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Accepted Practice to Test Bond Strength of Thermal Spray Coatings

Introduction The Thermal Spray Society (TSS) Accepted Practices Committee on Metallography and Mechanical Testing has prepared an Accepted Practice for Thermal Spray Coating Bond (Adhesion) Testing. This document defines and highlights critical aspects for representative and reproducible test results based on ASTM C633 and applicable industry standards. The Accepted Practices Committee, chaired by Dr. Tetyana Shmyreva of Rolls-Royce, is comprised of an international team (26 total members) of scientists and engineers from academia and industry, including notable organizations such as Rolls-Royce Corp. (US), Swinburne University (Australia), Sulzer Metco, and GKN Aerospace (EU). The charter of this committee is to provide the thermal spray industry with technical guidance for common evaluation techniques.

ASM Thermal Spray Society Accepted Practice to Test Bond Strength of Thermal Spray Coatings Sufficient adhesion bond between the coating and the surface of a coated part/material (often called the substrate) is critical for the coated part functionality. The coating integrity and durability also directly depends on the cohesion bond strength—bonding of coating internal elements such as the internal layers and individual lamellas. ASTM C633 ‘‘Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings’’ is the baseline and mandatory process to follow for tensile testing of bond strength of thermal spray coatings. The test is required as a condition of approval for new coatings and their suppliers, and is the core qualification test for coatings for aviation, oil & gas, automotive, power, marine, and many other industries. This practice clarifies details of ASTM C633 requirements and give examples of the best practice confirmed by hundreds of tests performed world-wide, adopted by numerous industrial standards, and requested to comply with international technical standardization and certification organizations such ISO, AS, SAE, and Nadcap.

Test Scope ASTM C633 test applies tensile stresses to coated system consisting of a coated sample (bond cap/substrate) glued to another cylindrical sample (mating cap) as shown in Fig. 1. As an alternative, the coating could be applied on a cylindrical ‘‘button’’ which is glued between two cylindrical samples as shown in Fig. 2. The load is applied in the direction perpendicular to the interface between the coating and the substrate. The level of tensile load is gradually increased from ‘‘0’’ to the load which results in the sample failure—the coating pulls out from the substrate or fractured in two pieces inside the coating. The coated surface is always a flat circle with a diameter 1 in. +0/0.005 in., and the sample dimensions are standardized by ASTM C633. The Bond Strength is calculated as the Load at sample failure divided on the coating failed area.

Bond Test Step by Step 1. 2. 3. 4. 5. 6.

Inspect coating quality on bond caps. (Coating chipping, cracks, delamination, separ