwealth Drive, Warrendale, PA 15096-0001, Tel: 877-606-7323 (inside USA and Canada) or 724-776-4970 (outside USA), w
14、ww.sae.org. AMS-QQ-C-320 Chromium Plating, Electrodeposited AMS 2447B Coating, Thermal Spray High Velocity Oxygen/Fuel Process AMS 2448 Application of Tungsten Carbide Coatings on Ultra High Strength Steels High Velocity Oxygen Fuel Process AMS 2449 Grinding of HVOF Sprayed Tungsten Carbide Coatings
15、 Applied to High Strength Steels AMS 2649 Etch Inspection of High Strength Steel Parts ARP 4462 Barkhausen Noise Inspection for Detecting Grinding Burns in High Strength Steel Parts AMS 7881 Tungsten Carbide-Cobalt Powder AMS 7882 Tungsten Carbide-Cobalt-Chrome Powder 2.1.2 ASME Publications Availab
16、le from ASME, 22 Law Drive, P.O. Box 2900, Fairfield, NJ 07007-2900, Tel: 973-882-1170, www.asme.org. ASME B46.1 Surface Texture 2.1.3 ASTM Publications Available from ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, Tel: 610-832-9585, www.astm.org. ASTM B 117 Standard Pr
17、actice for Operating Salt Spray (Fog) Apparatus ASTM E 1417 Standard Practice for Liquid Penetrant Examination ASTM E 1444 Standard Practice for Magnetic Particle Testing 2.1.4 ISO Publications Available from ANSI, 25 West 43rd Street, New York, NY 10036-8002, Tel: 212-642-4900, www.ansi.org. ISO 42
18、87 Geometrical Product Specifications - Surface Texture SAE INTERNATIONAL ARP5935 Page 2 of 7_ 2.2 Definitions HVOF (High Velocity Oxygenated Fuel): A combustion process in which the combustion gases are accelerated through a nozzle to supersonic velocities. Gas/particle temperatures can reach 4500
19、F (2482 C) with a particle velocity up to 2500 fps (762 m/s). SPALLING: Co-adhesive and adhesive failure of the coating material through cracking within the coating and/or debonding from the substrate surface. THERMAL SPRAY: A process in which thermal energy is used to create an aerosol of finely di
20、vided materials to be deposited in a molten or semi molten condition on to a prepared substrate to form a new surface deposit. Thermal spray processes include combustion, plasma, and arc/wire. FPI: Fluorescent penetrant inspection. Ra: Roughness average is a measurement of the average distance betwe
21、en the median line of the surface profile and its peaks and troughs. Rp: The maximum profile peak height measured from the median line. Rz: The average of the five maximum peak to trough separations for one evaluation length. Rmr: The symbol Rmr has two meanings. First, it is used generically as the
22、 abscissa of the bearing ratio curve. It is a percent bearing ratio. Second, as described in 3.6, Rmr as a parameter refers to the bearing ratio at a specified height. The most common way of specifying the height is to move over a certain percentage (the reference percent) on the bearing ratio curve
23、 and then to move down a certain depth (the slice depth). The bearing ratio at the resulting point is Rmr. The bearing ratio curve mathematically is the integral of the amplitude distribution function. It is a cumulative probability distribution. Ordinarily, the integral is performed from the highes
24、t peak downward, so each point on the bearing ratio curve has the physical significance of showing what linear fraction of a profile lies above a certain height. Rmr was formerly referred to as Tp, bearing length ratio. 3. RECOMMENDATIONS 3.1 Coating Materials The spray powder materials should be li
25、mited to either WC-17Co or WC-10Co-4Cr per AMS 7881 or AMS 7882. The choice of material should be made based partly on corrosion protection required. Coating thickness limitations are described in 3.5. 3.2 Spraying Equipment The HVOF gun should be capable of spraying coatings by injecting powdered m
26、aterials into a supersonic jet stream created by the combustion of oxygen and a fuel (typically hydrogen, kerosene, natural gas, etc.). When spraying, fixtures should be employed to maintain gun-to-target distances and traverse rates. Note: changing powders, guns, fuels, spray facilities, or environ
27、ment can alter the performance of the resulting coating and will require re-qualification of the process/component combination. 3.3 Spraying Process Heat treatment, stress relief, shot peening, and other cold working should be done prior to spraying. Prior to shot peening, the surface should have a
28、finish 125 Ra or better. The substrate surface should be grit blasted with coarse grit prior to spraying to assure good adhesion. SAE INTERNATIONAL ARP5935 Page 3 of 7_ Grinding of the substrate to provide for a tapered runout in the spray coating is not recommended because of the risk of grinding b
29、urns. However, if grinding is used, a temper etch should be done to inspect the surface for grinding burns. The as-sprayed coating should be kept to the minimum thickness required to achieve the final surface finish. The incident angle of the spray relative to the target surface should be kept close
30、 to 90 degrees whenever possible (slight angle is preferred not to exceed 5 degrees to prevent the coating from “bouncing” back into the flow stream). The spray angle should not be allowed to go below 45 degrees unless those low spray angles have been specifically evaluated for the application. Cont
31、rolling substrate temperature during spraying is critical to achieving high quality bond strengths and minimizing the risk of damage to the target part. Maximum allowable substrate temperature shall be in accordance with AMS 2448 and/or manufacturers recommendation. Surface cooling using air, N2, or
32、 CO2is permissible as long as it does not interfere with the spray path. A temperature measuring device such as an infrared pyrometer (i.e., IR Gun) should be used at all times during the spraying process to ensure that substrate temperatures do not exceed the recommended limits. One of the benefits
33、 of HVOF coating is, if applied correctly, the coating goes down with good compressive stresses which help prevent co-adhesive and adhesive type coating failures. Residual stresses should be checked using Almen type “N” strips. It is recommended that both sides of the “N” strip be grit blasted to mi
34、nimize curvature. One side of the “N” strip is then sprayed and the arc height is then measured using an Almen gage. It is recommended that screw type Almen holders be used in lieu of magnet type holders. See AMS 2448 for further details. 3.4 Grinding/Super-Finish A minimum depth diamond wheel grind
35、 per AMS 2449 is recommended. The edge of the coating should be tapered to the substrate in the same manner as chrome plate so as to minimize shear stress. Proper masking/shielding techniques should be employed to meet run-out requirements. Super-finishing is needed on WC HVOF coatings to achieve eq
36、uivalent seal life performance when compared to hard chrome plate and/or to minimize interface wear on bushings/bearings. Belts or stones have been successfully tested and either method will produce the required finish. 3.5 Coating Thickness On OEM applications, the final coating thickness should be
37、 0.003 to 0.005 in (0.076 to 0.127 mm). Repair applications may use higher thicknesses, but the risk of spalling is typically proportional to both the thickness, and the peak strain level. Extensive testing has demonstrated that for coatings up to 0.010 in (0.254 mm) finish thickness, the coating re
38、mains intact well above the proportional limit and exceeds the 0.2% offset yield strength. 3.6 Surface Finish There are three sets of applications for HVOF on landing gear equipment. Type 1 are surfaces that require sealing a hydraulic fluid with a sliding interface. Examples include strut and actua
39、tor piston barrels. To maximize seal life, superfinishing to 4 Ra or better is required. Recommended surface parameters include Rz of 40, Rp 8, Rmr 70 to 90% at C0 = 5% and C1 = 0.25 Rz. Note: Unlike chrome, Ra and bearing ratio Rmr do not change during service with HVOF applied WC coatings. Type 2
40、surfaces provide for limited rotational motion under high bearing stresses such as axle journals and trunnion pins. The recommended surface finish for the Type 2 applications is 8 Ra or better. This can be achieved with or without superfinishing. Type 3 surfaces provide for high pressure-velocity mo
41、tion under high bearing stresses. An example is a truck beam pivot. The recommended surface finish is 6 Ra or better. Superfinishing is preferred, but is not required. SAE INTERNATIONAL ARP5935 Page 4 of 7_ 3.7 Seal Types In Type 1 applications, self-lubricated PTFE seals have been shown to have sup
42、erior wear and leakage performance over elastomeric seals as long as wetting of the surface is permitted. However, some users have reported good performance with elastomeric contact seals on HVOF. Seal type selection is left to the preference of the user. 3.8 Fatigue Even though HVOF coatings are ha
43、rder and more brittle compared to chrome, testing has shown that the impact of HVOF spray coatings on substrate fatigue is less than that of hard chrome plate. Some data suggested it improved the fatigue life when compared to bare coupons. This performance is possible because bonding is different wi
44、th the substrate. Fatigue reduction factors relative to a bare substrate are left to the discretion of the user. To optimize fatigue performance, attention should be paid to surface preparation, spraying methods and other quality control parameters. Shot peening of the substrate prior to spraying is
45、 also recommended. See 3.3. 3.9 Friction Performance Testing has shown that HVOF coatings typically have improved friction performance over that of hard chrome plate. Friction should not limit or otherwise affect how and where HVOF coatings are used on landing gear equipment. 3.10 Wear Resistance So
46、me laboratory tests have shown occasional transfer of tungsten onto aluminum bronze alloy bushings. However, other testing and limited commercial fleet experience have shown that HVOF coatings have superior wear resistance to that of hard chrome plate. Wear should not limit or otherwise affect how a
47、nd where HVOF coatings are used on landing gear equipment. 3.11 Corrosion Resistance Corrosion testing to date has shown mixed results. However, both limited commercial fleet usage and testing of coupons in atmospheric conditions at Key West, Florida demonstrated that HVOF tungsten carbide coatings
48、outperformed hard chrome plate. Additional salt spray testing per ASTM B 117 also demonstrated that HVOF WC coatings outperformed hard chrome plate, and electrochemical corrosion tests have shown that WC-Co-Cr has better corrosion resistance than WC-Co. 3.12 Thermal Barrier Properties The thermal conductivity of chrome plate is not quite as high as that of pure chrome, but it is very similar to that of steels. Sintered (bulk) WC-Co is about as cond
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