Thermal spray tensile testing redundancy

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As part of the quality control requirements of thermal sprayed coatings, the adhesive tensile testing procedure is quite extensively used in plasma spray coatings and hvof spray coatings. In the thermal spray tensile testing procedure, tensile bond slugs are to be thermalsprayed using the same parameters as normally used in processing, then glued together with an epoxy which is subsequently cured and then the assembly is pulled apart in a tensile testing machine thus determining the bond strength of the thermal spray coating. It is imperative, obviously, to ensure that the epoxy itself pulls to the specified tensile requirements, otherwise the test results may be suspect. Tensile testing of thermal sprayed coatings is quite an established procedure and is widely used in several aerospace and non-aerospace applications of flame spray coatings quality control aspects. Thermal spray tensile test results in multi layer coatings can give insight into the proper bonding of the base coat to the substrate as well as bonding between base coat and top coat in dual coating systems. By nature, tensile test results for thermal sprayed coatings are highly amenable to statistical charting and hence can provide an additional tool for optimum process control. An added feature of the tensile test procedure which is widely used in releasing powders for plasma and hvof processes which allows for proper raw material control.

What is intriguing is the argument that I have heard over and over again to the effect that “Why should I perform tensile testing routinely in my thermal spray operations, since I cannot even remember the last time I failed such a test when the microstructure was acceptable.” I have also come across situations wherein this argument has gone beyond the argument stage and thermal spray quality control technicians have tended to postpone tensile testing and shipped hardware in the hopes that tensile testing will pass anyway and naive customers have allowed their suppliers to do so by way of providing release forms to allow them ship ahead of test results. Granted, tensile testing cannot be performed as rapidly as other testing procedures, but customers need to account for the added time in their production planning lead time cycles. Shipping hardware pending lab results is something that has never sat well with me for the following reason. Once the hardware is shipped, the customer assembles it in his operations, in a jet engine if it happens to be an aerospace situation, and then subsequent assemblies are carried out and then if the tensile test fails, the suspect hardware will have to be disassembled and replaced with a new component thereby spending more money rather than taking the simple alternative of assembling only after all testing results have proven satisfactory.

Additionally, coming back to the argument of never having failed a tensile test when the metallurgical microstructure has been satisfactory holds weaker grounds. Metallurgical microstructure as far as voids and porosity, oxide stringer levels and the like may be acceptable, but there may have been very bad grit blast contamination at the interface which could lead to poor bonding. What about the situation where the grit blast parameters were correct but the operator touched the grit blasted surface with a rag that is contaminated with oil and other foreign contaminants; that could lead to poor bonding. As mentioned above in multi-layered coatings, the cleanliness between bond coat and top coat is also a factor in proper bonding and this attribute may not be observable in an optical metallurgical mount. Hence, considering these factors, the argument that if the hardness levels and microstructural requirements are met, tensile testing should be waived does not hold credibility in my book. Tensile testing of thermal sprayed coatings is not a redundant test by any means and I certainly do not want to fly in an airplane which has components thermal sprayed by those that validate the arguments in favor of the tensile test redundancy group!