HVOF sprayed tungsten carbide (WC) thermal spray coatings are finding increasing acceptance for replacement of electroplated chromium on aircraft landing gear parts. In order to replace chrome plating by an HVOF WC coating, the latter should exhibit wear and fatigue characteristics at least as good as those of chrome plating. Sliding wear performance and fatigue life of tungsten carbide coated parts depend on morphology and phase composition of the coating which in turn depend on spray parameters such as powder characteristics, powder feed rate, gas flow rates, and spray distance. A Design of Experiments (DOE) approach for a Jet Kote™ HVOF spray system was used to identify optimal spray parameters for WC-Co and WC-Co-Cr coatings based on best sliding wear and best fatigue characteristics. Best sliding wear was defined as minimum block loss in the standard ASTM-G77 wear test in which a rotating WC coated ring is tested against a stationary Al-Ni-Bz block. Best fatigue characteristics were defined as compressive residual stresses in the coating in the range 250 to 450 MPa. Spray parameters found to strongly affect wear were powder feed rate, oxygen flow rate, powder size, and a powder type-powder size interaction. Spray parameters strongly affecting residual stresses include powder type, hydrogen flow rate, and powder feed rate. Some spray parameters were found to be orthogonal with respect to wear behavior and residual stress. This means some parameters can be adjusted to maximize one performance criterion (wear or residual stress) without adversely affecting the other.

Tungsten Carbide (WC) thermal spray coatings have had increased acceptance in commercial aircraft applications driven by the desire to replace chromium electroplate due to environmental and economic considerations [1]. In order to confidently replace chromium electroplate by WC thermal spray coatings, evaluation of wear and fatigue characteristics of the WC thermal spray coatings is necessary. For WC thermal spray coatings to replace chromium electroplate in aircraft applications, the coatings must demonstrate wear and fatigue characteristics as good as or better than those of chrome plating. Previous research in this area has shown that the fatigue life of the WC thermal spray coatings can be improved by inducing compressive stresses in the coating. This paper compares the wear characteristics of several types of WC thermal spray coatings with those of chromium electroplate in sliding wear tests following the "block-on-ring" procedures described in ASTM G77 standard. Wear results are interpreted in terms of coating residual stresses and in terms of X-ray diffraction and Scanning Electron Microscope (SEM) analysis.

One candidate alternative to chrome plating and hard anodizing is a tungsten carbide (WC) coating applied by the High Velocity Oxy-Fuel (HVOF) process. HVOF WC coatings are currently being evaluated in many service life tests, including fatigue. The purpose of this paper is to compare the fatigue life of HVOF WC coated specimens with the fatigue life of hard anodized and bare aluminum specimens. This work examines WC thermal spray coatings as candidates for replacement of hard chrome plating and hard anodizing in aircraft and helicopter applications such as landing gear. In fatigue testing, the results showed an expected fatigue deficit for hard anodizing as compared to bare aluminum. Paper includes a German-language abstract.

Tungsten carbide thermal spray coatings have been used for more than twenty years in the commercial aircraft industry in applications such as turbine blade and flap-track wear surfaces. Additionally, the evaluation of tungsten carbide (WC) coatings to replace chrome plating in other aircraft applications has been underway for several years. For example, WC coatings applied by the high velocity, oxy-fuel (HVOF) process are being evaluated for use on aircraft landing gear parts. One factor that affects the suitability of WC coatings is the fatigue life of the coated part. This study compares the fatigue life of electrodeposited chrome plated specimens to the fatigue life of WC HVOF-sprayed specimens on aircraft landing gear alloys. Fatigue tests were run on cantilever flat beam specimens coated on one side and subjected to bending fatigue loads. Residual stress levels for the coatings were determined using the Modified Layer Removal Method on rectangular residual stress specimens processed with the flat beam specimens. Also, the Young's modulus and Poisson's ratio of the coating were determined using the Cantilever Beam Bending Method performed on beam specimens that were processed with the fatigue specimens and the residual stress specimens. Results indicate that certain levels of residual stress in the coating can enhance the fatigue life of the parts. The fatigue lives in bending tests of several WC coated specimens are compared with the fatigue life of chrome plated specimens.

Tungsten caibide (WC) thermal spray coatings are being used for wear protection on selected components of aircraft. Tungsten carbide coatings are being used on aircraft flap tracks and fan and compressor blade mid-span dampers. However, a larger use of tungsten carbide coatings is being considered for other commercial aircraft applications where it would be used as a replacement for chrome plating. For instance, WC coatings are currently being tested on aircraft landing gear parts. One factor that affects the suitability of WC coatings for these applications is the fatigue life of the coated part. Coatings, whether chrome plating or thermal spray coating, can reduce the fatigue life of the part compared to an uncoated part. This study compares the fatigue life of uncoated 6061 aluminum specimens to the fatigue life of WC thermal sprayed coated 6061 aluminum specimens. The relation between the residual stress level in the coating and the fatigue life of the specimens is also investigated. Fatigue tests were run on cantilever flat beam specimens that were coated on one side. Specimens were cycled in bending so that the coatings experienced tensile fatigue stresses. Residual stress levels for each type of coating were determined using the Modified Layer Removal Method on specimens processed along with the cantilever flat beam specimens. Test results show that the fatigue life of the WC coated specimens is directly related to the level of compressive residual stress in the coating.